Climate Change Impacts 

in the United States

U.S. National Climate Assessment

U.S. Global Change Research Program
i CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Members of the National Guard 

lay sandbags to protect against 

Missouri River flooding.

Energy choices will affect the 

amount of future climate change.

Climate change is contributing 

to an increase in wildfires across 

the U.S. West.

Solar power use is increasing 

and is part of the solution to climate change.

Observed U.S. Temperature Change

The colors on the map show temperature changes over the past 22 years (1991-2012) compared to the 1901-1960 average for the contiguous 

U.S., and to the 1951-1980 average for Alaska and Hawaii. The bars on the graph show the average temperature changes for the U.S. by 

decade for 1901-2012 (relative to the 1901-1960 average). The far right bar (2000s decade) includes 2011 and 2012. The period from 2001 to 

2012 was warmer than any previous decade in every region. (Figure source: NOAA NCDC / CICS-NC). DoD photo by Staff Sgt. Michael Crane, U.S. Air Force ©Jim West/imagebroker/Corbis

©AP Photo/The Press-Enterprise/Terry Pierson

©Dennis Schroeder, NREL

Climate Change Impacts 

in the United States
ii CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Recommended Citation

Melillo, Jerry M., Terese (T.C.) Richmond, and Gary W. Yohe, Eds., 2014: Climate Change Impacts in the United States: 

The Third National Climate Assessment. U.S. Global Change Research Program, 841 pp. doi:10.7930/J0Z31WJ2.

This report was produced by an advisory committee chartered under the Federal Advisory 

Committee Act, for the Subcommittee on Global Change Research, and at the request of the 

U.S. Government. Therefore, the report is in the public domain. Some materials used in the 

report are copyrighted and permission was granted to the U.S. government for their publication 

in this report. For subsequent uses that include such copyrighted materials, permission for 

reproduction must be sought from the copyright holder. In all cases, credit must be given for 

copyrighted materials.

First published May 2014. PDF revised October 2014. See errata (available at http://nca2014.

globalchange.gov/downloads) for details.

Published in the United States of America

ISBN 9780160924026

Online at:

nca2014.globalchange.gov

Published by the U.S. Government Printing Office

Internet: bookstore.gpo.gov; Phone: toll free (866) 512-1800; DC area (202) 512-1800

Fax: (202) 512-2104 Mail: Stop IDCC, Washington, DC 20402-0001
iii CLIMATE CHANGE IMPACTS IN THE UNITED STATES

May 2014

Members of Congress:

On behalf of the National Science and Technology Council and the U.S. Global Change Research Program, we are pleased 

to transmit the report of the Third National Climate Assessment: Climate Change Impacts in the United States. As required by 

the Global Change Research Act of 1990, this report has collected, evaluated, and integrated observations and research on 

climate change in the United States. It focuses both on changes that are happening now and further changes that we can 

expect to see throughout this century.

This report is the result of a three-year analytical effort by a team of over 300 experts, overseen by a broadly constituted Federal 

Advisory Committee of 60 members. It was developed from information and analyses gathered in over 70 workshops and 

listening sessions held across the country. It was subjected to extensive review by the public and by scientific experts in and 

out of government, including a special panel of the National Research Council of the National Academy of Sciences. This 

process of unprecedented rigor and transparency was undertaken so that the findings of the National Climate Assessment 

would rest on the firmest possible base of expert judgment.

We gratefully acknowledge the authors, reviewers, and staff who have helped prepare this Third National Climate 

Assessment. Their work in assessing the rapid advances in our knowledge of climate science over the past several years has 

been outstanding. Their findings and key messages not only describe the current state of that science but also the current and 

future impacts of climate change on major U.S. regions and key sectors of the U.S. economy. This information establishes 

a strong base that government at all levels of U.S. society can use in responding to the twin challenges of changing our 

policies to mitigate further climate change and preparing for the consequences of the climate changes that can no longer be 

avoided. It is also an important scientific resource to empower communities, businesses, citizens, and decision makers with 

information they need to prepare for and build resilience to the impacts of climate change.

When President Obama launched his Climate Action Plan last year, he made clear that the essential information contained 

in this report would be used by the Executive Branch to underpin future policies and decisions to better understand and 

manage the risks of climate change. We strongly and respectfully urge others to do the same.

 

 

 Sincerely,

Dr. John P. Holdren 

Assistant to the President for Science and Technology 

Director, Office of Science and Technology Policy

Executive Office of the President 

Dr. Kathryn D. Sullivan

Under Secretary for Oceans and Atmosphere 

NOAA Administrator

U.S. Department of Commerce
iv CLIMATE CHANGE IMPACTS IN THE UNITED STATES

The National Climate Assessment assesses the science of climate change 

and its impacts across the United States, now and throughout this century. 

It documents climate change related impacts and responses for various 

sectors and regions, with the goal of better informing public and private 

decision-making at all levels. 

A team of more than 300 experts (see page 98), guided by a 60-member 

National Climate Assessment and Development Advisory Committee 

(listed on page vi) produced the full report – the largest and most diverse 

team to produce a U.S. climate assessment. Stakeholders involved in the 

development of the assessment included decision-makers from the public 

and private sectors, resource and environmental managers, researchers, 

representatives from businesses and non-governmental organizations, and 

the general public. More than 70 workshops and listening sessions were 

held, and thousands of public and expert comments on the draft report 

provided additional input to the process. 

The assessment draws from a large body of scientific peer-reviewed 

research, technical input reports, and other publicly available sources; all 

sources meet the standards of the Information Quality Act. The report was 

extensively reviewed by the public and experts, including a panel of the 

National Academy of Sciences, the 13 Federal agencies of the U.S. Global 

Change Research Program, and the Federal Committee on Environment, 

Natural Resources, and Sustainability.

NATIONAL CLIMATE ASSESSMENT

About the

HIGHLIGHTS

About the

The Highlights presents the major findings and selected highlights 

from Climate Change Impacts in the United States, the third National 

Climate Assessment.

The Highlights report is organized around the National Climate 

Assessment’s 12 Report Findings, which take an overarching view of 

the entire report and its 30 chapters. All material in the Highlights

report is drawn from the full report. The Key Messages from each of 

the 30 report chapters appear in boxes throughout this document. 

A 20-page Overview booklet is available online.

Online at:

nca2014.globalchange.gov

Climate Change Impacts 

in the United States

U.S. National Climate Assessment U.S. Global Change Research Program

Online at:

nca2014.globalchange.gov/highlights
CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Chair

Jerry Melillo, Marine Biological Laboratory

Vice-Chairs

Terese (T.C.) Richmond, Van Ness Feldman, LLP

Gary Yohe, Wesleyan University

Committee Members

Daniel Abbasi, GameChange Capital, LLC

E. Virginia Armbrust, University of Washington

Timothy (Bull) Bennett, Kiksapa Consulting, LLC

Rosina Bierbaum, University of Michigan and PCAST

Maria Blair, Independent

James Buizer, University of Arizona

Lynne M. Carter, Louisiana State University

F. Stuart Chapin III, University of Alaska

Camille Coley, Florida Atlantic University

Jan Dell, ConocoPhillips

Placido dos Santos, WestLand Resources, Inc.

Paul Fleming, Seattle Public Utilities

Guido Franco, California Energy Commission

Mary Gade, Gade Environmental Group 

Aris Georgakakos, Georgia Institute of Technology

David Gustafson, Monsanto Company

David Hales, Second Nature 

Sharon Hays, Computer Sciences Corporation

Mark Howden, CSIRO

Anthony Janetos, Boston University

Peter Kareiva, The Nature Conservancy

Rattan Lal, Ohio State University

Arthur Lee, Chevron Corporation

Jo-Ann Leong, Hawai‘i Institute of Marine Biology

Diana Liverman, University of Arizona and Oxford University

Rezaul Mahmood, Western Kentucky University

Edward Maibach, George Mason University

Michael McGeehin, RTI International

Susanne C. Moser, Susanne Moser Research & Consulting and 

Stanford University

Richard Moss, University of Maryland and PNNL

Philip Mote, Oregon State University

Jayantha Obeysekera, South Florida Water Management District

Marie O’Neill, University of Michigan

Lindene Patton, Zurich Financial Services

John Posey, East-West Gateway Council of Governments

Sara Pryor, Indiana University

Andrew Rosenberg, University of New Hampshire and Union of 

Concerned Scientists

Richard Schmalensee, Massachusetts Institute of Technology

Henry Schwartz, HGS Consultants, LLC

Joel Smith, Stratus Consulting

Donald Wuebbles, University of Illinois

Ex Officio Committee Members

Ko Barrett, U.S. Department of Commerce 

Katharine Batten, U.S. Agency for International Development 

Virginia Burkett, U.S. Department of the Interior 

Patricia Cogswell, U.S. Department of Homeland Security 

Gerald Geernaert, U.S. Department of Energy 

John Hall, U.S. Department of Defense 

Leonard Hirsch, Smithsonian Institution 

William Hohenstein, U.S. Department of Agriculture 

Patricia Jacobberger-Jellison, National Aeronautics and Space 

 Administration 

Thomas R. Karl, Subcommittee on Global Change Research, U.S. 

Department of Commerce

George Luber, U.S. Department of Health and Human Services 

C. Andrew Miller, U.S. Environmental Protection Agency 

Robert O’Connor, National Science Foundation 

Susan Ruffo, White House Council on Environmental Quality 

Arthur Rypinski, U.S. Department of Transportation

Trigg Talley, U.S. Department of State 

Designated Federal Officers

Cynthia J. Decker, NOAA

Kandis Y. Wyatt, NOAA

Federal National Climate Assessment and Development Advisory Committee (NCADAC) 

John Holdren, Assistant to the President for Science and Technology 

	 and Director, White House Office of Science and Technology Policy

Katharine Jacobs, Director, National Climate Assessment, White House 

	 Office of Science and Technology Policy (through December 2013)	

Thomas Armstrong, Director, U.S. Global Change Research Program 

	 National Coordination Office, White House Office of Science and 	 	

Technology Policy

Thomas R. Karl, Chair, Subcommittee on Global Change Research, 

U.S. Department of Commerce

Tamara Dickinson, Principal Assistant Director for Environment and 

	 Energy, White House Office of Science and Technology Policy

Fabien Laurier, Director, Third National Climate Assessment, White 

	 House Office of Science and Technology Policy

Glynis C. Lough, NCA Chief of Staff, U.S. Global Change Research 

Program

David Easterling, NCA Technical Support Unit Director, NOAA NCDC

Federal Executive Team

v
vi CLIMATE CHANGE IMPACTS IN THE UNITED STATES

National Climate Assessment Staff

USGCRP National Climate Assessment Coordination 

Office

Katharine Jacobs, Director, National Climate Assessment, White House Office of 

	 Science and Technology Policy (OSTP) (through December 2013) / University

of Arizona

Fabien Laurier, Director, Third National Climate Assessment, White House

	 OSTP (previously Deputy Director, USGCRP) (from December 2013)

Glynis Lough, NCA Chief of Staff, USGCRP / UCAR (from June 2012)

Sheila O’Brien, NCA Chief of Staff, USGCRP / UCAR (through May 2012)

Susan Aragon-Long, NCA Senior Scientist and Sector Coordinator, U.S.

Geological Survey 

Ralph Cantral, NCA Senior Scientist and Sector Coordinator, NOAA 

(through November 2012)

Tess Carter, Student Assistant, Brown University

Emily Therese Cloyd, NCA Public Participation and Engagement Coordinator, 

USGCRP / UCAR

Chelsea Combest-Friedman, NCA International Coordinator, Knauss Marine 

Policy Fellow, NOAA (February 2011-February 2012)

Alison Delgado, NCA Scientist and Sector Coordinator, Pacific Northwest 	 	

National Laboratory, Joint Global Change Research Institute, University of 

Maryland (from October 2012)

William Emanuel, NCA Senior Scientist and Sector Coordinator, Pacific 	 	

Northwest National Laboratory, Joint Global Change Research Institute, 

University of Maryland (June 2011-September 2012)

Matt Erickson, Student Assistant, Washington State University 

(July-October 2012)

Ilya Fischhoff, NCA Program Coordinator, USGCRP / UCAR 

Elizabeth Fly, NCA Coastal Coordinator, Knauss Marine Policy Fellow, NOAA 

	 (February 2013-January 2014)

Chelcy Ford, NCA Sector Coordinator, USFS (August-November 2011)

Wyatt Freeman, Student Assistant, George Mason University / UCAR 

(May-September 2012)

Bryce Golden-Chen, NCA Program Coordinator, USGCRP / UCAR

Nancy Grimm, NCA Senior Scientist and Sector Coordinator, NSF / Arizona 

State University (July 2011-September 2012)

Tess Hart, NCA Communications Assistant, USGCRP / UCAR (June-July 2011)

Melissa Kenney, NCA Indicators Coordinator, NOAA / University of Maryland

Fredric Lipschultz, NCA Senior Scientist and Regional Coordinator, NASA / 

Bermuda Institute of Ocean Sciences

Stuart Luther, Student Assistant, Arizona State University / UCAR 

(June-August 2011)

Julie Maldonado, NCA Engagement Assistant and Tribal Coordinator, 

USGCRP / UCAR

Krista Mantsch, Student Assistant, Indiana University / UCAR 

	 (May-September 2013)

Rebecca Martin, Student Assistant, Washington State University 

(June-August 2012)

Paul Schramm, NCA Sector Coordinator, Centers for Disease Control and 

Prevention (June-November 2010)

Technical Support Unit, National Climatic Data 

Center, NOAA/NESDIS 

David Easterling, NCA Technical Support Unit Director, NOAA National Climatic 

	 Data Center (from March 2013)

Anne Waple, NCA Technical Support Unit Director, NOAA NCDC / UCAR 

	 (through February 2013)

Susan Joy Hassol, Senior Science Writer, Climate Communication, LLC / 

Cooperative Institute for Climate and Satellites, North Carolina State University 

(CICS-NC) 

Paula Ann Hennon, NCA Technical Support Unit Deputy Director, CICS-NC

Kenneth Kunkel, Chief Scientist, CICS-NC 

Sara W. Veasey, Creative Director, NOAA NCDC 

Andrew Buddenberg, Software Engineer/Scientific Programmer, CICS-NC

Fred Burnett, Administrative Assistant, Jamison Professional Services, Inc. 

Sarah Champion, Scientific Data Curator and Process Analyst, CICS-NC

Doreen DiCarlo, Program Coordinator, CICS-NC (August 2011-April 2012)

Daniel Glick, Editor, CICS-NC

Jessicca Griffin, Lead Graphic Designer, CICS-NC

John Keck, Web Consultant, LMI, Inc. (August 2010 - September 2011)

Angel Li, Web Developer, CICS-NC

Clark Lind, Administrative Assistant, The Baldwin Group, Inc. 

(January-September 2012)

Liz Love-Brotak, Graphic Designer, NOAA NCDC

Tom Maycock, Technical Editor, CICS-NC

Janice Mills, Business Manager, CICS-NC

Deb Misch, Graphic Designer, Jamison Professional Services, Inc.

Julie Moore, Administrative Assistant, The Baldwin Group, Inc. 

(June 2010-January 2012)

Ana Pinheiro-Privette, Data Coordinator, CICS-NC (January 2012-July 2013)

Deborah B. Riddle, Graphic Designer, NOAA NCDC

April Sides, Web Developer, ERT, Inc. 

Laura E. Stevens, Research Scientist, CICS-NC

Scott Stevens, Support Scientist, CICS-NC

Brooke Stewart, Science Editor/Production Coordinator, CICS-NC

Liqiang Sun, Research Scientist/Modeling Support, CICS-NC

Robert Taylor, Student Assistant, UNC Asheville, CICS-NC

Devin Thomas, Metadata Specialist, ERT, Inc.

Teresa Young, Print Specialist, Team ERT/STG, Inc.

Review Editors 

Joseph Arvai, University of Calgary

Peter Backlund, University Corporation for Atmospheric Research

Lawrence Band, University of North Carolina

Jill S. Baron, U.S. Geological Survey / Colorado State University

Michelle L. Bell, Yale University

Donald Boesch, University of Maryland

Joel R. Brown, New Mexico State University

Ingrid C. (Indy) Burke, University of Wyoming

Gina Campoli, Vermont Agency of Transportation
vii CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Mary Anne Carroll, University of Michigan

Scott L. Collins, University of New Mexico

John Daigle, University of Maine

Ruth DeFries, Columbia University

Lisa Dilling, University of Colorado

Otto C. Doering III, Purdue University

Hadi Dowlatabadi, University of British Columbia

Charles T. Driscoll, Syracuse University

Hallie C. Eakin, Arizona State University

John Farrington, Woods Hole Oceanographic Institution

Chris E. Forest, Pennsylvania State University

Efi Foufoula-Georgiou, University of Minnesota

Adam Freed, The Nature Conservancy

Robert Fri, Resources for the Future

Stephen T. Gray, U.S. Geological Survey

Jay Gulledge, Oak Ridge National Laboratory

Terrie Klinger, University of Washington

Ian Kraucunas, Pacific Northwest National Laboratory

Larissa Larsen, University of Michigan

William J. Massman, U.S. Forest Service

Michael D. Mastrandrea, Stanford University

Pamela Matson, Stanford University

Ronald G. Prinn, Massachusetts Institute of Technology

J.C. Randolph, Indiana University

G. Philip Robertson, Michigan State University

David Robinson, Rutgers University

Dork Sahagian, Lehigh University

Christopher A. Scott, University of Arizona

Peter Vitousek, Stanford University

Andrew C. Wood, NOAA

United States Global Change Research Program

Thomas Armstrong (OSTP), Executive Director, USGCRP

Chris Weaver (OSTP / EPA), Deputy Executive Director, USGCRP

Subcommittee on Global Change Research 

Chair

Thomas Karl, U.S. Department of Commerce

Vice Chairs

Ann Bartuska, U.S. Department of Agriculture, Vice Chair, Adaptation Science

Gerald Geernaert, U.S. Department of Energy, Vice Chair, Integrated Modeling

Mike Freilich, National Aeronautics and Space Administration, Vice Chair, 

Integrated Observations

Roger Wakimoto, National Science Foundation, Vice-Chair 

Principals

John Balbus, U.S. Department of Health and Human Services

Katharine Batten, U.S. Agency for International Development

Joel Clement, U.S. Department of the Interior

Robert Detrick, U.S. Department of Commerce 

Scott L. Harper, U.S. Department of Defense

Leonard Hirsch, Smithsonian Institution

William Hohenstein, U.S. Department of Agriculture

Jack Kaye, National Aeronautics and Space Administration

Michael Kuperberg, U.S. Department of Energy

C. Andrew Miller, U.S. Environmental Protection Agency

Arthur Rypinski, U.S. Department of Transportation

Joann Roskoski, National Science Foundation

Trigg Talley, U.S. Department of State

Interagency National Climate Assessment 

Working Group 

Chair

Katharine Jacobs, White House Office of Science and Technology Policy 	 	

	 (through December 2013)

Fabien Laurier, White House Office of Science and Technology Policy

	 (from December 2013)

Vice-Chair

Virginia Burkett, U.S. Department of the Interior – U.S. Geological 

	 Survey (from March 2013)

Anne Waple, NOAA NCDC / UCAR (through February 2013)

National Aeronautics and Space Administration

Allison Leidner, Earth Science Division / Universities Space Research 

Association

National Science Foundation

Anjuli Bamzai, Directorate for Geosciences (through May 2011)

Eve Gruntfest, Directorate for Geosciences (January-November 2013)

Rita Teutonico, Directorate for Social, Behavioral, and Economic Sciences 

(through January 2011)

Smithsonian Institution

Leonard Hirsch, Office of the Undersecretary for Science

U.S. Department of Agriculture

Linda Langner, U.S. Forest Service (through January 2011)

Carolyn Olson, Office of the Chief Economist

Toral Patel-Weynand, U.S. Forest Service

Louie Tupas, National Institute of Food and Agriculture

Margaret Walsh, Office of the Chief Economist

U.S. Department of Commerce

Ko Barrett, National Oceanic and Atmospheric Administration 

	 (from February 2013)

David Easterling, National Oceanic and Atmospheric Administration – National 

	 Climatic Data Center (from March 2013)

Nancy McNabb, National Institute of Standards and Technology 

	 (from February 2013)

Adam Parris, National Oceanic and Atmospheric Administration

Anne Waple, NOAA NCDC / UCAR (through February 2013)
viii CLIMATE CHANGE IMPACTS IN THE UNITED STATES

U.S. Department of Defense

William Goran, U.S. Army Corps of Engineers

John Hall, Office of the Secretary of Defense

Katherine Nixon, Navy Task Force Climate Change (from May 2013)

Courtney St. John, Navy Task Force Climate Change (through August 2012)

U.S. Department of Energy

Robert Vallario, Office of Science

U.S. Department of Health and Human Services

John Balbus, National Institutes of Health

Paul Schramm, Centers for Disease Control and Prevention (through July 2011)

U.S. Department of Homeland Security

Mike Kangior, Office of Policy (from November 2011)

John Laws, National Protection and Programs Directorate (from May 2013)

U.S. Department of the Interior

Susan Aragon-Long, U.S. Geological Survey

Virginia Burkett, U.S. Geological Survey

Leigh Welling, National Park Service (through May 2011)

U.S. Department of State

David Reidmiller, Bureau of Oceans and International Environmental 

	 & Scientific Affairs

Kenli Kim, Bureau of Oceans and International Environmental 

	 & Scientific Affairs (from February 2013)

U.S. Department of Transportation

Arthur Rypinski, Office of the Secretary

Mike Savonis, Federal Highway Administration (through March 2011)

AJ Singletary, Office of the Secretary (through August 2010)

U.S. Environmental Protection Agency

Rona Birnbaum, Office of Air and Radiation

Anne Grambsch, Office of Research and Development

Lesley Jantarasami, Office of Air and Radiation

White House Council on Environmental Quality

Jeff Peterson (through July 2013)

Jamie Pool (from February 2013)

White House Office of Management and Budget

Stuart Levenbach (through May 2012)

White House Office of Science and Technology Policy

Katharine Jacobs, Environment and Energy Division (through December 2013)

Fabien Laurier, Environment and Energy Division (from December 2013)

With special thanks to former NOAA Administrator, Jane Lubchenco and former 

Associate Director of the Office of Science and Technology Policy, Shere Abbott
ix CLIMATE CHANGE IMPACTS IN THE UNITED STATES

CONTENTS ©Dave Martin/AP/AP/Corbis

STRATEGIES........................619

26. Decision Support ...........................620

27. Mitigation .......................................648

28. Adaptation.....................................670

29. Research Needs............................707

30. Sustained Assessment...................719

Climate Change and the

American People...............................1

About This Report ............................3

1. OVERVIEW..................................7

2. OUR CHANGING 

CLIMATE ...................................19

SECTORS ...........................68

3. Water ...........................................69

4. Energy ........................................113

5. Transportation .............................130

6. Agriculture...................................150

7. Forests........................................175

8. Ecosystems ................................195

9. Human Health .............................220

10. Energy, Water, and Land ..............257

11. Urban .........................................282

12. Indigenous Peoples ......................297

13. Land Use and 

Land Cover Change.....................318

14. Rural Communities.......................333

15. Biogeochemical Cycles ................350

REGIONS...............................369

16. Northeast .....................................371

17. Southeast.....................................396

18. Midwest........................................418

19. Great Plains ..................................441

20. Southwest ....................................462

21. Northwest ....................................487

22. Alaska ..........................................514

23. Hawaii and Pacific Islands.............537

24. Oceans.........................................557

25. Coasts..........................................579

APPENDICES

Appendix 1: Process............................727

Appendix 2: Information Quality ...........733

Appendix 3: Climate Science................735

Appendix 4: FAQs................................790

Appendix 5: Scenarios and Models.......821

Appendix 6: Future Assessment 

 Topics ..............................826 

Abbreviations and Acronyms ...............828

©John Fedele/Blend Images/Corbis

RESPONSE
1 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

CLIMATE CHANGE 

AND THE AMERICAN PEOPLE

Climate change, once considered an issue for a distant future, has moved firmly into the present. Corn producers in Iowa, oyster growers in Washington State, and maple syrup producers in Vermont are 

all observing climate-related changes that are outside of recent experience. So, too, are coastal planners 

in Florida, water managers in the arid Southwest, city dwellers from Phoenix to New York, and Native 

Peoples on tribal lands from Louisiana to Alaska. This National Climate Assessment concludes that the 

evidence of human-induced climate change continues to strengthen and that impacts are increasing 

across the country.

Americans are noticing changes all around them. Summers are longer and hotter, and extended periods 

of unusual heat last longer than any living American has ever experienced. Winters are generally shorter 

and warmer. Rain comes in heavier downpours. People are seeing changes in the length and severity of 

seasonal allergies, the plant varieties that thrive in their gardens, and the kinds of birds they see in any 

particular month in their neighborhoods. 

Other changes are even more dramatic. Residents of some coastal cities see their streets flood more 

regularly during storms and high tides. Inland cities near large rivers also experience more flooding, 

especially in the Midwest and Northeast. Insurance rates are rising in some vulnerable locations, and 

insurance is no longer available in others. Hotter and drier weather and earlier snowmelt mean that 

wildfires in the West start earlier in the spring, last later into the fall, and burn more acreage. In Arctic 

Alaska, the summer sea ice that once protected the coasts has receded, and autumn storms now cause 

more erosion, threatening many communities with relocation. 

Scientists who study climate change confirm that these observations are consistent with significant 

changes in Earth’s climatic trends. Long-term, independent records from weather stations, satellites, 

ocean buoys, tide gauges, and many other data sources all confirm that our nation, like the rest of the 

world, is warming. Precipitation patterns are changing, sea level is rising, the oceans are becoming more 

acidic, and the frequency and intensity of some extreme weather events are increasing. Many lines of 

independent evidence demonstrate that the rapid warming of the past half-century is due primarily to 

human activities. 

The observed warming and other climatic changes are triggering wide-ranging 

impacts in every region of our country and throughout our economy. Some of 

these changes can be beneficial over the short run, such as a longer growing 

season in some regions and a longer shipping season on the Great Lakes. But 

many more are detrimental, largely because our society and its infrastructure 

were designed for the climate that we have had, not the rapidly changing 

climate we now have and can expect in the future. In addition, climate change 

does not occur in isolation. Rather, it is superimposed on other stresses, 

which combine to create new challenges. 

©Ted Wood Photography
CLIMATE CHANGE AND THE AMERCIAN PEOPLE

2 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

This National Climate Assessment collects, integrates, and assesses 

observations and research from around the country, helping us to see 

what is actually happening and understand what it means for our lives, 

our livelihoods, and our future. This report includes analyses of impacts on 

seven sectors – human health, water, energy, transportation, agriculture, 

forests, and ecosystems – and the interactions among sectors at the 

national level. This report also assesses key impacts on all U.S. regions: 

Northeast, Southeast and Caribbean, Midwest, Great Plains, Southwest, 

Northwest, Alaska, Hawai‘i and the Pacific Islands, as well as the country’s 

coastal areas, oceans, and marine resources. 

Over recent decades, climate science has advanced significantly. Increased scrutiny has led to increased 

certainty that we are now seeing impacts associated with human-induced climate change. With each 

passing year, the accumulating evidence further expands our understanding and extends the record of 

observed trends in temperature, precipitation, sea level, ice mass, and many other variables recorded 

by a variety of measuring systems and analyzed by independent research groups from around the 

world. It is notable that as these data records have grown longer and climate models have become 

more comprehensive, earlier predictions have largely been confirmed. The only real surprises have been 

that some changes, such as sea level rise and Arctic sea ice decline, have outpaced earlier projections. 

What is new over the last decade is that we know with increasing certainty that climate change is 

happening now. While scientists continue to refine projections of the future, observations unequivocally 

show that climate is changing and that the warming of the past 50 years is primarily due to humaninduced emissions of heat-trapping gases. These emissions come mainly from burning coal, oil, and gas, 

with additional contributions from forest clearing and some agricultural practices. 

Global climate is projected to continue to change over this century and beyond, but there is still time to 

act to limit the amount of change and the extent of damaging impacts. 

This report documents the changes already 

observed and those projected for the 

future. It is important that these findings 

and response options be shared broadly to 

inform citizens and communities across our 

nation. Climate change presents a major 

challenge for society. This report advances 

our understanding of that challenge and 

the need for the American people to 

prepare for and respond to its far-reaching 

implications. 
3 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

ABOUT THIS REPORT

This report assesses the science of climate change and its impacts across the United States, now and throughout this century. 

It integrates findings of the U.S. Global Change Research Program 

(USGCRP)a with the results of research and observations from 

across the U.S. and around the world, including reports from the 

U.S. National Research Council. This report documents climate 

change related impacts and responses for various sectors and 

regions, with the goal of better informing public and private decision-making at all levels. 

REPORT REQUIREMENTS, PRODUCTION, AND APPROVAL

The Global Change Research Act1 requires that, every four years, 

the USGCRP prepare and submit to the President and Congress 

an assessment of the effects of global change in the United 

States. As part of this assessment, more than 70 workshops were 

held involving a wide range of stakeholders who identified issues 

and information for inclusion (see Appendix 1: Process). A team 

of more than 300 experts was involved in writing this report. Authors were appointed by the National Climate Assessment and 

Development Advisory Committee (NCADAC),b the federal advisory committee assembled for the purpose of conducting this 

assessment. The report was extensively reviewed and revised 

based on comments from the public and experts, including a 

panel of the National Academy of Sciences. The report was reviewed and approved by the USGCRP agencies and the federal 

Committee on Environment, Natural Resources, and Sustainability (CENRS). This report meets all federal requirements associated 

with the Information Quality Act (see Appendix 2: IQA), including 

those pertaining to public comment and transparency.

REPORT SOURCES

The report draws from a large body of scientific, peer-reviewed 

research, as well as a number of other publicly available sources. 

Author teams carefully reviewed these sources to ensure a reliable assessment of the state of scientific understanding. Each 

source of information was determined to meet the four parts of 

the IQA Guidance provided to authors: 1) utility, 2) transparency 

and traceability, 3) objectivity, and 4) integrity and security (see 

Appendix 2: IQA). Report authors made use of technical input reports produced by federal agencies and other interested parties 

in response to a request for information by the NCADAC;2 other peer-reviewed scientific assessments (including those of the 

Intergovernmental Panel on Climate Change); the U.S. National 

Climate Assessment’s 2009 report titled Global Climate Change 

Impacts in the United States;3 the National Academy of Science’s 

America’s Climate Choices reports;4 a variety of regional climate 

impact assessments, conference proceedings, and government 

statistics (such as population census and energy usage); and observational data. Case studies were also provided as illustrations 

of climate impacts and adaptation programs. 

 

a The USGCRP is made up of 13 Federal departments and agencies that carry out research and support the nation’s response to global change The 

USGCRP is overseen by the Subcommittee on Global Change Research (SGCR) of the National Science and Technology Council’s Committee on 

Environment, Natural Resources and Sustainability (CENRS), which in turn is overseen by the White House Office of Science and Technology Policy 

(OSTP). The agencies within USGCRP are: the Department of Agriculture, the Department of Commerce (NOAA), the Department of Defense, the 

Department of Energy, the Department of Health and Human Services, the Department of the Interior, the Department of State, the Department 

of Transportation, the Environmental Protection Agency, the National Aeronautics and Space Administration, the National Science Foundation, the 

Smithsonian Institution, and the U.S. Agency for International Development.

 

b The NCADAC is a federal advisory committee sponsored by the National Oceanic and Atmospheric Administration under the requirements of the 

Federal Advisory Committee Act.
4 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

ABOUT THIS REPORT

OVERARCHING PERSPECTIVES

Four overarching perspectives, derived from decades of observations, analysis, and experience, have helped to shape 

this report: 1) climate change is happening in the context of 

other ongoing changes across the U.S. and the globe; 2) climate change impacts can either be amplified or reduced by 

societal decisions; 3) climate change related impacts, vulnerabilities, and opportunities in the U.S. are linked to impacts 

and changes outside the United States, and vice versa; and 4) 

climate change can lead to dramatic tipping points in natural 

and social systems. These overarching perspectives are briefly 

discussed below.

Global Change Context 

Climate change is one of a number of global changes affecting 

society, the environment, and the economy; others include 

population growth, land-use change, air and water pollution, 

and rising consumption of resources by a growing and wealthier 

global population. This perspective has implications for assessments of climate change impacts and the design of research 

questions at the national, regional, and local scales. This assessment explores some of the consequences of interacting factors 

by focusing on sets of crosscutting issues in a series of six chapters: Energy, Water, and Land Use; Biogeochemical Cycles; Indigenous Peoples, Lands, and Resources; Urban Systems, Infrastructure, and Vulnerability; Land Use and Land Cover Change; 

and Rural Communities. The assessment also includes discussions of how climate change impacts cascade through different 

sectors such as water and energy, and affect and are affected 

by land-use decisions. These and other interconnections greatly stress society’s capacity to respond to climate-related crises 

that occur simultaneously or in rapid sequence.

A guide to the report

The report has eight major sections, outlined below:

• Overview and Report Findings: gives a high-level perspective on the full National Climate Assessment and sets out 

the report’s 12 key findings. The Overview synthesizes and summarizes the ideas that the authors consider to be 

of greatest importance to the American people.

• Our Changing Climate: presents recent advances in climate change science, which includes discussions of 

extreme weather events, observed and projected changes in temperature and precipitation, and the uncertainties 

associated with these projections. Substantial additional material related to this chapter can be found in the 

Appendices.

• Sectors: focuses on climate change impacts for seven societal and environmental sectors: human health, water, 

energy, transportation, agriculture, forests, and ecosystems and biodiversity; six additional chapters consider the 

interactions among sectors (such as energy, water, and land use) in the context of a changing climate.

• Regions: assesses key impacts on U.S. regions – Northeast, Southeast and Caribbean, Midwest, Great Plains, 

Southwest, Northwest, Alaska, and Hawai‘i and the U.S. affiliated Pacific Islands – as well as coastal areas, 

oceans, and marine resources. 

• Responses: assesses the current state of responses to climate change, including adaptation, mitigation, and 

decision support activities. 

• Research Needs: highlights major gaps in science and research to improve future assessments. New research is 

called for in climate science in support of assessments, climate impacts in regions and sectors, and adaptation, 

mitigation, and decision support. 

• Sustained Assessment Process: describes an initial vision for and components of an ongoing, long-term 

assessment process. 

• Appendices: Appendix 1 describes key aspects of the report process, with a focus on engagement; Appendix 

2 describes the guidelines used in meeting the terms of the Federal Information Quality Act; Appendix 3 

supplements the chapter on Our Changing Climate with an extended treatment of selected science issues; 

Appendix 4 provides answers to Frequently Asked Questions about climate change; Appendix 5 describes 

scenarios and models used in this assessment; and Appendix 6 describes possible topics for consideration in 

future assessments.
5 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

ABOUT THIS REPORT

Societal Choices

Because environmental, cultural, and socioeconomic systems 

are tightly coupled, climate change impacts can either be amplified or reduced by cultural and socioeconomic decisions. In 

many arenas, it is clear that societal decisions have substantial 

influence on the vulnerability of valued resources to climate 

change. For example, rapid population growth and development in coastal areas tends to amplify climate change related 

impacts. Recognition of these couplings, together with recognition of multiple sources of vulnerability, helps identify what 

information decision-makers need as they manage risks. 

International Context 

Climate change is a global phenomenon; the causes and the 

impacts involve energy-use, economic, and risk-management 

decisions across the globe. Impacts, vulnerabilities, and opportunities in the U.S. are related in complex and interactive 

ways with changes outside the United States, and vice versa. 

In order for U.S. concerns related to climate change to be addressed comprehensively, the international context must be 

considered. Foreign assistance, health, environmental quality 

objectives, and economic interests are all affected by climate 

changes experienced in other parts of the world. Although 

there is significantly more work to be done in this area, this 

report identifies some initial implications of global and international trends that can be more fully investigated in future 

assessments. 

Thresholds, Tipping Points, and Surprises

While some climate changes will occur slowly and relatively 

gradually, others could be rapid and dramatic, leading to unexpected breaking points in natural and social systems. Although 

they have potentially large impacts, these breaking points or 

tipping points are difficult to predict, as there are many uncertainties about future conditions. These uncertainties and 

potential surprises come from a number of sources, including 

insufficient data associated with low probability/high consequence events, models that are not yet able to represent all 

the interactions of multiple stresses, incomplete understanding of physical climate mechanisms related to tipping points, 

and a multitude of issues associated with human behavior, 

risk management, and decision-making. Improving our ability 

to anticipate thresholds and tipping points can be helpful in 

developing effective climate change mitigation and adaptation strategies (Ch. 2: Our Changing Climate; Ch. 29: Research 

Needs; and Appendices 3 and 4).

RISK MANAGEMENT FRAMEWORK

Authors were asked to consider the science and information 

needs of decision-makers facing climate change risks to infrastructure, natural ecosystems, resources, communities, and 

other things of societal value. They were also asked to consider opportunities that climate change might present. For each 

region and sector, they were asked to assess a small number 

of key climate-related vulnerabilities of concern based on 

the risk (considering likelihood and consequence) of impacts. 

They were also asked to address the most important information needs of stakeholders, and to consider the decisions 

stakeholders are facing. The criteria provided for identifying 

key vulnerabilities in each sector or region included magnitude, timing, persistence/reversibility, scale, and distribution 

of impacts, likelihood whenever possible, importance of impacts (based on the perceptions of relevant parties), and the 

potential for adaptation. Authors were encouraged to think 

about these topics from both a quantitative and qualitative 

perspective and to consider the influence of multiple stresses 

whenever possible. 

RESPONDING TO CLIMATE CHANGE

While the primary focus of this report is on the impacts of climate change in the United States, it also documents some of 

the actions society is taking or can take to respond. Responses 

to climate change fall into two broad categories. The first involves “mitigation” measures to reduce future climate change 

by reducing emissions of heat-trapping gases and particles, or 

increasing removal of carbon dioxide from the atmosphere. 

The second involves “adaptation” measures to improve society’s ability to cope with or avoid harmful impacts and take 

advantage of beneficial ones, now and in the future. At this 

point, both of these response activities are necessary to limit 

the magnitude and impacts of global climate change on the 

United States. 
6 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

ABOUT THIS REPORT

More effective mitigation measures can reduce the amount 

of climate change, and therefore reduce the need for future 

adaptation. This report underscores the effects of mitigation 

measures by comparing impacts resulting from higher versus lower emissions scenarios. This shows that choices made 

about emissions in the next few decades will have far-reaching consequences for climate change impacts throughout this 

century. Lower emissions will reduce the rate and lessen the 

magnitude of climate change and its impacts. Higher emissions 

will do the opposite.

While the report demonstrates the importance of mitigation 

as an essential part of the nation’s climate change strategy, it 

does not evaluate mitigation technologies or policies or undertake an analysis of the effectiveness of various approaches. 

The range of mitigation responses being studied includes, but 

is not limited to, policies and technologies that lead to more efficient production and use of energy, increased use of non-carbon-emitting energy sources such as wind and solar power, 

and carbon capture and storage.

Adaptation actions are complementary to mitigation actions. 

They are focused on moderating harmful impacts of current 

and future climate variability and change and taking advantage 

of possible opportunities. While this report assesses the current state of adaptation actions and planning across the country in a general way, the implementation of adaptive actions 

is still nascent. A comprehensive assessment of actions taken, 

and of their effectiveness, is not yet possible. This report documents some of the actions currently being pursued to address 

impacts such as increased urban heat extremes and air pollution, and describes the challenges decision-makers face in 

planning for and implementing adaptation responses. 

TRACEABLE ACCOUNTS: PROCESS AND CONFIDENCE

The “traceable accounts” that accompany each chapter: 1) 

document the process the authors used to reach the conclusions in their key messages; 2) provide additional information 

to reviewers and other readers about the quality of the information used; 3) allow traceability to resources; and 4) provide 

the level of confidence the authors have in the main findings 

of the chapters. The authors have assessed a wide range of 

information in the scientific literature and various technical 

reports. In assessing confidence, they have considered the 

strength and consistency of the observed evidence, the skill, 

range, and consistency of model projections, and insights from 

peer-reviewed sources.

When it is considered scientifically justified to report the 

likelihood of particular impacts within the range of possible 

outcomes, this report takes a plain-language approach to expressing the expert judgment of the author team based on 

the best available evidence. For example, an outcome termed 

“likely” has at least a two-thirds chance of occurring; an outcome termed “very likely” has more than a 90% chance. Key 

sources of information used to develop these characterizations 

are referenced. 
7 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

OVERVIEW AND 1 REPORT FINDINGS

Climate change is already affecting the American people in farreaching ways. Certain types of extreme weather events with 

links to climate change have become more frequent and/or intense, including prolonged periods of heat, heavy downpours, 

and, in some regions, floods and droughts. In addition, warming is causing sea level to rise and glaciers and Arctic sea ice 

to melt, and oceans are becoming more acidic as they absorb 

carbon dioxide. These and other aspects of climate change are 

disrupting people’s lives and damaging some sectors of our 

economy. 

Climate Change: 

Present and Future

Evidence for climate change abounds, from the top of the 

atmosphere to the depths of the oceans. Scientists and engineers from around the world have meticulously collected this 

evidence, using satellites and networks of weather balloons, 

thermometers, buoys, and other observing systems. Evidence 

of climate change is also visible in the observed and measured 

changes in location and behavior of species and functioning of 

ecosystems. Taken together, this evidence tells an unambiguous story: the planet is warming, and over the last half century, 

this warming has been driven primarily by human activity.

Multiple lines of independent evidence confirm that human 

activities are the primary cause of the global warming of the 

past 50 years. The burning of coal, oil, and gas, and clearing of 

forests have increased the concentration of carbon dioxide in 

the atmosphere by more than 40% since the Industrial Revolution, and it has been known for almost two centuries that this 

carbon dioxide traps heat. Methane and nitrous oxide emissions from agriculture and other human activities add to the 

atmospheric burden of heat-trapping gases. Data show that 

natural factors like the sun and volcanoes cannot have caused 

the warming observed over the past 50 years. Sensors on satellites have measured the sun’s 

output with great accuracy and 

found no overall increase during the past half century. Large 

volcanic eruptions during this 

period, such as Mount Pinatubo 

in 1991, have exerted a shortterm cooling influence. In fact, 

if not for human activities, global climate would actually have 

cooled slightly over the past 50 

years. The pattern of temperature change through the layers 

of the atmosphere, with warming near the surface and cooling 

higher up in the stratosphere, 

further confirms that it is the 

buildup of heat-trapping gases 

(also known as “greenhouse 

gases”) that has caused most 

of the Earth’s warming over the 

past half century. 

Coal-fired power plants emit heat-trapping carbon dioxide to the 

atmosphere.

© Frans Lanting/Corbis

These are just some of the indicators measured globally over many decades that show that the 

Earth’s climate is warming. White arrows indicate increasing trends; black arrows indicate decreasing 

trends. All the indicators expected to increase in a warming world are increasing, and all those 

expected to decrease in a warming world are decreasing. (Figure source: NOAA NCDC, based on 

data updated from Kennedy et al. 2010a).

Ten Indicators of a Warming World
8 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

1: OVERVIEW AND REPORT FINDINGS

Because human-induced warming is superimposed on 

a background of natural variations in climate, warming is not uniform over time. Short-term fluctuations 

in the long-term upward trend are thus natural and 

expected. For example, a recent slowing in the rate of 

surface air temperature rise appears to be related to 

cyclic changes in the oceans and in the sun’s energy 

output, as well as a series of small volcanic eruptions 

and other factors. Nonetheless, global temperatures 

are still on the rise and are expected to rise further.

U.S. average temperature has increased by 1.3°F to 

1.9°F since 1895, and most of this increase has occurred since 1970. The most recent decade was the 

nation’s and the world’s hottest on record, and 2012 

was the hottest year on record in the continental 

United States. All U.S. regions have experienced warming in recent decades, but the extent of warming has 

not been uniform. In general, temperatures are rising 

more quickly in the north. Alaskans have experienced 

some of the largest increases in temperature between 

1970 and the present. People living in the Southeast 

have experienced some of the smallest temperature 

increases over this period.

Temperatures are projected to rise another 2°F to 4°F 

in most areas of the United States over the next few decades. 

Reductions in some short-lived human-induced emissions that 

contribute to warming, such as black carbon (soot) and methane, could reduce some of the projected warming over the 

next couple of decades, because, unlike carbon dioxide, these 

gases and particles have relatively short atmospheric lifetimes.

The amount of warming projected beyond the next few decades is directly linked to the cumulative global emissions of 

heat-trapping gases and particles. By the end of this century, 

a roughly 3°F to 5°F rise is projected under a lower emissions 

scenario, which would require substantial reductions in emissions (referred to as the “B1 scenario”), and a 5°F to 10°F rise 

for a higher emissions scenario assuming continued increases 

in emissions, predominantly from fossil fuel combustion (referred to as the “A2 scenario”). These 

projections are based on results from 

16 climate models that used the two 

emissions scenarios in a formal intermodel comparison study. The range of 

model projections for each emissions 

scenario is the result of the differences 

in the ways the models represent key 

factors such as water vapor, ice and 

snow reflectivity, and clouds, which can 

either dampen or amplify the initial effect of human influences on temperature. The net effect of these feedbacks 

is expected to amplify warming. More 

information about the models and scenarios used in this report can be found 

in Appendix 5 of the full report.1

The green band shows how global average temperature would have changed 

over the last century due to natural forces alone, as simulated by climate 

models. The blue band shows model simulations of the effects of human and 

natural forces (including solar and volcanic activity) combined. The black line 

shows the actual observed global average temperatures. Only with the inclusion of human influences can models reproduce the observed temperature 

changes. (Figure source: adapted from Huber and Knutti 2012b). 

Separating Human and Natural 

Influences on Climate

Different amounts of heat-trapping gases released into the atmosphere by human activities produce different projected increases in 

Earth’s temperature. The lines on the graph 

represent a central estimate of global average temperature rise (relative to the 1901-

1960 average) for the two main scenarios 

used in this report. A2 assumes continued 

increases in emissions throughout this century, and B1 assumes significant emissions 

reductions, though not due explicitly to climate change policies. Shading indicates the 

range (5th to 95th percentile) of results from 

a suite of climate models. In both cases, 

temperatures are expected to rise, although 

the difference between lower and higher 

emissions pathways is substantial. (Figure 

source: NOAA NCDC / CICS-NC).

Projected Global Temperature Change
9 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

1: OVERVIEW AND REPORT FINDINGS

Prolonged periods of high temperatures and the persistence 

of high nighttime temperatures have increased in many locations (especially in urban areas) over the past half century. High 

nighttime temperatures have widespread impacts because 

people, livestock, and wildlife get no respite from the heat. In 

some regions, prolonged periods of high temperatures associated with droughts contribute to conditions that lead to larger 

wildfires and longer fire seasons. As expected in a warming 

climate, recent trends show that extreme heat is becoming 

more common, while extreme cold is becoming less common. 

Evidence indicates that the human influence on climate has already roughly doubled the probability of extreme heat events 

such as the record-breaking summer heat experienced in 2011 

in Texas and Oklahoma. The incidence of record-breaking high 

temperatures is projected to rise.2

Human-induced climate change means much more than just 

hotter weather. Increases in ocean and freshwater temperatures, frost-free days, and heavy downpours have all been 

documented. Global sea level has risen, and there have been 

large reductions in snow-cover extent, glaciers, and sea ice. 

These changes and other climatic changes have affected and 

will continue to affect human health, water supply, agriculture, 

transportation, energy, coastal areas, and many other sectors 

of society, with increasingly adverse 

impacts on the American economy 

and quality of life.3

Some of the changes discussed in 

this report are common to many regions. For example, large increases in 

heavy precipitation have occurred in 

the Northeast, Midwest, and Great 

Plains, where heavy downpours have 

frequently led to runoff that exceeded 

the capacity of storm drains and levees, and caused flooding events and 

accelerated erosion. Other impacts, 

such as those associated with the 

rapid thawing of permafrost in Alaska, 

are unique to a particular U.S. region. 

Permafrost thawing is causing extensive damage to infrastructure in our 

nation’s largest state.4

Some impacts that occur in one region 

ripple beyond that region. For example, the dramatic decline of summer 

sea ice in the Arctic – a loss of ice cover 

roughly equal to half the area of the 

continental United States – exacerbates global warming by reducing the 

reflectivity of Earth’s surface and increasing the amount of heat absorbed. 

Similarly, smoke from wildfires in one 

location can contribute to poor air quality in faraway regions, 

and evidence suggests that particulate matter can affect atmospheric properties and therefore weather patterns. Major 

storms and the higher storm surges exacerbated by sea level 

rise that hit the Gulf Coast affect the entire country through 

their cascading effects on oil and gas production and distribution.5

Water expands as it warms, causing global sea levels to rise; 

melting of land-based ice also raises sea level by adding water 

to the oceans. Over the past century, global average sea level 

has risen by about 8 inches. Since 1992, the rate of global sea 

level rise measured by satellites has been roughly twice the 

rate observed over the last century, providing evidence of acceleration. Sea level rise, combined with coastal storms, has 

increased the risk of erosion, storm surge damage, and flooding for coastal communities, especially along the Gulf Coast, 

the Atlantic seaboard, and in Alaska. Coastal infrastructure, 

including roads, rail lines, energy infrastructure, airports, port 

facilities, and military bases, are increasingly at risk from sea 

level rise and damaging storm surges. Sea level is projected to 

rise by another 1 to 4 feet in this century, although the rise in 

sea level in specific regions is expected to vary from this global 

average for a number of reasons. A wider range of scenarios, 

Percent changes in the amount of precipitation falling in very heavy events (the heaviest 1%) 

from 1958 to 2012 for each region. There is a clear national trend toward a greater amount 

of precipitation being concentrated in very heavy events, particularly in the Northeast and 

Midwest. (Figure source: updated from Karl et al. 2009c ).

Observed Change in Very Heavy Precipitation
10 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

1: OVERVIEW AND REPORT FINDINGS

from 8 inches to more than 6 feet by 2100, has 

been used in risk-based analyses in this report. 

In general, higher emissions scenarios that lead 

to more warming would be expected to lead 

to higher amounts of sea level rise. The stakes 

are high, as nearly five million Americans and 

hundreds of billions of dollars of property are 

located in areas that are less than four feet 

above the local high-tide level.6

In addition to causing changes in climate, increasing levels of carbon dioxide from the 

burning of fossil fuels and other human activities have a direct effect on the world’s oceans. 

Carbon dioxide interacts with ocean water to 

form carbonic acid, increasing the ocean’s acidity. Ocean surface waters have become 30% 

more acidic over the last 250 years as they have 

absorbed large amounts of carbon dioxide 

from the atmosphere. This ocean acidification 

makes water more corrosive, reducing the capacity of marine 

organisms with shells or skeletons made of calcium carbonate 

(such as corals, krill, oysters, clams, and crabs) to survive, grow, 

and reproduce, which in turn will affect the marine food chain.7

Widespread Impacts

Impacts related to climate change are already evident in many 

regions and sectors and are expected to become increasingly 

disruptive across the nation throughout this century and beyond. Climate changes interact with other environmental and 

societal factors in ways that can either moderate or intensify 

these impacts.

Pteropods, or “sea butterflies,” are eaten by a variety of marine species ranging from 

tiny krill to salmon to whales. The photos show what happens to a pteropod’s shell 

in seawater that is too acidic. On the left is a shell from a live pteropod from a region 

in the Southern Ocean where acidity is not too high. The shell on the right is from a 

pteropod in a region where the water is more acidic. (Figure source: (left) Bednaršek 

et al. 2012e

 (right) Nina Bednaršek).

Shells Dissolve in Acidified Ocean Water

The correlation between rising levels of carbon dioxide in the atmosphere (red) with 

rising carbon dioxide levels (blue) and falling pH in the ocean (green). As carbon 

dioxide accumulates in the ocean, the water becomes more acidic (the pH declines). 

(Figure source: modified from Feely et al. 2009d

).

As Oceans Absorb CO2

They Become More Acidic
11 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

1: OVERVIEW AND REPORT FINDINGS

Northeast Communities are affected by heat waves, more extreme precipitation events, and 

coastal flooding due to sea level rise and storm surge.

Southeast 

and 

Caribbean

Decreased water availability, exacerbated by population growth and land-use change, 

causes increased competition for water. There are increased risks associated with 

extreme events such as hurricanes.

Midwest

Longer growing seasons and rising carbon dioxide levels increase yields of some crops, 

although these benefits have already been offset in some instances by occurrence of 

extreme events such as heat waves, droughts, and floods.

Great Plains Rising temperatures lead to increased demand for water and energy and impacts on 

agricultural practices.

Southwest Drought and increased warming foster wildfires and increased competition for scarce 

water resources for people and ecosystems.

Northwest

Changes in the timing of streamflow related to earlier snowmelt reduce the supply of 

water in summer, causing far-reaching ecological and socioeconomic consequences.

Alaska

Rapidly receding summer sea ice, shrinking glaciers, and thawing permafrost cause 

damage to infrastructure and major changes to ecosystems. Impacts to Alaska Native 

communities increase.

Hawai‘i 

and Pacific 

Islands

Increasingly constrained freshwater supplies, coupled with increased temperatures, 

stress both people and ecosystems and decrease food and water security.

Coasts

Coastal lifelines, such as water supply infrastructure and evacuation routes, are 

increasingly vulnerable to higher sea levels and storm surges, inland flooding, and 

other climate-related changes.

Oceans

The oceans are currently absorbing about a quarter of human-caused carbon dioxide 

emissions to the atmosphere and over 90% of the heat associated with global 

warming, leading to ocean acidification and the alteration of marine ecosystems.

Observed and projected climate change impacts vary across the regions of the United States. Selected impacts emphasized in the 

regional chapters are shown below, and many more are explored in detail in this report. 
12 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

1: OVERVIEW AND REPORT FINDINGS

Some climate changes currently have beneficial effects for 

specific sectors or regions. For example, current benefits of 

warming include longer growing seasons for agriculture and 

longer ice-free periods for shipping on the Great Lakes. At the 

same time, however, longer growing seasons, along with higher temperatures and carbon dioxide levels, can increase pollen 

production, intensifying and lengthening the allergy season. 

Longer ice-free periods on the Great Lakes can result in more 

lake-effect snowfalls.

Sectors affected by climate changes include agriculture, water, 

human health, energy, transportation, forests, and ecosystems. 

Climate change poses a major challenge to U.S. agriculture 

because of the critical dependence of agricultural systems on 

climate. Climate change has the potential to both positively 

and negatively affect the location, timing, and productivity of 

crop, livestock, and fishery systems at local, national, and global 

scales. The United States produces nearly $330 billion per year 

in agricultural commodities. This productivity is vulnerable to 

direct impacts on crops and livestock from changing climate 

conditions and extreme weather events 

and indirect impacts through increasing 

pressures from pests and pathogens. 

Climate change will also alter the stability of food supplies and create new food 

security challenges for the United States 

as the world seeks to feed nine billion 

people by 2050. While the agriculture 

sector has proven to be adaptable to a 

range of stresses, as evidenced by continued growth in production and efficiency across the United States, climate 

change poses a new set of challenges.8

Water quality and quantity are being affected by climate 

change. Changes in precipitation and runoff, combined with 

changes in consumption and withdrawal, have reduced surface and groundwater supplies in many 

areas. These trends are expected to 

continue, increasing the likelihood of 

water shortages for many uses. Water quality is also diminishing in many 

areas, particularly due to sediment 

and contaminant concentrations after heavy downpours. Sea level rise, 

storms and storm surges, and changes 

in surface and groundwater use patterns are expected to compromise the 

sustainability of coastal freshwater 

aquifers and wetlands. In most U.S. regions, water resources managers and planners will encounter 

new risks, vulnerabilities, and opportunities that may not be 

properly managed with existing practices.9

Climate change affects human health in many ways. For example, increasingly frequent and intense heat events lead to 

more heat-related illnesses and deaths and, over time, worsen 

drought and wildfire risks, and intensify air pollution. Increasingly frequent extreme precipitation and associated flooding 

can lead to injuries and increases in waterborne disease. Rising sea surface temperatures have been linked with increasing 

levels and ranges of diseases. Rising sea levels intensify coastal 

flooding and storm surge, and thus exacerbate threats to public safety during storms. Certain groups of people are more vulnerable to the range of climate change related health impacts, 

including the elderly, children, the poor, and the sick. Others 

are vulnerable because of where they live, including those in 

floodplains, coastal zones, and some urban areas. Improving 

and properly supporting the public health infrastructure will 

be critical to managing the potential health impacts of climate 

change.10

 ©Ted Wood Photography

Increasing air and water temperatures, more intense precipitation 

and runoff, and intensifying droughts can decrease water quality 

in many ways. Here, middle school students in Colorado test 

water quality.

Climate change can exacerbate respiratory and asthma-related 

conditions through increases in pollen, ground-level ozone, and 

wildfire smoke.

©National Geographic

Certain groups of people are 

more vulnerable to the range of 

climate change related health 

impacts, including the elderly, 

children, the poor, and the sick.
13 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Climate change also affects the living world, including people, 

through changes in ecosystems and biodiversity. Ecosystems 

provide a rich array of benefits and services to humanity, including habitat for fish and wildlife, drinking water storage 

and filtration, fertile soils for growing crops, buffering against 

a range of stressors including climate change impacts, and 

aesthetic and cultural values. These 

benefits are not always easy to quantify, but they support jobs, economic 

growth, health, and human well-being. 

Climate change driven disruptions to 

ecosystems have direct and indirect 

human impacts, including reduced water supply and quality, the loss of iconic 

species and landscapes, effects on food 

chains and the timing and success of 

species migrations, and the potential for extreme weather and 

climate events to destroy or degrade the ability of ecosystems 

to provide societal benefits.11

Human modifications of ecosystems and landscapes often 

increase their vulnerability to damage from extreme weather 

events, while simultaneously reducing their natural capacity to 

moderate the impacts of such events. For example, salt marshes, reefs, mangrove forests, and barrier islands defend coastal 

ecosystems and infrastructure, such as roads and buildings, 

against storm surges. The loss of these natural buffers due to 

coastal development, erosion, and sea level rise increases the 

risk of catastrophic damage during or after extreme weather 

events. Although floodplain wetlands are greatly reduced 

from their historical extent, those that 

remain still absorb floodwaters and 

reduce the effects of high flows on 

river-margin lands. Extreme weather 

events that produce sudden increases 

in water flow, often carrying debris 

and pollutants, can decrease the natural capacity of ecosystems to cleanse 

contaminants.12

The climate change impacts being felt in the regions and sectors of the United States are affected by global trends and 

economic decisions. In an increasingly interconnected world, 

U.S. vulnerability is linked to impacts in other nations. It is thus 

difficult to fully evaluate the impacts of climate change on the 

United States without considering consequences of climate 

change elsewhere.

Response Options

As the impacts of climate change are becoming more prevalent, Americans face choices. Especially because of past emissions of long-lived heat-trapping gases, some additional climate change and related impacts are now unavoidable. This 

is due to the long-lived nature of many of these gases, as well 

as the amount of heat absorbed and retained by the oceans 

and other responses within the climate system. The amount of 

future climate change, however, will still largely be determined 

by choices society makes about emissions. Lower emissions of 

heat-trapping gases and particles mean less future warming 

and less-severe impacts; higher emissions mean more warming 

and more severe impacts. Efforts to limit emissions or increase 

carbon uptake fall into a category of response options known 

as “mitigation,” which refers to reducing the amount and speed 

of future climate change by reducing emissions of heat-trapping gases or removing carbon dioxide from the atmosphere.13

The other major category of response options is known as “adaptation,” and refers to actions to prepare for and adjust to 

new conditions, thereby reducing harm or taking advantage 

of new opportunities. Mitigation and adaptation actions are 

linked in multiple ways, including that effective mitigation reduces the need for adaptation in the future. Both are essential 

parts of a comprehensive climate change response strategy. 

The threat of irreversible impacts makes the timing of mitigation efforts particularly critical. This report includes chapters 

on Mitigation, Adaptation, and Decision Support that offer 

an overview of the options and activities being planned or 

implemented around the country as local, state, federal, and 

tribal governments, as well as businesses, organizations, and 

individuals begin to respond to climate change. These chapters conclude that while response actions are under development, current implementation efforts are insufficient to avoid 

increasingly negative social, environmental, and economic 

consequences.14

Large reductions in global emissions of heat-trapping gases, 

similar to the lower emissions scenario (B1) analyzed in this 

assessment, would reduce the risks of some of the worst impacts of climate change. Some targets called for in international climate negotiations to date would require even larger 

reductions than those outlined in the B1 scenario. Meanwhile, 

global emissions are still rising and are on a path to be even 

higher than the high emissions scenario (A2) analyzed in this 

report. The recent U.S. contribution to annual global emissions 

is about 18%, but the U.S. contribution to cumulative global 

emissions over the last century is much higher. Carbon dioxide 

lasts for a long time in the atmosphere, and it is the cumulative carbon emissions that determine the amount of global 

climate change. After decades of increases, U.S. CO2 emissions 

from energy use (which account for 97% of total U.S. emissions) 

declined by around 9% between 2008 and 2012, largely due to 

a shift from coal to less CO2-intensive natural gas for electricity 

production. Governmental actions in city, state, regional, and 

federal programs to promote energy efficiency have also contributed to reducing U.S. carbon emissions. Many, if not most 

of these programs are motivated by other policy objectives, 

but some are directed specifically at greenhouse gas emissions.

The amount of future climate 

change will still largely be determined by choices society makes 

about emissions. 
14 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

1: OVERVIEW AND REPORT FINDINGS

These U.S. actions and others that might be undertaken in the 

future are described in the Mitigation chapter of this report. 

Over the remainder of this century, aggressive and sustained 

greenhouse gas emission reductions by the United States and 

by other nations would be needed to reduce global emissions 

to a level consistent with the lower scenario (B1) analyzed in 

this assessment.15

With regard to adaptation, the pace and magnitude of observed and projected changes emphasize the need to be prepared for a wide variety and intensity of impacts. Because of 

the growing influence of human activities, the climate of the 

past is not a good basis for future planning. For example, building codes and landscaping ordinances could be updated to 

improve energy efficiency, conserve water supplies, protect 

against insects that spread disease (such as dengue fever), 

reduce susceptibility to heat stress, and improve protection 

against extreme events. The fact that climate change impacts 

are increasing points to the urgent need to develop and refine 

approaches that enable decision-making and increase flexibility and resilience in the face of ongoing and future impacts. 

Reducing non-climate-related stresses that contribute to existing vulnerabilities can also be an effective approach to climate 

change adaptation.16

Adaptation can involve considering local, state, regional, national, and international jurisdictional objectives. For example, 

in managing water supplies to adapt to a changing climate, the 

implications of international treaties should be considered in 

the context of managing the Great Lakes, the Columbia River, 

and the Colorado River to deal with increased drought risk. Both 

“bottom up” community planning and “top down” national 

strategies may help regions deal with impacts such as increases 

in electrical brownouts, heat stress, floods, and wildfires.17

Proactively preparing for climate change can reduce impacts 

while also facilitating a more rapid and efficient response to 

changes as they happen. Such efforts are beginning at the federal, regional, state, tribal, and local levels, and in the corporate and non-governmental sectors, to build adaptive capacity 

and resilience to climate change impacts. Using scientific information to prepare for climate changes in advance can provide 

economic opportunities, and proactively managing the risks 

can reduce impacts and costs over time.18

There are a number of areas where improved scientific information or understanding would enhance the capacity to estimate future climate change impacts. For example, knowledge 

of the mechanisms controlling the rate of ice loss in Greenland 

and Antarctica is limited, making it difficult for scientists to 

narrow the range of expected future sea level rise. Improved 

understanding of ecological and social responses to climate 

change is needed, as is understanding of how ecological and 

social responses will interact.19

A sustained climate assessment process could more efficiently 

collect and synthesize the rapidly evolving science and help 

supply timely and relevant information to decision-makers. 

Results from all of these efforts could continue to deepen our 

understanding of the interactions of human and natural systems in the context of a changing climate, enabling society to 

effectively respond and prepare for our future.20

The cumulative weight of the scientific evidence contained in 

this report confirms that climate change is affecting the American people now, and that choices we make will affect our future and that of future generations.

Cities providing transportation options including bike lanes, buildings designed with energy saving features such as green roofs, and 

houses elevated to allow storm surges to pass underneath are among the many response options being pursued around the country. ©John Sebastian Russo/San Francisco Chronicle/Corbis ©Proehl Studios/Corbis FEMA
15 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

1: OVERVIEW AND REPORT FINDINGS

These findings distill important results that arise from this National Climate Assessment. They do not represent a 

full summary of all of the chapters’ findings, but rather a synthesis of particularly noteworthy conclusions.

1. Global climate is changing and this is apparent across the United States in a 

wide range of observations. The global warming of the past 50 years is primarily 

due to human activities, predominantly the burning of fossil fuels.

Many independent lines of evidence confirm that human activities are affecting climate in 

unprecedented ways. U.S. average temperature has increased by 1.3°F to 1.9°F since record 

keeping began in 1895; most of this increase has occurred since about 1970. The most recent 

decade was the warmest on record. Because human-induced warming is superimposed on a 

naturally varying climate, rising temperatures are not evenly distributed across the country or 

over time.21

4. Impacts related to climate change are already evident in many sectors and 

are expected to become increasingly disruptive across the nation throughout this 

century and beyond.

Climate change is already affecting societies and the natural world. Climate change interacts 

with other environmental and societal factors in ways that can either moderate or intensify 

these impacts. The types and magnitudes of impacts vary across the nation and through 

time. Children, the elderly, the sick, and the poor are especially vulnerable. There is 

mounting evidence that harm to the nation will increase substantially in the future unless 

global emissions of heat-trapping gases are greatly reduced.24

3. Human-induced climate change is projected to continue, and it will accelerate 

significantly if global emissions of heat-trapping gases continue to increase.

Heat-trapping gases already in the atmosphere have committed us to a hotter future with 

more climate-related impacts over the next few decades. The magnitude of climate change 

beyond the next few decades depends primarily on the amount of heat-trapping gases that 

human activities emit globally, now and in the future.23

2. Some extreme weather and climate events have increased in recent decades, 

and new and stronger evidence confirms that some of these increases are related 

to human activities.

Changes in extreme weather events are the primary way that most people experience climate 

change. Human-induced climate change has already increased the number and strength of 

some of these extreme events. Over the last 50 years, much of the United States has seen an 

increase in prolonged periods of excessively high temperatures, more heavy downpours, and 

in some regions, more severe droughts.22

Report Findings
16 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

1: OVERVIEW AND REPORT FINDINGS

5. Climate change threatens human health and well-being in many ways, including 

through more extreme weather events and wildfire, decreased air quality, and 

diseases transmitted by insects, food, and water.

Climate change is increasing the risks of heat stress, respiratory stress from poor air quality, 

and the spread of waterborne diseases. Extreme weather events often lead to fatalities and 

a variety of health impacts on vulnerable populations, including impacts on mental health, 

such as anxiety and post-traumatic stress disorder. Large-scale changes in the environment 

due to climate change and extreme weather events are increasing the risk of the emergence 

or reemergence of health threats that are currently uncommon in the United States, such as 

dengue fever.25

8. Climate disruptions to agriculture have been increasing and are projected to 

become more severe over this century. 

Some areas are already experiencing climate-related disruptions, particularly due to extreme 

weather events. While some U.S. regions and some types of agricultural production will be 

relatively resilient to climate change over the next 25 years or so, others will increasingly suffer 

from stresses due to extreme heat, drought, disease, and heavy downpours. From mid-century 

on, climate change is projected to have more negative impacts on crops and livestock across 

the country – a trend that could diminish the security of our food supply.28 

7. Water quality and water supply reliability are jeopardized by climate change in 

a variety of ways that affect ecosystems and livelihoods.

Surface and groundwater supplies in some regions are already stressed by increasing demand 

for water as well as declining runoff and groundwater recharge. In some regions, particularly 

the southern part of the country and the Caribbean and Pacific Islands, climate change is 

increasing the likelihood of water shortages and competition for water among its many 

uses. Water quality is diminishing in many areas, particularly due to increasing sediment and 

contaminant concentrations after heavy downpours.27

6. Infrastructure is being damaged by sea level rise, heavy downpours, and 

extreme heat; damages are projected to increase with continued climate change. 

Sea level rise, storm surge, and heavy downpours, in combination with the pattern of continued 

development in coastal areas, are increasing damage to U.S. infrastructure including roads, 

buildings, and industrial facilities, and are also increasing risks to ports and coastal military 

installations. Flooding along rivers, lakes, and in cities following heavy downpours, prolonged 

rains, and rapid melting of snowpack is exceeding the limits of flood protection infrastructure 

designed for historical conditions. Extreme heat is damaging transportation infrastructure such 

as roads, rail lines, and airport runways.26
17 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

1: OVERVIEW AND REPORT FINDINGS

9. Climate change poses particular threats to Indigenous Peoples’ health, wellbeing, and ways of life. 

Chronic stresses such as extreme poverty are being exacerbated by climate change impacts 

such as reduced access to traditional foods, decreased water quality, and increasing exposure 

to health and safety hazards. In parts of Alaska, Louisiana, the Pacific Islands, and other 

coastal locations, climate change impacts (through erosion and inundation) are so severe that 

some communities are already relocating from historical homelands to which their traditions 

and cultural identities are tied. Particularly in Alaska, the rapid pace of temperature rise, ice 

and snow melt, and permafrost thaw are significantly affecting critical infrastructure and 

traditional livelihoods.29 

12. Planning for adaptation (to address and prepare for impacts) and mitigation 

(to reduce future climate change, for example by cutting emissions) is becoming 

more widespread, but current implementation efforts are insufficient to avoid 

increasingly negative social, environmental, and economic consequences.

Actions to reduce emissions, increase carbon uptake, adapt to a changing climate, and 

increase resilience to impacts that are unavoidable can improve public health, economic 

development, ecosystem protection, and quality of life.32

11. Ocean waters are becoming warmer and more acidic, broadly affecting ocean 

circulation, chemistry, ecosystems, and marine life. 

More acidic waters inhibit the formation of shells, skeletons, and coral reefs. Warmer waters 

harm coral reefs and alter the distribution, abundance, and productivity of many marine 

species. The rising temperature and changing chemistry of ocean water combine with other 

stresses, such as overfishing and coastal and marine pollution, to alter marine-based food 

production and harm fishing communities.31

10. Ecosystems and the benefits they provide to society are being affected by 

climate change. The capacity of ecosystems to buffer the impacts of extreme 

events like fires, floods, and severe storms is being overwhelmed.

Climate change impacts on biodiversity are already being observed in alteration of the timing 

of critical biological events such as spring bud burst and substantial range shifts of many 

species. In the longer term, there is an increased risk of species extinction. These changes 

have social, cultural, and economic effects. Events such as droughts, floods, wildfires, and 

pest outbreaks associated with climate change (for example, bark beetles in the West) are 

already disrupting ecosystems. These changes limit the capacity of ecosystems, such as 

forests, barrier beaches, and wetlands, to continue to play important roles in reducing the 

impacts of these extreme events on infrastructure, human communities, and other valued 

resources.30
18 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

OVERVIEW AND REPORT FINDINGS

References

Numbered references for the Overview indicate the chapters that 

provide supporting evidence for the reported conclusions.

1. Ch. 2.

2. Ch. 2, 3, 6, 9, 20.

3. Ch. 2, 3, 4, 5, 6, 9, 10, 12, 16, 20, 24, 25.

4. Ch. 2, 12, 16, 18, 19, 20, 21, 22, 23.

5. Ch. 2, 4, 12, 16, 17, 18, 19, 20, 22, 25.

6. Ch. 2, 4, 5, 10, 12, 16, 17, 20, 22, 25.

7. Ch. 2, 12, 23, 24, 25.

8. Ch. 2, 12, 13, 14, 18, 19.

9. Ch. 2, 3, 12, 16, 17, 18, 19, 20, 21, 23.

10. Ch. 2, 9, 11, 12, 13, 16, 18, 19, 20, 25.

11. Ch. 3, 6, 8, 12, 14, 23, 24, 25.

12. Ch. 3, 7, 8, 25.

13. Ch. 2, 26, 27.

14. Ch. 26, 27, 28.

15. Ch. 2, 4, 27.

16. Ch. 2, 3, 5, 9, 11, 12, 13, 25, 26, 27, 28.

17. Ch. 3, 7, 9, 10, 12, 18, 20, 21, 26, 28.

18. Ch. 28.

19. Ch. 29, Appendix 6.

20. Ch. 30.

21. Ch. 2, Appendices 3 and 4.

22. Ch. 2, 16, 17, 18, 19, 20, 23, Appendices 3 and 4.

23. Ch. 2, 27, Appendices 3 and 4.

24. Ch. 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 16, 17, 18, 19, 20, 21, 22, 23, 

24, 25.

25. Ch. 2, 6, 9, 11, 12, 16, 19, 20, 22, 23.

26. Ch. 2, 3, 5, 6, 11, 12, 16, 17, 18, 19, 20, 21, 22, 23, 25.

27. Ch. 2, 3, 12, 16, 17, 18, 19, 20, 21, 23.

28. Ch. 2, 6, 12, 13, 14, 18, 19.

29. Ch. 12, 17, 20, 21, 22, 23, 25.

30. Ch. 2, 3, 6, 7, 8, 10, 11, 14, 15, 19, 25.

31. Ch. 2, 12, 23, 24, 25.

32. Ch. 6, 7, 8, 9, 10, 13, 15, 25, 26, 27, 28.

Letter references refer to external sources

a. Kennedy, J. J., P. W. Thorne, T. C. Peterson, R. A. Reudy, P. 

A. Stott, D. E. Parker, S. A. Good, H. A. Titchner, and K. M. 

Willett, 2010: How do we know the world has warmed? State of 

the Climate in 2009. Bulletin of the American Meteorological Society, 

91, S26-27, doi:10.1175/BAMS-91-7-StateoftheClimate. [Available 

online at http://journals.ametsoc.org/doi/abs/10.1175/BAMS91-7-StateoftheClimate]

b. Huber, M., and R. Knutti, 2012: Anthropogenic and natural 

warming inferred from changes in Earth’s energy balance. Nature 

Geoscience, 5, 31-36, doi:10.1038/ngeo1327. [Available online at 

http://www.nature.com/ngeo/journal/v5/n1/pdf/ngeo1327.pdf]

c. Karl, T. R., J. T. Melillo, and T. C. Peterson, Eds., 2009: Global 

Climate Change Impacts in the United States. Cambridge University 

Press, 189 pp. [Available online at http://downloads.globalchange.

gov/usimpacts/pdfs/climate-impacts-report.pdf]

d. Feely, R. A., S. C. Doney, and S. R. Cooley, 2009: Ocean 

acidification: Present conditions and future changes in a highCO2

 world. Oceanography, 22, 36-47, doi:10.5670/oceanog.2009.95. 

[Available online at http://www.tos.org/oceanography/

archive/22-4_feely.pdf]

e. Bednaršek, N., G. A. Tarling, D. C. E. Bakker, S. Fielding, E. M. 

Jones, H. J. Venables, P. Ward, A. Kuzirian, B. Lézé, R. A. Feely, 

and E. J. Murphy, 2012: Extensive dissolution of live pteropods 

in the Southern Ocean. Nature Geoscience, 5, 881-885, doi:10.1038/

ngeo1635

PHOTO CREDITS

pg. 23–Person pumping gas: Charles Minshew/KOMU; People cooling 

off during heatwave: ©Julie Jacobson/AP/Corbis; Smog over city: 

©iStockPhoto.comDanielStein;Childblowingnose: ©Stockbyte/

Getty Images

pg. 24–Mosquito: ©James Gathany, CDC; Road washed out due to 

flooding: ©John Wark/AP/Corbis; Mountain stream: ©Dan 

Sherwood/Design Pics/Corbis; Farmer with corn: ©iStockPhoto.

com/ValentinRussanov

pg. 25–Person building house: ©Aaron Huey/National Geographic 

Society/Corbis; Bear: ©Chase Swift/Corbis; Manatee: US 

Fish and Wildlife Service; Person with solar panels: ©Dennis 

Schroeder, NREL
19

Climate Change Impacts in the United States

CHAPTER 2

OUR CHANGING CLIMATE

INFORMATION DRAWN FROM THIS CHAPTER IS INCLUDED IN THE HIGHLIGHTS REPORT AND IS IDENTIFIED BY THIS ICON

Recommended Citation for Chapter

Walsh, J., D. Wuebbles, K. Hayhoe, J. Kossin, K. Kunkel, G. Stephens, P. Thorne, R. Vose, M. Wehner, J. Willis, D. Anderson, 

S. Doney, R. Feely, P. Hennon, V. Kharin, T. Knutson, F. Landerer, T. Lenton, J. Kennedy, and R. Somerville, 2014: Ch. 2: Our 

Changing Climate. Climate Change Impacts in the United States: The Third National Climate Assessment, J. M. Melillo, Terese 

(T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 19-67. doi:10.7930/J0KW5CXT. 

On the Web: http://nca2014.globalchange.gov/report/our-changing-climate/introduction

First published May 2014. PDF revised October 2014. See errata (available at http://nca2014.globalchange.gov/downloads) for details.

Convening Lead Authors

John Walsh, University of Alaska Fairbanks

Donald Wuebbles, University of Illinois

Lead Authors

Katharine Hayhoe, Texas Tech University

James Kossin, NOAA National Climatic Data Center 

Kenneth Kunkel, CICS-NC, North Carolina State Univ., NOAA National Climatic Data Center

Graeme Stephens, NASA Jet Propulsion Laboratory

Peter Thorne, Nansen Environmental and Remote Sensing Center

Russell Vose, NOAA National Climatic Data Center

Michael Wehner, Lawrence Berkeley National Laboratory

Josh Willis, NASA Jet Propulsion Laboratory

Contributing Authors

David Anderson, NOAA National Climatic Data Center

Scott Doney, Woods Hole Oceanographic Institution

Richard Feely, NOAA Pacific Marine Environmental Laboratory

Paula Hennon, CICS-NC, North Carolina State Univ., NOAA National Climatic Data Center

Viatcheslav Kharin, Canadian Centre for Climate Modelling and Analysis, Environment Canada

Thomas Knutson, NOAA Geophysical Fluid Dynamics Laboratory

Felix Landerer, NASA Jet Propulsion Laboratory

Tim Lenton, Exeter University

John Kennedy, UK Meteorological Office

Richard Somerville, Scripps Institution of Oceanography, Univ. of California, San Diego
20 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

2 OUR CHANGING CLIMATE

Key Messages

1. Global climate is changing and this change is apparent across a wide range of observations. The 

 global warming of the past 50 years is primarily due to human activities.

2. Global climate is projected to continue to change over this century and beyond. The magnitude 

 of climate change beyond the next few decades depends primarily on the amount of heat- 

 trapping gases emitted globally, and how sensitive the Earth’s climate is to those emissions. 

3. U.S. average temperature has increased by 1.3°F to 1.9°F since record keeping began in 1895; 

 most of this increase has occurred since about 1970. The most recent decade was the nation’s 

 warmest on record. Temperatures in the United States are expected to continue to rise. Because 

 human-induced warming is superimposed on a naturally varying climate, the temperature rise 

 has not been, and will not be, uniform or smooth across the country or over time.

4. The length of the frost-free season (and the corresponding growing season) has been increasing 

 nationally since the 1980s, with the largest increases occurring in the western United States, 

 affecting ecosystems and agriculture. Across the United States, the growing season is projected 

 to continue to lengthen.

5. Average U.S. precipitation has increased since 1900, but some areas have had increases 

 greater than the national average, and some areas have had decreases. More winter and spring 

 precipitation is projected for the northern United States, and less for the Southwest, over this 

 century. 

6. Heavy downpours are increasing nationally, especially over the last three to five decades. 

 Largest increases are in the Midwest and Northeast. Increases in the frequency and intensity of 

 extreme precipitation events are projected for all U.S. regions. 

7. There have been changes in some types of extreme weather events over the last several 

 decades. Heat waves have become more frequent and intense, especially in the West. Cold 

 waves have become less frequent and intense across the nation. There have been regional 

 trends in floods and droughts. Droughts in the Southwest and heat waves everywhere are 

 projected to become more intense, and cold waves less intense everywhere. 

8. The intensity, frequency, and duration of North Atlantic hurricanes, as well as the frequency 

 of the strongest (Category 4 and 5) hurricanes, have all increased since the early 1980s. 

 The relative contributions of human and natural causes to these increases are still uncertain. 

 Hurricane-associated storm intensity and rainfall rates are projected to increase as the climate 

 continues to warm. 

9. Winter storms have increased in frequency and intensity since the 1950s, and their tracks have 

 shifted northward over the United States. Other trends in severe storms, including the intensity 

 and frequency of tornadoes, hail, and damaging thunderstorm winds, are uncertain and are being 

 studied intensively.

Continued
21 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

2: OUR CHANGING CLIMATE

This chapter summarizes how climate is changing, why it is 

changing, and what is projected for the future. While the focus 

is on changes in the United States, the need to provide context 

sometimes requires a broader geographical perspective. Additional geographic detail is presented in the regional chapters 

of this report. Further details on the topics covered by this 

chapter are provided in the Climate Science Supplement and 

Frequently Asked Questions Appendices.

Since the second National Climate Assessment was published 

in 2009,1 the climate has continued to change, with resulting 

effects on the United States. The trends described in the 2009 

report have continued, and our understanding of the data and 

ability to model the many facets of the climate system have increased substantially. Several noteworthy advances are mentioned in the box below.

The 12 key messages presented above are repeated below, 

together with supporting evidence for those messages. The 

discussion of each key message begins with a summary of recent variations or trends, followed by projections of the corresponding changes for the future.

Key Messages (Continued)

10. Global sea level has risen by about 8 inches since reliable record keeping began in 1880. It is 

 projected to rise another 1 to 4 feet by 2100. 

11. Rising temperatures are reducing ice volume and surface extent on land, lakes, and sea. This 

 loss of ice is expected to continue. The Arctic Ocean is expected to become essentially ice free 

 in summer before mid-century.

12. The oceans are currently absorbing about a quarter of the carbon dioxide emitted to the 

 atmosphere annually and are becoming more acidic as a result, leading to concerns about 

 intensifying impacts on marine ecosystems.

What’s new?

• Continued warming and an increased understanding of the U.S. temperature record, as well as multiple other 

sources of evidence, have strengthened our confidence in the conclusions that the warming trend is clear and 

primarily the result of human activities. For the contiguous United States, the last decade was the warmest on 

record, and 2012 was the warmest year on record.

• Heavy precipitation and extreme heat events are increasing in a manner consistent with model projections; the 

risks of such extreme events will rise in the future.

• The sharp decline in summer Arctic sea ice has continued, is unprecedented, and is consistent with humaninduced climate change. A new record for minimum area of Arctic sea ice was set in 2012.

• A longer and better-quality history of sea level rise has increased confidence that recent trends are unusual and 

human-induced. Limited knowledge of ice sheet dynamics leads to a broad range for projected sea level rise over 

this century.

• New approaches to building scenarios of the future have allowed for investigations of the implications of larger 

reductions in heat trapping gas emissions than examined previously. 
22 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

2: OUR CHANGING CLIMATE

Key Message 1: Observed Climate Change 

Global climate is changing and this change is apparent across a wide range of 

observations. The global warming of the past 50 years is primarily due to human activities. 

Climate is defined as long-term 

averages and variations in weather measured over a period of several decades. The Earth’s climate 

system includes the land surface, atmosphere, oceans, and 

ice. Many aspects of the global 

climate are changing rapidly, 

and the primary drivers of that 

change are human in origin. Evidence for changes in the climate 

system abounds, from the top of 

the atmosphere to the depths of 

the oceans (Figure 2.1).3 Scientists and engineers from around 

the world have compiled this evidence using satellites, weather 

balloons, thermometers at surface stations, and many other 

types of observing systems that 

monitor the Earth’s weather and 

climate. The sum total of this 

evidence tells an unambiguous 

story: the planet is warming. 

Temperatures at the surface, in the troposphere (the active 

weather layer extending up to about 5 to 10 miles above the 

ground), and in the oceans have all increased over recent 

decades (Figure 2.2). Consistent with our scientific understanding, the largest increases in temperature are occurring closer to the poles, especially in the Arctic. Snow and 

ice cover have decreased in most areas. Atmospheric water vapor is increasing in the lower atmosphere, because a 

warmer atmosphere can hold more water. Sea levels are also 

increasing (see Key Message 10). Changes in other climateReference periods for graphs

Many of the graphs in this report illustrate historical changes and future trends in climate compared to some reference period, with the choice of this period determined by the purpose of the graph and the availability of data. The 

great majority of graphs are based on one of two reference periods. The period 1901-1960 is used for graphs that 

illustrate past changes in climate conditions, whether in observations or in model simulations. The choice of 1960 as 

the ending date of this period was based on past changes in human influences on the climate system. Human-induced 

forcing exhibited a slow rise during the early part of the last century but then accelerated after 1960.2 Thus, these 

graphs highlight observed changes in climate during the period of rapid increase in human-caused forcing and also 

reveal how well climate models simulate these observed changes. The beginning date of 1901 was chosen because 

earlier historical observations are less reliable and because many climate model simulations begin in 1900 or 1901. 

The other commonly used reference period is 1971-2000, which is consistent with the World Meteorological Organization’s recommended use of 30-year periods for climate statistics. This is used for graphs that illustrate projected 

future changes simulated by climate models. The purpose of these graphs is to show projected changes compared to 

a period that people have recently experienced and can remember; thus, the most recent available 30-year period was 

chosen (the historical period simulated by the CMIP3 models ends in 1999 or 2000).

Figure 2.1. These are just some of the indicators measured globally over many decades 

that show that the Earth’s climate is warming. White arrows indicate increasing trends, 

and black arrows indicate decreasing trends. All the indicators expected to increase in a 

warming world are, in fact, increasing, and all those expected to decrease in a warming 

world are decreasing. (Figure source: NOAA NCDC based on data updated from Kennedy 

et al. 20103).

Ten Indicators of a Warming World
23 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

2: OUR CHANGING CLIMATE

relevant indicators such as growing season 

length have been observed in many areas. 

Worldwide, the observed changes in average conditions have been accompanied by 

increasing trends in extremes of heat and 

heavy precipitation events, and decreases 

in extreme cold.4

Natural drivers of climate cannot explain 

the recent observed warming. Over the 

last five decades, natural factors (solar 

forcing and volcanoes) alone would actually have led to a slight cooling (see Figure 

2.3).

5

The majority of the warming at the global 

scale over the past 50 years can only be 

explained by the effects of human influences,5,6,7 especially the emissions from 

burning fossil fuels (coal, oil, and natural 

gas) and from deforestation. The emissions from human influences that are 

affecting climate include heat-trapping 

gases such as carbon dioxide (CO2), methane, and nitrous oxide, and particles such 

as black carbon (soot), which has a warming influence, and sulfates, which have an 

overall cooling influence (see Appendix 3: 

Climate Science Supplement for further 

discussion).8,9 In addition to human-induced global climate change, local climate 

can also be affected by other human factors (such as crop irrigation) and natural 

variability (for example, Ashley et al. 2012; 

DeAngelis et al. 2010; Degu et al. 2011; Lo 

and Famiglietti 201310).

The conclusion that human influences are 

the primary driver of recent climate change 

is based on multiple lines of independent 

evidence. The first line of evidence is 

our fundamental understanding of how 

certain gases trap heat, how the climate 

system responds to increases in these 

gases, and how other human and natural 

factors influence climate. The second line 

of evidence is from reconstructions of past 

climates using evidence such as tree rings, 

ice cores, and corals. These show that 

global surface temperatures over the last 

several decades are clearly unusual, with 

the last decade (2000-2009) warmer than 

any time in at least the last 1300 years and 

perhaps much longer.11

Figure 2.2. Global annual average temperature (as measured over both land and 

oceans) has increased by more than 1.5°F (0.8°C) since 1880 (through 2012). Red bars 

show temperatures above the long-term average, and blue bars indicate temperatures 

below the long-term average. The black line shows atmospheric carbon dioxide (CO2) 

concentration in parts per million (ppm). While there is a clear long-term global warming 

trend, some years do not show a temperature increase relative to the previous year, 

and some years show greater changes than others. These year-to-year fluctuations in 

temperature are due to natural processes, such as the effects of El Niños, La Niñas, 

and volcanic eruptions. (Figure source: updated from Karl et al. 20091).

Global Temperature and Carbon Dioxide

Figure 2.3. Observed global average changes (black line), model simulations using 

only changes in natural factors (solar and volcanic) in green, and model simulations 

with the addition of human-induced emissions (blue). Climate changes since 1950 

cannot be explained by natural factors or variability, and can only be explained by 

human factors. (Figure source: adapted from Huber and Knutti29).

Separating Human and Natural Influences on Climate
24 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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The third line of evidence comes from using climate models to 

simulate the climate of the past century, separating the human 

and natural factors that influence climate. When the human 

factors are removed, these models show that solar and volcanic activity would have tended to slightly cool the earth, and 

other natural variations are too small to explain the amount 

of warming. Only when the human influences are included do 

the models reproduce the warming observed over the past 50 

years (see Figure 2.3).

Another line of evidence involves so-called “fingerprint” studies that are able to attribute observed climate changes to particular causes. For example, the fact that the stratosphere (the 

layer above the troposphere) is cooling while the Earth’s surface and lower atmosphere is warming is a fingerprint that the 

warming is due to increases in heat-trapping gases. In contrast, 

if the observed warming had been due to increases in solar 

output, Earth’s atmosphere would have warmed throughout 

its entire extent, including the stratosphere.6

In addition to such temperature analyses, scientific attribution of observed changes to human influence extends to many 

other aspects of climate, such as changing patterns in precipitation,12,13 increasing humidity,14,15 changes in pressure,16 and 

increasing ocean heat content.17 Further discussion of how we 

know the recent changes in climate are caused by human activity is provided in Appendix 3: Climate Science Supplement.

Natural variations in climate include the effects of cycles such 

as El Niño, La Niña and other ocean cycles; the 11-year sunspot 

cycle and other changes in energy from the sun; and the effects of volcanic eruptions. Globally, natural variations can be 

as large as human-induced climate change over timescales of 

up to a few decades. However, changes in climate at the global 

scale observed over the past 50 years are far larger than can be 

accounted for by natural variability. Changes in climate at the 

local to regional scale can be influenced by natural variability 

for multiple decades.18 This can affect the interpretation of climate trends observed regionally across the U.S. (see Appendix 

3: Climate Science Supplement).

Globally averaged surface air temperature has slowed its rate 

of increase since the late 1990s. This is not in conflict with our 

basic understanding of global warming and its primary cause. 

The decade of 2000 to 2009 was still the warmest decade on 

record. In addition, global surface air temperature does not always increase steadily. This time period is too short to signify a 

change in the warming trend, as climate trends are measured 

over periods of decades, not years.19,20,21,22 Such decade-long 

slowdowns or even reversals in trend have occurred before in 

the global instrumental record (for example, 1900-1910 and 

1940-1950; see Figure 2.2), including three decade-long periods since 1970, each followed by a sharp temperature rise.23

Nonetheless, satellite and ocean observations indicate that the 

Earth-atmosphere climate system has continued to gain heat 

energy.24

There are a number of possible contributions to the lower rate 

of increase over the last 15 years. First, the solar output during 

the latest 11-year solar cycle has been lower over the past 15 

years than the past 60 years. Second, a series of mildly explosive volcanoes, which increased stratospheric particles, likely 

had more of a cooling effect than previously recognized.25

Third, the high incidence of La Niña events in the last 15 years 

has played a role in the observed trends.20,26 Recent analyses27 suggest that more of the increase 

in heat energy during this period has been transferred to the deep ocean than previously. While 

this might temporarily slow the rate of increase in 

surface air temperature, ultimately it will prolong 

the effects of global warming because the oceans 

hold heat for longer than the atmosphere does. 

Climate models are not intended to match the 

real-world timing of natural climate variations – 

instead, models have their own internal timing 

for such variations. Most modeling studies do 

not yet account for the observed changes in solar 

and volcanic forcing mentioned in the previous 

paragraph. Therefore, it is not surprising that the 

timing of such a slowdown in the rate of increase 

in the models would be different than that observed, although it is important to note that such 

periods have been simulated by climate models, 

with the deep oceans absorbing the extra heat 

during those decades.

28 Oil used for transportation and coal used for electricity generation are the 

largest contributors to the rise in carbon dioxide that is the primary driver of 

observed changes in climate over recent decades.

©U.S. Geological Survey Department of the Interior

©Tom Mihalek/Reuters/Corbis
25 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Key Message 2: Future Climate Change

Global climate is projected to continue to change over this century and 

beyond. The magnitude of climate change beyond the next few decades 

depends primarily on the amount of heat-trapping gases emitted globally, 

and how sensitive the Earth’s climate is to those emissions. 

A certain amount of continued warming of the planet is projected to occur as a result of human-induced emissions to date; 

another 0.5°F increase would be expected over the next few 

decades even if all emissions from human activities suddenly 

stopped,30 although natural variability could still play an important role over this time period.

31 However, choices made 

now and in the next few decades will determine the amount of 

additional future warming. Beyond mid-century, lower levels 

of heat-trapping gases in scenarios with reduced emissions will 

lead to noticeably less future warming. Higher emissions levels

will result in more warming, and thus more severe impacts on 

human society and the natural world. 

Confidence in projections of future climate change has increased. The wider range of potential changes in global average temperature in the latest generation of climate model 

simulations32 used in the Intergovernmental Panel on Climate 

Change’s (IPCC) current assessment – versus those in the previous assessment8

 – is simply a result of considering more options 

for future human behavior. For example, one of the scenarios 

included in the IPCC’s latest assessment assumes aggressive 

emissions reductions designed to limit the global temperature 

increase to 3.6°F (2°C) above pre-industrial levels.33 This path 

would require rapid emissions reductions (more than 70% 

reduction in human-related emissions by 2050, and net negative emissions by 2100 – see the Appendix 3: Climate Science, 

Supplemental Message 5) sufficient to achieve heat-trapping 

gas concentrations well below those of any of the scenarios 

considered by the IPCC in its 2007 assessment. Such scenarios 

enable the investigation of climate impacts that would be 

avoided by deliberate, substantial reductions in heat-trapping 

gas emissions. 

Models used in the assessment

This report uses various projections from models of the physical processes affecting the Earth’s climate system, which 

are discussed further in Appendix 3: Climate Science Supplement. Three distinct sets of model simulations for past 

and projected changes in climate are used:

•	 Coupled Model Intercomparison Project, 3rd phase (CMIP3): global model analyses done for the Fourth 

Intergovernmental Panel on Climate Change (IPCC) assessment. Spatial resolutions typically vary from 125 

to 187 miles (at mid-latitudes); approximately 25 representations of different models (not all are used in all 

studies). CMIP3 findings are the foundation for most of the impact analyses included in this assessment.

•	 Coupled Model Intercomparison Project, 5th phase (CMIP5): newer global model analyses done for the 

Fifth IPCC assessment generally based on improved formulations of the CMIP3 models. Spatial resolutions 

typically vary from 62 to 125 miles; about 30 representations of different models (not all are used in all 

studies); this new information was not available in time to serve as the foundation for the impacts analyses 

in this assessment, and information from CMIP5 is primarily provided for comparison purposes.

•	 North American Regional Climate Change Assessment Program (NARCCAP): six regional climate model 

analyses (and limited time-slice analyses from two global models) for the continental U.S. run at about 30-

mile horizontal resolution. The analyses were done for past (1971-2000) and projected (2041-2070) time 

periods. Coarser resolution results from four of the CMIP3 models were used as the boundary conditions 

for the NARCCAP regional climate model studies, with each of the regional models doing analyses with 

boundary conditions from two of the CMIP3 models. 

The scenarios for future human-related emissions of the relevant gases and particles used in these models are further 

discussed in Appendix 3: Climate Science Supplement. The emissions in these scenarios depend on various assumptions about changes in global population, economic and technological development, and choices in transportation 

and energy use.
26 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Figure 2.4. Different amounts of heat-trapping gases released into the atmosphere by human activities produce different 

projected increases in Earth’s temperature. In the figure, each line represents a central estimate of global average 

temperature rise (relative to the 1901-1960 average) for a specific emissions pathway. Shading indicates the range (5th

to 95th percentile) of results from a suite of climate models. Projections in 2099 for additional emissions pathways are 

indicated by the bars to the right of each panel. In all cases, temperatures are expected to rise, although the difference 

between lower and higher emissions pathways is substantial. (Left) The panel shows the two main scenarios (SRES – 

Special Report on Emissions Scenarios) used in this report: A2 assumes continued increases in emissions throughout 

this century, and B1 assumes much slower increases in emissions beginning now and significant emissions reductions 

beginning around 2050, though not due explicitly to climate change policies. (Right) The panel shows newer analyses, 

which are results from the most recent generation of climate models (CMIP5) using the most recent emissions pathways 

(RCPs – Representative Concentration Pathways). Some of these new projections explicitly consider climate policies 

that would result in emissions reductions, which the SRES set did not.35 The newest set includes both lower and higher 

pathways than did the previous set. The lowest emissions pathway shown here, RCP 2.6, assumes immediate and rapid 

reductions in emissions and would result in about 2.5°F of warming in this century. The highest pathway, RCP 8.5, roughly 

similar to a continuation of the current path of global emissions increases, is projected to lead to more than 8°F warming 

by 2100, with a high-end possibility of more than 11°F. (Data from CMIP3, CMIP5, and NOAA NCDC).

Emissions Levels Determine Temperature Rises

Projections of future changes in precipitation show small increases in the global average but substantial shifts in where 

and how precipitation falls. Generally, areas closest to the 

poles are projected to receive more precipitation, while the 

dry subtropics (the region just outside the tropics, between 

23° and 35° on either side of the equator) expand toward the 

poles and receive less rain. Increases in tropical precipitation are projected during rainy seasons (such as monsoons), 

especially over the tropical Pacific. Certain regions, including 

the western U.S. (especially the Southwest1) and the Mediterranean, are presently dry and are expected to become drier. 

The widespread trend of increasing heavy downpours is expected to continue, with precipitation becoming less frequent 

but more intense.34 The patterns of the projected changes of 

precipitation do not contain the spatial details that characterize observed precipitation, especially in mountainous terrain, 

because the projections are averages from multiple models 

and because the effective resolution of global climate models 

is roughly 100-200 miles. 
27 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Figure 2.5. Projected change in average annual temperature over the period 2071-2099 (compared to the period 1970-1999) under a 

low scenario that assumes rapid reductions in emissions and concentrations of heat-trapping gases (RCP 2.6), and a higher scenario 

that assumes continued increases in emissions (RCP 8.5). (Figure source: NOAA NCDC / CICS-NC).

Projected Change in Average Annual Temperature

Figure 2.6. Projected change in average annual precipitation over the period 2071-2099 (compared to the period 1970-1999) under 

a low scenario that assumes rapid reductions in emissions and concentrations of heat-trapping gasses (RCP 2.6), and a higher 

scenario that assumes continued increases in emissions (RCP 8.5). Hatched areas indicate confidence that the projected changes 

are significant and consistent among models. White areas indicate that the changes are not projected to be larger than could be 

expected from natural variability. In general, northern parts of the U.S. (especially the Northeast and Alaska) are projected to receive 

more precipitation, while southern parts (especially the Southwest) are projected to receive less. (Figure source: NOAA NCDC / 

CICS-NC).

Projected Change in Average Annual Precipitation
28 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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One important determinant of how much climate will 

change is the effect of so-called “feedbacks” in the climate 

system, which can either dampen or amplify the initial effect of human influences on temperature. One important 

climate feedback is the loss of summer Arctic sea ice, allowing absorption of substantially more of the sun’s heat 

in the Arctic, increasing warming, and possibly causing 

changes in weather patterns over the United States. 

The observed drastic reduction in sea ice can also lead to 

a “tipping point” – a point beyond which an abrupt or irreversible transition to a different climatic state occurs. In 

this case, the dramatic loss of sea ice could tip the Arctic 

Ocean into a permanent, nearly ice-free state in summer, 

with repercussions that may extend far beyond the Arctic. 

Such potential “tipping points” have been identified in various components of the Earth’s climate system and could 

have important effects on future climate. The extent and 

magnitude of these potential effects are still unknown. 

These are discussed further in the Appendix 4: Frequently 

Asked Questions, under Question T.

Key Message 3: Recent U.S. Temperature Trends

U.S. average temperature has increased by 1.3°F to 1.9°F since record keeping 

began in 1895; most of this increase has occurred since about 1970. The most 

recent decade was the nation’s warmest on record. Temperatures in the United 

States are expected to continue to rise. Because human-induced warming 

is superimposed on a naturally varying climate, the temperature rise has not 

been, and will not be, uniform or smooth across the country or over time.

There have been substantial advances in our understanding of the U.S. temperature record since the 2009 

assessment (see Appendix 3: Climate Science, Supplemental Message 7 for more information). These advances confirm that the U.S. annually averaged temperature 

has increased by 1.3°F to 1.9°F since 1895.1,36,37,38 However, this increase was not constant over time. In particular, temperatures generally rose until about 1940, 

declined slightly until about 1970, then increased rapidly 

thereafter. The year 2012 was the warmest on record for 

the contiguous United States. Over shorter time scales 

(one to two decades), natural variability can reduce the 

rate of warming or even create a temporary cooling (see 

Appendix 3: Climate Science, Supplemental Message 3). 

The cooling in mid-century that was especially prevalent 

over the eastern half of the U.S. may have stemmed 

partly from such natural variations and partly from human influences, in particular the cooling effects of sulfate particles from coal-burning power plants,39 before 

these sulfur emissions were regulated to address health 

and acid rain concerns.

Climate sensitivity

“Climate sensitivity” is an important concept because it 

helps us estimate how much warming might be expected 

for a given increase in the amount of heat-trapping gases. 

It is defined as the amount of warming expected if carbon 

dioxide (CO2) concentrations doubled from pre-industrial 

levels and then remained constant until Earth’s temperature reached a new equilibrium over timescales of centuries to millennia. Climate sensitivity accounts for feedbacks in the climate system that can either dampen or 

amplify warming. The feedbacks primarily determining 

that response are related to water vapor, ice and snow reflectivity, and clouds.8

 Cloud feedbacks have the largest 

uncertainty. The net effect of these feedbacks is expected 

to amplify warming.8

Climate sensitivity has long been estimated to be in the 

range of 2.7°F to 8.1°F. As discussed in Appendix 3: Climate Science Supplement, recent evidence lends further 

confidence in this range.

Quantifying u.s. temperature rise

Quantifying long-term increases of temperature in the U.S. in 

a single number is challenging because the increase has not 

been constant over time. The increase can be quantified in 

a number of ways, but all of them show significant warming 

over the U.S. since the instrumental record began in 1895. 

For example, fitting a linear trend over the period 1895 to 

2012 yields an increase in the range of 1.3 to 1.9°F. Another 

approach, comparing the average temperature during the 

first decade of record with the average during the last decade 

of record, yields a 1.9°F increase. A third approach, calculating the difference between the 1901-1960 average and 

the past decade average yields a change of 1.5°F. Thus, the 

temperature increase cited in this assessment is described 

as 1.3°F to 1.9°F since 1895. Notably, however, the rate of 

rise in temperature over the past 4 to 5 decades has been 

greater than the rate over earlier decades.
29 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Figure 2.7. The colors on the map show temperature changes over the past 22 years (1991-2012) compared to the 1901-1960 

average, and compared to the 1951-1980 average for Alaska and Hawai‘i. The bars on the graphs show the average temperature 

changes by decade for 1901-2012 (relative to the 1901-1960 average) for each region. The far right bar in each graph (2000s 

decade) includes 2011 and 2012. The period from 2001 to 2012 was warmer than any previous decade in every region. (Figure 

source: NOAA NCDC / CICS-NC).

Observed U.S. Temperature Change

Since 1991, temperatures have averaged 1°F to 1.5°F higher 

than 1901-1960 over most of the United States, except for the 

Southeast, where the warming has been less than 1°F. On a 

seasonal basis, long-term warming has been greatest in winter 

and spring.

Warming is ultimately projected for all parts of the nation during this century. In the next few decades, this warming will be 

roughly 2°F to 4°F in most areas. By the end of the century, 

U.S. warming is projected to correspond closely to the level 

of global emissions: roughly 3°F to 5°F under lower emissions 

scenarios (B1 or RCP 4.5) involving substantial reductions in 

emissions, and 5°F to 10°F for higher emissions scenarios (A2 

or RCP 8.5) that assume continued increases in emissions; the 

largest temperature increases are projected for the upper Midwest and Alaska.

Future human-induced warming depends on both past and future emissions of heat-trapping gases and changes in the amount 

of particle pollution. The amount of climate change (aside from 

natural variability) expected for the next two to three decades 

is a combination of the warming already built into the climate 

system by the past history of human emissions of heat-trapping 

gases, and the expected ongoing increases in emissions of those 

gases. However, the magnitude of temperature increases over 

the second half of this century, both in the U.S. and globally, will 

be primarily determined by the emissions produced now and 

over the next few decades, and there are substantial differences 

between higher, fossil-fuel intensive scenarios compared to scenarios in which emissions are reduced. The most recent model 

projections of climate change due to human activities expand 

the range of future scenarios considered (particularly at the lower end), but are entirely consistent with the older model results. 

This consistency increases our confidence in the projections. 
30 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Figure 2.8. Maps show projected change in average surface air temperature in the later part of this century (2071-2099) relative 

to the later part of the last century (1970-1999) under a scenario that assumes substantial reductions in heat trapping gases (B1, 

left) and a higher emissions scenario that assumes continued increases in global emissions (A2, right). (See Appendix 3: Climate 

Science, Supplemental Message 5 for a discussion of temperature changes under a wider range of future scenarios for various 

periods of this century). (Figure source: NOAA NCDC / CICS-NC).

Projected Temperature Change

Figure 2.9. The largest uncertainty in 

projecting climate change beyond the 

next few decades is the level of heattrapping gas emissions. The most recent 

model projections (CMIP5) take into 

account a wider range of options with 

regard to human behavior, including a 

lower scenario than has been considered 

before (RCP 2.6). This scenario assumes 

rapid reductions in emissions – more than 

70% cuts from current levels by 2050 and 

further large decreases by 2100 – and 

the corresponding smaller amount of 

warming. On the higher end, the scenarios 

include one that assumes continued 

increases in emissions (RCP 8.5) and the 

corresponding greater amount of warming. 

Also shown are temperature changes for 

the intermediate scenarios RCP 4.5 (which 

is most similar to B1) and RCP 6.0 (which 

is most similar to A1B; see Appendix 3: 

Climate Science Supplement). Projections 

show change in average temperature in 

the later part of this century (2071-2099) 

relative to the late part of last century 

(1970-1999). (Figure source: NOAA NCDC 

/ CICS-NC). 

Newer simulations for projected temperature (cmip5 models)
31 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Figure 2.10. The frost-free season length, defined as the 

period between the last occurrence of 32°F in the spring 

and the first occurrence of 32°F in the fall, has increased in 

each U.S. region during 1991-2012 relative to 1901-1960. 

Increases in frost-free season length correspond to similar 

increases in growing season length. (Figure source: NOAA 

NCDC / CICS-NC).

Observed Increase in Frost-Free Season Length

Figure 2.11. The maps show projected increases in frost-free season length for the last three 

decades of this century (2070-2099 as compared to 1971-2000) under two emissions scenarios, 

one in which heat-trapping gas emissions continue to grow (A2) and one in which emissions 

peak in 2050 (B1). Increases in the frost-free season correspond to similar increases in the 

growing season. White areas are projected to experience no freezes for 2070-2099, and gray 

areas are projected to experience more than 10 frost-free years during the same period. (Figure 

source: NOAA NCDC / CICS-NC). 

Projected Changes in Frost-Free Season Length

Key Message 4: Lengthening Frost-free Season 

The length of the frost-free season (and the corresponding growing season) has 

been increasing nationally since the 1980s, with the largest increases occurring 

in the western United States, affecting ecosystems and agriculture. Across the 

United States, the growing season is projected to continue to lengthen.

The length of the frost-free season (and the corresponding 

growing season) is a major determinant of the types of plants 

and crops that do well in a particular region. The frost-free season length has been gradually increasing since the 1980s.40 The 

last occurrence of 32°F in the spring has been occurring earlier 

in the year, and the first occurrence of 32°F in the fall has been 

happening later. During 1991-2011, the average frost-free season was about 10 days longer than during 1901-1960. These 

observed climate changes have been mirrored by changes in 

the biosphere, including increases in forest productivity41,42

and satellite-derived estimates of the length of the growing 

season.

43 A longer growing season provides a longer period 

for plant growth and productivity and can slow the increase 

in atmospheric CO2 concentrations through increased CO2

uptake by living things and their environment.44 The longer 

growing season can increase the growth of beneficial plants 

(such as crops and forests) as well as undesirable ones (such 

as ragweed).

45 In some cases where moisture is limited, the 

greater evaporation and loss of moisture through plant transpiration (release of water from plant leaves) associated with a 

longer growing season can mean less productivity because of 

increased drying46 and earlier and longer fire seasons.

The lengthening of the frost-free season has been somewhat 

greater in the western U.S. than the eastern United States,

1

increasing by 2 to 3 weeks in the Northwest and Southwest, 

1 to 2 weeks in the Midwest, Great Plains, and Northeast, and 

slightly less than 1 week in the Southeast. These differences 

mirror the overall trend of 

more warming in the north 

and west and less warming 

in the Southeast.

In a future in which heattrapping gas emissions 

continue to grow, increases 

of a month or more in the 

lengths of the frost-free and 

growing seasons are projected across most of the 

U.S. by the end of the century, with slightly smaller 

increases in the northern 

Great Plains. The largest 

increases in the frost-free 

season (more than 8 weeks) 

are projected for the western U.S., particularly in high 

elevation and coastal areas. 

The increases will be con-
32 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Figure 2.12. The colors on the map show annual total precipitation changes for 1991-2012 compared to the 1901-1960 average, 

and show wetter conditions in most areas. The bars on the graphs show average precipitation differences by decade for 1901-2012 

(relative to the 1901-1960 average) for each region. The far right bar in each graph is for 2001-2012. (Figure source: adapted from 

Peterson et al. 201348).

Observed U.S. Precipitation Change

siderably smaller if heat-trapping gas emissions are reduced, 

although still substantial. These increases are projected to be 

much greater than the normal year-to-year variability experienced today. The projected changes also imply that the southern boundary of the seasonal freeze zone will move northward, with increasing frequencies of years without subfreezing temperatures in the most southern parts of the United States.

Key Message 5: U.S. Precipitation Change

Average U.S. precipitation has increased since 1900, but some areas have 

had increases greater than the national average, and some areas have 

had decreases. More winter and spring precipitation is projected for the 

northern United States, and less for the Southwest, over this century. 

Since 1900, average annual precipitation over the U.S. has increased by roughly 5%. This increase reflects, in part, the major 

droughts of the 1930s and 1950s, which made the early half 

of the record drier. There are important regional differences. 

For instance, precipitation since 1991 (relative to 1901-1960) 

increased the most in the Northeast (8%), Midwest (9%), and 

southern Great Plains (8%), while much of the Southeast and 

Southwest had a mix of areas of increases and decreases.47,48

While significant trends in average precipitation have been 

detected, the fraction of these trends attributable to human 

activity is difficult to quantify at regional scales because the 

range of natural variability in precipitation is large. Projected 

changes are generally small for central portions of the United 

States. However, if emissions of heat-trapping gases continue 

their upward trend, certain global patterns of precipitation 

change are projected to emerge that will affect northern and 
33 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

2: OUR CHANGING CLIMATE

southwestern areas of the United States. The northern U.S. 

is projected to experience more precipitation in the winter 

and spring (except for the Northwest in the spring), while the 

Southwest is projected to experience less, particularly in the 

spring. The contrast between wet and dry areas will increase 

both in the U.S. and globally – in other words, the wet areas 

will get wetter and the dry areas will get drier. As discussed in 

the next section, there has been an increase in the amount of 

precipitation falling in heavy events49 and this is projected to 

continue. 

The projected changes in the northern U.S. are a consequence 

of both a warmer atmosphere (which can hold more moisture than a colder one) and associated changes in large-scale 

Uncertainties in regional projections

On the global scale, climate model simulations show consistent projections of future conditions under a range of emissions 

scenarios. For temperature, all models show warming by late this century that is much larger than historical variations 

nearly everywhere. For precipitation, models are in complete agreement in showing decreases in precipitation in 

the subtropics and increases in precipitation at higher 

latitudes. 

Models unequivocally project large and historically unprecedented future warming in every region of the U.S. 

under all of the scenarios used in this assessment. The 

amount of warming varies substantially between higher 

versus lower scenarios, and moderately from model to 

model, but the amount of projected warming is larger 

than the model-to-model range.

The contiguous U.S. straddles the transition zone between 

drier conditions in the sub-tropics (south) and wetter conditions at higher latitudes (north). Because the precise 

location of this zone varies somewhat among models, projected changes in precipitation in central areas of the U.S. 

range from small increases to small decreases. A clear direction of change only occurs in Alaska and the far north 

of the contiguous U.S. where increases are projected and 

in the far Southwest where decreases are projected. 

Although this means that changes in overall precipitation 

are uncertain in many U.S. areas, there is a high degree 

of certainty that the heaviest precipitation events will increase everywhere, and by large amounts (Figure 2.13). 

This consistent model projection is well understood and is 

a direct outcome of the increase in atmospheric moisture 

caused by warming. There is also more certainty regarding 

dry spells. The annual maximum number of consecutive 

dry days is projected to increase in most areas, especially 

the southern and northwestern portions of the contiguous 

United States. Thus, both extreme wetness and extreme 

dryness are projected to increase in many areas.

Modeling methods that downscale (generate higher spatial resolution) climate projections from coarser global 

model output can reduce the range of projections to the 

extent that they incorporate better representation of certain physical processes (such as the influence of topography and 

convection). However, a sizeable portion of the range is a result of the variations in large-scale patterns produced by the 

global models and so downscaling methods do not change this.

Figure 2.13. Top panels show simulated changes in the average 

amount of precipitation falling on the wettest day of the year for 

the period 2070-2099 as compared to 1971-2000 under a scenario 

that assumes rapid reductions in emissions (RCP 2.6) and one 

that assumes continued emissions increases (RCP 8.5). Bottom 

panels show simulated changes in the annual maximum number of 

consecutive dry days (days receiving less than 0.04 inches (1 mm) 

of precipitation) under the same two scenarios. Simulations are 

from CMIP5 models. Stippling indicates areas where changes are 

consistent among at least 80% of the models used in this analysis. 

(Figure source: NOAA NCDC / CICS-NC).
34 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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weather patterns (which affect where precipitation occurs). 

The projected reduction in Southwest precipitation is a result of changes in large-scale weather patterns, including the 

northward expansion of the belt of high pressure in the subtropics, which suppresses rainfall. Recent improvements in understanding these mechanisms of change increase confidence 

in these projections.50 The patterns of the projected changes 

of precipitation resulting from human alterations of the climate are geographically smoother in these maps than what 

will actually be observed because: 1) the precise locations of 

natural increases and decreases differ from model to model, 

and averaging across models smooths these differences; and 

2) the resolution of current climate models is too coarse to 

capture fine topographic details, especially in mountainous 

terrain. Hence, there is considerably more confidence in the 

large-scale patterns of change than in local details.

In general, a comparison of the various sources of climate 

model data used in this assessment provides a consistent 

picture of the large-scale projected precipitation changes 

Figure 2.14. Projected change in seasonal precipitation for 2071-2099 (compared to 1970-1999) under an emissions scenario that 

assumes continued increases in emissions (A2). Hatched areas indicate that the projected changes are significant and consistent 

among models. White areas indicate that the changes are not projected to be larger than could be expected from natural variability. 

In general, the northern part of the U.S. is projected to see more winter and spring precipitation, while the southwestern U.S. is 

projected to experience less precipitation in the spring. (Figure source: NOAA NCDC / CICS-NC). 

Projected Precipitation Change by Season
35 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Newer simulations for projected precipitation change (cmip5 models)

Figure 2.15. Seasonal precipitation change for 2071-2099 (compared to 1970-1999) as projected by recent simulations that include 

a wider range of scenarios. The maps on the left (RCP 2.6) assume rapid reductions in emissions – more than 70% cuts from current 

levels by 2050 – and a corresponding much smaller amount of warming and far less precipitation change. On the right, RCP 8.5 

assumes continued increases in emissions, with associated large increases in warming and major precipitation changes. These would 

include, for example, large reductions in spring precipitation in the Southwest and large increases in the Northeast and Midwest. 

Rapid emissions reductions would be required for the more modest changes in the maps on the left. Hatched areas indicate that the 

projected changes are significant and consistent among models. White areas indicate that the changes are not projected to be larger 

than could be expected from natural variability. (Figure source: NOAA NCDC / CICS-NC). 

across the United States (see “Models Used in the Assessment”). Multi-model average changes in all three of these 

sources show a general pattern of wetter future conditions in 

the north and drier conditions in the south. The regional suite 

generally shows conditions that are somewhat wetter overall 

in the wet areas and not as dry in the dry areas. The general 

pattern agreement among these three sources, with the wide 

variations in their spatial resolution, provides confidence that 

this pattern is robust and not sensitive to the limited spatial 

resolution of the models. The slightly different conditions in 

the North American NARCCAP regional analyses for the U.S. 

appear to arise partially or wholly from the choice of the four 

CMIP3 global climate models used to drive the regional simulations. These four global models, averaged together, project 

average changes that are 2% wetter than the average of the 

suite of global models used in CMIP3. 

The patterns of precipitation change in the newer CMIP5 simulations are essentially the same as in the earlier CMIP3 and 

NARCCAP simulations used in impact analyses throughout this 

report, increasing confidence in our scientific understanding. 

The subtle differences between these two sets of projections 

are mostly due to the wider range of future scenarios considered in the more recent simulations. Thus, the overall picture 

remains the same: wetter conditions in the north and drier conditions in the Southwest in winter and spring. Drier conditions 

are projected for summer in most areas of the contiguous U.S. 

but, outside of the Northwest and south-central region, there 

is generally not high confidence that the changes will be large 

compared to natural variability. In all models and scenarios, 

a transition zone between drier (to the south) and wetter (to 

the north) shifts northward from the southern U.S. in winter to 

southern Canada in summer. Wetter conditions are projected 

for Alaska and northern Canada in all seasons. 
36 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Key Message 6: Heavy Downpours Increasing 

Heavy downpours are increasing nationally, especially over the last three to five decades. 

Largest increases are in the Midwest and Northeast. Increases in the frequency 

and intensity of extreme precipitation events are projected for all U.S. regions. 

Across most of the United States, the heaviest 

rainfall events have become heavier and more frequent. The amount of rain falling on the heaviest 

rain days has also increased over the past few decades. Since 1991, the amount of rain falling in very 

heavy precipitation events has been significantly 

above average. This increase has been greatest in 

the Northeast, Midwest, and upper Great Plains 

– more than 30% above the 1901-1960 average 

(see Figure 2.18). There has also been an increase 

in flooding events in the Midwest and Northeast 

where the largest increases in heavy rain amounts 

have occurred. 

Figure 2.16: One measure of 

a heavy precipitation event is 

a 2-day precipitation total that 

is exceeded on average only 

once in a five-year period, 

also known as a once-in-fiveyear event. As this extreme 

precipitation index for 1901-

2012 shows, the occurrence 

of such events has become 

m u c h m o r e c o m m o n i n 

recent decades. Changes are 

compared to the period 1901-

1960, and do not include Alaska or Hawai‘i. The 2000s decade (far right bar) 

includes 2001-2012. (Figure source: adapted from Kunkel et al. 201352). 

Observed U.S. Trend in Heavy Precipitation

Figure 2.17. Percent changes in the annual amount of precipitation falling in very heavy events, defined as the heaviest 1% of all daily 

events from 1901 to 2012 for each region. The far right bar is for 2001-2012. In recent decades there have been increases nationally, 

with the largest increases in the Northeast, Great Plains, Midwest, and Southeast. Changes are compared to the 1901-1960 average 

for all regions except Alaska and Hawai‘i, which are relative to the 1951-1980 average. (Figure source: NOAA NCDC / CICS-NC).

Observed Change in Very Heavy Precipitation
37 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Figure 2.18. The map shows percent increases in the amount 

of precipitation falling in very heavy events (defined as the 

heaviest 1% of all daily events) from 1958 to 2012 for each 

region of the continental United States. These trends are larger 

than natural variations for the Northeast, Midwest, Puerto Rico, 

Southeast, Great Plains, and Alaska. The trends are not larger 

than natural variations for the Southwest, Hawai‘i, and the 

Northwest. The changes shown in this figure are calculated 

from the beginning and end points of the trends for 1958 to 

2012. (Figure source: updated from Karl et al. 20091).

Observed Change in Very Heavy Precipitation

Figure 2.19. Maps show the increase in frequency of extreme daily precipitation events (a daily amount that now occurs once in 20 

years) by the later part of this century (2081-2100) compared to the later part of last century (1981-2000). Such extreme events are 

projected to occur more frequently everywhere in the United States. Under the rapid emissions reduction scenario (RCP 2.6), these 

events would occur nearly twice as often. For the scenario assuming continued increases in emissions (RCP 8.5), these events would 

occur up to five times as often. (Figure source: NOAA NCDC / CICS-NC). 

Projected Change in Heavy Precipitation Events

Warmer air can contain more water vapor than cooler air. 

Global analyses show that the amount of water vapor in the atmosphere has in fact increased over both land and oceans.14,51

Climate change also alters dynamical characteristics of the 

atmosphere that in turn affect weather patterns and storms. 

In the mid-latitudes, where most of the continental U.S. is located, there is an upward trend in extreme precipitation in the 

vicinity of fronts associated with mid-latitude storms.52 Locally, 

natural variations can also be important.53 

Projections of future climate over the U.S. suggest that the 

recent trend towards increased heavy precipitation events 

will continue. This is projected to occur even in regions where 

total precipitation is projected to decrease, such as the Southwest.52,54,55
38 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

2: OUR CHANGING CLIMATE©Bill Ross/Corbis ©Scott Olson/Getty Images

Key Message 7: Extreme Weather 

There have been changes in some types of extreme weather events over the last several 

decades. Heat waves have become more frequent and intense, especially in the West. Cold 

waves have become less frequent and intense across the nation. There have been regional 

trends in floods and droughts. Droughts in the Southwest and heat waves everywhere 

are projected to become more intense, and cold waves less intense everywhere.

Heat waves are periods of abnormally hot weather lasting days 

to weeks.48 Heat waves have generally become more frequent 

across the U.S. in recent decades, with western regions (including Alaska) setting records for numbers of these events in the 

2000s. Tree ring data suggests that the drought over the last 

decade in the western U.S. represents the driest conditions in 

800 years.1,56 Most other regions in the country had their highest number of short-duration heat waves in the 1930s, when 

the multi-year severe drought of the Dust Bowl period, combined with deleterious land-use practices,57 contributed to the 

intense summer heat through depletion of soil moisture and 

reduction of the moderating effects of evaporation.58 However, the recent prolonged (multi-month) extreme heat has been 

unprecedented since the start of reliable instrumental records 

in 1895. The recent heat waves and droughts in Texas (2011) 

and the Midwest (2012) set records for highest monthly average temperatures, exceeding in some cases records set in the 

1930s, including the highest monthly contiguous U.S. temperature on record (July 2012, breaking the July 1936 record) and 

the hottest summers on record in several states (New Mexico, 

Texas, Oklahoma, and Louisiana in 2011 and Colorado and 

Wyoming in 2012). For the spring and summer months, 2012 

had the second largest area of record-setting monthly average 

temperatures, including a 26-state area from Wyoming to the 

East Coast. The summer (June-August) temperatures of 2012 

ranked in the hottest 10% of the 118-year period of record in 

28 states covering the Rocky Mountain states, the Great Plains, 

the Upper Midwest, and the Northeast. The new records included both hot daytime maximum temperatures and warm 

nighttime minimum temperatures.59 Corresponding with this 

increase in extreme heat, the number of extreme cold waves 

has reached the lowest levels on record (since 1895). 

Many more high temperature records are being broken as 

compared to low temperature records over the past three to 

four decades – another indicator of a warming climate.60 The 

number of record low monthly temperatures has declined to 

the lowest levels since 1911, while the number of record high 

monthly temperatures has increased to the highest level since 

the 1930s. During this same period, there has been an increasing trend in persistently high nighttime temperature.1

 There 

are various reasons why low temperatures have increased 

more than high temperatures.61

In some areas, prolonged periods of record high temperatures 

associated with droughts contribute to dry conditions that are 

driving wildfires.62 The meteorological situations that cause 

heat waves are a natural part of the climate system. Thus 

the timing and location of individual events may be largely a 

natural phenomenon, although even these may be affected by 

human-induced climate change.63 However, there is emerging 

evidence that most of the increases of heat wave severity over 

the U.S. are likely due to human activity,64 with a detectable 

human influence in recent heat waves in the southern Great 

Plains1,65 as well as in Europe7,62 and Russia.60,66,67 The summer 

2011 heat wave and drought in Texas was primarily driven by 

precipitation deficits, but the human contribution to climate 

change approximately doubled the probability that the heat 

was record-breaking.68 So while an event such as this Texas 

heat wave and drought could be triggered by a naturally occurring event such as a deficit in precipitation, the chances for 

record-breaking temperature extremes has increased and will 
39 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

2: OUR CHANGING CLIMATE

Figure 2.20. Change in surface air temperature at the end of this century (2081-2100) relative to the turn of the last century (1986-2005) 

on the coldest and hottest days under a scenario that assumes a rapid reduction in heat trapping gases (RCP 2.6) and a scenario 

that assumes continued increases in these gases (RCP 8.5). This figure shows estimated changes in the average temperature of 

the hottest and coldest days in each 20-year period. In other words, the hottest days will get even hotter, and the coldest days will 

be less cold. (Figure source: NOAA NCDC / CICS-NC).

Projected Temperature Change of Hottest and Coldest Days

continue to increase as the global climate warms. Generally, 

the changes in climate are increasing the likelihood for these 

types of severe events.

The number of extremely hot days is projected to continue 

to increase over much of the United States, especially by late 

century. Summer temperatures are projected to continue rising, and a reduction of soil moisture, which exacerbates heat 

waves, is projected for much of the western and central U.S. in 

summer. Climate models project that the same summertime 

temperatures that ranked among the hottest 5% in 1950-1979 

will occur at least 70% of the time by 2035-2064 in the U.S. if 

global emissions of heat-trapping gases continue to grow (as in 

the A2 scenario).67 By the end of this century, what have previously been once-in-20-year extreme heat days (1-day events) 

are projected to occur every two or three years over most of 

the nation.69,70 In other words, what now seems like an extremely hot day will become commonplace. 
40 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Figure 2.21. Trend magnitude (triangle size) and direction (green = increasing trend, brown = 

decreasing trend) of annual flood magnitude from the 1920s through 2008. Local areas can be 

affected by land-use change (such as dams). Most significant are the increasing trend for floods in 

the Midwest and Northeast and the decreasing trend in the Southwest. (Figure source: Peterson 

et al. 201348).

There are significant trends Trends in Flood Magnitude

in the magnitude of river 

flooding in many parts of the 

United States. When averaged over the entire nation, 

however, the increases and 

decreases cancel each other 

out and show no national 

level trend.71 River flood 

magnitudes have decreased 

in the Southwest and increased in the eastern Great 

Plains, parts of the Midwest, 

and from the northern Appalachians into New England.48

Figure 2.21 shows increasing 

trends in floods in green and 

decreasing trends in brown. 

The magnitude of these 

trends is illustrated by the 

size of the triangles. 

These regional river flood 

trends are qualitatively consistent with trends in climate 

conditions associated with 

flooding. For example, average annual precipitation has increased in the Midwest and 

Northeast and decreased in the Southwest (Figure 2.12).48 Recent soil moisture trends show general drying in the Southwest 

and moistening in the Northeast and northern Great Plains and 

Midwest (Ch 3: Water, Figure 3.2). These trends are in general 

agreement with the flood trends. Although there is a strong 

national upward trend in extreme precipitation and not in river 

flooding, the regional variations are similar. Extreme precipitation has been increasing strongly in the Great Plains, Midwest, 

and Northeast, where river flooding increases have been observed, and there is little trend in the Southwest, where river 

flooding has decreased. An exact correspondence is not necessarily expected since the seasonal timing of precipitation 

events makes a difference in whether river flooding occurs. 

The increase in extreme precipitation events has been concentrated in the summer and fall52 when soil moisture is seasonally low and soils can absorb a greater fraction of rainfall. By 

contrast, many of the annual flood events occur in the spring 

when soil moisture is high. Thus, additional extreme rainfall 

events in summer and fall may not create sufficient runoff for 

the resulting streamflow to exceed spring flood magnitudes. 

However, these extreme precipitation events are often associated with local flash floods, a leading cause of death due to 

weather events (see “Flood Factors and Flood Types” in Ch. 3: 

Water).

Research into the effects of human-induced climate change on 

flood events is relatively new. There is evidence of a detectable human influence in recent flooding events in England and 

Wales13 and in other specific events around the globe during 

2011.48 In general, heavier rains lead to a larger fraction of 

rainfall running off and, depending on the surface conditions, 

more potential for flooding.

Higher temperatures lead to increased rates of evaporation, 

including more loss of moisture through plant leaves. Even in 

areas where precipitation does not decrease, these increases 

in surface evaporation and loss of water from plants lead to 

more rapid drying of soils if the effects of higher temperatures 

are not offset by other changes (such as in wind speed or humidity).72 As soil dries out, a larger proportion of the incoming heat from the sun goes into heating the soil and adjacent 

air rather than evaporating its moisture, resulting in hotter 

summers under drier climatic conditions.73 Under higher emissions scenarios, widespread drought is projected to become 

more common over most of the central and southern United 

States.56,74,75,76,77
41 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Figure 2.22. Average change in 

soil moisture compared to 1971-

2000, as projected for the middle 

of this century (2041-2070) and 

late this century (2071-2100) under 

two emissions scenarios, a lower 

scenario (B1) and a higher scenario 

(A2).75,77 The future drying of soils 

in most areas simulated by this 

sophisticated hydrologic model 

(Variable Infiltration Capacity or VIC 

model) is consistent with the future 

drought increases using the simpler 

Palmer Drought Severity Index 

(PDSI) metric. Only the western 

U.S. is displayed because model 

simulations were only run for this 

area. (Figure source: NOAA NCDC 

/ CICS-NC).

Projected Changes in Soil Moisture for the Western U.S.

Key Message 8: Changes in Hurricanes 

The intensity, frequency, and duration of North Atlantic hurricanes, as well as 

the frequency of the strongest (Category 4 and 5) hurricanes, have all increased 

since the early 1980s. The relative contributions of human and natural causes 

to these increases are still uncertain. Hurricane-associated storm intensity and 

rainfall rates are projected to increase as the climate continues to warm. 

There has been a substantial increase in most measures of 

Atlantic hurricane activity since the early 1980s, the period 

during which high-quality satellite data are available.

78,79 These 

include measures of intensity, frequency, and duration as well 

as the number of strongest (Category 4 and 5) storms. The ability to assess longer-term trends in hurricane activity is limited 

by the quality of available data. The historic record of Atlantic 

hurricanes dates back to the mid-1800s, and indicates other 

decades of high activity. However, there is considerable uncertainty in the record prior to the satellite era (early 1970s), 

and the further back in time one goes, the more uncertain the 

record becomes.79

The recent increases in activity are linked, in part, to higher 

sea surface temperatures in the region that Atlantic hurricanes 

form in and move through. Numerous factors have been shown 

to influence these local sea surface temperatures, including 

natural variability, human-induced emissions of heat-trapping 

gases, and particulate pollution. Quantifying the relative contributions of natural and human-caused factors is an active 

focus of research. Some studies suggest that natural variability, which includes the Atlantic Multidecadal Oscillation, is the 

dominant cause of the warming trend in the Atlantic since the 

1970s,80,81 while others argue that human-caused heat-trapping gases and particulate pollution are more important.82

Hurricane development, however, is influenced by more than 

just sea surface temperature. How hurricanes develop also 

depends on how the local atmosphere responds to changes 

in local sea surface temperatures, and this atmospheric response depends critically on the cause of the change.83 For 

example, the atmosphere responds differently when local sea 

surface temperatures increase due to a local decrease of particulate pollution that allows more sunlight through to warm 

the ocean, versus when sea surface temperatures increase 

more uniformly around the world due to increased amounts 

of human-caused heat-trapping gases.80,84 So the link between 

hurricanes and ocean temperatures is complex. Improving our 
42 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Figure 2.23. Recent variations of the Power Dissipation Index (PDI) in the North Atlantic and eastern North Pacific Oceans. 

PDI is an aggregate of storm intensity, frequency, and duration and provides a measure of total hurricane power over 

a hurricane season. There is a strong upward trend in Atlantic PDI, and a downward trend in the eastern North Pacific, 

both of which are well-supported by the reanalysis. Separate analyses (not shown) indicate a significant increase in 

the strength and in the number of the strongest hurricanes (Category 4 and 5) in the North Atlantic over this same time 

period. The PDI is calculated from historical data (IBTrACS92) and from reanalyses using satellite data (UW/NCDC & 

ADT-HURSAT93,94). IBTrACS is the International Best Track Archive for Climate Stewardship, UW/NCDC is the University 

of Wisconsin/NOAA National Climatic Data Center satellite-derived hurricane intensity dataset, and ADT-HURSAT is the 

Advanced Dvorak Technique–Hurricane Satellite dataset (Figure source: adapted from Kossin et al. 200793).

Observed Trends in Hurricane Power Dissipation

North Atlantic hurricanes have increased in intensity, frequency, and duration since 

the early 1980s.

understanding of the relationships between warming tropical 

oceans and tropical cyclones is another active area of research.

Changes in the average length and positions of Atlantic storm 

tracks are also associated with regional climate variability.85

The locations and frequency of storms striking land have been 

argued to vary in opposing ways than basin-wide frequency. 

For example, fewer storms have been observed to strike land 

during warmer years even though overall activity is higher than 

average,86 which may help to explain the lack of any clear trend 

in landfall frequency along the U.S. eastern and Gulf coasts.87,88

Climate models also project changes in hurricane tracks and 

where they strike land.89 The specific characteristics of the 

changes are being actively studied.

Other measures of Atlantic storm activity are projected to 

change as well.87,90,91 By late this century, models, on average, project a slight decrease in the annual number of tropical cyclones, but an increase in the number of 

the strongest (Category 4 and 5) hurricanes. 

These projected changes are based on an average of projections from a number of individual models, and they represent the most likely 

outcome. There is some uncertainty in this as 

the individual models do not always agree on 

the amount of projected change, and some 

models may project an increase where others 

project a decrease. The models are in better 

agreement when projecting changes in hurricane precipitation – almost all existing studies 

project greater rainfall rates in hurricanes in 

a warmer climate, with projected increases of 

about 20% averaged near the center of hurricanes. 

NOAA Environmental Visualization Lab
43 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

2: OUR CHANGING CLIMATE © John Zich/zrImages/Corbis

 © Roger Hill/ /Science Photo Library/Corbis

Key Message 9: Changes in Storms

Winter storms have increased in frequency and intensity since the 1950s, and 

their tracks have shifted northward over the United States. Other trends in severe 

storms, including the intensity and frequency of tornadoes, hail, and damaging 

thunderstorm winds, are uncertain and are being studied intensively.

Trends in the occurrences of storms, ranging from severe thunderstorms to winter storms to hurricanes, are subject to much 

greater uncertainties than trends in temperature and variables 

that are directly related to temperature (such as snow and ice 

cover, ocean heat content, and sea level). Recognizing that the 

impacts of changes in the frequency and intensity of these 

storms can easily exceed the impacts of changes in average 

temperature or precipitation, climate scientists are actively researching the connections between climate change and severe 

storms. There has been a sizeable upward trend in the number 

of storms causing large financial and other losses.95 However, 

there are societal contributions to this trend, such as increases 

in population and wealth.52

Severe Convective Storms

Tornadoes and other severe thunderstorm phenomena frequently cause as much annual property damage in the U.S. as 

do hurricanes, and often cause more deaths. Recent research 

has yielded insights into the connections between global 

warming and the factors that cause tornadoes and severe 

thunderstorms (such as atmospheric instability and increases 

in wind speed with altitude96). Although these relationships 

are still being explored, a recent study suggests a projected 

increase in the frequency of conditions favorable for severe 

thunderstorms.97

Winter Storms 

For the entire Northern Hemisphere, there is evidence of an 

increase in both storm frequency and intensity during the cold 

season since 1950,98 with storm tracks having shifted slightly 

towards the poles.99,100 Extremely heavy snowstorms increased 

in number during the last century in northern and eastern 

parts of the United States, but have been less frequent since 

2000.52,101 Total seasonal snowfall has generally decreased in 

southern and some western areas,102 increased in the northern 

Great Plains and Great Lakes region,102,103 and not changed in 

other areas, such as the Sierra Nevada, although snow is melting earlier in the year and more precipitation is falling as rain 

versus snow.104 Very snowy winters have generally been decreasing in frequency in most regions over the last 10 to 20 

years, although the Northeast has been seeing a normal number of such winters.105 Heavier-than-normal snowfalls recently 

observed in the Midwest and Northeast U.S. in some years, 

with little snow in other years, are consistent with indications 

of increased blocking (a large scale pressure pattern with little 

or no movement) of the wintertime circulation of the Northern 

Hemisphere.106 However, conclusions about trends in blocking 

have been found to depend on the method of analysis,107 so 

the assessment and attribution of trends in blocking remains 

an active research area. Overall snow cover has decreased in 

the Northern Hemisphere, due in part to higher temperatures 

that shorten the time snow spends on the ground.108
44 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Figure 2.24. Variation of winter storm frequency and intensity during the cold season (NovemberMarch) for high latitudes (60-90°N) and mid-latitudes (30-60°N) of the Northern Hemisphere over 

the period 1949-2010. The bar for each decade represents the difference from the long-term 

average. Storm frequencies have increased in middle and high latitudes, and storm intensities 

have increased in middle latitudes. (Figure source: updated from CCSP 2008109).

Variation of Storm Frequency and Intensity

during the Cold Season (November – March)

Key Message 10: Sea Level Rise

Global sea level has risen by about 8 inches since reliable record keeping 

began in 1880. It is projected to rise another 1 to 4 feet by 2100.

The oceans are absorbing over 90% of the increased atmospheric heat associated with emissions from human activity.110

Like mercury in a thermometer, water expands as it warms up 

(this is referred to as “thermal expansion”) causing sea levels 

to rise. Melting of glaciers and ice sheets is also contributing to 

sea level rise at increasing rates.111

Since the late 1800s, tide gauges throughout the world have 

shown that global sea level has risen by about 8 inches. A 

new data set (Figure 2.25) shows that this recent rise is much 

greater than at any time in at least the past 2000 years.112 Since 

1992, the rate of global sea level rise measured by satellites has 

been roughly twice the rate observed over the last century, 

providing evidence of additional acceleration.113
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Figure 2.25. Sea level change in the North Atlantic Ocean relative to the 

year 2000 based on data collected from North Carolina112 (red line, pink 

band shows the uncertainty range) compared with a reconstruction of global 

sea level rise based on tide gauge data from 1750 to present127 (blue line). 

(Figure source: Adapted from Kemp et al. 2011112).

North Atlantic Sea Level Change

Figure 2.26. Estimated, observed, and possible future 

amounts of global sea level rise from 1800 to 2100, 

relative to the year 2000. Estimates from proxy data112

(for example, based on sediment records) are shown 

in red (1800-1890, pink band shows uncertainty), tide 

gauge data are shown in blue for 1880-2009,113 and 

satellite observations are shown in green from 1993 to 

2012. 128 The future scenarios range from 0.66 feet to 

6.6 feet in 2100.123 These scenarios are not based on 

climate model simulations, but rather reflect the range of 

possible scenarios based on other scientific studies. The 

orange line at right shows the currently projected range 

of sea level rise of 1 to 4 feet by 2100, which falls within 

the larger risk-based scenario range. The large projected 

range reflects uncertainty about how glaciers and ice 

sheets will react to the warming ocean, the warming 

atmosphere, and changing winds and currents. As seen 

in the observations, there are year-to-year variations in the 

trend. (Figure source: Adapted from Parris et al. 2012,123

with contributions from NASA Jet Propulsion Laboratory).

Past and Projected Changes in Global Sea Level Rise

Projecting future rates of sea level rise is challenging. Even the most sophisticated climate models, 

which explicitly represent Earth’s physical processes, cannot simulate rapid changes in ice sheet 

dynamics, and thus are likely to underestimate 

future sea level rise. In recent years, “semi-empirical” methods have been developed to project 

future rates of sea level rise based on a simple statistical relationship between past rates of globally 

averaged temperature change and sea level rise. 

These models suggest a range of additional sea 

level rise from about 2 feet to as much as 6 feet by 

2100, depending on emissions scenario.

114,115,116,117

It is not clear, however, whether these statistical 

relationships will hold in the future, or that they 

fully explain historical behavior.118 Regardless of 

the amount of change by 2100, however, sea level 

rise is expected to continue well beyond this century as a result of both past and future emissions 

from human activities.

Scientists are working to narrow the range of sea level rise 

projections for this century. Recent projections show that for 

even the lowest emissions scenarios, thermal expansion of 

ocean waters119 and the melting of small mountain glaciers120

will result in 11 inches of sea level rise by 2100, even without 

any contribution from the ice sheets in Greenland and Antarctica. This suggests that about 1 foot of global sea level rise 

by 2100 is probably a realistic low end. On the high end, recent work suggests that 4 feet is plausible.22,115,121 In the context of risk-based analysis, some decision makers may wish to 

use a wider range of scenarios, from 8 inches to 6.6 feet by 

2100.

122,123 In particular, the high end of these scenarios may 

be useful for decision makers with a low tolerance for risk (see 

Figure 2.26 on global sea level rise).122,123 Although scientists 

cannot yet assign likelihood to any particular scenario, in general, higher emissions scenarios that lead to more warming 

would be expected to lead to higher amounts of sea level rise.

Nearly 5 million people in the U.S. live within 4 feet of the local high-tide level (also known as mean higher high water). In 

the next several decades, storm surges and high tides could 

combine with sea level rise and land subsidence to further increase flooding in many of these regions.124 Sea level rise will 

not stop in 2100 because the oceans take a very long time to 

respond to warmer conditions at the Earth’s surface. Ocean 

waters will therefore continue to warm and sea level will continue to rise for many centuries at rates equal to or higher 

than that of the current century.125 In fact, recent research 

has suggested that even present day carbon dioxide levels 

are sufficient to cause Greenland to melt completely over the 

next several thousand years.126
46 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Figure 2.27. Bars show decade averages of annual maximum Great Lakes ice 

coverage from the winter of 1962-1963, when reliable coverage of the entire 

Great Lakes began, to the winter of 2012-2013. Bar labels indicate the end 

year of the winter; for example, 1963-1972 indicates the winter of 1962-1963 

through the winter of 1971-1972. Only the most recent period includes the 

eleven years from 2003 to 2013. (Data updated from Bai and Wang, 2012130).

Ice Cover in the Great Lakes

Key Message 11: Melting Ice 

Rising temperatures are reducing ice volume and surface extent on land, 

lakes, and sea. This loss of ice is expected to continue. The Arctic Ocean is 

expected to become essentially ice free in summer before mid-century.

Rising temperatures across the U.S. have reduced lake ice, 

sea ice, glaciers, and seasonal snow cover over the last few 

decades.111 In the Great Lakes, for example, total winter ice 

coverage has decreased by 63% since the early 1970s.172 This 

includes the entire period since satellite data became available. When the record is extended back to 1963 using presatellite data,129 the overall trend is less negative because the 

Great Lakes region experienced several extremely cold winters 

in the 1970s. 

Sea ice in the Arctic has also decreased dramatically since the 

late 1970s, particularly in summer and autumn. Since the satellite record began in 1978, minimum Arctic sea ice extent (which 

occurs in early to mid-September) has decreased by more than 

40%.131 This decline is unprecedented in the historical record, 

and the reduction of ice volume and thickness is even greater. 

Ice thickness decreased by more than 50% from 1958-1976 to 

2003-2008,132 and the percentage of the March ice cover made 

up of thicker ice (ice that has survived a summer melt season) 

decreased from 75% in the mid-1980s to 45% in 2011.133 Recent 

analyses indicate a decrease of 36% in autumn sea ice volume 

over the past decade.134 The 2012 sea ice minimum broke the preceding record (set in 2007) 

by more than 200,000 square miles. Ice loss 

increases Arctic warming by replacing white, 

reflective ice with dark water that absorbs 

more energy from the sun. More open water 

can also increase snowfall over northern land 

areas135 and increase the north-south meanders of the jet stream, consistent with the occurrence of unusually cold and snowy winters 

at mid-latitudes in several recent years.106,135

Significant uncertainties remain at this time in 

interpreting the effect of Arctic ice changes on 

mid-latitudes.107

The loss of sea ice has been greater in summer 

than in winter. The Bering Sea, for example, has 

sea ice only in the winter-spring portion of the 

year, and shows no trend in surface area covered by ice over the past 30 years. However, 

seasonal ice in the Bering Sea and elsewhere in 

the Arctic is thin and susceptible to rapid melt 

during the following summer. 

The seasonal pattern of observed loss of Arctic 

sea ice is generally consistent with simulations 

by global climate models, in which the extent 

of sea ice decreases more rapidly in summer 

than in winter. However, the models tend to underestimate the 

amount of decrease since 2007. Projections by these models 

indicate that the Arctic Ocean is expected to become essentially ice-free in summer before mid-century under scenarios 

that assume continued growth in global emissions, although 

sea ice would still form in winter.136,137 Models that best match 

historical trends project a nearly sea ice-free Arctic in summer 

by the 2030s,138 and extrapolation of the present observed 

trend suggests an even earlier ice-free Arctic in summer.139

However, even during a long-term decrease, occasional temporary increases in Arctic summer sea ice can be expected 

over timescales of a decade or so because of natural variability.140 The projected reduction of winter sea ice is only about 

10% by 2030,141 indicating that the Arctic will shift to a more 

seasonal sea ice pattern. While this ice will be thinner, it will 

cover much of the same area now covered by sea ice in winter.

While the Arctic is an ocean surrounded by continents, Antarctica is a continent surrounded by ocean. Nearly all of the sea 

ice in the Antarctic melts each summer, and changes there are 

more complicated than in the Arctic. While Arctic sea ice has 
47 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

2: OUR CHANGING CLIMATE

Figure 2.28. Summer Arctic sea ice has declined dramatically since satellites began measuring it in 1979. The extent of sea ice in 

September 2012, shown in white in the top figure, was more than 40% below the median for 1979-2000. The graph on the bottom 

left shows annual variations in September Arctic sea ice extent for 1979-2013. It is also notable that the ice has become much 

thinner in recent years, so its total volume (bottom right) has declined even more rapidly than the extent.111 (Figure and data from 

National Snow and Ice Data Center).

Decline in Arctic Sea Ice Extent

been strongly decreasing, there has been a slight increase in 

sea ice in Antarctica.142 Explanations for this include changes 

in winds that directly affect ice drift as well as the properties 

of the surrounding ocean,143 and that winds around Antarctica 

may have been affected by stratospheric ozone depletion.144

Snow cover on land has decreased over the past several decades,145 especially in late spring.146 Each of five recent years 

(2008-2012) has set a new record for minimum snow extent 

in June in Eurasia, as did three of those five years in North 

America. 

The surface of the Greenland Ice Sheet has been experiencing 

summer melting over increasingly large areas during the past 

several decades. In the decade of the 2000s, the daily melt area 

summed over the warm season was double the corresponding 

amounts of the 1970s,147 culminating in summer surface melt 

that was far greater (97% of the Greenland Ice Sheet area) in 

2012 than in any year since the satellite record began in 1979. 

More importantly, the rate of mass loss from the Greenland 

Ice Sheet’s marine-terminating outlet glaciers has accelerated 

in recent decades, leading to predictions that the proportion 

of global sea level rise coming from Greenland will continue 

to increase.148 Glaciers terminating on ice shelves and on land 

are also losing mass, but the rate of loss has not accelerated 
48 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

2: OUR CHANGING CLIMATE

Figure 2.29. Model simulations of Arctic sea ice extent for September (1900-2100) 

based on observed concentrations of heat-trapping gases and particles (through 

2005) and four scenarios. Colored lines for RCP scenarios are model averages 

(CMIP5) and lighter shades of the line colors denote ranges among models for 

each scenario. Dotted gray line and gray shading denotes average and range of 

the historical simulations through 2005. The thick black line shows observed data 

for 1953-2012. These newer model (CMIP5) simulations project more rapid sea ice 

loss compared to the previous generation of models (CMIP3) under similar forcing 

scenarios, although the simulated September ice losses under all scenarios still 

lag the observed loss of the past decade. Extrapolation of the present observed 

trend suggests an essentially ice-free Arctic in summer before mid-century.139 The 

Arctic is considered essentially ice-free when the areal extent of ice is less than 

one million square kilometers. (Figure source: adapted from Stroeve et al. 2012136).

Projected Arctic Sea Ice Decline over the past decade.149 As discussed in Key 

Message 10, the dynamics of the Greenland 

Ice Sheet are generally not included in present global climate models and sea level rise 

projections.

Glaciers are retreating and/or thinning in 

Alaska and in the lower 48 states. In addition, permafrost temperatures are increasing over Alaska and much of the Arctic. 

Regions of discontinuous permafrost in 

interior Alaska (where annual average soil 

temperatures are already close to 32°F) are 

highly vulnerable to thaw. Thawing permafrost releases carbon dioxide and methane 

– heat-trapping gases that contribute to 

even more warming. Recent estimates suggest that the potential release of carbon 

from permafrost soils could add as much 

as 0.4oF to 0.6oF of warming by 2100.150

Methane emissions have been detected 

from Alaskan lakes underlain by permafrost,151 and measurements suggest potentially even greater releases from thawing 

methane hydrates in the Arctic continental 

shelf of the East Siberian Sea.152 However, 

the response times of Arctic methane hydrates to climate change are quite long 

relative to methane’s lifetime in the atmosphere (about a decade).153 More generally, 

the importance of Arctic methane sources 

relative to other methane sources, such as 

wetlands in warmer climates, is largely unknown. The potential for a self-reinforcing feedback between 

permafrost thawing and additional warming contributes additional uncertainty to the high end of the range of future warming. The projections of future climate shown throughout this report do not include the additional increase in temperature 

associated with this thawing. 

Key Message 12: Ocean Acidification 

The oceans are currently absorbing about a quarter of the carbon dioxide 

emitted to the atmosphere annually and are becoming more acidic as a result, 

leading to concerns about intensifying impacts on marine ecosystems.

As human-induced emissions of carbon dioxide (CO2) build up 

in the atmosphere, excess CO2 is dissolving into the oceans 

where it reacts with seawater to form carbonic acid, lowering 

ocean pH levels (“acidification”) and threatening a number of 

marine ecosystems.154 Currently, the oceans absorbs about a 

quarter of the CO2 humans produce every year.155 Over the 

last 250 years, the oceans have absorbed 560 billion tons of 

CO2, increasing the acidity of surface waters by 30%.156,157,158

Although the average oceanic pH can vary on interglacial timescales,156 the current observed rate of change is roughly 50 

times faster than known historical change.159,160 Regional factors such as coastal upwelling,161 changes in discharge rates 

from rivers and glaciers,162 sea ice loss,163 and urbanization164

have created “ocean acidification hotspots” where changes 

are occurring at even faster rates.

The acidification of the oceans has already caused a suppression of carbonate ion concentrations that are critical for marine 

calcifying animals such as corals, zooplankton, and shellfish. 

Many of these animals form the foundation of the marine food 
49 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

2: OUR CHANGING CLIMATE

Figure 2.30. The correlation between rising levels of CO2 in the atmosphere (red) at 

Mauna Loa and rising CO2 levels (blue) and falling pH (green) in the nearby ocean 

at Station Aloha. As CO2 accumulates in the ocean, the water becomes more acidic 

(the pH declines). (Figure source: modified from Feely et al. 2009157).

As Oceans Absorb CO2, They Become More Acidic 

Figure 2.31. Pteropods, or “sea butterflies,” are free-swimming sea snails about the size of a small pea. Pteropods 

are eaten by marine species ranging in size from tiny krill to whales and are an important source of food for North 

Pacific juvenile salmon. The photos above show what happens to a pteropod’s shell in seawater that is too acidic. 

The left panel shows a shell collected from a live pteropod from a region in the Southern Ocean where acidity is 

not too high. The shell on the right is from a pteropod collected in a region where the water is more acidic (Photo 

credits: (left) Bednaršek et al. 2012;168 (right) Nina Bednaršek).

Shells Dissolve in Acidified Ocean Water

web. Today, more than a billion people 

worldwide rely on food from the ocean 

as their primary source of protein. Ocean 

acidification puts this important resource 

at risk. 

Observations have shown that the northeastern Pacific Ocean, including the Arctic 

and sub-Arctic seas, is particularly susceptible to significant shifts in pH and calcium 

carbonate saturation levels. Recent analyses show that large areas of the oceans 

along the U.S. west coast,157,165 the Bering 

Sea, and the western Arctic Ocean158,166

will become difficult for calcifying animals 

within the next 50 years. In particular, animals that form calcium carbonate shells, 

including corals, crabs, clams, oysters, and 

tiny free-swimming snails called pteropods, could be particularly vulnerable, 

especially during the larval stage.167,168,169

Projections indicate that in higher emissions pathways, such as SRES A2 or RCP 

8.5, current pH could be reduced from the 

current level of 8.1 to as low as 7.8 by the 

end of the century.

158 Such large changes 

in ocean pH have probably not been experienced on the planet for the past 100 million years, and it 

is unclear whether and how quickly ocean life could adapt to 

such rapid acidification.159
50 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

2: OUR CHANGING CLIMATE

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60 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Process for Developing Key Messages

Development of the key messages involved discussions of the lead 

authors and accompanying analyses conducted via one in-person 

meeting plus multiple teleconferences and email exchanges from 

February thru September 2012. The authors reviewed 80 technical inputs provided by the public, as well as other published 

literature, and applied their professional judgment. 

Key message development also involved the findings from four special workshops that related to the latest scientific understanding 

of climate extremes. Each workshop had a different theme related 

to climate extremes, had approximately 30 attendees (the CMIP5 

meeting had more than 100), and the workshops resulted in a paper.55 The first workshop was held in July 2011, titled Monitoring 

Changes in Extreme Storm Statistics: State of Knowledge.52 The 

second was held in November 2011, titled Forum on Trends and 

Causes of Observed Changes in Heatwaves, Coldwaves, Floods, 

and Drought.48 The third was held in January 2012, titled Forum 

on Trends in Extreme Winds, Waves, and Extratropical Storms 

along the Coasts.98 The fourth, the CMIP5 results workshop, was 

held in March 2012 in Hawai‘i, and resulted in an analysis of 

CMIP5 results relative to climate extremes in the United States.55

The Chapter Author Team’s discussions were supported by targeted consultation with additional experts. Professional expertise and 

judgment led to determining “key vulnerabilities.” A consensusbased approach was used for final key message selection.

Key message #1 Traceable Account

Global climate is changing and this change is apparent across a wide range of observations. The 

global warming of the past 50 years is primarily due 

to human activities. 

Description of evidence base

The key message and supporting text summarizes extensive 

evidence documented in the climate science literature. Technical 

Input reports (82) on a wide range of topics were also reviewed; 

they were received as part of the Federal Register Notice 

solicitation for public input. 

2: OUR CHANGING CLIMATE

Evidence for changes in global climate arises from multiple 

analyses of data from in-situ, satellite, and other records 

undertaken by many groups over several decades.3 Changes 

in the mean state have been accompanied by changes in the 

frequency and nature of extreme events.4 A substantial body of 

analysis comparing the observed changes to a broad range of 

climate simulations consistently points to the necessity of invoking 

human-caused changes to adequately explain the observed 

climate system behavior.5,7 The influence of human impacts on the 

climate system has also been observed in a number of individual 

climate variables.6,12,13,14,15,16,17 A discussion of the slowdown in 

temperature increase with associated references (for example, 

Balmaseda et al. 2013; Easterling and Wehner 200919,27) is 

included in the chapter.

The Climate Science Supplement Appendix provides further 

discussion of types of emissions or heat-trapping gases and 

particles, and future projections of human-related emissions. 

Supplemental Message 4 of the Appendix provides further details 

on attribution of observed climate changes to human influence. 

New information and remaining uncertainties

Key remaining uncertainties relate to the precise magnitude and 

nature of changes at global, and particularly regional, scales, 

and especially for extreme events and our ability to simulate and 

attribute such changes using climate models. Innovative new 

approaches to climate data analysis, continued improvements in 

climate modeling, and instigation and maintenance of reference 

quality observation networks such as the U.S. Climate Reference 

Network (http://www.ncdc.noaa.gov/crn/) all have the potential to 

reduce uncertainties.

Assessment of confidence based on evidence 

There is very high confidence that global climate is changing and 

this change is apparent across a wide range of observations, given 

the evidence base and remaining uncertainties. All observational 

evidence is consistent with a warming climate since the late 

1800s.

There is very high confidence that the global climate change of 

the past 50 years is primarily due to human activities, given the 

evidence base and remaining uncertainties. Recent changes have 

SUPPLEMENTAL MATERIAL

TRACEABLE ACCOUNTS
2: OUR CHANGING CLIMATE

Traceable Accounts

61 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

been consistently attributed in large part to human factors across 

a very broad range of climate system characteristics. 

Key message #2 Traceable Account

Global climate is projected to continue to change 

over this century and beyond. The magnitude of 

climate change beyond the next few decades depends primarily on the amount of heat-trapping gases emitted globally, and how sensitive the Earth’s 

climate is to those emissions.

Description of evidence base

The key message and supporting text summarizes extensive 

evidence documented in the climate science peer-reviewed 

literature. Technical Input reports (82) on a wide range of topics 

were also reviewed; they were received as part of the Federal 

Register Notice solicitation for public input. 

Evidence of continued global warming is based on past observations 

of climate change and our knowledge of the climate system’s 

response to heat-trapping gases. Models have projected increased 

temperature under a number of different scenarios.8,32,33

That the planet has warmed is “unequivocal,”8 and is corroborated 

though multiple lines of evidence, as is the conclusion that the 

causes are very likely human in origin (see also Appendices 3 

and 4). The evidence for future warming is based on fundamental 

understanding of the behavior of heat-trapping gases in the 

atmosphere. Model simulations provide bounds on the estimates 

of this warming. 

New information and remaining uncertainties

The trends described in the 2009 report1 have continued, and our 

understanding of the data and ability to model the many facets of 

the climate system have increased substantially.

There are several major sources of uncertainty in making 

projections of climate change. The relative importance of these 

changes over time.

In the next few decades, the effects of natural variability will be 

an important source of uncertainty for climate change projections.

Uncertainty in future human emissions becomes the largest 

source of uncertainty by the end of this century.

Uncertainty in how sensitive the climate is to increased 

concentrations of heat-trapping gases is especially important 

beyond the next few decades. Recent evidence lends further 

confidence about climate sensitivity (see Appendix 3: Climate 

Science Supplement).

Uncertainty in natural climate drivers, for example how much solar 

output will change over this century, also affects the accuracy of 

projections.

Assessment of confidence based on evidence 

Given the evidence base and remaining uncertainties, confidence 

is very high that the global climate is projected to continue to 

change over this century and beyond.

The statement on the magnitude of the effect also has very high 

confidence. 

Key message #3 Traceable Account

U.S. average temperature has increased by 1.3°F 

to 1.9°F since record keeping began in 1895; most 

of this increase has occurred since about 1970. 

The most recent decade was the nation’s warmest on record. Temperatures in the United States 

are expected to continue to rise. Because humaninduced warming is superimposed on a naturally 

varying climate, the temperature rise has not been, 

and will not be, uniform or smooth across the country or over time.

Description of evidence base

The key message and supporting text summarizes extensive 

evidence documented in the climate science peer-reviewed 

literature. Technical Input reports (82) on a wide range of topics 

Confidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent 

results, well documented and 

accepted methods, etc.), high 

consensus

High

Moderate evidence (several sources, some consistency, methods 

vary and/or documentation limited, 

etc.), medium consensus

Medium

Suggestive evidence (a few 

sources, limited consistency, models incomplete, methods emerging, 

etc.), competing schools of thought

Low

Inconclusive evidence (limited 

sources, extrapolations, inconsistent findings, poor documentation 

and/or methods not tested, etc.), 

disagreement or lack of opinions 

among experts
2: OUR CHANGING CLIMATE

Traceable Accounts

62 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

were also reviewed; they were received as part of the Federal 

Register Notice solicitation for public input.

Evidence for the long-term increase in temperature is based on 

analysis of daily maximum and minimum temperature observations 

from the U.S. Cooperative Observer Network (http://www.nws.

noaa.gov/om/coop/). With the increasing understanding of U.S. 

temperature measurements, a temperature increase has been 

observed, and temperature is projected to continue rising.36,37,38

Observations show that the last decade was the warmest in over a 

century. A number of climate model simulations were performed 

to assess past, and to forecast future, changes in climate; 

temperatures are generally projected to increase across the United 

States.

The section entitled “Quantifying U.S. Temperature Rise” explains 

the rational for using the range 1.3°F to 1.9°F in the key message. 

All peer-reviewed studies to date satisfying the assessment 

process agree that the U.S. has warmed over the past century 

and in the past several decades. Climate model simulations 

consistently project future warming and bracket the range of 

plausible increases.

New information and remaining uncertainties

Since the 2009 National Climate Assessment,1 there have been 

substantial advances in our understanding of the U.S. temperature 

record (Appendix 3: Climate Science, Supplemental Message 

7).36,37,38

A potential uncertainty is the sensitivity of temperature trends to 

adjustments that account for historical changes in station location, 

temperature instrumentation, observing practice, and siting 

conditions. However, quality analyses of these uncertainties have 

not found any major issues of concern affecting the conclusions 

made in the key message (Appendix 3: Climate Science, 

Supplemental Message 7). (for example, Williams et al. 201238).

While numerous studies (for example, Fall et al. 2011; Vose 

et al. 2012; Williams et al. 201237,38) verify the efficacy of the 

adjustments, the information base can be improved in the future 

through continued refinements to the adjustment approach. Model 

biases are subject to changes in physical effects on climate; for 

example, model biases can be affected by snow cover and hence 

are subject to change as a warming climate changes snow cover. 

Assessment of confidence based on evidence

Given the evidence base and remaining uncertainties, confidence 

is very high in the key message. Because human-induced warming 

is superimposed on a naturally varying climate, the temperature 

rise has not been, and will not be, uniform or smooth across the 

country or over time. 

Key message #4 Traceable Account

The length of the frost-free season (and the corresponding growing season) has been increasing 

nationally since the 1980s, with the largest increases occurring in the western United States, affecting ecosystems and agriculture. Across the United 

States, the growing season is projected to continue 

to lengthen.

Description of evidence base

The key message and supporting text summarizes extensive 

evidence documented in the climate science peer-reviewed 

literature. Technical Input reports (82) on a wide range of topics 

were also reviewed; they were received as part of the Federal 

Register Notice solicitation for public input.

Nearly all studies to date published in the peer-reviewed literature 

(for example, Dragoni et al. 2011; EPA 2012; Jeong et al. 

201140,41,43) agree that the frost-free and growing seasons have 

lengthened. This is most apparent in the western United States. 

Peer-reviewed studies also indicate that continued lengthening 

will occur if concentrations of heat-trapping gases continue to rise. 

The magnitude of future changes based on model simulations is 

large in the context of historical variations. 

Evidence that the length of the frost-free season is lengthening 

is based on extensive analysis of daily minimum temperature 

observations from the U.S. Cooperative Observer Network. The 

geographic variations in increasing number of frost-free days are 

similar to the regional variations in mean temperature. Separate 

analysis of surface data also indicates a trend towards an earlier 

onset of spring.40,41,43,45

New information and remaining uncertainties

A key issue (uncertainty) is the potential effect on observed trends 

of climate monitoring station inhomogeneities (differences), 

particularly those arising from instrumentation changes. A second 

key issue is the extent to which observed regional variations (more 

lengthening in the west/less in the east) will persist into the future.

Local temperature biases in climate models contribute to the 

uncertainty in projections.

Viable avenues to improving the information base are to investigate 

the sensitivity of observed trends to potential biases introduced by 

station inhomogeneities and to investigate the causes of observed 

regional variations.

Assessment of confidence based on evidence

Given the evidence base and remaining uncertainties, confidence 

is very high that the length of the frost-free season (also referred 

to as the growing season) has been increasing nationally since 

the 1980s, with the largest increases occurring in the western 

U.S, affecting ecosystems, gardening, and agriculture. Given the 
2: OUR CHANGING CLIMATE

Traceable Accounts

63 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

evidence base, confidence is very high that across the U.S., the 

growing season is projected to continue to lengthen.

Key message #5 Traceable Account

Average U.S. precipitation has increased since 

1900, but some areas have had increases greater 

than the national average, and some areas have had 

decreases. More winter and spring precipitation is 

projected for the northern United States, and less 

for the Southwest, over this century.

Description of evidence base

The key message and supporting text summarizes extensive 

evidence documented in the climate science peer-reviewed 

literature. Technical Input reports (82) on a wide range of topics 

were also reviewed; they were received as part of the Federal 

Register Notice solicitation for public input.

Evidence of long-term change in precipitation is based on analysis 

(for example, Kunkel et al. 2013170) of daily observations from 

the U.S. Cooperative Observer Network. Published work shows the 

regional differences in precipitation.47,48 Evidence of future change 

is based on our knowledge of the climate system’s response to heattrapping gases and an understanding of the regional mechanisms 

behind the projected changes (for example, IPCC 20078). 

New information and remaining uncertainties

A key issue (uncertainty) is the sensitivity of observed precipitation 

trends to historical changes in station location, rain gauges, 

and observing practice. A second key issue is the ability of 

climate models to simulate precipitation. This is one of the 

more challenging aspects of modeling of the climate system, 

because precipitation involves not only large-scale processes 

that are well-resolved by models but small-scale process, 

such as convection, that must be parameterized in the current 

generation of global and regional climate models. However, our 

understanding of the physical basis for these changes has solidified 

and the newest set of climate model simulations (CMIP5) continues 

to show high-latitude increases and subtropical decreases in 

precipitation. For most of the contiguous U.S., studies171 indicate 

that the models currently do not detect a robust anthropogenic 

influence to observed changes, suggesting that observed changes 

are principally of natural origins. Thus, confident projections of 

precipitation changes are limited to the northern and southern 

areas of the contiguous U.S. that are part of the global pattern 

of observed and robust projected changes that can be related to 

anthropogenic forcing. Furthermore, for the first time in the U.S. 

National Climate Assessment, a confidence statement is made 

that some projected precipitation changes are deemed small. 

It is incorrect to attempt to validate or invalidate climate model 

simulations of observed trends in these regions and/or seasons, as 

such simulations are not designed to forecast the precise timing 

of natural variations.

Shifts in precipitation patterns due to changes in other sources 

of air pollution, such as sulfate aerosols, are uncertain and are an 

active research topic.

Viable avenues to improving the information base are to investigate 

the sensitivity of observed trends to potential biases introduced 

by station changes, and to investigate the causes of observed 

regional variations.

A number of peer-reviewed studies (for example, McRoberts and 

Nielsen-Gammon 2011; Peterson et al. 201347,48) document 

precipitation increases at the national scale as well as regionalscale increases and decreases. The variation in magnitude and 

pattern of future changes from climate model simulations is large 

relative to observed (and modeled) historical variations.

Assessment of confidence based on evidence

Given the evidence base and remaining uncertainties, confidence 

is high that average U.S. precipitation has increased since 1900, 

with some areas having had increases greater than the national 

average, and some areas having had decreases. 

Confidence is high, given the evidence base and uncertainties, 

that more winter and spring precipitation is projected for the 

northern U.S., and less for the Southwest, over this century in the 

higher emissions scenarios. Confidence is medium that humaninduced precipitation changes will be small compared to natural 

variations in all seasons over large portions of the U.S. in the lower 

emissions scenarios. Confidence is medium that human-induced 

precipitation changes will be small compared to natural variations 

in the summer and fall over large portions of the U.S. in the higher 

emissions scenarios. 

Key message #6 Traceable Account

Heavy downpours are increasing nationally, especially over the last three to five decades. Largest increases are in the Midwest and Northeast. 

Increases in the frequency and intensity of extreme 

precipitation events are projected for all U.S. regions.

Description of evidence base

The key message and supporting text summarizes extensive 

evidence documented in the climate science peer-reviewed 

literature. Technical Input reports (82) on a wide range of topics 

were also reviewed; they were received as part of the Federal 

Register Notice solicitation for public input.

Evidence that extreme precipitation is increasing is based primarily 

on analysis52,55,170 of hourly and daily precipitation observations 

from the U.S. Cooperative Observer Network, and is supported 

by observed increases in atmospheric water vapor.75 Recent 

publications have projected an increase in extreme precipitation 
2: OUR CHANGING CLIMATE

Traceable Accounts

64 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

events,52,137 with some areas getting larger increases1 and some 

getting decreases.54,55

Nearly all studies to date published in the peer-reviewed literature 

agree that extreme precipitation event number and intensity 

have risen, when averaged over the United States. The pattern 

of change for the wettest day of the year is projected to roughly 

follow that of the average precipitation, with both increases and 

decreases across the U.S. Extreme hydrologic events are projected 

to increase over most of the U.S.

New information and remaining uncertainties

A key issue (uncertainty) is the ability of climate models to 

simulate precipitation. This is one of the more challenging aspects 

of modeling of the climate system because precipitation involves 

not only large-scale processes that are well-resolved by models 

but also small-scale process, such as convection, that must be 

parameterized in the current generation of global and regional 

climate models.

Viable avenues to improving the information base are to perform 

some long, very high-resolution simulations of this century’s 

climate under different emissions scenarios.

Assessment of confidence based on evidence

Given the evidence base and uncertainties, confidence is high that 

heavy downpours are increasing in most regions of the U.S., with 

especially large increases in the Midwest and Northeast. 

Confidence is high that further increases in the frequency and 

intensity of extreme precipitation events are projected for most 

U.S. areas, given the evidence base and uncertainties. 

Key message #7 Traceable Account

There have been changes in some types of extreme weather events over the last several decades. Heat waves have become more frequent 

and intense, especially in the West. Cold waves 

have become less frequent and intense across the 

nation. There have been regional trends in floods 

and droughts. Droughts in the Southwest and heat 

waves everywhere are projected to become more 

intense, and cold waves less intense everywhere.

Description of evidence base

The key message and supporting text summarizes extensive 

evidence documented in the climate science peer-reviewed 

literature. Technical Input reports (82) on a wide range of topics 

were also reviewed; they were received as part of the Federal 

Register Notice solicitation for public input.

Analysis of U.S. temperature records indicates that record cold 

events are becoming progressively less frequent relative to 

record high events.60,170 There is evidence for the corresponding 

trends in a global framework.7,66 A number of publications have 

explored the increasing trend of heat waves.7,62,69 Additionally, 

heat waves observed in the southern Great Plains,1 Europe,7,62 and 

Russia60,66,67 have now been shown to have a higher probability of 

having occurred because of human-induced climate change. 

Some parts of the U.S. have been seeing changing trends for 

floods and droughts over the last 50 years, with some evidence for 

human influence.13,48,62 In the areas of increased flooding in parts 

of the Great Plains, Midwest, and Northeast, increases in both 

total precipitation and extreme precipitation have been observed 

and may be contributing to the flooding increases. However, when 

averaging over the entire contiguous U.S., there is no overall trend 

in flood magnitudes.71 A number of publications project drought 

as becoming a more normal condition over much of the southern 

and central U.S. (most recent references: Dai 2012; Hoerling et 

al. 2012; Wehner et al. 201175,76).

Analyses of U.S. daily temperature records indicate that low 

records are being broken at a much smaller rate than high records, 

and at the smallest rate in the historical record.60,170 However, 

in certain localized regions, natural variations can be as large or 

larger than the human induced change.

New information and remaining uncertainties

The key uncertainty regarding projections of future drought is 

how soil moisture responds to precipitation changes and potential 

evaporation increases. Most studies indicate that many parts of 

the U.S. will experience drier soil conditions but the amount of 

that drying is uncertain.

Natural variability is also an uncertainty affecting projections of 

extreme event occurrences in shorter timescales (several years 

to decades), but the changes due to human influence become 

larger relative to natural variability as the timescale lengthens. 

Stakeholders should view the occurrence of extreme events in the 

context of increasing probabilities due to climate change.

Continuation of long term temperature and precipitation 

observations is critical to monitoring trends in extreme weather 

events.

Assessment of confidence based on evidence

Given the evidence base and uncertainties, confidence is high for 

the entire key message.

Heat waves have become more frequent and intense, and confidence is high that heat waves everywhere are projected to become 

more intense in the future.

Confidence is high that cold waves have become less frequent and 

intense across the nation. 
2: OUR CHANGING CLIMATE

Traceable Accounts

65 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Confidence is high that there have been regional trends in floods 

and droughts.

Confidence is high that droughts in the Southwest are projected 

to become more intense.

Key message #8 Traceable Account

The intensity, frequency, and duration of North 

Atlantic hurricanes, as well as the frequency of the 

strongest (Category 4 and 5) hurricanes, have all 

increased since the early 1980s. The relative contributions of human and natural causes to these 

increases are still uncertain. Hurricane-associated 

storm intensity and rainfall rates are projected to 

increase as the climate continues to warm.

Description of evidence base

The key message and supporting text summarize extensive 

evidence documented in the climate science peer-reviewed 

literature. Technical Input reports (82) on a wide range of topics 

were also reviewed; they were received as part of the Federal 

Register Notice solicitation for public input.

Recent studies suggest that the most intense Atlantic hurricanes 

have become stronger since the early 1980s.93 While this is still the 

subject of active research, this trend is projected to continue.90,91

New information and remaining uncertainties

Detecting trends in Atlantic and eastern North Pacific hurricane 

activity is challenged by a lack of consistent historical data and 

limited understanding of all of the complex interactions between 

the atmosphere and ocean that influence hurricanes.87,88 

While the best analyses to date87,91 suggest an increase in 

intensity and in the number of the most intense hurricanes over 

this century, there remain significant uncertainties. 

Assessment of confidence based on evidence

Given the evidence base and remaining uncertainties:

High confidence that the intensity, frequency, and duration of 

North Atlantic hurricanes, as well as the frequency of the strongest 

(Category 4 and 5) hurricanes, have increased substantially since 

the early 1980s.

Low confidence in relative contributions of human and natural 

causes in the increases.

Medium confidence that hurricane intensity and rainfall rates are 

projected to increase as the climate continues to warm. 

Key message #9 Traceable Account

Winter storms have increased in frequency and 

intensity since the 1950s, and their tracks have 

shifted northward over the United States. Other 

trends in severe storms, including the intensity and 

frequency of tornadoes, hail, and damaging thunderstorm winds, are uncertain and are being studied intensively. 

Description of evidence base

The key message and supporting text summarize extensive 

evidence documented in the climate science peer-reviewed 

literature. Technical Input reports (82) on a wide range of topics 

were also reviewed; they were received as part of the Federal 

Register Notice solicitation for public input.

Current work98 has provided evidence of the increase in frequency 

and intensity of winter storms, with the storm tracks shifting 

poleward,99,100 but some areas have experienced a decrease in 

winter storm frequency.1 Although there are some indications 

of increased blocking (a large-scale pressure pattern with little 

or no movement) of the wintertime circulation of the Northern 

Hemisphere,106 the assessment and attribution of trends in 

blocking remain an active research area.107 Some recent research 

has provided insight into the connection of global warming to 

tornadoes and severe thunderstorms.96

New information and remaining uncertainties

Winter storms and other types of severe storms have greater 

uncertainties in their recent trends and projections, compared 

to hurricanes (Key Message 8). The text for this key message 

explicitly acknowledges the state of knowledge, pointing out “what 

we don’t know.” There has been a sizeable upward trend in the 

number of storm events causing large financial and other losses.95

Assessment of confidence based on evidence

Given the evidence base and remaining uncertainties:

Confidence is medium that winter storms have increased slightly 

in frequency and intensity, and that their tracks have shifted 

northward over the U.S.

Confidence is low on other trends in severe storms, including the 

intensity and frequency of tornadoes, hail, and damaging thunderstorm winds.
2: OUR CHANGING CLIMATE

Traceable Accounts

66 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Key message #10 Traceable Account

Global sea level has risen by about 8 inches since 

reliable record keeping began in 1880. It is projected to rise another 1 to 4 feet by 2100. 

Description of evidence base

The key message and supporting text summarize extensive 

evidence documented in the climate science peer-reviewed 

literature. Technical Input reports (82) on a wide range of topics 

were also reviewed; they were received as part of the Federal 

Register Notice solicitation for public input.

Nearly all studies to date published in the peer-reviewed literature 

agree that global sea level has risen during the past century, and 

that it will continue to rise over the next century. 

Tide gauges throughout the world have documented rising sea 

levels during the last 130 years. This rise has been further 

confirmed over the past 20 years by satellite observations, which 

are highly accurate and have nearly global coverage. Recent 

studies have shown current sea level rise rates are increasing112,123

and project that future sea level rise over the rest of this century 

will be faster than that of the last 100 years (Appendix 3: Climate 

Science, Supplemental Message 12).123

New information and remaining uncertainties

The key issue in predicting future rates of global sea level rise 

is to understand and predict how ice sheets in Greenland and 

Antarctica will react to a warming climate. Current projections of 

global sea level rise do not account for the complicated behavior 

of these giant ice slabs as they interact with the atmosphere, the 

ocean and the land. Lack of knowledge about the ice sheets and 

their behavior is the primary reason that projections of global sea 

level rise includes such a wide range of plausible future conditions. 

Early efforts at semi-empirical models suggested much higher 

rates of sea level rise (as much as 6 feet by 2100).115,117 More 

recent work suggests that a high end of 3 to 4 feet is more 

plausible.115,116,121 It is not clear, however, whether these statistical 

relationships will hold in the future or that they are appropriate in 

modeling past behavior, thus calling their reliability into question.118

Some decision-makers may wish to consider a broader range of 

scenarios such as 8 inches or 6.6 feet by 2100 in the context of 

risk-based analysis.122,123

Assessment of confidence based on evidence

Given the evidence and uncertainties, confidence is very high that 

global sea level has risen during the past century, and that it will 

continue to rise over this century, with medium confidence that 

global sea level rise will be in the range of 1 to 4 feet by 2100.

Key message #11 Traceable Account

Rising temperatures are reducing ice volume and 

surface extent on land, lakes, and sea. This loss of 

ice is expected to continue. The Arctic Ocean is 

expected to become essentially ice free in summer 

before mid-century.

Description of evidence base

The key message and supporting text summarize extensive 

evidence documented in the climate science peer-reviewed 

literature. Technical Input reports (82) on a wide range of topics 

were also reviewed; they were received as part of the Federal 

Register Notice solicitation for public input.

There have been a number of publications reporting decreases in 

ice on land147 and glacier recession. Evidence that winter lake ice 

and summer sea ice are rapidly declining is based on satellite data 

and is incontrovertible.111,172

Nearly all studies to date published in the peer-reviewed literature 

agree that summer Arctic sea ice extent is rapidly declining,131

with even greater reductions in ice thickness132,133 and volume,134

and that if heat-trapping gas concentrations continue to rise, an 

essentially ice-free Arctic ocean will be realized sometime during 

this century (for example, Stroeve et al. 2012136). September 

2012 had the lowest levels of Arctic ice in recorded history. Great 

Lakes ice should follow a similar trajectory. Glaciers will generally 

retreat, except for a small percentage of glaciers that experience 

dynamical surging.111 Snow cover on land has decreased over the 

past several decades.145 The rate of permafrost degradation is 

complicated by changes in snow cover and vegetation.

New information and remaining uncertainties

The rate of sea ice loss through this century is a key issue 

(uncertainty), which stems from a combination of large differences 

in projections between different climate models, natural climate 

variability and uncertainty about future rates of fossil fuel 

emissions. This uncertainty is illustrated in Figure 2.29, showing 

the CMIP5-based projections (adapted from Stroeve et al. 

2012136).

Viable avenues to improving the information base are determining 

the primary causes of the range of different climate model 

projections and determining which climate models exhibit the best 

ability to reproduce the observed rate of sea-ice loss.

Assessment of confidence based on evidence

Given the evidence base and uncertainties, confidence is very 

high that rising temperatures are reducing ice volume and extent 

on land, lakes, and sea, and that this loss of ice is expected to 

continue. 
2: OUR CHANGING CLIMATE

Traceable Accounts

67 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Confidence is very high that the Arctic Ocean is projected to 

become virtually ice-free in summer by mid-century.

Key message #12 Traceable Account

The oceans are currently absorbing about a quarter of the carbon dioxide emitted to the atmosphere 

annually and are becoming more acidic as a result, 

leading to concerns about intensifying impacts on 

marine ecosystems. 

Description of evidence base

The key message and supporting text summarize extensive 

evidence documented in the climate science peer-reviewed 

literature. Technical Input reports (82) on a wide range of topics 

were also reviewed; they were received as part of the Federal 

Register Notice solicitation for public input.

The oceans currently absorb a quarter of the CO2 the caused by 

human activities.155 Publications have shown that this absorption 

causes the ocean to become more acidic (for example, Doney et 

al. 2009154). Recent publications demonstrate the adverse effects 

further acidification will have on marine life.158,165,169

New information and remaining uncertainties

Absorption of CO2 of human origin, reduced pH, and lower 

calcium carbonate (CaCO3) saturation in surface waters, where 

the bulk of oceanic production occurs, are well verified from 

models, hydrographic surveys, and time series data.158 The key 

issue (uncertainty) is how future levels of ocean acidity will affect 

marine ecosystems.

Assessment of confidence based on evidence

Given the evidence base and uncertainties, confidence is very 

high that oceans are absorbing about a quarter of emitted CO2.

Very high for trend of ocean acidification; low-to-medium

for intensifying impacts on marine ecosystems. Our present 

understanding of projected ocean acidification impacts on marine 

organisms stems largely from short-term laboratory and mesocosm 

experiments, although there are also examples based on actual 

ocean observations; consequently, the response of individual 

organisms, populations, and communities of species to more 

realistic, gradual changes still has large uncertainties.
SECTORS

Cherry farmers in Michigan, insurance agents in Florida, and water managers in 

Arizona are among the millions of Americans already living with – and adapting to – a 

range of climate change impacts. Higher temperatures, rising sea levels, and more 

extreme precipitation events are altering the work of first responders, city planners, 

engineers, and others, influencing economic sectors from coast to coast. Agriculture, 

energy, transportation, and more, are all affected by climate change in concrete ways. 

American communities are contending with these changes now, and will be doing so 

increasingly in the future. 

Sectors of our economy do not exist in isolation. Forest management activities, for 

example, affect and are affected by water supply, changing ecosystems, impacts 

to biological diversity, and energy availability. Water supply and energy use are 

completely intertwined, since water is used to generate energy, and energy is required 

to pump, treat, and deliver water – which means that irrigation-dependent farmers 

and urban dwellers are linked as well. Human health is affected by water supply, 

agricultural practices, transportation systems, energy availability, and land use, among 

other factors – touching the lives of patients, nurses, county health administrators, 

and many others. Human social systems and communities are directly affected by 

extreme weather events and changes in natural resources such as water availability 

and quality; they are also affected both directly and indirectly by ecosystem health.

This report addresses some of these topics individually, focusing on the climaterelated risks and opportunities that occur within individual sectors, while others take a 

cross-sector approach. Single-sector chapters focus on:

Six crosscutting chapters address how climate change interacts with multiple sectors. 

These cover the following topics:

A common theme is that these sectors are interconnected in many ways. These 

intricate connections mean that changes in one sector are often amplified or reduced 

through links to other sectors. Another theme is how decisions can influence a 

cascade of events that affect individual and national vulnerability and/or resiliency 

to climate change across multiple sectors. This “systems approach” helps to reveal, 

for example, how adaptation and mitigation strategies are part of dynamic and 

interrelated systems. In this way, for example, adaptation plans for future coastal 

infrastructure are connected with the kinds of mitigation strategies that are – or 

are not – put into place today, since the amount of future sea level rise will differ 

according to various societal decisions about current and future emissions. These 

chapters also address the importance of underlying vulnerabilities and the ways they 

may influence risks associated with climate change. 

The chapters in the following section assess risks in the selected sectors, and include 

both observations of existing impacts associated with climate change, as well as 

projected impacts over the next several decades and beyond.

• Water resources

• Energy production and use

• Transportation

• Agriculture

• Forests

• Human health

• Ecosystems and biodiversity

• Energy, water, and land use

• Urban infrastructure and vulnerability

• Indigenous peoples, lands, and resources

• Land use and land cover

• Rural communities

• Biogeochemical cycles 

© Ted Wood Photography
69 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Climate Change Impacts in the United States

CHAPTER 3

WATER RESOURCES

INFORMATION DRAWN FROM THIS CHAPTER IS INCLUDED IN THE HIGHLIGHTS REPORT AND IS IDENTIFIED BY THIS ICON

Recommended Citation for Chapter

Georgakakos, A., P. Fleming, M. Dettinger, C. Peters-Lidard, Terese (T.C.) Richmond, K. Reckhow, K. White, and D. Yates, 

2014: Ch. 3: Water Resources. Climate Change Impacts in the United States: The Third National Climate Assessment, J. 

M. Melillo, Terese (T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 69-112. doi:10.7930/

J0G44N6T.

On the Web: http://nca2014.globalchange.gov/report/sectors/water

Convening Lead Authors

Aris Georgakakos, Georgia Institute of Technology

Paul Fleming, Seattle Public Utilities

Lead Authors

Michael Dettinger, U.S. Geological Survey

Christa Peters-Lidard, National Aeronautics and Space Administration

Terese (T.C.) Richmond, Van Ness Feldman, LLP

Ken Reckhow, Duke University

Kathleen White, U.S. Army Corps of Engineers

David Yates, University Corporation for Atmospheric Research
70 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Key Messages

1. Annual precipitation and river-flow increases are observed now in 

the Midwest and the Northeast regions. Very heavy precipitation 

events have increased nationally and are projected to increase in 

all regions. The length of dry spells is projected to increase in most 

areas, especially the southern and northwestern portions of the 

contiguous United States.

2. Short-term (seasonal or shorter) droughts are expected to intensify in 

most U.S. regions. Longer-term droughts are expected to intensify in 

large areas of the Southwest, southern Great Plains, and Southeast.

3. Flooding may intensify in many U.S. regions, even in areas where 

total precipitation is projected to decline. 

4. Climate change is expected to affect water demand, groundwater withdrawals, and aquifer 

recharge, reducing groundwater availability in some areas.

5. Sea level rise, storms and storm surges, and changes in surface and groundwater use patterns 

are expected to compromise the sustainability of coastal freshwater aquifers and wetlands.

6. Increasing air and water temperatures, more intense precipitation and runoff, and intensifying 

droughts can decrease river and lake water quality in many ways, including increases in 

sediment, nitrogen, and other pollutant loads.

7. Climate change affects water demand and the ways water is used within and across regions and 

economic sectors. The Southwest, Great Plains, and Southeast are particularly vulnerable to 

changes in water supply and demand.

8. Changes in precipitation and runoff, combined with changes in consumption and withdrawal, 

have reduced surface and groundwater supplies in many areas. These trends are expected to 

continue, increasing the likelihood of water shortages for many uses. 

9. Increasing flooding risk affects human safety and health, property, infrastructure, economies, 

and ecology in many basins across the United States.

10. In most U.S. regions, water resources managers and planners will encounter new risks, 

vulnerabilities, and opportunities that may not be properly managed within existing practices. 

11. Increasing resilience and enhancing adaptive capacity provide opportunities to strengthen water 

resources management and plan for climate change impacts. Many institutional, scientific, 

economic, and political barriers present challenges to implementing adaptive strategies.

3 WATER RESOURCES

This chapter contains three main sections: climate change impacts on the water cycle, climate change impacts on water resources 

use and management, and adaptation and institutional responses. Key messages for each section are summarized above.

Climate Change Impacts on Water Resources Use and Managment

Climate Change Impacts on the Water Cycle

Adaptation and Institutional Responses

© Dan Sherwood/Design Pics/Corbis
71 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

3: WATER RESOURCES

Climate Change Impacts on the Water Cycle

Water cycles constantly from the atmosphere to the land and 

the oceans (through precipitation and runoff) and back to the 

atmosphere (through evaporation and the release of water 

from plant leaves), setting the stage for all life to exist. The 

water cycle is dynamic and naturally variable, and societies 

and ecosystems are accustomed to functioning within this variability. However, climate change is altering the water cycle in 

multiple ways over different time scales and geographic areas, 

presenting unfamiliar risks and opportunities. 

Key Message 1: Changing Rain, Snow, and Runoff

Annual precipitation and river-flow increases are observed now in the Midwest and the 

Northeast regions. Very heavy precipitation events have increased nationally and are 

projected to increase in all regions. The length of dry spells is projected to increase in most 

areas, especially the southern and northwestern portions of the contiguous United States.

Annual average precipitation over the continental U.S. as 

a whole increased by close to two inches (0.16 inches per 

decade) between 1895 and 2011.1,2 In recent decades, annual average precipitation increases have been observed 

across the Midwest, Great Plains, the Northeast, and 

Alaska, while decreases have been observed in Hawai‘i 

and parts of the Southeast and Southwest (Ch. 2: Our 

Changing Climate, Figure 2.12). Average annual precipitation is projected to increase across the northern U.S., and 

decrease in the southern U.S., especially the Southwest. 

(Ch. 2: Our Changing Climate, Figures 2.14 and 2.15).

3

The number and intensity of very heavy precipitation

events (defined as the heaviest 1% of all daily events from 

1901 to 2012) have been increasing significantly across 

most of the United States. The amount of precipitation 

falling in the heaviest daily events has also increased 

in most areas of the United States (Ch. 2: Our Changing 

Climate, Figure 2.17). For example, from 1950 to 2007, 

daily precipitation totals with 2-, 5-, and 10-year average recurrence periods increased in the Northeast and 

western Great Lakes.4 Very heavy precipitation events are 

projected to increase everywhere (Ch. 2: Our Changing 

Climate, Figure 2.19).5 Heavy precipitation events that historically occurred once in 20 years are projected to occur 

as frequently as every 5 to 15 years by late this century.6

The number and magnitude of the heaviest precipitation 

events is projected to increase everywhere in the United 

States (Ch. 2: Our Changing Climate, Figure 2.13).

Dry spells are also projected to increase in length in most 

regions, especially in the southern and northwestern portions of the contiguous United States (Ch. 2: Our Changing 

Climate, Figure 2.13). Projected changes in total average 

annual precipitation are generally small in many areas, but 

both wet and dry extremes (heavy precipitation events 

The cycle of life is intricately joined with the cycle of water. 

— Jacques-Yves Cousteau

Figure 3.1. These projections, assuming continued increases in 

heat-trapping gas emissions (A2 scenario; Ch. 2: Our Changing 

Climate), illustrate: a) major losses in the water content of the 

snowpack that fills western rivers (snow water equivalent, or 

SWE); b) significant reductions in runoff in California, Arizona, 

and the central Rocky Mountains; and c) reductions in soil 

moisture across the Southwest. The changes shown are for 

mid-century (2041-2070) as percentage changes from 1971-

2000 conditions (Figure source: Cayan et al. 201318).

Projected Changes in Snow, Runoff, and Soil Moisture
72 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

3: WATER RESOURCES

and length of dry spells) are projected to increase substantially 

almost everywhere.

The timing of peak river levels has changed in response to 

warming trends. Snowpack and snowmelt-fed rivers in much 

of the western U.S. have earlier peak flow trends since the middle of the last century, including the past decade (Ch. 2: Our 

Changing Climate).7,8 This is related to declines in spring snowpack, earlier snowmelt-fed streamflow, and larger percentages 

of precipitation falling as rain instead of snow. These changes 

have taken place in the midst of considerable year-to-year 

variability and long-term natural fluctuations of the western 

U.S. climate, as well as other influences, such as the effects of 

dust and soot on snowpacks.7,9 There are both natural and human influences on the observed trends.

10,11 However, in studies specifically designed to differentiate between natural and 

human-induced causes, up to 60% of these changes have been 

attributed to human-induced climate warming,10 but only 

among variables that are more responsive to warming than to 

precipitation variability, such as the effect of air temperature 

on snowpack.

12

Other historical changes related to peak river-flow have been 

observed in the northern Great Plains, Midwest, and Northeast,13,14 along with striking reductions in lake ice cover (Ch. 2: 

Our Changing Climate).15,16

Permafrost is thawing in many parts of Alaska, a trend that not 

only affects habitats and infrastructure but also mobilizes subsurface water and reroutes surface water in ways not previously witnessed.17 Nationally, all of these trends are projected 

to become even more pronounced as the climate continues to 

warm (Figure 3.1).

Evapotranspiration (ET – the evaporation of moisture from soil, 

on plants and trees, and from water bodies; and transpiration, 

the use and release of water from plants), is the second largest 

component of the water cycle after precipitation. ET responds 

to temperature, solar energy, winds, atmospheric humidity, 

and moisture availability at the land surface and regulates 

amounts of soil moisture, groundwater recharge, and runoff.19

Transpiration comprises between 80% and 90% of total ET 

on land (Ch. 6: Agriculture).20 In snowy settings, sublimation 

of snow and ice (loss of snow and ice directly into water vapor without passing through a liquid stage) can increase these 

returns of water to the atmosphere, sometimes in significant 

amounts.21 These interactions complicate estimation and projection of regional losses of water from the land surface to the 

atmosphere.

Globally-averaged ET increased between 1982 and 1997 but 

stopped increasing, or has decreased, since about 1998.22 In 

North America, the observed ET decreases occurred in waterrich rather than water-limited areas. Factors contributing to 

these ET decreases are thought to include decreasing wind 

Figure 3.2. Changes in annual surface soil moisture per year over the period 1988 to 2010 based on multisatellite datasets. Surface soil moisture exhibits wetting trends in the Northeast, Florida, upper Midwest, and 

Northwest, and drying trends almost everywhere else. (Images provided by W. Dorigo35). 

Annual Surface Soil Moisture Trends
73 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

3: WATER RESOURCES

speed,23,24 decreasing solar energy at the land surface due to 

increasing cloud cover and concentration of small particles 

(aerosols),

25 increasing humidity,23 and declining soil moisture 

(Figure 3.2).26

Evapotranspiration projections vary by region,

27,28,29,30 but the 

atmospheric potential for ET is expected to increase; actual ET 

will be affected by regional soil moisture changes. Much more 

research is needed to confidently identify historical trends, 

causes, and implications for future ET trends.31 This represents a critical uncertainty in projecting the impacts of climate 

change on regional water cycles. 

Soil moisture plays a major role in the water cycle, regulating the exchange of water, energy, and carbon between the 

land surface and the atmosphere,22 the production of runoff, 

and the recharge of groundwater aquifers. Soil moisture is 

projected to decline with higher temperatures and attendant 

increases in the potential for ET in much of the country, especially in the Great Plains,

29 Southwest,18,32,33 and Southeast.28,34

Runoff and streamflow at regional scales declined during the 

last half-century in the Northwest.

36 Runoff and streamflow 

increased in the Mississippi Basin and Northeast, with no clear 

trends in much of the rest of the continental U.S.,37 although 

a declining trend is emerging in annual runoff in the Colorado 

River Basin.38 These changes need to be considered in the context of tree-ring studies in California’s Central Valley, the Colorado River and Wind River basins, and the southeastern U.S. 

that indicate that these regions have experienced prolonged, 

even drier and wetter conditions at various times in the past 

two thousand years.8,39,40 Human-caused climate change, when 

superimposed on past natural variability, may amplify these 

past extreme conditions. Projected changes in runoff for eight 

basins in the Northwest, northern Great Plains, and Southwest 

are illustrated in Figure 3.4.

Basins in the southwestern U.S. and southern Rockies (for example, the Rio Grande and Colorado River basins) are projected to experience gradual runoff declines during this century. 

Basins in the Northwest to north-central U.S. (for example, the 

Figure 3.3. Changes in seasonal surface soil moisture per year over the period 1988 to 2010 based on multi-satellite 

datasets.35 Seasonal drying is observed in central and lower Midwest and Southeast for most seasons (with the exception 

of the Southeast summer), and in most of the Southwest and West (with the exception of the Northwest) for spring 

and summer. Soil moisture in the upper Midwest, Northwest, and most of the Northeast is increasing in most seasons. 

(Images provided by W. Dorigo). 

Seasonal Surface Soil Moisture Trends
74 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

3: WATER RESOURCES

Columbia and the Missouri River basins) are projected to experience little change through the middle of this century, and 

increases by late this century. 

Projected changes in runoff differ by season, with cool season 

runoff increasing over the west coast basins from California to 

Washington and over the north-central U.S. (for example, the 

San Joaquin, Sacramento, Klamath, Missouri, and Columbia 

River basins). Basins in the southwestern U.S. and southern 

Rockies are projected to see little change to slight decreases in 

the winter months. 

Warm season runoff is projected to decrease substantially over 

a region spanning southern Oregon, the southwestern U.S., 

and southern Rockies (for example, the Klamath, Sacramento, 

San Joaquin, Rio Grande, and the Colorado River basins), and 

change little or increase slightly north of this region (for example, the Columbia and Missouri River basins).

In most of these western basins, these projected streamflow 

changes are outside the range of historical variability, especially by the 2050s and 2070s. The projected streamflow changes 

and associated uncertainties have water management implications (discussed below). 

Figure 3.4. Annual and seasonal streamflow projections based on the B1 (with substantial emissions reductions), A1B (with gradual 

reductions from current emission trends beginning around mid-century), and A2 (with continuation of current rising emissions trends) 

CMIP3 scenarios for eight river basins in the western United States. The panels show percentage changes in average runoff, with 

projected increases above the zero line and decreases below. Projections are for annual, cool, and warm seasons, for three future 

decades (2020s, 2050s, and 2070s) relative to the 1990s. (Source: U.S. Department of the Interior – Bureau of Reclamation 2011;41

Data provided by L. Brekke, S. Gangopadhyay, and T. Pruitt)

Streamflow Projections for River Basins in the Western U.S.
75 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Key Message 2: Droughts Intensify 

Short-term (seasonal or shorter) droughts are expected to intensify in most 

U.S. regions. Longer-term droughts are expected to intensify in large areas of 

the Southwest, southern Great Plains, and Southeast. 

Annual runoff and related river-flow are projected to decline in the Southwest42,43 and Southeast,34 and to increase 

in the Northeast, Alaska, Northwest, and upper Midwest regions,42,43,44,45 broadly mirroring projected precipitation patterns.

46 Observational studies47 have shown that decadal fluctuations in average temperature (up to 1.5°F) and precipitation 

changes of 10% have occurred in most areas of the U.S. during 

the last century. Fluctuations in river-flow indicate that effects 

of temperature are dominated by fluctuations in precipitation. 

Nevertheless, as warming affects water cycle processes, the 

amount of runoff generated by a given amount of precipitation 

is generally expected to decline.37

Droughts occur on time scales ranging from season-to-season 

to multiple years and even multiple decades. There has been 

no universal trend in the overall extent of drought across the 

continental U.S. since 1900. However, in the Southwest, widespread drought in the past decade has reflected both precipitation deficits and higher temperatures8

 in ways that resemble 

projected changes.

48 Long-term (multi-seasonal) drought conditions are also projected to increase in parts of the Southeast 

and possibly in Hawai‘i and the Pacific Islands (Ch. 23: Hawai‘i 

and Pacific Islands). Except in the few areas where increases 

in summer precipitation compensate, summer droughts (Ch. 

2: Our Changing Climate) are expected to intensify almost everywhere in the continental U.S.49 due to longer periods of dry 

weather and more extreme heat,

33 leading to more moisture 

loss from plants and earlier soil moisture depletion in basins 

where snowmelt shifts to earlier in the year.

50,51 Basins watered 

by glacial melt in the Sierra Nevada, Glacier National Park, and 

Alaska may experience increased summer river-flow in the 

next few decades, until the amounts of glacial ice become too 

small to contribute to river-flow.52,53

Key Message 3: Increased Risk of Flooding in Many Parts of the U.S.

Flooding may intensify in many U.S. regions, even in areas 

where total precipitation is projected to decline. 

There are various types of floods (see “Flood Factors and Flood 

Types”), some of which are projected to increase with continued climate change. Floods that are closely tied to heavy precipitation events, such as flash floods and urban floods, as well 

as coastal floods related to sea level rise and the resulting increase in storm surge height and inland impacts, are expected 

to increase. Other types of floods result from a more complex 

set of causes. For example, river floods are basin specific and 

dependent not only on precipitation but also on pre-existing 

soil moisture conditions, topography, and other factors, including important human-caused changes to watersheds and 

river courses across the United States.

54,55,56,57 

Significant changes in annual precipitation (Ch. 2: Our Changing 

Climate) and soil moisture (Figures 3.2 and 3.3), among other 

factors, are expected to affect annual flood magnitudes (Figure 3.5) in many regions.

58 River floods have been increasing in 

the Northeast and Midwest, and decreasing in the Southwest 

and Southeast.56,57,58,59 These decreases are not surprising, as 

short duration very heavy precipitation events often occur 

during the summer and autumn when rivers are generally low. 

However, these very heavy precipitation events can and do 

lead to flash floods, often exacerbated in urban areas by the 

effect of impervious surfaces on runoff. 

Heavy rainfall events are projected to increase, which is expected to increase the potential for flash flooding. Land cover, 

flow and water-supply management, soil moisture, and channel conditions are also important influences on flood generation55 and must be considered in projections of future flood 

risks. Region-specific storm mechanisms and seasonality also 

affect flood peaks.57 Because of this, and limited capacity to 

project future very heavy events with confidence, evaluations 

of the relative changes in various storm mechanisms may be 

useful.57,60,61 Warming is likely to directly affect flooding in 

many mountain settings, as catchment areas receive increasingly more precipitation as rain rather than snow, or more 

rain falling on existing snowpack.62 In some such settings, river 

flooding may increase as a result – even where precipitation 

and overall river flows decline (Ch. 2: Our Changing Climate). 
76 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Key Message 4: Groundwater Availability

Climate change is expected to affect water demand, groundwater withdrawals, 

and aquifer recharge, reducing groundwater availability in some areas.

Groundwater is the only perennial source of fresh water in 

many regions and provides a buffer against climate extremes. 

As such, it is essential to water supplies, food security, and ecosystems. Though groundwater occurs in most areas of the U.S., 

the capacity of aquifers to store water varies depending on the 

geology of the region. (Figure 3.6b illustrates the importance 

of groundwater aquifers.) In large regions of the Southwest, 

Great Plains, Midwest, Florida, and some other coastal areas, 

groundwater is the primary water supply. Groundwater aquifers in these areas are susceptible to the combined stresses 

of climate and water-use changes. For example, during the 

2006–2009 California drought, when the source of irrigation 

shifted from surface water to predominantly groundwater, 

groundwater storage in California’s Central Valley declined by 

an amount roughly equivalent to the storage capacity of Lake 

Mead, the largest reservoir in the United States.

64

Climate change impacts on groundwater storage are expected 

to vary from place to place and aquifer to aquifer. Although 

precise responses of groundwater storage and flow to climate 

change are not well understood nor readily generalizable, recent and ongoing studies65,66,67,68 provide insights on various 

underlying mechanisms: 

1) Precipitation is the key driver of aquifer recharge in waterlimited environments (like arid regions), while evapotranspiration (ET) is the key driver in energy-limited environments (like swamps or marshlands).

2) Climate change impacts on aquifer recharge depend on 

several factors, including basin geology, frequency and 

intensity of high-rainfall periods that drive recharge, seasonal timing of recharge events, and strength of groundwater-surface water interaction.

3) Changes in recharge rates are amplified relative to changes in total precipitation, with greater amplification for 

drier areas. 

With these insights in mind, it is clear that certain groundwater-dependent regions are projected to incur significant climate change related challenges. In some portions of the country, groundwater provides nearly 100% of the water supply 

(Figure 3.6b). Seasonal soil moisture changes are a key aquifer 

recharge driver and may provide an early indication of general 

aquifer recharge trends. Thus, the observed regional reductions in seasonal soil moisture for winter and spring (Figure 

3.3) portend adverse recharge impacts for several U.S. regions, 

especially the Great Plains, Southwest, and Southeast. 

Despite their critical national importance as water supply 

sources (see Figure 3.6), aquifers are not generally monitored 

Figure 3.5. Trend magnitude (triangle size) and direction (green = increasing trend, brown = 

decreasing trend) of annual flood magnitude from the 1920s through 2008. Flooding in local 

areas can be affected by multiple factors, including land-use change, dams, and diversions of 

water for use. Most significant are increasing trends for floods in Midwest and Northeast, and 

a decreasing trend in the Southwest. (Figure source: Peterson et al. 201363).

Trends in Flood Magnitude
77 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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in ways that allow for clear identification of climatic influences 

on groundwater recharge, storage, flows, and discharge. Nearly all monitoring is focused in areas and aquifers where variations are dominated by groundwater pumping, which largely 

masks climatic influences,

69 highlighting the need for a national 

framework for groundwater monitoring.

70

Generally, impacts of changing demands on groundwater systems, whether due directly to climate changes or indirectly 

through changes in land use or surface-water availability and 

management, are likely to have the most immediate effects on 

groundwater availability;67,71 changes in recharge and storage 

may be more subtle and take longer to emerge. Groundwater 

models have only recently begun to include detailed represenFigure 3.6. (a) Groundwater aquifers are found throughout the U.S., but they vary widely in terms of ability to store and recharge 

water. The colors on this map illustrate aquifer location and geology: blue colors indicate unconsolidated sand and gravel; yellow 

is semi-consolidated sand; green is sandstone; blue or purple is sandstone and carbonate‐rock; browns are carbonate-rock; red 

is igneous and metamorphic rock; and white is other aquifer types. (Figure source: USGS). (b) Ratio of groundwater withdrawals 

to total water withdrawals from all surface and groundwater sources by county. The map illustrates that aquifers are the main 

(and often exclusive) water supply source for many U.S. regions, especially in the Great Plains, Misssissippi Valley, east central 

U.S., Great Lakes region, Florida, and other coastal areas. Groundwater aquifers in these regions are prone to impacts due to 

combined climate and water-use change. (Data from USGS 2005).

Principal U.S. Groundwater Aquifers and Use
78 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

3: WATER RESOURCES

tations of groundwater recharge and interactions with surface-water and land-surface processes,50 with few projections 

of groundwater responses to climate change.

68,72 However, surface water declines have already resulted in larger groundwater 

withdrawals in some areas (for example, in the Central Valley 

of California and in the Southeast) and may be aggravated by 

climate change challenges.73 In many mountainous areas of the 

U.S., groundwater recharge is disproportionately generated 

from snowmelt infiltration, suggesting that the loss of snowpack will affect recharge rates and patterns.50,51,66,74 Models do 

not yet include dynamic representations of the groundwater 

reservoir and its connections to streams, the soil-vegetation 

system, and the atmosphere, limiting the understanding of the 

potential climate change impacts on groundwater and groundwater-reliant systems.75

As the risk of drought increases, groundwater can play a key 

role in enabling adaptation to climate variability and change. 

For example, groundwater can be augmented by surface water during times of high flow through aquifer recharge strategies, such as infiltration basins and injection wells. In addition, 

management strategies can be implemented that use surface 

water for irrigation and water supply during wet periods, and 

groundwater during drought, although these approaches face 

practical limitations within current management and institutional frameworks.

71,76 

Key Message 5: Risks to Coastal Aquifers and Wetlands

Sea level rise, storms and storm surges, and changes in surface and groundwater 

use patterns are expected to compromise the sustainability 

of coastal freshwater aquifers and wetlands.

With more than 50% of the nation’s population concentrated 

near coasts (Chapter 25: Coasts),77 coastal aquifers and wetlands are precious resources. These aquifers and wetlands, 

which are extremely important from a biological/biodiversity perspective (see Ch. 8: Ecosystems; Ch. 25: Coasts), may 

be particularly at risk due to the combined effects of inland 

droughts and floods, increased surface water impoundments 

and diversions, increased groundwater withdrawals, and accelerating sea level rise and greater storm surges.78,79 Estuaries 

are particularly vulnerable to changes in freshwater inflow and 

sea level rise by changing salinity and habitat of these areas.

Several coastal areas, including the Delaware, Susquehanna, 

and Potomac River deltas on the Northeast seaboard, most 

of Florida, the Apalachicola and Mobile River deltas and bays, 

the Mississippi River delta in Louisiana, and the delta of the 

Sacramento-San Joaquin rivers in northern California, are particularly vulnerable due to the combined effects of climate 

change and other human-caused stresses. In response, some 

coastal communities are among the nation’s most proactive in 

adaptation planning (Chapter 25: Coasts). 

Key Message 6: Water Quality Risks to Lakes and Rivers 

Increasing air and water temperatures, more intense precipitation and runoff, and intensifying 

droughts can decrease river and lake water quality in many ways, including increases in 

sediment, nitrogen, and other pollutant loads. 

Water temperature has been increasing in some rivers.80 The 

length of the season that lakes and reservoirs are thermally 

stratified (with separate density layers) is increasing with increased air and water temperatures.

81,82 In some cases, seasonal mixing may be eliminated in shallow lakes, decreasing 

dissolved oxygen and leading to excess concentrations of 

nutrients (nitrogen and phosphorous), heavy metals (such as 

mercury), and other toxins in lake waters.81,82

Lower and more persistent low flows under drought conditions 

as well as higher flows during floods can worsen water quality. 

Increasing precipitation intensity, along with the effects of wildfires and fertilizer use, are increasing sediment, nutrient, and 

contaminant loads in surface waters used by downstream water users84 and ecosystems. Mineral weathering products, like 

calcium, magnesium, sodium, and silicon and nitrogen loads85

have been increasing with higher streamflows.86 Changing land 

cover, flood frequencies, and flood magnitudes are expected 

to increase mobilization of sediments in large river basins.87

Increasing air and water temperatures, more intense precipitation and 

runoff, and intensifying droughts can decrease water quality in many 

ways. Here, middle school students in Colorado learn about water quality.© Ted Wood
79 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

3: WATER RESOURCES

Changes in sediment transport are expected to vary regionally 

and by land-use type, with potentially large increases in some 

areas,

88 resulting in alterations to reservoir storage and river 

channels, affecting flooding, navigation, water supply, and 

dredging. Increased frequency and duration of droughts, and 

associated low water levels, increase nutrient concentrations 

and residence times in streams, potentially increasing the likelihood of harmful algal blooms and low oxygen conditions.89

Concerns over such impacts and their potential link to climate 

change are rising for many U.S. regions including the Great 

Lakes,

90 Chesapeake Bay,91 and the Gulf of Mexico.85,86 Strategies aiming to reduce sediment, nutrient, and contaminant 

loads at the source remain the most effective management 

responses.92

Relationship between Historical and Projected Water Cycle Changes

Natural climate variations occur on essentially all time scales 

from days to millennia, and the water cycle varies in much the 

same way. Observations of changes in the water cycle over 

time include responses to natural hydroclimatic variability as 

well as other, more local, human influences (like dam building or land-use changes), or combinations of these influences 

with human-caused climate change. Some recent studies 

have attributed specific observed changes in the water cycle 

to human-induced climate change (for example, Barnett et al. 

200810). For many other water cycle variables and impacts, the 

observed and projected responses are consistent with those 

expected by human-induced climate change and other human influences. Research aiming to formally attribute these 

responses to their underlying causes is ongoing. 

Figure 3.7. The length of the season in which differences in lake temperatures with depth cause stratification (separate density 

layers) is increasing in many lakes. In this case, measurements show stratification has been increasing in Lake Tahoe (top left) since 

the 1960s and in Lake Superior (top right) since the early 1900s in response to increasing air and surface water temperatures (see 

also Ch. 18: Midwest). In Lake Tahoe, because of its large size (relative to inflow) and resulting long water-residence times, other 

influences on stratification have been largely overwhelmed, and warming air and water temperatures have caused progressive 

declines in near-surface density, leading to longer stratification seasons (by an average of 20 days), decreasing the opportunities 

for deep lake mixing, reducing oxygen levels, and causing impacts to many species and numerous aspects of aquatic ecosytems.

83

Similar effects are observed in Lake Superior,16 where the stratification season is lengthening (top right) and annual ice-covered 

area is declining (bottom); both observed changes are consistent with increasing air and water temperatures.

Observed Changes in Lake Stratification and Ice Covered Area
80 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

3: WATER RESOURCES

Flood factors and flood types

A flood is defined as any high flow, overflow, or inundation by water that causes or threatens damage.93 Floods are 

caused or amplified by both weather- and human-related factors. Major weather factors include heavy or prolonged 

precipitation, snowmelt, thunderstorms, storm surges from hurricanes, and ice or debris jams. Human factors include structural failures of dams and levees, inadequate drainage, and land cover alterations (such as pavement or 

deforestation) that reduce the capacity of the land surface to absorb water. Increasingly, humanity is also adding to 

weather-related factors, as human-induced warming increases heavy downpours, causes more extensive storm surges 

due to sea level rise, and leads to more rapid spring snowmelt.

Worldwide, from 1980 to 2009, floods caused more than 500,000 deaths and affected more than 2.8 billion 

people.94 In the U.S., floods caused 4,586 deaths from 1959 to 200595 while property and crop damage averaged 

nearly $8 billion per year (in 2011 dollars) over 1981 through 2011.93 The risks from future floods are significant, 

given expanded development in coastal areas and floodplains, unabated urbanization, land-use changes, and humaninduced climate change.94 

Major flood types include flash, urban, riverine, and coastal flooding: 

Flash floods occur in small and steep watersheds and waterways 

and can be caused by short-duration intense precipitation, dam 

or levee failure, or collapse of debris and ice jams. Snow cover 

and frozen ground conditions can exacerbate flash flooding during winter and early spring by increasing the fraction of precipitation that runs off. Flash floods develop within minutes or hours 

of the causative event, and can result in severe damage and loss 

of life due to high water velocity, heavy debris load, and limited 

warning. Most flood-related deaths in the U.S. are associated 

with flash floods.

Urban flooding can be caused by short-duration very heavy precipitation. Urbanization creates large areas of impervious surfaces 

(such as roads, pavement, parking lots, and buildings) and increases immediate runoff. Stormwater drainage removes excess 

surface water as quickly as possible, but heavy downpours can 

exceed the capacity of drains and cause urban flooding. 

Flash floods and urban 

flooding are directly 

linked to heavy precipitation and are expected 

to increase as a result 

of projected increases 

in heavy precipitation 

events. In mountainous 

watersheds, such increases may be partially offset in winter and 

spring due to projected 

snowpack reduction.

Riverine flooding occurs 

when surface water 

drains from a watershed into a stream or 

a river exceeds channel 

capacity, overflows the 

Riverine Flooding: In many regions, infrastructure is currently vulnerable to flooding, as demonstrated 

in these photos. Left: The Fort Calhoun Nuclear Power Plant in eastern Nebraska was surrounded 

by a Missouri River flood on June 8, 2011, that also affected Louisiana, Mississippi, Missouri, Illinois, 

Kentucky, Tennessee, and Arkansas (photo credit: Larry Geiger). Right: The R.M. Clayton sewage 

treatment plant in Atlanta, Georgia, September 23, 2009, was engulfed by floodwaters forcing it to 

shut down and resulting in the discharge of raw sewage into the Chattahoochee River (photo credit: 

Reuters/David Tulis). Flooding also disrupts road and rail transportation, and inland navigation.

Flash Flooding: Cave Creek, Arizona

(Photo credit: Tom McGuire).

Continued
81 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Climate Change Impacts on Water Resource Uses and Management

People use water for many different purposes and benefits. 

Our water use falls into five main categories: 1) municipal use, 

which includes domestic water for drinking and bathing; 2) agricultural use, which includes irrigation and cattle operations; 

3) industrial use, which includes electricity production from 

coal- or gas-fired power plants that require water to keep the 

machinery cool; 4) providing ecosystem benefits, such as supporting the water needs of plants and animals we depend on; 

and 5) recreational uses, such as boating and fishing. 

Water is supplied for these many uses from two main sources: 

•	 freshwater withdrawals (from streams, rivers, lakes, 

and aquifers), which supply water for municipal, industrial, agricultural, and recirculating thermoelectric 

plant cooling water supply;

•	 instream surface water flows, which support hydropower production, once-through thermoelectric plant 

cooling, navigation, recreation, and healthy ecosystems. 

Flood factors and flood types (continued)

banks, and inundates adjacent low lying areas. Riverine flooding is commonly associated with large watersheds and rivers, while flash and urban flooding occurs in smaller natural or urban watersheds. Because heavy precipitation is often 

localized, riverine flooding typically results from multiple heavy precipitation events over periods of several days, weeks, 

or even months. In large basins, existing soil moisture conditions and evapotranspiration rates also influence the onset 

and severity of flooding, as runoff increases with wetter soil and/or lower evapotranspiration conditions. Snow cover and 

frozen ground conditions can also exacerbate riverine flooding during winter and spring by increasing runoff associated 

with rain-on-snow events and by snowmelt, although these effects may diminish in the long term as snow accumulation 

decreases due to warming. Since riverine flooding depends on precipitation as well as many other factors, projections 

about changes in frequency or intensity are more uncertain than with flash and urban flooding. 

Coastal flooding is predominantly caused by storm surges that accompany hurricanes and other storms. Low storm 

pressure creates strong winds that create and push large sea water domes, often many miles across, toward the shore. 

The approaching domes can raise the water surface above normal tide levels (storm surge) by more than 25 feet, depending on various storm and shoreline factors. 

Inundation, battering waves, and floating debris 

associated with storm surge can cause deaths, 

widespread infrastructure damage (to buildings, 

roads, bridges, marinas, piers, boardwalks, and 

sea walls), and severe beach erosion. Stormrelated rainfall can also cause inland flooding 

(flash, urban, or riverine) if, after landfall, the 

storm moves slowly or stalls over an area. Inland 

flooding can occur close to the shore or hundreds of miles away and is responsible for more 

than half of the deaths associated with tropical 

storms.93 Climate change affects coastal flooding through sea level rise and storm surge, increases in heavy rainfall during hurricanes and 

other storms, and related increases in flooding in 

coastal rivers.

In some locations, early warning systems have helped reduce deaths, although property damage remains considerable 

(Ch. 28: Adaptation). Further improvements can be made by more effective communication strategies and better landuse planning.

94 

Hurricane Sandy coastal flooding in Mantoloking, N.J.

(Photo credit: New Jersey National Guard/Scott Anema).
82 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Key Message 7: Changes to Water Demand and Use

Climate change affects water demand and the ways water is used within and across regions 

and economic sectors. The Southwest, Great Plains, and Southeast are 

particularly vulnerable to changes in water supply and demand.

Climate change, acting concurrently with demographic, landuse, energy generation and use, and socioeconomic changes, is 

challenging existing water management practices by affecting 

water availability and demand and by exacerbating competition among uses and users (see Ch. 4: Energy; Ch. 6: Agriculture; 

Ch. 10: Energy, Water, and Land; Ch. 12: Indigenous Peoples; 

and Ch. 13: Land Use & Land Cover Change). In some regions, 

these current and expected impacts are hastening efficiency 

improvements in water withdrawal and use, the deployment 

of more proactive water management and adaptation approaches, and the reassessment of the water infrastructure 

and institutional responses.1

Water Withdrawals

Total freshwater withdrawals (including water that is withdrawn and consumed as well as water that returns to the original source) and consumptive uses have leveled off nationally since 1980 at 350 billion gallons of withdrawn water and 100 

billion gallons of consumptive water per day, despite the addition of 68 million people from 1980 to 2005 (Figure 3.8).

96

Irrigation and all electric power plant cooling withdrawals account for approximately 77% of total withdrawals, municipal 

and industrial for 20%, and livestock and aquaculture for 3%. 

Most thermoelectric withdrawals are returned back to rivers 

after cooling, while most irrigation withdrawals are consumed 

by the processes of evapotranspiration and plant growth. 

Thus, consumptive water use is dominated by irrigation (81%) 

followed distantly by municipal and industrial (8%) and the remaining water uses (5%). See Figure 3.9. 

Water sector withdrawals and uses vary significantly by region. 

There is a notable east-west water use pattern, with the largest regional withdrawals occurring in western states (where 

the climate is drier) for agricultural irrigation (Figure 3.10a,d). 

In the east, water withdrawals mainly serve municipal, industrial, and thermoelectric uses (Figure 3.10a,b,c). Irrigation is 

also dominant along the Mississippi Valley, in Florida, and in 

southeastern Texas. Groundwater withdrawals are especially 

Figure 3.8. Trends in total freshwater withdrawal (equal intense in parts of the Southwest, Southeast, Northwest, and 

to the sum of consumptive use and return flows to rivers) 

and population in the contiguous United States. This 

graph illustrates the remarkable change in the relationship 

between water use and population growth since about 

1980. Reductions in per capita water withdrawals are 

directly related to increases in irrigation efficiency for 

agriculture, more efficient cooling processes in electrical 

generation, and, in many areas, price signals, more 

efficient indoor plumbing fixtures and appliances, and 

reductions in exterior landscape watering, in addition to 

shifts in land-use patterns in some areas.97 Efficiency 

improvements have offset the demands of a growing 

population and have resulted in more flexibility in meeting 

water demand. In some cases these improvements 

have also reduced the flexibility to scale back water use 

in times of drought because some inefficiencies have 

already been removed from the system. With drought 

stress projected to increase in many U.S. regions, drought 

vulnerability is also expected to rise.1

U.S. Freshwater Withdrawal, Consumptive Use, 

and Population Trends

Figure 3.9. Total water withdrawals (groundwater and surface 

water) in the U.S. are dominated by agriculture and energy 

production, though the primary use of water for thermoelectric 

production is for cooling, where water is often returned to lakes 

and rivers after use (return flows). (Data from Kenny et al. 200996)

Freshwater Withdrawals by Sector
83 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

3: WATER RESOURCES

Figure 3.10. Based on the most recent USGS water withdrawal data (2005). This figure illustrates water withdrawals at the U.S. 

county level: (a) total withdrawals (surface and groundwater) in thousands of gallons per day per square mile; (b) municipal and 

industrial (including golf course irrigation) withdrawals as percent of total; (c) irrigation, livestock, and aquaculture withdrawals as 

percent of total; (d) thermoelectric plant cooling withdrawals as percent of total; (e) counties with large surface water withdrawals; 

and (f) counties with large groundwater withdrawals. The largest withdrawals occur in the drier western states for crop irrigation. 

In the east, water withdrawals mainly serve municipal, industrial, and thermoelectric uses. Groundwater withdrawals are intense in 

parts of the Southwest and Northwest, the Great Plains, Mississippi Valley, Florida and South Georgia, and near the Great Lakes 

(Figure source: Georgia Water Resources Institute, Georgia Institute of Technology; Data from Kenny et al. 2009;96 USGS 201398). 

U.S. Water Withdrawal Distribution
84 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

3: WATER RESOURCES

Great Plains, the Mississippi Valley, Florida and South Georgia, 

and near the Great Lakes (Figure 3.10f). Surface waters are 

most intensely used in all other U.S. regions. 

Per capita water withdrawal and use are decreasing due to 

many factors.

99 These include demand management, new 

plumbing codes, water-efficient appliances, efficiency improvement programs, and pricing strategies, especially in the 

municipal sector.100 Other factors contributing to decreasing 

per capita water use include changes from water-intensive 

manufacturing and other heavy industrial activities to serviceoriented businesses,101 and enhanced water-use efficiencies in 

response to environmental pollution legislation (in the industrial and commercial sector). In addition, replacement of older 

once-through-cooling electric power plants by plants that recycle their cooling water, and switching from flood irrigation to 

more efficient methods in the western United States102 have 

also contributed to these trends. 

Notwithstanding the overall national trends, regional water 

withdrawal and use are strongly correlated with climate;103

hotter and drier regions tend to have higher per capita usage, 

and water demand is affected by both temperature and precipitation on a seasonal basis (see also Ch. 28: Adaptation). 

Water demand is projected to increase as population grows, 

and will increase substantially more in some regions as a result 

of climate change. In the absence of climate change but in response to a projected population increase of 80% and a 245% 

increase in total personal income from 2005 to 2060, simulations under the A1B scenario indicate that total water demand 

in the U.S. would increase by 3%.99 Under these conditions, 

approximately half of the U.S. regions would experience an 

overall decrease in water demand, while the other half would 

experience an increase (Figure 3.11a). If, however, climate 

change projections based on the A1B emissions scenario (with 

gradual reductions from current emission trends beginning 

around mid-century) and three climate models are also factored in, the total water demand is projected to rise by an average of 26% over the same period (Figure 3.11b).99 Under the 

population increase scenario that also includes climate change, 

90% of the country is projected to experience a total demand 

increase, with decreases projected only in parts of the Midwest, Northeast and Southeast. Compared to an 8% increase in 

demand under a scenario without climate change, projections 

under the A2 emissions scenario (which assumes continued 

increases in global emissions) and three climate models over 

the 2005 to 2060 period result in a 34% increase in total water 

demand. By 2090, total water demand is projected to increase 

by 42% over 2005 levels under the A1B scenario and 82% under 

the higher A2 emissions scenario. 

Crop irrigation and landscape watering needs are directly affected by climate change, especially by projected changes in 

temperature, potential evapotranspiration, and soil moisture. 

Consequently, the projected climate change impacts on water 

demand are larger in the western states, where irrigation dominates total water withdrawals (see Figure 3.10). Uncertainties 

in the projections of these climate variables also affect water 

demand projections.99 However, it is clear that the impacts of 

projected population, socioeconomic, and climate changes 

amplify the effects on water demand in the Southwest and 

Southeast, where the observed and projected drying water cycle trends already make these regions particularly vulnerable. 

This vulnerability will be exacerbated by physical and operational limitations of water storage and distribution systems. 

River reservoirs and associated dams are usually designed to 

handle larger-than-historical streamflow variability ranges. 

Some operating rules and procedures reflect historical seasonal and interannual streamflow and water release patterns, 

while others include information about current and near-term 

conditions, such as snowpack depth and expected snowmelt 

volume. Climate change threatens to alter both the streamflow 

variability that these structures must accommodate and their 

opportunities to recover after doing so (due to permanent 

changes in average streamflow). Thus, as streamflow and demand patterns change, historically based operating rules and 

procedures could become less effective in balancing water 

supply with other uses.104

Some of the highest water demand increases under climate 

change are projected in U.S. regions where groundwater aquifers are the main water supply source (Figure 3.11b), including 

the Great Plains and parts of the Southwest and Southeast. 

The projected water demand increases combined with potentially declining recharge rates (see water cycle section) further 

challenge the sustainability of the aquifers in these regions. 

Power plant cooling is a critical national water use, because 

nearly 90% of the U.S. electrical energy is produced by thermoelectric power plants.105 Freshwater withdrawals per kilowatt 

hour have been falling in recent years due to the gradual replacement of once-through cooling of power plant towers with 

plants that recycle cooling water. Thermal plant cooling is principally supported by surface water withdrawals (Figure 3.10e,f) 

and has already been affected by climate change in areas 

where temperatures are increasing and surface water supplies 

are diminishing, such as the southern United States. Higher 

water temperatures affect the efficiency of electric generation 

and cooling processes. It also limits the ability of utilities to 

discharge heated water to streams from once-through cooled 

power systems due to regulatory requirements and concerns 

about how the release of warmer water into rivers and streams 

affects ecosystems and biodiversity (see Ch. 4: Energy).106
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3: WATER RESOURCES

Instream Water Uses

Hydropower contributes 7% of electricity generation nationwide, but provides up to 70% in the Northwest and 20% in California, Alaska, and the Northeast.107 Climate change is expected to affect hydropower directly through changes in runoff 

(average, extremes, and seasonality), and indirectly through 

increased competition with other water uses. Based on runoff 

projections, hydropower is expected to decline in the southern 

U.S. (especially the Southwest) and increase in the Northeast 

and Midwest (though actual gains or losses will depend on 

facility size and changes in runoff volume and timing). Where 

non-power water demands are expected to increase (as in the 

southern U.S.), hydropower generation, dependable capacity, 

and ancillary services are likely to decrease. Many hydropower 

facilities nationwide, especially in the Southeast, Southwest, 

and the Great Plains, are expected to face water availability 

constraints.

108 While some hydropower facilities may face water-related limitations, these could be offset to some degree 

by the use of more efficient turbines as well as innovative new 

hydropower technologies.

Inland navigation, most notably in the Great Lakes and the 

Missouri, Mississippi, and Ohio River systems, is particularly 

important for agricultural commodities (transported from the 

Midwest to the Gulf Coast and on to global food markets), coal, 

and iron ore.1,109 Navigation is affected by ice cover and by 

floods and droughts. Seasonal ice cover on the Great Lakes has 

been decreasing16 which may allow increased shipping.110 However, lake level declines are also possible in the long term, decreasing vessel draft and cargo capacity. Future lake levels may 

also depend on non-climate factors and are uncertain both in 

direction and magnitude (see Ch. 2: Our Changing Climate; Ch. 

5: Transportation; and Ch. 18: Midwest). Similarly, although 

the river ice cover period has been decreasing53 (extending 

the inland navigation season), seasonal ice cover changes111,112

could impede lock operations.112 Intensified floods are likely to 

hinder shipping by causing waterway closures and damaging or 

destroying ports and locks. Droughts have already been shown 

to decrease reliability of flows or channel depth, adversely 

impacting navigation (Ch. 5: Transportation). Both floods and 

droughts can disrupt rail and road traffic and increase shipping 

costs113 and result in commodity price volatility (Ch. 19: Great 

Plains). 

Recreational activities associated with water resources, including boating, fishing, swimming, skiing, camping, and wildlife 

watching, are strong regional and national economic drivers.

114

Recreation is sensitive to weather and climate,115 and climate 

change impacts to recreation can be difficult to project.116 Rising temperatures affect extent of snowcover and mountain 

snowpack, with impacts on skiing117 and snowmobiling.118 As 

the climate warms, changes in precipitation and runoff are 

expected to result in both beneficial (in some regions) and adverse impacts115 to water sports, with potential for considerable economic dislocation and job losses.118

Changing climate conditions are projected to affect water and 

wastewater treatment and disposal in ways that depend on 

system-specific and interacting attributes. For example, elevated stream temperatures, combined with lower flows, may 

require wastewater facilities to increase treatment to meet 

stream water quality standards.119 More intense precipitation 

and floods, combined with escalating urbanization and associated increasing impermeable surfaces, may amplify the likelihood of contaminated overland flow or combined sewer overFigure 3.11. The effects of climate change, primarily associated with increasing temperatures and potential 

evapotranspiration, are projected to significantly increase water demand across most of the United States. Maps show 

percent change from 2005 to 2060 in projected demand for water assuming (a) change in population and socioeconomic 

conditions based on the underlying A1B emissions scenario, but with no change in climate, and (b) combined changes 

in population, socioeconomic conditions, and climate according to the A1B emissions scenario (gradual reductions from 

current emission trends beginning around mid-century). (Figure source: Brown et al. 201399). 

Projected Changes in Water Withdrawals
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3: WATER RESOURCES

flows.

120 Moderate precipitation increases, however, could 

result in increased stream flows, improving capacity to dilute 

contaminants in some regions. Sea level rise and more frequent coastal flooding could damage wastewater utility infrastructure and reduce treatment efficiency (Ch. 25: Coasts).

121

Changes in streamflow temperature and flow regimes can 

affect aquatic ecosystem structure and function (see Ch. 8: 

Ecosystems). Water temperature directly regulates the physiology, metabolism, and energy of individual aquatic organisms, 

as well as entire ecosystems. Streamflow quantity influences 

the extent of available aquatic habitats, and streamflow variability regulates species abundance and persistence. Flow also 

influences water temperature, sediment, and nutrient concentrations.122 If the rate of climate change123 outpaces plant 

and animal species’ ability to adjust to temperature change, 

additional biodiversity loss may occur. Furthermore, climate 

change induced water cycle alterations may exacerbate existing ecosystem vulnerability, especially in the western United 

States124 where droughts and water shortages are likely to 

increase. But areas projected to receive additional precipitation, such as the northern Great Plains, may benefit. Lastly, hydrologic alterations due to human interventions have without 

doubt impaired riverine ecosystems in most U.S. regions and 

globally.125 The projected escalation of water withdrawals and 

uses (see Figure 3.11) threatens to deepen and widen ecosystem impairment, especially in southern states where climate 

change induced water cycle alterations are pointing toward 

drier conditions (see Ch. 8: Ecosystems). In these regions, balancing socioeconomic and environmental objectives will most 

likely require more deliberate management and institutional 

responses. 

Major Water Resource Vulnerabilities and Challenges 

Many U.S. regions are expected to face increased drought and flood vulnerabilities and exacerbated water management challenges. This section highlights regions where such issues are expected to be particularly intense. 

Key Message 8: Drought is Affecting Water Supplies 

Changes in precipitation and runoff, combined with changes in consumption and withdrawal, 

have reduced surface and groundwater supplies in many areas. These trends are expected 

to continue, increasing the likelihood of water shortages for many uses. 

Many southwestern and western watersheds, including 

the Colorado, Rio Grande,38,43,126 and Sacramento-San Joaquin,127,128 have recently experienced drier conditions. Even 

larger runoff reductions (about 10% to 20%) are projected 

over some of these watersheds in the next 50 years.48,129 Increasing evaporative losses, declining runoff and groundwater 

recharge, and changing groundwater pumpage are expected to 

affect surface and groundwater supplies65,66,67,71 and increase 

the risk of water shortages for many water uses. Changes in 

streamflow timing will exacerbate a growing mismatch between supply and demand (because peak flows are occurring 

earlier in the spring, while demand is highest in mid-summer) 

and will present challenges for the management of reservoirs, 

aquifers, and other water infrastructure.130 Rising stream 

temperatures and longer low flow periods may make electric 

power plant cooling water withdrawals unreliable, and may 

affect aquatic and riparian ecosystems by degrading habitats 

and favoring invasive, non-native species.131

Key Message 9: Flood Effects on People and Communities

Increasing flooding risk affects human safety and health, property, infrastructure, 

economies, and ecology in many basins across the U.S.

Flooding affects critical water, wastewater, power, transportation, and communications infrastructure in ways that are difficult to foresee and can result in interconnected and cascading failures (see “Flood Factors and Flood Types”). Very heavy 

precipitation events have intensified in recent decades in most 

U.S. regions, and this trend is projected to continue (Ch. 2: Our 

Changing Climate). Increasing heavy precipitation is an important contributing factor, but flood magnitude changes also depend on specific watershed conditions (including soil moisture, 

impervious area, and other human-caused alterations). 

Projected changes in flood frequency based on climate projections and hydrologic models have recently begun to emerge 

(for example, Das et al. 2012;60 Brekke et al. 2009;132 Raff et 

al. 2009;133 Shaw and Riha 2011;134 Walker et al. 2011135), and 

suggest that flood frequency and severity increases may occur 

in the Northeast and Midwest (Ch. 16: Northeast; Ch. 18: Midwest). Flooding and sea water intrusion from sea level rise and 

increasing storm surge threaten New York, Boston, Philadelphia, Virginia Beach, Wilmington, Charleston, Miami, Tampa, 

Naples, Mobile, Houston, New Orleans, and many other cities 

on U.S. coasts (Chapter 25: Coasts). 

The devastating toll of large floods (human life, property, environment, and infrastructure) suggests that proactive management measures could minimize changing future flood risks and 
87 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

3: WATER RESOURCES

consequences (Ch. 28: Adaptation). In coastal areas, sea level 

rise may act in parallel with inland climate changes to intensify 

water-use impacts and challenges (Ch. 12: Indigenous Peoples; 

Ch. 17: Southeast).136 Increasing flooding risk, both coastal and 

inland, could also exacerbate human health risks associated 

with failure of critical infrastructure,137,138 and an increase in 

both waterborne diseases (Ch. 9: Human Health)139 and airborne diseases.140

Changes in land use, land cover, development, and population 

distribution can all affect flood frequency and intensity. The nature and extent of these projected changes results in increased 

uncertainty and decreased accuracy of flood forecasting in 

both the short term133 and long term.141 This lack of certainty 

could hinder effective preparedness (such as evacuation planning) and the effectiveness of structural and non-structural 

flood risk reduction measures. However, many climate change 

projections are robust (Ch. 2: Our Changing Climate), and the 

long lead time needed for the planning, design, and construction of critical infrastructure that provides resilience to floods 

means that consideration of long-term changes is needed.

Effective climate change adaptation planning requires an integrated approach45,118,142 that addresses public health and 

safety issues (Ch. 28: Adaptation).143 Though numerous flood 

risk reduction measures are possible, including levees, landuse zoning, flood insurance, and restoration of natural floodplain retention capacity,144 economic and institutional conditions may constrain implementation. The effective use of 

these measures would require significant investment in many 

cases,145 as well as updating policies and methods to account 

for climate change42,146 in the planning, design, operation, and 

maintenance of flood risk reduction infrastructure.132,147 

Adaptation and Institutional Responses 

Key Message 10: Water Resources Management

In most U.S. regions, water resources managers and planners will encounter new risks, 

vulnerabilities, and opportunities that may not be properly managed within existing practices. 

Water managers and planners strive to balance water supply 

and demand across all water uses and users. The management 

process involves complex tradeoffs among water-use benefits, 

consequences, and risks. By altering water availability and 

demand, climate change is likely to present additional management challenges. One example is in the Sacramento-San 

Joaquin River Delta, where flooding, sea water intrusion, and 

changing needs for environmental, municipal, and agricultural 

water uses have created significant management challenges. 

This California Bay-Delta experience suggests that managing risks and sharing benefits requires re-assessment of very 

complex ecosystems, infrastructure systems, water rights, 

stakeholder preferences, and reservoir operation strategies – 

as well as significant investments. All of these considerations 

are subject to large uncertainties.54,148 To some extent, all U.S. 

regions are susceptible, but the Southeast and Southwest 

are highly vulnerable because climate change is projected to 

reduce water availability, increase demand, and exacerbate 

shortages (see “Water Management”). 

Recent assessments illustrate water management challenges 

facing California,127,129,149,150 the Southwest,130,151 Southeast (Ch. 

17: Southeast),

136,152 Northwest,153 Great Plains,154 and Great 

Lakes.155 A number of these assessments demonstrate that 

while expanding supplies and storage may still be possible 

in some regions, effective climate adaptation strategies can 

benefit from innovative management strategies. These strategies can include domestic water conservation programs that 

use pricing incentives to curb use; more flexible, risk-based, 

better-informed, and adaptive operating rules for reservoirs; 

the integrated use of combined surface and groundwater resources; and better monitoring and assessment of statewide 

water use.

129,149,156,157 Water management and planning would 

benefit from better coordination among public sectors at the 

national, state, and local levels (including regional partnerships 

and agreements), and the private sector, with participation of 

all relevant stakeholders in well-informed, fair, and equitable 

decision-making processes. Better coordination among hydrologists and atmospheric scientists, and among these scientists and the professional water management community, is 

also needed to facilitate more effective translation of knowledge from science to practice (Ch. 26: Decision Support; Ch. 

28: Adaptation).

158
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3: WATER RESOURCES

Water challenges in a southeast river basin

Figure 3.12. The Apalachicola-Chattahoochee-Flint (ACF) River Basin supports many water uses and users, including municipal, 

industrial, and agricultural water supply; flood management; hydroelectric and thermoelectric energy generation; recreation; 

navigation; fisheries; and a rich diversity of environmental and ecological resources. In recent decades, water demands have risen 

rapidly in the Upper Chattahoochee River (due to urban growth) and Lower Chattahoochee and Flint Rivers (due to expansion 

of irrigated agriculture). At the same time, basin precipitation, soil moisture, and runoff are declining, creating challenging water 

sharing tradeoffs for the basin stakeholders.159 The historical water demand and supply trends are expected to continue in the 

coming decades. Climate assessments for 50 historical (1960-2009) and future years (2050-2099) based on a scenario of 

continued increases in emissions (A2) for the Seminole and all other ACF sub-basins152 show that soil moisture is projected to 

continue to decline in all months, especially during the crop growing season from April to October (bottom right). Mean monthly 

runoff decreases (up to 20%, not shown) are also projected throughout the year and especially during the wet season from 

November to May. The projected soil moisture and runoff shifts are even more significant in the extreme values of the respective 

distributions. In addition to reduced supplies, these projections imply higher water demands in the agricultural and other sectors, 

exacerbating management challenges. These challenges are reflected in the projected response of Lake Lanier, the main ACF 

regulation project, the levels of which are projected (for 2050-2099) to be lower, by as much as 15 feet, than its historical (1960-

2009) levels, particularly during droughts (top right). Recognizing these critical management challenges, the ACF stakeholders 

are earnestly working to develop a sustainable and equitable management plan that balances economic, ecological, and social 

values.160 (Figure source: Georgia Water Resources Institute, Georgia Institute of Technology.

152).
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3: WATER RESOURCES

Key Message 11: Adaptation Opportunities and Challenges

Increasing resilience and enhancing adaptive capacity provide opportunities to 

strengthen water resources management and plan for climate change impacts. 

Many institutional, scientific, economic, and political barriers present 

challenges to implementing adaptive strategies. 

Climate adaptation involves both addressing the risks and leveraging the opportunities that may arise as a result of the climate impacts on the water cycle and water resources. Efforts 

to increase resiliency and enhance adaptive capacity may create opportunities for a wide-ranging public discussion of water 

demands, improved collaboration around water use, increased 

public support for scientific and economic information, and 

the deployment of new technologies supporting adaptation. In 

addition, adaptation can promote the achievement of multiple 

water resource objectives through improved infrastructure 

planning, integrated regulation, and planning and management approaches at regional, watershed, or ecosystem scales. 

Pursuing these opportunities may require assessing how current institutional approaches support adaptation in light of the 

anticipated impacts of climate change.161

Climate change will stress the nation’s aging water infrastructure to varying degrees by location and over time. Much of 

the country’s current drainage infrastructure is already overwhelmed during heavy precipitation and high runoff events, 

an impact that is projected to be exacerbated as a result of 

climate change, land-use change, and other factors. Large percentage increases in combined sewage overflow volumes, associated with increased intensity of precipitation events, have 

been projected for selected watersheds by the end of this 

century in the absence of adaptive measures.106,162 Infrastructure planning, especially for the long planning and operation 

horizons often associated with water resources infrastructure, 

can be improved by incorporating climate change as a factor 

in new design standards and in asset management and rehabilitation of critical and aging facilities, emphasizing flexibility, 

redundancy, and resiliency.106,132,163

Adaptation strategies for water infrastructure include structural and non-structural approaches. These may include changes 

in system operations and/or demand management changes, 

adopting water conserving plumbing codes, and improving 

flood forecasts, telecommunications, and early warning systems164 that focus on both adapting physical structures and 

innovative management.106,132,165 Such strategies could take 

advantage of conventional (“gray”) infrastructure upgrades 

(like raising flood control levees); adjustments to reservoir operating rules; new demand management and incentive strategies; land-use management that enhances adaptive capacity; 

protection and restoration at the scale of river basins, watersheds, and ecosystems; hybrid strategies that blend “green” 

infrastructure with gray infrastructure; and pricing strategies.1,106,132,166,167 Green infrastructure approaches that are 

increasingly being implemented by municipalities across the 

country include green roofs, rain gardens, roadside plantings, 

porous pavement, and rainwater harvesting (Ch. 28: Adaptation). These techniques typically utilize soils and vegetation 

in the built environment to absorb runoff close to where it 

falls, limiting flooding and sewer backups.168 There are numerous non-infrastructure related adaptation strategies, some of 

which could include promoting drought-resistant crops, flood 

insurance reform, and building densely developed areas away 

from highly vulnerable areas.

In addition to physical adaptation, capacity-building activities 

can build knowledge and enhance communication and collaboration within and across sectors.1,167,169 In particular, building 

networks, partnerships, and support systems has been identified as a major asset in building adaptive capacity (Ch. 26: 

Decision Support; Ch. 28: Adaptation).170

In addition to stressing the physical infrastructure of water 

systems, future impacts of climate change may reveal the 

weaknesses in existing water law regimes to accommodate 

novel and dynamic water management conditions. The basic 

paradigms of environmental and natural resources law are 

preservation and restoration, both of which are based on the 

assumption that natural systems fluctuate within an unchanging envelope of variability (“stationarity”).171 However, climate 

change is now projected to affect water supplies during the 

multi-decade lifetime of major water infrastructure projects in 

wide-ranging and pervasive ways.132 Under these circumstances, stationarity will no longer be reliable as the central assumption in water-resource risk assessment and planning.42,171 For 

example, in the future, water rights administrators may find it 

necessary to develop more flexible water rights systems conditioned to address the uncertain impacts of climate change.172

Agencies and courts may seek added flexibility in regulations 

and laws to achieve the highest and best uses of limited water 

resources and to enhance water management capacity in the 

context of new and dynamic conditions.132,173

In the past few years, many federal, state, and local agencies and tribal governments have begun to address climate 

change adaptation, integrating it into existing decision-making, planning, or infrastructure-improvement processes (Ch. 

28: Adaptation).43,174 Drinking water utilities are increasingly 

utilizing climate information to prepare assessments of their 

supplies,175 and utility associations and alliances, such as the 

Water Research Foundation and Water Utility Climate Alliance, 

have undertaken original research to better understand the 
90 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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implications of climate change on behalf of some of the largest 

municipal water utilities in the United States.119,156,176

The economic, social, and environmental implications of climate change induced water cycle changes are very significant, 

as is the cost of inaction. Adaptation responses need to address 

considerable uncertainties in the short-, medium-, and longterm; be proactive, integrated, and iterative; and be developed 

through well-informed stakeholder decision processes functioning within a flexible institutional and legal environment. 
91 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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3: WATER RESOURCES

Process for Developing Key Messages: 

The chapter author team engaged in multiple technical discussions 

via teleconferences from March – June 2012. These discussions followed a thorough review of the literature, which included an interagency prepared foundational document,1

 over 500 technical inputs 

provided by the public, as well as other published literature. The author team met in Seattle, Washington, in May 2012 for expert deliberation of draft key messages by the authors wherein each message 

was defended before the entire author team before this key message 

was selected for inclusion in the Chapter. These discussions were supported by targeted consultation with additional experts by the lead 

author of each message, and they were based on criteria that help 

define “key vulnerabilities.” Key messages were further refined following input from the NCADAC report integration team and authors 

of Ch. 2: Our Changing Climate.

Key message #1 Traceable Account

Annual precipitation and river-flow increases are 

observed now in the Midwest and the Northeast 

regions. Very heavy precipitation events have increased nationally and are projected to increase in 

all regions. The length of dry spells is projected to 

increase in most areas, especially the southern and 

northwestern portions of the contiguous United 

States. 

Description of evidence base

The key message and supporting chapter text summarizes extensive 

evidence documented in the inter-agency prepared foundational 

document,1 Ch. 2: Our Changing Climate, Ch. 20: Southwest, other 

technical input reports,

2

 and over 500 technical inputs on a wide 

range of topics that were received as part of the Federal Register Notice solicitation for public input.

Numerous peer-reviewed publications describe precipitation trends 

(Ch. 2: Our Changing Climate)4,7,8,34 and river-flow trends.

13,41 As discussed in Chapter 2, the majority of projections available from climate models (for example, Orlowsky and Seneviratne 2012;3

 Kharin 

et al. 20135) indicate small projected changes in total average annual 

precipitation in many areas, while heavy precipitation6 and the length 

of dry spells are projected to increase across the entire country. Projected precipitation responses (such as changing extremes) to increasing greenhouse gases are robust in a wide variety of models and 

depictions of climate.

The broad observed trends of precipitation and river-flow increases 

have been identified by many long-term National Weather Service 

(NWS)/National Climatic Data Center (NCDC) weather monitoring 

networks, USGS streamflow monitoring networks, and analyses of 

records therefrom (Ch. 2: Our Changing Climate;34,36,37). Ensembles 

of climate models3,42(see also Ch. 2: Our Changing Climate, Ch. 20: 

Southwest) are the basis for the reported projections. 

New information and remaining uncertainties

Important new evidence (cited above) confirmed many of the findings from the 2009 National Climate Assessment.177

Observed trends: Precipitation trends are generally embedded 

amidst large year-to-year natural variations and thus trends may be 

difficult to detect, may differ from site to site, and may be reflections 

of multi-decadal variations rather than external (human) forcings. 

Consequently, careful analyses of longest-term records from many 

stations across the country and addressing multiple potential explanations are required and are cornerstones of the evidentiary studies 

described above. 

Efforts are underway to continually improve the stability, placement, 

and numbers of weather observations needed to document trends; 

scientists also regularly search for other previously unanalyzed data 

sources for use in testing these findings. 

Projected trends: The complexity of physical processes that result 

in precipitation and runoff reduces abilities to represent or predict 

them as accurately as would be desired and with the spatial and temporal resolution required for many applications; however, as noted, 

the trends at the scale depicted in this message are very robust 

among a wide variety of climate models and projections, which lends 

confidence that the projections are appropriate lessons from current 

climate (and streamflow) models. Nonetheless, other influences not 

included in the climate change projections might influence future 

patterns of precipitation and runoff, including changes in land cover, 

water use (by humans and vegetation), and streamflow management.

Climate models used to make projections of future trends are continually increasing in number, resolution, and in the number of additional external and internal influences that might be confounding 

current projections. For example, much more of all three of these 

SUPPLEMENTAL MATERIAL

TRACEABLE ACCOUNTS
3: WATER RESOURCES

Traceable Accounts

106 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

directions for improvement are already evident in projection archives 

for the next IPCC assessment. 

Assessment of confidence based on evidence 

Observed trends have been demonstrated by a broad range of methods over the past 20+ years based on best available data; projected 

precipitation and river-flow responses to greenhouse gas increases 

are robust across large majorities of available climate (and hydrologic) models from scientific teams around the world.

Confidence is therefore judged to be high that annual precipitation 

and river-flow increases are observed now in the Midwest and the 

Northeast regions. 

Confidence is high that very heavy precipitation events have increased nationally and are projected to increase in all regions. 

Confidence is high that the length of dry spells is projected to increase 

in most areas, especially the southern and northwestern portions of 

the contiguous United States.

Key message #2 Traceable Account

Short-term (seasonal or shorter) droughts are expected to intensify in most U.S. regions. Longerterm droughts are expected to intensify in  large 

areas of the Southwest, southern Great Plains, and 

Southeast.

Description of evidence base

The key message and supporting chapter text summarizes extensive 

evidence documented in the inter-agency prepared foundational 

document,1 Ch. 16: Northeast, Ch 17: Southeast, Ch. 2: Our Changing Climate, Ch. 18: Midwest, Ch. 19: Great Plains, Ch. 20: Southwest, 

Ch. 21: Northwest, Ch. 23: Hawai‘i and Pacific Islands, and over 500 

technical inputs on a wide range of topics that were received as part 

of the Federal Register Notice solicitation for public input.

Projected drought trends derive directly from climate models in some 

studies (for example, Hoerling et al. 2012;8 Wehner et al. 2011;30 Gao 

et al. 2012;32 Gao et al. 2011;33), from hydrologic models responding 

to projected climate trends in others (for example, Georgakakos and 

Zhang 2011;38 Cayan et al. 2010;48), from considerations of the interactions between precipitation deficits and either warmer or cooler 

temperatures in historical (observed) droughts,48 and from combinations of these approaches (for example, Trenberth et al. 200449) in 

still other studies. 

New information and remaining uncertainties

Important new evidence (cited above) confirmed many of the findings from the 2009 National Climate Assessment.177

Warmer temperatures are robustly projected by essentially all climate models, with what are generally expected to be directly attendant increases in the potentials for greater evapotranspiration, or ET 

(although it is possible that current estimates of future ET are overly 

influenced by temperatures at the expense of other climate variables, 

like wind speed, humidity, net surface radiation, and soil moisture 

that might change in ways that could partly ameliorate rising ET demands). As a consequence, there is a widespread expectation that 

more water from precipitation will be evaporated or transpired in 

the warmer future, so that except in regions where precipitation increases more than ET increases, less overall water will remain on the 

landscape and droughts will intensify and become more common. 

Another widespread expectation is that precipitation variability will 

increase, which may result in larger swings in moisture availability, 

with swings towards the deficit side resulting in increased frequencies and intensities of drought conditions on seasonal time scales 

to times scales of multiple decades. An important remaining uncertainty, discussed in the supporting text for Key Message #1, is the 

extent to which the types of models used to project future droughts 

may be influencing results with a notable recent tendency for studies 

with more complete, more resolved land-surface models, as well as 

climate models, to yield more moderate projected changes.

Confidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent 

results, well documented and 

accepted methods, etc.), high 

consensus

High

Moderate evidence (several sources, some consistency, methods 

vary and/or documentation limited, 

etc.), medium consensus

Medium

Suggestive evidence (a few 

sources, limited consistency, models incomplete, methods emerging, 

etc.), competing schools of thought

Low

Inconclusive evidence (limited 

sources, extrapolations, inconsistent findings, poor documentation 

and/or methods not tested, etc.), 

disagreement or lack of opinions 

among experts
3: WATER RESOURCES

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107 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Other uncertainties derive from the possibility that changes in other 

variables or influences of CO2-fertilization and/or land cover change 

may also partly ameliorate drought intensification. Furthermore in 

many parts of the country, El Niño-Southern Oscillation (and other 

oceanic) influences on droughts and floods are large, and can overwhelm climate change effects during the next few decades. At present, however, the future of these oceanic climate influences remains 

uncertain. 

Assessment of confidence based on evidence 

Given the evidence base and remaining uncertainties: 

Confidence is judged to be medium-high that short-term (seasonal or 

shorter) droughts are expected to intensify in most U.S. regions. Confidence is high that longer-term droughts are expected to intensify in 

large areas of the Southwest, southern Great Plains, and Southeast.

Key message #3 Traceable Account

Flooding may intensify in many U.S. regions, even 

in areas where total precipitation is projected to decline. 

Description of evidence base

The key message and supporting chapter text summarizes extensive 

evidence documented in the inter-agency prepared foundational 

document,1 Ch. 16: Northeast, Ch 17: Southeast, Ch. 2: Our Changing Climate, Ch. 18: Midwest, Ch. 19: Great Plains, Ch. 20: Southwest, 

Ch. 21: Northwest, Ch. 23: Hawai‘i and Pacific Islands, and over 500 

technical inputs on a wide range of topics that were received as part 

of the Federal Register Notice solicitation for public input.

The principal observational bases for the key message are careful 

national-scale flood-trend analyses58 based on annual peak-flow records from a selection of 200 USGS streamflow gaging stations measuring flows from catchments that are minimally influenced by upstream water uses, diversions, impoundments, or land-use changes 

with more than 85 years of records, and analyses of two other subsets 

of USGS gages with long records (including gages both impacted by 

human activities and less so), including one analysis of 50 gages nationwide56 and a second analysis of 572 gages in the eastern United 

States.57 There is some correspondence among regions with significant changes in annual precipitation (Ch. 2: Our Changing Climate) 

and soil moisture (Figures 3.2 and 3.3), and annual flood magnitudes 

(Figure 3.5).58

Projections of future flood-frequency changes result from detailed hydrologic models (for example, Das et al. 2012;60 Raff et al. 

2009;133Walker et al. 2011135) of rivers that simulate responses to 

projected precipitation and temperature changes from climate models; such simulations have only recently begun to emerge in the peerreviewed literature.

New information and remaining uncertainties

Important new evidence (cited above) confirmed many of the findings from the 2009 National Climate Assessment.177

Large uncertainties remain in efforts to detect flood-statistic changes 

attributable to climate change, because a wide range of local factors 

(such as dams, land-use changes, river channelization) also affect 

flood regimes and can mask, or proxy for, climate change induced 

alterations. Furthermore, it is especially difficult to detect any kinds 

of trends in what are, by definition, rare and extreme events. Finally, 

the response of floods to climate changes are expected to be fairly 

idiosyncratic from basin to basin, because of the strong influences 

of within-storm variations and local, basin-scale topographic, soil 

and vegetation, and river network characteristics that influence the 

size and extent of flooding associated with any given storm or season.54,55,56,57

Large uncertainties still exist as to how well climate models can represent and project future extremes of precipitation. This has – until 

recently – limited attempts to make specific projections of future 

flood frequencies by using climate model outputs directly or as direct 

inputs to hydrologic models. However, precipitation extremes are expected to intensify as the atmosphere warms, and many floods result 

from larger portions of catchment areas receiving rain as snowlines 

recede upward. As rain runs off more quickly than snowfall this results in increased flood potential; furthermore, occasional rain-onsnow events exacerbates this effect. This trend is broadly expected to 

increase in frequency under general warming trends, particularly in 

mountainous catchments.62 Rising sea levels and projected increase 

in hurricane-associated storm intensity and rainfall rates provide 

first-principles bases for expecting intensified flood regimes in coastal settings (see Ch. 2: Our Changing Climate).

Assessment of confidence based on evidence 

Future changes in flood frequencies and intensities will depend on a 

complex combination of local to regional climatic influences, and the 

details of complex surface-hydrologic conditions in each catchment 

(for example, topography, land cover, and upstream management). 

Consequently, flood frequency changes may be neither simple nor 

regionally homogeneous, and basin by basin projections may need to 

be developed. Early results now appearing in the literature have most 

often projected intensifications of flood regimes, in large part as responses to projections of more intense storms and increasingly rainy 

(rather than snowy) storms in previously snow-dominated settings. 

Confidence in current estimates of future changes in flood frequencies and intensities is overall judged to be low.

Key message #4 Traceable Account

Climate change is expected to affect water demand, groundwater withdrawals, and aquifer recharge, reducing groundwater availability in some 

areas. 
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108 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Description of evidence base

The key message and supporting chapter text summarizes extensive 

evidence documented in the inter-agency prepared foundational 

document,

1

 regional chapters of the NCA, and over 500 technical 

inputs on a wide range of topics that were received as part of the 

Federal Register Notice solicitation for public input.

Several recent studies65,66,67,68,71,72 have evaluated the potential impacts of changes in groundwater use and recharge under scenarios 

including climate change, and generally they have illustrated the common-sense conclusion that changes in pumpage can have immediate 

and significant effects in the nation’s aquifers. This has certainly been 

the historical experience in most aquifers that have seen significant 

development; pumpage variations usually tend to yield more immediate and often larger changes on many aquifers than do historical 

climate variations on time scales from years to decades. Meanwhile, 

for aquifers in the Southwest, there is a growing literature of geochemical studies that fingerprint various properties of groundwater 

and that are demonstrating that most western groundwater derives 

preferentially from snowmelt, rather than rainfall or other sources.50,51,66,74 This finding suggests that much western recharge may be 

at risk of changes and disruptions from projected losses of snowpack, 

but as yet provides relatively little indication whether the net effects 

will be recharge declines, increases, or simply spatial redistribution.

New information and remaining uncertainties

The precise responses of groundwater storage and flow to climate 

change are not well understood, but recent and ongoing studies 

provide insights on underlying mechanisms.65,66,67 The observations 

and modeling evidence to make projections of future responses of 

groundwater recharge and discharge to climate change are thus far 

very limited, primarily because of limitations in data availability and 

in the models themselves. New forms and networks of observations 

and new modeling approaches and tools are needed to provide projections of the likely influences of climate changes on groundwater 

recharge and discharge. Despite the uncertainties about the specifics of climate change impacts on groundwater, impacts of reduced 

groundwater supply and quality would likely be detrimental to the 

nation.

Assessment of confidence based on evidence

Given the evidence base and remaining uncertainties, confidence 

is judged to be high that climate change is expected to affect water 

demand, groundwater withdrawals, and aquifer recharge, reducing 

groundwater availability in some areas. 

Key message #5 Traceable Account

Sea level rise, storms and storm surges, and 

changes in surface and groundwater use patterns 

are expected to compromise the sustainability of 

coastal freshwater aquifers and wetlands.

Description of evidence base

This message has a strong theoretical and observational basis, including considerable historical experience with seawater intrusion 

into many of the nation’s coastal aquifers and wetlands under the 

influence of heavy pumpage, some experience with the influences 

of droughts and storms on seawater intrusion, and experience with 

seepage of seawater into shallow coastal aquifers under storm and 

storm surge conditions that lead to coastal inundations with seawater. The likely influences of sea level rise on seawater intrusion into 

coastal (and island) aquifers and wetlands are somewhat less certain, 

as discussed below, although it is projected that sea level rise may 

increase opportunities for saltwater intrusion (see Ch. 25: Coasts).

New information and remaining uncertainties

There are few published studies describing the kinds of groundwater 

quality and flow modeling that are necessary to assess the real-world 

potentials for sea level rise to affect seawater intrusion.78 Studies in 

the literature and historical experience demonstrate the detrimental 

impacts of alterations to the water budgets of the freshwater lenses 

in coastal aquifers and wetlands around the world (most often by 

groundwater development), but few evaluate the impacts of sea level 

rise alone. More studies with real-world aquifer geometries and development regimes are needed to reduce the current uncertainty of 

the potential interactions of sea level rise and seawater intrusion. 

Assessment of confidence based on evidence 

Confidence is high that sea level rise, storms and storm surges, and 

changes in surface and groundwater use patterns are expected to 

compromise the sustainability of coastal freshwater aquifers and 

wetlands. 

Key message #6 Traceable Account

Increasing air and water temperatures, more intense precipitation and runoff, and intensifying 

droughts can decrease river and lake water quality in many ways, including increases in sediment, 

nitrogen, and other pollutant loads.

Description of evidence base

The key message and supporting chapter text summarizes extensive 

evidence documented in the inter-agency prepared foundational document,1

 Ch. 8: Ecosystems, Ch. 15: Biogeochemical Cycles, and over 

500 technical inputs on a wide range of topics that were reviewed as 

part of the Federal Register Notice solicitation for public input.

Thermal stratification of deep lakes and reservoirs has been observed 

to increase with increased air and water temperatures,1,81,82 and may 

be eliminated in shallow lakes. Increased stratification reduces mixing, resulting in reduced oxygen in bottom waters. Deeper set-up of 

vertical thermal stratification in lakes and reservoirs may reduce or 

eliminate a bottom cold water zone; this, coupled with lower oxygen 

concentration, results in a degraded aquatic ecosystem. 

Major precipitation events and resultant water flows increase watershed pollutant scour and thus increase pollutant loads.84 Fluxes 

of mineral weathering products (for example, calcium, magnesium, 
3: WATER RESOURCES

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109 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

sodium, and silicon) have also been shown to increase in response 

to higher discharge.86 In the Mississippi drainage basin, increased 

precipitation has resulted in increased nitrogen loads contributing 

to hypoxia in the Gulf of Mexico.85 Models predict and observations 

confirm that continued warming will have increasingly negative effects on lake water quality and ecosystem health.81

Future re-mobilization of sediment stored in large river basins will be 

influenced by changes in flood frequencies and magnitudes, as well as 

on vegetation changes in the context of climate and other anthropogenic factors.87 Model projections suggest that changes in sediment 

delivery will vary regionally and by land-use type, but on average 

could increase by 25% to 55%.88

New information and remaining uncertainties

It is unclear whether increasing floods and droughts cancel each 

other out with respect to long-term pollutant loads. 

It is also uncertain whether the absolute temperature differential 

with depth will remain constant, even with overall lake and reservoir 

water temperature increases. Further, it is uncertain if greater mixing 

with depth will eliminate thermal stratification in shallow, previously 

stratified lakes. Although recent studies of Lake Tahoe provide an example of longer stratification seasons,83 lakes in other settings and 

with other geometries may not exhibit the same response. 

Many factors influence stream water temperature, including air temperature, forest canopy cover, and ratio of baseflow to streamflow. 

Assessment of confidence based on evidence 

Given the evidence base, confidence is medium that increasing air 

and water temperatures, more intense precipitation and runoff, and 

intensifying droughts can decrease river and lake water quality in 

many ways, including increases in sediment, nitrogen, and pollutant 

loads.

Key message #7 Traceable Account

Climate change affects water demand and the 

ways water is used within and across regions and 

economic sectors. The Southwest, Great Plains, 

and Southeast are particularly vulnerable to changes in water supply and demand.

Description of evidence base

The key message and supporting chapter text summarizes extensive 

evidence documented in the inter-agency prepared foundational 

document,1 Ch. 2: Our Changing Climate, Ch. 17: Southeast, Ch. 19: 

Great Plains, Ch. 20: Southwest, Ch. 23: Hawai‘i and Pacific Islands, 

and many technical inputs on a wide range of topics that were received and reviewed as part of the Federal Register Notice solicitation 

for public input.

Observed Trends: Historical water withdrawals by sector (for example, municipal, industrial, agricultural, and thermoelectric) have 

been monitored and documented by USGS for over 40 years and 

represent a credible database to assess water-use trends, efficiencies, and underlying drivers. Water-use drivers principally include 

population, personal income, electricity consumption, irrigated area, 

mean annual temperature, growing season precipitation, and growing season potential evapotranspiration.

99 Water-use efficiencies 

are also affected by many non-climate factors, including demand 

management, plumbing codes, water efficient appliances, efficiency 

improvement programs, and pricing strategies;100 changes from water intensive manufacturing and other heavy industrial activities to 

service-oriented businesses,101 and enhanced water-use efficiencies 

in response to environmental pollution legislation; replacement of 

older once-through-cooling electric power plants by plants that recycle their cooling water; and switching from flood irrigation to more 

efficient methods in the western United States.

102 

Projected Trends and Consequences: Future projections have been 

carried out with and without climate change to first assess the water demand impacts of projected population and socioeconomic 

increases, and subsequently combine them with climate change induced impacts. The main findings are that in the absence of climate 

change total water withdrawals in the U.S. will increase by 3% in the 

coming 50 years,99 with approximately half of the U.S. experiencing 

a total water demand decrease and half an increase. If, however, climate change projections are also factored in, the demand for total 

water withdrawals is projected to rise by an average of 26%,99 with 

more than 90% of the U.S. projected to experience a total demand increase, and decreases projected only in parts of the Midwest, Northeast, and Southeast. When coupled with the observed and projected 

drying water cycle trends (see key messages in “Climate Change Impacts on the Water Cycle” section), the water demand impacts of projected population, socioeconomic, and climate changes intensify and 

compound in the Southwest and Southeast, rendering these regions 

particularly vulnerable in the coming decades. 

New information and remaining uncertainties

The studies of water demand in response to climate change and other 

stressors are very recent and constitute new information on their 

own merit.

99 In addition, for the first time, these studies make it possible to piece together the regional implications of climate change induced water cycle alterations in combination with projected changes 

in water demand. Such integrated assessments also constitute new 

information and knowledge building. 

Demand projections include various uncertain assumptions which 

become increasingly important in longer term (multi-decadal) projections. Because irrigation demand is the largest water demand 

component most sensitive to climate change, the most important 

climate-related uncertainties are precipitation and potential evapotranspiration over the growing season. Non-climatic uncertainties 

relate to future population distribution, socioeconomic changes, and 

water-use efficiency improvements. 
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110 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Assessment of confidence based on evidence 

Considering that (a) droughts are projected to intensify in large areas of the Southwest, Great Plains, and the Southeast, and (b) that 

these same regions have experienced and are projected to experience continuing population and demand increases, confidence that 

these regions will become increasingly vulnerable to climate change 

is judged to be high.

Key message #8 Traceable Account

Changes in precipitation and runoff, combined 

with changes in consumption and withdrawal, have 

reduced surface and groundwater supplies in many 

areas. These trends are expected to continue, increasing the likelihood of water shortages for many 

uses. 

Description of evidence base

The key message and supporting chapter text summarizes extensive 

evidence documented in the inter-agency prepared foundational 

document,1 Ch. 2: Our Changing Climate, Ch. 17: Southeast, Ch. 19: 

Great Plains, Ch. 20: Southwest, Ch. 23: Hawai‘i and Pacific Islands, 

and over 500 technical inputs on a wide range of topics that were 

received and reviewed as part of the Federal Register Notice solicitation for public input.

Observed Trends: Observations suggest that the water cycle in the 

Southwest, Great Plains, and Southeast has been changing toward 

drier conditions (Ch. 17: Southeast).130,151,152 Furthermore, paleoclimate tree-ring reconstructions indicate that drought in previous centuries has been more intense and of longer duration than the most 

extreme drought of the 20th and 21st centuries.40

Projected Trends and Consequences: Global Climate Model (GCM) 

projections indicate that this trend is likely to persist, with runoff 

reductions (in the range of 10% to 20% over the next 50 years) and 

intensifying droughts.48

The drying water cycle is expected to affect all human and ecological water uses, especially in the Southwest. Decreasing precipitation, 

rising temperatures, and drying soils are projected to increase irrigation and outdoor watering demand (which account for nearly 90% 

of consumptive water use) by as much as 34% by 2060 under the A2 

emissions scenario.99 Decreasing runoff and groundwater recharge 

are expected to reduce surface and groundwater supplies,66 increasing the annual risk of water shortages from 25% to 50% by 2060.130

Changes in streamflow timing will increase the mismatch of supply 

and demand. Earlier and declining streamflow and rising demands 

will make it more difficult to manage reservoirs, aquifers, and other 

water infrastructure.130

Such impacts and consequences have been identified for several 

southwestern and western river basins including the Colorado,38 Rio 

Grande,126 and Sacramento-San Joaquin.127,128,129

New information and remaining uncertainties

The drying climate trend observed in the Southwest and Southeast in 

the last decades is consistent across all water cycle variables (precipitation, temperature, snow cover, runoff, streamflow, reservoir levels, 

and soil moisture) and is not debatable. The debate is over whether 

this trend is part of a multi-decadal climate cycle and whether it will 

reverse direction at some future time. However, the rate of change 

and the comparative GCM assessment results with and without historical CO2 forcing (Ch. 2: Our Changing Climate) support the view 

that the observed trends are due to both factors acting concurrently.

GCMs continue to be uncertain with respect to precipitation, but they 

are very consistent with respect to temperature. Runoff, streamflow, 

and soil moisture depend on both variables and are thus less susceptible to GCM precipitation uncertainty. The observed trends and 

the general GCM agreement that the southern states will continue 

to experience streamflow and soil moisture reductions34,41 provides 

confidence that these projections are robust.

Assessment of confidence based on evidence 

Given the evidence base and remaining uncertainties, confidence is 

high that changes in precipitation and runoff, combined with changes 

in consumption and withdrawal, have reduced surface and groundwater supplies in many areas. Confidence is high that these trends are 

expected to continue, increasing the likelihood of water shortages for 

many uses. 

Key message #9 Traceable Account

Increasing flooding risk affects human safety and 

health, property, infrastructure, economies, and 

ecology in many basins across the U.S. 

Description of evidence base

The key message and supporting chapter text summarizes extensive 

evidence documented in the inter-agency prepared foundational 

document,

1

 Ch. 2: Our Changing Climate, Ch. 21: Northwest, Ch. 19: 

Great Plains, Ch. 18: Midwest, Ch. 16: Northeast, and over 500 technical inputs on a wide range of topics that were received as part of the 

Federal Register Notice solicitation for public input.

Observed Trends: Very heavy precipitation events have intensified 

in recent decades in most U.S. regions, and this trend is projected to 

continue (Ch. 2: Our Changing Climate). Increasing heavy precipitation is an important contributing factor for floods, but flood magnitude changes also depend on specific watershed conditions (including 

soil moisture, impervious area, and other human-caused alterations). 

There is, however, some correspondence among regions with significant changes in annual precipitation (Ch. 2: Our Changing Climate),

soil moisture (Figures 3.2 and 3.3), and annual flood magnitudes (Figure 3.5).58

Flooding and seawater intrusion from sea level rise and increasing storm surge threaten New York, Boston, Philadelphia, Virginia 

Beach, Wilmington, Charleston, Miami, Tampa, Naples, Mobile, 
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111 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Houston, New Orleans, and many other coastal cities (Chapter 25: 

Coasts). 

Projected Trends: Projections of future flood-frequency changes result from detailed hydrologic60,133,135 and hydraulic models of rivers 

that simulate responses to projected precipitation and temperature 

changes from climate models. 

Consequences: Floods already affect human health and safety and 

result in substantial economic, ecological, and infrastructure damages. Many cities are located along coasts and, in some of these cities 

(including New York, Boston, Miami, Savannah, and New Orleans), sea 

level rise is expected to exacerbate coastal flooding issues by backing 

up flood flows and impeding flood-management responses (see Ch. 

16: Northeast and Ch. 25: Coasts).

136

Projected changes in flood frequency and severity can bring new 

challenges in flood risk management. For urban areas in particular, 

flooding impacts critical infrastructure in ways that are difficult to 

foresee and can result in interconnected and cascading failures (for 

example, failure of electrical generating lines can cause pump failure, 

additional flooding, and failure of evacuation services). Increasing 

likelihood of flooding also brings with it human health risks associated with failure of critical infrastructure (Ch. 11: Urban),137 from waterborne disease that can persist well beyond the occurrence of very 

heavy precipitation (Ch. 9: Human Health),139 from water outages 

associated with infrastructure failures that cause decreased sanitary 

conditions,138 and from ecosystem changes that can affect airborne 

diseases (Ch. 8: Ecosystems).

140

New information and remaining uncertainties

Large uncertainties still exist as to how well climate models can represent and project future precipitation extremes. However, precipitation extremes are expected to intensify as the atmosphere warms, 

and many floods result from larger portions of catchment areas receiving rain as snowlines recede upward. As rain runs off more quickly 

than snowfall, this results in increased flood potential; furthermore 

occasional rain-on-snow events exacerbate this effect. This trend is 

broadly expected to increase in frequency under general warming 

trends, particularly in mountainous catchments.

62

Assessment of confidence based on evidence 

Future changes in flood frequencies and intensities will depend on a 

complex combination of local to regional climatic influences and on 

the details of complex surface-hydrologic conditions in each catchment (for example, topography, land cover, and upstream managements). Consequently, flood frequency changes may be neither 

simple nor regionally homogeneous, and basin by basin projections 

may need to be developed. Nonetheless, early results now appearing 

in the literature have most often projected intensifications of flood 

regimes, in large part as responses to projections of more intense 

storms and more rainfall runoff from previously snowbound catchments and settings.

Therefore, confidence is judged to be medium that increasing flooding risk affects human safety and health, property, infrastructure, 

economies, and ecology in many basins across the U.S. 

Key message #10 Traceable Account

In most U.S. regions, water resources managers 

and planners will encounter new risks, vulnerabilities, and opportunities that may not be properly 

managed within existing practices.

Description of evidence base

The key message and supporting chapter text summarizes extensive 

evidence documented in the inter-agency prepared foundational 

document,1 other chapters of the NCA, and over 500 technical inputs 

on a wide range of topics that were received as part of the Federal 

Register Notice solicitation for public input.

Observed and Projected Trends: Many U.S. regions are facing critical 

water management and planning challenges. Recent assessments illustrate water management challenges facing California,

127,128,129,149

the Southwest,130,151 Southeast (Ch. 17: Southeast),

136,152 Northwest,153 Great Plains,

154 and Great Lakes.155

The Sacramento-San Joaquin Bay Delta is already threatened by 

flooding, seawater intrusion, and changing needs for environmental, 

municipal, and agricultural water uses. Managing these risks and uses 

requires reassessment of a very complex system of water rights, levees, stakeholder consensus processes, reservoir system operations, 

and significant investments, all of which are subject to large uncertainties.54,148 Given the projected climate changes in the Sacramento-San Joaquin Bay Delta, adherence to historical management and 

planning practices may not be a long-term viable option,128,129 but the 

supporting science is not yet fully actionable,42 and a flexible legal 

and policy framework embracing change and uncertainty is lacking. 

The Apalachicola-Chattahoochee-Flint (ACF) River basin in Georgia, 

Alabama, and Florida supports a wide range of water uses and the 

regional economy, creating challenging water-sharing tradeoffs for 

the basin stakeholders. Climate change presents new stresses and 

uncertainties.152 ACF stakeholders are working to develop a management plan that balances economic, ecological, and social values.

160

New information and remaining uncertainties

Changes in climate, water demand, land use, and demography combine to challenge water management in unprecedented ways. This is 

happening with a very high degree of certainty in most U.S. regions. 

Regardless of its underlying causes, climate change poses difficult 
3: WATER RESOURCES

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112 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

challenges for water management because it invalidates stationarity 

– the perception that climate varies around a predictable mean based 

on the experience of the last century – and increases hydrologic variability and uncertainty. These conditions suggest that past management practices will become increasingly ineffective and that water 

management can benefit by the adoption of iterative, risk-based, and 

adaptive approaches. 

Assessment of confidence based on evidence 

The water resources literature is unanimous that water management 

should rely less on historical practices and responses and more on 

robust, risk-based, and adaptive decision approaches. 

Therefore confidence is very high that in most U.S. regions, water 

resources managers and planners will face new risks, vulnerabilities, 

and opportunities that may not be properly managed with existing 

practices. 

Key message #11 Traceable Account

Increasing resilience and enhancing adaptive capacity provide opportunities to strengthen water 

resources management and plan for climate change 

impacts. Many institutional, scientific, economic, 

and political barriers present challenges to implementing adaptive strategies. 

Description of evidence base

The key message and supporting chapter text summarizes extensive 

evidence documented in the inter-agency prepared foundational 

document1 and over 500 technical inputs on a wide range of topics 

that were received as part of the Federal Register Notice solicitation 

for public input.

There are many examples of adaptive strategies for water infrastructure106,132,164,165 as well as strategies for demand management, 

land-use and watershed management, and use of “green” infrastructure.1,106,132,166,167

Building adaptive capacity ultimately increases the ability to develop 

and implement adaptation strategies and is considered a no-regrets 

strategy.1,169 Building networks, partnerships, and support systems 

has been identified as a major asset in building adaptive capacity (Ch. 

26: Decision Support; Ch. 28: Adaptation).

170

Water utility associations have undertaken original research to better 

understand the implications of climate change on behalf of some of 

the largest municipal water utilities in the United States.

119,156,176

Challenges include “stationarity” no longer being reliable as the central assumption in water-resource planning,171 considerable uncertainties, insufficient actionable science ready for practical application, 

the challenges of stakeholder engagement, and a lack of agreement 

on “post-stationarity” paradigms on which to base water laws, regulations, and policies.42 Water administrators may find it necessary to 

develop more flexible water rights and regulations.132,172,173

New information and remaining uncertainties

Jurisdictions at the state and local levels are addressing climate 

change related legal and institutional issues on an individual basis. 

An ongoing assessment of these efforts may show more practical applications. 

Assessment of confidence based on evidence 

Confidence is very high that increasing resilience and enhancing 

adaptive capacity provide opportunities to strengthen water resources management and plan for climate change impacts.

Confidence is very high that many institutional, scientific, economic, 

and political barriers present challenges to implementing adaptive 

strategies.
113

Climate Change Impacts in the United States

CHAPTER 4

ENERGY SUPPLY AND USE

INFORMATION DRAWN FROM THIS CHAPTER IS INCLUDED IN THE HIGHLIGHTS REPORT AND IS IDENTIFIED BY THIS ICON

Recommended Citation for Chapter

Dell, J., S. Tierney, G. Franco, R. G. Newell, R. Richels, J. Weyant, and T. J. Wilbanks, 2014: Ch. 4: Energy Supply and Use. 

Climate Change Impacts in the United States: The Third National Climate Assessment, J. M. Melillo, Terese (T.C.) Richmond, 

and G. W. Yohe, Eds., U.S. Global Change Research Program, 113-129. doi:10.7930/J0BG2KWD.

On the Web: http://nca2014.globalchange.gov/report/sectors/energy

Convening Lead Authors

Jan Dell, ConocoPhillips

Susan Tierney, Analysis Group Consultants

Lead Authors

Guido Franco, California Energy Commission

Richard G. Newell, Duke University

Rich Richels, Electric Power Research Institute

John Weyant, Stanford University

Thomas J. Wilbanks, Oak Ridge National Laboratory
114 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Key Messages

1. Extreme weather events are affecting energy production and delivery facilities, causing supply 

disruptions of varying lengths and magnitudes and affecting other infrastructure that depends 

on energy supply. The frequency and intensity of certain types of extreme weather events are 

expected to change.

2. Higher summer temperatures will increase electricity use, causing higher summer peak loads, 

while warmer winters will decrease energy demands for heating. Net electricity use is projected 

to increase.

3. Changes in water availability, both episodic and long-lasting, will constrain different forms of 

energy production.

4. In the longer term, sea level rise, extreme storm surge events, and high tides will affect coastal 

facilities and infrastructure on which many energy systems, markets, and consumers depend.

5. As new investments in energy technologies occur, future energy systems will differ from today’s 

in uncertain ways. Depending on the character of changes in the energy mix, climate change will 

introduce new risks as well as opportunities. 

ENERGY 4 SUPPLY AND USE

The U.S. energy supply system is diverse and robust in its ability to provide a secure supply of energy with only occasional interruptions. However, projected impacts of climate change will 

increase energy use in the summer and pose additional risks 

to reliable energy supply. Extreme weather events and water 

shortages are already interrupting energy supply, and impacts 

are expected to increase in the future. Most vulnerabilities and 

risks to energy supply and use are unique to local situations; 

others are national in scope.

In addition to being vulnerable to the effects of climate change, 

electricity generation is a major source of the heat-trapping 

gases that contribute to climate change. Therefore, regulatory 

or policy efforts aimed at reducing emissions would also affect the energy supply system. See Ch. 10: Energy, Water, and 

Land, Key Message 2; and Ch. 27: Mitigation for more on this 

topic. This chapter focuses on impacts of climate change to the 

energy sector. 

The impacts of climate change in other countries will also affect U.S. energy systems through global and regional crossborder markets and policies. Increased energy demand within 

global markets due to industrialization, population growth, 

and other factors will influence U.S. energy costs through 

competition for imported and exported energy products. The 

physical impacts of climate change on future energy systems 

in the 25- to 100-year timeframe will depend on how those energy systems evolve. That evolution will be driven by multiple 

factors, including technology innovations and carbon emission 

constraints.

Adaptation actions can allow energy infrastructure to adjust 

more readily to climate change. Many investments toward 

adaptation provide short-term benefits because they address 

current vulnerabilities as well as future risks, and thus entail 

“no regrets.” Such actions can include a focus on increased efficiency of energy use as well as improvements in the reliability 

of production and transmission of energy. The general concept 

of adaptation is presented in Chapter 28: Adaptation. Energy infrastructure around the country has been 

compromised by extreme weather events.

©Gene Blevins
115 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

4: ENERGY SUPPLY AND USE

Key Message 1: Disruptions from Extreme Weather

Extreme weather events are affecting energy production and delivery facilities, causing 

supply disruptions of varying lengths and magnitudes and affecting other infrastructure 

that depends on energy supply. The frequency and intensity of certain types 

of extreme weather events are expected to change.

Much of America’s energy infrastructure is vulnerable to extreme weather events. Because so many components of U.S. 

energy supplies – like coal, oil, and electricity – move from 

one area to another, extreme weather events affecting energy 

infrastructure in one place can lead to supply consequences 

elsewhere.

Climate change has begun to affect the frequency, intensity, 

and length of certain types of extreme weather events.1,2,3

What is considered an extreme weather or climate event varies from place to place. Observed changes across most of the 

U.S. include increased frequency and intensity of extreme precipitation events, sustained summer heat, and in some regions, 

droughts and winter storms. The frequency of cold waves has 

decreased (Ch. 2: Our Changing Climate).

Projected climate changes include increases in various types 

of extreme weather events, particularly heat waves, wildfire, 

longer and more intense drought, more frequent and intense 

very heavy precipitation events, and extreme coastal high water due to heavy-precipitation storm events coupled with sea 

level rise. Extreme coastal high water will increasingly disrupt 

infrastructure services in some locations.4 The frequency of 

cold waves is expected to continue decreasing. Disruptions 

in services in one infrastructure system (such as energy) will 

lead to disruptions in one or more other infrastructures (such 

as communications and transportation) that depend on other 

affected systems. Infrastructure exposed to extreme weather 

and also stressed by age or by demand that exceeds designed 

levels is particularly vulnerable (see Ch. 11: Urban).

Like much of the nation’s infrastructure affected by major 

weather events with estimated economic damages greater 

than $1 billion,5,6 U.S. energy facilities and systems, especially 

those located in coastal areas, are vulnerable to extreme 

weather events. Wind and storm surge damage by hurricanes 

already causes significant infrastructure losses on the Gulf 

Coast.

In 2005, damage to oil and gas production and delivery infrastructure by Hurricanes Katrina and Rita affected natural gas, 

oil, and electricity markets in most parts of the United States. 4,7

Market impacts were felt as far away as New York and New 

England,

8,9

 highlighting the significant indirect economic impacts of climate-related events that 

go well beyond the direct damages 

to energy infrastructure.

Various aspects of climate change 

will affect and disrupt energy distribution and energy production systems. It is projected that wildfires 

will affect extensive portions of 

California’s electricity transmission 

grid.10 Extreme storm surge events 

at high tides are expected to increase,11 raising the risk of inundating energy facilities such as power 

plants, refineries, pipelines, and 

transmission and distribution networks. Rail transportation lines that 

carry coal to power plants, which 

produced 42% of U.S. electricity in 

2011, often follow riverbeds. More 

intense rainstorms can lead to river 

flooding that degrades or washes 

out nearby railroads and roadbeds, 

and increases in rainstorm intensity 

have been observed and are projected to continue.

Figure 4.1. A substantial portion of U.S. energy facilities is located on the Gulf Coast as 

well as offshore in the Gulf of Mexico, where they are particularly vulnerable to hurricanes 

and other storms and sea level rise. (Figure source: U.S. Government Accountability 

Office 2006).

Paths of Hurricanes Katrina and Rita Relative to 

Oil and Gas Production Facilities
116 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

4: ENERGY SUPPLY AND USE

By learning from previous events, offshore operations can be 

made more resilient to the impacts of hurricanes. During Hurricane Isaac in August 2012, the U.S. Bureau of Safety and Environmental Enforcement reported that oil and gas production 

was safely shut down and restarted within days of the event.12

The geographical diversification of energy sources away from 

hurricane-prone areas such as the Gulf of Mexico has reduced 

vulnerability to hurricanes. The U.S. Energy Information Administration (EIA) reports that the percentage of natural gas 

production from the Gulf of Mexico shifted from 20% in 2005 

to 7% in 2012.13 This is due to the development of shale gas 

production in other parts of the United States.

Key Message 2: Climate Change and Seasonal Energy Demands

Higher summer temperatures will increase electricity use, causing higher summer peak

 loads, while warmer winters will decrease energy demands for heating. Net 

electricity use is projected to increase.

Over the last 20 years, annual average temperatures typically 

have been higher than the long-term average; nationally, temperatures were above average during 12 of the last 14 summers (Ch. 2: Our Changing Climate).2

 These increased temperatures are already affecting the demand for energy needed to 

cool buildings in the United States.

Average temperatures have increased in recent decades. In 

response, the Energy Information Administration began using 10-year average weather data instead of 30-year average 

weather data in order to estimate energy demands for heating 

and cooling purposes. The shorter period is more consistent 

with the observed trend of warmer winters and summers,14

but is still not necessarily optimal for anticipating near-term 

temperatures.17

While recognizing that many factors besides climate change affect energy demand (including population changes, economic 

conditions, energy prices, consumer behavior, conservation 

programs, and changes in energy-using equipment), increases 

in temperature will result in increased energy use for cooling 

and decreased energy use for heating. These impacts differ 

among regions of the country and indicate a shift from predominantly heating to predominantly cooling in some regions 

with moderate climates. For example, in the Northwest, energy demand for cooling is projected to increase over the next 

century due to population growth, increased cooling degree 

days, and increased use of air conditioners as people adapt to 

higher temperatures.19 Population growth is also expected to 

increase energy demand for heating. However, the projected 

increase in energy demand for heating is about half as much 

when the effects of a warming climate are considered along 

with population growth.19

Demands for electricity for cooling are expected to increase 

in every U.S. region as a result of increases in average temperatures and high temperature extremes. 

The electrical grid handles virtually the entire cooling load, while the heating load is 

distributed among electricity, natural gas, 

heating oil, passive solar, and biofuel. In 

order to meet increased demands for peak 

electricity, additional generation and distribution facilities will be needed, or demand 

will have to be managed through a variety 

of mechanisms. Electricity at peak demand 

typically is more expensive to supply than 

at average demand.21 Because the balance 

between heating and cooling differs by location, the balance of energy use among 

delivery forms and fuel types will likely 

shift from natural gas and fuel oil used for 

heating to electricity used for air conditioning. In hotter conditions, more fuel and energy are required to generate and deliver 

electricity, so increases in air conditioning 

use and shifts from heating to cooling in regions with moderate climates will increase 

primary energy demands.4

Figure 4.2. The amount of energy needed to cool (or warm) buildings is proportional 

to cooling (or heating) degree days. The figure shows increases in population-weighted 

cooling degree days, which result in increased air conditioning use, and decreases 

in population-weighted heating degree days, meaning less energy required to heat 

buildings in winter, compared to the average for 1970-2000. Cooling degree days are 

defined as the number of degrees that a day’s average temperature is above 65oF, 

while heating degree days are the number of degrees a day’s average temperature 

is below 65oF. As shown, the increase in cooling needs is greater than the decrease 

in heating needs (Data from NOAA NCDC 201216).

Increase in Cooling Demand and Decrease in Heating Demand
117 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

4: ENERGY SUPPLY AND USE

Climate-related temperature shifts 

are expected to cause a net increase 

in residential electricity use.21,22 Increased electricity demands for cooling will exceed electricity savings resulting from lower energy demands 

for heating. One study examining 

state-level energy consumption, 

weather data, and high emission 

scenarios (A2 and A1FI; Appendix 3: 

Climate Science Supplement) found 

a net increase of 11% in residential 

energy demand.23 Another study 

reported annual increases in net 

energy expenditures for cooling and 

heating of about 10% ($26 billion in 

1990 U.S. dollars) by the end of this 

century for 4.5°F of warming, and 

22% ($57 billion in 1990 dollars) for 

overall warming of about 9°F.24 New 

energy-efficient technology could 

help to offset growth in demand.

Several studies suggest that if substantial reductions in emissions of 

heat-trapping gases were required, 

the electricity generating sector 

would switch to using alternative 

(non-fossil) fuel sources first, given 

the multiple options available to generate electricity from sources that do 

not emit heat-trapping gases, such as 

wind and solar power. Under these 

circumstances, electricity would 

displace direct use of fossil fuels for 

some applications, such as heating, 

to reduce overall emissions of heattrapping gases.25,26 The implications 

for peak electricity demand could be 

significant. In California, for example, 

the estimated increase in use of electricity for space heating would shift 

the peak in electricity demand from 

summer to winter.27 In addition, the 

fact that electricity from wind and 

solar is highly variable and may not 

be available when needed has the 

potential to decrease the reliability 

of the electricity system. However, 

some initial studies suggest that a 

well-designed electricity system 

with high penetration of renewable 

sources of energy should not decrease reliability (for example, Hand 

et al. 201228).

Figure 4.3. These maps show projected average changes in cooling degree days for two 

future time periods: 2021-2050 and 2070-2099 (as compared to the period 1971-2000). The 

top panel assumes climate change associated with continued increases in emissions of 

heat-trapping gases (A2), while the bottom panel assumes significant reductions (B1). The 

projections show significant regional variations, with the greatest increases in the southern 

United States by the end of this century under the higher emissions scenario. Furthermore, 

population projections suggest continued shifts toward areas that require air conditioning 

in the summer, thereby increasing the impact of temperature changes on increased energy 

demand.18 (Figure source: NOAA NCDC / CICS-NC).

Increase in Numbers of Cooling Degree Days

Table 4.1. Hotter and longer summers will increase the amount of electricity necessary to run air conditioning, 

especially in the Southeast and Southwest. Warmer winters will decrease the amount of natural gas required to 

heat buildings, especially in the Northeast, Midwest, and Northwest. Table information is adapted from multi-model 

means from 8 NARCCAP regional climate simulations for the higher emissions scenario (A2) considered in this 

report and is weighted by population. (Source: adapted from Regional Climate Trends and Scenarios reports20)

Changing Energy Use for Heating and Cooling Will Vary by Region

Consequences: Challenges and Opportunities

Region Cooling Heating

Physical Impacts - 

High Likelihood

Hotter and Longer Summers

Number of additional extreme hot days 

(> 95°F) and % increase in cooling degree days per year in 2041-2070 above 

1971-2000 level

Warmer Winters

Number of fewer extreme cold 

days (< 10°F) and % decrease in 

heating degree days per year in 

2041-2070 below 1971-2000 level

Northeast +10 days, +77% -12 days, -17%

Southeast +23 days, +43% -2 days, -19%

Midwest +14 days, +64% -14 days, -15%

Great Plains +22 days, +37% -4 days, -18%

Southwest +20 days, +44% -3 days, -20%

Northwest +5 days, +89% -7 days, -15%

Alaska Not studied Not studied

Pacific Islands Not studied Not studied
118 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

4: ENERGY SUPPLY AND USE

Key Message 3: Implications of Less Water for Energy Production

Changes in water availability, both episodic and long-lasting, 

will constrain different forms of energy production.

Producing energy from fossil fuels (coal, oil, and natural gas), 

nuclear power, biofuels, hydropower, and some solar power 

systems often requires adequate and sustainable supplies of 

water. Issues relatableted to water, including availability and 

restrictions on the temperature of cooling water returned to 

streams, already pose challenges to production from existing power plants and the ability to obtain permits to build 

new facilities (Ch. 10: Energy, Water, and Land).21,29,30

In the future, long-term precipitation changes, drought, and 

reduced snowpack are projected to alter water availability 

(Ch. 3: Water). Recent climate data indicate a national average 

increase in annual precipitation, owing to significant increases 

across the central and northeastern portions of the nation and 

a mix of increases and decreases elsewhere (Ch. 2: Our Changing Climate, Figure 2.12). Projected changes in precipitation are 

small in most areas of the United States, but vary both seasonally and regionally (Figure 4.4). The number of heavy downpours has generally increased and 

is projected to increase for all regions (Ch 2: Our Changing Climate, 

Figures 2.16, 2.17, 2.18, and 2.19). 

Different analyses of observed 

changes in dry spell length do not 

show clear trends,31 but longer dry 

spells are projected in southern 

regions and the Northwest (Ch. 2: 

Our Changing Climate, Figure 2.13) 

as a result of projected large-scale 

changes in circulation patterns.

Regional or seasonal water constraints, particularly in the Southwest and Southeast, will result 

from chronic or seasonal drought, 

growing populations, and increasing demand for water for various 

uses (Ch. 2: Our Changing Climate; Ch. 10: Energy, Water, and 

Land).29,32 Reduced availability of 

water for cooling, for hydropower, 

or for absorbing warm water discharges into water bodies without 

exceeding temperature limits, 

will continue to constrain power 

production at existing facilities and permitting of new power 

plants. Increases in water temperatures may reduce the efficiency of thermal power plant cooling technologies, potentially 

leading to warmer water discharge from some power plants, 

which in turn can affect aquatic life. Studies conducted during 

2012 indicate that there is an increasing likelihood of water 

shortages limiting power plant electricity production in many 

regions.21,33

Hydropower plants in the western United States depend 

on the seasonal cycle of snowmelt to provide steady output 

throughout the year. Expected reductions in snowpack in parts 

of the western U.S. will reduce hydropower production. There 

will also be increases in energy (primarily electricity) demand 

in order to pump water for irrigated agriculture and to pump 

and treat water for municipal uses.21

Figure 4.4. Climate change affects precipitation patterns as well as temperature patterns. The 

maps show projected changes in average precipitation by season for 2041–2070 compared to 

1971–1999, assuming emissions of heat-trapping gases continue to rise (A2 scenario). Note 

significantly drier conditions in the Southwest in spring and Northwest in summer, as well as 

significantly more precipitation (some of which could fall as snow) projected for northern areas 

in winter and spring. Hatched areas indicate that the projected changes are significant and 

consistent among models. White areas indicate that the changes are not projected to be larger 

than could be expected from natural variability. (Figure source: NOAA NCDC / CICS-NC).

Projected Changes in Seasonal Precipitation
119 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

4: ENERGY SUPPLY AND USE

The Electric Power Research Institute’s (EPRI) scenario-based 

technical projections of water demand in 2030 find that onequarter of existing power generation facilities (about 240,000 

megawatts) nationwide are in counties that face some type 

of water sustainability issue.34 Many regions face water sustainability concerns, with the most significant water-related 

stresses in the Southeast, Southwest, and Great Plains regions 

(Ch. 3: Water).34

Key Message 4: Sea Level Rise and Infrastructure Damage

In the longer term, sea level rise, extreme storm surge events, and high tides will 

affect coastal facilities and infrastructure on which many energy systems, 

markets, and consumers depend.

Significant portions of the nation’s energy production and delivery infrastructure are in lowlying coastal areas; these facilities include oil and 

natural gas production and delivery facilities, 

refineries, power plants, and transmission lines.

Global sea level has risen by about 8 inches since 

reliable record keeping began in 1880, affecting 

countries throughout the world, including the 

United States. The rate of rise increased in recent 

decades and is not expected to slow. Global average sea level is projected to rise 1 to 4 feet by 

2100 and is expected to continue to rise well beyond this century (Ch. 2: Our Changing Climate). 

Sea level change at any particular location can 

deviate substantially from this global average 

(Ch. 2: Our Changing Climate).35

Rising sea levels, combined with normal and 

potentially more intense coastal storms, an increase in very heavy precipitation events, and 

local land subsidence, threaten coastal energy 

equipment as a result of inundation, flooding, 

and erosion. This can be compounded in areas 

that are projected to receive more precipitation. 

In particular, sea level rise and coastal storms 

pose a danger to the dense network of Outer 

Continental Shelf marine and coastal facilities in 

the central Gulf Coast region.36 Many of California’s power plants are at risk from rising sea levels, which result in more extensive coastal storm 

flooding, especially in the low-lying San Francisco 

Bay area (Figure 4.5). Power plants and energy 

infrastructure in coastal areas throughout the 

United States face similar risks.

Figure 4.5. Rising sea levels will combine with storm surges and high tides to 

threaten power-generating facilities located in California coastal communities 

and around the San Francisco Bay. Sea level rise and more intense heavy 

precipitation events increase the risk of coastal flooding and damages to 

infrastructure (Ch. 3: Water). (Figure source: Sathaye et al. 201137).

California Power Plants Potentially at 

Risk from Sea Level Rise
120 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

4: ENERGY SUPPLY AND USE

Key Message 5: Future Energy Systems

As new investments in energy technologies occur, future energy systems will differ from 

today’s in uncertain ways. Depending on the character of changes in the energy mix, 

climate change will introduce new risks as well as opportunities. 

Countless aspects of the U.S. economy today are supported 

by reliable, affordable, and accessible energy supplies. Electricity and other forms of energy are necessary for telecommunications, water and sewer systems, banking, public safety, 

and more. Today’s energy systems vary significantly by region, 

however, with differences in climate-related impacts also introducing considerable variation by locale. Table 4.3 shows 

projected impacts of climate change on, and potential risks 

to, energy systems as they currently exist in different regions. 

Most vulnerabilities and risks for energy supply and use are 

unique to local situations, but others are national in scope. For 

example, biofuels production in three regions (Midwest, Great 

Plains, and Southwest) could be affected by the projected decrease in precipitation during the critical growing season in 

the summer months (Ch. 10: Energy, Water, and Land; Ch. 7: 

Forests).

One certainty about future energy systems is that they will be 

different than today’s, but in ways not yet known. Many uncertainties – financial, economic, regulatory, technological, and so 

on – will affect private and public consumption and investment 

decisions on energy fuels, infrastructure, and systems. Energy 

systems will evolve over time, depending upon myriad choices 

made by countless decision-makers responding to changing 

conditions in markets, technologies, policies, consumer preferences, and climate. A key challenge to understanding the nature and intensity of climate change impacts on future energy 

systems is the amount of uncertainty regarding future choices 

about energy technologies and their deployment. An evolving 

energy system is also an opportunity to develop an energy 

system that is more resilient and less vulnerable to climate 

change.

Very different future energy supply portfolios are possible 

depending upon key economic assumptions, including what 

climate legislation may look like,14,25,34 and whether significant 

changes in consumption patterns occur for a variety of other 

reasons. Renewable energy sources, including solar, wind, 

hydropower, biofuels, and geothermal are meeting a growing 

portion of U.S. demand, and there is the opportunity for this 

contribution to increase in the future (Ch. 6: Agriculture; Ch. 

7: Forests). This fundamental uncertainty about the evolving 

character of energy systems contributes another layer of complexity to understanding how climate change will affect energy 

systems. 

As they consider actions to enhance the resiliency of energy 

systems, decision-makers confront issues with current energy 

systems as well as possible future configurations. The systems 

will evolve and will be more resilient over time if actions tied 

to features of today’s systems do not make future systems less 

resilient as a result. For example, if moving toward biomass as 

an energy source involves more water-consumptive energy 

supplies that could be constrained by drier future climate conditions, then decisions about energy choices should be made 

with consideration of potential changes in climate conditions 

and the risks these changes present (See Ch. 26: Decision Support).

Because energy systems in the United States are not centrally 

planned, they tend to reflect energy decisions shaped by law, 

regulation, other policies, and economic, technological, and 

other factors in markets. Trends in use patterns may continue 

into the future; this is an opportunity to increase resilience but 

also a major uncertainty for energy utilities and policy makers. 

Energy infrastructure tends to be long-lived, so resiliency can 

be enhanced by more deliberate applications of risk-management techniques and information about anticipated climate 

impacts and trends.38

For example, risk-management approaches informed by evolving climate conditions could be used to project the value of 

research and development on, or investments in, construction 

of dikes and barriers for coastal facilities or for dry-cooling 

technologies for power plants in regions where water is already in short supply. Solar and wind electricity generation facilities could be sited in areas that are initially more expensive 

(such as offshore areas) but less subject to large reductions in 

power plant output resulting from climatic changes. Targets 

for installed reserve margins for electric generating capacity 

and capacity of power lines can be established using certain 

temperature expectations, but adjusted as conditions unfold 

over time.
121 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

4: ENERGY SUPPLY AND USE

Possible Climate Resilience and Adaptation Actions in Energy Sector

Possible Actions

Extreme 

Weather 

Events

Increase 

in Peak 

Energy 

Loads

Water 

Constraints 

on Energy 

Production

Sea 

Level 

Rise

Supply: System and Operational Planning

Diversifying supply chains X X X X

Strengthening and coordinating emergency response plans X X X

Providing remote/protected emergency-response coordination centers X

Developing flood-management plans or improving stormwater management X X

Developing drought-management plans for reduced cooling flows X

Developing hydropower management plans/policies addressing extremes X

Supply: Existing Equipment Modifications

Hardening/building redundancy into facilities X X

Elevating water-sensitive equipment or redesigning elevation of intake structures X X

Building coastal barriers, dikes, or levees X X

Improving reliability of grid systems through back-up power supply, intelligent 

controls, and distributed generation X X X

Insulating equipment for temperature extremes X

References to technical studies with case studies on many of these topics may be found in Wilbanks et al. 2012.4

Implementing dry (air-cooled) or low-water hybrid (or recirculating) cooling 

systems for power plants X

Adding technologies/systems to pre-cool water discharges X

Using non-fresh water supplies: municipal effluent, brackish or seawater X

Relocating vulnerable facilities X X X

Supply: New Equipment

Adding peak generation, power storage capacity, and distributed generation X X X X

Adding back-up power supply for grid interruptions X X X

Increasing transmission capacity within and between regions X X X X

Use: Reduce Energy Demand

Improving building energy, cooling-system and manufacturing efficiencies, 

and demand-response capabilities (for example, smart grid) X X

Setting higher ambient temperatures in buildings X X

Improving irrigation and water distribution/reuse efficiency X X

Allowing flexible work schedules to transfer energy use to off-peak hours X

A range of climate change impacts will affect future energy production. This table shows possible ways to anticipate and respond to 

these changes. Innovations in technologies may provide additional opportunities and benefits to these and other adaptation actions. 

Behavioral change by consumers can also promote resiliency.

Table 4.2 summarizes actions that can be taken to increase the ease with which energy systems can adjust to climate change. Many 

of these adaptation investments entail “no regrets” actions, providing short-term benefits because they address current vulnerabilities 

as well as future risks.

Key Challenges Addressed
122 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

4: ENERGY SUPPLY AND USE

Energy Supply: Summary of National and Regional Impacts, Challenges, and Opportunities

Consequencesa: Challenges and Opportunities

Fuel Extraction, Production 

and Refining Fuel 

Distribution

Transport/

Pipelines

Electricity Generation

Electricity 

Distribution

Hydrocarbonsb Biofuels Thermal Power Generationc

Physical

Impacts – 

High 

Likelihood

Increased Ambient 

Temperature of Air 

and Water

Increased 

Extremes in 

Water 

Availability

Coastal Erosion 

and Sea Level 

Rise

Increased 

Ambient Temperature of Air 

and Water

Increased 

Extremes in 

Water 

Availability

Coastal 

Erosion 

and Sea 

Level Rise

Hot Summer 

Periods

National 

Trend 

Summaryf 

Consequence

Decreased 

Production and 

Refining Capacity

Decreased 

Agricultural 

Yields

Damage to 

Facilities

Reduced 

Plant

Efficiency 

and Cooling 

Capacity

Interruptions 

to Cooling 

Systems

Damage to 

Facilities

Reduced Capacity/Damage 

to Lines

Key Indicator

(2071-2099 vs. 

1971-2000)

Mean Annual 

Temperatured

Summer

Precipitationd Sea level Risee

(2100)

Mean 

Annual 

Temperatured

Summer 

Precipitationd

Sea Level 

Risee

(2100)

# Days>90°Ff,g

(2055)

Northeast +4°F to 9°F -5% to +6%

1.6–3.9 ft

(0.5–1.2m)

+4°F to 9°F -5% to +6%

1.6–3.9 ft.

(0.5–1.2m)

+13 days

Southeast +3°F to 8°F -22% to +10% +3°F to 8°F -22% to +10% +31 days

Midwest +4°F to 10°F -22% to +7% +4°F to 10°F -22% to +7% +19 days

Great Plains +3°F to 9°F -27% to +5% +3°F to 9°F -27% to +5% +20 days

Southwest +4°F to 9°F -13% to +3% +4°F to 9°F -13% to +3% +24 days

Northwest +3°F to 8°F -34% to -4% +3°F to 8°F -34% to -4% +4 days

Alaska +4°F to 9°F +10% to +25% +4°F to 9°F +10% to 

+25% No Projection

Pacific 

Islands +2°F to 5°F

Range from 

little change to 

increases

+2°F to 5°F

Range from 

little change to 

increases

No Projection

Notes

a) Excludes extreme weather events.

b) Hydrocarbons include coal, oil, and gas including shales.

c) Thermal power generation includes power plants fired from nuclear, coal, gas, oil, biomass fuels, solar thermal, and geothermal energy.

d) CMIP3 15 GCM Models: 2070–2099 Combined Interquartile Ranges of SRES B1 and A2 (versus 1971–2000), incorporating uncertainties from both 

differences in model climate sensitivity and differences between B1 and A2 in emissions trajectories

e) Range of sea level rise for 2100 is the Low Intermediate to High Intermediate Scenario from “Sea Level Change Scenarios for the U.S. National 

Climate Assessment.”35 Range is similar to the 1 to 4 feet of sea level rise projected in Ch. 2: Our Changing Climate, Key Message 10. There will 

be regional variations in sea level rise, and this category of impacts does not apply for the Midwest region.

f) 2055 NARCCAP

g) References: 4,25

Table 4.3. Increased temperatures, changing precipitation patterns, and sea level rise will affect many sectors and regions, including 

energy production, agriculture yields, and infrastructure damage. Changes are also projected to affect hydropower, solar photovoltaic, 

and wind power, but the projected impacts are not well defined at this time.
123 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

1. IPCC, 2007: Climate Change 2007: The Physical Science Basis. Contribution 

of Working Group I to the Fourth Assessment Report of the Intergovernmental 

Panel on Climate Change. S. Solomon, D. Qin, M. Manning, Z. Chen, 

M. Marquis, K. B. Averyt, M. Tignor, and H. L. Miller, Eds. 

Cambridge University Press, 996 pp. [Available online at http://

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Photo Credits

Introduction to chapter; New Mexico mountains behind solar panels in 

top banner: ©Michael DeYoung/Blend Images/Corbis
127 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

SUPPLEMENTAL MATERIAL

TRACEABLE ACCOUNTS

4: ENERGY SUPPLY AND USE

Process for Developing Key Messages: 

The author team met bi-weekly by teleconference during the 

months of March through July 2012. Early in the development 

of key messages and a chapter outline, the authors reviewed all 

of the four dozen relevant technical input reports that were received in response to the Federal Register solicitation for public input. Selected authors participated in a U.S. Department of 

Energy (DOE) sponsored workshop on Energy Supply and Use, 

December 29-30, 2011, in Washington, D.C. The workshop was 

organized specifically to inform a DOE technical input report and 

this National Climate Assessment and to engage stakeholders in 

this process. The authors selected key messages based on the risk 

and likelihood of impacts, associated consequences, and available 

evidence. Relevance to decision support within the energy sector 

was also an important criterion.

The U.S. maintains extensive data on energy supply and use. The 

Energy Information Administration (EIA) of the U.S. Department 

of Energy is a primary organization in this activity, and data with 

quality control, quality assurance, and expert review are available 

through EIA Web pages (for example, EIA 2012, EIA 201339).

Key message #1 Traceable Account

Extreme weather events are affecting energy 

production and delivery facilities, causing supply 

disruptions of varying lengths and magnitudes and 

affecting other infrastructure that depends on energy supply. The frequency and intensity of certain 

types of extreme weather events are expected to 

change.

Description of evidence base

A series of NCA workshops reviewed potential influences of climate 

change thus far on the frequency and intensity of certain types 

of extreme events.3 Numerous past extreme events demonstrate 

damage to energy facilities and infrastructure. Data assembled 

and reviewed by the Federal Government summarize typical costs 

associated with damage to energy facilities by extreme events.5

State and regional reports as well as data provided by public utilities document specific examples.4,9,10,26

Damage to Gulf Coast energy facilities and infrastructure by Hurricanes Katrina and Rita in 2005 provides excellent examples to 

support this key message.8,9 Wildfire also damages transmission 

grids.10

The authors benefited from Agency-sponsored technical input reports summarizing relevant data and information on energy supply 

and use as well as urban systems and infrastructure.4,21,25 A number of other technical input reports were relevant as well. These 

were reviewed carefully, particularly with regard to the identification of key messages.

New information and remaining uncertainties

The information provided through a series of NCA workshops provided new (and current) evidence for influences of climate change 

on the frequency and intensity of extreme events. The summaries from those workshops provide succinct evidence that certain 

extreme events that damage energy facilities and infrastructure 

can be expected to increase in number and intensity with climate 

change (for example, Peterson et al. 20123). Documentation of 

damage to energy facilities and infrastructure continues to accumulate, increasing confidence in this key message.5,14

The regional and local character of extreme events varies substantially, and this variability is a source of significant uncertainty regarding the impacts of climate change and consequences in terms 

of damage to energy facilities by extreme events. Additionally, 

damage to energy infrastructure in a specific location can have 

far-reaching consequences for energy production and distribution, 

and synthesis of such indirect consequences for production and 

distribution does not yet support detailed projections.

Assessment of confidence based on evidence 

High. There is high consensus with moderate evidence that extreme weather events associated with climate change will increase 

disruptions of energy infrastructure and services in some locations.
4: ENERGY SUPPLY AND USE

Traceable Accounts

128 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Key message #2 Traceable Account

Higher summer temperatures will increase electricity use, causing higher summer peak loads, while 

warmer winters will decrease energy demands for 

heating. Net electricity use is projected to increase.

Description of evidence base

The key message and supporting text summarizes extensive evidence documented in the energy supply and use technical input.4

Global climate models simulate increases in summer temperatures, and the NCA climate scenarios2,20 describe this aspect of 

climate change projections for use in preparing this report (Ch. 2: 

Our Changing Climate). Data used by Kunkel et al.2 and Census 

Bureau population data, synthesized by the EIA,15 were the basis 

for calculating population-weighted heating and cooling degreedays over the historic period as well as projections assuming SRES 

B1 and A2 scenarios. 

The NCA climate scenarios2 project an increase in the number of 

cooling days and decrease in heating days, with peak electricity 

demand in some regions shifting from winter to summer27 and 

shifting to electricity needs for cooling instead of fossil fuels for 

heating.25,26,27

New information and remaining uncertainties

While there is little uncertainty that peak electricity demands will 

increase with warming by climate change, substantial regional 

variability is expected. Climate change projections do not provide 

sufficient spatial and temporal detail to fully analyze these consequences. Socioeconomic factors including population changes, 

economic conditions, and energy prices, as well as technological 

developments in electricity generation and industrial equipment, 

will have a strong bearing on electricity demands, specific to each 

region of the country. 

Assessment of confidence based on evidence 

High.Assuming specific climate change scenarios, the consequences for heating and cooling buildings are reasonably predictable, especially for the residential sector. With a shift to higher 

summer demands for electricity, peak demands for electricity 

can be confidently expected to increase.

Key message #3 Traceable Account

Changes in water availability, both episodic and 

long-lasting, will constrain different forms of energy production.

Description of evidence base

Climate scenarios prepared for the NCA2 describe decreases in 

precipitation under the SRES A2 scenario, with the largest decreases across the Northwest and Southwest in the spring and 

summer.

Technical input reports (for example, Wilbanks et al.4,21) summarize data and studies show that changes in water availability will 

affect energy production,33 and more specifically, that water shortages will constrain electricity production (Ch. 2: Our Changing Climate).29,32 The impacts of drought in Texas during 2011 are an 

example of the consequences of water shortages for energy production as well as other uses (Ch. 10: Energy, Water, and Land). 

Electric utility industry reports document potential consequences 

for operation of generating facilities.34 A number of power plants 

across the country have experienced interruptions due to water 

shortages.

New information and remaining uncertainties

An increasing number of documented incidents of interruptions in 

energy production due to water shortages provide strong evidence 

that decreased precipitation or drought will have consequences for 

energy production.21

There is little uncertainty that water shortages due to climate 

change will affect energy production. But uncertainty about 

changes in precipitation and moisture regimes simulated by global 

climate models is significantly higher than for simulated warming. Additionally, climate change simulations lack the spatial and 

temporal detail required to analyze the consequences for water 

availability at finer scales (for example, local and regional). FinerConfidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent results, well documented 

and accepted methods, etc.), 

high consensus

High

Moderate evidence (several 

sources, some consistency, 

methods vary and/or documentation limited, etc.), medium 

consensus

Medium

Suggestive evidence (a few 

sources, limited consistency, 

models incomplete, methods 

emerging, etc.), competing 

schools of thought

Low

Inconclusive evidence (limited sources, extrapolations, 

inconsistent findings, poor documentation and/or methods not 

tested, etc.), disagreement or 

lack of opinions among experts
4: ENERGY SUPPLY AND USE

Traceable Accounts

129 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

scale projections would be relevant to decisions about changes in 

energy facilities to reduce risk or adapt to water shortages associated with climate change.

Assessment of confidence based on evidence 

High. The evidence is compelling that insufficient water availability with climate change will affect energy production; however, 

simulations of climate change lack the detail needed to provide 

more specific information for decision support.

Key message #4 Traceable Account

In the longer term, sea level rise, extreme storm 

surge events, and high tides will affect coastal facilities and infrastructure on which many energy 

systems, markets, and consumers depend.

Description of evidence base

The sea level change scenario report prepared for the NCA (see 

also Ch. 2: Our Changing Climate)35 provides further information 

about sea level change. Extreme surge events at high tides are expected to increase,11 raising the risk of inundating energy facilities 

such as power plants, refineries, pipelines, and transmission and 

distribution networks (for example, Sathaye et al. 201310) Data 

available through the EIA (for example, EIA 201015 provide highquality information about the locations and distribution of energy 

facilities.

A substantial portion of the nation’s energy facilities and infrastructure are located along coasts or offshore, and sea level rise 

will affect these facilities (Ch. 25: Coasts; Ch. 17: Southeast; Ch. 

5: Transportation).4,10,21,36

New information and remaining uncertainties

Projections of sea level change are relatively uncertain compared 

to other aspects of climate change. More importantly, there will 

be substantial regional and local variability in sea level change, 

and facilities in locations exposed to more frequent and intense 

extreme wind and precipitation events will be at higher risk. Data 

and analyses to understand regional and local sea level change 

are improving, but substantial uncertainty remains and decision 

support for adaptation is challenged by these limitations.

Assessment of confidence based on evidence 

High. There is high confidence that increases in global mean sea 

level, extreme surge events, and high tides will affect coastal energy facilities; however, regional and local details are less certain. 

Key message #5 Traceable Account

As new investments in energy technologies occur, future energy systems will differ from today’s 

in uncertain ways. Depending on the character of 

changes in the energy mix, climate change will introduce new risks as well as opportunities. 

Description of evidence base

A number of studies describe U.S. energy system configurations 

in terms of supply and use assuming different scenarios of climate 

change, including SRES B1 and A2.14,25,34 A technical input report 

to the NCA by DOE4,21 provides details and updates earlier studies. 

The potential role of biofuels is described within chapters 6 and 7 

of this report (Ch. 6: Agriculture; Ch. 7: Forests).

New information and remaining uncertainties

Understanding of options for future energy supply and use within 

the U.S. improves, as the EIA and other organizations update data 

and information about U.S. energy systems as well as projections 

of the mix of primary energy under various assumptions about 

demographic, economic, and other factors. With additional data 

and better models, alternative energy mixes can be explored with 

respect to climate change adaptation and mitigation. But numerous factors that are very difficult to predict – financial, economic, 

regulatory, technological – affect the deployment of actual facilities and infrastructure. 

Assessment of confidence based on evidence 

High. Given the evidence about climate change impacts and remaining uncertainties associated with the future configuration of 

energy systems and infrastructure, there is high confidence that 

U.S. energy systems will evolve in ways that affect risk with respect to climate change and options for adaptation or mitigation.
130

Climate Change Impacts in the United States

CHAPTER 5

TRANSPORTATION

INFORMATION DRAWN FROM THIS CHAPTER IS INCLUDED IN THE HIGHLIGHTS REPORT AND IS IDENTIFIED BY THIS ICON

Recommended Citation for Chapter

Schwartz, H. G., M. Meyer, C. J. Burbank, M. Kuby, C. Oster, J. Posey, E. J. Russo, and A. Rypinski, 2014: Ch. 5: Transportation. Climate Change Impacts in the United States: The Third National Climate Assessment, J. M. Melillo, Terese (T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 130-149. doi:10.7930/J06Q1V53.

On the Web: http://nca2014.globalchange.gov/report/sectors/transportation

Convening Lead Authors

Henry G. Schwartz, HGS Consulting, LLC

Michael Meyer, Parsons Brinckerhoff

Lead Authors

Cynthia J. Burbank, Parsons Brinckerhoff

Michael Kuby, Arizona State University

Clinton Oster, Indiana University

John Posey, East-West Gateway Council of Governments

Edmond J. Russo, U.S. Army Corps of Engineers

Arthur Rypinski, U.S. Department of Transportation
131 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Key Messages

1. The impacts from sea level rise and storm surge, extreme weather events, higher temperatures 

and heat waves, precipitation changes, Arctic warming, and other climatic conditions are 

affecting the reliability and capacity of the U.S. transportation system in many ways. 

2. Sea level rise, coupled with storm surge, will continue to increase the risk of major coastal 

impacts on transportation infrastructure, including both temporary and permanent flooding of 

airports, ports and harbors, roads, rail lines, tunnels, and bridges.

3. Extreme weather events currently disrupt transportation networks in all areas of the country; 

projections indicate that such disruptions will increase.

4. Climate change impacts will increase the total costs to the nation’s transportation systems and 

their users, but these impacts can be reduced through rerouting, mode change, and a wide range 

of adaptive actions.

5 TRANSPORTATION

The U.S. economy depends on the personal and freight mobility provided by the country’s transportation system. Essential 

products and services like energy, food, manufacturing, and 

trade all depend in interrelated ways on the reliable functioning of these transportation components. Disruptions to transportation systems, therefore, can cause large economic and 

personal losses.1 The national transportation system is composed of four main components that are increasingly vulnerable to climate change impacts:

• fixed node infrastructure, such as ports, airports, and 

rail terminals;

• fixed route infrastructure, such as roads, bridges, pedestrian/bicycle trails and lanes, locks, canals/channels, 

light rail, subways, freight and commuter railways, and 

pipelines, with mixed public and private ownership and 

management;

• vehicles, such as cars, transit buses, and trucks; transit 

and railcars and locomotives; ships and barges; and aircraft – many privately owned; and

• the people, institutions, laws, policies, and information 

systems that convert infrastructure and vehicles into 

working transportation networks. 

Besides being affected by climate changes, transportation 

systems also contribute to changes in the climate through 

emissions. In 2010, the U.S. transportation sector accounted 

for 27% of total U.S. greenhouse gas emissions, with cars and 

trucks accounting for 65% of that total.2 Petroleum accounts 

for 93% of the nation’s transportation energy use.2 This means 

that policies and behavioral changes aimed at reducing greenhouse gas emissions will have significant implications for the 

various components of the transportation sector. 

Weather events influence the daily and seasonal operation 

of transport systems.3,4,5 Transportation systems are already 

experiencing costly climate change related impacts. Many inland states – for example, Vermont, Tennessee, Iowa, and Missouri – have experienced severe precipitation events, hail, and 

flooding during the past three years, damaging roads, bridges, 

and rail systems and the vehicles that use them. Over the coming decades, all regions and modes of transportation will be 

affected by increasing temperatures, more extreme weather 

events, and changes in precipitation. Concentrated transportation impacts are likely in 

Alaska and along seacoasts. 

Climate trends affect the design of transport infrastructure, which is expensive and 

designed for long life (typically 

50 to 100 years). The estimated value of U.S. transportation 

facilities in 2010 was $4.1 trillion.6

 As climatic conditions 

shift, portions of this infrastructure will increasingly be 

subject to climatic stresses 

that will reduce the reliability 

and capacity of transportation 

systems.7 Transportation systems are also vulnerable to 

interruptions in fuel and elec-

©Tom Mihalek/Reuters/Corbis
132 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

5: TRANSPORTATION

tricity supply, as well as communications disruptions – which 

are also subject to climatic stresses.7,8 For example, power outages resulting from Hurricane Katrina shut down three major 

petroleum pipelines for two days, and the systems operated at 

reduced capacities for two weeks.9

Climate change will affect transportation systems directly, 

through infrastructure damage, and indirectly, through changes in trade flows, agriculture, energy use, and settlement patterns. If, for instance, corn cultivation shifts northward in response to rising temperatures, U.S. agricultural products may 

flow to markets from different origins by different routes.10 If 

policy measures and technological changes reduce greenhouse 

gas emissions by affecting fuel types, there will likely be significant impacts on the transportation of energy supplies (such as 

pipelines and coal trains) and on the cost of transportation to 

freight and passenger users.11

Shifts in demographic trends, land-use patterns, and advances 

in transportation technology over the next few decades will 

have profound impacts on how the nation’s transportation system functions, its design, and its spatial extent. As transportation officials shape the future transportation system to address 

new demands, future climate conditions should be considered 

as part of the planning and decision-making process.

Disruptions to transportation system capacity and reliability 

can be partially offset by adaptations. Transportation systems 

as networks may use alternative routes around damaged elements or shift traffic to undamaged modes. Other adaptation 

actions include new infrastructure designs for future climate 

conditions, asset management programs, at-risk asset protection, operational changes, and abandoning/relocating infrastructure assets that would be too expensive to protect.12 As 

new and rehabilitated transportation systems are developed, 

climate change impacts should be routinely incorporated into 

the planning for these systems. 

There will be challenges in adapting transportation systems 

to climate related changes, particularly when factoring in projected growth in the transportation sector. A National Surface 

Transportation Policy and Revenue Commission in 2007 forecast the following annual average growth rates: average annual tonnage growth rates of 2.1% for trucks, 1.9% for rail, and 

1.2% for waterborne transportation, and an average annual 

passenger vehicle miles traveled growth rate of 1.82% through 

2035 and 1.72% through 2055.13

Key Message 1: Reliability and Capacity at Risk

The impacts from sea level rise and storm surge, extreme weather events, 

higher temperatures and heat waves, precipitation changes, Arctic warming,

 and other climatic conditions are affecting the reliability and capacity 

of the U.S. transportation system in many ways.

Global climate change has both gradual and extreme event implications. A gradually warming climate will accelerate asphalt 

deterioration and cause buckling of pavements and rail lines.14

Streamflows based on increasingly more frequent and intense 

rainfall instead of slower snowmelt could increase the likelihood of bridge damage from faster-flowing streams.15 However, less snow in some areas will reduce snow removal costs 

and extend construction seasons. 

Shifts in agricultural production 

patterns will necessitate changes in 

transportation routes and modes.16

Climate models project that extreme heat and heat waves will 

become more intense, longer lasting, and more frequent (Ch. 2: Our 

Changing Climate). By 2080-2100, 

average temperatures are expected to increase by 3°F to 6°F for 

the continental United States, assuming emissions reductions from 

current trends (B1 scenario), while 

continued increases in emissions 

(A2 scenario) would lead to an increase in average temperatures ranging from 5°F in Florida to 9°F in the upper Midwest.17

The impact on transportation systems not designed for such 

extreme temperatures would be severe. At higher temperatures, expansion joints on bridges and highways are stressed 

and some asphalt pavements deteriorate more rapidly.18 Rail 

Thawing alaska

Permafrost – soil saturated with frozen water – is a key feature of the Alaskan landscape. Frozen permafrost is a suitable base for transportation infrastructure such 

as roads and airfields. In rapidly warming Alaska, however, as permafrost thaws into 

mud, road shoulders slump, highway cuts slide, and runways sink. Alaska currently 

spends an extra $10 million per year repairing permafrost damage.25

A recent study, which examined potential climate damage to Alaskan public infrastructure using results from three different climate models,26 considered 253 

airports, 853 bridges, 131 harbors, 819 miles of railroad, 4,576 miles of paved 

road, and 5,000 miles of unpaved road that could be affected by climate change. 

The present value of additional public infrastructure costs due to climate change 

impacts was estimated at $5.6 to $7.6 billion through 2080, or 10% to 12% of 

total public infrastructure costs in Alaska. These costs might be reduced by 40% 

with strong adaptation actions.26
133 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

5: TRANSPORTATION

track stresses and track buckling will increase.14,19 High air 

temperatures can affect aircraft performance; lift-off limits 

at hot-weather and high-altitude airports will reduce aircraft 

operations.20

Construction crews may have to operate on altered time 

schedules to avoid the heat of the day, with greater safety 

risks for workers.21 The construction season may lengthen in 

many localities. Similarly, higher temperatures (and precipitation changes) are likely to affect transit ridership, bicycling, and 

walking.14,22

Climate change is most pronounced at high northern latitudes. 

Alaska has experienced a 3°F rise in average temperatures 

since 1949,23 double the rest of the country. Winter temperatures have risen by 6°F.23 On the North Slope, sea ice formerly 

provided protection to the shoreline against strong fall/winter 

winds and storms (see Ch. 12: Indigenous Peoples). Retreating ice reduces this protection, eroding the shoreline and 

endangering coastal villages. Thawing permafrost is causing 

pavement, runway, rail, and pipeline displacements, creating 

problems for operation and maintenance, and requiring reconstruction of key facilities. 

Arctic warming is also projected to allow the seasonal opening 

of the Northwest Passage to freight shipment.24 Global climate 

projections to 2100 show extensive open water areas during 

the summer around the Arctic basin. Retreat of Arctic sea ice 

has been observed in all seasons over the past five decades, 

with the most prominent retreat in summer.24 This has allowed 

a limited number of freighters, cruise ships, and smaller vessels 

to traverse the Northwest Passage for several years. 

Possible Future Flood Depths in Mobile, AL with Rising Sea Level

Figure 5.1. Many coastal areas in 

the United States, including the Gulf 

Coast, are especially vulnerable 

to sea level r ise impacts on 

transportation systems.11,27,28 This is 

particularly true when one considers 

the interaction among sea level rise, 

wave action, and local geology.29

This map shows that many parts of 

Mobile, Alabama, including critical 

roads, rail lines, and pipelines, would 

be exposed to storm surge under a 

scenario of a 30-inch sea level rise 

combined with a storm similar to 

Hurricane Katrina. Not all roads would 

be flooded if they merely run through 

low areas since some are built above 

flood levels. A 30-inch sea level rise 

scenario is within the range projected 

for global sea level rise (Ch. 2: Our 

Changing Climate, Key Message 10). 

(Figure source: U.S. Department of 

Transportation 201230).
134 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

5: TRANSPORTATION

Key Message 2: Coastal Impacts

Sea level rise, coupled with storm surge, will continue to increase the risk of major coastal 

impacts on transportation infrastructure, including both temporary and permanent flooding of 

airports, ports and harbors, roads, rail lines, tunnels, and bridges.

The transportation impacts of rising global sea level, which is 

expected to continue to rise by an additional 1 to 4 feet by 

2100 (see also Ch. 2: Our Changing Climate, Key Message 10),31

will vary widely by location and geography. When sea level 

rise is coupled with intense storms, the resulting storm surges 

will be greater, extend farther inland, and cause more extensive damage. Relative sea level rise will be greater along some 

coasts (such as Louisiana, Texas, and parts of the Chesapeake 

Bay), and this will have significant effects on transportation 

infrastructure, even without the coupling with storms, due 

to regional land subsidence (land sinking or settling) (Ch. 25: 

Coasts). Ports and harbors will need to be reconfigured to accommodate higher seas. Many of the nation’s largest ports are 

along the Gulf Coast, which is especially vulnerable due to a 

combination of sea level rise, storm surges, erosion, and land 

subsidence.11 Two additional impacts for ports include 1) as 

sea level rises, bridge clearance may not be adequate to allow safe passage of large vessels; 2) even if the elevation of 

port facilities is adequate, any main access road that is not elevated will become more frequently inundated, thus affecting 

port operations. In 2011, the United States imported 45% of all 

oil consumed, and 56% of those imports passed through Gulf 

Coast ports.32

More frequent disruptions and damage to roads, tracks, runways, and navigation channels are projected in coastal areas 

beyond the Gulf Coast. Thirteen of the nation’s 47 largest airports have at least one runway with an elevation within 12 feet 

of current sea levels.33 Most ocean-going ports are in low-lying 

coastal areas, including three of the most important for imports and exports: Los Angeles/Long Beach (which handles 31% 

of the U.S. port container movements) and the Port of South 

Louisiana and the Port of Galveston/Houston (which combined 

handle 25% of the tonnage handled by U.S. ports).34 Extreme 

floods and storms associated with climate change will lead to 

increased movement of sediment and buildup of sandy formations in channels. For example, many federally maintained 

navigation channels have deteriorated in recent years to dimensions less than those authorized, in part due to floods and 

storms, which resulted in reduced levels of service that affect 

navigation safety and reliability.35 Channels that are not well 

maintained and have less sedimentation storage volume will 

thus be more vulnerable to 

significant, abrupt losses in 

navigation service levels. 

Additional channel storage 

capacity that may be created by sea level rise will 

also increase water depths 

and increase sedimentation 

in some channels. (See Ch. 

25: Coasts for additional 

discussion of coastal transportation impacts.)

Airports Vulnerable to Storm Surge

Figure 5.2. Thirteen of the nation’s 47 largest airports have at least one runway with an elevation 

within the reach of moderate to high storm surge. Sea level rise will pose a threat to low-lying 

infrastructure, such as the airports shown here. (Data from Federal Aviation Administration 201233).
135 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

5: TRANSPORTATION

Key Message 3: Weather Disruptions

Extreme weather events currently disrupt transportation networks in all areas of the country; 

projections indicate that such disruptions will increase.

Changes in precipitation patterns, particularly 

more extreme precipitation events and drought, 

will affect transportation systems across the 

country. Delays caused by severe storms disrupt 

almost all types of transportation. Storm drainage systems for highways, tunnels, airports, and 

city streets could prove inadequate, resulting 

in localized flooding. Bridge piers are subject to 

scour as runoff increases stream and river flows, 

potentially weakening bridge foundations. Severe storms will disrupt highway traffic, leading 

to more accidents and delays. More airline traffic will be delayed or canceled. 

Inland waterways may well experience greater 

floods, with high flow velocities that are unsafe 

for navigation and that cause channels to shut 

down intermittently. Numerous studies indicate 

increasing severity and frequency of flooding 

throughout much of the Mississippi and Missouri 

River Basins.36 Increases in flood risk reflect both 

changing precipitation and changing land-use patterns.37 In the 

Upper Mississippi/Missouri Rivers, there have been two 300- 

to 500-year floods over the past 20 years.38 Drought increases 

the probability of wildfires, which affect visibility severely 

enough to close roads and airports. Drought can lower vessel 

drafts on navigable rivers and associated lock and dam pools. 

On the other hand, less ice formation on navigable waterways 

has the potential to increase seasonal windows for passage of 

navigation. 

The frequency of the strongest 

hurricanes (Category 4 and 5) 

in the Atlantic is expected to 

increase (see Ch. 2: Our Changing Climate, Key Message 8). 

As hurricanes approach landfall, they create storm surge, 

which carries water farther 

inland. The resulting flooding, 

wind damage, and bridge destruction disrupts virtually all 

transportation systems in the 

affected area. Many of the nation’s military installations are 

in areas that are vulnerable to 

extreme weather events, such 

as naval bases located in hurricane-prone zones.

Infrastructure around the country has been compromised by extreme weather 

events such as heavy downpours. Road and bridge damage are among the 

infrastructure failures that have occurred during these extreme events. 

© John Wark/ /AP/Corbis

Gulf Coast Transportation Hubs at Risk

Figure 5.3. Within this century, 2,400 miles of major roadway are projected to be inundated by 

sea level rise in the Gulf Coast region. The map shows roadways at risk in the event of a sea 

level rise of about 4 feet, which is within the range of projections for this region in this century 

(see also Ch. 2: Our Changing Climate, Key Message 10). In total, 24% of interstate highway 

miles and 28% of secondary road miles in the Gulf Coast region are at elevations below 4 feet. 

(Figure source: Kafalenos et al. 200839).
136 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

5: TRANSPORTATION

Hurricane sandy

On October 29, 2012, Hurricane Sandy 

dealt the transportation systems of New Jersey and New York and environs a massive 

blow (See also Ch.16: Northeast, “Hurricane Vulnerability”; Ch. 11: Urban “Hurricane Sandy”). The damages from Sandy are 

indicative of what powerful tropical storms 

and higher sea levels could bring on a more 

frequent basis in the future and were very 

much in line with vulnerability assessments 

conducted over the past four years.40,41,42 All 

tunnels and most bridges leading into New 

York City were closed during the storm. Storm 

tides of up to 14 feet43 flooded the Queens 

Midtown, Holland, and Carey (Brooklyn Battery) tunnels, which remained closed for at 

least one week (two weeks for the Carey Tunnel) while floodwaters were being pumped 

out and power restored. The three major 

airports (Kennedy, Newark, and LaGuardia) 

flooded, with LaGuardia absorbing the worst 

impact and closing for three days.44

Almost 7.5 million passengers per day ride 

the New York City subways and buses.45

Much of the New York City subway system 

below 34th Street was flooded, including 

all seven tunnels under the East River to 

Brooklyn and Queens. In addition to removing the floodwaters, all electrical signaling and power systems (the third rails) had to be cleaned, inspected, and 

repaired. Service on most Lower Manhattan subways was 

suspended for at least one week,46 as was the PATH system 

to New Jersey.47 Commuter rail service to New Jersey, Long 

Island, and northern suburbs, with more than 500,000 

passengers per day,45 was similarly affected for days or 

weeks with flooded tunnels, downed trees and large debris 

on tracks, and loss of electrical power.48 In addition, miles 

of local roads, streets, underpasses, parking garages, and 

bridges flooded and/or were badly damaged in the region, 

and an estimated 230,000 parked vehicles49 sustained 

water damage. Flooded roadways prevented the New York 

Fire Department from responding to a fire that destroyed 

more than 100 homes in Brooklyn’s Breezy Point neighborhood.50

Hurricane Sandy’s storm surge produced nearly four feet 

of floodwaters throughout the Port of New York and New 

Jersey, damaging electrical systems, highways, rail track, 

and port cargo; displacing hundreds of shipping containers; and causing ships to run aground.51 Floating debris, 

wrecks, and obstructions in the channel had to be cleared 

before the Port was able to reopen to incoming vessels 

within a week.52 Pleasure boats were damaged at marinas 

throughout the region. On a positive note, the vulnerability 

analyses prepared by the metropolitan New York authorities and referenced above provided a framework for efforts 

to control the damage and restore service more rapidly. 

Noteworthy are the efforts of the Metropolitan Transportation Authority to protect vital electrical systems and restore 

subway service to much of New York within four days.

The impacts of this extraordinary storm on one of the nation’s most important transportation nodes were felt across 

the country. Airline schedules throughout the United States 

and internationally were snarled; Amtrak rail service along 

the East Coast and as far away as Buffalo and Montreal was 

curtailed; and freight shipments in and out of the hurricane 

impact zone were delayed. The resultant direct costs to the 

community and indirect costs to the economy will undoubtedly rise into the tens of billions of dollars (See also Ch. 

11: Urban, “Hurricane Sandy”).

Figure 5.4. The nation’s busiest subway system sustained the worst 

damage in its 108 years of operation on October 29, 2012, as a result of 

Hurricane Sandy. Millions of people were left without service for at least one 

week after the storm, as the Metropolitan Transportation Authority rapidly 

worked to repair extensive flood damage (Photo credit: William Vantuono, 

Railway Age Magazine, 201246).

Hurricane Sandy Causes Flooding in 

New York City Subway Stations
137 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

5: TRANSPORTATION

Risks and Consequences

Risk is a function of both likelihood of impact and 

the consequences of that impact. Table 5.1 is an 

illustrative application of a risk matrix adapted 

from the Port Authority of New York and New Jersey. As shown, different types of climate-related 

incidents/events can have associated with them a 

likelihood of occurrence and a magnitude of the 

consequences if the incident does occur. 

In assessing consequences, the intensity of system 

use, as well as the existence or lack of alternative 

routes, must be taken into account. Disabling 

a transportation facility can have ripple effects 

across a network, with trunk (main) lines and hubs 

having the most widespread impacts.53 Any comprehensive assessment of the consequences of climate change would need to encompass the broad 

array of factors that influence the nation’s transportation system, and consider changes in population, society, technology, prices, regulation, and 

the economy that eventually affect transportation 

system performance.55 For example, the trend 

in recent years in the U.S. economy of adopting 

just-in-time logistics increases the vulnerability of 

businesses to day-to-day disruptions caused by 

weather and flooding. 

Key Message 4: Costs and Adaptation Options

Climate change impacts will increase the total costs to the nation’s transportation systems 

and their users, but these impacts can be reduced through rerouting, mode change, 

and a wide range of adaptive actions.

Adaptation strategies can be employed to reduce the impact of 

climate change related events and the resulting consequences 

(see Ch. 28: Adaptation). Consideration of adaptation strategies in the transportation sector is especially important in the 

following five areas:

• Transportation and land-use planning: deciding what 

infrastructure to build and where to build it, as well as 

planning for vulnerable areas of the community and impacts on specific population groups.

• Vulnerability and risk assessment: identifying existing 

vulnerable facilities and systems, together with the expected consequences.

• New infrastructure design: adapting new infrastructure 

designs that anticipate changing environmental and operational conditions.

• Asset management: adapting existing infrastructure 

and operations that respond to current and anticipated 

conditions, including changed maintenance practices 

and retrofits.

• Emergency response: anticipating expected disruptions 

from extreme weather events, and developing emergency response capability.

Adaptation takes place at multiple levels, from individual 

households and private businesses to federal, state, and local 

governments. The impacts associated with climate change are 

not new, since flooding, storm surge, and extreme heat have 

long been challenges. What is new is the changing frequency, 

intensity, and location/geography of impacts and hazards. 

Responding effectively to present and future environmental 

challenges enhances the resilience of communities. Examples 

Table 5.1 relates to overall national expectations based on Angel and Kunkel 

201054 and as postulated by chapter authors. This kind of matrix is likely to be 

most valuable and accurate if used at the state/regional/local levels. (Source: 

Matrix format adapted from McLaughlin et al. 201153).

Illustrative Risks of Climate-related Impacts

Likelihood of Occurrence

Low Medium High Virtually 

Certain

Subway and 

tunnel flooding

Increased 

widespread 

flooding of 

transportation 

facilities

Major localized flooding 

disrupts 

transportation 

systems

Inundation of

coastal assets 

due to storm 

surge

Increased rock/

mud slides 

blocking road 

and rail facilities

Train 

derailment due 

to rail buckling

Increased 

disruption of 

barge traffic 

due to flooding

Short-term 

road flooding 

and blocked 

culverts due to 

extreme events

Lower visibility 

from wildfires 

due to drought 

conditions

Northward shift 

of agricultural production 

places more 

demand and 

stress on roads 

and systems 

not prepared 

for higher 

volumes

Pavement 

heaving and 

reduced pavement life due to 

high temperatures

Inundation of 

local roads due 

to sea level rise

Reduced flight 

cancellations 

due to fewer 

blizzards

Reduced maintenance costs 

for highways 

and airports 

due to warmer 

winters

Reduced Great 

Lakes freezing, leading to 

longer shipping 

season

Longer seasonal opening 

of Northwest 

Passage

Magnitude of Consequences

Low Medium High Positive (beneficial)
138 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

5: TRANSPORTATION

Figure 5.5. Many projected climate change impacts and resulting consequences on transportation systems can be reduced through 

a combination of infrastructure modifications, improved information systems, and policy changes.

include improvements in storm water management, coastal 

zone management, and coastal evacuation plans. 

At the national level, the transportation network has some 

capability to adjust to climate-related disruptions due to the 

presence of network redundancy – multiple routes are often 

possible for long-distance travel, and more than one mode 

of transportation may be used for travel. However, in some 

cases, only one major route connects major destinations, such 

as Interstate 5 between Seattle and San Francisco; movements 

along such links are particularly vulnerable to disruption.

Disruptions to the nation’s inland water system from floods or 

droughts can, and has, totally disrupted barge traffic. Severe droughts throughout the upper Midwest 

in 2012 reduced flows in the Missouri and Mississippi Rivers to near record low levels, disrupting 

barge traffic. While alternative modes, such as rail 

and truck, may alleviate some of these disruptions, 

it is impractical to shift major product shipments 

such as Midwest grain to other modes of transportation – at least in the near term.57

While extreme weather events will continue to 

cause flight cancellations and delays, many weather 

delays from non-extreme events are compounded 

by existing inadequacies in the current national air 

traffic management system.58 Improvements in the 

air traffic system, such as those anticipated in the 

FAA’s NextGEN (www.faa.gov/nextgen/), should 

reduce weather-related delays.

At the state and local level, there is less resilience to be gained 

by alternative routing, and impacts may be more intense. For 

example, significant local and regional disruption and economic costs could result from the flooding of assets as diverse 

as New York’s subways, Iowa’s roads, San Francisco’s airports, 

and Vermont’s bridges. 

Climate change is one of many factors, and an increasingly important one, that many state, regional, and local agencies are 

considering as they plan for new and rehabilitated facilities. 

By incorporating climate change routinely into the planning 

process, governments can reduce the vulnerability to climate 

change impacts and take actions that enhance the resilience 

Role of Adaptive Strategies and Tactics in Reducing Impacts and Consequences

Winter storm-related closures of i-5 and

i-90 in washington state, 2007-2008

In December 2007, heavy rainfall west of I-5, combined with melting snow from the mountains, created extremely high floodwaters 

in western Washington State. Six-hour rainfall amounts were near 

a 100-year event for areas in Southwest Washington. High winds, 

heavy rains, mudslides, and falling trees made travel unsafe on highways. Downed power lines blocked roads, and, in many urban areas, 

rainwater overwhelmed drainage systems and flooded roadways.

The combined economic impact in the I-5 and I-90 corridors was 

estimated at almost $75 million, of which some $47 million was 

associated with the I-5 disruption and $28 million with the I-90 

corridor. Estimated highway damage from the winter storm was $18 

million for state routes and another $39 million for city and county 

roads.56
139 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

5: TRANSPORTATION

of the transportation system to adverse weather conditions. 

Governments at various levels are already taking action, as described below. 

Land-use planning can reduce risk by avoiding new development in flood-prone areas, conserving open space to enhance 

drainage, and relocating or abandoning structures or roads 

that have experienced repeated flooding. The National Flood 

Insurance Program encourages buyouts of repetitive loss 

structures and preservation of open space by reducing flood 

insurance rates for communities that adopt these practices.

An important step in devising an adaptation plan is 

to assess vulnerabilities (Ch. 26: Decision Support; 

Ch. 28: Adaptation). The Federal Highway Administration funded pilot projects in five coastal states to 

test a conceptual framework for evaluating risk.59

The framework identifies transportation assets, 

evaluates the likelihood of impact on specific assets, and assesses the seriousness of such impacts. 

Several state and local governments have conducted additional vulnerability assessments that identify potential impacts to transportation systems, 

especially in coastal areas. Detailed assessment 

work has been undertaken by New York City,40,42,60

California,61 Massachusetts,62 Washington,63 Florida, and Boston.64

Non-coastal states and regions have also begun to produce 

vulnerability assessments. Midwestern states, including Wisconsin66 Iowa,67 and Michigan,68 have addressed increasing risk 

of flooded roadways and other impacts. 

Transit systems are already implementing measures that reduce vulnerability to climate impacts, including rail buckling. 

Portland, Oregon’s transit agency has been installing expansion joints at vulnerable locations, improving reliability of rail 

Planning for climate change

Charlotte County exemplifies how local governments can incorporate aspects of climate change into transportation planning. The 

Metropolitan Planning Organization in Charlotte County-Punta 

Gorda, Florida conducted long-range scenario planning that integrated climate change projections.65 A “smart growth” scenario 

that concentrated growth in urban centers was compared with a 

“resilient growth” scenario that steered development away from 

areas vulnerable to sea level rise. Planners evaluated the scenarios 

based on projected transportation performance outcomes and selected a preferred scenario reflecting aspects of each alternative. 

Tropical storm irene devastates vermont transportation in august 2011

In August of 2011, Vermont was inundated with rain 

and massive flooding from Tropical Storm Irene (see 

also Ch.16: Northeast, “Hurricane Vulnerability”), 

closing down 146 segments of the state road system along with more than 200 bridges, and costing 

an estimated $175 to $200 million to rebuild state 

highways and bridges. An additional 2,000 or more 

municipal roads and nearly 1,000 culverts were damaged, and more than 200 miles of state-owned rail 

required repair.75

The volume of water was unprecedented, as was the 

power of the water in the rivers running through the 

state. Culverts and bridges were affected and slope 

stability was threatened as a result of the immense 

amount and power of water and subsequent flooding. 

When asked about the lessons learned, the Vermont Agency of Transportation (VTrans) indicated the importance of 

good maintenance of riverbeds as well as roads. VTrans is 

working with the Vermont Agency of Natural Resources, 

looking upstream and downstream at the structure of the 

rivers, recognizing that risk reduction may involve managing rivers as much as changing bridges or roadways. 

Rich Tetreault of VTrans emphasized that “Certainly we will 

be looking to right-size the bridges and culverts that need 

to be replaced ... Knowing that we do not have the funds to 

begin wholesale rebuilding of the entire highway network to 

withstand future flooding, we will also enhance our ability 

to respond” when future flooding occurs.74

Tropical Storm Impact on Vermont Road

Figure 5.6. Vermont Route 131, outside Cavendish, a week after Tropical 

Storm Irene unleashed severe precipitation and flooding that damaged 

many Vermont roads, bridges, and rail lines. (Photo credit: Vermont 

Agency of Transportation).
140 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

5: TRANSPORTATION

service.14 In New York, ventilation grates are being elevated to 

reduce the risk of flooding.40

Transportation agencies are incorporating climate change into 

ongoing design activities. For example, the Alaska Department 

of Transportation (DOT) spends more than $10 million annually on shoreline protection, relocations, and permafrost protection for roadways (see “Thawing Alaska”).25 In May 2011, 

the California Department of Transportation (Caltrans) issued 

guidance to their staff on whether and how to incorporate sea 

level rise into new project designs.69

States have begun to integrate climate impacts into Transportation Asset Management, a systematic process for monitoring 

the conditions of roads and transit facilities.18,70 Maryland is 

working to prioritize assets taking sea level rise and increased 

storm intensity into account and is developing a tool to track 

assets and assess vulnerability.71 Florida DOT continually monitors conditions on roads and bridges and is developing a statewide inventory and action plan for high-risk bridges.72 Among 

inland states, Michigan DOT has identified a wide range of operational and asset management changes to adjust to climate 

change.68 Planting street trees has been 

shown to reduce the urban heat island effect 

and reduce heat stress on pavement.73

Effective stormwater and stream/river management can reduce the risk of flooding for 

transportation infrastructure. Following 

Tropical Storm Irene, Vermont state agencies 

are working on stream and river management to reduce conditions that exacerbate 

flooding impacts on transportation.74

Effective asset management requires significant data and monitoring of transportation 

assets. Improved weather and road-condition information systems enable transportation system managers to anticipate and 

detect problems better and faster – enabling 

them to close systems if needed, alert motorists, and dispatch maintenance and snow-removal crews. 

As Michigan DOT has noted, an increase in lake-effect snows 

means that existing models used for snow and ice removal 

procedures are no longer reliable, requiring better monitoring 

and new models, as well as better roadway condition detection systems.68

Similarly, regular maintenance and cleaning of urban levee and 

culvert systems reduces the risk of roads and rails being inundated by flooding.

Extreme weather, such as hurricanes or intense storms, stresses transportation at precisely the time when smooth operation is critical. Effective evacuation planning, including early 

warning systems, coordination across jurisdictional boundaries, and creating multiple evacuation routes builds preparedness. Identifying areas with high concentrations of vulnerable 

and special-needs populations (including elderly, disabled, and 

transit-dependent groups) enhances readiness, as does identifying assets such as school buses or other transit vehicles that 

can be deployed for households that do not own vehicles. 

Storm surge on top of rising sea levels have damaged roads and other 

coastal infrastructure. 

AP Photo/The Virginian-Pilot, Steve Earley
141 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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and R. Zimmerman, 2011: Developing coastal adaptation to 

climate change in the New York City infrastructure-shed: Process, 

approach, tools, and strategies. Climatic Change, 106, 93-127, 

doi:10.1007/s10584-010-0002-8. [Available online at http://www.

ccrun.org/sites/ccrun/files/attached_files/2011_Rosenzweig_etal.

pdf]

61. California Natural Resources Agency, 2009: The California 

Climate Adaptation Strategy 2009: A Report to the Governor of 

the State of California in response to Executive Order S-13-2008, 

197 pp., California Natural Resources Agency,, Sacramento, CA. 

[Available online at http://resources.ca.gov/climate_adaptation/

docs/Statewide_Adaptation_Strategy.pdf]

62. Massachusetts Executive Office of Energy and Environmental 

Affairs and the Adaptation Advisory Committee, 2011: 

Massachusetts Climate Change Adaptation Report, 128 pp., 

Commonwealth of Massachusetts, Boston, MA. [Available online 

at http://www.mass.gov/eea/docs/eea/energy/cca/eea-climateadaptation-report.pdf]
5: TRANSPORTATION

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63. Adelsman, H., J. Ekrem, L. W. Binder, K. Cook, K. Cooper, L. M. 

Helbrecht, R. Jamison, J. D. Phillips, P. Pickett, C. L. Roalkvam, 

S. Salisbury, D. Siemann, and P. Speaks, 2012: Preparing for 

a Changing Climate. Washington State’s Integrated Climate 

Response Strategy. Publication No. 12-01-004. L. Geller, Ed., 

203 pp., State of Washington, Department of Ecology, Olympia, 

WA. [Available online at https://fortress.wa.gov/ecy/publications/

publications/1201004.pdf]

64. City of Boston, 2011: A Climate of Progress: City of Boston Climate 

Action Plan Update 2011, 43 pp, Boston, MA. [Available online 

at http://www.cityofboston.gov/Images_Documents/A%20

Climate%20of%20Progress%20-%20CAP%20Update%202011_

tcm3-25020.pdf]

65. CCMPO, 2010: Charlotte County-Punta Gorda Long Range 

Transportation Plan 2035: Summary Report. Charlotte CountyPunta Gorda Metropolitan Planning Organization, Punta Gorda, 

FL. [Available online at http://www.ccmpo.com/index.php/en/

plan/long-range-plan]

66. WICCI, 2011: Wisconsin’s Changing Climate Impacts and 

Adaptation. Wisconsin Initiative on Climate Change Impacts, 

Nelson Institute for Environmental Studies, University of 

Wisconsin-Madison, Wisconsin Department of Natural Resources, 

Madison, WI. [Available online at http://www.wicci.wisc.edu/

report/2011_WICCI-Report.pdf]

67. Iowa Climate Change Impacts Committee, 2011: Climate 

Change Impacts on Iowa 2010, 33 pp., Iowa Department of 

Natural Resources, Des Moines, IA. [Available online at http://

www.iowadnr.gov/portals/idnr/uploads/air/environment/

climatechange/complete_report.pdf?amp;tabid=1077]

68. Michigan Department of Transportation, 2011: Climate Change 

Adaptation Issues in Highway Operations. TRB Webinar. 

Transportation Research Board, Washington, D.C. 

69. Caltrans Climate Change Workshop, 2011: Guidance on 

Incorporating Sea-Level Rise: For Use in the Planning and 

Development of Project Initiation Documents.State government 

guidance report. 13 pp., California Department of Transportation, 

Sacramento, CA. [Available online at http://www.dot.ca.gov/ser/

downloads/sealevel/guide_incorp_slr.pdf]

70. Radow, L. J., and L. Neudorff, 2011: Transportation adaptation’s 

bearing on planning, systems management, operations, and 

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71. Slater, G. I., 2011: Transportation Climate Adaptation Activities 

in Maryland TRB 90th Annual Meeting (January 23-27, 2011), 

Washington, D.C., Transportation Research Board. 

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Exchange - December 2009. Schaumburg, IL, U.S. Department 

of Transportation, Federal Highway Administration. [Available 

online at http://www.fhwa.dot.gov/environment/climate_change/

adaptation/workshops_and_peer_exchanges/illinois_12_2009/]

73. Locke, D. H., J. M. Grove, J. W. T. Lu , A. Troy, J. P. M. O’NeilDunne, and B. D. Beck, 2010: Prioritizing preferable locations 

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trees Los Angeles canopy cover and benefit assessment. Landscape and 

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74. Tetreault, R., 2011: AASHTO Climate Brief, Interview. American 

Association of State Highway and Transportation Officials, 

Washington, D.C. 

75. VANR, cited 2012: Tropical Storm Irene: By the Numbers. Vermont 

Agency of National Resources. [Available online at http://www.

anr.state.vt.us/anr/climatechange/irenebythenumbers.html
146 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Process for Developing Key Messages 

In developing key messages, the chapter author team engaged, 

via teleconference, in multiple technical discussions from January 

through May 2012 as they reviewed numerous peer reviewed publications. Technical input reports (21) on a wide range of topics 

were also received and reviewed as part of the Federal Register 

Notice solicitation for public input. The author team’s review included a foundational Technical Input Report for the National Climate Assessment, “Climate Impacts and U.S. Transportation.”57

Other published literature and professional judgment were also 

considered as the chapter key messages were developed. The 

chapter author team met in St. Louis, MO, in April 2012 for expert 

deliberation and finalization of key messages.

Key Message #1 Traceable Account

The impacts from sea level rise and storm surge, 

extreme weather events, higher temperatures and 

heat waves, precipitation changes, Arctic warming, 

and other climatic conditions are affecting the reliability and capacity of the U.S. transportation system in many ways.

Description of evidence base

Climate impacts in the form of sea level rise, changing frequency 

of extreme weather events, heat waves, precipitation changes, 

Arctic warming, and other climatic conditions are documented in 

Ch. 2: Our Changing Climate of this report.

Climate can be described as the frequency distribution of weather 

over time. Existing weather conditions, flooding, and storm surge 

demonstrably affect U.S. transportation systems. By changing the 

frequency of these weather conditions, climate change will inevitably affect the reliability and capacity of U.S. transportations systems. This view is supported by multiple studies of the impacts of 

weather and climate change on particular transportation systems 

or particular regions.

An aggregate summary of impacts of climate change on U.S. 

transportation can be found in NRC 2008.7 A paper commissioned for NRC 2008 considers specific impacts of various forms 

of climate change on infrastructure, for example, possible future 

constraints on infrastructure.12 The effects of climate on transit 

systems are summarized in Hodges 2011.14 The impact of heat 

and other climate effects on rail systems are described by Hodges 

2011 and Rossetti 2002.14,19

Future impacts of sea level rise and other climatic effects on 

transportation systems in the Gulf Coast were examined by CCSP 

2008.11 The impacts of climate change on New York State, including its transportation system, were undertaken by Rosenzweig et 

al. 2011.60 Impacts of sea level rise on transportation infrastructure for the mid-Atlantic were also discussed in CCSP 2009 SAP 

4.1, Ch. 7.27

Weather impacts on road systems are discussed in “Climate Impacts and U.S. Transportation”57 and numerous other sources. 

Weather impacts on aviation operations are discussed in Kulesa 

200320 and numerous other sources.

In addition, the key message and supporting text summarize extensive evidence documented in “Climate Impacts and U.S. Transportation.”57

Additional peer-reviewed publications discuss the fact that Arctic 

warming is affecting existing Alaskan transportation infrastructure 

today, and is projected to allow the seasonal opening of the Northwest Passage to freight shipment.24

New information and remaining uncertainties

Recent changes in global sea level rise estimates documented in 

this report (Ch.2: Our Changing Climate, Key Message 10) have 

not been incorporated into existing regional studies of coastal 

areas. In addition, recent research by USGS on the interaction 

between sea level rise, wave action, and local geology have been 

incorporated in only a few studies.29

Specific estimates of climate change impacts on transportation 

are acutely sensitive to regional projections of climate change and, 

in particular, to the scale, timing, and type of predicted precipitation. New (CMIP5-based) regional climate projections will therefore affect most existing specific estimates of climate change 

impacts on transportation. Transportation planning in the face of 

uncertainties about regional-scale climate impacts presents particular challenges.

5: TRANSPORTATION

SUPPLEMENTAL MATERIAL

TRACEABLE ACCOUNTS
5: TRANSPORTATION

Traceable Accounts

147 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Impacts of climate on transportation system operations, including 

safety and congestion, both on road systems and in aviation, have 

been little studied to date. 

Future characteristics of society, such as land-use patterns, demographics, and the use of information technology to alter transportation patterns, and possible changes to the very nature of 

future transportation systems themselves all create uncertainty 

in evaluating climate impacts on the nation’s transportation networks. These societal changes will probably occur gradually, however, allowing the transportation systems to adapt. Adaptation can 

significantly ameliorate impacts on the transportation sector; however, evaluation of adaptation costs and strategies for the transportation sector is at a relatively early stage.

Assessment of confidence based on evidence 

Confidence is high that transportation systems will be affected 

by climate change, given current climate projections, particularly 

regarding sea level rise and extreme weather events.

Key Message #2 Traceable Account

Sea level rise, coupled with storm surge, will continue to increase the risk of major coastal impacts 

on transportation infrastructure, including both temporary and permanent flooding of airports, ports 

and harbors, roads, rail lines, tunnels, and bridges.

Description of evidence base

Estimates of global sea level rise are documented in Ch. 2: Our 

Changing Climate, Key Message 10 of this report. 

The prospective impact of sea level rise and storm surge on transportation systems is illustrated by the impact of recent hurricanes 

on U.S. coastlines. In addition, research on impacts of sea level 

rise and storm surge on transportation assets in particular regions 

of the United States demonstrate the potential for major coastal 

impacts (for example, CCSP 2008, Rosenzweig et al. 2011, and 

Suarez et al. 200511,28,60). Note that most existing literature on 

storm surge and sea level rise impacts on transportation systems 

is based on a global sea level rise of less than one meter (about 

3 feet). The most recent projections include a potentially greater 

rise in global sea level (Ch. 2: Our Changing Climate, Key Message 

10). 

In addition, the key message and supporting text summarize extensive evidence documented in “Climate Impacts and U.S. Transportation.”57

New information and remaining uncertainties

As noted above, new estimates of global sea level rise have overtaken most of the existing literature on transportation and sea 

level rise in the United States. In addition, it is not clear that the 

existing transportation literature reflects recent USGS work on interactions between sea level rise, wave action, and local geology.29

New global sea level rise estimates will enable the development 

of new regional estimates, as well as revision of regional coastal 

erosion and flood modeling. Such smaller scale estimates are important because transportation and other infrastructure impacts 

must necessarily be studied in a local context. 

Generally speaking, modeling of sea level rise impacts using existing USGS National Elevation Dataset (NED) data has well-understood limitations. Since NED data is freely and easily available, it 

is often used for preliminary modeling. More accurate and more 

recent elevation data may be captured via LIDAR campaigns, and 

this data collection effort will be necessary for accurate understanding of regional and local sea level rise and storm surge impacts.27

Accurate understanding of transportation impacts is specific to 

particular infrastructure elements, so detailed inventories of local 

and regional infrastructure must be combined with detailed and 

accurate elevation data and the best available predictions of local 

sea level rise and storm surge. Therefore, national assessments 

of sea level rise must be built on detailed local and regional assessments. 

Improved modeling is needed on the interactions among sea level 

rise, storm surge, tidal movement, and wave action to get a better 

understanding of the dynamics of the phenomena.

Confidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent results, well documented 

and accepted methods, etc.), 

high consensus

High

Moderate evidence (several 

sources, some consistency, 

methods vary and/or documentation limited, etc.), medium 

consensus

Medium

Suggestive evidence (a few 

sources, limited consistency, 

models incomplete, methods 

emerging, etc.), competing 

schools of thought

Low

Inconclusive evidence (limited sources, extrapolations, 

inconsistent findings, poor documentation and/or methods not 

tested, etc.), disagreement or 

lack of opinions among experts
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Traceable Accounts

148 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Assessment of confidence based on evidence

The authors have high confidence sea levels are rising and storm 

surge on top of these higher sea levels pose risks to coastal transportation infrastructure.

Key Message #3 Traceable Account

Extreme weather events currently disrupt transportation networks in all areas of the country; projections indicate that such disruptions will increase.

Description of evidence base

The key message and supporting text summarize extensive evidence documented in “Climate Impacts and U.S. Transportation.”57

Specific regional climate impacts can be identified in each NCA 

region of the country. Specific climate impacts on transportation 

by region include:

In Alaska, rising temperatures cause permafrost to melt, causing 

damage to roadbeds, airfields, pipelines, and other transportation 

infrastructure.25

In the Northeast, the Chesapeake region is likely to experience 

particularly severe local sea level rise due to geologic subsidence,27 and increased precipitation generally (see Ch. 2: Our 

Changing Climate, Key Message 5, and Ch.16: Northeast), along 

with an increased incidence of extreme weather events. The presence of large populations with associated transportation systems 

in coastal areas increases the potential impacts of sea level rise, 

storm surge, and precipitation-induced flooding.

The Southeast is subject to the interacting effects of sea level rise, 

increased precipitation, and other extreme events. The Southeast 

includes Virginia, so it shares the threat of regional sea level rise in 

the Chesapeake. In Louisiana, climate change poses a significant 

threat to transportation infrastructure of national significance.11

Midwest transportation infrastructure is subject to changing water levels on the Great Lakes.54 Barge traffic disruptions, due to 

flooding or drought on the Mississippi/Missouri/Ohio river system, 

might be induced by changes in precipitation patterns.

A major concern in the Southwest is that declining precipitation 

(see Ch. 2: Our Changing Climate, Key Message 5) may induce 

changes in the economy and society that will affect the transportation systems that serve this region. In the Southwest, rail and 

highway systems may be exposed to increased heat damage from 

the higher temperatures. San Francisco Bay, which encompasses 

two major airports and numerous key transportation links, is at 

risk for sea level rise and storm surge.61

Much of the economy of the Northwest is built around electricity 

and irrigation from a network of dams. The performance of this 

system may be affected by changing precipitation patterns, with 

potential consequences for agriculture and industry, and, consequently for transportation systems. In addition, the Seattle area 

may be affected by sea level rise.63

Many relevant and recent climate data and models predict more 

intense precipitation events in much of the U.S., especially the 

Great Plains, Midwest, Northeast, and Southeast, with decreased 

precipitation in parts of the Southwest and Southeast (see Ch. 2: 

Our Changing Climate, Key Message 5).

New information and remaining uncertainties

Recent data clearly show – and climate models further substantiate – an increase in the intensity of precipitation events throughout much of the U.S.

There is a need for a better definition of the magnitude of increased storm intensity so that accurate return frequency curves 

can be established. 

New regional climate model data from CMIP5 will have a significant impact on regional impact assessments.

Climate and impact data desired by transportation planners may 

be different from the projections generated by regional climate 

models. This presents a number of challenges:

Regional scale transportation impacts are often determined by 

flood risk and by water flows in rivers and streams. Flooding is, of 

course, linked to precipitation, but the linkage between precipitation and hydrology is very complex. Precipitation, as projected by 

climate models, is often difficult to convert into predictions of 

future flooding, which is what infrastructure designers need. 

Similarly, an ice storm would be an extreme event for a transportation planner, but the frequency of ice storms has not yet been derived from climate models. More generally, improved methods of 

deriving the frequency of infrastructure-affecting weather events 

from regional climate models may be helpful in assessing climate 

impacts on transportation systems.

There are uncertainties associated with the correlation between a 

warming climate and increased hurricane intensity.

In regions likely to see decreased precipitation, especially those 

areas subject to drought, stronger correlations to fire threat and 

lowered water levels in major waterways are needed as projections 

of climate models.

Planning tools and models can present a step-by-step process 

for connecting the risk of impact with specific planning strategies 

such as assessing the vulnerability of existing and proposed infrastructure and then identifying key adaptation practices to address 

the risk.
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Traceable Accounts

149 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Assessment of confidence based on evidence

Given the evidence base and remaining uncertainties, confidence 

is high that extreme weather events will affect transportation in all 

areas of the country.

Key Message #4 Traceable Account

Climate change impacts will increase the total 

costs to the nation’s transportation systems and 

their users, but these impacts can be reduced 

through rerouting, mode change, and a wide range 

of adaptive actions.

Description of evidence base

The economic cost of climate change to the transportation sector has been little studied. However, there is substantial evidence 

that costs will be significant. A recent study of climate change 

in New York indicated that a storm surge severe enough to flood 

Manhattan tunnels might cost as much as $100 billion.60 The 

actual experience of Hurricane Sandy, where multiple tunnels 

were flooded, attests to the scale of the costs and disruption that 

attend an event of this magnitude (See also Ch. 11: Urban; Box 

on Hurricane Sandy). A study of the risk to specific infrastructure 

elements in Alaska26 estimated the net present value of the extra 

cost from climate change at $2 to $4 billion through 2030, and 

$4 to $8 billion through 2080. 

The indirect evidence for significant costs from climate change 

impacts begin with the consequences of recent hurricanes, particularly on the Eastern seaboard, where Hurricane Irene, a rather 

minor storm, produced unexpectedly heavy infrastructure damage 

from heavy rains.75 The economic cost of infrastructure damage is 

often greater than the cost of repairing or replacing infrastructure.

In addition, a recent study of on-road congestion estimates the 

annual cost of highway congestion at about $100 billion,5 and 

the Federal Highway Administration estimates that weather accounts for about 15% of total delay.4

 Similarly, a recent study of 

aviation congestion indicates that the annual cost of airline delay 

is about $33 billion3 and that weather accounts for more than a 

third of airline delays. There is a strong circumstantial case to be 

made that increased frequency of extreme events (as defined by 

climate scientists) will produce increased traffic and aviation delays. Given the scale of current costs, even small changes in delay 

can have substantial economic costs. 

There is little published material on transportation adaptation 

costs and benefits in the literature, in part because “adaptation” 

is an abstraction (see Ch. 28: Adaptation). Climate change is statistical weather, and manifests itself as a change in the frequency 

of events that would still occur (but with lower frequency) in the 

absence of climate change. Transportation agencies decide to protect (or not) specific pieces of infrastructure based on a range of 

considerations, including age and condition, extent of current and 

future usage, and cost of protection, as well as changing weather 

patterns. The authors, however, are aware, that transportation 

systems have always been required to adapt to changing conditions, and that, in general, it is almost always far less expensive 

to protect useful infrastructure than to wait for it to collapse. This 

professional experience, based on examination of multitudes of 

individual engineering studies, is the basis for the conclusion in 

this report (for example, Caltrans Climate Change Workshop 2011, 

CCSP 2008, and Meyer 200811,12,69). 

There are numerous examples of actions taken by state and 

local governments to enhance resilience and reduce climate 

impact costs on transportation, including land-use planning to 

discourage development in vulnerable areas, establishment of 

design guidelines to reduce vulnerability to sea level rise, use of 

effective stormwater management techniques, and coordinated 

emergency response systems.7,69

New information and remaining uncertainties

There is relatively little information on the costs of climate change 

in the transportation sector, and less on the benefits of adaptation. Much of the available research is focused on the costs 

of replacing assets that are affected by extreme weather events, 

with far less effort devoted to both longer-term impacts of climate 

change on transportation systems (such as inundation of coastal 

roads due to sea level rise) and to the broader effects of disrupted 

facilities on network operations or on the community, for example, 

rerouting of traffic around bottlenecks or evacuation of sensitive 

populations from vulnerable areas.

Calculating climate impact and adaptation costs and benefits is an 

exceptionally complex problem, particularly at high levels of aggregation, since both costs and benefits accrue based on a multitude 

of location-specific events. In addition, all of the methodological 

issues that are confronted by any long-term forecasting exercise 

are present. The forecasting problem may be more manageable at 

the local and regional scales at which most transportation decisions are usually made.

Assessment of confidence based on evidence 

The authors have high confidence that climate impacts will be 

costly to the transportation sector, but are far less confident in 

assessing the exact magnitude of costs, based on the available 

evidence and their experience. The authors also have high confidence, based upon their experience, that costs may be significantly reduced by adaptation action, though, as noted, the magnitude 

of such potential reductions on a national scale would be difficult 

to determine. 
150

Climate Change Impacts in the United States

CHAPTER 6

AGRICULTURE

INFORMATION DRAWN FROM THIS CHAPTER IS INCLUDED IN THE HIGHLIGHTS REPORT AND IS IDENTIFIED BY THIS ICON

Recommended Citation for Chapter

Hatfield, J., G. Takle, R. Grotjahn, P. Holden, R. C. Izaurralde, T. Mader, E. Marshall, and D. Liverman, 2014: Ch. 6: Agriculture. Climate Change Impacts in the United States: The Third National Climate Assessment, J. M. Melillo, Terese (T.C.) 

Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 150-174. doi:10.7930/J02Z13FR.

On the Web: http://nca2014.globalchange.gov/report/sectors/agriculture

Convening Lead Authors 

Jerry Hatfield, U.S. Department of Agriculture

Gene Takle, Iowa State University

Lead Authors

Richard Grotjahn, University of California, Davis

Patrick Holden, Waterborne Environmental, Inc.

R. Cesar Izaurralde, Pacific Northwest National Laboratory

Terry Mader, University of Nebraska, Lincoln

Elizabeth Marshall, U.S. Department of Agriculture

Contributing Authors

Diana Liverman, University of Arizona
151 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Key Messages

1. Climate disruptions to agricultural production have increased in the past 40 years and are 

projected to increase over the next 25 years. By mid-century and beyond, these impacts will be 

increasingly negative on most crops and livestock.

2. Many agricultural regions will experience declines in crop and livestock production from 

increased stress due to weeds, diseases, insect pests, and other climate change induced 

stresses.

3. Current loss and degradation of critical agricultural soil and water assets due to increasing 

extremes in precipitation will continue to challenge both rainfed and irrigated agriculture unless 

innovative conservation methods are implemented. 

4. The rising incidence of weather extremes will have increasingly negative impacts on crop and 

livestock productivity because critical thresholds are already being exceeded. 

5. Agriculture has been able to adapt to recent changes in climate; however, increased innovation 

will be needed to ensure the rate of adaptation of agriculture and the associated socioeconomic 

system can keep pace with climate change over the next 25 years.

6. Climate change effects on agriculture will have consequences for food security, both in the U.S. 

and globally, through changes in crop yields and food prices and effects on food processing, 

storage, transportation, and retailing. Adaptation measures can help delay and reduce some of 

these impacts.

6 AGRICULTURE

The United States produces nearly $330 billion per year in agricultural commodities, with contributions from livestock accounting for roughly half of that value (Figure 6.1).1

 Production 

of all commodities will be vulnerable to direct impacts (from 

changes in crop and livestock development and yield due to 

changing climate conditions and extreme weather events) and 

indirect impacts (through increasing pressures from pests and 

pathogens that will benefit from a changing climate). The agricultural sector continually adapts to climate change through 

changes in crop rotations, planting times, genetic selection, 

fertilizer management, pest management, water management, 

and shifts in areas of crop production. These have proven to be 

effective strategies to allow previous agricultural production 

to increase, as evidenced by the continued growth in production and efficiency across the United States. 

Climate change poses a major challenge to U.S. agriculture 

because of the critical dependence of the agricultural system 

on climate and because of the complex role agriculture plays 

in rural and national social and economic systems (Figure 6.2). 

Climate change has the potential to both positively and negatively affect the location, timing, and productivity of crop, livestock, and fishery systems at local, national, and global scales. 

It will also alter the stability of food supplies and create new 

food security challenges for the United States as the world 

seeks to feed nine billion people by 2050. U.S. agriculture exists as part of the global economy and agricultural exports 

have outpaced imports as part of the overall balance of trade. 

However, climate change will affect the quantity of produce 

available for export and import as well as prices (Figure 6.3).

The cumulative impacts of climate change will ultimately 

depend on changing global market conditions as well as responses to local climate stressors, including farmers adjusting 

planting patterns in response to altered crop yields and crop 

species, seed producers investing in drought-tolerant varieties, 

and nations restricting trade to protect food security. Adaptive 

actions in the areas of consumption, production, education, 

and research involve seizing opportunities to avoid economic 

damages and decline in food quality, minimize threats posed 

by climate stress, and in some cases increase profitability.
152 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

6: AGRICULTURE

Key Message 1: Increasing Impacts on Agriculture

Climate disruptions to agricultural production have increased in the past 40 years 

and are projected to increase over the next 25 years. By mid-century and beyond, 

these impacts will be increasingly negative on most crops and livestock.

Impacts on Crop Production

Producers have many available strategies for adapting to the 

average temperature and precipitation changes projected (Ch. 

2: Our Changing Climate)2 for the next 25 years. These strategies include continued technological advancements, expansion 

of irrigated acreage, regional shifts in crop acreage and crop 

species, other adjustments in inputs and outputs, and changes 

in livestock management practices in response to changing climate patterns.3,4

 However, crop production projections often 

fail to consider the indirect impacts from weeds, insects, and 

diseases that accompany changes in both average trends and 

extreme events, which can increase losses significantly.2,5 By 

mid-century, when temperature increases are projected to 

be between 1.8°F and 5.4°F and precipitation extremes are 

further intensified, yields of major U.S. crops and farm profits 

are expected to decline.6,7 There have already been detectable impacts on production due to increasing temperatures.8

Over time, climate change is expected to increase the annual 

variation in crop and livestock production because of its effects on weather patterns and because of increases in some 

types of extreme weather events.9,10 Overall implications for 

production are for increased uncertainty in production totals, 

which affects both domestic and international markets and 

food prices. Recent analysis suggests that climate change has 

an outsized influence on year-to-year swings in corn prices in 

the United States.11

Figure 6.1. U.S. agriculture includes 

300 different commodities with 

a nearly equal division between 

crop and livestock products. This 

chart shows a breakdown of the 

monetary value of U.S. agriculture 

products by category. (Data from 

2007 Census of Agriculture, USDA 

National Agricultural Statistics 

Service 200812).

U.S. Agriculture

©iStockPhoto.com/small_frog
153 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

6: AGRICULTURE

Figure 6.2. Agricultural activity is distributed across the U.S. with market value and crop types varying by region. In 2010, the total 

market value was nearly $330 billion. Wide variability in climate, commodities, and practices across the U.S. will likely result in 

differing responses, both in terms of yield and management. (Figure source: USDA National Agricultural Statistics Service 200813).

Agricultural Distribution
154 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

6: AGRICULTURE

Plant response to climate change is dictated by complex 

interactions among carbon dioxide (CO2), temperature, 

solar radiation, and precipitation. Each crop species has 

a temperature range for growth, along with an optimum 

temperature.9 Plants have specific temperature tolerances, and can only be grown in areas where their temperature thresholds are not exceeded. As temperatures 

increase over this century, crop production areas may 

shift to follow the temperature range for optimal growth 

and yield of grain or fruit. Temperature effects on crop 

production are only one component; production over 

years in a given location is more affected by available soil 

water during the growing season than by temperature, 

and increased variation in seasonal precipitation, coupled 

with shifting patterns of precipitation within the season, 

will create more variation in soil water availability.

9,15

The use of a model to evaluate the effect of changing 

temperatures in the absence of changes in water availability reveals that crops in California’s Central Valley will 

respond differently to projected temperature increases, 

as illustrated in Figure 6.4. This example demonstrates 

one of the methods available for studying the potential 

effects of climate change on agriculture. 

Figure 6.3. U.S. agriculture exists in the context of global markets. 

Climate is among the important factors that affect these markets. 

For example, the increase in U.S. food exports in the 1970s is 

attributed to a combination of rising incomes in other nations, 

changes in national currency values and farm policies, and poor 

harvests in many nations in which climate was a factor. Through 

seasonal weather impacts on harvests and other impacts, climate 

change will continue to be a factor in global markets. The graph 

shows U.S. imports and exports for 1935-2011 in adjusted dollar 

values. (Data from USDA Economic Research Service 201214).

U.S. Agricultural Trade

Figure 6.4. Changes in climate through this 

century will affect crops differently because 

individual species respond differently to 

warming. This figure is an example of the 

potential impacts on different crops within 

the same geographic region. Crop yield 

responses for eight crops in the Central Valley 

of California are projected under two emissions 

scenarios, one in which heat-trapping gas 

emissions are substantially reduced (B1) and 

another in which these emissions continue to 

grow (A2). This analysis assumes adequate 

water supplies (soil moisture) and nutrients 

are maintained while temperatures increase. 

The lines show five-year moving averages for 

the period from 2010 to 2094, with the yield 

changes shown as differences from the year 

2009. Yield response varies among crops, 

with cotton, maize, wheat, and sunflower 

showing yield declines early in the period. 

Alfalfa and safflower showed no yield declines 

during the period. Rice and tomato do not 

show a yield response until the latter half of 

the period, with the higher emissions scenario 

resulting in a larger yield response. (Figure 

source: adapted from Lee et al. 2011

16

). 

Crop Yield Response to Warming in California’s Central Valley
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One critical period in which temperatures are a major factor is 

the pollination stage; pollen release is related to development 

of fruit, grain, or fiber. Exposure to high temperatures during 

this period can greatly reduce crop yields and increase the risk 

of total crop failure. Plants exposed to high nighttime temperatures during the grain, fiber, or fruit production period experience lower productivity and reduced quality.

15

 These effects 

have already begun to occur; high nighttime temperatures 

affected corn yields in 2010 and 2012 across the Corn Belt. 

With the number of nights with hot temperatures projected to 

increase as much as 30%, yield reductions will become more 

prevalent.

9

Temperature and precipitation 

changes will include an increase in 

both the number of consecutive dry 

days (days with less than 0.01 inches 

of precipitation) and the number of 

hot nights (Figure 6.5). The western 

and southern parts of the nation 

show the greatest projected increases in consecutive dry days, while the 

number of hot nights is projected to 

increase throughout the U.S. These 

increases in consecutive dry days 

and hot nights will have negative 

impacts on crop and animal production. High nighttime temperatures 

during the grain-filling period (the 

period between the fertilization of 

the ovule and the production of a 

mature seed in a plant) increase the 

rate of grain-filling and decrease the 

length of the grain-filling period, resulting in reduced grain yields. Exposure to multiple hot nights increases 

the degree of stress imposed on 

animals resulting in reduced rates of 

meat, milk, and egg production.

17

Though changes in temperature, CO2

concentrations, and solar radiation 

may benefit plant growth rates, this 

does not equate to increased production. Increasing temperatures cause 

cultivated plants to grow and mature 

more quickly. But because the soil 

may not be able to supply nutrients 

at required rates for faster growing 

plants, plants may be smaller, reducing grain, forage, fruit, or fiber production. Reduction in solar radiation 

in agricultural areas due to increased 

clouds and humidity in the last 60 

years

18

 is projected to continue

19

 and 

may partially offset the acceleration 

Figure 6.5. Many climate variables affect agriculture. The maps above show projected 

changes in key climate variables affecting agricultural productivity for the end of the century 

(2070-2099) compared to 1971-2000. Changes in climate parameters critical to agriculture 

show lengthening of the frost-free or growing season and reductions in the number of frost 

days (days with minimum temperatures below freezing), under an emissions scenario that 

assumes continued increases in heat-trapping gases (A2). Changes in these two variables are 

not identical, with the length of the growing season increasing across most of the United States 

and more variation in the change in the number of frost days. Warmer-season crops, such as 

melons, would grow better in warmer areas, while other crops, such as cereals, would grow 

more quickly, meaning less time for the grain itself to mature, reducing productivity.

9

 Taking 

advantage of the increasing length of the growing season and changing planting dates could 

allow planting of more diverse crop rotations, which can be an effective adaptation strategy. 

On the frost-free map, white areas are projected to experience no freezes for 2070-2099, 

and gray areas are projected to experience more than 10 frost-free years during the same 

period. In the lower left graph, consecutive dry days are defined as the annual maximum 

number of consecutive days with less than 0.01 inches of precipitation. In the lower right 

graph, hot nights are defined as nights with a minimum temperature higher than 98% of the 

minimum temperatures between 1971 and 2000. (Figure source: NOAA NCDC / CICS-NC).

Projected Changes in Key Climate Variables

Affecting Agricultural Productivity

©iStockPhoto/ valentinrussanov
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of plant growth due to higher temperatures and CO2 levels, 

depending on the crop. In vegetables, exposure to temperatures in the range of 1.8°F to 7.2°F above optimal moderately 

reduces yield, and exposure to temperatures more than 9°F 

to 12.6°F above optimal often leads to severe if not total production losses. Selective breeding and genetic engineering for 

both plants and animals provides some opportunity for adapting to climate change; however, development of new varieties 

in perennial specialty crops commonly requires 15 to 30 years 

or more, greatly limiting adaptive opportunity, unless varieties 

could be introduced from other areas. Additionally, perennial 

crops require time to reach their production potential. 

A warmer climate will affect growing conditions, and the lack 

of cold temperatures may threaten perennial crop production 

(Figure 6.6). Perennial specialty crops have a winter chilling 

requirement (typically expressed as hours when temperatures 

are between 32°F and 50°F) ranging from 200 to 2,000 cumulative hours. Yields decline if the chilling requirement is not 

completely satisfied, because flower emergence and viability 

is low.

20

 Projections show that chilling requirements for fruit 

and nut trees in California will not be met by the middle to the 

end of this century.

21

 For most of the Northeast, a 400-hour 

chilling requirement for apples is projected to continue to be 

met during this century, but crops with prolonged chilling reFigure 6.6. Many perennial plants (such as fruit trees and grape vines) require exposure to particular numbers of 

chilling hours (hours in which the temperatures are between 32°F and 50°F over the winter). This number varies 

among species, and many trees require chilling hours before flowering and fruit production can occur. With rising 

temperatures, chilling hours will be reduced. One example of this change is shown here for California’s Central Valley, 

assuming that observed climate trends in that area continue through 2050 and 2090. Under such a scenario, a rapid 

decrease in the number of chilling hours is projected to occur.

By 2000, the number of chilling hours in some regions was 30% lower than in 1950. Based on the A2 emissions 

scenario that assumes continued increases in heat-trapping gases relative to 1950, the number of chilling hours is 

projected to decline by 30% to 60% by 2050 and by up to 80% by 2100. These are very conservative estimates of 

the reductions in chilling hours because climate models project not just simple continuations of observed trends (as 

assumed here), but temperature trends rising at an increasing rate.21 To adapt to these kinds of changes, trees with 

a lower chilling requirement would have to be planted and reach productive age. 

Various trees and grape vines differ in their chilling requirements, with grapes requiring 90 hours, peaches 225, 

apples 400, and cherries more than 1,000.21 Increasing temperatures are likely to shift grape production for premium 

wines to different regions, but with a higher risk of extremely hot conditions that are detrimental to such varieties.24

The area capable of consistently producing grapes required for the highest-quality wines is projected to decline by 

more than 50% by late this century.24 (Figure source: adapted from Luedeling et al. 200921).

Reduced Winter Chilling Projected for California 
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quirements, such as plums and cherries (with chilling requirements of more than 700 hours), could be negatively affected, 

particularly in southern parts of the Northeast.

21,22

 Warmer 

winters can lead to early bud burst or bloom of some perennial 

plants, resulting in frost damage when cold conditions occur in 

late spring

15, as was the case with cherries in Michigan in 2012, 

leading to an economic impact of $220 million (Andresen 2012, 

personal communication).23 

The effects of elevated CO2 on grain and fruit yield and quality 

are mixed. Some experiments have documented that elevated 

CO2 concentrations can increase plant growth while increasing 

water use efficiency.25,26 The magnitude of CO2 growth stimulation in the absence of other stressors has been extensively 

analyzed for crop and tree species27,28 and is relatively well 

understood; however, the interaction with changing temperature, ozone, and water and nutrient constraints creates uncertainty in the magnitude of these responses.29 In plants such as 

soybean and alfalfa, elevated CO2 has been associated with 

reduced nitrogen and protein content, causing a reduction in 

grain and forage quality and reducing the ability of pasture and 

rangeland to support grazing livestock.30 The growth stimulation effect of increased atmospheric CO2 concentrations has 

a disproportionately positive impact on several weed species. 

This effect will contribute to increased risk of crop loss due to 

weed pressure.28,31

The advantage of increased water-use efficiency due to elevated CO2 in areas with limited soil water supply may be offset by 

other impacts from climate change. Rising average temperatures, for instance, will increase crop water demand, increasing 

the rate of water use by the crop. Rising temperatures coupled 

with more extreme wet and dry events, or seasonal shifts in 

precipitation, will affect both crop water demand and plant 

production. 

Impacts on Animal Production from Temperature Extremes

Animal agriculture is a major component of the U.S. agriculture 

system (Figure 6.1). Changing climatic conditions affect animal 

agriculture in four primary ways: 1) feed-grain production, 

availability, and price; 2) pastures and forage crop production 

and quality; 3) animal health, growth, and reproduction; and 

4) disease and pest distributions.32 The optimal environmental 

conditions for livestock production include temperatures and 

other conditions for which animals do not need to significantly 

alter behavior or physiological functions to maintain relatively 

constant core body temperature. 

Optimum animal core body temperature is often maintained 

within a 4°F to 5°F range, while deviations from this range can 

cause animals to become stressed. This can disrupt performance, production, and fertility, limiting the animals’ ability 

to produce meat, milk, or eggs. In many species, deviations in 

core body temperature in excess of 4°F to 5°F cause significant reductions in productive performance, while deviations 

of 9°F to 12.6°F often result in death.33 For cattle that breed 

during spring and summer, exposure to high temperatures 

reduces conception rates. Livestock and dairy production are 

more affected by the number of days of extreme heat than by 

increases in average temperature.34 Elevated humidity exacerbates the impact of high temperatures on animal health and 

performance. 

Animals respond to extreme temperature events (hot or cold) 

by altering their metabolic rates and behavior. Increases in 

extreme temperature events may become more likely for animals, placing them under conditions where their efficiency in 

meat, milk, or egg production is affected. Projected increases 

in extreme heat events (Ch. 2: Our Changing Climate, Key Message 7) will further increase the stress on animals, leading to 

the potential for greater impacts on production.34 Meat animals are managed for a high rate of weight gain (high metabolic rate), which increases their potential risk when exposed to 

high temperature conditions. Exposure to heat stress disrupts 

metabolic functions in animals and alters their internal temperature when exposure occurs. Exposure to high temperature 

events can be costly to producers, as was the case in 2011, 

when heat-related production losses exceeded $1 billion.35

Livestock production systems that provide partial or total shelter to reduce thermal environmental challenges can reduce 

the risk and vulnerability associated with extreme heat. In 

general, livestock such as poultry and swine are managed in 

housed systems where airflow can be controlled and housing 

temperature modified to minimize or buffer against adverse 

environmental conditions. However, management and energy 

costs associated with increased temperature regulation will 

increase for confined production enterprises and may require 

modification of shelter and increased water use for cooling. 
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Key Message 2: Weeds, Diseases, and Pests

Many agricultural regions will experience declines in crop and 

livestock production from increased stress due to weeds, diseases, 

insect pests, and other climate change induced stresses.

Weeds, insects, and diseases already have large negative impacts on agricultural production, and climate change has the 

potential to increase these impacts. Current estimates of losses in global crop production show that weeds cause the largest 

losses (34%), followed by insects (18%), and diseases (16%).36

Further increases in temperature and changes in precipitation 

patterns will induce new conditions that will affect insect populations, incidence of pathogens, and the geographic distribution of insects and diseases.15,37 Increasing CO2 boosts weed 

growth, adding to the potential for increased competition between crops and weeds.38 Several weed species benefit more 

than crops from higher temperatures and CO2 levels.28,31

One concern involves the northward spread of invasive weeds 

like privet and kudzu, which are already present in the southern states.39 Changing climate and changing trade patterns are 

likely to increase both the risks posed by, and the sources of, 

invasive species.40 Controlling weeds costs the U.S. more than 

$11 billion a year, with most of that spent on herbicides. Both 

herbicide use and costs are expected to increase as temperatures and CO2 levels rise.41 Also, the most widely used herbicide 

in the United States, glyphosate (also known as RoundUpTM and 

other brand names), loses its efficacy on weeds grown at CO2

levels projected to occur in the coming decades.42 Higher concentrations of the chemical and more frequent sprayings thus 

will be needed, increasing economic and environmental costs 

associated with chemical use.

Climate change effects on land-use patterns have the potential 

to create interactions among climate, diseases, and crops.37,43

How climate change affects crop diseases depends upon the 

effect that a combination of climate changes has on both the 

host and the pathogen. One example of the complexity of the 

interactions among climate, host, and pathogen is aflatoxin 

(Aspergillus flavus). Temperature and moisture availability are 

crucial for the production of this toxin, and both pre-harvest 

and post-harvest conditions are critical in understanding the 

impacts of climate change. High temperatures and drought 

stress increase aflatoxin production and at the same time 

reduce the growth of host plants. The toxin’s impacts are 

augmented by the presence of insects, creating a potential 

for climate-toxin-insect-plant interactions that further affect 

crop production.44 Earlier spring and warmer winter conditions 

are also expected to increase the survival and proliferation of 

disease-causing agents and parasites. 

Insects are directly affected by temperature and synchronize 

their development and reproduction with warm periods and 

are dormant during cold periods.45 Higher winter temperatures increase insect populations due to overwinter survival 

and, coupled with higher summer temperatures, increase 

reproductive rates and allow for multiple generations each 

year.46 An example of this has been observed in the European 

corn borer (Ostrinia nubialis) which produces one generation 

in the northern Corn Belt and two or more generations in the 

southern Corn Belt.47 Changes in the number of reproductive 

generations coupled with the shift in ranges of insects will alter 

insect pressure in a given region. 

Superimposed on these climate change related impacts on 

weed and insect proliferation will be ongoing land-use and 

land-cover changes (Ch. 13: Land Use & Land Cover Change). 

For example, northward movement of non-migratory butterflies in Europe and changes in the range of insects were associated with land-use patterns and climate change.48

Livestock production faces additional climate change related 

impacts that can affect disease prevalence and range. Regional 

warming and changes in rainfall distribution have the potential to change the distributions of diseases that are sensitive 

to temperature and moisture, such as anthrax, blackleg, and 

hemorrhagic septicemia, and lead to increased incidence of 

ketosis, mastitis, and lameness in dairy cows.33,49

These observations illustrate some of the interactions among 

climate change, land-use patterns, and insect populations. 

Weeds, insects, and diseases thus cause a range of direct and 

indirect effects on plants and animals from climate change, 

although there are no simple models to predict the potential 

interactions. Given the economic impact of these pests and 

the potential implications for food security, research is critical 

to further understand these dynamics. 
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Key Message 3: Extreme Precipitation and Soil Erosion

Current loss and degradation of critical agricultural soil and water assets due to 

increasing extremes in precipitation will continue to challenge both rainfed and 

irrigated agriculture unless innovative conservation methods are implemented.

Several processes act to degrade soils, including erosion, compaction, acidification, salinization, toxification, and net loss 

of organic matter (Ch. 15: Biogeochemical Cycles). Several of 

these processes, particularly erosion, will be directly affected 

by climate change. Rainfall’s erosive power is expected to increase as a result of increases in rainfall amount in northern 

portions of the United States (see Ch. 2: Our Changing Climate), accompanied by further increases in precipitation intensity.50 Projected increases in rainfall intensity that include 

more extreme events will increase soil erosion in the absence 

of conservation practices.51,52

Soil and water are essential resources for agricultural production, and both are subject to new conditions as climate changes. Precipitation and temperature affect the potential amount 

of water available, but the actual amount of available water 

also depends on soil type, soil water holding capacity, and the 

rate at which water filters through the soil (Figure 6.7 and 6.8). 

Such soil characteristics, however, are sensitive to changing 

climate conditions; changes in soil carbon content and soil loss 

will be affected by direct climate effects through changes in 

soil temperature, soil water availability, and the amount of 

organic matter input from plants.53

It is all about the water!

Soil is a critical component of agricultural systems, and the changing climate affects the amount, distribution, 

and intensity of precipitation. Soil erosion occurs when the rate of precipitation exceeds the ability of the soil to 

maintain an adequate infiltration rate. When this occurs, runoff from fields moves water and soil from the field 

into nearby water bodies. 

Water and soil that are lost from the field are no longer available to support crop growth. The increasing intensity 

of storms and the shifting of rainfall patterns toward more spring precipitation in the Midwest may lead to more 

scenes similar to this one (Figure 6.7). An analysis of the rainfall patterns across Iowa has shown there has not 

been an increase in total annual precipitation; however, there has been a large increase in the number of days 

with heavy rainfall (Figure 6.9). The increase in spring precipitation is evidenced by a decrease of three days 

in the number of workable days in the April to May period during 2001 through 2011 in Iowa compared to the 

period 1980-2000.15 To offset this increased precipitation, producers have been installing subsurface drainage to 

remove more water from the fields at a cost of $500 per acre (Figure 6.8). These are elaborate systems designed 

to move water from the landscape to allow agricultural operations to occur in the spring. Water erosion and runoff 

is only one portion of the spectrum of extreme precipitation. Wind erosion could increase in areas with persistent 

drought because of the reduction in vegetative cover. (Photo credit (left): USDA Natural Resources Conservation 

Service; Figure source (right): NOAA NCDC / CICS-NC).

Figure 6.7 Figure 6.8
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A few of the many important ecosystem services provided by 

soils include the provision of food, wood, fiber such as cotton, and raw materials; flood mitigation; recycling of wastes; 

biological control of pests; regulation of carbon and other 

heat-trapping gases; physical support for roads and buildings; 

and cultural and aesthetic values.54 Productive soils are characterized by levels of nutrients necessary for the production 

of healthy plants, moderately high levels of organic matter, a 

soil structure with good binding of the primary soil particles, 

moderate pH levels, thickness sufficient to store adequate water for plants, a healthy microbial community, and the absence 

of elements or compounds in concentrations that are toxic for 

plant, animal, and microbial life.

Changes in production practices can have more effect than 

climate change on soil erosion; however, changes in climate 

will exacerbate the effects of management practices that do 

not protect the soil surface from the forces of rainfall. Erosion 

is managed through maintenance of cover on the soil surface 

to reduce the effect of rainfall intensity. Studies have shown 

that a reduction in projected crop biomass (and hence the 

amount of crop residue that remains on the surface over the 

winter) will increase soil loss.57,58 Expected increases in soil erosion under climate change also will lead to increased off-site, 

non-point-source pollution. Soil conservation practices will 

therefore be an important element of agricultural adaptation 

to climate change.59

Rising temperatures and CO2 and shifting precipitation patterns will alter crop-water requirements, crop-water availability, crop productivity, and costs of water access across the 

agricultural landscape. Higher temperatures are projected to 

increase both evaporative losses from land and water surfaces 

and transpiration losses (through plant leaves) from non-crop 

land cover, potentially reducing annual runoff and streamflow 

for a given amount of precipitation. The resulting shift in crop 

health will, in turn, drive changes in cropland allocations and 

production systems.

Figure 6.9. Iowa is the nation’s top corn and soybean producing state. These crops are planted in the 

spring. Heavy rain can delay planting and create problems in obtaining a good stand of plants, both 

of which can reduce crop productivity. In Iowa soils with even modest slopes, rainfall of more than 

1.25 inches in a single day leads to runoff that causes soil erosion and loss of nutrients and, under 

some circumstances, can lead to flooding. The figure shows the number of days per year during 

which more than 1.25 inches of rain fell in Des Moines, Iowa. Recent frequent occurrences of such 

events are consistent with the significant upward trend of heavy precipitation events documented 

in the Midwest.51,55 (Figure source: adapted from Takle 201156).

 Increasing Heavy Downpours in Iowa

©Nati Harnik/AP/Corbis
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Key Message 4: Heat and Drought Damage

The rising incidence of weather extremes will have increasingly negative impacts on crop 

and livestock productivity because critical thresholds are already being exceeded. 

Climate change projections suggest an increase in extreme 

heat, severe drought, and heavy precipitation.60 Extreme climate conditions, such as dry spells, sustained droughts, and 

heat waves all have large effects on crops and livestock. The 

timing of extreme events will be critical because they may occur at sensitive stages in the life cycles of agricultural crops 

or reproductive stages for animals, diseases, and insects. Extreme events at vulnerable times could result in major impacts 

on growth or productivity, such as hot-temperature extreme 

weather events on corn during pollination. By the end of this 

century, the occurrence of very hot nights and the duration of 

periods lacking agriculturally significant rainfall are projected 

to increase. Recent studies suggest that increased average 

temperatures and drier conditions will amplify future drought 

severity and temperature extremes.

6,61,62 Crops and livestock 

will be at increased risk of exposure to extreme heat events. 

Projected increases in the occurrence of extreme heat events 

will expose production systems to conditions exceeding maximum thresholds for given species more frequently. Goats, 

sheep, beef cattle, and dairy cattle are the livestock species 

most widely managed in extensive outdoor facilities. Within 

physiological limits, animals can adapt to and cope with gradual thermal changes, though shifts in thermoregulation may 

result in a loss of productivity.63 Lack of prior conditioning to 

rapidly changing or adverse weather events, however, often 

results in catastrophic deaths in domestic livestock and losses 

of productivity in surviving animals.34

Key Message 5: Rate of Adaptation

Agriculture has been able to adapt to recent changes in climate; however, increased 

innovation will be needed to ensure the rate of adaptation of agriculture and the associated 

socioeconomic system can keep pace with climate change over the next 25 years.

There is emerging evidence about the economic impacts of 

climate change on agriculture and the potential for adaptive 

strategies.64 Much of the economic literature suggests that in 

the short term, producers will continue to adapt to weather 

changes and shocks as they always have, with changes in the 

timing of field operations, shifts in crops grown, and changing 

tillage or irrigation practices.64 In the longer term, however, existing adaptive technologies will likely not be sufficient to buffer the impacts of climate change without significant impacts 

to domestic producers, consumers, or both. New strategies 

for building long-term resilience include both new technologies and new institutions to facilitate appropriate, informed 

producer response to a changing climate. Furthermore, there 

are both public and private costs to adjusting agricultural production and infrastructure in a manner that enables adaptation.

2

 Limits to public investment and constraints on private 

investment could slow the speed of adaptation, yet potential 

constraints and limits are not well understood or integrated 

into economic impact assessments. The economic implications 

of changing biotic pressures on crops and livestock, and on the 

agricultural system as a whole, are not well understood, either 

in the short or long term.15 Adaptation may also be limited 

by the availability of inputs (such as land or water), changing 

prices of other inputs with climate change (such as energy and 

fertilizer), and by the environmental implications of intensifying or expanding agricultural production. 

Adaptation strategies currently used by U.S. farmers to cope 

with weather and climate changes include changing selection 

of crops, the timing of field operations, and the increasing use 

of pesticides to control increased pressure from pests. Technological innovation increases the tools available to farmers 

in some agricultural sectors. Diversifying crop rotations, integrating livestock with crop production systems, improving soil 

quality, minimizing off-farm flows of nutrients and pesticides, 

and other practices typically associated with sustainable agriculture also increase the resiliency of the agricultural system 

to productivity impacts of climate change.65,66 In the Midwest, ©iStockPhoto.com/shotbydave
162 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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there have been shifts in the distribution of crops and land-use 

change partially related to the increased demand for biofuels67

(see also Ch. 10: Energy, Water, and Land for more discussion 

on biofuels). In California’s Central Valley, an adaptation plan 

consisting of integrated changes in crop mix, irrigation methods, fertilization practices, tillage practices, and land management may be an effective approach to managing climate risk.68

These practices are available to all agricultural regions of the 

United States as potential adaptation strategies. 

Based on projected climate change impacts in some areas of 

the United States, agricultural systems may have to undergo 

more transformative changes to remain productive and profitable in the long term.65 Research and development of sustainable natural resource management strategies inform adaptation options for U.S. agriculture. More transformative adaptive 

strategies, such as conversion to integrated crop-livestock 

farming, may reduce environmental impacts, improve profitability and sustainability, and enhance ecological resilience to 

climate change in U.S. livestock production systems.69

There are many possible responses to climate change that will 

allow agriculture to adapt over the next 25 years; however, 

potential constraints to adaptation must be recognized and 

addressed. In addition to regional constraints on the availability of critical basic resources such as land and water, there are 

potential constraints related to farm financing and credit availability in the U.S. and elsewhere. Research suggests that such 

constraints may be significant, especially for small family farms 

with little available capital.22,64,70 In addition to the technical 

and financial ability to adapt to changing average conditions, 

farm resilience to climate change is also a function of financial 

capacity to withstand increasing variability in production and 

returns, including catastrophic loss.71 As climate change intensifies, “climate risk” from more frequent and intense weather 

events will add to the existing risks commonly managed by 

producers, such as those related to production, marketing, 

finances, regulation, and personal health and safety factors.72

The role of innovative management techniques and government policies as well as research and insurance programs will 

have a substantial impact on the degree to which the agricultural sector increases climate resilience in the longer term.

Modern agriculture has continually adapted to many changing 

factors, both within and outside of agricultural systems. As a 

result, agriculture in the U.S. over the past century has steadily 

increased productivity and integration into world markets. Although agriculture has a long history of successful adaptation 

to climate variability, the accelerating pace of climate change 

and the intensity of projected climate change represent new 

and unprecedented challenges to the sustainability of U.S. agriculture. In the short term, existing and evolving adaptation 

strategies will provide substantial adaptive capacity, protecting domestic producers and consumers from many of the 

impacts of climate change, except possibly the occurrence of 

protracted extreme events. In the longer term, adaptation will 

be more difficult and costly because the physiological limits 

of plant and animal species will be exceeded more frequently, 

and the productivity of crop and livestock systems will become 

more variable. 

Key Message 6: Food Security

Climate change effects on agriculture will have consequences for food 

security, both in the U.S. and globally, through changes in crop yields and food 

prices and effects on food processing, storage, transportation, and retailing. 

Adaptation measures can help delay and reduce some of these impacts.

Climate change impacts on agriculture will have consequences 

for food security both in the U.S. and globally. Food security 

includes four components: availability, stability, access, and 

utilization of food.73 Following this definition, in 2011, 14.9% 

of U.S. households did not have secure food supplies at some 

point during the year, with 5.7% of U.S. households experiencing very low food security.74 Food security is affected by a variety of supply and demand-side pressures, including economic 

conditions, globalization of markets, safety and quality of food, 

land-use change, demographic change, and disease and poverty.75,76

Within the complex global food system, climate change is expected to affect food security in multiple ways.77 In addition 

to altering agricultural yields, projected rising temperatures, 

changing weather patterns, and increases in frequency of 

extreme weather events will affect distribution of food- and 

©iStockPhoto.com/ mycola
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water-borne diseases as well as food trade and distribution.78

This means that U.S. food security depends not only on how 

climate change affects crop yields at the local and national 

level, but also on how climate change and changes in extreme 

events affect food processing, storage, transportation, and 

retailing, through the disruption of transportation as well as 

the ability of consumers to purchase food. And because about 

one-fifth of all food consumed in the U.S. is imported, our food 

supply and security can be significantly affected by climate 

variations and changes in other parts of the world. The import 

share has increased over the last two decades, and the U.S. 

now imports 13% of grains, 20% of vegetables (much higher in 

winter months), almost 40% of fruit, 85% of fish and shellfish, 

and almost all tropical products such as coffee, tea, and bananas (Figure 6.3).79 Climate extremes in regions that supply these 

products to the U.S. can cause sharp reductions in production 

and increases in prices.

In an increasingly globalized food system with volatile food 

prices, climate events abroad may affect food security in the 

U.S. while climate events in the U.S. may affect food security 

globally. The globalized food system can buffer the local impacts of weather events on food security, but can also increase 

the global vulnerability of food security by transmitting price 

shocks globally.80

The connections of U.S. agriculture and food security to global 

conditions are clearly illustrated by the recent food price spikes 

in 2008 and 2011 that highlighted the complex connections of 

climate, land use, demand, and markets. The doubling of the 

United Nations Food and Agriculture Organization (FAO) food 

price index over just a few months in 2010 was caused partly 

by weather conditions in food-exporting countries such as 

Australia, Russia, and the United States, but was also driven by 

increased demand for meat and dairy in Asia, increased energy 

costs and demand for biofuels, and commodity speculation in 

financial markets.81 

Adapting food systems to limit the impacts of climate extremes 

and changes involves strategies to maintain supply and manage demand as well as an understanding of how other regions 

of the world adapt their food systems in ways that might affect 

U.S. agricultural competitiveness, imports, and prices. Supplies 

can be maintained through adaptations such as reducing waste 

in the food system, making food distribution systems more 

resilient to climate risks, protecting food quality and safety in 

higher temperatures, and policies to ensure food access for 

disadvantaged populations and during extreme events (Ch. 28 

Adaptation).15,75,76,80,81
164 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

1. U.S. Census Bureau, 2012: The 2012 Statistical Abstract: 

Agriculture, 533-558 pp., U.S. Census Bureau, U.S Department 

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Photo Credits

Introduction to chapter; Iowa soybean field at sunset in top banner: 

©AgStock Images/Corbis
171 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Process for Developing Key Messages

A central component of the process was the development of a 

foundational technical input report (TIR), “Climate Change and 

Agriculture in the United States: An Assessment of Effects and 

Potential for Adaptation”.15 A public session conducted as part 

of the Tri-Societies (https://www.acsmeetings.org/home) meeting 

held in San Antonio, Texas, on Oct. 16-19, 2011, provided input 

to this report. 

The report team engaged in multiple technical discussions via 

teleconference, which included careful review of the foundational 

TIR15 and of approximately 56 additional technical inputs provided 

by the public, as well as other published literature and professional judgment. Discussions were followed by expert deliberation 

of draft key messages by the authors and targeted consultation 

with additional experts by the lead author of each message.

Key message #1 Traceable Account

Climate disruptions to agricultural production 

have increased in the past 40 years and are projected to increase over the next 25 years. By midcentury and beyond, these impacts will be increasingly negative on most crops and livestock.

Description of evidence base

The key message and supporting text summarize extensive evidence documented in the Agriculture TIR, “Climate Change and 

Agriculture in the United States: An Assessment of Effects and 

Potential for Adaptation.15 Additional Technical Input Reports (56) 

on a wide range of topics were also received and reviewed as part 

of the Federal Register Notice solicitation for public input.

Evidence that climate change has had and will have impacts on 

crops and livestock is based on numerous studies and is incontrovertible.6,7,8

The literature strongly suggests that carbon dioxide, temperature, 

and precipitation affect livestock and crop production. Plants 

have an optimal temperature range to which they are adapted, 

and regional crop growth will be affected by shifts in that region’s 

temperatures relative to each crop’s optimal range. Large shifts 

in temperature can significantly affect seasonal biomass growth, 

while changes in the timing and intensity of extreme temperature 

effects are expected to negatively affect crop development during 

critical windows such as pollination. Crop production will also be 

affected by changing patterns of seasonal precipitation; extreme 

precipitation events are expected to occur more frequently and 

negatively affect production levels. Livestock production is directly 

affected by extreme temperature as the animal makes metabolic 

adjustments to cope with heat stress.15 Further, production costs 

in confined systems markedly increase when climate regulation is 

necessary.

New information and remaining uncertainties

Important new evidence (cited above) confirmed many of the findings in the past Synthesis and Assessment Product on agriculture,82 which informed the 2009 National Climate Assessment.83

There is insufficient understanding of the effects on crop production of rising carbon dioxide, changing temperatures and more 

variable precipitation patterns.9 The combined effects on plant 

water demand and soil water availability will be critical to understanding regional crop response. The role of increasing minimum 

temperatures on water demand and growth and senescence rates 

of plants is an important factor. There is insufficient understanding of how prolonged exposure of livestock to high or cold temperatures affects metabolism and reproductive variables.26 For 

grazing animals, climate conditions during the growing season are 

critical in determining feed availability and quality on rangeland 

and pastureland.69

The information base can be enhanced by evaluating crop growth 

and livestock production models. This evaluation would further 

the understanding of the interactions of climate variables and 

the biological system. Better understanding of projected changes 

in precipitation will narrow uncertainty about future yield reductions.9,69

Assessment of confidence based on evidence 

There are a range of controlled environment and field studies that 

provide the evidence for these findings. Confidence in this key 

message is therefore judged to be high.

6: AGRICULTURE

SUPPLEMENTAL MATERIAL

TRACEABLE ACCOUNTS
6: AGRICULTURE

Traceable Accounts

172 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Key message #2 Traceable Account

Many agricultural regions will experience declines 

in crop and livestock production from increased 

stress due to weeds, diseases, insect pests, and 

other climate change induced stresses.

Description of evidence base

The key message and supporting text summarizes extensive evidence documented in the Agriculture TIR, “Climate Change and 

Agriculture in the United States: An Assessment of Effects and 

Potential for Adaptation”.15 Additional Technical Input Reports 

(56) on a wide range of topics were also received and reviewed 

as part of the Federal Register Notice solicitation for public input. 

Numerous peer-reviewed publications describe the direct effects 

of climate on the ecological systems within which crop and livestock operations occur. Many weeds respond more strongly to CO2

than do crops, and it is believed that the range of many diseases 

and pests (for both crop and livestock) will expand under warming conditions.28,31,40 Pests may have increased overwinter survival 

and fit more generations into a single year, which may also facilitate faster evolution of pesticide resistance. Changing patterns of 

pressure from weeds, other pests, and disease can affect crop and 

livestock production in ways that may be costly or challenging to 

address.9,15

New information and remaining uncertainties

Important new evidence (cited above) confirmed many of the findings in the past Synthesis and Assessment Product on agriculture,82 which informed the 2009 National Climate Assessment.83

In addition to extant species already in the U.S., exotic weeds, 

diseases, and pests have particular significance in that: 1) they 

can often be invasive (that is, arrive without normal biological/

ecological controls) and highly damaging; 2) with increasing international trade, there are numerous high-threat, high-impact 

species that will arrive on commodities from areas where some 

species even now are barely known to modern science, but which 

have the potential to emerge under a changed climate regime to 

pose significant risk of establishment in the U.S. and economic 

loss; and 3) can take advantage of “disturbances,” where climate 

variability acts as an additional ecological disturbance. Improved 

models and observational data related to how many agricultural 

regions will experience declines in animal and plant production 

from increased stress due to weeds, diseases, insect pests, and 

other climate change induced stresses will need to be developed. 

A key issue is the extent of the interaction between components 

of the natural biological system (for example, pests) and the economic biological system (for example, crop or animal). For insects, 

increased populations are a factor; however, their effect on the 

plant may be dependent upon the phenological stage of the plant 

when the insect is at specific phenological stages.15

To enhance our understanding of these issues will require a concerted effort to begin to quantify the interactions of pests and the 

economic crop or livestock system and how each system and their 

interactions are affected by climate.15

Assessment of confidence based on evidence 

The scientific literature is beginning to emerge; however, there are 

still some unknowns about the effects of biotic stresses, and there 

may well be emergent “surprises” resulting from departures from 

past ecological equilibria. Confidence is therefore judged to be 

medium that many agricultural regions will experience declines in 

animal and plant production from increased stress due to weeds, 

diseases, insect pests, and other climate change induced stresses. 

Key message #3 Traceable Account

Current loss and degradation of critical agricultural soil and water assets due to increasing extremes in precipitation will continue to challenge 

both rainfed and irrigated agriculture unless innovative conservation methods are implemented.

Description of evidence base

The key message and supporting text summarizes extensive evidence documented in the Agriculture TIR, “Climate Change and 

Agriculture in the United States: An Assessment of Effects and 

Potential for Adaptation.”15 Additional Technical Input Reports 

(56) on a wide range of topics were also received and reviewed 

as part of the Federal Register Notice solicitation for public input. 

Soil erosion is affected by rainfall intensity and there is evidence 

of increasing intensity in rainfall events even where the annual 

Confidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent 

results, well documented and 

accepted methods, etc.), high 

consensus

High

Moderate evidence (several sources, some consistency, methods 

vary and/or documentation limited, 

etc.), medium consensus

Medium

Suggestive evidence (a few 

sources, limited consistency, models incomplete, methods emerging, 

etc.), competing schools of thought

Low

Inconclusive evidence (limited 

sources, extrapolations, inconsistent findings, poor documentation 

and/or methods not tested, etc.), 

disagreement or lack of opinions 

among experts
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Traceable Accounts

173 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

mean is reduced.53 Unprotected soil surfaces will have increased 

erosion and require more intense conservation practices.58,59

Shifts in seasonality and type of precipitation will affect both timing and impact of water availability for both rainfed and irrigated 

agriculture. Evidence is strong that in the future there will be more 

precipitation globally, and that rain events will be more intense, 

even if separated by longer periods without rain.6

New information and remaining uncertainties

Important new evidence (cited above) confirmed many of the findings in the past Synthesis and Assessment Product on agriculture,82 which informed the 2009 National Climate Assessment.83

Both rainfed and irrigated agriculture will increasingly be challenged, based on improved models and observational data related 

to the effects of increasing precipitation extremes on loss and 

degradation of critical agricultural soil and water assets.51,52

Precipitation shifts are the most difficult to project, and uncertainty in regional projections increases with time into the future.61

To improve these projections will require enhanced understanding of shifts in timing, intensity, and magnitude of precipitation 

events. In the northern U.S., more frequent and severe winter and 

spring storms are projected, while there is a projected reduction in 

precipitation in the Southwest (see Ch. 2: Our Changing Climate).

Assessment of confidence based on evidence 

The precipitation forecasts are the limiting factor in these assessments; the evidence of the impact of precipitation extremes on 

soil water availability and soil erosion is well established. Confidence in this key message is therefore judged to be high.

Key message #4 Traceable Account

The rising incidence of weather extremes will 

have increasingly negative impacts on crop and 

livestock productivity because critical thresholds 

are already being exceeded. 

Description of evidence base

The key message and supporting text summarizes extensive evidence documented in the Agriculture TIR, “Climate Change and 

Agriculture in the United States: An Assessment of Effects and 

Potential for Adaptation”.15 Additional Technical Input Reports 

(56) on a wide range of topics were also received and reviewed 

as part of the Federal Register Notice solicitation for public input. 

Numerous peer-reviewed publications6,61,62 provide evidence that 

the occurrence of extreme events is increasing, and exposure 

of plants or animals to temperatures and soil water conditions 

(drought, water-logging, flood) outside of the biological range for 

the given species will cause stress and reduce production.6,61,62

The direct effects of an extreme event will depend upon the timing 

of the event relative to the growth stage of the biological system.

New information and remaining uncertainties

Important new evidence (cited above) confirmed many of the 

findings in the past Synthesis and Assessment Product on agriculture, 82 which informed the 2009 National Climate Assessment.83

One key area of uncertainty is the timing of extreme events during the phenological stage of the plant or the growth stage of the 

animal. For example, plants are more sensitive to extreme high 

temperatures during the pollination stage compared to vegetative 

growth stages.9 A parallel example for animals is relatively strong 

sensitivity to high temperatures during the conception phase.34

Milk and egg production are also vulnerable to temperature extremes. The effects of extreme combinations of weather variables 

must be considered, such as elevated humidity in concert with 

high temperatures.34

Other key uncertainties include inadequate precision in simulations of the timing of extreme events relative to short time periods 

of crop vulnerability, and temperatures close to key thresholds 

such as freezing.22 The uncertainty is amplified by the rarity of 

extreme events; this rarity means there are infrequent opportunities to study the impact of extreme events. In general, a shift 

of the distribution of temperatures can increase the frequency of 

threshold exceedance.15

The information base can be enhanced by improving the forecast 

of extreme events, given that the effect of extreme events on 

plants or animals is known.3,61

Assessment of confidence based on evidence 

There is high confidence in the effects of extreme temperature 

events on crops and livestock, and the agreement in the literature 

is good. 

Key message #5 Traceable Account

Agriculture has been able to adapt to recent 

changes in climate; however, increased innovation 

will be needed to ensure the rate of adaptation of 

agriculture and the associated socioeconomic system can keep pace with climate change over the 

next 25 years.

Description of evidence base

There is emerging evidence about the economic impacts of climate 

change on agriculture and the potential for adaptive strategies.64

In the case of crop production, much of the economic literature 

suggests that in the short term, producers will continue to adapt to 

weather changes and shocks as they always have, with changes in 

the timing of field operations, shifts in crops grown, and changing 

tillage or irrigation practices.64 In the longer term, however, existing adaptive technologies will likely not be sufficient to buffer the 

impacts of climate change without significant impacts to domestic 

producers, consumers, or both.
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174 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

New strategies for building long-term resilience include both 

new technologies and new institutions to facilitate appropriate, 

informed producer response to a changing climate. Furthermore, 

there are both public and private costs to adjusting agricultural 

production and infrastructure in a manner that enables adaptation.2

New information and remaining uncertainties

Limits to public investment and constraints on private investment 

could slow the speed of adaptation, yet potential constraints and 

limits are not well-understood or integrated into economic impact 

assessments. The economic implications of changing biotic pressures on crops and livestock, and on the agricultural system as a 

whole, are not well-understood, either in the short or long term.15

Adaptation may also be limited by availability of inputs (such as 

land or water), changing prices of other inputs with climate change 

(such as energy and fertilizer), and by the environmental implications of intensifying or expanding agricultural production. 

It is difficult to fully represent the complex interactions of the 

entire socio-ecological system within which agriculture operates, 

to assess the relative effectiveness and feasibility of adaptation 

strategies at various levels. Economic impact assessments require 

improved understanding of adaptation capacity and agricultural 

resilience at the system level, including the agri-ecosystem impacts related to diseases and pests. Economic impact assessments also require improved understanding of adaptation opportunities, economic resilience, and constraints to adaptation at the 

producer level.2,64 The economic value of ecological services, such 

as pollination services, is particularly difficult to quantify and incorporate into economic impact efforts.15

Assessment of confidence based on evidence 

Emerging evidence about adaptation of agricultural systems to 

changing climate is beginning to be developed. The complex interactions among all of the system components present a limitation to a complete understanding, but do provide a comprehensive 

framework for the assessment of agricultural responses to climate 

change. Given the overall and remaining uncertainty, there is medium confidence in this message.

Key message #6 Traceable Account

Climate change effects on agriculture will have 

consequences for food security, both in the U.S. 

and globally, through changes in crop yields and 

food prices and effects on food processing, storage, transportation, and retailing. Adaptation measures can help delay and reduce some of these 

impacts.

Description of evidence base

The relationships among agricultural productivity, climate change, 

and food security have been documented through ongoing investigations by the Food and Agriculture Organization,81,84 as well as 

the U.S. Department of Agriculture,85 and the National Research 

Council.77 There are many factors that affect food security, and 

agricultural yields are only one of them. Climate change is also 

expected to affect distribution of food- and waterborne diseases, 

and food trade and distribution.78

New information and remaining uncertainties

The components of food security derive from the intersection of 

political, physical, economic, and social factors. In many ways the 

impact of climate change on crop yields is the least complex of the 

factors that affect the four components of food security (availability, stability, access, and utilization). As the globalized food system 

is subject to conflicting pressures across scales, one approach 

to reducing risk is a “cross-scale problem-driven” approach to 

food security.76 This and other approaches to understanding and 

responding to the complexities of the global food system need additional research. Climate change will have a direct impact on crop 

and livestock production by increasing the variability in production 

levels from year to year, with varying effects across different regions. Climate change will also affect the distribution of food supplies as a result of disruptions in transportation routes. Addressing 

food security will require integration of multiple factors, including 

the direct and indirect impacts of climate change. 

Assessment of confidence based on evidence 

Given the evidence base and remaining uncertainty, there is high

confidence that climate change impacts will have consequences 

for food security both in the U.S. and globally through changes in 

crop yields and food prices, and very high confidence that other 

related factors, including food processing, storage, transportation, 

and retailing will also be affected by climate change. There is high

confidence that adaptation measures will help delay and reduce 

some of these impacts. 
Climate Change Impacts in the United States

CHAPTER 7

FORESTS

INFORMATION DRAWN FROM THIS CHAPTER IS INCLUDED IN THE HIGHLIGHTS REPORT AND IS IDENTIFIED BY THIS ICON

Recommended Citation for Chapter

Joyce, L. A., S. W. Running, D. D. Breshears, V. H. Dale, R. W. Malmsheimer, R. N. Sampson, B. Sohngen, and C. W. Woodall, 2014: Ch. 7: Forests. Climate Change Impacts in the United States: The Third National Climate Assessment, J. M. Melillo, 

Terese (T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 175-194. doi:10.7930/J0Z60KZC. 

On the Web: http://nca2014.globalchange.gov/report/sectors/forests

Convening Lead Authors

Linda A. Joyce, U.S. Forest Service

Steven W. Running, University of Montana

Lead Authors

David D. Breshears, University of Arizona

Virginia H. Dale, Oak Ridge National Laboratory

Robert W. Malmsheimer, SUNY Environmental Science and Forestry

R. Neil Sampson, Vision Forestry, LLC

Brent Sohngen, Ohio State University

Christopher W. Woodall, U.S. Forest Service

175
Key Messages

1. Climate change is increasing the vulnerability of many forests to ecosystem changes and tree 

 mortality through fire, insect infestations, drought, and disease outbreaks. 

2. U.S. forests and associated wood products currently absorb and store the equivalent of about 

 16% of all carbon dioxide (CO2) emitted by fossil fuel burning in the U.S. each year. Climate 

 change, combined with current societal trends in land use and forest management, is projected 

 to reduce this rate of forest CO2 uptake. 

3. Bioenergy could emerge as a new market for wood and could aid in the restoration of forests 

 killed by drought, insects, and fire. 

4. Forest management responses to climate change will be influenced by the changing nature of 

 private forestland ownership, globalization of forestry markets, emerging markets for bioenergy, 

 and U.S. climate change policy. 

7 FORESTS

Forests occur within urban areas, at the interface between 

urban and rural areas (wildland-urban interface), and in rural 

areas. Urban forests contribute to clean air, cooling buildings, 

aesthetics, and recreation in parks. Development in the 

wildland-urban interface is increasing because of the appeal 

of owning homes near or in the woods. In rural areas, market 

factors drive land uses among commercial forestry and land 

uses such as agriculture. Across this spectrum, forests provide 

recreational opportunities, cultural resources, and social 

values such as aesthetics.1 

Economic factors have historically influenced both the overall 

area and use of private forestland. Private entities (such as 

corporations, family forest owners, and tribes) own 56% 

of the forestlands in the United States. The remaining 44% 

of forests are on public lands: federal (33%), state (9%), and 

county and municipal government (2%).2 Market factors can 

influence management objectives for public lands, but societal 

values also influence objectives by identifying benefits such 

as environmental services not ordinarily provided through 

markets, like watershed protection and wildlife habitat. 

Different challenges and opportunities exist for public and for 

private forest management decisions, especially when climaterelated issues are considered on a national scale. For example, 

public forests typically carry higher levels of forest biomass, 

are more remote, and tend not to be as intensively managed as 

private forestlands.1

Forests provide opportunities to reduce future climate change 

by capturing and storing carbon, as well as by providing 

resources for bioenergy production (the use of forest-derived 

plant-based materials for energy production). The total 

amount of carbon stored in U.S. forest ecosystems and wood 

products (such as lumber and pulpwood) equals roughly 25 

years of U.S. heat-trapping gas emissions at current rates of 

emission, providing an important national “sink” that could 

grow or shrink depending on the extent of climate change, 

forest management practices, policy decisions, and other 

factors.3,4 For example, in 2011, U.S. forest ecosystems and 

the associated wood products industry captured and stored 

roughly 16% of all carbon dioxide emitted by fossil fuel burning 

in the United States.3 

Management choices for public, private, and tribal forests 

all involve similar issues. For example, increases in wildfire, 

disease, drought, and extreme events are projected for some 

regions (see also Ch. 16: Northeast; Ch. 20: Southwest; Ch. 

21: Northwest, Key Message 3; and Ch. 22: Alaska). At the 

same time, there is growing awareness that forests may play 

an expanded role in carbon management. Urban expansion 

fragments forests and may limit forest management options. 

Addressing climate change effects on forestlands requires 

considering the interactions among land-use practices, energy 

options, and climate change.5

176 CLIMATE CHANGE IMPACTS IN THE UNITED STATES
Key Message 1: Increasing Forest Disturbances

Climate change is increasing the vulnerability of many forests to ecosystem changes 

and tree mortality through fire, insect infestations, drought, and disease outbreaks. 

Insect and pathogen outbreaks, invasive species, wildfires, 

and extreme events such as droughts, high winds, ice 

storms, hurricanes, and landslides induced by storms8 are all 

disturbances that affect U.S. forests and their management 

(Figure 7.1). These disturbances are part of forest dynamics, 

are often interrelated, and can be amplified by underlying 

trends – for example, decades of rising average temperatures 

can increase damage to forests when a drought occurs.9

Disturbances that affect large portions of forest ecosystems 

occur relatively infrequently and in response to climate 

extremes. Changes in climate in the absence of extreme climate 

events (and the forest disturbances they trigger) may result in 

increased forest productivity, but extreme climate events can 

potentially overturn such patterns.10

Factors affecting tree death – such as drought, physiological 

water stress, higher temperatures, and/or pests and pathogens 

– are often interrelated, which means that isolating a single 

cause of mortality is rare.11,12,13 However, in western forests 

there have been recent large-scale die-off events due to one 

or more of these factors,14,15,16 and rates of tree mortality are 

well correlated with both rising temperatures and associated 

increases in evaporative water demand.17 In eastern forests, 

tree mortality at large spatial scales was more sensitive 

Figure 7.1. An example of the variability and distribution of major ecosystem 

disturbance types in North America, compiled from 2005 to 2009. Forest disturbance 

varies by topography, vegetation, weather patterns, climate gradients, and proximity 

to human settlement. Severity is mapped as a percent change in a satellite-derived 

Disturbance Index. White areas represent natural annual variability, orange 

represents moderate severity, and red represents high severity.

6

 Fire dominates 

much of the western forest ecosystems, and storms affect the Gulf Coast. Insect 

damage is widespread but currently concentrated in western regions, and timber 

harvest is predominant in the Southeast. (Figure source: modified from Goetz et 

al. 2012;7 Copyright 2012 American Geophysical Union).

Forest Ecosystem Disturbances

A Montana saw mill owner inspects a lodgepole 

pine covered in pitch tubes that show the tree 

trying, unsuccessfully, to defend itself against 

the bark beetle. The bark beetle is killing 

lodgepole pines throughout the western U.S.

Warmer winters allow more insects to survive 

the cold season, and a longer summer allows 

some insects to complete two life cycles in a 

year instead of one. Drought stress reduces 

trees’ ability to defend against boring insects. 

Above, beetle-killed trees in Rocky Mountain 

National Park in Colorado.

© Melanie Stetson Freeman/Getty Images © Pete McBride/National Geographic Creative

177 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

7: FORESTS
to foreststructure (age, tree size, and species composition) and 

air pollutants than climate over recent decades. Nonetheless, 

mortality of some eastern tree groups is related to rising 

temperature18 and is expected to increase as climate warms.19

Future disturbance rates in forests will depend on changes 

in the frequency of extreme events as well as the underlying 

changes in average climate conditions.9,20 Of particular concern 

is the potential for increased forest disturbance as the result 

of drought accompanied with warmer temperatures, which 

can cause both wildfire and tree death. Temperatures have 

generally been increasing and are projected to increase in the 

future (see Ch. 2: Our Changing Climate). Therefore, although 

it is difficult to predict trends in future extreme events,21

there is a high degree of confidence that future droughts will 

be accompanied by generally warmer conditions. Trees die 

faster when drought is accompanied by higher temperatures, 

so short droughts can trigger mortality if temperatures are 

higher.22 Short droughts occur more frequently than long 

droughts. Consequently, a direct effect of rising temperatures 

may be substantially greater tree mortality even with no 

change in drought frequency.22 

Given strong relationships between climate and fire, even 

when modified by land use and management, such as fuel 

treatments (Figure 7.2), projected climate changes suggest 

that western forests in the United States will be increasingly 

affected by large and intense fires that occur more 

frequently.16,23,24,25 These impacts are compounded by a legacy 

of fire suppression that has resulted in many U.S. forests 

becoming increasingly dense.26 Eastern forests are less likely 

to experience immediate increases in wildfire, unless a point is 

reached at which rising temperatures combine with seasonal 

dry periods, more protracted drought, and/or insect outbreaks 

to trigger wildfires – conditions that have been seen in Florida 

(see Ch. 17: Southeast).

Rising temperatures and CO2 levels can increase growth or 

alter migration of some tree species;1,27 however, the relationship between rising temperature and mortality is complex. For 

example, most functional groups show a decrease in mortality with higher summer temperatures (with the exception of 

northern groups), whereas warmer winters are correlated with 

higher mortality for some functional groups.18 Tree mortality 

is often the result of a combination of many factors; thus increases in pollutants, droughts, and wildfires will increase the 

probability of a tree dying (Figure 7.3). Under projected climate 

conditions, rising temperatures could work together with forest stand characteristics and these other stressors to increase 

mortality. Recent die-offs have been more severe than projected.11,14 As temperatures increase to levels projected for 

mid-century and beyond, eastern forests may be at risk of dieoff.19 New evidence indicates that most tree species can endure only limited abnormal water stress, reinforcing the idea that trees in wetter as well as semiarid forests are vulnerable 

to drought-induced mortality under warming climates.28

Figure 7.2. Forest management that selectively removes trees 

to reduce fire risk, among other objectives (a practice referred 

to as “fuel treatments”), can maintain uneven-aged forest 

structure and create small openings in the forest. Under some 

conditions, this practice can help prevent large wildfires from 

spreading. Photo shows the effectiveness of fuel treatments in 

Arizona’s 2002 Rodeo-Chediski fire, which burned more than 

400 square miles – at the time the worst fire in state history. 

Unburned area (left) had been managed with a treatment that 

removed commercial timber, thinned non-commercial-sized 

trees, and followed with prescribed fire in 1999. The right side 

of the photo shows burned area on the untreated slope below 

Limestone Ridge. (Photo credit: Jim Youtz, U.S. Forest Service).

Effectiveness of Forest Management

in Reducing Wildfire Risk

Climate change is contributing to increases in wildfires across 

the western U.S. and Alaska.

©Daryl Pederson/AlaskaStock/Corbis

178 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

7: FORESTS
Large-scale die-off and wildfire disturbance events could have 

potential impacts occurring at local and regional scales for 

timber production, flooding and erosion risks, other changes 

in water budgets, biogeochemical changes including carbon 

storage, and aesthetics.29,30,31 Rising disturbance rates can 

increase harvested wood output and potentially lower prices; 

however, higher disturbance rates could make future forest 

investments more risky (Figure 7.4). Western forests could 

also lose substantial amounts of carbon storage capacity. 

For example, an increase in wildfires, insect outbreaks, and 

droughts that are severe enough to alter soil moisture and 

nutrient contents can result in changes in tree density or 

species composition.10

Key Message 2: Changing Carbon Uptake

U.S. forests and associated wood products currently absorb and store the equivalent of 

about 16% of all carbon dioxide (CO2) emitted by fossil fuel burning in the U.S. each year. 

Climate change, combined with current societal trends in land use and forest 

management, is projected to reduce this rate of forest CO2 uptake. 

Climate-related Effects on Trees and Forest Productivity 

Forests within the United States grow across a wide range of 

latitudes and altitudes and occupy all but the driest regions. 

Current forest cover has been shaped by climate, soils, 

topography, disturbance frequency, and human activity.

Forest growth appears to be slowly accelerating (less than 1% 

per decade) in regions where tree growth is limited by low 

temperatures and short growing seasons that are gradually 

being altered by climate change (for species shifts, see Ch. 8: 

Ecosystems).32 Forest carbon storage appears to be increasing 

both globally and within the United States.33 Continental-scale 

satellite measurements document a lengthening growing 

season in the last thirty years, yet earlier spring growth may be 

negated by mid-summer drought.34

By the end of the century, snowmelt may occur a month 

earlier, but forest drought stress could increase by two 

months in the Rocky Mountain forests.35 In the eastern United 

States, elevated CO2 and temperature may increase forest 

growth and potentially carbon storage if sufficient water 

is available.1,31,36 Despite recent increases in forest growth, 

future net forest carbon storage is expected to decline due to 

accelerating mortality and disturbance. 

Figure 7.3. The figure shows a conceptual 

climate envelope analysis of forest vulnerability under current and projected future 

ranges of variability in climate parameters 

(temperature and precipitation, or alternatively drought duration and intensity). 

Climate models project increasing temperatures across the U.S. in coming decades, 

but a range of increasing or decreasing 

precipitation depending on region. Episodic 

droughts (where evaporation far exceeds 

precipitation) are also expected to increase 

in duration and/or intensity (see Ch. 2: 

Our Changing Climate). The overall result 

will be increased vulnerability of forests 

to periodic widespread regional mortality 

events resulting from trees exceeding their 

physiological stress thresholds.11 (Figure 

source: Allen et al. 201011). 

Forest Vulnerability to Changing Climate

179 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

7: FORESTS
Forest Carbon Sequestration and Carbon Management

From the onset of European settlement to the start of the 

last century, changes in U.S. forest cover due to expansion 

of agriculture, tree harvests, and settlements resulted in 

net emissions of carbon.37,38 More recently, with forests 

reoccupying land previously used for agriculture, technological 

advances in harvesting, and changes in forest management, 

U.S. forests and associated wood products now serve as a 

substantial carbon sink, capturing and storing more than 227.6 

million tons of carbon per year.3 The amount of carbon taken 

up by U.S. land is dominated by forests (Figure 7.5), which have 

annually absorbed 7% to 24% of fossil fuel carbon dioxide (CO2) 

emissions in the U.S. over the past two decades. The best 

estimate is that forests and wood products stored about 16% 

(833 teragrams, or 918.2 million short tons, of CO2 equivalent 

in 2011) of all the CO2 emitted annually by fossil fuel burning in 

the United States (see also “Estimating the U.S. Carbon Sink” in 

Ch. 15: Biogeochemical Cycles).3

The future role of U.S. forests in the carbon cycle 

will be affected by climate change through changes 

in disturbances (see Figures 7.3 and 7.4), as well 

as shifts in tree species, ranges, and productivity 

(Figure 7.6).19,38 Economic factors will affect any 

future carbon cycle of forests, as the age class 

and condition of forests are affected by the 

acceleration of harvesting,39,40 land-use changes 

such as urbanization,41 changes in forest types,42 and 

bioenergy development.41,43,44,45

Efforts in forestry to reduce atmospheric CO2

levels have focused on forest management and 

forest product use. Forest management strategies 

include land-use change to increase forest area 

(afforestation) and/or to avoid deforestation and 

optimizing carbon management in existing forests. 

Forest product-use strategies include the use of 

wood wherever possible as a structural substitute 

for steel and concrete, which require more carbon 

emissions to produce.38 The carbon emissions offset 

from using wood rather than alternate materials for 

a range of applications can be two or more times the 

carbon content of the product.47

Figure 7.4. Relative vulnerability of different forest regions to 

climate change is illustrated in this conceptual risk analysis 

diagram. Forest carbon exchange is the difference between 

carbon captured in photosynthesis and carbon released by 

respiration of vegetation and soils. Both photosynthesis and 

respiration are generally accelerated by higher temperatures, 

and slowed by water deficits, but the relative strengths 

of these controls are highly variable. Western forests are 

inherently limited by evaporation that exceeds precipitation 

during much of the growing season. Xeric (drier) eastern 

forests grow on shallow, coarse textured soils and experience 

water deficits during long periods without rain. Mesic (wetter) 

eastern forests experience severe water deficits only for 

relatively brief periods in abnormally dry years so the carbon 

exchanges are more controlled by temperature fluctuations. 

(Figure source: adapted from Vose et al. 20121).

Forests can be a Source – or a Sink – for Carbon

Figure 7.5. Forests are the largest component of the U.S. carbon sink, but 

growth rates of forests vary widely across the country. Well-watered forests 

of the Pacific Coast and Southeast absorb considerably more than the arid 

southwestern forests or the colder northeastern forests. Climate change 

and disturbance rates, combined with current societal trends regarding 

land use and forest management, are projected to reduce forest CO2

uptake in the coming decades.1 Figure shows average forest growth as 

measured by net primary production from 2000 to 2006. (Figure source: 

adapted from Running et al. 200446).

Forest Growth Provides an Important Carbon Sink

180 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

7: FORESTS
In the U.S., afforestation (active establishment or planting of 

forests) has the potential to capture and store a maximum of 

225 million tons of additional carbon per year from 2010 to 

211039,48 (an amount almost equivalent to the current annual 

carbon storage in forests). Tree and shrub encroachment into 

grasslands, rangelands, and savannas provides a large potential 

carbon sink that could exceed half of what existing U.S. forests 

capture and store annually.48

Expansion of urban and suburban areas is responsible for much 

of the current and expected loss of U.S. forestland, although 

these human-dominated areas often have extensive tree cover 

and potential carbon storage (see also Ch. 13: Land Use & Land 

Cover Change).41 In addition, the increasing prevalence of 

extreme conditions that encourage wildfires can convert some 

forests to shrublands and meadows25 or permanently reduce 

the amount of carbon stored in existing forests if fires occur 

more frequently.49

Carbon management on existing forests can include practices 

that increase forest growth, such as fertilization, irrigation, 

switching to fast-growing planting stock, shorter rotations, 

and weed, disease, and insect control.50 In addition, forest 

management can increase average forest carbon stocks by 

increasing the interval between harvests, by decreasing harvest 

intensity, or by focused density/species management.4,51 Since 

1990, CO2 emissions from wildland forest fires in the lower 48 

United States have averaged about 67 million tons of carbon 

per year.52,53 While forest management practices can reduce 

on-site carbon stocks, they may also help reduce future 

climate change by providing feedstock material for bioenergy 

production and by possibly avoiding future, potentially larger, 

wildfire emissions through fuel treatments (Figure 7.2).1

Figure 7.6. Historical, current, 

and projected annual rates of 

forest ecosystem and harvested 

wood product CO2 net emissions/

sequestration in the U.S. from 

1635 to 2055. In the top panel, 

the change in the historical annual 

carbon emissions (black line) in 

the early 1900s corresponds to the 

peak in the transformation of large 

parts of the U.S. from forested land 

to agricultural land uses. Green 

shading shows this decline in forest 

land area. In the bottom panel, 

future projections shown under 

higher (A2) and lower (B2 and 

A1B) emissions scenarios show 

forests as carbon sources (due to 

loss of forest area and accelerating 

disturbance rates) rather than sinks 

in the latter half of this century. 

The A1B scenario assumes similar 

emissions to the A2 scenario used 

in this report through 2050, and a 

slow decline thereafter. (Data from 

Birdsey 2006;37 USFS 2012;41EPA 

2013.53)

Forests and Carbon

181 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

7: FORESTS
Key Message 3: Bioenergy Potential

Bioenergy could emerge as a new market for wood and could aid in the 

restoration of forests killed by drought, insects, and fire.

Bioenergy refers to the use of plant-based material to produce 

energy, and comprises about 28% of the U.S. renewable energy 

supply (Ch. 10: Energy, Water, and Land). Forest resources 

potentially could produce bioenergy from 504 million acres of 

timberland and 91 million acres of other forested land (Figure 

7.7). Bioenergy from all sources, including agricultural and 

forests, could theoretically supply the equivalent of up to 30% 

of current U.S. petroleum consumption, but only if all relevant 

policies were optimized.45 The maximum projected potential 

for forest bioenergy ranges from 3% to 5% of total current U.S. 

energy consumption.54

Forest biomass energy could be one component of an overall 

bioenergy strategy to reduce emissions of carbon from fossil 

fuels,55 while also improving water quality56,57 and maintaining 

lands for timber production as an alternative to other 

socioeconomic options. Active biomass energy markets using 

wood and forest residues have emerged in the southern and 

northeastern United States, particularly in states that have 

adopted renewable fuel standards. The economic viability of 

using forests for bioenergy depends on regional context and 

circumstances, such as species type and prior management, 

land conditions, transport and storage logistics, conversion 

processes used to produce energy, distribution, and use.58 The 

environmental and socioeconomic consequences of bioenergy 

production vary greatly with region and intensity of human 

management. 

The potential for biomass energy to increase timber harvests 

has led to debates about whether forest biomass energy 

leads to higher carbon emissions.44,59 The debate on biogenic 

emissions regulations revolves around how to account for 

emissions related to biomass production and use.60 The forest 

carbon balance naturally changes over time and also depends 

on forest management scenarios. For 

example, utilizing natural beetle-killed 

forests will yield a different carbon 

balance than growing and harvesting a 

live, fast-growing plantation.

Markets for energy from biomass 

appear to be ready to grow in 

response to energy pricing, policy, 

and demand,44 although recent 

increases in the supply of natural gas 

have reduced the perceived urgency 

for new biomass projects. Further, 

because energy facilities typically buy 

the lowest quality wood at prices that 

rarely pay much more than cutting 

and hauling costs, they often require 

a viable saw timber market nearby to 

ensure an adequate, low-cost supply 

of material.61 Where it is desirable to 

remove dead wood after disturbances 

to thin forests or to dispose of 

residues, a viable bioenergy industry 

could finance such activities. However, 

the bioenergy market has yet to be 

made a profitable enterprise in most 

U.S. regions. 

Figure 7.7. Potential forestry bioenergy resources by 2030 at $80 per dry ton 

of biomass based on current forest area, production rates based on aggressive 

management for fast-growth, and short rotation bioenergy plantations. Units are 

oven dry tons (ODT) per square mile at the county level, where an ODT is 2,000 

pounds of biomass from which the moisture has been removed. Includes extensive 

material from existing forestland, such as residues, simulated thinnings, and some 

pulpwood for bioenergy, among other sources. (Figure source: adapted from U.S. 

Department of Energy 201145).

Location of Potential Forestry Biomass Resources

182 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

7: FORESTS
Key Message 4: Influences on Management Choices 

Forest management responses to climate change will be influenced by the changing 

nature of private forestland ownership, globalization of forestry markets, emerging 

markets for bioenergy, and U.S. climate change policy. 

Climate change will affect trees and forests in urban areas, 

the wildland-urban interface, and in rural areas. It will also 

challenge forest landowners managing forests for commercial 

products, energy development, environmental services such 

as watershed protection, or the conversion of forestland to 

developed and urban uses or agriculture. With increases in 

urbanization, the value of forests in and around urban areas in 

providing environmental services required by urban residents 

will increase.41 Potentially the greatest shifts in goods and 

environmental services produced from forests could occur 

in rural areas where social and economic factors will interact 

with the effects of climate change at landscape scales. 

Owner objectives, markets for forest products, crops and 

energy, the monetary value of private land, and policies 

governing private and public forestland all influence the 

actions taken to manage U.S. forestlands (56% privately 

owned, 44% public) (Figure 7.8). Ownership changes can bring 

changes in forest objectives. Among corporate owners (18% 

of all forestland), ownership has shifted from forest industry 

to investment management organizations that may or may not 

have active forest management as a primary objective. Noncorporate private owners, an aging demographic, manage 

38% of forestland. Their primary objectives are maintaining 

aesthetics and the privacy that the land provides as well as 

preserving the land as part of their family legacy.62

A significant economic factor facing private forest owners is the 

value of their forestlands for conversion to urban or developed 

uses. Economic opportunities from forests include wood 

products, non-timber forest products, recreation activities, 

and in some cases, environmental services.1,41 Less than 

1% of the volume of commercial trees from U.S. forestlands 

is harvested annually, and 92% of this harvest comes from 

private forestlands.2 Markets for wood products in the United 

States have been affected by increasingly competitive global 

markets,63 and timber prices are not projected to increase 

without substantial increases in wood energy consumption or 

other new timber demands.41 Urban conversions of forestland 

over the next 50 years could result in the loss of 16 to 31 million 

acres.41 The willingness of private forest owners to actively 

manage forests in the face of climate change will be affected 

primarily by market and policy incentives, not climate change 

itself.

The ability of public, private, and tribal forest managers to adapt 

to future climate change will be enhanced by their capacity 

to alter management regimes relatively rapidly in the face 

of changing conditions. The response to climate change may 

be greater on private forestlands where, in the past, owners 

have been highly responsive to market and policy signals.64

These landowners may be able to use existing or current 

forest management practices to reduce disturbance effects, 

increase the capture and storage of carbon, and modify plant 

species distributions under climate change. In addition, policy 

incentives, such as carbon pricing or cap and trade markets, 

could influence landowner choices. For human communities 

dependent upon forest resources, maintaining or enhancing 

their current resilience to change will influence their ability to 

respond to future stresses from climate change.65

On public, private, and tribal lands, management practices 

that can be used to reduce disturbance effects include 

altering tree planting and harvest strategies through species 

selection and timing; factoring in genetic variation; managing 

for reduced stand densities, which could reduce wildfire 

risk; reducing other stressors such as poor air quality; using 

forest management practices to minimize drought stress; 

and developing regional networks to mitigate impacts on 

ecosystem goods and services.1,30,66 Legally binding regulatory 

requirements may constrain adaptive management where 

plants, animals, ecosystems, and people are responding to 

climate change.67

Lack of fine-scale information about the possible effects of 

climate changes on locally managed forests limits the ability 

of managers to weigh these risks to their forests against the 

economic risks of implementing forest management practices 

such as adaptation and/or mitigation treatments. This 

knowledge gap will impede the implementation of effective 

management on public or private forestland in the face of 

climate change.

183 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

7: FORESTS
Figure 7.8. The figure shows forestland by ownership category in the contiguous U.S. in 2007.41 Western forests 

are most often located on public lands, while eastern forests, especially in Maine and in the Southeast, are more 

often privately held. (Figure source: U.S. Forest Service 201241).

Public and Private Forestlands

184 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

7: FORESTS
1. Vose, J. M., D. L. Peterson, and T. Patel-Weynand, Eds., 2012: Effects 

of Climatic Variability and Change on Forest Ecosystems: A Comprehensive 

Science Synthesis for the U.S. Forest Sector. General Technical Report 

PNW-GTR-870. U.S. Department of Agriculture, Forest Service, 

Pacific Northwest Research Station, 265 pp. [Available online at 

http://www.usda.gov/oce/climate_change/effects_2012/FS_

Climate1114%20opt.pdf]

2. Smith, W. B., P. D. Miles, C. H. Perry, and S. A. Pugh, 2009: Forest 

Resources of the United States, 2007. General Technical Report 

WO-78. 336 pp., U.S. Department of Agriculture. Forest Service, 

Washington, D.C. [Available online at http://www.fs.fed.us/nrs/

pubs/gtr/gtr_wo78.pdf]

3. EPA, 2013: Annex 3.12. Methodology for estimating net carbon 

stock changes in forest land remaining forest lands. Inventory of US 

Greenhouse Gas Emissions and Sinks: 1990-2011. EPA 430-R-13-001, 

U.S. Environmental Protection Agency, A-254 - A-303. [Available 

online at http://www.epa.gov/climatechange/Downloads/

ghgemissions/US-GHG-Inventory-2011-Annex_Complete_

Report.pdf]

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Haberl, H., K.-H. Erb, F. Krausmann, S. Running, T. D. Searchinger, 

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J. Stokes, and D. C. Erbach, 2005: Biomass as Feedstock for a 

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Zerbe, J. I., 2006: Thermal energy, electricity, and transportation 

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56. Dale, V. H., R. Lowrance, P. Mulholland, and G. P. Robertson, 

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57. Robertson, G. P., V. H. Dale, O. C. Doering, S. P. Hamburg, J. 

M. Melillo, M. M. Wander, W. J. Parton, P. R. Adler, J. N. Barney, 

R. M. Cruse, C. S. Duke, P. M. Fearnside, R. F. Follett, H. K. 

Gibbs, J. Goldemberg, D. J. Mladenoff, D. Ojima, M. W. Palmer, 

A. Sharpley, L. Wallace, K. C. Weathers, J. A. Wiens, and W. W. 

Wilhelm, 2008: Agriculture - Sustainable biofuels redux. Science, 

322, 49-50, doi:10.1126/science.1161525. 

58. Efroymson, R. A., V. H. Dale, K. L. Kline, A. C. McBride, 

J. M. Bielicki, R. L. Smith, E. S. Parish, P. E. Schweizer, 

and D. M. Shaw, 2013: Environmental indicators of biofuel 

sustainability: What about context? Environmental Management, 

51, 291-306, doi:10.1007/s00267-012-9907-5. 

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59. Bright, R. M., F. Cherubini, R. Astrup, N. Bird, A. L. Cowie, 

M. J. Ducey, G. Marland, K. Pingoud, I. Savolainen, and A. 

H. Strømman, 2012: A comment to “Large-scale bioenergy 

from additional harvest of forest biomass is neither 

sustainable nor greenhouse gas neutral”: Important insights 

beyond greenhouse gas accounting. Global Change Biolog y 

Bioenerg y 4, 617-619, doi:10.1111/j.1757-1707.2012.01190.x. 

Hudiburg, T. W., B. E. Law, C. Wirth, and S. Luyssaert, 2011: 

Regional carbon dioxide implications of forest bioenergy 

production. Nature Climate Change, 1, 419-423, doi:10.1038/

nclimate1264. 

Schulze, E. D., C. Körner, B. E. Law, H. Haberl, and S. 

Luyssaert, 2012: Large-scale bioenergy from additional 

harvest of forest biomass is neither sustainable nor 

greenhouse gas neutral. Global Change Biolog y Bioenerg y, 4, 611-

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carbon neutral? A comparative assessment of emissions 

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60. EPA, 2012: SAB Review of EPA’s Accounting Framework for 

Biogenic CO2 Emissions from Stationary Sources (September 

2011). EPA-SAB-12-011, 81 pp., U.S. Environmental 

Protection Agency, Washington, D.C. [Available online at 

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unsigned.pdf]

61. Galik, C. S., R. Abt, and Y. Wu, 2009: Forest biomass supply 

in the southeastern United States - implications for industrial 

roundwood and bioenergy production. Journal of Forestry, 107,

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62. Butler, B. J., 2008: Family forest owners of the United States, 

2006. A Technical Document Supporting the Forest Service 

2010 RPA Assessment. Gen. Tech. Rep. NRS-27, 72 pp., 

US Department of Agriculture, Forest Service, Northern 

Research Station, Newtown Square, PA. [Available online at 

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63. Ince, P. J., A. Schuler, H. Spelter, and W. Luppold, 2007: 

Globalization and Structural Change in the U.S. Forest 

Sector: An Evolving Context for Sustainable Forest 

Management. General Technical Report FPL-GTR-170, 62 

pp., U.S. Department of Agriculture, Forest Service, Forest 

Products Laboratory, Madison, WI. [Available online at 

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64. Wear, D. N., and J. P. Prestemon, 2004: Ch. 24: Timber 

market research, private forests, and policy rhetoric. General 

Technical Report SRS75, U.S. Department of Agriculture, 

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65. Wear, D., and L. A. Joyce, 2012: Climate change, human 

communities, and forests in rural, urban, and wildlandurban interace environments. Effects of Climatic Variability and 

Change on Forest Ecosystems: A Comprehensive Science Synthesis for 

the U.S. Forest Sector. General Technical Report PNW-GTR-870, 

J. M. Vose, D. L. Peterson, and T. Patel-Weynand, Eds., 

U.S. Department of Agriculture, Forest Service, Pacific 

Northwest Research Station, 265. [Available online at http://

www.usda.gov/oce/climate_change/effects_2012/FS_

Climate1114%20opt.pdf]

66. Joyce, L. A., G. M. Blate, J. S. Littell, S. G. McNulty, C. I. 

Millar, S. C. Moser, R. P. Neilson, K. O’Halloran, and D. L. 

Peterson, 2008: Ch. 3: National forests. Preliminary Review of 

Adaptation Options for Climate-Sensitive Ecosystems and Resources. 

A Report By the U.S. Climate Change Science Program and the 

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West, Eds., U.S. Environmental Protection Agency, 3-1 to 

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Millar, C. I., N. L. Stephenson, and S. L. Stephens, 2007: 

Climate change and forests of the future: Managing in the 

face of uncertainty. Ecological Applications, 17, 2145-2151, 

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67. Millar, C. I., and C. W. Swanston, 2012: Ch. 4: Strategies 

for adapting to climate change. Effects of Climatic Variability 

and Change on Forest Ecosystems: a comprehensive science synthesis 

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Department of Agriculture, U.S. Forest Service, Pacific 

Northwest Research Station, 125-192. [Available online at 

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68. McDowell, N. G., D. J. Beerling, D. D. Breshears, R. A. 

Fisher, K. F. Raffa, and M. Stitt, 2011: The interdependence of 

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69. Mildrexler, D. J., M. Zhao, F. A. Heinsch, and S. W. Running, 

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CO;2. 

70. CCSP, 2009: Thresholds of Climate Change in Ecosystems. A report 

by the U.S. Climate Change Science Program and the Subcommittee 

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I. Chapin. F.S., P. M. Groffman, G. R. Guntenspergen, A. K. 

Knapp, A. D. McGuire, P. J. Mulholland, D. P. C. Peters, D. 

D. Roby, and G. Sugihara, Eds. U.S. Geological Survey, 157 

pp. [Available online at http://digitalcommons.unl.edu/cgi/

viewcontent.cgi?article=1009&context=usgspubs]

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190 CLIMATE CHANGE IMPACTS IN THE UNITED STATES
SUPPLEMENTAL MATERIAL

TRACEABLE ACCOUNTS

Process for Developing Key Messages:

A central component of the process was a workshop held in July 

2011 by the U.S. Department of Agriculture Forest Service to 

guide the development of the technical input report (TIR). This 

session, along with numerous teleconferences, led to the foundational TIR, “Effects of Climatic Variability and Change on Forest 

Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector.”1

The chapter authors engaged in multiple technical discussions via 

teleconference between January and June 2012, which included 

careful review of the foundational TIR and of 58 additional technical inputs provided by the public, as well as other published 

literature and professional judgment. Discussions were followed 

by expert deliberation of draft key messages by the authors and 

targeted consultation with additional experts by the lead author of 

each message.

Key message #1 Traceable Account

Climate change is increasing the vulnerability of many forests to ecosystem changes and 

tree mortality through fire, insect infestations, 

drought, and disease outbreaks. 

Description of evidence base

The key message and supporting text summarizes extensive evidence documented in the TIR, “Effects of Climatic Variability and 

Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector.”1

 Technical input reports (58) on a 

wide range of topics were also received and reviewed as part of the 

Federal Register Notice solicitation for public input. 

Dale et al.8 addressed a number of climate change factors that will 

affect U.S. forests and how they are managed. This is supported 

by additional publications focused on effects of drought and by 

more large-scale tree die-off events,11,22 wildfire,16,23,25 insects 

and pathogens. 11,22 Other studies support the negative impact 

of climate change by examining the tree mortality rate due to rising temperatures,9,11,14,15,16,17,19,22 which is projected to increase in 

some regions.22

Although it is difficult to detect a trend in disturbances because 

they are inherently infrequent and it is impossible to attribute an 

individual disturbance event to changing climate, there is nonetheless much that past events, including recent ones, reveal about 

expected forest changes due to future climate. Observational17

and experimental22 studies show strong associations between forest disturbance and extreme climatic events and/or modifications 

in atmospheric evaporative demand related to warmer temperature. Regarding eastern forests, there are fewer observational or 

experimental studies, with Dietz and Moorcroft18 being the most 

comprehensive. 

Pollution and stand age are the most important factors in mortality. Tree survival increases with increased temperature in some 

groups. However, for other tree groups survival decreases with 

increased temperature.18 In addition, this study18 needs to be considered in the context that there have been fewer severe droughts 

in this region. However, physiological relationships suggest that 

trees will generally be more susceptible to mortality under an extreme drought, especially if it is accompanied by warmer temperatures.13,68 Consequently, it is misleading to assume that, because 

eastern forests have not yet experienced the types of large-scale 

die-off seen in the western forests, they are not vulnerable to such 

events if an extreme enough drought occurs. Although the effect 

of temperature on the rate of mortality during drought has only 

been shown for one species,22 the basic physiological relationships for trees suggest that warmer temperatures will exacerbate 

mortality for other species as well.13,68

Figure 7.1: This figure uses a figure from Goetz et al. 20127 which 

uses the MODIS Global Disturbance Index (MGDI) results from 

2005 to 2009 to illustrate the geographic distribution of major 

ecosystem disturbance types across North America (based on Mildrexler et al. 2007, 20096,69). The MGDI uses remotely sensed information to assess the intensity of the disturbance. Following the 

occurrence of a major disturbance, there will be a reduction in Enhanced Vegetation Index (EVI) because of vegetation damage; in 

contrast, Land Surface Temperature (LST) will increase because 

more absorbed solar radiation will be converted into sensible heat 

as a result of the reduction in evapotranspiration from less vegetation density. MGDI takes advantage of the contrast changes in 

EVI and LST following a disturbance to enhance the signal to ef7: FORESTS

191 CLIMATE CHANGE IMPACTS IN THE UNITED STATES
fectively detect the location and intensity of disturbances (http://

www.ntsg.umt.edu/project/mgdi). Moderate severity disturbance 

is mapped in orange and represents a 65%-100% divergence of 

the current-year MODIS Global Disturbance Index value from the 

range of natural variability, High severity disturbance (in red) signals a divergence of over 100%.7

New information and remaining uncertainties

Forest disturbances have large ecosystem effects, but high interannual variability in regional fire and insect activity makes detection 

of trends more difficult than for changes in mean conditions.20,21,70

Therefore, there is generally less confidence in assessment of future projections of disturbance events than for mean conditions 

(for example, growth under slightly warmer conditions).21

There are insufficient data on trends in windthrow, ice storms, 

hurricanes, and landslide-inducing storms to infer that these types 

of disturbance events are changing. 

Factors affecting tree death, such as drought, warmer temperatures, and/or pests and pathogens are often interrelated, which 

means that isolating a single cause of mortality is rare.11,12,13,17,22,68

Assessment of confidence based on evidence 

Very High. There is very high confidence that under projected 

climate changes there is high risk (high risk = high probability 

and high consequence) that western forests in the United States 

will be affected increasingly by large and intense fires that occur 

more frequently.16,23,25 This is based on the strong relationships 

between climate and forest response, shown observationally17 and 

experimentally.22 Expected responses will increase substantially 

to warming and also in conjunction with other changes such as 

an increase in the frequency and/or severity of drought and amplification of pest and pathogen impacts. Eastern forests are less 

likely to experience immediate increases in wildfire unless/until a 

point is reached at which warmer temperatures, concurrent with 

seasonal dry periods or more protracted drought, trigger wildfires.

Key message #2 Traceable Account

U.S. forests and associated wood products currently absorb and store the equivalent of about 

16% of all carbon dioxide (CO2) emitted by fossil 

fuel burning in the U.S. each year. Climate change, 

combined with current societal trends in land use 

and forest management, is projected to reduce this 

rate of forest CO2 uptake. 

Description of evidence base

The key message and supporting text summarizes extensive evidence documented in the TIR, “Effects of Climatic Variability and 

Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector.”1

 Technical input reports (58) on a 

wide range of topics were also received and reviewed as part of the 

Federal Register Notice solicitation for public input. 

A recent study3 has shown that forests are a big sink of CO2 nationally. However, the permanence of this carbon sink is contingent on forest disturbance rates, which are changing, and on economic conditions that may accelerate harvest of forest biomass.56

Market response can cause changes in the carbon source/sink 

dynamics through shifts in forest age,39,40 land-use changes and 

urbanization that reduce forested areas,41 forest type changes,42

and bioenergy development changing forest management.41,43,44,45

Additionally, publications have reported that fires can convert a 

forest into a shrubland or meadow,25 with frequent fires permanently reducing the carbon stock.49

New information and remaining uncertainties

That economic factors and societal choices will affect future carbon 

cycle of forests is known with certainty; the major uncertainties 

come from the future economic picture, accelerating disturbance 

rates, and societal responses to those dynamics.

Assessment of confidence based on evidence 

Based on the evidence and uncertainties, confidence is high that 

climate change, combined with current societal trends regarding 

land use and forest management, is projected to reduce forest 

CO2 uptake in the U.S. The U.S. has already seen large-scale 

shifts in forest cover due to interactions between forestland use 

and agriculture (for example, between the onset of European 

settlement to the present). There are competing demands for how 

forestland is used today. The future role of U.S. forests in the 

Confidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent results, well documented 

and accepted methods, etc.), 

high consensus

High

Moderate evidence (several 

sources, some consistency, 

methods vary and/or documentation limited, etc.), medium 

consensus

Medium

Suggestive evidence (a few 

sources, limited consistency, 

models incomplete, methods 

emerging, etc.), competing 

schools of thought

Low

Inconclusive evidence (limited sources, extrapolations, 

inconsistent findings, poor documentation and/or methods not 

tested, etc.), disagreement or 

lack of opinions among experts

7: FORESTS

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192 CLIMATE CHANGE IMPACTS IN THE UNITED STATES
carbon cycle will be affected by climate change through changes 

in disturbances (Key Message 1), growth rates, and harvest 

demands.

Key message #3 Traceable Account

Bioenergy could emerge as a new market for 

wood and could aid in the restoration of forests 

killed by drought, insects, and fire. 

Description of evidence base

The key message and supporting text summarize extensive evidence documented in the TIR, “Effects of Climatic Variability and 

Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector.”1

 Technical input reports (58) on a 

wide range of topics were also received and reviewed as part of the 

Federal Register Notice solicitation for public input. 

Studies have shown that harvesting forest bioenergy can prevent 

carbon emissions55 and replace a portion of U.S. energy consumption to help reduce future climate change. Some newer literature 

has explored how use of forest bioenergy can replace a portion of 

current U.S. energy production from oil.20,45 Some more recent 

publications have reported some environmental benefits, such 

as improved water quality56,57 and better management of timber 

lands,45 that can result from forest bioenergy implementation.

New information and remaining uncertainties

The implications of forest product use for bioenergy depends on 

regional context and circumstances, such as feedstock type and 

prior management, land conditions, transport and storage logistics, conversion processes used to produce energy, distribution 

and use.58

The potential for biomass energy to increase forest harvests 

has led to debates about whether biomass energy is net carbon 

neutral.59 The debate on biogenic emissions regulations revolves 

around how to account for emissions related to biomass production and use.60 Deforestation contributes to atmospheric CO2 concentration, and that contribution has been declining over time. 

The bioenergy contribution question is largely one of incentives 

for appropriate management. When forests have no value, they 

are burned or used inappropriately. Bioenergy can be produced 

in a way that provides more benefits than costs or vice versa. 

The market for energy from biomass appears to be ready to grow 

in response to energy pricing, policy, and demand; however, this 

industry is yet to be made a large-scale profitable enterprise in 

most regions of the United States.

Assessment of confidence based on evidence 

High. Forest growth substantially exceeds annual harvest for 

normal wood and paper products, and much forest harvest residue 

is now unutilized. Forest bioenergy will become viable if policy and 

economic energy valuations make it competitive with fossil fuels.

Key message #4 Traceable Account

Forest management responses to climate change 

will be influenced by the changing nature of private 

forestland ownership, globalization of forestry markets, emerging markets for bioenergy, and U.S. climate change policy. 

Description of evidence base

The key message and supporting text summarizes extensive evidence documented in the TIR, “Effects of Climatic Variability and 

Change on Forest Ecosystems: A Comprehensive Science Synthesis for the U.S. Forest Sector.”1

 Technical input reports (58) on a 

wide range of topics were also received and reviewed as part of the 

Federal Register Notice solicitation for public input. 

The forest management response to climate change in urban areas, the wildland-urban interface, and in rural areas has been 

studied from varying angles. The literature on urban forests identifies the value of those forests to clean air, aesthetics, and recreation and suggests that under a changing climate, urban communities will continue to enhance their environment with trees and 

urban forests.1,41 In the wildland-urban area and the rural areas, 

the changing composition of private forest landowners will affect 

the forest management response to climate change. Shifts in 

corporate owners to include investment organizations that may or 

may not have forest management as a primary objective has been 

described nationally.1,2 Family forest owners are an aging demographic; one in five acres of forestland is owned by someone who 

is at least 75 years of age.62 Multiple reasons for ownership are 

given by family forest owners, including the most commonly cited 

reasons of beauty/scenery, to pass land on to heirs, privacy, nature 

protection, and part of home/cabin. Many family forest owners feel 

it is necessary to keep the woods healthy but many are not familiar 

with forest management practices.62 Long-term studies of the forest sector in the southern United States document the adaptive 

response of forest landowners to market prices as they manage to 

supply wood and associated products from their forests;64 however prices are less of an incentive in other parts of the United 

States.1,41 Econometric approaches have been used to explore the 

economic activities in the forest sector, including interactions with 

other sectors such as agriculture, impact of climate change, and 

the potential for new markets with bioenergy.43,44 An earlier study 

explored the effects of globalization on forest management63 and 

a newer study looked at the effect of U.S. climate change policy.67

One of the biggest challenges is the lack of climate change information that results in inaction from many forest owners.62

New information and remaining uncertainties

Human concerns regarding the effects of climate change on 

forests and the role of adaptation and mitigation will be viewed 

from the perspective of the values that forests provide to human 

populations, including timber products, water, recreation, and 

aesthetic and spiritual benefits.1 Many people, organizations, in7: FORESTS

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193 CLIMATE CHANGE IMPACTS IN THE UNITED STATES
stitutions, and governments influence the management of U.S. 

forests. Economic opportunities influence the amount and nature 

of private forestland (and much is known quantitatively about this 

dynamic) and societal values have a strong influence on how public forestland is managed. However, it remains challenging to project exactly how humans will respond to climate change in terms 

of forest management. 

Climate change will alter known environmental and economic risks 

and add new risks to be addressed in the management of forests 

in urban areas, the wildland-urban interface, and rural areas. The 

capacity to manage risk varies greatly across landowners. While 

adaptation strategies provide a means to manage risks associated 

with climate change, a better understanding of risk perception 

by forest landowners would enhance the development and implementation of these management strategies. Identification of appropriate monitoring information and associated tools to evaluate 

monitoring data could facilitate risk assessment. Information and 

tools to assess environmental and economic risks associated with 

the impacts of climate change in light of specific management decisions would be informative to forestland managers and owners. 

Assessment of confidence based on evidence 

Given the evidence base and remaining uncertainty, there is 

medium confidence in this key message. Climate change and 

global and national economic events will have an integral impact 

on forest management, but it is uncertain to what magnitude. 

While forest landowners have shown the capacity to adapt to 

new economic conditions, potential changes in the international 

markets coincident with large-scale natural disturbances enhanced 

by climate change (fire, insects) could challenge this adaptive 

capacity. An important uncertainty is how people will respond to 

climate change in terms of forest management.

7: FORESTS

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194 CLIMATE CHANGE IMPACTS IN THE UNITED STATES
195

Climate Change Impacts in the United States

CHAPTER 8

ECOSYSTEMS,

BIODIVERSITY, AND ECOSYSTEM SERVICES

INFORMATION DRAWN FROM THIS CHAPTER IS INCLUDED IN THE HIGHLIGHTS REPORT AND IS IDENTIFIED BY THIS ICON

Recommended Citation for Chapter

Groffman, P. M., P. Kareiva, S. Carter, N. B. Grimm, J. Lawler, M. Mack, V. Matzek, and H. Tallis, 2014: Ch. 8: Ecosystems, 

Biodiversity, and Ecosystem Services. Climate Change Impacts in the United States: The Third National Climate Assessment, 

J. M. Melillo, Terese (T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 195-219. doi:10.7930/

J0TD9V7H. 

On the Web: http://nca2014.globalchange.gov/report/sectors/ecosystems

Convening Lead Authors

Peter M. Groffman, Cary Institute of Ecosystem Studies 

Peter Kareiva, The Nature Conservancy

Lead Authors

Shawn Carter, U.S. Geological Survey

Nancy B. Grimm, Arizona State University

Josh Lawler, University of Washington

Michelle Mack, University of Florida

Virginia Matzek, Santa Clara University

Heather Tallis, Stanford University
196 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Key Messages

1. Climate change impacts on ecosystems reduce their ability to improve water quality and regulate 

water flows.

2. Climate change, combined with other stressors, is overwhelming the capacity of ecosystems to 

buffer the impacts from extreme events like fires, floods, and storms.

3. Landscapes and seascapes are changing rapidly, and species, including many iconic species, 

may disappear from regions where they have been prevalent or become extinct, altering some 

regions so much that their mix of plant and animal life will become almost unrecognizable. 

4. Timing of critical biological events, such as spring bud burst, emergence from overwintering, and 

the start of migrations, has shifted, leading to important impacts on species and habitats.

5. Whole system management is often more effective than focusing on one species at a time, 

and can help reduce the harm to wildlife, natural assets, and human well-being that climate 

disruption might cause. 

ECOSYSTEMS, 8 BIODIVERSITY, AND ECOSYSTEM SERVICES

Climate change affects the living world, including people, 

through changes in ecosystems, biodiversity, and ecosystem 

services. Ecosystems entail all the living things in a particular 

area as well as the non-living things with which they interact, 

such as air, soil, water, and sunlight.1 Biodiversity refers to 

the variety of life, including the number of species, life forms, 

genetic types, and habitats and biomes (which are characteristic 

groupings of plant and animal species found in a particular 

climate). Biodiversity and ecosystems produce a rich array of 

benefits that people depend on, including fisheries, drinking 

water, fertile soils for growing crops, climate regulation, 

inspiration, and aesthetic and cultural values.2 These benefits 

are called “ecosystem services” – some of which, like 

food, are more easily quantified than others, such as 

climate regulation or cultural values. Changes in many 

such services are often not obvious to those who 

depend on them.

Ecosystem services contribute to jobs, economic 

growth, health, and human well-being. Although 

we interact with ecosystems and ecosystem 

services every day, their linkage to climate change 

can be elusive because they are influenced by so 

many additional entangled factors.3 Ecosystem 

perturbations driven by climate change have direct 

human impacts, including reduced water supply and 

quality, the loss of iconic species and landscapes, 

distorted rhythms of nature, and the potential for 

extreme events to overwhelm the regulating services 

of ecosystems. Even with these well-documented 

ecosystem impacts, it is often difficult to quantify human 

vulnerability that results from shifts in ecosystem processes 

and services. For example, although it is more straightforward 

to predict how precipitation will change water flow, it is much 

harder to pinpoint which farms, cities, and habitats will be at 

risk of running out of water, and even more difficult to say how 

people will be affected by the loss of a favorite fishing spot 

or a wildflower that no longer blooms in the region. A better 

understanding of how a range of ecosystem responses affects 

people – from altered water flows to the loss of wildflowers 

– will help to inform the management of ecosystems in a way 

that promotes resilience to climate change.

Forests absorb carbon dioxide and provide many other ecosystem services, 

such as purifying water and providing recreational opportunities.

© Michele Westmorland/Corbis
197 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Key Message 1: Water

Climate change impacts on ecosystems reduce their ability to 

improve water quality and regulate water flows.

Climate-driven factors that control water availability and 

quality are moderated by ecosystems. Land-based ecosystems

regulate the water cycle and are the source of sediment and 

other materials that make their way to aquatic ecosystems 

(streams, rivers, lakes, estuaries, oceans, groundwater). Aquatic 

ecosystems provide the critically important services of storing 

water, regulating water quality, supporting fisheries, providing 

recreation, and carrying water and materials downstream 

(Ch. 25: Coasts). Humans utilize, on average, the equivalent of 

more than 40% of renewable supplies of freshwater in more 

than 25% of all U.S. watersheds.4 Freshwater withdrawals are 

even higher in the arid Southwest, where the equivalent of 

76% of all renewable freshwater is appropriated by people.5

In that region, climate change has likely decreased and altered 

the timing of streamflow due to reduced snowpack and lower 

precipitation in spring, although the precipitation trends are 

weak due to large year-to-year variability, as well as geographic 

variation in the patterns (Ch. 3: Water; Ch. 20: Southwest).6

Depriving ecosystems of water reduces their ability to provide 

water to people as well as for aquatic plant and animal habitat 

(see Figure 8.1).

Habitat loss and local extinctions of fish and other aquatic 

species are projected from the combined effects of increased 

water withdrawal and climate change.7 In the U.S., 47% of 

trout habitat in the interior West would be lost by 2080 

under a scenario (A1B) that assumes similar emissions to the 

A2 scenario used in this report (Ch. 1: Overview, Ch. 2: Our 

Changing Climate) through 2050, and a slow decline thereafter.8

Across the entire U.S., precipitation amounts and intensity and 

associated river discharge are major drivers of water pollution 

in the form of excess nutrients, sediment, and dissolved organic 

carbon (DOC) (Ch. 3: Water).9 At high concentrations, nutrients 

that are required for life (such as nitrogen and phosphorus) can 

become pollutants and can promote excessive phytoplankton 

growth – a process known as eutrophication. Currently, many 

U.S. lakes and rivers are polluted (have concentrations above 

government standards) by excessive nitrogen, phosphorus, or 

sediment. There are well-established links among fertilizer use, 

nutrient pollution, and river discharge, and many studies show 

that recent increases in rainfall in several regions of the United 

States have led to higher nitrogen amounts carried by rivers 

(Northeast,10,11 California,12 and Mississippi Basin13,14). Over the 

past 50 years, due to both climate and land-use change, the 

Mississippi Basin is yielding an additional 32 million acre-feet 

of water each year – equivalent to four Hudson Rivers – laden 

with materials washed from its farmlands.15 This flows into the 

Gulf of Mexico, which is the site of the nation’s largest hypoxic 

(low oxygen) “dead” zone.4 The majority of U.S. estuaries are 

moderately to highly eutrophic.16

Links between discharge and sediment transport are well 

established,17 and cost estimates for in-stream and off-stream 

damages from soil erosion range from $2.1 to $10 billion 

per year.18,19 These estimates include costs associated with 

damages to, or losses of, recreation, water storage, navigation, 

commercial fishing, and property, but do not include costs of 

biological impacts.18 Sediment transport, with accompanying 

nutrients, can play a positive role in the shoreline dynamics 

of coastlines and the life cycles of coastal and marine plants 

and animals. However, many commercially and recreationally 

important fish species such as salmon and trout that lay their 

eggs in the gravel at the edges of streams are especially sensitive 

to elevated sediment fluxes in rivers.20 Sediment loading in 

lakes has been shown to have substantial detrimental effects 

on fish population sizes, community composition, 

and biodiversity.21

Dissolved organic carbon (DOC) fluxes to rivers and 

lakes are strongly driven by precipitation;22 thus 

in many regions where precipitation is expected 

to increase, DOC loading will also increase. 

Dissolved organic carbon is the substance that 

gives many rivers and lakes a brown, tea-colored 

look. Precipitation-driven increases in DOC 

concentration not only increase the cost of water 

treatment for municipal use,23 but also alter 

the ability of sunlight to act as nature’s water 

treatment plant. For example, Cryptosporidium, a 

pathogen potentially lethal to the elderly, babies, 

and people with compromised immune systems, is 

present in 17% of drinking water supplies sampled 

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in the United States.24 This pathogen is inactivated by doses 

of ultraviolet (UV) light equivalent to less than a day of sun 

exposure.25 Similarly, UV exposures reduce fungal parasites 

that infect Daphnia, a keystone aquatic grazer and food source 

for fish.26 Increasing DOC concentrations may thus reduce the 

ability of sunlight to regulate these UV-sensitive parasites. 

Few studies have projected the impacts of climate change 

on nitrogen, phosphorus, sediment, or DOC transport from 

the land to rivers. However, given the tight link between 

river discharge and all of these potential pollutants, areas 

of the United States that are projected to see increases 

in precipitation, and increases in intense rainfalls, like the 

Northeast, Midwest, and mountainous West,27 will also see 

increases in excess nutrients, DOC, and sediments transported 

to rivers. One of the few future projections available suggests 

that downstream and coastal impacts of increased nitrogen 

inputs could be profound for the Mississippi Basin. Under 

a scenario in which atmospheric CO2 reaches double preindustrial levels, a 20% increase in river discharge is expected 

to lead to higher nitrogen loads and a 50% increase in algae 

growth in the Gulf of Mexico, a 30% to 60% decrease in deepwater dissolved oxygen concentration, and an expansion of 

the dead zone.28 A recent comprehensive assessment10 shows 

that, while climate is an important driver, nitrogen carried by 

rivers to the oceans is most strongly driven by fertilizer inputs 

to the land. Therefore, in the highly productive agricultural 

systems of the Mississippi Basin, the ultimate impact of more 

precipitation on the expansion of the dead zone will depend on 

agricultural management practices in the Basin.14,29

Rising air temperatures can also lead to declines in water quality 

through a different set of processes. Some large lakes, including 

the Great Lakes, are warming rapidly.30 Warmer surface waters 

can stimulate blooms of harmful algae in both lakes and 

coastal oceans,9 which may include toxic cyanobacteria that 

are favored at higher temperatures.31 Harmful algal blooms, 

which are caused by many factors, including climate change, 

exact a cost in freshwater degradation of approximately $2.2 

billion annually in the United States alone.32

Figure 8.1. Climate change is projected to reduce the ability of ecosystems to supply water in some parts of the country. This is true 

in areas where precipitation is projected to decline, and even in some areas where precipitation is expected to increase. Compared 

to 10% of counties today, by 2050, 32% of counties will be at high or extreme risk of water shortages. Projections assume continued 

increases in greenhouse gas emissions through 2050 and a slow decline thereafter (A1B scenario). Numbers in parentheses indicate 

number of counties in each category. (Reprinted with permission from Roy et al., 2012.27 Copyright 2012 American Chemical Society).

Water Supplies Projected to Decline
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Key Message 2: Extreme Events

Climate change, combined with other stressors, is overwhelming the capacity of 

ecosystems to buffer the impacts from extreme events like fires, floods, and storms.

Ecosystems play an important role in “buffering” the effects 

of extreme climate conditions (floods, wildfires, tornadoes, 

hurricanes) on the movement of materials and the flow of energy through the environment.34 Climate change and human 

modifications often increase the vulnerability of ecosystems 

and landscapes to damage from extreme events while at the 

same time reducing their natural capacity to modulate the impacts of such events. Salt marshes, reefs, mangrove forests, 

and barrier islands provide an ecosystem service of defending 

coastal ecosystems and infrastructure against storm surges.35

Losses of these natural features – from coastal development, 

erosion, and sea level rise – render coastal ecosystems and infrastructure more vulnerable to catastrophic damage during or 

after extreme events (Ch. 25: Coasts).36 Floodplain wetlands, 

although greatly reduced from their historical extent, provide 

an ecosystem service of absorbing floodwaters and reducing 

the impact of high flows on river-margin lands. In the Northeast, even a small sea level rise (1.6 feet) would dramatically 

increase the numbers of people (47% increase) and property 

loss (73% increase) affected by storm surge in Long Island compared to present day storm surge impacts.37 Extreme weather 

events that produce sudden increases in water flow and the 

materials it carries can decrease the natural capacity of ecosystems to process pollutants, both by reducing the amount of 

time water is in contact with reactive sites and by removing or 

harming the plants and microbes that remove the pollutants.36

Warming and, in some areas, decreased precipitation (along 

with past forest fire suppression practices) have increased the 

risk of fires exceeding historical size, resulting in unprecedented social and economic challenges. Large fires put people living in the wildland-urban interface at risk for health problems 

and property loss. In 2011 alone, more than 8 million acres 

burned in wildfires, causing 15 deaths and property losses 

greater than $1.9 billion.38

Figure 8.2. Hurricanes illustrate the links among precipitation, discharge and nutrient loading to coastal 

waters. Hurricanes bring intense rainfall to coastal regions, and ensuing runoff leads to blooms of algae. 

These blooms contribute to dead zone formation after they die and decompose. Photo above shows 

Pamlico Sound, North Carolina, after Hurricane Floyd. Note light green area off the coast, which is new 

algae growth. The graph on the left shows a steep drop in salinity of ocean water due to the large influx 

of freshwater from rain after a series of hurricanes. Red arrows indicate Hurricanes Dennis, Floyd, and 

Irene, which hit sequentially during the 1999 hurricane season. The graph on the right shows a steep 

rise in the amount of surface chlorophyll after these hurricanes, largely due to increased algae growth. 

(Figure source: (top) NASA SeaWiFS; (bottom) Paerl et al. 200333).

The Aftermath of Hurricanes
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Key Message 3: Plants and Animals

Landscapes and seascapes are changing rapidly, and species, including many iconic species, 

may disappear from regions where they have been prevalent or become extinct, altering some 

regions so much that their mix of plant and animal life will become almost unrecognizable. 

Vegetation model projections suggest that much of the United 

States will experience changes in the composition of species 

characteristic of specific areas. Studies applying different 

models for a range of future climates project biome changes 

for about 5% to 20% of the land area of the U.S. by 2100.4,39

Many major changes, particularly in the western states and 

Alaska, will in part be driven by increases in fire frequency and 

severity. For example, the average time between fires in the 

Yellowstone National Park ecosystem is projected to decrease 

from 100 to 300 years to less than 30 years, potentially 

causing coniferous (pine, spruce, etc.) forests to be replaced 

by woodlands and grasslands.40 Warming has also led to novel 

wildfire occurrence in ecosystems where it has been absent 

in recent history, such as arctic Alaska and the southwestern 

deserts where new fires are fueled by non-native annual 

grasses (Ch. 20: Southwest; Ch. 22: Alaska). Extreme weather 

conditions linked to sea ice decline in 2007 led to the ignition 

of the Anaktuvuk River Fire, which burned more than 380 

square miles of arctic tundra that had not been disturbed by 

fire for more than 3,000 years.41 This one fire (which burned 

deeply into organic peat soils) released enough carbon to the 

atmosphere to offset all of the carbon taken up by the entire 

arctic tundra biome over the past quarter-century.42

In addition to shifts in species assemblages, there will also be 

changes in species distributions. In recent decades, in both land 

and aquatic environments, plants and animals have moved to 

higher elevations at a median rate of 36 feet (0.011 kilometers) 

per decade, and to higher latitudes at a median rate of 10.5 

miles (16.9 kilometers) per decade.43 As the climate continues 

to change, models and long-term studies project even greater 

shifts in species ranges.44 However, many species may not be 

able to keep pace with climate change for several reasons, for 

example because their seeds do not disperse widely or because 

they have limited mobility, thus leading, in some places, to 

local extinctions of both plants and animals. Both range shifts 

and local extinctions will, in many places, lead to large changes 

in the mix of plants and animals present in the local ecosystem, 

resulting in new communities that bear little resemblance to 

those of today.4,8,45,46

Some of the most obvious changes in the landscape are 

occurring at the boundaries between biomes. These include 

shifts in the latitude and elevation of the boreal (northern) 

forest/tundra boundary in Alaska;47 elevation shifts of the 

boreal and subalpine forest/tundra boundary in the Sierra 

Nevada, California;48 an elevation shift of the temperate 

broadleaf/conifer boundary in the Green Mountains, 

Vermont,49 the shift of temperate the shrubland/conifer forest 

boundary in Bandelier National Monument, New Mexico,50 and 

upslope shifts of the temperate mixed forest/conifer boundary 

in Southern California.51 All of these are consistent with recent 

climatic trends and represent visible changes, like tundra 

switching to forest, or conifer forest switching to broadleaf 

forest or even to shrubland.

As temperatures rise and precipitation patterns change, many 

fish species (such as salmon, trout, whitefish, and char) will be 

lost from lower-elevation streams, including a projected loss 

of 47% of habitat for all trout species in the western U.S. by 

2080.8 Similarly, in the oceans, transitions from cold-water fish 

communities to warm-water communities have occurred in 

commercially important harvest areas,52 with new industries 

developing in response to the arrival of new species.53 Also, 

warm surface waters are driving some fish species to deeper 

waters.54,55

Warming is likely to increase the ranges of several invasive 

plant species in the United States,56 increase the probability 

of establishment of invasive plant species in boreal forests 

in south-central Alaska, including the Kenai Peninsula,57 and 

expand the range of the hemlock wooly adelgid, an insect that 

has killed many eastern hemlocks in recent years.58 Invasive 

species costs to the U.S. economy are estimated at $120 

billion per year,59 including substantial impacts on ecosystem 

services. For instance, the yellow star-thistle, a wildland pest 

which is predicted to thrive with increased atmospheric CO2,

60

currently costs California ranchers and farmers $17 million in 

forage and control efforts61 and $75 million in water losses.62

Iconic desert species such as saguaro cactus are damaged or 

killed by fires fueled by non-native grasses, leading to a largescale transformation of desert shrubland into grassland in 

many of the familiar landscapes of the American West.63 Bark 

beetles have infested extensive areas of the western United 

States and Canada, killing stands of temperate and boreal 

conifer forest across areas greater than any other outbreak in 

the last 125 years.64 Climate change has been a major causal 

factor, with higher temperatures allowing more beetles to 

survive winter, complete two life cycles in a season rather than 

one, and to move to higher elevations and latitudes.64,65 Bark 

beetle outbreaks in the Greater Yellowstone Ecosystem are 

occurring in habitats where outbreaks either did not previously 

occur or were limited in scale.66

It is important to realize that climate change is linked to far more 

dramatic changes than simply altering species’ life cycles or 

shifting their ranges. Several species have exhibited population 

declines linked to climate change, with some declines so 
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severe that species are threatened with extinction.67 Perhaps 

the most striking impact of climate change is its effect on 

iconic species such as the polar bear, the ringed seal, and coral 

species (Ch. 22: Alaska; Ch. 24: Oceans). In 2008, the polar bear 

(Ursus maritimus) was listed as a threatened species, with the 

primary cause of its decline attributed to climate change.68 In 

2012, NOAA determined that four subspecies of the ringed 

seal (Phoca hispida) were threatened or endangered, with the 

primary threat being climate change.69 

Key Message 4: Seasonal Patterns

Timing of critical biological events, such as spring bud burst, emergence from overwintering, 

and the start of migrations, has shifted, leading to important impacts on species and habitats.

The effect of climate change on phenology – the pattern of 

seasonal life cycle events in plants and animals, such as timing 

of leaf-out, blooming, hibernation, and migration – has been 

called a “globally coherent fingerprint of climate change 

impacts” on plants and animals.70 Observed long-term trends 

towards shorter, milder winters and earlier spring thaws are 

altering the timing of critical spring events such as bud burst 

and emergence from overwintering. This can cause plants and 

animals to be so out of phase with their natural phenology that 

outbreaks of pests occur, or species cannot find food at the 

time they emerge.

Recent studies have documented an advance in the timing 

of springtime phenological events across species in response 

to increased temperatures.71 Long-term observations of lilac 

flowering indicate that the onset of spring has advanced 

one day earlier per decade across the northern hemisphere 

in response to increased winter and spring temperatures72

and by 1.5 days per decade earlier in the western United 

States.73 Other multi-decadal studies for plant species have 

documented similar trends for early flowering.74,75 In addition, 

plant-pollinator relationships may be disrupted by changes in 

nectar and pollen availability, as the timing of bloom shifts in 

response to temperature and precipitation.76,77

As spring is advancing and fall is being delayed in response 

to regional changes in climate,78 the growing season is 

lengthening. A longer growing season will benefit some crops 

and natural species, but there may be a timing mismatch 

between the microbial activity that makes nutrients available 

in the soil and the readiness of plants to take up those nutrients 

for growth.78,79 Where plant phenology is driven by day length, 

an advance in spring may exacerbate this mismatch, causing 

available nutrients to be leached out of the soil rather than 

absorbed and recycled by plants.80 Longer growing seasons 

also exacerbate human allergies. For example, a longer fall 

allows for bigger ragweed plants that produce more pollen 

later into the fall (see also Ch. 9: Health).81

Changes in the timing of springtime bird migrations are wellrecognized biological responses to warming, and have been 

documented in the western,82 midwestern,83 and eastern 

United States.84,85 Some migratory birds now arrive too late 

for the peak of food resources at breeding grounds because 

temperatures at wintering grounds are changing more slowly 

than at spring breeding grounds.86

In a 34-year study of an Alaskan creek, young pink salmon 

(Oncorhynchus gorbuscha) migrated to the sea increasingly 

earlier over time.87 In Alaska, warmer springs have caused 

earlier onset of plant emergence, and decreased spatial 

variation in growth and availability of forage to breeding 

caribou (Rangifer tarandus).

Key Message 5: Adaptation

Whole system management is often more effective than focusing on one species 

at a time, and can help reduce the harm to wildlife, natural assets, and 

human well-being that climate disruption might cause. 

Adaptation in the context of biodiversity and natural resource 

management is fundamentally about managing change, 

which is an inherent property of natural ecosystems.4,88,89

One strategy – adaptive management, which is a structured 

process of flexible decision-making under uncertainty that 

incorporates learning from management outcomes – has 

received renewed attention as a tool for helping resource 

managers make decisions relevant to whole systems in response 

to climate change.89,90 Other strategies tinclude assessments of 

vulnerability and impacts,91 and scenario planning,92 that can 

be assembled into a general planning process that is flexible 

and iterative. 

Guidance on adaptation planning for conservation has 

proliferated at the federal92,93,94 and state levels,95 and 

often emphasizes cooperation between scientists and 

managers.94,96,97 Ecosystem-based adaptation98,99 uses 

“biodiversity and ecosystem services as part of an overall 

adaptation strategy to help people adapt to the adverse 

effects of climate change.”99 An example is the explicit use of 
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storm-buffering coastal wetlands or mangroves rather than 

built infrastructure like seawalls or levies to protect coastal 

regions (Ch. 25: Coasts).100 An additional example is the use of 

wildlife corridors to connect fragmented wildlife habitat.101

Adaptation strategies to protect biodiversity include: 1) habitat 

manipulation, 2) conserving populations with higher genetic 

diversity or more flexible behaviors or morphologies, 3) replanting with species or ecotypes that are better suited for 

future climates, 4) managed relocation (sometimes referred to 

as assisted migration) to help move species and populations 

from current locations to those areas expected to become 

more suitable in the future, and 5) offsite conservation such as 

seed banking, biobanking, and captive breeding.92,94,96,97,102,103

Additional approaches focus on identifying and protecting 

features that are important for biodiversity and are less 

likely to be altered by climate 

change. The idea is to conserve 

the “stage” (the biophysical 

conditions that contribute to 

high levels of biodiversity) for 

whatever “actors” (species and 

populations) find those areas 

suitable in the future.104

One of the greatest challenges 

for adaptation in the face of 

climate change is the revision 

of management goals in 

fundamental ways. In particular, 

not only will climate change 

make it difficult to achieve 

existing conservation goals, it will 

demand that goals be critically 

examined and potentially altered 

in dramatic ways.102,105 Climate 

changes can also severely 

diminish the effectiveness of 

current strategies and require 

fresh approaches. For example, 

whereas establishing networks 

of nature reserves has been a 

standard approach to protecting 

species, fixed networks of 

reserve do not lend themselves 

to adjustments for climate 

change.105 Finally, migratory 

species and species with 

complex life histories cannot be 

simply addressed by defining 

preferred habitat and making vulnerability assessments. Often 

it could be specific life history stages that are the weak point in 

the species, and it is key to identify those weak links.106

While there is considerable uncertainty about how climate 

change will play out in particular locations, proactive measures 

can be taken to both plan for connectivity96,107 and to identify 

places or habitats that may in the future become valuable 

habitat as a result of climate change and vegetation shifts.108

It is important to note that when the Endangered Species Act 

(ESA) was passed in 1973, climate change was not a known 

threat or factor and was not considered in setting recovery 

goals or critical habitat designations.109 However, agencies are 

actively working to include climate change considerations in 

their ESA implementation activities. 

Figure 8.3. Iterative approaches to conservation planning require input and 

communication among many players to ensure flexibility in response to climate 

change. (Figure source: adapted from the National Wildlife Federation, 2013142).

Adaptation Planning and Implementation Framework
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Case study of the 2011 las conchas, new mexico fire

In the midst of severe drought in the summer of 2011, Arizona and New Mexico suffered the largest wildfires in their 

recorded history, affecting more than 694,000 acres. Some rare threatened and endangered species, like the Jemez 

salamander, were damaged by this unusually severe fire.110 Fires are often part of the natural disturbance regime, but 

if drought, poor management, and high temperatures combine, a fire can be so severe and widespread that species 

are damaged that otherwise might even be considered to be fire tolerant (such as spotted owls). Following the fires, 

heavy rainstorms led to major flooding and erosion, including at least ten debris flows. Popular recreation areas were 

evacuated and floods damaged the newly renovated, multi-million dollar U.S. Park Service Visitor Center at Bandelier 

National Monument. Sediment and ash eroded by the floods were washed downstream into the Rio Grande, which supplies 50% of the drinking water for Albuquerque, the largest city in New Mexico. Water withdrawals by the city from 

the Rio Grande were stopped entirely for a week and reduced for several months due to the increased cost of treatment. 

These fires provide an example of how forest ecosystems, biodiversity, and ecosystem services are affected by the impacts of climate change, other environmental stresses, and past management practices. Higher temperatures, reduced 

snowpack, and earlier onset of springtime are leading to increases in wildfire in the western United States,111 while 

extreme droughts are becoming more frequent.112 In addition, climate change is affecting naturally occurring bark 

beetles: warmer winter conditions allow these pests to breed more frequently and successfully.113,114 The dead trees 

left behind by bark beetles may make crown fires more likely, at least until needles fall from killed trees.114,115 Forest 

management practices also have made the forests more vulnerable to catastrophic fires. In New Mexico, even-aged, 

second-growth forests were hit hardest because they are much denser than naturally occurring forest and consequently 

consume more water from the soil and increase the availability of dry above-ground fuel.

Figure 8.4. Map of selected 

obser ved and projected 

biological responses to climate 

change across the United 

States. Case studies listed 

below correspond to observed 

responses (black icons on 

map) and projected responses 

(white icons on map, bold 

italicized statements). In 

general, because future 

climatic changes are projected 

to exceed those experienced 

in the recent past, projected 

biological impacts tend to be 

of greater magnitude than 

recent observed changes. 

Because the observations and 

projections presented here 

are not paired (that is, they 

are not for the same species 

or systems), that general 

difference is not illustrated. 

(Figure source: Staudinger et 

al., 20124).

Biological responses to climate change

Continued
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Biological responses to climate change (continued)

Continued

1. Mussel and barnacle beds have declined or disappeared along parts of the Northwest coast due to higher temperatures and drier conditions that have compressed habitable intertidal space.116

2. Northern flickers arrived at breeding sites earlier in the Northwest in response to temperature changes along migration routes, and egg laying advanced by 1.15 days for every degree increase in temperature, demonstrating that 

this species has the capacity to adjust their phenology in response to climate change.117

3. Conifers in many western forests have experienced mortality rates of up to 87% from warming-induced changes in 

the prevalence of pests and pathogens and stress from drought.118

4. Butterflies that have adapted to specific oak species have not been able to colonize new tree species when climate 

change-induced tree migration changes local forest types, potentially hindering adaptation.119

5. In response to climate-related habitat change, many small mammal species have altered their elevation ranges, 

with lower-elevation species expanding their ranges and higher-elevation species contracting their ranges.120

6. Northern spotted owl populations in Arizona and New Mexico are projected to decline during the next century and 

are at high risk for extinction due to hotter, drier conditions, while the southern California population is not projected to be sensitive to future climatic changes.121

7. Quaking aspen-dominated systems are experiencing declines in the western U.S. after stress due to climateinduced drought conditions during the last decade.122

8. Warmer and drier conditions during the early growing season in high-elevation habitats in Colorado are disrupting 

the timing of various flowering patterns, with potential impacts on many important plant-pollinator relationships.77

9. Population fragmentation of wolverines in the northern Cascades and Rocky Mountains is expected to increase as 

spring snow cover retreats over the coming century.123

10. Cutthroat trout populations in the western U.S. are projected to decline by up to 58%, and total trout habitat in the 

same region is projected to decline by 47%, due to increasing temperatures, seasonal shifts in precipitation, and 

negative interactions with non-native species.8

11. Comparisons of historical and recent first flowering dates for 178 plant species from North Dakota showed significant shifts occurred in over 40% of species examined, with the greatest changes observed during the two warmest 

years of the study.75

12. Variation in the timing and magnitude of precipitation due to climate change was found to decrease the nutritional 

quality of grasses, and consequently reduce weight gain of bison in the Konza Prairie in Kansas and the Tallgrass 

Prairie Preserve in Oklahoma.124 Results provide insight into how climate change will affect grazer population dynamics in the future. 

13. (a and b) Climatic fluctuations were found to influence mate selection and increase the probability of infidelity in 

birds that are normally socially monogamous, increasing the gene exchange and the likelihood of offspring survival.125

14. Migratory birds monitored in Minnesota over a 40-year period showed significantly earlier arrival dates, particularly 

in short-distance migrants, indicating that some species are capable of responding to increasing winter temperatures better than others.126

15. Up to 50% turnover in amphibian species is projected in the eastern U.S. by 2100, including the northern leopard 

frog, which is projected to experience poleward and elevational range shifts in response to climatic changes in the 

latter quarter of the century.127

16. Studies of black ratsnake (Elaphe obsoleta) populations at different latitudes in Canada, Illinois, and Texas suggest 

that snake populations, particularly in the northern part of their range, could benefit from rising temperatures if 

there are no negative impacts on their habitat and prey.128

17. Warming-induced hybridization was detected between southern and northern flying squirrels in the Great Lakes 

region of Ontario, Canada, and in Pennsylvania after a series of warm winters created more overlap in their habitat 

range, potentially acting to increase population persistence under climate change.129
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Biological responses to climate change (continued) © Stephen Dalton/Minden Pictures/Corbis © Jenny E. Ross/Corbis © Matthias Breiter/Minden Pictures/Corbis

© Jeffrey Rotman/Corbis

18. Some warm-water fishes have moved northwards, and some tropical and subtropical fishes in the northern Gulf of 

Mexico have increased in temperate ocean habitat.130 Similar shifts and invasions have been documented in Long 

Island Sound and Narragansett Bay in the Atlantic.131

19. Global marine mammal diversity is projected to decline at lower latitudes and increase at higher latitudes due to 

changes in temperatures and sea ice, with complete loss of optimal habitat for as many as 11 species by midcentury; seal populations living in tropical and temperate waters are particularly at risk to future declines.132

20. Higher nighttime temperatures and cumulative seasonal rainfalls were correlated with changes in the arrival times 

of amphibians to wetland breeding sites in South Carolina over a 30-year time period (1978-2008).133

21. Seedling survival of nearly 20 resident and migrant tree species decreased during years of lower rainfall in the 

Southern Appalachians and the Piedmont areas, indicating that reductions in native species and limited replacement by invading species were likely under climate change.134

22. Widespread declines in body size of resident and migrant birds at a bird-banding station in western Pennsylvania 

were documented over a 40-year period; body sizes of breeding adults were negatively correlated with mean regional temperatures from the preceding year.85

23. Over the last 130 years (1880-2010), native bees have advanced their spring arrival in the northeastern U.S. by an 

average of 10 days, primarily due to increased warming. Plants have also showed a trend of earlier blooming, thus 

helping preserve the synchrony in timing between plants and pollinators.135

24. In the Northwest Atlantic, 24 out of 36 commercially exploited fish stocks showed significant range (latitudinal and 

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25. Increases in maximum, and decreases in the annual variability of, sea surface temperatures in the North Atlantic 

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26. Changes in female polar bear reproductive success (decreased litter mass and numbers of yearlings) along the 

north Alaska coast have been linked to changes in body size and/or body condition following years with lower availability of optimal sea ice habitat.137

27. Water temperature data and observations of migration behaviors over a 34-year time period showed that adult pink 

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28. Warmer springs in Alaska have caused earlier onset of plant emergence, and decreased spatial variation in growth 

and availability of forage to breeding caribou. This ultimately reduced calving success in caribou populations.138

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heights, the duration of turbidity, and the amount of re-suspended sediment in the water; consequently, this will 

create potentially stressful conditions for coral reef communities.140
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216 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Process for Developing Key Messages

The key messages and supporting chapter text summarize extensive evidence documented in the Ecosystems Technical Input Report, Impacts of Climate Change on Biodiversity, Ecosystems, and 

Ecosystem Services: Technical Input to the 2013 National Climate 

Assessment.

4

 This foundational report evolved from a technical 

workshop held at the Gordon and Betty Moore Foundation in Palo 

Alto, CA, in January 2012 and attended by approximately 65 scientists. Technical inputs (127) on a wide range of topics related to 

ecosystems were also received and reviewed as part of the Federal 

Register Notice solicitation for public input. 

Key message #1 Traceable Account

Climate change impacts on ecosystems reduce 

their ability to improve water quality and regulate 

water flows.

Description of evidence base

The author team digested the contents of more than 125 technical 

input reports on a wide array of topics to arrive at this key message. The foundational Technical Input Report4

 was the primary 

source used. 

Studies have shown that increasing precipitation is already resulting in declining water quality in many regions of the country, particularly by increasing nitrogen loading.10,11,12,13,14 This is because 

the increases in flow can pick up and carry greater loads of nutrients like nitrogen to rivers.11,12,13,14

One model for the Mississippi River Basin, based on a doubling of 

CO2, projects that increasing discharge and nitrogen loading will 

lead to larger algal blooms in the Gulf of Mexico and a larger dead 

zone.28 The Gulf of Mexico is the recipient system for the Mississippi Basin, receiving all of the nitrogen that is carried downriver 

but not removed by river processes, wetlands, or other ecosystems.

Several models project that declining streamflow, due to the combined effects of climate change and water withdrawals, will cause 

local extinctions of fish and other aquatic organisms,7 particularly 

trout in the interior western U.S. (composite of 10 models, A1B 

8: ECOSYSTEMS, BIODIVERSITY, AND ECOSYSTEM SERVICES

scenario).8 The trout study8 is one of the few studies of impacts on 

fish that uses an emissions scenario and a combination of climate 

models. The researchers studied four different trout species. Although there were variations among species, their overall conclusion was robust across species for the composite model.

Water quality can also be negatively affected by increasing temperatures. There is widespread evidence that warmer lakes can 

promote the growth of harmful algal blooms, which produce toxins.31

New information and remaining uncertainties

Recent research has improved understanding of the relative importance of the effects of climate and human actions (for example, 

fertilization) on nitrogen losses from watersheds,10,12 and how the 

interactions between climate and human actions (for example, water withdrawals) will affect fish populations in the west.7,8

 However, 

few studies have projected the impacts of future climate change 

on water quality. Given the tight link between river discharge and 

pollutants, only areas of the U.S. that are projected to see increases in precipitation will see increases in pollutant transport 

to rivers. It is also important to note that pollutant loading – for 

example, nitrogen fertilizer use – is often more important as a 

driver of water pollution than climate.10,12

Assessment of confidence based on evidence 

Given the evidence base and uncertainties, there is high confidence that climate change impacts on ecosystems reduce their 

ability to improve water quality and regulate water flows.

It is well established that precipitation and associated river discharge are major drivers of water pollution in the form of excess 

nutrients, sediment, and dissolved organic carbon (DOC) transport 

into rivers. Increases in precipitation in many regions of the country are therefore contributing to declines in water quality in those 

areas. However, those areas of the country that will see reduced 

precipitation may experience water-quality improvement; thus, 

any lack of agreement on future water-quality impacts of climate 

change may be due to locational differences.

SUPPLEMENTAL MATERIAL

TRACEABLE ACCOUNTS
217 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

8: ECOSYSTEMS, BIODIVERSITY, AND ECOSYSTEM SERVICES

TRACEABLE ACCOUNTS

Key message #2 Traceable Account

Climate change, combined with other stressors, 

is overwhelming the capacity of ecosystems to 

buffer the impacts from extreme events like fires, 

floods, and storms.

Description of evidence base

The author team digested the contents of more than 125 technical 

input reports on a wide array of topics to arrive at this key message. The foundational Technical Input Report4

 was the primary 

source used. 

Fires: Climate change has increased the potential for extremely 

large fires with novel social, economic, and environmental impacts. 

In 2011, more than 8 million acres burned, with significant human mortality and property damage ($1.9 billion).38 Warming and 

decreased precipitation have made fire-prone ecosystems more 

vulnerable to “mega-fires” – large fires that are unprecedented 

in their social, economic, and environmental impacts. Large fires 

put people living in the urban-wildland interface at risk for health 

problems and property loss.

Floods: Natural ecosystems such as salt marshes, reefs, mangrove forests, and barrier islands defend coastal ecosystems and 

infrastructure against flooding due to storm surges. The loss of 

these natural features due to coastal development, erosion, and 

sea level rise render coastal ecosystems and infrastructure more 

vulnerable to catastrophic damage during or after extreme events 

(see Ch. 25: Coasts).36 Floodplain wetlands, which are also vulnerable to loss by inundation, absorb floodwaters and reduce the 

impact of high flows on river-margin lands. In the Northeast, a sea 

level rise of 1.6 feet (within the range of 1 to 4 feet projected for 

2100; Ch. 2: Our Changing Climate, Key Message 9) will dramatically increase impacts of storm surge on people (47% increase) 

and property loss (73% increase) in Long Island.37

Storms: Natural ecosystems have a capacity to buffer extreme 

weather events that produce sudden increases in water flow and 

materials. These events reduce the amount of time water is in contact with sites that support the plants and microbes that remove 

pollutants (Chapter 25: Coasts).36

New information and remaining uncertainties

A new analytical framework was recently developed to generate insights into the interactions among the initial state of ecosystems, 

the type and magnitude of disturbance, and effects of disturbance.34 Progress in understanding these relationships is critical 

for predicting how human activities and climate change, including 

extreme events like droughts, floods, and storms, will interact to 

affect ecosystems.

Uncertainties: The ability of ecosystems to buffer extreme events 

is extremely difficult to assess and quantify, as it requires understanding of complex ecosystem responses to very rare events. 

However, it is clear that the loss of this buffering ecosystem service is having important effects on coastal and fire-prone ecosystems across the United States. 

Assessment of confidence based on evidence 

Given the evidence base and uncertainties, there is high confidence that climate change, combined with other stressors, is overwhelming the capacity of ecosystems to buffer the impacts from 

extreme events like droughts, floods, and storms.

Ecosystem responses to climate change will vary regionally. For 

example, whether salt marshes and mangroves will be able to accrue sediment at rates sufficient to keep ahead of sea level rise 

and maintain their protective function will vary by region.

Climate has been the dominant factor controlling burned area 

during the 20th century, even during periods of fire suppression 

by forest management,40,111 and the area burned annually has increased steadily over the last 20 years concurrent with warming 

and/or drying climate. Warming and decreased precipitation have 

also made fire-prone ecosystems more vulnerable to “mega-fires” 

– large fires that are unprecedented in their social, economic, and 

environmental impacts. Large fires put people living in the urbanwildland interface at risk for health problems and property loss. 

In 2011 alone, 8.3 million acres burned in wildfires, causing 15 

deaths and property losses greater than $1.9 billion.38

Confidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent 

results, well documented and 

accepted methods, etc.), high 

consensus

High

Moderate evidence (several sources, some consistency, methods 

vary and/or documentation limited, 

etc.), medium consensus

Medium

Suggestive evidence (a few 

sources, limited consistency, models incomplete, methods emerging, 

etc.), competing schools of thought

Low

Inconclusive evidence (limited 

sources, extrapolations, inconsistent findings, poor documentation 

and/or methods not tested, etc.), 

disagreement or lack of opinions 

among experts
218 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

8: ECOSYSTEMS, BIODIVERSITY, AND ECOSYSTEM SERVICES

TRACEABLE ACCOUNTS

Key message #3 Traceable Account

Landscapes and seascapes are changing rapidly, and species, including many iconic species, 

may disappear from regions where they have been 

prevalent or become extinct, altering some regions 

so much that their mix of plant and animal life will 

become almost unrecognizable. 

Description of evidence base

The analysis for the Technical Input Report applied a range of 

future climate scenarios and projected biome changes across 5% 

to about 20% of the land area in the U.S. by 2100.4 Other analyses support these projections.39 Studies predict that wildfire will 

be a major driver of change in some areas, including Yellowstone 

National Park40 and the Arctic.41 These biome shifts will be associated with changes in species distributions.43

Evidence indicates that the most obvious changes will occur at 

the boundaries between ecosystems.47,48,49,51 Plants and animals 

are already moving to higher elevations and latitudes in response 

to climate change,43 with models projecting greater range shifts8,46

and local extinctions in the future, leading to new plant and animal 

communities that may be unrecognizable in some regions.4,45,46

One study on fish8 used global climate models (GCMs) simulating 

conditions in the 2040s and 2080s under the A1B emissions 

scenario, with the choice of models reflecting predictions of high 

and low climate warming as well as an ensemble of ten models. 

Their models additionally accounted for biotic interactions. In a 

second study, a 30-year baseline (1971-2000) and output from 

two GCMs under the A2 scenario (continued increases in global 

emissions) were used to develop climate variables that effectively 

predict present and future species ranges.46 Empirical data from 

the Sonoran Desert (n=39 plots) were used to evaluate species 

responses to past climate variability.

Iconic species: Wildfire is expected to damage and kill iconic desert species, including saguaro cactus.63 Bark beetle outbreaks, 

which have been exacerbated by climate change, are damaging 

extensive areas of temperate and boreal conifer forests that are 

characteristic of the western United States.64

New information and remaining uncertainties

In addition to the Technical Input Report, more than 20 new studies of observed and predicted effects of climate change on biomes 

and species distribution were incorporated in the assessment.

While changes in ecosystem structure and biodiversity, including 

the distribution of iconic species, are occurring and are highly 

likely to continue, the impact of these changes on ecosystem services is unclear, that is, there is uncertainty about the impact that 

loss of familiar landscapes will have on people.

Assessment of confidence based on evidence 

Based on the evidence base and uncertainties, confidence is high 

that familiar landscapes are changing so rapidly that iconic species may disappear from regions where they have been prevalent, 

altering some regions so much that their mix of plant and animal 

life will become almost unrecognizable. Many changes in species 

distribution have already occurred and will inevitably continue, 

resulting in the loss of familiar landscapes and the production of 

novel species assemblages. 

Key message #4 Traceable Account

Timing of critical biological events, such as spring 

bud burst, emergence from overwintering, and the 

start of migrations, has shifted, leading to important impacts on species and habitats.

Description of evidence base

The key message and supporting text summarizes extensive evidence documented in the Ecosystems Technical Input, Phenology 

as a bio-indicator of climate change impacts on people and ecosystems: Towards an integrated national assessment approach.

71

An additional 127 input reports, on a wide range of topics related 

to ecosystems, were also received and reviewed as part of the 

Federal Register Notice solicitation for public input. 

Many studies have documented an advance in springtime phenological events of species in response to climate warming. For 

example, long-term observations of lilac flowering indicate that the 

onset of spring has advanced one day earlier per decade across 

the northern hemisphere in response to increased winter and 

spring temperatures, and by 1.5 days per decade earlier in the 

western United States.72,73 Other multi-decadal studies for plant 

species have documented similar trends for early flowering.74,75

Evidence suggests that insect emergence from overwintering may 

become out of sync with pollen sources,77 and that the beginning 

of bird and fish migrations are shifting.82,83,84,85,86,87

New information and remaining uncertainties

In addition to the Ecosystems Technical Input71 many new studies have been conducted since the previous National Climate Assessment,141 contributing to our understanding of the impacts of 

climate change on phenological events. Many studies, in many 

areas, have shown significant changes in phenology, including 

spring bud burst, emergence from overwintering, and migration 

shifts.

A key uncertainty is “phase effects” where organisms are so out of 

phase with their natural phenology that outbreaks of pests occur, 

species emerge and cannot find food, or pollination is disrupted. 

This will vary with specific species and is therefore very difficult 

to predict.70
219 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

8: ECOSYSTEMS, BIODIVERSITY, AND ECOSYSTEM SERVICES

TRACEABLE ACCOUNTS

Assessment of confidence based on evidence 

Given the evidence base and uncertainties, there is very high confidence that the timing of critical events, such as spring bud burst, 

emergence from overwintering, and the start of migrations, has 

shifted, leading to important impacts on species and habitats. 

Key message #5 Traceable Account

Whole system management is often more effective than focusing on one species at a time, and 

can help reduce the harm to wildlife, natural assets, 

and human well-being that climate disruption might 

cause. 

Description of evidence base

Adaptation planning for conservation at federal92,93,94 and state 

levels,95 is focused on cooperation between scientists and managers.34,94,96,97 Development of ecosystem-based whole system management98 utilizes concepts about “biodiversity and ecosystem 

services to help people adapt to climate change.”99 An example 

is the use of coastal wetlands or mangroves rather than built infrastructure like seawalls or levees to protect coastal regions from 

storms (Chapter 25: Coasts).100

New information and remaining uncertainties

Adaptation strategies to protect biodiversity include: 1) habitat 

manipulations, 2) conserving populations with higher genetic diversity or more plastic behaviors or morphologies, 3) changing 

seed sources for re-planting to introduce species or ecotypes 

that are better suited for future climates, 4) managed relocation 

(sometimes referred to as assisted migration) to help move species 

and populations from current locations to those areas expected to 

become more suitable in the future, and 5) ex-situ conservation 

such as seed banking and captive breeding.92,94,96,97,102 Alternative 

approaches focus on identifying and protecting features that are 

important for biodiversity and are projected to be less altered by 

climate change. The idea is to conserve the physical conditions 

that contribute to high levels of biodiversity so that species and 

populations can find suitable areas in the future.104

Assessment of confidence based on evidence 

Given the evidence and remaining uncertainties, there is very high 

confidence that ecosystem-based management approaches are increasingly prevalent, and provide options for reducing the harm to 

biodiversity, ecosystems, and the services they provide to society. 

The effectiveness of these actions is much less certain, however.
220

Climate Change Impacts in the United States

CHAPTER 9

HUMAN HEALTH

INFORMATION DRAWN FROM THIS CHAPTER IS INCLUDED IN THE HIGHLIGHTS REPORT AND IS IDENTIFIED BY THIS ICON

Recommended Citation for Chapter

Luber, G., K. Knowlton, J. Balbus, H. Frumkin, M. Hayden, J. Hess, M. McGeehin, N. Sheats, L. Backer, C. B. Beard, K. 

L. Ebi, E. Maibach, R. S. Ostfeld, C. Wiedinmyer, E. Zielinski-Gutiérrez, and L. Ziska, 2014: Ch. 9: Human Health. Climate 

Change Impacts in the United States: The Third National Climate Assessment, J. M. Melillo, Terese (T.C.) Richmond, and G. 

W. Yohe, Eds., U.S. Global Change Research Program, 220-256. doi:10.7930/J0PN93H5.

On the Web: http://nca2014.globalchange.gov/report/sectors/human-health

Convening Lead Authors

George Luber, Centers for Disease Control and Prevention

Kim Knowlton, Natural Resources Defense Council and Mailman School of Public Health, Columbia University

Lead Authors

John Balbus, National Institutes of Health

Howard Frumkin, University of Washington

Mary Hayden, National Center for Atmospheric Research

Jeremy Hess, Emory University

Michael McGeehin, RTI International

Nicky Sheats, Thomas Edison State College

Contributing Authors

Lorraine Backer, Centers for Disease Control and Prevention

C. Ben Beard, Centers for Disease Control and Prevention

Kristie L. Ebi, ClimAdapt, LLC 

Edward Maibach, George Mason University

Richard S. Ostfeld, Cary Institute of Ecosystem Studies

Christine Wiedinmyer, National Center for Atmospheric Research

Emily Zielinski-Gutiérrez, Centers for Disease Control and Prevention

Lewis Ziska, United States Department of Agriculture
221 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Key Messages

1. Climate change threatens human health and well-being in many ways, including impacts from 

increased extreme weather events, wildfire, decreased air quality, threats to mental health, and 

illnesses transmitted by food, water, and disease-carriers such as mosquitoes and ticks. Some of 

these health impacts are already underway in the United States.

2. Climate change will, absent other changes, amplify some of the existing health threats the nation 

now faces. Certain people and communities are especially vulnerable, including children, the 

elderly, the sick, the poor, and some communities of color.

3. Public health actions, especially preparedness and prevention, can do much to protect people 

from some of the impacts of climate change. Early action provides the largest health benefits. As 

threats increase, our ability to adapt to future changes may be limited.

4. Responding to climate change provides opportunities to improve human health and well-being 

across many sectors, including energy, agriculture, and transportation. Many of these strategies 

offer a variety of benefits, protecting people while combating climate change and providing other 

societal benefits. 

9 HUMAN HEALTH

Climate change, together with other natural and human-made 

health stressors, influences human health and disease in numerous ways. Some existing health threats will intensify and 

new health threats will emerge. Not everyone is equally at risk. 

Important considerations include age, economic resources, 

and location. Preventive and adaptive actions, such as setting 

up extreme weather early warning systems and improving water infrastructure, can reduce the severity of these impacts, 

but there are limits to the effectiveness of such actions in the 

face of some projected climate change threats.

Climate change presents a global public health problem, with 

serious health impacts predicted to manifest in varying ways 

in different parts of the world. Public health in the U.S. can 

be affected by disruptions of physical, biological, and ecological systems, including disturbances originating in the U.S. 

and elsewhere. Health effects of these disruptions include 

increased respiratory and cardiovascular disease, injuries and 

premature deaths related to extreme weather events, changes 

in the prevalence and geographical distribution of food- and 

waterborne illnesses and other infectious diseases, and threats 

to mental health. 

Key weather and climate drivers of health impacts include 

increasingly frequent, intense, and longer-lasting extreme 

heat, which worsens drought, wildfire, and air pollution risks; 

increasingly frequent extreme precipitation, intense storms, 

and changes in precipitation patterns that lead to drought and 

ecosystem changes (Ch. 2: Our Changing Climate); and rising 

sea levels that intensify coastal flooding and storm surge (Ch. 

25: Coasts). Key drivers of vulnerability include the attributes 

of certain groups (age, socioeconomic status, race, current 

level of health – see Ch. 12: Indigenous Peoples for examples 

of health impacts on vulnerable populations) and of place 

(floodplains, coastal zones, and urban areas), as well as the resilience of critical public health infrastructure. Multi-stressor 

situations, such as impacts on vulnerable populations following 

natural disasters that also damage the social and physical infrastructure necessary for resilience and emergency response, 

are particularly important to consider when preparing for the 

impacts of climate change on human health.

 ©Jose Luis Pelaez, Inc./Blend Images/Corbis
222 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

9: HUMAN HEALTH

Key Message 1: Wide-ranging Health Impacts

Climate change threatens human health and well-being in many ways, including impacts from 

increased extreme weather events, wildfire, decreased air quality, threats to mental health, 

and illnesses transmitted by food, water, and disease-carriers such as mosquitoes and ticks. 

Some of these health impacts are already underway in the United States. 

Air Pollution

Climate change is projected to harm human health by increasing ground-level ozone and/or particulate matter air pollution 

in some locations. Ground-level ozone (a key component of 

smog) is associated with many health problems, such as diminished lung function, increased hospital admissions and 

emergency room visits for asthma, and increases in premature 

deaths.1,2,3 Factors that affect ozone formation include heat, 

concentrations of precursor chemicals, and methane emissions, while particulate matter concentrations are affected by 

wildfire emissions and air stagnation episodes, among other 

factors.4,5 By increasing these different factors, climate change 

is projected to lead to increased concentration of ozone and 

particulate matter in some regions.6,7,8,9 Increases in global 

temperatures could cause associated increases in premature 

deaths related to worsened ozone and particle pollution. Estimates made assuming no change in regulatory controls or 

population characteristics have ranged from 1,000 to 4,300 

additional premature deaths nationally per year by 2050 from 

combined ozone and particle health effects.10,11 There is less 

certainty in the responses of airborne particles to climate 

change than there is about the response of ozone. Health-related costs of the current effects of ozone air pollution exceeding national standards have been estimated at $6.5 billion (in 

2008 U.S. dollars) nationwide, based on a U.S. assessment of 

health impacts from ozone levels during 2000 to 2002.12,13

Allergens 

Climate change, resulting in more frost-free days 

and warmer seasonal air temperatures, can contribute to shifts in flowering time and pollen initiation from allergenic plant species, and increased 

CO2 by itself can elevate production of plant-based 

allergens.14,15,16,17,18,19 Higher pollen concentrations 

and longer pollen seasons can increase allergic 

sensitizations and asthma episodes,20,21,22 and 

diminish productive work and school days.19,22,23

Simultaneous exposure to toxic air pollutants can 

worsen allergic responses.24,25,26 Extreme rainfall 

and rising temperatures can also foster indoor air 

quality problems, including the growth of indoor 

fungi and molds, with increases in respiratory and 

asthma-related conditions.27 Asthma prevalence 

(the percentage of people who have ever been 

diagnosed with asthma and still have asthma) 

increased nationwide from 7.3% in 2001 to 8.4% 

in 2010. Asthma visits in primary care settings, 

emergency room visits, and hospitalizations were 

all stable from 2001 to 2009, and asthma death 

rates per 1,000 persons with asthma declined from 

2001 to 2009.28 To the extent that increased pollen 

exposures occur, patients and their physicians will 

face increased challenges in maintaining adequate 

asthma control. 

Figure 9.1. Projected increases in temperature, changes in wind patterns, and 

ecosystem changes will all affect future ground-level ozone concentrations. 

Climate projections using an increasing emissions scenario (A2) suggest 

that ozone concentrations in the New York metropolitan region will increase 

because of future climate change. This figure shows the estimated increase 

in ozone-related emergency room visits for children in New York in the 2020s 

(compared to the mid-1990s) resulting from climate change related increases 

in ozone concentrations. The results from this modeling exercise are shown 

as a percent change in visits specifically attributed to ozone exposure. For 

example, the 10.2% increase in Suffolk County represents five additional 

emergency room visits that could be attributed to increased ozone exposure 

over the baseline of 46 ozone-related visits from the mid-1990s. In 2010, an 

estimated 25.7 million Americans had asthma, which has become a problem 

in every state. (Figure source: Sheffield et al. 201114). 

Climate Change Projected to Worsen Asthma

©National Geographic Society
223 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

9: HUMAN HEALTH

Wildfires

Climate change is currently increasing the vulnerability of many forests 

to wildfire. Climate change is projected to increase the frequency of 

wildfire in certain regions of the United States (Ch. 7: Forests).17,29 Long 

periods of record high temperatures are associated with droughts 

that contribute to dry conditions and drive wildfires in some areas.30

Wildfire smoke contains particulate matter, carbon monoxide, nitrogen oxides, and various volatile organic compounds (which are ozone 

precursors)31 and can significantly reduce air quality, both locally and 

in areas downwind of fires.32,33 Smoke exposure increases respiratory 

and cardiovascular hospitalizations, emergency department visits, and 

medication dispensations for asthma, bronchitis, chest pain, chronic 

obstructive pulmonary disease (commonly known by its acronym, 

COPD), respiratory infections, and medical visits for lung illnesses.32,34,35

It has been associated with hundreds of thousands of deaths annually, in an assessment of the global health risks from landscape fire 

smoke.32,34,36,37 Future climate change is projected to increase wildfire 

risks and associated emissions, with harmful impacts on health.17,38,39,40

Figure 9.2. Ragweed pollen season length has increased in central North 

America between 1995 and 2011 by as much as 11 to 27 days in parts 

of the U.S. and Canada in response to rising temperatures. Increases in 

the length of this allergenic pollen season are correlated with increases 

in the number of days before the first frost. As shown in the figure, the 

largest increases have been observed in northern cities. (Data updated 

from Ziska et al. 201119; Photo credit: Lewis Ziska, USDA).

Ragweed Pollen Season Lengthens

Figure 9.3. Wildfires, which are projected to increase in 

some regions due to climate change, have health impacts 

that can extend hundreds of miles. Shown here, forest 

fires in Quebec, Canada, during July 2002 (red circles) 

resulted in up to a 30-fold increase in airborne fine particle 

concentrations in Baltimore, Maryland, a city nearly a 

thousand miles downwind. These fine particles, which are 

extremely harmful to human health, not only affect outdoor 

air quality, but also penetrate indoors, increasing the longdistance effects of fires on health.41 An average of 6.4 

million acres burned in U.S. wildfires each year between 

2000 and 2010, with 9.5 and 9.1 million acres burned in 

2006 and 2012, respectively.42 Total global deaths from 

the effects of landscape fire smoke have been estimated 

at 260,000 to 600,000 annually between the years 1997 

and 2006.37 (Figure source: Moderate Resolution Imaging 

Spectroradiometer (MODIS) instrument on the Terra 

satellite, Land Rapid Response Team, NASA/GSFC).

Wildfire Smoke has 

Widespread Health Effects

©Frans Lanting/Corbis
224 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

9: HUMAN HEALTH

Temperature Extremes

Extreme heat events have long threatened public health in 

the United States.43,44,45 Many cities, including St. Louis, Philadelphia, Chicago, and Cincinnati, have suffered dramatic increases in death rates during heat waves. Deaths result from 

heat stroke and related conditions,44,45,46 but also from cardiovascular disease, respiratory disease, and cerebrovascular 

disease.47,48 Heat waves are also associated with increased 

hospital admissions for cardiovascular, kidney, and respiratory disorders.48,49,50 Extreme summer heat is increasing in the 

United States (Ch. 2: Our Changing Climate, Key Message 7),51

and climate projections indicate that extreme heat events will 

be more frequent and intense in coming decades (Ch. 2: Our 

Changing Climate, Key Message 7).2,52,53,54 

Some of the risks of heat-related sickness and death have diminished in recent decades, possibly due to better forecasting, 

heat-health early warning systems, and/or increased access to 

air conditioning for the U.S. population.55 However, extreme 

heat events remain a cause of preventable death nationwide. 

Urban heat islands, combined with an aging population and 

increased urbanization, are projected to increase the vulnerability of urban populations to heat-related health impacts in 

the future (Ch. 11: Urban).56,57,58

Milder winters resulting from a warming climate can reduce 

illness, injuries, and deaths associated with cold and snow. 

Vulnerability to winter weather depends on many non-climate 

factors, including housing, age, and baseline health.59 While 

deaths and injuries related to extreme cold events are projected to decline due to climate change, these reductions are 

not expected to compensate for the increase in heat-related 

deaths.60,61

Precipitation Extremes: Heavy Rainfall, Flooding, and Droughts

The frequency of heavy precipitation events has already increased for the nation as a whole, and is projected to increase 

in all U.S. regions (Ch. 2: Our Changing Climate).54,62 Increases 

in both extreme precipitation and total precipitation have 

contributed to increases in severe flooding events in certain 

regions (see Ch. 2: Our Changing Climate, Figure 2.21). Floods 

are the second deadliest of all weather-related hazards in the 

United States, accounting for approximately 98 deaths per 

year,63 most due to drowning.64 Flash floods (see Ch. 3: Water, 

“Flood Factors and Flood Types”) and flooding associated with 

tropical storms result in the highest number of deaths.63

In addition to the immediate health hazards associated with 

extreme precipitation events when flooding occurs, other hazards can often appear once a storm event has passed. Elevated 

waterborne disease outbreaks have been reported in the weeks 

Figure 9.4. The maps show projected increases in the average temperature on the hottest days by late this century (2081-2100) 

relative to 1986-2005 under a scenario that assumes a rapid reduction in heat-trapping gases (RCP 2.6) and a scenario that assumes 

continued increases in these gases (RCP 8.5). The hottest days are those so hot they occur only once in 20 years. Across most of 

the continental United States, those days will be about 10oF to 15oF hotter in the future under the higher emissions scenario. (Figure 

source: NOAA NCDC / CICS-NC). 

Projected Temperature Change of Hottest Days
225 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

9: HUMAN HEALTH

following heavy rainfall,65 although other variables may affect 

these associations.66 Water intrusion into buildings can result 

in mold contamination that manifests later, leading to indoor 

air quality problems. Buildings damaged during hurricanes are 

especially susceptible to water intrusion. Populations living in 

damp indoor environments experience increased prevalence 

of asthma and other upper respiratory tract symptoms, such as 

coughing and wheezing67 as well as lower respiratory tract infections such as pneumonia, Respiratory Syncytial Virus (RSV), 

and RSV pneumonia (see Figure 9.7).68 

At the opposite end of precipitation extremes, drought also 

poses risks to public health and safety.69 Drought conditions 

may increase the environmental exposure to a broad set of 

health hazards including wildfires, dust storms, extreme heat 

events, flash flooding, degraded water quality, and reduced 

water quantity. Dust storms associated with drought conditions contribute to degraded air quality due to particulates 

and have been associated with increased incidence of Coccidioidomycosis (Valley fever), a fungal pathogen, in Arizona and 

California.70

Disease Carried by Vectors

Climate is one of the factors that influence the distribution of 

diseases borne by vectors (such as fleas, ticks, and mosquitoes, 

which spread pathogens that cause illness).71,72,73,74,75,76,77,78 The 

geographic and seasonal distribution of vector populations, 

and the diseases they can carry, depend not only on climate 

but also on land use, socioeconomic and cultural factors, pest 

control, access to health care, and human responses to disease 

risk, among other factors.72,73,79,80,81 Daily, seasonal, or year-toyear climate variability can sometimes result in vector/pathogen adaptation and shifts or expansions in their geographic 

ranges.73,74,81 Such shifts can alter disease incidence depending 

on vector-host interaction, host immunity, and pathogen evolution.71 North Americans are currently at risk from numerous 

vector-borne diseases, including Lyme,75,82,83,84 dengue fever,85

West Nile virus,86 Rocky Mountain spotted fever,87 plague, and 

tularemia.88 Vector-borne pathogens not currently found in the 

United States, such as chikungunya, Chagas disease, and Rift 

Valley fever viruses, are also threats. Climate change effects 

on the geographical distribution and incidence of vector-borne 

diseases in other countries where these diseases are already 

found can also affect North Americans, especially as a result 

of increasing trade with, and travel to, tropical and subtropical areas.74,81 Whether climate change in the U.S. will increase 

the chances of domestically acquiring diseases such as dengue 

fever is uncertain, due to vector-control efforts and lifestyle 

factors, such as time spent indoors, that reduce human-insect 

contact. 

Infectious disease transmission is sensitive to local, small-scale 

differences in weather, human modification of the landscape, 

the diversity of animal hosts,83 and human behavior that affects vector-human contact, among other factors. There is a 

need for finer-scale, long-term studies to help quantify the 

relationships among weather variables, vector range, and 

vector-borne pathogen occurrence, the consequences of shifting distributions of vectors and pathogens, and the impacts on 

human behavior. Enhanced vector surveillance and human disease tracking are needed to address these concerns. 

Transmission cycle of lyme disease

The development and survival of blacklegged ticks, their animal hosts, and the Lyme disease bacterium, Borrelia 

burgdorferi, are strongly influenced by climatic factors, especially temperature, precipitation, and humidity. Potential 

impacts of climate change on the transmission of Lyme disease include: 1) changes in the geographic distribution of 

the disease due to the increase in favorable habitat for ticks to survive off their hosts;89 2) a lengthened transmission 

season due to earlier onset of higher temperatures in the spring and later onset of cold and frost; 3) higher tick densities leading to greater risk in areas where the disease is currently observed, due to milder winters and potentially larger 

rodent host populations; and 4) changes in human behaviors, including increased time outdoors, which may increase 

the risk of exposure to infected ticks.

The Culex tarsalis mosquito is a vector that transmits West Nile 

Virus.

James Gathany, CDC
226 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

9: HUMAN HEALTH

Food- and Waterborne Diarrheal Disease

Diarrheal disease is a major public health issue in developing 

countries and, while not generally increasing in the United 

States, remains a persistent concern nonetheless. Exposure 

to a variety of pathogens in water and food causes diarrheal 

disease. Air and water temperatures, precipitation patterns, 

extreme rainfall events, and seasonal variations are all known 

to affect disease transmission.65,91,92 In the United 

States, children and the elderly are most vulnerable to 

serious outcomes, and those 

exposed to inadequately or 

untreated groundwater will 

be among those most affected.

In general, diarrheal diseases including Salmonellosis and Campylobacteriosis 

are more common when 

temperatures are higher,93,94

though patterns differ by 

place and pathogen. Diarrheal diseases have also 

been found to occur more 

frequently in conjunction 

with both unusually high 

and low precipitation.95 Sporadic increases in streamflow rates, often preceded 

by rapid snowmelt96 and changes in water treatment,97 have 

also been shown to precede outbreaks. Risks of waterborne 

illness and beach closures resulting from changes in the magnitude of recent precipitation (within the past 24 hours) and in 

lake temperature are expected to increase in the Great Lakes 

region due to projected climate change.98,99

Projected Change in Heavy Precipitation Events

Figure 9.6. Maps show the increase in frequency of extreme daily precipitation events (a daily amount 

that now occurs just once in 20 years) by the later part of this century (2081-2100) compared to the 

latter part of the last century (1981-2000). Such extreme events are projected to occur more frequently 

everywhere in the United States. Under a rapid emissions reduction scenario (RCP 2.6), these events 

would occur nearly twice as often. For a scenario assuming continued increases in emissions (RCP 

8.5), these events would occur up to five times as often. (Figure source: NOAA NCDC / CICS-NC). 

Figure 9.5. The maps show the current and projected probability of establishment of tick populations (Ixodes scapularis) that transmit 

Lyme disease. Projections are shown for 2020, 2050, and 2080. The projected expansion of tick habitat includes much of the eastern 

half of the country by 2080. For some areas around the Gulf Coast, the probability of tick population establishment is projected to 

decrease by 2080. (Figure source: adapted from Brownstein et al. 200590). 

Projected Changes in Tick Habitat
227 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

9: HUMAN HEALTH

Harmful Bloom of Algae

Figure 9.8. Remote sensing color image of harmful algal bloom in Lake Erie on 

October 9, 2011. The bright green areas have high concentrations of algae, which 

can be harmful to human health. The frequency and range of harmful blooms of algae 

are increasing.102,103 Because algal blooms are closely related to climate factors, 

projected changes in climate could affect algal blooms and lead to increases in 

water- and food-borne exposures and subsequent cases of illness.103 Other factors 

related to increases in harmful algal blooms include shifts in ocean conditions such 

as excess nutrient inputs.101,102,103 (Figure source: NASA Earth Observatory104).

Heavy Downpours are Increasing Exposure to Disease

Figure 9.7. Heavy downpours, which are increasing in the United States, have contributed to increases in heavy flood events 

(Ch. 2: Our Changing Climate, Key Message 6). The figure above illustrates how people can become exposed to waterborne 

diseases. Human exposures to waterborne diseases can occur via drinking water, as well as recreational waters.100,101,102,103

(Figure source: NOAA NCDC / CICS-NC).
228 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

9: HUMAN HEALTH

Food Security

Globally, climate change is expected to threaten food production and certain aspects of food quality, as well as food prices 

and distribution systems. Many crop yields are predicted to decline due to the combined effects of changes in rainfall, severe 

weather events, and increasing competition from weeds and 

pests on crop plants (Ch. 6: Agriculture, Key Message 6).105,106

Livestock and fish production is also projected to decline.107

Prices are expected to rise in response to declining food production and associated trends such as increasingly expensive 

petroleum (used for agricultural inputs such as pesticides and 

fertilizers).108

While the U.S. will be less affected than some other countries,109,110 the nation will not be immune. Health can be affected in several ways. First, Americans with particular dietary 

patterns, such as Alaska Natives, will confront shortages of key 

foods (Ch. 12: Indigenous Peoples, Key Message 1).111 Second, 

food insecurity increases with rising food prices.112 In such 

situations, people cope by turning to nutrient-poor but calorie-rich foods, and/or they endure hunger, with consequences 

ranging from micronutrient malnutrition to obesity.113 Third, 

the nutritional value of some foods is projected to decline. 

Elevated atmospheric CO2 is associated with decreased plant 

nitrogen concentration, and therefore decreased protein, in 

many crops, such as barley, sorghum, and soy.114 The nutrient 

content of crops is also projected to decline if soil nitrogen 

levels are suboptimal, with reduced levels of nutrients such as 

calcium, iron, zinc, vitamins, and sugars, although this effect is 

alleviated if sufficient nitrogen is supplied.115 Fourth, farmers 

are expected to need to use more herbicides and pesticides 

because of increased growth of pests116 and weeds117 as well 

as decreased effectiveness118 and duration119 of some of these 

chemicals (Ch. 6: Agriculture). Farmers, farmworkers, and 

consumers will thus sustain increased exposure to these substances and their residues, which can be toxic. These climate 

change impacts on the nutritional value of food exist within a 

larger context in which other factors, such as agricultural practices, food distribution systems, and consumer food choices, 

also play key roles. Adaptation activities can reduce the healthrelated impacts of some of the anticipated food security challenges (Ch. 6: Agriculture).

Mental Health and Stress-related Disorders

Mental illness is one of the major causes of suffering in the 

United States, and extreme weather events can affect mental health in several ways.120,121,122,123 First, following disasters, 

mental health problems increase, both among people with no 

history of mental illness, and those at risk – a phenomenon 

known as “common reactions to abnormal events.” These reactions may be short-lived or, in some cases, long-lasting.124

For example, research demonstrated high levels of anxiety 

and post-traumatic stress disorder among people affected by 

Hurricane Katrina,125 and similar observations have followed 

floods126 and heat waves.127 Some evidence suggests wildfires 

have similar effects.128 All of these events are increasingly fueled by climate change (see Ch. 2: Our Changing Climate). Other 

health consequences of intensely stressful exposures are also 

a concern, such as adverse birth outcomes including pre-term 

birth, low birth weight, and maternal complications.129 

Second, some patients with mental illness are especially 

susceptible to heat.130 Suicide rates vary with weather,131 rising with high temperatures,132 suggesting potential climate 

change impacts on depression and other mental illnesses. 

Dementia is a risk factor for hospitalization and death during heat waves.127,133 Patients with severe mental illness such 

as schizophrenia are at risk during hot weather because their 

medications may interfere with temperature regulation or 

even directly cause hyperthermia.134 Additional potential mental health impacts, less well understood, include the possible 

distress associated with environmental degradation135 and displacement,136 and the anxiety and despair that knowledge of 

climate change might elicit in some people (Ch. 12: Indigenous 

Peoples, Key Message 5).122

Key Message 2: Most Vulnerable at Most Risk

Climate change will, absent other changes, amplify some of the existing health threats 

the nation now faces. Certain people and communities are especially vulnerable, including 

children, the elderly, the sick, the poor, and some communities of color.

Climate change will increase the risk of climate-related illness 

and death for a number of vulnerable groups in the United 

States, as when Hurricane Katrina devastated New Orleans in 

2005. Children, primarily because of physiological and developmental factors, will disproportionately suffer from the effects of heat waves,47 air pollution, infectious illness, and trauma resulting from extreme weather events.14,16,18,22,138,139,140,141

The country’s older population also could be harmed more as 

the climate changes. Older people are at much higher risk of 

dying during extreme heat events.45,47,139,142 Pre-existing health 

conditions also make older adults susceptible to cardiac and 

respiratory impacts of air pollution26 and to more severe consequences from infectious diseases;143 limited mobility among 

older adults can also increase flood-related health risks.144 Lim-
229 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

9: HUMAN HEALTH

ited resources and an already high burden of chronic health 

conditions, including heart disease, obesity, and diabetes, will 

place the poor at higher risk of health impacts from climate 

change than higher income groups.26,47 Potential increases in 

food cost and limited availability of some foods will exacerbate 

current dietary inequalities and have significant health ramifications for the poorer segments of our population (Ch. 12: 

Indigenous Peoples, Key Message 1).110,145

Climate change will disproportionately affect low-income communities and some communities of color (Ch. 12: Indigenous 

Peoples, Key Message 2),139,149,151,152,153,154,155,156,157 raising environmental justice concerns. Existing health disparities153,158,159 

and other inequities160,161 increase vulnerability. Climate 

change related issues that have an equity component include 

heat waves, air quality, and extreme weather and climate 

events. For example, Hurricane Katrina demonstrated how 

vulnerable certain groups of people were to extreme weather 

events, because many low-income and of-color New Orleans 

residents were killed, injured, or had difficulty evacuating and 

recovering from the storm.154,155,156,161,162,163,164

Figure 9.9. A variety of factors can increase the vulnerability of a specific demographic group to health effects due to climate change. 

For example, older adults are more vulnerable to heat stress because their bodies are less able to regulate their temperature. Overall 

population growth is projected to continue to at least 2050, with older adults comprising an increasing proportion of the population. 

Similarly, there are an increasing number of people who are obese and have diabetes, heart disease, or asthma, which makes 

them more vulnerable to a range of climate-related health impacts. Their numbers are also rising. The poor are less able to afford 

the kinds of measures that can protect them from and treat them for various health impacts. (Data from CDC; Health E-Stat; U.S. 

Census Bureau 2010, 2012; and Akinbami et al. 2011137). 

Elements of Vulnerability to Climate Change
230 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

9: HUMAN HEALTH

Figure 9.10. This map illustrates the national scope of the dispersion of displaced people from Hurricane Katrina. 

It shows the location by zip code of the 800,000 displaced Louisiana residents who requested federal emergency 

assistance. The evacuees ended up dispersed across the entire nation, illustrating the wide-ranging impacts that 

can flow from extreme weather events, such as those that are projected to increase in frequency and/or intensity 

as climate continues to change (Ch. 2: Our Changing Climate, Key Message 8). (Figure source: Kent 2006150).

Katrina Diaspora

Societal system failures during extreme events

We have already seen multiple system failures during an extreme weather event in the United States, as when Hurricane 

Katrina struck New Orleans.146 Infrastructure and evacuation failures and collapse of critical response services during 

a storm is one example of multiple system failures. Another example is a loss of electrical power during a heat wave or 

wildfires, which can reduce food and water safety.147 Air conditioning has helped reduce illness and death due to extreme 

heat,148 but if power is lost, everyone is vulnerable. By their nature, such events can exceed our capacity to respond.79

In succession, these events severely deplete our resources needed to respond, from the individual to the national scale, 

but disproportionately affect the most vulnerable populations.149

Multiple climate stressors and health

Climate change impacts add to the cumulative stresses currently faced by vulnerable populations including children, 

the elderly, the poor, some communities of color, and people with chronic illnesses. These populations, and others living 

in certain places such as cities, floodplains, and coastlines, are more vulnerable not only to extreme events but also to 

ongoing, persistent climate-related threats. These threats include poor air quality, heat, drought, flooding, and mental 

health stress. Over time, the accumulation of these stresses will be increasingly harmful to these populations.
231 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

9: HUMAN HEALTH

Key Message 3: Prevention Provides Protection

Public health actions, especially preparedness and prevention, can do much to protect people 

from some of the impacts of climate change. Early action provides the largest health benefits. 

As threats increase, our ability to adapt to future changes may be limited.

Prevention is a central tenet of public health. Many conditions 

that are difficult and costly to treat when a patient gets to the 

doctor could be prevented before they occur at a fraction of 

the cost. Similarly, many of the larger health impacts associated with climate change can be prevented through early action 

at significantly lower cost than dealing with them after they occur.153,165 Early preventive interventions, such as early warnings 

for extreme weather, can be particularly cost-effective.166,167,168

As with many illnesses,169 once impacts are apparent, even the 

best adaptive efforts can be overwhelmed, and damage control becomes the priority.62

Activities that reduce carbon pollution often also provide cobenefits in the form of preventive health measures. For example, reliance on cleaner energy sources for electricity production174 and more efficient and active transport, like biking or 

walking,175 can have immediate public health benefits, through 

improved air quality and lowered rates of obesity, diabetes, 

and heart disease.176 Reducing carbon pollution also reduces 

long-term adverse climate-health impacts, thus producing cost 

savings in the near and longer term.176 Preventing exposures to 

other climate-sensitive impacts already apparent can similarly 

result in cost savings. For instance, heat wave early warning 

systems protect vulnerable groups very effectively and are 

much less expensive than treating and coping with heat illnesses. Systems that monitor for early outbreaks of disease are also 

typically much less expensive than treating communities once 

outbreaks take hold.12,49,177

Effective communication is a fundamental part of prevention. 

The public must understand risk in order to endorse proactive 

risk management. The public is familiar with the health risks 

of smoking, but not so for climate change. When asked about 

climate change impacts, Americans do not mention health 

impacts,178 and when asked about health impacts specifically, 

most believe it will affect people in a different time or place.179

But diverse groups of Americans find information on health 

impacts to be helpful once received, particularly information 

about the health benefits of mitigation (reducing carbon emissions) and adaptation.180

Determining which types of prevention to invest in (such as 

monitoring, early warning systems, and land-use changes that 

reduce the impact of heat and floods) depends on several 

factors, including health problems common to that 

particular area, vulnerable populations, the preventive health systems already in place, and the expected 

impacts of climate change.181 Local capacity to adapt 

is very important; unfortunately the most vulnerable 

populations also frequently have limited resources for 

managing climate-health risks. 

Overall, the capacity of the American public health and 

health care delivery systems faces many challenges.182

The cost of dealing with current health problems is 

diverting resources from preventing them in the first 

place. This makes the U.S. population more vulnerable.183,184 Without careful consideration of how to 

prevent future impacts, similar patterns could emerge 

regarding the health impacts from climate change. 

However, efforts to quantify and map vulnerability 

factors at the community level are underway.151,164,185

There are public health programs in some locations 

that address climate-sensitive health issues, and integrating such programs into the mainstream public 

health toolkit as adaptation needs increase would improve public health resilience to climate change.79,186,187

Given that these programs have demonstrated efficacy against current threats that are expected to worsen 

with climate change, it is prudent to invest in creating 

Large-scale environmental

change favors disease emergence

Climate change is causing large-scale changes in the environment, increasing the likelihood of the emergence or reemergence of unfamiliar disease threats.170 Factors include shifting ranges of disease-carrying pests, lack of immunity and 

preparedness, inadequate disease monitoring, and increasing 

global travel. Diseases including Lyme disease and dengue 

fever pose increasing health threats to the U.S. population; 

the number of U.S. patients hospitalized with dengue fever 

more than tripled from 2000 to 2007.171 Although most cases 

of dengue fever during that time period were acquired outside 

the contiguous United States, the introduction of infected 

people into areas where the dengue virus vector is established 

increases the risk of locally acquired cases. The public health 

system is not fully prepared to monitor or respond to these 

growing disease risks. The introduction of new diseases into 

non-immune populations has been and continues to be a major challenge in public health. There are concerns that climate 

change may provide opportunities for pathogens to expand or 

shift their geographic ranges.172,173
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9: HUMAN HEALTH

the strongest climate-health preparedness programs possible.153 One survey highlighted opportunities to address climate 

change preparedness activities and climate-health research181

before needs become more widespread. America’s Climate 

Choices: Adapting to the Impacts of Climate Choices (Table 3.5) 

provides examples of health adaptation options.187 

Key Message 4: Responses Have Multiple Benefits

Responding to climate change provides opportunities to improve human health and 

well-being across many sectors, including energy, agriculture, and transportation. Many 

of these strategies offer a variety of benefits, protecting people while combating 

climate change and providing other societal benefits. 

Policies and other strategies intended to reduce carbon pollution and mitigate climate change can often have independent influences on human health. For example, reducing CO2

emissions through renewable electrical power generation can 

reduce air pollutants like particles and sulfur dioxide. Efforts 

to improve the resiliency of communities and human infrastructure to climate change impacts can also improve human 

health. There is a growing recognition that the magnitude of 

health “co-benefits,” like reducing both pollution and cardiovascular disease, could be significant, both from a public health 

and an economic standpoint.176,188,189 Some climate change 

resilience efforts will benefit health, but potential co-harms 

should be considered when implementing these strategies. 

For example, although there are numerous benefits to urban 

greening, such as reducing the urban heat island effect while 

simultaneously promoting an active healthy lifestyle,159,190,191

the urban planting of certain allergenic pollen producing species22 could increase human pollen exposure and allergic illness. Increased pollen exposure has been linked to increased 

emergency department visits related to asthma and wheezing192 in addition to respiratory allergic illnesses such as allergic 

rhinitis or hay fever.193 The selective use of low to moderate 

pollen-producing species can decrease pollen exposure.194

Much of the focus of health co-benefits has been on reducing 

health-harming air pollution.6,174,175,195,196 One study projects 

that replacing 50% of short motor vehicle trips with bicycle 

use and the other 50% with other forms of transportation like 

walking or public transit would avoid nearly 1,300 deaths in 11 

midwestern metropolitan areas and create up to $8 billion in 

health benefits annually for the upper Midwest region.188 Such 

multiple-benefit actions can reduce heat-trapping gas emissions that lead to climate change, improve air quality by reducing vehicle pollutant emissions, and improve fitness and health 

through increased physical activity.99,197,198,199,200

Innovative urban design could create increased access to active transport.99 The compact geographical area found in cities 

presents opportunities to reduce energy use and emissions 

of heat-trapping gases and other air pollutants through active transit, improved building construction, provision of services, and infrastructure creation, such as bike paths and sidewalks.197,201 Urban planning strategies designed to reduce the 

urban heat island effect, such as green/cool roofs, increased 

green space, parkland and urban canopy, could reduce indoor 

temperatures, improve indoor air quality, and could produce 

additional societal co-benefits by promoting social interaction 

and prioritizing vulnerable urban populations.191,197

Patterns of change related to improving health can also have 

co-benefits in terms of reducing carbon pollution and mitigating climate change. Current U.S. dietary guidelines and many 

health professionals have recommended diets higher in fruits 

and vegetables and lower in red meat as a means of helping 

©Fotosearch
233 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

9: HUMAN HEALTH

to reduce the risk of cardiovascular disease and some cancers.199,202,203 These changes in food consumption, and related 

changes to food production, could have co-benefits in terms of 

reducing greenhouse gas emissions. While the greenhouse gas 

footprint of the production of other foods, compared to sources such as livestock, is highly dependent on a number of factors, production of livestock currently accounts for about 30% 

of the U.S. total emissions of methane.199,203,204 This amount of 

methane can be reduced somewhat by recovery methods such 

as the use of biogas digesters, but future changes in dietary 

practices, including those motivated by considerations other 

than climate change mitigation, could also have an effect on 

the amount of methane emitted to the atmosphere.205

In addition to producing health co-benefits,206 climate change 

prevention and preparedness measures could also yield positive equity impacts. For example, several studies have found 

that communities of color and poor communities experience 

disproportionately high exposures to air pollution.207,208 Climate change mitigation policies that improve local air quality 

thus have the potential to strongly benefit health in these communities. 

An area where adaptation policy could produce more equitable health outcomes is with respect to extreme weather 

events. As discussed earlier, Hurricane Katrina demonstrated 

that communities of color, poor communities, and certain other vulnerable populations (like new immigrant communities) 

are at a higher risk to the adverse effects of extreme weather events.152,155 These vulnerable populations could benefit 

from urban planning policies that ensure that new buildings, 

including homes, are constructed to resist extreme weather 

events.197
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234 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Photo Credits

Introduction to chapter; tourists walking close to misters keeping cool 

during heat wave in Las Vegas, Nevada, as shown in top banner: 

©Julie Jacobson/AP/Corbis
252 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

9: HUMAN HEALTH

Process for Developing Key Messages

The key messages were developed during technical discussions 

and expert deliberation at a two-day meeting of the eight chapter 

Lead Authors, plus Susan Hassol and Daniel Glick, held in Boulder, 

Colorado May 8-9, 2012; through multiple technical discussions 

via six teleconferences from January through June 2012, and an 

author team call to finalize the Traceable Account draft language 

on Oct 12, 2012; and through other various communications on 

points of detail and issues of expert judgment in the interim. The 

author team also engaged in targeted consultations during multiple exchanges with Contributing Authors, who provided additional 

expertise on subsets of the key message. These discussions were 

held after a review of the technical inputs and associated literature pertaining to human health, including a literature review,209

workshop reports for the Northwest and Southeast United States, 

and additional technical inputs on a variety of topics. 

Key message #1 Traceable Account

Climate change threatens human health and 

well-being in many ways, including impacts from 

increased extreme weather events, wildfire, decreased air quality, threats to mental health, and 

illnesses transmitted by food, water, and diseasescarriers such as mosquitoes and ticks. Some of 

these health impacts are already underway in the 

United States.

Description of evidence base

The key message and supporting text summarizes extensive evidence documented in several foundational technical inputs prepared for this chapter, including a literature review209 and workshop 

reports for the Northwest and Southeast United States. Nearly 60 

additional technical inputs related to human health were received 

and reviewed as part of the Federal Register Notice solicitation for 

public input. 

Air Pollution:

The effects of decreased ozone air quality on human health 

have been well documented concerning projected increases in 

ozone,6,7,9,11,39 even with uncertainties in projections owing to the 

complex formation chemistry of ozone and climate change, precursor chemical inventories, wildfire emission, stagnation episodes, 

methane emissions, regulatory controls, and population characteristics.4

 Ozone exposure leads to a number of health impacts.1,2

Allergens:

The effects of increased temperatures and atmospheric CO2 concentration have been documented concerning shifts in flowering 

time and pollen initiation from allergenic plants, elevated production of plant-based allergens, and health effects of increased pollen concentrations and longer pollen seasons.15,16,17,18,20,22,23,24,26,106

Additional studies have shown extreme rainfall and higher temperatures can lead to increased indoor air quality issues such as 

fungi and mold health concerns.27

Wildfire:

The effects of wildfire on human health have been well documented with increase in wildfire frequency17,29,39,40 leading to decreased 

air quality31,32,33 and negative health impacts.32,34,36

Temperature Extremes:

The effects of temperature extremes on human health have been 

well documented for increased heat waves,51,53,54 which cause 

more deaths,47,48 hospital admissions50 and population vulnerability.56,57

Precipitation Extremes - Heavy Rainfall, Flooding, and Droughts:

The effects of weather extremes on human health have been well 

documented, particularly for increased heavy precipitation, which 

has contributed to increases in severe flooding events in certain 

regions. Floods are the second deadliest of all weather-related 

hazards in the United States.63,64 Elevated waterborne disease 

outbreaks have been reported in the weeks following heavy rainfall,65 although other variables may affect these associations.66

Populations living in damp indoor environments experience increased prevalence of asthma and other upper respiratory tract 

symptoms.67

Disease Carried by Vectors:

Climate is one of the factors that influence the range of disease 

vectors; 73,74,76 a shift in the current range may increase 

interactions with people and affect human health.71 North 

Americans are currently at risk from a number of vector-borne 

diseases.75,82,83,85,86,87 There are some ambiguities on the relative 

SUPPLEMENTAL MATERIAL

TRACEABLE ACCOUNTS
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253 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

role and contribution of climate change among the range of factors 

that affect disease transmission dynamics.71,72,73,74,75,76 However,

observational studies are already underway and confidence is high 

based on scientific literature that climate change has contributed 

to the expanded range of certain disease vectors, including Ixodes

ticks which are vectors for Lyme disease in the United States.78,84,89

Food- and Waterborne Diarrheal Disease: 

There has been extensive research concerning the effects of climate 

change on water- and food-borne disease transmission.92,93,95,96,97

The current evidence base strongly supports waterborne diarrheal 

disease being both seasonal and sensitive to climate variability. 

There are also multiple studies associating extreme precipitation 

events with waterborne disease outbreaks.65 This evidence of 

responsiveness of waterborne disease to weather and climate, 

combined with evidence strongly suggesting that temperatures 

will increase and extreme precipitation events will increase in 

frequency and severity (Ch. 2: Our Changing Climate), provides 

a strong argument for climate change impacts on waterborne 

disease by analogy. There are multiple studies associating extreme 

precipitation events with waterborne disease outbreaks and strong 

climatological evidence for increasing frequency and intensity of 

extreme precipitation events in the future. The scientific literature 

modeling the projected impacts of climate change on waterborne 

disease is somewhat limited, however. Combined, we therefore 

have overall medium confidence in the impact of climate change 

on waterborne and food-borne disease.

Harmful Algal Blooms:

Because algal blooms are closely related to climate factors, 

projected changes in climate could affect algal blooms and lead 

to increases in food- and waterborne exposures and subsequent 

cases of illness.96,97,98,99,103 Harmful algal blooms have multiple 

exposure routes.100

Food Security:

Climate change is expected to have global impacts on both food 

production and certain aspects of food quality. The impact of 

temperature extremes, changes in precipitation and elevated 

atmospheric CO2, and increasing competition from weeds and pests 

on crop plants are areas of active research (Ch. 6: Agriculture, Key 

Message 6).105,106 The U.S. as a whole will be less affected than 

some other countries. However, the most vulnerable, including 

those dependent on subsistence lifestyles, especially Alaska 

Natives and low-income populations, will confront shortages of 

key foods. 

Mental Health and Stress-Related Disorders:

The effects of extreme weather on mental health have been 

extensively studied.120,122,123 Studies have shown the impacts of 

mental health problems after disasters,124 with extreme events 

like Hurricane Katrina,125 floods,126 heat waves,127 and wildfires128

having led to mental health problems. Further work has shown 

that some people with mental illnesses are especially vulnerable 

to heat. Suicide rates vary with weather,131,132 dementia is a risk 

factor for hospitalization and death during heat waves,127,133 and 

medications for schizophrenia may interfere with temperature 

regulation or even directly cause hyperthermia.134 Additional 

potential mental health impacts include distress associated with 

environmental degradation, displacement, and the knowledge of 

climate change.122,123,136

New information and remaining uncertainties

Important new evidence on heat-health effects44,45 confirmed 

many of the findings from a prior literature review. Uncertainties 

in the magnitude of projections of future climate-related morbidity and mortality can result from differences in climate model 

projections of the frequency and intensity of extreme weather 

events such as heat waves and other climate parameters such as 

precipitation. 

Efforts to improve the information base should address the coordinated monitoring of climate and improved surveillance of health 

effects.

Assessment of confidence based on evidence

Overall: Very High confidence. There is considerable consensus 

and a high quality of evidence in the published peer-reviewed literature that a wide range of health effects will be exacerbated by 

climate change in the United States. There is less agreement on 

the magnitude of these effects because of the exposures in question and the multi-factorial nature of climate-health vulnerability, 

with regional and local differences in underlying health susceptibilities and adaptive capacity. Other uncertainties include how 

much effort and resources will be put into improving the adapConfidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent 

results, well documented and 

accepted methods, etc.), high 

consensus

High

Moderate evidence (several sources, some consistency, methods 

vary and/or documentation limited, 

etc.), medium consensus

Medium

Suggestive evidence (a few 

sources, limited consistency, models incomplete, methods emerging, 

etc.), competing schools of thought

Low

Inconclusive evidence (limited 

sources, extrapolations, inconsistent findings, poor documentation 

and/or methods not tested, etc.), 

disagreement or lack of opinions 

among experts
9: HUMAN HEALTH

Traceable Accounts

254 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

tive capacity of public health systems to prepare in advance for 

the health effects of climate change, prevent harm to individual 

and community health, and limit associated health burdens and 

societal costs. 

Increased Ozone Exposure: Very High confidence. 

Allergens: High confidence.

Wildfires: Very High confidence. 

Thermal Extremes: Very High confidence. 

Extreme Weather Events: Very High confidence. 

Vector-borne Infectious Diseases: High or Very High confidence for 

shift in range of disease-carrying vectors. Medium confidence for 

whether human disease transmission will follow. 

Food- and Waterborne disease: Medium confidence. 

Harmful Algal Blooms: Medium confidence. 

Food Security: Medium confidence for food quality; High confidence 

for food security.

Threats to Mental Health: Very High confidence for post-disaster 

impacts; Medium confidence for climate-induced stress.

Key message #2 Traceable Account

Climate change will, absent other changes, amplify some of the existing health threats the nation 

now faces. Certain people and communities are especially vulnerable, including children, the elderly, 

the sick, the poor, and some communities of color.

Description of evidence base

The key message and supporting text summarizes extensive evidence documented in several foundational technical inputs prepared for this chapter, including a literature review209 and workshop reports for the Northwest and Southeast regions.210 Nearly 

60 additional technical inputs related to human health were received and reviewed as part of the Federal Register Notice solicitation for public input. 

Current epidemiological evidence on climate-sensitive health 

outcomes in the U.S. indicates that health impacts will differ 

substantially by location, pathway of exposure, underlying susceptibility, and adaptive capacity. These disparities in health 

impacts will largely result from differences in the distribution of 

individual attributes in a population that confers vulnerability (age, 

socioeconomic status, and race), attributes of place that reduce 

or amplify exposure (floodplain, coastal zone, and urban heat island), and the resilience of critical public health infrastructure. 

Amplification of existing health threats: The effects of extreme heat 

and heat waves, projected worsening air pollution and asthma, 

extreme rainfall and flooding, and displacement and injuries associated with extreme weather events, fueled by climate change, are 

already substantial public health issues. Trends projected under a 

changing climate are projected to exacerbate these health effects 

in the future.62

Children: The effects of climate change increase vulnerability of 

children to extreme heat, and increased health damage (morbidity, mortality) resulting from heat waves has been well documented.16,22,51,53,140 Extreme heat also causes more pediatric 

deaths,47,48 and more emergency room visits and hospital admissions.49,50 Adverse effects from increased heavy precipitation 

can lead to more pediatric deaths, waterborne diseases,66 and 

illness.141

The elderly: Heat stress is especially damaging to the health of 

older people,45,49,60,133,142,209 as are climate-sensitive increases in 

air pollution.

The sick: People and communities lacking the resources to adapt 

or to enhance mobility and escape health-sensitive situations are 

at relatively high risk.164

The poor: People and communities lacking the resources to adapt 

or to move and escape health-sensitive situations are at relatively 

high risk.164

Some communities of color: There are racial disparities in climate-sensitive exposures to extreme heat in urban areas, and 

in access to means of adaptation – for example air conditioning 

use.149,151,157,211 There are also racial disparities in withstanding, 

and recovering from, extreme weather events.155,162

Climate change will disproportionately impact low-income communities and some communities of color, raising environmental 

justice concerns.139,149,151,154,155,157,161,164 Existing health disparities153,158,159 and other inequities161 increase vulnerability. For 

example, Hurricane Katrina demonstrated how vulnerable these 

populations were to extreme weather events because many lowincome and of-color New Orleans residents were killed, injured, 

or had difficulty evacuating and recovering from the storm.155,162

Other climate change related issues that have an equity component include heat waves and air quality.139,149,154,164

New information and remaining uncertainties

Important new evidence45 confirmed findings from a prior literature 

review.139

The potential for specific climate-vulnerable communities to experience highly harmful health effects is not entirely clear in specific 

regions and on specific time frames due to uncertainties in rates of 

adaptation and uncertainties about the outcome of public health 

interventions currently being implemented that aim to address 

underlying health disparities and determinants of health.206 The 

public health community has not routinely conducted evaluations 

of the overall success of adaptation interventions or of particular 

elements of those interventions.
9: HUMAN HEALTH

Traceable Accounts

255 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Assessment of confidence based on evidence 

Given the evidence base and remaining uncertainties, confidence 

that climate change will amplify existing health threats: Very High.

Among those especially vulnerable are:

Children: Very High. 

The elderly: Very High.

The sick: Very High.

The poor: Very High.

Some communities of color: High.

Key message #3 Traceable Account

Public health actions, especially preparedness 

and prevention, can do much to protect people from 

some of the impacts of climate change. Early action provides the largest health benefits. As threats 

increase, our ability to adapt to future changes may 

be limited.

Description of evidence base

The key message and supporting text summarizes extensive 

evidence documented in several foundational technical inputs 

prepared for this chapter, including a literature review209 and 

workshop reports for the Northwest and Southeast United States. 

Nearly 60 additional technical inputs related to human health 

were received and reviewed as part of the Federal Register Notice 

solicitation for public input.

A number of studies have demonstrated that prevention activities 

that reduce carbon pollution, like using alternative energy sources174

and using active transportation like biking or walking,188 can lead to 

significant public health benefits, which can save costs in the near 

and long term.176 Health impacts associated with climate change 

can be prevented through early action at significantly lower cost 

than dealing with them after they occur. For example, heat wave 

early warning systems are much less expensive than treating heatrelated illnesses.165 Existing adaptation programs have improved 

public health resilience.9,153 One survey highlighted opportunities 

to address climate change preparedness activities and climatehealth research181 before needs become more widespread.

Considering U.S. public health in general, the cost-effectiveness 

of many prevention activities is well established.183 Some preventive actions are cost-saving, while others are deemed costeffective based on a pre-determined threshold. Early preventive 

interventions, such as early warnings for extreme weather, can be 

particularly cost-effective.166 However, there is less information on 

the cost-effectiveness of specific prevention interventions relevant 

to climate sensitive health threats (for example, heat early warning 

systems). Overall, we have high confidence that public health actions can do much to protect people from some of the impacts of 

climate change, and that early action provides the largest health 

benefits.

The inverse relationship between the magnitude of an impact and 

a community’s ability to adapt is well established and understood. 

Two extreme events, Hurricane Katrina and the European heat 

wave of 2003, illustrate this relationship well.167 Extreme events 

interact with social vulnerability to produce extreme impacts, 

and the increasing frequency of extreme events associated with 

climate change is prompting concern for impacts that may overwhelm adaptive capacity.62,173 This is equally true of the public 

health sector, specifically, leading to very high confidence that 

as threats increase, our ability to adapt to future changes may be 

limited. 

New information and remaining uncertainties

A key issue (uncertainty) is the extent to which the nation, states, 

communities and individuals will be able to adapt to climate 

change because this depends on the levels of local exposure 

to climate-health threats, underlying susceptibilities, and the 

capacities to adapt that are available at each scale. Overall, the 

capacity of the American public health and health care delivery 

systems faces many challenges.182 The cost of dealing with current 

health problems is diverting resources from preventing them in the 

first place. This makes the U.S. population more vulnerable.56,183

Steps for improving the information base on adaptation include 

undertaking a more comprehensive evaluation of existing climatehealth preparedness programs and their effectiveness in various 

jurisdictions (cities, counties, states, nationally).

Assessment of confidence based on evidence 

Overall, given the evidence base and remaining uncertainties: 

High. 

High: Public health actions, especially preparedness and 

prevention, can do much to protect people from some of the 

impacts of climate change. Prevention provides the most 

protection; but we do not as yet have a lot of post-implementation 

information with which to evaluate preparedness plans.

High: Early action provides the largest health benefits. There is 

evidence that heat-health early warning systems have saved lives 

and money in U.S. cities like Philadelphia, PA.165

Very High: Our ability to adapt to future changes may be limited.

Key message #4 Traceable Account

Responding to climate change provides opportunities to improve human health and well-being 

across many sectors, including energy, agriculture, 

and transportation. Many of these strategies offer 

a variety of benefits, protecting people while combating climate change and providing other societal 

benefits. 

Description of evidence base

The key message and supporting text summarizes extensive evidence documented in several foundational technical inputs prepared for this chapter, including a literature review209 and work-
9: HUMAN HEALTH

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256 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

shop reports for the Northwest and Southeast U.S. regions.210

Nearly 60 additional technical inputs related to human health 

were received and reviewed as part of the Federal Register Notice 

solicitation for public input. 

A number of studies have explored the opportunities available to 

improve health and well-being as a result of adapting to climate 

change,176 with many recent publications illustrating the benefit 

of reduced air pollution.6,174,175,195 Additionally, some studies have 

looked at the co-benefits to climate change and health of applying innovative urban design practices which reduce energy consumption and pollution while increasing public health,99,188,197,198

decrease vulnerability of communities to extreme events152,197 and 

reduce the disparity between different societal groups.206,207,212

New information and remaining uncertainties

More studies are needed to fully evaluate both the intended 

and unintended health consequences of efforts to improve the 

resiliency of communities and human infrastructure to climate 

change impacts. There is a growing recognition that the magnitude 

of these health co-benefits or co-harms could be significant, both 

from a public health and an economic standpoint. 176,188,189

Assessment of confidence based on evidence 

Given the evidence base and remaining uncertainties, confidence 

is Very High.
257

Climate Change Impacts in the United States

CHAPTER 10

ENERGY, WATER, AND LAND USE

INFORMATION DRAWN FROM THIS CHAPTER IS INCLUDED IN THE HIGHLIGHTS REPORT AND IS IDENTIFIED BY THIS ICON

Recommended Citation for Chapter

Hibbard, K., T. Wilson, K. Averyt, R. Harriss, R. Newmark, S. Rose, E. Shevliakova, and V. Tidwell, 2014: Ch. 10: Energy, 

Water, and Land Use. Climate Change Impacts in the United States: The Third National Climate Assessment, J. M. Melillo, 

Terese (T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 257-281. doi:10.7930/J0JW8BSF. 

On the Web: http://nca2014.globalchange.gov/report/sectors/energy-water-and-land

First published May 2014. PDF revised October 2014. See errata (available at http://nca2014.globalchange.gov/downloads) for details.

Convening Lead Authors

Kathy Hibbard, Pacific Northwest National Laboratory

Tom Wilson, Electric Power Research Institute

Lead Authors

Kristen Averyt, University of Colorado Boulder

Robert Harriss, Environmental Defense Fund

Robin Newmark, National Renewable Energy Laboratory

Steven Rose, Electric Power Research Institute 

Elena Shevliakova, Princeton University

Vincent Tidwell, Sandia National Laboratories 

© James Christensen/ Foto Natura/Corbis
258 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Energy, water, and land systems interact in many ways. Energy projects (energy production and delivery) require varying 

amounts of water and land; water projects (water supply and 

irrigation) require energy and land; and land-based activities 

(agriculture and forestry) depend upon energy and water. Increasing population and a growing economy intensify these 

interactions.

1

 Each sector is directly impacted by the others 

and by climate change, and each sector is a target for adaptation and mitigation efforts. Better understanding of the connections between and among energy, water, and land systems 

can improve our capacity to predict, prepare for, and mitigate 

climate change.

Challenges from climate change will arise from long-term, 

gradual changes, such as sea level rise, as well as from projected 

changes in weather extremes that have more sudden impacts. 

The independent implications of climate change for the 

energy, water, and land sectors have been studied extensively 

(see Ch. 4: Energy, Ch. 3: Water, and Ch. 13: Land Use & Land 

Cover Change). However, there are few analyses that capture 

the interactions among and competition for resources within 

these three sectors.1 Very little information is available to 

evaluate the implications for decision-making and planning, 

including legal, social, political, and other decisions.

Climate change is not the only factor driving changes. 

Other environmental and socioeconomic stressors interact 

with climate change and affect vulnerability and response 

strategies with respect to energy, water, and land systems. 

The availability and use of energy, water, and land resources 

and the ways in which they interact vary across the nation. 

Regions in the United States differ in their 1) energy mix (solar, 

wind, coal, geothermal, hydropower, nuclear, natural gas, 

petroleum, ethanol); 2) observed and projected precipitation 

and temperature patterns; 3) sources and quality of available 

water resources (for example, ground, surface, recycled); 4) 

technologies for storing, transporting, treating and using water; 

and 5) land use and land cover (see Ch. 13: Land Use & Land 

Cover Change). Decision-making processes for each sector also 

differ, and decisions often transcend scales, from local to state 

to federal, meaning that mitigation and adaptation options 

differ widely.

Given the many mitigation and adaptation opportunities available through the energy sector, a focus on energy is a useful 

Key Messages

1. Energy, water, and land systems interact in many ways. Climate change affects the individual 

sectors and their interactions; the combination of these factors affects climate change 

vulnerability as well as adaptation and mitigation options for different regions of the country.

2. The dependence of energy systems on land and water supplies will influence the development 

of these systems and options for reducing greenhouse gas emissions, as well as their climate 

change vulnerability.

3. Jointly considering risks, vulnerabilities, and opportunities associated with energy, water, and 

land use is challenging, but can improve the identification and evaluation of options for reducing 

climate change impacts.

ENERGY, 10 WATER, AND LAND USE

Figure 10.1. The interactions between and among the energy, 

water, land, and climate systems take place within a social and 

economic context. (Figure source: Skaggs et al. 20121).

Energy, Water, Land, and Climate Interactions
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10: ENERGY, WATER, AND LAND USE

way to highlight the interactions among energy, water, and 

land as well as intersections with climate and other stressors. 

For example, energy production already competes for water 

resources with agriculture, direct human uses, and natural systems. Climate-driven changes in land cover and land use are 

projected to further affect water quality and availability, increasing the competition for water needed for energy production. In turn, diminishing water quality and availability means 

that there will be a need for more energy to purify water and 

more infrastructure on land to store and distribute water. 

Stakeholders need to understand the interconnected nature of 

climate change impacts, and the value of assessments would 

be improved if risks and vulnerabilities were evaluated from a 

cross-sector standpoint.2

Key Message 1: Cascading Events

Energy, water, and land systems interact in many ways. Climate change affects the individual 

sectors and their interactions; the combination of these factors affects climate change 

vulnerability as well as adaptation and mitigation options for different regions of the country.

Energy production, land use, and water resources are linked 

in increasingly complex ways. In some parts of the country, 

electric utilities and energy companies compete with farmers 

and ranchers, other industries, and municipalities for water 

rights and availability, which are also constrained by interstate 

and international commitments. Private and public sector 

decision-makers must consider the impacts of strained water 

supplies on agricultural, ecological, industrial, urban, and public 

health needs. Across the country, these intertwined sectors 

will witness increased stresses due to climate changes that 

are projected to lower water quality and/or quantity in many 

regions and change heating and cooling electricity demands.

The links between and among energy, water, and land sectors 

mean that they are susceptible to cascading effects from one 

sector to the next. An example is found in the drought and 

heat waves experienced across much of the U.S. during the 

summers of 2011 and 2012. In 2011, drought spread across 

the south-central U.S., causing a series of energy, water, and 

land impacts that demonstrate the connections among these 

sectors. Texans, for example, experienced the hottest and 

driest summer on record. Summer average temperatures 

were 5.2°F higher than normal, and precipitation was lower 

than previous records set in 1956. The associated heat wave, 

with temperatures above 100°F for 40 consecutive days, 

together with drought, strained the region’s energy and water 

resources.3,4,5

These extreme climate events resulted in cascading effects 

across energy, water, and land systems. High temperatures 

caused increased demand for electricity for air conditioning, 

which corresponded to increased water withdrawal and 

consumption for electricity generation. Heat, increased 

evaporation, drier soils, and lack of rain led to higher irrigation 

demands, which added stress on water resources required for 

energy production. At the same time, low-flowing and warmer 

rivers threatened to suspend power plant production in several 

locations, reducing the options for dealing with the concurrent 

increase in electricity demand. 

The impacts on land resources and land use were dramatic. 

Drought reduced crop yields and affected livestock, costing 

Texas farmers and ranchers more than $5 billion, a 28% loss 

compared to average revenues of the previous four years.6

With increased feed costs, ranchers were forced to sell 

livestock at lower profit. Drought increased tree mortality,7

providing more fuel for record wildfires that burned 3.8 million 

acres (an area about the size of Connecticut) and destroyed 

2,763 homes.8

Figure 10.2. Map shows numbers of days with temperatures 

above 100°F during 2011. The black circles denote the 

location of observing stations recording 100°F days. The 

number of days with temperatures exceeding 100°F is 

expected to increase. The record temperatures and drought 

during the summer of 2011 represent conditions that will be 

more likely in the U.S. as climate change continues. When 

outdoor temperatures increase, electricity demands for 

cooling increase, water availability decreases, and water 

temperatures increase. Alternative energy technologies 

may require little water (for example, solar and wind) and 

can enhance resilience of the electricity sector, but still face 

land-use and habitat considerations. The projected increases 

in drought and heat waves provide an example of the ways 

climate changes will challenge energy, water, and land 

systems. (Figure source: NOAA NCDC, 2012).

 Coast-to-Coast 100-degree Days in 2011 
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Energy, water, and land interactions complicated and amplified 

the direct impacts on the electric sector. With electricity 

demands at all-time highs, water shortages threatened more 

than 3,000 megawatts of generating capacity – enough 

power to supply more than one million homes.9 As a result of 

the record demand and reduced supply, marginal electricity 

prices repeatedly hit $3,000 a megawatt hour, which is three 

times the maximum amount that generators can charge in 

deregulated electricity markets in the eastern United States.10

Competition for water also intensified. More than 16% of 

electricity production relied on cooling water from sources 

that shrank to historically low levels,9 and demands for water 

used to generate electricity competed with simultaneous 

demands for agriculture and other human activities. City and 

regional managers rationed water to farms and urban 

areas, and in some instances, water was trucked 

to communities that lacked sufficient supplies.11

As late as January 2012, customers of 1,010 Texas 

water systems were being asked to restrict water 

use; mandatory water restrictions were in place in 

647 water systems.12 At the same time, changing 

vegetation attributes, grazing, cropping, and 

wildfire compromised water quality and availability, 

increasing the amount of power required for water 

pumping and purification.

The Texas example shows how energy, land, water, 

and weather interacted in one region. Extreme 

weather events may affect other regions differently, 

because of the relative vulnerability of energy, water, 

and land resources, linkages, and infrastructure. 

For example, sustained droughts in the Northwest 

will affect how water managers release water from 

reservoirs, which in turn will affect water deliveries 

for ecosystem services, irrigation, recreation, 

and hydropower. Further complicating matters, 

hydropower is increasingly being used to balance 

variable wind generation in the Northwest, and 

seasonal hydroelectric restrictions have already 

created challenges to fulfilling this role. In the 

Midwest, drought poses challenges to meeting 

electricity demands because diminished water availability 

and elevated water temperatures reduce the efficiency of 

electricity generation by thermoelectric power plants. To 

protect water quality, federal and state regulations can require 

suspension of operations of thermoelectric power plants 

if water used to cool the power plants exceeds established 

temperature thresholds as it is returned to streams. 

Energy, land, water, and weather interactions are not limited 

to drought. For instance, 2011 also saw record flooding in the 

Mississippi basin. Floodwaters surrounded the Fort Calhoun 

nuclear power plant in Nebraska, shut down substations, and 

caused a wide range of energy, land, and water impacts (Ch. 

3: Water).

Interactions of Energy, Water, and Land Uses

Figure 10.4 depicts the current mix of energy, water, and land 

use within each U.S. region. The mixes reflect competition 

for water and land resources, but more importantly for the 

purposes here, the mixes reflect linkages across the energy, 

water, and land sectors as well as linkages to climate. For 

example, higher water withdrawal for thermoelectric power 

(power plants that use a steam cycle to generate electricity) 

generally reflects electric generation technology choices 

(often coal-, gas-, or nuclear-fired generation with open loop 

cooling) that assume the availability of large quantities of 

water. Therefore, the choice of energy technology varies based 

on the available resources in a region. Similarly, land-water 

linkages are evident in cropland and agricultural water use. 

The potential growth in renewable energy may strengthen the 

linkage between energy and land (see “Examples of Energy, 

Water, and Land Linkages”). Climate change affects each sector 

directly and indirectly. For instance, climate change affects 

water supplies, energy demand, and land productivity, all of 

which can affect sector-wide decisions. 

Figure 10.3. Graph shows average summer temperature and total rainfall 

in Texas from 1895 through 2012. The red dots illustrate the range of 

temperatures and rainfall observed over time. The record temperatures 

and drought during the summer of 2011 (large red dot) represent 

conditions far outside those that have occurred since the instrumental 

record began.4 An analysis has shown that the probability of such an event 

has more than doubled as a result of human-induced climate change3.

(Figure source: NOAA NCDC / CICS-NC).

 Texas Summer 2011: 

Record Heat and Drought 
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10: ENERGY, WATER, AND LAND USE

Figure 10.4. U.S. regions differ in the manner and intensity with which they use, or have available, energy, water, and 

land. Water bars represent total water withdrawals in billions of gallons per day (except Alaska and Hawai‘i, which are 

in millions of gallons per day); energy bars represent energy production for the region in 2012; and land represents land 

cover by type (green bars) or number of people (black and green bars). Only water withdrawals, not consumption, are 

shown (see Ch. 3: Water). Agricultural water withdrawals include irrigation, livestock, and aquaculture uses. (Data from 

EIA 201213 [energy], Kenny et al. 200914 [water], and USDA ERS 200715 [land]). 

Regional Water, Energy, and Land Use, with Projected Climate Change Impacts
262 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

10: ENERGY, WATER, AND LAND USE

Figure 10.4. U.S. regions differ in the manner and intensity with which they use, or have available, energy, water, and 

land. Water bars represent total water withdrawals in billions of gallons per day (except Alaska and Hawai‘i, which are 

in millions of gallons per day); energy bars represent energy production for the region in 2012; and land represents land 

cover by type (green bars) or number of people (black and green bars). Only water withdrawals, not consumption, are 

shown (see Ch. 3: Water). Agricultural water withdrawals include irrigation, livestock, and aquaculture uses. (Data from 

EIA 201213 [energy], Kenny et al. 200914 [water], and USDA ERS 200715 [land]). 

Regional Water, Energy, and Land Use, with Projected Climate Change Impacts
263 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

10: ENERGY, WATER, AND LAND USE

Figure 10.4. U.S. regions differ in the manner and intensity with which they use, or have available, energy, water, and 

land. Water bars represent total water withdrawals in billions of gallons per day (except Alaska and Hawai‘i, which are 

in millions of gallons per day); energy bars represent energy production for the region in 2012; and land represents land 

cover by type (green bars) or number of people (black and green bars). Only water withdrawals, not consumption, are 

shown (see Ch. 3: Water). Agricultural water withdrawals include irrigation, livestock, and aquaculture uses. (Data from 

EIA 201213 [energy], Kenny et al. 200914 [water], and USDA ERS 200715 [land]). 

Regional Water, Energy, and Land Use, with Projected Climate Change Impacts
264 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

10: ENERGY, WATER, AND LAND USE

Figure 10.4. U.S. regions differ in the manner and intensity with which they use, or have available, energy, water, and 

land. Water bars represent total water withdrawals in billions of gallons per day (except Alaska and Hawai‘i, which are 

in millions of gallons per day); energy bars represent energy production for the region in 2012; and land represents land 

cover by type (green bars) or number of people (black and green bars). Only water withdrawals, not consumption, are 

shown (see Ch. 3: Water). Agricultural water withdrawals include irrigation, livestock, and aquaculture uses. (Data from 

EIA 201213 [energy], Kenny et al. 200914 [water], and USDA ERS 200715 [land]). 

Regional Water, Energy, and Land Use, with Projected Climate Change Impacts
265 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Key Message 2: Options for Reducing Emissions and Climate Vulnerability 

The dependence of energy systems on land and water supplies will influence the 

development of these systems and options for reducing greenhouse gas emissions, 

as well as their climate change vulnerability.

Interactions among energy, water, and land resources have influenced and will continue to influence selection and operation of energy technologies. In some situations, land and water constraints also pose challenges to technology options for reducing 

Figure 10.5. Technology choices can significantly affect water and land use. These two panels show a selection of technologies. 

Ranges in water withdrawal/consumption reflect minimum and maximum amounts of water used for selected technologies. Carbon 

dioxide capture and storage (CCS) is not included in the figures, but is discussed in the text. The top panel shows water withdrawals 

for various electricity production methods. Some methods, like most conventional nuclear power plants that use “once-through” 

cooling systems, require large water withdrawals but return most of that water to the source (usually rivers and streams). For nuclear 

plants, utilizing cooling ponds can dramatically reduce water withdrawal from streams and rivers, but increases the total amount of 

water consumed. Beyond large withdrawals, once-through cooling systems also affect the environment by trapping aquatic life in 

intake structures and by increasing the temperature of streams.18 Alternatively, once-through systems tend to operate at slightly better 

efficiencies than plants using other cooling systems. The bottom panel shows water consumption for various electricity production 

methods. Coal-powered plants using recirculating water systems have relatively low requirements for water withdrawals, but consume 

much more of that water, as it is turned into steam. Water consumption is much smaller for various dry-cooled electricity generation 

technologies, including for coal, which is not shown. Although small in relation to cooling water needs, water consumption also 

occurs throughout the fuel and power cycle.19 (Figure source: Averyt et al. 201120).

Water Use for Electricity Generation by Fuel and Cooling Technology 
266 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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greenhouse gas emissions. For example, 

with the Southwest having most of the 

potential for deployment of concentrating solar technologies, facilities will 

need to be extremely water-efficient 

in order to compete for limited water 

resources. While wind farms avoid impacts on water resources, issues concerning land use, wildlife impacts, the 

environment, and aesthetics are often 

encountered. Raising crops to produce 

biofuels uses arable land and water that 

might otherwise be available for food 

production. This fact came into stark 

focus during the summer of 2012, when 

drought caused poor corn harvests, intensifying concerns about allocation of 

the harvest for food versus ethanol.16

Competition for water supplies is encouraging deployment of technologies 

that are less water-intensive than coal 

or nuclear power with once-through 

cooling. For example, wind, natural gas, 

photovoltaic (solar electric), and even 

thermoelectric generation with dry 

cooling use less water. Challenges in siting land- and water-intensive energy facilities are likely to intensify over time as 

competition for these resources grows. 

Considering the interactions among energy, water, and land systems presents 

opportunities for further identification 

and implementation of energy options 

that can reduce emissions, promote 

resilience, and improve sustainability. 

Every option for reducing greenhouse gas emissions involves 

tradeoffs that affect natural resources, socioeconomic systems, 

and the built environment. Energy system 

technologies vary widely in their carbon 

emissions and their use of water and land. 

As such, there are energy-water-land tradeoffs and synergies with respect to adaptation and mitigation. Each choice involves 

assessing the relative importance of the 

tradeoffs related to these resources in 

the context of both short- and long-term 

risks (see “Examples of Energy, Water, and 

Land Linkages” that describes four technologies that could play key roles). Figure 

10.5 provides a systematic comparison of 

water withdrawals and consumptive use, 

illustrating the wide variation across both 

electric generation technologies and the 

accompanying cooling technologies. Carbon dioxide capture and storage (CCS) is 

not included in the chart, but coal-fired 

Figure 10.6. The figure shows illustrative projections for 2030 of the total land-use 

intensity associated with various electricity production methods. Estimates consider 

both the footprint of the power plant as well as land affected by energy extraction. There 

is a relatively large range in impacts across technologies. For example, a change from 

nuclear to wind power could mean a significant change in associated land use. For 

each electricity production method, the figure shows the average of a most-compact 

and least-compact estimate for how much land will be needed per unit of energy. The 

figure uses projections from the Energy Information Administration Reference scenario 

for the year 2030, based on energy consumption by fuel type and power plant “capacity 

factors” (the ratio of total power generation to maximum possible power generation). 

The most-compact and least-compact estimates of biofuel land-use intensities reflect 

differences between current yield and production efficiency levels and those that are 

projected for 2030 assuming technology improvements.21 (Figure source: adapted from 

McDonald et al. 200921).

Projected Land-use Intensity in 2030 ©John Epperson/The Denver Post via Getty Image
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power plants (both evaporative cooling and dry cooling) fitted 

with CCS would consume twice as much water per unit of electricity generated as similar coal-fired facilities without CCS.17

Figure 10.6 shows projected land-use intensity in 2030 for various electricity production methods. Describing land use with a 

single number is valuable, but must be considered with care. For 

example, while wind generation can require significant amounts 

of land, it can co-exist with other activities such as farming and 

grazing, while other technologies may not be compatible with 

other land uses. Land and water influences on energy production capacity are expected to get stronger in the future, and 

greater resource scarcity will shape investment decisions.

Every adaptation and mitigation option involves tradeoffs in 

how it increases or decreases stress on energy systems and 

water and land resources. For a selected set of mitigation and 

adaptation measures, Table 10.1 provides a summary illustrating 

qualitatively how different technologies relate to energy, water, 

and land.1

Particularly relevant to climate change mitigation are the energy, water, and land risks associated with low-carbon electricity 

generation. For example, expansion of nuclear power and coal 

power with CCS are two measures that have been discussed as a 

potential part of a future decarbonized energy system.22,23 Both 

are also potentially water intensive and therefore have vulnerabilities related to climate impacts and competing water uses. 

Alternatively, renewable generation and combined cycle gas and 

coal have relatively modest water withdrawals (see also EPRI 

201124). Overall, energy, water, and land sector vulnerabilities 

are important factors to weigh in considering alternative electricity generation options and cooling systems. 

Bioenergy also presents opportunities for mitigation, but some 

potential bioenergy feedstocks are land and water intensive. 

Where land and water resources are limited, bioenergy may 

therefore be at risk of competing with other uses of land and 

water, and climate changes present additional challenges. Other 

mitigation options, such as afforestation (re-establishment of 

forests), forest management, agricultural soil management, 

and fertilizer management are also tied intimately into the interfaces among land availability, land management, and water 

resource quantity and quality.25

Some sector-specific mitigation and adaptation measures can 

provide opportunities to enhance climate mitigation or adaptation objectives in the other sectors. However, other measures may have negative impacts on mitigation or adaptation 

Table 10.1. Energy, water, and land sectoral impacts associated with a sample of climate mitigation and adaptation measures. Plus 

sign means a positive effect (reduced stress) on sector, minus sign means a negative effect (increased stress) on sector. Blank means 

effect not noted. Blue means consideration of energy extraction and power plant processes. It is important to keep in mind that this 

table only reflects physical synergies and tradeoffs. There are, of course, economic tradeoffs as well in the form of technology costs 

and societal concerns, such as energy security, food security, and water quality. Expansion of hybrid or dry-cooled solar technologies, 

versus wet, could help reduce water risks. For a more detailed description of the entries in the table, see Skaggs et al. 2012.1 Additional 

considerations regarding energy extraction, power plant processes, and energy use associated with irrigation were added to those 

reflected in Skaggs et al. 20121 (Adapted from Skaggs et al. 20121). 

Mitigation measures Water Land Energy

Switch from coal to natural gas fueled power plants + and – + and –

Expand CCS to fossil-fueled power plant – –

Expansion of nuclear power –

Expansion of wind + –

Expansion of solar thermal technologies (wet cooled) – –

Expansion of commercial scale photovoltaic + –

Expansion of hydropower + and – – +

Expansion of biomass production for energy + and – + and –

Adaptation measures Water Land Energy

Switch from once-through to recirculating cooling in thermoelectric power plants + and – -

Switch from wet to dry cooling at thermoelectric power plants + -

Desalinization + and – + + and –

New storage and conveyance of water + and – – –

Switch to drought-tolerant crops in drought vulnerable regions + – +

Increase transmission capacity to urban areas to reduce power outages 

during high demand periods – +
268 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

10: ENERGY, WATER, AND LAND USE

potential in other sectors. If such crosssector impacts are not considered, they 

can diminish the effectiveness of climate 

mitigation and adaptation actions.

For example, switching from coal- to natural-gas-fired electricity generation reduces the emissions associated with power 

generation. Depending on the situation, 

the switch to natural gas in the energy 

sector can either improve or reduce adaptive capacity in the water sector. Natural 

gas can reduce water use for thermoelectric cooling (gas-fired plants require less 

cooling water), but natural gas extraction 

techniques consume water, so water availability must be considered. In addition, 

gas production has the potential to affect 

land-based ecosystems by, for example, 

fragmenting habitat and inhibiting wildlife 

migration. Future improvements in natural 

gas technologies and water reuse may reduce the possibility of negative impacts on water supplies and 

enhance the synergies across the energy, water, and land interface. Incorporating consideration of such cross-sector interactions in planning and policy could affect sectoral decisions and 

decisions related to climate mitigation and adaptation. 

Changes in the availability of water and land due to climate 

change and other effects of human activities will affect location, design, choice, and operations of energy technologies 

in the future and, in some cases, constrain their deployment. 

Energy, water, and land linkages represent constraints, risks, 

and opportunities for private/public planning and investment 

decisions. “Examples of Energy, Water, and Land Linkages” below discusses four energy sector technologies that could contribute to reducing U.S. emissions of greenhouse gases and increasing energy security – natural gas from shale, solar power, 

biofuels, and CCS. These technologies were chosen to illustrate 

energy, water, and land linkages and other complexities for the 

design, planning, and deployment of our energy future.

© Michael DeYoung/ Corbis
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Examples of energy, water, and land linkages

Continued

Shale Natural Gas and Hydraulic Fracturing

The U.S. Energy Information Administration projects a 29% increase 

in U.S. natural gas production by 

2035, driven primarily by the economics of shale gas.13 As an energy 

source, natural gas (methane) can 

have a major advantage over coal 

and oil: when combusted, it emits 

less carbon dioxide per unit energy 

than other fossil fuels, and fewer pollutants like black carbon (soot) and 

mercury (see Ch. 27: Mitigation). An 

increase in natural gas consumption 

could lead to a reduction in U.S. 

greenhouse gas emissions compared 

to continued use of other fossil fuels. 

Disadvantages include the possibility that low-cost gas could supplant 

deployment of low-carbon generation 

technologies, such as nuclear power 

and renewable energy. In addition, 

the U.S. Environmental Protection Agency estimates that 6.9 million megatons of methane – with a 

global warming potential equivalent 

to 144.7 million megatons of CO2 – 

is emitted from the U.S. natural gas 

system through uncontrolled venting 

and leaks from drilling operations, 

pipelines, and storage tanks (see Ch. 

15: Biogeochemical Cycles; Ch. 27: 

Mitigation).26 There is considerable 

uncertainty about these estimates, 

and it is an active area of research. 

While technological improvements 

may reduce this leakage rate,26 leakage makes the comparison between 

natural gas and coal more complex 

from a climate perspective.27 For example, methane is a stronger greenhouse gas than carbon dioxide but has a much shorter atmospheric lifetime (see Ch. 15: Biogeochemical Cycles; Ch. 27: 

Mitigation; Appendix 3: Climate Science; Appendix 4: FAQs).

Recent reductions in natural gas prices are largely due to advances in hydraulic fracturing, which is a drilling method used 

to retrieve deep reservoirs of natural gas. Hydraulic fracturing injects large quantities of water, sand, and chemicals at high 

pressure into horizontally-drilled wells as deep as 10,000 feet below the surface in order to break the shale and extract 

natural gas.28 Questions about the water quantity necessary and the potential to affect water quality have produced national 

Figure 10.7. Hydraulic fracturing, a drilling method used to retrieve deep reservoirs 

of natural gas, uses large quantities of water, sand, and chemicals that are injected 

at high pressure into horizontally-drilled wells as deep as 10,000 feet below Earth’s 

surface. The pressurized mixture causes the rock layer to crack. Sand particles hold 

the fissures open so that natural gas from the shale can flow into the well. Questions 

about the water quantity necessary for this extraction method as well as the potential 

to affect water quality have produced national debate. (Figure source: NOAA NCDC).

Hydraulic Fracturing and Water Use
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10: ENERGY, WATER, AND LAND USE

Examples of energy, water, and land linkages (continued)

Continued

debate about this method. Federal government and state-led efforts are underway to identify, characterize, and if necessary, 

find approaches to address these issues (for example, EPA 2011; FracFocus 201229).

A typical shale gas well requires from two to four million gallons of water to drill and fracture (equivalent to the annual water 

use of 20 to 40 people in the U.S, or three to six Olympic-size swimming pools).28 The gas extraction industry has begun 

reusing water in order to lower this demand. However, with current technology, recycling water can require energy-intensive 

treatment, and becomes more difficult as salts and other contaminants build up in the water with each reuse.30 In regions 

where climate change leads to drier conditions, hydraulic fracturing could be vulnerable to climate change related reductions 

in water supply.

Shale gas development also requires land. To support the drilling and hydraulic fracturing process, a pad, which may be 

greater than five acres in size, is constructed.31 Land for new roads, compressor stations, pipelines, and water storage ponds 

are also required. 

The competition for water is expected to increase in the future. State and local water managers will need to assess how gas 

extraction competes with other priorities for water use, including electricity generation, irrigation, municipal supply, industry 

use, and livestock production. Collectively, such interactions between the energy and water resource sectors increase vulnerability to climate change, particularly in water-limited regions 

that are projected to, or become, significantly drier. 

Solar Power Generation

Solar energy technologies have the potential to satisfy a significant portion of U.S. electricity demand and reduce greenhouse gas emissions. The land and water requirements for 

solar power generation depend on the mix of solar technologies deployed. Small-scale (such as rooftop) installations are 

integrated into current land use and have minimal water requirements. In contrast, utility-scale solar technologies have 

significant land requirements and can – depending upon the 

specific generation and cooling technologies – also require 

significant water resources. For instance, utility-scale photovoltaic systems can require three to ten acres per megawatt 

(MW) of generating capacity32 and consume as much as five 

gallons of water per megawatt hour (MWh) of electricity production. Utility-scale concentrating solar systems can require 

up to 15 acres per MW33 and consume 1,040 gallons of water 

per MWh34 using wet cooling (and 97% less water with dry cooling). A recent U.S. Department of Energy study concluded 

that 14% of the U.S. demand for electricity could be met with solar power by 2030.34 To generate that amount of solar power 

would require rooftop installations plus about 0.9 million to 2.7 million acres, equivalent to about 1% to 4% of the land area 

of Arizona, for utility-scale solar power systems and concentrating solar power (CSP).34

Recognizing water limitations, most large-scale solar power systems now in planning or development are designed with dry 

cooling that relies on molten salt or other materials for heat transfer. However, while dry cooling systems reduce the need 

for water, they have lower plant thermal efficiencies, and therefore reduced production on hot days.35 Overall, as with other 

generation technologies, plant designs will have to carefully balance cost, operating issues, and water availability.

Biofuels

Biomass-based energy is currently the largest renewable energy source in the U.S., and biofuels from crops, grass, and 

trees are the fastest growing renewable domestic bioenergy sector.13 In 2011, approximately 40 million acres of cropland in 

the U.S. were used for ethanol production, roughly 16% of the land planted for the eight major field crops.37 The long-term 

environmental and social effects of biofuel production and use depend on many factors: the type of feedstock, manageFigure 10.8. Photovoltaic panels convert sunlight directly 

into electricity. Utility-sized solar power plants require 

large tracts of land. Photo shows Duke Energy’s 113-acre 

Blue Wing Solar Project in San Antonio, Texas, one of 

the largest photovoltaic solar farms in the country. (Photo 

credit: Duke Energy 201036).

Renewable Energy and Land Use
271 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

10: ENERGY, WATER, AND LAND USE

Examples of energy, water, and land linkages (continued)

ment practices used to produce them, fuel production and conversion technologies, prior land use, and land- and water-use 

changes caused by their production and use.38,39 Biofuels potentially can reduce greenhouse gas emissions by displacing 

fossil fuel consumption. Biofuels that comply with the Energy Independence and Security Act of 2007 are required to reduce 

greenhouse gas emissions relative to fossil fuels. In addition, biofuels also have the potential to provide net environmental 

benefits compared to fossil fuels. For example, ethanol is used as a gasoline additive to meet air quality standards, replacing 

a previous additive that leaked from storage tanks and contaminated groundwater.40 However, increases in corn production 

for biofuel has been cited as contributing to harmful algal blooms.38

Currently, most U.S. biofuels, primarily ethanol (from corn) and biodiesel (mainly from soy), are produced from edible parts 

of crops grown on rain-fed land. Consumptive water use over the life cycle of corn-grain ethanol varies widely, from 15 gallons of water per gallon of gasoline equivalent for rain-fed corn-based ethanol in Ohio, to 1,500 gallons of water per gallon of 

gasoline equivalent for irrigated corn-based ethanol in New Mexico. In comparison, producing and refining petroleum-based 

fuels uses 1.9 to 6.6 gallons of water per gallon of gasoline.38,41

The U.S. Renewable Fuels Standard (RFS) aims to expand production of cellulosic ethanol to at least 16 billion gallons per 

year by 2022. Cellulosic biofuels, derived from the entire plant rather than just the food portions, potentially have several 

advantages, such as fewer water quality impacts,42 less water consumption, and the use of forest-derived feedstocks.38 Cellulosic biofuels have not yet been produced in large volumes in the United States. The RFS target could require up to an 

additional 30 to 60 million acres of land, or alternatively be sourced from other feedstocks, such as forest and agricultural 

residues and municipal solid waste, but such supplies are projected to be inadequate for meeting the full cellulosic biofuel 

standard.38

Conversion of land not in cropland to crops for biofuel production may increase water consumption and runoff of fertilizers, 

herbicides, and sediment.43 The impacts of climate change, particularly in areas where water availability may decrease (see 

Ch. 2: Our Changing Climate, Ch. 3: Water, and Ch. 6: Agriculture), however, may make it increasingly difficult to raise crops 

in arid regions of the country. The use of crops that are better suited to arid conditions and are efficient in recycling nutrients, 

such as switchgrass for cellulosic ethanol, could lower the vulnerability of biofuel production to climate change.44 Another 

potential source of biomass for biofuel production is microalgae, but the existing technologies are still not carbon neutral, 

nor commercially viable.45

Carbon Capture and Storage

Carbon capture and storage (CCS) technologies have the potential to capture 90% of CO2 emissions from coal and natural 

gas combustion by industrial and electric sector facilities and thus allow continued use of low-cost fossil fuels in a carbonconstrained future.46 CCS captures CO2 post- or pre-fuel combustion and injects the CO2 into geologic formations for longterm storage. In addition, combining CCS with bioenergy applications represents one of a few potential options for actually 

removing CO2 from the atmosphere47 because carbon that was recently in the atmosphere and accumulated by growing 

plants can be captured and stored. 

CCS substantially increases the cost of building and operating a power plant, both through up-front costs and additional 

energy use during operation (referred to as “parasitic loads” or an energy penalty).46 Substantial amounts of water are also 

used to separate CO2 from emissions and to generate the required parasitic energy. With current technologies, CCS can increase water consumption 30% to 100%.48 Gasification technologies, where coal or biomass are converted to gases and CO2

is separated before combustion, reduce the energy penalty and water requirements, but currently at higher capital costs.49

As with other technologies, technology and design choices for CCS need to be balanced with water requirements and water 

availability. Climate change will influence the former via effects on energy demand and the latter via precipitation changes. 

CCS facilities themselves have relatively modest land demands compared to some other generation options. However, bioenergy use with CCS would imply a much stronger land linkage.

CCS facilities for electric power plants are currently operating at pilot scale, and a commercial scale demonstration project 

is under construction.50 Although the potential opportunities are large, many uncertainties remain, including cost, demonstration at scale, environmental impacts, and what constitutes a safe, long-term geologic repository for sequestering carbon 

dioxide.51
272 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

10: ENERGY, WATER, AND LAND USE

Key Message 3: Challenges to Reducing Vulnerabilities 

Jointly considering risks, vulnerabilities, and opportunities associated with energy, water, 

and land use is challenging, but can improve the identification and evaluation of options 

for reducing climate change impacts.

The complex nature of interactions among energy, water, and 

land systems, particularly in the context of climate change, 

does not lend itself to simple solutions. The energy, water, 

and land interactions themselves create vulnerabilities to 

competing resource demands. Climate change is an additional 

stressor. However, resource management decisions are often 

focused on just one of these sectors. Where the three sectors 

are tightly coupled, options for mitigating or adapting to 

climate change and consideration of the tradeoffs associated 

with technological or resource availability may be limited. 

The complex nature of water and energy systems are also 

highlighted in Chapter 3 (Water), which discusses water 

constraints in many areas of the U.S., and in Chapter 4 (Energy), 

where it is noted that there will be challenges across the nation 

for water quality to comply with thermal regulatory needs for 

energy production. 

A changing climate, particularly in areas projected to be warmer 

and drier, is expected to lead to drought and stresses on water 

supply, affecting energy, water, and land sectors in the United 

States. As the Texas drought of 2011 and 2012 illustrates, 

impacts to a particular sector, such as energy production, 

generate consequences for the others, such as water resource 

availability. Similarly, new energy development and production 

will require careful consideration of land and water sector 

resources. As a result, vulnerability to climate change depends 

on energy, water, and land linkages and on climate risks across 

all sectors, and decision-making is complex.

Figure 10.9. In many parts of the country, competing demands for water create stress in local and regional 

watersheds. Map shows a “water supply stress index” for the U.S. based on observations, with widespread 

stress in much of the Southwest, western Great Plains, and parts of the Northwest. Watersheds are 

considered stressed when water demand (from power plants, agriculture, and municipalities) exceeds 40% 

(water supply stress index of 0.4) of available supply. (Figure source: Averyt et al. 201120).

Water Stress in the U.S. 
273 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

10: ENERGY, WATER, AND LAND USE

The Columbia River Basin is one example of an area where risks, 

vulnerabilities, and opportunities are being jointly considered 

by a wide range of stakeholders and decision-makers (see Ch. 

28: Adaptation). The Columbia River, which crosses the U.S.-

Canada border, is the fourth largest river on the continent by 

volume, and it drives the production of more electricity than 

any other river in North America. Approximately 15% of the 

Columbia River Basin lies within British Columbia (Figure 10.10), 

but an average of 30% of the total average discharge originates 

from the Canadian portion of the watershed.52 To provide flood 

control for the U.S. and predicted releases for hydropower 

generation, the Columbia River system is managed through 

a treaty that established a cooperative agreement between 

the United States and Canada to regulate the river for these 

two uses.53 The basin also supports a range of other uses, such 

as navigation, tribal uses, irrigation, fish and wildlife habitat, 

recreation, and water resources for agricultural, industrial, and 

individual use. For all multi-use river basins, understanding 

the combined vulnerability of energy, water, and land use to 

climate change is essential to planning for water management 

and climate change adaptation.

A recent report projects a warmer annual, and drier summer,

climate for the Northwest (Ch. 21: Northwest; Ch. 2: Our 

Changing Climate, Figures 2.14 and 2.15; Appendix 3: Climate 

Science Supplement, Figures 21 and 22),54 potentially affecting 

both the timing and amounts of water availability. For example, 

if climate change reduces streamflow at certain times, fish and 

wildlife, as well as recreation, may be vulnerable.55 Climate 

change stressors will also increase the vulnerability of the 

region’s vast natural ecosystems and forests in multiple ways 

(see Ch. 7: Forests and Ch. 8: Ecosystems). Currently, only 30% of 

annual Columbia River Basin runoff can be stored in reservoirs.

56

Longer growing seasons might provide opportunities for 

greater agricultural production, but the projected warmer and 

drier summers could increase demand for water for irrigation, 

Figure 10.10. Agriculture is in yellow, forests are shades of green, shrublands are gray, and urban areas are in red. The river is 

used for hydropower generation, flood control, agriculture irrigation, recreation, support of forest and shrubland ecosystems, and 

fish and wildlife habitat. Climate change may impact the timing and supply of the water resources, affecting the multiple uses of 

this river system. (Figure source: Northwest Habitat Institute 1999).

The Columbia River Basin Land Use and Land Cover
274 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

10: ENERGY, WATER, AND LAND USE© James Christensen/ Foto Natura/Corbis

perhaps at the expense of other water uses 

due to storage limitations. Wetter winters 

might offset increased summer demands. 

However, the storage capacities of many 

water reservoirs with multiple purposes, 

including hydropower, were not designed 

to accommodate significant increases 

in winter precipitation. Regulations and 

operational requirements also constrain 

the ability to accommodate changing 

precipitation patterns (see Ch. 3: Water). 

Because of the complexity of interactions 

among energy, water, and land systems, 

considering the complete picture of climate 

impacts and potential adaptations can help 

provide better solutions. Adaptation to 

climate change occurs in large part locally 

or regionally, and conflicting stakeholder 

priorities, institutional commitments, 

and international agreements have the 

potential to complicate or even compromise 

adaption strategies with regard to energy, water, and land 

resources (see also Ch. 28: Adaptation). Effective adaptation to 

the impacts of climate change requires a better understanding 

of the interactions among the energy, water, and land resource 

sectors. Whether managing for water availability and quality in 

the context of energy systems, or land restrictions, or both, an 

improved dialog between the scientific and decision-making 

communities will be necessary to evaluate tradeoffs and 

compromises needed to manage and understand this complex 

system. This will require not only integrated and quantitative 

analyses of the processes that underlie the climate and natural 

systems, but also an understanding of decision criteria and risk 

analyses to communicate effectively with stakeholders and 

decision-makers.
275 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

10: ENERGY, WATER, AND LAND USE

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M. Loos, and L. A. Meyer, Eds. Intergovernmental Panel on 

Climate Change, Cambridge University Press, 442 pp. [Available 

online at http://www.ipcc.ch/pdf/special-reports/srccs/srccs_

wholereport.pdf]

48. Newmark, R. L., S. J. Friedmann, and S. A. Carroll, 2010: 

Water challenges for geologic carbon capture and sequestration. 

Environmental Management, 45, 651-661, doi:10.1007/s00267-010-

9434-1. [Available online at http://link.springer.com/content/pdf

/10.1007%2Fs00267-010-9434-1]

49. NETL, 2010: Cost and Performance Baseline for Fossil Energy 

Plants Volume 1: Bituminous Coal and Natural Gas to Electricity 

Revision 2, November 2010. DOE/NETL-2010/1397, 626 pp., 

National Energy Technology Laboratory, U.S. Department of 

Energy. [Available online at http://www.netl.doe.gov/File%20

Library/Research/Coal/ewr/water/BitBase_FinRep_Rev2.pdf] 

——, cited 2013: Gasifipedia: Advantages of Gasification – 

High Efficiency. National Energy Technology Laboratory, U.S. 

Department of Energy. [Available online at http://www.netl.doe.

gov/technologies/coalpower/gasification/gasifipedia/]

50. Mississippi Power, cited 2013: Mississippi Power revises dates, 

cost of Kemper plant project. [Available online at http://www.

mississippipower.com/kemper/news_oct29-2013.asp] 
10: ENERGY, WATER, AND LAND USE

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278 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

NETL, 2013: Demonstration of a Coal-Based Transport Gasifier, 

2 pp., National Energy Technology Laboratory, U.S. Department 

of Energy. [Available online at http://www.netl.doe.gov/

File%20Library/Research/Coal/major%20demonstrations/ccpi/

NT42391.pdf] 

51. USGS, 2013: National Assessment of Geologic Carbon Dioxide 

Storage Resources—Summary: U.S. Geological Survey Fact 

Sheet 2013–3020, 6 pp., U.S. Geological Survey Geologic Carbon 

Dioxide Storage Resources Assessment Team, Reston, VA. 

[Available online at http://pubs.usgs.gov/fs/2013/3020/pdf/

FS2013-3020.pdf]

52. Davidson, H. C., and R. K. Paisley, 2009: The Columbia River 

Basin: Issues & Driving forces within the Columbia River 

Basin with the potential to affect future transboundary water 

management. Final report for the Canadian Columbia River 

Forum., 50 pp., Canadian Columbia River Forum. [Available 

online at http://www.ccrf.ca/assets/docs/pdf/issues-drivingforces-ccrf-final-march-2009.pdf]

53. Center for Columbia River History, cited 2012: Treaty relating to 

cooperative development of the water resources of the Columbia 

River Basin (with Annexes). [Available online at http://www.ccrh.

org/comm/river/docs/cotreaty.htm]

54. Kunkel, K. E., L. E. Stevens, S. E. Stevens, L. Sun, E. Janssen, 

D. Wuebbles, K. T. Redmond, and J. G. Dobson, 2013: Regional 

Climate Trends and Scenarios for the U.S. National Climate 

Assessment: Part 6. Climate of the Northwest U.S. NOAA 

Technical Report NESDIS 142-6. 83 pp., National Oceanic and 

Atmospheric Administration, National Environmental Satellite, 

Data, and Information Service, Washington, D.C. [Available 

online at http://www.nesdis.noaa.gov/technical_reports/

NOAA_NESDIS_Tech_Report_142-6-Climate_of_the_

Northwest_U.S.pdf]

55. Dalton, M., P. Mote, J. A. Hicke, D. Lettenmaier, J. Littell, 

J. Newton, P. Ruggiero, and S. Shafer, 2012: A Workshop in 

Risk-Based Framing of Climate Impacts in the Northwest: 

Implementing the National Climate Assessment Risk-Based 

Approach 77 pp. [Available online at http://downloads.usgcrp.

gov/NCA/Activities/northwestncariskframingworkshop.pdf]

56. Bruce, J. P., H. Martin, P. Colucci, G. McBean, J. McDougall, 

D. Shrubsole, J. Whalley, R. Halliday, M. Alden, L. Mortsch, 

and B. Mills, 2003: Climate Change Impacts on Boundary and 

Transboundary Water Management; Report to the Climate 

Change Impacts Adaptation Program, 161 pp., Natural Resources 

Canada. [Available online at http://www.env.uwaterloo.ca/

research/aird/aird_pub/Climate%20Change%20Impacts%20

on%20Boundary%20and%20Transboundary%20Water%20

Management.pdf]

57. Karl, T. R., J. T. Melillo, and T. C. Peterson, Eds., 2009: Global 

Climate Change Impacts in the United States. Cambridge University 

Press, 189 pp. [Available online at http://downloads.globalchange.

gov/usimpacts/pdfs/climate-impacts-report.pdf]

58. DOE, 2009: Concentrating Solar Power Commercial Application 

Study: Reducing Water Consumption of Concentrating Solar 

Power Electricity Generation. Report to Congress., 24 pp., U.S. 

Department of Energy, Washington, D.C. [Available online at 

http://www1.eere.energy.gov/solar/pdfs/csp_water_study.pdf] 

EIA, 2011: Natural Gas Annual 2010. DOE/EIA-0131(10). U.S. 

Department of Energy, U.S. Energy Information Administration. 

[Available online at http://www.eia.gov/naturalgas/annual/pdf/

front_matter.pdf]

59. Mai, T., R. Wiser, D. Sandor, G. Brinkman, G. Heath, P. Denholm, 

D. J. Hostick, N. Darghouth, A. Schlosser, and K. Strzepek, 2012: 

Renewable Electricity Futures Study. Volume 1: Exploration 

of High-Penetration Renewable Electricity Futures. NREL/

TP-6A20-52409-1. M. M. Hand, S. Baldwin, E. DeMeo, J. M. 

Reilly, T. Mai, D. Arent, G. Porro, M. Meshek, and D. Sandor, 

Eds., 280 pp., National Renewable Energy Laboratory (NREL), 

Golden, CO. [Available online at http://www.nrel.gov/docs/

fy12osti/52409-1.pdf]
279 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

10: ENERGY, WATER, AND LAND USE

SUPPLEMENTAL MATERIAL

TRACEABLE ACCOUNTS

Process for Developing Key Messages 

The authors met for a one-day face-to-face meeting, and held 

teleconferences approximately weekly from March through August 2012. They considered a variety of technical input documents, including a Technical Input Report prepared through an 

interagency process,1 and 59 other reports submitted through the 

Federal Register Notice request for public input. The key messages were selected based on expert judgment, derived from the 

set of examples assembled to demonstrate the character and 

consequences of interactions among the energy, water, and land 

resource sectors.

Key message #1 Traceable Account

Energy, water, and land systems interact in many 

ways. Climate change affects the individual sectors and their interactions; the combination of these 

factors affects climate change vulnerability as well 

as adaptation and mitigation options for different 

regions of the country.

Description of evidence base

The key message and supporting text summarizes extensive evidence documented in the Technical Input Report (TIR): Climate 

and Energy-Water-Land System Interactions: Technical Report to 

the U.S. Department of Energy in Support of the National Climate 

Assessment.1 Technical input reports (59) on a wide range of topics were also received and reviewed as part of the Federal Register 

Notice solicitation for public input. 

The TIR1 incorporates the findings of a workshop, convened by the 

author team, of experts and stakeholders. The TIR summarizes 

numerous examples of interactions between specific sectors, such 

as energy and water or water and land use. A synthesis of these 

examples provides insight into how climate change impacts the 

interactions between these sectors.

The TIR1 shows that the character and significance of interactions among the energy, water, and land resource sectors vary 

regionally. Additionally, the influence of impacts on one sector for 

the other sectors will depend on the specific impacts involved. 

Climate change impacts will affect the interactions among sectors, 

but this may not occur in all circumstances.

The key message is supported by the National Climate Assessment Climate Scenarios (for example, Kunkel et al. 201354). Many 

of the historic trends included in the Climate Scenarios are based 

on data assembled by the Cooperative Observer Network of the 

National Weather Service (http://www.nws.noaa.gov/om/coop/). 

Regional climate outlooks are based on the appropriate regional 

chapter.

The Texas drought of 2011 and 2012 provides a clear example 

of cascading impacts through interactions among the energy, water, and land resource sectors.3,4,5,7,8,9 The U.S. Drought Monitor 

(http://droughtmonitor.unl.edu/) provides relevant historical data. 

Evidence also includes articles appearing in the public press11 and 

Internet media.6

New information and remaining uncertainties

The Texas drought of 2011 and 2012 demonstrates the occurrence of cascading impacts involving the energy, land, and water 

sectors; however, the Texas example cannot be generalized to all 

parts of the country or to all impacts of climate change (for example, see Chapter 3 for flooding and energy system impacts). The 

Technical Input Report1 provides numerous additional examples 

and a general description of interactions that underlie cascading 

impacts between these resource sectors.

There are no major uncertainties regarding this key message. 

There are major uncertainties, however, in the magnitude of impacts in how decisions in one sector might affect another. The 

intensity of interactions will be difficult to assess under climate 

change.

Assessment of confidence based on evidence 

Given the evidence base and remaining uncertainties, confidence 

is high. The primary limitation on the confidence assigned to this 

key message is with respect to its generality. The degree of interactions among the energy, water, and land sectors varies regionally as does the character and intensity of climate change.
10: ENERGY, WATER, AND LAND USE

Traceable Accounts

280 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Key message #2 Traceable Account

The dependence of energy systems on land and 

water supplies will influence the development of 

these systems and options for reducing greenhouse 

gas emissions, as well as their climate change vulnerability.

Description of evidence base

The key message and supporting text summarizes extensive evidence documented in the Technical Input Report (TIR): Climate 

and Energy-Water-Land System Interactions: Technical Report to 

the U.S. Department of Energy in Support of the National Climate 

Assessment.1 Technical input reports (59) on a wide range of topics were also received and reviewed as part of the Federal Register 

Notice solicitation for public input. 

Synthesis and Assessment Product 2.1 of the Climate Change 

Science Program,22 which informed the prior National Climate 

Assessment,57 describes relationships among different future 

mixtures of energy sources, and associated radiative forcing of 

climate change, as a context for evaluating emissions mitigation 

options.

Energy, water, and land linkages represent constraints, risks, and 

opportunities for private/public planning and investment decisions. There are evolving water and land requirements for four 

energy technologies: natural gas from shale,13 solar power,34 biofuels,38,39 and carbon dioxide capture and storage (CCS).47 Each 

Confidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent 

results, well documented and 

accepted methods, etc.), high 

consensus

High

Moderate evidence (several sources, some consistency, methods 

vary and/or documentation limited, 

etc.), medium consensus

Medium

Suggestive evidence (a few 

sources, limited consistency, models incomplete, methods emerging, 

etc.), competing schools of thought

Low

Inconclusive evidence (limited 

sources, extrapolations, inconsistent findings, poor documentation 

and/or methods not tested, etc.), 

disagreement or lack of opinions 

among experts

of these four technologies could contribute to reducing U.S. emissions of greenhouse gases. These technologies illustrate energy, 

water, and land linkages and other complexities for the design, 

planning, and deployment of our energy future.

Evidence for energy production and use are derived from U.S. 

government reports.58 The contributions of hydraulic fracturing to 

natural gas production are based on a brief article by the Energy 

Information Administration13 and a primer by the U.S. Department 

of Energy.28 Information about water and energy demands for 

utility-scale solar power facilities is derived from two major DOE 

reports.34,59 Distribution of U.S. solar energy resources is from 

Web-based products of the National Renewable Energy Laboratory (http://www.nrel.gov/gis/). On biofuels, there are government 

data on the scale of biomass-based energy,13 and studies on water 

and land requirements and other social and environmental aspects.38,39

New information and remaining uncertainties

There are no major uncertainties regarding this key message. 

Progress in development and deployment of the energy technologies described has tended to follow a pattern: potential constraints 

arise because of dependence on water and land resources, but 

then these constraints motivate advances in technology to reduced 

dependence or result in adjustments of societal priorities. There 

are uncertainties in how energy systems’ dependence on water will 

be limited by other resources, such as land; uncertainties about 

the effects on emissions and the development and deployment of 

future energy technologies; and uncertainties about the impacts 

of climate change on energy systems.

Assessment of confidence based on evidence 

Given the evidence base and remaining uncertainties, confidence 

is high. The primary limitation on confidence assigned to this 

key message is with respect to its generality and dependence on 

technological advances. Energy technology development has the 

potential to reduce water and land requirements, and to reduce 

vulnerability to climate change impacts. It is difficult to forecast 

success in this regard for technologies such as CCS that are still 

in early phases of development.

Key message #3 Traceable Account

Jointly considering risks, vulnerabilities, and opportunities associated with energy, water, and land 

use is challenging, but can improve the identification and evaluation of options for reducing climate 

change impacts.

Description of evidence base

The key message and supporting text summarizes extensive evidence documented in the Technical Input Report (TIR): Climate 

and Energy-Water-Land System Interactions: Technical Report to 

the U.S. Department of Energy in Support of the National Climate 

Assessment.1 Technical input reports (59) on a wide range of top-
10: ENERGY, WATER, AND LAND USE

Traceable Accounts

281 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

ics were also received and reviewed as part of the Federal Register 

Notice solicitation for public input. 

Interactions among energy, water, and land resource sectors can 

lead to stakeholder concerns that shape options for reducing vulnerability and thus for adapting to climate change. The Columbia 

River System provides a good example of an area where risks, 

vulnerabilities, and opportunities are being jointly considered.55,56

The 2011 Mississippi basin flooding, which shut down substations, provides another example of the interactions of energy, 

water, and land systems (Ch. 3: Water). For all multi-use river 

basins, understanding the combined vulnerability of energy, water, 

and land use to climate change is essential to planning for water 

management and climate change adaptation. 

New information and remaining uncertainties

There are no major uncertainties regarding this key message; 

however, it is highly uncertain the extent to which local, state 

and national policies will impact options to reduce vulnerability to 

climate change. 

Assessment of confidence based on evidence

Given the evidence base and remaining uncertainties, confidence 

is high. The primary limitation on confidence assigned to this key 

message is with respect to the explicit knowledge of the unique 

characteristics of each region with regards to impacts of climate 

change on energy, water, land, and the interactions among these 

sectors.
282

Climate Change Impacts in the United States

CHAPTER 11

URBAN SYSTEMS, INFRASTRUCTURE, 

AND VULNERABILITY

INFORMATION DRAWN FROM THIS CHAPTER IS INCLUDED IN THE HIGHLIGHTS REPORT AND IS IDENTIFIED BY THIS ICON

Recommended Citation for Chapter

Cutter, S. L., W. Solecki, N. Bragado, J. Carmin, M. Fragkias, M. Ruth, and T. J. Wilbanks, 2014: Ch. 11: Urban Systems, 

Infrastructure, and Vulnerability. Climate Change Impacts in the United States: The Third National Climate Assessment, J. 

M. Melillo, Terese (T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 282-296. doi:10.7930/

J0F769GR. 

On the Web: http://nca2014.globalchange.gov/report/sectors/urban

Convening Lead Authors

Susan L. Cutter, University of South Carolina

William Solecki, City University of New York

Lead Authors

Nancy Bragado, City of San Diego

JoAnn Carmin, Massachusetts Institute of Technology

Michail Fragkias, Boise State University

Matthias Ruth, Northeastern University

Thomas J. Wilbanks, Oak Ridge National Laboratory
283 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Key Messages

1. Climate change and its impacts threaten the well-being of urban residents in all U.S. regions. 

Essential infrastructure systems such as water, energy supply, and transportation will 

increasingly be compromised by interrelated climate change impacts. The nation’s economy, 

security, and culture all depend on the resilience of urban infrastructure systems.

2. In urban settings, climate-related disruptions of services in one infrastructure system will almost 

always result in disruptions in one or more other infrastructure systems.

3. Climate vulnerability and adaptive capacity of urban residents and communities are influenced 

by pronounced social inequalities that reflect age, ethnicity, gender, income, health, and 

(dis)ability differences. 

4. City government agencies and organizations have started adaptation plans that focus on 

infrastructure systems and public health. To be successful, these adaptation efforts require 

cooperative private sector and governmental activities, but institutions face many barriers to 

implementing coordinated efforts.

11

Climate change poses a series of interrelated challenges to the 

country’s most densely populated places: its cities. The United 

States is highly urbanized, with about 80% of its population 

living in cities and metropolitan areas. Many cities depend on 

infrastructure, like water and sewage systems, roads, bridges, 

and power plants, that is aging and in need of repair or replacement. Rising sea levels, storm surges, heat waves, and extreme 

weather events will compound these issues, stressing or even 

overwhelming these essential services.

Cities have become early responders to climate change challenges and opportunities due to two simple facts: first, urban 

areas have large and growing populations that are vulnerable 

for many reasons to climate variability and change; and second, cities depend on extensive infrastructure systems and the 

resources that support them. These systems are often connected to rural locations at great distances from urban centers.

The term infrastructure is used broadly and includes systems 

and assets that are essential for national and economic security, national public health or safety, or to the overall 

well-being of residents. These include energy, water and 

wastewater, transportation, public health, banking and 

finance, telecommunications, food and agriculture, and 

information technology, among others.

Urban dwellers are particularly vulnerable to disruptions 

in essential infrastructure services, in part because many 

of these infrastructure systems are reliant on each other. 

For example, electricity is essential to multiple systems, 

and a failure in the electrical grid can affect water treatment, transportation services, and public health. These 

infrastructure systems – lifelines to millions – will continue to be affected by various climate-related events 

and processes.

As climate change impacts increase, climate-related 

events will have large consequences for significant numbers of people living in cities or suburbs. Also at risk 

Heavy snowfalls during winter storms affect transportation systems and 

other urban infrastructure.

© FRANK POLICH/epa/Corbis

URBAN SYSTEMS,

INFRASTRUCTURE, AND VULNERABILITY
284 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

11: URBAN SYSTEMS, INFRASTRUCTURE, AND VULNERABILITY

from climate change are historic properties and sites as well 

as cultural resources and archeological sites. Vulnerability assessments and adaptation planning efforts could also include 

these irreplaceable resources. Changing conditions also create 

opportunities and challenges for urban climate adaptation (Ch. 

28: Adaptation), and many cities have begun planning to address these changes.

Key Message 1: Urbanization and Infrastructure Systems

Climate change and its impacts threaten the well-being of urban residents in all U.S. regions. 

Essential infrastructure systems such as water, energy supply, and transportation will 

increasingly be compromised by interrelated climate change impacts. The nation’s economy, 

security, and culture all depend on the resilience of urban infrastructure systems.

Direct and interacting effects of climate change will expose 

people who live in cities across the United States to multiple 

threats. Climate changes affect the built, natural, and social 

infrastructure of cities, from storm drains to urban waterways 

to the capacity of emergency responders. Climate change increases the risk, frequency, and intensity of certain extreme 

events like intense heat waves, heavy downpours, flooding from intense precipitation and coastal storm surges, and 

disease incidence related to temperature and precipitation 

changes. The vulnerability of urban dwellers multiplies when 

the effects of climate change interact with pre-existing urban 

stressors, such as deteriorating infrastructure, areas of intense 

poverty, and high population density. 

Three fundamental conditions define the key connections 

among urban systems, residents, and infrastructure.1,2 First, 

cities are dynamic, and are constantly being built and rebuilt 

through cycles of investment and innovation. Second, infrastructure in many cities has exceeded its design life and continues to age, resulting in an increasingly fragile system. At 

both local and national levels, infrastructure requires ongoing maintenance and investment to avoid a decline in service. 

Third, urban areas present tremendous 

social challenges, given widely divergent socioeconomic conditions and 

dynamic residence patterns that vary 

in different parts of each city. Heightened vulnerability of coastal cities and 

other metropolitan areas that are subject to storm surge, flooding, and other 

extreme weather or climate events will 

exacerbate impacts on populations and 

infrastructure systems.

Approximately 245 million people live in 

U.S. urban areas, a number expected to 

grow to 364 million by 2050.3 Paradoxically, as the economy and population 

of urban areas grew in past decades, 

the built infrastructure within cities 

and connected to cities deteriorated, 

becoming increasingly fragile and deficient.1,2

 Existing built infrastructure 

(such as buildings, energy, transportation, water, and sanitation systems) is expected to become more stressed in the next 

decades – especially when the impacts of climate change are 

added to the equation.4 As infrastructure is highly interdependent, failure in particular sectors is expected to have cascading effects on most aspects of affected urban economies. 

Further expansion of the U.S. urban landscape into suburban 

and exurban spaces is expected, and new climate adaptation 

and resiliency plans will need to account for this (Ch. 28: Adaptation).5

 Significant increases in the costs of infrastructure 

investments also are expected as population density becomes 

more diffuse.6

The vulnerability of different urban populations to hazards and 

risks associated with climate change depends on three characteristics: their exposure to particular stressors, their sensitivity 

to impacts, and their ability to adapt to changing conditions.8,9

Many major U.S. metropolitan areas, for example, are located 

on or near the coast and face higher exposure to particular climate impacts like sea level rise and storm surge, and thus may 

face complex and costly adaptation demands (Ch. 25: Coasts; 

Ch. 28: Adaptation). But as people begin to respond to new 

Coastal cities are vulnerable to sea level rise, storm surge, and related impacts. 

©Joe Raedle/Getty Images
285 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

11: URBAN SYSTEMS, INFRASTRUCTURE, AND VULNERABILITY

information about climate change through the urban development process, social and infrastructure vulnerabilities can be 

altered.10 For example, the City of New York conducted a comprehensive review of select building and construction codes 

and standards in response to increased climate change risk in 

order to identify adjustments that could be made to increase 

climate resilience. Climate change stressors will bundle with 

other socioeconomic and engineering stressors already connected to urban and infrastructure systems.1

Key Message 2: Essential Services are Interdependent

In urban settings, climate-related disruptions of services in one infrastructure system will 

almost always result in disruptions in one or more other infrastructure systems.

Urban areas rely on links to multiple jurisdictions through a 

complex set of infrastructure systems.11 For example, cities 

depend on other areas for supplies of food, materials, water, energy, and other inputs, and surrounding areas are destinations for products, services, 

and wastes from cities. If infrastructure and other 

connections among source areas and cities are disrupted by climate change, then the dependent urban area also will be affected.12 Moreover, the economic base of an urban area depends on regional 

comparative advantage; therefore, if competitors, 

markets, and/or trade flows are affected by climate 

change, a particular urban area is also affected.2

Urban vulnerabilities to climate change impacts are 

directly related to clusters of supporting resources 

and infrastructures located in other regions. For example, about half of the nation’s oil refineries are located in only four states.13 Experience over the past 

decade with major infrastructure disruptions, such 

as the 2011 San Diego blackout, the 2003 Northeast 

blackout, and Hurricane Irene in 2011, has shown 

that the greatest losses from disruptive events may be distant 

from where damages started.2 In another example, Hurricane 

Figure 11.1. Extreme weather events can affect multiple systems that provide services for millions of people in urban settings. The 

satellite images depict city lights on a normal night (left) and immediately following Hurricane Sandy (right). Approximately five million 

customers in the New York metropolitan region lost power. (Figure source: NASA Earth Observatory7). 

Blackout in New York and New Jersey after Hurricane Sandy

©Iwan Baan/Getty Images

A failure of the electrical grid can affect everything from water treatment to 

public health.
286 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

11: URBAN SYSTEMS, INFRASTRUCTURE, AND VULNERABILITY

Katrina disrupted oil terminal operations in southern Louisiana, not because of direct damage to port facilities, but because workers could not 

reach work locations through surface 

transportation routes and could not 

be housed locally because of disruption to potable water supplies, housing, and food shipments.14

Although infrastructures and urban 

systems are often considered individually – for example, transportation or water supply or wastewater/

drainage – they are usually highly 

interactive and interdependent.15

Such interdependencies can lead to 

cascading disruptions throughout 

urban infrastructures. These disruptions, in turn, can result in unexpected impacts on communication, 

water, and public health sectors, at 

least in the short term. On August 8, 

2007, New York City experienced an 

intense rainfall and thunderstorm 

event during the morning commute, 

where between 1.4 and 3.5 inches of rain fell within two 

hours.16 The event started a cascade of transit system failures 

– eventually stranding 2.5 million riders, shutting down much 

of the subway system, and severely disrupting the city’s bus 

system.16,17 The storm’s impact was unprecedented and, coupled with two other major system disruptions that occurred 

in 2004 and 2007, became the impetus for a full-scale assessment and review of transit procedures and policy in response 

to climate change.16,17,18

In August 2003, an electric power blackout that caused 50 million people in the U.S. Northeast and Midwest and Ontario, 

Canada, to lose electric power further 

illustrates the interdependencies of 

major infrastructure systems. The 

blackout caused significant indirect 

damage, such as shutdowns of water 

treatment plants and pumping stations. Other impacts included interruptions in communication systems 

for air travel and control systems for 

oil refineries. At a more local level, 

the lack of air conditioning and elevator access meant many urban residents were stranded in over-heating 

high-rise apartments. Similar cascading impacts have been observed from 

extreme weather events such as Hurricanes Katrina and Irene.2

 In fact, as 

urban infrastructures become more 

interconnected and more complex, 

the likelihood of large-scale cascading impacts will increase as risks to 

infrastructure increase.19

Figure 11.2. In urban settings, climate-related disruptions of services in one infrastructure 

system will almost always result in disruptions in one or more other systems. When power 

supplies that serve urban areas are interrupted after a major weather event, for example, 

public health, transportation, and banking systems may all be affected. This schematic 

drawing illustrates some of these connections. (Figure source: adapted from Wilbanks 

et al. 20122).

Urban Support Systems are Interconnected

Storm surges reach farther inland as they ride on top of sea levels that are higher due to 

warming.

©STAN HONDA/AFP/Getty Images
287 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

11: URBAN SYSTEMS, INFRASTRUCTURE, AND VULNERABILITY

Hurricane sandy: urban systems, infrastructure, and vulnerability

Sandy made landfall on the New Jersey shore 

just south of Atlantic City on October 29, 

2012, and became one of the most damaging storms to strike the continental United 

States. Sandy affected cities throughout the 

Atlantic seaboard, extending across the eastern United States to Chicago, Illinois, where 

it generated 20-foot waves on Lake Michigan 

and flooded the city’s Lake Shore Drive. The 

storm’s strength and resulting impact has 

been correlated with Atlantic Ocean water 

temperatures near the coast that were roughly 5 ̊F above normal, and with sea level rise 

along the region’s coastline as a result of a 

warming climate. 

Sandy caused significant loss of life as well 

as tremendous destruction of property and 

critical infrastructure. It disrupted daily life 

for millions of coastal zone residents across 

the New York-New Jersey metropolitan area, despite this being one of the best disaster-prepared coastal regions in the 

country. The death toll from Sandy in the metropolitan region exceeded 100, and the damage was estimated to be at 

least $65 billion.20,21 At its peak, the storm cut electrical power to more than 8.5 million customers.21

The death and injury, physical devastation, multi-day power, heat, and water outages, gasoline shortages, and cascade 

of problems from Sandy’s impact reveal what happens when the complex, integrated systems upon which urban life depends are stressed and fail. One example is what occurred after a Consolidated Edison electricity distribution substation 

in lower Manhattan ceased operation at approximately 9 PM Monday evening, when its flood protection barrier (designed 

to be 1.5 feet above the 10-foot storm surge of record) was overtopped by Sandy’s 14-foot storm surge. As the substation 

stopped functioning, it immediately caused a system-wide loss of power for more than 200,000 customers. Residents 

in numerous high-rise apartment buildings were left without heat and lights, and also without elevator service and water 

(which must be pumped to upper floors).

Sandy also highlighted the vast differences in vulnerabilities across the extended metropolitan region. Communities 

and neighborhoods on the coast were most vulnerable to the physical impact of the record storm surge. Many low- to 

moderate-income residents live in these areas and suffered damage to or loss of their homes, leaving tens of thousands 

of people displaced or homeless. As a specific sub-population, the elderly and infirm were highly vulnerable, especially 

those living in the coastal evacuation zone and those on upper floors of apartment buildings left without elevator service. 

These individuals had limited adaptive capacity because they could not easily leave their residences.

Even with the extensive devastation, the effects of the storm would have been far worse if local climate resilience strategies had not been in place. For example, the City of New York and the Metropolitan Transportation Authority worked aggressively to protect life and property by stopping the operation of the city’s subway before the storm hit and moving the 

train cars out of low-lying, flood-prone areas. At the height of the storm surge, all seven of the city’s East River subway 

tunnels flooded. Catastrophic loss of life would have resulted if there had been subway trains operating in the tunnels 

when the storm struck. The storm also fostered vigorous debate among local and state politicians, other decision-makers, 

and stakeholders about how best to prepare the region for future storms. Planning is especially important given the expectation of increases in flood frequency resulting from more numerous extreme precipitation events and riverine and 

street level flooding, and coastal storm surge flooding associated with accelerated sea level rise and more intense (yet 

not necessarily more numerous) tropical storms. 

Courtesy of NOAA
288 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

11: URBAN SYSTEMS, INFRASTRUCTURE, AND VULNERABILITY

Key Message 3: Social Vulnerability and Human Well-Being

Climate vulnerability and adaptive capacity of urban residents and communities are 

influenced by pronounced social inequalities that reflect age, ethnicity, gender, 

income, health, and (dis)ability differences. 

“Social vulnerability” describes characteristics of populations 

that influence their capacity to prepare for, respond to, and 

recover from hazards and disasters.22,23,24 Social vulnerability 

also refers to the sensitivity of a population to climate change 

impacts and how different people or groups are more or less 

vulnerable to those impacts.25 Those characteristics that most 

often influence differential impacts include socioeconomic 

status (wealth or poverty), age, gender, special needs, race, 

and ethnicity.26 Further, inequalities reflecting differences in 

gender, age, wealth, class, ethnicity, health, and disabilities 

also influence coping and adaptive capacity, especially to climate change and climate-sensitive hazards.27

The urban elderly are particularly sensitive to heat waves. 

They are often physically frail, have limited financial resources, 

and live in relative isolation in their apartments. They may 

not have adequate cooling (or heating), or may be unable to 

temporarily relocate to cooling stations. This combination led 

to a significant number of elderly deaths during the 1995 Chicago heat wave.28 Similarly, the impacts of Hurricane Katrina in 

New Orleans illustrated profound differences based on race, 

gender, and class where these social inequalities strongly influenced the capacity of residents to prepare for and respond to 

the events.29 It is difficult to assess the specific nature of vulnerability for particular groups of people. Urban areas are not 

homogeneous in terms of the social structures that influence 

inequalities. Also, the nature of the vulnerability is context 

specific, with both temporal and geographic determinants, 

and these also vary between and within urban areas.

Key Message 4: Trends in Urban Adaptation – Lessons from Current Adopters

City government agencies and organizations have started adaptation plans that focus on 

infrastructure systems and public health. To be successful, these adaptation efforts 

require cooperative private sector and governmental activities, but institutions 

face many barriers to implementing coordinated efforts.

City preparation efforts for climate change include planning 

for ways in which the infrastructure systems and buildings, 

ecosystem and municipal services, and residents will be affected. In the first large-scale analysis of U.S. cities, a 2011 survey showed that 58% of respondents are moving forward on 

climate adaptation (Ch. 28: Adaptation), defined as any activity 

to address impacts that climate change could have on a community. Cities are engaged in activities ranging from education 

and outreach to assessment, planning, and implementation, 

with 48% reporting that they are in the preliminary planning 

and discussion phases.30

Cities either develop separate strategic adaptation plans30,32 or 

integrate adaptation into community or general plans (as have 

Seattle, Washington; Portland, Oregon; Berkeley, California; 

and Homer, Alaska) (Ch. 28: Adaptation).1 Some climate action 

plans target certain sectors like critical infrastructure,24,33 and 

these have been effective in diverse contexts ranging from 

hazard mitigation and public-health planning to coastal-zone 

management and economic development. 

Cities have employed several strategies for managing adaptation efforts. For example, some approaches to climate adaptation planning require both intra- and inter-governmental 

agency and department coordination (“New York City Climate 

Action”) (Ch. 28: Adaptation). As a result, many cities focus on 

sharing information and examining what aspects of government operations will be affected by climate change impacts 

in order to gain support from municipal agency stakeholders 

and other local officials.34 Some cities also have shared climate 

change action experiences, both within the United States and 

internationally, as is the case with ongoing communication between decision-makers in New York City and London, England. 

National, state, and local policies play an important role in 

fostering and sustaining adaptation. There are no national 

regulations specifically designed to promote urban adaptation. 

However, existing federal policies, like the National Historic 

Preservation Act and National Environmental Policy Act – particularly through its impact assessment provision and evaluation criteria process – can provide incentives for adaptation 

strategies for managing federal property in urban areas.1,35

In addition, recent activities of federal agencies focused on 

promoting adaptation and resilience have been developed in 

partnership with cities like Miami and New York.36 Policies and 

planning measures at the local level, such as building codes, 

zoning regulations, land-use plans, water supply management, 

green infrastructure initiatives, health care planning, and disaster mitigation efforts, can support adaptation.1,2,37

Engaging the public in adaptation planning and implementation has helped to inform and educate the community at large 
289 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

11: URBAN SYSTEMS, INFRASTRUCTURE, AND VULNERABILITY

about climate change, while ensuring that information and 

ideas flow back to policymakers.38 Engagement can also help in 

identifying vulnerable populations39 and in mobilizing people 

to encourage policy changes and take individual actions to 

reduce and adapt to climate change.40 For instance, the Cambridge Climate Emergency Congress selected a demographically diverse group of resident delegates and engaged them in a 

deliberative process intended to express preferences and generate recommendations to inform climate action.41 In addition, 

the Boston Climate Action Leadership Committee was initiated 

by the Mayor’s office with the expectation that they would rely 

on public consultation to develop recommendations for updating the city’s climate action plan.42

There are many barriers to action at the city level. Proactive 

adaptation efforts require that anticipated climate changes 

and impacts are evaluated and addressed in the course of the 

planning process (Ch. 26: Decision 

Support; Ch. 28: Adaptation).43 This 

means that climate projections and 

impact assessment data must be 

available, but most U.S. cities are unable to access suitable data or perform desired analyses.36 To address

technical aspects of adaptation, 

cities are promoting cooperation 

with local experts, such as the New 

York City Panel on Climate Change, 

which brings together experts from 

academia and the public and private 

sectors to consider how the region’s 

critical infrastructure will be affected by, and can be protected from, 

future climate change.10,44 A further 

illustration comes from Chicago, 

where multi-departmental groups 

are focusing on specific areas identified in Chicago’s Climate Action 

Plan.45

Private sector involvement can be 

influential in promoting city-level 

adaptation (Ch. 28: Adaptation). 

Many utilities, for example, have asset management programs that address risk and vulnerabilities, which 

could also serve to address climate 

change. Yet to date there are limited 

examples of private sector interests 

working cooperatively with governments to limit risk. Instances where 

cooperation has taken place include 

property insurance companies1,46

and engineering firms that provide 

consulting services to cities. For 

example, firms providing infrastructure system plans have 

begun to account for projected changes in precipitation in 

their projects.47 With city and regional infrastructure systems, 

recent attention has focused on the potential role of private 

sector-generated smart technologies to improve early warning 

of extreme precipitation and heat waves, as well as establishing information systems that can inform local decision-makers 

about the status and efficiency of infrastructure.46,48

Uncertainty, in both the climate system and modeling techniques, is often viewed as a barrier to adaptation action (Ch. 

28: Adaptation).49 Urban and infrastructure managers, however, recognize that understanding of sources and magnitude 

of future uncertainty will continue to be refined,39 and that an 

incremental and flexible approach to planning that draws on 

both structural and nonstructural measures is prudent.44,46,50

Gaining the commitment and support of local elected officials 

Figure 11.3. Map shows areas in New York’s five boroughs that are projected to face 

increased flooding over the next 70 years, assuming an increased rate of sea level rise 

from the past century’s average. As sea level rises, storm surges reach farther inland. 

Map does not represent precise flood boundaries, but illustrates projected increases in 

areas flooded under various sea level rise scenarios. (Figure source: New York City Panel 

on Climate Change 201331). 

New York City and Sea Level Rise
290 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

11: URBAN SYSTEMS, INFRASTRUCTURE, AND VULNERABILITY

for adaptation planning and implementation is another important challenge.30 A compounding problem is that cities and city 

administrators face a wide range of other stressors demanding their attention, and have limited financial resources (see 

“Advancing Climate Adaptation in a Metropolitan Region”).46

Integrating climate change action in everyday city and infrastructure operations and governance (referred to as “mainstreaming”) is an important planning and implementation tool 

for advancing adaptation in cities (Ch. 28: Adaptation).44,46 By 

integrating climate change considerations into daily operations, these efforts can forestall the need to develop a new and 

isolated set of climate change-specific policies or procedures.39

This strategy enables cities and other government agencies 

to take advantage of existing funding sources and programs, 

and achieve co-benefits in areas such as sustainability, public 

health, economic development, disaster preparedness, and 

environmental justice. Pursuing low-cost, no-regrets options is 

a particularly attractive short-term strategy for many cities.39,46

Over the long term, responses to severe climate change impacts, such as sea level rise and greater frequency and intensity of other climate-related hazards, are of a scale and complexity that will likely require major expenditures and structural 

changes,1,46 especially in urban areas. When major infrastructure decisions must be made in order to protect human lives 

and urban assets, cities need access to the best available science, decision support tools, funding, and guidance. The Federal Government is seen by local officials to have an important 

role here by providing adaptation leadership and financial and 

technical resources, and by conducting and disseminating research (Ch. 28: Adaptation).36,39,46

Advancing climate adaptation in a

metropolitan region

Coordinating efforts across many jurisdictional boundaries is a major challenge for adaptation planning and 

practice in extended metropolitan regions and associated regional systems (Ch. 28: Adaptation). Regional 

government institutions may be well suited to address 

this challenge, as they cover a larger geographic scope 

than individual cities, and have potential to coordinate 

the efforts of multiple jurisdictions.1 California already 

requires metropolitan planning organizations to prepare 

Sustainable Communities Strategies (SCS) as part of the 

Regional Transportation Plan process.51 While its focus is 

on reducing emissions, SCS plans prepared to date have 

also introduced topics related to climate change impacts 

and adaptation.52 Examples of climate change vulnerabilities that could benefit from a regional perspective 

include water shortages, transportation infrastructure 

maintenance, loss of native plant and animal species, 

and energy demand.

New york city climate action

New York City leaders recognized that climate change represents a serious threat to critical infrastructure and responded with a comprehensive program to address climate change impacts and increase resilience.1,2

 The 2010 “Climate 

Change Adaptation in New York City: Building a Risk Management Response” report was prepared by the New York City 

Panel on Climate Change as a part of the city’s long-term sustainability plan.10 Major components of the process and 

program include:

• establishing multiple participatory processes to obtain broad public input, including a Climate Change Adaptation Task Force that included private and public stakeholders;46

• forming an expert technical advisory body, the New York City Panel on Climate Change (NPCC), to support the 

Task Force; 

• developing a Climate Change Assessment and Action Plan that helps improve responses to present-day climate 

variability as well as projected future conditions; 

• defining “Climate Protection Levels” to address the effectiveness of current regulations and design standards to 

respond to climate change impacts; and 

• producing adaptation assessment guidelines that recognize the need for flexibility to reassess and adjust strategies over time. The guidelines include a risk matrix and prioritization framework intended to become integral 

parts of ongoing risk management and agency operations.
291 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Recommendations of the Climate Action Leadership Committee 

and Community Advisory Committee, 53 pp., City of Boston 

Climate Action Leadership Committee, Boston, MA. [Available 

online at http://www.cityofboston.gov/Images_Documents/

BCA_full_rprt_r5_tcm3-19558.pdf] 

——, 2011: A Climate of Progress: City of Boston Climate Action 

Plan Update 2011, 43 pp., Boston, MA. [Available online at http://

www.cityofboston.gov/Images_Documents/A%20Climate%20

of%20Progress%20-%20CAP%20Update%202011_tcm3-25020.

pdf]

43. Hallegatte, S., and J. Corfee-Morlot, 2011: Understanding climate 

change impacts, vulnerability and adaptation at city scale: An 

introduction. Climatic Change, 104, 1-12, doi:10.1007/s10584-010-

9981-8. 

Howard, J., and G. Monbiot, 2009: Climate change mitigation 

and adaptation in developed nations: A critical perspective on 

the adaptation turn in urban climate planning. Planning for Climate 

Change: Strategies for Mitigation and Adaptation for Spatial Planners, S. 

Davoudi, J. Crawford, and A. Mehmood, Eds., Earthscan, 19-32. 

44. Rosenzweig, C., W. Solecki, S. A. Hammer, and S. Mehrotra, 2010: 

Cities lead the way in climate-change action. Nature, 467, 909-911, 

doi:10.1038/467909a. [Available online at http://ccrun.org/sites/

ccrun/files/attached_files/2010_Rosenzweig_etal_2.pdf]

45. City of Chicago, 2008: City of Chicago Climate Action Plan: 

Our City. Our Future, 57 pp. [Available online at http://

www.chicagoclimateaction.org/filebin/pdf/finalreport/

CCAPREPORTFINALv2.pdf]

46. NRC, 2010: Adapting to Impacts of Climate Change. America’s Climate 

Choices: Report of the Panel on Adapting to the Impacts of Climate Change.

National Research Council. The National Academies Press, 292 

pp. [Available online at http://www.nap.edu/catalog.php?record_

id=12783]

47. van der Tak, L., P. Pasteris, L. Traynham, C. Salas, T. Ajello, and 

B. Emily, 2010: Storm sewer infrastructure planning with climate 

change risk: The city of Alexandria Virginia case study. Water 

Practice & Technolog y, 5, doi:10.2166/wpt.2010.085. 

48. IBM News Room, 2009: IBM and Dubuque, Iowa Partner on 

Smarter City Initiative, September 17, 2009. IBM. [Available online 

at http://www-03.ibm.com/press/us/en/pressrelease/28420.wss]

49. Corfee-Morlot, J., I. Cochran, S. Hallegatte, and P. J. Teasdale, 

2011: Multilevel risk governance and urban adaptation policy. 

Climatic Change, 104, 169-197, doi:10.1007/s10584-010-9980-9. 

Mastrandrea, M. D., N. E. Heller, T. L. Root, and S. H. Schneider, 

2010: Bridging the gap: Linking climate-impacts research with 

adaptation planning and management. Climatic Change, 100, 87-101, 

doi:10.1007/s10584-010-9827-4. 

50. Carmin, J., and D. Dodman, 2013: Ch. 13: Engaging science 

and managing scientific uncertainty in urban climate adaptation 

planning. Successful Adaptation to Climate Change: Linking Science and 

Policy in a Rpaidly Changing World, S. Moser, and M. Boycoff, Eds., 

Routledge, 336. 

51. California Senate, 2008: Sustainable Communities and Climate 

Protection Act of 2008, SB 375, California State Senate. [Available 

online at http://www.leginfo.ca.gov/pub/07-08/bill/sen/sb_0351-

0400/sb_375_bill_20080930_chaptered.pdf]

52. SACOG, 2012: Metropolitan Transportation Plan Sustainable 

Communities Strategy 2035, 243 pp., Sacramento Area Council of 

Governments, Sacramento, CA. [Available online at http://www.

sacog.org/2035/files/MTP-SCS/Complete%20MTP-SCS%20

no%20appendices.pdf] 

SANDAG, 2011: Ch. 3: Forging a path toward more sustainable 

living: Sustainable communities strategy. 2050 Regional Transportation 

Plan, San Diego Association of Governments, 3-2 - 3-82. [Available 

online at http://www.sandag.org/uploads/2050RTP/F2050rtp_

all.pdf] 

SCAG, 2012: Regional Transportation Plan 2012-2035 Sustainable 

Communities Strategy Towards a Sustainable Future, 31-32 pp., 

Southern California Association of Governments, Los Angeles, CA. 

[Available online at http://rtpscs.scag.ca.gov/Documents/2012/

final/f2012RTPSCS.pdf]

53. Karl, T. R., J. T. Melillo, and T. C. Peterson, Eds., 2009: Global 

Climate Change Impacts in the United States. Cambridge University 

Press, 189 pp. [Available online at http://downloads.globalchange.

gov/usimpacts/pdfs/climate-impacts-report.pdf]
295 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Process for Developing Key Messages

In developing key messages, the report author team engaged in 

multiple technical discussions via teleconference. A consensus 

process was used to determine the final set of key messages, 

which are supported by extensive evidence documented in two 

Technical Report Inputs to the National Climate Assessment on urban systems, infrastructure, and vulnerability: 1) Climate Change 

and Infrastructure, Urban Systems, and Vulnerabilities: Technical 

Report for the U.S. Department of Energy in Support of the National Climate Assessment,

2 and 2) U.S. Cities and Climate Change: 

Urban, Infrastructure, and Vulnerability Issues.

1

Other Technical Input reports (56) on a wide range of topics were also received and 

reviewed as part of the Federal Register Notice solicitation for 

public input. 

Key message 1 Traceable Account

Climate change and its impacts threaten the wellbeing of urban residents in all U.S. regions. Essential infrastructure systems such as water, energy 

supply, and transportation will increasingly be compromised by interrelated climate change impacts. 

The nation’s economy, security, and culture all depend on the resilience of urban infrastructure systems.

Description of evidence base

Recent studies have reported that population and economic growth have made urban infrastructure more fragile and deficient,1,2

 with work projecting increased stresses due to climate 

change4 and increased costs of adaptation plans due to more extensive urban development.6

 Additionally, a few publications have 

assessed the main drivers of vulnerability8,9 and the effects of the 

amalgamation of climate change stresses with other urban and infrastructure stressors.1

New information and remaining uncertainties

Given that population trends and infrastructure assessments are 

well established and documented, the largest uncertainties are 

associated with the rate and extent of potential climate change.

Since the 2009 National Climate Assessment,53 recent publications have explored the driving factors of vulnerability in urban systems8,9

 and the effects of the combined effect of climate 

change and existing urban stressors.1

11: URBAN SYSTEMS, INFRASTRUCTURE, AND VULNERABILITY

Assessment of confidence based on evidence 

Given the evidence base and remaining uncertainties, confidence 

is very high that climate change and its impacts threaten the wellbeing of urban residents in all regions of the U.S.

Given the evidence base and remaining uncertainties, confidence 

is very high that essential local and regional infrastructure systems such as water, energy supply, and transportation will increasingly be compromised by interrelated climate change impacts. 

Key message 2 Traceable Account

In urban settings, climate-related disruptions of 

services in one infrastructure system will almost 

always result in disruptions in one or more other 

infrastructure systems.

Confidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent results, well documented 

and accepted methods, etc.), 

high consensus

High

Moderate evidence (several 

sources, some consistency, 

methods vary and/or documentation limited, etc.), medium 

consensus

Medium

Suggestive evidence (a few 

sources, limited consistency, 

models incomplete, methods 

emerging, etc.), competing 

schools of thought

Low

Inconclusive evidence (limited sources, extrapolations, 

inconsistent findings, poor documentation and/or methods not 

tested, etc.), disagreement or 

lack of opinions among experts

SUPPLEMENTAL MATERIAL

TRACEABLE ACCOUNTS
11: URBAN SYSTEMS, INFRASTRUCTURE, AND VULNERABILITY

Traceable Accounts

296 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Description of evidence base

The interconnections among urban systems and infrastructures 

have been noted in the past,19 with recent work expanding on 

this principle to assess the risks this interconnectivity poses. One 

study15 explored the misconception of independent systems, and 

stressed instead the interactive and interdependent nature of systems. The effects of climate change on one system ultimately affect systems that are dependent upon it.12 One of the foundational 

Technical Input Reports examined the economic effects from climate change and how they will affect urban areas.2

 Noted examples of this interconnectivity can be found in a number of publications concerning Hurricane Katrina,14 intense weather in New 

York City,16,17 and the vulnerability of U.S. oil refineries and electric power plants.2,13

New information and remaining uncertainties

Recent work has delved deeper into the interconnectivity of urban 

systems and infrastructure,2,12 and has expressed the importance 

of understanding these interactions when adapting to climate 

change.

The extensive number of infrastructure assessments has resulted 

in system interdependencies and cascade effects being well 

documented. Therefore, the most significant uncertainties are 

associated with the rate and extent of potential climate change.

Assessment of confidence based on evidence 

Given the evidence base and remaining uncertainties, confidence 

is very high that in urban settings, climate-related disruptions of 

services in one infrastructure system will almost always result in 

disruptions in one or more other infrastructure systems.

Key message 3 Traceable Account

Climate vulnerability and adaptive capacity of 

urban residents and communities are influenced 

by pronounced social inequalities that reflect age, 

ethnicity, gender, income, health, and (dis)ability 

differences. 

Description of evidence base

The topic of social vulnerability has been extensively studied,22,23,24

with some work detailing the social characteristics that are 

the most influential.26 More recent work has addressed the 

vulnerability of populations to climate change25 and how social 

inequalities influence capacity to adapt to climate change.27 Some 

empirical studies of U.S. urban areas were explored concerning 

these issues.9

New information and remaining uncertainties

Given that population trends and socioeconomic factors associated 

with vulnerability and adaptive capacity are well established and 

documented, the largest uncertainties are associated with the rate 

and extent of potential climate change.

Recent work has addressed the social vulnerabilities to climate 

change at a more detailed level than in the past,23,25 providing 

information on the constraints that social vulnerabilities can have 

on climate change adaptation.

Assessment of confidence based on evidence 

Given the evidence base and remaining uncertainties, confidence 

is very high that the climate vulnerability and adaptive capacity of 

urban residents and communities are influenced by pronounced 

social inequalities that reflect age, ethnicity, gender, income, 

health, and (dis)ability differences.

Key message 4 Traceable Account

City government agencies and organizations have 

started adaptation plans that focus on infrastructure systems and public health. To be successful, 

these adaptation efforts require cooperative private sector and governmental activities, but institutions face many barriers to implementing coordinated efforts.

Description of evidence base

Urban adaptation is already underway with a number of cities 

developing plans at the city30,32,33 and state levels,30 with some 

integrating adaptation into community plans1 and sharing 

information and assessing potential impacts.34 Some recent 

publications have explored how incentives and administrative and 

financial support can benefit climate adaptation through policy 

planning at the local level1,2,37 and by engaging the public.38,39,40

Barriers exist that can hinder the adaptation process, which 

has been demonstrated through publications assessing the 

availability of scientific data30,36 that is integral to the evaluation 

and planning process,43 uncertainty in the climate system and 

modeling techniques,49 and the challenges of gaining support and 

commitment from local officials.30,46

New information and remaining uncertainties

Besides uncertainties associated with the rate and extent of 

potential climate change, uncertainties emerge from the fact that, 

to date, there have been few extended case studies examining 

how U.S. cities are responding to climate change (<10 studies). 

Furthermore, only one large-scale survey of U.S. cites has been 

conducted for which results have been published and widely 

available.30

Assessment of confidence based on evidence 

Given the evidence base and remaining uncertainties, confidence 

is very high that city government agencies and organizations have 

started urban adaptation efforts that focus on infrastructure 

systems and public health.
297

Climate Change Impacts in the United States

CHAPTER 12

INDIGENOUS PEOPLES,

LAND, AND RESOURCES

INFORMATION DRAWN FROM THIS CHAPTER IS INCLUDED IN THE HIGHLIGHTS REPORT AND IS IDENTIFIED BY THIS ICON

Recommended Citation for Chapter

Bennett, T. M. B., N. G. Maynard, P. Cochran, R. Gough, K. Lynn, J. Maldonado, G. Voggesser, S. Wotkyns, and K. Cozzetto, 

2014: Ch. 12: Indigenous Peoples, Lands, and Resources. Climate Change Impacts in the United States: The Third National 

Climate Assessment, J. M. Melillo, Terese (T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 297-

317. doi:10.7930/J09G5JR1. 

On the Web: http://nca2014.globalchange.gov/report/sectors/indigenous-peoples

Convening Lead Authors

T.M. Bull Bennett, Kiksapa Consulting, LLC

Nancy G. Maynard, National Aeronautics and Space Administration and University of Miami

Lead Authors

Patricia Cochran, Alaska Native Science Commission

Robert Gough, Intertribal Council on Utility Policy

Kathy Lynn, University of Oregon

Julie Maldonado, American University, University Corporation for Atmospheric Research

Garrit Voggesser, National Wildlife Federation

Susan Wotkyns, Northern Arizona University

Contributing Author

Karen Cozzetto, University of Colorado at Boulder
298 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

12INDIGENOUS PEOPLES,

LAND, AND RESOURCES

Key Messages

1. Observed and future impacts from climate change threaten Native Peoples’ access to traditional 

foods such as fish, game, and wild and cultivated crops, which have provided sustenance as well 

as cultural, economic, medicinal, and community health for generations. 

2. A significant decrease in water quality and quantity due to a variety of factors, including 

climate change, is affecting drinking water, food, and cultures. Native communities’ 

vulnerabilities and limited capacity to adapt to water-related challenges are exacerbated by 

historical and contemporary government policies and poor socioeconomic conditions.

3. Declining sea ice in Alaska is causing significant impacts to Native communities, including 

increasingly risky travel and hunting conditions, damage and loss to settlements, food insecurity, 

and socioeconomic and health impacts from loss of cultures, traditional knowledge, and 

homelands.

4. Alaska Native communities are increasingly exposed to health and livelihood hazards from 

increasing temperatures and thawing permafrost, which are damaging critical infrastructure, 

adding to other stressors on traditional lifestyles.

5. Climate change related impacts are forcing relocation of tribal and indigenous communities, 

especially in coastal locations. These relocations, and the lack of governance mechanisms or 

funding to support them, are causing loss of community and culture, health impacts, and 

economic decline, further exacerbating tribal impoverishment. 

We humbly ask permission from all our relatives; our elders, our families, our children, the winged and the insects, 

the four-legged, the swimmers, and all the plant and animal nations, to speak. Our Mother has cried out to us. 

She is in pain. We are called to answer her cries. Msit No’Kmaq – All my relations! 

— Indigenous Prayer

The peoples, lands, and resources of indigenous communities 

in the United States, including Alaska and the Pacific Rim, face 

an array of climate change impacts and vulnerabilities that 

threaten many Native communities. The consequences of observed and projected climate change have and will undermine 

indigenous ways of life that have persisted for thousands of 

years. Key vulnerabilities include the loss of traditional knowledge in the face of rapidly changing ecological conditions, 

increased food insecurity due to reduced availability of traditional foods, changing water availability, Arctic sea ice loss, 

permafrost thaw, and relocation from historic homelands.1,2,3,4

Climate change impacts on many of the 566 federally recognized tribes and other tribal and indigenous groups in the U.S. 

are projected to be especially severe, since these impacts are 

compounded by a number of persistent social and economic 

problems.6,7 The adaptive responses to multiple social and 

ecological challenges arising from climate impacts on indigenous communities will occur against a complex backdrop of 

centuries-old cultures already stressed by historical events and 

contemporary conditions.8 Individual tribal responses will be 

grounded in the particular cultural and environmental heritage of each community, their social and geographical history, 

spiritual values, traditional ecological knowledge, and worldview. Furthermore, these responses will be informed by each 

group’s distinct political and legal status, which includes the 

legacy of more than two centuries of non-Native social and 

governmental institutional arrangements, relationships, policies, and practices. Response options will be informed by the 

often limited economic resources available to meet these challenges, as well as these cultures’ deeply ingrained relationships 

with the natural world.9,10,11,12
299 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

12: INDIGENOUS PEOPLES, LANDS, AND RESOURCES

The history and culture of many tribes and indigenous peoples 

are critical to understand before assessing additional climate 

change impacts. Most U.S. Native populations already face 

adverse socioeconomic factors such as extreme poverty; substandard and inadequate housing; a lack of health and community services, food, infrastructure, transportation, and 

education; low employment; and high fuel costs; as well as 

historical and current institutional and policy issues related 

to Native resources.7,11,12,13 The overwhelming driver of these 

adverse social indicators is pervasive poverty on reservations 

and in Native communities, as illustrated by an overall 28.4% 

poverty rate (36% for families with children) on reservations, 

compared with 15.3% nationally.13 Some reservations are far 

worse off, with more than 60% poverty rates and, in some 

cases, extremely low income levels (for example, Pine Ridge 

Reservation has the lowest per capita income in the U.S. at 

$1,535 per year).14 

These poverty levels result in problems such as: a critical housing shortage of well over two hundred thousand safe, healthy, 

and affordable homes;15 a homeless rate of more than 10% on 

reservations;16 a lack of electricity (more than 14% of reservation homes are without power, ten times the national average, and, on the Navajo Reservation, about 40% of homes 

have no electricity17); lack of running water in one-fifth of all 

reservation homes and for about one-third of people on the 

Navajo Reservation (compared with 1% of U.S. national households);18,19,20 and an almost complete lack of modern telecommunications – fewer than 50% of homes have phone service, 

fewer than 10% of residents have Internet access, and many 

reservations have no cell phone reception.21 In addition, Native 

populations are also vulnerable because their physical, mental, 

intellectual, social, and cultural well-being is traditionally tied 

to a close relationship with the natural world, and because of 

their dependence on the land and resources for basic needs 

such as medicine, shelter, and food.22,23 Climate changes will 

exacerbate many existing barriers to providing for these human needs, and in many cases will make adaptive responses 

more difficult.

Of the 5.2 million American Indians and Alaska Natives registered in the U.S. Census, approximately 1.1 million live on 

or near reservations or Native lands, located mostly in the 

Northwest, Southwest, Great Plains, and Alaska. Tribal lands 

include approximately 56 million acres (about 3% of U.S. lands) 

in the 48 contiguous states and 44 million acres (about 42% of 

Alaska’s land base) held by Alaska Native corporations.5 Most 

reservations are small and often remote or isolated, with a few 

larger exceptions such as the Navajo Reservation in Arizona, 

Utah, and New Mexico, which has 175,000 residents.5

Figure 12.1. Census data show that American Indian and Alaska Native populations are concentrated around, but are not limited 

to, reservation lands like the Hopi and Navajo in Arizona and New Mexico, the Choctaw, Chickasaw, and Cherokee in Oklahoma, 

and various Sioux tribes in the Dakotas and Montana. Not depicted in this graphic is the proportion of Native Americans who live 

off-reservation and in and around urban centers (such as Chicago, Minneapolis, Denver, Albuquerque, and Los Angeles) yet still 

maintain strong family ties to their tribes, tribal lands, and cultural resources. (Figure source: Norriset al. 20125).

 Indigenous Populations Extend beyond Reservation Lands
300 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

12: INDIGENOUS PEOPLES, LANDS, AND RESOURCES

House being built on Pine Ridge Reservation

Figure 12.2. From developing biomass energy projects on the Quinault Indian Nation in Washington and tribal and intertribal wind 

projects in the Great Plains,24 to energy efficiency improvement efforts on the Cherokee Indian Reservation in North Carolina and 

the sustainable community designs being pursued on the Lakota reservations in the Dakotas (see also Ch. 19: Great Plains),25

tribes are investigating ways to reduce future climate changes. The map shows only those initiatives by federally recognized tribes 

that are funded through the Department of Energy. (Figure source: U.S. Department of Energy 201126). 

Many Tribes, Many Climate 

Change Initiatives

© Aaron Huey/National Geographic Society/Corbis
301 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

12: INDIGENOUS PEOPLES, LANDS, AND RESOURCES

Native American, Alaska Native, and other indigenous communities across the U.S. share unique historical and cultural 

relationships with tribal or ancestral lands, significantly shaping their identities and adaptive opportunities.11 Some climate 

change adaptation opportunities exist on Native lands, and 

traditional knowledge can enhance adaptation and sustainability strategies. In many cases, however, adaptation options 

are limited by poverty, lack of resources, or – for some Native 

communities, such as those along the northern coast of Alaska 

constrained by public lands or on certain low-lying Pacific Islands – because there may be no land left to call their own. 

Conversely, for these same reasons, Native communities – especially in the Arctic – are also increasingly working to identify 

new economic opportunities associated with climate change 

and development activities (for example, oil and gas, mining, 

shipping, and tourism) and to optimize employment opportunities.1,27,28

Climate Change and Traditional Knowledge

Indigenous traditional knowledge has emerged in national and 

international arenas as a source of rich information for indigenous and non-indigenous climate assessments, policies, and 

adaptation strategies. Working Group II of the Intergovernmental Panel on Climate Change Fourth Assessment Report 

recognized traditional knowledge as an important information 

source for improving the understanding of climate change and 

other changes over time, and for developing comprehensive 

natural resource management and climate adaptation strategies.29

Traditional knowledge is essential to the economic and 

cultural survival of indigenous peoples, and, arguably, cultures 

throughout the world.30,31 Traditional knowledge has been 

defined as “a cumulative body of knowledge, practice, and 

belief, evolving by adaptive processes and handed down 

through generations by cultural transmission, about the 

relationship of living beings (including humans) with one 

another and with their environment.”

1,12,32 From an indigenous 

perspective, traditional knowledge encompasses all that is 

known about the world around us and how to apply that 

knowledge in relation to those beings that share the world.

12,33

As the elders of these communities – the “knowledge keepers” 

– pass away, the continued existence and viability of traditional 

knowledge is threatened. Programs are needed to help 

preserve the diverse traditional teachings and employ them to 

strive for balance among the physical, the spiritual, emotional, 

and intellectual – all things that encompass “wolakota,” 

meaning to be a complete human being.34

Many, if not all, indigenous resource managers believe their 

cultures already possess sufficient knowledge to respond to 

climate variation and change.30,35 However, there are elements 

of traditional knowledge that are increasingly vulnerable with 

changing climatic conditions,

4

 including cultural identities, 

ceremonies, and traditional ways of life.

36 The use of indigenous 

and traditional knowledge to address climate change issues 

in Indian country has been called “indigenuity” – indigenous 

knowledge plus ingenuity.33

Native cultures are directly tied to Native places and homelands, 

reflecting the indigenous perspective that includes the “power 

of place.”6,36,37 Many indigenous peoples regard all people, 

plants, and animals that share our world as relatives rather 

than resources. Language, ceremonies, cultures, practices, and 

food sources evolved in concert with the inhabitants, human 

and non-human, of specific homelands.

1,33 The wisdom and 

knowledge of Native people resides in songs, dances, art, 

language, and music that reflect these places. By regarding 

all things as relatives, not resources, natural laws dictate that 

people care for their relatives in responsible ways. “When you 

say, ‘my mother is in pain,’ it’s very different from saying ‘the 

earth is experiencing climate change.’”38,39 As climate change 

increasingly threatens these Native places, cultural identities, 

and practices, documenting the impacts on traditional lifestyles 

would strengthen adaptive strategies.

Traditional knowledge has developed tangible and reliable 

methods for recording historic weather and climate variability 

and their impacts on native societies.40 For example, tribal 

community historians (winter count keepers) on the northern 

Great Plains recorded pictographs on buffalo hides to 

remember the sequence of events that marked each year, 

dating back to the 1600s. These once-reliable methods are 

becoming increasingly more difficult to maintain and less 

reliable as time passes.41

There are recent examples, however, where traditional 

knowledge and western-based approaches are used together 

to address climate change and related impacts. For example, 

the Alaska Native Tribal Health Consortium chronicles climate 

change impacts on the landscape and on human health 

and also develops adaptation strategies.

1

 This Consortium 

employs western science, traditional ecological knowledge, 

and a vast network of “Local Environmental Observers” to 

develop comprehensive, community-scaled climate change 

health assessments.42 During a recent drought on the Navajo 

Reservation, traditional knowledge and western approaches 

were also applied together, as researchers worked with Navajo 

elders to observe meteorological and hydrological changes and 

other phenomena in an effort to assess and reduce disaster 

risks.43
302 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

12: INDIGENOUS PEOPLES, LANDS, AND RESOURCES

Key Message 1: Forests, Fires, and Food

Observed and future impacts from climate change threaten Native Peoples’ access to 

traditional foods such as fish, game, and wild and cultivated crops, which have provided 

sustenance as well as cultural, economic, medicinal, and community health for generations. 

Climate change impacts on forests and ecosystems are expected to have direct effects on culturally important plant and 

animal species, which will affect tribal sovereignty, culture, 

and economies.2,4 Warmer temperatures and more frequent 

drought are expected to cause dieback and tree loss of several 

tree and plant species (such as birch, brown ash, and sweet 

grass) important for Native artistic, cultural, and economic 

purposes, including tourism.22 Tribal access to valued resources 

is threatened by climate change impacts causing habitat degradation, forest conversion, and extreme changes in ecosystem 

processes.44

Observed impacts from both the causes and consequences of 

climate change, and added stressors such as extractive industry practices on or near Native lands, include species loss and 

shifts in species range.1,45,46,47 There have also been observed 

changes in the distribution and population density of wildlife 

species, contraction or expansion of some plant species’ range, 

and the northward migration of some temperate forest species.4,48 For example, moose populations in Maine and similar 

locations are expected to decline because of loss of preferred 

habitat and increased winter temperatures, which are enabling 

ticks to survive through the winter and causing damage from 

significant infestation of the moose.22

Loss of biodiversity, changes in ranges and abundance of culturally important native plants and animals, increases in invasive species, bark beetle damage to forests, and increased risk 

of forest fires have been observed in the Southwest, across 

much of the West, and in Alaska (see also Appendix 3: Climate 

Science Supplement, Figure 31; Ch. 7: Forests; Ch. 8: Ecosystems).4,30,48,49 Changes in ocean temperature and acidity affect 

distribution and abundance of important food sources, like fish 

and shellfish (Ch. 2: Our Changing Climate; Ch. 24: Oceans). 

Rising temperatures and hotter, drier summers are projected 

to increase the frequency and intensity of large wildfires (see 

Ch. 7: Forests).44 Warmer, drier, and longer fire seasons and 

increased forest fuel load will lead to insect outbreaks and the 

spread of invasive species, dry grasses, and other fuel sources 

(see Ch. 7: Forests). Wildfire threatens Native and tribal homes, 

safety, economies, culturally important species, medicinal 

plants, traditional foods, and cultural sites. “Fire affects the 

plants, which affect the water, which affects the fish, which affect terrestrial plants and animals, all of which the Karuk rely on 

for cultural perpetuity.”50

In interior Alaska, rural Native communities are experiencing 

new risks associated with climate change related wildfires in 

boreal forests and Arctic tundra (see also Ch. 22: Alaska).1,51

Reliance on local, wild foods and the isolated nature of these 

communities, coupled with their varied preparedness and limited ability to deal with wildfires, leaves many communities at 

an increased risk of devastation brought on by fires. While efforts are being made to better coordinate rural responses to 

wildfires in Alaska, current responses are limited by organization and geographic isolation.48

Indigenous peoples have historically depended on the gathering and preparation of a wide variety of local plant and animal 

species for food (frequently referred to as traditional foods), 

medicines, ceremonies, community cohesion, and economic 

health for countless generations.2,52 These include corn, beans, 

squash, seals, fish, shellfish, bison, bear, caribou, walrus, 

moose, deer, wild rice, cottonwood trees, and a multitude of 

native flora and fauna.2,45,47,49,52,53,54,55,56,57 A changing climate 

affects the availability, tribal access to, and health of these 

resources.1,2,4,47,57,58,59,60 This in turn threatens tribal customs, 

cultures, and identity. 

Medicinal and food plants are becoming increasingly difficult 

to find or are no longer found in historical ranges.2,56 For example, climate change and other environmental stressors are 

affecting the range, quality, and quantity of berry resources 

Harvesting traditional foods is important to Native Peoples’ culture, 

health, and economic well being. In the Great Lakes region, wild 

rice is unable to grow in its traditional range due to warming winters 

and changing water levels.

©Phil Schermeister/Corbis
303 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

12: INDIGENOUS PEOPLES, LANDS, AND RESOURCES

for the Wabanaki tribes in the Northeast.2,61 The Karuk people 

in California have experienced a near elimination of both salmonids and acorns, which comprise 50% of a traditional Karuk 

diet.62 In the Great Lakes region, wild rice is unable to grow in 

its traditional range due to warming winters and changing water levels, affecting the Anishinaabe peoples’ culture, health, 

and well-being.

54

Subsequent shifts from traditional lifestyles and diet, compounded by persistent poverty, food insecurity, the cost of 

non-traditional foods, and poor housing conditions have led to 

increasing health problems in communities, also increasing the 

risk to food and resource security.1,2,16 Climate change is likely 

to amplify other indirect effects to traditional foods and resources, including limited access to gathering places and hunting grounds and environmental pollution.4,57,59

Key Message 2: Water Quality and Quantity

A significant decrease in water quality and quantity due to a variety of factors, including 

climate change, is affecting drinking water, food, and cultures. Native communities’ 

vulnerabilities and limited capacity to adapt to water-related challenges are exacerbated by 

historical and contemporary government policies and poor socioeconomic conditions.

Native communities and tribes in different parts of the U.S. 

have observed changes in precipitation affecting their water 

resources. On the Colorado Plateau, tribes have been experiencing drought for more than a decade.63,64 Navajo elders have 

observed long-term decreases in annual snowfall over the past 

century, a transition from wet to dry conditions in the 1940s, 

and a decline in surface water features.20 Changes in long-term 

average temperature and precipitation have produced changes 

in the physical and hydrologic environment, making the Navajo 

Nation more susceptible to drought impacts, and some springs 

and shallow water wells on the Navajo Nation have gone dry.43

Southwest tribes have observed damage to their agriculture 

and livestock, the loss of springs and medicinal and culturally 

important plants and animals, and impacts on drinking water 

supplies.63,64,65,66 In the Northwest, tribal treaty rights to traditional territories and resources are being affected by the reduction of rainfall and snowmelt in the mountains, melting glaciers, rising temperatures, and shifts in ocean currents.52,58,67

In Hawai‘i, Native peoples have observed a shortening of the 

rainy season, increasing intensity of storms and flooding, and 

a rainfall pattern that has become unpredictable.38 In Alaska, 

water availability, quality, and quantity are threatened by the 

consequences of permafrost thaw, which has damaged community water infrastructure, as well as by the northward extension of diseases such as those caused by the Giardia parasite, 

a result of disease-carriers like beavers moving northward in 

response to rising temperatures.68 The impact 

of historical federal policies, such as the late 

1800s allotment policy and practices regarding 

Native access to treaty-protected resources,69

reverberate in current practices, such as states 

and the government permitting oil drilling and 

hydraulic fracturing on lands in and around 

reservations but outside of tribal jurisdiction 

(for example, a 2013 pipeline spill upstream of 

tribal reservations in Western North Dakota, 

and others). Such policies and practices exacerbate the threat to water quality and quantity 

for Native communities. 

Native American tribes have unique and significant adaptation needs related to climate impacts on water.66 There is little available data to 

establish baseline climatic conditions on tribal 

Human-caused stresses such as dam building have greatly reduced 

salmon on the Klamath River. 

Coal plant and fishermen, Navajo Reservation

©David McLain/Aurora Photos

©Ted Wood Photography
304 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

12: INDIGENOUS PEOPLES, LANDS, AND RESOURCES

lands, and many tribes do not have sufficient capacity to monitor changing conditions.63 Without 

scientific monitoring, tribal decision-makers lack 

the data needed to quantify and evaluate current 

conditions and emerging trends in precipitation, 

streamflow, and soil moisture, and to plan and 

manage resources accordingly.10,64,66 However, 

some existing efforts to document climate impacts on water resources could be replicated in 

other regions to assess hydrologic vulnerabilities.58

Water infrastructure is in disrepair or lacking on 

some reservations.43,70 Approximately 30% of 

people on the Navajo Nation are not served by 

municipal systems and must haul water to meet 

their daily needs.19,43 Longer-term impacts of this 

lack of control over water access are projected to 

include loss of traditional agricultural crops.19,43

Furthermore, there is an overall lack of financial 

resources to support basic water infrastructure 

on tribal lands.63 Uncertainty associated with 

undefined tribal water rights make it difficult to 

determine strategies to deal with water resource 

issues.70 Potential impacts to treaty rights and 

water resources exist, such as a reduction of 

groundwater and drinking water availability and 

water quality decline, including impacts from oil 

and natural gas extraction and sea level rise-induced saltwater intrusion into coastal freshwater 

aquifers (see also Ch. 3: Water).7 New datasets 

on climate impacts on water in many locations 

throughout Indian Country, such as the need to 

quantify available water and aquifer monitoring, 

will be important for improved adaptive planning.

Key Message 3: Declining Sea Ice

Declining sea ice in Alaska is causing significant impacts to Native communities, including 

increasingly risky travel and hunting conditions, damage and loss to settlements, 

food insecurity, and socioeconomic and health impacts from loss of cultures, 

traditional knowledge, and homelands.

“...since the late 1970s, communities along the coast of the 

northern Bering and Chukchi Seas have noticed substantial 

changes in the ocean and the animals that live there. While 

we are used to changes from year-to-year in weather, hunting 

conditions, ice patterns, and animal populations, the past two 

decades have seen clear trends in many environmental factors. 

If these trends continue, we can expect major, perhaps irreversible, impacts to our communities....” 

– C. Pungowiyi, personal communication71

Scientists across the Arctic have documented rising regional 

temperatures over the past few decades at twice the global 

rate, and indigenous Arctic communities have observed these 

changes in their daily lives.1 This temperature increase – which 

is expected to continue with future climate change – is accompanied by significant reductions in sea ice thickness and 

extent, increased permafrost thaw, more extreme weather 

and severe storms, and changes in seasonal ice melt/freeze 

of lakes and rivers, water temperature, sea level rise, flooding 

patterns, erosion, and snowfall timing and type (see also Ch. 2: 

Figure 12.3. On the Arizona portion of the Navajo Nation, recurring 

drought and rising temperatures have accelerated growth and movement 

of sand dunes. Map above shows range and movement of Great Falls 

Dune Field from 1953 to 2010. Moving and/or growing dunes can threaten 

roads, homes, traditional grazing areas, and other tribal assets. (Figure 

source: Redsteer et al. 201155).

Sand Dune Expansion
305 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

12: INDIGENOUS PEOPLES, LANDS, AND RESOURCES

Our Changing Climate).71,72,73,74,75

These climate-driven changes in 

turn increase the number of serious problems for Alaska Native 

populations, which include injury 

from extreme or unpredictable 

weather and thinning sea ice, 

which can trap people far from 

home; changing snow and ice 

conditions that limit safe hunting, 

fishing, or herding practices; malnutrition and food insecurity from 

lack of access to subsistence food; 

contamination of food and water; increasing economic, mental, 

and social problems from loss of 

culture and traditional livelihood; 

increases in infectious diseases; 

and the loss of buildings and infrastructure from permafrost erosion and thawing, resulting in the 

relocation of entire communities 

(Ch. 22: Alaska).1,68,71,75,76

Alaska Native Inupiat and Yup’ik 

experts and scientists have observed stronger winds than in previous decades,71,75,78 observations 

Figure 12.4. In August and September 2012, sea ice covered less of the Arctic Ocean than any time since the beginning of 

reliable satellite measurements (1979). The long-term retreat of sea ice has occurred faster than climate models had predicted. 

The average minimum extent of sea ice for 1979-2000 was 2.59 million square miles. The image on the left shows Arctic minimum 

sea ice extent in 1984, which was about the average minimum extent for 1979-2000. The image on the right shows that the 

extent of sea ice had dropped to 1.32 million square miles at the end of summer 2012. Alaska Native coastal communities rely 

on sea ice for many reasons, including its role as a buffer against coastal erosion from storms. (Figure source: NASA Earth 

Observatory 201277).

Sea Ice Cover Reaches Record Low

Figure 12.5. Dramatic reductions in Arctic sea ice and changes in its timing and composition 

affect the entire food web, including many Inupiaq communities that continue to rely heavily 

on subsistence hunting and fishing. (Figure source: NOAA NCDC).

Arctic Marine Food Web
306 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

12: INDIGENOUS PEOPLES, LANDS, AND RESOURCES

that are consistent with scientific findings showing changing 

Arctic wind patterns, which in turn influence loss of sea ice and 

shifts in North American and European weather.79 They also 

observe accelerated melting of ice and snow, and movement 

of ice and marine mammals far beyond accessible range for 

Native hunters.1 Thinning sea ice, earlier ice break-up, increasing temperatures, and changes in precipitation (for example, 

in the timing and amount of snow) also cause changes in critical feeding, resting, breeding, and denning habitats for arctic 

mammals important as subsistence foods, like polar bears, 

walrus, and seals.1,73,75,80

Key Message 4: Permafrost Thaw

Alaska Native communities are increasingly exposed to health and livelihood hazards from 

increasing temperatures and thawing permafrost, which are damaging critical infrastructure, 

adding to other stressors on traditional lifestyles.

The increased thawing of permafrost 

(permanently frozen soil) along the 

coasts and rivers is an especially potent threat to Alaska Native villages 

because it causes serious erosion, 

flooding, and destruction of homes, 

buildings, and roads from differential 

settlement, slumping, and/or collapse of underlying base sediments 

(see Ch. 2: Our Changing Climate; 

Ch.22: Alaska, Key Message 3).81 This 

loss of infrastructure is further exacerbated by loss of land-fast sea ice, 

sea level rise, and severe storms.1,82,83

At this time, more than 30 Native villages in Alaska (such as Newtok and 

Shishmaref) are either in need of, or 

in the process of, relocating their entire village.1,84

Serious public health issues arise due 

to damaged infrastructure caused 

by these multiple erosion threats. 

Among them are loss of clean water 

for drinking and hygiene, saltwater 

intrusion, and sewage contamination 

that could cause respiratory and gastrointestinal infections, pneumonia, 

and skin infections.1,76,82,85 In addition, permafrost thaw is causing food 

insecurity in Alaska Native communities due to the thawing of ice cellars 

or ice houses used for subsistence 

food storage. This in turn leads to 

food contamination and sickness as 

well as dependence upon expensive, 

less healthy, non-traditional “storebought” foods.

1,85,86

Figure 12.6. The maps show projected ground temperature at a depth of 3.3 feet assuming 

continued increases in emissions (A2 scenario) and assuming a substantial reduction 

in emissions (B1 scenario). Blue shades represent areas below freezing at a depth of 

3.3 feet and yellow and red shades represent areas above freezing at that depth (see 

Ch. 22: Alaska for more details). Many Alaska Natives depend on permafrost for ice 

cellars to store frozen food, and replacing these cellars with electricity-driven freezers 

is expensive or otherwise infeasible. Permafrost thawing also affects infrastructure like 

roads and utility lines. (Figure source: Permafrost Lab, Geophysical Institute, University 

of Alaska Fairbanks).

Thawing Permafrost in Alaska
307 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

12: INDIGENOUS PEOPLES, LANDS, AND RESOURCES

Key Message 5: Relocation

Climate change related impacts are forcing relocation of tribal and indigenous communities, 

especially in coastal locations. These relocations, and the lack of governance mechanisms 

or funding to support them, are causing loss of community and culture, health impacts, and 

economic decline, further exacerbating tribal impoverishment. 

Native peoples are no strangers to relocation and its consequences on their communities. Many eastern and southeastern tribal communities were forced to relocate to Canada or 

the western Great Lakes in the late 1700s and early 1800s and, 

later, to Oklahoma, compelling them to adjust and adapt to 

new and unfamiliar landscapes, subsistence resources, and climatic conditions. Forced relocations have continued into more 

recent times as well.87 Now, many Native peoples in Alaska and 

other parts of the coastal United States, such as the Southeast 

and Pacific Northwest, are facing relocation as a consequence 

of climate change and additional stressors, such as food insecurity and unsustainable development and extractive practices on or near Native lands; such forms of displacement are 

leading to severe livelihood, health, and socio-cultural impacts 

on the communities.1,3,23,38,45,88,89,90,91

For example, Newtok, a traditional Yup’ik village in Alaska, is 

experiencing accelerated rates of erosion caused by the combination of decreased Arctic sea ice, thawing permafrost, and 

extreme weather events (Ch. 22: Alaska).1,3 As a result, the 

community has lost critical basic necessities and infrastructure. While progress has been made toward relocation, limitations of existing federal and state statutes and regulations 

have impeded their efforts, and the absence of legal authority 

and a governance structure to facilitate relocation are significant barriers to the relocation of Newtok and other Alaska Native villages.3,88,92 Tribal communities in coastal Louisiana are 

experiencing climate change induced rising sea levels, along 

with saltwater intrusion, subsidence, and intense erosion and 

land loss due to oil and gas extraction, levees, dams, and other 

river management techniques, forcing them to either relocate 

or try to find ways to save their land.3,45 Tribal communities 

in Florida are facing potential displacement due to the risk of 

rising sea levels and saltwater intrusion inundating their reservation lands.93 The Quileute tribe in northern Washington is 

responding to increased winter storms and flooding connected 

with increased precipitation by relocating some of their village homes and buildings to higher ground within 772 acres of 

Olympic National Park that has been transferred to them; the 

Hoh tribe is also looking at similar options for relocation.90,94,95

Native Pacific Island communities, including those in Hawai‘i 

and the U.S. affiliated Pacific Islands, are also being forced to 

consider relocation plans due to increasing sea level rise and 

storm surges.38,96 While many Native communities are not necessarily being forced to relocate, they are experiencing other 

social and cultural forms of displacement. For example, rising 

sea levels are expected to damage Native coastal middens 

(sites reflecting past human activity such as food preparation) 

as well as Wabanaki coastal petroglyphs, leading to loss of culture and connection to their past for Northeast tribes.22

Currently, the U.S. lacks an institutional framework to relocate entire communities. National, state, local, and tribal government agencies lack the legal authority and the technical, 

organizational, and financial capacity to implement relocation processes for communities forcibly displaced by climate 

change.3,12 New governance institutions, frameworks, and 

funding mechanisms are needed to specifically respond to the 

increasing necessity for climate change induced relocation.3,88

To be effective and culturally appropriate, it is important that 

such institutional frameworks recognize the sovereignty of 

tribal governments and that any institutional development 

stems from significant engagement with tribal representatives.12

 “In Indigenous cultures, it is understood that ecosystems are 

chaotic, complex, organic, in a constant state of flux, and filled 

with diversity. No one part of an ecosystem is considered more 

important than another part and all parts have synergistic roles 

to play. Indigenous communities say that ‘all things are connected’ – the land to the air and water, the earth to the sky, the 

plants to the animals, the people to the spirit.”

– Patricia Cochran, Inupiat Leader97

Rising temperatures are causing damage in Native villages in Alaska 

as sea ice declines and permafrost thaws. Resident of Selawik, 

Alaska, and his granddaughter survey a water line sinking into the 

thawing permafrost, August 2011.

©Michael Brubaker
308 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

12: INDIGENOUS PEOPLES, LANDS, AND RESOURCES

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314 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

12: INDIGENOUS PEOPLES, LANDS, AND RESOURCES

Process for Developing Key Messages: 

A central component of the assessment process was participation 

by members of the Chapter Author Team in a number of climate 

change meetings attended by indigenous peoples and other interested parties, focusing on issues relevant to tribal and indigenous 

peoples. These meetings included:

Oklahoma Inter-Tribal Meeting on Climate Variability and Change 

held on December 12, 2011, at the National Weather Center, Norman, OK, attended by 73 people.56

Indigenous Knowledge and Education (IKE) Hui Climate Change 

and Indigenous Cultures forum held in January 2012 in Hawai‘i 

and attended by 36 people.38

Alaska Forum on the Environment held from February 6-10, 2012, 

at the Dena’ina Convention Center in Anchorage, Alaska, and attended by about 1400 people with approximately 30 to 60 people 

per session.27

Stories of Change: Coastal Louisiana Tribal Communities’ Experiences of a Transforming Environment, a workshop held from January 22-27, 2012, in Pointe-au-Chien, Louisiana, and attended by 

47 people.45

American Indian Alaska Native Climate Change Working Group 

2012 Spring Meeting held from April 23–24, 2012, at the Desert 

Diamond Hotel-Casino in Tucson, Arizona, and attended by 80 

people.98

First Stewards Symposium. First Stewards: Coastal Peoples Address Climate Change. National Museum of the American Indian, 

Washington DC. July 17-20, 2012.30

In developing key messages, the Chapter Author Team engaged 

in multiple technical discussions via teleconferences from August 

2011 to March 2012 as they reviewed more than 200 technical 

inputs provided by the public, as well as other published literature and professional judgment. Subsequently, the Chapter Author 

Team teleconferenced weekly between March and July 2012 for 

expert deliberations of draft key messages by the authors. Each 

key message was defended by the entire author team before being 

selected for inclusion in the chapter report. These discussions 

were supported by targeted consultation with additional experts 

by the lead author of each message.

Key message #1 Traceable Account

Observed and future impacts from climate 

change threaten Native Peoples’ access to traditional foods such as fish, game, and wild and cultivated crops, which have provided sustenance as 

well as cultural, economic, medicinal, and community health for generations. 

Description of evidence base

The key message and supporting chapter text summarize extensive evidence documented in more than 200 technical input reports on a wide range of topics that were received and reviewed 

as part of the Federal Register Notice solicitation for public input.

Numerous peer-reviewed publications describe loss of biodiversity, impacts on culturally important native plants and animals, 

increases in invasive species, bark beetle damage to forests, and 

increased risk of forest fires that have been observed across the 

United States.4,7,22,49,52,58

Climate drivers associated with this key message are also discussed in Ch. 2: Our Changing Climate.

There are also many relevant and recent peer-reviewed publications1,2,4,48,52,58,66 describing the northward migration of the boreal 

forest and changes in the distribution and density of wildlife species that have been observed.

Observed impacts on plant and animal species important to 

traditional foods, ceremonies, medicinal, cultural and economic 

well-being, including species loss and shifts in species range, are 

well-documented.1,2,4,6,7,22,45,46,47,52

New information and remaining uncertainties

A key uncertainty is how indigenous people will adapt to climate 

change, given their reliance on local, wild foods and the isolated 

nature of some communities, coupled with their varied preparedness and limited ability to deal with wildfires. Increased wildfire 

SUPPLEMENTAL MATERIAL

TRACEABLE ACCOUNTS
12: INDIGENOUS PEOPLES, LANDS, AND RESOURCES

Traceable Accounts

315 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

occurrences may affect tribal homes, safety, economy, culturally 

important species, medicinal plants, traditional foods, and cultural sites.

There is uncertainty as to the extent that climate change will affect Native American and Alaska Natives’ access to traditional 

foods such as salmon, shellfish, crops, and marine mammals, 

which have provided sustenance as well as cultural, economic, 

medicinal, and community health for countless generations.

Assessment of confidence based on evidence 

Based on the evidence and remaining uncertainties, confidence is 

very high that observed and future impacts from climate change, 

such as increased frequency and intensity of wildfires, higher temperatures, changes in sea ice, and ecosystem changes, such as 

forest loss and habitat damage, are threatening Native American 

and Alaska Natives’ access to traditional foods such as salmon, 

shellfish, crops, and marine mammals, which have provided sustenance as well as cultural, economic, medicinal, and community 

health for countless generations. 

Key message #2 Traceable Account

A significant decrease in water quality and quantity due to a variety of factors, including climate 

change, is affecting drinking water, food, and cultures. Native communities’ vulnerabilities and limited capacity to adapt to water-related challenges 

are exacerbated by historical and contemporary 

government policies and poor socioeconomic conditions.

Description of evidence base

The key message and supporting chapter text summarizes extensive evidence documented in more than 200 technical input 

reports on a wide range of topics that were received and reviewed 

as part of the Federal Register Notice solicitation for public input.

There are numerous examples of tribal observations of changes 

in precipitation, rainfall patterns, and storm intensity and impacts on surface water features, agriculture, grazing, medicinal 

and culturally important plants and animals, and water resources.2,4,6,7,43,52,55,58,63,64,65,66

Examples of ceremonies are included in the Oklahoma Inter-Tribal 

Meeting on Climate Variability and Change Meeting Summary Report.56 Water is used for some ceremonies, so it can be problematic when there is not enough at the tribe’s disposal.52,56,66 More 

than one tribe at the meeting also expressed how heat has been a 

problem during ceremonies because the older citizens cannot go 

into lodges that lack air conditioning.56

New information and remaining uncertainties

There is limited data to establish baseline climatic conditions on 

tribal lands, and many tribes do not have sufficient capacity to 

monitor changing conditions.10,52,63,66 Without monitoring, tribal 

decision-makers lack the data needed to quantify and evaluate the 

current conditions and emerging trends in precipitation, streamflow, and soil moisture, and to plan and manage resources accordingly.10,52,64,66

Water infrastructure is in disrepair or lacking on some reservations.43,70 There is an overall lack of financial resources to support 

basic water infrastructure on tribal lands, such as is found in the 

Southwest.63

Tribes that rely on water resources to maintain their cultures, religions, and life ways are especially vulnerable to climate change. 

Monitoring data is needed to establish baseline climatic conditions 

and to monitor changing conditions on tribal lands. Uncertainty 

associated with undefined tribal water rights makes it difficult to 

determine strategies to deal with water resource issues.70

Assessment of confidence based on evidence 

Based on the evidence and remaining uncertainties, confidence is 

very high that decreases in water quality and quantity are affectConfidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent results, well documented 

and accepted methods, etc.), 

high consensus

High

Moderate evidence (several 

sources, some consistency, 

methods vary and/or documentation limited, etc.), medium 

consensus

Medium

Suggestive evidence (a few 

sources, limited consistency, 

models incomplete, methods 

emerging, etc.), competing 

schools of thought

Low

Inconclusive evidence (limited sources, extrapolations, 

inconsistent findings, poor documentation and/or methods not 

tested, etc.), disagreement or 

lack of opinions among experts
12: INDIGENOUS PEOPLES, LANDS, AND RESOURCES

Traceable Accounts

316 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

ing Native Americans and Alaska Natives’ drinking water supplies, 

food, cultures, ceremonies, and traditional ways of life. Based 

upon extensive evidence, there is very high confidence that Native communities’ vulnerabilities and lack of capacity to adapt to 

climate change are exacerbated by historical and contemporary 

federal and state land-use policies and practices, political marginalization, legal issues associated with tribal water rights, water 

infrastructure deficiencies, and poor socioeconomic conditions.

Key message #3 Traceable Account

Declining sea ice in Alaska is causing significant 

impacts to Native communities, including increasingly risky travel and hunting conditions, damage 

and loss to settlements, food insecurity, and socioeconomic and health impacts from loss of cultures, 

traditional knowledge, and homelands.

Description of evidence base

The key message and supporting chapter text summarizes extensive evidence documented in more than 200 technical input 

reports on a wide range of topics that were received and reviewed 

as part of the Federal Register Notice solicitation for public input.

Evidence that summer sea ice is rapidly declining is based on 

satellite data and other observational data and is incontrovertible. 

The seasonal pattern of observed loss of Arctic sea ice is generally 

consistent with simulations by global climate models, in which the 

extent of sea ice decreases more rapidly in summer than in winter  

(Ch. 2: Our Changing Climate). Projections by these models indicate that the Arctic Ocean is projected to become virtually ice-free 

in summer before mid-century, and models that best match historical trends project a nearly sea ice-free Arctic in summer by the 

2030s.74 Extrapolation of the present observed trends suggests 

an even earlier ice-free Arctic in summer. (Ch. 2: Our Changing 

Climate and Ch. 22: Alaska).

Sea ice loss is altering marine ecosystems; allowing for greater 

ship access and new development; increasing Native community 

vulnerabilities due to changes in sea ice thickness and extent; 

destroying housing, village sanitation and other infrastructure 

(including entire villages); and increasing food insecurity due to 

lack of access to subsistence food and loss of cultural traditions. 

Evidence for all these impacts of sea ice loss is well-documented in field studies, indigenous knowledge, and scientific literature.1,2,3,71,73,75,78

New information and remaining uncertainties

A key uncertainty is how indigenous peoples will be able to maintain historical subsistence ways of life, which include hunting, 

fishing, harvesting, and sharing, and sustain the traditional relationship with the environment given the impacts from sea ice 

decline and changes. Increased sea ice changes and declines are 

already causing increasingly hazardous hunting and traveling conditions along ice edges; damage to homes and infrastructure from 

erosion; changes in habitat for subsistence foods and species, 

with overall impacts on food insecurity and for species necessary for medicines, ceremonies, and other traditions.1

 The effects 

of sea ice loss are exacerbated by other climate change driven 

impacts such as changes in snow and ice, weather, in-migration 

of people, poverty, lack of resources to respond to changes, and 

contamination of subsistence foods.1,2

Additional observations and monitoring are needed to more adequately document ice and weather changes. 

Assessment of confidence based on evidence 

Based on the evidence and remaining uncertainties, there is very 

high confidence that loss of sea ice is affecting the traditional 

life ways of Native communities in a number of important ways, 

such as more hazardous travel and hunting conditions along the 

ice edge; erosion damage to homes, infrastructure, and sanitation 

facilities (including loss of entire villages); changes in ecosystem 

habitats and, therefore, impacts on food security; and socioeconomic and health impacts from cultural and homeland losses. 

Key message #4 Traceable Account

Alaska Native communities are increasingly exposed to health and livelihood hazards from increasing temperatures and thawing permafrost, 

which are damaging critical infrastructure, adding 

to other stressors on traditional lifestyles.

Description of evidence base

The key message and supporting chapter text summarizes extensive evidence documented in more than 200 technical input 

reports on a wide range of topics that were received and reviewed 

as part of the Federal Register Notice solicitation for public input.

Given the evidence base and uncertainties, confidence is high that 

rising temperatures are thawing permafrost and that this thawing 

is expected to continue (Ch. 2: Our Changing Climate) Permafrost 

temperatures are increasing over Alaska and much of the Arctic.  

Regions of discontinuous permafrost (where annual average soil 

temperatures of already close to 32°F) are highly vulnerable to 

thaw (Ch. 2: Our Changing Climate).81

There are also many relevant and recent peer-reviewed publications1,3,82,83 describing the impact of permafrost thaw on Alaska 

Native villages. Over 30 Native villages in Alaska are in need of 

relocation or are in the process of being moved. Recent work1,84,85

documents public health issues such as contamination of clean 

water for drinking and hygiene and food insecurity through thawing of ice cellars for subsistence food storage.

New information and remaining uncertainties

Improved models and observational data (see Ch. 22: Alaska) 

confirmed many of the findings from the prior 2009 Alaska as-
12: INDIGENOUS PEOPLES, LANDS, AND RESOURCES

Traceable Accounts

317 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

sessment chapter, which informed the 2009 National Climate 

Assessment.99

A key uncertainty is how indigenous peoples in Alaska will be able 

to sustain traditional subsistence life ways when their communities and settlements on the historical lands of their ancestors are 

collapsing due to permafrost thawing, flooding, and erosion combined with loss of shore-fast ice, sea level rise, and severe storms, 

especially along the coasts and rivers.1

Another uncertainty is how indigenous communities can protect 

the health and welfare of the villagers from permafrost-thawcaused public health issues of drinking water contamination, loss 

of traditional food storage, and potential food contamination.1

It is uncertain how Native communities will be able to effectively 

relocate and maintain their culture, particularly because there are 

no institutional frameworks, legal authorities, or funding to implement relocation for communities forced to relocate.1,3,12

Assessment of confidence based on evidence

Based on the evidence and remaining uncertainties, confidence is 

very high that Alaska Native communities are increasingly exposed 

to health and livelihood hazards from permafrost thawing and increasing temperatures, which are causing damage to roads, water 

supply and sanitation systems, homes, schools, ice cellars, and 

ice roads, and threatening traditional lifestyles. 

Key message #5 Traceable Account

Climate change related impacts are forcing relocation of tribal and indigenous communities, especially in coastal locations. These relocations, and 

the lack of governance mechanisms or funding to 

support them, are causing loss of community and 

culture, health impacts, and economic decline, further exacerbating tribal impoverishment. 

Description of evidence base

The key message and supporting chapter text summarizes extensive evidence documented in more than 200 technical input 

reports on a wide range of topics that were received and reviewed 

as part of the Federal Register Notice solicitation for public input.

There is well-documented evidence that tribal communities 

are vulnerable to coastal erosion that could force them to relocate.1,3,23,38,88,89 For example, tribal communities in Alaska, such 

as Newtok, Kivalina, and Shishmaref, are experiencing accelerated rates of erosion caused by the combination of decreased 

Arctic sea ice, thawing permafrost, and extreme weather events, 

resulting in loss of basic necessities and infrastructure (see also 

Ch. 22: Alaska).1,3,88,91

Tribal communities in coastal Louisiana are experiencing climateinduced rising sea levels, along with saltwater intrusion and intense erosion and land loss due to oil and gas extraction and river 

management, forcing them to either relocate or try to find ways to 

save their land (see also Ch. 25: Coasts and Ch. 17 Southeast).3,45

Tribal communities in Florida are facing potential displacement 

due to the risk of rising sea levels and saltwater intrusion inundating their reservation lands.93 The Quileute tribe in northern Washington is relocating some of their village homes and buildings to 

Olympic National Park in response to increased winter storms and 

flooding connected with increased precipitation; the Hoh tribe is 

also considering similar options.90,94

Native Pacific Island communities are being forced to consider 

relocation plans due to increasing sea level rise and storm surges 

(see also Ch. 23: Hawai‘i and Pacific Islands).38

New information and remaining uncertainties

A key uncertainty is the extent to which the combination of other 

impacts (for example, erosion caused by dredging for oil pipelines 

or second-order effects from adaptation-related development projects) will coincide with sea level rise and other climate-related 

issues to increase the rate at which communities will need to relocate.1,3,38

Another key uncertainty is how communities will be able to effectively relocate, maintain their communities and culture, and 

reduce the impoverishment risks that often go along with relocation.1,3,38 The United States lacks an institutional framework to 

relocate entire communities, and national, state, local, and tribal 

government agencies lack the legal authority and the technical, 

organizational, and financial capacity to implement relocation processes for communities forcibly displaced by climate change.3,12

Assessment of confidence based on evidence

Based on the evidence, there is very high confidence that tribal 

communities in Alaska, coastal Louisiana, Pacific Islands, and 

other coastal locations are being forced to relocate due to sea level 

rise, coastal erosion, melting permafrost, and/or extreme weather 

events. There is very high confidence that these relocations and 

the lack of governance mechanisms or funding to support them 

are causing loss of community and culture, health impacts, and 

economic decline, further exacerbating tribal impoverishment.
318

Climate Change Impacts in the United States

CHAPTER 13

LAND USE AND LAND COVER CHANGE

INFORMATION DRAWN FROM THIS CHAPTER IS INCLUDED IN THE HIGHLIGHTS REPORT AND IS IDENTIFIED BY THIS ICON

Recommended Citation for Chapter

Brown, D. G., C. Polsky, P. Bolstad, S. D. Brody, D. Hulse, R. Kroh, T. R. Loveland, and A. Thomson, 2014: Ch. 13: Land 

Use and Land Cover Change. Climate Change Impacts in the United States: The Third National Climate Assessment, J. M. 

Melillo, Terese (T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 318-332. doi:10.7930/

J05Q4T1Q. 

On the Web: http://nca2014.globalchange.gov/report/sectors/land-use-and-land-cover-change

Convening Lead Authors

Daniel G. Brown, University of Michigan

Colin Polsky, Clark University

Lead Authors

Paul Bolstad, University of Minnesota

Samuel D. Brody, Texas A&M University at Galveston

David Hulse, University of Oregon

Roger Kroh, Mid-America Regional Council

Thomas R. Loveland, U.S. Geological Survey

Allison Thomson, Pacific Northwest National Laboratory
319 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Key Messages

1. Choices about land-use and land-cover patterns have affected and will continue to affect how 

 vulnerable or resilient human communities and ecosystems are to the effects of climate change.

2. Land-use and land-cover changes affect local, regional, and global climate processes.

3. Individuals, businesses, non-profits, and governments have the capacity to make land-use 

 decisions to adapt to the effects of climate change.

4. Choices about land use and land management may provide a means of reducing atmospheric 

 greenhouse gas levels.

LAND USE 13 AND LAND COVER CHANGE

In addition to emissions of heat-trapping greenhouse gases 

from energy, industrial, agricultural, and other activities, humans also affect climate through changes in land use (activities taking place on land, like growing food, cutting trees, or 

building cities) and land cover (the physical characteristics of 

the land surface, including grain crops, trees, or concrete).1 For 

example, cities are warmer than the surrounding countryside 

because the greater extent of paved areas in cities affects how 

water and energy are exchanged between the land and the atmosphere. This increases the exposure of urban populations to 

the effects of extreme heat events. Decisions about land use 

and land cover can therefore affect, positively or negatively, 

how much our climate will change and what kind of vulnerabilities humans and natural systems will face as a result.

The impacts of changes in land use and land cover cut across all 

regions and sectors of the National Climate Assessment. Chapters addressing each region discuss land-use and land-cover 

topics of particular concern to specific regions. Similarly, chapters addressing sectors examine specific land-use matters. In 

particular, land cover and land use are a major focus for sectors 

such as agriculture, forests, rural and urban communities, and 

Native American lands. By contrast, the key messages of this 

chapter are national in scope and synthesize the findings of 

other chapters regarding land cover and land use.

Land uses and land covers change over time in response to 

evolving economic, social, and biophysical conditions.2 Many 

of these changes are set in motion by individual landowners 

and land managers and can be quantified from satellite measurements, aerial photographs, on-the-ground observations, 

and reports from landowners and users.3,4 Over the past few 

decades, the most prominent land changes within the U.S. 

have been changes in the amount and kind of forest cover 

due to logging practices and development in the Southeast 

and Northwest and to urban expansion in the Northeast and 

Southwest.

Because humans control land use and, to a large extent, land 

cover, individuals, businesses, non-profit organizations, and 

governments can make land decisions to adapt to and/or reduce the effects of climate change. Often the same land-use 

decision can serve both aims. Adaptation options (those aimed 

at coping with the effects of climate change) include varying 

the local mix of vegetation and concrete to reduce heat in cities or elevating homes to reduce exposure to sea level rise or 

flooding. Land-use and land-cover-related options for mitigating climate change (reducing the speed and amount of climate 

change) include expanding forests to accelerate removal of 

carbon from the atmosphere, modifying the way cities are built 

and organized to reduce energy and motorized transportation 

demands, and altering agricultural management practices to 

increase carbon storage in soil.

Despite this range of climate change response options, there 

are three main reasons why private and public landowners 

may choose not to modify land uses and land covers for climate adaptation or mitigation purposes. First, land decisions Land-use and land-cover changes affect climate processes: Above, 

development along Colorado’s Front Range.

©Ted Wood Photography
320 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

13: LAND USE AND LAND COVER CHANGE

Figure 13.1. Map shows regional differences in land cover. These patterns affect climate and will be affected 

by climate change. They also influence the vulnerability and resilience of communities to the effects of 

climate change (Figure source: USGS Earth Resources Observation and Science (EROS) Center). (See 

Table 13.2 for definitions of mechanically and non-mechanically disturbed.)

U.S. Land-Cover Composition in 2000

are influenced not only by climate but also by economic, cultural, legal, or other considerations. In many cases, climatebased land-change efforts to adapt to or reduce climate 

change meet with resistance because current practices are too 

costly to modify and/or too deeply entrenched in local societies and cultures. Second, certain land uses and land covers are 

simply difficult to modify, regardless of desire or intent. For instance, the number of homes constructed in floodplains or the 

amount of irrigated agriculture can be so deeply rooted that 

they are difficult to change, no matter how much those practices might impede our ability to respond to climate change. 

Finally, the benefits of land-use decisions made by individual 

landowners with specific adaptation or mitigation goals do not 

always accrue to those landowners or even to their communities. Therefore, without some institutional intervention (such 

as incentives or penalties), the motivations for such decisions 

can be weak.

Recent Trends

In terms of land area, the U.S. remains a predominantly rural 

country, especially as its population increasingly gravitates 

towards urban areas. In 1910, only 46% of the U.S. population lived in urban areas, but by 2010 that figure had climbed 

to more than 81%.5 In 2006 (the most recent year for which 

these data are available), more than 80% of the land cover in 

the lower 48 states was dominated by shrub/scrub vegetation, 

grasslands, forests, and agriculture.6,7 Forests and grasslands, 

which include acreage used for timber production and grazing, 

account for more than half of all U.S. land use by area (Table 

13.1), about 63% of which is in private ownership, though their 

distribution and ownership patterns vary regionally.4 Agricultural land uses are carried out on 18% of U.S. surface area. Developed or built-up areas covered only about 5% of the country’s land surface, with the greatest concentrations of urban 

areas in the Northeast, Midwest, and Southeast. This apparently small percentage of developed area belies its rapid expansion and does not include development that is dispersed in 

a mosaic among other land uses (like agriculture and forests). 

In particular, low-density housing developments (suburban 

and exurban areas), which are not well-represented in commonly used satellite measurements, have rapidly expanded 

throughout the U.S. over the last 60 years or so.8,9 Based on 

Census data, areas settled at suburban and exurban densities 

(1 house per 1 to 40 acres on average) cover more than 15 

times the land area settled at urban densities (1 house per acre 

or less) and covered five times more land area in 2000 than in 

1950.8

Despite these rapid changes in developed land covers, the vast 

size of the country means that total land-cover changes in the 

U.S. may appear deceptively modest. Since 1973, satellite data 

show that the overall rate of land-cover changes nationally has 

averaged about 0.33% per year. Yet this small rate of change 

has produced a large cumulative impact. Between 1973 and 

2000, 8.6% of the area of the lower 48 states experienced landcover change, an area roughly equivalent to the combined land 

area of California and Oregon.1
321 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

13: LAND USE AND LAND COVER CHANGE

Table 13.1. Circa-2001 land-cover statistics for the National Climate Assessment regions of the United States based on the National Land Cover Dataset,7

and overall United States land-use statistics—circa 2007.4

Land Cover 

Class Northeast Southeast Midwest Great Plains Southwest Northwest Alaska Hawaii United States

Land Use 

Class (ca 

2007) 

United 

States 

(ca 

2007)

Agriculture 10.9% 23.0% 49.0% 29.7% 5.0% 10.0% 0.0% 4.0% 18.6% Cropland 18.0%

Grassland, 

Shrub/Scrub, 

Moss, Lichen 

3.4% 7.8% 2.9% 50.5% 65.7% 42.8% 44.9% 33.3% 39.2%

 Grassland, 

Pasture, 

and Range 

27.1%

Forest 52.4% 38.7% 23.7% 10.7% 19.9% 37.7% 22.4% 22.0% 23.2%a Forest 29.7%a

Barren 0.8% 0.3% 0.2% 0.5% 3.7% 1.5% 7.7% 11.2% 2.6% Special 

Useb 13.8%

Developed, 

Built-Up 9.6% 7.7% 8.0% 4.0% 2.7% 3.0% 0.1% 6.7% 4.0% Urban 2.7%

Water, Ice, 

Snow 14.9% 7.3% 10.4% 1.9% 1.7% 3.2% 18.5% 21.7% 7.4%

 Miscellaneousc 8.7%

Wetlands 8.0% 15.2% 5.8% 2.7% 0.7% 1.3% 6.4% 0.3% 5.0%

a

 Definitional differences in the way certain categories are defined, such as the special uses distinction in the USDA Economic Research Service land use estimates, make direct comparisons between land use and land cover challenging. For example, forest land use (29.7%) exceeds forest cover (23.2%). Forest use 

definitions include lands where trees have been harvested and may be replanted, while forest cover is a measurement of the presence of trees.

b

 Special uses represent rural transportation, rural parks and wildlife, defense and industrial, plus miscellaneous farm and other special uses.

c

 Miscellaneous uses represent unclassified uses such as marshes, swamps, bare rock, deserts, tundra plus other uses not estimated, classified, or inventoried.

Table 13.2. Percentage change in land-cover type between 1973 and 2000 for the contiguous U.S. National Climate Assessment regions. These figures do 

not indicate the total amount of changes that have occurred, for example when increases in forest cover were offset by decreases in forest cover, and when 

cropland taken out of production was offset by other land being put into agricultural production. Data from USGS Land Cover Trends Project; Sleeter et al. 2013.10

Land Cover Type Northeast Southeast Midwest Great Plains Southwest Northwest

Grassland/Shrubland 0.73 0.31 0.59 1.55 -0.28 0.35

Forest -2.02 -2.51 -0.93 -0.71 -0.49 2.39

Agriculture -0.85 -1.62 -1.38 -1.60 -0.37 -0.35

Developed 1.36 2.28 1.34 0.43 0.51 0.51

Mining 0.14 -0.05 0.02 0.07 0.10 0.03

Barren 0.00 -0.01 0.00 0.00 0.00 0.00

Snow/Ice 0.00 0.00 0.00 0.00 0.00 0.00

Water 0.03 0.45 0.08 0.23 0.03 -0.02

Wetland -0.05 -0.69 -0.05 -0.13 -0.02 0.03

Mechanically Disturbeda 0.66 1.76 0.32 0.11 0.07 0.07

Non-mechanically Disturbedb 0.00 0.07 0.01 0.06 0.46 1.78

a

 Land in an altered and often un-vegetated state that, because of disturbances by mechanical means, is in transition from one cover type to another. Mechanical 

disturbances include forest clear-cutting, earthmoving, scraping, chaining, reservoir drawdown, and other similar human-induced changes.

b

 Land in an altered and often un-vegetated state that because of disturbances by non-mechanical means, is in transition from one cover type to another. Nonmechanical disturbances are caused by fire, wind, floods, animals, and other similar phenomena.

These national-level annual rates of land changes mask 

considerable geographic variability in the types, rates, and 

causes of change.3 Between 1973 and 2000, the Southeast 

region had the highest rate of change, due to active forest 

timber harvesting and replanting, while the Southwest region had the lowest rate of change. 
322 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

13: LAND USE AND LAND COVER CHANGE

Projections 

Future patterns of land use and land cover will interact with climate changes to affect human communities and ecosystems. 

At the same time, future climate changes will also affect how 

and where humans live and use land for various purposes.

National-scale analyses suggest that the general historical 

trends of land-use and land-cover changes (described above) 

will continue, with some important regional differences. These 

projections all assume continued population growth based 

on assumed or statistically modeled rates of birth, death, and 

migration,11 which will result in changes in land use and land 

cover that are spread unevenly across the country. Urban land 

covers are projected to increase in the lower 48 states by 73% 

to 98% (to between 10% and 12% of land area versus less than 

6% in 1997) by 2050, using low versus high growth assumptions, respectively. The slowest rate of increase is in the Northeast region, because of the high level of existing development 

and relatively low rates of population growth, and the highest 

rate is in the Northwest. In terms of area, the Northwest has 

the smallest projected increase in urban area (approximately 

4.2 million acres) and the Southeast the largest (approximately 

27.5 million acres).12

Changes in development density will have an impact on how 

population is distributed and affects land use and land cover. 

Some of the projected changes in developed areas will depend 

on assumptions about changes in household size and how concentrated urban development will be. Higher population density means less land is converted from forests or grasslands, 

but results in a greater extent of paved area. Projections based 

on estimates of housing-unit density allow the assessment of 

impacts of urban land-use growth by density class. Increases 

in low-density exurban areas will result in a greater area affected by development and are expected to increase commuting times and infrastructure costs. 

The areas projected to experience exurban development will 

have less density of impervious surfaces (like asphalt or concrete). While about one-third of exurban areas are covered 

by impervious surfaces,13 urban or suburban areas are about 

one-half concrete and asphalt. Impervious surfaces have a 

wide range of environmental impacts and thus represent a 

key means by which developed lands modify the movement of 

water, energy, and living things. For example, areas with more 

impervious surfaces like parking lots and roads tend to experience more rapid runoff, greater risk of flooding, and higher 

temperatures from the urban heat-island effect. 

Projections of both landuse and land-cover changes 

will depend to some degree 

on rates of population and 

economic growth. In general, scenarios that assume 

continued high growth produce more rapid increases 

in developed areas of all 

densities and in areas covered by impervious surfaces 

(paved areas and buildings) 

by 2050.12,13

Land-use scenarios project 

that exurban and suburban 

areas will expand nationally by 15% to 20% between 

2000 and 2050,13 based on 

high- and low-growth scenarios respectively. Landcover projections by Wear12

show that both cropland 

and forest are projected 

to decline most relative 

to 1997 (by 6% to 7%, respectively, by 2050) under 

a scenario of high population and economic growth 

Figure 13.2. Projected percentages in each housing-unit density category for 2050 compared with 

2010, assuming demographic and economic growth consistent with the high-growth emissions 

scenario (A2). (Data from U.S. EPA Integrated Climate and Land Use Scenarios).

Projections of Settlement Densities 

(2010-2050)
323 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

13: LAND USE AND LAND COVER CHANGE

and least (by 4% and 6%, 

respectively) under lowergrowth scenarios. More 

forest than cropland is 

projected to be lost in the 

Northeast and Southeast, 

whereas more cropland 

than forest is projected 

to be lost in the Midwest 

and Great Plains.14 Some of 

these regional differences 

are due to the current mix 

of land uses, others to the 

differential rates of urbanization in these different 

regions.

Key Message 1: Effects on Communities and Ecosystems

Choices about land-use and land-cover patterns have affected and will continue 

to affect how vulnerable or resilient human communities and ecosystems are 

to the effects of climate change. 

Decisions about land-use and land-cover change by individual 

landowners and land managers are influenced by demographic 

and economic trends and social preferences, which unfold at 

global, national, regional, and local scales. Policymakers can 

directly affect land use and land cover. For example, Congress 

can declare an area as federally protected wilderness, or local 

officials can set aside portions of a town for industrial development and create tax benefits for companies to build there. 

Climate factors typically play a secondary role in land decisions, if they are considered at all. Nonetheless, land-change 

decisions may affect the vulnerabilities of individuals, households, communities, businesses, non-profit organizations, 

and ecosystems to the effects of climate change.15 A farmer’s 

choice of crop rotation in response to price signals affects his 

or her farm income’s susceptibility to drought, for example. 

Such choices, along with changes in climate can also affect the 

farm’s demand for water for irrigation. Similarly, a developer’s 

decision to build new homes in a floodplain may affect the new 

homeowners’ vulnerabilities to flooding events. A decision to 

include culverts underneath a coastal roadway may facilitate 

migration of a salt marsh inland as sea level rises.

The combination of residential location choices with wildfire occurrence dramatically illustrates how the interactions 

between land use and climate processes can affect climate 

change impacts and vulnerabilities. Low-density (suburban 

and exurban) housing patterns in the U.S. have expanded and 

are projected to continue to expand.13 One result is a rise in the 

amount of construction in forests and other wildlands16 that in 

turn has increased the exposure of houses, other structures, 

and people to damages from wildfires, which are increasing. 

The number of buildings lost in the 25 most destructive fires 

in California history increased significantly in the 1990s and 

2000s compared to the previous three decades.17 These losses 

are one example of how changing development patterns can 

interact with a changing climate to create dramatic new risks. 

In the western United States, increasing frequencies of large 

wildfires and longer wildfire durations are strongly associated 

with increased spring and summer temperatures and an earlier 

Figure 13.3. Projected percentages in each land-cover category for 2050 compared with 2010, 

assuming demographic and economic growth consistent with the high-growth emissions scenario 

(A2) (Data from USDA).

Projected Land Covers (2010-2050)
324 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

13: LAND USE AND LAND COVER CHANGE

Figure 13.4. Many forested areas in the U.S. have experienced 

a recent building boom in what is known as the “wildland-urban 

interface.” This figure shows the number of buildings lost from the 25 

most destructive wildland-urban interface fires in California history 

from 1960 to 2007 (Figure source: Stephens et al. 200917).

Building Loss by Fires at 

California Wildland-Urban Interfaces

©Elmer Frederick Fischer/Corbis

Construction near forests and wildlands is growing. Here, 

wildfire approaches a housing development.

spring snowmelt.18 The effects on property loss of increases in 

the frequency and sizes of fires under climate change are also 

projected to increase in the coming decades because so many 

more people will have moved into increasingly fire-prone places (Ch. 2: Our Changing Climate; Ch. 7: Forests).

Key Message 2: Effects on Climate Processes 

Land-use and land-cover changes affect local, regional, and global climate processes.

Land use and land cover play critical roles in the interaction 

between the land and the atmosphere, influencing climate at 

local, regional, and global scales.19 There is growing evidence 

that land use, land cover, and land management affect the U.S. 

climate in several ways:

• Air temperature and near-surface moisture are changed 

in areas where natural vegetation is converted to agriculture.20,21 This effect has been observed in the Great Plains 

and the Midwest, where overall dew point temperatures 

or the frequency of occurrences of extreme dew point 

temperatures have increased due to converting land to 

agricultural use.21,22,23 This effect has also been observed 

where the fringes of California’s Central Valley are being 

converted from natural vegetation to agriculture.24 Other 

areas where uncultivated and conservation lands are being returned to cultivation, for example from restored 

grassland into biofuel production, have also experienced 

temperature shifts. Regional daily maximum temperatures were lowered due to forest clearing for agriculture 

in the Northeast and Midwest, and then increased in the 

Northeast following regrowth of forests due to abandonment of agriculture.25

• Conversion of rain-fed cropland to irrigated agriculture 

further intensifies the impacts of agricultural conversion 

on temperature. For example, irrigation in California has 

been found to reduce daily maximum temperatures by up 

to 9°F.26 Model comparisons suggest that irrigation cools 

temperatures directly over croplands in California’s Central Valley by 5°F to 13°F and increases relative humidity by 

9% to 20%.27 Observational data-based studies found similar impacts of irrigated agriculture in the Great Plains.22,28

• Both observational and modeling studies show that introduction of irrigated agriculture can alter regional precipitation.29,30 It has been shown that irrigation in the Ogallala 

aquifer portion of the Great Plains can affect precipitation 

as far away as Indiana and western Kentucky.30

• Urbanization is having significant local impacts on weather 

and climate. Land-cover changes associated with urban-
325 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

13: LAND USE AND LAND COVER CHANGE

ization are creating higher air temperatures compared to 

the surrounding rural area.31,32 This is known as the “urban 

heat island” effect (see Ch. 9: Human Health). Urban landscapes are also affecting formation of convective storms 

and changing the location and amounts of precipitation 

compared to pre-urbanization.32,33

• Land-use and land-cover changes are affecting global 

atmospheric concentrations of greenhouse gases. The 

impact is expected to be most significant in areas with 

forest loss or gain, where the amount of carbon that can 

be transferred from the atmosphere to the land (or from 

the land to the atmosphere) is modified. Even in relatively 

un-forested areas, this effect can be significant. A recent 

USGS report suggests that from 2001 to 2005 in the Great 

Plains between 22 to 106 million metric tons of carbon 

were stored in the biosphere due to changes in land use 

and climate.34 Even with these seemingly large numbers, 

U.S. forests absorb only 7% to 24% (with a best estimate of 

16%) of fossil fuel CO2 emissions (see Ch. 15: Biogeochemical Cycles, “Estimating the U.S. Carbon Sink”).

Key Message 3: Adapting to Climate Change 

Individuals, businesses, non-profits, and governments have the capacity to 

make land-use decisions to adapt to the effects of climate change.

Land-use and land-cover patterns may be modified to adapt 

to anticipated or observed effects of a changed climate. These 

changes may be either encouraged or mandated by government (whether at federal or other levels), or undertaken by 

private initiative. In the U.S., even though land-use decisions 

are highly decentralized and strongly influenced by Constitutional protection of private property, the Supreme Court has 

also defined a role for government input into some land-use 

decisions.35 Thus on the one hand farmers may make private 

decisions to plant different crops in response to changing 

growing conditions and/or market prices. On the other hand, 

homeowners may be compelled to respond to policies, zoning, 

or regulations (at national, state, county, or municipal levels) 

by elevating their houses to reduce flood impacts associated 

with more intense rainfall events and/or increased impervious 

surfaces.

Land-use and land-cover changes are thus rarely the product 

of a single factor. Land-use decision processes are influenced 

not only by the biophysical environment, but also by markets, 

laws, technology, politics, perceptions, and culture. Yet there 

is evidence that climate adaptation considerations are playing 

an increasingly large role in land decisions, even in the absence 

of a formal federal climate policy. Motivations typically include 

avoiding or reducing negative impacts from extreme weather 

events (such as storms or heat waves) or from slow-onset hazards (such as sea level rise) (see Ch. 12: Indigenous Peoples).

For example, New Orleans has, through a collection of private and public initiatives, rebuilt some of the neighborhoods 

damaged by Hurricane Katrina with housing elevated six feet 

or even higher above the ground and with roofs specially designed to facilitate evacuation.36 San Francisco has produced 

a land-use plan to reduce impacts from a rising San Francisco 

Bay.37 A similar concern has prompted collective action in four 

Miami-area counties and an array of San Diego jurisdictions, 

to name just two locales, to shape future land uses to comply 

with regulations linked to sea level rise projections.36,38 Chicago 

has produced a plan for limiting the number of casualties, especially among the elderly and homeless, during heat waves 

(Ch. 9: Human Health).36 Deeper discussion of the factors 

commonly influencing adaptation decisions at household, municipal, state, and federal levels is provided in Chapter 28 (Ch. 

28: Adaptation) of this report; Chapters 26 (Ch. 26: Decision 

Support) and 27 (Ch. 27: Mitigation) treat the related topics of 

Decision Support and Mitigation, respectively. 

Key Message 4: Reducing Greenhouse Gas Levels 

Choices about land use and land management may provide a means 

of reducing atmospheric greenhouse gas levels. 

Choices about land use and land management affect the 

amount of greenhouse gases entering and leaving the atmosphere and, therefore, provide opportunities to reduce climate 

change (Ch. 15: Biogeochemical Cycles; Ch. 27: Mitigation).39

Such choices can affect the balance of these gases directly, 

through decisions to preserve or restore carbon in standing 

vegetation (like forests) and soils, and indirectly, in the form of 

land-use policies that affect fossil fuel emissions by influencing energy consumption for transportation and in buildings. 

Additionally, as crops are increasingly used to make fuel, the 

potential for reducing net carbon emissions through replacement of fossil fuels represents a possible land-based carbon 

emissions reduction strategy, albeit one that is complicated by 

many natural and economic interactions that will determine 

the ultimate effect of these strategies on emissions (Ch. 7: Forests; Ch. 6: Agriculture).
326 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

13: LAND USE AND LAND COVER CHANGE

Land-cover change and management accounts for about onethird of all carbon released into the atmosphere by people 

globally since 1850. The primary source related to land use 

has been the conversion of native vegetation like forests and 

grasslands to croplands, which in turn has released carbon 

from vegetation and soil into the atmosphere as carbon dioxide (CO2).40 Currently, an estimated 16% of CO2 going into the 

atmosphere is due to land-related activities globally, with the 

remainder coming from fossil fuel burning and cement manufacturing.40 In the United States, activities related to land use 

are effectively balanced with respect to CO2: as much CO2 is 

released to the atmosphere by land-use activities as is taken 

up by and stored in, for example, vegetation and soil. The regrowth of forests and increases of conservation-related forest 

and crop management practices have also increased carbon 

storage. Overall, setting aside emissions due to burning fossil fuels, in the U.S. and the rest of North America, land cover 

takes up more carbon than it releases. This has happened as 

a result of more efficient forest and agricultural management 

practices, but it is not clear if this rate of uptake can be increased or if it will persist into the future. The projected declines in forest area (Figure 13.3) put these carbon stores at 

risk. Additionally, the rate of carbon uptake on a given acre of 

forest can vary with weather, making it potentially sensitive to 

climate changes.41

Opportunities to increase the net uptake of carbon from the atmosphere by the land include42 increasing the amount of area 

in ecosystems with high carbon content (by converting farms 

to forests or grasslands); increasing the rate of carbon uptake 

in existing ecosystems (through fertilization); and reducing carbon loss from existing ecosystems (for example, through no-till 

farming).43 Because of these effects, policies specifically aimed 

at increasing carbon storage, either directly through mandates 

or indirectly through a market for carbon offsets, may be used 

to encourage more land-based carbon storage.44

The following uncertainties deserve further investigation: 1) 

the effects of these policies or actions on the balance of other 

greenhouse gases, like methane and nitrous oxide; 2) the degree of permanence these carbon stores will have in a changing 

climate (especially through the effects of disturbances like fires 

and plant pests45); 3) the degree to which increases in carbon 

storage can be attributed to any specific policy, or whether or 

not they may have occurred without any policy change; and 4) 

the possibility that increased carbon storage in one location 

might be partially offset by releases in another. All of these 

specific mitigation options present implementation challenges, as the decisions must be weighed against competing objectives. For example, retiring farmland to sequester carbon may 

be difficult to achieve if crop prices rise,46 such as has occurred 

in recent years in response to the fast-growing market for biofuels. Agricultural research and development that increases 

the productivity of the sector presents the possibility of reducing demand for agricultural land and may serve as a powerful 

greenhouse gas mitigation strategy, although the ultimate net 

effect on greenhouse gas emissions is uncertain.47

Land-use decisions in urban areas also present carbon reduction options. Carbon storage in urban areas can reach densities 

as high as those found in tropical forests, with most of that 

carbon found in soils, but also in vegetation, landfills, and the 

structures and contents of buildings.48 Urban and suburban 

areas tend to be net sources of carbon to the atmosphere, 

whereas exurban and rural areas tend to be net sinks.49 Effects of urban development patterns on carbon storage and 

emissions due to land and fossil fuel use are topics of current 

research and can be affected by land-use planning choices. 

Many cities have adopted land-use plans with explicit carbon 

goals, typically targeted at reducing carbon emissions from the 

often intertwined activities of transportation and energy use. 

This trend, which includes major cities such as Los Angeles,50

Chicago,51 and New York City52 as well as small towns, such as 

Homer, Alaska,53 has occurred even in the absence of a formal 

federal climate policy. 
327 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

13: LAND USE & LAND COVER CHANGE

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5, 551-556, doi:10.1038/ngeo1529. [Available online at http://

ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/33148/

LawBeverlyForestryReductionCarbonUptake.pdf?sequence=1]

42. Izzaurralde, R. C., W. M. Post, and T. O. West, 2013: Ch. 13: 

Managing carbon: Ecological limits and constraints. Land Use and 

the Carbon Cycle: Advances in Integrated Science, Management and Policy, 

D. G. Brown, D. T. Robinson, N. H. French, and B. C. Reed, Eds., 

Cambridge University Press, 331-358. 

43.	 Cambardella, C. A., and J. L. Hatfield, 2013: Ch. 15: Soil carbon

dynamics in agricultural systems. Land Use and the Carbon Cycle: 

Advances in Integrated Science, Management and Policy, D. G. Brown, 

D. T. Robinson, N. H. French, and B. C. Reed, Eds., Cambridge 

University Press, 381-401. 

44. Jones, C. A., C. J. Nickerson, and N. Cavallaro, 2013: Ch. 16: U.S. 

Policies and greenhouse gas mitigation in agriculture. Land Use and 

the Carbon Cycle: Advances in Integrated Science, Management and Policy, 

D. G. Brown, D. T. Robinson, N. H. French, and B. C. Reed, Eds., 

Cambridge University Press, 403-430. 

Pearson, T., and S. Brown, 2013: Ch. 17: Opportunities and 

challenges for offsetting greenhouse gas emissions with forests. 

Land Use and the Carbon Cycle: Advances in Integrated Science, Management 

and Policy, D. G. Brown, D. T. Robinson, N. H. French, and B. C. 

Reed, Eds., Cambridge University Press, 431-454. 

45.	 Hurteau, M. D., 2013: Ch. 14: Effects of wildland fire management

on forest carbon stores. Land Use and the Carbon Cycle: Advances in 

Integrated Science, Management and Policy, D. G. Brown, D. T. Robinson, 

N. H. French, and B. C. Reed, Eds., Cambridge University Press,

359-380. 

46. Lubowski, R. N., A. J. Plantinga, and R. N. Stavins, 2008: What 

drives land-use change in the United States? A national analysis 

of landowner decisions. Land Economics, 84, 529-550, doi:10.3368/

le.84.4.529. 

47. Jones, C. A., C. J. Nickerson, and P. W. Heisey, 2013: New uses of old 

tools? Greenhouse gas mitigation with agriculture sector policies. 

Applied Economic Perspectives and Policy, 35, 398-434, doi:10.1093/

aepp/ppt020. 

48. Churkina, G., D. G. Brown, and G. Keoleian, 2010: Carbon stored 

in human settlements: The conterminous United States. Global 

Change Biolog y, 16, 135-143, doi:10.1111/j.1365-2486.2009.02002.x. 

49. Zhao, T., M. W. Horner, and J. Sulik, 2011: A geographic approach 

to sectoral carbon inventory: Examining the balance between 

consumption-based emissions and land-use carbon sequestration 

in Florida. Annals of the Association of American Geographers, 101, 752-

763, doi:10.1080/00045608.2011.567936. 

50. EnvironmentLA, cited 2012: ClimateLA: City of Los 

Angeles. [Available online at http://environmentla.org/ead_

GreenLAClimateLA.htm]

51. City of Chicago, cited 2012: Chicago Green Homes Program: 

City of Chicago. [Available online at http://www.cityofchicago.

org/city/en/depts/bldgs/provdrs/chicago_green_homesprogram.

html]

52. NYCDEP, 2011: NYC Green Infrastructure Plan, 154 pp., New 

York City Department of Environmental Protection, New York, 

New York. [Available online at http://www.nyc.gov/html/

dep/pdf/green_infrastructure/NYCGreenInfrastructurePlan_

LowRes.pdf]

53. City of Homer, 2007: City of Homer Climate Action Plan: Reducing 

the Threat of Global Climate Change Through Government 

and Community Efforts, 44 pp., City of Homer, Homer, Alaska. 

[Available online at http://www.cityofhomer-ak.gov/sites/default/

files/fileattachments/climate_action_plan.pdf]

Photo Credits

Introduction to chapter; California Valley Solar Ranch in top banner: 

©Proehl Studios/Corbis
330 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

13: LAND USE AND LAND COVER CHANGE

Process for Developing Key Messages: 

The author team benefited from a number of relevant technical 

input reports. One report described the findings of a three-day 

workshop held from November 29 to December 1, 2011, in Salt 

Lake City, in which a number of the chapter authors participated.2 

Findings of the workshop provided a review of current issues and 

topics as well as the availability and quality of relevant data. In 

addition, from December 2011 through June 2012 the author 

team held biweekly teleconferences. Key messages were identified during this period and discussed in two phases, associated 

with major chapter drafts. An early draft identified a number of 

issues and key messages. Based on discussions with National Climate Assessment (NCA) leadership and other chapter authors, the 

Land Use and Land Cover Change authors identified and reached 

consensus on a final set of four key messages and organized most 

of the chapter to directly address these messages. The authors 

selected key messages based on the consequences and likelihood 

of impacts, the implied vulnerability, and available evidence. Relevance to decision support, mitigation, and adaptation was also 

an important criterion for the selection of key messages for the 

cross-cutting and foundational topic of this chapter.

The U.S. acquires, produces, and distributes substantial data 

that characterize the nation’s land cover and land use. Satellite 

observations, with near complete coverage over the landscape 

and consistency for estimating change and trends, are particularly 

valuable. Field inventories, especially of agriculture and forestry, 

provide very reliable data products that describe land cover as well 

as land-use change. Together, remote sensing and field inventory 

data, as well as related ecological and socioeconomic data, allow 

many conclusions about land-use and land-cover change with very 

high confidence.

Key message #1 Traceable Account

Choices about land-use and land-cover patterns 

have affected and will continue to affect how vulnerable or resilient human communities and ecosystems are to the effects of climate change.

Description of evidence base

The influences of climate on vegetation and soils, and thus on land 

cover and land use, are relatively well understood, and a number 

of well-validated mathematical models are used to investigate 

potential consequences of climate change for ecosystem processes, 

structure, and function. Given scenarios about socioeconomic 

factors or relevant models, some aspects of land-use and landcover change can also be analyzed and projected into the future 

based on assumed climate change. During a workshop convened to 

review land-use and land-cover change for the NCA, participants 

summarized various studies from different perspectives, including 

agriculture and forestry as well as socioeconomic issues such as 

flood insurance.2

Residential exposure to wildfire is an excellent example supporting 

this key message and is well documented in the literature.16,17,18

New information and remaining uncertainties

Steadily accumulating field and remote sensing observations as 

well as inventories continue to increase confidence in this key 

message. A recent study by the EPA13 provides relevant projections 

of housing density and impervious surface under alternative 

scenarios of climate change. 

While there is little uncertainty about the general applicability 

of this key message, the actual character and consequences of 

climate change as well as its interactions with land cover and 

land use vary significantly between locations and circumstances. 

Thus the specific vulnerabilities resulting from the specific ways in 

which people, both as individuals and as collectives, will respond 

to anticipated or observed climate change impacts are less well 

understood than the biophysical dimensions of this problem.

Assessment of confidence based on evidence 

Very High. Observed weather and climate impacts and 

consequences for land cover and land use, basic understanding 

of processes and analyses using models of those processes, as 

well as substantial literature are consistent in supporting this key 

message.

Key message#2 Traceable Account

Land-use and land-cover changes affect local, regional, and global climate processes.

13: LAND USE AND LAND COVER CHANGE

SUPPLEMENTAL MATERIAL

TRACEABLE ACCOUNTS
13: LAND USE AND LAND COVER CHANGE

Traceable Accounts

331 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Description of evidence base

The dependence of weather and climate processes on land surface 

properties is reasonably well understood in terms of the biophysical 

processes involved. Most climate models represent land-surface 

conditions and processes, though only recently have they begun to 

incorporate these conditions dynamically to represent changes in 

the land surface within a model run. Regional weather models are 

increasingly incorporating land surface characteristics. Extensive 

literature – as well as textbooks – documents this understanding,

as do models of land surface processes and properties. A Technical 

Input report to the National Climate Assessment1 summarizes the 

literature and basic understanding of interactions between the 

atmosphere and land surface that influence climate. 

Examples are provided within the chapter to demonstrate 

that land-use and land-cover change are affecting U.S. 

climate.20,24,25,27,31,32,33,34

New information and remaining uncertainties

While there is little uncertainty about this key message in general, 

the heterogeneity of the U.S. landscape and associated processes,

as well as regional and local variations in atmospheric processes,

make it difficult to analyze or predict the character of land use 

and land cover influences on atmospheric processes at all scales.

Assessment of confidence based on evidence 

Very High. The basic processes underlying the biophysics of 

interactions between the land surface and atmosphere are well 

understood. A number of examples and field studies are consistent 

in demonstrating effects of land use and land-cover change on the 

climate of the United States.

Key message #3 Traceable Account

Individuals, businesses, non-profits, and governments have the capacity to make land-use decisions to adapt to the effects of climate change.

Description of evidence base

The key message is supported by well-understood aspects of 

land-use planning and management, including the legal roles of 

government and citizens and management practices such as zoning 

and taxation. Participants in the NCA workshop (Nov 29-Dec 1, 

2011, in Salt Lake City) on land use and land cover presented and 

discussed a number of examples showing the influences of landuse decisions on climate change adaptation options.2

 The chapter 

describes specific examples of measures to adapt to climate 

change, further supporting this key message.36,37,38

New information and remaining uncertainties

Experience with climate change adaptation measures involving 

land-use decisions is accumulating rapidly.

36,37,38

Although there is little uncertainty that land-use decisions can 

enable adaptation to climate change, the information about 

climate change, at scales where such decisions are made, is 

generally lacking.

Assessment of confidence based on evidence 

Very High. The aspects of land-use planning that can enable 

climate change adaptation are well understood and examples 

demonstrate where actions are being taken.

Key message #4 Traceable Account

Choices about land use and land management 

provide a means of reducing atmospheric greenhouse gas levels.

Description of evidence base

The evidence base for this key message includes scientific studies 

on the carbon cycle at both global and local scales (summarized 

in Izzauralde et al. 2013; Hurteau 2013; and Cambardella and 

Hatfield 2013).42,43,45 The evidence base also includes policy 

studies on the costs and benefits and feasibilities of various 

actions to reduce carbon emissions from land-based activities 

and/or to increase carbon storage in the biosphere through landbased activities (summarized in Jones et al. 2013; and Pearson 

and Brown 2013).

44 Foundational studies are summarized in the 

NCA Technical Input documents.1,2

New information and remaining uncertainties

A major study by the U.S. Geological Survey is estimating carbon 

stocks in vegetation and soils of the U.S., and this inventory will 

Confidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent 

results, well documented and 

accepted methods, etc.), high 

consensus

High

Moderate evidence (several sources, some consistency, methods 

vary and/or documentation limited, 

etc.), medium consensus

Medium

Suggestive evidence (a few 

sources, limited consistency, models incomplete, methods emerging, 

etc.), competing schools of thought

Low

Inconclusive evidence (limited 

sources, extrapolations, inconsistent findings, poor documentation 

and/or methods not tested, etc.), 

disagreement or lack of opinions 

among experts
13: LAND USE AND LAND COVER CHANGE

Traceable Accounts

332 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

clarify the potential for capturing greenhouse gasses by land-use 

change (an early result is reported in Sohl et al. 201214).

There is little uncertainty behind the premise that specific land 

uses affect the carbon cycle. There are, however, scientific 

uncertainties regarding the magnitudes of effects resulting from 

specific actions designed to leverage this linkage for mitigation. For 

example, uncertainties are introduced regarding the permanence 

of specific land-based stores of carbon, the incremental value of 

specific management or policy decisions to increase terrestrial 

carbon stocks beyond changes that would have occurred in the 

absence of management, and the possibility for decreases in 

carbon storage in another location that offset increases resulting 

from specific actions at a given location. Also, we do not yet know 

how natural processes might alter the amount of carbon storage 

expected to occur with management actions. There are further 

uncertainties regarding the political feasibilities and economic 

efficacy of policy options to use land-based activities to reduce 

the concentration of greenhouse gases in the atmosphere.

Assessment of confidence based on evidence 

Given the evidence base and uncertainties, there is medium 

confidence that land use and land management choices can 

reduce the amount of greenhouse gases in the atmosphere. 
333

Climate Change Impacts in the United States

CHAPTER 14

RURAL COMMUNITIES

INFORMATION DRAWN FROM THIS CHAPTER IS INCLUDED IN THE HIGHLIGHTS REPORT AND IS IDENTIFIED BY THIS ICON

Recommended Citation for Chapter

Hales, D., W. Hohenstein, M. D. Bidwell, C. Landry, D. McGranahan, J. Molnar, L. W. Morton, M. Vasquez, and J. Jadin, 

2014: Ch. 14: Rural Communities. Climate Change Impacts in the United States: The Third National Climate Assessment, J. 

M. Melillo, Terese (T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 333-349. doi:10.7930/

J01Z429C. 

On the Web: http://nca2014.globalchange.gov/report/sectors/rural-communities

Convening Lead Authors

David Hales, Second Nature

William Hohenstein, U.S. Department of Agriculture

Lead Authors

Marcie D. Bidwell, Mountain Studies Institute

Craig Landry, East Carolina University

David McGranahan, U.S. Department of Agriculture

Joseph Molnar, Auburn University

Lois Wright Morton, Iowa State University

Marcela Vasquez, University of Arizona

Contributing Authors

Jenna Jadin, U.S. Department of Agriculture 
334 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Key Messages

1. Rural communities are highly dependent upon natural resources for their livelihoods and 

social structures. Climate change related impacts are currently affecting rural communities. 

These impacts will progressively increase over this century and will shift the locations where 

rural economic activities (like agriculture, forestry, and recreation) can thrive.  

2. Rural communities face particular geographic and demographic obstacles in responding to 

and preparing for climate change risks. In particular, physical isolation, limited economic 

diversity, and higher poverty rates, combined with an aging population, increase the vulnerability 

of rural communities. Systems of fundamental importance to rural populations are already 

stressed by remoteness and limited access. 

3. Responding to additional challenges from climate change impacts will require significant 

adaptation within rural transportation and infrastructure systems, as well as health and 

emergency response systems. Governments in rural communities have limited institutional 

capacity to respond to, plan for, and anticipate climate change impacts.

14 RURAL COMMUNITIES

More than 95% of U.S. land area is classified as rural, but is 

home to just 19% of the population (see also Ch. 13: Land Use 

& Land Cover Change).1 Rural America’s importance to the 

country’s economic and social well-being is disproportionate 

to its population, as rural areas provide natural resources that 

much of the rest of the United States depends on for food, energy, water, forests, recreation, national character, and quality 

of life.2 Rural economic foundations and community cohesion 

are intricately linked to these natural systems, which are inherently vulnerable to climate change. Urban areas that depend 

on goods and services from rural areas will also be affected by 

climate change driven impacts across the countryside.

Warming trends, climate volatility, extreme weather events, 

and environmental change are already affecting the economies and cultures of rural areas. Many rural communities face considerable risk to their infrastructure, livelihoods, and quality of life from observed and projected 

climate shifts (Ch. 12: Indigenous Peoples). These changes 

will progressively increase volatility in food commodity 

markets, shift the ranges of plant and animal species, and, 

depending on the region, increase water scarcity, exacerbate flooding and coastal erosion, and increase the intensity and frequency of wildfires across the rural landscape. 

Climate changes will severely challenge many rural communities, shifting locations where particular economic 

activities are capable of thriving. Changes in the timing of 

seasons, temperatures, and precipitation will alter where 

commodities, value-added crops, and recreational activities are best suited. Because many rural communities are less 

diverse than urban areas in their economic activities, changes 

in the viability of one traditional economic sector will place disproportionate stresses on community stability. 

Climate change impacts will not be uniform or consistent 

across rural areas, and some communities may benefit from 

climate change. In the short term, the U.S. agricultural system 

is expected to be fairly resilient to climate change due to the

system’s flexibility to engage in adaptive behaviors such as expansion of irrigated acreage, regional shifts in acreage for specific crops, crop rotations, changes to management decisions 

(such as choice and timing of inputs and cultivation practices), 

and altered trade patterns compensating for yield changes (Ch. © Perry Conway/Corbis
335 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

14: RURAL COMMUNITIES

6: Agriculture; Key Message 5).4 Recreation, tourism, and leisure activities in 

some regions will benefit from shifts in 

temperature and precipitation. 

Negative impacts from projected climate changes, however, will ripple 

throughout rural America. Agricultural 

systems in some areas may need to undergo more transformative changes to 

keep pace with future climate change 

(Ch. 6: Agriculture, Key Message 5). In 

lakes and riparian areas, warming is 

projected to increase the growth of 

algae and invasive species, particularly 

in areas already facing water quality impairments.5

 Mountain species and cold 

water fish, such as salmon, are expected to face decreasing range sizes due to 

warming, while ranges could expand for 

some warm water fish, such as bass.6

Alaska, with its reliance on commercial 

and subsistence fishing catch, is particularly vulnerable. Warmer weather and 

higher water temperatures will reduce 

salmon harvests, creating hardships for 

the rural communities and tribes that 

depend upon these catches (Ch. 12: 

Indigenous Peoples, Key Message 1).7

Communities in Guam and American 

Samoa, which depend on fish for 25% to 

69% of their protein, are expected to be 

particularly hard hit as climate change 

alters the composition of coral reef ecosystems.8

Across the United States, rural areas 

provide ecosystem services – like carbon absorption in forests, water filtration in wetlands, wildlife habitat in prairies, and environmental flows in rivers 

and streams – whose value tends to be 

overlooked. Preserving these ecosystem services sustains the quality of life 

in rural communities and also benefits 

those who come to rural communities 

for second homes, tourism, and other 

amenities. They also provide urban residents with vital resources – like food, 

energy, and fresh water – that meet essential needs. This layered connection 

between rural areas and populous urban centers suggests that maintaining 

the health of rural areas is a national, 

and not simply a local, concern.

Figure 14.1. Although the majority of the U.S. population lives in urban areas, most of 

the country is still classified as rural. In this map, counties are classified as rural if they 

do not include any cities with populations of 50,000 or more. (Figure source: USDA 

Economic Research Service 20133).

Rural Counties

Figure 14.2. Much of the rural United States depends on agriculture, mining, and 

manufacturing. Climate changes will affect each region and each economic sector in 

complex and interrelated ways. The economic dependence classification used in the 

map indicates the largest share of earnings and employment in each county. (Figure 

source: USDA Economic Research Service 20133

).

Economic Dependence Varies 

by Region
336 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

14: RURAL COMMUNITIES

Key Message 1: Rural Economies

Rural communities are highly dependent upon natural resources for their livelihoods and 

social structures. Climate change related impacts are currently affecting rural communities. 

These impacts will progressively increase over this century and will shift the locations where 

rural economic activities (like agriculture, forestry, and recreation) can thrive.  

Rural America has already experienced some of the 

impacts of climate change related weather effects, including crop and livestock loss from severe drought and 

flooding,9 infrastructure damage to levees and roads 

from extreme storms,10 shifts in planting and harvesting 

times in farming communities,11 and large-scale losses 

from fires and other weather-related disasters.12 These 

impacts have profound effects, often significantly affecting the health and well-being of rural residents as well 

as their communities, and are amplified by the essential 

economic link that many of these communities have to 

their natural resource base.

Rural communities are often characterized by their 

natural resources and associated economic activity. 

Dominant economic drivers include agriculture, forestry, 

mining, energy, outdoor recreation, and tourism. In addition, many rural areas with pleasant climates and appealing 

landscapes are increasingly reliant on second-home owners 

and retirees for their tax base and community activities. 

Nationally, fewer than 7% of rural workers are directly employed in agriculture, but the nation’s two million farms occupy more than 40% of U.S. land mass – and many rural 

© STR/Reuters/Corbis

River flood waters illustrate threats rural areas face in a changing climate.

Figure 14.3. The left map shows that if emissions continue to increase (A2 scenario), the U.S. growing season (or frost-free season) 

will lengthen by as much as 30 to 80 days by the end of the century (2070-2099 as compared to 1971-2000). The right map shows 

a reduction in the number of frost days (days with minimum temperatures below freezing) by 20 to 80 days in much of the United 

States in the same time period. While changes in the growing season may have positive effects for some crops, reductions in the 

number of frost days can result in early bud-bursts or blooms, consequently damaging some perennial crops grown in the United 

States (See also Ch. 6: Agriculture). White areas are projected to experience no freezes for 2070-2099, and gray areas are projected 

to experience more than 10 freeze-free years during the same period. (Figure source: NOAA NCDC / CICS-NC).

Growing Season Lengthens
337 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

14: RURAL COMMUNITIES

communities rely extensively on farming and ranching (Ch. 6 

Agriculture; Ch. 13 Land Use & Land Cover Change).13 Farmers 

are responding to climate change by shifting cropping patterns 

and altering the timing of planting and harvesting. This may 

result in additional use of herbicides and pesticides with the 

accompanying human exposure to additional health risks.14

Changes in rainfall, temperature, and extreme weather events 

will increase the risk of poor yields and reduced crop profitability. For example, the increased frequency and intensity of 

heavy downpours will accelerate soil erosion rates, increasing 

deposition of nitrogen and phosphorous into water bodies and 

diminishing water quality.15

Many areas will face increasing competition for water among 

household, industrial, agricultural, and urban users (Ch. 3: Water).16 Reduced surface water will place more stress on surface 

water systems as well as groundwater systems (Ch. 3: Water; 

Key Message 4). In-stream flow requirements for the maintenance of environmental resources are an equally important 

water demand. While irrigated cropland is an important and 

growing component of the farm economy,17 water withdrawals necessary for generating electricity in thermal power 

plants are already roughly equal to irrigation withdrawals.18

As climate change increases water scarcity in some regions, 

there will be increased competition for water between 

energy production and agriculture.19 Mining also requires 

large quantities of water, and 

scarcity resulting from drought 

associated with climate change 

may affect operations. Changes in seasonality and intensity 

of precipitation will increase 

costs of runoff containment. 

Climate change impacts on 

forestry have important implications for timber and forestamenity-based rural communities. Shifting forest range 

and composition, as well as 

increased attacks from pests 

and diseases, will have negative effects on biodiversity 

and will increase wildfire risks 

(Ch. 7: Forests).8,20 Shifts in the 

distribution and abundance of 

many economically important 

tree species would affect the 

pulp and wood industry. As 

ranges shift and the distribution of plant species in forests 

changes, the range of other 

forest-dependent animal species will also change, causing additional economic and sociocultural impacts. 

Tourism contributes significantly to rural economies. Changes 

in the length and timing of seasons, temperature, precipitation, and severe weather events can have a direct impact on 

tourism and recreation activities by influencing visitation patterns and tourism-related economic activity. 

Climate change impacts on tourism and recreation will vary 

significantly by region. For instance, some of Florida’s top tourist attractions, including the Everglades and Florida Keys, are 

threatened by sea level rise,21 with estimated revenue losses 

of $9 billion by 2025 and $40 billion by the 2050s. The effects 

of climate change on the tourism industry will not be exclusively negative. In Maine, coastal tourism could increase due to 

warmer summer months, with more people visiting the state’s 

beaches.22 Employing a Tourism Climatic Index (Figure 14.4) 

that accounts for temperature, precipitation, sunshine, and 

wind, one study finds that conditions conducive for outdoor 

recreation will be shifting northward with climate change, 

though it is unclear whether absolute conditions or relative 

weather conditions will be more important in influencing future tourist behaviors.23

Climate Change Impacts on Summertime Tourism

Figure 14.4. Tourism is often climate-dependent as well as seasonally dependent. Increasing 

heat and humidity – projected for summers in the Midwest, Southeast, and parts of the Southwest 

by mid-century (compared to the period 1961-1990) – is likely to create unfavorable conditions 

for summertime outdoor recreation and tourism activity. The figures illustrate projected changes 

in climatic attractiveness (based on maximum daily temperature and minimum daily relative humidity, average daily temperature and relative humidity, precipitation, sunshine, and wind speed) 

in July for much of North America. In the coming century, the distribution of these conditions is 

projected to shift from acceptable to unfavorable across most of the southern Midwest and a portion of the Southeast, and from very good or good to acceptable conditions in northern portions 

of the Midwest, under a high emissions scenario (A2a). (Figure source: Nicholls et al. 200524).
338 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

14: RURAL COMMUNITIES

Climate change will also influence the distribution and composition of plants and animals across the United States. Hunting, 

fishing, bird watching, and other wildlife-related activities will 

be affected as habitats shift and relationships among species 

change.25 Cold-weather recreation and tourism will be adversely affected by climate change. Snow accumulation in the 

western United States has decreased, and is expected to continue to decrease, as a result of observed and projected warming. Reduced snow accumulation also reduces the amount of 

spring snowmelt, decreasing warm-season runoff in mid- to 

high-latitude regions.

Similar changes to snowpack are expected in the Northeast.26

Adverse impacts on winter sports are projected to be more 

pronounced in the Northeast and Southwest regions of the 

United States.8 Coastal areas will be adversely affected by sea 

level rise and increased severity of storms.

22,27 Changing environmental conditions, such as wetland loss and beach erosion 

in coastal areas28 and increased risk of natural hazards such as 

wildfire, flash flooding, storm surge, river flooding, drought, 

and extremely high temperatures can alter the character and 

attraction of rural areas as tourist destinations. 

The implications of climate change on communities that are 

dependent on resource extraction (coal, oil, natural gas, and 

mining) have not been well studied. Attributes of economic 

development in these communities, such as cyclical growth, 

transient workforce, rapid development, pressure on infrastructure, and lack of economic diversification suggest that 

these communities could face challenges in adapting to climate change.13,29,30

Key Message 2: Responding to Risks

Rural communities face particular geographic and demographic obstacles in responding 

to and preparing for climate change risks. In particular, physical isolation, limited economic 

diversity, and higher poverty rates, combined with an aging population, increase the 

vulnerability of rural communities. Systems of fundamental importance to rural populations 

are already stressed by remoteness and limited access. 

Relatively rapid changes in demographics, economic activity, and climate are particularly challenging in rural communities, where local, agrarian values often run generations deep. 

Changing rural demographics, influenced by new immigration 

patterns, fluctuating economic conditions, and evolving community values add to these challenges – especially with regard 

to climate changes. 

Modern rural populations are generally older, less affluent, and less educated than their urban counterparts. 

Rural areas are characterized by higher 

unemployment, more dependence on 

government transfer payments, less 

diversified economies, and fewer social and economic resources needed 

for resilience in the face of major 

changes.8,31 In particular, the combination of an aging population and poverty increases the vulnerability of rural 

communities to climate fluctuations.

There has been a trend away from 

manufacturing, resource extraction, 

and farming to amenity-based economic activity in many rural areas of 

the United States.32 Expanding amenity-based economic activities in rural 

areas include recreation and leisure, 

e-commuting residents, tourism, and 

second home and retirement home 

development. This shift has stressed 

traditional cultural values33 and put 

pressure on infrastructure34 and natuFigure 14.5. Census data show significant population decreases in many rural areas, 

notably in the Great Plains. Many rural communities’ existing vulnerabilities to climate 

change, including physical isolation, reduced services like health care, and an aging 

population, are projected to increase as population decreases. (Figure source: USDA 

Economic Research Service 20133).

Many Rural Areas are

 Losing Population
339 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

14: RURAL COMMUNITIES

ral amenities35 that draw people to rural areas. Changes in climate and weather are likely to increase these stresses. Rural 

components of transportation systems are particularly vulnerable to risks from flooding and sea level rise.

36 Since rural areas 

often have fewer transportation options and fewer infrastructure redundancies, any disruptions in road, rail, or air transport 

will deeply affect rural communities. 

Power and communication outages resulting from extreme 

events often take longer to repair in rural areas, contributing 

to the isolation and vulnerability of elderly residents who may 

not have cell phones. The lack of cellular coverage in some rural areas can create problems for emergency response during 

power failures.37

In some parts of the country there has been a recent trend in 

Hispanic population growth in rural regions that have not been 

traditional migrant destinations. New Hispanic immigrants 

are often highly segregated residentially and isolated from 

mainstream institutions,38 making them more vulnerable to 

changes in climate. Low wages, unstable work, language barriers, and inadequate housing are critical obstacles to managing 

climate risk.

Rural communities rely on various transportation modes, both 

for export and import of critical goods (Ch. 5: Transportation). 

Climate changes will result in increased erosion and maintenance costs for local road and rail systems, as well as changes 

in streamflows and predictability that will result in increased 

maintenance costs for waterways. More frequent disruption 

of shipping is projected, with serious economic consequences. 

For example, in 2010, about 40 million tons of cereal grains 

were shipped by water to Louisiana, while less than 4 million 

tons traveled by rail.10 While rail can help ameliorate smallscale or off-peak capacity limitations on the Mississippi River, 

it seems unlikely that the rail system can fully replace the river 

system in the event of a prolonged harvest-time disruption. 

Events that affect both rail and barge traffic would be particularly damaging to rural communities that depend upon these 

systems to get commodities to market.

Health and emergency response systems also face additional 

demands from substantial direct and indirect health risks associated with global climate changes. Indirect risks, particularly 

those posed by emerging and reemerging infectious diseases, 

are more difficult to assess, but pose looming threats to economically challenged communities where health services are 

limited. Direct threats (such as extreme heat, storm events, 

and coastal and riparian flooding) tend to be more associated 

with specific local vulnerabilities, so the risks are somewhat 

easier to assess.39

The socioeconomic and demographic characteristics of rural 

areas interact with climate change to create health concerns 

that differ from those of urban and suburban communities. 

Older populations with lower income and educational levels in 

rural areas spend a larger proportion of their income on health 

care than their urban counterparts. Moreover, health care access declines as geographic isolation increases. Overall, rural 

residents already have higher rates of age-adjusted mortality, 

disability, and chronic disease than do urban populations.40

These trends are likely to be exacerbated by climate change 

(Ch. 9: Human Health). 

Governments in rural areas are generally ill-prepared to respond quickly and effectively to large-scale events, although 

individuals and voluntary associations often show significant 

resilience. Health risks are exacerbated by limitations in the 

health service systems characteristic of rural areas, including 

the distance between rural residents and health care providers 

and the reduced availability of medical specialists.

The effects of climate change on mental health merit special 

consideration. Rural residents are already at a heightened risk 

from mental health issues because of the lack of access to 

mental health providers. The adverse impact of severe weather disasters on mental health is well established,41 and there is 

emerging evidence that climate change in the form of increasing heat waves and droughts has harmful effects on mental 

health (Ch. 9: Human Health, Key Message 1). Droughts often 

result in people relocating to seek other employment, causing a loss of home and social networks. Studies have shown 

that springtime droughts in rural areas cause a decrease in 

life satisfaction.42 The primary care physicians who form the 

backbone of rural health care often have heavy caseloads and 

lack specialized training in mental health issues.40 Additionally, 

patients referred to mental health specialists often experience 

significant delays.43

The frequency and distribution of infectious diseases is also 

projected to increase with rising temperatures and associated 

seasonal shifts. Increased rates of mutation and increased 

resistance to drugs and other treatments are already evident 

in the behavior of infectious disease-causing bacteria and viruses.44 In addition, changes in temperature, surface water, humidity, and precipitation affect the distribution and abundance 

of disease-carriers and intermediate hosts, and result in larger 

distributions for many parasites and diseases. Rural residents 

who spend significant time outdoors have an increased risk of 

exposure to these disease-carriers, like ticks and mosquitoes 

(Ch. 9: Human Health). 
340 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

14: RURAL COMMUNITIES

Key Message 3: Adaptation

Responding to additional challenges from climate change impacts will require significant 

adaptation within rural transportation and infrastructure systems, as well as health and 

emergency response systems. Governments in rural communities have limited institutional 

capacity to respond to, plan for, and anticipate climate change impacts.

Climate variability and increases in temperature, extreme 

events (such as storms, floods, heat waves, and droughts), and 

sea level rise are expected to have widespread impacts on the 

provision of services from state, regional, local, and tribal governments. Emergency management, energy use and distribution systems, transportation and infrastructure planning, and 

public health will all be affected. 

Rural governments often depend heavily on volunteers to 

meet community challenges like fire protection or flood response. In addition, rural communities have limited locally 

available financial resources to help deal with the effects of 

climate change. Small community size tends to make services 

expensive or available only by traveling some distance. 

Local governance structures tend to de-emphasize planning 

capacity, compared to urban areas. While 73% of metropolitan 

counties have land-use planners, only 29% of rural counties 

not adjacent to a metropolitan county had one or more planners. Moreover, rural communities are not equipped to deal 

with major infrastructure expenses.45

Communities across the United States are experiencing infrastructure losses, water scarcity, unpredictable water availability, and increased frequency and intensity of wildfires. However, local authorities often do not explicitly associate these 

observed changes with climate, and responses rarely take 

climate disruption into account. Even in communities where 

there is increasing awareness of climate change and interest 

in comprehensive adaptation planning, lack of funding, human 

resources, access to information, training, and expertise provide significant barriers for many rural communities.46

If rural communities are to respond adequately to future climate changes, they will likely need help assessing their risks 

and vulnerabilities, prioritizing and coordinating projects, 

funding and allocating financial and human resources, and deploying information-sharing and decision support tools (Ch. 26: 

Decision Support). There is still little systematic research on 

the vulnerability of rural communities and there is a need for 

additional empirical research in this area. Impacts due to climate change will cross community and regional lines, making 

solutions dependent upon meaningful participation of numerous stakeholders from federal, state, local, and tribal governments, science and academia, the private sector, non-profit 

organizations, and the general public (Ch. 28: Adaptation, Key 

Message 3). 

Effective adaptation measures are closely tied to specific local conditions and needs and take into account existing social 

networks.47,48 The economic and social diversity of rural communities affects the ability of both individuals and communities to adapt to climate changes, and underscores the need 

to assess climate change impacts on a local basis. The quality 

and availability of natural resources, legacies of past use, and 

changing industrial needs affect the economic, environmental, 

and social conditions of rural places and are critical factors to 

be assessed.13,30,49 Successful adaptation to climate change 

requires balancing immediate needs with long-term development goals, as well as development of local-level capacities to 

deal with climate change.48,50

Potential national climate change mitigation responses (Ch. 

27: Mitigation) – especially those that require extensive use of 

land, such as permanent reforestation, constructing large solar 

or wind arrays, hydroelectric generation, and biofuel cropping 

– are also likely to significantly affect rural communities, with 

both positive and negative effects.51 As with the development 

of rural resource-intensive economic activities, where national 

or multi-national companies tend to wield ownership and control, local residents and communities are unlikely to be the 

primary investors in or beneficiaries of this kind of new economic activity. For example, mitigation policies that affect coal 

production could have a substantial economic impact on many 

rural communities, as could policies to promote production of 

non-fossil-fuel energy such as wind.

Decisions regarding adaptation responses for both urban and 

rural populations can occur at various scales (federal, state, local, tribal, private sector, and individual) but need to take interdependencies into account. Many decisions that significantly 

affect rural communities may not be under the control of local 

governments or rural residents. Given that timing is a critical 

aspect of adaptation, as well as mitigation, engaging rural residents early in decision processes about investments in public 

infrastructure, protection of shorelines, changes in insurance 

provision, or new management initiatives can influence individual behavior and choice in ways that enhance positive outcomes of adaptation and mitigation. 

Box: Local Responses to Climate Change in the San Juan Mountains
341 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

14: RURAL COMMUNITIES

Local responses to climate change in the san juan mountains

The San Juan Mountains region straddles the southern 

edge of the Southern Rocky Mountains and the 

northeastern tip of the arid Southwest. The high mountain 

headwaters of the Rio Grande, San Juan, and major 

tributaries of the Upper Colorado River are critical water 

towers for five states: Texas, Nevada, California, Arizona, 

and New Mexico. The diversity of the landforms, high 

plateaus, steep mountains, deep canyons, and foothills 

leads to a complex and diverse mix of coniferous and 

deciduous forested landscapes.52 County populations in 

the area range from 700 to 51,000 people. Population 

changes between 2000 and 2010 ranged from a 25% 

decline to an 86% increase. Public lands account for 

69% of the land base.53 Over half of the local economies 

are dependent upon natural resources to support tourism, 

minerals and natural gas extraction, and second home 

development. 

Average annual temperatures in the San Juan Mountains 

have risen 1.1oF in only three decades,54 a rate of warming 

greater than any other region of the United States except 

Alaska.55 The timing of snowmelt has shifted two weeks 

earlier between 1978 and 2007, and this earlier seasonal 

release of water resources is of particular concern to all 

western states.56 Current challenges for the region include 

changes in forests due to pests and diseases, intensive 

recreation use, fire management for natural and prescribed 

fires, and increasing development in the wildland-urban 

interface. Communities are vulnerable to changes from a 

warmer and drier climate that would affect the frequency 

and intensity of wildfires, shift vegetation and range of 

forest types, and increase pressures on water supplies. 

In response, the San Juan Climate Initiative drew 

together stakeholders, including natural resource 

managers, community planners, elected officials, 

industry representatives, resource users, citizens, 

non-profit organizations, and scientists. By combining 

resources and capabilities, stakeholders have been able 

to accomplish much more together than if they had 

worked independently. For example, local governments 

developed a plan to reduce greenhouse gas emissions 

and identify strategies for adaptation, signing the U.S. 

Mayor’s Climate Protection Agreement in 2009. Climate 

modelers at University of Colorado and National Center 

for Atmospheric Research analyzed regional trends in 

temperature, precipitation, snowpack, and streamflow. 

Researchers at Mountain Studies Institute, University of 

Colorado, and Fort Lewis College are partnering with San 

Juan National Forest to monitor alpine plant communities 

and changes in climate across the region, and to document 

carbon resources. San Juan National Forest is developing 

strategies for adapting to climate changes in the region 

related to drought, wildfire, and other potential effects. La 

Plata County is leading an effort to plan for sustainable 

transportation and food networks that will be less 

dependent upon carbon-based fuels, while the Mountain 

Studies Institute is leading citizen science programs to 

monitor changes to sensitive species like the American 

pika.

© Cody Duncan/Aurora Open/Corbis

Hiker in the San Juan mountains, Colorado.
342 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

14: RURAL COMMUNITIES

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40.	 Jones, C. A., T. S. Parker, M. Ahearn, A. K. Mishra, and J. N.

Variyam, 2009: Health Status and Health Care Access of Farm

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41.	 Salcioglu, E., M. Basoglu, and M. Livanou, 2007: Post-traumatic

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42.	 Hart, C. R., H. L. Berry, and A. M. Tonna, 2011: Improving the

mental health of rural New South Wales communities facing

drought and other adversities. Australian Journal of Rural Health, 19,

231-238, doi:10.1111/j.1440-1584.2011.01225.x.

43.	 Rost, K., J. Fortney, M. Zhang, J. Smith, and G. R. Smith, Jr.,

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doi:10.1111/j.1748-0361.1999.tb00752.x.

44.	 Alanis, A. J., 2005: Resistance to antibiotics: Are we in the postantibiotic era? Archives of medical research, 36, 697-705, doi:10.1016/j.

arcmed.2005.06.009.

45.	 Kraybill, D. S., and L. Lobao, 2001: The Emerging Roles of

County Governments in Rural America: Findings from a Recent

National Survey. American Agricultural Economics Association

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bitstream/20697/1/sp01kr01.pdf]

46.	 Romsdahl, R. J., L. Atkinson, and J. Schultz, 2013: Planning for

climate change across the US Great Plains: Concerns and insights

from government decision-makers. Journal of Environmental Studies 

and Sciences, 3, 1-14, doi:10.1007/s13412-012-0078-8.

47.	 Berkes, F., 2007: Understanding uncertainty and reducing vulnerability: Lessons from resilience thinking. Natural Hazards, 41, 283-

295, doi:10.007/s11069-006-9036-7. 

	 Ostrom, E., 2009: A general framework for analyzing sustainability

of social-ecological systems. Science, 325, 419-422, doi:10.1126/science.1172133. [Available online at http://www.era-mx.org/biblio/

Ostrom,%202009.pdf]

48.	 Nelson, D. R., 2011: Adaptation and resilience: Responding to a

changing climate. Wiley Interdisciplinary Reviews: Climate Change, 2,

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49.	 Adger, W. N., and D. R. Nelson, 2010: Ch. 5: Fair decision making

in a new climate of risk. Climate Change, Ethics and Human Security, 

K. O’Brien, A. L. St Clair, and B. Kristoffersen, Eds., Cambridge

University Press, 83-94. 

	 Bark, R. H., and K. L. Jacobs, 2009: Indian water rights settlements and water management innovations: The role of the Arizona Water Settlements Act. Water Resources Research, 45, W05417,

doi:10.1029/2008WR007130. 

	 Flora, C. B., Ed., 2001: Interactions Between Agroecosystems and Rural 

Communities. CRC Press, 296 pp. 

	 Oliver-Smith, A., 2006: Disasters and forced migration in the 21st

Century. Social Science Research Council Understanding Katrina: Perspectives from the Social Sciences Social Science Research Council. [Available online at http://forums.ssrc.org/understandingkatrina/disasters-and-forced-migration-in-the-21st-century/] 

	 Peacock, W. G., and C. Girard, 1997: Ch. 9: Ethnic and racial inequalities in hurricane damage and insurance settlements. Hurricane Andrew: Ethnicity, Gender, and the Sociolog y of Disasters, Routledge,

171-190. [Available online at http://www.routledge.com/books/

details/9780415168113/] 

	 Peguero, A. A., 2006: Latino disaster vulnerability the dissemination of hurricane mitigation information among Florida’s homeowners. Hispanic Journal of Behavioral Sciences, 28, 5-22,

doi:10.1177/0739986305284012. 

	 Vásquez-León, M., 2009: Hispanic farmers and farmworkers:

Social networks, institutional exclusion, and climate vulnerability in Southeastern Arizona. American Anthropologist, 111, 289-301,

doi:10.1111/j.1548-1433.2009.01133.x.

50.	 Furman, C., C. Roncoli, T. Crane, and G. Hoogenboom, 2011:

Beyond the “fit”: Introducing climate forecasts among organic

farmers in Georgia (United States). Climatic Change, 109, 791-799,

doi:10.1007/s10584-011-0238-y. 

	 O’Brien, K., 2009: Ch. 10: Do values subjectively define the limits

to climate change adaptation. Adapting to climate change. Thresholds, 

values, governance, W. N. Adger, I. Lorenzoni, and K. L. O’Brien,

Eds., Cambridge University Press, 164-180. [Available online at

http://www.sv.uio.no/iss/personer/vit/karenob/obrien_chapter10_values1.pdf]

51.	 van der Horst, D., 2007: NIMBY or not? Exploring the relevance

of location and the politics of voiced opinions in renewable energy

siting controversies. Energ y Policy, 35, 2705-2714, doi:10.1016/j.enpol.2006.12.012. 

	 Lovich, J. E., and J. R. Ennen, 2011: Wildlife conservation and

solar energy development in the desert southwest, United States.

BioScience, 61, 982-992, doi:10.1525/bio.2011.61.12.8. 

	 Abbasi, S. A., and N. Abbasi, 2000: The likely adverse environmental impacts of renewable energy sources. Applied Energ y, 65,

121-144, doi:10.1016/S0306-2619(99)00077-X.

52.	 Romme, W. H., M. L. Floyd, and D. Hanna, 2009: Historical Range 

of Variability and Current Landscape Condition Analysis: South Central 

Highlands Section, Southwestern Colorado & Northwestern New Mexico.

Colorado Forest Restoration Institute at Colorado State University

and Region 2 of the U.S. Forest Service, 256 pp.
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53.	 USFS, 2008: Ch. 3: Timber management and wood products. San 

Juan National Forest Land and Resource Management Plan Revision Draft 

Environmental Impact Statement San Juan Public Lands Center, 3208.

54.	 Rangwala, I., and J. R. Miller, 2010: Twentieth century temperature

trends in Colorado's San Juan Mountains. Arctic, Antarctic, and Alpine Research, 42, 89-97, doi:10.1657/1938-4246-42.1.89. [Available

online at http://www.bioone.org/doi/pdf/10.1657/1938-4246-

42.1.89]

55.	 Ray, A. J., J. J. Barsugli, K. B. Averyt, K. Wolter, M. Hoerling, N.

Doesken, B. Udall, and R. S. Webb, 2008: Climate Change in Colorado: A Synthesis to Support Water Resources Management and

Adaptation. Report for the Colorado Water Conservation Board,

58 pp., University of Colorado, Boulder, CO. [Available online at

http://wwa.colorado.edu/publications/reports/WWA_ClimateChangeColoradoReport_2008.pdf]

56.	 Clow, D. W., 2010: Changes in the timing of snowmelt and streamflow in Colorado: A response to recent warming. Journal of Climate, 

23, 2293-2306, doi:10.1175/2009JCLI2951.1.

57.	 Hauser, R., and J. Jadin, 2012: Rural Communities Workshop

Technical Report to the 2013 National Climate Assessment, 38

pp. [Available online at http://data.globalchange.gov/report/ncatechreport-rural-2012]

58.	 Kunkel, K. E., L. E. Stevens, S. E. Stevens, L. Sun, E. Janssen,

D. Wuebbles, and J. G. Dobson, 2013: Regional Climate Trends

and Scenarios for the U.S. National Climate Assessment: Part 9.

Climate of the Contiguous United States. NOAA Technical Report NESDIS 142-9. 85 pp., National Oceanic and Atmospheric

Administration, National Environmental Satellite, Data, and Information Service, Washington, D.C. [Available online at http://

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Tech_Report_142-9-Climate_of_the_Contiguous_United_States.

pdf]
347 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Process for Developing Key Message: 

The key messages were initially developed at a meeting of the 

authors in Charleston, South Carolina, in February 2012. This 

initial discussion was supported by a series of conference calls 

from March through June, 2012. These ensuing discussions were 

held after a thorough review of the technical inputs and associated literature, including the Rural Communities Workshop Report 

prepared for the NCA57 and additional technical inputs on a variety 

of topics. 

Key message #1 Traceable Account

Rural communities are highly dependent upon 

natural resources for their livelihoods and social 

structures. Climate change related impacts are currently affecting rural communities. These impacts 

will progressively increase over this century and 

will shift the locations where rural economic activities (like agriculture, forestry, and recreation) can 

thrive.

Description of evidence base

The key message and supporting text summarizes extensive evidence documented in the Rural Communities Workshop Report.57

Thirty one technical input reports on a wide range of topics were 

also received and reviewed as part of the Federal Register Notice 

solicitation for public input. 

Evidence that the impacts of climate change are increasing is 

compelling and widespread. This evidence is based on historical 

records and observations and on global climate models, including those driven by B1 (substantial emissions reduction) and A2 

(continued increases in global emissions) scenarios. This evidence 

is clearly summarized and persuasively referenced in the “Our 

Changing Climate” chapter of this Assessment and in the Scenarios developed for the NCA.58

The dependency of rural communities on their natural resources 

has been demonstrated,13 with a number of studies showing that 

climate change results in crop and livestock loss,9 infrastructure 

damage to levees and roads,10 shifts in agriculture practices,11

and losses due to disasters.12 A number of publications project 

these impacts to increase, with effects on the natural environment8,15,20 and increased competition for water between agriculture and energy.19 Studies have projected that tourism locations 

in the Everglades and Florida Keys are threatened.21 Meanwhile, 

Maine’s tourism could increase,22 which coincides with a projected 

northern shift in outdoor recreation.23 Hunting, fishing, and bird 

watching will be affected by beach erosion and wetland loss,28 and 

changing plant and animal habitats and inter-species relationships 

(see also Ch. 8: Ecosystems). Outdoor recreation and tourism in 

many areas in the U.S. are affected by early snowpack melt.8,26

New information and remaining uncertainties

Key remaining uncertainties relate to the precise magnitude, timing, and location of impacts at regional and local scales.

Assessment of confidence based on evidence 

(See confidence level key on next page)

Given the evidence and uncertainties, there is very high confidence that rural communities are highly dependent on natural 

resources that are expected to be affected by climate change, 

especially the many communities that rely on farming, forestry or 

tourism for their livelihoods.

Given the evidence and uncertainties, there is high confidence 

that climate change is currently affecting rural communities.

Given the evidence and uncertainties, there is very high confidence that impacts will increase (see Ch 2: Our Changing Climate).

Given the evidence and uncertainties, there is high confidence 

about shifts in locations of economic activities.

Key message #2 Traceable Account

Rural communities face particular geographic 

and demographic obstacles in responding to and 

preparing for climate change risks. In particular, 

physical isolation, limited economic diversity, and 

higher poverty rates, combined with an aging population, increase the vulnerability of rural communities. Systems of fundamental importance to rural 

populations are already stressed by remoteness 

and limited access.

14: RURAL COMMUNITIES

SUPPLEMENTAL MATERIAL

TRACEABLE ACCOUNTS
14: RURAL COMMUNITIES

Traceable Accounts

348 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Description of evidence base

The key message and supporting text summarizes extensive evidence documented in the Rural Communities Workshop Report.57

Thirty one technical input reports on a wide range of topics were 

also received and reviewed as part of the Federal Register Notice 

solicitation for public input. 

With studies showing that rural communities are already 

stressed,33,34,35 a number of publications have explored the barriers of rural communities to preparing and responding to climate 

change.8,31 Some studies provide in-depth looks at the obstacles 

created by limited economic diversity32 and an aging population.40

New information and remaining uncertainties

Projecting the interactions of these variables with each other and 

applying this analysis to local or regional realities is complex at 

best, with uncertainties at every level of analysis. 

Assessment of confidence based on evidence

Given the evidence and uncertainties, there is high confidence that 

the obstacle of physical isolation will hamper some communities’ 

ability to adapt or have an adequate response during extreme 

events.

Given the evidence and uncertainties, there is high confidence 

that the obstacle of limited economic diversity will hinder rural 

communities’ ability to adapt. 

Given the evidence and uncertainties, there is high confidence 

that the obstacle of higher poverty rates will significantly increase 

vulnerability of many communities from adapting properly.

Given the evidence and uncertainties, there is high confidence 

that the obstacle of an aging population will hinder some rural 

communities and prevent them from having an adequate response. 

Given the evidence and uncertainties, there is high confidence that 

fundamental systems in rural communities are already stressed by 

remoteness and limited access.

Key message #3 Traceable Account

Responding to additional challenges from climate 

change impacts will require significant adaptation 

within rural transportation and infrastructure systems, as well as health and emergency response 

systems. Governments in rural communities have 

limited institutional capacity to respond to, plan for, 

and anticipate climate change impacts.

Description of evidence base

The key message and supporting text summarize extensive evidence documented in the Rural Communities Workshop Report.57

Thirty one technical input reports on a wide range of topics were 

also received and reviewed as part of the Federal Register Notice 

solicitation for public input. 

Rural communities are not equipped to deal with major infrastructure expenses.45 Work has been performed illustrating the need 

to tie adaptation measures to specific local conditions and needs 

and take into account existing social networks.47,48 Publications 

have shown that there are a number of critical factors to be assessed, including the quality and availability of natural resources, 

legacies of past use of resources, and changing industrial needs 

that affect economic, environmental, and social conditions.13,30,49

Additionally, studies have expressed the requirement of accounting for both near- and long-term needs for climate change adaptation to be successful.50

New information and remaining uncertainties

It is difficult to fully capture the complex interactions of the entire 

socioeconomic-ecological system within which the effects of climate change will interact, especially in regard to local and regional 

impacts. Impact assessments and adaptation strategies require 

improved understanding of capacity and resilience at every level, 

international to local. The policy context in which individuals and 

communities will react to climate effects is vague and uncertain. 

Identification of informational needs alone indicates that adaptation will be expensive.

Assessment of confidence based on evidence

Given the evidence and uncertainties, there is high confidence 

that rural communities have limited capacity to respond to imConfidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent results, well documented 

and accepted methods, etc.), 

high consensus

High

Moderate evidence (several 

sources, some consistency, 

methods vary and/or documentation limited, etc.), medium 

consensus

Medium

Suggestive evidence (a few 

sources, limited consistency, 

models incomplete, methods 

emerging, etc.), competing 

schools of thought

Low

Inconclusive evidence (limited sources, extrapolations, 

inconsistent findings, poor documentation and/or methods not 

tested, etc.), disagreement or 

lack of opinions among experts
14: RURAL COMMUNITIES

Traceable Accounts

349 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

pacts, because of their remoteness, age, lack of diversity, and 

other reasons described in the text.

Given the evidence and uncertainties, there is high confidence 

that rural communities have limited capacity to plan for impacts, 

as explained in the text.

Given the evidence and uncertainties, there is high confidence 

that rural communities will have limited capacity to anticipate impacts because of the lack of infrastructure and expertise available 

in rural communities.

Given the evidence and uncertainties, there is high confidence 

that significant climate change adaptation is needed for transportation in rural communities.

Given the evidence and uncertainties, there is high confidence 

that significant climate change adaptation is needed for health 

care and emergency response in rural communities, so that rural 

communities can handle climate change impacts.
350

Climate Change Impacts in the United States

CHAPTER 15

BIOGEOCHEMICAL CYCLES

INFORMATION DRAWN FROM THIS CHAPTER IS INCLUDED IN THE HIGHLIGHTS REPORT AND IS IDENTIFIED BY THIS ICON

Recommended Citation for Chapter

Galloway, J. N., W. H. Schlesinger, C. M. Clark, N. B. Grimm, R. B. Jackson, B. E. Law, P. E. Thornton, A. R. Townsend, and 

R. Martin, 2014: Ch. 15: Biogeochemical Cycles. Climate Change Impacts in the United States: The Third National Climate 

Assessment, J. M. Melillo, Terese (T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 350-368. 

doi:10.7930/J0X63JT0. 

On the Web: http://nca2014.globalchange.gov/report/sectors/biogeochemical-cycles

Convening Lead Authors

James N. Galloway, University of Virginia 

William H. Schlesinger, Cary Institute of Ecosystem Studies

Lead Authors

Christopher M. Clark, U.S. Environmental Protection Agency

Nancy B. Grimm, Arizona State University

Robert B. Jackson, Duke University 

Beverly E. Law, Oregon State University

Peter E. Thornton, Oak Ridge National Laboratory

Alan R. Townsend, University of Colorado Boulder 

Contributing Author

Rebecca Martin, Washington State University Vancouver
351 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Key Messages

1. Human activities have increased atmospheric carbon dioxide by about 40% over 

pre-industrial levels and more than doubled the amount of nitrogen available to ecosystems. 

Similar trends have been observed for phosphorus and other elements, and these changes have 

major consequences for biogeochemical cycles and climate change. 

2. In total, land in the United States absorbs and stores an amount of carbon equivalent to about 

17% of annual U.S. fossil fuel emissions. U.S. forests and associated wood products account 

for most of this land sink. The effect of this carbon storage is to partially offset warming from 

emissions of CO2 and other greenhouse gases.

3. Altered biogeochemical cycles together with climate change increase the vulnerability of 

biodiversity, food security, human health, and water quality to changing climate. However, 

natural and managed shifts in major biogeochemical cycles can help limit rates of climate 

change.

15 BIOGEOCHEMICAL CYCLES

Biogeochemical cycles involve the fluxes of chemical elements 

among different parts of the Earth: from living to non-living, 

from atmosphere to land to sea, and from soils to plants. They 

are called “cycles” because matter is always conserved and 

because elements move to and from major pools via a variety of two-way fluxes, although some elements are stored in 

locations or in forms that are differentially accessible to living 

things. Human activities have mobilized Earth elements and 

accelerated their cycles – for example, more than doubling the 

amount of reactive nitrogen that has been added to the biosphere since pre-industrial times.1,2 Reactive nitrogen is any nitrogen compound that is biologically, chemically, or radiatively 

active, like nitrous oxide and ammonia, but not nitrogen gas 

(N2). Global-scale alterations of biogeochemical cycles are occurring, from human activities both in the U.S. and elsewhere, 

with impacts and implications now and into the future. Global carbon dioxide emissions are the most significant driver of 

human-caused climate change. But human-accelerated cycles 

of other elements, especially nitrogen, phosphorus, and sulfur, also influence climate. These elements can affect climate 

directly or act as indirect factors that alter the carbon cycle, 

amplifying or reducing the impacts of climate change. 

Climate change is having, and will continue to have, impacts 

on biogeochemical cycles, which will alter future impacts on 

climate and affect our capacity to cope with coupled changes 

in climate, biogeochemistry, and other factors.

Key Message 1: Human-Induced Changes

Human activities have increased atmospheric carbon dioxide by about 40% over pre-industrial 

levels and more than doubled the amount of nitrogen available to ecosystems. Similar trends 

have been observed for phosphorus and other elements, and these changes have major 

consequences for biogeochemical cycles and climate change. 

The human mobilization of carbon, nitrogen, and phosphorus 

from the Earth’s crust and atmosphere into the environment 

has increased 36, 9, and 13 times, respectively, compared 

to geological sources over pre-industrial times.3 Fossil fuel 

burning, land-cover change, cement production, and the 

extraction and production of fertilizer to support agriculture 

are major causes of these increases.4 Carbon dioxide (CO2) 

is the most abundant of the heat-trapping greenhouse gases 

that are increasing due to human activities, and its production 

dominates atmospheric forcing of global climate change.5

However, methane (CH4) and nitrous oxide (N2O) have higher 

greenhouse-warming potential per molecule than CO2, and 

both are also increasing in the atmosphere. In the U.S. and 

Europe, sulfur emissions have declined over the past three 

decades, especially since the mid-1990s, because of efforts 

to reduce air pollution.6 Changes in biogeochemical cycles of 

carbon, nitrogen, phosphorus, and other elements – and the 

coupling of those cycles – can influence climate. In turn, this 
15: BIOGEOCHEMICAL CYCLES

352 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

can change atmospheric composition in other ways that affect 

how the planet absorbs and reflects sunlight (for example, 

by creating small particles known as aerosols that can reflect 

sunlight). 

State of the Carbon Cycle 

The U.S. was the world’s largest producer of human-caused 

CO2 emissions from 1950 until 2007, when it was surpassed by 

China. U.S. emissions account for approximately 85% of North 

American emissions of CO2

7

 and 18% of global emissions.

8,9

Ecosystems represent potential “sinks” for CO2, which are 

places where carbon can be stored over the short or long term 

(see “Estimating the U.S. Carbon Sink”). At the continental 

scale, there has been a large and relatively consistent increase 

in forest carbon stocks over the last two decades,10 due to 

recovery from past forest harvest, net increases in forest area, 

improved forest management regimes, and faster growth driven 

by climate or fertilization by CO2 and nitrogen.7,11 The largest 

rates of disturbance and “regrowth sinks” are in southeastern, 

south central, and Pacific northwestern regions.11 However, 

emissions of CO2 from human activities in the U.S. continue 

to increase and exceed ecosystem CO2 uptake by more than 

three times. As a result, North America remains a net source of 

CO2 into the atmosphere7 by a substantial margin.

Sources and Fates of Reactive Nitrogen 

The nitrogen cycle has been dramatically altered by human 

activity, especially by the use of nitrogen fertilizers, which 

have increased agricultural production over the past half 

century.1,2 Although fertilizer nitrogen inputs have begun 

to level off in the U.S. since 1980,12 human-caused reactive 

nitrogen inputs are now at least five times greater than those 

from natural sources.13,14,15,16 At least some of the added 

nitrogen is converted to nitrous oxide (N2O), which adds to the 

greenhouse effect in Earth’s atmosphere.

An important characteristic of reactive nitrogen is its legacy. 

Once created, it can, in sequence, travel throughout the 

environment (for example, from land to rivers to coasts, 

sometimes via the atmosphere), contributing to environmental 

problems such as the formation of coastal low-oxygen “dead 

zones” in marine ecosystems in summer. These problems 

persist until the reactive nitrogen is either captured and stored 

in a long-term pool, like the mineral layers of soil or deep ocean 

sediments, or converted back to nitrogen gas.17,18 The nitrogen 

cycle affects atmospheric concentrations of the three most 

important human-caused greenhouse gases: carbon dioxide, 

methane, and nitrous oxide. Increased available nitrogen 

stimulates the uptake of carbon dioxide by plants, the release 

of methane from wetland soils, and the production of nitrous 

oxide by soil microbes.

Figure 15.1. The release of carbon dioxide from fossil fuel burning in North America (shown here for 2010) 

vastly exceeds the amount that is taken up and temporarily stored in forests, crops, and other ecosystems 

(shown here is the annual average for 2000-2006). (Figure source: King et al. 20127).

Major North American Carbon Dioxide Sources and Sinks
15: BIOGEOCHEMICAL CYCLES

353 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Phosphorus and other elements 

The phosphorus cycle has been greatly transformed in the 

United States,

19 primarily from the use of phosphorus fertilizers 

in agriculture. Phosphorus has no direct effects on climate, 

but does have indirect effects, such as increasing carbon sinks 

by fertilizing plants. Emissions of sulfur, as sulfur dioxide, can 

reduce the growth of plants and stimulate the leaching of soil 

nutrients needed by plants.20

Key Message 2: Sinks and Cycles

In total, land in the United States absorbs and stores an amount of carbon equivalent to 

about 17% of annual U.S. fossil fuel emissions. U.S. forests and associated wood products 

account for most of this land sink. The effect of this carbon storage is to partially offset 

warming from emissions of CO2 and other greenhouse gases. 

Considering the entire atmospheric CO2 budget, the temporary 

net storage on land is small compared to the sources: more 

CO2 is emitted than can be taken up (see “Estimating the 

U.S. Carbon Sink”).7,21,22,23 Other elements and compounds 

affect that balance by direct and indirect means (for example, 

nitrogen stimulates carbon uptake [direct] and nitrogen 

decreases the soil methane sink [indirect]). The net effect on 

Earth’s energy balance from changes in major biogeochemical 

cycles (carbon, nitrogen, sulfur, and phosphorus) depends 

upon processes that directly affect how the planet absorbs 

or reflects sunlight, as well as those that indirectly affect 

concentrations of greenhouse gases in the atmosphere. 

Figure 15.2. Once created, a molecule of reactive nitrogen has a cascading impact on people and ecosystems as it contributes 

to a number of environmental issues. Molecular terms represent oxidized forms of nitrogen primarily from fossil fuel combustion 

(such as nitrogen oxides, NOx), reduced forms of nitrogen primarily from agriculture (such as ammonia, NH3), and organic 

forms of nitrogen (Norg) from various processes. NOy is all nitrogen-containing atmospheric gases that have both nitrogen and 

oxygen, other than nitrous oxide (N2O). NHx is the sum of ammonia (NH3) and ammonium (NH4). (Figure source: adapted from 

EPA 2011;13 Galloway et al. 2003;17 with input from USDA. USDA contributors were Adam Chambers and Margaret Walsh). 

Human Activities that Form Reactive Nitrogen

and Resulting Consequences in Environmental Reservoirs
15: BIOGEOCHEMICAL CYCLES

354 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Carbon

In addition to the CO2 effects described above, other carbon-containing compounds affect climate change, such as 

methane and volatile organic compounds (VOCs). As the most 

abundant non-CO2 greenhouse gas, methane is 20 to 30 times 

more potent than CO2 over a century timescale. It accounted 

for 9% of all human-caused greenhouse gas emissions in the 

United States in 2011,8 and its atmospheric concentration today is more than twice that of pre-industrial times.24,25 Methane has an atmospheric lifetime of about 10 years before it is 

oxidized to CO2, but it has about 25 times the global warming 

potential of CO2. An increase in methane concentration in the 

industrial era has contributed to warming in many ways.26

Methane also has direct and indirect effects on climate because of its influences on atmospheric chemistry. Increases in 

atmospheric methane and VOCs are expected to deplete concentrations of hydroxyl radicals, causing methane to persist in 

the atmosphere and exert its warming effect for longer periods.25,27 The hydroxyl radical is the most important “cleaning 

agent” of the troposphere (the active weather layer extending 

up to about 5 to 10 miles above the ground), where it is formed 

by a complex series of reactions involving ozone and ultraviolet 

light.3

Nitrogen and Phosphorus

The climate effects of an altered nitrogen cycle are substantial 

and complex.4,28,29,30,31 Carbon dioxide, methane, and nitrous 

oxide contribute most of the human-caused increase in climate 

forcing, and the nitrogen cycle affects atmospheric concentrations of all three gases. Nitrogen cycling processes regulate 

ozone (O3) concentrations in the troposphere and stratosphere, and produce atmospheric aerosols, all of which have 

additional direct effects on climate. Excess reactive nitrogen 

also has multiple indirect effects that simultaneously amplify 

and mitigate changes in climate. Changes in ozone and organic 

aerosols are short-lived, whereas changes in carbon dioxide 

and nitrous oxide have persistent impacts on the atmosphere. 

Figure 15.3. Figure shows how climate change will affect U.S. reactive nitrogen emissions, in Teragrams (Tg) 

CO2 equivalent, on a 20-year (top) and 100-year (bottom) global temperature potential basis. Positive values 

on the vertical axis depict warming; negative values reflect cooling. The height of the bar denotes the range of 

uncertainty, and the white line denotes the best estimate. The relative contribution of combustion (dark brown) 

and agriculture (green) is denoted by the color shading. (Figure source: adapted from Pinder et al. 201228).

Nitrogen Emissions
15: BIOGEOCHEMICAL CYCLES

355 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

The strongest direct effect of an altered nitrogen 

cycle is through emissions of nitrous oxide (N2O), 

a long-lived and potent greenhouse gas that is increasing steadily in the atmosphere.25,26 Globally, 

agriculture has accounted for most of the atmospheric rise in N2O.32,33 Roughly 60% of agricultural 

N2O derives from elevated soil emissions resulting 

from the use of nitrogen fertilizer. Animal waste 

treatment accounts for about 30%, and the remaining 10% comes from crop-residue burning.34

The U.S. reflects this global trend: around 75% to 

80% of U.S. human-caused N2O emissions are due 

to agricultural activities, with the majority being 

emissions from fertilized soil. The remaining 20% is 

derived from a variety of industrial and energy sectors.35,36 While N2O currently accounts for about 

6% of human-caused warming,26 its long lifetime in 

the atmosphere and rising concentrations will increase N2O-based climate forcing over a 100-year 

time scale.33,37,38

Excess reactive nitrogen indirectly exacerbates changes in 

climate by several mechanisms. Emissions of nitrogen oxides 

(NOx) increase the production of tropospheric ozone, which is 

a greenhouse gas.39 Elevated tropospheric ozone may reduce 

CO2 uptake by plants and thereby reduce the terrestrial CO2

sink.40 Nitrogen deposition to ecosystems can also stimulate 

the release of nitrous oxide and methane and decrease methane uptake by soil microbes.41

However, excess reactive nitrogen also mitigates changes in 

greenhouse gas concentrations and climate through several 

intersecting pathways. Over short time scales, NOx and ammonia emissions lead to the formation of atmospheric aerosols, 

which cool the climate by scattering or absorbing incoming radiation and by affecting cloud cover.26,42 In addition, the presence of NOx in the lower atmosphere increases the formation 

of sulfate and organic aerosols.43 At longer time scales, NOx 

can increase rates of methane oxidation, thereby reducing the 

lifetime of this important greenhouse gas. 

One of the dominant effects of reactive nitrogen on climate 

stems from how it interacts with ecosystem carbon capture 

and storage, and thus, the carbon sink. As mentioned previously, addition of reactive nitrogen to natural ecosystems can increase carbon storage as long as other factors are not limiting 

plant growth, such as water and nutrient availability.44 Nitrogen deposition from human sources is estimated to contribute 

to a global net carbon sink in land ecosystems of 917 to 1,830 

million metric tons (1,010 to 2,020 million tons) of CO2 per year. 

These are model-based estimates, as comprehensive, observationally-based estimates at large spatial scales are hindered 

by the limited number of field experiments. This net land sink 

represents two components: 1) an increase in vegetation 

growth as nitrogen limitation is alleviated by human-caused 

nitrogen deposition, and 2) a contribution from the influence 

of increased reactive nitrogen availability on decomposition. 

While the former generally increases with increased reactive 

nitrogen, the net effect on decomposition in soils is not clear. 

The net effect on total ecosystem carbon storage was an average of 37 metric tons (41 tons) of carbon stored per metric ton 

of nitrogen added in forests in the U.S. and Europe.45

When all direct and indirect links between reactive nitrogen 

and climate in the U.S. are added up, a recent estimate suggests 

a modest reduction in the rate of warming in the near term 

(next several decades), but a progressive switch to greater net 

warming over a 100-year timescale.28,29 That switch is due to 

a reduction in nitrogen oxide (NOx) emissions, which provide 

modest cooling effects, a reduction in the nitrogen-stimulated 

CO2 storage in forests, and a rising importance of agricultural 

nitrous oxide emissions. Current policies tend to reinforce this 

switch. For example, policies that reduce nitrogen oxide and 

sulfur oxide emissions have large public health benefits, but 

also reduce the indirect climate mitigation co-benefits by reducing carbon storage and aerosol formation.

Changes in the phosphorus cycle have no direct effects on 

climate, but phosphorus availability constrains plant and microbial activity in a wide variety of land- and water-based ecosystems.46,47 Changes in phosphorus availability due to human 

activity can therefore have indirect impacts on climate and 

the emissions of greenhouse gases in a variety of ways. For example, in land-based ecosystems, phosphorus availability can 

limit both CO2 storage and decomposition46,48 as well as the 

rate of nitrogen accumulation.49 In turn, higher nitrogen inputs 

can alter phosphorus cycling via changes in the production and 

activity of enzymes that release phosphorus from decaying 

organic matter,

50 creating another mechanism by which rising 

nitrogen inputs can stimulate carbon uptake.
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356 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Other Effects: Sulfate Aerosols

In addition to the aerosol effects from nitrogen mentioned 

above, there are both direct and indirect effects on climate 

from other aerosol sources. Components of the sulfur cycle 

exert a cooling effect through the formation of sulfate aerosols 

created from the oxidation of sulfur dioxide (SO2) emissions.26

In the United States, the dominant source of sulfur dioxide is 

coal combustion. Sulfur dioxide emissions rose until 1980, but 

have since decreased by more than 50% following a series of 

air-quality regulations and incentives focused on improving human health and the environment, as well as reductions in the 

delivered price of low-sulfur coal.51 That decrease in emissions 

has had a marked effect on U.S. climate forcing: between 1970 

and 1990, sulfate aerosols caused cooling, primarily over the 

eastern U.S., but since 1990, further reductions in sulfur dioxide emissions have reduced the cooling effect of sulfate aerosols by half or more.42 Continued declines in sulfate aerosol 

cooling are projected for the future,

42 particularly if coal continues to be replaced by natural gas (which contains far fewer 

sulfur impurities) for electricity generation. Here, as with nitrogen oxide emissions, the environmental and socioeconomic 

tradeoffs are important to recognize: lower sulfur dioxide and 

nitrogen oxide emissions remove some climate cooling agents, 

but improve ecosystem health and save lives.16,31,52

Three low-concentration industrial gases are particularly potent for trapping heat: nitrogen trifluoride (NF3), sulfur hexafluoride (SF6), and trifluoromethyl sulfur pentafluoride (SF5CF3). 

None currently makes a major contribution to climate forcing, 

but since their emissions are increasing and their effects last 

for millennia, continued monitoring is important.

Key Message 3: Impacts and Options

Altered biogeochemical cycles together with climate change increase the vulnerability of 

biodiversity, food security, human health, and water quality to changing climate. 

However, natural and managed shifts in major biogeochemical cycles can help 

limit rates of climate change.

Climate change alters key aspects of biogeochemical cycling, 

creating the potential for feedbacks that alter both warming 

and cooling processes into the future. For example, as 

soils warm, the rate of decomposition will increase, adding 

more CO2 to the atmosphere. In addition, both climate and 

biogeochemistry interact strongly with environmental and 

ecological concerns, such as biodiversity loss, freshwater and 

marine eutrophication (unintended fertilization of aquatic 

ecosystems that leads to water quality problems), air pollution, 

human health, food security, and water resources. Many of 

the latter connections are addressed in other sections of this 

assessment, but we summarize some of them here because 

consideration of mitigation and adaptation options for changes 

in climate and biogeochemistry often requires this broader 

context. 

Climate-Biogeochemistry Feedbacks

Both rising temperatures and changes in water availability can 

alter climate-relevant biogeochemical processes. For example, 

as summarized above, nitrogen deposition drives temperate 

forest carbon storage, both by increasing plant growth and 

by slowing organic-matter decomposition.53 Higher temperatures will counteract soil carbon storage by increasing decomposition rates and subsequent emission of CO2 via microbial 

respiration. However, that same increase in decomposition 

accelerates the release of reactive nitrogen (and phosphorus) 

from organic matter, which in turn can fuel additional plant 

growth.44 Temperature also has direct effects on net primary 

productivity (the total amount of CO2 stored by a plant through 

photosynthesis minus the amount released through respiration). The combined effects on ecosystem carbon storage will 

depend on the extent to which nutrients constrain both net 

primary productivity and decomposition, on the extent of 

warming, and on whether any simultaneous changes in water 

availability occur.54

Similarly, natural methane sources are sensitive to variations 

in climate; ice core records show a strong correlation between 

methane concentrations and warmer, wetter conditions.55

Thawing permafrost in polar regions is of particular concern 

because it stores large amounts of methane that could potentially be released to the atmosphere.

Biogeochemistry, Climate, and Interactions with Other Factors

Societal options for addressing links between climate and biogeochemical cycles must often be informed by connections 

to a broader context of global environmental changes. For 

example, both climate change and nitrogen deposition can 

reduce biodiversity in water- and land-based ecosystems. The 

greatest combined risks are expected to occur where critical 

loads are exceeded.56,57 A critical load is defined as the input 

rate of a pollutant below which no detrimental ecological effects occur over the long-term according to present knowledge.57 Although biodiversity is often shown to decline when 

nitrogen deposition is high due to fossil fuel combustion and 

agricultural emissions,57,58 the compounding effects of multi-
15: BIOGEOCHEMICAL CYCLES

357 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

ple stressors are difficult to predict. Warming and changes in 

water availability have been shown to interact with nitrogen in 

additive or synergistic ways to exacerbate biodiversity loss.59

Unfortunately, very few multi-factorial studies have been done 

to address this gap. 

Human induced acceleration of the nitrogen and phosphorus 

cycles already causes widespread freshwater and marine eutrophication,60,61 a problem that is expected to worsen under a 

warming climate.61,62 Without efforts to reduce future climate 

change and to slow the acceleration of biogeochemical cycles, 

existing climate changes will combine with increasing inputs 

of nitrogen and phosphorus into freshwater and estuarine 

ecosystems. This combination of changes is projected to have 

substantial negative effects on water quality, human health, 

inland and coastal fisheries, and greenhouse gas emissions.18,61

Similar concerns – and opportunities for the simultaneous 

reduction of multiple environmental problems (known as 

“co-benefits”) – exist in the realms of air pollution, human 

health, and food security. For example, methane, volatile organic compounds, and nitrogen oxide emissions all contribute 

to the formation of tropospheric ozone, which is a greenhouse 

gas and has negative consequences for human health and crop 

and forest productivity.37,63,64 Rates of ozone formation are accelerated by higher temperatures, creating a reinforcing cycle 

between rising temperatures and continued human alteration 

of the nitrogen and carbon cycles.65 Rising temperatures also 

work against some of the benefits of air pollution control.64

Some changes will trade gains in one arena for declines in others. For example, lowered NOx, NHx, and SOx emissions remove 

cooling agents from the atmosphere, but improve air quality.16,31 Recent analyses suggest that targeting reductions in 

compounds like methane and black carbon aerosols that have 

both climate and air-pollution consequences can achieve significant improvements in not only the rate of climate change, 

but also in human health.31 Finally, reductions in excess nitrogen and phosphorus from agricultural and industrial activities 

can potentially reduce the rate and impacts of climate change, 

while simultaneously addressing concerns in biodiversity, water quality, food security, and human health.66

Figure 15.4. Top panel shows the impact of the alteration of the carbon cycle alone on radiative forcing. The bottom panel shows the 

impacts of the alteration of carbon, nitrogen, and sulfur cycles on radiative forcing. SO2 and NH3 increase aerosols and decrease 

radiative forcing. NH3 is likely to increase plant biomass, and consequently decrease forcing. NOx is likely to increase the formation of 

tropospheric ozone (O3) and increase radiative forcing. Ozone has a negative effect on plant growth/biomass, which might increase 

radiative forcing. CO2 and NH3 act synergistically to increase plant growth, and therefore decrease radiative forcing. SO2 is likely 

to reduce plant growth, perhaps through the leaching of soil nutrients, and consequently increase radiative forcing. NOx is likely to 

reduce plant growth directly and through the leaching of soil nutrients, therefore increasing radiative forcing. However, it could act 

as a fertilizer that would have the opposite effect.

Many Factors Combine to Affect Biogeochemical Cycles
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358 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Estimating the u.s. carbon sink

Any natural or engineered process that temporarily 

or permanently removes and stores carbon dioxide 

(CO2) from the atmosphere is considered a carbon 

“sink.” Temporary (10 to 100 years) CO2 sinks at 

the global scale include absorption by plants as they 

photosynthesize, as well as CO2 dissolution into the 

ocean. Forest biomass and soils in North America 

offer large temporary carbon sinks in the global 

carbon budget; however, the spatial distribution, 

longevity, and mechanisms controlling these sinks 

are less certain.67 Understanding these processes is 

critical for predicting how ecosystem carbon sinks 

will change in the future, and potentially for managing the carbon sink as a mitigation strategy for 

climate change. 

Both inventory (measurement) and modeling techniques have been used to estimate land-based carbon sinks at a 

range of scales in both time and space. For inventory methods, carbon stocks are measured at a location at two points 

in time, and the amount of carbon stored or lost can be estimated over the intervening time period. This method is 

widely used to estimate the amount of carbon stored in forests in the United States over timescales of years to decades. Terrestrial biosphere models estimate carbon sinks by modeling a suite of processes that control carbon cycling 

dynamics, such as photosynthesis (CO2 uptake by plants) and respiration (CO2 release by plants, animals, and microorganisms in soil and water). Fieldbased data and/or remotely sensed 

data are used as inputs and also to 

validate these models. Estimates of 

the land-based carbon sink can vary 

depending on the data inputs and 

how different processes are modeled.22 Atmospheric inverse models 

use information about atmospheric 

CO2 concentrations and atmospheric 

transport (like air currents) to estimate the terrestrial carbon sink.68

This approach can provide detailed 

information about carbon sinks over 

time. However, because atmospheric 

CO2 is well-mixed and monitoring 

sites are widely dispersed, these 

models estimate fluxes over large areas and it is difficult to identify processes responsible for the sink from 

these data.22 Recent estimates using 

atmospheric inverse models show 

that global land and ocean carbon 

sinks are stable or even increasing 

globally.69

Figure 15.5. Figure shows growth in fossil fuel CO2 emissions (black line) and 

forest and total land carbon sinks in the U.S. for 1990–2010 (green and orange 

lines; from EPA 201221) and for 2003 (symbols; from the first State of the Carbon 

Cycle Report67). Carbon emissions are significantly higher than the total land sink’s 

capacity to absorb and store them. (Data from EPA 2012 and CCSP 200721,67).

U.S. Carbon Sinks Absorb a Fraction of CO2 Emissions

Continued

Table 15.1. Carbon (C) sinks and uncertainty estimated by Pacala et al. for the 

first State of the Carbon Cycle Report.23 Forests take up the highest percentage 

of carbon of all land-based carbon sinks. Due to a number of factors, there are 

high degrees of uncertainty in carbon sink estimates.

Land Area C sink (Tg C/y)

(95% CI) Method

Forest -256 (+/- 50%) inventory, modeled

Wood products -57 (+/- 50%) inventory

Woody encroachment -120 (+/- >100%) inventory

Agricultural soils -8 (+/- 50%) modeled

Wetlands -23 (+/- >100%) inventory

Rivers and reservoirs -25 (+/- 100%) inventory

Net Land Sink -489 (+/- 50%) inventory
15: BIOGEOCHEMICAL CYCLES

359 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Estimating the u.s. carbon sink (continued)

The U.S. Environmental Protection Agency (EPA) conducts an annual inventory of U.S. greenhouse gas emissions and 

sinks as part of the nation’s commitments under the Framework Convention on Climate Change. Estimates are based on 

inventory studies and models validated with field-based data (such as the CENTURY model) in accordance with the Intergovernmental Panel on Climate Change (IPCC) best practices.70 An additional comprehensive assessment, The First State 

of the Carbon Cycle Report (SOCCR), provides estimates for carbon sources and sinks in the U.S. and North America 

around 2003.67 This assessment also utilized inventory and field-based terrestrial biosphere models, and incorporated 

additional land sinks not explicitly included in EPA assessments. 

Data from these assessments suggest that the U.S. carbon sink has been variable over the last two decades, but still 

absorbs and stores a small fraction of CO2 emissions. The forest sink comprises the largest fraction of the total land sink 

in the United States, annually absorbing 7% to 24% (with a best estimate of 16%) of fossil fuel CO2 emissions during 

the last two decades. Because the U.S. Forest Service has conducted detailed forest carbon inventory studies, the uncertainty surrounding the estimate for the forest sink is lower than for most other components (see Pacala et al. 2007, 

Table 223). The role of lakes, reservoirs, and rivers in the carbon budget, in particular, has been difficult to quantify and 

is rarely included in national budgets.71 The IPCC guidelines for estimating greenhouse gas sources or sinks from lakes, 

reservoirs, or rivers are included in the “wetlands” category, but only for lands converted to wetlands. These ecosystems 

are not included in the EPA’s estimates of the total land sink. Rivers and reservoirs were estimated to be a sink in the 

State of the Carbon Cycle analysis,23 but recent studies suggest that inland waters may actually be an important source 

of CO2 to the atmosphere.72 It is important to note that these two methods use different datasets, different models, and 

different methodologies to estimate land-based carbon sinks in the United States. In particular, we note that the EPA 

Inventory, consistent with IPCC Guidelines for national inventories, includes only carbon sinks designated as humancaused, while the SOCCR analysis does not make this distinction. 

Figure 15.6. Changes in CO2 emissions and land-based sinks in two recent decades, showing amongyear variation (vertical lines: minimum and maximum estimates among years; boxes: 25th and 75th

quartiles; horizontal line: median). Total CO2 emissions, as well as total CO2 emissions from fossil 

fuels, have risen; land-based carbon sinks have increased slightly, but at a much slower pace. (Data 

from EPA 2012 and CCSP 200721,67).

U.S. Carbon Sources and Sinks from 1991 to 2000 and 2001 to 2010
360 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

15: BIOGEOCHEMICAL CYCLES

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Photo Credits

Introduction to chapter; Serene forest in top banner: ©Corbis/Royalty 

Free
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Traceable Accounts

366 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Process for Developing Key Messages 

The key messages and supporting text summarize extensive evidence documented in two technical input reports submitted to 

the NCA: 1) a foundational report supported by the Departments 

of Energy and Agriculture: Biogeochemical Cycles and Biogenic 

Greenhouse Gases from North American Terrestrial Ecosystems: 

A Technical Input Report for the National Climate Assessment,30

and 2) an external report: The Role of Nitrogen in Climate Change 

and the Impacts of Nitrogen-Climate Interactions on Terrestrial and 

Aquatic Ecosystems, Agriculture, and Human Health in the United 

States: A Technical Report Submitted to the U.S. National Climate 

Assessment.

4 The latter report was supported by the International 

Nitrogen Initiative, a National Science Foundation grant, and the 

David and Lucille Packard Foundation. 

Author meetings and workshops were held regularly for the foundational report,30 including a workshop at the 2011 Soil Science 

Society of America meeting. A workshop held in July 2011 at 

the USGS John Wesley Powell Center for Analysis and Synthesis in Fort Collins, CO, focused on climate-nitrogen actions and 

was summarized in the second primary source.4 An additional 15 

technical input reports on various topics were also received and 

reviewed as part of the Federal Register Notice solicitation for 

public input. 

The entire author team for this chapter conducted its deliberations by teleconference from April to June 2012, with three major 

meetings resulting in an outline and a set of key messages. The 

team came to expert consensus on all of the key messages based 

on their reading of the technical inputs, other published literature, 

and professional judgment. Several original key messages were 

later combined into a broader set of statements while retaining 

most of the original content of the chapter. Major revisions to the 

key messages, chapter, and traceable accounts were approved 

by authors; further minor revisions were consistent with the messages intended by the authors.

Key message #1 Traceable Account

Human activities have increased atmospheric 

carbon dioxide by about 40% over pre-industrial 

levels and more than doubled the amount of nitrogen available to ecosystems. Similar trends have 

been observed for phosphorus and other elements, 

and these changes have major consequences for 

biogeochemical cycles and climate change. 

Description of evidence base

The author team evaluated technical input reports (17) on biogeochemical cycles, including the two primary sources.4,31 In particular, 

one report4 focused on changes in the nitrogen cycle and was comprehensive. Original literature was consulted for changes in other 

biogeochemical cycles. The foundational report30 updated several 

aspects of our understanding of the carbon balance in the United 

States. 

Publications have shown that human activities have altered biogeochemical cycles. A seminal paper comparing increases in the global 

fluxes of carbon (C), nitrogen (N), sulfur (S), and phosphorous (P) was 

published in 200073 and was recently updated.

3

 Changes observed in 

the nitrogen cycle1,17,18 show anthropogenic sources to be far greater 

than natural ones.14,36,47 For phosphorus, the effect of added phosphorus on plants and microbes is well understood.

19,46,47 Extensive 

research shows that increases in CO2 are the strongest human impact 

forcing climate change, mainly because the concentration of CO2 is so 

much greater than that of other greenhouse gases.5,7,73

New information and remaining uncertainties

The sources of C, N, and P are from well-documented processes, such 

as fossil fuel burning and fertilizer production and application. The 

flux from some processes is well known, while others have significant 

remaining uncertainties. 

Some new work has synthesized the assessment of global and national CO2 emissions7

 and categorized the major CO2 sources and sinks.4,30

Annual updates of CO2 emissions and sink inventories are done by 

EPA (for example, EPA 20138). 

Advances in the knowledge of the nitrogen cycle have quantified that 

human-caused reactive nitrogen inputs are now at least five times 

greater than natural inputs.4,13,14

Assessment of confidence based on evidence 

High confidence. Evidence for human inputs of C, N, and P come from 

academic, government, and industry sources. The data show substantial agreement.

15: BIOGEOCHEMICAL CYCLES

SUPPLEMENTAL MATERIAL

TRACEABLE ACCOUNTS
15: BIOGEOCHEMICAL CYCLES

Traceable Accounts

367 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

The likelihood of continued dominance of CO2 over other greenhouse 

gases as a driver of global climate change is also judged to be high, 

because its concentration is an order of magnitude higher and its rate 

of change is well known. 

Key message #2 Traceable Account

In total, land in the United States absorbs and 

stores an amount of carbon equivalent to about 17% 

of annual U.S. fossil fuel emissions. U.S. forests 

and associated wood products account for most of 

this land sink. The effect of this carbon storage is 

to partially offset warming from emissions of CO2 

and other greenhouse gases.

Description of evidence base

The author team evaluated technical input reports (17) on biogeochemical cycles, including the two primary sources.

4,30 The “Estimating the U.S. Carbon Sink” section relies on multiple sources of data 

that are described therein. 

Numerous studies of the North American and U.S. carbon sink have 

been published in reports and the scientific literature. Estimates 

of the percentage of fossil fuel CO2 emissions that are captured by 

forest, cropland, and other lands vary from a low of 7% to a high of 

about 24%, when the carbon storage is estimated from carbon inventories.7,22,36 The forest sink has persisted in the U.S. as forests that 

were previously cut have regrown. Further studies show that carbon 

uptake can be increased to some extent by a fertilization effect with 

reactive nitrogen44,45 and phosphorus,46,47,48 both nutrients that can 

limit the rate of photosynthesis. The carbon sink due to nitrogen fertilization is projected to lessen in the future as controls on nitrogen 

emissions come into play.28

While carbon uptake by ecosystems has a net cooling effect, trace 

gases emitted by ecosystems have a warming effect that can offset 

the cooling effect of the carbon sink.26 The most important of these 

gases are methane and nitrous oxide (N2O), the concentrations of 

which are projected to rise.

25,26,33,37,38

New information and remaining uncertainties

The carbon sink estimates have very wide margins of error. The percent of U.S. CO2 emissions that are stored in ecosystems depends on 

which years are used for emissions and whether inventories, ecosystem process models, atmospheric inverse models, or some combination of these techniques are used to estimate the sink size (see 

“Estimating the U.S. Carbon Sink”). The inventories are continually 

updated (for example, EPA 20138), but there is a lack of congruence 

on which of the three techniques is most reliable. A recent paper that 

uses atmospheric inverse modeling suggests that the global land and 

ocean carbon sinks are stable or increasing.69

While known to be significant, continental-scale fluxes and sources 

of the greenhouse gases N2O and CH4 are based on limited data and 

are potentially subject to revision. Recent syntheses28 evaluate the 

dynamics of these two important gases and project future changes. 

Uncertainties remain high.

Assessment of confidence based on evidence 

We have very high confidence that the value of the forest carbon sink 

lies within the range given, 7% to 24% (with a best estimate of 16%) 

of annual U.S. greenhouse gas emissions. There is wide acceptance 

that forests and soils store carbon in North America, and that they 

will continue to do so into the near future. The exact value of the sink 

strength is very poorly constrained, however, and knowledge of the 

projected future sink is low. As forests age, their capacity to store 

carbon in living biomass will necessarily decrease,10 but if other, unknown sinks are dominant, ecosystems may continue to be a carbon 

sink.

We have high confidence that the combination of ecosystem carbon 

storage of human-caused greenhouse gas emissions and potential 

warming from other trace gases emitted by ecosystems will ultimately result in a net warming effect. This is based primarily on one 

recent synthesis,

28 which provides ranges for multiple factors and describes the effects of propagating uncertainties. However, the exact 

amount of warming or cooling produced by various gases is not yet 

well known, because of the interactions of multiple factors. 

Confidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent results, well documented 

and accepted methods, etc.), 

high consensus

High

Moderate evidence (several 

sources, some consistency, 

methods vary and/or documentation limited, etc.), medium 

consensus

Medium

Suggestive evidence (a few 

sources, limited consistency, 

models incomplete, methods 

emerging, etc.), competing 

schools of thought

Low

Inconclusive evidence (limited sources, extrapolations, 

inconsistent findings, poor documentation and/or methods not 

tested, etc.), disagreement or 

lack of opinions among experts
15: BIOGEOCHEMICAL CYCLES

Traceable Accounts

368 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Key message #3 Traceable Account

Altered biogeochemical cycles together with 

climate change increase the vulnerability of biodiversity, food security, human health, and water 

quality to changing climate. However, natural and 

managed shifts in major biogeochemical cycles can 

help limit rates of climate change.

Description of evidence base

The author team evaluated technical input reports (17) on biogeochemical cycles, including the two primary sources.

4,30

The climate–biogeochemical cycle link has been demonstrated 

through numerous studies on the effects of reactive nitrogen and 

phosphorus on forest carbon uptake and storage, and decomposition 

of organic matter;44,53 temperature effects on ecosystem productivity;54 and sensitivity of natural methane emissions to climate variation.55

Where the nitrogen and phosphorus cycles are concerned, a number 

of publications have reported effects of excess loading on ecosystem 

processes60,61 and have projected these effects to worsen.

61,62 Additionally, studies have reported the potential for future climate change 

and increasing nitrogen and phosphorus loadings to have an additive 

effect and the need for remediation.18,61 The literature suggests that 

co-benefits are possible from addressing the environmental concerns 

of both nutrient loading and climate change.4,31,64,65,66

New information and remaining uncertainties

Scientists are still investigating the impact of nitrogen deposition 

on carbon uptake and of sulfur and nitrogen aerosols on radiative 

forcing.

Recent work has shown that more than just climate change aspects 

can benefit from addressing multiple environmental concerns (air/

water quality, biodiversity, food security, human health, and so on)

Assessment of confidence based on evidence

High. We have a high degree of confidence that climate change will 

affect biogeochemical cycles through its effects on ecosystem structure and function (species composition and productivity). Similarly, 

there is high confidence that altered biogeochemical cycles will affect climate change, as for example in the increased rates of carbon 

storage in forests and soils that often accompany excess nitrogen 

deposition.
369 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

REGIONS

American

Samoa

Marshall

Islands

Puerto Rico

U.S. Virgin

Islands

Northern

Mariana

Islands

Guam

Hawai‘i

Republic

of Palau

Federated

States of

Micronesia

Washington, DC Washington, DC

Alaska Alaska

Northwest Northwest

Southwest Southwest

Great Plains Great Plains

Midwest Midwest

Southeast

and 

Caribbean

Southeast

and 

Caribbean

Northeast Northeast

Hawai‘i

and

Pacific Islands

From the Rocky Mountains to the Shenandoah Valley, the Great Lakes to the Gulf 

of Mexico, our country’s landscapes and communities vary dramatically. But amidst 

our geographical and economic diversity, we share many common attributes and 

challenges. One common challenge facing every U.S. region is a new and dynamic 

set of realities resulting from our changing climate.

The evidence can be found in every region, and impacts are visible in every state. 

Some of the most dramatic changes are in Alaska, where average temperatures 

have increased more than twice as fast as the rest of the country. The rapid 

decline of Arctic sea ice cover in the last decade is reshaping that region. In 

the Southwest, a combination of increased temperatures and reductions in 

annual precipitation are already affecting forests and diminishing water supplies. 

Meanwhile, that region’s population continues to grow at double-digit rates, 

increasing the stress on water supplies. In various regions, evidence of climate 

change is apparent in ecosystem changes, such as species moving northward, 

increases in invasive species and insect outbreaks, and changes in the length of 

the growing season. In many cities, impacts to the urban environment are closely 

linked to the changing climate, with increased flooding, greater incidence of heat 

waves, and diminished air 

quality. Along most of our 

coastlines, increasing sea 

levels and associated threats to 

coastal areas and infrastructure 

are becoming a common 

experience.

For all U.S. regions, warming 

in the future is projected 

to be very large compared 

to historical variations. 

Precipitation patterns will be 

altered as well, with some 

regions becoming drier and 

some wetter. The exact location 

of some of these future 

changes is not easy to pinpoint, 

because the continental 

U.S. straddles a transition 

zone between projected drier 

conditions in the sub-tropics 

(south) and wetter conditions at higher latitudes (north). As a result, projected 

precipitation changes in the northernmost states (which will get wetter) and 

southernmost states (which will get drier) are more certain than those for the 

central areas of the country. The heaviest precipitation events are projected to 

increase everywhere, and by large amounts. Extended dry spells are also projected 

to increase in length.

©iStockPhoto.com/Andrew Johnson
Regional differences in climate change impacts provide opportunities as well as challenges. A changing climate requires 

alterations in historical agricultural practices, which, if properly anticipated, can have some benefits. Warmer winters mean 

reductions in heating costs for those in the northern portions of the country. Well-designed adaptation and mitigation actions 

that take advantage of regional conditions can significantly enhance the nation’s resilience in the face of multiple challenges, 

which include many factors in addition to climate change.

The regions defined in this report intentionally follow state lines (see Figure 1 and Table 1), but landscape features such as 

forests and mountain ranges do not follow these artificial boundaries. The array of distinct landscapes within each region 

required difficult choices of emphasis for the authors. The chapters that follow provide a summary of changes and impacts 

that are observed and anticipated in each of the eight regions of the United States, as well as on oceans and coasts.

For more information about the regional climate histories and projections1 and sea level rise scenarios2 developed for the 

National Climate Assessment, and used throughout this report, see Ch. 2: Our Changing Climate and Appendix 5: Scenarios 

and Model

References

1. Kunkel, K. E., L. E. Stevens, S. E. Stevens, L. Sun, E. Janssen, D. Wuebbles, and J. G. Dobson, 2013: Regional Climate Trends and Scenarios for 

the U.S. National Climate Assessment: Part 9. Climate of the Contiguous United States. NOAA Technical Report NESDIS 142-9. 85 pp., National 

Oceanic and Atmospheric Administration, National Environmental Satellite, Data, and Information Service, Washington, D.C. [Available online 

at http://www.nesdis.noaa.gov/technical_reports/NOAA_NESDIS_Tech_Report_142-9-Climate_of_the_Contiguous_United_States.pdf]

2. Parris, A., P. Bromirski, V. Burkett, D. Cayan, M. Culver, J. Hall, R. Horton, K. Knuuti, R. Moss, J. Obeysekera, A. Sallenger, and J. Weiss, 2012: 

Global Sea Level Rise Scenarios for the United States National Climate Assessment. NOAA Tech Memo OAR CPO-1, 37 pp., National Oceanic 

and Atmospheric Administration, Silver Spring, MD. [Available online at http://scenarios.globalchange.gov/sites/default/files/NOAA_SLR_r3_0.

pdf]

Table 1: Composition of NCA Regions

Region Composition

Northeast Connecticut, Delaware, Maine, Maryland, Massachusetts, New Hampshire, New Jersey, New York, 

Pennsylvania, Rhode Island, Vermont, West Virginia, District of Columbia,

Southeast and 

Caribbean

Alabama, Arkansas, Florida, Georgia, Kentucky, Louisiana, Mississippi, North Carolina, South 

Carolina, Tennessee, Virginia, Puerto Rico, U.S. Virgin Islands

Midwest Illinois, Indiana, Iowa, Michigan, Minnesota, Missouri, Ohio, Wisconsin

Great Plains Kansas, Montana, Nebraska, North Dakota, Oklahoma, South Dakota, Texas, Wyoming

Northwest Idaho, Oregon, Washington

Southwest Arizona, California, Colorado, Nevada, New Mexico, Utah

Alaska Alaska

Hawai‘i and U.S. 

Pacific Islands

Hawai‘i, Commonwealth of the Northern Mariana Islands, Federated States of Micronesia, Republic 

of the Marshall Islands, Republic of Palau, Territory of American Samoa, Territory of Guam

©iStockPhoto.com/Andrew Johnson

REGIONS

370 CLIMATE CHANGE IMPACTS IN THE UNITED STATES
Climate Change Impacts in the United States

CHAPTER 16

NORTHEAST

INFORMATION DRAWN FROM THIS CHAPTER IS INCLUDED IN THE HIGHLIGHTS REPORT AND IS IDENTIFIED BY THIS ICON

Recommended Citation for Chapter

Horton, R., G. Yohe, W. Easterling, R. Kates, M. Ruth, E. Sussman, A. Whelchel, D. Wolfe, and F. Lipschultz, 2014: Ch. 16: 

Northeast. Climate Change Impacts in the United States: The Third National Climate Assessment, J. M. Melillo, Terese (T.C.) 

Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 16-1-nn.

On the Web: http://nca2014.globalchange.gov/report/regions/northeast

First published May 2014. PDF revised October 2014. See errata (available at http://nca2014.globalchange.gov/downloads) for details.

Convening Lead Authors

Radley Horton, Columbia University

Gary Yohe, Wesleyan University

Lead Authors

William Easterling, Pennsylvania State University

Robert Kates, University of Maine

Matthias Ruth, Northeastern University

Edna Sussman, Fordham University School of Law 

Adam Whelchel, The Nature Conservancy

David Wolfe, Cornell University

Contributing Author

Fredric Lipschultz, NASA and Bermuda Institute of Ocean Sciences

371
372 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Key Messages

16NORTHEAST

1. Heat waves, coastal flooding, and river flooding will pose a growing challenge to the 

region’s environmental, social, and economic systems. This will increase the vulnerability 

of the region’s residents, especially its most disadvantaged populations. 

2. Infrastructure will be increasingly compromised by climate-related hazards, including sea 

level rise, coastal flooding, and intense precipitation events.

3. Agriculture, fisheries, and ecosystems will be increasingly compromised over the 

next century by climate change impacts. Farmers can explore new crop options, but 

these adaptations are not cost- or risk-free. Moreover, adaptive capacity, which varies 

throughout the region, could be overwhelmed by a changing climate. 

4. While a majority of states and a rapidly growing number of municipalities have begun to 

incorporate the risk of climate change into their planning activities, implementation of 

adaptation measures is still at early stages. 

Sixty-four million people are concentrated in the Northeast. 

The high-density urban coastal corridor from Washington, D.C., 

north to Boston is one of the most developed environments in 

the world. It contains a massive, complex, and long-standing 

network of supporting infrastructure. The region is home to 

one of the world’s leading financial centers, the nation’s capital, and many defining cultural and historical landmarks. 

The region has a vital rural component as well. The Northeast 

includes large expanses of sparsely populated but ecologically and agriculturally important areas. Much of the Northeast landscape is dominated by forest, but the region also has 

grasslands, coastal zones, beaches and dunes, and wetlands, 

and it is known for its rich marine and freshwater fisheries. 

These natural areas are essential to recreation and tourism 

sectors and support jobs through the sale of timber, maple syrup, and seafood. They also contribute important 

ecosystem services to broader populations – protecting 

water supplies, buffering shorelines, and sequestering 

carbon in soils and vegetation. The twelve Northeastern 

states have more than 180,000 farms, with $17 billion in 

annual sales.1 The region’s ecosystems and agricultural 

systems are tightly interwoven, and both are vulnerable 

to a changing climate. 

Although urban and rural regions in the Northeast have 

profoundly different built and natural environments, 

both include populations that have been shown to be 

highly vulnerable to climate hazards and other stresses. 

Both also depend on aging infrastructure that has already 

been stressed by climate hazards including heat waves, 

as well as coastal and riverine flooding due to a combination of 

sea level rise, storm surge, and extreme precipitation events.

The Northeast is characterized by a diverse climate.2 Average 

temperatures in the Northeast generally decrease to the north, 

with distance from the coast, and at higher elevations. Average 

annual precipitation varies by about 20 inches throughout the 

Northeast with the highest amounts observed in coastal and 

select mountainous regions. During winter, frequent storms 

bring bitter cold and frozen precipitation, especially to the 

north. Summers are warm and humid, especially to the south. 

The Northeast is often affected by extreme events such as ice 

storms, floods, droughts, heat waves, hurricanes, and major 

storms in the Atlantic Ocean off the northeast coast, referred 

to as nor’easters. However, variability is large in both space and © Frank Siteman/Science Faction/Corbis
373 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

16: NORTHEAST

time. For example, parts of southern New 

England that experienced heavy snows in 

the cold season of 2010-2011 experienced 

little snow during the cold season of 2011-

2012. Of course, even a season with low 

totals can feature costly extreme events; 

snowfall during a 2011 pre-Halloween 

storm that hit most of the Northeast, when 

many trees were still in leaf, knocked out 

power for up to 10 days for thousands of 

households.

Observed Climate Change

Between 1895 and 2011, temperatures 

in the Northeast increased by almost 2 ̊F 

(0.16 ̊F per decade), and precipitation increased by approximately five inches, or 

more than 10% (0.4 inches per decade).3

Coastal flooding has increased due to a rise 

in sea level of approximately 1 foot since 

1900. This rate of sea level rise exceeds 

the global average of approximately 8 inches (see Ch. 2: Our 

Changing Climate, Key Message 10; Ch. 25: Coasts), due primarily to land subsidence,4

 although recent research suggests 

that changes in ocean circulation in the North Atlantic – specifically, a weakening of the Gulf Stream – may also play a role.5

The Northeast has experienced a greater recent increase in extreme precipitation than any other region in the United States; 

between 1958 and 2010, the Northeast saw more than a 70% 

increase in the amount of precipitation falling in very heavy 

events (defined as the heaviest 1% of all daily events) (see Ch. 

2: Our Changing Climate, Figure 2.18).7

© ADAM HUNGER/Reuters/Corbis

Sea Level is Rising

Figure 16.1. (Map) Local sea level trends in the Northeast region. Length of time series for each arrow varies 

by tide gauge location. (Figure source: NOAA6). (Graph) Observed sea level rise in Philadelphia, PA, has 

significantly exceeded the global average of 8 inches over the past century, increasing the risk of impacts to 

critical urban infrastructure in low-lying areas. Over 100 years (1901-2012), sea level increased 1.2 feet (Data 

from Permanent Service for Mean Sea Level).
374 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

16: NORTHEAST

Projected Climate Change

As in other areas, the amount of warming in the Northeast 

will be highly dependent on global emissions of heat-trapping 

gases. If emissions continue to increase (as in the A2 scenario), 

warming of 4.5F° to 10°F is projected by the 2080s; if global 

emissions were reduced substantially (as in the B1 scenario), 

projected warming ranges from about 3°F to 6°F by the 2080s.3

Under both emissions scenarios, the frequency, intensity, and 

duration of heat waves is expected to increase, with larger increases under higher emissions (Ch. 2: Our Changing Climate). 

Much of the southern portion of the region, including the 

majority of Maryland and Delaware, and southwestern West 

Virginia and New Jersey, are projected by mid-century to experience many more days per year above 90°F compared to 

the end of last century under continued increases in emissions 

(Figure 16.2, A2 scenario). This will affect the region’s vulnerable populations, infrastructure, agriculture, and ecosystems.

The frequency, intensity, and duration of cold air outbreaks is 

expected to decrease as the century progresses, although some 

research suggests that loss of Arctic sea ice could indirectly reduce this trend by modifying the jet stream and mid-latitude 

weather patterns.8,9

Projections of precipitation changes are less certain than projections of temperature increases.3

 Winter and spring precipitation is projected to increase, especially but not exclusively in 

the northern part of the region (Ch. 2: Our Changing Climate, 

Key Messages 5 and 6).3,10 A range of model projections for the 

end of this century under a higher emissions scenario (A2), averaged over the region, suggests about 5% to 20% (25th to 75th

percentile of model projections) increases in winter precipitation. Projected changes in summer and fall, and for the entire 

year, are generally small at the end of the century compared to 

natural variations (Ch. 2: Our Changing Climate, Key Message 

5).3 The frequency of heavy downpours is projected to continue to increase as the century progresses (Ch. 

2: Our Changing Climate, Key Message 6). Seasonal drought risk is also projected to increase in 

summer and fall as higher temperatures lead to 

greater evaporation and earlier winter and spring 

snowmelt.11

Global sea levels are projected to rise 1 to 4 feet 

by 2100 (Ch. 2: Our Changing Climate, Key Message 10),12 depending in large part on the extent 

to which the Greenland and West Antarctic Ice 

Sheets experience significant melting. Sea level 

rise along most of the coastal Northeast is expected to exceed the global average rise due to 

local land subsidence, with the possibility of even 

greater regional sea level rise if the Gulf Stream 

weakens as some models suggest.5,13 Sea level 

rise of two feet, without any changes in storms, 

would more than triple the frequency of dangerous coastal flooding throughout most of the 

Northeast.14

Although individual hurricanes cannot be directly 

attributed to climate change, Hurricanes Irene 

and Sandy nevertheless provided “teachable moments” by demonstrating the region’s vulnerability to extreme weather events and the potential 

for adaptation to reduce impacts.

Projected Increases in the Number of Days over 90°F 

Figure 16.2. Projected number of days per year with a maximum temperature 

greater than 90°F averaged between 2041 and 2070, compared to 1971-2000 

(Historical Climate), assuming continued increases in global emissions (A2) 

and substantial reductions in future emissions (B1). (Figure source: NOAA 

NCDC / CICS-NC). 
375 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

16: NORTHEAST

Two recent events contrast existing vulnerability to extreme events: Hurricane Irene, which produced a broad swath of 

very heavy rain (greater than five inches in total and sometimes two to three inches per hour in some locations) from 

southern Maryland to northern Vermont from August 27 to 29, 2011; and Hurricane Sandy, which caused massive 

coastal damage from storm surge and flooding along the Northeast coast from October 28 to 30, 2012. 

Rainfall associated with Irene led to hydrological extremes in the region. These heavy rains were part of a broader pattern 

of wet weather preceding the storm (rainfall totals for August and September exceeded 25 inches across much of 

the Northeast) that left the region predisposed to extreme 

flooding from Irene; for example, the Schoharie Creek in 

New York experienced a 500-year flood.15 

In anticipation of Irene, the New York City mass transit 

system was shut down, and 2.3 million coastal residents 

in Delaware, New Jersey, and New York faced mandatory 

evacuations. However, it was the inland impacts, especially in upstate New York and in central and southern 

Vermont, that were most severe. Ironically, many New 

York City residents fled to inland locations, which were 

harder hit. Flash flooding washed out roads and bridges, undermined railroads, brought down trees and power lines, flooded homes and businesses, and damaged 

floodplain forests. In Vermont, more than 500 miles of 

roadways and approximately 200 bridges were damaged, 

with estimated rebuilding costs of $175 to $250 million. Hazardous wastes were released in a number of areas, and 17 municipal wastewater treatment plants were 

breached by floodwaters. Agricultural losses included 

damage to barn structures and flooded fields of crops. 

Many towns and villages were isolated for days due to 

infrastructure impacts from river flooding (see also Ch. 

5: Transportation, “Tropical Storm Irene Devastates Vermont Transportation in August 2011”).2

 Affected residents suffered from increased allergen exposure due to 

mold growth in flooded homes and other structures and 

were exposed to potentially harmful chemicals and pathogens in their drinking water. In the state of Vermont, cleaning 

up spills from aboveground hazardous waste tanks cost an estimated $1.75 million. Septic systems were also damaged 

from high groundwater levels and river or stream erosion, including 17 septic system failures in the state of Vermont.17

Sandy was responsible for about 150 deaths, approximately half of which occurred in the Northeast.18 Damages, concentrated in New Jersey, New York, and Connecticut, were estimated at $60 to $80 billion, making Sandy the second 

most costly Atlantic Hurricane in history behind Katrina.19 It is also estimated that 650,000 homes were damaged or 

destroyed, and that 8.5 million people were without power.18 Floodwaters inundated subway tunnels in New York City (see 

also Ch. 5: Transportation, “Hurricane Sandy”). Sandy also caused significant damage to the electrical grid and overwhelmed sewage treatment plants.18 In New Jersey, repairs to damaged power and gas lines are expected to cost about 

$1 billion, and repairs to waste, water, and sewer systems are expected to cost $3 billion. 

Many of these vulnerabilities to coastal flooding and sea level rise (Ch. 2: Our Changing Climate, Key Message 10) and 

intensifying storms (Ch. 2: Our Changing Climate, Key Messages 8 and 9) – including the projected frequency of flooding of tunnels and airports – were documented as early as 2001 in a report developed in support of the 2000 National 

Climate Assessment.20 Despite such reports, the observed vulnerability was a surprise to many coastal residents, which 

suggests improved communication is needed. 

Flooding and Hurricane Irene 

Figure 16.3. Hurricane Irene over the Northeast on August 

28, 2011. The storm, which brought catastrophic flooding 

rains to parts of the Northeast, took 41 lives in the United 

States, and the economic cost was estimated at $16 billion.16

(Figure source: MODIS instrument on NASA’s Aqua satellite).

Hurricane vulnerability

Continued
376 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

16: NORTHEAST

Over the last decade, cities, states, and agencies in 

the New York metropolitan region took steps to reduce 

their vulnerability to coastal storms.21 In 2008, New 

York City convened a scientific body of experts – the 

New York City Panel on Climate Change (NPCC) – and 

formed a Climate Adaptation Task Force comprised 

of approximately 40 agencies, private sector companies, and regional groups. A process, approach, and 

tools for climate change adaptation were developed 

and documented in New York City11,22 and New York 

State.23 In 2012, the NPCC and Climate Adaptation 

Task Force were codified into New York City law, a 

key step towards institutionalizing climate science, 

impact, and adaptation assessment into long-term 

planning.24

These initiatives led to adaptation efforts, including 

elevating infrastructure, restoring green spaces, and 

developing evacuation plans that helped reduce damage and save lives during Irene and Sandy (also see 

discussion of Hurricane Sandy in Ch. 11: Urban). As 

rebuilding and recovery advances,24 decision-making based on current and projected risks from such 

events by a full set of stakeholders and participants 

in the entire Northeast could dramatically improve resilience across the region.

Coastal Flooding Along New Jersey’s Shore

Figure 16.4. Predictions of coastal erosion prior to Sandy’s 

arrival provided the region’s residents and decision-makers with 

advance warning of potential vulnerability. The map shows three 

bands: collision of waves with beaches causing erosion on the 

front of the beach; overwash that occurs when water reaches 

over the highest point and erodes from the rear, which carries 

sand inland; and inundation, when the shore is severely eroded 

and new channels can form that lead to permanent flooding. 

The probabilities are based on the storm striking at high tide. 

For New Jersey, the model estimated that 21% of the shoreline 

had more than a 90% chance of experiencing inundation. These 

projections were realized, and made the New Jersey coastline 

even more vulnerable to the nor’easter that followed Hurricane 

Sandy by only 10 days. (Figure source: ESRI and USGS 201225).

Hurricane vulnerability
377 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

16: NORTHEAST

Key Message 1: Climate Risks to People

Heat waves, coastal flooding, and river flooding will pose a growing challenge to the region’s 

environmental, social, and economic systems. This will increase the vulnerability of the 

region’s residents, especially its most disadvantaged populations. 

Urban residents have unique and multifaceted vulnerabilities 

to heat extremes. Northeastern cities, with their abundance 

of concrete and asphalt and relative lack of vegetation, tend to 

have higher temperatures than surrounding regions (the “urban heat island” effect). During extreme heat events, nighttime 

temperatures in the region’s big cities are generally several degrees higher26 than surrounding regions, leading to increased 

heat-related death among those less able to recover from the 

heat of the day.27 Since the hottest days in the Northeast are 

often associated with high concentrations of ground-level 

ozone and other pollutants,28 the combination of heat stress 

and poor air quality can pose a major health risk to vulnerable groups: young children, the elderly, and those with preexisting health conditions including asthma.29 Vulnerability is 

further increased as key infrastructure, including electricity for 

potentially life-saving air conditioning, is more likely to fail precisely when it is most needed – when demand exceeds available supply. Significant investments may be required to ensure 

that power generation keeps up with rising demand associated with rising temperatures.30 Finally, vulnerability to heat 

waves is not evenly distributed throughout urban areas; 

outdoor versus indoor air temperatures, air quality, baseline 

health, and access to air conditioning are all dependent on 

socioeconomic factors.29 Socioeconomic factors that tend 

to increase vulnerability to such hazards include race and 

ethnicity (being a minority), age (the elderly and children), 

gender (female), socioeconomic status (low income, status, 

or poverty), and education (low educational attainment). 

The condition of human settlements (type of housing and 

construction, infrastructure, and access to lifelines) and the 

built environment are also important determinants of socioeconomic vulnerability, especially given the fact that these 

characteristics influence potential economic losses, injuries, 

and mortality.31

Increased health-related impacts and costs, such as premature death and hospitalization due to even modest increases 

in heat, are predicted in the Northeast’s urban centers (Ch. 

9: Human Health).32 One recent study projected that temperature changes alone would lead to a 50% to 91% increase 

in heat-related deaths in Manhattan by the 2080s (relative 

Urban Heat Island

Figure 16.5. Surface temperatures in New York City on a 

summer’s day show the “urban heat island,” with temperatures 

in populous urban areas being approximately 10°F higher than 

the forested parts of Central Park. Dark blue reflects the colder 

waters of the Hudson and East Rivers. (Figure source: Center for 

Climate Systems Research, Columbia University).

©Richard Drew/AP/AP/Corbis
378 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

16: NORTHEAST

to a 1980s baseline).33 Increased ground-level ozone due to 

warming is projected to increase emergency department visits 

for ozone-related asthma in children (0 to 17 years of age) by 

7.3% by the 2020s (given the A2 scenario) relative to a 1990 

baseline of approximately 650 visits in the New York metropolitan area.34

Heat wave research has tended to focus on urban areas, but 

vulnerability to heat may also become a major issue in rural 

areas and small towns because air conditioning is currently not 

prevalent in parts of the rural Northeast where heat waves 

have historically been rare. Some areas of northern New England, near the Canadian border, are projected to shift from 

having less than five to more than 15 days per year over 90°F 

by the 2050s under the higher emissions scenario (A2) of heattrapping gases.3

 It should be noted that winter heating needs, a 

significant expense for many Northeastern residents, are likely 

to decrease as the century progresses.35

The impacts of climate change on public health will extend beyond the direct effects of temperature on human physiology. 

Changing distributions of temperature, precipitation, and carbon dioxide could affect the potency of plant allergens,36 and 

there has been an observed increase of 13 to 27 days in the 

ragweed pollen season at latitudes above 44°N.36

Vector-borne diseases are an additional concern. Most occurrences of Lyme disease in United States are in the Northeast, 

especially Connecticut.37 While it is unclear how climate change 

will impact Lyme disease,38 several studies in the Northeast 

have linked tick activity and Lyme disease incidence to climate, 

specifically abundant late spring and early summer moisture.39

West Nile Virus (WNV) is another vector-borne disease that 

may be influenced by changes in climate. Suitable habitat for 

the Asian Tiger Mosquito, which can transmit West Nile and 

other vector-borne diseases, is expected to increase in the 

Northeast from the current 5% to 16% in the next two decades 

and from 43% to 49% by the end of the century, exposing more 

than 30 million people to the threat of dense infestations by 

this species.40

Many Northeast cities, including New York, Boston, and Philadelphia, are served by combined sewer systems that collect 

and treat both stormwater and municipal wastewater. During 

heavy rain events, combined systems can be overwhelmed 

and untreated water may be released into local water bodies. 

In Connecticut, the risk for contracting a stomach illness while 

swimming significantly increased after a one inch precipitation 

event,41 and studies have found associations between diarrheal illness among children and sewage discharge in Milwaukee.42

More frequent heavy rain events could therefore increase the 

incidence of waterborne disease.

Historical settlement patterns and ongoing investment in 

coastal areas and along major rivers combine to increase the 

vulnerabilities of people in the Northeast to sea level rise and 

coastal storms. Of the Northeast’s population of 64 million,43

approximately 1.6 million people live within the Federal Emergency Management Agency’s (FEMA) 100-year coastal flood 

zone, with the majority – 63% of those at risk – residing in New 

York and New Jersey.44 As sea levels rise, populations in the 

current 1-in-100-year coastal flood zone (defined as the area 

with at least a 1% chance of experiencing a coastal flood in a 

given year) will experience more frequent flooding, and populations that have historically fallen outside the 1-in-100-year 

flood zone will find themselves in that zone. People living in 

coastal flood zones are vulnerable to direct loss of life and injury associated with tropical storms and nor’easters. Flood damage to personal property, businesses, and public infrastructure 

can also result (see Key Message 2). 

This risk is not limited to the 1-in-100-year flood zone; in the 

Mid-Atlantic part of the region alone, estimates suggest that 

between 450,000 and 2.3 million people are at risk from a 

three foot sea level rise,45 which is in the range of projections 

for this century. 

Throughout the Northeast, populations are also concentrated 

along rivers and their flood plains. In mountainous regions, including much of West Virginia and large parts of Pennsylvania, 

New York, Vermont, and New Hampshire, more intense precipitation events (Ch. 2: Our Changing Climate)3

 will mean greater 

flood risk, particularly in valleys, where people, infrastructure, 

and agriculture tend to be concentrated. 
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Key Message 2: Stressed Infrastructure

Infrastructure will be increasingly compromised by climate-related hazards, 

including sea level rise, coastal flooding, and intense precipitation events.

Disruptions to services provided by public and private infrastructure in the Northeast both interrupt commerce and 

threaten public health and safety (see also Ch. 11: Urban).46

In New York State, two feet of sea level rise is estimated (absent adaptation investment) to flood or render unusable 212 

miles of roads, 77 miles of rail, 3,647 acres of airport facilities, 

and 539 acres of runways.47 Port facilities, such as in Maryland 

(primarily Baltimore), also have flooding impact estimates: 298 

acres, or 32% of the overall port facilities in the state.47 These 

impacts have potentially significant economic ramifications. 

For example, in 2006 alone the Port of Baltimore generated 

more than 50,200 jobs, $3.6 billion in personal income, $1.9 

billion in business revenues, and $388 million in state, county, and municipal tax.48 The New York City Panel on Climate 

Change highlighted a broader range of climate impacts on 

infrastructure sectors (see Table 16.1).11 Although this study 

focused specifically on New York City, these impacts are applicable throughout the region. Predicted impacts of coastal 

flooding on infrastructure were largely borne out by Hurricane 

Sandy; sea level rise will only increase these vulnerabilities.

The more southern states within the region, including Delaware 

and Maryland, have a highly vulnerable land area because of a 

higher rate of sea level rise and relatively flat coastlines compared to the northern tier. The northern states, including Massachusetts, Rhode Island, and Connecticut, have less land area 

exposed to a high inundation risk because of a lower relative 

sea level rise and because of their relatively steep coastal terrain.49 Still, low-lying coastal metropolitan areas in New England have considerable infrastructure at risk. In Boston alone, 

cumulative damage to buildings and building contents, as well 

as the associated emergency costs, could potentially be as high 

as $94 billion between 2000 and 2100, depending on the sea 

level rise scenario and which adaptive actions are taken.50

Table 16.1. Impacts of sea level rise and coastal floods on critical coastal infrastructure by sector. Sources: Horton and Rosenzweig 2010,51 Zimmerman 

and Faris 2010,52 and Ch. 25: Coasts.

Communications Energy Transportation Water and Waste

Higher average sea level

• Increased saltwater encroachment and damage to 

low-lying communications 

infrastructure not built to 

withstand saltwater exposure

• Increased rates of coastal 

erosion and/or permanent 

inundation of low-lying areas, 

causing increased maintenance costs and shortened 

replacement cycles

• Cellular tower destruction or 

loss of function

• Increased coastal erosion 

rates and/or permanent 

inundation of low-lying areas, 

threatening coastal power 

plants

• Increased equipment damage 

from corrosive effects of 

saltwater encroachment, resulting in higher maintenance 

costs and shorter replacement cycles

• Increased saltwater encroachment and damage to 

infrastructure not built to 

withstand saltwater exposure

• Increased coastal erosion 

rates and/or permanent 

inundation of low-lying areas, 

resulting in increased maintenance costs and shorter 

replacement cycles

• Decreased clearance levels 

under bridges

• Increased saltwater encroachment and damage to 

water and waste infrastructure not built to withstand 

saltwater exposure

• Increased release of pollution 

and contaminant runoff from 

sewer systems, treatment 

plants, brownfields, and 

waste storage facilities

• Permanent inundation of lowlying areas, wetlands, piers, 

and marine transfer stations

• Increased saltwater infiltration into freshwater distribution systems

More frequent and intense coastal flooding

• Increased need for emergency management actions 

with high demand on communications infrastructure

• Increased damage to communications equipment and 

infrastructure in low-lying 

areas

• Increased need for emergency management actions

• Exacerbated flooding of lowlying power plants and equipment, as well as structural 

damage to infrastructure due 

to wave action

• Increased use of energy to 

control floodwaters

• Increased number and 

duration of local outages 

due to flooded and corroded 

equipment

• Increased need for emergency management actions

• Exacerbated flooding of 

streets, subways, tunnel and 

bridge entrances, as well as 

structural damage to infrastructure due to wave action

• Decreased levels of service 

from flooded roadways; 

increased hours of delay 

from congestion during street 

flooding episodes

• Increased energy use for 

pumping

• Increased need for emergency management actions

• Exacerbated street, basement, and sewer flooding, 

leading to structural damage 

to infrastructure 

• Episodic inundation of lowlying areas, wetlands, piers, 

and marine transfer stations
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In the transportation sector (see also Ch. 5: Transportation), 

many of the region’s key highways (including I-95) and rail systems (including Amtrak and commuter rail networks) span areas that are prone to coastal flooding. In addition to temporary 

service disruptions, storm surge flooding can severely undermine or disable critical infrastructure along coasts, including 

subway systems, wastewater treatment plants, and electrical 

substations. Saltwater corrosion can damage sensitive and 

critical electrical equipment, such as electrical substations 

for energy distribution and signal equipment for rail systems; 

corrosion also accelerates rust damage on rail lines. Saltwater 

also threatens groundwater supplies and damages wastewater 

treatment plants.

Key Message 3: Agricultural and Ecosystem Impacts

Agriculture, fisheries, and ecosystems will be increasingly compromised over the next century 

by climate change impacts. Farmers can explore new crop options, but these adaptations are 

not cost- or risk-free. Moreover, adaptive capacity, which varies throughout the region, 

could be overwhelmed by a changing climate. 

Farmers in the Northeast are already experiencing consequences of climate change. In addition to direct crop damage 

from increasingly intense precipitation events, wet springs 

can delay planting for grain and vegetables in New York, for 

example, and subsequently delay harvest dates and reduce 

yields.53 This is an issue for agriculture nationally,54 but is particularly acute for the Northeast, where heavy rainfall events 

have increased more than in any other region of the country 

(Ch. 2: Our Changing Climate, Key Message 6).7 In the future, 

farmers may also face too little water in summer to meet increased crop water demand as summers become hotter and 

growing seasons lengthen.55,56 Increased frequency of summer 

heat stress is also projected, which can negatively affect crop 

yields and milk production.57

Despite a trend toward warmer winters, the risk of frost and 

freeze damage continues, and has paradoxically increased over 

the past decade (see also Ch. 8: Ecosystems). These risks are 

exacerbated for perennial crops in years with variable winter 

temperatures. For example, midwinter-freeze damage cost 

wine grape growers in the Finger Lakes region of New York millions of dollars in losses in the winters of 2003 and 2004.58 This 

was likely due to de-hardening of the vines during an unusually 

warm December, which increased susceptibility to cold damage just prior to a subsequent hard freeze. Another avenue for 

cold damage, even in a relatively warm winter, is when there 

is an extended warm period in late winter or early spring causing premature leaf-out or bloom, followed by a damaging frost 

event, as occurred throughout the Northeast in 200759 and 

again in 2012 when apple, grape, cherry, and other fruit crops 

were hard hit.60

Increased weed and pest pressure associated with longer 

growing seasons and warmer winters will be an increasingly important challenge; there are already examples of earlier arrival 

and increased populations of some insect pests such as corn 

earworm.57 Furthermore, many of the most aggressive weeds, 

such as kudzu, benefit more than crop plants from higher atmospheric carbon dioxide, and become more resistant to herbicide control.61 Many weeds respond better than most cash 

crops to increasing carbon dioxide concentrations, particularly 

“invasive” weeds with the so-called C3 photosynthetic pathway, and with rapid and expansive growth patterns, including 

large allocations of below-ground biomass, such as roots.62 Research also suggests that glyphosate (for example, Roundup), 

the most widely-used herbicide in the United States, loses its NOAA

Coney Island after Hurricane Irene

Figure 16.6. Flooded subway tracks in Coney 

Island after Hurricane Irene. (Photo credit: 

Metropolitan Transportation Authority of the 

State of New York 2011).
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efficacy on weeds grown at the increased carbon dioxide levels 

likely to occur in the coming decades.63 To date, all weed/crop 

competition studies where the photosynthetic pathway is the 

same for both species favor weed growth over crop growth as 

carbon dioxide is increased.61

Effects of rising temperatures on the Northeast’s ecosystems 

have already been clearly observed (see also Ch. 8: Ecosystems). Further, changes in species distribution by elevation are 

occurring; a Vermont study found an upslope shift of 299 to 

390 feet in the boundary between northern hardwoods and 

boreal forest on the western slopes of the Green Mountains 

between 1964 and 2004.64 Wildflowers65 and woody perennials are blooming earlier 66 and migratory birds are arriving 

sooner.67 Because species differ in their ability to adjust, asynchronies (like a mismatch between key food source availability 

and migration patterns) can develop, increasing species and 

ecosystem vulnerability. Several bird species have expanded 

their ranges northward68 as have some invasive insect species, 

such as the hemlock woolly adelgid,69 which has devastated 

hemlock trees. Warmer winters and less snow cover in recent 

years have contributed to increased deer populations70 that 

degrade forest understory vegetation.71

As ocean temperatures continue to rise, the range of suitable 

habitat for many commercially important fish and shellfish 

species is projected to shift northward. For example, cod and 

lobster fisheries south of Cape Cod are projected to have significant declines.72 Although suitable habitats will be shrinking 

for some species (such as coldwater fish like brook trout) and 

expanding for others (such as warmwater fish like bass), it is 

difficult to predict what proportion of species will be able to 

move or adapt as their optimum climate zones shift.73 As each 

species responds uniquely to climate change, disruptions of important species interactions (plants and pollinators; predators 

and prey) can be expected. For example, it is uncertain what 

forms of vegetation will move into the Adirondack Mountains 

when the suitable habitat for spruce-fir forests disappears.74

Increased productivity of some northern hardwood trees in 

the Northeast is projected (due to longer growing seasons and 

assuming a significant benefit from higher atmospheric carbon 

dioxide), but summer drought and other extreme events may 

offset potential productivity increases.75 Range shifts in traditional foods gathered from the forests by Native American 

communities, such as Wabanaki berries in the Northeast, can 

have negative health and cultural impacts (Ch. 12: Indigenous 

Peoples).76 

In contrast, many insect pests, pathogens, and invasive plants 

like kudzu appear to be highly and positively responsive to recent and projected climate change.77 Their expansion will lead 

to an overall loss of biodiversity, function, and resilience of 

some ecosystems. 

The Northeast’s coastal ecosystems and the species that inhabit them are highly vulnerable to rising seas (see also Ch. 

25: Coasts, Key Message 4). Beach and dune erosion, both a 

cause and effect of coastal flooding, is also a major issue in 

the Northeast.78,79 Since the early 1800s, there has been an 

estimated 39% decrease in marsh coverage in coastal New 

England; in the metropolitan Boston area, marsh coverage is 

estimated to be less than 20% of its late 1700s value.80 Impervious urban surfaces and coastal barriers such as seawalls limit 

the ability of marshes to expand inland as sea levels rise.81

The Chesapeake Bay is the largest U.S. estuary, with a drainage basin that extends over six states. It is a critical and 

highly integrated natural and economic system threatened by changing land-use patterns and a changing climate – 

including sea level rise, higher temperatures, and more intense precipitation events. The ecosystem has a central role 

in the economy, including providing sources of food for people and the region’s other inhabitants, and cooling water 

for the energy sector. It also provides critical ecosystem services. 

As sea levels rise, the Chesapeake Bay region is expected to experience an increase in coastal flooding and drowning 

of estuarine wetlands. The lower Chesapeake Bay is especially at risk due to high rates of sinking land (known as 

subsidence).82 Climate change and sea level rise are also likely to cause a number of ecological impacts, including 

declining water quality and clarity, increases in harmful algae and low oxygen (hypoxia) events, decreases in a number 

of species including eelgrass and seagrass beds, and changing interactions among trophic levels (positions in the food 

chain) leading to an increase in subtropical fish and shellfish species in the bay.83

The chesapeake bay
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Key Message 4: Planning and Adaptation

While a majority of states and a rapidly growing number of municipalities have begun to 

incorporate the risk of climate change into their planning activities, implementation 

of adaptation measures is still at early stages. 

Of the 12 states in the Northeast, 11 have developed adaptation plans for several sectors and 10 have released, or plan to 

release, statewide adaptation plans.84 Given the interconnectedness of climate change impacts and adaptation, multi-state 

coordination could help to ensure that information is shared 

efficiently and that emissions reduction and adaptation strategies do not operate at cross-purposes. 

Local and state governments in the Northeast have been 

leaders and incubators in utilizing legal and regulatory opportunities to foster climate change policies.85 The Regional 

Greenhouse Gas Initiative (RGGI) was the first market-based 

regulatory program in the U.S. aimed at reducing greenhouse 

gas emissions; it is a cooperative effort among nine northeastern states.86 Massachusetts became the first state to officially 

incorporate climate change impacts into its environmental 

review procedures by adopting legislation that directs agencies to “consider reasonably foreseeable climate 

change impacts, including additional greenhouse 

gas emissions, and effects, such as predicted sea 

level rise.”87 In addition, Maine, Massachusetts, 

and Rhode Island have each adopted some form 

of “rolling easement” to ensure that wetlands or 

dunes migrate inland as sea level rises and reduce the risk of loss of life and property.45

Northeast cities have employed a variety of 

mechanisms to respond to climate change, including land-use planning, provisions to protect 

infrastructure, regulations related to the design 

and construction of buildings, and emergency 

preparation, response, and recovery.91 While 

significant progress has been made, local governments still face limitations of legal authority, 

geographic jurisdiction, and resource constraints 

that could be addressed through effective engagement and support from higher levels of government. 

Keene, New Hampshire, has been a pilot community for ICLEI’s Climate Resilient Communities 

program for adaptation planning92 – a process 

implemented through innovative community engagement methods.93 The Cape Cod Commission 

is another example in New England; the Commission has drafted model ordinances to help 

communities incorporate climate into zoning 

decision-making. Farther south, New York City 

has taken numerous steps to implement PlaNYC, 

a far-reaching sustainability plan for the city, including amending the construction code and the 

zoning laws and the implementation of measures 

focused on developing adaptation strategies to 

protect the City’s public and private infrastructure from the effects of climate change;24 some 

major investments in protection have even been 

conceptualized.

Connecticut Coastline and Expanding Salt Marshes

Figure 16.7. The Nature Conservancy’s adaptation decision-support 

tool (www.coastalresilience.org)88 depicts building-level impacts due 

to inundation (developed land cover, yellow areas) and potential marsh 

advancement zones (undeveloped land cover – currently forest, grass, 

and agriculture – blue areas) using downscaled sea level rise projections 

(52 inches by 2080s depicted) along the Connecticut and New York 

coasts. (Figure source: Ferdaña et al. 2010,90 Beck et al. 201389).
383 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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One widely used adaptation-planning template is the eightstep iterative approach developed by the New York City Panel 

on Climate Change; it was highlighted in the contribution of the 

National Academy of Science’s Adaptation Panel to America’s 

Climate Choices and adopted by the Committee on America’s 

Climate Choices. It describes a procedure that decision-makers 

at all levels can use to design a flexible adaptation pathway to 

address infrastructure and other response issues through inventory and assessment of risk. The key, with respect to infrastructure, is to link adaptation strategies with capital improvement cycles and adjustment of plans to incorporate emerging 

climate projections11,94 – but the insights are far more general 

than that (see the Adaptation Panel Report95).

In most cases, adaptation requires information and tools 

coupled to a decision-support process steered by strong leadership, and there are a growing number of examples in the 

Northeast. At the smaller, municipal scale, coastal pilot projects in Maryland,96 Delaware,97 New York, and Connecticut90

are underway. 

Research and outreach efforts are underway in the region to 

help farmers find ways to cope with a rapidly changing climate, 

take advantage of a longer growing season, and reduce greenhouse gas emissions,56,98 but unequal access to capital and 

information for strategic adaptation and mitigation remain a 

challenge. Financial barriers can constrain farmer adaptation.99

Even relatively straightforward adaptations such as changing varieties are not always a low-cost option. Seed for new 

stress-tolerant varieties is sometimes expensive or regionally 

unavailable, and new varieties often require investments in 

new planting equipment or require adjustment in a wide range 

of farming practices. Investment in irrigation and drainage 

systems are relatively expensive options, and a challenge for 

farmers will be determining when the frequency of yield losses 

due to summer water deficits or flooding has or will become 

frequent enough to warrant such capital investments.

Regional activities in the Northeast are also being linked to federal efforts. For example, NASA’s Agency-wide Climate Adaptation Science Investigator Workgroup (CASI) brings together 

NASA facilities managers with NASA climate scientists in local 

Climate Resilience Workshops. This approach was in evidence 

at the Goddard Space Flight Center in Maryland, where scientists helped institutional managers address energy and stormwater management vulnerabilities.

Storm Surge Barrier

Figure 16.8. Conceptual design of a storm surge barrier in New York City. (Figure source: Jansen and Dircke 2009).
384 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Maine’s culverts: an adaptation case study

Culverts and the structures they protect are receiving increasing attention, since they are vulnerable to damage during the 

types of extreme precipitation events that are occurring with increasing frequency in the Northeast (Ch. 2: Our Changing 

Climate, Key Message 6; Ch. 5: Transportation). For instance, severe storms in the Northeast that were projected in the 

1950s to occur only once in 100 years, now are projected to occur once every 60 years.100

The Maine Department of Transportation manages more than 97,000 culverts, but individual property owners or small 

towns manage even more; Scarborough, Maine, for example, has 2,127 culverts. When 71 town managers and officials 

in coastal Maine were surveyed as part of the statewide Sustainability Solutions Initiative, culverts, with their 50 to 65 

year expected lifespan, emerged atop a wish list 

for help in adapting to climate change.101

A research initiative that mapped decisions by 

town managers in Maine to sources of climate information, engineering design, mandated requirements, and calendars identified the complex, 

multi-jurisdictional challenges of widespread adaptation for even such seemingly simple actions 

as using larger culverts to carry water from major 

storms.101 To help towns adapt culverts to expected climate change over their lifetimes, the Sustainability Solutions Initiative is creating decision 

tools to map culvert locations, schedule maintenance, estimate needed culvert size, and analyze 

replacement needs and costs.

 ©Esperanza Stancioff, UMaine Extension and Maine Sea Grant
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Association Section of Environment Energy and Resources. 

Alternative Dispute Resolution Committee Newsletter, 9. [Available 

online at http://apps.americanbar.org/environ/committees/adr/

newsletter/may10/ADR_May10.pdf]
16: NORTHEAST

References

391 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

94. New York State, 2011: Responding to Climate Change in New York 

State: The ClimAID Integrated Assessment for Effective Climate Change 

Adaptation in New York State. Vol. 1244, Blackwell Publishing, 649 

pp. [Available online at http://onlinelibrary.wiley.com/doi/10.1111/

j.1749-6632.2011.06331.x/pdf]

95. NRC, 2010: Adapting to Impacts of Climate Change. America’s Climate 

Choices: Report of the Panel on Adapting to the Impacts of Climate Change.

National Research Council. The National Academies Press, 292 

pp. [Available online at http://www.nap.edu/catalog.php?record_

id=12783]

96. Cole, W. D., 2008: Sea level rise: Technical guidance for Dorchester 

County Maryland Eastern Shore Resource Conservation and 

Development Council, 55 pp., Maryland Eastern Shore Resource 

Conservation & Development Council, Annapolis, MD. [Available 

online at http://www.dnr.state.md.us/dnrnews/pdfs/Dorchester.

pdf] 

Maryland Department of Natural Resources, 2008: Ch. 5: Phase 

I: Sea level rise and coastal storms. Report of the Maryland 

commission on climate change adaptation and response working 

group. Comprehensive Strateg y for Reducing Maryland’s Vulnerability 

to Climate Change, Maryland Department of Natural Resources, 

Maryland Department of Planning, 32. [Available online at http://

www.mde.state.md.us/assets/document/Air/ClimateChange/

Chapter5.pdf]

97. Delaware Coastal Programs, 2011: Preparing for Sea Level 

Rise: Development of a Sea level Rise Initiative. Sea Level Rise 

Initiative Project Compendium. [Available online at http://

www.dnrec.delaware.gov/coastal/Documents/SeaLevelRise/

SLRCompSept2011.pdf]

98. Wolfe, D. W., J. Beem-Miller, L. Chambliss, and A. Chatrchyan, 

2011: Climate Change Facts: Farm Energy, Carbon, and 

Greenhouse Gases, 4 pp., Cornell Climate Change Program Work 

Team. [Available online at http://files.campus.edublogs.org/blogs.

cornell.edu/dist/8/90/files/2009/09/farm_energy-zhts0p.pdf]

99. Adger, W. N., S. Agrawala, M. M. Q. Mirza, C. Conde, K. O’Brien, 

J. Pulhin, R. Pulwarty, B. Smit, and K. Takahashi, 2007: Ch. 17: 

Assessment of adaptation practices, options, constraints and 

capacity. Climate Change 2007: Impacts, Adaptation and Vulnerability. 

Contribution of Working Group II to the Fourth Assessment Report of the 

Intergovernmental Panel on Climate Change, M. L. Parry, O. F. Canziani, 

J. P. Palutikof, P. J. van der Linden, and C. E. Hanson, Eds., 

Cambridge University Press, 717-743. 

Wolfe, D. W., 2012: Ch. 2: Climate change solutions from the 

agronomy perspective. Handbook of Climate Change and Agroecosystems: 

Global and Regional Aspects and Implications, D. Hillel, and C. 

Rosenzweig, Eds., Imperial College Press, 11-30. 

100. DeGaetano, A. T., 2009: Time-dependent changes in extremeprecipitation return-period amounts in the continental United 

States. Journal of Applied Meteorolog y and Climatolog y, 48, 2086-2099, 

doi:10.1175/2009jamc2179.1. [Available online at http://journals.

ametsoc.org/doi/pdf/10.1175/2009JAMC2179.1]

101. Jain, S., E. Stancioff, and A. Gray, cited 2012: Coastal Climate 

Adaptation in Maine’s Coastal Communities: Governance Mapping 

for Culvert Management. [Available online at http://umaine.

edu/maineclimatenews/archives/spring-2012/coastal-climateadaptation-in-maines-coastal-communities/]

102. Lenton, T., A. Footitt, and A. Dlugolecki, 2009: Major Tipping 

Points in the Earth’s Climate System and Consequences for 

the Insurance Sector, 89 pp., World Wide Fund for Nature, 

Gland, Switzerland and Munich, Germany. [Available online at 

http://assets.worldwildlife.org/publications/386/files/original/

Major_Tipping_Points_in_the_Earth’s_Climate_System_and_

Consequences_for_the_Insurance_Sector.pdf?1345749585]

103. NPCC, 2009: Climate Risk Information, 74 pp., New York City 

Panel on Climate Change. [Available online at http://www.nyc.

gov/html/om/pdf/2009/NPCC_CRI.pdf]
392 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

16: NORTHEAST

SUPPLEMENTAL MATERIAL

TRACEABLE ACCOUNTS

Process for Developing Key Messages: 

Results of the Northeast Regional Climate assessment workshop 

that was held on November 17-18, 2011, at Columbia University, 

with approximately 60 attendees, were critically important in our 

assessment. The workshop was the beginning of the process that 

led to the foundational Technical Input Report (TIR).2 That 313-

page report consisted of seven chapters by 13 lead authors and 

more than 60 authors in total. Public and private citizens or institutions who service and anticipate a role in maintaining support 

for vulnerable populations in Northeast cities and communities 

indicated that they are making plans to judge the demand for adaptation services. These stakeholder interactions were surveyed 

and engaged in the preparation of this chapter. We are confident 

that the TIR authors made a vigorous attempt to engage various 

agencies at the state level and non-governmental organizations 

(NGOs) that have broader perspectives. 

The author team engaged in multiple technical discussions via 

teleconferences, which included careful review of the foundational 

TIR2 and approximately 50 additional technical inputs provided 

by the public, as well as the other published literature and professional judgment. Discussions were followed by expert deliberation 

of draft key messages by the authors and targeted consultation 

with additional experts by the lead author of each key message.

Key message #1 Traceable Account

Heat waves, coastal flooding, and river flooding 

will pose a growing challenge to the region’s environmental, social, and economic systems. This will 

increase the vulnerability of the region’s residents, 

especially its most disadvantaged populations. 

Description of evidence base

The key message and supporting text summarizes extensive 

evidence documented in the Northeast Technical Input Report.2

Nearly 50 Technical Input reports, on a wide range of topics, were 

also received and reviewed as part of the Federal Register Notice 

solicitation for public input. 

Numerous peer-reviewed publications (including many that are not 

cited) describe increasing hazards associated with sea level rise 

and storm surge, heat waves, and intense precipitation and river 

flooding for the Northeast. For sea level rise (SLR), the authors 

relied on the NCA SLR scenario12 and research by the authors 

on the topic (for example, Horton et al. 201051). Recent work26

summarizes the literature on heat islands and extreme events. For 

a recent study on climate in the Northeast,3 the authors worked 

closely with the region’s state climatologists on both the climatology and projections. 

The authors also considered many recent peer-reviewed publications29,32,34,44 that describe how human vulnerabilities to climate 

hazards in the region can be increased by socioeconomic and 

other factors. Evaluating coupled multi-system vulnerabilities is 

an emerging field; as a result, additional sources including white 

papers3 have informed this key message as well.

To capture key issues, concerns, and opportunities in the region, 

various regional assessments were also consulted, such as PlaNYC 

(http://www.nyc.gov/html/planyc2030) and Boston’s Climate 

Plan (http://www.cityofboston.gov/Images_Documents/A%20Climate%20of%20Progress%20-%20CAP%20Update%202011_

tcm3-25020.pdf). 

New information and remaining uncertainties

Important new evidence (cited above) confirmed many of the findings from a prior Northeast assessment10 (see http://nca2009.

globalchange.gov/northeast). 

The evidence included results from improved models and updated 

observational data (for example, Liu et al. 2012; Parris et al. 

2012; Sallenger et al. 20125,9,12). The current assessment included insights from stakeholders collected in a series of distributed 

engagement meetings that confirm its relevance and significance 

for local decision-makers; examples include a Northeast Listening 

Session in West Virginia, a kickoff meeting in New York City, and 

New York City Panel on Climate Change meetings. 

There is wide diversity of impacts across the region driven by both 

exposure and sensitivity that are location and socioeconomic context specific. Future vulnerability will be influenced by changes in 

demography, economics, and policies (development and climate 

driven) that are difficult to predict and dependent on international 

and national considerations. Another uncertainty is the potential 
16: NORTHEAST

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393 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

for adaptation strategies (and to a lesser extent mitigation) to reduce these vulnerabilities.

There are also uncertainties associated with the character of the 

interconnections among systems, and the positive and negative 

synergies. For example, a key uncertainty is how systems will 

respond during extreme events and how people will adjust their 

short- to long-term planning to take account of a dynamic climate. 

Such events are, by definition, manifestations of historically rare 

and therefore relatively undocumented climatology which represent uncertainty in the exposure to climate risk. Nonetheless, 

these events are correlated, when considered holistically, with 

climate change driven to some degree by human interference with 

the climate system. There are uncertainties in exposure. 

There are also uncertainties associated with sensitivity to future 

changes driven to some (potentially significant) degree by non-climate stressors, including background health of the human population and development decisions. Other uncertainties include how 

much effort will be put into making systems more resilient and the 

success of these efforts. Another critical uncertainty is associated 

with the climate system itself.

Assessment of confidence based on evidence 

Given the evidence base and remaining uncertainties, confidence 

is: 

Very high for sea level rise and coastal flooding as well as heat 

waves.

High for intense precipitation events and riverine flooding. 

Very high for both added stresses on environmental, social, and 

economic systems and for increased vulnerability, especially for 

populations that are already most disadvantaged.

Key message #2 Traceable Account

Infrastructure will be increasingly compromised by climaterelated hazards, including sea level rise, coastal flooding, and intense precipitation events.

Description of evidence base

The key message summarizes extensive evidence documented 

in the Northeast Technical Input Report (TIR).2 Technical Input 

reports (48) on a wide range of topics were also received and 

reviewed as part of the Federal Register Notice solicitation for 

public input. 

To capture key issues, concerns and opportunities in the region, 

various regional assessments were also consulted, such as PlaNYC 

(http://www.nyc.gov/html/planyc2030) and Boston’s Climate 

Plan (http://www.cityofboston.gov/Images_Documents/A%20Climate%20of%20Progress%20-%20CAP%20Update%202011_

tcm3-25020.pdf). 

In addition, a report by the U.S. Department of Transportation47

provided extensive documentation that augmented an NGO 

report.102 Other sources that support this key message include 

Horton and Rosenzweig, 2010, Rosenzweig et al. 2011, and Zimmerman and Faris, 2010.23,51,52

New information and remaining uncertainties

Important new evidence (cited above) confirmed many of the findings from the prior Northeast assessment: (http://nca2009.globalchange.gov/northeast) which informed the prior NCA.10

The new sources above relied on improved models that have been 

calibrated to new observational data across the region.

It is important to note, of course, that there is wide diversity across 

the region because both exposure and sensitivity are location- and 

socioeconomic-context-specific. The wisdom derived from many 

previous assessments by the National Academy of Sciences, the 

New York Panel on Climate Change, and the 2009 National Climate Assessment10,11,95 indicates that future vulnerability at any 

specific location will be influenced by changes in demography, 

economics, and policy. These changes are difficult to predict at 

local scales even as they also depend on international and national 

considerations. The potential for adaptation strategies (and to a 

lesser extent mitigation) to reduce these vulnerabilities is yet another source of uncertainty that expands as the future moves into 

the middle of this century.

Confidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent 

results, well documented and 

accepted methods, etc.), high 

consensus

High

Moderate evidence (several sources, some consistency, methods 

vary and/or documentation limited, 

etc.), medium consensus

Medium

Suggestive evidence (a few 

sources, limited consistency, models incomplete, methods emerging, 

etc.), competing schools of thought

Low

Inconclusive evidence (limited 

sources, extrapolations, inconsistent findings, poor documentation 

and/or methods not tested, etc.), 

disagreement or lack of opinions 

among experts
16: NORTHEAST

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394 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Assessment of confidence based on evidence 

We have very high confidence in projected sea level rise and 

increased coastal flooding, and high confidence for increased 

intense precipitation events. This assessment of confidence is 

based on our review of the literature and submitted input and has 

been defended internally and externally in conversation with local 

decision-makers and representatives of interested NGOs, as well 

as the extensive interactions with stakeholders across the region 

reported in the Northeast TIR.2 

Very high confidence that infrastructure will be increasingly compromised, based on the clear evidence of impacts on current infrastructure from hazards such as Hurricane Irene, and from the 

huge deficit of needed renewal identified by a diverse engineering 

community.46

Key message #3 Traceable Account

Agriculture, fisheries, and ecosystems will be increasingly compromised over the next century by 

climate change impacts. Farmers can explore new 

crop options, but these adaptations are not cost- or 

risk-free. Moreover, adaptive capacity, which varies 

throughout the region, could be overwhelmed by a 

changing climate.

Description of evidence base

The key message summarizes extensive evidence documented in 

the Northeast Technical Input Report.2 Technical Input reports 

(48) on a wide range of topics were also received and reviewed 

as part of the Federal Register Notice solicitation for public input. 

The Traceable Account for Key Message 1 provides the evidence 

base on sea level rise, flooding, and precipitation. 

Various regional assessments were also consulted to capture key 

issues, concerns and opportunities in the region with particular 

focus on managed (agriculture and fisheries) and unmanaged 

(ecosystems) systems (for example, Buonaiuto et al. 2011; Wolfe 

et al. 201156,70,78). 

Species and ecosystem vulnerability have been well documented 

historically in numerous peer-reviewed papers in addition to the 

ones cited in the TIR.2 There have also been many examples of impacts on agriculture of climate variability and change in the Northeast (for example, Wolfe et al. 200857). Most note that there is 

potential for significant benefits associated with climate changes 

to partially offset expected negative outcomes for these managed 

systems (for example, Hatfield et al. 201154)

New information and remaining uncertainties

Important new evidence (cited above, plus Najjar et. al. 2010,83

for example) confirmed many of the findings from the prior Northeast assessment (http://nca2009.globalchange.gov/northeast)

which informed the 2009 NCA.10

These new sources also relied on improved models that have been 

calibrated to new observational data across the region.

Agriculture, fisheries, and ecosystems in the Northeast are strongly linked to climate change and to other changes occurring outside 

the region and beyond the boundaries of the United States. These 

changes can influence the price of crops and agricultural inputs 

such as fertilizer, for example, as well as the abundance of ecosystem and agricultural pests and the abundance and range of 

fish stocks. Other uncertainties include imprecise understandings 

of how complex ecosystems will respond to climate- and nonclimate-induced changes and the extent to which organisms may 

be able to adapt to a changing climate.

Assessment of confidence based on evidence 

Based on our assessment, we have very high confidence for climate impacts (especially sea level rise and storm surge) on ecosystems; and we have high confidence for climate impacts on 

agriculture (reduced to some degree, compared to our level of 

confidence about ecosystems, by uncertainty about the efficacy 

and implementation of adaptation options). Confidence in fisheries changes is high since confidence in both ocean warming and 

fish sensitivity to temperature is high.

Key message #4 Traceable Account

While a majority of states and a rapidly growing 

number of municipalities have begun to incorporate 

the risk of climate change into their planning activities, implementation of adaptation measures is still 

at early stages. 

Description of evidence base

The key message relies heavily on extensive evidence documented 

in the Northeast Technical Input Report (TIR).2 Technical Input 

reports (48) on a wide range of topics were also received and 

reviewed as part of the Federal Register Notice solicitation for 

public input. Many of the key references cited in the TIR reflected 

experiences and processes developed in iterative stakeholder engagement concerning risk management94,103 that have been heavily cited and employed in new venues – local communities like 

Keane (NH) and New York City, for example. 

Various regional assessments were also consulted to capture key 

issues, concerns and opportunities in the region (for example, for 

Delaware, Maine, Maryland, and Long Island, NY). In addition, 

there have been agency and government white paper reports describing proposed adaptation strategies based on climate impact 

assessments.11,90 We discovered that 10 of the 12 states in the 

Northeast have statewide adaptation plans in place or under development (many plans can be found at: http://georgetownclimate.

org/node/3324). 
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395 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

New information and remaining uncertainties

That most Northeast states have begun to plan for adaptation is 

a matter of record. That few adaptation plans have been implemented is confirmed in Technical Inputs submitted to the National 

Climate Assessment process as well as prior assessments (http://

nca2009.globalchange.gov/northeast), which informed the 2009 

NCA.10

Key uncertainties looking forward include: 1) the extent to which 

proposed adaptation strategies will be implemented given a range 

of factors including competing demands and limited funding; 2) 

the role of the private sector and individual action in adaptation, 

roles which can be difficult to document; 3) the extent of the 

federal role in adaptation planning and implementation; and 4) 

how changes in technology and the world economy may change 

the feasibility of specific adaptation strategies.11

Assessment of confidence based on evidence 

This Key Message is simply a statement of observed fact, so confidence language is not applicable.
396

Climate Change Impacts in the United States

CHAPTER 17

SOUTHEAST AND THE CARIBBEAN

INFORMATION DRAWN FROM THIS CHAPTER IS INCLUDED IN THE HIGHLIGHTS REPORT AND IS IDENTIFIED BY THIS ICON

Recommended Citation for Chapter

Carter, L. M., J. W. Jones, L. Berry, V. Burkett, J. F. Murley, J. Obeysekera, P. J. Schramm, and D. Wear, 2014: Ch. 17: 

Southeast and the Caribbean. Climate Change Impacts in the United States: The Third National Climate Assessment, J. M. 

Melillo, Terese (T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 396-417. doi:10.7930/J0NP22CB. 

On the Web: http://nca2014.globalchange.gov/report/regions/southeast

First published May 2014. PDF revised October 2014. See errata (available at http://nca2014.globalchange.gov/downloads) for details.

Convening Lead Authors

Lynne M. Carter, Louisiana State University

James W. Jones, University of Florida

Lead Authors

Leonard Berry, Florida Atlantic University

Virginia Burkett, U.S. Geological Survey

James F. Murley, South Florida Regional Planning Council

Jayantha Obeysekera, South Florida Water Management District

Paul J. Schramm, Centers for Disease Control and Prevention

David Wear, U.S. Forest Service 
397 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

SOUTHEAST 17 AND THE CARIBBEAN

The Southeast and Caribbean are exceptionally vulnerable to 

sea level rise, extreme heat events, hurricanes, and decreased 

water availability. The geographic distribution of these impacts 

and vulnerabilities is uneven, since the region encompasses 

a wide range of natural system types, from the Appalachian 

Mountains to the coastal plains. It is also home to more than 

80 million people1 and draws millions of visitors 

every year. In 2009, Puerto Rico hosted 3.5 million tourists who spent $3.5 billion.2

 In 2012, Louisiana and Florida alone hosted more than 115 million visitors.3

The region has two of the most populous metropolitan areas in the country (Miami and Atlanta) 

and four of the ten fastest-growing metropolitan 

areas.1 Three of these (Palm Coast, FL, Cape Coral-Fort Myers, FL, and Myrtle Beach area, SC) are 

along the coast and are vulnerable to sea level rise 

and storm surge. Puerto Rico has one of the highest population densities in the world, with 56% of 

the population living in coastal municipalities.4

The Gulf and Atlantic coasts are major producers 

of seafood and home to seven major ports5 that 

are also vulnerable. The Southeast is a major energy producer of coal, crude oil, and natural gas, and is the 

highest energy user of any of the National Climate Assessment 

regions.5

The Southeast’s climate is influenced by many factors, including latitude, topography, and proximity to the Atlantic Ocean 

Key Messages

1. Sea level rise poses widespread and continuing threats to both natural and built environments 

and to the regional economy. 

2. Increasing temperatures and the associated increase in frequency, intensity, and duration of 

extreme heat events will affect public health, natural and built environments, energy, agriculture, 

and forestry.

3. Decreased water availability, exacerbated by population growth and land-use change, will 

continue to increase competition for water and affect the region’s economy and 

unique ecosystems.

Figure 17.1. This map summarizes the number of times each state has been 

affected by weather and climate events over the past 30 years that have 

resulted in more than a billion dollars in damages. The Southeast has been 

affected by more billion-dollar disasters than any other region. The primary 

disaster type for coastal states such as Florida is hurricanes, while interior 

and northern states in the region also experience sizeable numbers of 

tornadoes and winter storms. For a list of events and the affected states, see: 

http://www.ncdc.noaa.gov/billions/events.

6

 (Figure source: NOAA NCDC).

Billion Dollar Weather/Climate Disasters

© Richard H. Cohen/Corbis
398 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

17: SOUTHEAST AND THE CARIBBEAN

and the Gulf of Mexico. Temperatures generally decrease 

northward and into mountain areas, while precipitation decreases with distance from the Gulf and Atlantic coasts. The 

region’s climate also varies considerably over seasons, years, 

and decades, largely due to natural cycles such as the El NiñoSouthern Oscillation (ENSO – periodic changes in ocean surface temperatures in the Tropical Pacific Ocean), the semi-permanent high pressure system over Bermuda, differences in 

atmospheric pressure over key areas of the globe, and landfalling tropical weather systems.7

 These cycles alter the occurrences of hurricanes, tornadoes, droughts, flooding, freezing 

winters, and ice storms, contributing to climate and weather 

disasters in the region that have exceeded the total number of 

billion dollar disasters experienced in all other regions of the 

country combined (see Figure 17.1). 

Observed and Projected Climate Change

Average annual temperature during the last century across the 

Southeast cycled between warm and cool periods (see Figure 

17.3, black line). A warm peak occurred during the 1930s and 

1940s followed by a cool period in the 1960s and 1970s. Temperatures increased again from 1970 to the present by an average of 2°F, with higher average temperatures during summer 

months. There have been increasing numbers of days above 

95°F and nights above 75°F, and decreasing numbers of extremely cold days since 1970.11 The Caribbean also exhibits a 

trend since the 1950s, with increasing numbers of very warm 

days and nights, and with daytime maximum temperatures 

above 90°F and nights above 75°F.4 Daily and five-day rainfall 

intensities have also increased.5 Also, summers have been either increasingly dry or extremely wet.11 For the Caribbean, 

precipitation trends are unclear, with some regions experiencing smaller annual amounts of rainfall and some increasing amounts.4

 Although the number of major tornadoes has 

increased over the last 50 years, there is no statistically significant trend (Ch 2: Our Changing Climate, Key Message 9).11,12

This increase may be attributable to better reporting of tornadoes. The number of Category 4 and 5 hurricanes in the Atlantic basin has increased substantially since the early 1980s 

compared to the historical record that dates back to the mid1880s (Ch. 2: Our Changing Climate, Key Message 8). This can 

Stories of change: coastal louisiana tribal communities

Climate change impacts, especially sea level 

rise and related increases in storm surges pulsing farther inland, will continue to exacerbate 

ongoing land loss already affecting Louisiana 

tribes. Four Native communities in Southeast 

Louisiana (Grand Bayou Village, Grand Caillou/Dulac, Isle de Jean Charles, and Pointeau-Chien) have already experienced significant 

land loss. Management of river flow has deprived the coastal wetlands of the freshwater 

and sediment that they need to replenish and 

persist. Dredging of canals through marshes for 

oil and gas exploration and pipelines has led to 

erosion and intense saltwater intrusion, resulting in additional land loss. Due to these and 

other natural and man-made problems, Louisiana has lost 1,880 square miles of land in the 

last 80 years.8 This combination of changes has 

resulted in a cascade of losses of sacred places, 

healing plants, habitat for important wildlife, 

food security,9 and in some cases connectivity 

with the mainland. Additional impacts include 

increased inundation of native lands, further travel to reach traditional fishing grounds, reduced connections among 

family members as their lands have become more flood-prone and some have had to move, and declining community 

cohesiveness as heat requires more indoor time.10 (For more specifics, see Ch. 12: Indigenous Peoples). Numerous 

other impacts from increases in temperature, sea level rise, land loss, erosion, subsidence, and saltwater intrusion 

amplify these existing problems. 

Figure 17.2. Aerial photos of Isle de Jean Charles in Louisiana taken 45 

years apart shows evidence of the effects of rising seas, sinking land, 

and human development. The wetlands adjacent to the Isle de Jean 

Charles community (about 60 miles south of New Orleans) have been 

disappearing rapidly since the photo on the left was taken in 1963. By 

2008, after four major hurricanes, significant erosion, and alteration of 

the surrounding marsh for oil and gas extraction, open water surrounds 

the greatly reduced dry land. See Ch. 25: Coasts for more information. 

(Photo credit: USGS). 

Shrinking Lands for Tribal Communities
399 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

17: SOUTHEAST AND THE CARIBBEAN

be attributed to both natural variability and climate 

change. 

Temperatures across the 

Southeast and Caribbean 

are expected to increase 

during this century, with 

shorter-term (year-to-year 

and decade-to-decade) 

fluctuations over time due 

to natural climate variability (Ch. 2: Our Changing Climate, Key Message 

3).4 Major consequences of 

warming include significant 

increases in the number 

of hot days (95°F or above) 

and decreases in freezing 

events. Although projected increases for some parts of the 

region by the year 2100 are generally smaller than for other 

regions of the United States, projected increases for interior 

states of the region are larger than coastal regions by 1°F to 

2°F. Regional average increases are in the range of 4°F to 8°F 

(combined 25th to 75th percentile range for A2 and B1 emissions 

scenarios) and 2°F to 5°F for Puerto Rico.11

Projections of future precipitation patterns are 

less certain than projections for temperature increases.11 Because the Southeast is located in the 

transition zone between projected wetter conditions to the north and drier conditions to the 

southwest, many of the model projections show 

only small changes relative to natural variations. 

However, many models do project drier conditions in the far southwest of the region and wetter conditions in the far northeast of the region, 

consistent with the larger continental-scale pattern of wetness and dryness (Ch. 2: Our Changing Climate, Key Message 5).11 For the Caribbean, 

it is equally difficult to project the magnitude of 

precipitation changes, although the majority of 

models show future decreases in precipitation 

are likely, with a few areas showing increases. In 

general, annual average decreases are likely to 

be spread across the entire region.4 Projections 

further suggest that warming will cause tropical 

storms to be fewer in number globally, but stronger in force, with more Category 4 and 5 storms 

(Ch. 2: Our Changing Climate, Key Message 8).13

On top of the large increases in extreme precipitation observed during last century and early 

this century (Ch. 2: Our Changing Climate, Figures 2.16, 2.17, and 2.18), substantial further increases are projected as this century progresses 

(Ch. 2: Our Changing Climate, Figure 2.19).

Figure 17.3. Observed annual average temperature 

for the Southeast and projected temperatures 

assuming substantial emissions reductions (lower 

emissions, B1) and assuming continued growth 

in emissions (higher emissions, A2).11 For each 

emissions scenario, shading shows the range of 

projections and the line shows a central estimate. 

The projections were referenced to observed 

temperatures for the period 1901-1960. The region 

warmed during the early part of last century, cooled 

for a few decades, and is now warming again. The 

lack of an overall upward trend over the entire 

period of 1900-2012 is unusual compared to the 

rest of the U.S. and the globe. This feature has 

been dubbed the “warming hole” and has been 

the subject of considerable research, although a 

conclusive cause has not been identified. (Figure 

source: adapted from Kunkel et al. 201311).

Southeast Temperature: Observed and Projected

Figure 17.4. Projected average number of days per year with maximum 

temperatures above 95°F for 2041-2070 compared to 1971-2000, assuming 

emissions continue to grow (A2 scenario). Patterns are similar, but less 

pronounced, assuming a reduced emissions scenario (B1). (Figure source: 

NOAA NCDC / CICS-NC).

Projected Change in Number of Days Over 95°F
400 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

17: SOUTHEAST AND THE CARIBBEAN

Key Message 1: Sea Level Rise Threats

Sea level rise poses widespread and continuing threats to both 

natural and built environments and to the regional economy. 

Global sea level rise over the past century averaged approximately eight inches (Ch. 2: Our Changing Climate, Key Message 

10),14,15 and that rate is expected to accelerate through the end 

of this century.16 Portions of the Southeast and Caribbean are 

highly vulnerable to sea level rise.4,5 How much sea level rise is 

experienced in any particular place depends on whether and 

how much the local land is sinking (also called subsidence) or 

rising, and changes in offshore currents.16,17

Large numbers of cities, roads, railways, ports, airports, oil and 

gas facilities, and water supplies are at low elevations and potentially vulnerable to the impacts of sea level rise. New Orleans (with roughly half of its population living below sea level19), Miami, Tampa, Charleston, and Virginia Beach are among 

those most at risk.20 As a result of current sea level rise, the 

coastline of Puerto Rico around Rincón is being eroded at a 

rate of 3.3 feet per year.4

According to a recent study co-sponsored by a regional utility, coastal counties and parishes in Alabama, Mississippi, Louisiana, and Texas, with a population of approximately 12 million, assets of about $2 trillion, and producers of $634 billion in 

annual gross domestic product, already face significant losses 

that annually average $14 billion from hurricane winds, land 

subsidence, and sea level rise. Future losses for the 2030 timeframe could reach $18 billion (with no sea level rise or change 

in hurricane wind speed) to $23 billion (with a nearly 3% increase in hurricane wind speed and just under 6 inches of sea 

level rise). Approximately 50% of the increase in the estimated 

losses is related to climate change. The study identified $7 billion in cost-effective adaptation investments that could reduce 

estimated annual losses by about 30% in the 2030 timeframe.21

The North Carolina Department of Transportation is raising the 

roadbed of U.S. Highway 64 across the Albemarle-Pamlico Peninsula by four feet, which includes 18 inches to allow for highFigure 17.5. Projected average number of days per year with temperatures less 

than 32°F for 2041-2070 compared to 1971-2000, assuming emissions continue 

to grow (A2 scenario). Patterns are similar, but less pronounced, assuming a 

reduced emissions scenario (B1). (Figure source: NOAA NCDC / CICS-NC).

Projected Change in Number of Nights Below 32°F
401 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

17: SOUTHEAST AND THE CARIBBEAN

er future sea levels.22 Louisiana State Highway 1, heavily used 

for delivering critical oil and gas resources from Port Fourchon, 

is literally sinking, resulting in more frequent and more severe flooding during high tides and storms.8

 The Department 

of Homeland Security estimated that a 90-day shutdown of this road would cost the nation $7.8 billion.23

Sea level rise increases pressure on utilities – such as 

water and energy – by contaminating potential freshwater supplies with saltwater. Such problems are amplified during extreme dry periods with little runoff. 

Uncertainties in the scale, timing, and location of climate change impacts can make decision-making difficult, but response strategies, especially those that 

try to anticipate possible unintended consequences, 

can be more effective with early planning. Some utilities in the region are already taking sea level rise into 

account in the construction of new facilities and are 

seeking to diversify their water sources.24

There is an imminent threat of increased inland 

flooding during heavy rain events in low-lying coastal 

areas such as southeast Florida, where just inches of 

sea level rise will impair the capacity of stormwater 

drainage systems to empty into the ocean.24 Drainage 

problems are already being experienced in many 

locations during seasonal high tides, heavy rains, 

and storm surge events. Adaptation options that 

are being assessed in this region include the redesign and improvement of storm drainage canals, 

flood control structures, and stormwater pumps. 

As temperatures and sea levels increase, changes in marine and coastal systems are expected to 

affect the potential for energy resource development in coastal zones and the outer continental 

shelf. Oil and gas production infrastructure in bays 

and coves that are protected by barrier islands, for 

example, are likely to become increasingly vulnerable to storm surge as sea level rises and barrier islands deteriorate along the central Gulf Coast. The 

capacity for expanding and maintaining onshore 

and offshore support facilities and transportation 

networks is also apt to be affected.25

Sea level rise and storm surge can have impacts far 

beyond the area directly affected. Homes and infrastructure in low areas are increasingly prone to 

flooding during tropical storms. As a result, insurance costs may increase or coverage may become 

unavailable26 and people may move from vulnerable areas, stressing the social and infrastructural 

capacity of surrounding areas. This migration also 

happens in response to extreme events such as 

Hurricane Katrina, when more than 200,000 migrants were temporarily housed in Houston and 42% indicated 

they would try to remain there (Ch. 9: Human Health, Figure 

9.10).27

Homes and infrastructure in low-lying areas are increasingly vulnerable to 

flooding due to storm surge as sea level rises.

Figure 17.6. The map shows the relative risk that physical changes will occur 

as sea level rises. The Coastal Vulnerability Index used here is calculated 

based on tidal range, wave height, coastal slope, shoreline change, landform 

and processes, and historical rate of relative sea level rise. The approach 

combines a coastal system’s susceptibility to change with its natural ability 

to adapt to changing environmental conditions, and yields a relative measure 

of the system’s natural vulnerability to the effects of sea level rise. (Data 

from Hammar-Klose and Thieler 200118). 

Vulnerability to Sea Level Rise
402 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

17: SOUTHEAST AND THE CARIBBEAN

Furthermore, because income is a key indicator of climate vulnerability, people that have limited economic resources are 

more likely to be adversely affected by climate change impacts 

such as sea level rise. In the Gulf region, nearly 100% of the 

“most socially vulnerable people live in areas unlikely to be 

protected from inundation,” bringing equity issues and environmental justice into coastal planning efforts.28 

Ecosystems of the Southeast and Caribbean are exposed to 

and at risk from sea level rise, especially tidal marshes and 

swamps. Some tidal freshwater forests are already retreating, 

while mangrove forests (adapted to coastal conditions) are expanding landward.29 The pace of sea level rise will increasingly 

lead to inundation of coastal wetlands in the region. Such a 

crisis in land loss has occurred in coastal Louisiana for several 

decades, with 1,880 square miles having been lost since the 

1930s as a result of natural and man-made factors.8,30 With tidal wetland loss, protection of coastal lands and people against 

storm surge will be compromised. 

Reduction of wetlands also increases the 

potential for losses of important fishery habitat. Additionally, ocean warming 

could support shifts in local species composition, invasive or new locally viable 

species, changes in species growth rates, 

shifts in migratory patterns or dates, and 

alterations to spawning seasons.4,31 Any 

of these could affect the local or regional 

seafood output and thus the local economy.

In some southeastern coastal areas, 

changes in salinity and water levels due 

to a number of complex interactions (including subsidence, availability of sediment, precipitation, and sea level rise) 

can happen so fast that local vegetation 

cannot adapt quickly enough and those 

areas become open water.32 Fire, hurricanes, and other disturbances have similar effects, causing ecosystems to cross 

thresholds at which dramatic changes 

occur over short time frames.33

The impacts of sea level rise on agriculture derive from decreased freshwater 

availability, land loss, and saltwater intrusion. Saltwater intrusion is projected 

to reduce the availability of fresh surface 

and groundwater for irrigation, thereby 

limiting crop production in some areas.34

Agricultural areas around Miami-Dade 

County and southern Louisiana with 

shallow groundwater tables are at risk of 

increased inundation and future loss of cropland with a projected loss of 37,500 acres in Florida with a 27-inch sea level 

rise,35 which is well within the 1- to 4-foot range of sea level 

rise projected by 2100 (Ch. 2: Our Changing Climate, Key Message 10).

There are basically three types of adaptation options to rising sea levels: protect (such as building levees or other “hard” 

methods), accommodate (such as raising structures or using 

“soft” or natural protection measures such as wetlands restoration), and retreat.15,32 Individuals and communities are using 

all of these strategies. However, regional cooperation among 

local, state, and federal governments can greatly improve the 

success of adapting to impacts of climate change and sea level rise. An excellent example is the Southeast Florida Regional 

Compact. Through collaboration of county, state, and federal 

agencies, a comprehensive action plan was developed that includes hundreds of actions and special Adaptation Action Areas.37

Figure 17.7. Highway 1 in southern Louisiana is the only road to Port Fourchon, whose 

infrastructure supports 18% of the nation’s oil and 90% of the nation’s offshore oil and 

gas production. Flooding is becoming more common on Highway 1 in Leeville (inset 

photo from flooding in 2004), on the way to Port Fourchon. See also Ch. 25: Coasts, 

Figure 25.5. (Figure and photo sources: Louisiana Department of Transportation and 

Development; State of Louisiana 20128).

Highway 1 to Port Fourchon: 

Vulnerability of a Critical Link for U.S. Oil
403 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

17: SOUTHEAST AND THE CARIBBEAN

Key Message 2: Increasing Temperatures

Increasing temperatures and the associated increase in frequency, 

intensity, and duration of extreme heat events will affect public health, 

natural and built environments, energy, agriculture, and forestry.

The negative effects of heat on human cardiovascular, cerebral, and respiratory systems are well established (Ch. 9: Human Health)(for example: Kovats and Hajat 2008; O’Neill and 

Ebi 200938). Atlanta, Miami, New Orleans, and Tampa have already had increases in the number of days with temperatures 

exceeding 95°F, during which the number of deaths is above 

average.39 Higher temperatures also contribute to the formation of harmful air pollutants and allergens.40 Ground-level 

ozone is projected to increase in the 19 largest urban areas of 

the Southeast, leading to an increase in deaths.41 A rise in hospital admissions due to respiratory illnesses, emergency room 

visits for asthma, and lost school days is expected.42

The climate in many parts of the Southeast and Caribbean is 

suitable for mosquitoes carrying malaria and yellow and dengue fevers. The small island states in the Caribbean already 

have a high health burden from climate-sensitive disease, including vector-borne and zoonotic (animal to human) diseases.43 It is still uncertain how regional climate changes will affect 

vector-borne and zoonotic disease transmissions. While higher 

temperatures are likely to shorten both development and incubation time,44 vectors (like disease-carrying insects) also need 

Figure 17.8. Sea level rise presents major challenges to 

South Florida’s existing coastal water management system 

due to a combination of increasingly urbanized areas, aging 

flood control facilities, flat topography, and porous limestone 

aquifers. For instance, South Florida’s freshwater well field 

protection areas (left map: pink areas) lie close to the current 

interface between saltwater and freshwater (red line), which 

will shift inland with rising sea level, affecting water managers’ 

ability to draw drinking water from current resources. Coastal 

water control structures (right map: yellow circles) that were 

originally built about 60 years ago at the ends of drainage 

canals to keep saltwater out and to provide flood protection 

to urbanized areas along the coast are now threatened by sea 

level rise. Even today, residents in some areas such as Miami 

Beach are experiencing seawater flooding their streets (lower 

photo). (Maps from The South Florida Water Management 

District.36 Photo credit: Luis Espinoza, Miami-Dade County 

Department of Regulatory and Economic Resources).

South Florida: Uniquely Vulnerable to Sea Level Rise

Figure 17.9. Miami-Dade County staff leading workshop on 

incorporating climate change considerations in local planning. 

(Photo credit: Armando Rodriguez, Miami-Dade County).

Local Planning
404 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

17: SOUTHEAST AND THE CARIBBEAN

the right conditions for breeding (water), for dispersal (vegetation and humidity), and access to susceptible vertebrate hosts 

to complete the disease transmission cycle.5 While these transmission cycles are complex, increasing temperatures have the 

potential to result in an expanded region with more favorable 

conditions for transmission of these diseases.45,46

Climate change is expected to increase harmful algal blooms 

and several disease-causing agents in inland and coastal waters, which were not previously problems in the region.47,48,49

For instance, higher sea surface temperatures are associated 

with higher rates of ciguatera fish poisoning,48,50 one of the 

most common hazards from algal blooms in the region.51 The 

algae that causes this food-borne illness is moving northward, 

following increasing sea surface temperatures.52 Certain species of bacteria (Vibrio, for example) that grow in warm coastal 

waters and are present in Gulf Coast shellfish can cause infections in humans. Infections are now frequently reported both 

earlier and later by one month than traditionally observed.53

Coral reefs in the Southeast and Caribbean, as well as worldwide, are susceptible to climate change, especially warming 

waters and ocean acidification, whose impacts are exacerbated when coupled with other stressors, including disease, runoff, over-exploitation, and invasive species.4,5

An expanding population and regional land-use changes have 

reduced land available for agriculture and forests faster in the 

Southeast than in any other region in the contiguous United 

States.54 Climate change is also expected to change the unwanted spread and locations of some non-native plants, which 

will result in new management challenges.55

Heat stress adversely affects dairy and livestock production.56

Optimal temperatures for milk production are between 40oF 

and 75oF, and additional heat stress could shift dairy production northward.57 A 10% decline in livestock yield is projected 

across the Southeast with a 9oF increase in temperatures (applied as an incremental uniform increase in temperature between 1990 and 2060), related mainly to warmer summers.58

Summer heat stress is projected to reduce crop productivity, 

especially when coupled with increased drought (Ch. 6: Agriculture). The 2007 drought cost the Georgia agriculture industry $339 million in crop losses,59 and the 2002 drought cost the 

agricultural industry in North Carolina $398 million.5 A 2.2oF increase in temperature would likely reduce overall productivity 

for corn, soybeans, rice, cotton, and peanuts across the South 

– though rising CO2 levels could partially offset these decreases based on a crop yield simulation model.60 In Georgia, climate projections indicate corn yields could decline by 15% and 

wheat yields by 20% through 2020.61 In addition, many fruit 

crops from long-lived trees and bushes require chilling periods 

and may need to be replaced in a warming climate.60

Adaptation for agriculture involves decisions at many scales, 

from infrastructure investments (like reservoirs) to management decisions (like cropping patterns).62 Dominant adaptation strategies include altering local planting choices to better 

match new climate conditions62 and developing heat-tolerant 

crop varieties and breeds of livestock.5,57 Most critical for effective adaptation is the delivery of climate risk information to 

decision-makers at appropriate temporal and spatial scales57,62

and a focus on cropping systems that increase water-use efficiency, shifts toward irrigation, and more precise control of 

irrigation delivery (see also Ch. 28: Adaptation, Table 28.6).5,57

The southeastern U.S. (data include Texas and Oklahoma, not 

Puerto Rico) leads the nation in number of wildfires, averaging 45,000 fires per year,63 and this number continues to increase.64,65 Increasing temperatures contribute to increased 

fire frequency, intensity, and size,63 though at some level of fire 

frequency, increased fire frequency would lead to decreased 

fire intensity. Lightning is a frequent initiator of wildfires,66 and 

the Southeast currently has the greatest frequency of lightning strikes of any region of the country.67 Increasing temperatures and changing atmospheric patterns may affect the number of lightning strikes in the Southeast, which could influence 

air quality, direct injury, and wildfires. Drought often correlates with large wildfire events, as seen with the Okeefenokee 

(2007) and Florida fires (1998). The 1998 Florida fires led to 

Figure 17.10. Ground-level ozone is an air pollutant that is 

harmful to human health and which generally increases with 

rising temperatures. The map shows projected changes in 

average annual ground level ozone pollution concentration 

in 2050 as compared to 2001, using a mid-range emissions 

scenario (A1B, which assumes gradual reductions from current 

emissions trends beginning around mid-century). (Figure 

source: adapted from Tagaris et al. 200942).

Ground-level Ozone
405 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

17: SOUTHEAST AND THE CARIBBEAN

losses of more than $600 million.68 Wildfires also affect human 

health through reduced air quality and direct injuries.68,69,70 Expanding population and associated land-use fragmentation 

will limit the application of prescribed burning, a useful adaptive strategy.65 Growth management could enhance the ability 

to pursue future adaptive management of forest fuels.

Forest disturbances caused by insects and pathogens are altered by climate changes due to factors such as increased tree 

stress, shifting phenology, and altered insect and pathogen 

lifecycles.71 Current knowledge provides limited insights into 

specific impacts on epidemics, associated tree growth and 

mortality, and economic loss in the Southeast, though the 

overall extent and virulence of some insects and pathogens 

have been on the rise (for example, Hemlock Woolly Adelgid 

in the Southern Appalachians), while recent declines in southern pine beetle (Dendroctonus frontalis Zimmerman) epidemics in Louisiana and East Texas have been attributed to rising 

temperatures.72 Due to southern forests’ vast size and the high 

cost of management options, adaptation strategies are limited, 

except through post-epidemic management responses – for 

example, sanitation cuts and species replacement.

The Southeast has the existing power plant capacity to produce 32% of the nation’s electricity.73 Energy use is approximately 27% of the U.S. total, more than any other region.5

 Net 

energy demand is projected to increase, largely due to higher 

temperatures and increased use of air conditioning. This will 

potentially stress electricity generating capacity, distribution 

infrastructure, and energy costs. Energy costs are of particular 

concern for lower income households, the elderly, and other 

vulnerable communities, such as native tribes.5,10 Long periods 

of extreme heat could also damage roadways by softening asphalt and cause deformities of railroad tracks, bridge joints, 

and other transportation infrastructure.74

Increasing temperatures will affect many facets of life in the 

Southeast and Caribbean region. For each impact there could 

be many possible responses. Many adaptation responses are 

described in other chapters in this document. For examples, 

please see the sector chapter of interest and Ch. 28: Adaptation.

Key Message 3: Water Availability

Decreased water availability, exacerbated by population growth and 

land-use change, will continue to increase competition for water 

and affect the region’s economy and unique ecosystems.

Water resources in the Southeast are abundant and support 

heavily populated urban areas, rural communities, unique ecosystems, and economies based on agriculture, energy, and 

tourism. The region also experiences extensive droughts, such 

as the 2007 drought in Atlanta, Georgia, that created water 

conflicts among three states.11,75 In northwestern Puerto Rico, 

water was rationed for more than 200,000 people during the 

winter and spring of 1997-1998 because of low reservoir levels.76 Droughts are one of the most frequent climate hazards 

in the Caribbean, resulting in economic losses.77 Water supply 

and demand in the Southeast and Caribbean are influenced by 

many changing factors, including climate (for example, temperature increases that contribute to increased transpiration 

from plants and evaporation from soils and water bodies), 

population, and land use.4,5 While change in projected precipitation for this region has high uncertainty (Ch. 2: Our Changing 

Climate), there is still a reasonable expectation that there will 

be reduced water availability due to the increased evaporative 

losses resulting from rising temperatures alone.

With projected increases in population, the conversion of rural 

areas, forestlands, and wetlands into residential, commercial, 

industrial, and agricultural zones is expected to intensify.54 The 

continued development of urbanized areas will increase water 

demand, exacerbate saltwater intrusion into freshwater aquifers, and threaten environmentally sensitive wetlands bordering urban areas.24

Additionally, higher sea levels will accelerate saltwater intrusion into freshwater supplies from rivers, streams, and groundwater sources near the coast. The region’s aquaculture industry also may be compromised by climate-related stresses on 

groundwater quality and quantity.78 Porous aquifers in some 

areas make them particularly vulnerable to saltwater intrusion.36,79 For example, officials in the city of Hallandale Beach, 

Florida, have already abandoned six of their eight drinking water wells.80

With increasing demand for food and rising food prices, irrigated agriculture will expand in some states. Also, population 

expansion in the region is expected to increase domestic water demand. Such increases in water demand by the energy, 

agricultural, and urban sectors will increase the competition 

for water, particularly in situations where environmental water 

needs conflict with other uses.5

As seen from Figure 17.11, the net water supply availability in 

the Southeast is expected to decline over the next several decades, particularly in the western part of the region.82 Analysis 

of current and future water resources in the Caribbean shows 
406 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

17: SOUTHEAST AND THE CARIBBEAN

many of the small islands would be exposed to severe water 

stress under all climate change scenarios.83

New freshwater well fields may have to be established inland 

to replenish water supply lost from existing wells closer to the 

ocean once they are 

compromised by saltwater intrusion. Programs to increase water-use efficiency, reuse 

of wastewater, and water storage capacity are 

options that can help 

alleviate water supply 

stress. 

The Southeast and Caribbean, which has a 

disproportionate number of the fastest-growing metropolitan areas in the country and 

important economic 

sectors located in lowlying coastal areas, is 

particularly vulnerable 

to some of the expected impacts of climate 

change. The most severe and widespread 

impacts are likely to 

be associated with sea 

level rise and changes 

in temperature and precipitation, which ultimately affect 

water availability. Changes in land use and land cover, more 

rapid in the Southeast and Caribbean than most other areas 

of the country, often interact with and serve to amplify the 

effects of climate change on regional ecosystems.

Figure 17.11. Left: Projected trend in Southeast-wide annual water yield (equivalent to water availability) due to climate change. The 

green area represents the range in predicted water yield from four climate model projections based on the A1B and B2 emissions 

scenarios. Right: Spatial pattern of change in water yield for 2010-2060 (decadal trend relative to 2010). The hatched areas are 

those where the predicted negative trend in water availability associated with the range of climate scenarios is statistically significant 

(with 95% confidence). As shown on the map, the western part of the Southeast region is expected to see the largest reductions in 

water availability. (Figure source: adapted from Sun et al. 201382).

Trends in Water Availability

Figure 17.12. The Apalachicola-Chattahoochee-Flint River Basin in Georgia 

exemplifies a place where many water 

uses are in conflict, and future climate 

change is expected to exacerbate this 

conflict.84 The basin drains 19,600 square 

miles in three states and supplies water for 

multiple, often competing, uses, including 

irrigation, drinking water and other municipal uses, power plant cooling, navigation, 

hydropower, recreation, and ecosystems. 

Under future climate change, this basin 

is likely to experience more severe water 

supply shortages, more frequent emptying 

of reservoirs, violation of environmental 

flow requirements (with possible impacts to 

fisheries at the mouth of the Apalachicola), 

less energy generation, and more competition for remaining water. Adaptation 

options include changes in reservoir storage and release procedures and possible 

phased expansion of reservoir capacity.84,85 Additional adaptation options could 

include water conservation and demand 

management. (Figure source: Georgakakos et al. 201084).

A Southeast River Basin 

Under Stress
407 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

17: SOUTHEAST AND THE CARIBBEAN

Water recycling

Because of Clayton County, Georgia’s, innovative water 

recycling project during the 2007-2008 drought, they 

were able to maintain reservoirs at near capacity and an 

abundant supply of water while neighboring Lake Lanier, 

the water supply for Atlanta, was at record lows. Clayton 

County developed a series of constructed wetlands used 

to filter treated water that recharges groundwater and 

supplies surface reservoirs. They have also implemented 

efficiency and leak detection programs81 (for additional 

specific information see the Clayton County Water Authority website at: http://www.ccwa.us/).

©Clayton County Water Authority
408 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

17: SOUTHEAST AND THE CARIBBEAN

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Zimmerman, 2001: Confronting Climate Change in the Gulf Coast 

Region: Prospects for Sustaining Our Ecological Heritage, 82 pp., 

Union of Concerned Scientists, Ecological Society of America, 

Cambridge, MA/Washington, D.C. [Available online at http://

www.ucsusa.org/assets/documents/global_warming/gulfcoast.

pdf]

79. Obeysekera, J., M. Irizarry, J. Park, J. Barnes, and T. Dessalegne, 

2011: Climate change and its implications for water resources 

management in south Florida. Stochastic Environmental Research and 

Risk Assessment, 25, 495-516, doi:10.1007/s00477-010-0418-8. 

80. Berry, L., F. Bloetscher, N. Hernández Hammer, M. Koch-Rose, D. 

Mitsova-Boneva, J. Restrepo, T. Root, and R. Teegavarapu, 2011: 

Florida Water Management and Adaptation in the Face of Climate 

Change, 68 pp., Florida Climate Change Task Force. [Available 

online at http://floridaclimate.org/docs/water_managment.pdf]

81. Hewes, W., and K. Pitts, 2009: Natural Security: How Sustainable 

Water Strategies Are Preparing Communities for a Changing 

Climate, 112 pp., American Rivers, Washington, D.C. [Available 

online at http://www.americanrivers.org/assets/pdfs/reports-andpublications/natural-security-report.pdf]

82. Sun, G., S. Arumugam, P. V. Caldwell, P. A. Conrads, A. P. 

Covich, J. Cruise, J. Feldt, A. P. Georgakakos, R. T. McNider, S. 

G. McNulty, D. A. Marion, V. Misra, T. C. Rasmussen, L. Romolo, 

and A. Terando, 2013: Impacts of climate change and variability on 

water resources in the Southeast USA. Climate of the Southeast United 

States: Variability, Change, Impacts, and Vulnerability, K. T. Ingram, K. 

Dow, L. Carter, and J. Anderson, Eds., Island Press, 210-236. 

83. UNEP, 2008: Climate Change in the Caribbean and the Challenge of 

Adaptation, 92 pp., United Nations Environment Programme, 

Regional Office for Latin America and the Caribbean, Panama 

City, Panama. [Available online at http://www.pnuma.org/deat1/

pdf/Climate_Change_in_the_Caribbean_Final_LOW20oct.pdf]

84. Georgakakos, A. P., F. Zhang, and H. Yao, 2010: Climate Variability 

and Change Assessment for the ACF River Basin, Southeast US. 

Georgia Water Resources Institute (GWRI) Technical Report 

sponsored by NOAA, USGS, and Georgia EPD, 321 pp., Georgia 

Institute of Technology, Atlanta, GA. 
17: SOUTHEAST AND THE CARIBBEAN

References

413 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

85. Georgakakos, A., and F. Zhang, 2011: Climate Change Scenario 

Assessment for ACF, OOA, SO, ACT, TN, and OSSS Basins in 

Georgia. Georgia Water Resources Institute (GWRI) Technical 

Report, 229 pp., Georgia Institute of Technology, Atlanta, Georgia, 

USA. 

Georgakakos, K. P., N. E. Graham, F.-Y. Cheng, C. Spencer, 

E. Shamir, A. P. Georgakakos, H. Yao, and M. Kistenmacher, 

2012: Value of adaptive water resources management in northern 

California under climatic variability and change: Dynamic 

hydroclimatology. Journal of Hydrolog y, 412-413, 47-65, doi:10.1016/j.

jhydrol.2011.04.032. 

86. IPCC, 2007: Climate Change 2007: The Physical Science Basis. Contribution 

of Working Group I to the Fourth Assessment Report of the Intergovernmental 

Panel on Climate Change. S. Solomon, D. Qin, M. Manning, Z. Chen, 

M. Marquis, K. B. Averyt, M. Tignor, and H. L. Miller, Eds. 

Cambridge University Press, 996 pp. [Available online at http://

www.ipcc.ch/publications_and_data/publications_ipcc_fourth_

assessment_report_wg1_report_the_physical_science_basis.htm]

87. Caldwell, P. V., G. Sun, S. G. McNulty, E. C. Cohen, and J. A. Moore 

Myers, 2012: Impacts of impervious cover, water withdrawals, and 

climate change on river flows in the Conterminous US. Hydrolog y 

and Earth System Sciences Discussions, 9, 4263-4304, doi:10.5194/

hessd-9-4263-2012. [Available online at http://www.hydrol-earthsyst-sci.net/16/2839/2012/hess-16-2839-2012.pdf]
414 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

17: SOUTHEAST AND THE CARIBBEAN

Process for Developing Key Messages

A central component of the process was the Southeast Regional 

Climate Assessment Workshop that was held on September 26-

27, 2011, in Atlanta, with approximately 75 attendees. This workshop began the process leading to a foundational Technical Input 

Report (TIR). That 341-page foundational “Southeast Region 

Technical Report to the National Climate Assessment”5 comprised 

14 chapters from over 100 authors, including all levels of government, non-governmental organizations, and business. 

The writing team held a 2-day meeting in April 2012 in Ft. Lauderdale, engaged in multiple teleconference and webinar technical discussions, which included careful review of the foundational 

TIR,5 nearly 60 additional technical inputs provided by the public, 

and other published literature and professional judgment. Discussions were followed by expert deliberation of draft key messages 

by the authors, and targeted consultation with additional experts 

by the Southeast chapter writing team and lead author of each 

key message.

Key message #1 Traceable Account

Sea level rise poses widespread and continuing 

threats to both natural and built environments and 

to the regional economy.

Description of evidence base

The key message and supporting text summarize extensive evidence documented in the Southeast Technical Input Report.5

 A 

total of 57 technical inputs on a wide range of southeast-relevant 

topics (including sea level rise) were also received and reviewed 

as part of the Federal Register Notice solicitation for public 

input.

Evidence that the rate of sea level rise has increased is based 

on satellite altimetry data and direct measurements such as tide 

gauges (Ch. 2: Our Changing Climate, Key Message 10). Numerous peer-reviewed publications describe increasing hazards associated with sea level rise and storm surge, heat waves, and intense 

precipitation for the Southeast.5 For sea level rise, the authors 

relied on the NCA Sea Level Change Scenario16 and detailed discussion in the foundational TIR.5

Evidence that sea level rise is a threat to natural and human environments is documented in detail within the foundational TIR5

and other technical inputs, as well as considerable peer-reviewed 

literature (for example, Campanella 2010).19 Field studies document examples of areas that are being flooded more regularly, 

saltwater intrusion into fresh water wells,80 and changes from 

fresh to saltwater in coastal ecosystems (for example, freshwater 

marshes) causing them to die,32 and increases in vulnerability of 

many communities to coastal erosion. Economic impacts are seen 

in the cost to avoid flooded roads, buildings, and ports;23 the need 

to drill new fresh water wells;80 and the loss of coastal ecosystems 

and their storm surge protection. 

New information and remaining uncertainties

Tremendous improvement has been made since the last Intergovernmental Panel on Climate Change evaluation of sea level rise in 

2007,86 with strong evidence of mass loss of Greenland icecap and 

glaciers worldwide (Ch. 2: Our Changing Climate). Improved analyses of tide gauges, coastal elevations, and circulation changes in 

offshore waters have also provided new information on accelerating rates of rise (Ch. 2: Our Changing Climate, Figure 2.26). These 

have been documented in the NCA Sea Level Change Scenario 

publication.16

Uncertainties in the rate of sea level rise through this century 

stems from a combination of large differences in projections 

among different climate models, natural climate variability, uncertainties in the melting of land-based glaciers and the Antarctic and Greenland ice sheets especially, and uncertainties about 

future rates of fossil fuel emissions. A further key uncertainty is 

the rate of vertical land movement at specific locations. The two 

factors – sea level rise and subsidence – when combined, increase 

the impact of global sea level rise in any specific area. A third 

area of uncertainty is where and what adaptive plans and actions 

are being undertaken to avoid flooding and associated impacts on 

people, communities, facilities, infrastructure, and ecosystems.

Assessment of confidence based on evidence 

Sea level is expected to continue to rise for several centuries, even 

if greenhouse gas emissions are stabilized, due to the time it takes 

for the ocean to absorb heat energy from the atmosphere. Because sea levels determine the locations of human activities and 

SUPPLEMENTAL MATERIAL

TRACEABLE ACCOUNTS
17: SOUTHEAST AND THE CARIBBEAN

Traceable Accounts

415 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

ecosystems along the coasts, increases in sea level and in the rate 

of rise will nearly certainly have substantial impacts on natural and 

human systems along the coastal area. What specific locations 

will be impacted under what specific levels of sea level rise needs 

to be determined location-by-location. However, given that many 

locations are already being affected by rising seas, more and more 

locations will be impacted as sea levels continue to rise. Confidence in this key message is therefore judged to be very high. 

 Key message #2 Traceable Account

Increasing temperatures and the associated increase in frequency, intensity, and duration of extreme heat events will affect public health, natural 

and built environments, energy, agriculture, and 

forestry.

Description of evidence base

The key message and supporting text summarize extensive evidence documented in the Southeast Technical Input Report.5

Technical inputs (57) on a wide range of topics were also received 

and reviewed as part of the Federal Register Notice solicitation for 

public input. 

Numerous peer-reviewed publications describe increasing hazards 

associated with heat events and rising temperatures for the Southeast. The authors of a report on the Southeast climate11 worked 

closely with the region’s state climatologists on both the climatology and projections for temperature and associated heat events. 

Evidence of rising temperatures and current impacts38,39 is based 

on an extensive set of field measurements. 

There is considerable evidence of the effects of high air temperatures across a wide range of natural and managed systems in the 

Southeast. Increased temperatures affect human health and hospital admissions.38,40,42

Rising water temperatures also increase risks of bacterial infection 

from eating Gulf Coast shellfish53 and increase algal blooms that 

have negative human health effects.47,48 There is also evidence 

that there will be an increase in favorable conditions for mosquitoes that carry diseases.46 Higher temperatures are detrimental 

to natural and urban environments, through increased wildfires in 

natural areas and managed forests63,64,65,70 and increased invasiveness of some non-native plants.55 High temperatures also contribute to more roadway damage and deformities of transportation 

infrastructure such as railroad tracks and bridges (Ch. 5: Transportation).74 In addition, high temperatures increase net energy 

demand and costs, placing more stress on electricity generating 

plants and distribution infrastructure.

Increasing temperatures in the Southeast cause more stresses on 

crop and livestock agricultural systems. Heat stress reduces dairy 

and livestock production56 and also reduces yields of various crops 

grown in this region (corn, soybean, peanuts, rice, and cotton).60,61

New information and remaining uncertainties

Since 2007, studies on impacts of higher temperatures have increased in many areas. Most of the publications cited above concluded that increasing temperatures in the Southeast will result in 

negative impacts on human health, the natural and built environments, energy, agriculture, and forestry.

A key issue (uncertainty) is the detailed mechanistic responses, 

including adaptive capacities and/or resilience, of natural and 

built environments, the public health system, energy systems, 

agriculture, and forests to increasing temperatures and extreme 

heat events. 

Another uncertainty is how combinations of stresses, for example 

lack of water in addition to extreme heat, will affect outcomes. 

There is a need for more monitoring to document the extent and 

location of vulnerable areas (natural and human), and then research to assess how those impacts will affect productivity of key 

food and forest resources and human well-being. There is also 

a need for research that develops or identifies more resilient, 

adapted systems.

Assessment of confidence based on evidence

Increasing Temperatures: There is high confidence in documentation that projects increases in air temperatures (but not in the precise amount) and associated increases in the frequency, intensity, 

Confidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent results, well documented 

and accepted methods, etc.), 

high consensus

High

Moderate evidence (several 

sources, some consistency, 

methods vary and/or documentation limited, etc.), medium 

consensus

Medium

Suggestive evidence (a few 

sources, limited consistency, 

models incomplete, methods 

emerging, etc.), competing 

schools of thought

Low

Inconclusive evidence (limited sources, extrapolations, 

inconsistent findings, poor documentation and/or methods not 

tested, etc.), disagreement or 

lack of opinions among experts
17: SOUTHEAST AND THE CARIBBEAN

Traceable Accounts

416 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

and duration of extreme heat events. Projections for increases in 

temperature are more certain in the Southeast than projections of 

changes in precipitation. 

Impacts of increasing temperatures: Rising temperatures and the 

substantial increase in duration of high temperatures (for either 

the low [B1] or high [A2] emissions scenarios) above critical 

thresholds will have significant impacts on the population, agricultural industries, and ecosystems in the region. There is high

confidence in documentation that increases in temperature in the 

Southeast will result in higher risks of negative impacts on human 

health, agricultural, and forest production; on natural systems; 

on the built environment; and on energy demand. There is lower 

confidence in the magnitude of these impacts, partly due to lack 

of information on how these systems will adapt (without human 

intervention) or be adapted (by people) to higher temperatures, 

and partly due to the limited knowledge base on the wide diversity 

that exists across this region in climates and human and natural 

systems.

Key message #3 Traceable Account

Decreased water availability, exacerbated by 

population growth and land-use change, will continue to increase competition for water and affect 

the region's economy and unique ecosystems.

Description of evidence base

The key message and supporting text summarize extensive 

evidence documented in the Southeast Technical Input Report 

(TIR).5 Technical inputs (57) on a wide range of topics were also 

received and reviewed as part of the Federal Register Notice solicitation for public input. 

Chapter 2, Our Changing Climate, describes evidence for drought 

and precipitation in its key messages. Numerous salient studies 

support the key message of decreased water availability, as summarized for the Southeast in the TIR.5

Evidence for the impacts on the region’s economy and unique 

ecosystems is also detailed in the TIR5 and the broader literature 

surveyed by the authors.77

New information and remaining uncertainties

Many studies have been published since 2007 documenting increasing demands for water in the Southeast due to increases in 

populations and irrigated agriculture, in addition to water shortages due to extensive droughts.5,11 There is also new evidence of 

losses in fresh water wells near coastlines due to saltwater intrusion79,80 and of continuing conflicts among states for water use, 

particularly during drought periods.5,84

It is a virtual certainty that population growth in the Southeast 

will continue in the future and will be accompanied by a significant change in patterns of land use, which is projected to include 

a larger fraction of urbanized areas, reduced agricultural areas, 

and reduced forest cover.54 With increasing population and human 

demand, competition for water among the agriculture, urban, and 

environment sectors is projected to continue to increase. However, 

the projected population increases for the lower (B1) versus higher 

(A2) emissions scenarios differ significantly (33% versus 151%).11

Consequently, the effect of climate change on urban water demand for the lower emissions scenario is projected to be much 

lower than for that of the higher emissions scenario. Land-use 

change will also alter the regional hydrology significantly. Unless 

measures are adopted to increase water storage, availability of 

freshwater during dry periods will decrease, partly due to drainage 

and other human activities. 

Projected increase in temperature will increase evaporation, and 

in areas (the western part of the region87) where precipitation 

is projected to decrease in response to climate change, the net 

amount of water supply for human and environmental uses may 

decrease significantly. 

Along the coastline of the Southeast, accelerated intrusion of saltwater due to sea level rise will impact both freshwater well fields 

and potentially freshwater intakes in rivers and streams connected 

to the ocean. Although sea level rise (SLR) corresponding to the 

higher emissions scenario is much higher (twice as much), even 

the SLR for the lower emissions scenario will increasingly impact 

water supply availability in low-lying areas of the region, as these 

areas are already being impacted by SLR and land subsidence. 

Projections of specific spatial and temporal changes in precipitation in the Southeast remain highly uncertain and it is important 

to know with a reasonable confidence the sign and the magnitude 

of this change in various parts of the large Southeast region.

For the Southeast, there are no reliable projections of evapotranspiration, another major factor that determines water yield. This 

adds to uncertainty about water availability.

There are inadequate regional studies at basin scales to determine 

the future competition for water supply among sectors (urban, agriculture, and environment).

There is a need for more accurate information on future changes 

in drought magnitude and frequency.

Assessment of confidence based on evidence

There is high confidence in each aspect of the key message: it is 

virtually certain that the water demand for human consumption 

in the Southeast will increase as a result of population growth. 

The past evidence of impacts during droughts and the projected 

changes in drivers (land-use change, population growth, and 
17: SOUTHEAST AND THE CARIBBEAN

Traceable Accounts

417 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

climate change) suggest that there is a high confidence of the 

above assessment of future water availability. However, without 

additional studies, the resilience and the adaptive capacity of the 

socioeconomic and environmental systems are not known.

Water supply is critical for sustainability of the region, particularly 

in view of increasing population and land-use changes. Climate 

models’ precipitation projections are uncertain. Nonetheless, the 

combined effects of possible decreases in precipitation, increasing evaporation losses due to warming, and increasing demands 

for water due to higher populations (under either lower [B1] or 

higher [A2] emissions scenarios) will have a significant impact on 

water availability for all sectors. 
418

Climate Change Impacts in the United States

CHAPTER 18

MIDWEST

INFORMATION DRAWN FROM THIS CHAPTER IS INCLUDED IN THE HIGHLIGHTS REPORT AND IS IDENTIFIED BY THIS ICON

Recommended Citation for Chapter

Pryor, S. C., D. Scavia, C. Downer, M. Gaden, L. Iverson, R. Nordstrom, J. Patz, and G. P. Robertson, 2014: Ch. 18: Midwest. Climate Change Impacts in the United States: The Third National Climate Assessment, J. M. Melillo, Terese (T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 418-440. doi:10.7930/J0J1012N. 

On the Web: http://nca2014.globalchange.gov/report/regions/midwest

Convening Lead Authors

Sara C. Pryor, Indiana University

Donald Scavia, University of Michigan

Lead Authors

Charles Downer, U.S. Army Engineer Research and Development Center

Marc Gaden, Great Lakes Fishery Commission

Louis Iverson, U.S. Forest Service

Rolf Nordstrom, Great Plains Institute

Jonathan Patz, University of Wisconsin

G. Philip Robertson, Michigan State University
419 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Key Messages

18MIDWEST

1. In the next few decades, longer growing seasons and rising carbon dioxide levels will 

increase yields of some crops, though those benefits will be progressively offset by extreme 

weather events. Though adaptation options can reduce some of the detrimental effects, in the 

long term, the combined stresses associated with climate change are expected to decrease 

agricultural productivity. 

2. The composition of the region’s forests is expected to change as rising temperatures drive

habitats for many tree species northward. The role of the region’s forests as a net absorber of 

carbon is at risk from disruptions to forest ecosystems, in part due to climate change. 

3. Increased heat wave intensity and frequency, increased humidity, degraded air quality, and 

reduced water quality will increase public health risks. 

4. The Midwest has a highly energy-intensive economy with per capita emissions of greenhouse

gases more than 20% higher than the national average. The region also has a large and 

increasingly utilized potential to reduce emissions that cause climate change. 

5. Extreme rainfall events and flooding have increased during the last century, and these trends 

are expected to continue, causing erosion, declining water quality, and negative impacts on 

transportation, agriculture, human health, and infrastructure.

6. Climate change will exacerbate a range of risks to the Great Lakes, including changes in the range 

and distribution of certain fish species, increased invasive species and harmful blooms of algae, 

and declining beach health. Ice cover declines will lengthen the commercial navigation season.

The Midwest has a population of more than 61 million people 

(about 20% of the national total) and generates a regional 

gross domestic product of more than $2.6 trillion (about 19% 

of the national total).1 The Midwest is home to expansive agricultural lands, forests in the north, the Great Lakes, substantial 

industrial activity, and major urban areas, including eight of the 

nation’s 50 most populous cities. The region has experienced 

shifts in population, socioeconomic changes, air and water 

pollution, and landscape changes, and exhibits multiple vulnerabilities to both climate variability and climate change. 

In general, climate change will tend to amplify existing climaterelated risks from climate to people, ecosystems, and infrastructure in the Midwest (Ch. 10: Energy, Water, and Land). 

Direct effects of increased heat stress, flooding, drought, and 

late spring freezes on natural and managed ecosystems may 

be multiplied by changes in pests and disease prevalence, increased competition from non-native or opportunistic native 

species, ecosystem disturbances, land-use change, landscape 

fragmentation, atmospheric pollutants, and economic shocks 

such as crop failures or reduced yields due to extreme weather 

events. These added stresses, when taken collectively, are 

projected to alter the ecosystem and socioeconomic patterns 

and processes in ways that most people in the region would 

consider detrimental. Much of the region’s fisheries, recreation, tourism, and commerce depend on the Great Lakes and 

expansive northern forests, which already face pollution and 

invasive species pressure that will be exacerbated by climate 

change.

Most of the region’s population lives in cities, which are particularly vulnerable to climate change related flooding and lifethreatening heat waves because of aging infrastructure and 

other factors. Climate change may also augment or intensify 

other stresses on vegetation encountered in urban environments, including increased atmospheric pollution, heat island 

effects, a highly variable water cycle, and frequent exposure to 

new pests and diseases. Some cities in the region are already 

engaged in the process of capacity building or are actively 

building resilience to the threats posed by climate change. The 

region’s highly energy-intensive economy emits a disproportionately large amount of the gases responsible for warming 
18: MIDWEST

420 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

the climate (called greenhouse gases or heat-trapping gases). 

But as discussed below, it also has a large and increasingly realized potential to reduce these emissions.

The rate of warming in the Midwest has markedly accelerated 

over the past few decades. Between 1900 and 2010, the average Midwest air temperature increased by more than 1.5°F 

(Figure 18.1). However, between 1950 and 2010, the average 

temperature increased twice as quickly, and between 1980 and 

2010, it increased three times as quickly as it did from 1900 to 

2010.1 Warming has been more rapid at night and during winter. These trends are consistent with expectations of increased 

concentrations of heat-trapping gases and observed changes 

in concentrations of certain particles in the atmosphere.1,2

The amount of future warming will depend on changes in the 

atmospheric concentration of heat-trapping gases. Projections 

for regionally averaged temperature increases by the middle 

of the century (2046-2065) relative to 1979-2000 are approximately 3.8°F for a scenario with substantial emissions reductions (B1) and 4.9°F with continued growth in global emissions 

(A2). The projections for the end of the century (2081-2100) 

are approximately 5.6°F for the lower emissions scenario and 

8.5°F for the higher emissions scenario (see Ch. 2: Our Changing Climate, Key Message 3).3

In 2011, 11 of the 14 U.S. weather-related disasters with damages of more than $1 billion affected the Midwest.5

 Several types 

of extreme weather events have already increased in frequency 

and/or intensity due to climate change, and further increases 

are projected (Ch. 2: Our Changing Climate, Key Message 7).6

Key Message 1: Impacts to Agriculture

In the next few decades, longer growing seasons and rising carbon dioxide levels will 

increase yields of some crops, though those benefits will be progressively offset by 

extreme weather events. Though adaptation options can reduce some of the detrimental 

effects, in the long term, the combined stresses associated with climate change 

are expected to decrease agricultural productivity. 

Agriculture dominates Midwest land use, with more than twothirds of land designated as farmland.3

 The region accounts 

for about 65% of U.S. corn and soybean production,7 mostly 

from non-irrigated lands.1 Corn and soybeans constitute 85% 

of Midwest crop receipts, with high-value crops such as fruits 

and vegetables making up most of the remainder.8 Corn and 

soybean yields increased markedly (by a factor of more than 5) 

over the last century largely due to technological innovation, 

but are still vulnerable to year-to-year variations in weather 

conditions.9

The Midwest growing season lengthened by almost two weeks 

since 1950, due in large part to earlier occurrence of the last 

spring freeze.10 This trend is expected to continue,3,11 though 

the potential agricultural consequences are complex and 

vary by crop. For corn, small long-term average temperature 

increases will shorten the duration of reproductive development, leading to yield declines,12 even when offset by carbon 

dioxide (CO2) stimulation.13 For soybeans, yields have a two in 

three chance of increasing early in this century due to CO2 fertilization, but these increases are projected to be offset later in 

the century by higher temperature stress14 (see Figure 18.2 for 

projections of increases in the frost-free season length and the 

number of summer days with temperatures over 95°F).

Future crop yields will be more strongly influenced by anomalous weather events than by changes in average temperature 

or annual precipitation (Ch. 6: Agriculture). Cold injury due to 

a freeze event after plant budding can decimate fruit crop production,15 as happened in 2002, and again in 2012, to Michigan’s $60 million tart cherry crop. Springtime cold air outbreaks 

(at least two consecutive days during which the daily average 

surface air temperature is below 95% of the simulated average 

wintertime surface air temperature) are projected to continue 

to occur throughout this century.16 As a result, increased productivity of some crops due to higher temperatures, longer 

growing seasons, and elevated CO2 concentrations could be 

offset by increased freeze damage.17 Heat waves during polFigure 18.1. Annual average temperatures (red line) across 

the Midwest show a trend towards increasing temperature. 

The trend (dashed line) calculated over the period 1895-2012 

is equal to an increase of 1.5°F. (Figure source: updated from 

Kunkel et al. 20134).

Temperatures are Rising in the Midwest
18: MIDWEST

421 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

lination of field crops such as corn and soybean also 

reduce yields (Figure 18.3).12 Wetter springs may reduce crop yields and profits,18 especially if growers 

are forced to switch to late-planted, shorter-season 

varieties. A recent study suggests the volatility of 

U.S. corn prices is more sensitive to near-term climate change than to energy policy influences or to 

use of agricultural products for energy production, 

such as biofuel.19

Agriculture is responsible for about 8% of U.S. heattrapping gas emissions,20 and there is tremendous 

potential for farming practices to reduce emissions 

or store more carbon in soil.21 Although large-scale 

agriculture in the Midwest historically led to decreased carbon in soils, higher crop residue inputs 

and adoption of different soil management techniques have reversed this trend. Other techniques, 

such as planting cover crops and no-till soil management, can further increase CO2 uptake and reduce 

energy use.22,23 Use of agricultural best management practices can also improve water quality by 

reducing the loss of sediments and nutrients from 

farm fields. Methane released from animals and 

their wastes can be reduced by altered diets and 

methane capture systems, and nitrous oxide production can be reduced by judicious fertilizer use24

and improved waste handling.21 In addition, if biofuel crops are grown sustainably,25 they offer emissions reduction opportunities by substituting for 

fossil fuel-based energy (Ch. 10: Energy, Water, and 

Land). 

Figure 18.2. Projected increase in annual average temperatures (top left) 

by mid-century (2041-2070) as compared to the 1971-2000 period tell 

only part of the climate change story. Maps also show annual projected 

increases in the number of the hottest days (days over 95°F, top right), 

longer frost-free seasons (bottom left), and an increase in cooling degree 

days (bottom right), defined as the number of degrees that a day’s average 

temperature is above 65°F, which generally leads to an increase in energy 

use for air conditioning. Projections are from global climate models that 

assume emissions of heat-trapping gases continue to rise (A2 scenario). 

(Figure source: NOAA NCDC / CICS-NC). 

Projected Mid-Century Temperature Changes 

in the Midwest

Figure 18.3. Crop yields are very sensitive to temperature and rainfall. They are especially sensitive to high temperatures during the 

pollination and grain filling period. For example, corn (left) and soybean (right) harvests in Illinois and Indiana, two major producers, 

were lower in years with average maximum summer (June, July, and August) temperatures higher than the average from 1980 to 

2007. Most years with below-average yields are both warmer and drier than normal.26,27 There is high correlation between warm and 

dry conditions during Midwest summers28 due to similar meteorological conditions and drought-caused changes.29 (Figure source: 

Mishra and Cherkauer 201026).

Crop Yields Decline under Higher Temperatures
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422 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Key Message 2: Forest Composition

The composition of the region’s forests is expected to change as rising temperatures drive 

habitats for many tree species northward. The role of the region’s forests as a net absorber 

of carbon is at risk from disruptions to forest ecosystems, in part due to climate change. 

The Midwest is characterized by a rich diversity of native species juxtaposed on one of the world’s most productive agricultural systems.30 The remnants of intact natural ecosystems in 

the region,31 including prairies, forests, streams, and wetlands, 

are rich with varied species.32 The combined effects of climate 

change, land-use change, and increasing numbers of invasive 

species are the primary threats to Midwest natural ecosystems.33 Species most vulnerable to climate change include 

those that occur in isolated habitats; live near their physiological tolerance limits; have specific habitat requirements, low 

reproductive rates, or limited dispersal capability; are dependent on interactions with specific other species; and/or have 

low genetic variability.34

Among the varied ecosystems of the region, forest systems 

are particularly vulnerable to multiple stresses. The habitat 

ranges of many iconic tree species such as paper birch, quaking aspen, balsam fir, and black spruce are projected to decline 

substantially across the northern Midwest as they shift northward, while species that are common farther south, including 

several oaks and pines, expand their ranges northward into 

the region (Figure 18.4).35,36 There is considerable variability in 

the likelihood of a species’ habitat changing and the adaptability of the species with regard to climate change.37 Migration 

to accommodate changed habitat is expected to be slow for 

many Midwest species, due to relatively flat topography, high 

latitudes, and fragmented habitats including the Great Lakes 

barrier. To reach areas that are 1.8°F cooler, species in mountainous terrains need to shift 550 feet higher in altitude (which 

can be achieved in only a few miles), whereas species in flat 

terrain like the Midwest must move as much as 90 miles north 

to reach a similarly cooler habitat.38

Although global forests currently capture and store more carbon each year than they emit,39 the ability of forests to act as 

large, global carbon absorbers (“sinks”) may be reduced by 

projected increased disturbances from insect outbreaks,40 forest fire,41 and drought,42 leading to increases in tree mortality and carbon emissions. Some regions may even shift from 

being a carbon sink to being an atmospheric carbon dioxide 

source,43,44 though large uncertainties exist, such as whether 

projected disturbances to forests will be chronic or episodic.45

Midwest forests are more resilient to forest carbon losses than 

most western forests because of relatively high moisture availability, greater nitrogen deposition (which tends to act as a 

fertilizer), and lower wildfire risk.43,46

Forest Composition Shifts

Figure 18.4. As climate changes, species can often adapt by changing their ranges. Maps show current and projected future 

distribution of habitats for forest types in the Midwest under two emissions scenarios, a lower scenario that assumes reductions 

in heat-trapping gas emissions (B1), and a very high scenario that assumes continued increases in emissions (A1FI). Habitats for 

white/red/jack pine, maple/beech/birch, spruce/fir, and aspen/birch forests are projected to greatly decline from the northern forests, 

especially under higher emissions scenarios, while various oak forest types are projected to expand.37 While some forest types 

may not remain dominant, they will still be present in reduced quantities. Therefore, it is more appropriate to assess changes on an 

individual species basis, since all species within a forest type will not exhibit equal responses to climate change. (Figure source: 

Prasad et al. 200737). 
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423 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Key Message 3: Public Health Risks

Increased heat wave intensity and frequency, increased humidity, 

degraded air quality, and reduced water quality will increase public health risks. 

The frequency of major heat waves in the Midwest has increased over the last six decades.47 For the United States, mortality increases 4% during heat waves compared with non-heat 

wave days.48 During July 2011, 132 million people across the 

U.S. were under a heat alert – and on July 20 of that year, the 

majority of the Midwest experienced temperatures in excess 

of 100°F. Heat stress is projected to increase as a result of both 

increased summer temperatures and humidity.49,50 One study 

projected an increase of between 166 and 2,217 excess deaths 

per year from heat wave-related mortality in Chicago alone by 

2081-2100.51 The lower number assumes a climate scenario 

with significant reductions in emissions of greenhouse gases 

(B1), while the upper number assumes a scenario under which 

emissions continue to increase (A2). These projections are significant when compared to recent Chicago heat waves, where 

114 people died from the heat wave of 1999 and about 700 

died from the heat wave of 1995.52 Heat response plans and 

early warning systems save lives, and from 1975 to 2004, mortality rates per heat event declined.53 However, many municipalities lack such plans.54

More than 20 million people in the Midwest experience air 

quality that fails to meet national ambient air quality standards.1

 Degraded air quality due to human-induced emissions55 and increased pollen season duration56 are projected 

to be amplified with higher temperatures,57 and pollution and 

pollen exposures, in addition to heat waves, can harm human 

health (Ch. 9: Human Health). Policy options exist (for example, 

see “Alternative Transportation Options Create Multiple Benefits”) that could reduce emissions of both heat-trapping gases 

and other air pollutants, yielding benefits for human health 

and fitness. Increased temperatures and changes in precipitation patterns could also increase the vulnerability of Midwest 

residents to diseases carried by insects and rodents (Ch. 9: Human Health).58

Alternative transportation options create multiple benefits

Figure 18.5. Annual reduction in the number of premature deaths (left) and annual change in the number of cases with acute 

respiratory symptoms (right) due to reductions in particulate matter and ozone caused by reducing automobile exhaust. 

The maps project health benefits if automobile trips shorter than five miles (round-trip) were eliminated for the 11 largest 

metropolitan areas in the Midwest. Making 50% of these trips by bicycle just during four summer months would save 1,295 

lives and yield savings of more than $8 billion per year from improved air quality, avoided mortality, and reduced health care 

costs for the upper Midwest alone. (Figure source: Grabow et al. 2012; reproduced with permission from Environmental 

Health Perspectives59). 

The transportation sector produces one-third of U.S. greenhouse gas emissions, and automobile exhaust also contains 

precursors to fine particulate matter (PM2.5) and ground-level ozone (O3), which pose threats to public health. Adopting 

a low-carbon transportation system with fewer automobiles, therefore, could have immediate health “co-benefits” of 

both reducing climate change and improving human health via both improved air quality and physical fitness. 

Reducing Emissions, Improving Health
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424 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Key Message 4: Fossil-Fuel Dependent Electricity System

The Midwest has a highly energy-intensive economy with per capita emissions of greenhouse 

gases more than 20% higher than the national average. The region also has a large and 

increasingly utilized potential to reduce emissions that cause climate change. 

The Midwest is a major exporter of electricity to other U.S. regions and has a highly energy-intensive economy (Ch. 10: Energy, Water, and Land, Figure 10.4). Energy use per dollar of gross 

domestic product is approximately 20% above the national 

average, and per capita greenhouse gas emissions are 22% 

higher than the national average due, in part, to the reliance on 

fossil fuels, particularly coal for electricity generation.1 A large 

range in seasonal air temperature causes energy demand for 

both heating and cooling, with the highest demand for winter 

heating. The demand for heating in major midwestern cities is 

typically five to seven times that for cooling,1 although this is 

expected to shift as a result of longer summers, more frequent 

heat waves, and higher humidity, leading to an increase in the 

number of cooling degree days. This increased demand for 

cooling by the middle of this century is projected to exceed 10 

gigawatts (equivalent to at least five large conventional power 

plants), requiring more than $6 billion in infrastructure investments.60 Further, approximately 95% of the electrical generating infrastructure in the Midwest is susceptible to decreased 

efficiency due to higher temperatures.60

Climate change presents the Midwest’s energy sector with a 

number of challenges, in part because of its current reliance on 

coal-based electricity1 and an aging, less-reliable electric distribution grid61 that will require significant reinvestment even 

without additional adaptations to climate change.62

Increased use of natural gas in the Midwest has the potential 

to reduce emissions of greenhouse gases. The Midwest also 

has potential to produce energy from zero- and low-carbon 

sources, given its wind, solar, and biomass resources, and 

potential for expanded nuclear power. The Midwest does not 

have the highest solar potential in the country (that is found 

in the Southwest), but its potential is nonetheless vast, with 

some parts of the Midwest having as good a solar resource as 

Florida.63 More than one-quarter of national installed wind energy capacity, one-third of biodiesel capacity, and more than 

two-thirds of ethanol production are located in the Midwest 

(see also Ch. 4: Energy and Ch. 10: Energy, Water, and Land).1

Progress toward increasing renewable energy is hampered by 

electricity prices that are distorted through a mix of direct and 

indirect subsidies and unaccounted-for costs for conventional 

energy sources.64

Key Message 5: Increased Rainfall and Flooding

Extreme rainfall events and flooding have increased during the last century, and these trends 

are expected to continue, causing erosion, declining water quality, and negative impacts 

on transportation, agriculture, human health, and infrastructure.

Precipitation in the Midwest is greatest in the east, declining 

towards the west. Precipitation occurs about once every seven 

days in the western part of the region and once every three 

days in the southeastern part.65 The 10 rainiest days can contribute as much as 40% of total precipitation in a given year.65

Generally, annual precipitation increased during the past 

century (by up to 20% in some locations), with much of the 

increase driven by intensification of the heaviest rainfalls.65,66

This tendency towards more intense precipitation events is 

projected to continue in the future.67

Model projections for precipitation changes are less certain 

than those for temperature.3,4 Under a higher emissions scenario (A2), global climate models (GCMs) project average winter and spring precipitation by late this century (2071-2099) to 

increase 10% to 20% relative to 1971-2000, while changes in 

summer and fall are not expected to be larger than natural variations. Projected changes in annual precipitation show increases larger than natural variations in the north and smaller in the 

south (Ch. 2: Our Changing Climate, Key Message 5).4 Regional 

climate models (RCMs) using the same emissions scenario also 

project increased spring precipitation (9% in 2041-2062 relative to 1979-2000) and decreased summer precipitation (by an 

average of about 8% in 2041-2062 relative to 1979-2000) particularly in the southern portions of the Midwest.3

 Increases 

in the frequency and intensity of extreme precipitation are 

projected across the entire region in both GCM and RCM simulations (Figure 18.6), and these increases are generally larger 

than the projected changes in average precipitation.3,4

Flooding can affect the integrity and diversity of aquatic ecosystems. Flooding also causes major human and economic consequences by inundating urban and agricultural land and by disrupting navigation in the region’s roads, rivers, and reservoirs 

(see Ch. 5: Transportation, Ch. 9: Human Health, and Ch. 11: 

Urban). For example, the 2008 flooding in the Midwest caused 

24 deaths, $15 billion in losses via reduced agricultural yields, 

and closure of key transportation routes.1 Water infrastructure 

for flood control, navigation, and other purposes is susceptible 

to climate change impacts and other forces because the de-
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425 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

signs are based upon historical patterns of precipitation and 

streamflow, which are no longer appropriate guides.

Snowfall varies across the region, comprising less than 10% of 

total precipitation in the south, to more than half in the north, 

with as much as two inches of water available in the snowpack 

at the beginning of spring melt in the northern reaches of the 

river basins.68 When this amount of snowmelt is combined 

with heavy rainfall, the resulting flooding can be widespread 

and catastrophic (see “Cedar Rapids: A Tale of Vulnerability 

and Response”).69 Historical observations indicate declines in 

the frequency of high magnitude snowfall years over much of 

the Midwest,70 but an increase in lake effect snowfall.71 These 

divergent trends and their inverse relationships with air temperatures make overall projections of regional impacts of the associated snowmelt 

extremely difficult. Large-scale flooding 

can also occur due to extreme precipitation 

in the absence of snowmelt (for example, 

Rush Creek and the Root River, Minnesota, 

in August 2007 and multiple rivers in southern Minnesota in September 2010).72 These 

warm-season events are projected to increase in magnitude. Such events tend to 

be more regional and less likely to cover as 

large an area as those that occur in spring, 

in part because soil water storage capacity 

is typically much greater during the summer. 

Changing land use and the expansion of 

urban areas are reducing water infiltration into the soil and increasing surface 

runoff. These changes exacerbate impacts 

caused by increased precipitation intensity. 

Many major Midwest cities are served by 

combined storm and sewage drainage systems. As surface area has been increasingly 

converted to impervious surfaces (such as 

asphalt) and extreme precipitation events 

have intensified, combined sewer overflow 

has degraded water quality, a phenomenon 

expected to continue to worsen with increased urbanization and climate change.75

The U.S. Environmental Protection Agency 

(EPA) estimates there are more than 800 

billion gallons of untreated combined sewage released into the nation’s waters annually.76 The Great Lakes, which provide drinking water to more than 40 million people 

and are home to more than 500 beaches,75

have been subject to recent sewage overflows. For example, stormwater across the 

city of Milwaukee recently showed high human fecal pathogen levels at all 45 outflow 

locations, indicating widespread sewage contamination.77 One 

study estimated that increased storm events will lead to an increase of up to 120% in combined sewer overflows into Lake 

Michigan by 2100 under a very high emissions scenario (A1FI),75

leading to additional human health issues and beach closures. 

Municipalities may be forced to invest in new infrastructure 

to protect human health and water quality in the Great Lakes, 

and local communities could face tourism losses from fouled 

nearshore regions.

Increased precipitation intensity also increases erosion, damaging ecosystems and increasing delivery of sediment and subsequent loss of reservoir storage capacity. Increased storminduced agricultural runoff and rising water temperatures 

When it Rains, it Pours

Figure 18.6. Precipitation patterns affect many aspects of life, from agriculture 

to urban storm drains. These maps show projected changes for the middle of the 

current century (2041-2070) relative to the end of the last century (1971-2000) 

across the Midwest under continued emissions (A2 scenario). Top left: the changes 

in total annual average precipitation. Across the entire Midwest, the total amount 

of water from rainfall and snowfall is projected to increase. Top right: increase in 

the number of days with very heavy precipitation (top 2% of all rainfalls each year). 

Bottom left: increases in the amount of rain falling in the wettest 5-day period over 

a year. Both (top right and bottom left) indicate that heavy precipitation events will 

increase in intensity in the future across the Midwest. Bottom right: change in the 

average maximum number of consecutive days each year with less than 0.01 inches 

of precipitation. An increase in this variable has been used to indicate an increase 

in the chance of drought in the future. (Figure source: NOAA NCDC / CICS-NC).
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426 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

have increased non-point source pollution problems in recent 

years.78 This has led to increased phosphorus and nitrogen 

loading, which in turn is contributing to more and prolonged 

occurrences of low-oxygen “dead zones” and to harmful, 

lengthy, and dense algae growth in the Great Lakes and other 

Midwest water bodies.79 (Such zones and their causes are also 

discussed in Ch. 25: Coasts, Ch. 15: Biogeochemical Cycles, and 

Ch. 3: Water, Key Message 6). Watershed planning can be used 

to reduce water quantity and quality problems due to changing 

climate and land use.

While there was no apparent change in drought duration in the 

Midwest region as a whole over the past century,80 the average 

number of days without precipitation is projected to increase 

in the future. This could lead to agricultural drought and suppressed crop yields.9

 This would also increase thermoelectric 

power plant cooling water temperatures and decrease cooling 

efficiency and plant capacity because of the need to avoid discharging excessively warm water (see also Ch. 4: Energy, and 

Ch. 10: Energy, Water, and Land).60

Key Message 6: Increased Risks to the Great Lakes

Climate change will exacerbate a range of risks to the Great Lakes, including changes 

in the range and distribution of certain fish species, increased invasive species and 

harmful blooms of algae, and declining beach health. Ice cover declines 

will lengthen the commercial navigation season.

The Great Lakes, North America’s largest freshwater feature, 

have recently recorded higher water temperatures and less 

ice cover as a result of changes in regional climate (see also 

Ch. 2: Our Changing Climate, Key Message 11). Summer surface water temperatures in Lakes Huron increased 5.2°F and 

in Lake Ontario, 2.7°F, between 1968 and 2002,81 with smaller 

increases in Lake Erie.81,82 Due to the reduction in ice cover, 

the temperature of surface waters in Lake Superior during the 

summer increased 4.5°F, twice the rate of increase in air temperature.83 These lake surface temperatures are projected to 

rise by as much as 7°F by 2050 and 12.1°F by 2100.84,85 Higher 

temperatures, increases in precipitation, and lengthened 

growing seasons favor production of blue-green and toxic algae that can harm fish, water quality, habitats, and aesthetics,79,84,86 and could heighten the impact of invasive species 

already present.87

In the Great Lakes, the average annual maximum ice coverage 

during 2003-2013 was less than 43% compared to the 1962-

2013 average of 52%,88 lower than any other decade during 

the period of measurements (Figure 18.7), although there is 

substantial variability from year to year. During the 1970s, 

which included several extremely cold winters, maximum ice 

coverage averaged 67%. Less ice, coupled with more frequent 

and intense storms (as indicated by some analyses of historical 

wind speeds),89 leaves shores vulnerable to erosion and flooding and could harm property and fish habitat.84,90 Reduced ice 

cover also has the potential to lengthen the shipping season.91

The navigation season increased by an average of eight days 

between 1994 and 2011, and the Welland Canal in the St. Lawrence River remained open nearly two weeks longer. Increased 

shipping days benefit commerce but could also increase shoreline scouring and bring in more invasive species.91,92

Cedar rapids: a tale of vulnerability and response

Cedar Rapids, Des Moines, Iowa City, and Ames, Iowa, have all suffered 

multi-million-dollar losses from floods since 1993. In June 2008, a record 

flood event exceeded the once-in-500-year flood level by more than 5 feet, 

causing $5 to $6 billion in damages from flooding, or more than $40,000 

per resident of the city of Cedar Rapids.73 The flood inundated much of the 

downtown, damaging more than 4,000 structures, including 80% of government offices, and displacing 25,000 people.74 The record flood at Cedar 

Rapids was the result of low reservoir capacity and extreme rainfall on soil 

already saturated from unusually wet conditions. Rainfall amounts comparable to those in 1993 (8 inches over two weeks) overwhelmed a flood 

control system designed largely for a once-in-100-year flood event. Such 

events are consistent with observations and projections of wetter springs 

and more intense precipitation events (see Figure 18.6). With the help of 

more than $3 billion in funding from the federal and state government, 

Cedar Rapids is recovering and has taken significant steps to reduce future 

flood damage, with buyouts of more than 1,000 properties, and numerous 

buildings adapted with flood protection measures.

©American Red Cross/Flickr
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427 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Changes in lake levels can also influence the amount of cargo 

that can be carried on ships. On average, a 1000-foot ship sinks 

into the water by one inch per 270 tons of cargo;93 thus if a ship 

is currently limited by water depth, any lowering of lake levels 

will result in a proportional reduction in the amount of cargo 

that it can transport to Great Lakes ports. However, current 

estimates of lake level changes are uncertain, even for continued increases in global greenhouse gas emissions (A2 scenario). The most recent projections suggest a slight decrease or 

even a small rise in levels.94 Recent studies have also indicated 

that earlier approaches to computing evapotranspiration estimates from temperature may have overestimated evaporation 

losses.94,95,96,97 The recent studies, along with the large spread 

in existing modeling results, indicate that projections of Great 

Lakes water levels represent evolving research and are still 

subject to considerable uncertainty (see Appendix 3: Climate 

Science Supplemental Message 8).

Figure 18.7. Bars show decade averages of annual maximum 

Great Lakes ice coverage from the winter of 1962-1963, when 

reliable coverage of the entire Great Lakes began, to the winter 

of 2012-2013. Bar labels indicate the end year of the winter; for 

example, 1963-1972 indicates the winter of 1962-1963 through 

the winter of 1971-1972. The most recent period includes the 

eleven years from 2003 to 2013. (Data updated from Bai and 

Wang, 201288).

Ice Cover in the Great Lakes
428 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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92. Hellmann, J. J., J. E. Byers, B. G. Bierwagen, and J. S. Dukes, 

2008: Five potential consequences of climate change for invasive 

species. Conservation Biolog y, 22, 534-543, doi:10.1111/j.1523-

1739.2008.00951.x. [Available online at http://onlinelibrary.wiley.

com/doi/10.1111/j.1523-1739.2008.00951.x/pdf] 

Smith, A. L., N. Hewitt, N. Klenk, D. R. Bazely, N. Yan, S. Wood, 

I. Henriques, J. I. MacLellan, and C. Lipsig-Mummé, 2012: Effects 

of climate change on the distribution of invasive alien species in 

Canada: A knowledge synthesis of range change projections in a 

warming world. Environmental Reviews, 20, 1-16, doi:10.1139/a11-

020. 

93. Sousounis, P., and J. M. Bisanz, Eds., 2000: Preparing for a Changing 

Climate. The Potential Consequences of Climate Variability and Change: 

Great Lakes. University of Michigan, Atmospheric, Oceanic and 

Space Sciences Department, 116 pp. [Available online at http://

www.gcrio.org/NationalAssessment/greatlakes/greatlakes.pdf]

94. IUGLSB, 2012: Lake Superior Regulation: Addressing Uncertainty 

in Upper Great Lakes Water Levels. Final Report to the International 

Joint Commission. March 2012, 236 pp., International Upper Great 

Lakes Study Board, Ottawa, ON [Available online at http://www.

ijc.org/iuglsreport/wp-content/report-pdfs/Lake_Superior_

Regulation_Full_Report.pdf]

95. Angel, J. R., and K. E. Kunkel, 2010: The response of Great 

Lakes water levels to future climate scenarios with an emphasis 

on Lake Michigan-Huron. Journal of Great Lakes Research, 36, 51-58, 

doi:10.1016/j.jglr.2009.09.006. 

96. Hayhoe, K., J. VanDorn, T. Croley, II, N. Schlegal, and D. 

Wuebbles, 2010: Regional climate change projections for Chicago 

and the US Great Lakes. Journal of Great Lakes Research, 36, 7-21, 

doi:10.1016/j.jglr.2010.03.012. [Available online at http://www.

bioone.org/doi/pdf/10.1016/j.jglr.2010.03.012]

97. MacKay, M., and F. Seglenieks, 2012: On the simulation of 

Laurentian Great Lakes water levels under projections of global 

climate change. Climatic Change, 117, 55-67, doi:10.1007/s10584-

012-0560-z. 

Lofgren, B. M., T. S. Hunter, and J. Wilbarger, 2011: Effects of 

using air temperature as a proxy for potential evapotranspiration in 

climate change scenarios of Great Lakes basin hydrology. Journal of 

Great Lakes Research, 37, 744-752, doi:10.1016/j.jglr.2011.09.006. 

98.	 Winkler, J., J. Andresen, and J. Hatfield, Eds., 2012: Midwest Technical 

Input Report: Prepared for the US National Climate Assessment. 236 pp. 

99. Zhang, F., J. M. Chen, Y. Pan, R. A. Birdsey, S. Shen, W. Ju, and L. 

He, 2012: Attributing carbon changes in conterminous US forests 

to disturbance and non-disturbance factors from 1901 to 2010. 

Journal of Geophysical Research, 117, doi:10.1029/2011JG001930. 

100. Pryor, S. C., Ed., 2013: Climate Change in the Midwest: Impacts, Risks, 

Vulnerability and Adaptation. Indiana University Press, 288 pp. 

101. Wang, J., X. Bai, H. Hu, A. Clites, M. Colton, and B. Lofgren, 2012: 

Temporal and spatial variability of Great Lakes ice cover, 1973-

2010. Journal of Climate, 25, 1318-1329, doi:10.1175/2011JCLI4066.1. 

102. Karl, T. R., J. T. Melillo, and T. C. Peterson, Eds., 2009: Global 

Climate Change Impacts in the United States. Cambridge University 

Press, 189 pp. [Available online at http://downloads.globalchange.

gov/usimpacts/pdfs/climate-impacts-report.pdf]

Photo Credits

Introduction to chapter; midwest farm in top banner: ©iStock.com/

Georga_Burga
435 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

18: MIDWEST

SUPPLEMENTAL MATERIAL

TRACEABLE ACCOUNTS

Process for Developing Key Messages: 

The assessment process for the Midwest Region began with a 

workshop was that was held July 25, 2011, in Ann Arbor, Michigan. Ten participants discussed the scope and authors for a foundational Technical Input Report (TIR) report entitled “Midwest 

Technical Input Report.”98 The report, which consisted of nearly 

240 pages of text organized into 13 chapters, was assembled by 

23 authors representing governmental agencies, non-governmental organizations (NGOs), tribes, and other entities. 

The Chapter Author Team engaged in multiple technical discussions via teleconferences that permitted a careful review of the 

foundational TIR98 and of approximately 45 additional technical 

inputs provided by the public, as well as the other published literature, and professional judgment. The Chapter Author Team 

convened teleconferences and exchanged extensive emails to define the scope of the chapter for their expert deliberation of input 

materials and to generate the chapter text and figures. Each expert drafted key messages, initial text and figure drafts and traceable accounts that pertained to their individual fields of expertise. 

These materials were then extensively discussed by the team and 

were approved by the team members. 

Key message #1 Traceable Account

In the next few decades, longer growing seasons and rising carbon dioxide levels will increase 

yields of some crops, though those benefits will be 

progressively offset by extreme weather events. 

Though adaptation options can reduce some of the 

detrimental effects, in the long term, the combined 

stresses associated with climate change are expected to decrease agricultural productivity. 

Description of evidence base

The key message and supporting text summarize extensive evidence documented in the Technical Input Report.98 Technical 

input reports on a wide range of topics were also received and 

reviewed as part of the Federal Register Notice solicitation for 

public input. 

Evidence for altered growing seasons across the U.S. are discussed in Chapter 2 (Our Changing Climate, Key Message 4) and 

its Traceable Accounts. “Climate Trends and Scenarios for the 

U.S. National Climate Assessment”4 and its references provide 

specific details for the Midwest. Evidence for longer growing seasons in the Midwest is based on regional temperature records and 

is incontrovertible, as is evidence for increasing carbon dioxide 

concentrations.

U.S. Department of Agriculture data tables provide evidence for 

the importance of the eight Midwest states for U.S. agricultural 

production.8 Evidence for the effect of future elevated carbon dioxide concentrations on crop yields is based on scores of greenhouse 

and field experiments that show a strong fertilization response 

for C3 plants such as soybeans and wheat and a positive but not 

as strong a response for C4 plants such as corn. Observational 

data, evidence from field experiments, and quantitative modeling 

are the evidence base of the negative effects of extreme weather 

events on crop yield: early spring heat waves followed by normal 

frost events have been shown to decimate Midwest fruit crops; 

heat waves during flowering, pollination, and grain filling have 

been shown to significantly reduce corn and wheat yields; more 

variable and intense spring rainfall has delayed spring planting in 

some years and can be expected to increase erosion and runoff; 

and floods have led to crop losses.12,13,14

New information and remaining uncertainties

Key issues (uncertainties) are: a) the rate at which grain yield improvements will continue to occur, which could help to offset the 

overall negative effect of extreme events at least for grain crops 

(though not for individual farmers); and b) the degree to which 

genetic improvements could make some future crops more tolerant of extreme events such as drought and heat stress. Additional 

uncertainties are: c) the degree to which accelerated soil carbon 

loss will occur as a result of warmer winters and the resulting effects on soil fertility and soil water availability; and d) the potential 

for increased pest and disease pressure as southern pests such 

as soybean rust move northward and existing pests better survive 

milder Midwest winters.
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436 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Assessment of confidence based on evidence

Because nearly all studies published to date in the peer-reviewed 

literature agree that Midwest crops benefit from CO2 fertilization 

and some benefit from a longer growing season, there is very high 

confidence in this component of the key message. 

Studies also agree that full benefits of climate change will be offset partly or fully by more frequent heat waves, early spring thaws 

followed by freezing temperatures, more variable and intense rainfall events, and floods. Again, there is very high confidence in this 

aspect. 

There is less certainty (high) about pest effects and about the 

potential for adaptation actions to significantly mitigate the risk 

of crop loss. 

Key Message #2 Traceable Account

The composition of the region’s forests is expected to change as rising temperatures drive habitats 

for many tree species northward. The role of the 

region’s forests as a net absorber of carbon is at 

risk from disruptions to forest ecosystems, in part 

due to climate change.

Description of evidence

The key message and supporting text summarize extensive evidence documented in the Technical Input Report.98 Technical 

inputs on a wide range of topics were also received and reviewed 

as part of the Federal Register Notice solicitation for public input. 

Evidence for increased temperatures and altered growing seasons 

across the U.S. is discussed in Chapter 2 (Our Changing Climate, 

Key Messages 3 and 4) and its Traceable Accounts. “Climate 

Trends and Scenarios for the U.S. National Climate Assessment,”4

with its references, provides specific details for the Midwest. Evidence that species have been shifting northward or ascending in 

altitude has been mounting for numerous species, though less 

so for long-lived trees. Nearly all studies to date published in the 

peer-reviewed literature agree that many of the boreal species of 

the north will eventually retreat northward. The question is when. 

Multiple models and paleoecological evidence show these trends 

have occurred in the past and are projected to continue in the 

future.36

The forests of the eastern United States (including the Midwest) 

have been accumulating large quantities of carbon over the past 

century,23 but evidence shows this trend is slowing in recent decades. There is a large amount of forest inventory data supporting 

the gradual decline in carbon accumulation throughout the eastern United States,99 as well as evidence of increasing disturbances 

and disturbance agents that are reducing overall net productivity 

in many of the forests.

New information and remaining uncertainties

A key issue (uncertainty) is the rate of change of habitats and for 

organisms adapting or moving as habitats move. The key questions are: How much will the habitats change (what scenarios 

and model predictions will be most correct)? As primary habitats 

move north, which species will be able to keep up with changing 

habitats on their own or with human intervention through assisted 

migration, management of migration corridors, or construction or 

maintenance of protected habitats within species’ current landscapes? 

Viable avenues to improving the information base are determining 

which climate models exhibit the best ability to reproduce the 

historical and potential future change in habitats, and determining 

how, how fast, and how far various species can move or adapt. 

An additional key source of uncertainty is whether projected disturbances to forests are chronic or episodic in nature.45

Assessment of confidence based on evidence

There is very high confidence in this key message, given the evidence base and remaining uncertainties.

Confidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent results, well documented 

and accepted methods, etc.), 

high consensus

High

Moderate evidence (several 

sources, some consistency, 

methods vary and/or documentation limited, etc.), medium 

consensus

Medium

Suggestive evidence (a few 

sources, limited consistency, 

models incomplete, methods 

emerging, etc.), competing 

schools of thought

Low

Inconclusive evidence (limited sources, extrapolations, 

inconsistent findings, poor documentation and/or methods not 

tested, etc.), disagreement or 

lack of opinions among experts
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437 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Key Message #3 Traceable Account

Increased heat wave intensity and frequency, increased humidity, degraded air quality, and reduced 

water quality will increase public health risks.

Description of evidence

The key message and supporting text summarize extensive evidence documented in the Technical Input Report.98 Technical 

inputs on a wide range of topics were also received and reviewed 

as part of the Federal Register Notice solicitation for public input. 

Evidence for extreme weather such as heat waves across the U.S. 

are discussed in Chapter 2 (Our Changing Climate, Key Message 

7) and its Traceable Accounts. Specific details for the Midwest are 

in “Climate Trends and Scenarios for the U.S. National Climate 

Assessment”4 with its references. A recent book100 also contains 

chapters detailing the most current evidence for the region. 

Heat waves: The occurrence of heat waves in the recent past has 

been well-documented,1,15,49 as have health outcomes (particularly 

with regards to mortality). Projections of thermal regimes indicate 

increased frequency of periods with high air temperatures (and 

high apparent temperatures, which are a function of both air temperature and humidity). These projections are relatively robust and 

consistent between studies. 

Humidity: Evidence on observed and projected increased humidity 

can be found in a recent study.49

Air quality: In 2008, in the region containing North Dakota, South 

Dakota, Nebraska, Kansas, Minnesota, Iowa, Missouri, Wisconsin, 

Illinois, Michigan, Indiana, and Ohio, over 26 million people lived 

in counties that failed the National Ambient Air Quality Standards 

(NAAQS) for PM2.5 (particles with diameter below 2.5 microns), 

and over 24 million lived in counties that failed the NAAQS for 

ozone (O3).1 Because not all counties have air quality measurement stations in place, these data must be considered a lower 

bound on the actual number of counties that violate the NAAQS. 

Given that the NAAQS were designed principally with the goal of 

protecting human health, failure to meet these standards implies a 

significant fraction of the population live in counties characterized 

by air quality that is harmful to human health. While only relatively 

few studies have sought to make detailed air quality projections for 

the future, those that have1 generally indicate declining air quality 

(see uncertainties below). 

Water quality: The EPA estimates there are more than 800 billion gallons of untreated combined sewage released into the nation’s waters annually.76 Combined sewers are designed to capture 

both sanitary sewage and stormwater. Combined sewer overflows 

lead to discharge of untreated sewage as a result of precipitation events, and can threaten human health. While not all urban 

areas within the Midwest have combined sewers for delivery to 

wastewater treatment plants, many do (for example, Chicago and 

Milwaukee), and such systems are vulnerable to combined sewer 

overflows during extreme precipitation events. Given projected 

increases in the frequency and intensity of extreme precipitation 

events in the Midwest (Chapter 2: Our Changing Climate, Key Message 6),75 it appears that sewer overflow will continue to constitute 

a significant current health threat and a critical source of climate 

change vulnerability for major urban areas within the Midwest. 

New information and remaining uncertainties

Key issues (uncertainties) are: Human health outcomes are contingent on a large number of non-climate variables. For example, 

morbidity and mortality outcomes of extreme heat are strongly 

determined by a) housing stock and access to air-conditioning in 

residences; b) existence and efficacy of heat wave warning and 

response plans (for example, foreign-language-appropriate communications and transit plans to public cooling centers, especially 

for the elderly); and c) co-stressors (for example, air pollution). 

Further, heat stress is dictated by apparent temperature, which 

is a function of both air temperature and humidity. Urban heat 

islands tend to exacerbate elevated temperatures and are largely 

determined by urban land use and human-caused heat emissions. 

Urban heat island reduction plans (for example, planted green 

roofs) represent one ongoing intervention. Nevertheless, the occurrence of extreme heat indices will increase under all climate 

scenarios. Thus, in the absence of policies to reduce heat-related 

illness/death, these impacts will increase in the future.

Air quality is a complex function not only of physical meteorology 

but emissions of air pollutants and precursor species. However, 

since most chemical reactions are enhanced by warmer temperatures, as are many air pollutant emissions, warmer temperatures 

may lead to worsening of air quality, particularly with respect to 

tropospheric ozone (see Ch. 9: Human Health). Changes in humidity are more difficult to project but may amplify the increase in 

heat stress due to rising temperatures alone.49

Combined sewer overflow is a major threat to water quality in some 

midwestern cities now. The tendency towards increased magnitude of extreme rain events (documented in the historical record 

and projected to continue in downscaling analyses) will cause an 

increased risk of waterborne disease outbreaks in the absence of 

infrastructure overhaul. However, mitigation actions are available, 

and the changing structure of cities (for example, reducing impervious surfaces) may offset the impact of the changing climate.

Assessment of confidence based on evidence

In the absence of concerted efforts to reduce the threats posed 

by heat waves, increased humidity, degraded air quality and degraded water quality, climate change will increase the health risks 

associated with these phenomena. However, these projections are 

contingent on underlying assumptions regarding socioeconomic 

conditions and demographic trends in the region. Confidence is 

therefore high regarding this key message. 
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438 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Key Message #4 Traceable Account

The Midwest has a highly energy-intensive economy with per capita emissions of greenhouse gases 

more than 20% higher than the national average. 

The region also has a large and increasingly utilized 

potential to reduce emissions that cause climate 

change. 

Description of evidence

The key message and supporting text summarize extensive evidence documented in the Technical Input Report.98 Technical 

inputs on a wide range of topics were also received and reviewed 

as part of the Federal Register Notice solicitation for public input. 

The Midwest’s disproportionately large reliance on coal for electricity generation and the energy intensity of its agricultural and 

manufacturing sectors are all well documented in both government and industry records, as is the Midwest’s contribution to 

greenhouse gases.1 The region’s potential for zero- and lowercarbon energy production is also well documented by government 

and private assessments. Official and regular reporting by state 

agencies and non-governmental organizations demonstrates the 

Midwest’s progress toward a decarbonized energy mix (Ch. 4: Energy; Ch. 10: Energy, Water, and Land).1

There is evidence that the Midwest is steadily decarbonizing its 

electricity generation through a combination of new state-level 

policies (for example, energy efficiency and renewable energy 

standards) and will continue to do so in response to low natural 

gas prices, falling prices for renewable electricity (for example, 

wind and solar), greater market demand for lower-carbon energy 

from consumers, and new EPA regulations governing new power 

plants. Several midwestern states have established Renewable 

Portfolio Standards (see https://www.misoenergy.org/WhatWeDo/

StrategicInitiatives/Pages/RenewablePortfolioStandards.aspx).

New information and remaining uncertainties

There are four key uncertainties. The first uncertainty is the net 

effect of emerging EPA regulations on the future energy mix of the 

Midwest. Assessments to date suggest a significant number of 

coal plants will be closed or repowered with lower-carbon natural 

gas; and even coal plants that are currently thought of as “must 

run” (to maintain the electric grid’s reliability) may be able to 

be replaced in some circumstances with the right combination 

of energy efficiency, new transmission lines, demand response, 

and distributed generation. A second key uncertainty is whether 

or not natural gas prices will remain at their historically low levels. 

Given that there are really only five options for meeting electricity 

demand – energy efficiency, renewables, coal, nuclear, and natural gas – the replacement of coal with natural gas for electricity 

production would have a significant impact on greenhouse gas 

emissions in the region. Third is the uncertain future for federal 

policies that have spurred renewable energy development to date, 

such as the Production Tax Credit for wind. While prices for both 

wind and solar continue to fall, the potential loss of tax credits 

may dampen additional market penetration of these technologies. 

A fourth uncertainty is the net effect of climate change on energy 

demand, and the cost of meeting that new demand profile. Research to date suggests the potential for a significant swing from 

the historically larger demand for heating in the winter to more 

demand in the summer instead, due to a warmer, more humid 

climate.3

Assessment of confidence based on evidence

There is no dispute about the energy intensity of the midwestern 

economy, nor its disproportionately large contribution of greenhouse gas emissions. Similarly, there is broad agreement about 

the Midwest’s potential for—and progress toward—lower-carbon 

electricity production. There is therefore very high confidence in 

this statement. 

Key Message #5 Traceable Account

Extreme rainfall events and flooding have increased during the last century, and these trends 

are expected to continue, causing erosion, declining 

water quality, and negative impacts on transportation, agriculture, human health, and infrastructure.

Description of evidence

The key message and supporting text summarize extensive evidence documented in the Technical Input Report.98 Technical 

inputs on a wide range of topics were also received and reviewed 

as part of the Federal Register Notice solicitation for public input. 

Evidence for extreme weather and increased precipitation across 

the U.S. are discussed in Chapter 2 (Our Changing Climate, Key 

Messages 5, 6, and 7) and its Traceable Accounts. Specific details for the Midwest are detailed in “Climate Trends and Scenarios 

for the U.S. National Climate Assessment”4 with its references. A 

recent book100 also contains chapters detailing the most current 

evidence for the region. 

There is compelling evidence that annual total precipitation has 

been increasing in the region, with wetter winters and springs, 

drier summers, an increase in extreme precipitation events, and 

changes in snowfall patterns. These observations are consistent 

with climate model projections. Both the observed trends and climate models suggest these trends will increase in the future. 

Recent records also indicate evidence of a number of high-impact 

flood events in the region. Heavy precipitation events cause increased kinetic energy of surface water and thus increase erosion. 

Heavy precipitation events in the historical records have been 

shown to be associated with discharge of partially or completely 

untreated sewage due to the volumes of water overwhelming combined sewer systems that are designed to capture both domestic 

sewage and stormwater.
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439 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Climate downscaling projections tend to indicate an increase in 

the frequency and duration of extreme events (both heavy precipitation and meteorological drought) in the future.

An extensive literature survey and synthetic analysis is presented 

in chapters in a recent book100 for impacts on water quality, transportation, agriculture, health, and infrastructure.

New information and remaining uncertainties

Precipitation is much less readily measured or modeled than air 

temperature.3 Thus both historical tendencies and projections 

for precipitation are inherently less certain than for temperature. 

Most regional climate models still have a positive bias in precipitation frequency but a negative bias in terms of precipitation amount 

in extreme events.

Flood records are very heterogeneous and there is some ambiguity 

about the degree to which flooding is a result of atmospheric conditions.69 Flooding is not solely the result of incident precipitation 

but is also a complex function of the preceding conditions such 

as soil moisture content and extent of landscape infiltration. A key 

issue (uncertainty) is the future distribution of snowfall. Records 

indicate that snowfall is decreasing in the southern parts of the 

region, along with increasing lake effect snow. Climate models 

predict these trends will increase. There is insufficient knowledge 

about how this change in snowfall patterns will affect flooding and 

associated problems, but it is projected to affect the very large 

spring floods that typically cause the worst flooding in the region. 

In addition, recent data and climate predictions indicate drier 

summer conditions, which could tend to offset the effects of higher intensity summer storms by providing increased water storage 

in the soils. The relative effects of these offsetting trends need to 

be assessed. To determine future flooding risks, hydrologic modeling is needed that includes the effects of the increase in extreme 

events, changing snow patterns, and shifts in rainfall patterns. 

Adaptation measures to reduce soil erosion and combined sewer 

overflow (CSO) events are available and could be widely adopted.

The impacts of increased magnitude of heavy precipitation events 

on water quality, agriculture, human health, transportation, and 

infrastructure will be strongly determined by the degree to which 

the resilience of such systems is enhanced (for example, some 

cities are already implementing enhanced water removal systems).

Assessment of confidence based on evidence

There have been improvements in agreement between observed 

precipitation patterns and model simulations. Also an increase in 

extreme precipitation events is consistent with first-order reasoning and increased atmospheric water burdens due to increased air 

temperature. Recent data suggest an increase in flooding in the 

region but there is uncertainty about how changing snow patterns 

will affect flood events in the future. Thus there is high confidence 

in increases in high-magnitude rainfall events and extreme precipitation events, and that these trends are expected to continue. 

There is medium confidence that, in the absence of substantial 

adaptation actions, the enhancement in extreme precipitation and 

other tendencies in land use and land cover result in a projected 

increase in flooding. There is medium confidence that, in the absence of major adaptation actions, the enhancement in extreme 

precipitation will tend to increase the risk of erosion, declines in 

water quality, and negative impacts on transportation, agriculture, 

human health, and infrastructure.3

Key Message #6 Traceable Account

Climate change will exacerbate a range of risks 

to the Great Lakes, including changes in the range 

and distribution of certain fish species, increased 

invasive species and harmful blooms of algae, 

and declining beach health. Ice cover declines will 

lengthen the commercial navigation season.

Description of evidence

The key message and supporting text summarize extensive evidence documented in the Technical Input Report.98 Technical 

inputs on a wide range of topics were also received and reviewed 

as part of the Federal Register Notice solicitation for public input. 

Evidence for changes in ice cover due to increased temperatures 

across the U.S. are discussed in Chapter 2 (Our Changing Climate, 

Key Message 11) and its Traceable Accounts. Specific details for 

the Midwest are detailed in “Climate Trends and Scenarios for the 

U.S. National Climate Assessment”4 with its references. A recent 

book100 also contains chapters detailing the most current evidence 

for the region. 

Altered fish communities: Warmer lakes and streams will certainly 

provide more habitat for warmwater species as conditions in northern reaches of the basin become more suitable for warmwater fish 

and as lakes and streams are vacated by cool- and coldwater species.84 Habitat for coldwater fish, though not expected to disappear, will shrink substantially, though it could also expand in some 

areas, such as Lake Superior. Whether climate change expands 

the range of any type of fish is dependent on the availability of 

forage fish, as higher temperatures also necessitate greater food 

intake.

Increased abundances of invasive species: As climate change alters water temperatures, habitat, and fish communities, conditions that once were barriers to alien species become conduits for 

establishment and spread.84 This migration will alter drastically 

the fish communities of the Great Lakes basin. Climate change is 

also projected to heighten the impact of invasive species already 

present in the Great Lakes basin. Warmer winter conditions, for 

instance, have the potential to benefit alewife, round gobies, ruffe, 

sea lamprey, rainbow smelt, and other non-native species. These 

species have spread rapidly throughout the basin and have already 

inflicted significant ecological and economic harm. 
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440 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Declining beach health and harmful algal blooms: Extreme events 

increase runoff, adding sediments, pollutants, and nutrients to 

the Great Lakes. The Midwest has experienced rising trends in 

precipitation and runoff. Agricultural runoff, in combination with 

increased water temperatures, has caused considerable non-point 

source pollution problems in recent years, with increased phosphorus and nitrogen loadings from farms contributing to more 

frequent and prolonged occurrences of anoxic “dead zones” and 

harmful, dense algae growth for long periods. Stormwater runoff 

that overloads urban sewer systems during extreme events adds 

to increased levels of toxic substances, sewage, and bacteria in 

the Great Lakes, affecting water quality, beach health, and human 

well-being. Increased storm events caused by climate change will 

lead to an increase in combined sewer overflows.84

Decreased ice cover: Increasingly mild winters have shortened the 

time between when a lake freezes and when it thaws.101 Scientists 

have documented a relatively constant decrease in Great Lakes ice 

cover since the 1970s, particularly for Lakes Superior, Michigan, 

Huron, and Ontario. The loss of ice cover on the Great Lakes has 

both ecological and economic implications. Ice serves to protect 

shorelines and habitat from storms and wave power. Less ice—

coupled with more frequent and intense storms—leaves shores 

vulnerable to erosion and flooding and could harm property and 

fish habitat.

Water levels: The 2009 NCA102 included predictions of a significant drop in Great Lakes levels by the end of the century, based 

on methods of linking climate models to hydrologic models. These 

methods have been significantly improved by fully coupling the 

hydrologic cycle among land, lake, and atmosphere.97 Without accounting for that cycle of interactions, a study96 concluded that 

increases in precipitation would be negated by increases in winter evaporation from less ice cover and by increases in summer 

evaporation and evapotranspiration from warmer air temperatures, 

under a scenario of continued increases in global emissions (SRES 

A2 scenario). Declines of 8 inches to 2 feet have been projected 

by the end of this century, depending on the specific lake in question.96 A recent comprehensive assessment,94 however, has concluded that with a continuation of current rising emissions trends 

(A2), the lakes will experience a slight decrease or even a rise in 

water levels; the difference from earlier studies is because earlier 

studies tended to overstress the amount of evapotranspiration expected to occur. The range of potential future lake levels remains 

large and includes the earlier projected decline. Overall, however, 

scientists project an increase in precipitation in the Great Lakes 

region (with extreme events projected to contribute to this increase), which will contribute to maintenance of or an increase 

in Great Lakes water levels. However, water level changes are not 

predicted to be uniform throughout the basin.

Shipping: Ice cover is expected to decrease dramatically by the 

end of the century, possibly lengthening the shipping season and, 

thus, facilitating more shipping activity. Current science suggests 

water levels in the Great Lakes are projected to fall slightly or 

might even rise over the short run. However, by causing even a 

small drop in water levels, climate change could make the costs 

of shipping increase substantially. For instance, for every inch of 

draft a 1000-foot ship gives up, its capacity is reduced by 270 

tons.93 Lightened loads today already add about $200,000 in 

costs to each voyage. 

New information and remaining uncertainties

Key issues (uncertainties) are: Water levels are influenced by the 

amount of evaporation from decreased ice cover and warmer air 

temperatures, by evapotranspiration from warmer air temperatures, and by potential increases in inflow from more precipitation. 

Uncertainties about Great Lakes water levels are high, though 

most models suggest that the decrease in ice cover will lead to 

slightly lower water levels, beyond natural fluctuations.

The spread of invasive species into the system is near-certain (given the rate of introductions over the previous 50 years) without major policy and regulatory changes. However, the changes in Great 

Lakes fish communities are based on extrapolation from known 

fishery responses to projected responses to expected changing 

conditions in the basin. Moreover, many variables beyond water 

temperature and condition affect fisheries, not the least of which 

is the availability of forage fish. Higher water temperatures necessitate greater food intake, yet the forage base is changing rapidly 

in many parts of the Great Lakes basin, thus making the projected 

impact of climate change on fisheries difficult to discern with very 

high certainty. 

Assessment of confidence based on evidence

Peer-reviewed literature about the effects of climate change are in 

broad agreement that air and surface water temperatures are rising and will continue to do so, that ice cover is declining steadily, 

and that precipitation and extreme events are on the rise. For 

large lake ecosystems, these changes have well-documented effects, such as effects on algal production, stratification (change 

in water temperature with depth), beach health, and fisheries. Key 

uncertainties exist about Great Lakes water levels and the impact 

of climate change on fisheries. 

A qualitative summary of climate stressors and coastal margin 

vulnerabilities for the Great Lakes is given in a technical input 

report.84 We have high confidence that the sum of these stressors 

will exceed the risk posed by any individual stressor. However, 

quantifying the cumulative impacts of those stressors is very challenging. 

Given the evidence and remaining uncertainties, there is very high

confidence in this key message, except high confidence for lake 

levels changing, and high confidence that declines in ice cover will 

continue to lengthen the commercial navigation season. There is 

limited information regarding exactly how invasive species may 

respond to changes in the regional climate, resulting in medium

confidence for that part of the key message.
441

Climate Change Impacts in the United States

CHAPTER 19

GREAT PLAINS

INFORMATION DRAWN FROM THIS CHAPTER IS INCLUDED IN THE HIGHLIGHTS REPORT AND IS IDENTIFIED BY THIS ICON

Recommended Citation for Chapter

Shafer, M., D. Ojima, J. M. Antle, D. Kluck, R. A. McPherson, S. Petersen, B. Scanlon, and K. Sherman, 2014: Ch. 19: Great 

Plains. Climate Change Impacts in the United States: The Third National Climate Assessment, J. M. Melillo, Terese (T.C.) 

Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 441-461. doi:10.7930/J0D798BC. 

On the Web: http://nca2014.globalchange.gov/report/regions/great-plains

Convening Lead Authors

Dennis Ojima, Colorado State University

Mark Shafer, Oklahoma Climatological Survey

Lead Authors

John M. Antle, Oregon State University

Doug Kluck, National Oceanic and Atmospheric Administration

Renee A. McPherson, University of Oklahoma

Sascha Petersen, Adaptation International

Bridget Scanlon, University of Texas

Kathleen Sherman, Colorado State University
442 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Key Messages

19GREAT PLAINS

1. Rising temperatures are leading to increased demand for water and energy. In parts of the region, 

this will constrain development, stress natural resources, and increase competition for water 

among communities, agriculture, energy production, and ecological needs.

2. Changes to crop growth cycles due to warming winters and alterations in the timing and magnitude 

of rainfall events have already been observed; as these trends continue, they will require new 

agriculture and livestock management practices.

3. Landscape fragmentation is increasing, for example, in the context of energy development 

activities in the northern Great Plains. A highly fragmented landscape will hinder adaptation of 

species when climate change alters habitat composition and timing of plant development cycles.

4. Communities that are already the most vulnerable to weather and climate extremes will be 

stressed even further by more frequent extreme events occurring within an already highly variable 

climate system.

5. The magnitude of expected changes will exceed those experienced in the last century. Existing 

adaptation and planning efforts are inadequate to respond to these projected impacts.

The Great Plains is a diverse region where climate and water 

are woven into the fabric of life. Day-to-day, month-to-month, 

and year-to-year changes in the weather can be dramatic and 

challenging for communities and their commerce. The region 

experiences multiple climate and weather hazards, including 

floods, droughts, severe storms, tornadoes, 

hurricanes, and winter storms. In much of 

the Great Plains, too little precipitation falls 

to replace that needed by humans, plants, 

and animals. These variable conditions in 

the Great Plains already stress communities and cause billions of dollars in damage; 

climate change will add to both stress and 

costs.

The people of the Great Plains historically 

have adapted to this challenging climate. Although projections suggest more frequent 

and more intense droughts, severe rainfall events, and heat waves, communities 

and individuals can reduce vulnerabilities 

through the use of new technologies, community-driven policies, and the judicious 

use of resources. Adaptation (means of coping with changed conditions) and mitigation 

(reducing emissions of heat-trapping gases 

to reduce the speed and amount of climate change) choices 

can be locally driven, cost effective, and beneficial for local 

economies and ecosystem services.

USFWS
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19: GREAT PLAINS

Significant climate-related challenges are expected to involve 

1) resolving increasing competition among land, water, and energy resources; 2) developing and maintaining sustainable agricultural systems; 3) conserving vibrant and diverse ecological 

systems; and 4) enhancing the resilience of the region’s people 

to the impacts of climate extremes. These growing challenges 

will unfold against a changing backdrop that includes a growing 

urban population and declining rural population, new economic factors that drive incentives for crop and energy production, 

advances in technology, and shifting policies such as those related to farm and energy subsidies.

The Great Plains region features relatively flat plains that increase in elevation from sea level to more than 5,000 feet at 

the base of mountain ranges along the Continental Divide. 

Forested mountains cover western Montana and Wyoming, 

extensive rangelands spread throughout the Plains, marshes 

extend along Texas’ Gulf Coast, and desert landscapes distinguish far west Texas.1

 A highly diverse climate results from the 

region’s large north-south extent and change of elevation. This 

regional diversity also means that climate change impacts will 

vary across the region. 

Great Plains residents already must contend with weather 

challenges from winter storms, extreme heat and cold, severe 

thunderstorms, drought, and flood-producing rainfall. Texas’ 

Gulf Coast averages about three tropical storms or hurricanes 

every four years,2 generating coastal storm surge and sometimes bringing heavy rainfall and damaging winds hundreds of 

miles inland. The expected rise in sea level will result in the 

potential for greater damage from storm surge along the Gulf 

Coast of Texas (see Ch. 25: Coasts).

Annual average temperatures range from less than 40°F in the 

mountains of Wyoming and Montana to more than 70°F in 

South Texas, with extremes ranging from -70°F in Montana to 

121°F in North Dakota and Kansas.3 Summers are long and hot 

in the south; winters are long and often severe in the north. 

North Dakota’s increase in annual temperature over the past 

130 years is the fastest in the contiguous U.S. and is mainly 

driven by warming winters.4

The region has a distinct north-south gradient in average temperature patterns, with a hotter south and colder north (Figure 19.1). Average annual precipitation greater than 50 inches 

supports lush vegetation in eastern Texas and Oklahoma. For 

most places, however, average rainfall is less than 30 inches, 

with some of Montana, Wyoming, and far west Texas receiving 

less than 15 inches a year. Across much of the region, annual 

water loss from transpiration by plants and from evaporation 

is higher than annual precipitation, making these areas particularly susceptible to droughts.

Projected climate change

For an average of seven days per year, maximum temperatures 

reach more than 100°F in the Southern Plains and about 95°F 

in the Northern Plains (Figure 19.2). These high temperatures 

are projected to occur much more frequently, even under a 

Figure 19.1. The region has a distinct north-south gradient in average temperature patterns 

(left), with a hotter south and colder north. For precipitation (right), the regional gradient runs 

west-east, with a wetter east and a much drier west. Averages shown here are for the period 

1981-2010. (Figure source: adapted from Kunkel et al. 20134).

Temperature and Precipitation Distribution in the Great Plains
444 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

19: GREAT PLAINS

scenario of substantial reductions in heat-trapping gas (also 

called greenhouse gas) emissions (B1), with days over 100°F 

projected to double in number in the north and quadruple in 

the south by mid-century (Ch. 2: Our Changing Climate, Key 

Message 7).

4

 Similar increases are expected in the number of 

nights with minimum temperatures higher than 80°F in the 

south and 60°F in the north (cooler in mountain regions; see 

Figure 19.3). These increases in extreme heat will have many 

negative consequences, including increases in surface water 

losses, heat stress, and demand for air conditioning.5 These 

negative consequences will more than offset the benefits of 

warmer winters, such as lower winter heating demand, less 

cold stress on humans and animals, and a longer growing season, which will be extended by mid-century an average of 24 

days relative to the 1971-2000 average.

4,5 More overwintering 

insect populations are also expected.5

Figure 19.3. The number of nights with the warmest 

temperatures is projected to increase dramatically. By midcentury (2041-2070), the projected change in number of 

nights exceeding those warmest temperatures is greatest in 

the south for both the lower emissions scenario (B1) and for 

the higher emissions scenario (A2). (Figure source: NOAA 

NCDC / CICS-NC).

 Projected Change in Number of Warm Nights

Figure 19.2. The number of days with the hottest temperatures 

is projected to increase dramatically. By mid-century (2041-

2070), the projected change in the number of days exceeding 

those hottest temperatures is greatest in the western areas 

and Gulf Coast for both the lower emissions scenario (B1) and 

for the higher emissions scenario (A2). (Figure source: NOAA 

NCDC / CICS-NC).

Projected Change in Number of Hot Days

The historical (1971-2000) distribution of temperature for the 

hottest 2% of days (about seven days a year) echoes the distinct 

north-south gradient in average temperatures.

The historical (1971-2000) distribution of temperature for 

the warmest 2% of nights (about seven days a year) echoes 

the distinct north-south gradient in average temperatures. 

Historical Temperature on the

7 Hottest Days of the Year

Historical Temperature on the

7 Warmest Nights of the Year
445 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

19: GREAT PLAINS

Winter and spring precipitation is projected to increase in the 

northern states of the Great Plains region under the A2 scenario, relative to the 1971-2000 average. In central areas, changes 

are projected to be small relative to natural variations (Ch. 2: 

Our Changing Climate, Key Message 5).4 Projected changes in 

summer and fall precipitation are small except for summer 

drying in the central Great Plains, although the exact locations 

of this drying are uncertain. The number of days with heavy 

precipitation is expected to increase by mid-century, especially in the north (Ch. 2: Our Changing Climate, Key Message 6). 

Large parts of Texas and Oklahoma are projected to see longer 

dry spells (up to 5 more days on average by mid-century). By 

contrast, changes are projected to be minimal in the north (Ch. 

2: Our Changing Climate, Key Message 7).4

Figure 19.5. Current regional trends of a drier south and 

a wetter north are projected to become more pronounced 

by mid-century (2041-2070 as compared to 1971-2000 

averages). Maps show the maximum annual number of 

consecutive days in which limited (less than 0.01 inches) 

precipitation was recorded on average from 1971 to 2000 

(top), projected changes in the number of consecutive dry 

days assuming substantial reductions in emissions (B1), 

and projected changes if emissions continue to rise (A2). 

The southeastern Great Plains, which is the wettest portion 

of the region, is projected to experience large increases in 

the number of consecutive dry days. (Figure source: NOAA 

NCDC / CICS-NC).

Projected Change in Number of 

Consecutive Dry Days

Figure 19.4. The number of days with the heaviest 

precipitation is not projected to change dramatically. By 

mid-century (2041-2070), the projected change in days 

exceeding those precipitation amounts remains greatest 

in the northern area for both the lower emissions scenario 

(B1) and for the higher emissions scenario (A2). (Figure 

source: NOAA NCDC / CICS-NC). 

 Projected Change in Number of Heavy 

Precipitation Days

The historical (1971-2000) distribution of the greatest 2% 

of daily precipitation (about seven days a year) echoes 

the regional west-east gradient in average precipitation.

Historical Amount of Precipitation on the

7 Wettest Days of the Year
446 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

19: GREAT PLAINS

Key Message 1: Energy, Water and Land Use

Rising temperatures are leading to increased demand for water and energy. In parts of the 

region, this will constrain development, stress natural resources, and increase competition for 

water among communities, agriculture, energy production, and ecological needs.

Energy, water, and land use are inherently interconnected,

6

and climate change is creating a new set of challenges for these 

critical sectors (Ch. 2: Our Changing Climate; Ch. 10: Energy, 

Water, and Land).

7,8,9 The Great Plains is rich with energy resources, primarily from coal, oil, and natural gas, with growing 

wind and biofuel industries.

10 Texas produces 16% of U.S. energy (mostly from crude oil and natural gas), and Wyoming provides an additional 14% (mostly from coal). North Dakota is the 

second largest producer of oil in the Great Plains, behind Texas. 

Nebraska and South Dakota rank third and fifth in biofuel production, and five of the eight Great Plains states have more 

than 1,000 megawatts of installed wind generation capacity, 

with Texas topping the list.11 More than 80% of the region’s 

land area is used for agriculture, primarily cropland, pastures,

and rangeland. Other land uses include forests, urban and rural 

development, transportation, conservation, and industry.

Significant amounts of water are used to produce energy7,12

and to cool power plants.13 Electricity is consumed to collect, 

purify, and pump water. Although hydraulic fracturing to release oil and natural gas is a small component of total water 

use,

14 it can be a significant proportion of water use in local 

and rural groundwater systems. Energy facilities, transmission 

lines, and wind turbines can fragment both natural habitats 

and agriculture lands (Ch. 10: Energy, Water, and Land).

5

The trend toward more dry days and 

higher temperatures across the south 

will increase evaporation, decrease 

water supplies, reduce electricity transmission capacity, and increase cooling 

demands. These changes will add stress 

to limited water resources and affect 

management choices related to irrigation, municipal use, and energy generation.

15 In the Northern Plains, warmer 

winters may lead to reduced heating 

demand while hotter summers will 

increase demand for air conditioning, 

with the summer increase in demand 

outweighing the winter decrease (Ch. 4: 

Energy, Key Message 2).15

Changing extremes in precipitation are 

projected across all seasons, including 

higher likelihoods of both increasing 

heavy rain and snow events4 and more 

intense droughts (Ch. 2: Our Changing Climate, Key Messages 

5 and 6).16 Winter and spring precipitation and very heavy precipitation events are both projected to increase in the northern 

portions of the area, leading to increased runoff and flooding 

that will reduce water quality and erode soils. Increased snowfall, rapid spring warming, and intense rainfall can combine to 

produce devastating floods, as is already common along the 

Red River of the North. More intense rains will also contribute 

to urban flooding. 

Increased drought frequency and intensity can turn marginal 

lands into deserts. Reduced per capita water storage will continue to increase vulnerability to water shortages.17 Federal 

and state legal requirements mandating water allocations for 

ecosystems and endangered species add further competition 

for water resources.

Diminishing water supplies and rapid population growth are 

critical issues in Texas. Because reservoirs are limited and have 

high evaporation rates, San Antonio has turned to the Edwards 

Aquifer as a major source of groundwater storage. Nineteen 

water districts joined to form a Regional Water Alliance for sustainable water development through 2060. The alliance creates a competitive market for buying and selling water rights 

and simplifies transfer of water rights.

© Jim West/imagebroker/Corbis
447 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Key Message 2: Sustaining Agriculture

Changes to crop growth cycles due to warming winters and alterations in the timing and 

magnitude of rainfall events have already been observed; as these trends continue, 

they will require new agriculture and livestock management practices

The important agricultural sector in the Great Plains, with a 

total market value of about $92 billion (the most important being crops at 43% and livestock at 46%),

18 already contends with 

significant climate variability (Ch. 6: Agriculture). Projected 

changes in climate, and human responses to it, will affect aspects of the region’s agriculture, from the many crops that rely 

solely on rainfall, to the water and land required for increased 

energy production from plants, such as fuels made from corn 

or switchgrass (see Ch. 10: Energy, Water, and Land).

Water is central to the region’s productivity. The High Plains 

Aquifer, including the Ogallala, is a primary source for irrigation.19 In the Northern Plains, rain recharges 

this aquifer quickly, but little recharge occurs 

in the Southern Plains.20,21

Projected changes in precipitation and temperature have both positive and negative 

consequences to agricultural productivity in 

the Northern Plains. Projected increases in 

winter and spring precipitation in the Northern Plains will benefit agricultural productivity 

by increasing water availability through soil 

moisture reserves during the early growing 

season, but this can be offset by fields too wet 

to plant. Rising temperatures will lengthen 

the growing season, possibly allowing a second annual crop in some places and some 

years. Warmer winters pose challenges.22,23,24

For example, some pests and invasive weeds 

will be able to survive the warmer winters.25

Winter crops that leave dormancy earlier are 

susceptible to spring freezes.26 Rainfall events 

already have become more intense,

27 increasing erosion and nutrient runoff, and projections are that the frequency and severity of 

these heavy rainfall events will increase.4,28

The Northern Plains will remain vulnerable 

to periodic drought because much of the projected increase in precipitation is expected to 

occur in the cooler months while increasing 

temperatures will result in additional evapotranspiration.

In the Central and Southern Plains, projected declines in precipitation in the south 

and greater evaporation everywhere due to 

higher temperatures will increase irrigation 

demand and exacerbate current stresses on 

agricultural productivity. Increased water withdrawals from 

the Ogallala Aquifer and High Plains Aquifer would accelerate 

ongoing depletion in the southern parts of the aquifers and 

limit the ability to irrigate.21,29 Holding other aspects of production constant, the climate impacts of shifting from irrigated to 

dryland agriculture would reduce crop yields by about a factor of two.30 Under these climate-induced changes, adaptation 

of agricultural practices will be needed, however, there may 

be constraints on social-ecological adaptive capacity to make 

these adjustments (see also Ch. 28: Adaptation). 

Figure 19.6. Irrigation in western Kansas, Oklahoma, and Texas supports crop 

development in semiarid areas. Declining aquifer levels threaten the ability 

to maintain production. Some aquifer-dependent regions, like southeastern 

Nebraska, have seen steep rises in irrigated farmland, from around 5% to more 

than 40%, during the period shown. (Figure source: reproduced from Atlas of 

the Great Plains by Stephen J. Lavin, Clark J. Archer, and Fred M. Shelley by 

permission of the University of Nebraska. Copyright 2011 by the Board of Regents 

of the University of Nebraska33).

Increases in Irrigated Farmland in the Great Plains
448 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

19: GREAT PLAINS

The projected increase in high temperature extremes and heat 

waves will negatively affect livestock and concentrated animal 

feeding operations.31 Shortened dormancy periods for winter 

wheat will lessen an important source of feed for the livestock 

industry. Climate change may thus result in a northward shift 

of crop and livestock production in the region. In areas projected to be hotter and drier in the future, maintaining agriculture 

on marginal lands may become too costly.

Adding to climate change related stresses, growing water demands from large urban areas are also placing stresses on limited water supplies. Options considered in some areas include 

groundwater development and purchasing water rights from 

agricultural areas for transfer to cities.32

During the droughts of 2011 and 2012, ranchers liquidated 

large herds due to lack of food and water. Many cattle were 

sold to slaughterhouses; others were relocated to other pastures through sale or lease. As herds are being rebuilt, there 

is an opportunity to improve genetic stock, as those least 

adapted to the drought conditions were the first to be sold or 

relocated. Some ranchers also used the drought as an opportunity to diversify their portfolio, managing herds in both Texas 

and Montana. 

Key Message 3: Conservation and Adaptation 

Landscape fragmentation is increasing, for example, in the context of energy development 

activities in the northern Great Plains. A highly fragmented landscape will hinder 

adaptation of species when climate change alters habitat 

composition and timing of plant development cycles.

Land development for energy production, land transformations on the fringes of urban areas, and economic pressures 

to remove lands from conservation easements pose threats to 

natural systems in the Great Plains.34 Habitat fragmentation 

is already a serious issue that inhibits the ability of species to 

migrate as climate variability and change alter local habitats.35

Lands that remain out of production are susceptible to invasion from non-native plant species.

Many plant and animal species are responding to rising temperatures by adjusting their ranges at increasingly greater 

rates.36 These adjustments may also require movement of 

species that have evolved to live in very specific habitats, 

which may prove increasingly difficult for these species. The 

historic bison herds migrated to adapt to climate, disturbance, 

and associated habitat variability,

37 but modern land-use patterns, roads, agriculture, and structures inhibit similar largescale migration.38 In the playa regions of the southern Great 

Plains, agricultural practices have modified more than 70% 

of seasonal lakes larger than 10 acres, and these lakes will be 

further altered under warming conditions.

39,40 These changes 

in seasonal lakes will further affect bird populations41 and fish 

populations42 in the region. 

Observed climate-induced changes have been linked to changing timing of flowering, increases in wildfire activity and pest 

outbreaks, shifts in species distributions, declines in the abundance of native species, and the spread of invasive species (Ch. 

8: Ecosystems). From Texas to Montana, altered flowering patterns due to more frost-free days have increased the length of 

pollen season for ragweed by as many as 16 days over the period from 1995 to 2009.

43 Earlier snowmelt in Wyoming from 

1961 to 2002 has been related to the American pipit songbird 

laying eggs about 5 days earlier.44 During the past 70 years, 

observations indicate that winter wheat is flowering 6 to 10 

days earlier as spring temperatures have risen.23 Some species 

may be less sensitive to changes in temperature and precipitation, causing first flowering dates to change for some species but not for others.22 Even small shifts in timing, however, 

can disrupt the integrated balance of ecosystem functions like 

predator-prey relationships, mating behavior, or food availability for migrating birds.

In addition to climate changes, the increase in atmospheric CO2

concentrations may offset the drying effects from warming 

by considerable improvements in plant water-use efficiency, 

which occur as CO2 concentrations increase.45 However, nutrient content of the grassland communities may be decreased 

under enriched CO2 environments, affecting nutritional quality 

of the grasses and leaves eaten by animals.

The interaction of climate and land-use changes across the 

Great Plains promises to be challenging and contentious. Opportunities for conservation of native grasslands, including 

species and processes, depend primarily and most immediately on managing a fragmented network of untilled prairie. Restoration of natural processes, conservation of remnant species 

and habitats, and consolidation/connection of fragmented 

areas will facilitate conservation of species and ecosystem 

services across the Great Plains. However, climate change will 

complicate current conservation efforts as land fragmentation 

continues to reduce habitat connectivity. The implementation 

of adaptive management approaches provides robust options 

for multiple solutions.
449 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

19: GREAT PLAINS

Sage grouse and climate change

Habitat fragmentation inhibits the ability of species such as the Greater Sage Grouse, a candidate for Endangered 

Species Act protections, to migrate in response to climate change. Its current habitat is threatened by energy development, agricultural practices, and urban development. Rapid expansion of oil and gas fields in North Dakota, Wyoming,

and Montana and development of wind farms from North Dakota through Texas are opening new lands to development 

and contributing to habitat fragmentation of important core Sage Grouse habitat.46 The health of Sage Grouse habitat 

is associated with other species’ health as well.47 Climate change projections also suggest a shift in preferred habitat 

locations and increased susceptibility to West Nile Virus.48

Figure 19.7. Comparing estimates of Greater Sage Grouse distribution from before settlement of the 

area (light green: prior to about 1800) with the current range (dark green: 2000) shows fragmentation 

of the sagebrush habitat required by this species. Over the last century, the sagebrush ecosystem 

has been altered by fire, invasion by new plant species, and conversion of land to agriculture, causing 

a decline in Sage Grouse populations. (Figure source: adapted from Aldridge et al. 2008.49 Photo 

credit: U.S. Fish and Wildlife Service, Wyoming Ecological Services).

Historical and Current Range of Sage Grouse Habitat
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Key Message 4: Vulnerable Communities

Communities that are already the most vulnerable to weather and climate extremes 

will be stressed even further by more frequent extreme events occurring 

within an already highly variable climate system.

The Great Plains is home to a geographically, economically, and 

culturally diverse population. For rural and tribal communities, 

their remote locations, sparse development, limited local services, and language barriers present greater challenges in responding to climate extremes. Working-age people are moving 

to urban areas, leaving a growing percentage of elderly people 

in rural communities (see also Ch. 14: Rural Communities). 

Overall population throughout the region is stable or declining, with the exception of substantial increases in urban Texas, 

tribal communities, and western North Dakota, related in large 

part to rapid expansion of energy development.50 Growing urban areas require more water, expand into forests and cropland, fragment habitat, and are at a greater risk of wildfire – all 

factors that interplay with climate. 

Populations such as the elderly, low-income, and non-native 

English speakers face heightened climate vulnerability. Public 

health resources, basic infrastructure, adequate housing, and 

effective communication systems are often lacking in comFigure 19.8. Demographic shifts continue to reshape 

communities in the Great Plains, with many central Great Plains 

communities losing residents. Rural and tribal communities 

will face additional challenges in dealing with climate change 

impacts due to demographic changes in the region (Ch. 14: 

Rural Communities; Ch. 12: Indigenous Peoples). Figure shows 

population change from 2000 to 2010. (Figure source: U.S. 

Census Bureau 201057).

Population Change in the Great Plains 

Figure 19.9. Tribal populations in the Great Plains are 

concentrated near large reservations, like various Sioux 

tribes in South Dakota and Blackfeet and Crow reservations 

in Montana; and in Cherokee, Chickasaw, Choctaw, and 

other tribal lands in Oklahoma (Figure source: reproduced 

from Atlas of the Great Plains by Stephen J. Lavin, Clark J. 

Archer, and Fred M. Shelley by permission of the University 

of Nebraska. Copyright 2011 by the Board of Regents of the 

University of Nebraska33).

Tribal Populations in the Great Plains
451 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

19: GREAT PLAINS

munities that are geographically, politically, and economically 

isolated.51 Elderly people are more vulnerable to extreme heat, 

especially in warmer cities and communities with minimal air 

conditioning or sub-standard housing.52 Language barriers for 

Hispanics may impede their ability to plan for, adapt to, and 

respond to climate-related risks.53

The 70 federally recognized tribes in the Great Plains are diverse in their land use, with some located on lands reserved 

from their traditional homelands, and others residing within 

territories designated for their relocation, as in Oklahoma (see 

also Ch. 12: Indigenous Peoples). While tribal communities 

have adapted to climate change for centuries, they are now 

constrained by physical and political boundaries.54 Traditional 

ecosystems and native resources no longer provide the support they used to.55 Tribal members have reported the decline or disappearance of culturally important animal species, 

changes in the timing of cultural ceremonies due to earlier 

onset of spring, and the inability to locate certain types of ceremonial wild plants.56

Key Message 5: Opportunities to Build Resilience 

The magnitude of expected changes will exceed those experienced in the last century. 

Existing adaptation and planning efforts are inadequate to respond 

to these projected impacts.

The Great Plains is an integrated system. Changes in one part, 

whether driven by climate or by human decisions, affect other 

parts. Some of these changes are already underway, and many 

pieces of independent evidence project that ongoing climaterelated changes will ripple throughout the region.

Many of these challenges will cut across sectors: water, land 

use, agriculture, energy, conservation, and livelihoods. Competition for water resources will increase within alreadystressed human and ecological systems, particularly in the 

Southern Plains, affecting crops, energy production, and how 

well people, animals, and plants can thrive. The region’s ecosystems, economies, and communities will be further strained 

by increasing intensity and frequency of floods, droughts, and 

heat waves that will penetrate into the lives and livelihoods 

of Great Plains residents. Although some communities and 

Oglala lakota respond to climate change

The Oglala Lakota tribe in South Dakota is incorporating climate change adaptation and mitigation planning as they consider long-term sustainable development planning. Their Oyate Omniciye plan is a partnership built around six livability principles related to transportation, housing, economic competitiveness, 

existing communities, federal investments, and local values. Interwoven with 

this is a vision that incorporates plans to reduce future climate change and 

adapt to future climate change, while protecting cultural resources.58

©Aaron Huey
452 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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states have made efforts to plan for these projected changes, 

the magnitude of the adaptation and planning efforts do not 

match the magnitude of the expected changes. 

Successful adaptation of human and natural systems to climate change would benefit from:

• recognition of and commitment to addressing these 

challenges;

• regional-scale planning and local-to-regional implementation;8,59

• mainstreaming climate planning into existing natural 

resource, public health, and emergency management 

processes;60

• renewed emphasis on restoration of ecological systems 

and processes;61

• recognition of the value of natural systems to sustaining 

life;62,63

• sharing information among decision-makers; and

• enhanced alignment of social and ecological goals.64

Communities already face tradeoffs in efforts to make efficient and sustainable use of their resources. Jobs, infrastructure, and tax dollars that come with fossil fuel extraction or renewable energy production are important, especially for rural 

communities. There is also economic value in the conversion of 

native grasslands to agriculture. Yet the tradeoffs among this 

development, the increased pressure on water resources, and 

the effects on conservation need to be considered if the region 

is to develop climate-resilient communities. 

Untilled prairies used for livestock grazing provide excellent 

targets for native grassland conservation. Partnerships among 

many different tribal, federal, state, local, and private landowners can decrease landscape fragmentation and help manage the connection between agriculture and native habitats. 

Soil and wetland restoration enhances soil stability and health, 

water conservation, aquifer recharge, and food sources for 

wildlife and cattle. Healthy species and ecosystem services 

support social and economic systems where local products, 

tourism, and culturally significant species accompany largescale agriculture, industry, and international trade as fundamental components of society. 

Although there is tremendous adaptive potential among the 

diverse communities of the Great Plains, many local government officials do not yet recognize climate change as a problem that requires proactive planning.60,65 Positive steps toward 

greater community resilience have been achieved through 

local and regional collaboration and increased two-way communication between scientists and local decision-makers (see 

Ch. 28: Adaptation). For example, the Institute for Sustainable 

Communities conducts Climate Leadership Academies that 

promote peer learning and provides direct technical assistance 

to communities in a five-state region in the Southwest as part 

of their support of the Western Adaptation Alliance.66 Other 

regions have collaborated to share information, like the Southeast Florida Regional Compact 2012. Programs such as NOAA’s 

Regional Integrated Sciences and Assessments (RISA) support 

scientists working directly with communities to help build capacity to prepare for and adapt to both climate variability and 

climate change.67 Climate-related challenges can be addressed 

with creative local engagement and prudent use of community 

assets.68 These assets include social networks, social capital, 

indigenous and local knowledge, and informal institutions.
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Future climate change projections include more 

precipitation in the Northern Great Plains and 

less in the Southern Great Plains. In 2011, such 

a pattern was strongly manifest, with exceptional 

drought and recording-setting temperatures in 

Texas and Oklahoma and flooding in the Northern 

Great Plains. 

Many locations in Texas and Oklahoma experienced 

more than 100 days over 100oF. Both states set 

new records for the hottest summer since record 

keeping began in 1895. Rates of water loss due in 

part to evaporation were double the long-term average. The heat and drought depleted water resources 

and contributed to more than $10 billion in direct 

losses to agriculture alone. These severe water 

constraints strained the ability to meet electricity 

demands in Texas during 2011 and into 2012, a 

problem exacerbated by the fact that Texas is nearly 

isolated from the national electricity grid. 

These recent temperature extremes were attributable in part to human-induced climate change (approximately 20% of the heat wave magnitude and 

a doubling of the chance that it would occur).69 In 

the future, average temperatures in this region are 

expected to increase and will continue to contribute 

to the intensity of heat waves (Ch. 2: Our Changing 

Climate, Key Messages 3 and 7). 

By contrast to the drought in the Southern Plains, the Northern Plains were exceptionally wet in 2011, with Montana 

and Wyoming recording all-time wettest springs and the Dakotas and Nebraska not far behind. Record rainfall and 

snowmelt combined to push the Missouri River and its tributaries beyond their banks and leave much of the Crow Reservation in Montana underwater. The Souris River near Minot, North Dakota, crested at four feet above its previous record, with a flow five times greater than any in the past 30 years. Losses from the flooding were estimated at $2 billion. 

The summer of 2011

Figure 19.10. In 2011, cities including Houston, Dallas, Austin, 

Oklahoma City, and Wichita, among others, all set records for the 

highest number of days recording temperatures of 100oF or higher in 

those cities’ recorded history. The black circles denote the location 

of observing stations recording 100oF days. (Figure source: NOAA 

NCDC 20123). 

Days Above 100oF in Summer 2011 

A Texas State Park police officer walks across a cracked 

lakebed in August 2011. This lake once spanned more 

than 5,400 acres.

© Tony Gutierrez/ /AP/Corbis

Increases in heavy downpours contribute to flooding.

© LANE HICKENBOTTOM/Reuters/Corbis
454 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Joyce, L. A., G. M. Blate, S. G. McNulty, C. I. Millar, S. Moser, 

R. P. Neilson, and D. L. Peterson, 2009: Managing for multiple 

resources under climate change: National forests. Environmental 

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60. Romsdahl, R. J., L. Atkinson, and J. Schultz, 2013: Planning for 

climate change across the US Great Plains: Concerns and insights 

from government decision-makers. Journal of Environmental Studies 

and Sciences, 3, 1-14, doi:10.1007/s13412-012-0078-8. 

61. Eriksen, S., and K. Brown, 2011: Sustainable adaptation to 

climate change. Climate and Development, 3, 3-6, doi:10.3763/

cdev.2010.0064. [Available online at http://www.tandfonline.com/

doi/pdf/10.3763/cdev.2010.0064] 

Eriksen, S. H., and K. O’Brien, 2007: Vulnerability, poverty and 

the need for sustainable adaptation measures. Climate Policy, 7, 337-

352, doi:10.1080/14693062.2007.9685660. 

Eriksen, S. K., P. Aldunce, C. S. Bahinipati, R. D’Almeida Martins, 

J. I. Molefe, C. Nhemachena, K. O’Brien, F. Olorunfemi, J. 

Park, L. Sygna, and K. Ulsrud, 2011: When not every response 

to climate change is a good one: Identifying principles of 

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cdev.2010.0060. [Available online at http://www.tandfonline.com/

doi/pdf/10.3763/cdev.2010.0060] 

McNeeley, S. M., 2012: Examining barriers and opportunities 

for sustainable adaptation to climate change in Interior 

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0158-x. [Available online at http://link.springer.com/content/

pdf/10.1007%2Fs10584-011-0158-x] 

O'Brien, K., and R. Leichenko, 2008: Human Security, 

Vulnerability and Sustainable Adaptation. Human Development 

Report 2007/2008, 48 pp., United Nations Development Program. 

[Available online at http://hdr.undp.org/en/reports/global/

hdr2007-2008/papers/o'brien_karen%20and%20leichenko_robin.

pdf]

62. Berkes, F., and C. Folke, 1998: Linking Social and Ecological Systems: 

Management Practices and Social Mechanisms for Building Resilience.

University of Cambridge, 476 pp. 

63. Gunderson, L. H., and C. S. Holling, Eds., 2002: Panarchy: 

Understanding Transformations in Human and Natural Systems. Island 

Press, 508 pp. 

Tschakert, P., O. T. Coomes, and C. Potvin, 2007: Indigenous 

livelihoods, slash-and-burn agriculture, and carbon stocks in 

Eastern Panama. Ecolog y Economics, 60, 807-820, doi:10.1016/j.

ecolecon.2006.02.001. 

Walker, B., and J. A. Meyers, 2004: Thresholds in ecological and 

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Assessment of the Climate-Related Needs of Oklahoma Decision 

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we_work/usa/article/waa/]

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Regional Integrated Sciences and Assessments (RISA) Program: 

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Assessment of Global Climate Change, C. G. Knight, and J. Jäger, Eds., 

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books.google.com/books?id=B8O31ILKKOMC]

68. Ostrom, E., 1990: Governing the Commons: The Evolution of Institutions 

for Collective Action. Cambridge University Press, 280 pp. 

69. Hoerling, M., M. Chen, R. Dole, J. Eischeid, A. Kumar, J. W. 

Nielsen-Gammon, P. Pegion, J. Perlwitz, X.-W. Quan, and T. 

Zhang, 2013: Anatomy of an extreme event. Journal of Climate, 26,

2811–2832, doi:10.1175/JCLI-D-12-00270.1. [Available online at 

http://journals.ametsoc.org/doi/pdf/10.1175/JCLI-D-12-00270.1]
459 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

19: GREAT PLAINS

Traceable Accounts

Process for Developing Key Messages: 

A central component of the assessment process was the Great 

Plains Regional Climate assessment workshop that was held in 

August 2011 in Denver, CO, with approximately 40 attendees. 

The workshop began the process leading to a foundational 

Technical Input Report (TIR), the Great Plains Regional Climate Assessment Technical Report.5

 The TIR consists of 18 

chapters assembled by 37 authors representing a wide range 

of inputs including governmental agencies, non-governmental 

organizations, tribes, and other entities. 

The chapter author team engaged in multiple technical discussions via regular teleconferences. These included careful 

review of the foundational TIR8 and of approximately 50 additional technical inputs provided by the public, as well as the 

other published literature, and professional judgment. These 

discussions were followed by expert deliberation of draft key 

messages by the authors during an in-person meeting in Kansas City in April 2012, wherein each message was defended 

before the entire author team prior to the key message being 

selected for inclusion in the report. These discussions were 

supported by targeted consultation with additional experts by 

the lead author of each message, and they were based on 

criteria that help define “key vulnerabilities”.

Key message #1 Traceable Account

Rising temperatures are leading to increased 

demand for water and energy. In parts of the 

region, this will constrain development, stress 

natural resources, and increase competition for 

water among communities, agriculture, energy 

production, and ecological needs.

Description of evidence base

The key message and supporting text summarizes extensive 

evidence documented in the Technical Input Report.5 Technical inputs (47) on a wide range of topics were also received 

and reviewed as part of the Federal Register Notice solicitation 

for public input. 

Temperatures are rising across the United States (Ch. 2: Our 

Changing Climate, Key Message 3 and its Traceable Account). 

Specific details for the Great Plains are provided in the Regional 

Climate Trends and Scenarios for the U.S. National Climate Assessment4

 with its references.

Rising temperatures impact energy and water (Ch.10: Energy, 

Water, and Land; Ch. 4: Energy). Publications have explored the 

projected increase in water competition and stress for natural resources7,13,14,17 and the fragmentation of natural habitats and agricultural lands.8

 These sources provided numerous references that 

were drawn from to lead to this key message.

New information and remaining uncertainties

A key uncertainty is the exact rate and magnitude of the projected 

changes in precipitation, because high inter-annual variability may 

either obscure or highlight the long-term trends over the next few 

years. 

Confidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent results, well documented 

and accepted methods, etc.), 

high consensus

High

Moderate evidence (several 

sources, some consistency, 

methods vary and/or documentation limited, etc.), medium 

consensus

Medium

Suggestive evidence (a few 

sources, limited consistency, 

models incomplete, methods 

emerging, etc.), competing 

schools of thought

Low

Inconclusive evidence (limited sources, extrapolations, 

inconsistent findings, poor documentation and/or methods not 

tested, etc.), disagreement or 

lack of opinions among experts

SUPPLEMENTAL MATERIAL

TRACEABLE ACCOUNTS
19: GREAT PLAINS

Traceable Accounts

460 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Also unknown is ecological demand for water. Water use by native 

and invasive species under current climate needs to be quantified so that it can be modeled under future scenarios to map 

out potential impact envelopes. There is also uncertainty over the 

projections of changes in precipitation due to difficulty of modeling projections of convective precipitation, which is the primary 

source of water for most of the Great Plains.

Assessment of confidence based on evidence 

Very High for all aspects of the key message. The relationship 

between increased temperatures and higher evapotranspiration 

is well established. Model projections of higher temperatures are 

robust. Confidence is highest for the southern Great Plains, where 

competition among sectors, cities, and states for future supply is 

already readily apparent, and where population growth (demandside) and projected increases in precipitation deficits are greatest.

Key message #2 Traceable Account

Changes to crop growth cycles due to warming 

winters and alterations in the timing and magnitude 

of rainfall events have already been observed; as 

these trends continue, they will require new agriculture and livestock management practices.

Description of evidence base

The key message and supporting text summarize extensive evidence documented in the Great Plains Technical Input Report.5 

Technical inputs (47) on a wide range of topics were also received 

and reviewed as part of the Federal Register Notice solicitation for 

public input. 

Evidence for altered precipitation across the U.S. is discussed in 

Ch. 2: Our Changing Climate, Key Message 5 and 6 and their 

Traceable Accounts. Specific details for the Great Plains, such 

as warming winters and altered rainfall events are in the Climate 

Trends and Scenarios for the U.S. National Climate Assessment4 

with its references. 

Limitations of irrigation options in the High Plains aquifer have 

been detailed.21 The impacts of shifting from irrigated to rain-fed 

agriculture have also been detailed.30 Studies document negative 

impacts on livestock production through the Great Plains.31

New information and remaining uncertainties

A key issue (uncertainty) is rainfall patterns. Although models 

show a general increase in the northern Great Plains and a decrease in the southern Great Plains, the diffuse gradient between 

the two leaves uncertain the location of greatest impacts on the 

hydrologic cycle. Timing of precipitation is critical to crop planting, development and harvesting; shifts in seasonality of precipitation therefore need to be quantified. Rainfall patterns will similarly 

affect forage production, particularly winter wheat that is essential 

to cattle production in the southern Great Plains.

Assessment of confidence based on evidence 

The general pattern of precipitation changes and overall increases 

in temperature are robust. The implications of these changes are 

enormous, although assessing changes in more specific locations 

is more uncertain. Our assessment is based on the climate projections and known relationships to crops (for example, corn not 

being able to “rest” at night due to high minimum temperatures), 

but pinpointing where these impacts will occur is difficult. Additionally, other factors that influence productivity, such as genetics, 

technological change, economic incentives, and federal and state 

policies, can alter or accelerate the impacts. Given the evidence 

and remaining uncertainties, agriculture and livestock management practices will need to adjust to these changes in climate 

and derived aspects although specific changes are yet to be determined. Overall, confidence is high. 

Key message #3 Traceable Account

Landscape fragmentation is increasing, for example, in the context of energy development activities 

in the northern Great Plains. A highly fragmented 

landscape will hinder adaptation of species when 

climate change alters habitat composition and timing of plant development cycles.

Description of evidence base 

The key message and supporting text summarize extensive evidence documented in the Great Plains Technical Input Report.5

Technical inputs (47) on a wide range of topics were also received 

and reviewed as part of the Federal Register Notice solicitation for 

public input. 

A number of publications have explored the changes in habitat 

composition,39 plant distribution and development cycles 22,23,43

and animal distributions.36,38,44

New information and remaining uncertainties 

In general, the anticipated carbon dioxide enrichment, warming, 

and increase in precipitation variability influence vegetation primarily by affecting soil-water availability to plants. This is especially important as the transition between water surplus and water 

deficit (based on precipitation minus evapotranspiration) occurs 

across the Great Plains, with eastern areas supporting more biomass than western areas, especially given the current east-to-west 

difference in precipitation and the vegetation it supports.1 These 

effects are evident in experiments with each of the individual aspects of climate change.45 It is difficult to project, however, all 

of the interactions with all of the vegetative species of the Great 

Plains, so as to better manage ecosystems.

Several native species have been in decline due to habitat fragmentation, including quail, ocelots, and lesser prairie chickens.46

Traditional adaptation methods of migration common to the Great 

Plains, such as bison herds had historically done, are less of an 

option as animals are confined to particular locations due to habitat fragmentation. As habitats change due to invasive species of 
19: GREAT PLAINS

Traceable Accounts

461 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

plant and animals and as climate change reduces viability of native vegetation, the current landscapes may be incapable of supporting these wildlife populations.38

Assessment of confidence based on evidence 

Confidence is very high that landscape is already fragmented and 

will continue to become more fragmented as energy exploration 

expands into less suitable agriculture lands that have not been 

developed as extensively. The effects of carbon dioxide and water 

availability on individual species are well known, but there is less 

published research on the interaction among different species. 

Evidence for the impact of climate change on species is very 

high, but specific adaptation strategies used by these species are 

less certain. Because of the more limited knowledge on adaptation strategies, we rate this key message overall has having high

confidence. Our assessment is based upon historical methods, 

such as migration, used by species across the Great Plains to 

adapt to previous changes in climate and habitats and the incompatibility of those methods with current land-use practices.

Key message #4 Traceable Account

Communities that are already the most vulnerable 

to weather and climate extremes will be stressed 

even further by more frequent extreme events occurring within an already highly variable climate 

system.

Description of evidence base

The key message and supporting text summarize extensive 

evidence documented in the Technical Input Report.5 Technical inputs (47) on a wide range of topics were also received and 

reviewed as part of the Federal Register Notice solicitation for 

public input. 

Extreme events are documented for the nation (Ch. 2: Our Changing Climate, Key Message 7), and for the region in the Climate 

Trends and Scenarios for the U.S. National Climate Assessment.4

There are a few studies documenting the vulnerability of communities in remote locations with sparse infrastructure, limited 

local services, and aging populations (Ch. 14: Rural Communities),51 with some areas inhibited by language barriers.53 Changes 

in the tribal communities have been documented on a number of 

issues.54,55,56,58

New information and remaining uncertainties 

A key issue (uncertainty) is how limited financial resources will 

be dedicated to adaptation actions and the amount of will and 

attention that will be paid to decreasing vulnerability and increasing resilience throughout the region. Should the awareness 

of damage grow great enough, it may overcome the economic 

incentives for development and change perspectives, allowing 

for increased adaptive response. But if current trends continue, 

more vulnerable lands may be lost. Thus the outcome on rural 

and vulnerable populations is largely unknown.

Assessment of confidence based on evidence 

Extensive literature exists on vulnerable populations, limited resources and ability to respond to change. However, because the 

expected magnitude of changes is beyond previous experience and 

societal response is unknown, so the overall confidence is high.

Key message #5 Traceable Account

The magnitude of expected changes will exceed 

those experienced in the last century. Existing adaptation and planning efforts are inadequate to respond to these projected impacts. 

Description of evidence base

The key message and supporting text summarize extensive 

evidence documented in the Great Plains Technical Input Report.5

 Technical inputs (47) on a wide range of topics were also 

received and reviewed as part of the Federal Register Notice 

solicitation for public input. 

A number of publications have looked at the requirements for adaptation of human and natural systems to climate change. These 

requirements include large- and small-scale planning,8,59,62 emphasis on restoring ecological systems and processes,61 realizing 

the importance of natural systems,62,63 and aligning the social and 

ecological goals.64

New information and remaining uncertainties 

No clear catalog of ongoing adaptation activities exists for the 

Great Plains region. Initial steps towards such a catalog have 

been supported by the National Climate Assessment in association with NOAA’s Regional Integrated Sciences and Assessments 

teams. The short-term nature of many planning activities has 

been described.65 Until a systematic assessment is conducted, 

most examples of adaptation are anecdotal. However, stresses in 

physical and social systems are readily apparent, as described in 

the other key messages. How communities, economic sectors, 

and social groups will respond to these stresses needs further 

study.

Assessment of confidence based on evidence

Climate trends over the past century, such as North Dakota 

warming more than any other state in the contiguous U.S., 

coupled with evidence of ecological changes and projections for 

further warming indicates very high confidence that climate patterns will be substantially different than those of the preceding 

century. While systematic evidence is currently lacking, emerging studies point toward a proclivity toward short-term planning 

and incremental adjustment rather than long-term strategies for 

evolving agricultural production systems, habitat management, 

water resources and societal changes. Evidence suggests that 

adaptation is ad hoc and isolated and will likely be inadequate to 

address the magnitude of social, economic, and environmental 

challenges that face the region. Overall confidence is medium.
462

Climate Change Impacts in the United States

CHAPTER 20

SOUTHWEST

INFORMATION DRAWN FROM THIS CHAPTER IS INCLUDED IN THE HIGHLIGHTS REPORT AND IS IDENTIFIED BY THIS ICON

Recommended Citation for Chapter

Garfin, G., G. Franco, H. Blanco, A. Comrie, P. Gonzalez, T. Piechota, R. Smyth, and R. Waskom, 2014: Ch. 20: Southwest. 

Climate Change Impacts in the United States: The Third National Climate Assessment, J. M. Melillo, Terese (T.C.) Richmond, 

and G. W. Yohe, Eds., U.S. Global Change Research Program, 462-486. doi:10.7930/J08G8HMN.

On the Web: http://nca2014.globalchange.gov/report/regions/southwest

Convening Lead Authors

Gregg Garfin, University of Arizona

Guido Franco, California Energy Commission

Lead Authors

Hilda Blanco, University of Southern California

Andrew Comrie, University of Arizona

Patrick Gonzalez, National Park Service

Thomas Piechota, University of Nevada, Las Vegas 

Rebecca Smyth, National Oceanic and Atmospheric Administration

Reagan Waskom, Colorado State University
463 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

20SOUTHWEST

Key Messages

1. Snowpack and streamflow amounts are projected to decline in parts of the Southwest, decreasing 

surface water supply reliability for cities, agriculture, and ecosystems.

2. The Southwest produces more than half of the nation’s high-value specialty crops, which are 

irrigation-dependent and particularly vulnerable to extremes of moisture, cold, and heat. Reduced 

yields from increasing temperatures and increasing competition for scarce water supplies will 

displace jobs in some rural communities. 

3. Increased warming, drought, and insect outbreaks, all caused by or linked to climate change, 

have increased wildfires and impacts to people and ecosystems in the Southwest. Fire models 

project more wildfire and increased risks to communities across extensive areas.

4. Flooding and erosion in coastal areas are already occurring even at existing sea levels and 

damaging some California coastal areas during storms and extreme high tides. Sea level rise 

is projected to increase as Earth continues to warm, resulting in major damage as wind-driven 

waves ride upon higher seas and reach farther inland.

5. Projected regional temperature increases, combined with the way cities amplify heat, will pose 

increased threats and costs to public health in southwestern cities, which are home to more 

than 90% of the region’s population. Disruptions to urban electricity and water supplies will 

exacerbate these health problems.

The Southwest is the hottest and driest region in the 

United States, where the availability of water has defined 

its landscapes, history of human settlement, and modern 

economy. Climate changes pose challenges for an already 

parched region that is expected to get hotter and, in its 

southern half, significantly drier. Increased heat and changes 

to rain and snowpack will send ripple effects throughout 

the region’s critical agriculture sector, affecting the lives and 

economies of 56 million people – a population that is expected 

to increase 68% by 2050, to 94 million.1 Severe and sustained 

drought will stress water sources, already over-utilized in many 

areas, forcing increasing competition among farmers, energy 

producers, urban dwellers, and plant and animal life for the 

region’s most precious resource.

The region’s populous coastal cities face rising sea levels, 

extreme high tides, and storm surges, which pose particular 

risks to highways, bridges, power plants, and sewage treatment 

plants. Climate-related challenges also increase risks to critical 

port cities, which handle half of the nation’s incoming shipping 

containers.

Agriculture, a mainstay of the regional and national economies, 

faces uncertainty and change. The Southwest produces more 

than half of the nation’s high-value specialty crops, including 

certain vegetables, fruits, and nuts. The severity of future 

impacts will depend upon the complex interaction of pests, 

water supply, reduced chilling periods, and more rapid changes 

in the seasonal timing of crop development due to projected 

warming and extreme events.

Climate changes will increase stress on the region’s rich 

diversity of plant and animal species. Widespread tree death 

© Momatiuk - Eastcott/Corbis
464 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

20: SOUTHWEST

and fires, which already have caused billions of dollars in 

economic losses, are projected to increase, forcing wholesale 

changes to forest types, landscapes, and the communities that 

depend on them (see also Ch. 7: Forests). 

Tourism and recreation, generated by the Southwest’s 

winding canyons, snow-capped peaks, and Pacific Ocean 

beaches, provide a significant economic force that also faces 

climate change challenges. The recreational economy will be 

increasingly affected by reduced streamflow and a shorter 

snow season, influencing everything from the ski industry to 

lake and river recreation.

Observed and Projected Climate Change

The Southwest is already experiencing the impacts of climate 

change. The region has heated up markedly in recent decades, 

and the period since 1950 has been hotter than any comparably 

long period in at least 600 years (Ch. 2: Our Changing Climate, 

Key Message 3).2,3,4 The decade 2001-2010 was the warmest in 

the 110-year instrumental record, with temperatures almost 

2°F higher than historic averages, with fewer cold air outbreaks 

and more heat waves.4 Compared to relatively uniform regional 

temperature increases, precipitation trends vary considerably 

across the region, with portions experiencing decreases and 

others experiencing increases (Ch. 2: Our Changing Climate, Key 

Message 5).4 There is mounting evidence that the combination 

of human-caused temperature increases and recent drought 

has influenced widespread tree mortality,6,7 increased fire 

occurrence and area burned,8 and forest insect outbreaks 

(Ch. 7: Forests).9 Human-caused temperature increases and 

drought have also caused earlier spring snowmelt and shifted 

runoff to earlier in the year.10

Regional annual average temperatures are projected to rise 

by 2.5°F to 5.5°F by 2041-2070 and by 5.5°F to 9.5°F by 2070-

2099 with continued growth in global emissions (A2 emissions 

scenario), with the greatest increases in the summer and fall 

(Figure 20.1). If global emissions are substantially reduced (as 

in the B1 emissions scenario), projected temperature increases 

are 2.5°F to 4.5°F (2041-2070), and 3.5°F to 5.5°F (2070-2099). 

Summertime heat waves are projected to become longer 

and hotter, whereas the trend of decreasing wintertime cold 

air outbreaks is projected to continue (Ch. 2: Our Changing 

Climate, Key Message 7).11,12 These changes will directly affect 

urban public health through increased risk of heat stress, and 

urban infrastructure through increased risk of disruptions to 

electric power generation.13,14,15,16 Rising temperatures also 

have direct impacts on crop yields and productivity of key 

regional crops, such as fruit trees. 

Figure 20.1. Maps show projected changes in average, as compared to 1971-1999. 

Top row shows projections assuming heat-trapping gas emissions continue to rise 

(A2). Bottom row shows projections assuming substantial reductions in emissions 

(B1). (Figure source: adapted from Kunkel et al. 201317).

Projected Temperature Increases
465 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

20: SOUTHWEST

Projections of precipitation changes are less certain than those 

for temperature.

17,18 Under a continuation of current rising 

emissions trends (A2), reduced winter and spring precipitation 

is consistently projected for the southern part of the Southwest by 2100 as part of the general global precipitation reduction in subtropical areas. In the northern part of the region, 

projected winter and spring precipitation changes are smaller 

than natural variations. Summer and fall changes are also 

smaller than natural variations throughout the region (Ch. 2: 

Our Changing Climate, Key Message 5).17 An increase in winter 

flood hazard risk in rivers is projected due to increases in flows 

of atmospheric moisture into California’s coastal ranges and 

the Sierra Nevada (Ch. 3: Water).19 These “atmospheric rivers” 

have contributed to the largest floods in California history20

and can penetrate inland as far as Utah and New Mexico.

The Southwest is prone to drought. Southwest paleoclimate 

records show severe mega-droughts at least 50 years long.21

Future droughts are projected to be substantially hotter, 

and for major river basins such as the Colorado River Basin, 

drought is projected to become more frequent, intense, and 

longer lasting than in the historical record.18 These drought 

conditions present a huge challenge for regional management 

of water resources and natural hazards such as wildfire. In light 

of climate change and water resources treaties with Mexico, 

discussions will need to continue into the future to address 

demand pressures and vulnerabilities of groundwater and 

surface water systems that are shared along the border.

Key Message 1: Reduced Snowpack and Streamflows

Snowpack and streamflow amounts are projected to decline in parts of the Southwest, 

decreasing surface water supply reliability for cities, agriculture, and ecosystems.

Winter snowpack, which slowly melts and releases water in 

spring and summer, when both natural ecosystems and people 

have the greatest needs for water, is key to the Southwest’s 

hydrology and water supplies. Over the past 50 years across 

most of the Southwest, there has been less late-winter 

precipitation falling as snow, earlier snowmelt, and earlier 

arrival of most of the year’s streamflow.26,27 Streamflow totals 

in the Sacramento-San Joaquin, the Colorado, the Rio Grande, 

and in the Great Basin were 5% to 37% lower between 2001 

and 2010 than the 20th century average flows.4 Projections 

of further reduction of late-winter and spring snowpack and 

subsequent reductions in runoff and soil moisture28,29 pose 

increased risks to the water supplies needed to maintain the 

Southwest’s cities, agriculture, and ecosystems. 

Temperature-driven reductions in snowpack are compounded 

by dust and soot accumulation on the surface of snowpack. 

This layer of dust and soot, transported by winds from lowland 

regions, increases the amount of the sun’s energy absorbed 

by the snow. This leads to earlier snowmelt and evaporation 

– both of which have negative implications for water supply, 

alpine vegetation, and forests.30,31 The prospect of more 

lowland soil drying out from drought and human disturbances 

(like agriculture and development) makes regional dust a 

potent future risk to snow and water supplies.

In California, drinking water infrastructure needs are estimated 

at $4.6 billion annually over the next 10 years, even without 

considering the effects of climate change.32 Climate change 

will increase the cost of maintaining and improving drinking 

Vulnerabilities of native nations and border cities

The Southwest’s 182 federally recognized tribes and communities in its U.S.-Mexico border region share particularly 

high vulnerabilities to climate changes such as high temperatures, drought, and severe storms. Tribes may face loss of 

traditional foods, medicines, and water supplies due to declining snowpack, increasing temperatures, and increasing 

drought (see also Ch 12: Indigenous Peoples).22 Historic land settlements and high rates of poverty – more than double 

that of the general U.S. population23 – constrain tribes’ abilities to respond effectively to climate challenges. 

Most of the Southwest border population is concentrated in eight pairs of fast-growing, adjacent cities on either side of 

the U.S.-Mexico border (like El Paso and Juárez) with shared problems. If the 24 U.S. counties along the entire border 

were aggregated as a 51st state, they would rank near the bottom in per capita income, employment rate, insurance 

coverage for children and adults, and high school completion.24 Lack of financial resources and low tax bases for generating resources have resulted in a lack of roads and safe drinking water infrastructure, which makes it more daunting 

for tribes and border populations to address climate change issues. These economic pressures increase vulnerabilities 

to climate-related health and safety risks, such as air pollution, inadequate erosion and flood control, and insufficient 

safe drinking water.25
466 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

20: SOUTHWEST

water infrastructure, because expanded wastewater 

treatment and desalinating water for drinking are 

among the key strategies for supplementing water 

supplies. 

Conservation efforts have proven to reduce water 

use, but are not projected to be sufficient if current 

trends for water supply and demand continue.41

Large water utilities are currently attempting to 

understand how water supply and demand may 

change in conjunction with climate changes, and 

which adaptation options are most viable.42,43

Figure 20.2. Snow water equivalent (SWE) 

refers to the amount of water held in a volume 

of snow, which depends on the density of the 

snow and other factors. Figure shows projected 

snow water equivalent for the Southwest, 

as a percentage of 1971-2000, assuming 

continued increases in global emissions (A2 

scenario). The size of bars is in proportion to 

the amount of snow each state contributes to 

the regional total; thus, the bars for Arizona are 

much smaller than those for Colorado, which 

contributes the most to region-wide snowpack. 

Declines in peak SWE are strongly correlated 

with early timing of runoff and decreases in 

total runoff. For watersheds that depend on 

snowpack to provide the majority of the annual 

runoff, such as in the Sierra Nevada and in 

the Upper Colorado and Upper Rio Grande 

River Basins, lower SWE generally translates 

to reduced reservoir water storage. (Data from 

Scripps Institution of Oceanography).

Projected Snow Water Equivalent

The southwest’s renewable potential to produce energy with less water

The Southwest’s abundant geothermal, wind, and solar power-generation resources could help transform the region’s 

electric generating system into one that uses substantially more renewable energy. This transformation has already 

started, driven in part by renewable energy portfolio standards adopted by five of six Southwest states, and renewable 

energy goals in Utah. California’s law limits imports of baseload electricity generation from coal and oil and mandates 

reduction of heat-trapping greenhouse gas emissions to 1990 levels by 2020.33

As the regional climate becomes hotter and, in parts of the Southwest, drier, there will be less water available for the 

cooling of thermal power plants (Ch. 2: Our Changing Climate),34 which use about 40% of the surface water withdrawn in 

the United States.35 The projected warming of water in rivers and lakes will reduce the capacity of thermal power plants, 

especially during summer when electricity demand skyrockets.36 Wind and solar photovoltaic installations could substantially reduce water withdrawals. A large increase in the portion of power generated by renewable energy sources may be 

feasible at reasonable costs,37,38 and could substantially reduce water withdrawals (Ch. 10: Energy, Water, and Land).39

© Peter Essick/Getty Images
467 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

20: SOUTHWEST

Key Message 2: Threats to Agriculture 

The Southwest produces more than half of the nation’s high-value specialty crops, which 

are irrigation-dependent and particularly vulnerable to extremes of moisture, cold, and heat. 

Reduced yields from increasing temperatures and increasing competition for scarce 

water supplies will displace jobs in some rural communities. 

Farmers are renowned for adapting to yearly changes in the 

weather, but climate change in the Southwest could happen 

faster and more extensively than farmers’ ability to adapt. 

The region’s pastures are rain-fed (non-irrigated) and highly 

susceptible to projected drought. Excluding Colorado, more 

than 92% of the region’s cropland is irrigated, and agricultural 

uses account for 79% of all water withdrawals in the 

region.44,45,46 A warmer, drier climate is projected to accelerate 

current trends of large transfers of irrigation water to urban 

areas,47,48,49 which would affect local agriculturally dependent 

economies.

California produces about 95% of U.S. apricots, almonds, 

artichokes, figs, kiwis, raisins, olives, cling peaches, dried 

plums, persimmons, pistachios, olives, and walnuts, in 

addition to other high-value crops.50 Drought and extreme 

weather affect the market value of fruits and vegetables 

more than other crops because they have high water content 

and because sales depend on good visual appearance.51 The 

combination of a longer frost-free season, less frequent cold 

air outbreaks, and more frequent heat waves accelerates crop 

ripening and maturity, reduces yields of corn, tree fruit, and 

wine grapes, stresses livestock, and increases agricultural 

water consumption.52,53 This combination of climate changes 

is projected to continue and intensify, possibly requiring a 

northward shift in crop production, displacing existing growers 

and affecting farming communities.54,55

Winter chill periods are projected to fall below the duration 

necessary for many California trees to bear nuts and fruits, 

which will result in lower yields.56 Warm-season vegetable crops 

grown in Yolo County, one of California’s biggest producers, 

may not be viable under hotter climate conditions.54,57 Once 

temperatures increase beyond optimum growing thresholds, 

further increases in temperature, like those projected for the 

decades beyond 2050, can cause large decreases in crop yields 

and hurt the region’s agricultural economy.

Figure 20.3. Major shifts in how electricity is produced can lead to large reductions in heat-trapping gas emissions. 

Shown is an illustrative scenario in which different energy combinations could, by 2050, achieve an 80% reduction 

of heat-trapping gas emissions from 1990 levels in the electricity sector in the Southwest. For each state, that mix 

varies, with the circle representing the average hourly generation in megawatts (the number above each circle) from 

10 potential energy sources. CCS refers to carbon capture and storage. (Data from Wei et al. 2012, 201338,40). 

Scenario for Greenhouse Gas Emissions Reductions in the Electricity Sector
468 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

20: SOUTHWEST

Key Message 3: Increased Wildfire

Increased warming, drought, and insect outbreaks, all caused by or linked to climate change, 

have increased wildfires and impacts to people and ecosystems in the Southwest. Fire 

models project more wildfire and increased risks to communities across extensive areas.

Fire naturally shapes southwestern landscapes. Indeed, many 

Southwest ecosystems depend on periodic wildfire to maintain 

healthy tree densities, enable seeds to germinate, and reduce 

pests.58 Excessive wildfire destroys homes, exposes slopes to 

erosion and landslides, threatens public health, and causes 

economic damage.59,60 The $1.2 billion in damages from the 

2003 Grand Prix fire in southern California illustrates the high 

cost of wildfires.60

Beginning in the 1910s, the Federal Government developed a 

national policy of attempting to extinguish every fire, which 

allowed wood and other fuels to over-accumulate61 and urban 

development to encroach on fire-prone areas. These changes 

have also contributed to increasing fire risk.

Increased warming due to climate change,3 drought, insect 

infestations,62 and accumulation of woody fuels and nonnative grasses63,64 make the Southwest vulnerable to increased 

wildfire. Climate outweighed other factors in determining 

burned area in the western U.S. from 1916 to 2003,65 a finding 

confirmed by 3000-year long reconstructions of southwestern 

fire history.66,67,68 Between 1970 and 2003, warmer and drier 

conditions increased burned area in western U.S. mid-elevation 

conifer forests by 650% (Ch. 7: Forests, Key Message 1).8

Drought and increased temperatures due to climate change 

have caused extensive tree death across the Southwest.7,69

In addition, winter warming due to climate change has 

exacerbated bark beetle outbreaks by allowing more beetles, 

which normally die in cold weather, to survive and reproduce.70

Wildfire and bark beetles killed trees across 20% of Arizona 

and New Mexico forests from 1984 to 2008.62

Numerous fire models project more wildfire as climate change 

continues.64,71,72,73,74 Models project a doubling of burned area 

in the southern Rockies,73 and up to a 74% increase in burned 

area in California,74 with northern California potentially 

experiencing a doubling under a high emissions scenario 

toward the end of the century. Fire contributes to upslope 

shifting of vegetation, spread of invasive plants after extensive 

and intense fire, and conversion of forests to woodland or 

grassland.63,75

Figure 20.4. The frost-free season is defined as 

the period between the last occurrence of 32°F 

in spring and the first occurrence of 32°F in the 

subsequent fall. The chart shows significant 

increases in the number of consecutive frostfree days per year in the past three decades 

compared to the 1901-2010 average. Increased 

frost-free season length, especially in already 

hot and moisture-stressed regions like the 

Southwest, is projected to lead to further heat 

stress on plants and increased water demands 

for crops. Higher temperatures and more frostfree days during winter can lead to early bud burst 

or bloom of some perennial plants, resulting in 

frost damage when cold conditions occur in late 

spring (see Ch. 6: Agriculture); in addition, with 

higher winter temperatures, some agricultural 

pests can persist year-round, and new pests 

and diseases may become established.47 (Figure 

source: Hoerling et al. 20134).

Longer Frost-Free Season Increases Stress on Crops © AP Photo/The Press-Enterprise, Terry Pierson
469 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

20: SOUTHWEST

Historical and projected climate change makes two-fifths (40%) 

of the region vulnerable to these shifts of major vegetation 

types or biomes; notably threatened are the conifer forests of 

southern California and sky islands of Arizona.71

Prescribed burning, mechanical thinning, and retention of large 

trees can help some southwestern forest ecosystems adapt to 

climate change.68,76 These adaptation measures also reduce 

emissions of the gases that cause climate change because 

long-term storage of carbon in large trees can outweigh shortterm emissions from prescribed burning.61,77

Key Message 4: Sea Level Rise and Coastal Damage

Flooding and erosion in coastal areas are already occurring even at existing sea levels and 

damaging some California coastal areas during storms and extreme high tides. Sea level rise 

is projected to increase as Earth continues to warm, resulting in major damage as 

wind-driven waves ride upon higher seas and reach farther inland.

In the last 100 years, sea level has risen along the California 

coast by 6.7 to 7.9 inches.78 In the last decade, high tides on 

top of this sea level rise have contributed to new damage to 

infrastructure, such as the inundation of Highway 101 near San 

Francisco and backup of seawater into the San Francisco Bay 

Area sewage systems.

Although sea level along the California coast has been relatively 

constant since 1980, both global and relative Southwest sea 

levels are expected to increase at accelerated rates.78,79,80

During the next 30 years, the greatest impacts will be seen 

during high tides and storm events. Rising sea level will allow 

more wave energy to reach farther inland and extend high tide 

periods, worsening coastal erosion on bluffs and beaches and 

increasing flooding potential.18,81,82,83,84

The result will be impacts to the nation’s largest ocean-based 

economy, which is estimated at $46 billion annually.85,86 If 

adaptive action is not taken, coastal highways, bridges, and 

other transportation infrastructure (such as the San Francisco 

and Oakland airports) are at increased risk of flooding with 

a 16-inch rise in sea level in the next 50 years,5 an amount 

consistent with the 1 to 4 feet of expected global increase in 

sea level (see Ch. 2: Our Changing Climate, Key Message 10). 

In Los Angeles, sea level rise 

poses a threat to groundwater 

supplies and estuaries,82,87

by potentially contaminating 

groundwater with seawater, 

or increasing the costs to 

protect coastal freshwater 

aquifers.88

Projected increases in 

extreme coastal flooding as 

a result of sea level rise will 

increase human vulnerability 

to coastal flooding events. 

Currently, 260,000 people 

in California are at risk from 

what is considered a oncein-100-year flood.82 With 

a sea level rise of about 

three feet (in the range of 

projections for this century – 

Ch. 2: Our Changing Climate, 

Key Message 10)78,80 and at 

current population densities, 

420,000 people would be at 

risk from the same kind of 

100-year flood event,85 based 

on existing exposure levels. 

Highly vulnerable populations 

Figure 20.5. King tides, which typically 

happen twice a year as a result of 

a gravitational alignment of the sun, 

moon, and Earth, provide a preview 

of the risks rising sea levels may 

present along California coasts in 

the future. While king tides are the 

extreme high tides today, with projected 

future sea level rise, this level of water 

and flooding will occur during regular 

monthly high tides. During storms and 

future king tides, more coastal flooding 

and damage will occur. The King Tide 

Photo Initiative encourages the public 

to visually document the impact of 

rising waters on the California coast, 

as exemplified during current king tide 

events. Photos show water levels along 

the Embarcadero in San Francisco, 

California during relatively normal tides 

(top), and during an extreme high tide 

or “king tide” (bottom). (Photo credit: 

Mark Johnsson).

Coastal Risks Posed by Sea Level Rise and High Tides
470 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

20: SOUTHWEST

– people less able to prepare, respond, or recover from natural 

disaster due to age, race, or income – make up approximately 

18% of the at-risk population (Ch. 25: Coasts).85,89

The California state government, through its Ocean and Coastal 

Resources Adaptation Strategy, along with local governments, 

is using new sea level mapping and information about social 

vulnerability to undertake coastal adaptation planning. NOAA 

has created an interactive map showing areas that would 

be affected by sea level rise (http://www.csc.noaa.gov/slr/

viewer/#). 

Key Message 5: Heat Threats to Health 

Projected regional temperature increases, combined with the way cities amplify heat, will 

pose increased threats and costs to public health in southwestern cities, which are home to 

more than 90% of the region’s population. Disruptions to urban electricity and water 

supplies will exacerbate these health problems.

The Southwest has the highest percentage of its population 

living in cities of any U.S. region. Its urban population rate, 

92.7%, is 12% greater than the national average.90 Increasing 

metropolitan populations already pose challenges to providing 

adequate domestic water supplies, and the combination of 

increased population growth and projected increased risks 

to surface water supplies will add further challenges.91,92

Tradeoffs are inevitable between conserving water to help 

meet the demands of an increasing population and providing 

adequate water for urban greenery to reduce increasing urban 

temperatures. 

Urban infrastructures are especially vulnerable because of 

their interdependencies; strains in one system can cause 

disruptions in another (Ch. 11: Urban, Key Message 2; Ch. 9: 

Human Health).16,93 For example, an 11-minute power system 

disturbance in September 2011 cascaded into outages that left 

1.5 million San Diego residents without power for 12 hours;94

the outage disrupted pumps and water service, causing 1.9 

million gallons of sewage to spill near beaches.95 Extensive use 

of air conditioning to deal with high temperatures can quickly 

increase electricity demand and trigger cascading energy 

system failures, resulting in blackouts or brownouts.14,15

Figure 20.6. The projected increase in heat waves in Southwest cities (Ch. 2: Our Changing Climate, Key Message 7) increases 

the chances that a chain of escalating effects could lead to serious increases in illness and death due to heat stress. The top of the 

figure provides some of the links in that chain, while the bottom of the figure provides adaptation and improved governance options 

that can reduce this vulnerability and improve the resilience of urban infrastructure and community residents. 

Urban Heat and Public Health
471 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

20: SOUTHWEST

Heat stress, a recurrent health problem for urban residents, has 

been the leading weather-related cause of death in the United 

States since 1986, when record keeping began96 – and the 

highest rates nationally are found in Arizona.97 The effects of 

heat stress are greatest during heat waves lasting several days 

or more, and heat waves are projected to increase in frequency, 

duration, and intensity,11,13,98 become more humid,11 and cause 

a greater number of deaths.99 Already, severe heat waves, such 

as the 2006 ten-day California event, have resulted in high 

mortality, especially among elderly populations.100 In addition, 

evidence indicates a greater likelihood of impacts in less 

affluent neighborhoods, which typically lack shade trees and 

other greenery and have reduced access to air conditioning.101

Exposure to excessive heat can also aggravate existing human 

health conditions, like for those who suffer from respiratory or 

heart disease.99 Increased temperatures can reduce air quality, 

because atmospheric chemical reactions proceed faster in 

warmer conditions. The outcome is that heat waves are often 

accompanied by increased ground-level ozone,102 which can 

cause respiratory distress. Increased temperatures and longer 

warm seasons will also lead to shifts in the distribution of 

disease-transmitting mosquitoes (Ch. 9: Human Health, Key 

Message 1).97
472 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

20: SOUTHWEST

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482 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

SUPPLEMENTAL MATERIAL

TRACEABLE ACCOUNTS

20: SOUTHWEST

Process for Developing Key Messages

A central component of the assessment process was the Southwest Regional Climate assessment workshop that was held August 

1-4, 2011, in Denver, CO with more than 80 participants in a 

series of scoping presentations and workshops. The workshop began the process leading to a foundational Technical Input Report 

(TIR) report.103 The TIR consists of nearly 800 pages organized 

into 20 chapters that were assembled by 122 authors representing a wide range of inputs, including governmental agencies, nongovernmental organizations, tribes, and other entities. The report 

findings were described in a town hall meeting at the American 

Geophysical Union’s annual fall meeting in 2011, and feedback 

was collected and incorporated into the draft. 

The chapter author team engaged in multiple technical discussions 

through more than 15 biweekly teleconferences that permitted a 

careful review of the foundational TIR103 and of approximately 125 

additional technical inputs provided by the public, as well as the 

other published literature and professional judgment. The chapter 

author team then met at the University of Southern California on 

March 27-28, 2012, for expert deliberation of draft key messages 

by the authors. Each key message was defended before the entire 

author team prior to the key message being selected for inclusion. 

These discussions were supported by targeted consultation with 

additional experts by the lead author of each message, and they 

were based on criteria that help define “key vulnerabilities, which 

include magnitude, timing, persistence and reversibility, likelihood 

and confidence, potential for adaptation, distribution, and importance of the vulnerable system.”104

Key message #1 Traceable Account

Snowpack and streamflow amounts are projected to decline in parts of the Southwest, decreasing 

surface water supply reliability for cities, agriculture, and ecosystems. 

Description of evidence base

The key message was chosen based on input from the extensive 

evidence documented in the Southwest Technical Input Report103

and additional technical input reports received as part of the 

Federal Register Notice solicitation for public input, as well as 

stakeholder engagement leading up to drafting the chapter. 

Key Message 5 in Chapter 2, Our Changing Climate, also provides 

evidence for declining precipitation across the United States, and 

a regional study17 discusses regional trends and scenarios for the 

Southwest. 

Over the past 50 years, there has been a reduction in the amount 

of snow measured on April 1 as a proportion of the precipitation 

falling in the corresponding water-year (October to September), 

which affects the timing of snowfed rivers. The implication 

of this finding is that the lower the proportion of April 1 snow 

water equivalent in the water-year-to-date precipitation, the more 

rapid the runoff, and the earlier the timing of center-of-mass 

of streamflow in snowfed rivers.26,27 For the “recent decade” 

(2001 to 2010), snowpack evidence is from U.S. Department of 

Agriculture (USDA) Natural Resources Conservation Service snow 

course data, updated through 2010. One study4 has analyzed 

streamflow amounts for the region’s four major river basins, the 

Colorado, Sacramento-San Joaquin, Great Basin (Humboldt River, 

NV), and the Rio Grande; data are from the U.S. Department of 

the Interior – Bureau of Reclamation, California Department of 

Water Resources, U.S. Geological Survey, and the International 

Boundary and Water Commission (U.S. Section), respectively. 

These data are backed by a rigorous detection and attribution 

study.10 Projected trends18 make use of downscaled climate 

parameters for 16 global climate models (GCMs), and hydrologic 

projections for the Colorado River, Rio Grande, and SacramentoSan Joaquin River System. 

Based on GCM projections, downscaled and run through the 

variable infiltration capacity (VIC) hydrological model,105 there 

are projected reductions in spring snow accumulation and total 

annual runoff, leading to reduced surface water supply reliability 

for much of the Southwest, with greater impacts occurring during 

the second half of this century.18,28

Future flows in the four major Southwest rivers are projected to 

decline as a result of a combination of increased temperatures, 

increased evaporation, less snow, and less persistent snowpack. 

These changes have been projected to result in decreased surface 

water supplies, which will have impacts for allocation of water 

resources to major uses, such as urban drinking water, agriculture, 

and ecosystem flows.
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Traceable Accounts

483 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

New information and remaining uncertainties

Different model simulations predict different levels of snow 

loss. These differences arise because of uncertainty in climate 

change warming and precipitation projections due to differences 

among GCMs, uncertainty in regional downscaling, uncertainty 

in hydrological modeling, differences in emissions, aerosols, 

and other forcings, and because differences in the hemispheric 

and regional-scale atmospheric circulation patterns produced by 

different GCMs produce different levels of snow loss in different 

model simulations.

In addition to the aforementioned uncertainties in regional 

climate and hydrology projections, projection of future surface 

water supply reliability includes at least the following additional 

uncertainties: 1) changes in water management, which depend on 

agency resources and leadership and cooperation of review boards 

and the public;106 2) management responses to non-stationarity;107

3) legal, economic, and institutional options for augmenting 

existing water supplies, adding underground water storage and 

recovery infrastructure, and fostering further water conservation 

(for example, Udall 2013108); 4) adjudication of unresolved water 

rights; and 5) local, state, regional, and national policies related 

to the balance of agricultural, ecosystem, and urban water use (for 

example, Reclamation 201143).

Assessment of confidence based on evidence

There is high confidence in the continued trend of declining 

snowpack and streamflow in parts of the Southwest given the 

evidence base and remaining uncertainties. 

For the impacts on water supply, there is high confidence that 

reduced surface water supply reliability will affect the region’s 

cities, agriculture, and ecosystems. 

Key message #2 Traceable Account

The Southwest produces more than half of the 

nation’s high-value specialty crops, which are irrigation-dependent and particularly vulnerable to extremes of moisture, cold, and heat. Reduced yields 

from increasing temperatures and increasing competition for scarce water supplies will displace jobs 

in some rural communities.

Description of evidence base

Increased competition for scarce water was presented in the 

first key message and in the foundational Technical Input Report 

(TIR).103 U.S. temperatures, including those for the Southwest 

region, have increased and are expected to continue to rise (Ch. 2: 

Our Changing Climate, Key Message 3). Heat waves have become 

more frequent and intense and droughts are expected to become 

more intense in the Southwest (Ch. 2: Our Changing Climate, Key 

Message 7). The length of the frost-free season in the Southwest 

has been increasing, and frost-free season length is projected to 

increase (Ch. 2: Our Changing Climate, Key Message 4). A regional 

study17 discusses the trends and scenarios in the Southwest for 

moisture, cold, heat, and their extremes. 

There is abundant evidence of irrigation dependence and 

vulnerability of high-value specialty crops to extremes of moisture, 

cold, and heat, including, prominently, the 2009 National Climate 

Assessment109 and the foundational TIR.103 Southwest agricultural 

production statistics and irrigation dependence of that production 

is delineated in the USDA 2007 Census of Agriculture45 and the 

USDA Farm and Ranch Irrigation Survey.46

Reduced Yields. Even under the most conservative emissions 

scenarios evaluated (the combination of SRES B1emissions 

scenario with statistically downscaled winter chill projections 

from the HADCM3 climate model), one study56 projected that 

required winter chill periods will fall below the number of hours 

that are necessary for many of the nut- and fruit-bearing trees 

of California, and yields are projected to decline as a result. A 

second study54 found that California wheat acreage and walnut 

acreage will decline due to increased temperatures. Drought and 

extreme weather may have more effect on the market value of 

fruits and vegetables, as opposed to other crops, because fruits 

and vegetables have high water content and because consumers 

expect good visual appearance and flavor.51 Extreme daytime 

and nighttime temperatures have been shown to accelerate crop 

ripening and maturity, reduce yield of crops such as corn, fruit 

trees, and vineyards, cause livestock to be stressed, and increase 

water consumption in agriculture.

53

Confidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent 

results, well documented and 

accepted methods, etc.), high 

consensus

High

Moderate evidence (several sources, some consistency, methods 

vary and/or documentation limited, 

etc.), medium consensus

Medium

Suggestive evidence (a few 

sources, limited consistency, models incomplete, methods emerging, 

etc.), competing schools of thought

Low

Inconclusive evidence (limited 

sources, extrapolations, inconsistent findings, poor documentation 

and/or methods not tested, etc.), 

disagreement or lack of opinions 

among experts
20: SOUTHWEST

Traceable Accounts

484 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Irrigation water transfers to urban. Warmer, drier future scenarios 

portend large transfers of irrigation water to urban areas even 

though agriculture will need additional water to meet crop demands, 

affecting local agriculturally-dependent economies.55 In particular 

areas of the Southwest (most notably lower-central Arizona), a 

significant reduction in irrigated agriculture is already underway 

as land conversion occurs near urban centers.48 Functioning water 

markets, which may require legal and institutional changes, can 

enable such transfers and reduce the social and economic impacts 

of water shortages to urban areas.47 The economic impacts of 

climate change on Southwest fruit and nut growers are projected 

to be substantial and will result in a northward shift in production 

of these crops, displacing growers and affecting communities. 

New information and remaining uncertainties

Competition for water is an uncertainty. The extent to which 

water transfers take place depends on whether complementary 

investments in conveyance or storage infrastructure are made. 

Currently, there are legal and institutional restrictions limiting 

water transfers across state and local jurisdictions. It is uncertain 

whether infrastructure investments will be made or whether 

institutional innovations facilitating transfers will develop. 

Institutional barriers will be greater if negative third-party 

effects of transfers are not adequately addressed. Research 

that would improve the information base to inform future water 

transfer debates includes: 1) estimates of third party impacts, 2) 

assessment of institutional mechanisms to reduce those impacts, 

3) environmental impacts of water infrastructure projects, and 4) 

options and costs of mitigating those environmental impacts.

Extremes and phenology. A key uncertainty is the timing of 

extreme events during the phenological stage of the plant or the 

growth cycle of the animal. For example, plants are more sensitive 

to extreme high temperatures and drought during the pollination 

stage compared to vegetative growth stages. 

Genetic improvement potential. Crop and livestock reduction 

studies by necessity depend on assumptions about adaptive 

actions by farmers and ranchers. However, agriculture has proven 

to be highly adaptive in the past. A particularly high uncertainty 

is the ability of conventional breeding and biotechnology to keep 

pace with the crop plant and animal genetic improvements needed 

for adaptation to climate-induced biotic and abiotic stresses. 

Assessment of confidence based on evidence

Although evidence includes studies of observed climate and 

weather impacts on agriculture, projections of future changes 

using climate and crop yield models and econometric models show 

varying results depending on the choice of crop and assumptions 

regarding water availability. For example, projections of 2050 

California crop yields show reductions in field crop yields, based 

on assumptions of a 21% decline in agricultural water use, shifts 

away from water-intensive crops to high-value specialty crops, 

and development of a more economical means of transferring 

water from northern to southern California.47 Other studies, 

using projections of a dry, warmer future for California, and an 

assumption that water will flow from lower- to higher-valued uses 

(such as urban water use), generated a 15% decrease in irrigated 

acreage and a shift from lower- to higher-valued crops.49

Because net reductions in the costs of water shortages depend 

on multiple institutional responses, it is difficult as yet to locate 

a best estimate of water transfers between zero and the upper 

bound. Water scarcity may also be a function of tradeoffs between 

economic returns from agricultural production and returns for 

selling off property or selling water to urban areas (for example, 

Imperial Valley transfers to San Diego).

Given the evidence base and remaining uncertainties, confidence 

is high in this key message. 

Key Message #3 Traceable Account

Increased warming, drought, and insect outbreaks, all caused by or linked to climate change, 

have increased wildfires and impacts to people and 

ecosystems in the Southwest. Fire models project 

more wildfire and increased risks to communities 

across extensive areas.

Description of evidence base

Increased warming and drought are extensively described in the 

foundational Technical Input Report (TIR).103 U.S. temperatures 

have increased and are expected to continue to rise (Ch. 2: Our 

Changing Climate, Key Message 3). There have been regional 

changes in droughts, and there are observed and projected 

changes in cold and heat waves and droughts (Ch. 2: Our 

Changing Climate, Key Message 7) for the nation. A study for the 

Southwest17 discusses trends and scenarios in both cold waves 

and heat waves. 

Analyses of weather station data from the Southwest have detected 

changes from 1950 to 2005 that favor wildfire, and statistical 

analyses have attributed the changes to anthropogenic climate 

change. The changes include increased temperatures,3 reduced 

snowpack,27 earlier spring warmth,30 and streamflow.10 These 

climate changes have increased background tree mortality rates 

from 1955 to 2007 in old-growth conifer forests in California, 

Colorado, Utah, and the northwestern states7 and caused extensive 

piñon pine mortality in Arizona, Colorado, New Mexico, and Utah 

between 1989 and 2003.69

Climate factors contributed to increases in wildfire in the previous 

century. In mid-elevation conifer forests of the western United 

States, increases in spring and summer temperatures, earlier 

snowmelt, and longer summers increased fire frequency by 400% 

and burned area by 650% from 1970 to 2003.8 Multivariate 

analysis of wildfire across the western U.S. from 1916 to 2003 
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485 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

indicates that climate was the dominant factor controlling 

burned area, even during periods of human fire suppression.65

Reconstruction of fires of the past 400 to 3000 years in the 

western U.S.66 and in Yosemite and Sequoia National Parks in 

California67,68 confirm that temperature and drought are the 

dominant factors explaining fire occurrence.

Four different fire models project increases in fire frequency 

across extensive areas of the Southwest in this century.71,72,73,74

Multivariate statistical generalized additive models64,72 project 

extensive increases across the Southwest, but the models project 

decreases when assuming that climate alters patterns of net 

primary productivity. Logistic regressions74 project increases 

across most of California, except for some southern parts of the 

state, with average fire frequency increasing 37% to 74%. Linear 

regression models project up to a doubling of burned area in the 

southern Rockies by 2070 under emissions scenarios B1 or A2.73

The MC1 dynamic global vegetation model projects increases 

in fire frequencies on 40% of the area of the Southwest from 

2000 to 2100 and decreases on 50% of the areas for emissions 

scenarios B1 and A2.71

Excessive wildfire destroys homes, exposes slopes to erosion 

and landslides, and threatens public health, causing economic 

damage.59,60 Further impacts to communities and various 

economies (local, state, and national) have been projected.74

New information and remaining uncertainties

Uncertainties in future projections derive from the inability of 

models to accurately simulate all past fire patterns, and from 

the different GCMs, emissions scenarios, and spatial resolutions 

used by different fire model projections. Fire projections depend 

highly on the spatial and temporal distributions of precipitation 

projections, which vary widely across GCMs. Although models 

generally project future increases in wildfire, uncertainty remains 

on the exact locations. Research groups continue to refine the fire 

models.

Assessment of confidence based on evidence

There is high confidence in this key message given the extensive 

evidence base and discussed uncertainties. 

Key message #4 Traceable Account

Flooding and erosion in coastal areas are already 

occurring even at existing sea levels and damaging some California coastal areas during storms 

and extreme high tides. Sea level rise is projected 

to increase as Earth continues to warm, resulting 

in major damage as wind-driven waves ride upon 

higher seas and reach farther inland. 

Description of evidence base

The key message and supporting text summarizes extensive 

evidence documented in the Technical Input Report.103 Several 

studies document potential coastal flooding, erosion, and winddriven wave damages in coastal areas of California due to sea level 

rise (for example, Bromirski et al. 2012; Heberger et al. 2011, and 

Revell et al. 201181,82). Global sea level has risen, and further rise 

of 1 to 4 feet is projected by 2100 (Ch. 2: Our Changing Climate, 

Key Message 10). 

All of the scientific approaches to detecting sea level rise come to 

the conclusion that a warming planet will result in higher sea levels. 

In addition, numerous recent studies78,80 produce much higher sea 

level rise projections for the rest of this century as compared to 

the projections in the most recent report of the Intergovernmental 

Panel on Climate Change83 for the rest of this century. 

New information and remaining uncertainties

There is strong recent evidence from satellites such as GRACE110

and from direct observations that glaciers and ice caps worldwide 

are losing mass relatively rapidly, contributing to the recent 

increase in the observed rate of sea level rise. 

Major uncertainties are associated with sea level rise projections, 

such as the behavior of ice sheets with global warming and the 

actual level of global warming that the Earth will experience in 

the future.78,80 Regional sea level rise projections are even more 

uncertain than the projections for global averages because local 

factors such as the steric component (changes in the volume of 

water with changes in temperature and salinity) of sea level rise 

at regional levels and the vertical movement of land have large 

uncertainties.78 However, it is virtually certain that sea levels will 

go up with a warming planet as demonstrated in the paleoclimatic 

record, modeling, and from basic physical arguments.

Assessment of confidence based on evidence

Given the evidence, especially since the last IPCC report,83 there 

is very high confidence the sea level will continue to rise and that 

this will entail major damage to coastal regions in the Southwest. 

There is also very high confidence that flooding and erosion in 

coastal areas are already occurring even at existing sea levels and 

damaging some areas of the California coast during storms and 

extreme high tides. 

Key message #5 Traceable Account

Projected regional temperature increases, combined with the way cities amplify heat, will pose 

increased threats and costs to public health in 

southwestern cities, which are home to more than 

90% of the region’s population. Disruptions to urban electricity and water supplies will exacerbate 

these health problems.

Description of evidence base

There is excellent agreement regarding the urban heat island 

effect and exacerbation of heat island temperatures by increases 

in regional temperatures caused by climate change. There is 
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486 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

abundant evidence of urban heat island effect for some Southwest 

cities (for example, Sheridan et al.98), as well as several studies, 

some from outside the region, of the public health threats of urban 

heat to residents (for example, Ch. 9: Human Health, Ostro et al. 

2009, 200199,100). Evidence includes observed urban heat island 

studies and modeling of future climates, including some climate 

change modeling studies for individual urban areas (for example, 

Phoenix and Los Angeles). There is wide agreement in Southwest 

states that increasing temperatures combined with projected 

population growth will stress urban water supplies and require 

continued water conservation and investment in new water supply 

options. There is substantial agreement that disruption to urban 

electricity may cause cascading impacts, such as loss of water, 

and that projected diminished supplies will pose challenges for 

urban cooling (for example, the need for supplemental irrigation 

for vegetation-based cooling). However, there are no studies on 

urban power disruption induced by climate change.

With projected surface water losses, and increasing water demand 

due to increasing temperatures and population, water supply 

in Southwest cities will require greater conservation efforts 

and capital investment in new water supply sources.92 Several 

southwestern states, including California, New Mexico, and 

Colorado have begun to study climate impacts to water resources, 

including impacts in urban areas.91

The interdependence of infrastructure systems is well established, 

especially the dependence of systems on electricity and 

communications and control infrastructures, and the potential 

cascading effects of breakdowns in infrastructure systems.16

The concentration of infrastructures in urban areas adds to the 

vulnerability of urban populations to infrastructure breakdowns. 

This has been documented in descriptions for major power 

outages such as the Northeast power blackout of 2003, or the 

recent September 2011 San Diego blackout.94

A few references point to the role of urban power outages in 

threatening public health due to loss of air conditioning14 and 

disruption to water supplies.94

New information and remaining uncertainties

Key uncertainties include the intensity and spatial extent of 

drought and heat waves. Uncertainty is also associated with 

quantification of the impact of temperature and water availability 

on energy generation, transmission, distribution, and consumption 

– all of which have an impact on possible disruptions to urban 

electricity. Major disruptions are contingent on a lack of operator 

response and/or adaptive actions such as installation of adequate 

electricity-generating capacity to serve the expected enhanced 

peak electricity demand. Thus a further uncertainty is the extent 

to which adaptation actions are taken.

Assessment of confidence based on evidence

The urban heat island effect is well demonstrated and hence 

projected climate-induced increases to heat will increase exposure 

to heat-related illness. Electricity disruptions are a key uncertain 

factor, and potential reductions in water supply not only may 

reduce hydropower generation, but also availability of water for 

cooling of thermal power plants. 

Based on the substantial evidence and the remaining uncertainties, 

confidence in each aspect of the key message is high. 
487

Climate Change Impacts in the United States

CHAPTER 21

NORTHWEST

INFORMATION DRAWN FROM THIS CHAPTER IS INCLUDED IN THE HIGHLIGHTS REPORT AND IS IDENTIFIED BY THIS ICON

Recommended Citation for Chapter

Mote, P., A. K. Snover, S. Capalbo, S. D. Eigenbrode, P. Glick, J. Littell, R. Raymondi, and S. Reeder, 2014: Ch. 21: Northwest. Climate Change Impacts in the United States: The Third National Climate Assessment, J. M. Melillo, Terese (T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 487-513. doi:10.7930/J04Q7RWX.

On the Web: http://nca2014.globalchange.gov/report/regions/northwest

Convening Lead Authors

Philip Mote, Oregon State University

Amy K. Snover, University of Washington

Lead Authors

Susan Capalbo, Oregon State University

Sanford D. Eigenbrode, University of Idaho

Patty Glick, National Wildlife Federation

Jeremy Littell, U.S. Geological Survey 

Richard Raymondi, Idaho Department of Water Resources

Spencer Reeder, Cascadia Consulting Group 
488 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

21NORTHWEST

Key Messages

1. Changes in the timing of streamflow related to changing snowmelt are already observed and will 

continue, reducing the supply of water for many competing demands and causing far-reaching 

ecological and socioeconomic consequences.

2. In the coastal zone, the effects of sea level rise, erosion, inundation, threats to infrastructure and 

habitat, and increasing ocean acidity collectively pose a major threat to the region.

3. The combined impacts of increasing wildfire, insect outbreaks, and tree diseases are already 

causing widespread tree die-off and are virtually certain to cause additional forest mortality by 

the 2040s and long-term transformation of forest landscapes. Under higher emissions scenarios, 

extensive conversion of subalpine forests to other forest types is projected by the 2080s.

4. While the agriculture sector’s technical ability to adapt to changing conditions can offset some 

adverse impacts of a changing climate, there remain critical concerns for agriculture with respect 

to costs of adaptation, development of more climate resilient technologies and management, and 

availability and timing of water.

With craggy shorelines, volcanic mountains, and high sage 

deserts, the Northwest’s complex and varied topography 

contributes to the region’s rich climatic, geographic, social, 

and ecologic diversity. Abundant natural resources – timber, 

fisheries, productive soils, and plentiful water – remain 

important to the region’s economy.

Snow accumulates in mountains, melting in spring to power 

both the region’s rivers and economy, creating enough 

hydropower (40% of national total)1 to export 2 to 6 million 

megawatt hours per month.2 Snowmelt waters crops in the 

dry interior, helping the region produce tree fruit (number 

one in the world) and almost $17 billion worth of agricultural 

commodities, including 55% of potato, 15% of wheat, and 11% 

of milk production in the United States.3

Seasonal water patterns shape the life cycles of the region’s 

flora and fauna, including iconic salmon and steelhead, and 

forested ecosystems, which cover 47% of the landscape.4

Along more than 4,400 miles of coastline, regional economic 

centers are juxtaposed with diverse habitats and ecosystems 

that support thousands of species of fish and wildlife, including 

commercial fish and shellfish resources valued at $480 million 

in 2011.5

Adding to the influence of climate, human activities have 

altered natural habitats, threatened species, and extracted so 

much water that there are already conflicts among multiple 

users in dry years. More recently, efforts have multiplied to 

balance environmental restoration and economic growth while 

evaluating climate risks. As conflicts and tradeoffs increase, 

the region’s population continues to grow, and the regional 

consequences of climate change continue to unfold. The need 

to seek solutions to these conflicts is becoming increasingly 

urgent.

The Northwest’s economy, infrastructure, natural systems, 

public health, and vitally important agriculture sector all face 

important climate change related risks. Those risks – and 

possible adaptive responses – will vary significantly across the 

region.6 Impacts on infrastructure, natural systems, human 

health, and economic sectors, combined with issues of social 

and ecological vulnerability, will play out quite differently in 

largely natural areas, like the Cascade Range or Crater Lake 

National Park, than in urban areas like Seattle and Portland 

(Ch. 11: Urban),7 or among the region’s many Native American 

tribes, like the Umatilla or the Quinault (Ch. 12: Indigenous 

Peoples).8

As climatic conditions diverge from those that determined 

patterns of development and resource use in the last century, 

and as demographic, economic, and technological changes 

also stress local systems, efforts to cope with climate change 

would benefit from an evolving, iterative risk management 

approach.9
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Observed Climate Change

Temperatures increased across the region from 1895 to 2011, 

with a regionally averaged warming of about 1.3°F.10 While 

precipitation has generally increased, trends are small as 

compared to natural variability. Both increasing and decreasing 

trends are observed among various locations, seasons, and 

time periods of analysis (Ch. 2: Our Changing Climate, Figure 

2.12). Studies of observed changes in extreme precipitation 

use different time periods and definitions of “extreme,” but 

none find statistically significant changes in the Northwest.11

These and other climate trends include contributions from 

both human influences (chiefly heat-trapping gas emissions) 

and natural climate variability, and consequently are not 

projected to be uniform or smooth across the country or over 

time (Ch. 2: Our Changing Climate, Key Message 3). They are 

also consistent with expected changes due to human activities 

(Ch. 2: Our Changing Climate, Key Message 1).

Projected Climate Change

An increase in average annual temperature of 3.3°F to 9.7°F 

is projected by 2070 to 2099 (compared to the period 1970 

to 1999), depending largely on total global emissions of heattrapping gases. The increases are projected to be largest in 

summer. This chapter examines a range of scenarios, including 

ones where emissions increase and then decline, leading to 

lower (B1 and RCP 4.5) and medium (A1B) total emissions, 

and scenarios where emissions continue to rise with higher 

totals (A2, A1FI, and RCP 8.5 scenarios). Change in annual 

average precipitation in the Northwest is projected to be 

within a range of an 11% decrease to a 12% increase for 2030 

to 2059 and a 10% decrease to an 18% increase for 2070 to 

209912 for the B1, A1B, and A2 scenarios (Ch. 2: Our Changing 

Climate). For every season, some models project decreases 

and some project increases (Ch. 2: Our Changing Climate, 

Key Message 5),

10,12 yet one aspect of seasonal changes in 

precipitation is largely consistent across climate models: for 

scenarios of continued growth in global heat-trapping gas 

emissions, summer precipitation is projected to decrease by 

as much as 30% by the end of the century (Ch. 2: Our Changing 

Climate).10,12 Northwest summers are already dry and although 

a 10% reduction (the average projected change for summer) is 

a small amount of precipitation, unusually dry summers have 

many noticeable consequences, including low streamflow west 

of the Cascades13 and greater extent of wildfires throughout 

the region.14 Note that while projected temperature increases 

are large relative to natural variability, the relatively small 

projected changes in precipitation are likely to be masked by 

natural variability for much of the century.15

Ongoing research on the implications of these and other 

changes largely confirms projections and analyses made over 

the last decade, while providing more information about how 

climate impacts are likely to vary from place to place within 

the region. In addition, new areas of concern, such as ocean 

acidification, have arisen.

Key Message 1: Water-related Challenges

Changes in the timing of streamflow related to changing snowmelt have been observed and 

will continue, reducing the supply of water for many competing demands and causing farreaching ecological and socioeconomic consequences.

Description of Observed and Projected Changes

Observed regional warming has been linked to changes in the 

timing and amount of water availability in basins with significant 

snowmelt contributions to streamflow. Since around 1950, 

area-averaged snowpack on April 1 in the Cascade Mountains 

decreased about 20%,16 spring snowmelt occurred 0 to 30 

days earlier depending on location,17 late winter/early spring 

streamflow increases ranged from 0% to greater than 20% as a 

fraction of annual flow,18,19 and summer flow decreased 0% to 

15% as a fraction of annual flow,17 with exceptions in smaller 

areas and shorter time periods.20

Hydrologic response to climate change will depend upon the 

dominant form of precipitation in a particular watershed, as 

well as other local characteristics including elevation, aspect, 

geology, vegetation, and changing land use.22 The largest responses are expected to occur in basins with significant snow 

accumulation, where warming increases winter flows and advances the timing of spring melt.18,23 By 2050, snowmelt is pro-

© Dan Sherwood/Design Pics/Corbis
490 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

21: NORTHWEST

jected to shift three to four weeks earlier than 

the 20th century average, and summer flows 

are projected to be substantially lower, even 

for an emissions scenario that assumes substantial emissions reductions (B1).24 In some 

North Cascade rivers, a significant fraction 

(10% to 30%) of late summer flow originates 

as glacier melt;25 the consequences of eventual 

glacial disappearance are not well quantified. 

Basins with a significant groundwater component may be less responsive to climate change 

than indicated here.26

Changes in river-related flood risk depends 

on many factors, but warming is projected to 

increase flood risk the most in mixed basins 

(those with both winter rainfall and late spring 

snowmelt-related runoff peaks) and remain 

largely unchanged in snow-dominant basins.27

Regional climate models project increases 

of 0% to 20% in extreme daily precipitation, 

depending on location and definition of 

“extreme” (for example, annual wettest day). 

Figure 21.1. Reduced June flows in many Northwest snow-fed rivers is a 

signature of warming in basins that have a significant snowmelt contribution. 

The fraction of annual flow occurring in June increased slightly in rain-dominated 

coastal basins and decreased in mixed rain-snow basins and snowmeltdominated basins over the period 1948 to 2008.21 The high flow period is in June 

for most Northwest river basins; decreases in summer flows can make it more 

difficult to meet a variety of competing human and natural demands for water. 

(Figure source: adapted from Fritze et al. 201121).

Observed Shifts in Streamflow Timing

Figure 21.2. (Left) Projected increased winter flows and decreased summer flows in many Northwest rivers will cause widespread 

impacts. Mixed rain-snow watersheds, such as the Yakima River basin, an important agricultural area in eastern Washington, will see 

increased winter flows, earlier spring peak flows, and decreased summer flows in a warming climate. Changes in average monthly 

streamflow by the 2020s, 2040s, and 2080s (as compared to the period 1916 to 2006) indicate that the Yakima River basin could 

change from a snow-dominant to a rain-dominant basin by the 2080s under the A1B emissions scenario (with eventual reductions 

from current rising emissions trends). (Figure source: adapted from Elsner et al. 2010)24. 

(Right) Natural surface water availability during the already dry late summer period is projected to decrease across most of the 

Northwest. The map shows projected changes in local runoff (shading) and streamflow (colored circles) for the 2040s (compared 

to the period 1915 to 2006) under the same scenario as the left figure (A1B).29 Streamflow reductions such as these would stress 

freshwater fish species (for instance, endangered salmon and bull trout) and necessitate increasing tradeoffs among conflicting 

uses of summer water. Watersheds with significant groundwater contributions to summer streamflow may be less responsive to 

climate change than indicated here.26

Future Shift in Timing of Stream Flows Reduced Summer Flows
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Averaged over the region, the number of days with more 

than one inch of precipitation is projected to increase 13% in 

2041 to 2070 compared with 1971 to 2000 under a scenario 

that assumes a continuation of current rising emissions trends 

(A2),10 though these projections are not consistent across 

models.28 This increase in heavy downpours could increase 

flood risk in mixed rain-snow and rain-dominant basins, and 

could also increase stormwater management challenges in 

urban areas. 

Consequences and Likelihoods of Changes

Reservoir systems have multiple objectives, including irrigation, 

municipal and industrial use, hydropower production, flood 

control, and preservation of habitat for aquatic species. 

Modeling studies indicate, with near 100% likelihood and for 

all emissions scenarios, that reductions in summer flow will 

occur by 2050 in basins with significant snowmelt (for example, 

Elsner et al. 201024). These reduced flows will require more 

tradeoffs among objectives of the whole system of reservoirs,30

especially with the added challenges of summer increases 

in electric power demand for cooling31 and additional water 

consumption by crops and forests.10,32 For example, reductions 

in hydropower production of as much as 20% by the 2080s

could be required to preserve in-stream flow targets for fish 

in the Columbia River basin.33 Springtime irrigation diversions 

increased between 1970 and 2007 in the Snake River basin, as 

earlier snowmelt led to reduced spring soil moisture.34 In the 

absence of human adaptation, annual hydropower production 

is much more likely to decrease than to increase in the Columbia 

River basin; economic impacts of hydropower changes could 

be hundreds of millions of dollars per year.35

Region-wide summer temperature increases and, in certain 

basins, increased river flooding and winter flows and 

decreased summer flows, will threaten many freshwater 

species, particularly salmon, steelhead, and trout.27 Rising 

temperatures will increase disease and/or mortality in several 

iconic salmon species, especially for spring/summer Chinook 

and sockeye in the interior Columbia and Snake River basins.36

Some Northwest streams30 and lakes have already warmed 

over the past three decades, contributing to changes such as 

earlier Columbia River sockeye salmon migration37 and earlier 

blooms of algae in Lake Washington.38 Relative to the rest of 

the United States, Northwest streams dominated by snowmelt 

runoff appear to be less sensitive, in the short term, to warming 

due to the temperature buffering provided by snowmelt and 

groundwater contributions to those streams.39 However, as 

snowpack declines, the future sensitivity to warming is likely to 

increase in these areas.40 By the 2080s, suitable habitat for the 

four trout species of the interior western U.S. is projected to 

decline 47% on average, compared to the period 1978-1997.41

As species respond to climate change in diverse ways, there is 

potential for ecological mismatches to occur – such as in the 

timing of the emergence of predators and their prey.38

Adaptive Capacity and Implications for Vulnerability 

The ability to adapt to climate changes is strengthened 

by extensive water resources infrastructure, diversity of 

institutional arrangements,42 and management agencies that 

are responsive to scientific input. However, over-allocation 

of existing water supply, conflicting objectives, limited 

management flexibility caused by rigid water allocation and 

operating rules, and other institutional barriers to changing 

operations continue to limit progress towards adaptation in 

many parts of the Columbia River basin.43,44 Vulnerability to 

projected changes in snowmelt timing is probably highest in 

basins with the largest hydrologic response to warming and 

lowest management flexibility – that is, fully allocated, midelevation, temperature-sensitive, mixed rain-snow watersheds 

with existing conflicts among users of summer water. Regional 

power planners have expressed concerns over the existing 

hydroelectric system’s potential inability to provide adequate 

summer electricity given the combination of climate change, 

demand growth, and operating constraints.1 Vulnerability 

is probably lowest where hydrologic change is likely to be 

smallest (in rain-dominant basins) and where institutional 

arrangements are simple and current natural and human 

demands rarely exceed current water availability.43,45,46

The adaptive capacity of freshwater ecosystems also varies 

and, in managed basins, will depend on the degree to which 

 NOAA

© James Christensen/ Foto Natura/Minden Pictures/Corbis
492 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

21: NORTHWEST

the need to maintain streamflows and water quality for fish 

and wildlife is balanced with human uses of water resources. 

In highly managed rivers, release of deeper, colder water 

from reservoirs could offer one of the few direct strategies to 

lower water temperatures downstream.47 Actions to improve 

stream habitat, including planting trees for shade, are being 

tested. Some species may be able to change behavior or take 

advantage of cold-water refuges.48

Key Message 2: Coastal Vulnerabilities

In the coastal zone, the effects of sea level rise, erosion, inundation, threats to infrastructure 

and habitat, and increasing ocean acidity collectively pose a major threat to the region.

With diverse landforms (such as beaches, rocky shorelines, 

bluffs, and estuaries), coastal and marine ecosystems, and 

human uses (such as rural communities, dense urban areas, 

international ports, and transportation), the Northwest coast 

will experience a wide range of climate impacts.

Description of Observed and Projected Changes

Global sea levels have risen about 8 inches since 1880 and 

are projected to rise another 1 to 4 feet by 2100 (Ch. 2: 

Our Changing Climate, Key Message 10). Many local and 

regional factors can modify the global trend, including 

vertical land movement, oceanic winds and circulation, 

sediment compaction, subterranean fluid withdrawal (such as 

groundwater and natural gas), and other geophysical factors 

such as the gravitational effects of major ice sheets and glaciers 

on regional ocean levels.

Much of the Northwest coastline is rising due to a geophysical 

force known as “tectonic uplift,” which raises the land surface. 

Because of this, apparent sea level rise is less than the currently 

observed global average. However, a major earthquake along 

the Cascadia subduction zone, expected within the next few 

hundred years, would immediately reverse centuries of uplift 

and, based on historical evidence, increase relative sea level 

40 inches or more.49,50 On the other hand, some Puget Sound 

locations are currently experiencing subsidence (where land is 

sinking or settling) and could see the reverse effect, witnessing 

immediate uplift during a major earthquake and lowered 

relative sea levels.51,52

Taking into account many of these factors and considering 

a wider range of emissions scenarios than are used in this 

assessment (Appendix 5: Scenarios and Models), a recent 

Figure 21.3. Projected relative sea level rise for the 

latitude of Newport, Oregon (relative to the year 2000) 

is based on a broader suite of emissions scenarios 

(ranging from B1 to A1FI) and a more detailed and 

regionally-focused calculation than those generally 

used in this assessment (see Ch. 2: Our Changing 

Climate).50 The blue area shows the range of relative 

sea level rise, and the black line shows the projection, 

which incorporates global and regional effects of 

warming oceans, melting land ice, and vertical land 

movements.50 Given the difficulty of assigning likelihood 

to any one possible trajectory of sea level rise at this 

time, a reasonable risk assessment would consider 

multiple scenarios within the full range of possible 

outcomes shown, in conjunction with long- and shortterm compounding effects, such as El Niño-related 

variability and storm surge. (Data from NRC 201250).

Projected Relative Sea Level Rise for the Latitude of Newport, Oregon

© Macduff Everton/Corbis
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evaluation calculated projected sea level rise and ranges for 

the years 2030, 2050, and 2100 (relative to 2000) based on 

latitude for Washington, Oregon, and California (see Figure 

21.3).50 In addition to long-term climate-driven changes in 

sea level projected for the Northwest, shorter-term El Niño 

conditions can increase regional sea level by about 4 to 12 

inches for periods of many months.50,53

Northwest coastal waters, some of the most productive on the 

West Coast,54 have highly variable physical and ecological conditions as a result of seasonal and year-to-year changes in upwelling of deeper marine water that make longer-term changes 

difficult to detect. Coastal sea surface temperatures have increased55 and summertime fog has declined between 1900 

and the early 2000s, both of which could be consequences of 

weaker upwelling winds.56 Projected changes include increasing but highly variable acidity,57,58,59 increasing surface water 

temperature (2.2°F from the period 1970 to 1999 to the period 

2030 to 2059),60 and possibly changing storminess.61 Climate 

models show inconsistent projections for the future of Northwest coastal upwelling.12,62

Consequences and Likelihoods of Changes

In Washington and Oregon, more than 140,000 acres of 

coastal lands lie within 3.3 feet in elevation of high tide.63 As 

sea levels continue to rise, these areas will be inundated more 

frequently. Many coastal wetlands, tidal flats, and beaches will 

probably decline in quality and extent as a result of sea level 

rise, particularly where habitats cannot shift inland because 

of topographical limitations or physical barriers resulting from 

human development. Species such as shorebirds and forage 

fish (small fish eaten by larger fish, birds, or mammals) would 

be harmed, and coastal infrastructure and communities would 

be at greater risk from coastal storms.64

Ocean acidification threatens culturally and commercially 

significant marine species directly affected by changes in ocean 

chemistry (such as oysters) and those affected by changes in 

the marine food web (such as Pacific salmon65). Northwest 

coastal waters are among the most acidified worldwide, 

especially in spring and summer with coastal upwelling58,59,66

combined with local factors in estuaries.57,58

Increasing coastal water temperatures and changing ecological 

conditions may alter the ranges, types, and abundances of 

marine species.67,68 Recent warm periods in the coastal ocean, 

for example, saw the arrival of subtropical and offshore marine 

species from zooplankton to top predators such as striped 

marlin, tuna, and yellowtail more common to the Baja area.69

Warmer water in regional estuaries (such as Puget Sound) 

may contribute to a higher incidence of harmful blooms of 

algae linked to paralytic shellfish poisoning,70 and may result 

in adverse economic impacts from beach closures affecting 

recreational harvesting of shellfish such as razor clams.71

Toxicity of some harmful algae appears to be increased by 

acidification.72

Many human uses of the coast – for living, working, and 

recreating – will also be negatively affected by the physical 

and ecological consequences of climate change. Erosion, 

inundation, and flooding will threaten public and private 

property along the coast; infrastructure, including wastewater 

treatment plants;7,73 stormwater outfalls;74,75 ferry terminals;76

and coastal road and rail transportation, especially in 

Puget Sound.77 Municipalities from Seattle74 and Olympia,75

Washington, to Neskowin, Oregon, have mapped risks from 

the combined effects of sea level rise and other factors.

Figure 21.4. Areas of Seattle projected by Seattle Public 

Utilities to be below sea level during high tide (Mean Higher 

High Water) and therefore at risk of flooding or inundation 

are shaded in blue under three levels of sea level rise,78

assuming no adaptation. (High [50 inches] and medium 

[13 inches] levels are within the range projected for the 

Northwest by 2100; the highest level [88 inches] includes the 

compounding effect of storm surge, derived from the highest 

observed historical tide in Seattle79). Unconnected inland 

areas shown to be below sea level may not be inundated, but 

could experience problems due to areas of standing water 

caused by a rise in the water table and drainage pipes backed 

up with seawater. (Figure source: Seattle Public Utilities80).

Rising Sea Levels

and Changing Flood Risks in Seattle
494 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

21: NORTHWEST

Adaptive Capacity and Implications for Vulnerability

Human activities have increased the vulnerability of many 

coastal ecosystems, by degrading and eliminating habitat81 and 

by building structures that, along with natural bluffs, thwart 

inland movement of many remaining habitats. In Puget Sound, 

for example, seawalls, bulkheads, and other structures have 

modified an estimated one-third of the shoreline,82 though 

some restoration has occurred. Human responses to erosion 

and sea level rise, especially shoreline armoring, will largely 

determine the viability of many shallow-water and estuarine 

ecosystems.68,82,83 In communities with few alternatives to 

existing coastal transportation networks, such as on parts of 

Highway 101 in Oregon, sea level rise and storm surges will 

pose an increasing threat to local commerce and livelihoods. 

Finally, there are few proven options for ameliorating projected 

ocean acidification.84

Key Message 3: Impacts on Forests

The combined impacts of increasing wildfire, insect outbreaks, and tree diseases are

 already causing widespread tree die-off and are virtually certain to cause additional 

forest mortality by the 2040s and long-term transformation of forest landscapes. Under 

higher emissions scenarios, extensive conversion of subalpine forests to other 

forest types is projected by the 2080s.

Evergreen coniferous forests are a prominent feature of 

Northwest landscapes, particularly in mountainous areas. 

Forests support diverse fish and wildlife species, promote 

clean air and water, stabilize soils, and store carbon. They 

support local economies and traditional tribal uses and provide 

recreational opportunities. 

Figure 21.5. In Washington’s Nisqually River Delta, estuary restoration on a large scale to assist salmon 

and wildlife recovery provides an example of adaptation to climate change and sea level rise. After a century 

of isolation behind dikes (left), much of the Nisqually National Wildlife Refuge was reconnected with tidal 

flow in 2009 by removal of a major dike and restoration of 762 acres (right), with the assistance of Ducks 

Unlimited and the Nisqually Indian Tribe. This reconnected more than 21 miles of historical tidal channels and 

floodplains with Puget Sound.85 A new exterior dike was constructed to protect freshwater wetland habitat for 

migratory birds from tidal inundation and future sea level rise. Combined with expansion of the authorized 

Refuge boundary, ongoing acquisition efforts to expand the Refuge will enhance the ability to provide diverse 

estuary and freshwater habitats despite rising sea level, increasing river floods, and loss of estuarine habitat 

elsewhere in Puget Sound. This project is considered a major step in increasing estuary habitat and recovering 

the greater Puget Sound estuary. (Photo credits: (left) Jesse Barham, U.S. Fish and Wildlife Service; (right) 

Jean Takekawa, U.S. Fish and Wildlife Service). 

Adapting the Nisqually River Delta to Sea Level Rise
495 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

21: NORTHWEST

Description of Observed and Projected Changes

Climate change will alter Northwest forests by increasing 

wildfire risk and insect and tree disease outbreaks, and 

by forcing longer-term shifts in forest types and species 

(see Ch 7: Forests). Many impacts will be driven by 

water deficits, which increase tree stress and mortality, 

tree vulnerability to insects, and fuel flammability. 

The cumulative effects of disturbance – and possibly 

interactions between insects and fires – will cause the 

greatest changes in Northwest forests.86,87 A similar 

outlook is expected for the Southwest region (see Ch. 

20: Southwest, Key Message 3). 

Although wildfires are a natural part of most Northwest 

forest ecosystems, warmer and drier conditions have 

helped increase the number and extent of wildfires in 

western U.S. forests since the 1970s.14,87,88,89 This trend 

is expected to continue under future climate conditions. 

By the 2080s, the median annual area burned in the 

Northwest would quadruple relative to the 1916 to 

2007 period to 2 million acres (range of 0.2 to 9.8 million 

acres) under the A1B scenario. Averaged over the region, 

this would increase the probability that 2.2 million acres 

would burn in a year from 5% to nearly 50%.14 Within 

the region, this probability will vary substantially with 

sensitivity of fuels to climatic conditions and local variability 

in fuel type and amount, which are in turn a product of forest 

type, effectiveness of fire suppression, and land use. For 

example, in the Western Cascades, the year-to-year variability 

in area burned is difficult to attribute to climate conditions, 

while fire in the eastern Cascades and other specific vegetation 

zones is responsive to climate.14 How individual fires behave in 

the future and what impacts they have will depend on factors 

we cannot yet project, such as extreme daily weather and 

forest fuel conditions.

Higher temperatures and drought stress are contributing to 

outbreaks of mountain pine beetles that are increasing pine 

mortality in drier Northwest forests.90,91 This trend is projected 

to continue with ongoing warming.14,92,93,94 Between now and 

the end of this century, the elevation of suitable beetle habitat 

is projected to increase as temperature increases, exposing 

higher-elevation forests to the pine beetle, but ultimately 

limiting available area as temperatures exceed the beetles’ 

optimal temperatures.14,92,93 As a result, the proportion of 

Northwest pine forests where mountain pine beetles are most 

likely to survive is projected to first increase (27% higher in 

2001 to 2030 compared to 1961 to 1990) and then decrease 

(about 49% to 58% lower by 2071 to 2100).92 For many tree 

species, the most climatically suited areas will shift from their 

current locations, increasing vulnerability to insects, disease, 

and fire in areas that become unsuitable. Eighty-five percent of 

the current range of three species that are host to pine beetles 

is projected to be climatically unsuitable for one or more of 

those species by the 2060s,14,95 while 21 to 38 currently existing 

plant species may no longer find climatically appropriate 

habitat in the Northwest by late this century.96

Consequences and Likelihoods of Changes

The likelihood of increased disturbance (fire, insects, diseases, 

and other sources of mortality) and altered forest distribution 

are very high in areas dominated by natural vegetation, and 

the resultant changes in habitat would affect native species 

and ecosystems. Subalpine forests and alpine ecosystems are 

especially at risk and may undergo almost complete conversion 

to other vegetation types by the 2080s (A2 and B1;104 A2;105

Ensemble A2, B1, B2;106). While increased area burned can 

be statistically estimated from climate projections, changes 

in the risk of very large, high-intensity, stand-replacing fires 

cannot yet be predicted, but such events could have enormous 

impacts for forest-dependent species.88 Increased wildfire 

could exacerbate respiratory and cardiovascular illnesses in 

nearby populations due to smoke and particulate pollution 

(Ch. 9: Human Health).107,108

These projected forest changes will have moderate economic 

impacts for the region as a whole, but could significantly affect 

local timber revenues and bioenergy markets.109

Figure 21.6. Forest mortality due to fire and insect activity is already 

evident in the Northwest. Continued changes in climate in coming 

decades are expected to increase these effects. Trees killed by a 

fire (left side of watershed) and trees killed by mountain pine beetle 

and spruce beetle infestations (orange and gray patches, right 

side of watershed) in subalpine forest in the Pasayten Wilderness, 

Okanogan Wenatchee National Forest, Washington, illustrates how 

cumulative disturbances can affect forests. (Photo credit: Jeremy 

Littell, USGS).

Forest Mortality 
496 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Adaptive Capacity and Implications for Vulnerability

Ability to prepare for these changes varies with land ownership 

and management priorities. Adaptation actions that decrease 

forest vulnerability exist, but none is appropriate across all of 

the Northwest’s diverse climate threats, land-use histories, 

and management objectives.86,110 Surface and canopy thinning 

can reduce the occurrence and effects of high severity fire in 

currently low severity fire systems, like drier eastern Cascades 

forests,111 but may be ineffective in historically high-severityfire forests, like the western Cascades, Olympics, and some 

subalpine forests. It is possible to use thinning to reduce tree 

mortality from insect outbreaks,86,112 but not on the scale of 

the current outbreaks in much of the West.

Key Message 4: Adapting Agriculture

While the agriculture sector’s technical ability to adapt to changing conditions can offset 

some adverse impacts of a changing climate, there remain critical concerns for agriculture 

with respect to costs of adaptation, development of more climate resilient technologies 

and management, and availability and timing of water.

Agriculture provides the economic and cultural foundation 

for Northwest rural populations and contributes substantively 

to the overall economy. Agricultural commodities and food 

production systems contributed 3% and 11% of the region’s 

gross domestic product, respectively, in 2009.113 Although the 

overall consequences of climate change will probably be lower 

Figure 21.7.

(Top) Insects and fire have cumulatively 

affected large areas of the Northwest and 

are projected to be the dominant drivers 

of forest change in the near future. Map 

shows areas recently burned (1984 

to 2008)97,98 or affected by insects or 

disease (1997 to 2008).99

(Middle) Map indicates the increases in 

area burned that would result from the 

regional temperature and precipitation 

changes associated with a 2.2°F global 

warming100 across areas that share broad 

climatic and vegetation characteristics.101

Local impacts will vary greatly within 

these broad areas with sensitivity of fuels 

to climate.14

(Bottom) Projected changes in the 

probability of climatic suitability for 

mountain pine beetles for the period 

2001 to 2030 (relative to 1961 to 1990), 

where brown indicates areas where pine 

beetles are projected to increase in the 

future and green indicates areas where 

pine beetles are expected to decrease 

in the future. Changes in probability of 

survival are based on climate-dependent 

factors important in beetle population 

success, including cold tolerance,102

spring precipitation,103 and seasonal heat 

accumulation.91,92

Insects and Fire in Northwest Forests
497 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

21: NORTHWEST

in the Northwest than in certain other regions, sustainability 

of some Northwest agricultural sectors is threatened by soil 

erosion114 and water supply uncertainty, both of which could 

be exacerbated by climate change.

Description of Observed and Projected Changes

Northwest agriculture’s sensitivity to climate change stems 

from its dependence on irrigation water, a specific range 

of temperatures, precipitation, and growing seasons, and 

the sensitivity of crops to temperature extremes. Projected 

warming will reduce the availability of irrigation water in 

snowmelt-fed basins and increase the probability of heat 

stress to field crops and tree fruit. Some crops will benefit 

from a longer growing season115 and/or higher atmospheric 

carbon dioxide, at least for a few decades.115,116 Longer-term 

consequences are less certain. Changes in plant diseases, 

pests, and weeds present additional potential risks. Higher 

average temperatures generally can exacerbate pest pressure 

through expanded geographic ranges, earlier emergence 

or arrival, and increased numbers of pest generations (for 

example, Ch. 6: Agriculture).117 Specifics differ among pathogen 

and pest species and depend upon multiple interactions (Ch. 

6: Agriculture)118 preventing region-wide generalizations. 

Research is needed to project changes in vulnerabilities to pest, 

disease, and weed complexes for specific cropping systems in 

the Northwest.

Consequences of Changes

Because much of the Northwest has low annual precipitation, 

many crops require irrigation. Reduction in summer flows in 

snow-fed rivers (see Figure 21.2), coupled with warming that 

could increase agricultural and other demands, potentially 

produces irrigation water shortages.108 The risk of a watershort year – when Yakima basin junior water rights holders are 

allowed only 75% of their water right amount – is projected 

to increase from 14% in the late 20th century to 32% by 2020 

and 77% by 2080, assuming no adaptation and under the A1B 

scenario.46

Assuming adequate nutrients and excluding effects of 

pests, weeds, and diseases, projected increases in average 

temperature and hot weather episodes and decreases in 

summer soil moisture would reduce yields of spring and winter 

wheat in rain-fed production zones of Washington State by 

the end of this century by as much as 25% relative to 1975 

to 2005. However, carbon dioxide fertilization should offset 

these effects, producing net yield increases as great as 33% 

by 2080.115 Similarly, for irrigated potatoes in Washington 

State, carbon dioxide fertilization is projected to mostly offset 

direct climate change related yield losses, although yields are 

still projected to decline by 2% to 3% under the A1B emissions 

scenario.115 Higher temperatures could also reduce potato 

tuber quality.119

Irrigated apple production is projected to increase in 

Washington State by 6% in the 2020s, 9% in the 2040s, and 

16% in the 2080s (relative to 1975 to 2005) when offsetting 

effects of carbon dioxide fertilization are included.115 However, 

because tree fruit requires chilling to ensure uniform flowering 

and fruit set and wine grape varieties have specific chilling 

requirements for maturation,120 warming could adversely 

affect currently grown varieties of these commodities. Most 

published projections of climate change impacts on Northwest 

agriculture are limited to Washington State and have focused 

on major commodities, although more than 300 crops are 

grown in the region. More studies are needed to identify the 

implications of climate change for additional cropping systems 

and locations within the region. The economic consequences 

for Northwest agriculture will be influenced by input and 

output prices driven by global economic conditions as well as 

by regional and local changes in productivity.

Adaptive Capacity and Implications for Vulnerability

Of the four areas of concern discussed here, agriculture is 

perhaps best positioned to adapt to climate trends without 

explicit planning and policy, because it already responds to 

annual climate variations and exploits a wide range of existing 

climates across the landscape.121 Some projected changes 

in climate, including warmer winters, longer annual frostfree periods, and relatively unchanged or increased winter 

precipitation, could be beneficial to some agriculture systems. 

Nonetheless, rapid climate change could present difficulties. 

Adaptation could occur slowly if substantial investments or 

significant changes in farm operations and equipment are 

required. Shifts to new varieties of wine grapes and tree 

fruit, if indicated, and even if ultimately more profitable, are 

necessarily slow and expensive. Breeding for drought- and 

heat-resistance requires long-term effort. Irrigation water 

shortages that necessitate shifts away from more profitable 

commodities could exact economic penalties.108
498 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Photo Credits

Introduction to chapter; Bear River Migratory Bird Refuge, Oregon, in 

top banner: ©USFWS, Bryant Olsen
507 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Process for Developing Key Messages

The authors and several dozen collaborators undertook a risk 

evaluation of the impacts of climate change in the Northwest that 

informed the development of the four key messages in this chapter (see also Ch. 26: Decision Support). This process considered 

the combination of impact likelihood and the consequences for 

the region’s economy, infrastructure, natural systems, human 

health, and the economically-important and climate sensitive regional agriculture sector (see Dalton et al. 20136

 for details). The 

qualitative comparative risk assessment underlying the key messages in the Northwest chapter was informed by the Northwest 

Regional Climate Risk Framing workshop (December 2, 2011, in 

Portland, OR). The workshop brought together stakeholders and 

scientists from a cross-section of sectors and jurisdictions within 

the region to discuss and rank the likelihood and consequences for 

key climate risks facing the Northwest region and previously identified in the Oregon Climate Change Adaptation Framework.122 The 

approach consisted of an initial qualitative likelihood assessment 

based on expert judgment and consequence ratings based on the 

conclusions of a group of experts and assessed for four categories: 

human health, economy, infrastructure, and natural systems.123

This initial risk exercise was continued by the lead author team of 

the Northwest chapter, resulting in several white papers that were 

1) condensed and synthesized into the Northwest chapter, and 2) 

expanded into a book-length report on Northwest impacts.6 The 

NCA Northwest chapter author team engaged in multiple technical discussions via regular teleconferences and two all-day meetings. These included careful review of the foundational technical 

input report123 and approximately 80 additional technical inputs 

provided to the NCA by the public, as well additional published 

literature. They also drew heavily from two state climate assessment reports.124

The author team identified potential regional impacts by 1) working forward from drivers of regional climate impacts (for example, 

changes in temperature, precipitation, sea level, ocean chemistry, and storms), and 2) working backward from affected regional 

sectors (for example, agriculture, natural systems, and energy). 

The team identified and ranked the relative consequences of each 

impact for the region’s economy, infrastructure, natural systems, 

and the health of Northwest residents. The likelihood of each 

impact was also qualitatively ranked, allowing identification of 

the impacts posing the highest risk, that is, likelihood × consequence, to the region as a whole. The key regionally consequential 

risks thus identified are those deriving from projected changes 

in streamflow timing (in particular, warming-related impacts in 

watersheds where snowmelt is an important contributor to flow); 

coastal consequences of the combined impact of sea level rise 

and other climate-related drivers; and changes in Northwest forest ecosystems. The Northwest chapter therefore focuses on the 

implications of these risks for Northwest water resources, key 

aquatic species, coastal systems, and forest ecosystems, as well 

as climate impacts on the regionally important, climate-sensitive 

agricultural sector.

Each author produced a white paper synthesizing the findings in 

his/her sectoral area, and a number of key messages pertaining 

to climate impacts in that area. These syntheses were followed by 

expert deliberation of draft key messages by the authors wherein 

each key message was defended before the entire author team 

before this key message was selected for inclusion in the report. 

These discussions were supported by targeted consultation with 

additional experts by the lead author of each message, and they 

were based on criteria that help define “key vulnerabilities,” including likelihood of climate change and relative magnitude of its 

consequences for the region as a whole, including consequences 

for the region’s economy, human health, ecosystems, and infrastructure.123

Though the risks evaluated were aggregated over the whole region, 

it was recognized that impacts, risks, and appropriate adaptive 

responses vary significantly in local settings. For all sectors, the 

focus on risks of importance to the region’s overall economy, ecology, built environment, and health is complemented, where space 

allows, by discussion of the local specificity of climate impacts, 

vulnerabilities and adaptive responses that results from the heterogeneity of Northwest physical conditions, ecosystems, human 

institutions and patterns of resource use. 

Key message #1 Traceable Account

Changes in the timing of streamflow related to 

changing snowmelt are already observed and will 

continue, reducing the supply of water for many 

SUPPLEMENTAL MATERIAL

TRACEABLE ACCOUNTS
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Traceable Accounts

508 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

competing demands and causing far-reaching 

ecological and socioeconomic consequences.

Description of evidence base

This message was selected because of the centrality of the 

water cycle to many important human and natural systems of the 

Northwest: hydropower production and the users of this relatively 

inexpensive electricity; agriculture and the communities and 

economies dependent thereon, and; coldwater fish, including 

several species of threatened and endangered salmon, the tribal 

and fishing communities and ecosystems that depend on them,

and the adjustments in human activities and efforts necessary 

to restore and protect them. Impacts of water-cycle changes on 

these systems, and any societal adjustments to them, will have 

far-reaching ecological and socioeconomic consequences.

Evidence that winter snow accumulation will decline under 

projected climate change is based on 20th century observations 

and theoretical studies of the sensitivity of Northwest snowpack to 

changes in precipitation and temperature. There is good agreement 

on the physical role of climate in snowpack development, and 

projections of the sign of future trends are consistent (many 

studies). However, climate variability creates disagreement over 

the magnitude of current and near-term future trends.

Evidence that projected climate change would shift the timing and 

amount of streamflow deriving from snowmelt is based on 20th

century observations of climate and streamflow and is also based 

on hydrologic model simulation of streamflow responses to climate 

variability and change. There is good agreement on the sign of 

trends (many studies), though the magnitude of current and nearterm future trends is less certain because of climate variability.

Evidence that declining snowpack and changes in the timing of 

snowmelt-driven streamflow will reduce water supply for many 

competing and time-sensitive demands is based on: 

•	 hydrologic simulations, driven by future climate 

projections, that consistently show reductions in spring 

and summer flows in mixed rain-snow and some snowdominant watersheds; 

•	 documented competition among existing water uses 

(irrigation, power, municipal, and in-stream flows) and 

inability for all water systems to meet all summer water 

needs all of the time, especially during drier years; 

•	 empirical and theoretical studies that indicate increased 

water demand for many uses under climate change; and

•	 policy and institutional analyses of the complex legal 

and institutional arrangements governing Northwest 

water management and the challenges associated with 

adjusting water management in response to changing 

conditions. 

Evidence for far-reaching ecological and socioeconomic 

consequences of the above is based on:

•	 model simulations showing negative impacts of projected 

climate and altered streamflow on many water resource 

uses at scales ranging from individual basins (for 

example, Skagit, Yakima) to the region (for example, 

Columbia River basin);

•	 model simulations of future agricultural water allocation 

in the Yakima46 and the Snake River Basin,

32 showing 

increased likelihood of water curtailments for junior 

water rights holders;

•	 model and empirical studies documenting sensitivity of 

coldwater fish to water temperatures, sensitivity of water 

temperature to air temperature, and projected warming 

of summer stream temperatures;

•	 regional and extra-regional dependence on Northwestproduced hydropower; and

•	 legal requirements to manage water resources for 

threatened & endangered fish as well as for human uses.

Evidence that water users in managed mixed rain-snow basins 

are likely to be the most vulnerable to climate change and less 

vulnerable in rain-dominated basins is based on: 

•	 observed, theoretical, and simulated sensitivity of 

watershed hydrologic response to warming by basin type;

•	 historical observations and modeled simulations of 

tradeoffs required among water management objectives 

under specific climatic conditions;

•	 analyses from water management agencies of potential 

system impacts and adaptive responses to projected 

future climate; and 

•	 institutional and policy analyses documenting sources 

and types of management rigidity (for example, difficulty 

adjusting management practices to account for changing 

conditions).

New information and remaining uncertainties

A key uncertainty is the degree to which current and future 

interannual and interdecadal variations in climate will enhance or 

obscure long-term anthropogenic climate trends. 

Uncertainty over local groundwater or glacial inputs and other local 

effects may cause overestimates of increased stream temperature 

based solely on air temperature. However, including projected 

decreases in summer streamflow would increase estimates of 

summer stream temperature increases above those based solely 

on air temperature.

Uncertainty in how much increasing temperatures will affect crop 

evapotranspiration affects future estimates of irrigation demand.
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Traceable Accounts

509 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Uncertainty in future population growth and changing per capita 

water use affects estimates of future municipal demand and 

therefore assessments of future reliability of water resource 

systems.

A major uncertainty is the degree to which water resources 

management operations of regulated systems can be adjusted 

to account for climate-driven changes in the amount and timing 

of streamflow, and how competing resource objectives will be 

accommodated or prioritized. Based on current institutional inertia, 

significant changes are unlikely to occur for several decades.

There is uncertainty in economic assessment of the impacts 

of hydrologic changes on the Northwest because much of the 

needed modeling and analysis is incomplete. Economic impacts 

assessment would require quantifying both potential behavioral 

responses to future climate-affected economic variables (prices of 

inputs and products) and to climate change itself. Some studies 

have sidestepped the issue of behavioral response to these and 

projected economic impacts based on future scenarios that do 

not consider adaptation, which lead to high estimates of “costs” 

or impacts.

Assessment of confidence based on evidence and agreement 

or, if defensible, estimates of the likelihood of impact or consequence

Confidence is very high based on strong strength of evidence and 

high level of agreement among experts.

See specifics under “description of evidence” above.

Key message #2 Traceable Account

In the coastal zone, the effects of sea level rise, 

erosion, inundation, threats to infrastructure and 

habitat, and increasing ocean acidity collectively 

pose a major threat to the region.

Description of evidence base

Given the extent of the coastline, the importance of coastal 

systems to the region’s ecology, economy, and identity, and the 

difficulty of adapting in response, the consequences of sea level 

rise, ocean acidification, and other climate driven changes in ocean 

conditions and coastal weather are expected to be significant and 

largely negative, which is why this message was included.

Evidence for observed global (eustatic) sea level rise and regional 

sea level change derives from satellite altimetry and coastal tide 

gauges. Evidence for projected global sea level rise is described 

in Ch. 2: Our Changing Climate, in the recent NRC report50 that 

includes a detailed discussion of the U.S. West Coast, and Parris 

et al. 2012.125

Evidence of erosion associated with coastal storms is based on 

observations of storm damage in some areas of the Northwest. 

Evidence for erosion and inundation associated with projected 

sea level rise is based on observations and mapping of coastal 

elevations and geospatial analyses of the extent and location of 

inundation associated with various sea level rise and storm surge 

scenarios.

Evidence for climate change impacts on coastal infrastructure 

derives from geospatial analyses (mapping infrastructure locations 

likely to be affected by various sea level rise scenarios, storm 

surge scenarios and/or river flooding scenario), such as those 

undertaken by various local governments to assess local risks of 

flooding for the downtown area (Olympia), of sea level rise and 

storm surge for marine shoreline inundation and risk to public 

utility infrastructure (Seattle – highest observed tide from NOAA 

tide gauge added to projected sea levels), and of sea level rise for 

wastewater treatment plants and associated infrastructure (King 

County). Vulnerability of coastal transportation infrastructure to 

climate change has been assessed by combining geospatial risk 

analyses with expert judgment of asset sensitivity to climate risk 

and criticality to the transportation system in Washington State 

and by assessing transportation infrastructure exposure to climate 

risks associated with sea level rise and river flooding in the region 

as a whole. 

Confidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent 

results, well documented and 

accepted methods, etc.), high 

consensus

High

Moderate evidence (several sources, some consistency, methods 

vary and/or documentation limited, 

etc.), medium consensus

Medium

Suggestive evidence (a few 

sources, limited consistency, models incomplete, methods emerging, 

etc.), competing schools of thought

Low

Inconclusive evidence (limited 

sources, extrapolations, inconsistent findings, poor documentation 

and/or methods not tested, etc.), 

disagreement or lack of opinions 

among experts
21: NORTHWEST

Traceable Accounts

510 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Evidence for impacts of climate change on coastal habitat is 

based on:

•	 model-based studies of projected impacts of sea level 

rise on tidal habitat showing significant changes in the 

composition and extent of coastal wetland habitats in 

Washington and Oregon; 

•	 observations of extent and location of coastal armoring 

and other structures that would potentially impede inland 

movement of coastal wetlands;

•	 observed changes in coastal ocean conditions 

(upwelling, nutrients, and sea surface temperatures); 

biogeographical, physiological, and paleoecological 

studies indicating a historical decline in coastal 

upwelling; and global climate model projections of future 

increases in sea surface temperatures;

•	 modeled projections for increased risk of harmful algal 

blooms (HABs) in Puget Sound associated with higher air 

and water temperatures, reduced streamflow, low winds, 

and small tidal variability (i.e., these conditions offer a 

favorable window of opportunity for HABs); and

•	 observed changes in the geographic ranges, migration 

timing, and productivity of marine species due to 

changes in sea surface temperatures associated with 

cyclical events, such as the interannual El Niño Southern 

Oscillation and the inter-decadal Pacific Decadal 

Oscillation and North Pacific Gyre Oscillation.

Evidence for historical increases in ocean acidification is from 

observations of changes in coastal ocean conditions, which 

also indicate high spatial and temporal variability. Evidence for 

acidification’s effects on various species and the broader marine 

food web is still emerging but is based on observed changes in 

abundance, size, and mortality of marine calcifying organisms and 

laboratory based and in situ acidification experiments.

Evidence for marine species responses to climate change derives 

from observations of shifts in marine plankton, fish, and seabird 

species associated with historical changes in ocean conditions, 

including temperature and availability of preferred foods. 

Evidence for low adaptive capacity is from observations of extent of 

degraded or fragmented coastal habitat, existence of few options 

for mitigating changes in marine chemical properties, observed 

extent of barriers to inland habitat migration, narrow coastal 

transportation corridors, and limited transportation alternatives 

for rural coastal towns. Evidence for low adaptive capacity is 

also based on the current limitations (both legal and political) of 

local and state governments to restrict and/or influence shoreline 

modifications on private lands.

New information and remaining uncertainties

There is significant but well-characterized uncertainty about 

the rate and extent of future sea level rise at both the global 

and regional/sub-regional scales. However, there is virtually no 

uncertainty in the direction (sign) of global sea level rise. There 

is also a solid understanding of the primary contributing factors 

and mechanisms causing sea level rise. Other details concerning 

uncertainty in global sea level rise are treated elsewhere (for 

example, NRC 201250) and in Ch. 2: Our Changing Climate). 

Regional uncertainty in projected Northwest sea level rise results 

primarily from global factors such as ice sheet mass balance and 

local vertical land movement (affecting relative sea level rise). 

An accurate determination of vertical land deformation requires 

a sufficient density of monitoring sites (for example, NOAA tide 

gauges and permanent GPS sites that monitor deformation) to 

capture variations in land deformation over short spatial scales, 

and in many Northwest coastal locations such dense networks 

do not exist. There is a general trend, however, of observed uplift 

along the northwestern portion of the Olympic Peninsula and of 

subsidence within the Puget Sound region (GPS data gathered 

from PBO data sets -- http://pbo.unavco.org/data/gps; see also 

Chapman and Melbourne 200951).

There is also considerable uncertainty about potential impacts of 

climate change on processes that influence storminess and affect 

coastal erosion in the Northwest. These uncertainties relate to 

system complexity and the limited number of studies and lack 

of consensus on future atmospheric and oceanic conditions that 

will drive changes in regional wind fields. Continued collection 

and assessment of meteorological data at ocean buoy locations 

and via remote sensing should improve our understanding of these 

processes.

Uncertainty in future patterns of sediment delivery to the coastal 

system limit projections of future inundation, erosion, and changes 

in tidal marsh. For example, substantial increases in riverine 

sediment delivery, due to climate-related changes in the amount 

and timing of streamflow, could offset erosion and/or inundation 

projected from changes in sea level alone. However, there are 

areas in the Northwest where it is clear that man-made structures 

have interrupted sediment supply and there is little uncertainty 

that shallow water habitat will be lost.

Although relatively well-bounded, uncertainty over the rate of 

projected relative sea level rise limits our ability to assess whether 

any particular coastal habitat will be able to keep pace with future 

changes through adaptation (for example, through accretion).

The specific implications of the combined factors of sea level 

rise, coastal climate change, and ocean acidification for coastal 

ecosystems and specific individual species remain uncertain 

due to the complexity of ecosystem response. However, there is 

general agreement throughout the peer-reviewed literature that 

negative impacts for a number of marine calcifying organisms are 

projected, particularly during juvenile life stages.
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511 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Projections of future coastal ocean conditions (for example, 

temperature, nutrients, pH, and productivity) are limited, in part, 

by uncertainty over future changes in upwelling – climate model 

scenarios show inconsistent projections for likely future upwelling 

conditions. Considerable uncertainty also remains in whether, 

and how, higher average ocean temperatures will influence 

geographical ranges, abundances, and diversity of marine species, 

although evidence of changes in pelagic fish species ranges and in 

production associated with Pacific Ocean temperature variability 

during cyclical events have been important indicators for potential 

species responses to climate change in the future. Consequences 

from ocean acidification for commercial fisheries and marine 

food web dynamics are potentially very high – while the trend 

of increasing acidification is very likely, the rate of change and 

spatial variability within coastal waters are largely unknown and 

are the subject of ongoing and numerous nascent research efforts. 

Additional uncertainty surrounds non-climate contributors 

to coastal ocean chemistry (for example, riverine inputs, 

anthropogenic carbon, and nitrogen point and non-point source 

inputs) and society’s ability to mitigate these inputs.

Assessment of confidence based on evidence and agreement 

or, if defensible, estimates of the likelihood of impact or consequence

There is very high confidence in the global upward trend of 

sea level rise (SLR) and ocean acidification (OA). There is high

confidence that SLR over the next century will remain under 

an upper bound of approximately 2 meters. Projections for SLR 

and OA at specific locations are much less certain (medium to 

low) because of the high spatial variability and multiple factors 

influencing both phenomena at regional and sub-regional scales.

There is medium confidence in the projections of species response 

to sea level rise and increased temperatures, but low confidence in 

species response to ocean acidification. Uncertainty in upwelling 

changes result in low confidence for projections of future change 

that depend on specific coastal ocean temperatures, nutrient 

contents, dissolved oxygen content, stratification, and other 

factors.

There is high confidence that significant changes in the type and 

distribution of coastal marsh habitat are likely, but low confidence 

in our current ability to project the specific location and timing of 

changes.

There is high confidence in the projections of increased erosion 

and inundation.

There is very high confidence that ocean acidity will continue to 

increase.

Key message #3 Traceable Account

The combined impact of increasing wildfire, insect outbreaks, and tree diseases are already causing widespread tree die-off and are virtually certain 

to cause additional forest mortality by the 2040s 

and long-term transformation of forest landscapes. 

Under higher emissions scenarios, extensive conversion of subalpine forests to other forest types is 

projected by the 2080s.

Description of evidence base

Evidence that the area burned by fire has been high, relative to 

earlier in the century, since at least the 1980s is strong. Peerreviewed papers based on federal fire databases (for example, 

National Interagency Fire Management Integrated Database 

[NIFMID], 1970/1980-2011) and independent satellite data 

(Monitoring Trends in Burn Severity [MTBS], 1984-2011) indicate 

increases in area burned.98,126

Evidence that the interannual variation in area burned was at least 

partially controlled by climate during the period 1980-2010 is also 

strong. Statistical analysis has shown that increased temperature 

(related to increased potential evapotranspiration, relative 

humidity, and longer fire seasons) and decreased precipitation 

(related to decreased actual evapotranspiration, decreased 

spring snowpack, and longer fire seasons) are moderate to strong 

(depending on forest type) correlates to the area and number of 

fires in the Pacific Northwest. Projections of area burned with 

climate change are documented in peer-reviewed literature, and 

different approaches (statistical modeling and dynamic global 

vegetation modeling) agree on the order of magnitude of those 

changes for Pacific Northwest forests, though the degree of 

increase depends on the climate change scenario and modeling 

approach.

Evidence from aerial disease and detection surveys jointly 

coordinated by the U.S. Forest Service and state level governments 

supports the statement that the area of forest mortality caused by 

insect outbreaks (including the mountain pine beetle) and by tree 

diseases is increasing.

Evidence that mountain pine beetle and spruce bark beetle 

outbreaks are climatically controlled is from a combination of 

laboratory experiments and mathematical modeling reported 

in peer-reviewed literature. Peer-reviewed future projections 

of climate have been used to develop projections of mountain 

pine beetle and spruce beetle habitat suitability based on these 

models, and show increases in the area of climatically suitable 

habitat (particularly at mid- to high elevations) by the mid-21st

century, but subsequent (late 21st century) declines in suitable 

habitat, particularly at low- to mid-elevation. There is considerable 

spatial variability in the patterns of climatically suitable habitat.
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512 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Evidence for long-term changes in the distribution of vegetation 

types and tree species comes from statistical species models, 

dynamic vegetation models, and other approaches and uses the 

correlation between observed climate and observed vegetation 

distributions to model future climatic suitability. These models 

agree broadly in their conclusions that future climates will be 

unsuitable for historically present species over significant areas of 

their ranges and that broader vegetation types will likely change, 

but the details depend greatly on climate change scenario, location 

within the region, and forest type.

Evidence that subalpine forests are likely to undergo almost 

complete conversion to other vegetation types is moderately 

strong (relatively few studies, but good agreement) and comes 

from dynamic global vegetation models that include climate, 

statistical models that relate climate and biome distribution, and 

individual statistical species distribution models based on climatic 

variables. The fact that these three different approaches generally 

agree about the large decrease in area of subalpine forests despite 

different assumptions, degrees of “mechanistic” simulation, and 

levels of ecological hierarchy justifies the key message.

New information and remaining uncertainties

The key uncertainties are primarily the timing and magnitude 

of future projected changes in forests, rather than the direction 

(sign) of changes. 

The rate of expected change is affected by the rate of climate 

change – higher emissions scenarios have higher impacts earlier 

in studies that consider multiple scenarios. Most impacts analyses 

reported in the literature and synthesized here use emissions 

scenario A1B or A2. Projections of changes in the proportion of 

Northwest pine forests where mountain pine beetles are likeliest 

to survive and of potential conversion of subalpine forests used 

scenario A2.

Statistical fire models do not include changes in vegetation that 

occur in the 21st century due to disturbance (such as fire, insects, 

and tree diseases) and other factors such as land-use change and 

fire suppression changes. As conditions depart from the period 

used for model training, projections of future fire become more 

uncertain, and by the latter 21st century (beyond about the 2060s 

to 2080s), statistical models may over-predict area burned. 

Despite this uncertainty, the projections from statistical models 

are broadly similar to those from dynamic global vegetation models 

(DGVMs), which explicitly simulate changes in future vegetation. 

A key difference is for forest ecosystems where fire has been rare 

since the mid 20th century, such as the Olympic Mountains and 

Oregon coast range, and statistical models are comparatively 

weak. In these systems, statistical fire models likely underestimate 

the future area burned, whereas DGVMs may capably simulate 

future events that are outside the range of the statistical model’s 

capability. In any case, an increase in forest area burned is 

nearly ubiquitous in these studies regardless of method, but the 

amount of increase and the degree to which it varies with forest 

type is less certain. However, fire risk in any particular location or 

at any particular time is beyond the capability of current model 

projections. In addition, the statistical model approaches to future 

fire cannot address fundamental changes in fire behavior due to 

novel extreme weather patterns, so conclusions about changes in 

fire severity are not necessarily warranted.

Only a few insects have had sufficient study to understand their 

climatic linkages, and future insect outbreak damage from other 

insects, currently unstudied, could increase the estimate of future 

areas of forest mortality due to insects. 

Fire-insect interactions and diseases are poorly studied – the 

actual effects on future landscapes could be greater if diseases 

and interactions were considered more explicitly.

For subalpine forests, what those forests become instead of 

subalpine forests is highly uncertain – different climate models 

used to drive the same dynamic global vegetation model agree 

about loss of subalpine forests, but disagree about what will 

replace them. In addition, statistical approaches that consider 

biome level and species level responses without the ecological 

process detail of DGVMs show similar losses, but do not agree 

on responses, which depend on climate scenarios. Because these 

statistical models simulate neither the regeneration of seedlings 

nor the role of disturbances, the future state of the system is 

merely correlative and based on the statistical relationship 

between climate and historical forest distribution. 

Assessment of confidence based on evidence and agreement 

or, if defensible, estimates of the likelihood of impact or consequence

The observed effects of climate on fires and insects combined 

with the agreement of future projections across modeling efforts 

warrants very high confidence that increased disturbance will 

increase forest mortality due to area burned by fire, and increases 

in insect outbreaks also have very high confidence until at least 

the 2040s in the Northwest. The timing and nature of the rates 

and the sources of mortality may change, but current estimates 

may be conservative for insect outbreaks due to the unstudied 

impacts of other insects. But in any case, the rate of projected 

forest disturbance suggests that changes will be driven by 

disturbance more than by gradual changes in forest cover or 

species composition. After mid-21st century, uncertainty about 

the interactions between disturbances and landscape response 

limits confidence to high because total area disturbed could begin 

to decline as most of the landscape becomes outside the range of 

historical conditions. The fact that different modeling approaches 

using a wide variety of climate scenarios indicate similar losses of 

subalpine forests justifies high confidence; however, comparatively 

little research that simulates ecological processes of both 

disturbance and regeneration as a function of climate, so there is 

low confidence on what will replace them. 
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513 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Key message #4 Traceable Account

While agriculture’s technical ability to adapt to 

changing conditions can offset some of the adverse 

impacts of a changing climate, there remain critical 

concerns for agriculture with respect to costs of 

adaptation, development of more climate resilient 

technologies and management, and availability and 

timing of water.

Description of evidence base

Northwest agriculture’s sensitivity to climate change stems from 

its dependence on irrigation water, adequate temperatures, 

precipitation and growing seasons, and the sensitivity of crops to 

temperature extremes. Projected warming trends based on global 

climate models and emissions scenarios potentially increase 

temperature-related stress on annual and perennial crops in the 

summer months. 

Evidence for projected impacts of warming on crop yields consists 

primarily of published studies using crop models indicating 

increasing vulnerability with projected warming over 1975-2005 

baselines. These models also project that thermal-stress-related 

losses in agricultural productivity will be offset or overcompensated 

by fertilization from accompanying increases in atmospheric CO2. 

These models have been developed for key commodities including 

wheat, apples, and potatoes. Longer term, to end of century, 

models project crop losses from temperature stress to exceed the 

benefits of CO2 fertilization. 

Evidence for the effects of warming on suitability of parts of the 

region for specific wine grape and tree fruit varieties are based 

on well-established and published climatic requirements for these 

varieties. 

Evidence for negative impacts of increased variability of 

precipitation on livestock productivity due to stress on range and 

pasture consists of a few economic studies in states near the 

region; relevance to Northwest needs to be established. 

Evidence for negative impacts of warming on dairy production 

in the region is based on a published study examining projected 

summer heat-stress on milk production.

Evidence for reduction in available irrigation water is based on 

peer-reviewed publications and state and federal agency reports 

utilizing hydrological models and precipitation and snowpack 

projections. These are outlined in more detail in the traceable 

account for Key Message 1 of this chapter. Increased demands 

for irrigation water with warming are based on cropping systems 

models and projected increases in acres cultivated. These 

projections, coupled with those for water supply, indicate that 

some areas will experience increased water shortages. Water 

rights records allow predictions of the users most vulnerable to 

the effects of these shortages.

Projections for surface water flows include decreases in summer 

flow related to changes in snowpack dynamics and reductions in 

summer precipitation. Although these precipitation projections are 

less certain than those concerning temperatures, they indicate that 

water shortages for irrigation will be more frequent in some parts 

of the region, based especially on a Washington State Department 

of Ecology-sponsored report that considered the Columbia basin. 

Other evidence for these projected changes in water is itemized in 

Key Message 1 of this chapter.

Evidence that agriculture has a high potential for autonomous 

adaptation to climate change, assuming adequate water availability, 

is inferred primarily from the wide range of production practices 

currently being used across the varied climates of the region.

New information and remaining uncertainties

Although increasing temperatures can affect the distribution of 

certain pest, weed, and pathogen species, existing models are 

limited. Without more comprehensive studies, it is not possible 

to project changes in overall pressure from these organisms, so 

overall effects remain uncertain. Some species may be adversely 

affected by warming directly or through enhancement of their 

natural enemy base, while others become more serious threats.

Uncertainty exists in models in how increasing temperatures will 

impact crop evapotranspiration, which affects future estimates of 

irrigation demand (Key Message 1 of this chapter). 

Shifting international market forces including commodity prices 

and input costs, adoption of new crops, which may have different 

heat tolerance or water requirements, and technological advances 

are difficult or impossible to project, but may have substantial 

effects on agriculture’s capacity to adapt to climate change. 

Estimates of changes in crop yields as a result of changing 

climate and CO2 are based on very few model simulations, so the 

uncertainty has not been well quantified. 

Assessment of confidence based on evidence and agreement 

or, if defensible, estimates of the likelihood of impact or consequence

Confidence is very high based on strong strength of evidence and 

high level of agreement among experts.

See specifics under “description of evidence” above.
514

Climate Change Impacts in the United States

CHAPTER 22

ALASKA

INFORMATION DRAWN FROM THIS CHAPTER IS INCLUDED IN THE HIGHLIGHTS REPORT AND IS IDENTIFIED BY THIS ICON

Recommended Citation for Chapter

Chapin, F. S., III, S. F. Trainor, P. Cochran, H. Huntington, C. Markon, M. McCammon, A. D. McGuire, and M. Serreze, 2014: 

Ch. 22: Alaska. Climate Change Impacts in the United States: The Third National Climate Assessment, J. M. Melillo, Terese 

(T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 514-536. doi:10.7930/J00Z7150. 

On the Web: http://nca2014.globalchange.gov/report/regions/alaska

Convening Lead Authors

F. Stuart Chapin III, University of Alaska Fairbanks

Sarah F. Trainor, University of Alaska Fairbanks

Lead Authors

Patricia Cochran, Alaska Native Science Commission

Henry Huntington, Huntington Consulting

Carl Markon, U.S. Geological Survey

Molly McCammon, Alaska Ocean Observing System

A. David McGuire, U.S. Geological Survey and University of Alaska Fairbanks

Mark Serreze, University of Colorado
515 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Key Messages

22ALASKA

1. Arctic summer sea ice is receding faster than previously projected and is expected to virtually

disappear before mid-century. This is altering marine ecosystems and leading to greater ship

access, offshore development opportunity, and increased community vulnerability to 

coastal erosion.

2. Most glaciers in Alaska and British Columbia are shrinking substantially. This trend is expected 

to continue and has implications for hydropower production, ocean circulation patterns, 

fisheries, and global sea level rise. 

3. Permafrost temperatures in Alaska are rising, a thawing trend that is expected to continue, 

causing multiple vulnerabilities through drier landscapes, more wildfire, altered wildlife habitat, 

increased cost of maintaining infrastructure, and the release of heat-trapping gases that increase 

climate warming.

4. Current and projected increases in Alaska’s ocean temperatures and changes in ocean chemistry 

are expected to alter the distribution and productivity of Alaska’s marine fisheries, which lead 

the U.S. in commercial value.

5. The cumulative effects of climate change in Alaska strongly affect Native communities, which are 

highly vulnerable to these rapid changes but have a deep cultural history of adapting to change.

Alaska is the United States’ only Arctic region. Its marine, tundra, boreal (northern) forest, and rainforest ecosystems differ 

from most of those in other states and are relatively intact. 

Alaska is home to millions of migratory birds, hundreds of 

thousands of caribou, some of the world’s largest salmon runs, 

a significant proportion of the nation’s marine mammals, and 

half of the nation’s fish catch.1

Energy production is the main driver of the state’s economy, 

providing more than 80% of state government revenue and 

thousands of jobs.2 Continuing pressure for oil, gas, and mineral development on land and offshore in ice-covered waters 

increases the demand for infrastructure, placing additional 

stresses on ecosystems. Land-based energy exploration will be 

affected by a shorter season when ice roads are viable, yet reduced sea ice extent may create more opportunity for offshore 

development. Climate also affects hydropower generation.3

Mining and fishing are the second and third largest industries 

in the state, with tourism rapidly increasing since the 1990s.2

Fisheries are vulnerable to changes in fish abundance and dis-

 © Bryan F. Peterson/CORBIS
516 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

22: ALASKA

tribution that result from both climate change and fishing pressure. Tourism might respond positively to warmer springs and 

autumns4 but negatively to less favorable conditions for winter 

activities and increased summer smoke from wildfire.5

Alaska is home to 40% (229 of 566) of the federally recognized 

tribes in the United States.6 The small number of jobs, high cost 

of living, and rapid social change make rural, predominantly Native, communities highly vulnerable to climate change through 

impacts on traditional hunting and fishing and cultural connection to the land and sea. Because most of these communities 

are not connected to the state’s road system or electrical grid, 

the cost of living is high, and it is challenging to supply food, 

fuel, materials, health care, and other services. Climate impacts on these communities are magnified by additional social 

and economic stresses. However, Alaskan Native communities 

have for centuries dealt with scarcity and high environmental 

variability and thus have deep cultural reservoirs of flexibility 

and adaptability. 

Observed Climate Change

Over the past 60 years, Alaska has warmed more than twice as 

rapidly as the rest of the United States, with state-wide average 

annual air temperature increasing by 3°F and average winter 

temperature by 6°F, with substantial year-to-year and regional 

variability.7 Most of the warming occurred around 1976 during a shift in a long-lived climate pattern (the Pacific Decadal 

Oscillation [PDO]) from a cooler pattern to a warmer one. The 

PDO has been shown to alternate over time between warm 

and cool phases. The underlying long-term warming trend has 

moderated the effects of the more recent shift of the PDO to 

its cooler phase in the early 2000s.8 The overall warming has 

involved more extremely hot days and fewer extremely cold 

days (Ch. 2: Our Changing Climate, Key Message 7).7,9 

Because of its cold-adapted features and rapid warming, climate change impacts on Alaska are already pronounced, including earlier spring snowmelt, reduced sea ice, widespread 

glacier retreat, warmer permafrost, drier landscapes, and more 

extensive insect outbreaks and wildfire, as described below.

Projected Climate Change

Average annual temperatures in Alaska are projected to rise by an additional 2°F to 4°F by 2050. 

If global emissions continue to increase during 

this century, temperatures can be expected to 

rise 10°F to 12°F in the north, 8°F to 10°F in the 

interior, and 6°F to 8°F in the rest of the state. 

Even with substantial emissions reductions, Alaska is projected to warm by 6°F to 8°F in the north 

and 4°F to 6°F in the rest of the state by the end 

of the century (Ch. 2: Our Changing Climate, Key 

Message 3).7,10

Annual precipitation is projected to increase, 

especially in northwestern Alaska,7 as part of 

the broad pattern of increases projected for 

high northern latitudes. Annual precipitation increases of about 15% to 30% are projected for 

the region by late this century if global emissions 

continue to increase (A2). All models project increases in all four seasons.7

However, increases in 

evaporation due to higher air temperatures and 

longer growing seasons are expected to reduce 

water availability in most of the state.11

The length of the growing season in interior Alaska has increased 45% over the last century12 and 

that trend is projected to continue.13 This could 

improve conditions for agriculture where moisture is adequate, but will reduce water storage 

and increase the risks of more extensive wildfire 

and insect outbreaks across much of Alaska.14,15

Alaska Will Continue to Warm Rapidly 

Figure 22.1. Northern latitudes are warming faster than more temperate 

regions, and Alaska has already warmed much faster than the rest of the 

country. Maps show changes in temperature, relative to 1971-1999, projected 

for Alaska in the early, middle, and late parts of this century, if heat-trapping 

gas (also known as greenhouse gas) emissions continue to increase (higher 

emissions, A2), or are substantially reduced (lower emissions, B1). (Figure 

source: adapted from Stewart et al. 20137).
517 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

22: ALASKA

Changes in dates of snowmelt and freeze-up would influence 

seasonal migration of birds and other animals, increase the 

likelihood and rate of northerly range expansion of native and 

non-native species, alter the habitats of both ecologically important and endangered species, and affect ocean currents.16

Key Message 1: Disappearing Sea Ice

Arctic summer sea ice is receding faster than previously projected and is 

expected to virtually disappear before mid-century. This is altering marine 

ecosystems and leading to greater ship access, offshore development 

opportunity, and increased community vulnerability to coastal erosion.

Arctic sea ice extent and thickness have declined substantially, 

especially in late summer (September), when there is now only 

about half as much sea ice as at the beginning of the satellite record in 1979 (Ch. 2: Our Changing Climate, Key Message 

11).17,18 The seven Septembers with the lowest ice extent all 

occurred in the past seven years. As sea ice declines, it becomes thinner, with less ice build-up over multiple years, and 

therefore more vulnerable to further melting.18 Models that 

best match historical trends project northern waters that are 

virtually ice-free by late summer by the 2030s.19,20 Within the 

general downward trend in sea ice, there will be time periods 

with both rapid ice loss and temporary recovery,21 making it 

challenging to predict short-term changes in ice conditions.

Reductions in sea ice increase the amount of the sun’s energy 

that is absorbed by the ocean. This leads to a self-reinforcing 

climate cycle, because the warmer ocean melts more ice, leaving more dark open water that gains even more heat. In autumn 

and winter, there is a strong release of this extra ocean heat 

back to the atmosphere. This is a key driver of the observed increases in air temperature in the Arctic.23 This strong warming 

linked to ice loss can influence atmospheric circulation and patterns of precipitation, both within and beyond the Arctic (for 

example, Porter et al. 201224). There is growing evidence that 

this has already occurred25 through more evaporation from the 

ocean, which increases water vapor in the lower atmosphere26

and autumn cloud cover west and north of Alaska.27

With reduced ice extent, the Arctic Ocean is more accessible 

for marine traffic, including trans-Arctic shipping, oil and gas 

Figure 22.2. Average September extent of Arctic sea ice in 1980 

(second year of satellite record and year of greatest September 

sea ice extent; outer red boundary), 1998 (about halfway through 

the time series; outer pink boundary) and 2012 (recent year of 

record and year of least September sea ice extent; outer white 

boundary). September is typically the month when sea ice is 

least extensive. Inset is the complete time series of average 

September sea ice extent (1979-2013). (Figure source: NSIDC 

2012; Data from Fetterer et al. 201322). 

Declining Sea Ice Extent

Figure 22. 3. Reductions in sea ice alter food availability for many 

species from polar bear to walrus, make hunting less safe for 

Alaska Native hunters, and create more accessibility for Arctic 

Ocean marine transport, requiring more Coast Guard coverage. 

(Photo credits: (top left) G. Carleton Ray; (bottom left) Daniel 

Glick; (right) Patrick Kelley).

Sea Ice Loss Brings Big Changes to Arctic Life
518 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

22: ALASKA

exploration, and tourism.28 This facilitates access to the substantial deposits of oil and natural gas under the seafloor in 

the Beaufort and Chukchi seas, as well as raising the risk to 

people and ecosystems from oil spills and other drilling and 

maritime-related accidents. A seasonally ice-free Arctic Ocean 

also increases sovereignty and security concerns as a result of 

potential new international disputes and increased possibilities 

for marine traffic between the Pacific and Atlantic Oceans.10

Polar bears are one of the most sensitive Arctic marine mammals to climate warming because they spend most of their 

lives on sea ice.29 Declining sea ice in northern Alaska is associated with smaller bears, probably because of less successful 

hunting of seals, which are themselves ice-dependent and so 

are projected to decline with diminishing ice and snow cover.30

Although bears can give birth to cubs on sea ice, increasing 

numbers of female bears now come ashore in Alaska in the 

summer and fall31 and den on land.32 In Hudson Bay, Canada, 

the most studied population in the Arctic, sea ice is now absent 

for three weeks longer than just a few decades ago, resulting in 

less body fat, reduced survival of both the youngest and oldest 

bears,33 and a population now estimated to be in decline34 and 

projected to be in jeopardy.35 Similar polar bear population declines are projected for the Beaufort Sea region.36

Walrus depend on sea ice as a platform for giving birth, nursing, and resting between dives to the seafloor, where they 

feed.37 In recent years, when summer sea ice in the Chukchi 

Sea retreated over waters that were too deep for walrus to 

feed,38 large numbers of walrus abandoned the ice and came 

ashore. The high concentration of animals results in increased 

competition for food and can lead to stampedes when animals 

are startled, resulting in trampling of calves.39 This movement 

to land first occurred in 2007 and has happened three times 

since then, suggesting a threshold change in walrus ecology.

Living on the front lines of climate change

“Not that long ago the water was far from our village and could not be easily seen from our homes. Today the weather is changing and is slowly taking away our village. Our boardwalks are warped, some of our buildings tilt, the land is sinking and falling 

away, and the water is close to our homes. The infrastructure that supports our village is compromised and affecting the health 

and well-being of our community members, especially our children.” 

 – Alaska Department of Commerce and Community and Economic Development, 201244

Newtok, a Yup’ik Eskimo community on the seacoast of western Alaska, is on the front lines of climate change. Between 

October 2004 and May 2006, three storms accelerated the erosion and repeatedly “flooded the village water supply, 

caused raw sewage to be spread throughout the community, displaced residents from homes, destroyed subsistence 

food storage, and shut down essential utilities.”45 The village 

landfill, barge ramp, sewage treatment facility, and fuel storage 

facilities were destroyed or severely damaged.46 The loss of the 

barge landing, which delivered most supplies and heating fuel, 

created a fuel crisis. Saltwater is intruding into the community 

water supply. Erosion is projected to reach the school, the largest building in the community, by 2017.

Recognizing the increasing danger from coastal erosion, Newtok has worked for a generation to relocate to a safer location. However, current federal legislation does not authorize 

federal or state agencies to assist communities in relocating, 

nor does it authorize them to repair or upgrade storm-damaged 

infrastructure in flood-prone locations like Newtok.42 Newtok 

therefore cannot safely remain in its current location nor can 

it access public funds to adapt to climate change through relocation.

Newtok’s situation is not unique. At least two other Alaskan 

communities, Shishmaref and Kivalina, also face immediate 

threat from coastal erosion and are seeking to relocate, but 

have been unsuccessful in doing so. Many of the world’s largest cities are coastal and are also exposed to climate change 

induced flood risks.47

Figure 22.4. Residents in Newtok, Alaska are living with 

the effects of climate change, with thawing permafrost, 

tilting houses, sinking boardwalks, in conjunction with 

aging fuel tanks and other infrastructure that cannot be 

replaced because of laws that prevent public investment 

in flood-prone localities. (Photo credit: F. S. Chapin III).

Newtok, Alaska
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22: ALASKA

With the late-summer ice edge located farther north than it 

used to be, storms produce larger waves and more coastal erosion.10 An additional contributing factor is that coastal bluffs 

that were “cemented” by ice-rich permafrost are beginning 

to thaw in response to warmer air and ocean waters, and are 

therefore more vulnerable to erosion.40 Standard defensive 

adaptation strategies to protect coastal communities from 

erosion, such as use of rock walls, sandbags, and riprap, have 

been largely unsuccessful.41 Several coastal communities are 

seeking to relocate to escape erosion that threatens infrastructure and services but, because of high costs and policy 

constraints on use of federal funds for community relocation, 

only one Alaskan village has begun to relocate (see also Ch. 12: 

Indigenous Peoples).42,43

Key Message 2: Shrinking Glaciers

Most glaciers in Alaska and British Columbia are shrinking substantially. This 

trend is expected to continue and has implications for hydropower production, 

ocean circulation patterns, fisheries, and global sea level rise. 

Alaska is home to some of the largest glaciers and fastest loss 

of glacier ice on Earth.48,49,50 This rapid ice loss is primarily a 

result of rising temperatures (for example, Arendt et al. 2002, 

200951,52,53; Ch. 2: Our Changing Climate, Key Message 11). Loss 

of glacial volume in Alaska and neighboring British Columbia, 

Canada, currently contributes 20% to 30% as much surplus 

freshwater to the oceans as does the Greenland Ice Sheet – 

about 40 to 70 gigatons per year,49,54,55,56 comparable to 10% 

of the annual discharge of the Mississippi River.57 Glaciers 

continue to respond to climate warming for years to decades 

after warming ceases, so ice loss is expected to continue, even 

if air temperatures were to remain at current levels. The global 

decline in glacial and ice-sheet volume is predicted to be one 

of the largest contributors to global sea level rise during this 

century (Ch. 2: Our Changing Climate, Key Message 10).58,59

Water from glacial landscapes is also recognized as an important source of organic carbon,60,61 phosphorus,62 and iron63 that 

contribute to high coastal productivity, so changes in these inputs could alter critical nearshore fisheries.61,64

Glaciers supply about half of the total freshwater input to the 

Gulf of Alaska.65 Glacier retreat currently increases river discharge and hydropower potential in south central and southeast Alaska, but over the longer term might reduce water input 

to reservoirs and therefore hydropower resources.3

Photo by glaciologist William O. Field, United States Geologicl Survey

Photo by glaciologist Bruce F. Molnia, United States Geologicl Survey

On the left is a photograph of Muir Glacier in Alaska taken on August 13, 1941; on the right, a photograph taken from the same vantage 

point on August 31, 2004. Total glacial mass has declined sharply around the globe, adding to sea level rise. (Left photo by glaciologist 

William O. Field; right photo by geologist Bruce F. Molnia of the United States Geological Survey.)
520 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

22: ALASKA

Key Message 3: Thawing Permafrost

Permafrost temperatures in Alaska are rising, a thawing trend that is expected 

to continue, causing multiple vulnerabilities through drier landscapes, more 

wildfire, altered wildlife habitat, increased cost of maintaining infrastructure, 

and the release of heat-trapping gases that increase climate warming.

Alaska differs from most of the rest of the U.S. in having permafrost – frozen ground that restricts water drainage and 

therefore strongly influences landscape water balance and the 

design and maintenance of infrastructure. Permafrost near the 

Alaskan Arctic coast has warmed 4°F to 5°F at 65 foot depth66,67

since the late 1970s and 6°F to 8°F at 3.3 foot depth since the 

mid-1980s.68 In Alaska, 80% of land is underlain by permafrost, and of this, more than 70% is vulnerable to subsidence 

upon thawing because of ice content that is either variable, 

moderate, or high.69 Thaw is already occurring in interior and 

southern Alaska and in northern Canada, where permafrost 

temperatures are near the thaw point.70 Models project that 

permafrost in Alaska will continue to thaw,71,72 and some models project that near-surface permafrost will be lost entirely 

from large parts of Alaska by the end of the century.73

Uneven sinking of the ground in response to permafrost thaw is estimated to add between $3.6 

and $6.1 billion (10% to 20%) to current costs of 

maintaining public infrastructure such as buildings, pipelines, roads, and airports over the next 

20 years.74 In rural Alaska, permafrost thaw will 

likely disrupt community water supplies and sewage systems,75,76,77 with negative effects on human health.78 The period during which oil and gas 

exploration is allowed on tundra has decreased 

by 50% since the 1970s as a result of permafrost 

vulnerability.11

On average, lakes have decreased in area in the 

last 50 years in the southern two-thirds of Alaska,80,81,82 due to a combination of permafrost thaw, 

greater evaporation in a warmer climate, and increased soil organic accumulation during a longer 

season for plant growth. In some places, however, 

lakes are getting larger because of lateral permafrost degradation.81 Future permafrost thaw 

will likely increase lake area in areas of continuous permafrost and decrease lake area in places 

where the permafrost zone is more fragmented.71

A continuation of the current drying of Alaskan 

lakes and wetlands could affect waterfowl management nationally because Alaska accounts for 

81% of the National Wildlife Refuge System and 

provides breeding habitat for millions of migratory birds that winter in more southerly regions 

of North America and on other continents.83 Wetland loss would also reduce waterfowl harvest in Alaska, where 

it is an important food source for Alaska Natives and other rural residents.

Both wetland drying and the increased frequency of warm 

dry summers and associated thunderstorms have led to more 

large fires in the last ten years than in any decade since recordkeeping began in the 1940s.14 In Alaskan tundra, which was 

too cold and wet to support extensive fires for approximately 

the last 5,000 years,84 a single large fire in 2007 released as 

much carbon to the atmosphere as had been absorbed by the 

entire circumpolar Arctic tundra during the previous quartercentury.85 Even if climate warming were curtailed by reducing 

heat-trapping gas (also known as greenhouse gas) emissions 

(as in the B1 scenario), the annual area burned in Alaska is proThe Big Thaw

Figure 22.5. Projections for average annual ground temperature at a depth of 

3.3 feet over time if emissions of heat-trapping gases continue to grow (higher 

emissions scenario, A2), and if they are substantially reduced (lower emissions 

scenario, B1). Blue shades represent areas below freezing at a depth of 3.3 

feet, and yellow and red shades represent areas above freezing at that depth, 

based on the GIPL 1.0 model. (Figure source: Permafrost Lab, Geophysical 

Institute, University of Alaska Fairbanks).
521 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

22: ALASKA

jected to double by mid-century and to triple by 

the end of the century,86 thus fostering increased 

emissions of heat-trapping gases, higher temperatures, and increased fires. In addition, thick 

smoke produced in years of extensive wildfire 

represents a human health risk (Ch. 9: Human 

Health). More extensive and severe wildfires 

could shift the forests of Interior Alaska during 

this century from dominance by spruce to broadleaf trees for the first time in the past 4,000 to 

6,000 years.87,88

Wildfire has mixed effects on habitat. It generally improves habitat for berries, mushrooms, 

and moose,58,89 but reduces winter habitat for 

caribou because lichens, a key winter food 

source for caribou, require 50 to 100 years to recover after wildfire.90 These habitat changes are 

nutritionally and culturally significant for Alaska 

Native Peoples.89,91 In addition, exotic plant species that were introduced along roadways are 

now spreading onto river floodplains and recently burned forests,92 potentially changing the 

suitability of these lands for timber production 

and wildlife. Some invasive species are toxic to 

moose, on which local people depend for food.93

Changes in terrestrial ecosystems in Alaska and 

the Arctic may be influencing the global climate 

system. Permafrost soils throughout the entire 

Arctic contain almost twice as much carbon as 

the atmosphere.94 Warming and thawing of 

these soils increases the release of carbon dioxide and methane through increased decomposition. Thawing permafrost also delivers organicrich soils to lake bottoms, where decomposition 

in the absence of oxygen releases additional 

methane.95 Extensive wildfires also release carbon that contributes to climate warming.86,96

The capacity of the Yukon River Basin in Alaska 

and adjacent Canada to store carbon has been 

substantially weakened since the 1960s by the 

combination of warming and thawing of permafrost and by increased wildfire.97 Expansion of 

tall shrubs and trees into tundra makes the surface darker and rougher, increasing absorption 

of the sun’s energy and further contributing to 

warming.98 This warming is likely stronger than 

the potential cooling effects of increased carbon 

dioxide uptake associated with tree and shrub 

expansion.99 The shorter snow-covered seasons in Alaska further increase energy absorption by the land surface, an effect 

only slightly offset by the reduced energy absorption of highly 

reflective post-fire snow-covered landscapes.99 This spectrum 

of changes in Alaskan and other high-latitude terrestrial ecosystems jeopardizes efforts by society to use ecosystem carbon management to offset fossil fuel emissions.94,100

Figure 22.7. Progressive drying of lakes in northern forest wetlands in the 

Yukon Flats National Wildlife Refuge, Alaska. Foreground orange area was 

once a lake. Mid-ground lake once extended to the shrubs. (Photo credit: 

May-Le Ng).

Drying Lakes and Changing Habitat

Figure 22.6. Effects of permafrost thaw on houses in interior Alaska (2001, 

top left), roads in eastern Alaska (1982, top right), and the estimated costs 

(with and without climate change) of replacing public infrastructure in Alaska, 

assuming a mid-range emissions scenario (A1B, with some decrease from 

current emissions growth trends). (Photo credits: (top left) Larry Hinzman; (top 

right) Joe Moore. Figure source: adapted from Larsen and Goldsmith 200779). 

Mounting Expenses from Permafrost Thawing
522 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

22: ALASKA

Key Message 4: Changing Ocean Temperatures and Chemistry

Current and projected increases in Alaska’s ocean temperatures and changes 

in ocean chemistry are expected to alter the distribution and productivity of 

Alaska’s marine fisheries, which lead the U.S. in commercial value.

Ocean acidification, rising ocean temperatures, declining sea 

ice, and other environmental changes interact to affect the 

location and abundance of marine fish, including those that 

are commercially important, those used as food by other species, and those used for subsistence.101,102,103 These changes 

have allowed some near-surface fish species such as salmon 

to expand their ranges northward along the Alaskan coast.104

In addition, non-native species are invading Alaskan waters 

more rapidly, primarily through ships releasing ballast waters 

and bringing southerly species to Alaska.10,105 These species 

introductions could affect marine ecosystems, including the 

feeding relationships of fish important to commercial and subsistence fisheries.

Overall habitat extent is expected to change as well, though 

the degree of the range migration will depend upon the life history of particular species. For example, reductions in seasonal 

sea ice cover and higher surface temperatures may open up 

new habitat in polar regions for some important fish species, 

such as cod, herring, and pollock.106 However, continued presence of cold bottom-water temperatures on the Alaskan continental shelf could limit northward migration into the northern 

Bering Sea and Chukchi Sea off northwestern Alaska.107 In addition, warming may cause reductions in the abundance of some 

species, such as pollock, in their current ranges in the Bering 

Sea108 and reduce the health of juvenile sockeye salmon, potentially resulting in decreased overwinter survival.109 If ocean 

warming continues, it is unlikely that current fishing pressure 

on pollock can be sustained.110 Higher temperatures are also 

likely to increase the frequency of early Chinook salmon migrations, making management of the fishery by multiple user 

groups more challenging.111

The changing temperature and chemistry of the Arctic Ocean 

and Bering Sea are likely changing their role in global ocean 

circulation and as carbon sinks for atmospheric CO2 respectively, 

although the importance of these changes in the global 

carbon budget remains unresolved. The North Pacific Ocean 

is particularly susceptible to ocean acidification (see also Ch. 

2: Our Changing Climate, Key Message 12; Ch. 24: Oceans).112

Acidifying changes in ocean chemistry have potentially widespread impacts on the marine food web, including commercially important species.

Ocean acidification in alaska

Ocean waters globally have become 30% more acidic due to absorption of large amounts of human-produced carbon 

dioxide (CO2) from the atmosphere. This CO2 interacts with ocean water to form carbonic acid that lowers the ocean’s 

pH (ocean acidification). The polar ocean is particularly prone to acidification because of low temperature113,114 and 

low salt content, the latter resulting from the large freshwater input from melting sea ice115 and large rivers. Acidity 

reduces the capacity of key plankton species and shelled animals to form and maintain shells and other hard parts, 

and therefore alters the food available to important fish species.113,116 The rising acidity will have particularly strong 

societal effects on the Bering Sea on Alaska’s west coast because of its high-productivity commercial and subsistence 

fisheries.102,117

Shelled pteropods, which are tiny planktonic snails near the base of the food chain, respond quickly to acidifying 

conditions and are an especially critical link in high-latitude food webs, as commercially important species such as 

pink salmon depend heavily on them for food.118 A 10% decrease in the population of pteropods could mean a 20% 

decrease in an adult pink salmon’s body weight.119 Pteropod consumption by juvenile pink salmon in the northern Gulf 

of Alaska varied 45% between 1999 and 2001, although the reason for this variation is unknown.120

At some times of year, acidification has already reached a critical threshold for organisms living on Alaska’s continental shelves.121 Certain algae and animals that form shells (such as clams, oysters, and crab) use carbonate minerals 

(aragonite and calcite) that dissolve below that threshold. These organisms form a crucial component of the marine 

food web that sustains life in the rich waters off Alaska’s coasts. In addition, Alaska oyster farmers are now indirectly 

affected by ocean acidification impacts farther south because they rely on oyster spat (attached oyster larvae) from 

Puget Sound farmers who are now directly affected by the recent upwelling of acidic waters along the Washington and 

Oregon coastline (Ch. 24: Oceans; Ch. 21: Northwest).122
523 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

22: ALASKA

Key Message 5: Native Communities 

The cumulative effects of climate change in Alaska strongly affect 

Native communities, which are highly vulnerable to these rapid changes 

but have a deep cultural history of adapting to change.

With the exception of oil-producing regions in the north, rural 

Alaska is one of the most extensive areas of poverty in the U.S. 

in terms of household income, yet residents pay the highest 

prices for food and fuel.123 Alaska Native Peoples, who are the 

most numerous residents of this region, depend economically, 

nutritionally, and culturally on hunting and fishing for their 

livelihoods.124,125,126 Hunters speak of thinning sea and river ice 

that makes harvest of wild foods more dangerous,127 changes 

to permafrost that alter spring run-off patterns, a northward 

shift in seal and fish species, and rising sea levels with more extreme tidal fluctuations (see Ch. 12: Indigenous Peoples).128,129

Responses to these changes are often constrained by regulations.77,129 Coastal erosion is destroying infrastructure. Impacts 

of climate change on river ice dynamics and spring flooding are 

threats to river communities but are complex, and trends have 

not yet been well documented.130

Major food sources are under stress due to many factors, 

including lack of sea ice for marine mammals.131 Thawing of 

near-surface permafrost beneath lakes and ponds that provide 

drinking water cause food and water security challenges for villages. Sanitation and health problems also result from deteriorating water and sewage systems, and ice cellars traditionally 

used for storing food are thawing (see also Ch. 12: Indigenous 

Peoples).75,78 Warming also releases human-caused pollutants, 

such as poleward-transported mercury and organic pesticides, 

from thawing permafrost and brings new diseases to Arctic 

plants and animals, including subsistence food species, posing 

new health challenges, especially to rural communities.132 Positive health effects of warming include a longer growing season 

for gardening and agriculture.10,133

Development activities in the Arctic (for example, oil and gas, 

minerals, tourism, and shipping) are of concern to Indigenous 

communities, from both perceived threats and anticipated 

benefits.126 Greater levels of industrial activity might alter the 

distribution of species, disrupt subsistence activities, increase 

the risk of oil spills, and create various social impacts. At the 

same time, development provides economic opportunities, if 

it can be harnessed appropriately.134

Alaska Native Elders say, “We must prepare to adapt.” However, the implications of this simple instruction are multi-faceted. Adapting means more than adjusting hunting technologies 

and foods eaten. It requires learning how to garner information from a rapidly changing environment. Permanent infrastructure and specified property rights increasingly constrain 

people’s ability to safely use their environment for subsistence 

and other activities. 

Traditional knowledge now facilitates adaptation to climate 

change as a framework for linking new local observations with 

western science.124,135 The capacity of Alaska Natives to survive 

for centuries in the harshest of conditions reflects their resilience.91 Communities must rely not only on improved knowledge of changes that are occurring, but also on support from 

traditional and other institutions – and on strength from within 

– in order to face an uncertain future.124

Figure 22.8: One effect of the reduction in Alaska sea ice is that storm surges that used to be buffered by the ice are now causing 

more shoreline damage. Photos show infrastructure damage from coastal erosion in Tuntutuliak (left) and Shishmaref, Alaska (right). 

(Photo credits: (left) Alaska Department of Environmental Conservation; (right) Ned Rozell).

Alaska Coastal Communities Damaged
524 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Process for developing key messages

A central component of the assessment process was the Alaska 

Regional Climate assessment workshop that was held September 

12-15, 2012, in Anchorage with approximately 20 attendees; it 

began the process leading to a foundational Technical Input Report (TIR).10 The report consists of 148 pages of text, 45 figures, 

8 tables, and 27 pages of references. Public and private citizens 

or institutions were consulted and engaged in its preparation and 

expert review by the various agencies and non-governmental organizations (NGOs) represented by the 11-member TIR writing team. 

The key findings of the report were presented at the Alaska Forum 

on the Environment and in a regularly scheduled, monthly webinar by the Alaska Center for Climate Assessment and Policy, with 

feedback then incorporated into the report.

The chapter author team engaged in multiple technical discussions via regular teleconferences. These included careful expert 

review of the foundational TIR10 and of approximately 85 additional technical inputs provided by the public, as well as the other 

published literature and professional judgment. These discussions 

were followed by expert deliberation of draft key messages by the 

writing team in a face-to-face meeting before each key message 

was selected for inclusion in the Report. These discussions were 

supported by targeted consultation with additional experts by the 

lead author of each message, and they were based on criteria that 

help define “key vulnerabilities” (Ch. 26: Decision Support).

Key message #1 Traceable Account

Arctic summer sea ice is receding faster than 

previously projected and is expected to virtually 

disappear before mid-century. This is altering marine ecosystems and leading to greater ship access, 

offshore development opportunity, and increased 

community vulnerability to coastal erosion.

Description of evidence base

The key message and supporting chapter text summarize extensive 

evidence documented in the Alaska TIR.10 Technical input reports 

(85) on a wide range of topics were also received and reviewed as 

part of the Federal Register Notice solicitation for public input. 

Although various models differ in the projected rate of sea ice 

loss, more recent CMIP5 models20 that most accurately reconstruct historical sea ice loss project that late-summer sea ice will 

virtually disappear by the 2030s, leaving only remnant sea ice. 

Evidence is strong about the impacts of sea ice loss.10 Because 

the sea ice cover plays such a strong role in human activities and 

Arctic ecosystems, loss of the ice cover is nearly certain to have 

substantial impacts.17

New information and remaining uncertainties

Important new evidence confirmed many of the findings from a 

prior Alaska assessment (http://nca2009.globalchange.gov/alaska), which informed the 2009 NCA.136

Evidence from improved models (for example, Wang and Overland 

201220) and updated observational data from satellite, especially 

new results, clearly show rapid decline in not only extent but also 

mass and thickness of multi-year ice,18 information that was not 

available in prior assessments. 

Nearly all studies to date published in the peer-reviewed literature agree that summer Arctic sea ice extent is rapidly declining 

and that, if heat-trapping gas concentrations continue to rise, an 

essentially ice-free summer Arctic ocean will be realized before 

mid-century. However, there remains uncertainty in the rate of 

sea ice loss, with the models that most accurately project historical sea ice trends currently suggesting nearly ice-free conditions 

sometime between 2021 and 2043 (median 2035).20 Uncertainty 

across all models stems from a combination of large differences in 

projections among different climate models, natural climate variability, and uncertainty about future rates of fossil fuel emissions. 

Ecosystems: There is substantial new information that ocean acidification, rising ocean temperatures, declining sea ice, and other 

environmental changes are affecting the location and abundance 

of marine fish, including those that are commercially important, 

those used as food by other species, and those used for subsistence.101,102 However, the relative importance of these potential 

causes of change is highly uncertain.

Offshore oil and gas development: A key uncertainty is the price of 

fossil fuels. Viable avenues for improving the information base inSUPPLEMENTAL MATERIAL

TRACEABLE ACCOUNTS
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Traceable Accounts

534 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

clude determining the primary causes of variation among different 

climate models and determining which climate models exhibit the 

best ability to reproduce the observed rate of sea ice loss.

Coastal erosion: There is new information that lack of sea ice 

causes storms to produce larger waves and more coastal erosion.10

An additional contributing factor is that coastal bluffs that were 

“cemented” by permafrost are beginning to thaw in response to 

warmer air and ocean waters, and are therefore more vulnerable 

to erosion.40 Standard defensive adaptation strategies to protect 

coastal communities from erosion such as use of rock walls, sandbags, and riprap have been largely unsuccessful.41 There remains 

considerable uncertainty, however, about the spatial patterns of 

future coastal erosion.

Assessment of confidence based on evidence 

Given the evidence base and remaining uncertainties:

Very high confidence for summer sea ice decline. High confidence 

for summer sea ice disappearing by mid-century.

Very high confidence for altered marine ecosystems, greater ship 

access, and increased vulnerability of communities to coastal erosion.

High confidence regarding offshore development opportunity. 

Key message #2 Traceable Account

Most glaciers in Alaska and British Columbia are 

shrinking substantially. This trend is expected to 

continue and has implications for hydropower production, ocean circulation patterns, fisheries, and 

global sea level rise.

Description of evidence base

The key message and supporting chapter text summarize extensive evidence documented in the Alaska Technical Input Report.10

Technical input reports (85) on a wide range of topics were also 

received and reviewed as part of the Federal Register Notice solicitation for public input. 

Evidence that glaciers in Alaska and British Columbia are shrinking 

is strong and is based on field studies,56 energy balance models,59

LIDAR remote sensing,51,52 and satellite data, especially new lines 

of evidence from the Gravity Recovery and Climate Experiment 

(GRACE) satellite.48,52,55

Evidence is also strong that Alaska ice mass loss contributes to 

global sea level rise,58 with latest results permitting quantitative 

evaluation of losses globally.49

Numerous peer-reviewed publications describe implications of 

recent increases, but likely longer-term declines, in water input 

from glacial rivers to reservoirs and therefore hydropower resources.3,10,65

Glacial rivers account for 47% of the freshwater input to the Gulf 

of Alaska65 and are an important source of organic carbon,60,61

phosphorus,62 and iron63 that contribute to the high productivity of 

near-shore fisheries.61,64 Therefore, it is projected that the changes 

in discharge of glacial rivers will affect ocean circulation patterns 

and major U.S. and locally significant fisheries. 

New information and remaining uncertainties

Important new evidence confirmed many of the findings from a 

prior Alaska assessment (http://nca2009.globalchange.gov/alaska), which informed the 2009 NCA.136

As noted above, major advances from GRACE and other datasets 

now permit analyses of glacier mass loss that were not possible 

previously. 

Key uncertainties remain related to large year-to-year variation, 

the spatial distribution of snow accumulation and melt, and the 

quantification of glacier calving into the ocean and lakes. Although 

most large glaciated areas of the state are regularly measured 

observationally, extrapolation to unmeasured areas carries uncertainties due to large spatial variability. 

Although there is broad agreement that near-shore circulation in 

the Gulf of Alaska is influenced by the magnitude of freshwater 

inputs, little is known about the mechanisms by which near-term 

increases and subsequent longer-term decreases in glacier runoff 

Confidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent results, well documented 

and accepted methods, etc.), 

high consensus

High

Moderate evidence (several 

sources, some consistency, 

methods vary and/or documentation limited, etc.), medium 

consensus

Medium

Suggestive evidence (a few 

sources, limited consistency, 

models incomplete, methods 

emerging, etc.), competing 

schools of thought

Low

Inconclusive evidence (limited sources, extrapolations, 

inconsistent findings, poor documentation and/or methods not 

tested, etc.), disagreement or 

lack of opinions among experts
22: ALASKA

Traceable Accounts

535 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

(as the glaciers disappear) will affect the structure of the Alaska 

Coastal Current and smaller-scale ocean circulation, both of which 

have feedback on fisheries. 

The magnitude and timing of effects on hydropower production 

depend on changes in glacial mass, as described above.

Assessment of confidence based on evidence 

High confidence that glacier mass loss in Alaska and British Columbia is high, contributing 20% to 30% as much to sea level rise 

as does shrinkage of the Greenland Ice Sheet.

High confidence that due to glacier mass loss there will be related 

impacts on hydropower production, ocean circulation, fisheries, 

and global sea level rise. 

Key message #3 Traceable Account

Permafrost temperatures in Alaska are rising, a 

thawing trend that is expected to continue, causing 

multiple vulnerabilities through drier landscapes, 

more wildfire, altered wildlife habitat, increased 

cost of maintaining infrastructure, and the release 

of heat-trapping gases that increase climate warming.

Description of evidence base

The key message and supporting chapter text summarize extensive evidence documented in the Alaska Technical Input Report.10

Technical input reports (85) on a wide range of topics were also 

received and reviewed as part of the Federal Register Notice solicitation for public input. 

Previous evidence that permafrost is warming66 has been confirmed and enhanced by more recent studies.70 The most recent 

modeling efforts (for example, Avis et al. 2011; Jafarov et al. 

201271,73) extend earlier results72 and project that permafrost will 

be lost from the upper few meters from large parts of Alaska by 

the end of this century. 

Evidence that permafrost thaw leads to drier landscapes81,82 is 

beginning to accumulate, especially as improved remote sensing 

tools are applied to assess more remote regions.71

Satellite data has expanded the capacity to monitor wildfire across 

the region, providing additional evidence of wildfire extent.87 This 

new evidence has led to increased study that is beginning to reveal 

impacts on ecosystems and wildlife habitat, but much more work 

is needed to understand the extent of natural resilience. 

Impacts of permafrost thaw on the maintenance of infrastructure11,74,75,76,77 is currently moderate but rapidly accumulating. Evidence that permafrost thaw will jeopardize efforts to offset fossil 

fuel emissions is suggestive (Ch. 2: Our Changing Climate).94,100

New information and remaining uncertainties

Important new evidence confirmed many of the findings from a 

prior Alaska assessment (http://nca2009.globalchange.gov/alaska), which informed the 2009 NCA.136

This evidence included results from improved models and updated 

observational data. The assessment included insights from stakeholders collected in a series of distributed engagement meetings 

that confirm the relevance and significance of the key message for 

local decision-makers. 

Key uncertainties involve: 1) the degree to which increases in 

evapotranspiration versus permafrost thaw are leading to drier 

landscapes; 2) the degree to which it is these drier landscapes 

associated with permafrost thaw, versus more severe fire weather 

associated with climate change, that is leading to more wildfire; 3) 

the degree to which the costs of the maintenance of infrastructure 

are associated with permafrost thaw caused by climate change 

versus disturbance of permafrost due to other human activities; 

and 4) the degree to which climate change is causing Alaska to 

be a sink versus a source of greenhouse gases to the atmosphere.

Assessment of confidence based on evidence 

Very high confidence that permafrost is warming.

High confidence that landscapes in interior Alaska are getting 

drier, although the relative importance of different mechanisms is 

not completely clear. 

Medium confidence that thawing permafrost results in more wildfires. There is high confidence that wildfires have been increasing 

in recent decades, even if it is not clear whether permafrost thaw 

or hotter and drier weather is more important. 

High confidence that climate change will lead to increased maintenance costs in future decades. Low confidence that climate 

change has led to increased maintenance costs of infrastructure 

in recent decades.

Very high confidence that ecological changes will cause Alaska to 

become a source of greenhouse gases to the atmosphere, even 

though evidence that Alaska is currently a carbon source is only 

suggestive. 

Key message #4 Traceable Account

Current and projected increases in Alaska’s ocean 

temperatures and changes in ocean chemistry are 

expected to alter the distribution and productivity 

of Alaska’s marine fisheries, which lead the U.S. in 

commercial value.

Description of evidence base

The key message and supporting chapter text summarize extensive evidence documented in the Alaska Technical Input Report.10
22: ALASKA

Traceable Accounts

536 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Technical input reports (85) on a wide range of topics were also 

received and reviewed as part of the Federal Register Notice solicitation for public input. 

Numerous peer-reviewed publications describe evidence that 

ocean temperatures are rising and ocean chemistry, especially 

pH, is changing.10 New observational data from buoys and ships 

document increasing acidity and aragonite under-saturation (that 

is, the tendency of calcite and aragonite in shells to dissolve) in 

Alaskan coastal waters.

Accumulating strong evidence suggests that these changes in 

ocean temperature and chemistry, including pH, will likely affect 

major Alaska marine fisheries, although the relative importance of 

these changes and the exact nature of response of each fishery 

are uncertain.101,102,103

Alaska’s commercial fisheries account for roughly 50 percent of 

the United States’ total wild landings. Alaska led all states in both 

volume and ex-vessel value of commercial fisheries landings in 

2009, with a total of 1.84 million metric tons worth $1.3 billion.1

New information and remaining uncertainties

Important new evidence confirmed many of the findings from a 

prior Alaska assessment (http://nca2009.globalchange.gov/alaska), which informed the 2009 NCA.136

The new evidence included results from improved models and 

updated observational data. The assessment included insights 

from stakeholders collected in a series of distributed engagement 

meetings that confirm the relevance and significance of the key 

message for local decision-makers. 

A key uncertainty is what the actual impacts of rising temperatures and changing ocean chemistry, including an increase in 

ocean acidification, will be on a broad range of marine biota and 

ecosystems. More monitoring is needed to document the extent 

and location of changes. Additional research is needed to assess 

how those changes will affect the productivity of key fishery resources and their food and prey base.

Assessment of confidence based on evidence 

Given the evidence base and remaining uncertainties:

High confidence of increased ocean temperatures and changes in 

chemistry. 

Medium confidence that fisheries will be affected.

Key message #5 Traceable Account

The cumulative effects of climate change in Alaska strongly affect Native communities, which are 

highly vulnerable to these rapid changes but have a 

deep cultural history of adapting to change.

Description of evidence base

The key message and supporting chapter text summarize extensive evidence documented in the Alaska Technical Input Report.10

Technical input reports (85) on a wide range of topics were also 

received and reviewed as part of the Federal Register Notice solicitation for public input. 

Evidence exists in recorded local observational accounts as well as 

in the peer-reviewed scientific literature of the cumulative effects 

of climate-related environmental change on Native communities 

in Alaska; these effects combine with other socioeconomic stressors to strain rural Native communities (Ch. 12: Indigenous Peoples).124,125,126,131 Increasing attention to impacts of climate change 

is revealing new aspects, such as impacts to health and hunter 

safety (for example, Baffrey and Huntington 2010; Brubaker et al. 

201178,134). There is also strong evidence for the cultural adaptive 

capacity of these communities and peoples over time.91,130,135

New information and remaining uncertainties

Important new evidence confirmed many of the findings from a 

prior Alaska assessment (http://nca2009.globalchange.gov/alaska), which informed the 2009 NCA.136

The precise mechanisms by which climate change affects Native communities are poorly understood, especially in the context 

of rapid social, economic, and cultural change. Present day responses to environmental change are poorly documented. More 

research is needed on the ways that Alaska Natives respond to 

current biophysical climate change and to the factors that enable 

or constrain contemporary adaptation.

Alaska Native communities are already being affected by climateinduced changes in the physical and biological environment, from 

coastal erosion threatening the existence of some communities, to 

alterations in hunting, fishing, and gathering practices that undermine the intergenerational transfer of culture, skill, and wisdom. 

At the same time, these communities have a long record of adaptation and flexibility. Whether such adaptability is sufficient to 

address the challenges of climate change depends both on the 

speed of climate-induced changes and on the degree to which 

Native communities are supported rather than constrained in the 

adaptive measures they need to make.124

Assessment of confidence based on evidence 

There is high confidence that cumulative effects of climate change 

in Alaska strongly affect Native communities, which are highly 

vulnerable to these rapid changes but have a deep cultural history 

of adapting to change.
537

Climate Change Impacts in the United States

CHAPTER 23

HAWAI‘I 

AND U.S. AFFILIATED PACIFIC ISLANDS

INFORMATION DRAWN FROM THIS CHAPTER IS INCLUDED IN THE HIGHLIGHTS REPORT AND IS IDENTIFIED BY THIS ICON

Recommended Citation for Chapter

Leong, J.-A., J. J. Marra, M. L. Finucane, T. Giambelluca, M. Merrifield, S. E. Miller, J. Polovina, E. Shea, M. Burkett, J. 

Campbell, P. Lefale, F. Lipschultz, L. Loope, D. Spooner, and B. Wang, 2014: Ch. 23: Hawai‘i and U.S. Affiliated Pacific 

Islands. Climate Change Impacts in the United States: The Third National Climate Assessment, J. M. Melillo, Terese (T.C.) 

Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 537-556. doi:10.7930/J0W66HPM. 

On the Web: http://nca2014.globalchange.gov/report/regions/hawaii-and-pacific-islands

Convening Lead Authors

Jo-Ann Leong, University of Hawai‘i

John J. Marra, National Oceanic and Atmospheric Administration

Lead Authors

Melissa L. Finucane, East-West Center

Thomas Giambelluca, University of Hawai‘i

Mark Merrifield, University of Hawai‘i

Stephen E. Miller, U.S. Fish and Wildlife Service

Jeffrey Polovina, National Oceanic and Atmospheric Administration

Eileen Shea, National Oceanic and Atmospheric Administration

Contributing Authors

Maxine Burkett, University of Hawai‘i

John Campbell, University of Waikato

Penehuro Lefale, Meteorological Service of New Zealand Ltd. 

Fredric Lipschultz, NASA and Bermuda Institute of Ocean Sciences

Lloyd Loope, U.S. Geological Survey

Deanna Spooner, Pacific Island Climate Change Cooperative

Bin Wang, University of Hawai‘i 
 

Key Messages

HAWAI‘I 23AND U.S. AFFILIATED PACIFIC ISLANDS

1. Warmer oceans are leading to increased coral bleaching events and disease outbreaks in coral

reefs, as well as changed distribution patterns of tuna fisheries. Ocean acidification will reduce 

coral growth and health. Warming and acidification, combined with existing stresses, will strongly 

affect coral reef fish communities. 

2. Freshwater supplies are already constrained and will become more limited on many islands.

Saltwater intrusion associated with sea level rise will reduce the quantity and quality of freshwater 

in coastal aquifers, especially on low islands. In areas where precipitation does not increase, 

freshwater supplies will be adversely affected as air temperature rises.

 3. Increasing temperatures, and in some areas reduced rainfall, will stress native Pacific Island plants 

and animals, especially in high-elevation ecosystems with increasing exposure to invasive species, 

increasing the risk of extinctions. 

4. Rising sea levels, coupled with high water levels caused by storms, will incrementally increase 

coastal flooding and erosion, damaging coastal ecosystems, infrastructure, and agriculture, and 

negatively affecting tourism.

5. Mounting threats to food and water security, infrastructure, health, and safety are expected to 

lead to increasing human migration, making it increasingly difficult for Pacific Islanders to sustain 

the region’s many unique customs, beliefs, and languages. 

The U.S. Pacific Islands region (Figure 23.1) 

is vast, comprising more than 2,000 islands 

spanning millions of square miles of ocean. 

The largest group of islands in this region, the 

Hawaiian Archipelago, is located nearly 2,400 

miles from any continental landmass, which 

makes it one of the most remote archipelagos on the globe.1

 The Hawaiian Islands support fewer than 2 million people, yet provide 

vital strategic capabilities to U.S. defense – 

and the islands’ biodiversity is important to 

the world. Hawai‘i and the U.S. affiliated Pacific Islands are at risk from climate changes 

that will affect nearly every aspect of life. 

Rising air and ocean temperatures, shifting 

rainfall patterns, changing frequencies and 

intensities of storms and drought, decreasing baseflow in streams, rising sea levels, and 

changing ocean chemistry will affect ecosystems on land and in the oceans, as well as 

local communities, livelihoods, and cultures. 

Low islands are particularly at risk.

 © Michael Wells/fstop/Corbis

538 CLIMATE CHANGE IMPACTS IN THE UNITED STATES
539 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

23: HAWAI‘I AND U.S. AFFILIATED PACIFIC ISLANDS

The Pacific Islands include volcanic islands, islands of continental crust, atolls 

(formed by coral reefs), limestone islands, and islands of mixed geologic 

origin, with tremendous landscape diversity. In the Hawaiian High Islands, as 

many as 10 ecozones – from alpine systems to tropical rainforests – exist within 

a 25 mile span.3,4 Isolation and landscape 

diversity in Hawai‘i brings about some of 

the highest concentrations of native species, found nowhere else in the world.4

Several U.S. Pacific Islands are marine 

biodiversity hotspots, with the greatest 

diversity found in the Republic of Palau, 

and the highest percentage of native reef 

fishes in Hawai‘i.5 These islands provide 

insights into evolution and adaptation, 

concepts important for predicting the 

impacts of climate change on ecosystems. Their genetic diversity also holds 

the potential for developing natural 

products and processes for biomedical 

and industrial use. 

The Pacific Islands region includes demographically, culturally, and economically 

varied communities of diverse indigenous Pacific Islanders, intermingled with 

immigrants from many countries. At least 

20 languages are spoken in the region. 

Pacific Islanders recognize the value and 

relevance of their cultural heritage and 

systems of traditional knowledge; their 

laws emphasize the long-term multigenerational connection 

with their lands and resources.6 Tourism contributes prominently to the gross domestic product of most island jurisdictions, as does the large U.S. military presence. Geographic 

remoteness means that the costs of air transport and shipping 

profoundly influence island economies. Natural resources are 

limited, with many communities relying on agriculture and 

ecosystems (such as coral reefs, open oceans, streams, and 

forests) for sustenance and revenue. 

Key Message 1: Changes to Marine Ecosystems

Warmer oceans are leading to increased coral bleaching events and disease outbreaks in 

coral reefs, as well as changed distribution patterns of tuna fisheries. Ocean acidification will 

reduce coral growth and health. Warming and acidification, combined with existing stresses, 

will strongly affect coral reef fish communities. 

Ocean temperatures in the Pacific region exhibit strong yearto-year and decadal fluctuations, but since the 1950s, they 

have also exhibited a warming trend, with temperatures from 

the surface to a depth of 660 feet rising by as much as 3.6°F.7

Future sea surface temperatures are projected to increase 

1.1°F (compared to the 1990 levels) by 2030, 1.8°F by 2055, 

and 2.5°F by 2090 under a scenario that assumes substantial 

reductions in emissions (B1), or 1.7°F by 2030, 2.3°F by 2055, 

and 4.7°F by 2090 under a scenario that assumes continued 

increases in emissions (A2).8

Bleaching events (as a result of higher ocean temperatures) can 

weaken or kill corals. At least three mass bleaching episodes 

have occurred in the northwestern Hawaiian Islands in the last 

decade.9 Incidences of coral bleaching have been recorded in 

Figure 23.1. The U.S. Pacific Islands region includes our 50th state, Hawai‘i, as well as 

the Territories of Guam, American Samoa, the Commonwealth of the Northern Mariana 

Islands (CNMI), the Republic of Palau (RP), the Federated States of Micronesia 

(FSM), and the Republic of the Marshall Islands (RMI). Citizens of Guam and CNMI 

are U.S. citizens, and citizens of American Samoa are U.S. nationals. Through the 

Compacts of Free Association, citizens of RP, FSM, and RMI have the right to travel 

to the U.S. without visas to maintain “habitual residence” and to pursue education 

and employment. The map shows three sub-regions used in this assessment and 

the islands that comprise the Pacific Remote Islands National Monument. Shaded 

areas indicate each island’s Exclusive Economic Zone (EEZ) (Figure source: Keener 

et al. 20122). 

U.S. Pacific Islands Region 
540 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

23: HAWAI‘I AND U.S. AFFILIATED PACIFIC ISLANDS

Micronesia and American Samoa,10 testing the resilience of 

these reefs. Coral disease outbreaks have also been reported 

in the Hawaiian archipelago,11 American Samoa,12,13 the Marshall Islands, and Palau,14 correlated with periods of unusually 

high water temperatures.15 Despite uncertainties, advanced 

modeling techniques project a large decline in coral cover in 

the Hawaiian Archipelago during this century. However, there 

are significant differences in the projected time frames and 

geographic distribution of these declines, even under a single 

climate change scenario.16 By 2100, assuming ongoing increases in emissions of heat-trapping gases (A2 scenario), continued 

loss of coral reefs and the shelter they provide will result in 

extensive losses in both numbers and species of reef fishes.17

Even with a substantial reduction in emissions (B1 scenario), 

reefs could be expected to lose as much as 40% of their reefassociated fish. Coral reefs in Hawai‘i provide an estimated 

$385 million in goods and services annually,18 which could be 

threatened by these impacts. 

Ocean acidification is also taking place in the region, which 

adds to ecosystem stress from increasing temperatures. Ocean 

acidity has increased by about 30% since the pre-industrial 

era and is projected to further increase by 37% to 50% from 

present levels by 2100 (Ch. 2: Our Changing Climate, Key Message 12).19 The amount of calcium carbonate, the biologically 

important mineral critical to reef-building coral and to calcifying algae, will decrease as a result of ocean acidification. By 

2035 to 2060, levels of one form of the mineral (aragonite) are 

projected to decline enough to reduce coral growth and survival around the Pacific, with continuing declines thereafter.20

Crustose coralline algae, an inconspicuous but important component of reefs that help reefs to form and that act as critical 

surfaces on which other living things grow, are also expected 

to exhibit reduced growth and survival.21,22 Ocean acidification 

reduces the ability of corals to build reefs and also increases 

erosion,23 leading to more fragile reef habitats. These changes 

are projected to have a strong negative impact on the econoFigure 23.2. The Pacific Islands include “high” volcanic islands, such as that on the left, that reach nearly 14,000 feet above sea level, 

and “low” atolls and islands, such as that on the right, that peak at just a few feet above present sea level. (Left) Ko‘olau Mountains 

on the windward side of Oahu, Hawai‘i (Photo credit: kstrebor via Flickr.com). (Right) Laysan Island, Papahānaumokuākea Marine 

National Monument (Photo credit: Andy Collins, NOAA).

“High” and “Low” Pacific Islands Face Different Threats

The Pacific region is subject to various patterns of climate variability. The effects of the El Niño-Southern Oscillation 

(ENSO) and other patterns of oceanic and atmospheric variability on the region are significant. They include large 

variations in sea surface temperatures, the strength and persistence of the trade winds, the position of jet streams and 

storm tracks, and the location and intensity of rainfall.8,29,30 The ENSO-related extremes of El Niño and La Niña generally 

persist for 6 to 18 months and change phase roughly every 3 to 7 years.8,31 The Pacific Decadal Oscillation (PDO) and 

the Interdecadal Pacific Oscillation (IPO) are patterns that operate over even longer time horizons and also influence 

the weather and climate of the region.31,32 Such dramatic short-term variability (the “noise”) can obscure the long-term 

trend (the “signal”).33 Despite the challenges of distinguishing natural climate variability from climate change, there are 

several key indicators of observed change that serve as a basis for monitoring and evaluating future change.2

El niño and other patterns of climate variability
541 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

23: HAWAI‘I AND U.S. AFFILIATED PACIFIC ISLANDS

mies and well-being of island communities, with loss of coral 

biodiversity and reduced resilience.24

Similarly, there will be large impacts to the economically 

important tuna fishery in the Pacific Island region. Surface 

chlorophyll data obtained by satellites indicate less favorable 

conditions resulting in reduced productivity for tuna in the 

subtropical South and North Pacific26 due to warming. This 

trend is projected to continue under future climate change.27

One fishery model, coupled with a climate model, forecasts 

that the overall western and central Pacific fishery catch for 

skipjack tuna would initially increase by about 19% by 2035, 

though there would be no change for bigeye tuna. However, 

by 2100, skipjack catch would decline by 8% and bigeye catch 

would decline by 27% if emissions continue to rise (A2 scenario); geographic variations 

are projected within the region.28

These changes to both corals 

and fish pose threats to communities, cultures, and ecosystems of the Pacific Islands both 

directly through their impact 

on food security and indirectly 

through their impact on economic sectors including fisheries and tourism. 

Figure 23.3. Ocean waters have already become more acidic from absorbing carbon dioxide 

from the atmosphere. As this absorption lowers pH, it reduces the amount of calcium carbonate, 

which is critical for many marine species to reproduce and grow. Maps show projections of 

the saturation state of aragonite (the form of calcium carbonate used by coral and many other 

species) if CO2 levels were stabilized at 380 ppm (a level that has already been exceeded), 450 

ppm (middle map), and 500 ppm (bottom map), corresponding approximately to the years 2005, 

2030, and 2050, assuming a decrease in emissions from the current trend (scenario A1B). As 

shown on the maps, many areas that are adequate will become marginal. Higher emissions 

will lead to many more places where aragonite concentrations are “marginal” or “extremely 

marginal” in much of the Pacific. (Figure source: Burke et al. 201125). 

Increased Acidification Decreases Suitable Coral Habitat

© Ron Dahlquist/Corbis

Increasing ocean temperature and acidity threaten coral reef 

ecosystems.
542 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

23: HAWAI‘I AND U.S. AFFILIATED PACIFIC ISLANDS

Key Message 2: Decreasing Freshwater Availability

Freshwater supplies are already constrained and will become more limited on many islands. 

Saltwater intrusion associated with sea level rise will reduce the quantity and quality of 

freshwater in coastal aquifers, especially on low islands. In areas where precipitation does not 

increase, freshwater supplies will be adversely affected as air temperature rises.

In Hawai‘i, average precipitation, average stream discharge, 

and stream baseflow have been trending downward for nearly 

a century, especially in recent decades, but with high variability 

due to cyclical climate patterns such as ENSO and the PDO (see 

“El Niño and other Patterns of Climate Variability”).34,35,36 For 

the Western North Pacific, a decline of 15% in annual rainfall 

has been observed in the eastern-most islands in the Micronesia region, and slight upward trends in precipitation have been 

seen for the western-most islands with high ENSO-related variability.7

 In American Samoa, no trends in average rainfall are 

apparent, but there is very limited available data.7,37 

Projections of precipitation are less certain than those for temperature.2,38 For Hawai‘i, a scenario based on statistical downscaling projects a 5% to 10% reduction for the wet season and 

a 5% increase in the dry season for the end of this century.39

Projections for late this century from global models for the 

region give a range of results. Generally they predict annual 

rainfall to either change little or to increase by up to 5% for the 

main Hawaiian Islands and to change little or decrease up to 

10% in the Northwestern Hawaiian Islands. They also project 

increases in the Micronesia region (Ch. 2: Our Changing Climate, Figure 2.6),40 though there is low confidence in all these 

projections. 

Climate change impacts on freshwater resources in the Pacific 

Islands will vary across the region. Different islands will be affected by different factors, including natural variability patterns that affect storms and precipitation (like El Niño and La 

Niña events), as well as climate trends that are strongly influenced by specific geographic locations. For example, surface 

air temperature has increased and is expected to continue to 

rise over the entire region.41 In Hawai‘i, the rate of increase 

has been greater at high elevations.41 In Hawai‘i and the Central North Pacific, projected annual surface air temperature 

increases range from 1.5°F by 2055 (relative to 1971-2000) 

under a scenario of substantial emissions reduction (B1), to 

3.5°F assuming continued increases in emissions (A2).40,42 In 

the Western North Pacific, the projected increases by 2055 are 

1.9°F for the B1 scenario and 2.6°F for the A2 scenario.8 In the 

central South Pacific, projected annual surface air temperature 

increases by 2055 are 1.9°F (B1) and 2.5°F (A2).8

On most islands, increased temperatures coupled with decreased rainfall and increased drought will reduce the amount 

of freshwater available for drinking and crop irrigation.43 Climate 

change impacts on freshwater resources in the region will also vary 

because of differing island size and 

topography, which affect water 

storage capability and susceptibility to coastal flooding. Low-lying 

islands will be particularly vulnerable due to their small land mass, 

geographic isolation, limited potable water sources, and limited 

agricultural resources.44 Also, as 

sea level rises over time, increasing 

saltwater intrusion from the ocean 

during storms will exacerbate the 

situation (Figure 23.6).45,46 These 

are only part of a cascade of climate change related impacts that 

will increase the pressures on, and 

threats to, the social and ecosystem sustainability of these island 

communities.47

Observed Changes in Annual Rainfall in the Western North Pacific

Figure 23.4. Islands in the western reaches of the Pacific Ocean are getting slightly more 

rainfall than in the past, while islands more to the east are getting drier (measured in change 

in inches of monthly rainfall per decade over the period 1950-2010). Darker blue shading 

indicates that conditions are wetter, while darker red shading indicates drier conditions. 

The size of the dot is proportional to the size of the trend on the inset scale. (Figure source: 

Keener et al. 20122).
543 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

23: HAWAI‘I AND U.S. AFFILIATED PACIFIC ISLANDS

Key Message 3: Increased Stress on Native Plants and Animals

Increasing temperatures, and in some areas reduced rainfall, will stress native Pacific Island 

plants and animals, especially in high-elevation ecosystems with increasing exposure to 

invasive species, increasing the risk of extinctions. 

Projected climate changes will significantly alter the distribution and abundance of many native marine, terrestrial, and 

freshwater species in the Pacific Islands. The vulnerability of 

coral reef and ocean ecosystems was discussed earlier. Landbased and freshwater species that exist in high-elevation 

ecosystems in high islands, as well as low-lying coastal ecosystems on all islands, are especially vulnerable. Existing climate 

zones on high islands are generally projected to shift upslope 

in response to climate change.48 The ability of native species 

to adapt to shifting habitats will be affected by ecosystem discontinuity and fragmentation, as well as the survival or extinction of pollinators and seed dispersers. Some (perhaps many) 

invasive plant species will have a competitive edge over native 

species, as they disproportionately benefit from increased carbon dioxide, disturbances from extreme weather and climate 

events, and an ability to invade higher elevation habitats as climates warm.49 Hawaiian high-elevation alpine ecosystems on 

Hawai‘i and Maui islands are already beginning to show strong 

signs of higher temperatures and increased drought.50 For example, the number of Haleakalā silversword, a rare plant that 

is an integral component of the alpine ecosystem in Haleakalā 

National Park in Maui and is found nowhere else on the planet, 

has declined dramatically over the past two decades.51 Many 

of Hawai‘i’s native forest birds, marvels of evolution largely 

limited to high-elevation forests due to predators and diseases, are increasingly vulnerable as rising temperatures allow 

mosquitoes carrying diseases like avian malaria to thrive at 

higher elevations and thereby reduce the extent of safe bird 

habitat.48,52

On high islands like Hawai‘i, decreases in precipitation and 

baseflow are already indicating impacts on freshwater ecosystems and aquatic species.35,37 Many Pacific Island freshwater 

fishes and invertebrates have oceanic larval stages in which 

they seasonally return to high island streams to aid reproduction.53 Changes in stream flow and oceanic conditions that 

affect larval growth and survival will alter the ability of these 

species to maintain viable stream populations. 

Key Message 4: Sea Level Rising

Rising sea levels, coupled with high water levels caused by tropical and extra-tropical storms, 

will incrementally increase coastal flooding and erosion, damaging coastal ecosystems, 

infrastructure, and agriculture, and negatively affecting tourism.

Global average sea level has risen by about 8 inches since 

1900,54 with recent satellite observations indicating an increased rate of rise over the past two decades (1.3 inches per 

decade) (see also Ch. 2: Our Changing Climate, Key Message 

10). 55 Recent regional sea level trends in the western tropical 

Pacific are higher56,57 than the global average, due in part to 

changing wind patterns associated with natural climate variability.58,59 Over this century, sea level in the Pacific is expected 

to rise at about the same rate as the projected increase in global average sea level, with regional variations associated with 

ocean circulation changes and the Earth’s response to other 

large-scale changes, such as melting glaciers and ice sheets 

as well as changing water storage in lakes and reservoirs.60,61

For the region, extreme sea level events generally occur when 

high tides combine with changes in water levels due to storms, 

ENSO (see “El Niño and other Patterns of Climate Variability”), 

and other variations.54,55,56,57,58,59,60

Rising sea levels will escalate the threat to coastal structures 

and property, groundwater reservoirs, harbor operations, airports, wastewater systems, shallow coral reefs, sea grass beds, 

intertidal flats and mangrove forests, and other social, ecoFigure 23.5. Warming at high elevations could alter the 

distribution of native plants and animals in mountainous 

ecosystems and increase the threat of invasive species. The 

threatened, endemic ‘ahinahina, or Haleakalā silversword 

(Argyroxiphium sandwicense subsp. macrocephalum), shown 

here in full bloom on Maui, Hawaiian Islands, is one example. 

(Photo credit: Forest and Kim Starr). 

Native Plants at Risk
544 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

23: HAWAI‘I AND U.S. AFFILIATED PACIFIC ISLANDS

nomic, and natural resources. Impacts will vary with location 

depending on how regional sea level variability combines with 

increases of global average sea level.62 On low islands, critical 

public facilities and infrastructure as well as private commercial and residential property are especially vulnerable. Agricultural activity will also be affected, as sea level rise decreases 

the land area available for farming45 and periodic flooding 

increases the salinity of groundwater. Coastal and nearshore 

environments will progressively be affected as sea levels rise 

and high wave events alter low islands’ size and shape. Based 

on extrapolation from results in American Samoa, sea level rise 

could cause future reductions of 10% to 20% in total regional 

mangrove area over the next century.63 This would in turn reduce the nursery areas and feeding grounds for fish species, 

habitat for crustaceans and invertebrates, shoreline protection 

and wave dampening, and water filtration provided by mangroves.

64 Pacific seabirds that breed on low-lying atolls will lose 

large segments of their breeding populations65 as their habitat 

is increasingly and more extensively covered by seawater. 

Impacts to the built environment on low-lying portions of 

high islands, where nearly all airports are located and where 

each island’s road network is 

sited,66 will be nearly as profound as those experienced 

on low islands. Islands with 

more developed built infrastructure will experience 

more economic impacts 

from tourism loss. In Hawai‘i, 

for example, where tourism 

comprises 26% of the state’s 

economy, damage to tourism 

infrastructure could have 

large economic impacts –the 

loss of Waikīkī Beach alone 

could lead to an annual loss 

of $2 billion in visitor expenditures.67

Figure 23.8. Map shows large variations across the Pacific Ocean in sea level trends for 1993-

2010. The largest sea level increase has been observed in the western Pacific. (Figure source: 

adapted from Merrifield 201157 by permission of American Meteorological Society).

Higher Sea Level Rise in Western Pacific

Figure 23.6. Taro crops destroyed by encroaching saltwater 

at Lukunoch Atoll, Chuuk State, FSM. Giant swamp taro is a 

staple crop in Micronesia that requires a two- to three-year 

growing period from initial planting to harvest. After a saltwater 

inundation from a storm surge or very high tide, it may take two 

years of normal rainfall to flush brackish water from a taro patch, 

resulting in a five-year gap before the next harvest if no further 

saltwater intrusion takes place. (Photo credit: John Quidachay, 

USDA Forest Service). 

Saltwater Intrusion Destroys Crops Residents of Low-lying Islands at Risk

Figure 23.7. Republic of the Marshall Islands, with a land area 

of just 1.1 square miles and a maximum elevation of 10 feet, 

may be among the first to face the possibility of climate change 

induced human migration as sea level continues to rise. (Photo 

credit: Darren Nakata). 
545 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

23: HAWAI‘I AND U.S. AFFILIATED PACIFIC ISLANDS

Key Message 5: Threats to Lives, Livelihoods, and Cultures

Mounting threats to food and water security, infrastructure, and public health and safety 

are expected to lead to increasing human migration from low to high elevation islands and 

continental sites, making it increasingly difficult for Pacific Islanders to sustain the 

region’s many unique customs, beliefs, and languages.

All of the climate change impacts described above will have 

an impact on human communities in Pacific Islands. Because 

Pacific Islands are almost entirely dependent upon imported 

food, fuel, and material, the vulnerability of ports and airports 

to extreme events, sea level rise, and increasing wave heights 

is of great concern. Climate change is expected to have serious effects on human health, for example by increasing the 

incidence of dengue fever (Ch. 9: Human Health).68 In addition, 

sea level rise and flooding are expected to overwhelm sewer 

systems and threaten public sanitation. 

The traditional lifestyles and cultures of indigenous communities in all Pacific Islands will be seriously affected by climate 

change (see also Chapter 12: Indigenous Peoples). Sea level 

rise and associated flooding is expected to destroy coastal 

artifacts and structures69 or even the entire land base associated with cultural traditions.70 Drought threatens traditional 

food sources such as taro and breadfruit, and coral death from 

warming-induced bleaching will threaten subsistence fisheries 

in island communities.46 Climate change related environmental 

deterioration for communities at or near the coast, coupled 

with other socioeconomic or political motivations, is expected 

to lead individuals, families, or communities to consider moving to new locations. Depending on the scale and distance of 

the migration, a variety of challenges face the migrants and the 

communities receiving them. Migrants need to establish themselves in their new community, find employment, and access 

services, while the receiving community’s infrastructure, labor 

market, commerce, natural resources, and governance structures need to absorb a sudden burst of population growth.

Adaptation Activities

Adaptive capacity in the region varies and reflects the histories 

of governance, the economies, and the geographical features 

of the island/atoll site. High islands can better support larger 

populations and infrastructure, attract industry, foster institutional growth, and thus bolster adaptive capacity;2

 but these 

sites have larger policy or legal hurdles that complicate coastal 

planning.71 Low islands have a different set of challenges. Climate change related migration, for example, is particularly 

relevant to the low island communities in the Republic of the 

Marshall Islands (RMI) and the Federated States of Micronesia (FSM), and presents significant practical, cultural, and legal 

challenges.72

In Hawai‘i, state agencies have drafted a framework for climate 

change adaptation by identifying sectors affected by climate 

change and outlining a process for coordinated statewide adaptation planning.73 Both Hawai‘i and American Samoa specifically consider climate change in their U.S. Federal Emergency 

Management Agency (FEMA) hazard mitigation plans, and the 

Commonwealth of Northern Mariana Islands lists climate variability as a possible hazard related to extreme climate events.74

The U.S. Pacific Island Freely Associated States (which includes 

the Republic of Palau, FSM, and RMI; Figure 23.1) have worked 

with regional organizations to develop plans and access international resources. Each of these jurisdictions has developed 

a status report on integrating climate-related hazard information in disaster risk reduction planning and has developed 

plans for adaptation to climate-related disaster risks.75 Overall, 

there is very little research on the effectiveness of alternative 

adaptation strategies for Pacific Islands and their communities. 

The regional culture of communication and collaboration provides a strong foundation for adaptation planning and will be 

important for building resilience in the face of the changing 

climate. 
546 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

23: HAWAI‘I AND US AFFILIATED PACIFIC ISLANDS 

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in Hawai‘i: A policy tool kit for state and local governments, 76 pp., 

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552 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

23: HAWAI‘I AND US AFFILIATED PACIFIC ISLANDS 

Process for Developing Key Messages

 A central component of the assessment process was convening 

three focus area workshops as part of the Pacific Islands Regional 

Climate Assessment (PIRCA). The PIRCA is a collaborative effort 

aimed at assessing the state of climate knowledge, impacts, and 

adaptive capacity in Hawai‘i and the U.S. Affiliated Pacific Islands. 

These workshops included representatives from the U.S. federal 

agencies, universities, as well as international participants from 

other national agencies and regional organizations. The workshops 

led to the formulation of a foundational Technical Input Report 

(TIR).2 The report consists of nearly 140 pages, with almost 300 

references, and was organized into 5 chapters by 11 authors. 

The chapter author team engaged in multiple technical discussions via regular teleconferences that permitted a careful review of 

the foundational TIR2 and of approximately 23 additional technical inputs provided by the public, as well as the other published 

literature, and professional judgment. These discussions included 

a face-to-face meeting held on July 9, 2012. These discussions 

were supported by targeted consultation among the lead and contributing authors of each message. There were several iterations 

of review and comment on draft key messages and associated 

content.

Key message #1 Traceable Account

Warmer oceans are leading to increased coral 

bleaching events and disease outbreaks in coral 

reefs, as well as changed distribution patterns of 

tuna fisheries. Ocean acidification will reduce coral 

growth and health. Warming and acidification, combined with existing stresses, will strongly affect 

coral reef fish communities.

Description of evidence base

The key message was chosen based on input from the extensive evidence documented in the Hawai‘i Technical Input Report2

 and additional technical inputs received as part of the 

Federal Register Notice solicitation for public input, as well as 

stakeholder engagement leading up to drafting the chapter. 

Ocean warming: There is ample evidence that sea-surface temperatures have already risen throughout the region based on clear 

observational data, with improved data with the advent of satellite and in situ (ARGO & ship-based) data.7

 Assessment of the 

literature for the region by other governmental bodies (such as 

Australian Bureau of Meteorology [ABOM] and the Commonwealth 

Scientific and Industrial Research Organization [CSIRO]) point to 

continued increases under both B1 and A2 scenarios.8

Ocean acidification: Globally, the oceans are currently absorbing 

about a quarter of the carbon dioxide emitted to the atmosphere 

annually, and becoming more acidic as a result (Ch. 2: Our Changing Climate, Key Message 12). Historical and current observations 

of aragonite saturation state (Ωar) for the Pacific Ocean show a 

decrease from approximately 4.9 to 4.8 in the Central North Pacific (Hawaiian Islands); in the Western North Pacific (Republic 

of Marshall Islands, Commonwealth of Northern Mariana Islands, 

Federated States of Micronesia, Republic of Palau, Guam), it has 

declined from approximately 4.5 to 3.9 in 2000, and to 4.1 in 

the Central South Pacific (American Samoa) (this chapter: Figure 

23.3; Ch. 24: Oceans and Marine Resources).19 Projections from 

CMIP3 models indicate the annual maximum aragonite saturation 

state will reach values below 3.5 by 2035 in the waters of the 

Republic of the Marshall Islands (RMI), by 2030 in the Federated States of Micronesia (FSM), by 2040 in Palau, and by 2060 

around the Samoan archipelago. These values are projected to 

continue declining thereafter.2 The recently published Reefs at 

Risk Revisited25 estimates aragonite saturation state (as an indicator of ocean acidification) for CO2 stabilization levels of 380 ppm, 

450 ppm, and 500 ppm, which correspond approximately to the 

years 2005, 2030, and 2050 under the A1B emissions scenario 

(which assumes similar emissions to the A2 scenario through 

2050 and a slow decline thereafter) (Figure 4.4 from Keener et 

al. 20122). 

Bleaching events: These have been well-documented in extensive 

literature worldwide due to increasing temperatures, with numerous studies in Hawai‘i and the Pacific Islands.9,10

SUPPLEMENTAL MATERIAL

TRACEABLE ACCOUNTS
23: HAWAI‘I AND U.S. AFFILIATED PACIFIC ISLANDS

Traceable Accounts

553 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Disease outbreaks: Reports of coral diseases have been proliferating in the past years,11,13 but few have currently been adequately 

described, with causal organisms identified (for example, fulfill 

Koch’s Postulates). 

Reduced growth: There is abundant evidence from laboratory experiments that lower seawater pH reduces calcification rates in 

marine organisms (for example, Feely et al. 200919). However, actual measurements on the effects of ocean acidification on coral 

reef ecosystems in situ or in complex mesocosms are just now 

becoming available, and these measurements show that there are 

large regional and diel variability in pH and pCO2.

76 The role of diel 

and regional variability on coral reef ecosystems requires further 

investigation.

Distribution patterns of coastal and ocean fisheries: Evidence of 

the effects of ocean acidification on U.S. fisheries in Hawai‘i and 

the Pacific Islands is currently limited (Lehodey et al. 2011)28 but 

there is accumulating evidence for ecosystem impacts. 

New information and remaining uncertainties 

New information: Since the 2009 National Climate Assessment,77

considerable effort has been employed to understand the impacts 

of ocean acidification (OA) on marine ecosystems, including recent ecosystem-based efforts.22,28 Studies of OA impacts on organisms has advanced considerably, with careful chemistry using 

worldwide standard protocols making inroads into understanding 

a broadening range of organisms. 

However, predicting the effect of ocean acidification on marine 

organisms and marine coral reef ecosystems remains the key issue 

of uncertainty. The role of community metabolism and calcification in the face of overall reduction in aragonite saturation state 

must be investigated. 

Understanding interactions between rising temperatures and OA 

remains a challenge. For example, high temperatures simultaneously cause coral bleaching, as well as affect coral calcification 

rates, with both impacts projected to increase in the future. 

Assessment of confidence based on evidence

There is very high confidence that ocean acidification and 

decreased aragonite saturation is taking place and is projected 

to continue. There is high confidence that ocean warming is 

taking place and is projected to continue; there is medium

confidence that the thermal anomalies will lead to continued 

coral bleaching and coral disease outbreaks.

Key message #2 Traceable Account

Freshwater supplies are already constrained and 

will become more limited on many islands. Saltwater intrusion associated with sea level rise will 

reduce the quantity and quality of freshwater in 

coastal aquifers, especially on low islands. In areas 

where precipitation does not increase, freshwater 

supplies will be adversely affected as air temperature rises.

Description of evidence base

There is abundant and definitive evidence that air temperature has 

increased and is projected to continue to increase over the entire 

region,8,41,78 as there is globally (Ch. 2: Our Changing Climate, Key 

Message 3).

In Hawai‘i and the Central North Pacific (CNP), projected annual 

surface air temperature increases are 1.0°F to 2.5°F by 2035, 

relative to 1971-2000.40,42 In the Western North Pacific (WNP), 

the projected increases are 2.0°F to 2.3°F by 2030, 6.1°F to 

8.5°F by 2055, and 4.9°F to 9.2°F by 2090.8 In the central South 

Pacific (CSP), projected annual surface air temperature increases 

are 1.1°F to 1.3°F by 2030, 1.8°F to 2.5°F by 2055, and 2.5°F 

to 4.9°F by 2090.8 (Please note that the islands that comprise the 

U.S. Pacific Islands Region are shown in Figure 23.1).

In Hawai‘i, mean precipitation, average stream discharge, and 

stream baseflow have been trending downward for nearly a cenConfidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent 

results, well documented and 

accepted methods, etc.), high 

consensus

High

Moderate evidence (several sources, some consistency, methods 

vary and/or documentation limited, 

etc.), medium consensus

Medium

Suggestive evidence (a few 

sources, limited consistency, models incomplete, methods emerging, 

etc.), competing schools of thought

Low

Inconclusive evidence (limited 

sources, extrapolations, inconsistent findings, poor documentation 

and/or methods not tested, etc.), 

disagreement or lack of opinions 

among experts
23: HAWAI‘I AND U.S. AFFILIATED PACIFIC ISLANDS

Traceable Accounts

554 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

tury, especially in recent decades and with high variability related 

to El Niño-Southern Oscillation (ENSO) and Pacific Decadal Oscillation (PDO).34,35 For the WNP, a decline of 15% in annual rainfall 

has been observed in the eastern-most islands in the Micronesia 

region and slight upward trends in precipitation have been seen 

for the western-most islands, with high ENSO-related variability.8

In American Samoa, no trends in average rainfall are apparent 

based on the very limited available data.8,37

For the region as a whole, models disagree about projected changes in precipitation. Mostly models predict increases in mean annual rainfall and suggest a slight dry season decrease and wet 

season increase in precipitation.8 However, based on statistical 

downscaling, one study39 projected a 5% to 10% reduction in precipitation for the wet season and a 5% increase in the dry season 

for Hawai‘i by the end of this century.

On most islands, increased temperatures coupled with decreased 

rainfall and increased drought will reduce the amount of freshwater for drinking and crop irrigation.43 Atolls will be particularly 

vulnerable due to their low elevation, small land mass, geographic 

isolation, and limited potable water sources and agricultural resources.44 The situation will also be exacerbated by the increased 

incidence of intrusion of saltwater from the ocean during storms 

as the mean sea level rises over time (Key Message 4, this chapter; Ch. 2: Our Changing Climate, Key Message 10).2

New information and remaining uncertainties

Climate change impacts on freshwater resources in the Pacific 

Islands region will vary because of differing island size and height, 

which affect water storage capability and susceptibility to coastal 

inundation. The impacts will also vary because of natural phase 

variability (for example, ENSO and PDO) in precipitation and 

storminess (tropical and extra-tropical storms) as well as longterm trends, both strongly influenced by geographic location.

Climate model simulations produce conflicting assessments as to 

how the tropical Pacific atmospheric circulation will respond in the 

future to climate change.

Assessment of confidence based on evidence

Freshwater systems are inherently fragile in many Pacific Islands. 

Historical observations show strong evidence of a decreasing trend 

for rainfall in Hawai‘i and many other Pacific Islands (Ch. 2: Our 

Changing Climate).2 There is abundant and definitive evidence 

that air temperature has increased and will continue to increase. 

All of the scientific approaches to detecting sea level rise come 

to the conclusion that a warming planet will result in higher sea 

levels. Based on the evidence base and remaining uncertainties, 

we have high confidence in the key message. 

Key message #3 Traceable Account

Increasing temperatures, and in some areas reduced rainfall, will stress native Pacific Island 

plants and animals, especially in high-elevation ecosystems with increasing exposure to invasive species, increasing the risk of extinctions.

Description of evidence base

In Hawai‘i and the Central North Pacific (CNP), projected annual 

surface air temperature increases are 1.0°F to 2.5°F by 2035, relative to 1971-2000.40,42 In the Western North Pacific (WNP), the 

projected increases are 2.0°F to 2.3°F by 2030, 6.1°F to 8.5°F 

by 2055, and 4.9°F to 9.2°F by 2090.8 In the Central South 

Pacific (CSP), projected annual surface air temperature increases 

are 1.1°F to 1.3°F by 2030, 1.8°F to 2.5°F by 2055, and 2.5°F to 

4.9°F by 2090.8 In Hawai‘i the rate of increase has been greater 

at high elevations.41 (Please note that the islands that comprise 

the U.S. Pacific Islands Region are shown in Figure 23.1). 

In Hawai‘i mean precipitation, average stream discharge, and 

stream baseflow have been trending downward for nearly a century, especially in recent decades and with high ENSO and PDOrelated variability.34,35,36 Projects based on statistical downscaling39 suggest the most likely precipitation scenario for Hawai‘i for 

the 21st century to be a 5% to 10% reduction for the wet season 

and a 5% increase in the dry season.

On high islands like Hawai‘i, decreases in precipitation and baseflow35 are already indicating that there will be impacts on freshwater ecosystems and aquatic species, and on water-intensive 

sectors such as agriculture and tourism.

Hawaiian high-elevation alpine ecosystems on Hawai‘i and 

Maui islands are already beginning to show strong signs of increased drought and warmer temperatures.50 Demographic 

data for the Haleakalā silversword, a unique (endemic to upper 

Haleakalāvolcano) and integral component of the alpine ecosystem in Haleakalā National Park, Maui, have recorded a severe 

decline in plant numbers over the past two decades.51 Many of 

Hawai‘i’s endemic forest birds, marvels of evolution largely limited 

to high-elevation forests by predation and disease, are increasingly vulnerable as rising temperatures allow the disease-vectoring 

mosquitoes to thrive upslope and thereby reduce the extent of safe 

bird habitat.48,52

New information and remaining uncertainties 

Climate change impacts in the Pacific Islands region will vary because of differing island size and height. The impacts will also 

vary because of natural phase variability (for example, El NiñoSouthern Oscillation and Pacific Decadal Oscillation) in precipitation and storminess (tropical and extra-tropical storms) as well as 

long-term trends, both strongly influenced by geographic location.
23: HAWAI‘I AND U.S. AFFILIATED PACIFIC ISLANDS

Traceable Accounts

555 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Climate model simulations produce conflicting assessments as to 

how the tropical Pacific atmospheric circulation will respond in the 

future to climate change.2,8

Climate change ecosystem response is poorly understood.2

Assessment of confidence based on evidence

Terrestrial and marine ecosystems are already being impacted by 

local stressors, such as coastal development, land-based sources 

of pollution, and invasive species.2,25 There is abundant and definitive evidence that air temperature has increased and will continue to increase. Historical observations show strong evidence of 

a decreasing trend for rainfall in Hawai‘i and many other Pacific 

Islands.2 Given the evidence base and remaining uncertainties, 

confidence is high in this key message. 

Key message #4 Traceable Account

Rising sea levels, coupled with high water levels 

caused by tropical and extra-tropical storms, will incrementally increase coastal flooding and erosion, 

damaging coastal ecosystems, infrastructure, and 

agriculture, and negatively affecting tourism. 

Description of evidence base

All of the scientific approaches to detecting sea level rise come to the 

conclusion that a warming planet will result in higher sea levels. 

Recent studies give higher sea level rise projections than those 

projected in 2007 by the Intergovernmental Panel on Climate 

Change29 for the rest of this century (Ch. 2: Our Changing Climate, 

Key Message 10).55

Sea level is rising and is expected to continue to rise. Over the 

past few decades, global mean sea level, as measured by satellite 

altimetry, has been rising at an average rate of twice the estimated 

rate for the previous century, based on tide gauge measurements,55

with models suggesting that global sea level will rise significantly 

over the course of this century. Regionally, the highest increases 

have been observed in the western tropical Pacific.56 However, the 

current high rates of regional sea level rise in the western tropical 

Pacific are not expected to persist, as regional sea level will fall 

in response to a change in phase of natural variability.62 Regional 

variations in sea level at interannual and interdecadal time scales 

are generally attributed to changes in prevailing wind patterns associated with El Niño-Southern Oscillation (ENSO) as well as the 

Pacific Decadal Oscillation (PDO) and low frequency components 

of the Southern Oscillation Index (SOI).59

For the region, extreme sea level events generally occur when 

high tides combine with some non-tidal residual change in water level. In the major typhoon zones (Guam and Commonwealth 

of the Northern Mariana Islands), storm-driven surges can cause 

coastal flooding and erosion regardless of tidal state. Wave-driven 

inundation events are a major concern for all islands in the region. 

At present, trends in extreme levels tend to follow trends in mean 

sea level.

Increasing mean water levels and the possibility of more frequent 

extreme water level events, and their manifestation as flooding 

and erosion, will threaten coastal structures and property, groundwater reservoirs, harbor operations, airports, wastewater systems, 

sandy beaches, coral reef ecosystems, and other social and economic resources. Impacts will vary with location, depending on 

how natural sea level variability combines with modest increases 

of mean levels.62

On low-lying atolls, critical public facilities and infrastructure as 

well as private commercial and residential property are especially 

vulnerable.62 Agricultural activity will also be affected, as sea level 

rise decreases the land area available for farming45 and episodic 

inundation increases salinity of groundwater resources. Impacts to 

the built environment on low-lying portions of high islands will be 

much the same as those experienced on low islands. Islands with 

more developed built infrastructure will experience more economic impacts from tourism loss. One report stated: “Our analyses estimate that nearly $2.0 billion in overall visitor expenditures could 

be lost annually due to a complete erosion of Waikīkī Beach.”67

Coastal and nearshore environments (sandy beaches, shallow 

coral reefs, seagrass beds, intertidal flats, and mangrove forests) 

and the vegetation and terrestrial animals in these systems will 

progressively be affected as sea level rise and high wave events alter atoll island size and shape and reduce habitat features necessary for survival. Based on extrapolation from results in American 

Samoa, sea level rise could cause future reductions of 10%–20% 

of total regional mangrove area over the next century.63 Further, 

atoll-breeding Pacific seabirds will lose large segments of their 

breeding populations65 as their habitat is increasingly and more 

extensively inundated.

Major uncertainties 

Sea levels in the Pacific Ocean will continue to rise with global sea 

level. Models provide a range of predictions, with some suggesting 

that global warming may raise global sea level considerably over 

the course of this century. The range of predictions is large due 

in part to unresolved physical understanding of various processes, 

notably ice sheet dynamics. 

Changes in prevailing wind patterns associated with natural climate cycles such as ENSO and the PDO affect regional variations 

in sea level at interannual and interdecadal time scales. Sea level 

at specific locales will continue to respond to changes in phase of 

these natural climate cycles. The current high rates of regional sea 

level rise in the western tropical Pacific are not expected to persist 

over time, falling once the trade winds begin to weaken. 

Future wind wave conditions are difficult to project with confidence given the uncertainties regarding future storm conditions. 
23: HAWAI‘I AND U.S. AFFILIATED PACIFIC ISLANDS

Traceable Accounts

556 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Assessment of confidence based on evidence

Evidence for global sea level rise is strong (Ch. 25: Coasts; Ch. 2: 

Our Changing Climate). Confidence is therefore very high. Modeling studies have yielded conflicting results as to how ENSO and 

other climate modes will vary in the future. As a result, there is 

low confidence in the prediction of future climate states and their 

subsequent influence on regional sea level.62 Recent assessments 

of future extreme conditions generally place low confidence on 

region-specific projections of future storminess.61

For aspects of the key message concerning impacts, confidence 

is high. 

Key message #5 Traceable Account

Mounting threats to food and water security, infrastructure, and public health and safety are expected to lead to increasing human migration from 

low to high elevation islands and continental sites, 

making it increasingly difficult for Pacific Islanders 

to sustain the region’s many unique customs, beliefs, and languages.

Description of evidence base

Climate change threatens communities, cultures, and ecosystems 

of the Pacific Islands both directly through impact on food and 

water security, for example, as well as indirectly through impacts 

on economic sectors including fisheries and tourism. 

On most islands, increased temperatures, coupled with decreased 

rainfall and increased drought, will lead to an additional need for 

freshwater resources for drinking and crop irrigation.43 This is 

particularly important for locations in the tropics and subtropics 

where observed data and model projections suggest that, by the 

end of this century, the average growing season temperatures will 

exceed the most extreme seasonal temperatures recorded from 

1900 to 2006. Atolls will be particularly vulnerable due to their 

low elevation, small land mass, geographic isolation, and limited 

potable water sources and agricultural resources.44 The situation 

will also be exacerbated by the increased incidence of intrusion of 

saltwater from the ocean during storms as the mean sea level rises 

over time. These are but part of a cascade of impacts that will increase the pressures on, and threats to, the social and ecosystem 

sustainability of these island communities.47 On high islands like 

Hawai‘i, decreases in precipitation and baseflow35 are already indicating that there will be impacts on freshwater ecosystems and 

aquatic species and on water-intensive sectors such as agriculture 

and tourism.

Increasing mean oceanic and coastal water levels and the possibility of more frequent extreme water level events with flooding 

and erosion will escalate the threat to coastal structures and property, groundwater reservoirs, harbor operations, airports, wastewater systems, sandy beaches, coral reef ecosystems, and other 

social and economic resources. Impacts will vary with location 

depending on how natural sea level variability combines with modest increases of mean levels.62 On low-lying atolls, critical public 

facilities and infrastructure as well as private commercial and 

residential property are especially vulnerable. Agricultural activity 

will also be affected, as sea level rise decreases the land area 

available for farming45 and episodic inundation increases salinity 

of groundwater resources. 

With respect to cultural resources, impacts will extend from the 

loss of tangible artifacts and structures69 to the intangible loss of 

a land base and the cultural traditions that are associated with it.70

New information and remaining uncertainties 

Whenever appraising threats to human society, it is uncertain the 

degree to which societies will successfully adapt to limit impact. 

For island communities, though, the ability to migrate is very limited, and the ability to adapt is especially limited. Depending on the 

scale and distance of the migration, a variety of challenges face 

the migrants and the communities receiving them. Migrants need 

to establish themselves in their new community, find employment, 

and access services, while the receiving community’s infrastructure, labor market, commerce, natural resources, and governance 

structures need to absorb a sudden burst of population growth.

Assessment of confidence based on evidence

Evidence for climate change and impacts is strong, but highly variable from location to location. One can be highly confident that 

climate change will continue to pose varied threats in the region. 

Adaptive capacity is also highly variable among the islands, so 

the resulting situation will play out differently in different places. 

Confidence is therefore medium. 
557

Climate Change Impacts in the United States

CHAPTER 24

OCEANS AND MARINE RESOURCES

INFORMATION DRAWN FROM THIS CHAPTER IS INCLUDED IN THE HIGHLIGHTS REPORT AND IS IDENTIFIED BY THIS ICON

Recommended Citation for Chapter

Doney, S., A. A. Rosenberg, M. Alexander, F. Chavez, C. D. Harvell, G. Hofmann, M. Orbach, and M. Ruckelshaus, 2014: Ch. 

24: Oceans and Marine Resources. Climate Change Impacts in the United States: The Third National Climate Assessment, 

J. M. Melillo, Terese (T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 557-578. doi:10.7930/

J0RF5RZW. 

On the Web: http://nca2014.globalchange.gov/report/regions/oceans

Convening Lead Authors

Scott Doney, Woods Hole Oceanographic Institution

Andrew A. Rosenberg, Union of Concerned Scientists

Lead Authors

Michael Alexander, National Oceanic and Atmospheric Administration

Francisco Chavez, Monterey Bay Aquarium Research Institute

C. Drew Harvell, Cornell University

Gretchen Hofmann, University of California Santa Barbara

Michael Orbach, Duke University

Mary Ruckelshaus, Natural Capital Project
558 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

OCEANS 24 AND MARINE RESOURCES

Key Messages

1. The rise in ocean temperature over the last century will persist into the future, with continued 

large impacts on climate, ocean circulation, chemistry, and ecosystems.

2. The ocean currently absorbs about a quarter of human-caused carbon dioxide emissions to the 

atmosphere, leading to ocean acidification that will alter marine ecosystems in dramatic yet 

uncertain ways.

3. Significant habitat loss will continue to occur due to climate change for many species and areas, 

including Arctic and coral reef ecosystems, while habitat in other areas and for other species 

will expand. These changes will consequently alter the distribution, abundance, and productivity 

of many marine species.

4. Rising sea surface temperatures have been linked with increasing levels and ranges of diseases 

in humans and marine life, including corals, abalones, oysters, fishes, and marine mammals.

5. Climate changes that result in conditions substantially different from recent history may 

significantly increase costs to businesses as well as disrupt public access and enjoyment of 

ocean areas.

6. In response to observed and projected climate impacts, some existing ocean policies, practices, 

and management efforts are incorporating climate change impacts. These initiatives can serve 

as models for other efforts and ultimately enable people and communities to adapt to changing 

ocean conditions.

As a nation, we depend on the oceans for seafood, recreation 

and tourism, cultural heritage, transportation of goods, and, increasingly, energy and other critical resources. The U.S. Exclusive Economic Zone extends 200 nautical miles seaward from 

the coasts, spanning an area about 1.7 times the land area of 

the continental U.S. and encompassing waters along the U.S. 

East, West, and Gulf coasts, around Alaska and Hawai‘i, and 

including the U.S. territories in the Pacific and Caribbean. This 

vast region is host to a rich diversity of marine plants and animals and a wide range of ecosystems, from tropical coral reefs 

to Arctic waters covered with sea ice. 

Oceans support vibrant economies and coastal communities 

with numerous businesses and jobs. More than 160 million 

people live in the coastal watershed counties of the United 

States, and population in this zone is expected to grow in the 

future. The oceans help regulate climate, absorb carbon dioxide (an important greenhouse, or heat-trapping, gas), and 

strongly influence weather patterns far into the continental 

interior. Ocean issues touch all of us in both direct and indirect 

ways.1,2,3

Changing climate conditions are already affecting these valuable marine ecosystems and the array of resources and services we derive from the sea. Some climate trends, such as rising 

seawater temperatures and ocean acidification, are common 

across much of the coastal areas and open ocean worldwide. 

The biological responses to climate change often vary from 

region to region, depending on the different combinations of 

species, habitats, and other attributes of local systems. Data 

records for the ocean are often shorter and less complete than 

those on land, and for many biological variables it is still difficult to discern long-term ocean trends from natural variability.4

©iStockPhoto.com Frank P.J. van Haalen
559 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Key Message 1: Rising Ocean Temperatures

The rise in ocean temperature over the last century will persist into the future, with continued 

large impacts on climate, ocean circulation, chemistry, and ecosystems. 

Cores from corals, ocean sediments, ice records, and other indirect temperature measurements indicate the recent rapid increase of ocean temperature is the greatest that has occurred 

in at least the past millennium and can only be reproduced by 

climate models with the inclusion of human-caused sources of 

heat-trapping gas emissions.5,6 The ocean is a critical reservoir 

for heat within Earth’s climate system, and because of seawater’s large heat storing capacity, small changes in ocean temperature reflect large changes in ocean heat storage. Direct 

measurements of ocean temperatures show warming beginning in about 1970 down to at least 2,300 feet, with stronger 

warming near the surface leading to increased thermal stratification (or layering) of the water column.

7,8 Sea surface temperatures in the North Atlantic and Pacific, including near U.S. 

coasts, have also increased since 1900.9,10 In conjunction with a 

warming climate, the extent and thickness of Arctic sea ice has 

decreased rapidly over the past four decades.11,12 Models that 

best match historical trends project seasonally ice-free northern 

waters by the 2030s.13

Climate-driven warming reduces vertical mixing of ocean water 

that brings nutrients up from deeper water, leading to potential 

impacts on biological productivity. Warming and altered ocean 

circulation are also expected to reduce the supply of oxygen 

to deeper waters, leading to future expansion of sub-surface 

low-oxygen zones.15 Both reduced nutrients at the surface and 

reduced oxygen at depth have the potential to change ocean 

productivity.

14 Satellite observations indicate that warming of 

the upper ocean on year-to-year timescales leads to reductions in the biological productivity of tropical and subtropical 

(the region just outside the tropics) oceans and expansion of 

the area of surface waters with very low quantities of phytoplankton (microscopic marine 

plants) biomass.16 Ecosystem models suggest that the 

same patterns of productivity 

change will occur over the next 

century as a consequence of 

warming during this century, 

perhaps also with increasing 

productivity near the poles.17

These changes can affect ecosystems at multiple levels of 

the food web, with consequent 

changes for fisheries and other 

important human activities 

that depend on ocean productivity.4,18

Other changes in the physical 

and chemical properties of the 

ocean are also underway due 

to climate change. These include rising sea level,19 changes 

in upper ocean salinity (including reduced salinity of Arctic 

surface waters) resulting from 

altered inputs of freshwater 

and losses from evaporation, 

changes in wave height from 

changes in wind speed, and 

changes in oxygen content at 

various depths – changes that 

will affect marine ecosystems 

and human uses of the ocean 

in the coming years.4

Figure 24.1. Sea surface temperatures for the ocean surrounding the U.S. and its territories have 

warmed by more than 0.9°F over the past century (top panel). There is significant variation from 

place to place, with the ocean off the coast of Alaska, for example, warming far more rapidly than 

other areas (bottom panel). The gray shading on the map denotes U.S. land territory and the 

regions where the U.S. has rights over the exploration and use of marine resources, as defined 

by the U.S. Exclusive Economic Zone (EEZ). (Figure source: adapted from Chavez et al. 201114).

Observed Ocean Warming
560 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

24: OCEANS AND MARINE RESOURCES

While the long-term global pattern is clear, there is considerable variability in the effects of climate change regionally and 

locally because oceanographic conditions are not uniform and 

are strongly influenced by natural climate fluctuations. Trends 

during short periods of a decade or so can be dominated by 

natural variability.25 For example, the high incidence of La Niña 

events in the last 15 years has played a role in the observed 

temperature trends.26

Analyses27 suggest that 

more of the increase in 

heat energy during this 

period has been transferred to the deep ocean 

(see also Ch. 2: Our Changing Climate). While this 

might temporarily slow 

the rate of increase in surface air temperature, ultimately it will prolong the 

effects of global warming 

because the oceans hold 

heat for longer than the 

atmosphere does.

Interactions with processes in the atmosphere 

and on land, such as rainfall patterns and runoff, 

also vary by region and 

are strongly influenced 

by natural climate fluctuations, resulting in additional local variation in 

the observed effects in 

the ocean. 

Marine ecosystems are 

also affected by other human-caused local and regional disturbances such 

as overfishing, coastal 

habitat loss, and pollution, and climate change 

impacts may exacerbate 

the effects of these other 

human factors.

Figure 24.2. As heat-trapping gases, primarily carbon dioxide (CO2) (panel A), have increased over 

the past decades, not only has air temperature increased worldwide, but so has the temperature of the 

ocean’s surface (panel B). The increased ocean temperature, combined with melting of glaciers and 

ice sheets on land, is leading to higher sea levels (panel C). Increased air and ocean temperatures 

are also causing the continued, dramatic decline in Arctic sea ice during the summer (panel D). 

Additionally, the ocean is becoming more acidic as increased atmospheric CO2 dissolves into it (panel 

E). (CO2 data from Etheridge 2010,20 Tans and Keeling 2012,21 and NOAA NCDC 2012;22 SST data 

from NOAA NCDC 201222 and Smith et al. 2008;10 Sea level data from CSIRO 201223 and Church 

and White 2011;19 Sea ice data from University of Illinois 2012;24 pH data from Doney et al. 20124).

Ocean Impacts of Increased Atmospheric Carbon Dioxide
561 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

24: OCEANS AND MARINE RESOURCES

Key Message 2: Ocean Acidification Alters Marine Ecosystems

The ocean currently absorbs about a quarter of human-caused carbon dioxide 

emissions to the atmosphere, leading to ocean acidification that will alter 

marine ecosystems in dramatic yet uncertain ways. 

Atmospheric carbon dioxide (CO2) has risen by about 40% 

above pre-industrial levels.21,28 The ocean absorbs about a 

quarter of human-caused emissions of carbon dioxide annually, thereby changing seawater chemistry and decreasing pH 

(making seawater more acidic) (Ch. 2: Our Changing Climate, 

Key Message 12).3,29 Surface ocean pH has declined by 0.1 

units, equivalent to a 30% increase in ocean acidity, since preindustrial times.30 Ocean acidification will continue in the future due to the interaction of atmospheric carbon dioxide and 

ocean water. Regional differences in ocean pH occur as a result 

of variability in regional or local conditions, such as upwelling that brings subsurface waters up to the surface.31 Locally, 

coastal waters and estuaries can also exhibit acidification as 

the result of pollution and excess nutrient inputs.

More acidic waters create repercussions along the marine food chain. For example, calcium carbonate is a 

skeletal component of a wide variety of organisms in the 

oceans, including corals. The chemical changes caused 

by the uptake of CO2 make it more difficult for these living things to form and maintain calcium carbonate shells 

and skeletal components and increases erosion of coral 

reefs,

32 resulting in alterations in marine ecosystems 

that will become more severe as present-day trends in 

acidification continue or accelerate (Ch. 22: Alaska; Ch. 

23: Hawai‘i and Pacific Islands).

33,34,35 Tropical corals are 

particularly susceptible to the combination of ocean 

acidification and ocean warming, which would threaten 

the rich and biologically diverse coral reef habitats.

Over 90% of seafood consumed in the U.S. is imported, and 

more than half of the imported seafood comes from aquaculture (fish and shellfish farming).1

 While only 1% of U.S. seafood 

comes from domestic shellfish farming, the industry is locally 

important. In addition, shellfish have historically been an important cultural and food resource for indigenous peoples 

along our coasts (Ch. 12: Indigenous Peoples, Key Message 1). 

Increased ocean acidification, low-oxygen events, and rising 

temperatures are already affecting shellfish aquaculture operations. Higher temperatures are predicted to increase aquaculture potential in poleward regions, but decrease it in the 

tropics.37 Acidification, however, will likely reduce growth and 

survival of shellfish stocks in all regions.

34

Figure 24.3. The 36-day-old clams in the photos are a single species, Mercenaria mercenaria, grown in the 

laboratory under varying levels of carbon dioxide (CO2) in the air. CO2 is absorbed from the air by ocean water, 

acidifying the water and thus reducing the ability of juvenile clams to grow their shells. As seen in the photos, where 

CO2 levels rise progressively from left to right, 36-day-old clams (measured in microns) grown under elevated 

CO2 levels are smaller than those grown under lower CO2 levels. The highest CO2 level, about 1500 parts per 

million (ppm; far right), is higher than most projections for the end of this century but could occur locally in some 

estuaries. (Figure source: Talmage and Gobler 201036).

Ocean Acidification Reduces Size of Clams

Pteropods, or “sea butterflies,” are eaten by a variety of marine species 

ranging from tiny krill to salmon to whales. The photos show what happens 

to a pteropod’s shell in seawater that is too acidic. On the left is a shell 

from a live pteropod from a region in the Southern Ocean where acidity 

is not too high. The shell on the right is from a pteropod in a region where 

the water is more acidic. (Photo credits: (left) Bednaršek et al. 2012;105

(right) Nina Bednaršek).
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Key Message 3: Habitat Loss Affects Marine Life

Significant habitat loss will continue to occur due to climate change for many species and 

areas, including Arctic and coral reef ecosystems, while habitat in other areas and 

for other species will expand. These changes will consequently alter the 

distribution, abundance, and productivity of many marine species. 

Species have responded to climate change in part by shifting where they live.45 Such range shifts result in ecosystem 

changes, including the relationships between species and their 

connection to habitat, because different species respond to 

changing conditions in different ways. This means that ocean 

ecosystems are changing in complex ways, with accompanying 

changes in ecosystem functions (such as nutrient cycling, productivity of species, and predator-prey relationships). Overall 

habitat extent is expected to change as well, though the degree of range migration will depend upon the life history of 

particular species. For example, reductions in seasonal sea-ice 

cover and higher surface temperatures may open up new habitats in polar regions for some important fish species, such as 

cod, herring, and pollock.46 However, the continuing presence 

of cold bottom-water temperatures on the Alaskan Continental shelf could limit northward migration into the northern 

Bering Sea and Chukchi Sea.47 In addition, warming may cause 

reductions in the abundance of some species, such as pollock, 

in their current ranges in the Bering Sea.48 For other ice-dependent species, including several marine mammals such as 

polar bears, walruses, and many seal species, the loss of their 

critically important habitat will result in population declines.49

Additionally, climate extremes can facilitate biological invasions by a variety of mechanisms such as increased movement 

or transport of invasive species, and decreased resilience of 

native species, so that climate change could increase existing 

impacts from human transport.50 These changes will result in 

changing interactions among species with consequences that 

are difficult to predict. Tropical species and ecosystems may 

encounter similar difficulties in migrating poleward as success 

The impacts of ocean acidification on west coast aquaculture

Ocean acidification has already changed the way shellfish farmers on the West Coast conduct business. For oyster 

growers, the practical effect of the lowering pH of ocean water has not only been to make the water more acidic, but 

also more corrosive to young shellfish raised in aquaculture facilities. Growers at Whiskey Creek Hatchery, in Oregon’s 

Netarts Bay, found that low pH seawater during spawning reduced growth in mid-stage larval (juvenile) Pacific oysters.38

Hatcheries in Washington State have also experienced losses of spat (oyster larvae that have attached to a surface and 

begun to develop a shell) due to water quality issues that include other human-caused effects like dredging and pollution.39 Facilities like the Taylor Shellfish Farms hatchery on Hood Canal have changed their production techniques to 

respond to increasing acidification in Puget Sound.

These impacts bring to light a potential challenge: existing natural variation may interact with human-caused changes 

to produce unanticipated results for shell-forming marine life, especially in coastal regions.40 As a result, there is an 

increasing need for information about water chemistry conditions, such as data obtained through the use of sensor networks. In the case of Whiskey Creek, instruments installed in collaboration with ocean scientists created an “early warning” system that allows oyster growers to choose the time they take water into the hatchery from the coastal ocean. This 

allows them to avoid the lower-pH water related to upwelling and the commensurate loss of productivity in the hatchery. 

From a biological perspective, these kinds of preventative measures can help produce higher-quality oysters. Studies 

on native Olympia oysters (Ostrea lurida) show that there is a “carry-over” effect of acidified water – oysters exposed 

to acidic conditions while in the juvenile stage continue to grow slower in later life stages.41 Research on some oyster 

species such as Pacific oyster (Crassostrea gigas), the commercially important species in U.S. west coast aquaculture, 

shows that specially selected strains can be more resistant to acidification.42

Overall, economically important species such as oysters, mussels, and sea urchins are highly vulnerable to changes 

in ocean conditions brought on by climate change and rising atmospheric CO2 levels. Sea temperature and acidification are expected to increase; the acidity of surface seawater is projected to nearly double by the end of this century. 

Some important cultured species may be influenced in larval and juvenile developing stages, during fertilization, and 

as adults,43 resulting in lower productivity. Action groups, such as the California Current Acidification Network (C-CAN), 

are working to address the needs of the shellfish industry – both wild and aquaculture-based fisheries – in the face of 

ocean change. These efforts bring scientists from across disciplines together with aquaculturists, fishermen, the oceanographic community, and state and federal decision-makers to ensure a concerted, standardized, and cost-effective 

approach to gaining new understanding of the impact of acidification on ecosystems and the economy.44
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24: OCEANS AND MARINE RESOURCES

of some key species such as corals may be limited by adequate 

bottom substrate, water clarity, and light availability.51

Climate change impacts such as increasing ocean temperatures can profoundly affect production of natural stocks of fish 

by changing growth, reproduction, survival, and other critical 

characteristics of fish stocks and ecosystems. For species that 

migrate to freshwater from the sea, like salmon, some published studies indicate earlier start of spawning migration, 

warming stream temperatures, and extirpation in southern extent of range, all of which can affect productivity.4,52 To remain 

within their normal temperature range, some fish stocks are 

moving poleward and to deeper water.53,54 Fishery productivity 

is predicted to decline in the lower 48 states, but increase in 

parts of Alaska.55 However, projections based only on temperature may neglect important food web effects. Fishing costs 

are predicted to increase as fisheries transition to new species 

and as processing plants and fishing jobs shift poleward.18 The 

cumulative impact of such changes will be highly variable on 

regional scales because of the combination of factors – some 

acting in opposite directions. Some areas will benefit from 

range expansions of valuable species or increases in productivity, while others will suffer as species move away from previously productive areas. 

Coral reef ecosystem collapse

Recent research indicates that 75% of the world’s coral reefs are threatened due to the interactive effects of climate 

change and local sources of stress, such as overfishing, nutrient pollution, and disease.

56,57 In Florida, all reefs are 

rated as threatened, with significant impacts on valuable ecosystem services they provide.58 Caribbean coral cover has 

decreased 80% in less than three decades.59 These declines have in turn led to a flattening of the three dimensional 

structure of coral reefs and hence a decrease in the capacity of coral reefs to provide shelter and other resources for 

other reef-dependent ocean life.

60

The relationship between coral and zooxanthellae (algae vital for reef-building corals) is disrupted by higher than usual 

temperatures and results in a condition where the coral is still alive, but devoid of all its color (bleaching). Bleached 

corals can later die or become infected with disease.61,62 Thus, high temperature events alone can kill large stretches 

of coral reef, although 

cold water and poor 

water quality can 

also cause localized 

bleaching and death. 

Evidence suggests that 

relatively pristine reefs,

with fewer human impacts and with intact 

fish and associated 

invertebrate communities, are more resilient 

to coral bleaching and 

disease.63

Figure 24.4. A colony of star coral (Montastraea faveolata) off the southwestern coast of Puerto Rico 

(estimated to be about 500 years old) exemplifies the effect of rising water temperatures. Increasing 

disease due to warming waters killed the central portion of the colony (yellow portion in A), followed 

by such high temperatures that bleaching - or loss of symbiotic algae from coral - occurred from the 

surrounding tissue (white area in B). The coral then experienced more disease in the bleached area 

on the periphery (C) that ultimately killed the colony (D). (Photo credit: Ernesto Weil). 

Warming Seas Are a Double-blow to Corals
564 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

24: OCEANS AND MARINE RESOURCES

Key Message 4: Rising Temperatures Linked to Diseases

Rising sea surface temperatures have been linked with increasing levels and 

ranges of diseases in humans and in marine life, including corals, abalones, 

oysters, fishes, and marine mammals. 

There has been a significant increase in reported incidences of 

disease in corals, urchins, mollusks, marine mammals, turtles, 

and echinoderms (a group of some 70,000 marine species including sea stars, sea urchins, and sand dollars) over the last 

several decades.64,65,66,67 Increasing disease outbreaks in the 

ocean affecting ecologically important species, which provide 

critically important habitat for other species such as corals,

65,68

algae,69 and eelgrass,

70 have been linked with rising temperatures. Disease increases mortality and can reduce abundance 

for affected populations as well as fundamentally change ecosystems by changing habitat or species relationships. For example, loss of eelgrass beds due to disease can reduce critical 

nursery habitat for several species of commercially important 

fish.70,71

The complexity of the host/environment/pathogen interaction 

makes it challenging to separate climate warming from the 

myriad of other causes facilitating increased disease outbreaks 

in the ocean. However, three categories of disease-causing 

pathogens are unequivocally related to warming oceans.

Firstly, warmer winters due to climate change can increase 

the overwinter survival and growth rates of pathogens.

67 A 

disease-causing parasite in oysters that proliferates at high 

water temperatures and high salinities spread northward up 

the eastern seaboard as water temperatures warmed during 

the 1990s.72 Growth rates of coral disease lesions increased 

with winter and summer warming from 1996 to 2006.

62 Winter 

warming in the Arctic is resulting in increased incidence of a 

salmon disease in the Bering Sea and is now thought to be a 

cause of a 57% decline of Yukon Chinook salmon.73

Secondly, increasing disease outbreaks in ecologically important species like coral, eelgrass, and abalone have been 

linked with temperatures that are higher than the long-term 

averages. The spectacular biodiversity of tropical coral reefs 

is particularly vulnerable to warming because the corals that 

form the foundational reef structure live very near the upper 

temperature limit at which they thrive. The increasing frequency of record hot temperatures has caused widespread coral 

bleaching66 and disease outbreaks65 and is a principal factor 

contributing to the International Union for the Conservation 

of Nature listing a third of the reef-building corals as vulnerable, endangered, or critically endangered 74 and the National 

Oceanic and Atmospheric Administration proposing to list 66 

species of corals under the Endangered Species Act.75,76 In the 

Chesapeake Bay, eelgrass died out almost completely during 

the record-hot summers of 2005 and 2010,77 and the California 

black abalone has been driven to the edge of extinction by a 

combination of warming water and bacterial disease.78

Thirdly, there is evidence that increased water temperature is 

responsible for the enhanced survival and growth of certain 

marine bacteria that make humans sick.78 Increases in growth 

of Vibrio parahaemolyticus (a pathogenic bacterial species) 

during the warm season are responsible for human illnesses 

associated with oysters harvested from the Gulf of Mexico79

and northern Europe.80 Vibrio vulnificus, which is responsible 

for the overwhelming majority of reported seafood-related 

deaths in the United States,81 is also a significant and growing 

source of potentially fatal wound infections associated with 

recreational swimming, fishing-related cuts, and seafood handling, and is most frequently found in water with a temperature above 68°F.

79,81,82

Key Message 5: Economic Impacts of Marine-related Climate Change 

Climate changes that result in conditions substantially different from recent 

history may significantly increase costs to businesses as well 

as disrupt public access and enjoyment of ocean areas.

Altered environmental conditions due to climate change will 

affect, in both positive and negative ways, human uses of the 

ocean, including transportation, resource use and extraction, 

leisure and tourism activities and industries, in the nearshore 

and offshore areas. Climate change will also affect maritime 

security and governance. Arctic-related national security concerns and threats to national sovereignty have also been a 

recent focus of attention for some researchers.83,84 With sea 

ice receding in the Arctic as a result of rising temperatures, 

global shipping patterns are already changing and will continue to change considerably in the decades to come.84,85 The 

increase in maritime traffic could make disputes over the legal 

status of sea lines-of-communication and international straits 

more pointed, but mechanisms exist to resolve these disputes 

peacefully through the Law of the Sea Convention and other 

customary international laws. 

Resource use for fisheries, aquaculture, energy production, 

and other activities in ocean areas will also need to adjust to 

changing ocean climate conditions. In addition to the shift in 
565 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

24: OCEANS AND MARINE RESOURCES

habitat of living resources discussed above, changing ocean 

and weather conditions due to human-induced climate change 

make any activities at sea more difficult to plan, design, and 

operate.

In the United States, the healthy natural services (such as fishing and recreation) and cultural resources provided by the 

ocean also play a large economic role in our tourism industry. 

Nationally in 2010, 2.8% of gross domestic product, 7.52 million jobs, and $1.11 trillion in travel and recreational total sales 

are supported by tourism.86 In 2009-2010, nine of the top ten 

states and U.S. territories and seven of the top ten cities visited 

by overseas travelers were coastal, including the Great Lakes. 

Changes in the location and distribution of marine resources 

(such as fish, healthy reefs, and marine mammals) due to climate change will affect the recreational industries and all the 

people that depend on reliable access to these resources in 

predictable locales. For example, as fish species shift poleward 

or to deeper waters,54,87 these fish may be less accessible to 

recreational fishermen. Similar issues will also affect commercial fishing.

Similarly, new weather conditions differing from the historical 

pattern will pose a challenge for tourism, boating, recreational 

fishing, diving, and snorkeling, all of which rely on highly predictable, comfortable water and air temperatures and calm waters. For example, the strength of hurricanes and the number of 

strong (Category 4 and 5) hurricanes are projected to increase 

over the North Atlantic (Ch. 2: Our Changing Climate). Changes 

in wind patterns88 and wave heights have been observed89 and 

are projected to continue to change in the future.90 This means 

that the public will not be able to rely on recent experience in 

planning leisure and tourism activities.91,92 As weather patterns 

change and air and sea surface temperatures rise, preferred 

locations for recreation and tourism also may change. In addition, infrastructure such as marinas, marine supply stores, 

boardwalks, hotels, and restaurants that support leisure activities and tourism will be negatively affected by sea level rise. 

They may also be affected by increased storm intensity and 

changing wave heights,92 as well as elevated storm surge due 

to sea level rise and other expected effects of a changing climate; these impacts will vary significantly by region.

93

Key Message 6: Initiatives Serve as a Model

In response to observed and projected climate impacts, some existing ocean policies, 

practices, and management efforts are incorporating climate change impacts. These 

initiatives can serve as models for other efforts and ultimately enable people 

and communities to adapt to changing ocean conditions.

Climate considerations can be integrated into planning, restoration, design of marine protected areas, fisheries management, and aquaculture practices to enhance ocean resilience 

and adaptive capacity. Many existing sustainable-use strategies, such as ending overfishing, establishing protected areas,

and conserving habitat, are known to increase resilience. Analyses of fishery management and climate scenarios suggest that 

adjustments to harvest regimes (especially reducing harvest 

rates of over-exploited species) can improve catch stability 

under changing climate conditions. These actions could have a 

greater effect on biological and economic performance in fisheries than impacts due to warming over the next 25 years.

94

The stability of international ocean and fisheries treaties, particularly those covering commercially exploited and critical 

species, might be threatened as the ocean changes.

95

The fact that the climate is changing is beginning to be incorporated into existing management strategies. New five-year 

strategies for addressing flooding, shoreline erosion, and 

coastal storms have been developed by most coastal states 

under their Coastal Zone Management Act programs.3 Many 

of these plans are explicitly taking into account future climate 

scenarios as part of their adaptation initiatives. The North Pacific Fishery Management Council and NOAA have declared a 

moratorium on most commercial fisheries in the U.S. Arctic 

pending sufficient understanding of the changing productivity of these fishing grounds as they become increasingly icefree. Private shellfish aquaculture operations are changing 

their business plans to adapt to ocean acidification.38,39 These 

changes include monitoring and altering the timing of spat 

settlement dependent on climate change induced conditions, 

as well as seeking alternative, acid-resistant strains for culturing. Marine protected areas in the National Marine Sanctuary 

(NMS) System are gradually preparing climate impact reports 

and climate adaptation action plans under their Climate Smart 

Sanctuary Initiative.96

Additionally, there is promise in restoring key habitats to provide a broad suite of benefits that can reduce climate impacts 

with relatively little ongoing maintenance costs (see Ch. 25: 

Coasts; Ch. 28: Adaptation). For example, if in addition to sea 

level rise, an oyster reef or mangrove restoration strategy also 

included fish habitat benefits for commercial and recreational 

uses and coastal protection services, the benefits to surrounding communities could multiply quickly. Coral-reef-based tourism can be more resilient to climate change impacts through 

protection and restoration, as well as reductions of pollution 

and other habitat-destroying activities. Developing alternative 

livelihood options as part of adaptation strategies for marine 

food-producing sectors can help reduce economic and social 

impacts of a changing climate.
566 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

24: OCEANS AND MARINE RESOURCES

Climate impacts on new england fisheries

Fishing in New England has been associated with bottom-dwelling fish for more than 

400 years, and is a central part of the region’s cultural identity and social fabric. Atlantic 

halibut, cod, haddock, flounders, hakes, pollock, plaice, and soles are included under 

the term “groundfish.” The fishery is pursued by both small boats (less than 50 feet long) 

that are typically at sea for less than a day, and by large boats (longer than 50 feet) that 

fish for a day to a week at a time. These vessels use home ports in more than 100 coastal communities from Maine to New Jersey, and the landed value from fisheries in New 

England and the 

Mid-Atlantic in 

2010 was nearly 

$1.2 billion.

76 Captains and crew are 

often second- or 

third-generation 

fishermen who have 

learned the trade 

from their families.

From 1982 to 

2006, sea surface temperature in the coastal waters of the Northeast warmed by close to twice the 

global rate of warming over this period.97 Long-term 

monitoring of bottom-dwelling fish communities in 

New England revealed that the abundance of warmwater species increased, while cool-water species decreased.54,98 A recent study suggests that many species in this community have shifted their geographic 

distributions northward by up to 200 miles since 

1968, though substantial variability among species 

also exists.54 The northward shifts of these species 

are reflected in the fishery as well: landings and landed value of these species have shifted towards northern states such as Massachusetts and Maine, while 

southern states have seen declines (see Figure 24.5). 

The economic and social impacts of these changes 

depend in large part on the response of the fishing 

communities in the region.99 Communities have a 

range of strategies for coping with the inherent uncertainty and variability of fishing, including diversification among species and livelihoods, but climate 

change imposes both increased variability and sustained change that may push these fishermen beyond 

their ability to cope.100 Larger fishing boats can follow 

the fish to a certain extent as they shift northward, 

while smaller inshore boats will be more likely to 

leave fishing or switch to new species.100 Long-term 

viability of fisheries in the region may ultimately depend on a transition to new species that have shifted 

from regions farther south.18

Figure 24.5. Ocean species are shifting northward along U.S. 

coastlines as ocean temperatures rise. As a result, over the 

past 40 years, more northern ports have gradually increased 

their landings of four marine species compared to the earlier 

pattern of landed value. While some species move northward 

out of an area, other species move in from the south. This kind of 

information can inform decisions about how to adapt to climate 

change. Such adaptations take time and have costs, as local 

knowledge and equipment are geared to the species that have 

long been present in an area. (Figure source: adapted from Pinsky 

and Fogerty 2012101). 

Fisheries Shifting North

iStockPhoto.com/©mayo5
567 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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85. Cressey, D., 2007: Arctic melt opens Northwest passage. Nature 

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Stewart, E. J., S. E. L. Howell, D. Draper, J. Yackel, and A. Tivy, 

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Mastrandrea, K. J. Mach, G.-K. Plattner, S.K. Allen, M. Tignor, 

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Ianelli, J. N., A. B. Hollowed, A. C. Haynie, F. J. Mueter, and N. 

A. Bond, 2011: Evaluating management strategies for eastern 

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in the Southern Ocean. Nature Geoscience, 5, 881-885, doi:10.1038/

ngeo1635.

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575 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

24: OCEANS AND MARINE RESOURCES

Traceable Accounts

Process for Developing Key Messages: 

A central component of the assessment process was the Oceans 

and Marine Resources Climate assessment workshop that was 

held January 23-24, 2012, at the National Oceanographic and 

Atmospheric Administration (NOAA) in Silver Spring, MD, and simultaneously, via web teleconference, at NOAA in Seattle, WA. In 

the workshop, nearly 30 participants took part in a series of scoping presentations and breakout sessions that began the process 

leading to a foundational Technical Input Report (TIR) entitled 

“Oceans and Marine Resources in a Changing Climate: Technical 

Input to the 2013 National Climate Assessment.”102 The report, 

consisting of nearly 220 pages of text organized into 7 sections 

with numerous subsections and more than 1200 references, was 

assembled by 122 authors representing governmental agencies, 

non-governmental organizations, tribes, and other entities. 

The chapter author team engaged in multiple technical discussions via teleconferences that permitted a careful review of the 

foundational TIR102 and of approximately 25 additional technical 

inputs provided by the public, as well as the other published literature, and professional judgment. The chapter author team met at 

Conservation International in Arlington, VA on 3-4 May 2012 for 

expert deliberation of draft key messages by the authors, wherein 

each message was defended before the entire author team before 

the key message was selected for inclusion in the report. These 

discussions were supported by targeted consultation with additional experts by the lead author of each message to help define 

“key vulnerabilities.”

Key message #1 Traceable Account

The rise in ocean temperature over the last century will persist into the future, with continued large 

impacts on climate, ocean circulation, chemistry, 

and ecosystems.

Description of evidence base

The key message is supported by extensive evidence documented 

in Sections 2 and 3 of the Oceans Technical Input Report102 and 

in the additional technical inputs received as part of the Federal 

Register Notice solicitation for public input, as well as stakeholder 

engagement leading up to drafting the chapter. 

Relevant and recent peer-reviewed publications,5,7,8 including 

many others that are cited therein, describe evidence that ocean 

temperature has risen over the past century. This evidence base 

includes direct and indirect temperature measurements, paleoclimate records, and modeling results.

There are also many relevant and recent peer-reviewed publications describing changes in physical and chemical ocean properties that are underway due to climate change.11,14

New information and remaining uncertainties

Important new information since the last National Climate Assessment103 includes the latest update to a data set of ocean temperatures.7

There is accumulating new information on all of these points with 

regard to physical and chemical changes in the ocean and resultant impacts on marine ecosystems. Both measurements and 

model results are continuing to sharpen the picture. 

A significant area of uncertainty remains with regard to the region-by-region impacts of warming, acidification, and associated 

changes in the oceans. Regional and local conditions mean that 

impacts will not be uniform around the U.S. coasts or internationally. Forecasting of regional changes is still an area of very active 

research, though the overall patterns for some features are now 

clear. 

Large-scale and recurring climate phenomena (such as the El Niño 

Southern Oscillation, the Pacific Decadal Oscillation, and the Atlantic Multidecadal Oscillation) cause dramatic changes in biological productivity and ecosystem structure and make it difficult to 

discern climate-driven trends.

Current time series of biological productivity are restricted to 

a handful of sites around the globe and to a few decades, and 

global, comprehensive satellite time series of ocean color are even 

shorter, beginning in 1997. Based on an analysis of different in 

situ datasets, one research group suggested a decline of 1% per 

year over the past century, but these findings may be an artifact 

SUPPLEMENTAL MATERIAL

TRACEABLE ACCOUNTS
576 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

of limited data and have been widely debated.14,104 However, the 

few in situ time series mostly indicate increases in biological productivity over the past 20 years, but with clear links to regional 

changes in climate.14

Assessment of confidence based on evidence

Confidence that the ocean is warming and acidifying, and that sea level is rising is very high. Changes in other physical and chemical properties such as ocean circulation, wave heights, oxygen minimums, and 

salinity are of medium confidence. For ecosystem changes, there is 

high confidence that these are occurring and will persist and likely 

grow in the future, though the details of these changes are highly 

geographically variable. 

Key message #2 Traceable Account

The ocean currently absorbs about a quarter of 

human-caused carbon dioxide emissions to the atmosphere, leading to ocean acidification that will 

alter marine ecosystems in dramatic yet uncertain 

ways.

Description of evidence base

The key message is supported by extensive evidence documented 

in the Oceans Technical Input Report102 and additional technical 

inputs received as part of the Federal Register Notice solicitation 

for public input, as well as stakeholder engagement leading up to 

drafting the chapter. 

Numerous references provide evidence for the increasing acidity 

(lower pH) of oceans around the world (Ch. 2: Our Changing Climate, Key Message 12).3,31

There is a rapid growth in peer-reviewed publications describing 

how ocean acidification will impact ecosystems,33,34 but to date 

evidence is largely based on studies of calcification rather than 

growth, reproduction, and survival of organisms. For these latter 

effects, available evidence is from laboratory studies in low pH 

conditions, rather than in situ observations.35

New information and remaining uncertainties

The interplay of environmental stressors may result in “surprises” 

where the synergistic impacts may be more deleterious or more 

beneficial than expected. Such synergistic effects create complexities in predicting the outcome of the interplay of stressors 

on marine ecosystems. Many, but not all, calcifying species are 

affected by increased acidity in laboratory studies. How those responses will cascade through ecosystems and food webs is still 

uncertain. Although studies are underway to expand understanding of ocean acidification on all aspects of organismal physiology, 

much remains to be learned. 

Assessment of confidence based on evidence

Confidence is very high that carbon dioxide emissions to the atmosphere are causing ocean acidification, and high that this will alter 

marine ecosystems. The nature of those alterations is unclear, 

however, and predictions of most specific ecosystem changes 

have low confidence at present, but with medium confidence for 

coral reefs.

Key message #3 Traceable Account

Significant habitat loss will continue to occur due 

to climate change for many species and areas, including Arctic and coral reef ecosystems, while 

habitat in other areas and for other species will 

expand. These changes will consequently alter the 

distribution, abundance, and productivity of many 

marine species.

Description of evidence base

The key message is supported by extensive evidence documented 

in the Oceans Technical Input Report102 and additional technical 

inputs received as part of the Federal Register Notice solicitation 

for public input, as well as stakeholder engagement leading up to 

drafting the chapter. 

Many peer-reviewed publications56,70 describe threats to coral 

reefs induced by global change.

There are also many relevant and recent peer-reviewed publications53,54,87 that discuss impacts on marine species and resources 

of habitat change that is induced by climate change. 

Confidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent 

results, well documented and 

accepted methods, etc.), high 

consensus

High

Moderate evidence (several sources, some consistency, methods 

vary and/or documentation limited, 

etc.), medium consensus

Medium

Suggestive evidence (a few 

sources, limited consistency, models incomplete, methods emerging, 

etc.), competing schools of thought

Low

Inconclusive evidence (limited 

sources, extrapolations, inconsistent findings, poor documentation 

and/or methods not tested, etc.), 

disagreement or lack of opinions 

among experts

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Traceable Accounts
577 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

New information and remaining uncertainties

Regional and local variation is, again, a major component of the 

remaining uncertainties. Different areas, habitats, and species are 

responding differently and have very different adaptive capacities. 

Those species that are motile will certainly respond differently, or 

at least at a different rate, by changing distribution and migration 

patterns, compared to species that do not move, such as corals. 

Although it is clear that some fish stocks are moving poleward and 

to deeper water, how far they will move and whether most species will move remains unclear. A key uncertainty is the extent to 

which various areas will benefit from range expansions of valuable 

species or increases in productivity, while other areas will suffer 

as species move away from previously productive areas. The loss 

of critically important habitat due to climate change will result in 

changes in species interactions that are difficult to predict. 

Assessment of confidence based on evidence

There is very high confidence that habitat and ecosystems are 

changing due to climate change, but that change is not unidirectional by any means. Distribution, abundance, and productivity 

changes are species and location dependent and may be increasing or decreasing in a complex pattern. 

Key message #4 Traceable Account

Rising sea surface temperatures have been linked 

with increasing levels and ranges of diseases in humans and in marine life, including corals, abalones, 

oysters, fishes, and marine mammals.

Description of evidence base

The key message is supported by extensive evidence in the Oceans 

Technical Input Report102 and additional technical inputs received 

as part of the Federal Register Notice solicitation for public input, as well as stakeholder engagement leading up to drafting the 

chapter. 

As noted in the chapter, the references document increased levels 

and ranges of disease coincident with rising temperatures.64,65,66,67

New information and remaining uncertainties

The interactions among host, environment, and pathogen are complex, which makes it challenging to separate warming due to climate change from other causes of disease outbreaks in the ocean.

Assessment of confidence based on evidence

There is high confidence that disease outbreaks and levels are 

increasing, and that this increase is linked to increasing temperatures. Again, there is substantial local to regional variation but the 

overall pattern seems consistent. 

Key message #5 Traceable Account

Climate changes that result in conditions substantially different from recent history may significantly increase costs to businesses as well as disrupt 

public access and enjoyment of ocean areas.

Description of evidence base

The key message is supported by extensive evidence documented 

in the Oceans Technical Input Report102 and additional technical 

inputs received as part of the Federal Register Notice solicitation 

for public input, as well as stakeholder engagement leading up to 

drafting the chapter. 

Many peer-reviewed publications describe the predicted impacts 

of climate change on tourism and recreation industries and their 

associated infrastructure.91,92

New information and remaining uncertainties

Given the complexity of transportation, resource use and extraction, and leisure and tourism activities, there are large uncertainties in impacts in specific locales or for individual activities. Some 

businesses and communities may be able to adapt rapidly, others 

less so. Infrastructure impacts of climate change will also be an 

important part of the ability of businesses, communities, and the 

public to adapt. 

Assessment of confidence based on evidence

As with many other impacts of climate change, the evidence that 

change is occurring is very strong but the resultant impacts are 

still uncertain. For all of these human uses, and the associated 

costs and disruption, the evidence is suggestive and confidence 

medium on the effects of the ongoing changes in ocean conditions. 

Key message #6 Traceable Account

In response to observed and projected climate 

impacts, some existing ocean policies, practices, 

and management efforts are incorporating climate 

change impacts. These initiatives can serve as 

models for other efforts and ultimately enable people and communities to adapt to changing ocean 

conditions.

Description of evidence base

The key message is supported by extensive evidence documented 

in the Oceans Technical Input Report102 and additional technical 

inputs reports received as part of the Federal Register Notice 

solicitation for public input, as well as stakeholder engagement 

leading up to drafting the chapter. 

Scenarios suggest that adjustments to fish harvest regimes can 

improve catch stability under increased climate variability. These 

actions could have a greater effect on biological and economic 

performance in fisheries than impacts due to warming over the 

next 25 years.

94

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Traceable Accounts
578 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

New information and remaining uncertainties

Efforts are underway to enhance the development and deployment 

of science in support of adaptation, to improve understanding and 

awareness of climate-related risks, and to enhance analytic capacity to translate understanding into planning and management 

activities. While critical knowledge gaps exist, there is a wealth of 

climate- and ocean-related science pertinent to adaptation.102

Assessment of confidence based on evidence 

There is high confidence that adaptation planning will help mitigate the impacts of changing ocean conditions. But there is much 

work to be done to craft local solutions to the set of emerging 

issues in ocean and coastal areas. 

24: OCEANS AND MARINE RESOURCES

Traceable Accounts
579

Climate Change Impacts in the United States

CHAPTER 25

COASTAL ZONE DEVELOPMENT 

AND ECOSYSTEMS

INFORMATION DRAWN FROM THIS CHAPTER IS INCLUDED IN THE HIGHLIGHTS REPORT AND IS IDENTIFIED BY THIS ICON

Recommended Citation for Chapter

Moser, S. C., M. A. Davidson, P. Kirshen, P. Mulvaney, J. F. Murley, J. E. Neumann, L. Petes, and D. Reed, 2014: Ch. 25: 

Coastal Zone Development and Ecosystems. Climate Change Impacts in the United States: The Third National Climate Assessment, J. M. Melillo, Terese (T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, , 579-618. 

doi:10.7930/J0MS3QNW. 

On the Web: http://nca2014.globalchange.gov/report/regions/coasts

Convening Lead Authors

Susanne C. Moser, Susanne Moser Research & Consulting, Stanford University

Margaret A. Davidson, National Oceanic and Atmospheric Administration

Lead Authors

Paul Kirshen, University of New Hampshire

Peter Mulvaney, Skidmore, Owings & Merrill LLP

James F. Murley, South Florida Regional Planning Council

James E. Neumann, Industrial Economics, Inc.

Laura Petes, National Oceanic and Atmospheric Administration

Denise Reed, The Water Institute of the Gulf
580 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

COASTAL ZONE 25 DEVELOPMENT AND ECOSYSTEMS

Key Messages

1. Coastal lifelines, such as water supply and energy infrastructure and evacuation routes, are 

increasingly vulnerable to higher sea levels and storm surges, inland flooding, erosion, and other 

climate-related changes.

2. Nationally important assets, such as ports, tourism and fishing sites, in already-vulnerable coastal 

locations, are increasingly exposed to sea level rise and related hazards. This threatens to disrupt 

economic activity within coastal areas and the regions they serve and results in significant costs 

from protecting or moving these assets.

3. Socioeconomic disparities create uneven exposures and sensitivities to growing coastal risks and 

limit adaptation options for some coastal communities, resulting in the displacement of the most 

vulnerable people from coastal areas.

4. Coastal ecosystems are particularly vulnerable to climate change because many have already 

been dramatically altered by human stresses; climate change will result in further reduction or 

loss of the services that these ecosystems provide, including potentially irreversible impacts.

5. Leaders and residents of coastal regions are increasingly aware of the high vulnerability of 

coasts to climate change and are developing plans to prepare for potential impacts on citizens, 

businesses, and environmental assets. Significant institutional, political, social, and economic 

obstacles to implementing adaptation actions remain.

Figure 25.1. U.S. population 

growth in coastal watershed counties has been most significant 

over the past 40 years in urban 

centers such as Puget Sound, 

San Francisco Bay, southern California, Houston, South Florida and 

the northeast metropolitan corridor. A coastal watershed county 

is defined as one where either 1) 

at a minimum, 15% of the county’s 

total land area is located within a 

coastal watershed, or 2) a portion 

of or an entire county accounts for 

at least 15% of a coastal USGS 

8-digit cataloging unit.1 Residents 

in these coastal areas can be considered “the U.S. population that 

most directly affects the coast.”1

We use this definition of “coastal” 

throughout the chapter unless 

otherwise specified. (Data from 

U.S. Census Bureau). 

Population Change in U.S. Coastal Watershed Counties

(1970-2010)
581 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

25: COASTAL ZONE DEVELOPMENT AND ECOSYSTEMS

Each year, more than 1.2 million people move to the coast, collectively adding the equivalent of nearly one San Diego, or more 

than three Miami’s, to the Great Lakes or open-ocean coastal 

watershed counties and parishes of the United States. As a result, 164 million Americans – more than 50% of the population 

– now live in these mostly densely populated areas1,2 (Figure 

25.1) and help generate 58% of the national gross domestic 

product (GDP).3 People come – and stay – for the diverse and 

growing employment opportunities in recreation and tourism, commerce, energy and mineral production, vibrant urban 

centers, and the irresistible beauty of our coasts.4 Residents, 

combined with the more than 180 million tourists that flock to 

the coasts each year,5,6 place heavy demands on the unique 

natural systems and resources that make coastal areas so 

attractive and productive.7

Meanwhile, public agencies and officials are charged with balancing the needs of economic vitality and public safety, while 

sustaining the built and natural environments in the face of 

risks from well-known natural hazards such as storms, flooding, 

and erosion.8 Although these risks play out in different ways 

along the United States’ more than 94,000 miles of coastline,9

all coasts share one simple fact: no other region concentrates 

so many people and so much economic activity on so little 

land, while also being so relentlessly affected by the sometimes 

violent interactions of land, sea, and air.

Humans have heavily altered the coastal environment through 

development, changes in land use, and overexploitation of 

resources. Now, the changing climate is imposing additional 

stresses,10 making life on the coast more challenging (Figure 

25.2). The consequences will ripple through the entire nation, 

which depends on the productivity and vitality of coastal regions.

Events like Superstorm Sandy in 2012 have illustrated that 

public safety and human well-being become jeopardized by 

the disruption of crucial lifelines, such as water, energy, and 

evacuation routes. As climate continues to change, repeated 

disruption of lives, infrastructure functions, and nationally and 

internationally important economic activities will pose intolerable burdens on people who are already most vulnerable 

and aggravate existing impacts on valuable and irreplaceable 

natural systems. Planning long-term for these changes, while 

balancing different and often competing demands, are vexing 

challenges for decision-makers (Ch. 26: Decision Support).

Coastal resilience defined

Resilience means different things to different disciplines 

and fields of practice. In this chapter, resilience generally refers to an ecological, human, or physical system’s 

ability to persist in the face of disturbance or change and 

continue to perform certain functions.11 Natural or physical systems do so through absorbing shocks, reorganizing 

after disturbance, and adapting;12 social systems can also 

consciously learn.13

Figure 25.2. Sea level rise is not just a problem of the future, but is already affecting coastal communities such 

as Charleston, South Carolina, and Olympia in South Puget Sound through flooding during high tides. (Photo credits: 

(left) NOAA Coastal Services Center; (right) Ray Garrido, January 6, 2010, reprinted with permission by the Washington 

Department of Ecology).

Flooding During High Tides
582 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

25: COASTAL ZONE DEVELOPMENT AND ECOSYSTEMS

Climate-related Drivers of Coastal Change

The primary climatic forces affecting the coasts are changes in 

temperature, sea and water levels, precipitation, storminess, 

ocean acidity, and ocean circulation.7

• Sea surface temperatures are rising14 and are expected to 

rise faster over the next few decades,15 with significant regional variation, and with the possibility for more intense 

hurricanes as oceans warm (Ch. 2: Our Changing Climate).

• Global average sea level is rising and has been doing so 

for more than 100 years (Ch.2: Our Changing Climate), 

and greater rates of sea level rise are expected in the 

future.16 Higher sea levels cause more coastal erosion, 

changes in sediment transport and tidal flows, more frequent flooding from higher storm surges, landward migration of barrier shorelines, fragmentation of islands, and 

saltwater intrusion into aquifers and estuaries.7,17,18,19

• Rates of sea level rise are not uniform along U.S. coasts20,21

and can be exacerbated locally by land subsidence or reduced by uplift.22,23 Along the shorelines of the Great 

Lakes, lake level changes are uncertain (Ch. 18: Midwest), 

but erosion and sediment migration will be exacerbated 

by increased lakeside storm events, tributary flooding, and increased wave action due to loss of ice cover.24

• Patterns of precipitation change are affecting coastal areas in complex ways (Ch. 2: Our Changing Climate). In regions where precipitation increases, coastal areas will see heavier runoff from inland areas, with the already observed trend 

toward more intense rainfall events continuing to increase 

the risk of extreme runoff and flooding. Where precipitation is expected to decline and droughts to increase, freshwater inflows to the coast will be reduced (Ch. 3: Water).

• There has been an overall increase in storm activity near 

the Northeast and Northwest coastlines since about 

1980.25 Winter storms have increased slightly in frequency 

and intensity and their storm tracks have shifted northward.26 The most intense tropical storms have increased 

in intensity in the last few decades.27 Future projections 

suggest increases in hurricane rainfall and intensity (with 

a greater number of the strongest – Category 4 and 5 – 

hurricanes), a slight decrease in the frequency of tropical 

cyclones, and possible shifts in storm tracks, though the 

details remain uncertain (Ch. 2: Our Changing Climate).

• Marine ecosystems are being threatened by climate change 

and ocean acidification. The oceans are absorbing more 

carbon dioxide as the concentration in the atmosphere 

increases, resulting in ocean acidification, which threatens coral reefs and shellfish.28,29,30 Coastal fisheries are 

also affected by rising water temperatures31 and climaterelated changes in oceanic circulation (Ch. 24: Oceans).32,33

Wetlands and other coastal habitats are threatened by sea 

level rise, especially in areas of limited sediment supply 

or where barriers prevent onshore migration.34 The combined effects of saltwater intrusion, reduced precipitation, 

and increased evapotranspiration will elevate soil salinities 

and lead to an increase in salt-tolerant vegetation35,36 and 

the dieback of coastal swamp forests.37

None of these changes operate in isolation. The combined effects of climate changes with other human-induced stresses 

makes predicting the effects of climate change on coastal 

systems challenging. However, it is certain that these factors 

will create increasing hazards to the coasts’ densely populated 

areas.38,39,40

Jacques Descloitres, MODIS Rapid Response Team, NASA/GSFC
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25: COASTAL ZONE DEVELOPMENT AND ECOSYSTEMS

Figure 25.3. The amount of sea level rise (SLR) by 2050 will vary along different stretches of the U.S. coastline and under 

different SLR scenarios, mostly due to land subsidence or uplift (Ch.2: Our Changing Climate).16 The panels show feet of sea level

above 1992 levels at different tide gauge stations based on a) an 8 inch SLR and b) a 1.24 foot SLR by 2050. The flood level 

that has a 1% chance of occurring in any given year (“return level”) is similarly projected to differ by region as a result of varying 

storm surge risk. Panel c) shows return levels for a 1.05 foot SLR above mean high tide by 2050. Finally, panel d) shows how a 

1.05 foot SLR by 2050 could cause the level of flooding that occurs during today’s 100-year storm to occur more frequently by 

mid-century, in some regions as often as once a decade or even annually. ( F i g u r e source: replicated Tebaldi et al. 201223

analysis with NCA sea level rise scenarios16 for panels a) and b); data/ensemble SLR projections used for panels c) and d) 

from Tebaldi et al. 201223; all estimates include the effect of land subsidence).

Projected Sea Level Rise and Flooding by 2050
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25: COASTAL ZONE DEVELOPMENT AND ECOSYSTEMS

Figure 25.4. (a) Social Vulnerability, (b) Probability of Shoreline Erosion

(a) Social Vulnerabilty Index (SoVI) at the Census tract level for counties along the coast. The Social Vulnerability 

Index provides a quantitative, integrative measure for comparing the degree of vulnerability of human populations 

across the nation. A high SoVI (dark pink) typically indicates some combination of high exposure and high sensitivity to 

the effects of climate change and low capacity to deal with them. Specific index components and weighting are unique 

to each region (North Atlantic, South Atlantic, Gulf, Pacific, Great Lakes, Alaska, and Hawai‘i). All index components 

are constructed from readily available Census data and include measures of poverty, age, family structure, location 

(rural versus urban), foreign-born status, wealth, gender, Native American status, and occupation.41,42

(b) Probability of Shoreline Erosion greater than 3.3 feet per year for counties along the coast. Probability is based on 

historical conditions only and does not reflect the possibility of acceleration due to increasing rates of sea level rise.43
585 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

25: COASTAL ZONE DEVELOPMENT AND ECOSYSTEMS

Figure 25.4. (c) Climate-Related Threats

(c) Regional Threats from Climate Change are compiled from technical input reports, the regional chapters in this report, 

and from scientific literature. For related information, see http://data.globalchange.gov/report/regional-differences-2012
586 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

25: COASTAL ZONE DEVELOPMENT AND ECOSYSTEMS

Figure 25.4. (d) Adaptation Activities

(d) Examples of Adaptation Activities in Coastal Areas of the U.S. and Affiliated Island States are compiled from 

technical input reports, the regional chapters in this report, and scientific literature. For related information, see 

http://data.globalchange.gov/report/coastal-adaptation-examples-2012
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Key Message 1: Coastal Lifelines at Risk

Coastal lifelines, such as water supply and energy infrastructure and evacuation routes, 

are increasingly vulnerable to higher sea levels and storm surges, inland flooding, 

erosion, and other climate-related changes.

Key coastal vulnerabilities arise from complex interactions among climate change and other physical, 

human, and ecological factors. These vulnerabilities 

have the potential to fundamentally alter life at the 

coast and disrupt coast-dependent economic activities.

Coastal infrastructure is exposed to climate 

change impacts from both the landward and ocean 

sides.44,45,46,47,48 Some unique characteristics increase 

the vulnerability of coastal infrastructure to climate 

change (Ch. 11: Urban).7,49 For instance, many coastal 

regions were settled long ago, making much of the 

infrastructure older than in other locations.50 Also, 

inflexibility of some coastal, water-dependent infrastructure, such as onshore gas and oil facilities, port 

facilities, thermal power plants, and some bridges, 

makes landward relocation difficult (Figure 25.5), 

and build-up of urban and industrial areas inland 

from the shoreline can inhibit landward relocation.7

Infrastructure is built to certain site-specific design 

standards (such as the once-in-10-year, 24-hour 

rainstorm or the once-in-100-year flood) that take 

account of historical variability in climate, coastal, 

and hydrologic conditions. Impacts exceeding these 

standards can shorten the expected lifetime, increase maintenance costs, and decrease services. 

In general, higher sea levels, especially when combined with 

inland changes from flooding and erosion, will result in accelerated infrastructure impairment, with associated indirect 

effects on regional economies and a need for infrastructure 

upgrades, redesign, or relocation.7,44,45,46,51

The more than 60,000 miles of coastal roads52 are essential for 

human activities in coastal areas (Ch. 5: Transportation), especially in case of evacuations during coastal emergencies.53,54

Population growth to date and expected additional growth 

place increasing demands on these roads, and climate change 

will decrease their functionality unless adaptation measures 

are taken.55,56 Already, many coastal roads are affected during 

storm events57 and extreme high tides.58 Moreover, as coastal 

bridges, tunnels, and roads are built or redesigned, engineers 

must account for inland and coastal changes, including drainage flooding, thawing permafrost, higher groundwater levels, 

erosion, and increasing saturation of roadway bases.59 During 

Hurricane Katrina, many bridges failed because they had only 

been designed for river flooding but were also unexpectedly 

exposed to storm surges.55,60

Wastewater management and drainage systems constitute 

critical infrastructure for coastal businesses and residents (Ch. 

3: Water). Wastewater treatment plants are typically located 

at low elevations to take advantage of gravity-fed sewage collection. Increased inland and coastal flooding make such plants 

more vulnerable to disruption, while increased inflows will reduce treatment efficiency.47,61,62 Drainage systems – designed 

using mid-1900s rainfall records – will become overwhelmed 

in the future with increased rainfall intensity over more impervious surfaces, such as asphalt and concrete.27,63,64,65 Sea level 

rise will increase pumping requirements for coastal wastewater treatment plants, reduce outlet capacities for drainage systems, and increasingly infiltrate sewer lines, while salt water 

intrusion into coastal aquifers will affect coastal water supplies 

and salt fronts will advance farther up into coastal rivers, affecting water supply intakes (Ch. 3: Water).19,66 Together, these 

impacts increase the risks of urban flooding, combined sewer 

overflows, deteriorating coastal water quality, and human health 

impacts (Ch. 11: Urban; Ch. 9: Human Health).67,68,69

Figure 25.5. This “mock-up” shows the existing Highway LA-1 and 

Leeville Bridge in coastal Louisiana (on the right) with a planned new, 

elevated bridge that would retain functionality under future, higher sea 

level conditions (center left). (Current sea level and sinking bridge are 

shown here.) A 7-mile portion of the planned bridge has been completed 

and opened to traffic in December 2011. (Figure source: Greater 

Lafourche Port Commission, reprinted with permission).

Adapting Coastal Infrastructure

to Sea Level Rise and Land Loss
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Coastal water infrastructure adaptation options include (but 

are not limited to): 

• integrating both natural landscape features and humanengineered, built infrastructure to reduce stormwater 

runoff and wave attack, including, where feasible, creative use of dredge material from nearby coastal locations in the build-up of wetlands and berms (Figure 25.6);

• constructing seawalls around wastewater treatment 

plants and pump stations;

• pumping effluent to higher elevations to keep up with 

sea level rise;

• pumping freshwater into coastal aquifers to reduce infiltration of saltwater; and 

• reusing water after treatment to replace diminished water supplies due to sea level rise.70

Technical and financial feasibility may limit how well and how 

long coastal infrastructure can be protected in place before 

it needs to be moved or abandoned. One group estimated 

that nationwide adaptation costs to utilities for wastewater 

systems alone could range between $123 billion and $252 billion by 2050 and, while not specific to coastal systems, gives 

a sense of the magnitude of necessary expenditures to avert 

climate change impacts.71 

The nation’s energy infrastructure, such as power plants, oil 

and gas refineries, storage tanks, transformers, and electricity transmission lines, are often located directly in the coastal 

floodplain.48,72 Roughly two-thirds of imported oil enters the 

U.S. through Gulf of Mexico ports,55 where it is refined and 

then transported inland. Unless adaptive measures are taken, 

storm-related flooding, erosion, and permanent inundation 

from sea level rise will disrupt these refineries (and related underground infrastructure) and, in turn, will constrain the supply 

of refined products to the rest of the nation (Ch. 4: Energy; Ch. 

10: Energy, Water, and Land) (Figure 25.5).73

Coastal communities have a variety of options to protect, replace, and redesign existing infrastructure, including flood 

proofing and flood protection through dikes, berms, pumps, 

integration of natural landscape features, elevation, more frequent upgrades, or relocation.74 Relocation of large coastal 

infrastructure away from the coastline can be very expensive 

and, for some facilities such as port installations, impossible 

due to the need for direct access to the shoreline. In most instances, the addition of new flood-proofed infrastructure in 

high-hazard zones has been viewed as a more cost-effective 

near-term option than relocation.75 In these cases, significantly 

higher removal costs may be incurred later when sea level is 

higher or if the facility needs to be abandoned altogether in 

the future. This suggests that adaptation options are best assessed in a site-specific context, comprehensively weighing 

social, economic, and ecological considerations over multiple 

timeframes. A combination of gray and green infrastructure 

is increasingly recognized as a potentially cost-effective approach67,76 to reducing risks to communities and economies 

while preserving or restoring essential ecosystems and thus 

their benefits to human welfare (Figure 25.6).7,77

Figure 25.6. A coastal ecosystem restoration project in New York 

City integrates revegetation (a form of green infrastructure) with 

bulkheads and riprap (gray or built infrastructure). Investments 

in coastal ecosystem conservation and restoration can protect 

coastal waterfronts and infrastructure, while providing additional 

benefits, such as habitat for commercial and recreational fish, 

birds, and other animal and plant species, that are not offered by 

built infrastructure. (Photo credit: Department of City Planning, 

New York City, reprinted with permission).

Ecosystem Restoration

Assessing flood exposure of critical facilities and roads

NOAA’s Critical Facilities Flood Exposure Tool provides an initial assessment of the risk to a community’s critical facilities and roads within the “100-year” flood zone established by the Federal Emergency Management Agency (FEMA) 

(the 100-year flood zone is the areal extent of a flood that has a 1% chance of occurring or being exceeded in any given 

year). The tool helps coastal managers quickly learn which facilities may be at risk – providing information that can be 

used to increase flood risk awareness and to inform a more detailed analysis and ultimately flood risk reduction measures. The critical facilities tool was initially created to assist Mississippi/Alabama Sea Grant in conducting its “Coastal 

Resiliency Index: A Community Self-Assessment” workshops and is now available for communities nationwide. For 

additional information see: http://www.csc.noaa.gov/digitalcoast/tools/criticalfacilities.
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Key Message 2: Economic Disruption

Nationally important assets, such as ports, tourism, and fishing sites, in already-vulnerable 

coastal locations, are increasingly exposed to sea level rise and related hazards. This 

threatens to disrupt economic activity within coastal areas and the regions they serve and 

results in significant costs from protecting or moving these assets.

In 2010, economic activity in shoreline counties accounted 

for approximately 66 million jobs and $3.4 trillion in wages78

through diverse industries and commerce. In many instances, 

economic activity is fundamentally dependent on the physical and ecological characteristics of the coast. These features 

provide the template for coastal economic activities, including 

natural protection from waves, access to beaches, flat land for 

port development and container storage, and wetlands that 

support fisheries and provide flood protection.

More than 5,790 square miles and more than $1 trillion of 

property and structures are at risk of inundation from sea level 

rise of two feet above current sea level – an elevation which 

could be reached by 2050 under a high rate of sea level rise 

of approximately 6.6 feet by 2100,16 20 years later assuming a 

lower rate of rise (4 feet by 2100) (Ch. 2: Our Changing Climate), 

and sooner in areas of rapid land subsidence.79,80 Roughly half 

of the vulnerable property value is located in Florida, and the 

most vulnerable port cities are Miami, Greater New York, New 

Orleans, Tampa-St. Petersburg, and Virginia Beach.38,45,79,81

Although comprehensive national estimates are not yet available, regional studies are indicative of the potential risk: the 

incremental annual damage of climate change to capital assets 

in the Gulf region alone could be $2.7 to $4.6 billion by 2030, 

and $8.3 to $13.2 billion by 2050; about 20% of these at-risk 

assets are in the oil and gas industry.82 Investing approximately 

$50 billion for adaptation over the next 20 years could lead to 

approximately $135 billion in averted losses over the lifetime of 

adaptive measures.82,83

More than $1.9 trillion in imports came through U.S. ports in

2010, with commercial ports directly supporting more than 13 

million jobs78 and providing 90% of consumer goods.84 Ports 

damaged during major coastal storms can be temporarily or 

permanently replaced by other modes of freight movement, 

but at greater cost (Ch. 5: Transportation). The stakes are high 

and resources exist for ports to take proactive adaptation 

steps, such as elevating and interconnecting port- and land-based 

infrastructure or developing offsite storage capability (off-dock intermodal yards) for goods and related emergency response procedures.85 However, a recent survey showed that most U.S. ports 

have not yet taken actions to adapt their operations to rising 

seas, increased flooding, and the potential for more extreme 

coastal storms.86

Coastal recreation and tourism comprises the largest and 

fastest-growing sector of the U.S. service industry, accounting 

for 85% of the $700 billion annual tourism-related revenues,5,88

making this sector particularly vulnerable to increased impacts 

from climate change.89 Historically, development of immediate 

shoreline areas with hotels, vacation rentals, and other tourism-related establishments has frequently occurred without 

adequate regard for coastal hazards, shoreline dynamics (for 

example, inlet migration), or ecosystem health.90 Hard shoreline protection against the encroaching sea (like building sea 

walls or riprap) generally aggravates erosion and beach loss 

and causes negative effects on coastal ecosystems, undermining the attractiveness of beach tourism. Thus, “soft protection,” 

such as beach replenishment or conservation and restoration 

of sand dunes and wetlands, is increasingly preferred to “hard 

protection” measures. Increased sea level rise means sand replenishment would need to be undertaken more frequently, 

and thus at growing expense.34,91,92,93

Natural shoreline protection features have some capacity to 

adapt to sea level rise and storms (Figure 25.6) and can also 

provide an array of ecosystem services benefits94 that may 

offset some maintenance costs. A challenge ahead is the need 

to integrate climate considerations (for example, temperature 

change and sea level rise) into coastal ecosystem restoration 

and conservation efforts,95 such as those underway in the Gulf 

of Mexico, Chesapeake Bay, and Sacramento-San Joaquin Delta, to ensure that these projects have long-term effectiveness.

U.S. oceanic and Great Lakes coasts are important centers for 

commercial and recreational fishing due to the high productivity of coastal ecosystems. In 2009, the U.S. seafood industry 

supported approximately 1 million full- and part-time jobs and 

©Boston Globe via Getty Images
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25: COASTAL ZONE DEVELOPMENT AND ECOSYSTEMS

Figure 25.7. Ports are deeply interconnected with inland areas through the goods imported and exported each year. Climate 

change impacts on ports can thus have far-reaching implications for the nation’s economy. These maps show the exports and 

imports in 2010 (in tons/year) and freight flows (in trucks per day) from four major U.S. ports to other U.S. areas designated in the 

U.S. Department of Transportation’s Freight Analysis Framework (FAF): Los Angeles, Houston, New York/New Jersey, and Seattle. 

Note: Highway Link Flow less than 5 FAF Trucks/Day are not shown. (Figure source: U.S. Department of Transportation, Federal 

Highway Administration, Office of Freight Management and Operations, Freight Analysis Framework, version 3.4, 2012).87

Coast-to-Inland Economic Connections

generated $116 billion in sales and $32 billion in income.96 Recreational fishing also contributes to the economic engine of the 

coasts, with some 74 million saltwater fishing trips along U.S. 

coasts in 2009 generating $50 billion in sales and supporting 

over 327,000 jobs.96 Climate change threatens to disrupt fishing 

operations through direct and indirect impacts to fish stocks 

(for example, temperature-related shifts in species ranges, 

changes in prey availability, and loss of coastal nursery habitat) 

as well as storm-related disruptions of harbor installations (Ch. 

24: Oceans).
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Key Message 3: Uneven Social Vulnerability

Socioeconomic disparities create uneven exposures and sensitivities to growing coastal 

risks and limit adaptation options for some coastal communities, resulting in the 

displacement of the most vulnerable people from coastal areas.

In 2010, almost 2.8% of the U.S. population, or more than 8.6 million Americans, lived within the area subject to coastal floods 

that have at least a 1% chance of occurring in any one year.97,98

More than 120 million Americans live in counties that border 

the open ocean or Great Lakes coasts and/or have a 100-year 

coastal floodplain within them.98 Two trends will place even 

more people at risk in the future: 1) the expansion of the floodplain as sea level rises, and 2) the continuing immigration of 

people to coastal areas.

By 2100, the fraction of the U.S. population living in coastal 

counties is expected to increase by 50% (46.2 million) to 144% 

(131.2 million) depending on alternative projections of future 

housing.99 While specific population projections for future 100-

year flood zones are only available for some locations,100 many 

of these new arrivals can be expected to locate in high-hazard 

areas. Thus, coastal population densities, along with increasing 

economic development, will continue to be an important factor 

in the overall exposure to climate change.3,7,39,101

Despite persistent beliefs that living on the coast is reserved 

for the wealthy,79,102 there are large social disparities in coastal 

areas that vary regionally.41,103 Full understanding of risk for 

coastal communities requires consideration of social vulnerability factors limiting people’s ability to adapt. These factors include lower income; minority status; low educational 

achievement; advanced age; income 

dependencies; employment in lowpaying service, retail, and other sectors, 

as well as being often place-bound; 

less economically and socially mobile; 

and much less likely to be insured than 

wealthy property owners (see panel (a) 

in Figure 25.4).104

For example, in California, an estimated 

260,000 people are currently exposed 

to a 100-year flood; this number could 

increase to 480,000 by 2100 as a result 

of a 4.6 foot sea level rise alone (roughly 

equivalent to the high end of the 1 to 4 

foot range of sea level rise projections, 

Ch.2: Our Changing Climate).38 Approximately 18% of those exposed to high 

flood risk by the end of this century also 

are those who currently fall into the 

“high social vulnerability” category.81

This means that while many coastal 

property owners at the shorefront tend 

to be less socially vulnerable, adjacent populations just inland 

are often highly vulnerable.

The range of adaptation options for highly socially vulnerable 

populations is limited.81 Native communities in Alaska, Louisiana, and other coastal locations already face this challenge 

today (see “Unique Challenges for Coastal Tribes” and Ch. 12: 

Indigenous Peoples).105,106 As sea level rises faster and coastal 

storms, erosion, and inundation cause more frequent or widespread threats, relocation (also called (un)managed retreat 

or realignment), while not a new strategy in dynamic coastal 

environments, may become a more pressing option. In some 

instances relocation may become unavoidable, and for poorer 

populations sooner than for the wealthy. Up to 50% of the 

areas with high social vulnerability face the prospect of unplanned displacement under the 1 to 4 foot range of projected 

sea level rise (Ch.2: Our Changing Climate), for several key 

reasons: they cannot afford expensive protection measures 

themselves, public expense is not financially justified (often 

because social, cultural, and ecological factors are not considered), or there is little social and political support for a more 

orderly retreat process. By contrast, only 5% to 10% of the low 

social vulnerability areas are expected to face relocation.41 This 

suggests that climate change could displace many socially vulnerable individuals and lead to significant social disruptions in 

some coastal areas.107,108,109

Unique challenges for coastal tribes

Coastal Native American and Native Alaskan people, with their traditional dependencies upon natural resources and specific land areas, exhibit unique 

vulnerabilities. Tribal adaptation options can be limited because tribal land

boundaries are typically bordered by non-reservation lands, and climate

change could force tribes to abandon traditionally important locations, certain

cultural practices, and natural resources on which they depend (Ch. 12: Indigenous Peoples).110 Coastal food sources are also threatened, including salmon 

and shellfish. Climate change could affect other food species as well, worsening 

already existing health problems such as obesity, diabetes, and cancer.

Tribes pride themselves, however, for their experience and persistence in 

adapting to challenging situations. Some tribes are exploring unique adaptation approaches. In Louisiana’s Isle de Jean Charles, for example, the BiloxiChitimacha-Choctaw Indian community partnered with a local academic center 

and a religious congregation to work toward relocating scattered tribal members 

with those seeking a communal safe haven, while working to save their ancestral 

land – aiming for community and cultural restoration and for the redevelopment 

of traditional livelihoods.108,111
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Key Message 4: Vulnerable Ecosystems

Coastal ecosystems are particularly vulnerable to climate change because many have already 

been dramatically altered by human stresses; climate change will result in further reduction or 

loss of the services that these ecosystems provide, including potentially irreversible impacts.

Coastal ecosystems provide a suite of valuable benefits (ecosystem services) on which humans depend, including reducing the impacts from floods, buffering from storm surge and 

waves, and providing nursery habitat for important fish and 

other species, water filtration, carbon storage, and opportunities for recreation and enjoyment (Figure 25.8).95,112,113

However, many of these ecosystems and the services they 

provide are rapidly being degraded by human impacts, including pollution, habitat destruction, and the spread of invasive 

species. For example, 75% of U.S. coral reefs in the Atlantic, 

Caribbean, and Gulf of Mexico are already in “poor” or “fair” 

condition;114,115 all Florida reefs are currently rated as “threatened.”116 Coastal barrier ecosystems continue to be degraded 

by human development, even in cases where development has 

slowed (for example, Crawford et al. 2013; Feagin et al. 2010b 

117). Coastal wetlands are being lost at high rates in southeastern Louisiana (Figure 25.9).118 In addition, the incidence of lowoxygen “dead zones” in coastal waters has increased 30-fold in 

the U.S. since 1960, with over 300 coastal water bodies now 

experiencing stressful or lethal oxygen levels (Ch. 8: Ecosystems).119

These existing stresses on coastal ecosystems will be exacerbated by climate change effects, such as increased ocean 

temperatures that lead to coral bleaching,30 altered river flows 

affecting the health of estuaries,121 and acidified waters threatening shellfish.122 Climate change affects the survival, reproduction, and health of coastal plants and animals in different 

ways. For example, changes in the timing of seasonal events 

(such as breeding and migration), shifts in species distributions 

and ranges, changes in species interactions, and declines in 

biodiversity all combine to produce fundamental changes in 

ecosystem character, distribution, and functioning.28 Species 

with narrow physiological tolerance to change, low genetic 

diversity, specialized resource requirements, and poor competitive abilities are particularly vulnerable.123,124 Where the 

rate of climate change exceeds the pace at which plants and 

animals can acclimate or adapt, impacts on coastal ecosystems 

will be profound.35,125,126 For example, high death rates of East 

Coast intertidal mussels at their southern range boundary have 

occurred because of rising temperatures between 1956 and 

2007.127 The presence of physical barriers (for example, hardened shorelines or reduced sediment availability) and other 

non-climatic stressors (such as pollution, habitat destruction, 

and invasive species) will further exacerbate the ecological impacts of climate change and limit the ability of these ecosystems to adapt.128,129,130 Onshore migration of coastal marshes 

as sea level rises is often limited by bulkheads or roads (a phenomenon often called “coastal squeeze”), ultimately resulting 

in a reduction in wetland area.35,126,128,131,132,133

Of particular concern is the potential for coastal ecosystems 

to cross thresholds of rapid change (“tipping points”), beyond 

which they exist in a dramatically altered state or are lost entirely from the area; in some cases, these changes will be irreversible.134 These unique, “no-analog” environments present 

serious challenges to resource managers, who are confronted 

with conditions never seen before.135,136,137 The ecosystems 

most susceptible to crossing such tipping points are those that 

have already lost some of their resilience due to degradation 

or depletion by non-climatic stressors.138 Certain coastal ecosystems are already rapidly changing as a result of interactions 

between climatic and non-climatic factors, and others have 

already crossed tipping points. Eelgrass in the Chesapeake Bay 

died out almost completely during the record-hot summer of 

2005, when temperatures exceeded the species’ tolerance 

threshold of 86°F,139 and subsequent recovery has been poor.140

Severe low-oxygen events have emerged as a new phenomenon in the Pacific Northwest due to changes in the timing and 

duration of coastal upwelling.32,141 These have led to high mortality of Dungeness crabs33 and the temporary disappearance 

of rockfish,32 with consequences for local fisheries. Reducing 

non-climatic stressors at the local scale can potentially prevent 

crossing some of these tipping points.142
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Figure 25.8. Coastal ecosystems provide a suite of valuable benefits (ecosystem services) on which humans depend 

for food, economic activities, inspiration, and enjoyment. This schematic illustrates many of these services situated 

in a Pacific or Caribbean island setting, but many of them can also be found along mainland coastlines.

Coastal Ecosystem Services
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Key Message 5: The State of Coastal Adaptation

Leaders and residents of coastal regions are increasingly aware of the high vulnerability 

of coasts to climate change and are developing plans to prepare for potential impacts on 

citizens, businesses, and environmental assets. Significant institutional, political, social, and 

economic obstacles to implementing adaptation actions remain.

Considerable progress has been made since the last National 

Climate Assessment in both coastal adaptation science and 

practice (Figure 25.4, panel (d)), though significant gaps in understanding, planning, and implementation remain.20,143,144,145

U.S. coastal managers pay increasing attention to adaptation, 

but are mostly still at an early stage of building their capacities 

for adaptation rather than implementing structural or policy 

changes (Ch. 28: Adaptation).20,146,147 Although many non-structural (land-use planning, fiscal, legal, and educational) and 

structural adaptation tools are available through the Coastal 

Zone Management Act, Coastal Barriers Resources Act, and 

other frameworks, and while coastal managers are well familiar 

with these historical approaches to shoreline protection, they 

are less familiar with some of the more innovative approaches 

to coastal adaptation, such as rolling easements, ecosystembased adaptation, or managed realignment.109,131,144,148 Federal, 

state, and local management approaches have also been 

found to be at odds at times,149 making successful integration 

of adaptation more difficult.145 There is only limited evidence 

of more substantial (“transformational”) adaptation occurring, 

that is, of adaptations that are “adopted at a much larger scale, 

that are truly new to a particular region or resource system, 

and that transform places and shift locations,”150 such as relocation of communities in coastal Alaska and Louisiana (Ch. 

22: Alaska).83,109,150,151 Although more research is needed, reasons for the limited transformational adaptation to date may 

include the relatively early stage of recognizing climate change 

and sea level rise risks, the perception that impacts are not yet 

severe enough, and the fact that social objectives can still be 

met.152

Coastal leaders and populations, however, are increasingly concerned about climate-related impacts and support the development of adaptation plans,153,154,155 but support for development 

restrictions or managed retreat is limited.156,157,158 Economic 

interests and population trends tend to favor continued (re)development and in-fill in near-shore locations. Current disaster 

recovery practices frequently promote rapid rebuilding on-site 

with limited consideration for future conditions159 despite clear 

evidence that more appropriate siting and construction can 

substantially reduce future losses.160,161

Enacting measures that increase resilience in the face of current hazards, while reducing long-term risks due to climate 

change, continues to be challenging.162,163,164 This is particularly difficult in coastal flood zones that are subject to a 1% 

Figure 25.9. These maps show expected future land change in coastal Louisiana under two different sea level rise scenarios 

without protection or restoration actions. Red indicates a transition from land (either wetlands or barrier islands) to open water. 

Green indicates new land built over previously open water. Land loss is influenced by factors other than sea level rise, including 

subsidence, river discharge and sediment load, and precipitation patterns. However, all these factors except sea level rise 

were held constant for this analysis. The panel on the left shows land change with a sea level rise of 10.6 inches between 

2010 and 2060, while the one on the right assumes 31.5 inches of sea level rise for the same period. These amounts of 

sea level rise are within the projected ranges for this time period (Ch. 2: Our Changing Climate). (Figure source: State of 

Louisiana, reprinted with permission120).

Projected Land Loss from Sea Level Rise in Coastal Louisiana
595 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

25: COASTAL ZONE DEVELOPMENT AND ECOSYSTEMS

or greater chance of flooding in any given year, including those 

areas that experience additional hazards from wave action. According to FEMA and policy/property data maintained by the 

National Flood Insurance Program’s (NFIP) Bureau and Statistical Agent, nearly half of the NFIP’s repetitive flood losses occur 

in those areas.165,166 A robust finding is that the cost of inaction 

is 4 to 10 times greater than the cost associated with preventive 

hazard mitigation.79,160 Even so, prioritizing expenditures now 

whose benefits accrue far in the future is difficult.167 Moreover, 

cumulative costs to the economy of responding to sea level 

rise and flooding events alone could be as high as $325 billion 

by 2100 for 4 feet of sea level rise, with $130 billion expected 

to be incurred in Florida and $88 billion in the North Atlantic 

region.80 The projected costs associated with one foot of sea 

level rise by 2100 are roughly $200 billion. These figures only 

cover costs of beach nourishment, hard protective structures, 

and losses of inundated land and property where protection 

is not warranted, but exclude losses of valuable ecosystem 

services, as well as indirect losses from business disruption, 

lost economic activity, impacts on economic growth, or other 

non-market losses.80,168,169 Such indirect losses, even in regions 

generally well prepared for disaster events, can be substantial 

(in the case of Superstorm Sandy, followed by a nor’easter, in 

fall 2012, insured losses and wider economic damages added 

up to at least $65 billion).170 Sequences of extreme events that 

occur over a short period not only reduce the time available 

for natural and social systems to recover and for adaptation 

measures to be implemented, but also increase the cumulative 

effect of back-to-back extremes compared to the same events 

occurring over a longer period.164,171 The cost of managed retreat requires further assessment.

Property insurance can serve as an important mode of financial adaptation to climate risks,172 but the full potential of leveraging insurance rates and availability has not yet been realized.

7,173,174 The Government Accountability Office (GAO) listed 

the National Flood Insurance Program as a “high-risk area” for 

the first time in 2006, indicating its significance in terms of 

federal fiscal exposure (nearly $1.3 trillion in 2012).175 In the 

context of identifying climate change as a high risk to federal operations, the GAO in 2013 singled out the NFIP again, recognizing 

growing risks and liabilities due to climate change and sea level 

rise and the increase in erosion and flooding they entail.176 While 

insured assets in coastal areas represent only a portion of this 

total liability, taxpayers are responsible, via the NFIP, for more 

than $510 billion of insured assets in the coastal Special Flood 

Hazard Area (SFHA) alone.53,177 A number of reforms in the NFIP 

have been enacted in 2012 to ensure that the program is more 

fiscally sound and hazard mitigation is improved, though various challenges remain.178

Climate adaptation efforts that integrate hazard mitigation, 

natural resource conservation, and restoration of coastal 

ecosystems can enhance ecological resilience and reduce the 

exposure of property, infrastructure, and economic activities 

to climate change impacts (Figure 25.6).113,179 Yet, the integration and translation of scientific understanding of the benefits 

provided by ecosystems into engineering design and hazard 

management remains challenging.180 Moreover, interdependencies among functioning infrastructure types and coastal 

uses require an integrated approach across scientific disciplines 

and levels of government, but disconnected scientific efforts 

and fragmented governance at the managerial, financial, and 

regulatory levels, and narrow professional training, job descriptions, and agency missions pose significant barriers (Ch. 11: Urban; Ch. 28: Adaptation).145,181,182 Adaptation efforts to date that 

have begun to connect across jurisdictional and departmental 

boundaries and create innovative solutions are thus extremely 

encouraging.7,145,183,184
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Photo Credits

Introduction to chapter; Florida coast line in top banner: ©Joe Raedle 

via Getty Images News
613 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

25: COASTAL ZONE DEVELOPMENT AND ECOSYSTEMS

Process for Developing Key Messages 

A central component of the assessment process was a Chapter 

Lead Authors meeting held in St. Louis, Missouri in April 2012. 

The key messages were initially developed at this meeting. Key 

vulnerabilities were operationally defined as those challenges 

that can fundamentally undermine the functioning of human and 

natural coastal systems. They arise when these systems are highly 

exposed and sensitive to climate change and (given present or 

potential future adaptive capacities) insufficiently prepared or able 

to respond. The vulnerabilities that the team decided to focus on 

were informed by ongoing interactions of the author team with 

coastal managers, planners, and stakeholders, as well as a review 

of the existing literature. In addition, the author team conducted 

a thorough review of the technical input reports (TIR) and associated literature, including the coastal zone foundational TIR 

prepared for the National Climate Assessment (NCA).7 Chapter 

development was supported by numerous chapter author technical 

discussions via teleconference from April to June 2012.

Key message #1 Traceable Account

Coastal lifelines, such as water supply and energy infrastructure and evacuation routes, are increasingly vulnerable to higher sea levels and storm 

surges, inland flooding, erosion, and other climaterelated changes.

Description of evidence base

Coastal infrastructure is defined here to include buildings, roads, 

railroads, airports, port facilities, subways, tunnels, bridges, water 

supply systems, wells, sewer lines, pump stations, wastewater 

treatment plants, water storage and drainage systems, port

facilities, energy production and transmission facilities on land and 

offshore, flood protection systems such as levees and seawalls, 

and telecommunication equipment. Lifelines are understood in 

the common usage of that term in hazards management.

The key message and supporting text summarize extensive 

evidence documented in the coastal zone technical input report7 

as well as a technical input report on infrastructure.48 Technical 

input reports (68) on a wide range of topics were also received 

and reviewed as part of the Federal Register Notice solicitation for 

public input, along with the extant scientific literature. Additional 

evidence is provided in other chapters on hurricanes (Ch. 2: Our 

Changing Climate, Key Message 8), global sea level rise (Ch. 2: Our 

Changing Climate, Key Message 10), water supply vulnerabilities 

(Ch. 3: Water); key coastal transportation vulnerabilities (Ch. 5: 

Transportation), and energy-related infrastructure (Ch. 4: Energy). 

This key message focuses mainly on water supply and energy 

infrastructure and evacuation routes, as these constitute critical 

lifelines.

The evidence base for exposure, sensitivity, and adaptive capacity 

to higher sea levels and storm surges is very strong, both from 

empirical observation and historical experience and from studies 

projecting future impacts on critical coastal infrastructure. There 

are numerous publications concerning the effects of sea level rise 

and storm surges on roadways, coastal bridges, and supply of refined 

products.7,38,40,64,93,147,162 The information on roadways came from 

various reports (for example, DOT 2012; Transportation Research 

Board 2011; NPCC 2009, 201055,56,184) and other publications (for 

example, State of Louisiana 201283). The impact on coastal bridges 

is documented in U.S. Department of Transportation reports.55,59 A 

number of publications explored the impacts on supply of refined 

oil-based products such as gasoline.73

The evidence base is moderate for the interaction of inland 

and coastal flooding. There are many and recent publications 

concerning impacts to wastewater treatment plants47,61 and 

drainage systems.18,27,64,65,70 These impacts lead to increased risk 

of urban flooding and disruption of essential services to urban 

residents.

New information and remaining uncertainties

The projected rate of sea level rise (SLR) is fully accounted for 

through the use of common scenarios. We note, however, that 

there is currently limited impacts literature yet that uses the 

lowest or highest 2100 scenario and none that specifically use 

the broader range of SLR (0.2 to 2 meters, or 0.7 to 6.6 feet, by 

2100) 16 and NCA land-use scenarios (60% to 164% increase in 

urban and suburban land area).185

The severity and frequency of storm damage in any given location 

cannot yet be fully accounted for due to uncertainties in projecting 

future extratropical and tropical storm frequency, intensity, and 

SUPPLEMENTAL MATERIAL

TRACEABLE ACCOUNTS
25: COASTAL ZONE DEVELOPMENT AND ECOSYSTEMS

Traceable Accounts

614 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

changes in storm tracks for different regions (Ch. 2: Our Changing 

Climate).

7

The timely implementation and efficacy of adaptation measures,

including planned retreat, in mitigating damages is accounted for 

in the underlying literature (for example, by varying assumptions 

about the timing of implementation of adaptation measures and the 

type of adaptation measures) such as hard protection, elevation, 

relocation, or protection through wetlands and dunes in front of 

the infrastructure in question) (for example, Aerts and Botzen 

2012; Biging et al. 2012; Bloetscher et al. 2011; Heberger et al. 

2009; Irish et al. 2010; Kirshen et al. 201118,38,44,45,47). However,

such studies can only test the sensitivity of conclusions to these 

assumptions; they do not allow statements about what is occurring 

on the ground.

Additional uncertainties arise from the confluence of climate 

change impacts from the inland and ocean side, which have yet to 

be studied in an integrated fashion across different coastal regions 

of the United States.

Assessment of confidence based on evidence

Given the evidence base, the large quantity of infrastructure 

(water-related infrastructure, energy infrastructure, and the 

60,000 miles of coastal roads) in the U.S. coastal zone, and the 

directional trend at least of sea level rise and runoff associated 

with heavy precipitation events, we have very high confidence that 

these types of infrastructure in the coastal zone are increasingly 

vulnerable.

Key message #2 Traceable Account

Nationally important assets, such as ports, tourism and fishing sites, in already-vulnerable coastal 

locations, are increasingly exposed to sea level rise 

and related hazards. This threatens to disrupt economic activity within coastal areas and the regions 

they serve and results in significant costs from protecting or moving these assets.

Description of evidence base

The key message and supporting text summarize extensive evidence 

documented in the coastal zone technical input report.7 Technical 

input reports (68) on a wide range of topics were also received 

and reviewed as part of the Federal Register Notice solicitation for 

public input, as well as the extant scientific literature.

The evidence base for increased exposure to assets is strong. 

Many publications have assessed at-risk areas (for example, 

Biging et al. 2012; Cooley et al. 2012; Heberger et al. 2009; 

Neumann et al. 2010a38,45,79,81). Highly reliable economic activity 

information is available from recurring surveys conducted by the 

National Oceanographic and Atmospheric Administration (NOAA) 

and others, and asset exposure is conclusively demonstrated by 

historical information (from storm and erosion damage), elevation 

data (in Geographic Information System (GIS)-based, LIDAR, and 

other forms), and numerous vulnerability and adaptation studies 

of the built environment. Further evidence is provided in technical 

input reports and other NCA chapters on infrastructure and urban 

systems (Ch. 11: Urban),48 transportation (Ch. 5: Transportation),55

and energy (Ch. 4: Energy). A number of studies in addition to 

the ones cited in the text, using various economic assumptions, 

aim to assess the cost of protecting or relocating coastal assets 

and services. Many publications and reports explore the cost of 

replacing services offered by ports,55,91 though one study186 notes 

that few ports are implementing adaptation practices to date. 

The economic consequences of climate change on tourism are 

supported by a number of recent studies.89,90,91,93 The threats of 

climate change on fishing have been explored in the coastal zone 

technical input report.7

Additional evidence comes from empirical observation: public 

statements by private sector representatives and public 

officials indicate high awareness of economic asset exposure 

and a determination to see those assets protected against an 

encroaching sea, even at high cost (New York City, Miami Dade 

County, San Francisco airport, etc.). The economic value of 

exposed assets and activities is frequently invoked when they 

get damaged or interrupted during storm events (for example, 

Hallegattee 2012169). Threats to economic activity are also 

consistently cited as important to local decision-making in the 

coastal context (for example, Titus et al. 2009109).

Confidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent 

results, well documented and 

accepted methods, etc.), high 

consensus

High

Moderate evidence (several sources, some consistency, methods 

vary and/or documentation limited, 

etc.), medium consensus

Medium

Suggestive evidence (a few 

sources, limited consistency, models incomplete, methods emerging, 

etc.), competing schools of thought

Low

Inconclusive evidence (limited 

sources, extrapolations, inconsistent findings, poor documentation 

and/or methods not tested, etc.), 

disagreement or lack of opinions 

among experts
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615 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

New information and remaining uncertainties

The projected rate of sea level rise is fully accounted for through 

the use of common scenarios. We note, however, that there 

is currently limited impacts literature that uses the lowest or 

highest scenario for 2100, and no studies that specifically use 

the broader range of SLR (0.7 to 6.6 feet,) and NCA land-use 

scenarios (60% to 164% increase in urban and suburban land 

area).185

The projected severity and frequency of storm damage in any given 

location cannot yet be fully accounted for due to uncertainties 

in projecting future extratropical and tropical storm frequency, 

intensity, and changes in storm tracks for different regions.7

The timely implementation and efficacy of adaptation measures, 

including planned retreat, in mitigating damages are accounted for 

in the underlying literature (for example, by varying assumptions 

about the timing of implementation of adaptation measures, the 

type of adaptation measures, and other economic assumptions 

such as discount rates). However, such studies can only test the 

sensitivity of conclusions to these assumptions; they do not allow 

statements about what is occurring on the ground. Well-established 

post-hoc assessments160 suggest that hazard mitigation action is 

highly cost-effective (for every dollar spent, four dollars in damages 

are avoided). A more recent study suggests an even greater costeffectiveness.79

Assessment of confidence based on evidence

Given the evidence base, the well-established accumulation of 

economic assets and activities in coastal areas, and the directional 

trend of sea level rise, we have very high confidence in the main 

conclusion that resources and assets that are nationally important 

to economic productivity are threatened by SLR and climate 

change.

While there is currently no indication that the highest-value assets 

and economic activities are being abandoned in the face of sea 

level rise and storm impacts, we have very high confidence that 

the cost of protecting these assets in place will be high, and that 

the cost will be higher the faster sea level rises relative to land.

We have very high confidence that adequate planning 

and arrangement for future financing mechanisms, timely 

implementation of hazard mitigation measures, and effective 

disaster response will keep the economic impacts and adaptation 

costs lower than if these actions are not taken.

We are not able to assess timing or total cost of protecting or 

relocating economic assets with any confidence at this time, due 

to uncertainties in asset-specific elevation above sea level, in the 

presence and efficacy of protective measures (at present and in the 

future), in the feasibility of relocation in any particular case, and 

uncertainties in future storm surge heights and storm frequencies.

Key message #3 Traceable Account

Socioeconomic disparities create uneven exposures and sensitivities to growing coastal risks and 

limit adaptation options for some coastal communities, resulting in the displacement of the most vulnerable people from coastal areas.

Description of evidence base

The key message and supporting text summarize extensive 

evidence documented in the coastal zone technical input report.7

Technical input reports (68) on a wide range of topics were also 

received and reviewed as part of the Federal Register Notice 

solicitation for public input, along with the extant literature.

Evidence base is moderate: assessment of the social vulnerability to 

coastal impacts of climate change is a comparatively new research 

focus in the United States, and clearly an advance since the prior 

NCA.187 There are currently multiple published, peer-reviewed 

studies, by different author teams, using different vulnerability 

metrics, which all reach the same conclusion: economically and 

socially vulnerable individuals and communities face significant 

coastal risks and have a lower adaptive capacity than less socially 

vulnerable populations. Studies have shown that the U.S. coastal 

population is growing 99 and have assessed the importance of this 

population for climate change exposure.39,101 The social factors that 

play key roles in coastal vulnerability are detailed in numerous 

publications.81,104,188

There is an additional body of evidence emerging in the literature 

that also supports this key message, namely the growing 

literature on “barriers to adaptation,” particularly from studies 

conducted here in the United States.7,81,105,145,189 This literature 

reports on the limitations poorer communities face at present in 

beginning adaptation planning, and on the challenges virtually 

all communities face in prioritizing adaptation and moving from 

planning to implementation of adaptation options.

There is empirical evidence for how difficult it is for small, less 

wealthy communities (for example, the Native communities 

in Alaska or southern Louisiana) to obtain federal funds to 

relocate from eroding shorelines.107,108 Eligibility criteria (positive 

benefit-cost ratios) make it particularly difficult for low-income 

communities to obtain such funds; current federal budget 

constraints limit the available resources to support managed retreat 

and relocation.166,173 The recent economic hardship has placed 

constraints even on the richer coastal communities in the U.S. in 

developing and implementing adaptation strategies, for example 

in California.145 While the economic situation, funding priorities, or 

institutional mechanisms to provide support to socially vulnerable 

communities will not remain static over time, there is no reliable 

scientific evidence for how these factors may change in the future.
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616 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

New information and remaining uncertainties

The body of research on this topic is largely new since the prior 

NCA in 2009.187 Each of the peer-reviewed studies discusses 

data gaps and methodological limitations, as well as the particular 

challenge of projecting demographic variables – a notoriously 

difficult undertaking – forward in time. While methods for 

population projections are well established (typically using housing 

projections), those, in turn, depend on more difficult to make 

assumptions about fertility, migration, household size, and travel 

times to urban areas. The conclusion is limited by uneven coverage 

of in-depth vulnerability studies; although those that do exist are 

consistent with and confirm the conclusions of a national study.41

This latter study was extended by applying the same approach, 

data sources, and methodology to regions previously not covered, 

thus closing important informational gaps (Hawai‘i, Alaska, the 

Great Lakes region). Data gaps remain for most coastal locations 

in the Pacific Islands, Puerto Rico, and other U.S. territories.

The most important limit on understanding is the current inability 

to project social vulnerability forward in time. While some social 

variables are more easily predicted (for example, age and gender 

distribution) than others (for example, income distribution, ethnic 

composition, and linguistic abilities), the predictive capability 

declines the further out projections aim (beyond 2030 or 2050). 

Further, it is particularly difficult to project these variables in 

specific places subject to coastal hazards, as populations are 

mobile over time, and no existing model reliably predicts placebased demographics at the scale important to these analyses.

Assessment of confidence based on evidence

We have high confidence in this conclusion, as it is based on 

well-accepted techniques, replicated in several place-based case 

studies, and on a nationwide analysis, using reliable Census data. 

Consistency in insights and conclusions in these studies, and in 

others across regions, sectors, and nations, add to the confidence. 

The conclusion does involve significant projection uncertainties, 

however, concerning where socially vulnerable populations will 

be located several decades from now. Sensitivity analysis of this 

factor, and overall a wider research base is needed, before a higher 

confidence assessment can be assigned.

Key message #4 Traceable Account

Coastal ecosystems are particularly vulnerable to 

climate change because many have already been 

dramatically altered by human stresses; climate 

change will result in further reduction or loss of the 

services that these ecosystems provide, including 

potentially irreversible impacts.

Description of evidence base

The key message and supporting text summarize extensive 

evidence documented in the coastal zone technical input report.7

Technical input reports (68) on a wide range of topics were also 

received and reviewed as part of the Federal Register Notice 

solicitation for public input, along with the extant literature.

Evidence base is strong for this part of the key message: “Coastal 

ecosystems are particularly vulnerable to climate change because 

many have already been dramatically altered by human stresses.”

The degradation and depletion of coastal systems due to 

human stresses (for example, pollution, habitat destruction, and 

overharvesting) has been widely documented throughout the U.S. 

and the world.68,115,116,118,119 The degree of degradation varies 

based on location and level of human impact. However, evidence 

of degradation is available for all types of U.S. coastal ecosystems, 

from coral reefs to seagrasses and rocky shores. Human stresses 

can be direct (for example, habitat destruction due to dredging 

of bays) or indirect (for example, food web disruption due to 

overfishing). There is also consistent evidence that ecosystems 

degraded by human activities are less resilient to changes in 

climatic factors, such as water temperature, precipitation, and 

sea level rise (for example, Gedan et al. 2009; Glick et al. 2011; 

Williams and Grosholz 2008128,129,130).

Evidence base is strong: “climate change will result in further 

reduction or loss of the services that these ecosystems provide.”

The impacts of changing coastal conditions (for example, changes 

associated with altered river inflows, higher temperatures, and the 

effects of high rates of relative sea level rise) on coastal ecosystems 

and their associated services have been extensively documented 

through observational and empirical studies, including recent 

publications.28,121,122,123,129,133 Many models of coastal ecosystem 

responses to climatic factors have been well-validated with field 

data. Given the existing knowledge of ecosystem responses, future 

climate projections, and the interactions with non-climatic stressors 

that further exacerbate climatic impacts, evidence is strong of the 

potential for further reduction and/or loss of ecosystem services.

Evidence is suggestive: “including potentially irreversible impacts.”

Severe impacts (for example, mass coral bleaching events and 

rapid species invasions) have been extensively documented for 

U.S. coastal ecosystems. Many experts have suggested that some 

of these impacts may be irreversible134 and never before seen 

conditions have been documented.136,137 Recovery may or may not 

be possible in different instances; this depends on factors that are 

not well-understood, such as the adaptive capacity of ecosystems, 

future projections of change that consider interactions among 

multiple climatic and non-climatic human alterations of systems, 

the dynamics and persistence of alternative states that are 

created after a regime shift has occurred, and whether or not the 

climatic and/or non-climatic stressors that lead to impacts will be 

ameliorated.32,33,138,139,140,141

New information and remaining uncertainties

Since the 2009 NCA,187 new studies have added weight to 

previously established conclusions. The major advance lies in the 

examination of tipping points for species and entire ecosystems 
25: COASTAL ZONE DEVELOPMENT AND ECOSYSTEMS

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617 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

(for example, Barnosky et al. 2012; Folke et al. 2004; Foti et 

al. 2013; Hoegh-Guldberg and Bruno 2010134,135,137,138). Existing 

uncertainties and future research needs were identified through 

reviewing the NCA technical inputs and other peer-reviewed, 

published literature on these topics, as well as through our own 

identification and assessment of knowledge gaps.

Key uncertainties in our understanding of ecosystem impacts of 

climate change in coastal areas are associated with:

•	 the interactive effects and relative contributions 

of multiple climatic and non-climatic stressors on 

coastal organisms and ecosystems;

•	 how the consequences of multiple stressors for 

individual species combine to affect community- and 

ecosystem-level interactions and functions;

•	 the projected magnitude of coastal ecosystem change 

under different scenarios of temperature change, sea 

level rise, and land-use change, particularly given 

the potential for feedbacks and non-linearities in 

ecosystem responses;

•	 the potential adaptive capacity of coastal organisms 

and ecosystems to climate change;

•	 trajectories, timeframes, and magnitudes of coastal 

ecosystem recovery;

•	 the dynamics and persistence of alternative states 

that are created after ecosystem regime shifts have 

occurred; and

•	 the potential and likelihood for irreversible climaterelated coastal ecosystem change.

In general, relatively little work to date has been conducted 

to project future coastal ecosystem change under integrative 

scenarios of temperature change, sea level rise, and changes in 

human uses of, and impacts to, coastal ecosystems (for example, 

through land-use change). Advancing understanding and 

knowledge associated with this key uncertainty, as well as the 

others included in the above list, would be fostered by additional 

research.

Assessment of confidence based on evidence

We have very high confidence that coastal ecosystems are 

particularly vulnerable to climate change because they have already 

been dramatically altered by human stresses, as documented in 

extensive and conclusive evidence.

We have very high confidence that climate change will result in 

further reduction or loss of the services that these ecosystems 

provide, as there is extensive and conclusive evidence related to 

this vulnerability.

We have high confidence that climatic change will include 

“potentially irreversible impacts.” Site-specific evidence of 

potentially irreversible impacts exists in the literature. This 

vulnerability is frequently identified by studies of coastal 

ecosystems. However, methods, research, and models are still 

being developed for understanding, documenting, and predicting 

potentially irreversible impacts across all types of coastal 

ecosystems.

Key message #5 Traceable Account

Leaders and residents of coastal regions are increasingly aware of the high vulnerability of coasts 

to climate change, and are developing plans to prepare for potential impacts on citizens, businesses, 

and environmental assets. Significant institutional, 

political, social, and economic obstacles to implementing adaptation actions remain.

Description of evidence base

The key message and supporting text summarize extensive 

evidence documented in the coastal zone technical input report.7

Technical input reports (68) on a wide range of topics were also 

received and reviewed as part of the Federal Register Notice 

solicitation for public input, along with the extant literature.

Evidence base is moderate to strong: the results on which this 

key message relies are based on case studies, direct observation 

and “lessons learned” assessments from a wide range of efforts, 

surveys, and interview studies in ongoing adaptation efforts around 

the country.154 There has been some planning for remediating 

climate change impacts, including recent publications144,153,163,164

and there are publications on the lower social acceptance 

of certain adaptation option (for example, Finzi Hart et al. 

2012; Peach 2012144,158) and on the many barriers that affect 

adaptation.145,181,182

In addition, there is confirming evidence of very similar findings 

from other locations outside the U.S. (some, from Canada, were 

also submitted as technical input reports to the NCA), such as the 

United Kingdom, continental Europe, Australia, and others.157,181

New information and remaining uncertainties

Adaptation is a rapidly spreading policy and planning focus across 

coastal America. This was not previously captured or assessed in 

the 2009 NCA187 and is thus a major advance in understanding, 

including what adaptation activities are underway, what impedes 

them, and how coastal stakeholders view and respond to these 

emerging adaptation activities.

Given the local nature of adaptation (even though it frequently 

involves actors from all levels of government), it is difficult to 

systematically track, catalog, or assess progress being made on 

adaptation in coastal America. The difficulty, if not impossibility, 

of comprehensively tracking such progress has been previously 

acknowledged.20 This conclusion is reiterated in the Adaptation 

chapter (Ch. 28) of this report.
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618 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

While the findings and integrative key message stand on strong 

evidence, some uncertainties remain about U.S. coastal regions’ 

adaptive capacity, the level of adoption of hazard mitigation and 

other adaptation strategies, and the extent and importance of 

barriers to adaptation.

Possibly the least well-understood aspect about coastal adaptation 

is how and when to undertake large-scale, transformational 

adaptation. Aside from the mentioned examples of relocation, no 

other examples exist at the present time, and further research is 

required to better understand how major institutional, structural, 

or social transformation might occur and what would be involved to 

realize such options.

Assessment of confidence based on evidence

We have very high confidence in this key message, as it is primarily 

based on studies using well-accepted social science research 

techniques (for example, surveys, interviews, and participant 

observation), replicated in several place-based case studies, 

and on a nationwide compilation of adaptation case studies. 

Consistency in insights and conclusions in these studies, and in 

others across regions, sectors, and nations, add to the confidence.

As described above, a comprehensive catalogue of all adaptation 

efforts, and of related challenges and lessons learned, is difficult 

if not impossible to ever obtain. Nevertheless, the emerging 

insights and evidence from different regions of the country 

provide considerable confidence that the situation is reasonably 

well captured in the documents relied on here. The coastal 

stakeholders represented among the authors of the foundational 

technical input report7 confirmed the conclusions from their longterm experience in coastal management and direct involvement in 

adaptation efforts locally.

Moreover, evidence from other regions outside the U.S. adds 

weight to the conclusions drawn here.
People make choices every day about risks and benefits in their lives, weighing 

experience, information, and judgment as they consider the impacts of their 

decisions on themselves and the people around them. Similarly, people make 

choices that alter the magnitude of impacts resulting from current and future 

climate change. Using science-based information to anticipate future changes can 

help society make better decisions about how to reduce risks and protect people, 

places, and ecosystems from climate change impacts. Decisions made now and in 

the future will influence society’s resilience to impacts of future climate change. 

In recognition of the significance of these decisions, the National Climate 

Assessment presents information that is useful for a wide variety of decisions 

across regions and sectors, at multiple scales, and over multiple time frames. For 

the first time, the National Climate Assessment includes chapters on Decision 

Support, Mitigation, and Adaptation, in addition to identifying research needs 

associated with these topics. 

As with other sections of this report, the linkages across and among these chapters 

are extremely important. There are direct connections between mitigation decisions 

(about whether and how to manage emissions of heat-trapping gases) and how 

much climate will change in the future. The amount of change that occurs will in 

turn dictate the amount of adaptation that will be required. 

In the Decision Support chapter, a variety of approaches to bridge the gap 

between scientific understanding and decision-making are discussed, leading to 

the conclusion that there are many opportunities to help scientists understand the 

needs of decision-makers, and also to help decision-makers use available tools 

and information to reduce the risks of climate change. The Mitigation chapter 

describes emissions trajectories and assesses the state of mitigation activities. 

Policies already enacted and other factors lowered U.S. emissions in recent years, 

but achievement of a global emissions path consistent with the lower scenario (B1) 

analyzed in this assessment will require strenuous action by all major emitters. The 

Adaptation chapter assesses current adaptation activities across the United States 

in the public and private sectors, and concludes that although a lot of adaptation 

planning is being done, implementation lags significantly behind the scale of 

anticipated changes. 

This report concludes with chapters on Research Needs to improve future climate 

and global change assessments and on the Sustained Assessment Process, which 

describes the rationale for ongoing assessment activity to achieve greater efficiency 

and better scientific and societal outcomes.

RESPONSE STRATEGIES

©Don Mason/Blend Images/Corbis

©Najlah Feanny/Corbis©Denise Applewhite
620

Climate Change Impacts in the United States

CHAPTER 26

DECISION SUPPORT

CONNECTING SCIENCE, RISK PERCEPTION, AND DECISIONS

INFORMATION DRAWN FROM THIS CHAPTER IS INCLUDED IN THE HIGHLIGHTS REPORT AND IS IDENTIFIED BY THIS ICON

Recommended Citation for Chapter

Moss, R., P. L. Scarlett, M. A. Kenney, H. Kunreuther, R. Lempert, J. Manning, B. K. Williams, J. W. Boyd, E. T. Cloyd, L. 

Kaatz, and L. Patton, 2014: Ch. 26: Decision Support: Connecting Science, Risk Perception, and Decisions. Climate Change 

Impacts in the United States: The Third National Climate Assessment, J. M. Melillo, Terese (T.C.) Richmond, and G. W. Yohe, 

Eds., U.S. Global Change Research Program, 620-647. doi:10.7930/J0H12ZXG. 

On the Web: http://nca2014.globalchange.gov/report/response-strategies/decision-support

Convening Lead Authors

Richard Moss, Joint Global Change Research Institute, Pacific Northwest National Laboratory, University of Maryland

P. Lynn Scarlett, The Nature Conservancy

Lead Authors

Melissa A. Kenney, University of Maryland

Howard Kunreuther, University of Pennsylvania

Robert Lempert, RAND Corporation

Jay Manning, Cascadia Law Group

B. Ken Williams, The Wildlife Society

Contributing Authors

James W. Boyd, Resources for the Future

Emily T. Cloyd, University Corporation for Atmospheric Research

Laurna Kaatz, Denver Water

Lindene Patton, Zurich North America
621 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

DECISION SUPPORT: 26 CONNECTING SCIENCE, RISK PERCEPTION, AND DECISIONS

Key Messages

1. Decisions about how to address climate change can be complex, and responses will require 

a combination of adaptation and mitigation actions. Decision-makers – whether individuals, 

public officials, or others – may need help integrating scientific information into adaptation and 

mitigation decisions.

2. To be effective, decision support processes need to take account of the values and goals of the 

key stakeholders, evolving scientific information, and the perceptions of risk.

3. Many decision support processes and tools are available. They can enable decision-makers to 

identify and assess response options, apply complex and uncertain information, clarify tradeoffs, 

strengthen transparency, and generate information on the costs and benefits of different choices.

4. Ongoing assessment processes should incorporate evaluation of decision support tools, their 

accessibility to decision-makers, and their application in decision processes in different sectors 

and regions.

5. Steps to improve collaborative decision processes include developing new decision support tools 

and building human capacity to bridge science and decision-making.

After a long period of relative stability in the climate system, climate conditions are changing and are projected to continue to 

change (Ch. 2: Our Changing Climate). As a result, historically successful strategies for managing climate-sensitive resources and 

infrastructure will become less effective over time. Although 

decision-makers routinely make complex decisions under uncertain conditions, decision-making in the context of climate 

change can be especially challenging due to a number of factors. 

These include the rapid pace of changes in some physical and 

human systems, long time lags between human activities and 

response of the climate system, the high economic and political 

stakes, the number and diversity of potentially affected stakeholders, the need to incorporate uncertain scientific information of varying confidence levels, and the values of stakeholders 

and decision-makers.1,2,3 The social, economic, psychological, 

and political dimensions of these decisions underscore the need 

for ways to improve communication of scientific information 

and uncertainties and to help decision-makers assess risks and 

opportunities.

Extensive literature and practical experience offer means to help 

improve decision-making in the context of climate variability and 

change. The decision-support literature includes topics such as 

decision-making frameworks, decision support tools, and decision support processes. These approaches can help evaluate the 

costs and benefits of alternative actions, communicate relative 

amounts of risk associated with different options, and consider 

the role of alternative institutions and governance structures. 

In particular, iterative decision processes that incorporate improving scientific information and learning though periodic reviews of decisions over time are helpful in the context of rapid 

changes in environmental conditions.3,4 Some of the approaches 

described in this chapter can also help overcome barriers to the 

use of existing tools and improve communications among scientists, decision-makers, and the public.5,6

Focus of this chapter

This chapter introduces decision-making frameworks 

that are useful for considering choices about climate 

change responses through the complementary strategies 

of adaptation and mitigation. It also includes numerous 

examples in which decision support tools are being 

employed in making adaptation and mitigation decisions. It 

focuses on the processes that promote sustained interaction 

between decision-makers and the scientific/technical 

community. This chapter reviews the state of knowledge 

and practice in the context of managing risk. Extensive 

literature makes clear that in many cases, decisions aided 

by the types of approaches described here prove more 

successful than unaided decisions.3,7 Because of space 

limitations, the chapter describes some general classes of 

tools but does not assess specific decision support tools. 
622 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

26: DECISION SUPPORT: CONNECTING SCIENCE, RISK PERCEPTION, AND DECISIONS

What are the decisions and who are the decision-makers?

Decisions about climate change adaptation and mitigation are 

being made in many settings (Table 26.1). For example:

• The Federal Government is engaged in decisions that 

affect climate policy at the national and international 

level; makes regulatory decisions (for example, setting 

efficiency standards for vehicles); and makes decisions 

about infrastructure and technologies that may reduce 

risks associated with climate change for its own facilities 

and activities.

• State, tribal, and local governments are involved in 

setting policy about both emissions and adaptation activities in a variety of applications, including land use, 

renewable portfolio and energy efficiency standards, 

and investments in infrastructure and technologies that 

increase resilience to extreme weather events.

• Private-sector companies have initiated strategies to 

respond both to the risks to their investments and the 

business opportunities associated with preparing for a 

changing climate.

• Non-governmental organizations have been active in 

supporting decisions that integrate both adaptation 

and mitigation considerations, often in the context of 

promoting sustainability within economic sectors, communities, and ecosystems.

Individuals make decisions on a daily basis that affect their contributions to greenhouse gas emissions, their preparedness for 

extreme events, and the health and welfare of their families.

8

Many decisions involve decision-makers and stakeholders at 

multiple scales and in various sectors. Effective decision support must link and facilitate interactions across different decision networks.

9

What is decision support?

Decision support refers to “organized efforts to produce, disseminate, and facilitate the use of data and information” to 

improve decision-making.3 It includes processes, decision support tools, and services. Some examples include methods for 

assessing tradeoffs among options, scenarios of the future 

used for exploring the impacts of alternative decisions, vulnerability and impacts assessments, maps of projected climate 

impacts, and tools that help users locate, organize, and display 

data in new ways. Outcomes of effective decision support processes include building relationships and trust that can support 

longer-term problem-solving capacity between knowledge 

producers and users; providing information that users regard 

as credible, useful, and actionable; and enhancing the quality 

of decisions.

3

 Decision support activities that facilitate wellstructured decision processes can result in consensus about 

defining the problems to be addressed, objectives and options 

for consideration, criteria for evaluation, potential opportunities and consequences, and tradeoffs (Figure 26.1). 

Table 26.1. Examples of Decisions at Different Scales

Individuals

↓

↓

Organizations

↓

↓

Communities

↓

↓

↓

National Governments

↓

↓

↓

International Institutions

A farmer decides whether to adopt no-till agricultural practices.

A private firm decides whether to invest in solar or wind energy.

A city develops a plan to increase resiliency to coastal floods in light of projections for sea 

level rise.

A government agency plans incentives for renewable energy to meet greenhouse gas reduction goals.

A national government develops its positions for international climate negotiations, including 

what commitments the government should make with respect to reducing greenhouse gas 

emissions.

A United Nations agency designs a long-term strategy to manage increased flows of refugees 

who are migrating in part due to desertification related to climate change.
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26: DECISION SUPPORT: CONNECTING SCIENCE, RISK PERCEPTION, AND DECISIONS

Boundary Processes: Collaboration among Decision-Makers, Scientists, and Stakeholders

Incorporating the implications of climate 

change in decision-making requires consideration of scientific insights as well as 

cultural and social considerations, such as 

the values of those affected and cultural 

and organizational characteristics. Chapter 28 (Adaptation) addresses how some 

of these factors might be addressed in the 

context of adaptation. The importance of 

both scientific information and societal 

considerations suggests the need for the 

public, technical experts, and decisionmakers to engage in mutual shared learning and shared production of relevant 

knowledge.3,10 A major challenge in these 

engagements is communicating scientific 

information about the risks and uncertainties of potential changes in climate.

11

Efforts to facilitate interactions among 

technical experts and members of the 

public and decision-makers are often 

referred to as “boundary processes” 

(Figure 26.2). Boundary processes and associated tools include, for example, joint 

fact finding, structured decision-making, 

Decision-Making Elements and Outcomes

Figure 26.1. Decisions take place within a complex context. Decision support processes and tools can help structure decisionmaking, organize and analyze information, and build consensus around options for action.

Figure 26.2. Boundary processes facilitate the flow of information and sharing of 

knowledge between decision-makers and scientists/technical experts. Processes 

that bring these groups together and help translate between different areas of 

expertise can provide substantial benefits. 

Boundary Processes Linking

Decision-Makers and Scientific/Technical Experts
624 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

26: DECISION SUPPORT: CONNECTING SCIENCE, RISK PERCEPTION, AND DECISIONS

collaborative adaptive management, and computer-aided 

collaborative simulation, each of which engages scientists, 

stakeholders, and decision-makers in ongoing dialog about 

understanding the policy problem and identifying what information and analyses are necessary to evaluate decision options.

12,13,14 The use of these kinds of processes is increasing 

in decision settings involving complex scientific information 

and multiple – sometimes competing – societal values and 

goals. Well-designed boundary processes improve the match 

between the availability of scientific information and capacity 

to use it and result in scientific information that is perceived as 

useful and applicable.

Though boundary processes developed to support climaterelated decisions vary in their design, they all involve bringing together scientists, decision-makers, and citizens to collaborate in the scoping, conduct, and employment of technical 

and scientific studies to improve decision-making. Boundary 

processes can involve establishing specialized institutions, 

sometimes referred to as boundary organizations, to provide 

a forum for interaction amongst scientists and decision-makers.15 One such boundary activity is the National Oceanic and 

Atmospheric Administration’s (NOAA) Regional Integrated Science and Assessment (RISA) Program. Interdisciplinary RISA 

teams are largely based at universities and engage regional, 

state, and local governments, non-governmental organizations, and private sector organizations to address issues of 

concern to decision-makers and planners at the regional level. 

RISA teams help to build bridges across the scientist, decisionmaker, and stakeholder divide.

16 Effective engagement may 

also occur through less formal approaches by incorporating 

boundary processes that bring scientists, stakeholders, and 

decision-makers together within a specific decision-making 

setting rather than relying on an independent boundary organization. Sustained conversations among scientists, decisionmakers, and stakeholders are often necessary to frame issues 

and identify, generate, and use relevant information.17

Some analysts have emphasized the importance of boundary 

processes that are collaborative and iterative.

18 In one example, federal, state, and local agencies, water users, and other 

stakeholders are using a collaborative process to manage the 

Platte River to meet species protection goals and the needs 

of other water users. The Platte River Recovery Implementation Program brings together participants on an ongoing basis 

to help set goals, choose management options, and generate 

information about the effectiveness of their actions.19 Scientists engaged in the process do not make policy decisions, but 

they engage directly with participants to help them frame scientific questions relevant to management choices, understand 

available information, design monitoring systems to assess 

outcomes of management actions, and generate new knowledge tailored to addressing key decision-maker questions. The 

process has helped participants move beyond disagreements 

about the water-flow needs of the endangered species and 

move to action. Through monitoring, participants will evaluate 

whether the water flows and other management practices are 

achieving the goals for species recovery set out in the Platte 

River Recovery Implementation Plan.

In a number of other examples, boundary processes involve 

the use of computer simulation models.14 Scientists, stakeholders, and decision-makers develop a shared understanding of the problem and potential solutions by jointly designing models that reflect their values, interests, and analytical 

needs. The U.S. Army Corps of Engineers has developed this 

type of boundary process in their “shared vision planning.”

20

A comprehensive website provides a history of the process, 

demonstrations and case studies, and tools and techniques for 

implementing the process.21

Recently, the International Joint Commission used the shared 

vision planning process in decisions about how to regulate 

water levels in both the Lake Ontario-St. Lawrence River system22 and in the Upper Great Lakes.23,24 Both studies engaged 

hundreds of participants from the United States and Canada 

in discussions about water level management options and the 

impacts of those options on ecosystems; recreational boating 

and tourism; hydropower; commercial navigation; municipal, 

industrial, and domestic water use; and the coastal zone. The 

models used in the studies incorporated information about 

ecosystem responses, shoreline dynamics, economics, and 

lake hydrology, and the potential operating plans were tested 

using multiple climate change scenarios. Although the shared 

vision planning process did not ultimately lead to consensus on 

a single recommended plan in the Lake Ontario-St. Lawrence 

River Study, the process did help improve participants’ understanding of the system and develop a shared vision of possible 

futures.22,25 Building on lessons from the Lake Ontario-St. Lawrence River Study, the Upper Great Lakes Study’s use of shared 

vision planning did result in a single recommended plan.24

Courtesy of Lynn Laws, Iowa State University 2013
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Using a Decision-Making Framework

The term “adaptive management” is used here to refer to a 

specific approach in which decisions are adjusted over time 

to reflect new scientific information and decision-makers 

learn from experience. The National Research Council (NRC) 

contrasts the processes of “adaptive management” and “deliberation with analysis.”

3

 Both can be used as part of an “iterative adaptive risk management framework” that is useful for 

decisions about adaptation and ways to reduce future climate 

change, especially given uncertainties and ongoing advances 

in scientific understanding.

8,26 Iterative adaptive risk management emphasizes learning by doing and continued adaptation 

to improve outcomes. It is especially useful when the likelihood of potential outcomes is very uncertain.

An idealized iterative adaptive risk management process includes clearly defining the issue, establishing decision criteria, 

identifying and incorporating relevant information, evaluating 

options, and monitoring and revisiting effectiveness (Figure 

26.3). The process can be used in situations of varying complexity, and while it can be more difficult for complex decisions,27

the incorporation of an iterative approach makes it possible 

to adjust decisions as information improves. Iterative adaptive 

risk management can be undertaken through collaborative 

processes that facilitate incorporation of stakeholder values in 

goal-setting and review of decision options.28 Examples of the 

process and decision support tools that are helpful at its different stages are included in subsequent sections of this chapter.

Defining the Issue and Establishing Decision Criteria

An initial step in a well-structured decision process is to identify the context of the decision and factors that will affect choices – making sure that the questions are posed properly from 

scientific, decision-maker, and stakeholder (or public) perspectives (corresponding to the first two steps in Figure 26.3). An 

important challenge is identifying the stakeholders and how 

to engage them in decision-making processes. There are often 

many categories of stakeholders, including those directly and 

indirectly affected by, or interested in, the outcomes of decisions, as well as the decision-makers, scientists, and elected 

officials.29 Other important considerations often overlooked 

but critical to defining the issue are:

• understanding the goals and values of the participants 

in the decision process; 

• identifying risk perceptions and the sense of urgency of 

the parties involved in the decision; 

• being clear about the time frame of the decision (shortversus long-term options relative to current and future 

risk levels) – and when the decision must be reached;

• acknowledging the scale and degree of controversy associated with the risks and opportunities as well as the 

alternatives;

• assessing the distribution of benefits or losses associated with current conditions and the alternatives being 

considered;

• reaching out to communities that will be affected but 

may lack ready access to the process (for example, considering environmental justice issues);

• recognizing the diverse interests of the participants;

• recognizing when neutral facilitators or trained science 

translators are needed to support the process; and

• understanding legal or institutional constraints on options.

Identifying and agreeing on decision criteria – metrics that help 

participants judge the outcomes of different decision options 

– can be extremely helpful in clarifying the basis for reaching a 

decision. Based on the relevant objectives, decision criteria can 

be established that reflect constraints and values of decisionmakers and affected parties. Criteria can be quantitative (for 

example, obtaining a particular rate of return on investment) 

or qualitative (for example, maintaining a community’s character or culture). If the issue identified is to reduce the risks 

associated with climate change, decision criteria might include 

minimizing long-term costs and maximizing public safety. Related sections below provide information on tools for valuing 

and comparing options and outcomes and provide a basis for 

using decision criteria. 

Decision framing and establishment of decision criteria can 

be facilitated using various methods, including brainstorming, community meetings, focus groups, surveys, and problem 

Figure 26.3. This illustration highlights several stages of a wellstructured decision-making process. (Figure source: adapted 

from NRC 20108 and Willows and Connell 200326).

Decision-Making Framework
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26: DECISION SUPPORT: CONNECTING SCIENCE, RISK PERCEPTION, AND DECISIONS

mapping;3,29 selecting among techniques requires consideration of a number of context-specific issues.

30 There are a variety of techniques for organizing, weighting information, and 

making tradeoffs for the goals that are important for a decision,31,32 several of which are discussed in more detail in the 

section “Examples of Decision Support Tools and Methods.”

Accessing Information 

Developing a solid base of information to support decisionmaking is ideally a process of matching user needs with available information, including observations, models, and decision 

support tools. In some cases, needed information does not 

exist in the form useful to decision makers, thus requiring the 

capacity for synthesis of currently available information into 

new data products and formats. For decisions in the context of 

climate change and variability, it is critical to consult information that helps clarify the risks and opportunities to allow for 

appropriate planning and management. An example of information systems that synthesize data and products to support 

mitigation and adaptation decisions is the National Integrated 

Drought Information System (NIDIS), a federal, interagency effort to supply information about drought impacts and risks as 

well as decision support tools to allow sectors and communities to prepare for the effects of drought.33 Learning from the 

successes of such efforts, the National Climate Assessment 

(NCA) is currently developing an indicator system to track climate changes as well as physical, natural, and societal impacts, 

vulnerabilities, and responses.34 This effort is building on existing indicator efforts, such as the U.S. Environmental Protection 

Agency’s (EPA) Climate Change Indicators,35 NASA Vital Signs,36

and NOAA indicator products,37 as well as 

identifying when new data, information, and 

indicator products are needed.

Information technology systems and data 

analytics can harness vast data sources, facilitating collection, storage, access, analysis, 

visualization, and collaboration by scientists, 

analysts, and decision-makers. Such technologies allow for rapid scenario building 

and testing using many different variables, 

enhancing capacity to measure the physical 

impacts of climate change. These technologies are managing an increasing volume of 

data from satellite instruments, in situ (direct) measurement networks, and increasingly detailed and high-resolution models.38

“Information Technology Supports Adaptation Decision-Making” below highlights use of an open platform data system that facilitated collaboration across multiple 

public and private sector entities in analyzing climate risk and 

adaptation economics along the U.S. Gulf Coast.

While progress is being made in development of data management and information systems, multiple challenges remain. 

Specific issues highlighted in the recent USGCRP National 

Global Change Research Plan38 include data permanence, 

volume, transparency, quality control, and access. For data 

on socioeconomic systems – important for evaluating vulnerabilities, adaptation, and mitigation – privacy, confidentiality, 

and integration with broader systems of environmental data 

are important issues.38 Experience with adaptation and mitigation decisions is often an excellent source of information and 

knowledge but is difficult to access and validate. Several organizations have been developing knowledge management 

systems for integrating this highly dispersed information and 

providing it to a network of practitioners (for example, CAKE 

201239). Addressing these and other challenges is essential for 

making progress in establishing a sustained assessment process and meeting the challenge of informing decision-making.40

Assessing, Perceiving, and Managing Risk 

Making effective climate-related decisions requires balance 

among actions intended to manage, reduce, and transfer risk. 

Risks are threats to life, health and safety, the environment, 

economic well-being, and other things of value. Risks are often 

evaluated in terms of how likely they are to occur (probability) 

and the damages that would result if they did happen (consequences). As noted by the Intergovernmental Panel on Climate 

Change,

45 human choices affect the risks associated with climate variability and change. Such choices include how to manage our ecosystems and agriculture, where to live, and how to 

build resilient infrastructure. Choices regarding a portfolio of 

actions to address the risks associated with climate variability 

Information technology supports

adaptation decision-making

Entergy (a regional electric utility), Swiss Re (a reinsurance company), 

and the Economics of Climate Adaptation Working Group (a partnership 

between several public and private organizations) integrated natural 

catastrophe weather models with economic data to develop damage 

estimates related to climate change adaptation.41 An extension of this 

work is the first comprehensive analysis of climate risks and adaptation 

economics along the U.S. Gulf Coast.42 Another example is a simplified model, developed with support from EPA, to look at flooding risks 

associated with coastal exposure in southern Maine.43 Use of an “open 

platform” system that allows multiple users to input and access data 

resulted in spreadsheets, graphs, and three-dimensional imagery displayed on contour maps downscaled to the city and county level for 

local decision-makers to access.44
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26: DECISION SUPPORT: CONNECTING SCIENCE, RISK PERCEPTION, AND DECISIONS

and change are most effective when they take 

into consideration the range of factors affecting 

human behavior, including people’s perception 

of risk, the relative importance of those risks, 

and the socioeconomic context.

45,46 The process 

shown in Figure 26.4 is designed to help take such 

factors into consideration.

The next few sections describe the “integrate, 

evaluate, and decide” steps in Figure 26.3, which 

aim to help decision-makers choose risk management strategies. While a full quantitative risk 

analysis is not always possible, the concept of risk 

assessment coupled with understanding of risk 

perception provides a powerful framework for 

decision-makers to evaluate alternative options 

for managing the risks that they face today and 

in the future.47 As described below, methods such 

as multiple criteria analysis, valuation of both 

risks and opportunities, and scenarios can help to 

combine experts’ assessment of climate change 

risks with public perception of these risks, both 

influenced by the diverse values people bring 

to these questions48 and in support of risk management strategies more likely to achieve both 

public support and their desired objectives.

46 To 

illustrate how this framework can be applied to 

resource management decisions, we use an example of coastal risk management decisions in 

the context of climate change.

49

Risk Assessment 

Risk assessment includes studies that estimate the likelihood 

of specific sets of events occurring and/or their potential consequences.

50 Experts often prov ide quantitative information 

regarding the nature of the climate change risk and the degree 

of uncertainty surrounding their estimates. Risk assessment 

focuses on the likelihood of negative consequences but does 

not exclude the possibility that there may also be beneficial 

consequences.

There are four basic elements for assessing risk – hazard, inventory, vulnerability, and loss.

51 This generalized approach 

to risk assessment is useful for a variety of types of decisions. 

The first element focuses on the risk of a hazard as a function 

of climate change, including interactions of climate effects 

with other factors. In the context of the coastal community 

example, the community is concerned with the likelihood of 

future hurricanes and the impacts that sea level rise may have 

on damage to the residential development from future hurricanes. There is likely to be considerable uncertainty about 

maximum storm surge and sea level from hurricanes during 

the next 50 to 70 years. The second element identifies the 

inventory of properties, people, and the environment at risk. 

To inventory structures, for instance, requires evaluating their 

location, physical dimensions, and construction quality.

Evaluating both the hazard and its impacts on the inventory 

often requires an appropriate treatment of uncertainty. In 

some cases a probabilistic treatment may prove sufficient. For 

instance, in the coastal community example, decision-makers 

may have sufficient confidence in estimates of the return frequency of extreme storms (for example, that the once-in-ahundred-years storm is and will remain a once-in-a-hundredyears storm) to base their choices largely on these estimates. 

If such probabilistic estimates are not available, or if decisionmakers lack sufficient confidence in those that are available, 

they may find it useful to consider a range of scenarios and 

seek risk management strategies robust across these ranges of 

estimates.49,52,53 

Together, the hazard and inventory elements enable calculation of the damage vulnerability of the structures, people, and 

environment at risk. The vulnerability component enables estimation of the human, property, and environmental losses from 

different climate change scenarios by integrating biophysical 

information on climate change and other stressors with soFigure 26.4. This figure highlights the importance of incorporating 

both experts’ assessment of the climate change risk and general public 

perceptions of this risk in developing risk management strategies for reducing 

the negative impacts of climate change. As indicated by the arrows, how 

the public perceives risk should be considered when experts communicate 

data on the risks associated with climate change so the public refines its 

understanding of these risks. As the arrows indicate, the general public’s 

views must also be considered in addition to experts’ judgments when 

developing risk management strategies that achieve decision-makers’ 

desired objectives. Climate change policies that are implemented will, in turn, 

affect both expert assessment and public perception of this risk in the future, 

as indicated by the feedback loop from risk management to these two boxes.

Linking Risk Assessment and Risk Perception

with Risk Management of Climate Change
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26: DECISION SUPPORT: CONNECTING SCIENCE, RISK PERCEPTION, AND DECISIONS

cioeconomic and environmental information.

54 These assessments typically involve evaluation of exposure, sensitivity, and 

adaptive capacity for current and projected conditions. Quantitative indicators are increasingly used to diagnose potential 

vulnerabilities under different scenarios of socioeconomic and 

environmental change55 and to identify priorities and readiness for adaptation investments.

56 In the case of a coastal residential development, the design of the facility will influence 

its ability to reduce damage from hurricanes and injuries or 

fatalities from hurricane storm surge and sea level rise. Decisions may involve determining whether to elevate the facility 

so it is above ten feet, how much this adaptation measure will 

cost, and the reduction in the impact of future hurricanes on 

damage to the facility and on the residents in the building, as a 

function of different climate change scenarios.

Risk Perception in Climate Change Decision-Making

The concept of risk perception refers to individual, group, and 

public views and attitudes toward risks, where risks are understood as threats to life, health and safety, the environment, 

economic well-being, and other things of value. Risk perception encompasses perspectives on various dimensions of risks, 

including their severity, scope, incidence, timing, controllability, and origins or causes. The knowledge base regarding risk 

perception includes research in psychology, social psychology, 

sociology, decision science, and health-related disciplines (see 

“Factors Affecting Attitudes Toward Risk”).

As noted in “Factors Affecting Attitudes Towards Risk,” many 

factors influence risk. Social scientists and psychologists have 

studied people’s concerns about climate change risks and 

found that many individuals view hazards for which they have 

little personal knowledge and experience as highly risky.72 On 

the other hand, seeing climate change as a simple and gradual change from current to future values on variables such as 

average temperatures and precipitation may make it seem 

controllable.73

The effects of risk perception on decision-making have also 

been studied extensively and support a number of conclusions 

that need to be considered in decision support processes. The 

decision process of non-experts with respect to low-probability, high-consequence events differs from that of experts.74

Non-experts tend to focus on short time horizons, seeking 

to recoup investments over a short period of time, in which 

case future impacts from climate change are not given much 

weight in actions taken today. This is a principal reason why 

there is a lack of interest in undertaking adaptation measures 

with upfront investments costs where the benefits accrue over 

Factors affecting attitudes towards risk

Extensive literature indicates that a range of factors shape risk perceptions. For example, psychological risk dimensions have been shown to influence people’s perceptions of health and safety risks across numerous studies in multiple 

countries.57 People also often use common “mental shortcuts,” such as availability and representativeness, to organize 

a wide range of experiences and information.58 How risks are framed is also important – for example, as numbers versus 

percentages and worst-case formulations versus more probable events.59 Recent research has emphasized the role of 

emotions in the perception of risk.60,61

Other factors explored in the literature center on perceived characteristics of specific risks, such as whether the risks are 

familiar or unfamiliar; prosaic or perceived as catastrophic (“dread” risks); reversible or irreversible; and voluntarily assumed or imposed.62 Risk perception is also influenced by the social characteristics of individuals and groups, including 

gender, race, and socioeconomic status.61,63 Experiences with specific risks are also important, such as being affected 

by a hazard (for discussions, see Figner and Weber 2011;64 NRC 2006;64 Tierney et al. 200166) and experiencing near 

misses or false alarms.67

Risk perceptions do not exist as isolated perceptions, but are linked to other individual and group perceptions and beliefs and to psychosocial factors, such as fatalism, locus of control (the degree to which people feel they have control 

over their own lives and outcomes), and religiosity,65,66 as well as to more general worldviews. Research has also focused 

on people’s mental models regarding the causality and effects of different risks.68

Still other research focuses on how risk information is mediated through organizations and institutions and how mediation processes influence individual and group risk perceptions. For example, the “social amplification of risk” framework 

stresses the importance of the media and other institutions in shaping risk perceptions, such as by making risks seem 

more or less threatening.69 Perceptions are also related to people’s trust in the institutions that manage risk; loss of 

trust can lead to feelings of disloyalty regarding organizations that produce risks and institutions charged with managing 

them, which can in turn amplify individual and public concerns.70 Additionally, perceptions are linked to individual and 

group attitudes concerning sources of risk information, including official and media sources. These factors include the 

perceived legitimacy, credibility, believability, and consistency of information sources.71
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26: DECISION SUPPORT: CONNECTING SCIENCE, RISK PERCEPTION, AND DECISIONS

a long period of time.75 In the context of the coastal residential development, elevating the structure will reduce expected 

damages from hurricanes, resulting in smaller annual insurance premiums. Long-term loans that spread the costs of this 

action over time can make the option financially attractive, if 

the savings on the insurance premiums outweigh the costs of 

the loan payments.

There is also a tendency for decision-makers to treat a lowprobability event as if it had no chance of occurring because 

it is below their threshold level of concern (such as a 1 in 100 

chance of a damaging disaster occurring next year). As shown 

by empirical research, stretching the time horizon over which 

information is communicated can make a difference in risk perception.76 In the case of the coastal residential development,

community leaders may pay more attention to the need for 

adaptation measures if the likelihood of inundation by a future 

hurricane is presented over a 25-year or 50-year horizon (for 

example, the facility may flood 5 times in 25 years) rather than 

as a risk on annual basis (for example, there is a 20% chance of 

flooding in any given year).

Risk Management Strategies

In general, an effective response to the current and future risks 

from climate variability and change will require a portfolio of 

different types of actions, ranging from those intended to 

manage, reduce, and transfer risk to those intended to provide 

additional information on risks and the effectiveness of various actions for addressing it (see “Value of Information”). For 

instance, in the coastal community example, decision-makers 

might better manage risk through changes in building codes 

intended to reduce the impact of flooding on structures, might 

share risk by appropriate adjustments in flood insurance rates, 

and might reduce risk via land-use policies that shift development towards higher ground and via participating in and advocating for greenhouse gas emission reduction policies that may 

reduce future levels of sea level rise.

To facilitate these strategies given the uncertainty associated with the likelihood and consequences of climate change, 

“robust decision-making” may be a useful tool for evaluating alternative options and risk management strategies. One 

study reviews the application of a range of decision-making 

approaches to assessing options for mitigating or adapting to 

the impacts of climate change.77 In the context of the coastal 

residential development, the choice of adaptation measures 

to reduce the likelihood of future water-related damage may 

require using such an approach. To illustrate, consider two adaptation measures, elevating a building and flood-proofing it, 

to reduce the chances of severe water damage from hurricane 

storm surge coupled with sea level rise. Measure 1 (elevation) 

may perform extremely well based on specific estimates of the 

likelihood of different climate change conditions that will affect storm surge and sea level rise, but it may perform poorly 

if those estimates turn out to be mistaken. Measure 2 (floodproofing) may have a lower expected benefit than elevation 

but much less variance in its outcomes and thus be the preferred choice of the community.49

Turning to risk management strategies, public agencies, private firms, and individuals have incentives, information, and 

options available to adapt to emerging conditions due to climate change. These options may include ensuring continuity 

of service or fulfillment of agency responsibilities, addressing 

procurement or supply chain issues, preserving market share, 

or holding the line on agency or private-sector production 

costs. Commercially available mechanisms such as insurance 

can also play a role in providing protection against losses due 

to climate change.78 However, insurers may be unwilling to provide coverage against such losses due to the uncertainty of the 

risks and lack of clarity on the liability issues associated with 

global climate change.79 In these cases, public sector involvement through public education programs, economic incentives (subsidies and fines), and regulations and standards may 

be relevant options. Criteria for evaluating risk management 

strategies can include impacts on resource allocation, equity 

and distributional impacts, ease of implementation, and justification.

Implementation, Continued Monitoring, and Evaluation of Decisions 

The implementation phase of a well-structured decision process involves an ongoing cycle of setting goals, taking action, 

learning from experience, and monitoring to evaluate the consequences of undertaking specific actions, as shown on the 

left-hand side of Figure 26.3. This cycle offers the potential for 

policy and outcome improvement through time. Ongoing evalValue of information

A frequently asked question when making complex decisions is: “When does the addition of more information 

contribute to decision-making so that the benefit of obtaining this information exceeds the expense of collecting, processing, or waiting for it?” In a decision context, 

the value of information often is defined as the expected 

additional benefit from additional information, relative 

to what could be expected without that information.80,81

Even though decision-makers often cite a lack of information as a rationale for not making timely decisions, 

delaying a decision to obtain more information does not 

always lead to different or better decisions.82,83
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uation can focus on how the system responds to the decision, 

leading to better future decisions, as well as on how different 

stakeholders respond, resulting in improvements in future 

decision-making processes. The need for social and technical 

learning to inform decision-making is likely to increase in the 

face of pressures on social and resource systems from climate 

change. However, the relative effectiveness of monitoring and 

assessment in producing social and technical learning depends 

on the nature of the problem, the amount and kind of uncertainty and risk associated with climate change, and the design 

of the monitoring and evaluation efforts.

Examples of Decision Support Tools and Methods

While decision frameworks vary in their details, they generally 

incorporate most or all of the steps outlined above. To support 

decision-making across these steps, various technical tools and 

methods, developed in both the public and private sectors, can 

assist stakeholders and decision-makers in meeting their objectives and clarify where there are value differences or varying tolerances for risk and uncertainty. Many of these tools 

and methods are applicable throughout the decision-making 

process, from framing through assessment of options through 

evaluation of outcomes. Several of the tools and methods – 

data management systems and scientific assessments – help 

to expand the relevant information and provide a means of 

managing large amounts of data. Three other tools described 

below – comparative tradeoff methods, scenario planning, 

and integrated assessment models – are particularly useful 

in assisting stakeholders and decision-makers in identifying 

and evaluating different options for managing risks associated 

with climate change. The following discussion describes these 

approaches; examples are provided in “Example Decision Support Tools.”

Example decision support tools

Many decision support tools apply climate science and other information to specific decisions and issues; several online 

clearinghouses describe these tools and provide case studies of their use (for example, CAKE 2012;39 CCSP 2005;84 NatureServe 201285). Typically, these applications integrate observed or modeled data on climate and a resource or system 

to enable users to evaluate the potential consequences of options for management, investment, and other decisions. 

These tools apply to many types of decisions; examples of decisions and references for further information are provided 

in Table 26.2.

Table 26.2. Examples of Decisions and Tools Used

Topic Example Decision(s) Further Information and Case Studies

Water resources

Making water supply decisions in the context of changes in precipitation, increased 

temperatures, and changes in water quality, 

quantity, and water use

Means et al. 2010;86 International Upper 

Great Lakes Study 2012;24 State of Washington 2012;87 “Denver Water Case Study” 

(below); Ch. 3: Water

Infrastructure

Designing and locating energy or transportation facilities in the coastal zone to limit the 

impacts of sea level rise

Ch. 11: Urban; Ch. 10: Energy, Water, and 

Land 

Ecosystems and biodiversity

Managing carbon capture and storage, fire, 

invasive species, ecosystems, and ecosystem services

Byrd et al. 2011;88 Labiosa et al. 2009;89

USGS 2012a, 2012b, 2012c;90,91 Figure 26.5

Human health

Providing public health warnings in response 

to ecosystem changes or degradation, air 

quality, or temperature issues

Ch. 9: Human Health

Regional climate change 

response planning

Develop plans to reduce emissions of greenhouse gases in multiple economic sectors 

within a state

“Washington State’s Climate Action Team” 

(below)

Continued
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Valuing the Effects of Different Decisions

Understanding costs and benefits of different decisions requires understanding people’s preferences and developing 

ways to measure outcomes of those decisions relative to 

preferences. This “valuation” process is used to help rank alternative actions, illuminate tradeoffs, and enlighten public 

discourse.31 In the context of climate change, the process of 

measuring the economic values or non-monetary benefits of 

different outcomes involves managers, scientists, and stakeholders and a set of methods to help decision-makers evaluate 

the consequences of climate change decisions.92 Although values are defined differently by different individuals and groups 

and can involve different metrics – for example, monetary values and non-monetary benefit measures93 – in all cases, valuation is used to assess the relative importance to the public 

or specific stakeholders of different impacts. Such valuation 

assessments can be used as inputs into iterative adaptive risk 

management assessments (which has advantages in a climate 

context because of its ability to address uncertainty) or more 

traditional cost-benefit analyses, if appropriate.

Some impacts ultimately are reflected in changes in the value 

of activities within the marketplace and in dollars94 – for example, the impacts of increased temperatures on commercial 

crop yields.

95 Other evaluations use non-monetary benefit 

measures such as biodiversity measures96 or soil conservation 

and water services.97

Valuation methods can provide input to a range of decisions, 

including cost-benefit analysis of new or existing regulations98

or government projects;99 assessing the implications of landuse changes;100 transportation investments and other planning 

efforts;101,102 developing metrics for ecosystem services; and 

stakeholder and conflict resolution processes.103

Many available and widely applied decision-making tools can be used to support management in response to climate 

extremes or seasonal fluctuations. Development of decision support resources focused on decadal or multi-decadal investment decisions is in a relatively early stage but is evolving rapidly and shared through the types of clearinghouses 

discussed above.

Example decision support tools (continued)

Figure 26.5. The Santa Cruz Watershed Ecosystem Portfolio Model is a regional land-use planning tool that integrates 

ecological, economic, and social information and values relevant to decision-makers and stakeholders. The tool is a mapbased set of evaluation tools for planners and stakeholders, and is meant to help in balancing disparate interests within a 

regional context. Projections for climate change can be added to tools such as this one and used to simulate impacts of climate 

change and generate scenarios of climate change sensitivity; such an application is under development for this tool (Figure 

source: USGS 201290).

Land-use Planning Tool for the Upper Santa Cruz Watershed
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Comparative Tradeoff Methods

Once their consequences are valued or otherwise described,

alternative options are often compared against the objectives 

or decision criteria. In such cases, approaches such as listing 

the pros and cons,

104 cost-benefit analysis,105 multi-criteria 

methods,

80 or robust decision methods106 can be useful. Multicriteria methods provide a way to compare options by considering the positive and negative consequences for each of the 

objectives without having to choose a single valuation method 

for all the attributes important to decision-makers.31 This approach allows for consequences to be evaluated using criteria 

most relevant for a given objective.

107 The options can then be 

compared directly by considering the relative importance of 

each objective for the particular decision. 

Integrated Assessment Models 

Integrated Assessment Models are tools for modeling interactions across climate, environmental, and socioeconomic systems.108 In particular, integrated assessment models can be 

used to provide information that informs tradeoffs analyses, 

often by simulating the potential consequences of alternative 

decisions. Integrated assessment models typically include representations of climate, economics, energy, and other technology systems, as well as demographic trends and other factors 

that can be used in scenario development and uncertainty 

quantification.109 They are useful in national and global policy 

decisions about emissions targets, timetables, and the implications of different technologies for emissions management.110

These models are now being extended to additional domains 

such as water resources and ecosystem services to inform a 

broader range of tradeoff analyses and to finer resolutions to 

support regional decision-making.111

Scenarios and Scenario Planning 

Scenarios are depictions of possible futures or plausible conditions given a set of assumptions; they are not predictions. 

Scenarios enable decision makers to consider uncertainties in 

future conditions and explore how alternate decisions could 

shape the futures or perform under uncertainty. One approach 

to building scenarios begins with identifying any changes over 

time that might occur in climate and socioeconomic factors (for 

example, population growth and changes in water availability), 

and then using these projections to help decision-makers rank 

the desirability of alternative decision options to respond to 

these changes.112 This works well when decision-makers agree 

on the definition of the problem and scientific evidence.53,113 A 

second approach is widely used in robust decision-making and 

decision-scaling approaches. It begins with a specific decision 

under consideration by a specific community of users and then 

poses questions relevant to these decisions (for example, “how 

can we build a vibrant economy in our community in light of 

uncertainty about population growth and water supply?”) to 

organize information about future climate and socioeconomic 

conditions (for example, Robinson 1988114). 

Scenario planning often combines quantitative science-based 

scenarios with participatory “visioning” processes used by 

communities and organizations to explore desired futures.115

It can also facilitate participatory learning and development of 

a common understanding of problems or decisions. There are 

many different approaches, from a single workshop that uses 

primarily qualitative approaches to more complex exercises 

that integrate qualitative and quantitative methods with visualization and/or simulation techniques over multiple workshops 

or meetings. Common elements include scoping and problem 

definition; group development of qualitative (and, optionally, 

quantitative) scenarios and analyses that explore interactions 

of key driving forces, uncertainties, and decision options.

Scenario planning has been useful for water managers such as 

Denver Water, which has also used “robust decision-making” 

to assess policies that perform well across a wide range of future conditions, in the face of uncertainty and unknown probabilities (see “Denver Water Case Study”). Other examples of 

the use of scenario planning include:

• National Park Service, to consider potential climate 

change impacts and identify adaptation needs and priorities in several parks or regions116

• California State Coastal Conservancy, to plan tidal marsh 

restoration and planning in the San Francisco estuary in 

the face of climate change and sea level rise117

• Urban Ecology Research Lab at the University of Washington, for planning adaptation to preserve ecosystem 

services in the Snohomish Basin118

• A group of agencies and organizations considering the 

impacts of climate change on ecosystems in the Florida 

Everglades119

The National Climate Assessment has developed and used 

a number of different types of scenarios and approaches in 

preparation of this report (see Appendix 5: Scenarios and 

models).120

NOAA/NCDC
633 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

26: DECISION SUPPORT: CONNECTING SCIENCE, RISK PERCEPTION, AND DECISIONS

Scientific Assessments

Ongoing assessments of the state of knowledge allow for iterative improvements in understanding over time and can provide 

opportunities to work directly with decision-makers to understand their needs for information.123 A sustained assessment 

process (Ch. 30: Sustained Assessment)40 can be designed to 

support the adaptation and mitigation information needs of 

decision-makers, with ongoing improvements in data quality 

and utility over time. This report represents one such type of 

Climate change is one of the biggest challenges facing the Denver 

Water system. Due to recent and 

anticipated effects of climate variability and change on water availability, Denver Water faces the 

challenge of weighing alternative 

response strategies and is looking 

at developing options to help meet 

more challenging future conditions. 

Denver Water is using scenario 

planning in its long-range planning 

process (looking out to 2050) to 

consider a range of plausible future scenarios (Figure 26.6). This 

approach contrasts with its traditional approach of planning for 

a single future based on demand 

projections and should better prepare the utility and enhance its 

ability to adapt to changing and 

uncertain future conditions.

Denver Water is assessing multiple scenarios based on several 

potential water system challenges, 

including climate change, demographic and water-use changes, 

and economic and regulatory 

changes. The scenario planning 

strategy includes “robust decisionmaking,” which focuses on keeping as many future options open as 

possible while trying to ensure reliability of current supplies.

Scenario planning was chosen as a way to plan for multiple possible futures, given the degree of uncertainty associated 

with many variables, particularly demographic change and potential changes in precipitation. This method is easy to 

understand and has gained acceptance across the utility. It is a good complement to more technical, detailed analytical 

approaches. 

The next step for Denver Water is to explore a more technical approach to test their existing plan and identified options 

against multiple climate change scenarios. Following a modified robust decision-making approach,121 Denver Water will 

test and hedge its plan and options until those options demonstrate that they can sufficiently handle a range of projected 

climate conditions.

Denver water case study

Figure 26.6: Scenario planning is an important component of decision-making. This “cone 

of uncertainty” is used to depict potential futures in Denver Water’s scenario planning 

exercises. (Figure source: adapted from Waage 2010122).

Scenario Planning

©Denver Water
634 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

26: DECISION SUPPORT: CONNECTING SCIENCE, RISK PERCEPTION, AND DECISIONS

assessment. The Intergovernmental Panel on Climate Change 

(IPCC) has prepared assessments of the state of the science 

related to climate change, impacts and adaptation, and mitigation since the late 1980s. Numerous additional assessments 

have been prepared for a variety of national and international 

bodies focused on issues such as biodiversity, ecosystem services, global change impacts in the Arctic, and many others.

Incorporating Recent Scientific Advances and Translating Science for Decision-Making

While decision support is not necessarily constrained by a lack 

of tools, a number of barriers restrict application of existing 

and emerging science and technology in adaptation and mitigation decisions.3,8,129 In cases where tools exist, decision-makers 

may be 1) unaware of tools; 2) overwhelmed by the number of 

tools; 3) hesitant to use tools that are not appraised or updated 

and maintained with new information; or 4) require training 

in how to use tools.

8,130 Recent scientific developments could 

help address some of these barriers, but are not yet incorporated into decision support tools.65 For example, individual climate models can provide very different projections of future 

climate conditions for a given region, and the divergence of 

these projections can make it seem impossible to reach a decision. But comparing different models and constructing climate 

model “ensembles” can highlight areas of agreement across 

large numbers of models and model runs, and can also be used 

to develop ranges and other forms of quantification of uncertainty (for further discussion, see Ch. 2: Our Changing Climate 

and Appendix 3: Climate Science Supplement). While results 

from these activities can prove difficult to present in formats 

that could help decision-makers,

131 new approaches to visualization and decision support can make such ensembles useful 

for decision-making.132

There is also a need for “science translators” who can help 

decision-makers efficiently access and properly use data and 

tools that would be helpful in making more informed decisions in the context of climate change.

3,4,8,83,133 The culture of 

research in the United States often perpetuates a belief that 

basic and applied research need to be kept separate, though 

Between 2000 and 2007, pioneering work by the University of Washington’s Climate Impacts Group (a NOAA RISA) tailored national climate models to the Pacific Northwest and produced, for the first time, specific information about likely 

adverse impacts to virtually every part of Washington’s economy and environment if carbon dioxide concentrations in the 

atmosphere were not quickly stabilized.124 The localized impacts predicted from these models were significant.

In February of 2007, Governor Christine Gregoire issued Executive Order 07-02, establishing the Climate Action Team 

(CAT).125 Its charge was to develop a plan to achieve dramatic, climate-stabilizing reductions in emissions of greenhouse 

gases according to goals established in the Executive Order. The CAT was a 29-member team that included representatives of industry, utilities, environmental advocacy groups, Native American tribes, state and municipal governments, and 

elected officials. 

The CAT met four to five times a year for two years. Between meetings, technical consultants, including boundary organizations such as the Climate Impacts Group, provided detailed analyses of the issues that were on the next CAT agenda. 

Technical experts were recruited to provide direct testimony to the CAT. Professional facilitators helped run the meetings, 

decipher the technical testimony, and keep the CAT on track to meet its obligations. All CAT meetings were open to the 

public, and public testimony was accepted. To assist in this effort, five subcommittees were created to develop proposals for achieving emissions reductions in the following parts of the economy: the built environment, agriculture, forestry, 

transportation, and energy generation. Similarly, adaptation groups were formed to develop recommendations for dealing 

with impacts that could not be avoided. These Preparation/Adaptation Working Groups focused on forest health, farmlands, human health, and coastal infrastructure and resources.

The CAT and the working groups were well supported with science and technical expertise. The CAT issued its first report, 

on reducing greenhouse gases, at the close of 2007.126 It was well received by the legislature, and a significant number 

of its recommendations were implemented in the 2008 session.127

In 2008, the CAT continued its work. The focus shifted to whether Washington should join the Western Climate Initiative 

(WCI), a state and provincial organization that was developing a regional, economy-wide cap and trade system for carbon 

emissions. The same high-quality professional facilitation was provided at all meetings. Several highly qualified technical 

experts provided technical support. 

With this support, the CAT produced another set of recommendations.128 The centerpiece recommendation was that Washington join the WCI’s regional cap and trade program. This time, the combination of a weakening economy and political 

dynamics trumped the CAT’s findings, and resulted in a decision not to implement its recommendations.

Washington state’s climate action team: uses and limits to decision support
635 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

26: DECISION SUPPORT: CONNECTING SCIENCE, RISK PERCEPTION, AND DECISIONS

it has been demonstrated that research motivated by “considerations of use” can also make fundamental advances in scientific understanding and theory.

134 The U.S. climate research 

effort has been strongly encouraged to improve integration of 

social and ecological sciences and to develop the capacity for 

decision support to help address the need to effectively incorporate advances in climate science into decision-making.135

Research to Improve Decision Support 

There are a number of areas where scientific knowledge needs 

to be expanded or tools further developed to take advantage of existing insight. The National Research Council (NRC) 

identifies a research agenda both for decision support (such 

as identifying specific information needs) and on decision support (such as improving tools for risk assessment and management).3

 A number of studies assess approaches and identify 

needed research and development (for example, Arvai et al. 

2006136). A subset of the opportunities and needs identified by 

the NRC seem particularly relevant in the context of the National Climate Assessment, including:

• A comprehensive analysis of the state of decision support for adaptation and mitigation, including assessment of processes, tools, and applications, and development of a knowledge-sharing platform will facilitate 

wide public access to these resources. 

• Comparisons of different adaptation and mitigation options will be improved by investments in understanding 

how the effects of climate change and response options 

can be valued and compared, especially for non-market 

ecosystem goods and services101,137 and those impacts 

and decisions that have an effect over long time scales.

• Improvements in risk management require closing the 

gap between expert and public understanding of risk 

and building the institutions and processes needed for 

managing persistent risks over the long term.

• Probabilistic forecasts or other information regarding 

consequential climate extremes/events have the potential to be very useful for decision-makers, if used with 

improving information on the consequences of climate 

change and appropriate decision support tools.

• Better methods for assessing and communicating scientific confidence and uncertainty in the context of specific decisions would be very useful in supporting risk 

management strategies.

• Improvements in processes that effectively link scientists with decision-makers and the public in resource 

management settings and developing criteria to evaluate their effectiveness would enhance knowledge building and understanding. 
636 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Carmichael, J. Robinson, and S. Cohen, 2011: Future visioning of 

local climate change: A framework for community engagement 

and planning with scenarios and visualisation. Futures, 43, 400-412, 

doi:10.1016/j.futures.2011.01.009. 

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Weeks, D., P. Malone, and L. Welling, 2011: Climate change 

scenario planning: A tool for managing parks into uncertain 

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117. Moore, S. S., N. E. Seavy, and M. Gerhart, 2013: Scenario Planning 

for Climate Change Adaptation. A Guidance for Resource 

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118. Alberti, M., M. Russo, and K. Tenneson, 2013: Snohomish Basin 

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Well-Being. Health Synthesis. Island Press, 53 pp. 

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id=11139]
646 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

sions about adaptation and ways to reduce future climate change, 

especially given uncertainties and advances in scientific understanding.

8,26

Well-designed decision support processes, especially those in which 

there is a good match between the availability of scientific information and the capacity to use it, can result in more effective outcomes 

based on relevant information that is perceived as useful and applicable.6

New information and remaining uncertainties

N/A

Assessment of confidence based on evidence and agreement 

or, if defensible, estimates of the likelihood of impact or consequence

N/A

Key message #2 Traceable Account

To be effective, decision support processes need 

to take account of the values and goals of the key 

stakeholders, evolving scientific information, and 

the perceptions of risk.

Description of evidence base

This message emphasizes that making a decision is more than picking 

the right tool and adopting its outcome. It is a process that should 

involve stakeholders, managers, and decision-makers to articulate 

and frame the decision, develop options, consider consequences 

(positive and negative), evaluate tradeoffs, make a decision, implement, evaluate, learn, and reassess.

1,8

 Oftentimes having an inclusive, 

transparent decision process increases buy-in, regardless of whether 

a particular stakeholder’s preferred option is chosen.3 Decisions 

about investment in adaptation and mitigation measures occur in the 

context of uncertainty and high political and economic stakes, complicating the evaluation of information and its application in decisionmaking.3,8

 Decisions involve both scientific information and values 

– for example, how much risk is acceptable and what priorities and 

preferences are addressed.2

New information and remaining uncertainties

 N/A

Assessment of confidence based on evidence 

N/A

Process for Developing Key Messages

During March-June 2012, the author team engaged in multiple technical discussions via teleconference (6 telecons) and email and in a 

day-long in-person meeting (April 27, 2012, in Washington, D.C.). Authors reviewed over 50 technical inputs provided by the public and a 

wide variety of technical and scholarly literature related to decision 

support, including reports from the National Research Council that 

provided recent syntheses of the field (America’s Climate Choices series, especially the reports Informing an Effective Response to Climate 

Change8 and Informing Decisions in a Changing Climate3). During the 

in-person meeting, authors reflected on the body of work informing 

the chapter and drafted a number of candidate critical messages that 

could be derived from the literature. Following the meeting, authors 

ranked these messages and engaged in expert deliberation via teleconference and email discussions in order to agree on a small number 

of key messages for the chapter.

Key message #1 Traceable Account

Decisions about how to address climate change 

can be complex, and responses will require a combination of adaptation and mitigation actions. Decision-makers – whether individuals, public officials, 

or others – may need help integrating scientific information into adaptation and mitigation decisions.

Description of evidence base

The sensitivity of the climate system to human activities, the extent 

to which mitigation policies are implemented, and the effects of other demographic, social, ecological, and economic changes on vulnerability also contribute to uncertainty in decision-making. 

Uncertainties can make decision-making in the context of climate 

change especially challenging for several reasons, including the rapid 

pace of changes in physical and human systems, the lags between 

climate change and observed effects, the high economic and political 

stakes, the number and diversity of potentially affected stakeholders, 

the need to incorporate scientific information of varying confidence 

levels, and the values of stakeholders and decision-makers.2,3

An iterative decision process that incorporates constantly improving 

scientific information and learning through periodic reviews of decisions over time is helpful in the context of rapid changes in environmental conditions.3,4

 The National Research Council has concluded 

that an “iterative adaptive risk management” framework, in which 

decisions are adjusted over time to reflect new scientific information 

and decision-makers learn from experience, is appropriate for deci26: DECISION SUPPORT

SUPPLEMENTAL MATERIAL

TRACEABLE ACCOUNTS
26: DECISION SUPPORT: CONNECTING SCIENCE, RISK PERCEPTION, AND DECISIONS

Traceable Accounts

647 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Key message #3 Traceable Account

Many decision support processes and tools are 

available. They can enable decision-makers to 

identify and assess response options, apply complex and uncertain information, clarify tradeoffs, 

strengthen transparency, and generate information 

on the costs and benefits of different choices.

Description of evidence base

Many decision support tools have been developed to support adaptive management in specific sectors or for specific issues. These tools 

include: risk assessments; geographic information system (GIS)-based 

analysis products; targeted projections for high-consequence events 

such as fires, floods, or droughts; vulnerability assessments; integrated assessment models; decision calendars; scenarios and scenario 

planning; and others.3,8,84 Many of these tools have been validated 

scientifically and evaluated from the perspective of users. They are 

described in the sector and regional chapters of this assessment. In 

addition, a variety of clearing houses and data management systems 

provide access to decision support information and tools (for example, CAKE 2012; NatureServe 201239,85).

There are many tools, some of which we discuss in the chapter, that 

are currently being used to make decisions that include a consideration of climate change and variability, or the impacts or vulnerabilities that would result from such changes.

Also important is the creation of a well-structured and transparent 

decision process that involves affected parties in problem framing, 

establishing decision criteria, fact finding, deliberation, and reaching 

conclusions.1,8,26 These aspects of decision-making are often overlooked by those who focus more on scientific inputs and tools, but 

given the high stakes and remaining uncertainties, they are crucial for 

effective decision-making on adaptation and mitigation.

New information and remaining uncertainties

 N/A

Assessment of confidence based on evidence 

N/A

Key message #4 Traceable Account

Ongoing assessment processes should incorporate evaluation of decision support tools, their 

accessibility to decision-makers, and their application in decision processes in different sectors and 

regions.

Description of evidence base

As part of a sustained assessment, it is critical to understand the state 

of decision support, including what is done well and where we need 

to improve. At this point in time, there is a lack of literature that provides a robust evidence base to allow us to conduct this type of national, sector-scale assessment. Developing an evidence base would 

allow for a movement from case studies to larger-scale assessment 

across decision support and would allow us to better understand how 

to better utilize what decision support is available and understand 

what needs to be improved to support adaptation and mitigation decisions in different sectors and regions.

New information and remaining uncertainties

 N/A

Assessment of confidence based on evidence 

N/A

Key message #5 Traceable Account

Steps to improve collaborative decision processes include developing new decision support tools 

and building human capacity to bridge science and 

decision-making.

Description of evidence base

There are many challenges in communicating complex scientific information to decision makers and the public,11 and while “translation” of 

complex information is one issue, there are many others. Defining the 

scope and scale of the relevant climate change problem can raise both 

scientific and social questions. These questions require both scientific 

insights and consideration of values and social constructs, and require 

that participants engage in mutual learning and the co-production of 

relevant knowledge.10 Boundary processes that are collaborative and 

iterative18 among scientists, stakeholders, and decision-makers, such 

as joint fact finding and collaborative adaptive management, foster 

ongoing dialogue and increasing participants’ understanding of policy 

problems and information and analysis necessary to evaluate decision options.12,13 Analysis of the conditions that contribute to their 

effectiveness of boundary processes is an emerging area of study.13

A large body of literature notes that the ability of decision-makers 

to use data and tools has not kept pace with the rate at which new 

tools are developed, pointing to a need for “science translators” who 

can help decision-makers efficiently access and properly use data and 

tools that would be helpful in making more informed decisions in the 

context of climate change.3,4,8,83,133 The U.S. climate research effort 

has been strongly encouraged to improve integration of social and 

ecological sciences and to develop the capacity for decision support 

to help address the need to effectively incorporate advances in climate science into decision-making.135

New information and remaining uncertainties

 N/A

Assessment of confidence based on evidence 

N/A
648

Climate Change Impacts in the United States

CHAPTER 27

MITIGATION

INFORMATION DRAWN FROM THIS CHAPTER IS INCLUDED IN THE HIGHLIGHTS REPORT AND IS IDENTIFIED BY THIS ICON

Recommended Citation for Chapter

Jacoby, H. D., A. C. Janetos, R. Birdsey, J. Buizer, K. Calvin, F. de la Chesnaye, D. Schimel, I. Sue Wing, R. Detchon, J. 

Edmonds, L. Russell, and J. West, 2014: Ch. 27: Mitigation. Climate Change Impacts in the United States: The Third National 

Climate Assessment, J. M. Melillo, Terese (T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 

648-669. doi:10.7930/J0C8276J.

On the Web: http://nca2014.globalchange.gov/report/response-strategies/mitigation

Convening Lead Authors

Henry D. Jacoby, Massachusetts Institute of Technology

Anthony C. Janetos, Boston University

Lead Authors

Richard Birdsey, U.S. Forest Service

James Buizer, University of Arizona

Katherine Calvin, Pacific Northwest National Laboratory, University of Maryland

Francisco de la Chesnaye, Electric Power Research Institute

David Schimel, NASA Jet Propulsion Laboratory

Ian Sue Wing, Boston University

Contributing Authors

Reid Detchon, United Nations Foundation

Jae Edmonds, Pacific Northwest National Laboratory, University of Maryland

Lynn Russell, Scripps Institution of Oceanography, University of California, San Diego

Jason West, University of North Carolina
649 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

27MITIGATION

Mitigation refers to actions that reduce the human contribution to the planetary greenhouse effect. Mitigation actions 

include lowering emissions of greenhouse gases like carbon dioxide and methane, and particles like black carbon (soot) that 

have a warming effect. Increasing the net uptake of carbon 

dioxide through land-use change and forestry can make a contribution as well. As a whole, human activities result in higher 

global concentrations of greenhouse gases and to a warming 

of the planet – and the effect is increased by various self-reinforcing cycles in the Earth system (such as the way melting 

sea ice results in more dark ocean water, which absorbs more 

heat, and leads to more sea ice loss). Also, the absorption of 

increased carbon dioxide by the oceans is leading to increased 

ocean acidity with adverse effects on marine ecosystems. 

Four mitigation-related topics are assessed in this chapter. 

First, it presents an overview of greenhouse gas emissions and 

their climate influence to provide a context for discussion of 

mitigation efforts. Second, the chapter provides a survey of 

activities contributing to U.S. emissions of carbon dioxide and 

other greenhouse gases. Third, it provides a summary of current government and voluntary efforts to manage these emissions. Finally, there is an assessment of the adequacy of these 

efforts relative to the magnitude of the climate change threat 

and a discussion of preparation for potential future action. 

Key Messages

1. Carbon dioxide is removed from the atmosphere by natural processes at a rate that is roughly 

half of the current rate of emissions from human activities. Therefore, mitigation efforts that 

only stabilize global emissions will not reduce atmospheric concentrations of carbon dioxide, 

but will only limit their rate of increase. The same is true for other long-lived greenhouse 

gases.

2. To meet the lower emissions scenario (B1) used in this assessment, global mitigation actions 

would need to limit global carbon dioxide emissions to a peak of around 44 billion tons per year 

within the next 25 years and decline thereafter. In 2011, global emissions were around 34 billion 

tons, and have been rising by about 0.9 billion tons per year for the past decade. Therefore, the 

world is on a path to exceed 44 billion tons per year within a decade.

3. Over recent decades, the U.S. economy has emitted a decreasing amount of carbon dioxide per

dollar of gross domestic product. Between 2008 and 2012, there was also a decline in the total

amount of carbon dioxide emitted annually from energy use in the United States as a result of 

a variety of factors, including changes in the economy, the development of new energy 

production technologies, and various government policies. 

4. Carbon storage in land ecosystems, especially forests, has offset around 17% of annual U.S. 

fossil fuel emissions of greenhouse gases over the past several decades, but this carbon “sink” 

may not be sustainable. 

5. Both voluntary activities and a variety of policies and measures that lower emissions are 

currently in place at federal, state, and local levels in the United States, even though there is 

no comprehensive national climate legislation. Over the remainder of this century, aggressive 

and sustained greenhouse gas emission reductions by the United States and by other nations 

would be needed to reduce global emissions to a level consistent with the lower scenario (B1) 

analyzed in this assessment. 
650 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

27: MITIGATION

While the chapter presents a brief overview of mitigation issues, it does not provide a comprehensive discussion of policy 

options, nor does it attempt to review or analyze the range of 

technologies available to reduce emissions.

These topics have also been the subject of other assessments, 

including those by the National Academy of Sciences1 and the 

U.S. Department of Energy.2 Mitigation topics are addressed 

throughout this report (see Ch. 4: Energy, Key Message 5; Ch. 

5: Transportation, Key Message 4; Ch. 7: Forests, Key Message 

4; Ch. 9: Human Health, Key Message 4; Ch. 10: Energy, Water, 

and Land, Key Messages 1, 2, 3; Ch. 13: Land Use & Land Cover 

Change, Key Messages 2, 4; Ch. 15: Biogeochemical Cycles, Key 

Message 3; Ch. 26: Decision Support, Key Messages 1, 2, 3; Appendix 3: Climate Science Supplemental Message 5; Appendix 

4: FAQs N, S, X, Y, Z).

Emissions, Concentrations, and Climate Forcing

Setting mitigation objectives requires knowledge of the Earth 

system processes that determine the relationship among 

emissions, atmospheric concentrations and, ultimately, climate. Human-caused climate change results mainly from the 

increasing atmospheric concentrations of greenhouse gases.3

These gases cause radiative “forcing” – an imbalance of heat 

trapped by the atmosphere compared to an equilibrium state. 

Atmospheric concentrations of greenhouse gases are the result of the history of emissions and of processes 

that remove them from the atmosphere; for example, by “sinks” like growing forests.4

 The fraction of 

emissions that remains in the atmosphere, which is 

different for each greenhouse gas, also varies over 

time as a result of Earth system processes.

The impact of greenhouse gases depends partly 

on how long each one persists in the atmosphere.5

Reactive gases like methane and nitrous oxide are 

destroyed chemically in the atmosphere, so the 

relationships between emissions and atmospheric 

concentrations are determined by the rate of those 

reactions. The term “lifetime” is often used to describe the speed with which a given gas is removed 

from the atmosphere. Methane has a relatively 

short lifetime (largely removed within a decade or 

so, depending on conditions), so reductions in emissions can lead to a fairly rapid decrease in concentrations as the gas is oxidized in the atmosphere.6

Nitrous oxide has a much longer lifetime, taking 

more than 100 years to be substantially removed.7

Other gases in this category include industrial gases, 

like those used as solvents and in air conditioning, 

some of which persist in the atmosphere for hundreds or thousands of years.

Carbon dioxide (CO2) does not react chemically 

with other gases in the atmosphere, so it does not, 

strictly speaking, have a “lifetime.”8 Instead, the relationship between emissions and concentrations 

from year to year is determined by patterns of release (for example, through burning of fossil fuels) 

and uptake (for example, by vegetation and by the 

ocean).9 Once CO2 is emitted from any source, a 

portion of it is removed from the atmosphere over 

time by plant growth and absorption by the oceans, 

after which it continues to circulate in the land-atmosphereocean system until it is finally converted into stable forms in 

soils, deep ocean sediments, or other geological repositories 

(Figure 27.1). 

Of the carbon dioxide emitted from human activities in a year, 

about half is removed from the atmosphere by natural processes within a century, but around 20% continues to circuHuman Activities and the Global Carbon 

Dioxide Budget

Figure 27.1. Figure shows human-induced changes in the global carbon 

dioxide budget roughly since the beginning of the Industrial Revolution. 

Emissions from fossil fuel burning are the dominant cause of the steep rise 

shown here from 1850 to 2012. (Global Carbon Project 2010, 201210).
651 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

27: MITIGATION

late and to affect atmospheric concentrations for thousands 

of years.11 Stabilizing or reducing atmospheric carbon dioxide

concentrations, therefore, requires very deep reductions in future emissions – ultimately approaching zero – to compensate 

for past emissions that are still circulating in the Earth system. 

Avoiding future emissions, or capturing and storing them in 

stable geological storage, would prevent carbon dioxide from 

entering the atmosphere, and would have very long-lasting effects on atmospheric concentrations.

In addition to greenhouse gases, there can be climate effects 

from fine particles in the atmosphere. An example is black carbon (soot), which is released from coal burning, diesel engines, 

cooking fires, wood stoves, wildfires, and other combustion 

sources. These particles have a warming influence, especially 

when they absorb solar energy low in the atmosphere.12 Other 

particles, such as those formed from sulfur dioxide released 

during coal burning, have a cooling effect by reflecting some 

of the sun’s energy back to space or by increasing the brightness of clouds (see: Ch. 2: Our Changing Climate; Appendix 3: 

Climate Science Supplement; and Appendix 4: FAQs). 

The effect of each gas is related to both how long it lasts in the 

atmosphere (the longer it lasts, the greater its influence) and 

its potency in trapping heat. The warming influence of different gases can be compared using “global warming potentials” 

(GWP), which combine these two effects, usually added up 

over a 100-year time period. Global warming potentials are 

referenced to carbon dioxide – which is defined as having a 

GWP of 1.0 – and the combined effect of multiple gases is denoted in carbon dioxide equivalents, or CO2-e.

The relationship between emissions and concentrations of 

gases can be modeled using Earth System Models.4 Such models apply our understanding of biogeochemical processes that 

remove greenhouse gas from the atmosphere to predict their 

future concentrations. These models show that stabilizing CO2

emissions would not stabilize its atmospheric concentrations 

but instead result in a concentration that would increase at a 

relatively steady rate. Stabilizing atmospheric concentrations

of CO2 would require reducing emissions far below presentday levels. Concentration and emissions scenarios, such as the 

recently developed Representative Concentration Pathways 

(RCPs) and scenarios developed earlier by the Intergovernmental Panel on Climate Change’s (IPCC) Special Report on 

Emissions Scenarios (SRES), are used in Earth System Models 

to study potential future climates. The RCPs span a range of 

atmospheric targets for use by climate modelers,13,14 as do the 

SRES cases. These global analyses form a framework within 

which the climate contribution of U.S. mitigation efforts can be 

assessed. In this report, special attention is given to the SRES 

A2 scenario (similar to RCP 8.5), which assumes continued increases in emissions, and the SRES B1 scenario (close to RCP 

4.5), which assumes a substantial reduction of emissions (Ch. 

2: Our Changing Climate; Appendix 5: Scenarios and Models).

Section 1: U.S. Emissions and Land-Use Change

Industrial, Commercial, and Household Emissions

U.S. greenhouse gas emissions, not accounting for uptake by 

land use and agriculture (see Figure 27.3), rose to as high as 

7,260 million tons CO2-e in 2007, and then fell by about 9% 

between 2008 and 2012.19 Several factors contributed to the 

decline, including the reduction in energy use in response to 

the 2008-2010 recession, the displacement of coal in electric 

generation by lower-priced natural gas, and the effect of federal and state energy and environmental policies.20

Geoengineering

Geoengineering has been proposed as a third option for addressing climate change in addition to, or alongside, 

mitigation and adaptation. Geoengineering refers to intentional modifications of the Earth system as a means to address climate change. Three types of activities have been proposed: 1) carbon dioxide removal (CDR), which boosts 

CO2 removal from the atmosphere by various means, such as fertilizing ocean processes and promoting land-use 

practices that help take up carbon, 2) solar radiation management (SRM), which reflects a small percentage of 

sunlight back into space to offset warming from greenhouse gases,15 and 3) direct capture and storage of CO2 from 

the atmosphere.16

Current research suggests that SRM or CDR could diminish the impacts of climate change. However, once undertaken, sudden cessation of SRM would exacerbate the climate effects on human populations and ecosystems, and 

some CDR might interfere with oceanic and terrestrial ecosystem processes.17 SRM undertaken by itself would not 

slow increases in atmospheric CO2 concentrations, and would therefore also fail to address ocean acidification. 

Furthermore, existing international institutions are not adequate to manage such global interventions. The risks associated with such purposeful perturbations to the Earth system are thus poorly understood, suggesting the need for 

caution and comprehensive research, including consideration of the implicit moral hazards.18
652 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

27: MITIGATION

Carbon dioxide made up 84% of U.S. greenhouse gas emissions 

in 2011. Forty-one percent of these emissions were attributable to liquid fuels (petroleum), followed closely by solid fuels 

(principally coal in electric generation), and to a lesser extent 

by natural gas.20 The two dominant production sectors responsible for these emissions are electric power generation (coal 

and gas) and transportation (petroleum). Flaring and cement 

manufacture together account for less than 1% of the total. If 

emissions from electric generation are allocated to their various end-uses, transportation is the largest CO2 source, contributing a bit over one-third of the total, followed by industry at 

slightly over a quarter, and residential use and the commercial 

sector at around one-fifth each.

A useful picture of historical patterns of carbon dioxide emissions can be constructed by decomposing the cumulative 

change in emissions from a base year into the contributions of 

five driving forces: 1) decline in the CO2 content of energy use, 

as with a shift from coal to natural gas in electric generation, 2) 

reduction in energy intensity – the energy needed to produce 

each unit of gross domestic product (GDP) – which results from 

substitution responses to energy prices, changes in the composition of the capital stock, and both autonomous and priceinduced technological change, 3) changes in the structure of 

the economy, such as a decline in energy-intensive industries 

and an increase in services that use less energy, 4) growth in 

per capita GDP, and 5) rising population. 

Over the period 1963-2008, annual U.S. carbon dioxide emissions slightly more than doubled, because growth in emissions 

potential attributable to increases in population and GDP per 

person outweighed reductions contributed by lowered energy 

and carbon intensity and changes in economic structure (Figure 27.2). Each series in the figure illustrates the quantity of 

cumulative emissions since 1963 that would have been generated by the effect of the associated driver. By 2008, fossil fuel 

burning had increased CO2 emissions by 2.7 billion tons over 

1963 levels. However, by itself the observed decline in energy 

would have reduced emissions by 1.8 billion tons, while the 

observed increase in per capita GDP would have increased 

emissions by more than 5 billion tons.

After decades of increases, CO2 emissions from energy use 

(which account for 97% of total U.S. emissions) declined by 

around 9% between 2008 and 2012, largely due to a shift 

from coal to less CO2-intensive natural gas for electricity 

production.19 Trends in driving forces shown in Figure 

27.2 are expected to continue in the future, though their 

relative contributions are subject to significant uncertainty. The reference case projection by the U.S. Energy 

Information Administration (EIA) shows their net effect 

being a slower rate of CO2 emissions growth than in the 

past, with roughly constant energy sector emissions to 

2040.22 It must be recognized, however, that emissions 

from energy use rise and fall from year to year, as the 

aforementioned driving forces vary.

The primary non-CO2 gas emissions in 2011 were methane (9% of total CO2-e emissions), nitrous oxide (5%), 

and a set of industrial gases (2%). U.S. emissions of each 

of these gases have been roughly constant over the past 

half-dozen years.22 Emissions of methane and nitrous oxide have been roughly constant over the past couple of 

decades, but there has been an increase in the industrial 

gases as some are substituted for ozone-destroying substances controlled by the Montreal Protocol.23

Yet another warming influence on the climate system 

is black carbon (soot), which consists of fine particles 

that result mainly from incomplete combustion of fossil 

fuels and biomass. Long a public health concern, black 

carbon particles absorb solar radiation during their short 

life in the atmosphere (days to weeks). When deposited 

on snow and ice, these particles darken the surface and 

reduce the reflection of incoming solar radiation back to 

space. These particles also influence cloud formation in 

ways yet poorly quantified.24

Figure 27.2. This graph depicts the changes in carbon dioxide 

(CO2) emissions over time as a function of five driving forces: 1) the 

amount of CO2 produced per unit of energy (CO2 intensity); 2) the 

amount of energy used per unit of gross domestic product (energy 

intensity); 3) structural changes in the economy; 4) per capita income; 

and 5) population. Although CO2 intensity and especially energy 

intensity have decreased significantly and the structure of the 

U.S. economy has changed, total CO2 emissions have continued 

to rise as a result of the growth in both population and per capita 

income. (Baldwin and Sue Wing, 201321).

Drivers of U.S. Fossil Emissions
653 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

27: MITIGATION

Land Use, Forestry, and Agriculture

The main stocks of carbon in its various 

biological forms (plants and trees, dead 

wood, litter, soil, and harvested products) 

are estimated periodically and their rate of 

change, or flux, is calculated as the average 

annual difference between two time periods. Estimates of carbon stocks and fluxes 

for U.S. lands are based on land inventories augmented with data from ecosystem 

studies and production reports.25,26

U.S. lands were estimated to be a net sink 

of between approximately 640 and 1,074 

million tons CO2-e in the late 2000s.26,27

Estimates vary depending on choice of 

datasets, models, and methodologies (see 

Ch. 15: Biogeochemical Cycles, “Estimating the U.S. Carbon Sink,” for more discussion). This net land sink effect is the result 

of sources (from crop production, livestock 

production, and grasslands) and sinks (in 

forests, urban trees, and wetlands). Sources of carbon have been relatively stable over the last two decades, but sinks have been more variable. Long-term trends 

suggest significant emissions from forest clearing in the early 

1900s followed by a sustained period of net uptake from forest regrowth over the last 70 years.28 The amount of carbon 

taken up by U.S. land sinks is dominated by forests, which have 

annually absorbed 7% to 24% (with a best estimate of about 

16%) of fossil fuel CO2 emissions in the U.S. over the past two 

decades.20

The persistence of the land sink depends on the relative effects of several interacting factors: recovery from historical 

land-use change, atmospheric CO2 and nitrogen deposition, 

natural disturbances, and the effects of climate variability 

and change – particularly drought, wildfires, and changes in 

the length of the growing season. Deforestation continues to 

cause an annual loss of 877,000 acres (137,000 square miles) 

of forested land, offset by a larger area gain of new forest of 

about 1.71 million acres (268,000 square miles) annually.29

Since most of the new forest is on relatively low-productivity 

lands of the Intermountain West, and much of the deforestation occurs on high-productivity lands in the East, recent landuse changes have decreased the potential for future carbon 

storage.30 The positive effects of increasing carbon dioxide 

concentration and nitrogen deposition on carbon storage are 

not likely to be as large as the negative effects of land-use 

change and disturbances.31 In some regions, longer growing 

seasons associated with climate change may increase annual 

productivity.32 Droughts and other disturbances, such as fire 

and insect infestations, have already turned some U.S. land regions from carbon sinks into carbon sources (see Ch. 13: Land 

Use & Land Cover Change and Ch. 15: Biogeochemical Cycles).31

The current land sink may not be sustainable for more than a 

few more decades,33 though there is a lack of consistency in 

published results about the relative effects of disturbance and 

other factors on net land-use emissions.31,34

Section 2: Activities Affecting Emissions

Early and large reductions in global emissions would be necessary to achieve the lower emissions scenarios (such as the 

lower B1 scenario; see Ch. 2: Our Changing Climate) analyzed 

in this assessment. The principal types of national actions that 

could effect such changes include putting a price on emissions, 

setting regulations and standards for activities that cause 

emissions, changing subsidy programs, and direct federal expenditures. Market-based approaches include cap and trade 

programs that establish markets for trading emissions permits, 

analogous to the Clean Air Act provisions for sulfur dioxide reductions. None of these price-based measures has been implemented at the national level in the United States, though cap 

and trade systems are in place in California and in the Northeast’s Regional Greenhouse Gas Initiative. Moreover, a wide 

range of governmental actions are underway at federal, state, 

regional, and city levels using other measures, and voluntary 

efforts, that can reduce the U.S. contribution to total global 

emissions. Many, if not most of these programs are motivated 

by other policy objectives – energy, transportation, and air pollution – but some are directed specifically at greenhouse gas 

emissions, including: 

• reduction in CO2 emissions from energy end-use and 

infrastructure through the adoption of energy-efficient 

Figure 27.3 Graph shows annual average greenhouse gas emissions from land use 

including livestock and crop production, but does not include fossil fuels used in 

agricultural production. Forests are a significant “sink” that absorbs carbon dioxide 

from the atmosphere. All values shown are for 2008, except wetlands, which are 

shown for 2003. (Pacala et al. 2007;27 USDA 201126).

Sources and Sinks in U.S. Agriculture and Forests
654 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

27: MITIGATION

components and systems – 

including buildings, vehicles, 

manufacturing processes, 

appliances, and electric grid 

systems;

• reduction of CO2 emissions 

from energy supply through 

the promotion of renewables 

(such as wind, solar, and bioenergy), nuclear energy, and 

coal and natural gas electric 

generation with carbon capture and storage; and

• reduction of emissions of 

non-CO2 greenhouse gases 

and black carbon; for example, by lowering methane emissions from energy 

and waste, transitioning to 

climate-friendly alternatives to hydrofluorocarbons 

(HFCs), cutting methane and 

nitrous oxide emissions from 

agriculture, and improving 

combustion efficiency and 

means of particulate capture.

Federal Actions

The Federal Government has implemented a number of measures that promote energy efficiency, clean technologies, and 

alternative fuels.35 A sample of these actions is provided in 

Table 27.1 and they include greenhouse gas regulations, other 

rules and regulations with climate co-benefits, various standards and subsidies, research and development, and federal 

procurement practices. 

The U.S. Environmental Protection Agency (EPA) has a 40-

year history of regulating the concentration and deposition of 

criteria pollutants (six common air pollutants that affect human health). A 2012 Supreme Court decision upheld the EPA’s 

finding that greenhouse gases “endanger public health and 

welfare.”36 This ruling added the regulation of greenhouse gas 

emissions to the Agency’s authority under the Clean Air Act. 

Actions taken and proposed under the new authority have focused on road transport and electric power generation. 

The U.S. Department of Energy (DOE) provides most of the 

funding for a broad range of programs for energy research, 

Programs underway that reduce carbon dioxide emissions include the promotion of solar, nuclear, 

and wind power and efficient vehicles © Don Mason/Blend Images/Corbis

© Joseph Sohm/Visions of America/Corbis

 © Layne Kennedy/Corbis

© Carlos Osorio/AP/Corbis

© REBECCA COOK/Reuters/Corbis

© Jerome Levitch/CorbisFlickr Solar Panels Department of Energy
655 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

27: MITIGATION

development, and demonstration. DOE also has the authority 

to regulate the efficiency of appliances and building codes for 

manufactured housing. In addition, most of the other federal 

agencies – including the Departments of Defense, Housing and 

Urban Development, Transportation, and Agriculture – have 

programs related to greenhouse gas mitigation. 

The Administration’s Climate Action Plan37 builds on these activities with a broad range of mitigation, adaptation, and preparedness measures. The mitigation elements of the plan are 

in part a response to the commitment made during the 2010 

Cancun Conference of the Parties of the United Nations Framework Convention on Climate Change to reduce U.S. emissions 

of greenhouse gases by 17% below 2005 levels by 2020. Actions proposed in the Plan include: 1) limiting carbon emissions 

from both new and existing power plants, 2) continuing to 

increase the stringency of fuel economy standards for automobiles and trucks, 3) continuing to improve energy efficiency 

in the buildings sector, 4) reducing the emissions of non-CO2

greenhouse gases through a variety of measures, 5) increasing 

federal investments in cleaner, more efficient energy sources 

for both power and transportation, and 6) identifying new approaches to protect and restore our forests and other critical 

landscapes, in the presence of a changing climate. 

City, State, and Regional Actions

Jurisdiction for greenhouse gases and energy policies is shared 

between the federal government and the states.1 For example, 

states regulate the distribution of electricity and natural gas to 

consumers, while the Federal Energy Regulatory Commission 

regulates wholesale sales and transportation of natural gas 

and electricity. In addition, many states have adopted climate 

initiatives as well as energy policies that reduce greenhouse 

gas emissions. For a survey of many of these state activities, 

see Table 27.2. Many cities are taking similar actions. 

The most ambitious state activity is California’s Global Warming Solutions Act (AB 32), a law that sets a state goal to reduce 

greenhouse gas emissions to 1990 levels by 2020. The state 

program caps emissions and uses a market-based system of 

trading in emissions credits (cap and trade), as well as a number of regulatory actions. The most well-known, multi-state 

effort has been the Regional Greenhouse Gas Initiative (RGGI), 

formed by ten northeastern and Mid-Atlantic states (though 

New Jersey exited in 2011). RGGI is a cap and trade system 

applied to the power sector with revenue from allowance 

auctions directed to investments in efficiency and renewable 

energy. 

Voluntary Actions 

Corporations, individuals, and non-profit organizations have 

initiated a host of voluntary actions. The following examples 

give the flavor of the range of efforts:

• The Carbon Disclosure Project has the largest global collection of self-reported climate change and water-use 

information. The system enables companies to measure, 

disclose, manage, and share climate change and wateruse information. Some 650 U.S. signatories include banks, 

pension funds, asset managers, insurance companies, and 

foundations.

• Many local governments are undertaking initiatives to reduce greenhouse gas emissions within and outside of their 

organizational boundaries.38 For example, over 1,055 municipalities from all 50 states have signed the U.S. Mayors 

Climate Protection Agreement,39 and many of these communities are actively implementing strategies to reduce 

their greenhouse gas footprint.

• Under the American College and University Presidents’ 

Climate Commitment (ACUPCC), 679 institutions have 

pledged to develop plans to achieve net-neutral climate 

emissions through a combination of on-campus changes 

and purchases of emissions reductions elsewhere.

• Voluntary compliance with efficiency standards developed by industry and professional associations, such as 

the building codes of the American Society of Heating, 

Refrigerating and Air-Conditioning Engineers (ASHRAE), is 

widespread.

 © Denise Applewhite© Najlah Feanny/Corbis© Proehl Studios/Corbis
656 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

27: MITIGATION

• Federal voluntary programs include Energy STAR, a labeling program that identifies energy efficient products for 

use in residential homes and commercial buildings and 

plants, and programs and partnerships devoted to reducing methane emissions from fossil fuel production and 

landfill sources and high GWP emissions from industrial 

activities and agricultural conservation programs.

Costs of Emissions Reductions

The national cost of achieving U.S. emissions reductions over 

time depends on the level of reduction sought and the particular measures employed. Studies of price-based policies, 

such as a cap and trade system, indicate that a 50% reduction 

in emissions by 2050 could be achieved at a cost of a year or 

two of projected growth in gross domestic product over the 

period (for example, Paltsev et al. 2009; EIA 200940). However, 

because of differences in analysis method, and in assumptions 

about economic growth and technology change, cost projections vary considerably even for a policy applying price penalties.41 Comparisons of emissions reduction by prices versus 

regulations show that a regulatory approach can cost substantially more than a price-based policy.42

Section 3: Preparation for Potential Future Mitigation Action

To meet the emissions reduction in the lower (B1) scenario 

used in this assessment (Ch. 2: Our Changing Climate) under 

reasonable assumptions about managing costs, annual global 

CO2 emissions would need to peak at around 44 billion tons 

within the next 25 years or so and decline steadily for the rest 

of the century. At the current rate of emissions growth, the 

world is on a path to exceed the 44 billion ton level within a decade (see “Emissions Scenarios and RCPs”). Thus achievement 

of a global emissions path consistent with the B1 scenario will 

require strenuous action by all major emitters. 

Policies already enacted and other factors lowered U.S. emissions in recent years. The Annual Energy Outlook prepared by 

the EIA, which previously forecasted sustained growth in emissions, projected in 2013 that energy-related U.S. CO2 emissions would remain roughly constant for the next 25 years.22

Co-benefits for air pollution and human health

Actions to reduce greenhouse gas emissions can yield co-benefits for objectives apart from climate change, such 

as energy security, health, ecosystem services, and biodiversity.43,44 The co-benefits for reductions in air pollution 

have received particular attention. Because air pollutants and greenhouse gases share common sources, particularly 

from fossil fuel combustion, actions to reduce greenhouse gas emissions also reduce air pollutants. While some 

greenhouse gas reduction measures might increase other emissions, broad programs to reduce greenhouse gases 

across an economy or a sector can reduce air pollutants markedly.14,45 (Unfortunately for climate 

mitigation, cutting sulfur dioxide pollution from 

coal burning also reduces the cooling influence of 

reflective particles formed from these emissions in 

the atmosphere.46)

There is significant interest in quantifying the air 

pollution and human health co-benefits of greenhouse gas mitigation, particularly from the public 

health community,44,47 as the human health benefits can be immediate and local, in contrast to 

the long-term and widespread effects of climate 

change.48 Many studies have found that monetized 

health and pollution control benefits can be of 

similar magnitude to abatement costs (for example, Nemet et al. 2010; Burtraw et al. 200348,49). 

Methane reductions have also been shown to generate health benefits from reduced ozone.50 Similarly, in developing nations, reducing black carbon from household 

cook stoves substantially reduces air pollution-related illness and death.51 Ancillary health benefits in developing 

countries typically exceed those in developed countries for a variety of reasons.48 But only in very few cases are these 

ancillary benefits considered in analyses of climate mitigation policies.

© iStockPhoto.com_Daniel Stein
657 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

27: MITIGATION

Moreover, through the President’s Climate Action Plan, the 

Administration has committed to additional measures not yet 

reflected in the EIA’s projections, with the goal of reducing 

emissions about 17% below 2005 levels by 2020. Still, additional and stronger U.S. action, as well as strong action by other 

major emitters, will be needed to meet the long-term global 

emission reductions reflected in the B1 scenario. 

Achieving the B1 emissions path would require substantial decarbonization of the global economy by the end of this century, 

implying a fundamental transformation of the global energy 

system. Details of the energy mix along the way differ among 

analyses, but the implied involvement by the U.S. can be seen 

in studies carried out under the U.S. Climate Change Science 

Program54 and the Energy Modeling Forum.55,56 In these studies, direct burning of coal without carbon capture is essentially 

excluded from the power system, and the same holds for natural gas toward the end of the century – to be replaced by some 

combination of coal or gas with carbon capture and storage, 

nuclear generation, and renewables. Biofuels and electricity 

are projected to substitute for oil in the transport sector. A substantial component of the task is accomplished with demand 

reduction, through efficiency improvement, conservation, and 

shifting to an economy less dependent on energy services.

The challenge is great enough even starting today, but delay by 

any of the major emitters makes meeting any such target even 

more difficult and may rule out some of the more ambitious 

goals.54,55 A study of the climate change threat and potential 

responses by the U.S. National Academies therefore concludes 

that there is “an urgent need for U.S. action to reduce greenhouse emissions.”57 The National Research Council (NRC) goes 

on to suggest alternative national-level strategies that might 

be followed, including an economy-wide system of prices on 

greenhouse gas emissions and a portfolio of possible regulatory measures and subsidies. Deciding these matters will be a 

continuing task, and U.S. Administrations and Congress face a 

long series of choices about whether to take additional mitigation actions and how best to do it. Two supporting activities will help guide this process: opening future technological 

options and development of ever-more-useful assessments of 

the cost effectiveness and benefits of policy choices.

Many technologies are potentially available to accomplish 

emissions reduction. They include ways to increase the efficiency of fossil energy use and facilitate a shift to low-carbon 

energy sources, sources of improvement in the cost and performance of renewables (for example, wind, solar, and bioenergy) and nuclear energy, ways to reduce the cost of carbon 

capture and storage, means to expand terrestrial sinks through 

management of forests and soils and increased agricultural 

productivity,2 and phasing down HFCs. In addition to the research and development carried out by private sector firms 

with their own funds, the Federal Government traditionally 

supports major programs to advance these technologies. This 

support is accomplished in part by credits and deductions in 

the tax code, and in part by federal expenditure. For example, 

the 2012 federal budget devoted approximately $6 billion to 

clean energy technologies.58 Success in these ventures, lowering the cost of greenhouse gas reduction, can make a crucial 

contribution to future policy choices.1

Because they are in various stages of market maturity, the 

costs and effectiveness of many of these technologies remain 

uncertain: continuing study of their performance is important 

to understanding their role in future mitigation decisions.59 In 

addition, evaluation of broad policies and particular mitigation 

measures requires frameworks that combine information from 

a range of disciplines. Study of mitigation in the near future 

can be done with energy-economic models that do not assume large changes in the mix of technologies or changes in 

the structure of the economy. Analysis over the time spans relevant to stabilization of greenhouse gas concentrations, however, requires Integrated Assessment Models, which consider 

all emissions drivers and policy measures that affect them, 

and that take account of how they are related to the larger 

economy and features of the climate system.54,55,60 This type 

of analysis is also useful for exploring the relations between 

mitigation and measures to adapt to a changing climate.

Continued development of these analytical capabilities can 

help support decisions about national mitigation and the U.S. 

position in international negotiations. In addition, as shown 

Emissions scenarios and RCPs

The Representative Concentration Pathways (RCPs) 

specify alternative limits to human influence on the 

Earth’s energy balance, stated in watts per square meter 

(W/m2) of the Earth’s surface.13,52 The A2 emissions scenario implies atmospheric concentrations with radiative 

forcing slightly lower than the highest RCP, which is 8.5 

W/m2. The lower limits, at 6.0, 4.5 and 2.6 W/m2, imply 

ever-greater mitigation efforts. The B1 scenario (rapid 

emissions reduction) is close to the 4.5 W/m2 RCP53 and 

to a similar case (Level 2) analyzed in a previous federal 

study.54 Those assessments find that, to limit the economic costs, annual global CO2 emissions from fossil 

fuels and industrial sources like cement manufacture, 

need to peak by 2035 to 2040 at around 44 billion 

tons of CO2, and decline thereafter. The scale of the 

task can be seen in the fact that these global emissions 

were already at 34 billion tons CO2 in 2011, and over 

the previous decade they rose at around 0.92 billion 

tons of CO2 per year.10 The lowest RCP would require 

an even more rapid turnaround and negative net emissions – that is, removing more CO2 from the air than is 

emitted globally – in this century.52
658 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

27: MITIGATION

above, mitigation is being undertaken by individuals and firms 

as well as by city, state, and regional governments. The capacity for mitigation from individual and household behavioral 

changes, such as increasing energy end-use efficiency with 

available technology, is known to be large.63 Although there 

is capacity, there is not always broad acceptance of those behavioral changes, nor is there sufficient understanding of how 

to design programs to encourage such changes.64 Behavioral 

and institutional research on how such choices are made and 

the results evaluated would be extremely beneficial. For many 

of these efforts, understanding of cost and effectiveness is 

limited, as is understanding of aspects of public support and 

institutional performance; so additional support for studies 

of these activities is needed to ensure that resources are efficiently employed. 

Section 4: Research Needs • Engineering and scientific research is needed on the development of cost-effective energy use technologies (devices, systems, and control strategies) and energy supply 

technologies that produce little or no CO2 or other greenhouse gases.

• Better understanding of the relationship between emissions and atmospheric greenhouse gas concentrations is 

needed to more accurately predict how the atmosphere 

and climate system will respond to mitigation measures.

• The processes controlling the land sink of carbon in the 

U.S. require additional research, including better monitoring and analysis of economic decision-making about the 

fate of land and how it is managed, as well as the inherent 

ecological processes and how they respond to the climate 

system.

• Uncertainties in model-based projections of greenhouse 

gas emissions and of the effectiveness and costs of policy 

measures need to be better quantified. Exploration is 

needed of the effects of different model structures, assumptions about model parameter values, and uncertainties in input data.

• Social and behavioral science research is needed to inform 

the design of mitigation measures for maximum participation and to prepare a consistent framework for assessing 

cost effectiveness and benefits of both voluntary mitigation efforts and regulatory and subsidy programs. 

Interactions between adaptation and mitigation

There are various ways in which mitigation efforts and adaptation measures are interdependent (see Ch. 28: Adaptation). For example, the use of plant material as a substitute for petroleum-based transportation fuels or directly as a substitute for burning coal or gas for electricity generation has received substantial attention.61 But land used for mitigation 

purposes is potentially not available for food production, even as the global demand for agricultural products continues 

to rise.62 Conversely, land required for adaptation strategies, like setting aside wildlife corridors or expanding the extent 

of conservation areas, is potentially not available for mitigation involving the use of plant material, or active management practices to enhance carbon storage in vegetation or soils. These possible interactions are poorly understood but 

potentially important, especially as climate change itself affects vegetation and ecosystem productivity and carbon storage. Increasing agricultural productivity to adapt to climate change can also serve to mitigate climate change.
659 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

27: MITIGATION

Table 27.1. A number of existing federal laws and regulations target ways to reduce future climate change by decreasing greenhouse gas emissions 

emitted by human activities.

Sample Federal Mitigation Measures

Greenhouse Gas Regulations

Emissions Standards for Vehicles and Engines

-- For light-duty vehicles, rules establishing standards for 2012-2016 model years and 2017-2025 model years.

-- For heavy- and medium-duty trucks, a rule establishing standards for 2014-2018 model years. 

Carbon Pollution Standard for New Power Plants

-- A proposed rule setting limits on CO2 emissions from future power plants. 

Stationary Source Permitting

-- A rule setting greenhouse gas emissions thresholds to define when permits under the New Source Review Prevention of 

Significant Deterioration and Title V Operating Permit programs are required for new and modified industrial facilities. 

Greenhouse Gas Reporting Program

-- A program requiring annual reporting of greenhouse gas data from large emission sources and suppliers of products that emit 

greenhouse gases when released or combusted. 

Other Rules and Regulations with Climate Co-Benefits

Oil and Natural Gas Air Pollution Standards

-- A rule revising New Source Performance Standards and National Emission Standards for Hazardous Air Pollutants for certain 

components of the oil and natural gas industry. 

Mobile Source Control Programs

-- Particle control regulations affecting mobile sources (especially diesel engines) that reduce black carbon by controlling direct 

particle emissions. 

-- The requirement to blend increasing volumes of renewable fuels.

National Forest Planning

-- Identification and evaluation of information relevant to a baseline assessment of carbon stocks.

-- Reporting of net carbon stock changes on forestland. 

Standards and Subsidies

Appliance and Building Efficiency Standards

-- Energy efficiency standards and test procedures for residential, commercial, industrial, lighting, and plumbing products.

-- Model residential and commercial building energy codes, and technical assistance to state and local governments, and nongovernmental organizations.

Financial Incentives for Efficiency and Alternative Fuels and Technology

-- Weatherization assistance for low-income households, tax incentives for commercial and residential buildings and efficient 

appliances, and support for state and local efficiency programs.

-- Tax credits for biodiesel and advanced biofuel production, alternative fuel infrastructure, and purchase of electric vehicles.

-- Loan guarantees for innovative energy or advanced technology vehicle production and manufacturing; investment and production 

tax credits for renewable energy.

Funding of Research, Development, Demonstration, and Deployment

-- Programs on clean fuels, energy end-use and infrastructure, CO2 capture and storage, and agricultural practices.

Federal Agency Practices and Procurement

-- Executive orders and federal statutes requiring federal agencies to reduce building energy and resource consumption intensity and 

to procure alternative fuel vehicles.

-- Agency-initiated programs in most departments oriented to lowering energy use and greenhouse gas emissions.
660 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

27: MITIGATION

Table 27.2. Most states and Native communities have implemented programs to reduce greenhouse gases or adopt increased 

energy efficiency goals.

State Climate and Energy Initiatives

Examples of greenhouse gas policies include:

Greenhouse Gas Reporting and Registries

http://www.c2es.org/us-states-regions/policy-maps/ghg-reporting65

Greenhouse Gas Emissions Targets

http://www.c2es.org/us-states-regions/policy-maps/emissions-targets66

CO2 Controls on Electric Power plants

http://www.edf.org/sites/default/files/state-ghg-standards-03132012.pdf67

Low-Carbon Fuel Standards

 http://www.c2es.org/us-states-regions/policy-maps/low-carbon-fuel-standard68

Climate Action Plans

http://www.c2es.org/us-states-regions/policy-maps/action-plan69

Cap and Trade Programs

http://arb.ca.gov/cc/capandtrade/capandtrade.htm70

Regional Agreements

http://www.c2es.org/us-states-regions/regional-climate-initiatives#WCI71

Tribal Communities

http://www.epa.gov/statelocalclimate/tribal72

States have also taken a number of energy measures, motivated in part by greenhouse gas concerns. For example:

Renewable Portfolio Standards

http://www.dsireusa.org/documents/summarymaps/RPS_map.pdf73

Energy Efficiency Resource Standards

 http://www.dsireusa.org/documents/summarymaps/EERS_map.pdf74

Property Tax Incentives for Renewables

 http://www.dsireusa.org/documents/summarymaps/75
661

27: MITIGATION

References

CLIMATE CHANGE IMPACTS IN THE UNITED STATES

References

1. NRC, 2010: Limiting the Magnitude of Future Climate Change. 

America’s Climate Choices. Panel on Limiting the Magnitude of 

Future Climate Change. National Research Council, Board on 

Atmospheric Sciences and Climate, Division of Earth and Life 

Studies. The National Academies Press, 276 pp. [Available online at 

http://www.nap.edu/catalog.php?record_id=12785]

2. DOE, 2011: Report of the First Quadrennial Technology Review, 168 

pp., U.S. Department of Energy, Washington, D.C. [Available online 

at http://energy.gov/sites/prod/files/QTR_report.pdf]

3. IPCC, 2007: Climate Change 2007: The Physical Science Basis. Contribution 

of Working Group I to the Fourth Assessment Report of the Intergovernmental 

Panel on Climate Change. S. Solomon, D. Qin, M. Manning, Z. Chen, M. 

Marquis, K. B. Averyt, M. Tignor, and H. L. Miller, Eds. Cambridge 

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65. C2ES, cited 2013: Greenhouse Gas Reporting and Registries. Center 

for Climate and Energy Solutions. [Available online at http://www.

c2es.org/us-states-regions/policy-maps/ghg-reporting]

66. ——, cited 2013: Greenhouse Gas Emissions Targets. Center for 

Climate and Energy Solutions. [Available online at http://www.c2es.

org/us-states-regions/policy-maps/emissions-targets]

67. EDF, 2012: States Have Led the Way in Curbing Carbon Pollution 

from New Power Plants, 1 pp., Environmental Defense Fund. 

[Available online at http://www.edf.org/sites/default/files/state-ghgstandards-03132012.pdf]

68. C2ES, cited 2013: Low Carbon Fuel Standard. Center for Climate 

and Energy Solutions. [Available online at http://www.c2es.org/usstates-regions/policy-maps/low-carbon-fuel-standard]

69. ——, cited 2013: Climate Action Plans. Center for Climate and 

Energy Solutions. [Available online at http://www.c2es.org/usstates-regions/policy-maps/action-plan]

70. CEPA, cited 2013: Cap-and-Trade Program. California Environmental 

Protection Agency. [Available online at http://arb.ca.gov/cc/

capandtrade/capandtrade.htm]

71. C2ES, cited 2013: Multi-State Climate Initiatives. Center for Climate 

and Energy Solutions. [Available online at http://www.c2es.org/usstates-regions/regional-climate-initiatives#WCI]

72. EPA, cited 2013: Tribal Climate and Energy Information. U.S. 

Environmental Protection Agency. [Available online at http://www.

epa.gov/statelocalclimate/tribal]

73. DOE, 2013: Database of State Incentives for Renewables & Efficiency. 

Renewable Portfolio Standard Policies, 1 pp., U.S. Department of 

Energy. [Available online at http://www.dsireusa.org/documents/

summarymaps/RPS_map.pdf]

74. ——, 2013: Database of State Incentives for Renewables & Efficiency. 

Energy Efficiency Resource Standards., 1 pp., U.S. Department of 

Energy. [Available online at http://www.dsireusa.org/documents/

summarymaps/EERS_map.pdf]

75. ——, 2013: Database of State Incentives for Renewables & Efficiency. 

Property Tax Incentives for Renewables, 1 pp., U.S. Department of 

Energy. [Available online at http://www.dsireusa.org/documents/

summarymaps/PropertyTax_map.pdf]

76. NRC, 2011: Climate Stabilization Targets: Emissions, Concentrations, and 

Impacts over Decades to Millennia. National Research Council. The 

National Academies Press, 298 pp. [Available online at http://www.

nap.edu/catalog.php?record_id=12877]
27: MITIGATION

References

666 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

77. van Vuuren, D. P., J. Edmonds, M. Kainuma, K. Riahi, A. Thomson, 

K. Hibbard, G. C. Hurtt, T. Kram, V. Krey, and J. F. Lamarque, 2011: 

The representative concentration pathways: An overview. Climatic 

Change, 109, 5-31, doi:10.1007/s10584-011-0148-z. [Available online 

at http://link.springer.com/content/pdf/10.1007%2Fs10584-011-

0148-z.pdf]

78. EIA, 2011: International Energy Outlook 2011. U.S. Energy 

Information Administration, Washington, D.C. [Available online at 

http://www.eia.gov/forecasts/archive/ieo11/]

79. Metcalf, G. E., 2008: An empirical analysis of energy intensity 

and its determinants at the state level. The Energ y Journal, 29, 1-26, 

doi:10.5547/ISSN0195-6574-EJ-Vol29-No3-1. [Available online at 

http://works.bepress.com/cgi/viewcontent.cgi?article=1005&contex

t=gilbert_metcalf] 

Sue Wing, I., 2008: Explaining the declining energy intensity of the 

US economy. Resource and Energ y Economics, 30, 21-49, doi:10.1016/j.

reseneeco.2007.03.001. 
667 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

27: MITIGATION

SUPPLEMENTAL MATERIAL

TRACEABLE ACCOUNTS

Process for Developing Key Messages: 

Evaluation of literature by Coordinating Lead Authors

Key message #1 Traceable Account

Carbon dioxide is removed from the atmosphere 

by natural processes at a rate that is roughly half 

of the current rate of emissions from human activities. Therefore, mitigation efforts that only stabilize global emissions will not reduce atmospheric 

concentrations of carbon dioxide, but will only limit 

their rate of increase. The same is true for other 

long-lived greenhouse gases.

Description of evidence base

The message is a restatement of conclusions derived from the 

peer-reviewed literature over nearly the past 20 years (see Section 

1 of chapter). Publications have documented the long lifetime of 

CO2 in the atmosphere, resulting in long time lags between action 

and reduction,9,11,76 and Earth System Models have shown that 

stabilizing emissions will not immediately stabilize atmospheric 

concentrations, which will continue to increase.4

New information and remaining uncertainties

There are several important uncertainties in the current carbon 

cycle, especially the overall size, location, and dynamics of the 

land-use sink9,11 and technological development and performance. 

Simulating future atmospheric concentrations of greenhouse gases 

requires both assumptions about economic activity, stringency of 

any greenhouse gas emissions control, and availability of technologies, as well as a number of assumptions about how the changing 

climate system affects both natural and anthropogenic sources.

Assessment of confidence based on evidence 

Very High. Observations of changes in the concentrations of greenhouse gases are consistent with our understanding of the broad 

relationships between emissions and concentrations.

Key message #2 Traceable Account

To meet the lower emissions scenario (B1) used 

in this assessment, global mitigation actions would 

need to limit global carbon dioxide emissions to a 

peak of around 44 billion tons per year within the 

next 25 years and decline thereafter. In 2011, global emissions were around 34 billion tons, and have 

been rising by about 0.9 billion tons per year for 

the past decade. Therefore, the world is on a path 

to exceed 44 billion tons per year within a decade.

Description of evidence base

A large number of emissions scenarios have been modeled, with 

a number of publications showing what would be required to limit 

CO213,53,54,77 to any predetermined limit. At current concentrations 

and rate of rise, the emissions of CO2 would need to peak around 

Confidence Level

Very High

Strong evidence (established 

theory, multiple sources, consistent results, well documented 

and accepted methods, etc.), 

high consensus

High

Moderate evidence (several 

sources, some consistency, 

methods vary and/or documentation limited, etc.), medium 

consensus

Medium

Suggestive evidence (a few 

sources, limited consistency, 

models incomplete, methods 

emerging, etc.), competing 

schools of thought

Low

Inconclusive evidence (limited sources, extrapolations, 

inconsistent findings, poor documentation and/or methods not 

tested, etc.), disagreement or 

lack of opinions among experts
27: MITIGATION

Traceable Accounts

668 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

44 billion tons within the next 25 years in order to stabilize concentrations as in the B1 scenario. Given the rate of increase in 

recent years,10 this limit is expected to be surpassed.78

New information and remaining uncertainties

Uncertainties about the carbon cycle could affect these calculations, but the largest uncertainties are the assumptions made 

about the strength and cost of greenhouse gas emissions policies.

Assessment of confidence based on evidence 

The confidence in the conclusion is high. This is a contingent 

conclusion, though – we do not have high confidence that the 

current emission rate will be sustained. However, we do have high 

confidence that if we do choose to limit concentrations as in the 

B1 scenario, emissions will need to peak soon and then decline. 

Key message #3 Traceable Account

Over recent decades, the U.S. economy has emitted a decreasing amount of carbon dioxide per dollar of gross domestic product. Between 2008 and 

2012, there was also a decline in the total amount 

of carbon dioxide emitted annually from energy 

use in the United States as a result of a variety of 

factors, including changes in the economy, the development of new energy production technologies, 

and various government policies.

Description of evidence base

Trends in greenhouse gas emissions intensity are analyzed and 

published by governmental reporting agencies.20,23,26 Published, 

peer-reviewed literature cited in Section 2 of the Mitigation Chapter supports the conclusions about why these trends have occurred.79

New information and remaining uncertainties

Economic and technological forecasts are highly uncertain.

Assessment of confidence based on evidence 

High. The statement is a summary restatement of published analyses by government agencies and interpretation from the reviewed 

literature.

Key message #4 Traceable Account

Carbon storage in land ecosystems, especially 

forests, has offset around 17% of annual U.S. fossil fuel emissions of greenhouse gases over the 

past several decades, but this carbon “sink” may 

not be sustainable.

Description of evidence base

Underlying data come primarily from U.S. Forest Service Forest 

Inventory and Analysis (FIA) plots, supplemented by additional 

ecological data collection efforts. Modeling conclusions come 

from peer-reviewed literature. All references are in Section 2 of 

the Mitigation Chapter. Studies have shown that there is a large 

land-use carbon sink in the United States.26,27,28 Many publications attribute this sink to forest re-growth, and the sink is projected to decline as a result of forest aging30,31,33 and factors like 

drought, fire, and insect infestations31 reducing the carbon sink of 

these regions.

New information and remaining uncertainties

FIA plots are measured extremely carefully over long time periods, 

but do not cover all U.S. forested land. Other U.S. land types 

must have carbon content estimated from other sources. Modeling 

relationships between growth and carbon content, and taking CO2

and climate change into account have large scientific uncertainties associated with them.

Assessment of confidence based on evidence 

High. Evidence of past trends is based primarily on government 

data sources, but these also have to be augmented by other data 

and models in order to incorporate additional land-use types. Projecting future carbon content is consistent with published models, 

but these have intrinsic uncertainties associated with them.

Key message #5 Traceable Account

Both voluntary activities and a variety of policies 

and measures that lower emissions are currently in 

place at federal, state, and local levels in the United States, even though there is no comprehensive 

national climate legislation. Over the remainder of 

this century, aggressive and sustained greenhouse 

gas emission reductions by the United States and 

by other nations would be needed to reduce global 

emissions to a level consistent with the lower scenario (B1) analyzed in this assessment.

Description of evidence base

The identification of state, local, regional, federal, and voluntary 

programs that will have an effect of reducing greenhouse gas emissions is a straightforward accounting of both legislative action and 

announcements of the implementation of such programs. Some 

of the programs include the Carbon Disclosure Project (CDP), the 

American College and University Presidents’ Climate Commitment 

(ACUPCC), U.S. Mayors Climate Protection Agreement,39 and 

many other local government initiatives.38 Several states have also 

adapted climate policies including California’s Global Warming 

Solutions Act (AB 32) and the Regional Greenhouse Gas Initiative 

(RGGI). The assertion that they will not lead to a reduction of US 

CO2 emissions is supported by calculations from the U.S. Energy 

Information Administration.

New information and remaining uncertainties

The major uncertainty in the calculation about future emissions 

levels is whether a comprehensive national policy will be implemented.
27: MITIGATION

Traceable Accounts

669 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Assessment of confidence based on evidence 

Very High. There is recognition that the implementation of voluntary programs may differ from how they are originally planned, 

and that institutions can always choose to leave voluntary programs (as is happening with RGGI, noted in the chapter). The 

statement about the future of U.S. CO2 emissions cannot be taken 

as a prediction of what will happen – it is a conditional statement 

based on an assumption of no comprehensive national legislation 

or regulation.
670

Climate Change Impacts in the United States

CHAPTER 28

ADAPTATION

INFORMATION DRAWN FROM THIS CHAPTER IS INCLUDED IN THE HIGHLIGHTS REPORT AND IS IDENTIFIED BY THIS ICON

Recommended Citation for Chapter

Bierbaum, R., A. Lee, J. Smith, M. Blair, L. M. Carter, F. S. Chapin, III, P. Fleming, S. Ruffo, S. McNeeley, M. Stults, L. 

Verduzco, and E. Seyller, 2014: Ch. 28: Adaptation. Climate Change Impacts in the United States: The Third National Climate Assessment, J. M. Melillo, Terese (T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program,

670-706. doi:10.7930/J07H1GGT. 

On the Web http://nca2014.globalchange.gov/report/response-strategies/adaptation

Convening Lead Authors

Rosina Bierbaum, University of Michigan

Arthur Lee, Chevron Corporation

Joel Smith, Stratus Consulting

Lead Authors

Maria Blair, Independent

Lynne M. Carter, Louisiana State University

F. Stuart Chapin III, University of Alaska Fairbanks

Paul Fleming, Seattle Public Utilities

Susan Ruffo, The Nature Conservancy

Contributing Authors

Shannon McNeeley, Colorado State University

Missy Stults, University of Michigan

Laura Verduzco, Chevron Corporation

Emily Seyller, University Corporation for Atmospheric Research
671 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

28ADAPTATION

Key Messages

1. Substantial adaptation planning is occurring in the public and private sectors and at all levels of 

government; however, few measures have been implemented and those that have appear to be 

incremental changes.

2. Barriers to implementation of adaptation include limited funding, policy and legal impediments, 

and difficulty in anticipating climate-related changes at local scales.

3. There is no “one-size fits all” adaptation, but there are similarities in approaches across regions 

and sectors. Sharing best practices, learning by doing, and iterative and collaborative processes 

including stakeholder involvement, can help support progress.

4. Climate change adaptation actions often fulfill other societal goals, such as sustainable 

development, disaster risk reduction, or improvements in quality of life, and can therefore be 

incorporated into existing decision-making processes.

5. Vulnerability to climate change is exacerbated by other stresses such as pollution, habitat 

fragmentation, and poverty. Adaptation to multiple stresses requires assessment of the composite 

threats as well as tradeoffs among costs, benefits, and risks of available options. 

6. The effectiveness of climate change adaptation has seldom been evaluated, because actions have 

only recently been initiated and comprehensive evaluation metrics do not yet exist. 

Over the past few years, the focus moved from the question 

“Is climate changing?” to the equally important question: “Can 

society manage unavoidable changes and avoid unmanageable 

changes?”1,2 Research demonstrates that both mitigation 

(efforts to reduce future climate changes) and adaptation 

(efforts to reduce the vulnerability of society to climate change 

impacts) are needed in order to minimize the damages from 

human-caused climate change and to adapt to the pace and 

ultimate magnitude of changes that will occur.3,4,5 

Adaptation and mitigation are closely linked; adaptation 

efforts will be more difficult, more costly, and less likely to 

succeed if significant mitigation actions are not taken.2,6 The 

study and application of adaptation in the climate change 

realm is nascent compared to the many analyses of mitigation 

policies and practices to reduce emissions. Uncertainties 

about future socioeconomic conditions as well as future 

climate changes can make it difficult to arrive at adaptation 

decisions now. However, the pace and magnitude of projected 

change emphasize the need to be prepared for a wide range 

and intensity of climate impacts in the future. Planning and 

managing based on the climate of the last century means 

that tolerances of some infrastructure and species will be 

exceeded.5,7,8 For example, building codes and landscaping 

ordinances will likely need to be updated not only for energy 

efficiency but also to conserve water supplies, protect against 

disease vectors, reduce susceptibility to heat stress, and 

improve protection against extreme events.5,9 Although there 

is uncertainty about future conditions, research indicates that 

intelligent adaptive actions can still be taken now.10,11 Climate 

change projections have inherent uncertainties, but it is still 

important to develop, refine, and deploy tools and approaches 

that enable iterative decision-making and increase flexibility 

and robustness of climate change responses (Ch. 2: Our 

Changing Climate).12

Climate change affects human health, natural ecosystems, 

built environments, and existing social, institutional, and 

legal arrangements. Adaptation considerations include 

local, state, regional, national, and international issues. For 

example, the implications of international arrangements 

need to be considered in the context of managing the Great 

Lakes, the Columbia River, and the Colorado River to deal 

with drought.13,14 Both “bottom up” community planning and 

“top down” national strategies11 may help regions deal with 

impacts such as increases in electrical brownouts, heat stress, 

floods, and wildfires. Such a mix of approaches will require 
672 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

28: ADAPTATION

cross-boundary coordination at multiple levels as operational 

agencies integrate adaptation planning into their programs. 

Adaptation actions can be implemented reactively, after 

changes in climate occur, or proactively, to prepare for projected 

changes.11 Proactively preparing can reduce the harm from 

certain climate change impacts, such as increasingly intense 

extreme events, shifting zones for agricultural crops, and rising 

sea levels, while also facilitating a more rapid and efficient 

response to changes as they happen. This chapter highlights 

efforts at the federal, regional, state, tribal, and local levels, 

as well as initiatives in the corporate and non-governmental 

sectors to build adaptive capacity and resilience in response to 

climate change. While societal adaptation to climate variability

is as old as civilization itself,15 the focus of this chapter is on 

preparing for unprecedented human-induced climate change 

through adaptation. A map of illustrative adaptation activities 

and four detailed case examples that highlight ongoing 

adaptation activity across the U.S. are provided in Section 4 of 

this chapter. 

Adaptation Activities in the United States

Federal Government

Federal leadership, guidance, information, and support are 

vital to planning for and implementing adaptation actions at all 

scales and in all affected sectors of society (Table 28.1).11,18,19,20

Several new federal climate adaptation initiatives and 

strategies have been developed in recent years, including: 

• Executive Order (EO) 13514, requiring federal agencies to 

develop recommendations for strengthening policies and 

programs to adapt to the impacts of climate change;21

• the release of President Obama’s Climate Action Plan in June 

2013, which has as one of its three major pillars, preparing 

the United States for the impacts of climate change, including 

building stronger and safer communities and infrastructure, 

protecting the economy and natural resources, and using 

sound science to manage climate impacts;22

• the creation of an Interagency Climate Change Adaptation 

Task Force (ICCATF) (now the Council on Climate Preparedness and Resilience, per Executive Order 1365323) that led to 

the development of national principles for adaptation and 

Adaptation key terms definitions*

Adapt, Adaptation: Adjustment in natural or human systems to a new or changing environment that exploits beneficial 

opportunities or moderates negative effects.

Adaptive Capacity: The potential of a system to adjust to climate change (including climate variability and extremes) 

to moderate potential damages, take advantage of opportunities, and cope with the consequences.

Mitigation: Technological change and substitutions that reduce resource inputs and emissions per unit of output. 

Although several social, economic, and technological actions would reduce emissions, with respect to climate 

change, mitigation means implementing actions to reduce greenhouse gas emissions or increase the amount of 

carbon dioxide absorbed and stored by natural and man-made carbon sinks (see Ch. 27: Mitigation).

Multiple Stressors: Stress that originates from different sources that affect natural, managed, and socioeconomic 

systems and can cause impacts that are compounded and sometimes unexpected. An example would be when 

economic or market stress combines with drought to negatively impact farmers.

Resilience: A capability to anticipate, prepare for, respond to, and recover from significant multi-hazard threats with 

minimum damage to social well-being, the economy, and the environment.

Risk: A combination of the magnitude of the potential consequence(s) of climate change impact(s) and the likelihood 

that the consequence(s) will occur.

Vulnerability: The degree to which a system is susceptible to, or unable to cope with, adverse effects of climate 

change, including climate variability and extremes. Vulnerability is a function of the character, magnitude, and rate 

of climate variation to which a system is exposed, its sensitivity, and its adaptive capacity.

*Definitions adapted from (IPCC 2007; 16 NRC 2007,17 201011).
673 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

28: ADAPTATION

is leading to crosscutting and government-wide adaptation 

policies; 

• the development of three crosscutting national adaptation 

strategies focused on integrating federal, and often state, 

local, and tribal efforts on adaptation in key sectors: 1) the 

National Action Plan: Priorities for Managing Freshwater Resources in a Changing Climate;24 2) the National Fish, Wildlife 

and Plants Climate Adaptation Strategy;25 and 3) a priority 

objective on resilience and adaptation in the National Ocean 

Policy Implementation Plan;26

• a new decadal National Global Change Research Plan (2012–

2021) that includes elements related to climate adaptation, 

such as improving basic science, informing decisions, improving assessments, and communicating with and educating the 

public;27

• the development of several interagency and agency-specific 

groups focused on adaptation, including a “community of 

practice” for federal agencies that are developing and implementing adaptation plans, an Adaptation Science Workgroup 

inside the U.S. Global Change Research Program (USGCRP), 

and several agency specific climate change and adaptation 

task forces; and 

• a November 2013 Executive Order entitled “Preparing the 

United States for the Impacts of Climate Change” that, among 

other things, calls for the modernizing of federal programs to 

support climate resilient investments, managing lands and 

waters for climate preparedness and resilience, the creation 

of a Council on Climate Preparedness and Resilience, and the 

creation of a State, Local, and Tribal Leaders Task Force on 

Climate Preparedness and Resilience.23

Federal agencies are all required to plan for adaptation. Actions 

include coordinated efforts at the White House, regional and 

cross-sector efforts, agency-specific adaptation plans, as well 

as support for local-level adaptation planning and action. Table 

28.1 lists examples, but is not intended as a comprehensive list.

Table 28.1. Examples of Individual Federal Agency Actions to 

Promote, Implement, and Support Adaptation at Multiple Scales*

Agency Component Action Description

All Federal Agencies

Developed Adaptation Plans as 

part of their annual Strategic Sustainability Performance Plans

The 2012 Strategic Sustainability Performance 

Plans for Federal agencies contain specific 

sections on adaptation. Agencies are required 

to evaluate climate risks and vulnerabilities to 

manage both short- and long-term effects on 

missions and operations.

Department of Health and Human 

Services (HHS)

Centers for Disease 

Control and Prevention 

(CDC)

Climate-Ready States and Cities 

Initiative

Through their first climate change cooperative 

agreements in 2010, CDC awarded $5.25 million to ten state and local health departments 

to assess risks and develop programs to address 

climate change related challenges.

Department of Agriculture (USDA) Integrating climate change objectives into plans and networks

USDA is using existing networks such as the 

Cooperative Extension Service, the Natural 

Resource Conservation Districts, and the Forest 

Service’s Climate Change Resource Center to 

provide climate services to rural and agricultural 

stakeholders.

USDA Forest Service

Developed a National Roadmap 

for Responding to Climate Change 

and a Guidebook for Developing 

Adaptation Options, among many 

resources

The National Roadmap was developed in 2010 

to identify short- and long-term actions to reduce 

climate change risks to the nation’s forests and 

grasslands. The Guidebook builds on this previous work and provides science-based strategic 

and tactical approaches to adaptation. 

Department of Commerce (DOC) NOAA

Supporting research teams and 

local communities on adaptationrelated issues and develops tools 

and resources

Through the Regional Integrated Sciences 

and Assessments (RISAs) program, develop 

collaboration between researchers and managers to better manage climate risks. Through 

the Regional Climate Centers (RCCs) and the 

Digital Coast partnership, deliver science to 

support decision-making.

Department of Defense (DoD) Developed a DoD Climate 

Change Adaptation Roadmap 

DoD released its initial Department-level 

Climate Change Adaptation Roadmap in 2012. 

The Roadmap identifies four goals that serve 

as the foundation for guiding the Department’s 

response to climate change that include using 

a robust decision making approach based on 

the best available science.
674 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

28: ADAPTATION

Table 28.1. Examples of Individual Federal Agency Actions to 

Promote, Implement, and Support Adaptation at Multiple Scales*

 DoD

U.S. Army Corps of 

Engineers (USACE), 

Civil Works Program

Developed climate change 

adaptation plan; making progress 

in priority areas including 

vulnerability assessments and 

development of policy and 

guidance

The USACE Civil Works Program initial climate 

change adaptation plan in 2011 has a goal to 

reduce vulnerabilities and improve resilience 

of water resources infrastructure impacted by 

climate change. Vulnerability assessments and 

pilot projects are in progress. Other guidance is 

underway. 

DoD Department of the Navy

Developed road maps for 

adaptation in the Arctic and across 

the globe

The Navy Arctic Roadmap (November 2009) 

promotes maritime security and naval readiness 

in a changing Arctic. The Climate Change 

Roadmap (May 2010) examines broader issues 

of climate change impacts on Navy missions 

and capabilities globally.

Department of Energy (DOE)

Develop higher spatial and 

temporal scales of climate 

projections and integrate 

adaptation and climate 

considerations into integrated 

assessments

Develops community-based, high-resolution 

(temporal and spatial) models for climate 

projections and integrated assessment models 

that increasingly reflect multi-sectoral processes 

and interactions, multiple stressors, coupled 

impacts, and adaptation potential. 

DOE

Developed climate change 

adaptation plan, and completed 

comprehensive study of 

vulnerabilities to the energy sector 

of climate change and extreme 

weather

The 2013 DOE Report “U.S. Energy Sector 

Vulnerabilities to Climate Change and Extreme 

Weather” examines current and potential future 

impacts of climate trends and identifies activities 

underway and potential opportunities to 

enhance energy system climate preparedness 

and resilience.

Department of Homeland Security 

(DHS)

Federal Emergency 

Management Agency 

(FEMA)

Works with communities across 

the Nation to help them prioritize 

their activities to reduce risks

FEMA released a Climate Change Adaptation 

Policy Statement establishing the Agency’s 

approach to supporting the Department in 

ensuring resilience to disasters in the face of 

climate change. FEMA’s action areas focus 

on developing actionable “future risk” tools, 

enabling state and local adaptation, and building 

resilience capabilities.

Department of the Interior (DOI) Fish and Wildlife Service 

(FWS)

Developed a FWS climate 

change strategic plan (2010) 

and established a network 

of Landscape Conservation 

Cooperatives (LCCs)

Established a framework to help ensure the 

sustainability of fish, wildlife, plants, and 

habitats in the face of climate change. Created 

a network of 22 LCCs to promote shared 

conservation goals, approaches, and resource 

management planning and implementation 

across the United States.

DOI U.S. Geological Survey 

(USGS)

Established a network of Climate 

Science Centers (CSCs)

DOI operates a National Climate Change and 

Wildlife Center and eight regional CSCs, which 

provide scientific information and tools that land, 

water, wildlife, and cultural resource managers 

and other stakeholders can apply to anticipate, 

monitor, and adapt to climate change.

DOI National Park Service 

(NPS)

Climate Change Response 

Strategy (2010), Climate Change 

Action Plan (2012), and Green 

Parks Plan (2012)

NPS actions span climate change science, 

adaptation, mitigation, and communication 

across national parks, including exhibits for park 

visitors, providing climate trend information for 

all national parks, risk screening and adaptation 

for coastal park units, and implementing 

scenario planning tools.

DOI Bureau of Land 

Management (BLM)

Rapid Ecoregional Assessments 

(REAs)

REAs synthesize information about resource 

conditions and trends within an ecoregion; 

assess impacts of climate change and 

other stressors; map areas best-suited for 

future development; and establish baseline 

environmental conditions, against which to 

gauge management effectiveness.

(Continued)
675 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

28: ADAPTATION

Federal agencies can be particularly helpful in facilitating 

climate adaptation by:

• fostering the stewardship of public resources and maintenance of federal facilities, services, and operations such as 

defense, emergency management, transportation, and ecosystem conservation in the face of a changing climate;11,28,29,30

• providing usable information and financial support for adaptation;11,20,30

• facilitating the dissemination of best practices and supporting a clearinghouse to share data, resources, and lessons 

learned;11,20,31

• dealing with and anticipating impacts that cross geopolitical 

boundaries, assisting in disaster response, and supporting 

flexible regulatory frameworks;11,30

• ensuring the establishment of federal policies that allow for 

“flexible” adaptation efforts and take steps to avoid unintended consequences;30,32 and 

• building public awareness.33

Table 28.1. Examples of Individual Federal Agency Actions to 

Promote, Implement, and Support Adaptation at Multiple Scales*

Department of Transportation (DOT) Federal Highway 

Administration (FHWA)

Developed Risk Assessment 

Model for transportation decisions

DOT worked with five local and state 

transportation authorities to develop a 

conceptual Risk Assessment Model to identify 

which assets are: a) most exposed to climate 

change threats and/or b) associated with the 

most serious potential consequences of climate 

change threats. Completed November 2011.

DOT

Comprehensive study of climate 

risks to Gulf Coast transportation 

infrastructure followed by in-depth 

study of Mobile, AL

Phase 1 of the 2008 study assessed 

transportation infrastructure vulnerability to 

climate change impacts across the Gulf. Phase 

2, to be completed in 2013, focuses on Mobile, 

AL. This effort will develop transferable tools for 

transportation planners.

Environmental Protection Agency 

(EPA)

Established the Climate Ready 

Estuaries program, the Climate 

Ready Water Utilities initiative, 

and a tribal climate change 

adaptation planning training 

program

These selected EPA initiatives provide 

resources and tools to build the capacity of 

coastal managers, water utilities, and tribal 

environmental professionals to plan for and 

implement adaptation strategies.

National Aeronautics and Space 

Administration (NASA)

Initiated NASA’s Climate 

Adaptation Science Investigator 

(CASI) Workgroup to partner 

NASA scientists, engineers, and 

institutional stewards

The CASI team builds capacity to address 

climate change at NASA facilities by 

downscaling facility-specific climate hazard 

information and projections; conducting 

customized climate research for each location; 

and leading resilience and adaptation 

workshops that spur community-based 

responses.

*Material provided in table is derived directly from Agency representatives and Agency websites. These are select examples and should not be considered all-inclusive.

(Continued)
676 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

28: ADAPTATION

States

States have become important actors in national climate 

change related efforts. State governments can create policies 

and programs that encourage or discourage adaptation at 

other governance scales (such as counties or regions)34 through 

regulation and by serving as laboratories for innovation.35,36

Although many of these actions are not specifically designed to 

address climate change, they often include climate adaptation 

components.

Many state-level climate change-specific adaptation 

actions focus on planning. As of 2013, fifteen states had 

completed climate adaptation plans; four states were in the 

process of writing their plans; and seven states had made 

recommendations to create state-wide adaptation plans.37

In addition to formal adaptation plans, numerous states 

have created sector-specific plans that consider long-term 

climate change (Figure 28.1). For example, at least 16 states 

have biodiversity conservation plans that focus on preparing 

for long-term changes in climate.38 In addition to planning, 

some states have created legislation and/or programs that 

are either directly or indirectly targeted at reducing climate 

vulnerabilities (Table 28.2).

Figure 28.1. Status of State Climate Adaptation Plans. (Figure source: redrawn from C2ES 201337). 

Status of State Climate Adaptation Plans
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Tribal Governments 

Tribal governments have been particularly active in assessing 

and preparing for the impacts of climate change (see Ch. 12: 

Indigenous Peoples). For example: 

• Adaptation planning in Point Hope, Alaska, emphasizes strategies for enhancing community health.49

• In Newtok, Alaska, the village council is leading a land-acquisition and planning effort to relocate the community, because 

climate change induced coastal erosion has destroyed essential infrastructure, making the current village site unsafe.50

• The Tulalip Tribes in Washington State are using traditional 

knowledge gleaned from elders, stories, and songs and 

combining this knowledge with downscaled climate data 

to inform decision-making.51 Also in Washington State, the 

Swinomish Indian Tribal Community integrated climate 

change into decision-making in major sectors of the Swinomish Community, such as education, fisheries, social services, 

and human health.52

• The Haudenosaunee Confederacy in the northeastern U.S. is 

addressing climate impacts by preserving a native food base 

through seed-banking (Ch. 12: Indigenous Peoples).51

Table 28.2. Examples of State-Level Adaptation Activities*

State Adaptation Action

Alaska Alaska Climate Change Impact Mitigation Program provides funds for hazard impact assessments to evaluate climate change related 

impacts, such as coastal erosion and thawing permafrost.39

California Building standards mandating energy and water efficiency savings, advancing both adaptation and mitigation; State Adaptation Plan 

calls for 20% reduction in per capita water use.40

Florida

Law supporting low water use landscaping techniques.41

Hawaii

Water code that calls for integrated management, preservation, and enhancement of natural systems.42

Kentucky Action Plan to Respond to Climate Change in Kentucky: A Strategy of Resilience, which identifies six goals to protect ecosystems and 

species in a changing climate.43

Louisiana Comprehensive Master Plan for a Sustainable Coast 2012 includes both protection and restoration activities addressing land loss from 

sea level rise, subsidence, and other factors over the next 50 years.44

Maine The Maine Sand Dune Rules require that structures greater than 2,500 square feet be set back at a distance that is calculated based 

on the future shoreline position and considering two feet of sea level rise over the next 100 years.45

Maryland Passed Living Shorelines Act to reduce hardened shorelines throughout the state;46 passed “Building Resilience to Climate Change” 

policy which establishes practices and procedures related to facility siting and design, new land investments, habitat restoration, 

government operations, research and monitoring, resource planning, and advocacy. 

Montana Maintains a statewide climate change website to help stakeholders access relevant and timely climate information, tools, and resources.

New 

Mexico The Active Water Resource Management program allows for temporary water rights changes in real time in case of drought.47

Pennsylvania Enacted polices to encourage the use of green infrastructure and ecosystem-based approaches for managing storm water and flooding.9

Rhode 

Island Requires public agencies considering land-use applications to accommodate a 3- to 5-foot rise in sea level.

Texas Coordinated response to drought through National Integrated Drought Information System (NIDIS); RISAs (Southern Climate Impacts 

Planning Program [SCIPP], Climate Assessment for the Southwest [CLIMAS]); and state and private sector partners through anticipatory planning and preparedness (for example, implemented in 2011 drought).48

*This list contains selected examples of state-level adaptation activities and should not be considered all-inclusive.
678 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Local and Regional Governments

Most adaptation efforts to date have occurred at local and 

regional levels.53,54,55,56,57 Primary mechanisms that local 

governments are using to prepare for climate change include 

land-use planning; provisions to protect infrastructure and 

ecosystems; regulations related to the design and construction 

of buildings, roads, and bridges; and emergency preparation, 

response, and recovery (Table 28.3).9,45,56,58

According to a recent survey of 298 U.S. local governments, 

59% indicated they are engaged in some form of adaptation 

planning.59 Local adaptation planning and actions are unfolding 

in municipalities of varying sizes and in diverse geographical 

areas. Communities such as Keene, New Hampshire; New 

York City, New York; King County, Washington; and Chicago, 

Illinois are vanguards in the creation of climate adaptation 

strategies.9,11,60 In addition to local government action, 

regional agencies and regional aggregations of governments 

are becoming significant climate change adaptation actors.8,57

Table 28.3. Examples of Local and Regional Adaptation Activities*

Local or Regional Government Adaptation Action

Satellite Beach, FL Collaboration with the Indian River Lagoon National Estuary Program led to efforts to try to incorporate 

sea level rise projections and policies into the city’s comprehensive growth management plan.54

Portland, OR Updated the city code to require on-site stormwater management for new development and 

re-development. Provides a downspout disconnection program to help promote on-site 

stormwater management .

61

Lewes, DE In partnership with Delaware Sea Grant, ICLEI-Local Governments for Sustainability, the University of 

Delaware, and state and regional partners, the City of Lewes undertook a stakeholder-driven process 

to understand how climate adaptation could be integrated into the hazard mitigation planning process. 

Recommendations for integration and operational changes were adopted by the City Council and are currently being implemented.62

Groton, CT Partnered with federal, state, regional, local, non-governmental, and academic partners through the 

EPA’s Climate Ready Estuaries program to assess vulnerability to and devise solutions for sea level 

rise.63

San Diego Bay, CA Five municipalities partnered with the port, the airport, and more than 30 organizations with direct interests in the Bay’s future to develop the San Diego Bay Sea Level Rise Adaptation Strategy. The strategy 

identified key vulnerabilities for the Bay and adaptation actions that can be taken by individual agencies, 

as well as through regional collaboration.9

Chicago, IL Through a number of development projects, the city has added 55 acres of permeable surfaces since 

2008 and has more than four million square feet of green roofs planned or completed.64

King County, WA Created King County Flood Control District in 2007 to address increased impacts from flooding through 

activities such as maintaining and repairing levees and revetments, acquiring repetitive loss properties, 

and improving countywide flood warnings.65

New York City, NY Through a partnership with the Federal Emergency Management Agency (FEMA), the city is updating 

FEMA Flood Insurance Rate Maps based on more precise elevation data. The new maps will help stakeholders better understand their current flood risks and allow the city to more effectively plan for climate 

change.66

Southeast Florida Climate Change Compact Joint commitment among Broward, Miami-Dade, Palm Beach, and Monroe Counties to partner in reducing heat-trapping gas emissions and adapting to climate impacts, including adaptation in transportation, 

water resources, natural resources, agriculture, and disaster risk reduction. Notable policies emerging 

from the Compact include regional collaboration to revise building codes and land development regulations to discourage new development or post-disaster redevelopment in vulnerable areas.67

Phoenix, AZ; Boston, MA; Philadelphia, PA; 

and New York, NY 

Climate change impacts are being integrated into public health planning and implementation activities 

that include creating more community cooling centers, neighborhood watch programs, and reductions in 

the urban heat island effect.9,68,69

Boulder, CO; New York, NY; and Seattle, WA Water utilities in these communities are using climate information to assess vulnerability and inform 

decision-making.61

City of Philadelphia In 2006, the Philadelphia Water Department began a program to develop a green stormwater infrastructure, intended to convert more than one-third of the city’s impervious land cover to “Greened Acres”: 

green facilities, green streets, green open spaces, green homes, etc., along with stream corridor restoration and preservation.5

*This table includes select examples of local and regional adaptation activities and should not be considered all-inclusive.
679 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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There is no one-size-fits-all adaptation solution to the challenges of adapting to climate change impacts, as solutions 

will differ depending on context, local circumstance, and 

scale as well as on local culture and internal capacity.

9,31

Non-governmental and Private Sector

Many non-governmental entities have been significant 

actors in the national effort to prepare for climate change 

by providing assistance that includes planning guidance, 

implementation tools, contextualized climate information, 

best practice exchange, and help with bridging the sciencepolicy divide to a wide array of stakeholders (Table 28.4).70,71

The Nature Conservancy, for example, established the 

Canyonlands Research Center in Monticello, Utah, to 

facilitate research and develop conservation applications for 

resource issues under the multi-stresses of climate change 

and land-use demands in the Colorado Plateau region.72

With regard to the private sector, evidence from 

organizations such as the Carbon Disclosure Project (CDP) 

and the Securities and Exchange Commission’s (SEC) Climate 

Change 10-K Disclosure indicate that a growing number 

of companies are beginning to actively address risks from 

climate change (Table 28.5).73 The World Business Council for 

Sustainable Development (WBCSD) and the Center for Climate 

and Energy Solutions (C2ES) have identified three types of 

risks driving private sector adaptation efforts, including risks 

to core operations, the value chain, and broader changes in the 

economy and infrastructure (see Figure 28.2).74,75,76

This analysis is supported by responses to the 2011 CDP, and 

suggests that companies are concerned about how changes in 

the climate will impact issues such as feedstock, water supply 

and quality, infrastructure, core operations, supply chains, and 

customers’ ability to use (and their need for) services.73

Some companies are taking action to not only avoid risk, but to 

explore potential opportunities that may emerge in a changing 

climate, such as developing new products and services, developing or expanding existing consulting services, expanding into 

new operational territories, extending growing seasons and 

hours of operation, and responding to increased demand for 

existing products and services.73,75,77,78

This one-acre stormwater wetland was constructed in Philadelphia to 

treat stormwater runoff in an effort to improve drinking water quality while 

minimizing the impacts of storm-related flows on natural ecosystems.

Table 28.4. Examples of Non-governmental Adaptation Efforts and Services*

Types of Adaptation Efforts and Services Examples of Organizations Providing Services

Adaptation planning assistance, including creation of guides, tools, and templates Center for Climate Strategies, ICLEI-Local Governments for Sustainability, International Institute 

for Sustainable Development, Natural Resources Defense Council, The Nature Conservancy, World 

Resources Institute, World Wildlife Fund 

Networking and best practice exchange C40 Cities Climate Leadership Group, Adaptation Network, Center for Clean Air Policy, Climate 

Adaptation Knowledge Exchange, ICLEI-Local Governments for Sustainability, Institute for Sustainable Communities, Urban Sustainability Directors Network, World Business Council for Sustainable 

Development

Climate information providers Union of Concerned Scientists, Urban Climate Change Research Network, Stockholm Environment 

Institute–U.S. Center

Policy, legal, and institutional support Center for Climate and Energy Solutions (formerly Pew Center on Global Climate Change), Georgetown Climate Center

Aggregation of adaptation-pertinent information Carbon Disclosure Project, Climate Adaptation Knowledge Exchange, Georgetown Climate Center

*This list contains examples of non-governmental organizations providing the identified services and should not be considered all-inclusive or a validation of actions 

claimed by the organizations.

©Lewis Cook for PWD
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Table 28.5. Examples of Private Sector Actions to Adapt to Climate Risks 

as Reported to the Carbon Disclosure Project*

Company Sector Climate Risk Examples of Actions Undertaken

Coca-Cola 

Company

Consumer 

Staples

Changes in physical climate 

parameters; Changes in other 

climate-related developments

Coca-Cola is working around the world to replenish the water used in finished 

beverages by participating in locally relevant water projects that support communities and nature. Since 2005, the Coca-Cola system has engaged in more 

than 320 projects in 86 countries. The range of community projects includes 

watershed protection; expanding community drinking water and sanitation access; 

water for productive use, such as agricultural water efficiency; and education and 

awareness programs. (http://www.thecoca-colacompany.com/citizenship/conservation_partnership.html) 

ConAgra 

Foods, Inc.

Consumer 

Staples

Company experienced 

weather-related sourcing 

challenges, such as delayed 

tomato harvesting due to 

unseasonably cool weather, 

and difficulty sourcing other 

vegetables due to above 

normal precipitation.

As part of its business continuity planning, ConAgra Foods has analyzed its supply risk to develop strategic partnerships with suppliers, minimize sole-sourced 

ingredients, and identify alternate suppliers and contract manufacturers to minimize production disruptions in the instance of an unexpected disruption in supply. 

(http://company.conagrafoods.com/phoenix.zhtml?c=202310&p=Policies_Environment) 

Constellation 

Brands

Consumer 

Staples

Changes in physical climate 

parameters; Changes in other 

climate-related developments

Constellation has already taken adaptation actions, particularly in California where 

water availability is an issue, to manage or adapt to these risks. Constellation is working with numerous organizations to help fund industry-based research to determine 

potential climate change impacts on vineyard production. 

Munich Re Reinsurance

Changes in regulation; 

Changes in physical climate 

parameters; Changes in other 

climate-related developments

Since 2007, a Group-wide climate change strategy covering all aspects of climate 

change – for example, weather-related impacts, regulatory impacts, litigation and 

health risks, etc. – has supported their core corporate strategy. The strategy is 

based on five pillars: mitigation, adaptation, research, in-house carbon dioxide 

reduction, and advocacy. (http://www.munichre.com/en/group/focus/climate_change/

default.aspx) 

Pacific Gas 

and Electric 

Company 

(PG&E)

Utilities

Changes in regulation; 

changes in physical climate 

parameters; Changes in other 

climate-related developments

PG&E’s adaptation strategies for potential increased electricity demand include 

expanded customer energy efficiency and demand response programs and 

improvements to its electric grid. PG&E is proactively tracking and evaluating the 

potential impacts of reductions to Sierra Nevada snowpack on its hydroelectric 

system and has developed adaptation strategies to minimize them. Strategies 

include maintaining higher winter carryover reservoir storage levels, reducing 

conveyance flows in canals and flumes in response to an increased portion of precipitation falling as rain, and reducing discretionary reservoir water releases during 

the late spring and summer. PG&E is also working with both the U.S. Geological 

Survey (USGS) and the California Department of Water Resources to begin using 

the USGS Precipitation-Runoff Modeling System (PRMS) watershed model, to 

help manage reservoirs on watersheds experiencing mountain snowpack loss. 

(http://www.pge.com/about/environment/commitment/) 

SC Johnson & 

Son, Inc.

Household 

Products

Changes in physical climate 

parameters

SC Johnson is adjusting to the various physical risks that climate change imposes 

through a diversified supplier and global manufacturing base. In March 2009, SC 

Johnson announced a broad ingredient communication program. SC Johnson 

assesses risks along each ingredient’s supply chain to ensure that the company 

is sourcing from a geographically diverse supplier base. In addition to evaluating 

product ingredients, SC Johnson has also diversified its operations around the 

world, allowing it to maintain business continuity in the face of a regional climate 

change related disruption. (http://www.scjohnson.com/en/commitment/overview.

aspx) 

Spectra 

Energy, Inc. Energy

Changes in regulation; 

Changes in physical climate 

parameters; Changes in other 

climate-related developments

Spectra Energy uses a corporate-wide risk analysis framework to ensure the 

oversight and management of its four major risk categories: financial, strategic, operational, and legal risks. Physical risks posed by climate change fall within these 

categories and the company uses risk management committees to ensure that all 

material risks are identified, evaluated, and managed prior to financial approvals of 

major projects. (http://www.spectraenergy.com/Sustainability/) 

* This list contains examples of private sector actions to adapt to climate risks as reported to the Carbon Disclosure Project and should not be considered all-inclusive 

or a validation of actions claimed by the organizations.
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Section 1: Adaptation Process 

General patterns in adaptation processes are beginning to 

emerge, with similarities discernible across sectors, systems, 

and scales.53,78,79

This is not a stepwise or linear process; various stages can be 

occurring simultaneously, in a different order, or be omitted 

completely. However, as shown clockwise in Figure 28.3, 

the process generally involves characterizing vulnerability, 

developing options, implementing actions, monitoring 

outcomes, and reevaluating strategies. Each of these is 

described in more detail below.

Identifying and Understanding Risk, 

Vulnerabilities, and Opportunities

Most adaptation actions are currently in the initial phase, 

with many actors focusing on identifying the relevant climate 

risks and conducting current and future risk and vulnerability 

assessments of their assets and resources.8,11,59,80,81,82 In 2011, 

only 13% of 298 U.S. municipalities surveyed had completed 

vulnerability or risk assessments, but 42% expected to complete 

an assessment in the future.59 At least 21 state fish and wildlife 

agencies have undertaken climate vulnerability assessments 

or recently completed an assessment of a particular species, 

habitat, or both.38 Multiple qualitative and quantitative 

methods are used to understand climate vulnerability and 

risk, including case studies and analogue analyses, scenario 

analyses, sensitivity analyses, monitoring of key species, and 

peer information sharing.8,28,83,84

Figure 28.3. Generalized Adaptation Process 

(Figure source: adapted from NRC 201011).

Adaptation Process

Figure 28.2. “Risk Disk” depicts three pathways by which 

risks posed by climate change can affect business, such 

as through core operations, the value chain, and broader 

changes in the economy and infrastructure. (Figure source: 

redrawn from C2ES 200874).

Effects of Climate Change on...
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28: ADAPTATION

Planning, Assessing, and Selecting Options

Once risks and vulnerabilities are understood, the next stage 

typically involves identifying, evaluating, and selecting options 

for responding to and managing existing and future changes 

in the climate.28 Decision support planning methods and 

associated tools help to identify flexible and context-relevant 

adaptation activities for implementation.11,79 Participatory 

approaches support the integration of stakeholder perspectives 

and context-specific information into decision-making.85,86

This approach can include having community members and 

governing institutions work collectively to define the problem 

and design adaptation strategies that are robust while being 

sensitive to stakeholder values.86,87 Moreover, regional 

collaboration has emerged as an effective strategy for defining 

common approaches to reducing potential threats, selecting 

metrics for tracking purposes, and creating governance 

structures to help navigate political challenges.67,88 As discussed 

above, a number of government and other organizations have 

developed plans with identified adaptation options.

Common approaches to adaptation planning include 

“mainstreaming” or integrating climate adaptation into 

existing management plans (for example, hazard mitigation, 

ecosystem conservation, water management, public health, 

risk contingency, and energy) or developing stand-alone 

adaptation plans.68,82,89,90

Many frameworks, tools, and approaches have emerged to help 

decision-makers make decisions in light of both uncertainty and 

the need to achieve multiple societal goals.7,79 Some of these, 

however, are specific to particular localities or resources, are 

not easy to use by the intended audiences, do not adequately 

evaluate tradeoffs, and require sophisticated knowledge 

of climate change.91 In general, these approaches promote 

options that allow reversibility, preserve future options, can 

tolerate a variety of impacts, and are flexible, such that midcourse adjustments are possible.32,92 Among these approaches 

are Robust Decision Making (RDM), Iterative Risk Management 

(IRM), Adaptive Management or Co-Management, Portfolio 

Management, and Scenario Planning (see Ch. 26: Decision 

Support for more on decision frameworks, processes, and 

tools).7,11,28,54,93,94,95,96,97

Implementation

There is little peer-reviewed literature on adaptation actions, 

or evaluations of their successes and failures.11,36,81,98 Many 

of the documents submitted as part of this Third National 

Climate Assessment (NCA) process indicate that adaptation 

actions are being implemented for a variety of reasons. Often, 

these are undertaken with an aim toward reducing current 

vulnerabilities to hazards or extreme weather events, such as 

forest thinning and fuel treatments that reduce fire hazards in 

national forests or through the diversification of supply chain 

sourcing in the private sector.72,73 Additionally, an increasing 

movement toward mainstreaming climate adaptation concerns 

into existing processes means that discerning unique climate 

adaptation activities will be a challenge.82,99

Monitoring and Evaluation

There is little literature evaluating the effectiveness of 

adaptation actions.9,72,79,86 Evaluation and monitoring efforts, 

to date, have focused on the creation of process-based rather 

than outcome-based indicators.86,90 A number of efforts are 

underway to create indicators related to climate adaptation,27

including work by the National Climate Assessment and 

Development Advisory Committee Indicators Working Group100

and the U.S. Environmental Protection Agency.101 Part of 

monitoring should include accounting for costs of adaptation. 

To be sure, this may be difficult to account for because of 

challenges in attribution of climate events to climate change 

versus climate variability. A few studies summarize projected 

future costs of adaptation.102,103

Revise Strategies/Processes and Information Sharing

Uncertainty about future climate as well as population growth, 

economic development, response strategies, and other 

social and demographic issues can stymie climate adaptation 

activity.95,104,105 Through iterative processes, however, 

stakeholders can regularly evaluate the appropriateness of 

planned and implemented activities and revise them as new 

information becomes available.11,28,84 Additionally, the sharing 

of best practices and lessons learned can be pivotal means to 

advancing understanding and uptake of climate adaptation 

activity.82,86 The use of established information-sharing 

networks, such as regional climate initiatives, are illustrations 

of the types of networks that have supported stakeholder 

adaptation activity to-date.9,76,79,86
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Section 2: Barriers to Adaptation and Examples of Overcoming Barriers

Despite emerging recognition of the necessity of climate 

change adaptation, many barriers still impede efforts to 

build local, regional, and national-level resilience. Barriers 

are obstacles that can delay, divert, or temporarily block 

the adaptation process,106 and include difficulties in using 

climate change projections for decision-making; lack of 

resources to begin and sustain adaptation efforts; lack of 

coordination and collaboration within and across political and 

natural system boundaries as well as within organizations; 

institutional constraints; lack of leadership; and divergent risk 

perceptions/cultures and values (Table 28.6).11,20,107 Barriers are 

distinguished from physical or ecological limits to adaptation, 

such as physiological tolerance of species to changing climatic 

conditions that cannot be overcome (except with technology 

or some other physical intervention).8,54,108

Despite barriers, individuals within and across sectors and 

regions are organizing to collectively overcome barriers and 

adapt to climate change. In many cases, lessons learned from 

initial programs help inform future adaptation strategies. 

Figure 28.4 highlights ongoing climate adaptation activities that 

have overcome some of these barriers in different regions led 

Table 28.6. Summary of Adaptation Barriers

Barrier Specific Examples

Climate Change Information and Decision-Making

References:

7,8,10,11,14,17,31,32,42,59,68,69,72,82,90,93,104,109,110,111,112

• Uncertainty about future climate impacts and difficulty in interpreting the cause of 

individual weather events

• Disconnect between information providers and information users

• Fragmented, complex, and often confusing information

• Lack of climate education for professionals and the public

• Lack of usability and accessibility of existing information

• Mismatch of decision-making timescales and future climate projections

Lack of Resources to Begin and Sustain Adaptation Efforts

References: 8,13,42,51,54,59,81,82,111,112,113,114

• Lack of financial resources / no dedicated funding

• Limited staffing capacity

• Underinvestment in human dimensions research

Fragmentation of Decision-Making

References: 8,14,31,32,51,68,115,116

• Lack of coordination within and across agencies, private companies, and nongovernmental organizations

• Uncoordinated and fragmented research efforts

• Disjointed climate related information

• Fragmented ecosystem and jurisdictional boundaries

Institutional Constraints

References: 8,13,42,51,54,97,113,117,118,119

• Lack of institutional flexibility

• Rigid laws and regulations

• No legal mandate to act

• Use of historical data to inform future decisions

• Restrictive management procedures

• Lack of operational control or influence

Lack of Leadership

References: 30,96,112,113,119,120,121

• Lack of political leadership

• Rigid and entrenched political structures

• Polarization

Divergent Risk Perceptions, Cultures, and Values

References: 51,71,82,116,117,120,122

• Conflicting values/risk perceptions

• Little integration of local knowledge, context, and needs with traditional scientific 

information

• Cultural taboos and conflict with cultural beliefs

• Resistance to change due to issues such as risk perception
684 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

28: ADAPTATION

by state, local, and private actors in the United States. It is not 

a comprehensive compilation of national adaptation activity, 

but is intended to identify some of the variety of adaptation 

efforts taking place across the country. 

In addition, Section 4 of this chapter provides four in-depth 

case studies of climate adaptation strategies at different 

scales, with multiple stakeholders, and tackling different 

challenges. Each of these case studies highlights the different 

ways stakeholders are approaching adaptation. 

• Through the creation of the National Integrated Drought Information System (NIDIS), the Federal Government, in partnership with the National Drought Mitigation Center (NDMC), 

states, tribes, universities, and others, has improved capacity 

to proactively manage and respond to drought-related risks 

and impacts through: 1) the provision of drought early warning information systems with local/regional input on extent, 

onset, and severity; 2) a web-based drought portal featuring 

the U.S. Drought Monitor and other visualization tools; 3) coordination of research in support and use of these systems; 

and 4) leveraging of existing partnerships, forecasting, and 

assessment programs. 

• In the Colorado River Basin, water resource managers, government leaders, federal agencies, tribes, universities, nongovernmental organizations (NGOs), and the private sector 

are collaborating on strategies for managing water under a 

changing climate through partnerships like the Western Governors’ Association (WGA) and WestFAST (Western Federal 

Agency Support Team). 

• In Wisconsin, the Northern Institute of Applied Climate Science and the U.S. Forest Service, working with multiple partners, initiated a “Climate Change Response Framework” integrating climate-impacts science with forest management.

• In Cape Cod, Massachusetts, the U.S. Department of Transportation’s Volpe Center worked with federal, regional, state, 

and local stakeholders to integrate climate change mitigation and adaptation considerations into existing and future 

transportation, land-use, coastal, and hazard-mitigation processes. 

Adaptation Activity

Continued
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Figure 28.4. Adaptation Activity

1. The State of Hawai‘i, Office of Planning, in cooperation with university, private, state, and federal scientists and others, has 

drafted a framework for climate change adaptation that identifies sectors affected by climate change, and outlines a process for 

coordinated statewide adaptation planning.123

2. One of the priorities of the Hawai‘i State Plan is preserving water sources through forest conservation, as indicated in their “Rain 

Follows The Forest” report.124

3. New England Federal Partners is a multi-agency group formed to support the needs of the states, tribes, and communities of the 

New England Region and to facilitate and enable informed decision-making on issues pertaining to coastal and marine spatial 

planning, climate mitigation, and climate adaptation throughout the region.125

4. Philadelphia is greening its combined sewer infrastructure to protect rivers, reduce greenhouse gas emissions, improve air quality, 

and enhance adaptation to a changing climate.126

5. Keene, NH, developed a Comprehensive Master Plan that emphasizes fostering walkable, mixed-use neighborhoods by putting 

services, jobs, homes, arts and culture, and other community amenities within walking distance of each other. The plan also 

calls for sustainable site and building designs that use resources efficiently. These strategies were identified in the city’s 2007 

Adaptation Plan as ways to build resilience while reducing greenhouse gas emissions.127

6. New York City has created a Green Infrastructure Plan and is committed to goals that include the construction of enough green 

infrastructure throughout the city to manage 10% of the runoff from impervious surfaces by 2030.128

7. Lewes, DE, undertook an intensive stakeholder process to integrate climate change into the city’s updated hazard mitigation plan.62

8. Local governments and tribes throughout Alaska, such as those in Homer, are planting native vegetation and changing the coastal 

surface, moving inland or away from rivers, and building riprap walls, seawalls or groins, which are shore-protection structures 

built perpendicular to the shoreline (see also: Ch. 22:Alaska; Ch. 12: Indigenous Peoples).129

9. Alaskan villages are physically being relocated because of climate impacts such as sea level rise and erosion; these include 

Newtok, Shishmaref, Kivalina, and dozens of other villages.130

10.Cedar Falls, Iowa, passed legislation in 2009 that includes a new floodplain ordinance that expands zoning restrictions from the 

100-year floodplain to the 500-year floodplain, because this expanded floodplain zone better reflects the flood risks experienced 

by the city during the 2008 floods.131

11. In January 2011, the Michigan Department of Community Health (MDCH) released the Michigan Climate and Health Adaptation 

Plan, which has a goal of “preparing the public health system in Michigan to address the public health consequences of climate 

change in a coordinated manner.” In September 2010, MDCH received three years’ funding to implement this plan as part of the 

Climate-Ready States and Cities Initiative of CDC.132

12.Chicago was one of the first cities to officially integrate climate adaptation into a citywide climate adaptation plan. Since its release, 

a number of strategies have been implemented to help the city manage heat, protect forests, and enhance green design, such 

as their work on green roofs.64

13.Grand Rapids, MI, recently released a sustainability plan that integrates future climate projections to ensure that the economic, 

environmental, and social strategies embraced are appropriate for today as well as the future.133

14.Tulsa, OK, has a three-pronged approach to reducing flooding and managing stormwater: a) prevent new problems by looking 

ahead and avoiding future downstream problems from new development (for example, requiring on-site stormwater detention); 

b) correct existing problems and learn from disasters to reduce future disasters (for example, through watershed management 

and the acquisition and relocation of buildings in flood-prone areas); and c) act to enhance the safety, environment, and quality 

of life of the community through public awareness, an increase in stormwater quality, and emergency management.134

15.Firewise Communities USA is a nationwide program of the National Fire Protection Association and is co-sponsored by USDA 

Forest Service, DOI, and the National Association of State Foresters. According to the Texas Forest Service, there are more than 

20 recognized Texas Firewise Communities. The Texas Forest Service works closely with communities to help them to reach 

Firewise Community status and offers a variety of awareness, educational, informational, and capacity-building efforts, such as 

Texas Wildscapes, a program that assists in choosing less fire-friendly plants.135

Continued
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28: ADAPTATION

16.After the heavy rainfall events of 2004 that resulted in significant erosion on his farms, Dan Gillespie, a farmer with the Natural 

Resources Conservation Service in Norfolk, NE, began experimenting with adding cover crops to the no-till process. It worked 

so well in reducing erosion and increasing crop yields that he is now sharing his experience with other farmers. (http://www.lenrd.

org/projects-programs/; http://www.notill.org/)

136

17. Point Reyes National Seashore is preparing for climate change by removing two dams that are barriers to water flow and fish 

migration. This change restores ecological continuity for anadromous fish (those that migrate from the sea to fresh water to 

spawn), creating a more resilient ecosystem.137

18.Western Adaptation Alliance is a group of eleven cities in five states in the Intermountain West that share lessons learned in 

adaptation planning, develop strategic thinking that can be applied to specific community plans, and join together to generate 

funds to support capacity building, adaptation planning, and vulnerability assessment.138

19.Navajo Nation used information on likely changes in future climate to help inform their drought contingency plan.139

20.California Department of Health and the Natural Resources Defense Council collaborated to create the Public Health Impacts 

of Climate Change in California: Community Vulnerability Assessment and Adaptation Strategies report, which is being used to 

inform public health preparedness activities in the state.140

21. State of Idaho successfully integrated climate adaptation into the state’s Wildlife Management Plan. (http://fishandgame.idaho.

gov/public/wildlife/cwcs/)

8

22.The Rising Tides Competition was held in 2009 by the San Francisco Bay Conservation and Development Commission to elicit 

ideas for how the Bay could respond to sea level rise.141

23.Flagstaff, Arizona, created a resilience strategy and passed a resilience policy, as opposed to a formal adaptation plan, as a 

means to institutionalize adaptation efforts in city government operations.142

24.The Olympic National Forest and Olympic National Park were sites of case studies looking at how to adapt management of federal 

lands to climate change. Sensitivity assessments, review of management activities and constraints, and adaptation workshops 

in the areas of hydrology and roads, fish, vegetation, and wildlife were all components of the case study process.143

25.King County Flood Control District was reformed to merge multiple flood management zones into a single county entity for funding 

and policy oversight for projects and programs – partly in anticipation of increased stormwater flows due to climate change.144

26.The Water Utilities Climate Alliance has been working with member water utilities to ensure that future weather and climate 

considerations are integrated into short- and long-term water management planning. (http://www.wucaonline.org/html/)

90

27. Seattle’s RainWatch program uses an early warning precipitation forecasting tool to help inform decisions about issues such as 

drainage operations. (http://www.atmos.washington.edu/SPU/)

19

28.City of Portland and Multnomah County created a Climate Action Plan that includes indicators to help them gauge progress in 

planning and implementing adaptation actions.145

29.In 2010, the state of Louisiana launched a $10 million program to assist communities that had been affected by Hurricanes 

Gustav and Ike in becoming more resilient to future environmental problems. Twenty-nine communities from around the state 

were awarded resiliency development funds. The Coastal Sustainability Studio at Louisiana State University started working in 

2012 with all 29 funded communities, as well as many that did not receive funds, to develop peer-learning networks, develop 

best practices, build capacity to implement plans, and develop planning tools and a user-inspired and useful website to increase 

community resiliency in the state.146

30.U.S. Fish and Wildlife Service and The Nature Conservancy are cooperating in a pilot adaptation project to address erosion 

and saltwater intrusion, among other issues, in the Alligator River Refuge. This project incorporates multiple agencies, native 

knowledge, community involvement, local economics, and technical precision.147

31. North and South Carolina are actively working to revise their state wildlife strategies to include climate adaptation.82

32.The Southeast Florida Climate Change Compact is a collaboration of the four southernmost counties in Florida (Monroe, Broward, 

Palm Springs, and Miami-Dade) focusing on enhancing regional resilience to climate change and reducing regional greenhouse 

gas emissions.67
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28: ADAPTATION

Section 3: Next Steps

Adaptation to climate change is in a nascent stage. The 

Federal Government is beginning to develop institutions 

and practices necessary to cope with climate change, 

including efforts such as regional climate centers within the 

U.S. Department of Agriculture, the National Oceanic and 

Atmospheric Administration (a division of the U.S. Department 

of Commerce), and the U.S. Department of the Interior. While 

the Federal Government provides financial assistance in 

federally-declared disasters, it is also enabling and facilitating 

early adaptation within states, regions, local communities, 

and the public and private sectors.11 The approaches include 

working to limit current institutional constraints to effective 

adaptation, funding pilot projects, providing useful and 

usable adaptation information – including disseminating best 

practices and helping develop tools and techniques to evaluate 

successful adaptation. 

Despite emerging efforts, the pace and extent of adaptation 

activities are not proportional to the risks to people, property, 

infrastructure, and ecosystems from climate change; 

important opportunities available during the normal course 

of planning and management of resources are also being 

overlooked. A number of state and local governments are 

engaging in adaptation planning, but most have not taken 

action to implement the plans.107 Some companies in the 

private sector and numerous non-governmental organizations 

have also taken early action, particularly in capitalizing on the 

opportunities associated with facilitating adaptive actions. 

Actions and collaborations have occurred across all scales. At 

the same time, barriers to effective implementation continue 

to exist (see Section 2). 

One of the overarching key areas of focus for global change 

research is enabling research and development to advance 

adaptation across scales, sectors, and disciplines. This includes 

social science research for overcoming the barriers identified 

in Section 2, such as strategies that foster coordination, better 

communication, and knowledge sharing amongst fragmented 

governing structures and stakeholders. Research on the 

kinds of information that users desire and how to deliver that 

information in contextually appropriate ways and research on 

decision-making in light of uncertainty about climate change 

and other considerations will be equally important. In addition 

to these areas, emerging areas of emphasis include: 

• Costs and Benefits of Adaptation: Methodologies to evaluate 

the relevant costs of adaptation options, as well as the costs 

of inaction, need to be developed.6,102

• A Compendium of Adaptation Practices: A central and 

streamlined database of adaptation options implemented at 

different scales in space and time is needed. Information on 

the adaptation actions, how effective they were, what they 

cost, and how monitoring and evaluation were conducted 

should be part of the aggregated information.11,20,31

• Adaptation and Mitigation Interactions: Research and analysis on the growing and competing demands for land, water, 

and energy and how mitigation actions could affect adaptation options, and vice versa.4,27,81,148

• Critical Adaptation Thresholds: Research to identify critical 

thresholds beyond which social and/or ecological systems 

are unable to adapt to climate change. This should include 

analyzing historical and geological records to develop models 

of “breakpoints”.2,31,149

• Adaptation to Extreme Events: Research on preparedness 

and response to extreme events such as droughts, floods, 

intense storms, and heat waves in order to protect people, 

ecosystems, and infrastructure. Increased attention must be 

paid to how extreme events and variability may change as 

climate change proceeds, and how that affects adaptation 

actions.11,150

Effective adaptation will require ongoing, flexible, transparent, inclusive, and iterative decision-making processes, collaboration across scales of government and sectors, and the 

continual exchange of best practices and lessons learned. All 

stakeholders have a critical role to play in ensuring the preparedness of our society to extreme events and long-term 

changes in climate.

Section 4: Case Studies

Illustrative Case One: National Integrated Drought Information System 

NIDIS (National Integrated Drought Information System), 

originally proposed by the Western Governors’ Association 

(WGA) and established by Congress in 2006,151 is a federallycreated entity that improves the nation’s capacity to 

proactively manage drought-related risks across sectors, 

regions, and jurisdictions. It was created by Congress to 

“enable the Nation to move from a reactive to a more 

proactive approach to managing drought risks and impacts.” 

NIDIS has successfully brought together government partners 

and research organizations to advance a warning system for 

drought-sensitive areas.

The creation of NIDIS involved many years of development and 

coordination among federal, state, local, regional, and tribal 

partners with the help of Governors’ associations and Senate 

and Congressional leaders. NIDIS provides: 1) drought early 

warning information systems with regional detail concerning 

onset and severity; 2) a web-based portal (www.drought.gov); 
688 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

28: ADAPTATION

3) coordination of federal research in support of and use of 

these systems; and 4) leveraging of existing partnerships and of 

forecasting and assessment programs. NIDIS currently supports 

work on water supply and demand, wildfire risk assessment 

and management, and agriculture. Regional drought early 

warning system pilot projects have been established to 

illustrate the benefits of improved knowledge management, 

improved use of existing and new information products, and 

coordination and capacity development for early warning 

systems. These prototype systems are in the Upper Colorado 

Basin, the Apalachicola-Chattahoochee-Flint River Basin in 

the Southeast, the Four Corners region in the Southwest, and 

California. The NIDIS Outlook in the Upper Colorado Basin 

provides early warning information every week, for example, 

that is utilized by a variety of users from federal agencies, 

water resource management, and the recreation industry. 

The Western Governors’ Association, the U.S. Congress, 

and others have formally acknowledged that NIDIS provides 

a successful example of achieving effective federal-state 

partnerships by engaging both leadership and the public, and 

establishing an authoritative basis for integrating monitoring 

and research to support risk management. Some of NIDIS’s 

keys to success include:

• Usable Technology and Information for Decision 

Support: The production of the U.S. Drought 

Monitor map, which integrates multiple indicators and indices from many data sources, was 

developed before NIDIS was established and has 

become a useful visual decision support tool for 

monitoring and characterizing drought onset, 

severity, and persistence. NIDIS has engaged regional and local experts in refining the regional 

details of this national product and in “ground 

truthing” maps via email discussions and webinars (Figure 28.5). 

• Financial Assistance: Federal funding was allocated to NOAA specifically for NIDIS, but leveraged in kind by other agencies and partners.

• Institutional/Partnerships: Effective collaborations, partnerships, and coordination with 

NOAA, WGA, USDA, DOI, and USGS as well as 

local, regional, state, and tribal partners and 

with the National Drought Mitigation Center at 

the University of Nebraska, Lincoln, have led to 

multi-institutional “buy-in.”

• Institutional/Policy: The NIDIS Act was oriented 

toward the improvement of coordination across 

federal agencies and with regional organizations, 

universities, and states. It focused on the application of technology, including the Internet, and on 

impact assessments for decision support. A key aspect of NIDIS is the development of an ongoing regional outlook forum 

based on the above information to build awareness of the 

drought hazard and to embed information in planning and 

practice (in partnership with the National Drought Mitigation 

Center, the Regional Integrated Sciences and Assessments 

(RISA), and other research-based boundary organizations) to 

reduce risks and impacts associated with drought.

• Leadership and Champions: NIDIS supporters worked at all 

levels over more than two decades (1990s and 2000s) to establish the NIDIS Act, including political groups (WGA, Southern Governors’ Association, National Governors Association, 

and U.S. Senators and Representatives), scientific leaders, 

and federal agencies (NOAA, USDA, DOI).

• Risk Perceptions: Whereas drought had been considered primarily a western issue in previous decades, drought is now 

regularly affecting the southern, southeastern, and northeastern parts of the country and response strategies are 

needed. During the 2012 drought, more than 63% of the contiguous U.S. by the end of July was classified as experiencing 

moderate to exceptional drought, and more than 3,200 heat 

records were broken in June 2012 alone.152

Figure 28.5. U.S. Drought Monitor Map accessed on August 20, 2012. 

The U.S. Drought Monitor is produced in partnership between the national 

Drought Mitigation Center at the University of Nebraska-Lincoln, the 

United States Department of Agriculture, and the National Oceanic and 

Atmospheric Administration. Map courtesy of NDMC-UNL.

U.S. Drought Monitor

August 14, 2012
689 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

28: ADAPTATION

Illustrative Case Two: Adaptive Governance in the Colorado River Basin

The Colorado River supplies water and valuable ecosystem 

services to 33 million people and is vulnerable to climate 

change because of decreases in mountain snowpack and water 

availability, increased competition among water users, fires, 

drought, invasive species, and extended extreme heat events, 

among other threats.13,153 The 1922 Colorado River Compact, 

which allocates water among seven U.S. states and Mexico, 

was agreed upon in a particularly wet time period;154 thus the 

river water is already over-allocated for current conditions. 

Given the likelihood of having less water because of climate 

change, resource managers and government leaders are 

increasingly recognizing that water must be managed with 

flexibility to respond to the projected impacts and the range 

of possible future climates (see Ch. 2: Our Changing Climate; 

Ch. 3: Water).13,155 Multiple actors across multiple disciplines, 

scales of governance (including tribal, local, state, and federal), 

non-governmental organizations, and the private sector are 

organizing and working together to address these concerns 

and the relationship between climate and other stresses in the 

basin.

The Western Governors’ Association (WGA) spearheaded 

adaptation efforts to enable federal, state, tribal, local, and 

private sector partners to address a range of issues, including 

climate change.13,155,156 For example, the Western Federal 

Agency Support Team (WestFAST), which was established 

in 2008, created a partnership between the Western States 

Water Council (WSWC) and 11 federal agencies with water 

management responsibilities in the western United States. 

The agencies created a work plan in 2011 to address three key 

areas: 1) climate change; 2) water availability, water use, and 

water reuse; and 3) water quality. To date they have produced 

the WestFAST Water-Climate Change Program Inventory, the 

Federal Agency Summary, and a Water Availability Studies 

Inventory (http://www.westgov.org/wswc/WestFAST.htm). 

The WSWC and the USACE produced the Western States 

Watershed Study (WSWS), which demonstrated how federal 

agencies could work collaboratively with western states 

on planning activities.157 In 2009, the WGA also adopted a 

policy resolution titled “Supporting the Integration of Climate 

Change Adaptation Science in the West” that created a Climate 

Adaptation Work Group composed of western state experts in 

air quality, forest management, water resources, and wildlife 

management. Other important adaptation actions were the 

SECURE Water Act in 2009, the Reclamation Colorado River 

Basin water supply and demand study, and the creation of 

NIDIS to support stakeholders in coping with drought.151,158

Illustrative Case Three: Climate Change Adaptation in Forests

Northern Wisconsin’s climate has warmed over the past 50 

years, and windstorms, wildfires, insect outbreaks, and floods 

are projected to become more frequent in this century.160 The 

resulting impacts on forests, combined with fragmented and 

complex forest ownership, create management challenges 

that extend across ownership boundaries, creating the need 

for a multi-stakeholder planning process.161

To address these concerns, the Northern Institute of Applied 

Climate Science, the USDA’s Forest Service, and many other 

partners initiated the Climate Change Response Framework 

to incorporate scientific research on climate change impacts 

into on-the-ground management. Originally developed as a 

pilot project for all-lands conservation in northern Wisconsin, 

it has expanded to cover three ecological regions (Northwoods 

[Figure 28.6], Central Hardwoods, and Central Appalachians) 

across eight states in the Midwest and Northeast. The 

Framework uses a collaborative and iterative approach to 

provide information and resources to forest owners and 

managers across a variety of private and public organizations. 

Several products were developed through the Framework in 

northern Wisconsin:

1. Vulnerability and mitigation assessments summarized the 

observed and projected changes in the northern Wisconsin 

climate, projected changes in forest composition and carbon 

stocks across a range of potential climates, and assessed 

related vulnerabilities of forest ecosystems in northern Wisconsin.160

2. Forest Adaptation Resources: Climate Change Tools and Approaches for Land Managers162 was developed to help managers identify management tactics that facilitate adaptation. 

A “menu” of adaptation strategies and approaches for planning, implementing, and monitoring adaptation activities 

was synthesized into an adaptation workbook from a broad 

set of literature and refined based on feedback from regional 

scientists and managers.163

3. A series of adaptation demonstrations was initiated to showcase ground-level implementation. The Framework and 

adaptation workbook provide a common process shared 

by diverse landowners and a formal network that supports 

Figure 28.6.

Northwoods Climate 

Change Response 

Framework Region 

(Figure Source: 

USDA Forest Service 

2012159).
690 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

28: ADAPTATION

cross-boundary discussion about different management objectives, ecosystems, and associated adaptation tactics.

From the beginning, the Framework has taken an adaptive 

management approach in its adaptation planning and projects. 

Lessons learned include: 

• Define the purpose and scope of the Framework and its components early, but allow for refinement to take advantage of 

new opportunities.

• Begin projects with a synthesis of existing information to 

avoid duplicating efforts. 

• Plan for the extra time necessary to implement true collaboration.

• Carefully match the skills, commitment, and capacity of people and organizations to project tasks.

• Maintain an atmosphere of trust, positivity, and sense of adventure, rather than dwelling on failures.

• Acknowledge and work with uncertainty, rather than submit 

to “uncertainty paralysis.”

• Recognize the necessity of effective communication among 

people with different goals, disciplinary backgrounds, vocabulary, and perspectives on uncertainty.

• Integrate the ecological and socioeconomic dimensions early 

by emphasizing the many ways that communities value and 

depend on forests.

• Use technology to increase efficiency of internal communication and collaboration, as well as outreach.

The Framework brings scientists and land managers together 

to assess the vulnerability of ecosystems based on scientific 

information and experience in order to plan adaptation actions 

that meet management goals. On-the-ground implementation 

has just begun, and an increased focus on demonstrations, 

monitoring, and evaluation will inform future adaptation 

efforts. 

Illustrative Case Four: Transportation, Land Use, and Climate Change – Integrating 

Climate Adaptation and Mitigation in Cape Cod, Massachusetts

Cape Cod, Massachusetts, a region of scenic beauty and 

environmental significance, is currently affected by sea level 

rise, coastal erosion, and localized flooding – impacts that are 

likely to be exacerbated by climate change.164,165 To address 

these concerns and help meet the state’s greenhouse gas (GHG) 

reduction target (25% reduction based on 1990 levels by 2020), 

the U.S. Department of Transportation’s Volpe Center worked 

with federal, regional, state, and local stakeholders to integrate 

climate change into existing and future transportation, landuse, coastal zone, and hazard mitigation planning through an 

initiative called the Transportation, Land Use, and Climate 

Change Pilot Project.164,166

The process was initiated through an expert elicitation held 

in mid-2010 to identify areas on Cape Cod that are or could 

potentially be vulnerable to sea level rise, flooding, and 

erosion. The Volpe Center then used a geographic information 

system (GIS) software tool to develop and evaluate a series of 

transportation and land-use scenarios for the Cape under future 

development projections.165,167 All scenarios were evaluated 

against a series of criteria that included: 1) reduction in vehicle 

miles traveled; 2) reduced heat-trapping gas emissions; 3) 

reduction in transportation energy use; 4) preservation of 

natural/existing ecosystems; 5) reduction in percentage of 

new population in areas identified as vulnerable to climate 

change impacts; and 6) increased regional accessibility to 

transportation.164

Once the preliminary scenarios were developed, a workshop 

was convened in which community and transportation 

planners, environmental managers, and Cape Cod National 

Seashore stakeholders selected areas for development and 

transit improvements to accommodate new growth while 

meeting the goals of reduced heat-trapping gas emissions, 

increased resilience to climate change, and the conservation 

of natural systems.165 Through interactive visualization tools, 

participants were able to see in real-time the impacts of 

their siting decisions, allowing them to evaluate synergies 

and potential tradeoffs of their choices and to highlight areas 

where conflict could or already does exist, such as increasing 

density of development in areas already or likely to be 

vulnerable to climate change.168 As a result, the stakeholders 

developed a refined transportation and land-use scenario 

that will support the region’s long-range transportation 

planning as well as other local, regional, and state plans. 

This updated scenario identifies strategies that have climate 

adaptation and mitigation value, helping to ensure that the 

region simultaneously reduces its heat-trapping gas footprint 

while building resilience to existing and future changes in 

climate.164,165 The overall success of the pilot project stemmed 

from the intensive stakeholder interaction at each phase of the 

project (design, implementation, and evaluation).
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28: ADAPTATION

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Pierce, T. P. Barnett, G. Bala, A. Mirin, A. W. Wood, C. Bonfils, 

B. D. Santer, and T. Nozawa, 2009: Detection and attribution of 

streamflow timing changes to climate change in the western United 

States. Journal of Climate, 22, 3838-3855, doi:10.1175/2009jcli2470.1. 

[Available online at http://journals.ametsoc.org/doi/

abs/10.1175/2009JCLI2470.1] 

Pierce, D. W., T. P. Barnett, H. G. Hidalgo, T. Das, C. Bonfils, B. 

D. Santer, G. Bala, M. D. Dettinger, D. R. Cayan, A. Mirin, A. 

W. Wood, and T. Nozawa, 2008: Attribution of declining western 

US snowpack to human effects. Journal of Climate, 21, 6425-6444, 

doi:10.1175/2008JCLI2405.1. [Available online at http://journals.

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Seager, R., and G. A. Vecchi, 2010: Greenhouse warming and 

the 21st century hydroclimate of southwestern North America. 

Proceedings of the National Academy of Sciences, 107, 21277-21282, 

doi:10.1073/pnas.0910856107. [Available online at http://www.

pnas.org/content/107/50/21277.full.pdf]

154. Gray, S. T., J. J. Lukas, and C. A. Woodhouse, 2011: Millenniallength records of streamflow from three major Upper Colorado 

River tributaries. JAWRA Journal of the American Water Resources 

Association, 47, 702-712, doi:10.1111/j.1752-1688.2011.00535.x. 

[Available online at http://onlinelibrary.wiley.com/doi/10.1111/

j.1752-1688.2011.00535.x/pdf] 

Woodhouse, C. A., S. T. Gray, and D. M. Meko, 2006: Updated 

streamflow reconstructions for the Upper Colorado River Basin. 

Water Resources Research, 42, doi:10.1029/2005WR004455. 

155. Brown, C., 2010: The end of reliability. Journal of Water Resources 

Planning and Management, 136, 143-145, doi:10.1061/(ASCE)

WR.1943-5452.65. 

156. Western Governors’ Association, 2006: Water Needs and Strategies 

for a Sustainable Future 26 pp., Western Governors’ Association, 

Western States Water Council, Denver, CO. 

——, 2008: Water Needs and Strategies for a Sustainable Future: Next 

Steps. Western Governors’ Association, 37 pp. 

——, 2010: Water Needs and Strategies for a Sustainable Future: 

2010 Progress Report. Western Governors’ Association and 

Western States Water Council, Denver, CO. [Available online 

at http://www.westgov.org/wswc/wswc_2010_complete%20

-compressed.pdf]

157. USACE, 2009: Western States Watershed Study: Report to 

the Western States Water Council 42 pp., U.S. Army Corps of 

Engineers. [Available online at http://www.westgov.org/wswc/

wsws%20main%20report_jan09.pdf]

158. Reclamation, 2011: Reclamation Managing Water in the West. 

SECURE Water Act Section 9503(c) - Reclamation Climate 

Change and Water 2011. P. Alexander, L. Brekke, G. Davis, S. 

Gangopadhyay, K. Grantz, C. Hennig, C. Jerla, D. Llewellyn, 

P. Miller, T. Pruitt, D. Raff, T. Scott, M. Tansey, and T. Turner, 

Eds., 226 pp., U.S. Department of the Interior, U.S. Bureau of 

Reclamation, Denver, CO. [Available online at http://www.usbr.

gov/climate/SECURE/docs/SECUREWaterReport.pdf] 

——, 2011: Reclamation Managing Water in the West: Interim 

Report No. 1, Colorado River Basin Water Supply and Demand 

Study, Status Report. U.S Department of the Interior, Bureau of 

Reclamation, Denver, CO. [Available online at http://www.usbr.

gov/lc/region/programs/crbstudy/Report1/StatusRpt.pdf]

159. USFS, cited 2012: Northern Institute of Applied Climate Science: 

Climate Change Response Framework. U.S. Department of 

Agriculture, U.S. Forest Service. [Available online at http://nrs.

fs.fed.us/niacs/climate/framework/]

160. Swanston, C. W., M. Janowiak, L. R. Iverson, L. R. Parker, D. 

J. Mladenoff, L. Brandt, P. Butler, M. St. Pierre, A. M. Prasad, 

S. Matthews, M. P. Peters, and D. Higgins, 2011: Ecosystem 

Vulnerability Assessment and Synthesis: A Report From the 

Climate Change Response Framework Project in Northern 

Wisconsin. Gen. Tech. Rep. NRS-82, 142 pp., U.S. Department of 

Agriculture, Forest Service, Northern Research Station, Newtown 

Square, PA. [Available online at http://www.fs.fed.us/nrs/pubs/

gtr/gtr_nrs82.pdf]

161. Joyce, L. A., G. M. Blate, S. G. McNulty, C. I. Millar, S. Moser, 

R. P. Neilson, and D. L. Peterson, 2009: Managing for multiple 

resources under climate change: National forests. Environmental 

Management, 44, 1022-1032, doi:10.1007/s00267-009-9324-6. 

Miles, P. D., 2010: Forest Inventory EVALIDator web-application 

version 4.01 beta. U.S. Department of Agriculture, Forest Service, 

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tmattribute.jsp] 

WDNR, 2009: Forest Ownership and Parcelization. Wisconsin 

Department of Natural Resources, Madison, WI. 
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162. Swanston, C., and M. Janowiak, Eds., 2012: Forest Adaptation 

Resources: Climate Change Tools and Approaches for Land 

Managers. General Technical Report NRS-87, 121 pp., U.S. 

Department of Agriculture, Forest Service, Newtown Square, PA. 

[Available online at http://www.nrs.fs.fed.us/pubs/gtr/gtr_nrs87.

pdf]

163. Butler, P., M. Janowiak, L. Brandt, and C. Swanston, 2011: Lessons 

learned from the Climate Change Response Framework Project in 

Northern Wisconsin: Newtown Square, PA, USDA Forest Service. 

24 pp. [Available online at http://www.nrs.fs.fed.us/niacs/localresources/docs/LESSONS_LEARNED_from_the_CCRFP.pdf] 

Janowiak, M. K., P. R. Butler, C. W. Swanston, L. R. Parker, M. 

J. St. Pierre, and L. A. Brandt, 2012: Adaptation workbook. Forest 

Adaptation Resources: Climate Change Tools and Approaches for Land 

Managers. General Technical Report NRS-87, C. Swanston, and M. 

Janowiak, Eds., U.S. Department of Agriculture, Forest Service,

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nrs87.pdf]

164. DOT, 2011: Interagency Transportation, Land Use, and Climate 

Change Cape Cod Pilot Project: Cape Cod Commission Action 

Plan, 22 pp., U.S. Department of Transportation: Federal Highway 

Administration, John A. Volpe National Transportation Systems 

Center. [Available online at http://www.volpe.dot.gov/sites/volpe.

dot.gov/files/docs/ccc_action_plan.pdf]

165. ——, 2011: Interagency Transportation, Land Use, and Climate 

Change Cape Cod Pilot Project. One-Pager., 20 pp., U.S. 

Department of Transportation: Federal Highway Administration, 

John A. Volpe National Transportation Systems Center, 

Washington, D.C. [Available online at http://www.volpe.dot.

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Project%20One%20Pager_092811.pdf]

166. Commonwealth of Massachusetts, 2004: Massachusetts Climate 

Protection Plan, 54 pp., Boston, MA. 

167. Esri, 2011: Climate Change Scenario Planning for Cape Cod: A 

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American Planning Association, 26 pp. [Available online at http://

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704 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

28: ADAPTATION

SUPPLEMENTAL MATERIAL

TRACEABLE ACCOUNTS

Process for Developing Key Messages 

A central component of the process were bi-weekly technical discussions held from October 2011 to June 2012 via teleconference 

that focused on collaborative review and summary of all technical 

inputs relevant to adaptation (130+) as well as additional published literature, the iterative development of key messages, and 

the final drafting of the chapter. An in-person meeting was held 

in Washington, D.C., in June 2012. Meeting discussions were followed by expert deliberation of draft key messages by the authors 

and targeted consultation with additional experts by the lead author of each key message. Consensus was reached on all key messages and supporting text.

Key message #1 Traceable Account

Substantial adaptation planning is occurring in 

the public and private sectors and at all levels of 

government; however, few measures have been 

implemented and those that have appear to be incremental changes.

Description of evidence base

The key message and supporting text summarize extensive 

evidence documented in the peer-reviewed literature as well as 

the more than 130 technical inputs received and reviewed as part 

of the Federal Register Notice solicitation for public input.

Numerous peer-reviewed publications indicate that a growing 

number of sectors, governments at all scales, and private and 

non-governmental actors are starting to undertake adaptation 

activity.9,13 Much of this activity is focused on planning with 

little literature documenting implementation of activities.8,11,82

Supporting this statement is also plentiful literature that profiles 

barriers or constraints that are impeding the advancement of 

adaptation activity across sectors, scales, and regions.42,68

Additional citations are used in the text of the chapter to 

substantiate this key message. 

New information and remaining uncertainties

n/a

Assessment of confidence based on evidence 

n/a

Key message #2 Traceable Account

Barriers to implementation of adaptation include 

limited funding, policy and legal impediments, and 

difficulty in anticipating climate-related changes at 

local scales.

Description of evidence base

The key message and supporting text summarize extensive 

evidence documented in the peer reviewed literature as well as 

the more than 130 technical inputs received and reviewed as 

part of the Federal Register Notice solicitation for public input. 

A significant quantity of reviewed literature profiles barriers or 

constraints that are impeding the advancement of adaptation 

activity across sectors, scales, and regions.11,20,42,68

Numerous peer-reviewed documents describe adaptation barriers 

(see Table 28.6). Moreover, additional citations are used in the 

text of the chapter to substantiate this key message. 

New information and remaining uncertainties

n/a

Assessment of confidence based on evidence

n/a

Key message #3 Traceable Account

There is no “one-size fits all” adaptation, but 

there are similarities in approaches across regions 

and sectors. Sharing best practices, learning by 

doing, and iterative and collaborative processes including stakeholder involvement, can help support 

progress.

Description of evidence base

The key message and supporting text summarize extensive 

evidence documented in the peer-reviewed literature as well as 

the more than 130 technical inputs received and reviewed as part 

of the Federal Register Notice solicitation for public input.
28: ADAPTATION

Traceable Accounts

705 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Literature submitted for this assessment, as well as additional 

literature reviewed by the author team, fully supports the concept 

that adaptations will ultimately need to be selected for their 

local applicability based on impacts, timing, political structure, 

finances, and other criteria.11,90 Similarities do exist in the types 

of adaptation being implemented, although nuanced differences 

do make most adaptation uniquely appropriate for the specific 

implementer. The selection of locally and context-appropriate 

adaptations is enhanced by iterative and collaborative processes 

in which stakeholders directly engage with decision-makers and 

information providers.11,20,28 While there are no “one-size fits all” 

adaptation strategies, evidence to date supports the message that 

the sharing of best practices and lessons learned are greatly aiding 

in adaptation progress across sectors, systems, and governance 

systems.82,86

Additional citations are used in the text of the chapter to 

substantiate this key message. 

New information and remaining uncertainties

n/a 

Assessment of confidence based on evidence

n/a

Key message #4 Traceable Account

Climate change adaptation actions often fulfill 

other societal goals, such as sustainable development, disaster risk reduction, or improvements in 

quality of life, and can therefore be incorporated 

into existing decision-making processes.

Description of evidence base

The key message and supporting text summarize extensive 

evidence documented in the peer-reviewed literature as well as 

the more than 130 technical inputs received and reviewed as part 

of the Federal Register Notice solicitation for public input. 

Literature submitted for this assessment, as well as additional 

literature reviewed by the author team, supports the message that 

a significant amount of activity that has climate adaptation value 

is initiated for reasons other than climate preparedness and/or has 

other co-benefits in addition to increasing preparedness to climate 

and weather impacts.11,20,82,86,116 In recognition of this and other 

factors, a movement has emerged encouraging the integration of 

climate change considerations into existing decision-making and 

planning processes (i.e., mainstreaming).5,11,40 The case studies 

discussed in the chapter amplify this point.

Additional citations are used in the text of the chapter to 

substantiate this key message. 

New information and remaining uncertainties

n/a 

Assessment of confidence based on evidence

n/a

Key message #5 Traceable Account

Vulnerability to climate change is exacerbated by 

other stresses such as pollution, habitat fragmentation, and poverty. Adaptation to multiple stresses 

requires assessment of the composite threats as 

well as tradeoffs amongst costs, benefits, and 

risks of available options. 

Description of evidence base

The key message and supporting text summarize extensive 

evidence documented in the peer-reviewed literature as well as 

the more than 130 technical inputs received and reviewed as part 

of the Federal Register Notice solicitation for public input.

Climate change is only one of a multitude of stresses affecting 

social, environmental, and economic systems. Activity to date and 

literature profiling those activities support the need for climate 

adaptation activity to integrate the concerns of multiple stresses 

in decision-making and planning.16,17,32 As evidenced by activities 

to date, integrating multiple stresses into climate adaptation 

decision-making and vice versa will require the assessment of 

tradeoffs amongst costs, benefits, the risks of available options, 

and the potential value of outcomes.5,90,111

Additional citations are used in the text of the chapter to 

substantiate this key message. 

New information and remaining uncertainties

n/a

Assessment of confidence based on evidence

n/a

Key message #6 Traceable Account

The effectiveness of climate change adaptation 

has seldom been evaluated, because actions have 

only recently been initiated and comprehensive 

evaluation metrics do not yet exist. 

Description of evidence base

The key message and supporting text summarize extensive 

evidence documented in the peer-reviewed literature as well as 

the more than 130 technical inputs received and reviewed as part 

of the Federal Register Notice solicitation for public input.

Numerous peer-reviewed publications indicate that no 

comprehensive adaptation evaluation metrics exist, meaning 

that no substantial body of literature or guidance materials 
28: ADAPTATION

Traceable Accounts

706 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

exist on how to thoroughly evaluate the success of adaptation 

activities.11,81,110 This is an emerging area of research. A challenge 

of creating adaptation evaluation metrics is the growing interest 

in mainstreaming; this means that separating out adaptation 

activities from other activities could prove difficult. 

Additional citations are used in the text of the chapter to 

substantiate this key message. 

New information and remaining uncertainties

n/a 

Assessment of confidence based on evidence

n/a
707

Climate Change Impacts in the United States

CHAPTER 29

RESEARCH NEEDS FOR CLIMATE AND 

GLOBAL CHANGE ASSESSMENTS

Recommended Citation for Chapter

Corell, R. W., D. Liverman, K. Dow, K. L. Ebi, K. Kunkel, L. O. Mearns, and J. Melillo, 2014: Ch. 29: Research Needs 

for Climate and Global Change Assessments. Climate Change Impacts in the United States: The Third National Climate 

Assessment, J. M. Melillo, Terese (T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 707-718. 

doi:10.7930/J03R0QR3. 

On the Web: http://nca2014.globalchange.gov/report/response-strategies/research-needs

First published May 2014. PDF revised October 2014. See errata (available at http://nca2014.globalchange.gov/downloads) for details.

Convening Lead Authors

Robert W. Corell, Florida International University and the GETF Center for Energy and Climate Solutions 

Diana Liverman, University of Arizona

Lead Authors

Kirstin Dow, University of South Carolina

Kristie L. Ebi, ClimAdapt, LLC

Kenneth Kunkel, CICS-NC, North Carolina State Univ., NOAA National Climatic Data Center

Linda O. Mearns, National Center for Atmospheric Research

Jerry Melillo, Marine Biological Laboratory
708 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

RESEARCH NEEDS FOR CLIMATE AND 29GLOBAL CHANGE ASSESSMENTS

Overview 

This chapter identifies key areas of research to provide foundational understanding and advance climate assessments. Many 

of these research topics overlap with those needed for advancing scientific understanding of climate and its impacts and for 

informing a broader range of relevant decisions.

The research areas and activities discussed in this chapter were 

identified during the development of the regional and sectoral 

technical input reports, from the contributions of over 250 National Climate Assessment (NCA) chapter authors and experts, 

and from input from reviewers. The five high-level research 

goals, five foundational cross-cutting research capabilities, 

and more specific research elements described in this chapter 

also draw from a variety of previous reports and assessments. 

These lists are provided as recommendations to the Federal 

Government. Priority activities for global change research 

across 13 federal agencies are coordinated by the U.S. Global 

Change Research Program, which weighs all activities within 

the more than $2 billion annual climate science portfolio relative to one another, considering agency missions, priorities, 

and budgets.

The last National Climate Assessment report, released by the 

U.S. Global Change Research Program (USGCRP) in 2009, recommended research on: 1) climate change impacts on ecosystems, the economy, health, and the built environment; 2) projections of climate change and extreme events at local scales; 

3) decision-relevant information on climate change and its 

impacts; 4) thresholds that could lead to abrupt changes in climate or ecosystems; 5) understanding the ways to reduce the 

rate and magnitude of climate change through mitigation; and 

6) understanding how society can adapt to climate change.1

Some of these topics have received continued or increased 

attention in the last five years – such as ecosystem impacts, 

downscaled climate projections, and mitigation options – but 

the current assessment finds that significant knowledge gaps 

remain for all of the research priorities identified in 2009. This 

conclusion is reinforced by the findings of many subsequent 

reviews by the National Research Council (NRC) and others 

who have continued to identify these as priorities. For example, the NRC’s America’s Climate Choices Panel on Advancing the Science of Climate Change and the Panel on Informing 

Effective Decisions and Actions2,3 highlighted several priorities 

that are relevant to climate assessments (see “Cross-Cutting 

Themes for the New Era of Climate Change Research Identified by America’s Climate Choices”). These included the need 

for a more comprehensive, interdisciplinary, use-inspired, and 

integrated research enterprise that combines fundamental 

understanding of climate change and response choices, that 

improves understanding of human-environment systems; that 

supports effective adaptation and mitigation responses, and 

that provides better observing systems and projections. In recognition of fiscal limitations, it is clear that research agencies 

and partners will need to work together to leverage resources 

and ensure coordinated and collaborative approaches. 

Research goals and cross-cutting capabilities

Five Research Goals

• Improve understanding of the climate system 

and its drivers

• Improve understanding of climate impacts and 

vulnerability

• Increase understanding of adaptation pathways

• Identify the mitigation options that reduce the 

risk of longer-term climate change

• Improve decision support and integrated assessment

Five Foundational Cross-Cutting Research Capabilities

• Integrate natural and social science, engineering, 

and other disciplinary approaches

• Ensure availability of observations, monitoring, 

and infrastructure for critical data collection and 

analysis

• Build capacity for climate assessment through 

training, education, and workforce development

• Enhance the development and use of scenarios

• Promote international research and collaboration
709 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

29. RESEARCH NEEDS FOR CLIMATE AND GLOBAL CHANGE ASSESSMENTS

The U.S. Global Change Research Program’s 2012-2021 Strategic Plan5 lists a number of strategic goals and objectives for 

advancing science, informing decisions, conducting sustained 

assessments, and communicating and educating about global 

change. The plan includes research priorities to understand 

Earth system components, their interactions, vulnerability and 

resilience; advance observations, modeling, and information 

management; and evaluate assessment processes and products. 

This chapter focuses specifically on the research identified 

through the National Climate Assessment process as needed to 

improve climate assessments. It is not intended to cover the full range of goals and related 

research priorities of the USGCRP and other 

groups, but instead to focus on research that 

will improve ongoing assessments. Therefore, 

many USGCRP priorities for climate change 

and global change science more broadly are 

not reflected here. The chapter does, however, directly support the USGCRP Strategic 

Plan’s sustained assessment activities (see 

“Goal 3 of the USGCRP Strategic Plan”).

This chapter is not intended to prescribe a 

specific research agenda but summarizes the 

research needs and gaps that emerged during 

development of this Third National Climate 

Assessment report that are relevant to the development of future USGCRP research plans.

During the development of this report, the authors were concerned that several important topics could not be comprehensively covered. In addition, several commenters noted the absence of these topics and felt that they were critical to consider 

in future reports. These include analyses of the economic costs 

of climate change impacts (and the associated benefits of mitigation and adaptation strategies); the implications of climate 

change for U.S. national security as a topic integrated with 

other regional and sectoral discussions; and the interactions 

of adaptation and mitigation options, including consideration 

of the co-benefits and potential unintended consequences of 

particular decisions. 

Cross-cutting themes for the new era of climate change

research identified by America’s Climate Choices

Research to Improve Understanding of Human-Environment Systems

1. Climate forcings, feedbacks, responses, and thresholds in the Earth system

2. Climate-related human behaviors and institutions

Research to Support Effective Responses to Climate Change

3. Vulnerability and adaptation analyses of coupled human-environment systems

4. Research to support strategies for limiting climate change

5. Effective information and decision support systems

Research Tools and Approaches to Improve Both Understanding and Responses

6. Integrated climate observing systems

7. Improved projections, analyses, and assessments

Source: America’s Climate Choices, Advancing the Science of Climate Change, 

National Academy of Sciences 2010, p. 92.4

Goal 3 of the usgcrp strategic plan

Conduct Sustained Assessments: Build sustained assessment capacity 

that improves the Nation’s ability to understand, anticipate, and respond to global change impacts and vulnerabilities.

The USGCRP will conduct and participate in national and international 

assessments to evaluate past, current, and likely future scenarios of 

global change and their impacts, as well as how effectively science 

is being used to support and inform the United States’ response to 

change. The USGCRP will integrate emerging scientific understanding 

of the Earth system into assessments and identify critical gaps and 

limitations in scientific understanding. It will also build a standing capacity to conduct national assessments and support those at regional 

levels. The USGCRP will evaluate progress in responding to change 

and identify science and stakeholder needs for further progress. The 

program will use this regular assessment to inform its priorities.
710 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

29. RESEARCH NEEDS FOR CLIMATE AND GLOBAL CHANGE ASSESSMENTS

Research Goals

Research Goal 1: Improve understanding of the climate system and its drivers 

Research investments across a broad range of disciplines are 

critically important to building understanding of, and in some 

cases reducing uncertainties related to, the physical and human-induced processes that govern the evolution of the climate system. This assessment demonstrates the continued 

need for high quality data and observations, analysis of Earth 

system processes and changes, and modeling that increases 

understanding and projections of climate change across scales. 

Social science research is also essential to improved understanding and modeling of the drivers of climate change, such 

as energy use and land-use change, as well as understanding 

impacts (see Research Goal 2). Assessing a changing climate 

requires understanding the role of feedbacks, thresholds, 

extreme events, and abrupt changes and exploring a range 

of scenarios (see Cross-Cutting Research Capabilities section) 

that drive changes in the climate system. 

This assessment reveals several research needs including:

• Continue efforts to improve the understanding, modeling, and projections of climate changes, especially at 

the regional scale, including driving forces of emissions 

and land-use change, changes in temperature, precipitation, soil moisture, runoff, groundwater, evapotranspiration, permafrost, ice and snow cover, sea level 

change, and ocean processes and chemistry; 

• Improve characterization of important sources of 

uncertainty, including feedbacks and possible thresholds in the climate system associated with changes in 

clouds, land and sea ice, aerosols (tiny particles in the 

atmosphere), greenhouse gases, land use and land cover, emissions scenarios, and ocean dynamics; 

• Develop indicators that allow for timely reporting and 

enhanced public understanding of climate changes and 

that allow anticipation and attribution of changes, including abrupt changes and extreme events in the context of a changing climate; and

• Advance understanding of the interactions of climate 

change and natural variability at multiple time scales, 

including seasonal to decadal changes (and consideration of climate oscillations including the El Niño Southern Oscillation, Pacific Decadal Oscillation, and the 

North Atlantic Oscillation), and extreme events (such as 

hurricanes, droughts, and floods).

Research Goal 2: Improve understanding of climate impacts and vulnerability

Assessing the implications of climate change for the U.S. relies not just on studies of the threats associated with changing 

weather patterns due to climate change and emerging chronic 

stresses such as sea level rise, but also on studies of who or 

what is exposed and sensitive to those threats, their underlying vulnerability, the associated costs, and adaptive capacity. 

The detailed sectoral and regional chapters of this assessment 

show that considerable progress has been made in understanding the extent to which natural and human systems in the 

U.S. are vulnerable to climate change and how these vulnerabilities combine with climatic trends and exposures to create 

impacts, but there is still a need to build capacity for assessing 

vulnerability.

This assessment suggests related research goals and activities 

including:

• Maintain and enhance research and development of 

data collection and analyses to monitor and attribute 

ongoing and emerging climate impacts across the United 

States, including changes in ecosystems, pests and pathogens, disaster losses, water resources, oceans, and social, 

urban, and economic systems. Priorities include ensuring 

enhanced geographic coverage of impacts research; the 

assessment of economic costs and benefits, as well as 

comparative studies of alternative response options; social science research focused on impacts; and the use of 

geospatial data systems;

• Assess the impacts of climatic extremes, high-end temperature scenarios, and abrupt climate change on ecosystems, health, food, water, energy, infrastructure, and 

other critical sectors, and improve modeling capabilities 

to better project and understand the vulnerability and 

resilience of human systems and ecosystems to climate 

change and other stresses such as land-use change and 

pollution;

• Increase the understanding of how climate uncertainties 

combine with socioeconomic and ecological uncertainties and identify improved ways to communicate the combined outcomes;

• Develop measurement tools and valuation methods

for documenting the economic consequences of climate 

changes;

• Expand climate impact analyses to focus on understudied but significant economic sectors such as natural resources and energy development (for example, mining, 
711 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

29. RESEARCH NEEDS FOR CLIMATE AND GLOBAL CHANGE ASSESSMENTS

oil, gas, and timber); manufacturing; infrastructure, land 

development, and urban areas; finance and other services; retail; and human health and well-being; and

• Investigate how climate impacts are affected by, or increase inequity in, patterns of vulnerability of particular 

population groups within the U.S. and abroad (for example, children, the elderly, the poor, and natural resource 

dependent communities).

Research Goal 3: Increase understanding of adaptation pathways 

This assessment and others, including the America’s Climate 

Choices Adapting to the Impacts of Climate Change report2

and Chapter 4 (on adaptation and mitigation options and responses) of the Intergovernmental Panel on Climate Change’s 

(IPCC) AR4 Synthesis Report,

6

 identifies a broad set of research 

needs for understanding and implementing adaptation. These 

include research on adaptation processes, adaptive capacity, 

adaptation option identification, implementation and evaluation, and adaptive management of risks and opportunities.

Important needs include research on the limits to, timing of, 

and tradeoffs in adaptation, and understanding of how adaptation interacts with mitigation activities, other stresses, and 

broader sustainability issues.

This assessment suggests research activities to:

•	 Identify the best practices for adaptation planning,

implementation, and evaluation across federal, 

state, and local agencies, tribal entities, private firms, 

non-governmental organizations, and local communities. This requires the rigorous and comparative analysis of the effectiveness of iterative risk management, 

adaptation strategies and decision support tools (for 

example, in terms of stakeholder views, institutional 

structures including regional centers and multi-agency programs, cost/benefit, assessment against stated 

goals or social and ecological indicators, model validation, and use of relevant information, including traditional knowledge); and

•	 Understand the institutional and behavioral barriers 

to adaptation and how to overcome them, including 

revisions to legal codes, building and infrastructure 

standards, urban planning, and policy practices.

Research Goal 4: Identify the mitigation options that reduce the risk of longer-term climate change

The severity of climate change impacts in the U.S. and the 

need for adapting to them over the longer term will depend 

on the success of efforts to reduce or sequester heat-trapping 

greenhouse gas (GHG) emissions, particularly those associated 

with the burning of fossil fuels but also those associated with 

changes in land use. Managing the consequences of climate 

change over this century depends on reducing concentrations 

of greenhouse gases, including short-lived climate pollutants 

such as black carbon (soot).

While such efforts are necessarily worldwide, the U.S. produces a significant share of global greenhouse gases and can 

assist and influence other countries to reduce their emissions. 

Assessments can play a significant role in providing a better 

information base from which to analyze mitigation options. 

Therefore, the mitigation section of this assessment (Ch. 27: 

Mitigation) noted the importance of research to understand 

and develop emission reductions through: 1) identifying climate and global change scenarios and their impacts; 2) providing a range of options for reducing the risks to climate and 

global change; and 3) developing options that allow joint mitigation-adaptation strategies, such as buildings that are more 

energy efficient and resilient to climate change impacts. 

More generally, the America’s Climate Choices report on Limiting the Magnitude of Climate Change3

 recommended that the 

U.S. promptly develop and implement appropriate strategies 

to reduce GHG emissions and identified important research 

needs, including the need to study the feasibility, costs, and 

consequences of different mitigation options. In addition, the 

report recommended research to support new technologies 

and the effective deployment of existing options, research into 

how best to monitor emissions and adherence to international 

policies, and research into how human behavior and institutions enable mitigation.3

This Third National Climate Assessment also suggests research 

activities to:

•	 Develop information that supports analysis of new 

technologies for energy production and use, carbon 

capture and storage, agricultural and land-use practices, and other technologies that could reduce or 

offset greenhouse gas emissions; research into the 

policy mechanisms that could be used to foster their 

development and implementation; analyses of the 

costs, benefits, tradeoffs, and synergies associated 

with different actions and combinations of actions; 

and improved understanding of the potential and 

risks of geoengineering;

•	 Investigate the co-benefits, interactions, feedbacks,

and tradeoffs between adaptation and mitigation

at the local and regional level, for example, in sectors such as agriculture, forestry, energy, health, and 
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29. RESEARCH NEEDS FOR CLIMATE AND GLOBAL CHANGE ASSESSMENTS

the built environment. This involves, as a priority, the 

assessment of the economics of impacts, mitigation,

and adaptation;

•	 Improve understanding of the effectiveness and 

timescales of mitigation measures through deepened understanding of the relationship between the 

fate of human-induced and natural carbon emissions, 

uptake by the terrestrial biosphere and oceans, and 

atmospheric concentrations; and

•	 Identify the critical social, cultural, institutional, 

economic, and behavioral processes that present 

barriers and opportunities for mitigation at the federal and international levels and by individuals, state 

and local governments, and corporations.

Research Goal 5: Improve decision support and integrated assessment 

For assessments to be useful to policy makers, they need to 

provide integrated results that can be used in decision-making. 

Research can develop tools that facilitate decision-making and 

the integration of knowledge.

Critical gaps in knowledge for decision support include the issues that affect the capacity of agencies, individuals, and communities to access and use the best available scientific information in support of decision-making, including the need to 

assess the ability of existing institutions, legal, and regulatory 

structures to respond to highly interdependent climate impacts. There are instances where policy barriers, institutional 

capacity or structure, or conflicting laws and regulations can 

create barriers to effective decisions. For instance, Chapter 12 

(Indigenous Peoples) notes that there is no institutional framework for addressing village relocation in response to climate 

change in Alaska,7 and Chapter 3 (Water) points out that existing water management institutions may be inadequate in the 

context of rapidly changing conditions. These instances point 

to research to evaluate whether the existing legal and regulatory structures, largely developed to address specific issues in 

isolation, can adequately respond to the highly interconnected 

issues associated with climate change. Decision support and 

integrated assessment also require research into the behavioral and other factors that influence individual decisions.

 Assessments can benefit from research activities that:

•	 Identify decision-maker needs within regions and 

sectors, and support the development of research 

methods, tools, and information systems and models 

for managing carbon, establishing early warning systems, providing climate and drought information services, and analyzing the legal, regulatory, and policy 

approaches that support adaptation and mitigation 

efforts in the context of a changing climate;

•	 Develop tools to support risk-based decision processes, including tools to identify risk management 

information needs, develop transferable vulnerability assessment techniques, and evaluate alternative 

adaptation options. In addition, tools are needed 

to improve understanding of consumption patterns 

and environmental consequences; effective resource 

management institutions; iterative risk management 

strategies; and social learning, cognition, and adaptive processes;

•	 Improve, fill gaps, and enhance research efforts to 

evaluate the effectiveness, costs, and benefits of 

mitigation and adaptation actions, including economic and non-economic metrics that evaluate the 

costs of action, inaction, and residual impacts. Focus is also needed on the development of methods 

and baseline information supporting evaluation of 

completed and ongoing adaptation, mitigation, and 

assessment efforts that will foster adaptive learning; 

and

•	 Develop, test, and expand integrated assessment 

models that link decisions about emissions with impacts under different development pathways and 

ways to categorize uncertainties in the supporting 

data.
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29. RESEARCH NEEDS FOR CLIMATE AND GLOBAL CHANGE ASSESSMENTS

Foundational Cross-Cutting Research Capabilities to Support Future Climate Assessments

This assessment identifies a set of five foundational cross-cutting research capabilities that 

are essential for advancing our ability to continue to conduct climate and global change 

assessments and for addressing the five research goals.

1. Integrate natural and social sciences, engineering, and other disciplinary approaches

Continued advances in comprehensive and useful climate assessments will rely on additional interdisciplinary research. 

Understanding of the coupled human-environment system is 

enriched by combining research from natural and social sciences with research and experience from the engineering, law, 

and business professions.

Because human activities and decisions are influencing many 

Earth system processes, models and observations of natural 

and social changes at planetary, regional, and local scales are 

needed to understand how climate is changing, its impacts on 

people and environments, and how human responses feedback on the Earth system.

Building experienced interdisciplinary research teams that are 

able to understand each other’s theories, methods, and language as well as the needs of stakeholders will allow for more 

rapid and effective assessments. 

Interdisciplinary research is needed, for example, to:

• Understand how hydrological drivers of water supply 

interact with changing patterns of water demand and 

evolving water management practices to increase risks of 

drought, or influence the effectiveness of adaptation and 

mitigation options;

• Understand climate change in the context of multiple 

stresses on Earth, ecological, and human systems;

• Bring together economic and quantitative assessment of 

climate impacts and policies with other more qualitative 

assessments that include non-market and cultural values; 

and

• Integrate the understanding of human behavior, engineering, and genomics to expand the range of choice in 

responding to climate change by providing and thoroughly 

evaluating new options for adaption and mitigation that 

improve economic development, energy, health, and food 

security.

2. Ensure availability of observations, monitoring, and infrastructure for critical data collection and analysis

Our understanding and ability to assess changes in climate 

and other global processes is based on a comprehensive and 

sustained system of observations that document the history 

of climate, socioeconomic, and related changes at spatial and 

time scales relevant to global, regional, and sectoral needs. 

The most recent USGCRP Strategic Plan5 states that to advance 

scientific knowledge of an integrated natural and human 

Earth system, an interoperable and integrated observational, 

monitoring, and data access capability is also essential. This 

observational capability is needed to gain the fundamental 

scientific understanding of essential status, trends, variability,

and changes in the Earth system. It should include the physical, chemical, biological, and human components of the Earth 

system over multiple space and time scales.

To attain their full value, observational systems must provide 

data that are responsive to the needs of decision-makers in 

government, industry, and society. These needs include observations and data that can inform the nation’s strategies to 

respond to climate and global change, including, for example, 

efforts to limit emissions, monitor public health, capture 

and store carbon, monitor changes in ocean processes, and 

implement adaptation strategies. This will require establishing explicit baseline conditions, specifying spatial detail and 

temporal frequency of observations, including social data, and 

setting standards for metadata (information about collected 

data), interoperability, and regulatory and voluntary reporting, 

such as those outlined in the Informing an Effective Response 

to Climate Change Panel Report of the National Research 

Council’s Americas Climate Choices series.8 These data need to 

be openly and widely available in order to support the best and 

most comprehensive science and for use in decision-making by 

a range of stakeholders.

This assessment shows that enhanced research and development will be necessary to ensure that the scope and integration 

of relevant scientific data improves overall utility for decisionmakers, including better ways to communicate metadata, data 

quality, and uncertainties. The observations must include critical geophysical variables such as temperature, precipitation, 

sea level changes, ocean circulation, atmospheric composition, 

and hydrology; the essential parameters that describe the biosphere; and social science information on drivers, impacts, and 

responses to climate and other global changes. More comprehensive and integrated data capabilities are needed to document the processes and patterns that drive natural and social 

feedbacks and better describe the mechanisms of abrupt 

change. Progress is needed in particular for data-poor regions,
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29. RESEARCH NEEDS FOR CLIMATE AND GLOBAL CHANGE ASSESSMENTS

focusing on inadequately documented socioeconomic, ecological, and health-related factors, and under-observed regional 

and sectoral data. There are opportunities to take advantage 

of citizen science observations where appropriate; monitor 

system resilience and robustness; and attend to physical and 

social systems that are not currently observed with sufficient 

temporal or spatial resolution to enable vulnerability analysis 

and decision support at regional and sectoral scales. More explicitly, strategic integration of our nation’s observations, monitoring, and data capabilities should be considered in order to: 

• Sustain and integrate the nation’s capacity to observe 

long-term changes in the Earth system and improve 

fundamental understanding of the complex causes and 

consequences of global change, including integration of 

essential socioeconomic, health, and ecological observations;

• Maintain and enhance advanced modeling capability,

including high-performance computing infrastructure, 

improvements in analysis of large and complex data sets, 

comprehensive Earth system and integrated assessment 

models, reanalysis, verification, and model comparisons;

• Better integrate observations and modeling to advance 

scientific understanding about past, present, and future 

climate within government, industry, and civil society; and

• Develop more fully the components and structure of a 

national climate and global change indicator system to 

support assessment that includes indicators of climate 

change, impacts, vulnerabilities, opportunities, and preparedness as well as trends and changes in land use, air 

and water pollution, water supply and demand, extreme 

events, diseases, public health, and agronomic data, 

coastal and ocean conditions (such as marine ecosystem 

health, ocean acidity, sea level, and salinity), cryosphere 

data (such as snow, sea ice conditions, ice sheets and glacier melt rates), and changes in public attitudes and understanding of climate change. All of these are important 

to assessing climate change, and should eventually be better coordinated at local, as well as national and regional 

levels in collaboration with local agencies.

3. Build capacity for climate assessment through training, education, and workforce development 

Building human capacity for improved assessments requires 

expansion of skills within the existing public and private sectors and developing a much larger workforce that excels at 

critical and interdisciplinary thinking. Useful capacities include 

the ability to facilitate and communicate research and practice, manage collaborative processes to allow for imaginative 

analysis and solutions, develop sustainable technologies to 

reduce climate risks, and build tools for decision-making in an 

internationally interdependent world.

A deeper understanding of the processes and impacts of climate change, disaster risk reduction, energy policy impacts, 

ecosystem services and biodiversity, poverty reduction, food 

security, and sustainable consumption requires new approaches to training and curriculum, as well as research to evaluate 

the effectiveness of different approaches to research and 

teaching.

Assessments will benefit from activities that:

• Strengthen approaches to education about climate, impacts, and responses including developing and evaluating 

the best ways to educate in the fields of science (natural 

and social), technology, engineering, and mathematics 

and related fields of study (such as business, law, medicine, and other relevant professional disciplines). Ideally, 

such training would include a deeper understanding of the 

climate system, natural resources, adaptation and energy 

policy options, and economic sustainability, and would 

build capacity at colleges and institutions, including minority institutions such as tribal colleges; and

• Identify increasingly effective approaches to developing a more climate-informed society that understands 

and can participate in assessments, including alternative 

media and methods for communication; this could also include a program to certify climate interpreters to actively 

assist decision-makers and policymakers to understand 

and use climate scenarios.

8
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29. RESEARCH NEEDS FOR CLIMATE AND GLOBAL CHANGE ASSESSMENTS

4. Enhance the development and use of scenarios

Scenarios are “coherent, internally consistent and plausible descriptions of possible future states of the world”9

 that provide 

reasoned projections of energy and land use, future population levels, economic activity, the structure of governance, social values, and patterns of technological change. They survey, 

integrate, and synthesize science, within and among scientific 

disciplines and across sectors and regions. Such scenarios are 

essential tools that enable projections of emissions, climate, 

vulnerabilities, and global change. They are indispensable for 

linking science and decision-making and for assessing choices 

about America’s climate future.

Stakeholders and scientists within this assessment identified 

a need for more fully developed scenario-building capabilities 

that better enable assessments at regional and sectoral scales 

in timeframes of relevance to policy and decision-making and 

that more effectively reflect climate and global change at 

these scales.

Achieving capacity in scenario development will:

• Enhance understanding of how and why climate may 

change and its implications, especially at the regional 

scale. For example, a set of scenarios can be used to better 

understand the way energy, land use, and policy choices 

create alternative emissions pathways; how changes at 

global scales can be downscaled to estimate local climate 

possibilities; how various socioeconomic development 

pathways increase or decrease climate vulnerability; and 

to assess alternative strategies for reducing emissions and 

implementing adaptation; and

• Develop new methods, tools, and skills for applying scenarios to policy development at local levels in order to 

broaden society’s understanding of a changing climate 

and to analyze the full range of policy choices. In addition, improve capabilities in integrated assessment modeling to inform policy analysis and allow stakeholders to 

co-produce information and explore options for local and 

national decisions.

5. Promote international research and collaboration

Research efforts in support of climate assessment are very 

dependent on the international research community. International teams conduct Earth system monitoring and analysis using observing systems that cannot be funded and maintained 

by any one country alone. Many of the impacts of climate 

change in the U.S. are closely linked to how climate affects 

other parts of the world. There is general understanding that 

impacts of climate change on U.S. socioeconomic systems are 

mediated or amplified through globally connected commodity 

chains and prices; more detailed research on climate change 

and its impacts elsewhere is needed to provide accurate assessments of what could happen to U.S. regional and local 

economies. The U.S. has the capacity to leverage investments 

in collaborative international climate and global change scientific research efforts, examples of which include IGBP (International Geosphere-Biosphere Programme), WCRP (World 

Climate Research Programme), DIVERSITAS (an international 

program of biodiversity science), IHDP (International Human 

Dimensions Programme) (as they evolve into or in affiliation 

with the new Future Earth program), and IGFA (International 

Group of Funding Agencies for Global Change Research). 

Supporting international collaborative research will:

• Contribute to international systems of data collection, 

monitoring, indicators, and modeling that closely track 

and project changes in Earth system dynamics, climate, 

human drivers, and climate impacts that are needed for 

national and international assessments;

• Assess the implications of climate change for globally 

shared common resources such as the oceans, polar regions, and migratory species; and

• Fill important gaps in understanding of how climate 

change in other countries affects U.S. food, energy, 

health, manufacturing, and national security.
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29. RESEARCH NEEDS FOR CLIMATE AND GLOBAL CHANGE ASSESSMENTS

Conclusions

This chapter summarizes research recommendations across a 

broad range of topics – research that the assessment authors 

deem essential to support future assessments. The authors 

recognize that federal agencies and others are making progress on many of these research areas and that sustained assessment is included in the goals of the USGCRP.

While the research goals discussed in this chapter are not 

ranked, the objectives listed below can be used as criteria 

for prioritizing these activities. The nation’s federal research 

investments in support of the sustained assessment strategy 

should be designed to enhance the nation’s ability to limit 

climate-related risk and increase the utility of scientific understanding in supporting decisions.

• Promote understanding of the fundamental behavior of 

the Earth’s climate and environmental systems: The consequences of climate variability and change will require 

enhanced investment in use-inspired research using both 

fundamental and applied analysis, providing a foundation 

for the nation’s sustained assessment process;

• Promote understanding of the socioeconomic impacts of 

a changing climate: Provide comprehensive understanding, including the development of indicators of the impacts and consequences of climate variability and change 

for regions and sectors within the United States; 

• Build capacity to assess risks and consequences: Support 

improved, timely, and accessible estimations and projections of climate and other global change risks, their consequences and relevance for stakeholders, associated costs 

and benefits, and interactions with other stresses;

• Support research that enables infrastructure for analysis: Sustain and enhance critical infrastructure, including 

observations and data essential to monitoring trends, projecting climate risks, and evaluating the effectiveness of 

responses in decision-making and policy implementation;

• Build decision-support capacity: Build the knowledge 

base essential for decision support including developing 

and evaluating climate mitigation and adaptation solutions, technology innovation, institutions, and behavioral 

change; and

• Support engagement of the private sector and investment communities: Develop strategies to leverage federal 

research investments by engaging the private sector more 

fully in research and technology development, including 

partnerships with the nation’s universities and scientific 

research institutions, to address critical gaps in knowledge 

and to build the nation’s future scientific, technical, and 

sustained assessment capacities.

• Leverage private sector, university, and international resources and partnerships: Take advantage of topics and 

expertise where the U.S. can leverage and complement 

private sector and university capabilities, obtain return 

on research investments, and lead internationally on research investment efforts; build capacity through education and training; support humanitarian response; and 

fill critical gaps in global knowledge of relevance to the 

United States.
717 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

1. Karl, T. R., J. T. Melillo, and T. C. Peterson, Eds., 2009: Global 

Climate Change Impacts in the United States. Cambridge University 

Press, 189 pp. [Available online at http://downloads.globalchange.

gov/usimpacts/pdfs/climate-impacts-report.pdf]

2. NRC, 2010: Adapting to Impacts of Climate Change. America’s Climate 

Choices: Report of the Panel on Adapting to the Impacts of Climate Change.

National Research Council. The National Academies Press, 292 

pp. [Available online at http://www.nap.edu/catalog.php?record_

id=12783]

3. ——, 2010: Limiting the Magnitude of Future Climate Change. America’s 

Climate Choices. Panel on Limiting the Magnitude of Future Climate Change.

National Research Council, Board on Atmospheric Sciences 

and Climate, Division of Earth and Life Studies. The National 

Academies Press, 276 pp. [Available online at http://www.nap.edu/

catalog.php?record_id=12785]

4. ——, 2010: Advancing the Science of Climate Change. America’s Climate 

Choices: Panel on Advancing the Science of Climate Change. National 

Research Council. The National Academies Press, 528 pp. 

[Available online at http://www.nap.edu/catalog.php?record_

id=12782]

5. USGCRP, 2012: The National Global Change Research Plan 

2012–2021: A Strategic Plan for the U.S. Global Change Research 

Program. 132 pp., The U.S. Global Change Research Program, 

Washington, D.C. [Available online at http://downloads.

globalchange.gov/strategic-plan/2012/usgcrp-strategic-plan-2012.

pdf]

6. IPCC, 2007: Adaptation and mitigation options and responses, 

and the inter-relationship with sustainable development, at global 

and regional levels. Climate Change 2007: Synthesis Report. Contribution 

of Working Groups I, II and III to the Fourth Assessment Report of the 

Intergovernmental Panel on Climate Change, Core Writing Team, R. K. 

Pachauri, and A. Reisinger, Eds., IPCC, 56-62. [Available online at 

http://www.ipcc.ch/pdf/assessment-report/ar4/syr/ar4_syr.pdf]

7. Bronen, R., 2011: Climate-induced community relocations: 

Creating an adaptive governance framework based in human rights 

doctrine. NYU Review Law & Social Change, 35, 357-408. [Available 

online at http://socialchangenyu.files.wordpress.com/2012/08/

climate-induced-migration-bronen-35-2.pdf]

8. NRC, 2010: Informing an Effective Response to Climate Change. America’s 

Climate Choices: Panel on Informing Effective Decisions and Actions Related to 

Climate Change. National Research Council, Board on Atmospheric 

Sciences and Climate, Division on Earth and Life Studies, National 

Academies Press, 348 pp. [Available online at http://www.nap.

edu/catalog.php?record_id=12784]

9. IPCC, 2007: Climate Change 2007: Impacts, Adaptation and Vulnerability. 

Contribution of Working Group II to the Fourth Assessment Report of the 

Intergovernmental Panel on Climate Change. M. L. Parry, O. F. Canziani, 

J. P. Palutikof, P. J. van der Linden, and C. E. Hanson, Eds. 

Cambridge University Press, 976 pp. 

10. NRC, 2011: America’s Climate Choices. National Research Council. 

The National Academies Press, 144 pp. [Available online at http://

www.nap.edu/catalog.php?record_id=12781]

Photo Credits

Introduction to chapter; Midwest farm in top banner: ©Michael 

DeYoung/Blend Images/Corbis

References

29: RESEARCH NEEDS FOR CLIMATE AND GLOBAL CHANGE 

ASSESSMENTS
718 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

Chapter Process: 

The author team asked each of the other chapter author teams to identify important gaps in knowledge and key research needs in the 

course of writing their chapters, particularly in the context of the needs for research to support future assessments. In addition to the 

lists provided by each chapter author team, the team also drew on analyses from over 100 technical and public review suggestions and a 

wide variety of technical and scholarly literature, especially the U.S. Global Change Research Program’s Strategic Plan5 and the National 

Research Council’s America’s Climate Choices reports,2,3,4,8,10 to compile a list of potential research needs. Using expert deliberation, 

including a number of teleconference meetings and email conversations among author team members, the author team agreed on highpriority research needs, organized under five research goals.

29: RESEARCH NEEDS FOR CLIMATE AND GLOBAL CHANGE 

ASSESSMENTS

SUPPLEMENTAL MATERIAL

TRACEABLE ACCOUNTS
719

Climate Change Impacts in the United States

CHAPTER 30

SUSTAINED ASSESSMENT: A NEW VISION 

FOR FUTURE U.S. ASSESSMENTS

Recommended Citation for Chapter

Hall, J. A., M. Blair, J. L. Buizer, D. I. Gustafson, B. Holland, S. C. Moser, and A. M. Waple, 2014: Ch. 30: Sustained Assessment: A New Vision for Future U.S. Assessments. Climate Change Impacts in the United States: The Third National Climate 

Assessment, J. M. Melillo, Terese (T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 719-726. 

doi:10.7930/J000001G. 

On the Web: http://nca2014.globalchange.gov/report/response-strategies/sustained-assessment

Convening Lead Authors

John A. Hall, U.S. Department of Defense

Maria Blair, Independent

Lead Authors

James L. Buizer, University of Arizona

David I Gustafson, Monsanto Company

Brian Holland, ICLEI – Local Governments for Sustainability

Susanne C. Moser, Susanne Moser Research & Consulting and Stanford University

Anne M. Waple, Second Nature and University Corporation for Atmospheric Research
720 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

30SUSTAINED ASSESSMENT:

A NEW VISION FOR FUTURE U.S. ASSESSMENTS

A primary goal of the U.S. National Climate Assessment 

(NCA) is to help the nation anticipate, mitigate, and adapt to 

impacts from global climate change, including changes in climate variability, in the context of other national and global 

change factors. Since 1990, when Congress authorized the 

U.S. Global Change Research Program (USGCRP) through the 

Global Change Research Act1 and required periodic updates on 

climate science and its implications, researchers from many 

fields have observed significant climate change impacts in every region of the United States. The accelerating pace of these 

changes (for example, the recent rapid reductions observed in 

the extent and thickness of Arctic sea ice), as well as scenariobased projections for future climate changes and effects, is 

articulated in this third NCA.

Based on recommendations stemming from the National 

Research Council (NRC), USGCRP in its most recent strategic 

plan2 identified the rationale and benefits of implementing a 

sustained assessment process. In response, a vision for a new 

approach to assessments took shape as the third NCA report 

was being prepared. The vision includes an ongoing process of 

working to understand and evaluate the nation’s vulnerabilities 

to climate variability and change and its capacity to respond. A 

sustained assessment, in addition to producing quadrennial assessment reports as required by law, recognizes that the ability 

to understand, predict, assess, and respond to rapid changes in 

the global environment requires ongoing efforts to integrate 

new knowledge and experience. It accomplishes this by: 1) advancing the science needed to improve the assessment process 

and its outcomes, building associated foundational knowledge, 

and collecting relevant data; 2) developing targeted scientific 

reports and other products that respond directly to the needs 

of federal agencies, state and local governments, tribes, other 

decision-makers, and end users; 3) creating a framework for 

continued interactions between the assessment partners and 

stakeholders and the scientific community; and 4) supporting 

the capacity of those engaged in assessment activities to maintain such interactions.

To provide decision-makers with more timely, concise, and 

useful information, a sustained assessment process would 

include both ongoing, extensive engagement with public and 

private partners and targeted, scientifically rigorous reports 

that address concerns in a timely fashion. A growing body of 

assessment literature has guided and informed the development of this approach to a sustained assessment.3,4,5

The envisioned sustained assessment process includes continuing and expanding engagement with scientists and other 

professionals from government, academia, business, and nongovernmental organizations. These partnerships broaden the 

knowledge base from which conclusions can be drawn. In addition, sustained engagement with decision-makers and end 

users helps scientists understand what information society 

wants and needs, and it provides mechanisms for researchers 

to receive ongoing feedback on the utility of the tools and data 

they provide.

An ongoing process that supports these forms of outreach 

and engagement allows for more comprehensive and insightful evaluation of climate changes across the nation, including 

how decision-makers and end users are responding to these 

changes. The most thoughtful and robust responses to climate 

change can be made only when these complex issues, including the underlying science and its many implications for the 

nation, are documented and communicated in a way that both 

scientists and non-scientists can understand.

This sustained assessment process will lead to better outcomes 

for the people of the United States by providing more relevant, 

comprehensible, and usable knowledge to guide decisions related to climate change at local, regional, and national scales. 

Additional details about the components of the sustained assessment process are provided in “Preparing the Nation for 

Change: Building a Sustained National Climate Assessment 

Process,” the first special report of the National Climate Assessment and Development Advisory Committee.6

Contributions of a Sustained Assessment Process

A sustained assessment process will not only include producing the quadrennial assessment reports required by the 1990 

GCRA, but it also will enable many other important outcomes. 

A well-designed and executed sustained assessment process 

will:

1. Increase the nation’s capacity to measure and evaluate 

the impacts of and responses to further climate change 

in the United States, locally, regionally, and nationally.

2. Improve the collection of assessment-related critical 

data, access to those data, and the capacity of users to 

work with datasets – including their use in decision support tools – relevant to their specific issues and interests. This includes periodically assessing how users are 

applying such data.
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30. SUSTAINED ASSESSMENT: A NEW VISION FOR FUTURE U.S. ASSESSMENTS

3. Support the creation of the first integrated suite of national indicators of climate-related trends across a variety of important climate drivers and responses.

4. Catalyze the production of targeted, in-depth special assessment reports on sectoral topics (for example, agriculture), cross-sectoral topics (for example, the connection between water and energy production), regional 

topics, and other topics that will help inform Americans’ 

climate choices about mitigation and adaptation. These 

reports will generate new insights about climate change, 

its impacts, and the effectiveness of societal responses. 

In addition, a second report category, referred to as 

foundational reports, will focus on improvements to 

specific aspects of the process (for example, scenarios 

and indicators) to reinforce the foundation for the overarching, but necessarily more constrained, quadrennial 

assessment reports. 

5. Facilitate the creation of, support, and leverage a network of scientific, decision-maker, and user communities for extended dialog and engagement regarding 

climate change.

6. Provide a systematic way to identify gaps in knowledge 

and uncertainties faced by the scientific community and 

by U.S. domestic and international partners and to assist in setting priorities for their resolution.

7. Enhance integration with other assessment efforts such 

as the Intergovernmental Panel on Climate Change and 

modeling efforts such as the Coupled Model Intercomparison Project.

8. Develop and apply tools to evaluate progress and guide 

improvements in processes and products over time. 

This will support an iterative approach to managing risks 

and opportunities associated with changing global and 

national conditions.

Assessments facilitate the collection of different kinds of information that can be integrated to yield new and useful scientific 

insights. The vision for the sustained assessment process is to 

continue to build knowledge about human and natural systems 

and their interactions to better understand the risks and opportunities of global change at multiple spatial and temporal 

scales. The sustained assessment process also can help define 

the range of information needs of decision-makers and end 

users relative to adaptation and mitigation, as well as the associated costs of impacts and benefits of response actions. 

Moreover, it is by its very nature a continuous process, uniquely positioned to support an iterative, risk-based approach to 

adaptation.

Finally, although a sustained assessment process allows for 

ongoing improvements in products and processes, it also 

requires underlying support systems. These can include access to observational data sources, support networks, and 

information management systems such as the Global Change 

Information System (GCIS; see section on “Data Collection, 

Access, and Analysis”). Other fundamental support for assessments includes various types of integrated and vulnerability 

assessment models, climate model intercomparison projects, 

data streams (for example, emissions data and socioeconomic 

data), processes for building scenarios and deploying them at 

critical junctures in the assessment process, and evaluation approaches.

Assessment Capacity

Scientific assessments require substantial scientific expertise 

and judgment, involving skills atypical of those required for 

routine research.4,5 Assessment capacity includes engaging 

knowledgeable and experienced people, developing networks 

to promote interactions, identifying and mentoring new scientific talent, and building in-depth understanding of a variety of 

economic, technical, and scientific topics. Building and maintaining capacity through all of these approaches is therefore 

critical to the smooth and efficient functioning of the assessment process.

Sustained interactions among scientists and stakeholders have 

consistently been shown to improve the utility and effectiveness of assessment processes and outcomes5

 and to facilitate 

the development of decision support tools.7 A sustained assessment provides the necessary coordination and infrastructure needed to maintain an ongoing dialog among producers 

and users of information so that decision-makers can manage 

risks and take advantage of opportunities more efficiently. 

This provides the capacity and flexibility to react to, and take 

advantage of, rapidly advancing developments in decision and 

climate science and changing conditions to inform robust decision-making and improve the utility and timeliness of future 

quadrennial assessment reports.

Data Collection, Access, and Analysis

Credible scientific information is needed on an ongoing basis 

to support fundamental understanding of the climate system 

and its interactions with ecological, economic, and social systems – and for the development of adaptation and mitigation strategies. Improved systems for data access can more 

effectively meet the requests of stakeholders for accessible, 

relevant, and timely information. An ongoing process can build 

a more complete information base relevant to climate change 

related impacts and vulnerabilities, and it can result in more 

sophisticated scientific analyses that support the mandated 
722 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

30. SUSTAINED ASSESSMENT: A NEW VISION FOR FUTURE U.S. ASSESSMENTS

quadrennial assessment reports in a more efficient and effective manner. Selecting which data to collect and analyze is a 

critical component of assessments of change. In addition, for 

certain assessment-related purposes, use of traditional knowledge may be appropriate and require different analytical approaches.

The sustained assessment process will facilitate the development and maintenance of a web-based assessment information discovery, access, and retrieval system that facilitates easy 

access to a range of information for those who need it, in a 

timely and authoritative manner (the GCIS of the USGCRP). A 

major short-term goal is to provide transparent and highlylinked access to the data used to support conclusions in the 

third NCA report, but this is only the first step in a much larger 

effort. Initially targeted audiences include assessment practitioners across various sectors and governmental levels.

Indicators

Indicators are measurements or calculations that represent 

important features of the status, trends, or performance of a 

system (such as the economy, agriculture, natural ecosystems, 

or Arctic sea ice cover). Indicators are used to identify and 

communicate changing conditions to inform both research and 

management decisions.8 The NCA indicator system is intended 

to focus on key aspects of change – as well as vulnerabilities, 

impacts, and states of preparedness – to inform decisionmakers and the public. In the context of ongoing assessment 

activities, these indicators can be tracked to provide timely, 

authoritative, and climate-relevant measurements regarding 

the status, rates of change, and trends of key physical, ecological, and societal variables.

Special and Foundational Reports

As currently envisioned, the sustained assessment process 

also paves the way for additional types of assessment-related reports that can help inform local, regional, and sectoral 

mitigation and adaptation activities and provide a foundation 

for more useful and more comprehensive quadrennial assessment reports. Completing in-depth assessments of national 

or regional importance and providing a constantly improving 

foundation for the quadrennial assessment reports provides 

for significant flexibility and enhanced policy relevance. Special topical assessment reports can investigate emerging issues 

of concern or help decision-makers understand the tradeoffs 

among different courses of action. Moreover, these types of 

assessments can encompass a more holistic, multi-disciplinary, 

and integrated approach that considers various types of data 

analyses that may not have been previously attempted. These 

more focused reports that emerge from ongoing assessment 

activities can blend the objectives of incorporating the latest 

science with responding relatively quickly to the most pressing stakeholder and government needs. Finally, foundational 

reports also can be produced on scenarios of climate change, 

sea level rise, demography, land-use change, and other issues 

critical to the assessment process.

A Network to Foster Partnerships, Encourage Engagement, and Develop Solutions

The USGCRP has long recognized the importance of partnerships, effective two-way communication, and ongoing and 

meaningful engagement.2 The five NRC America’s Climate 

Choices reports published in 2010 and 2011 also underscore 

the essential nature of this engagement (for example, NRC 

20109). Partnerships and engagement strategies among federal and non-federal participants are needed to: 1) communicate effectively about the assessment, including its products 

and processes and their relevance as actionable information;10

2) encourage participation and knowledge sharing; 3) create 

opportunities for meaningful engagement of end users and 

public and private decision-makers to inform the substance of 

the assessment; and 4) offer opportunities for input, direction, 

review, and feedback.

An important component of the new sustained assessment vision is NCAnet: a “network of networks” that helps to foster 

engagement in the NCA process and communicate products 

to a broader audience (for additional details about NCAnet, 

please see Appendix 1: Process). This network of partner organizations, including private sector, government, non-governmental organizations, and professional societies, leverages 

resources and facilitates communication and partnerships. By 

its first meeting in January 2012, NCAnet consisted of over 

three dozen partner organizations. Much of the network’s 

subsequent growth to over 100 partner organizations (as of 

fall 2013) has been driven by the partners’ own outreach and 

interest in building a community around the practice of assessment. NCAnet can assist in developing and supporting diverse 

science capabilities and assessment competencies within and 

outside of the Federal Government.
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30. SUSTAINED ASSESSMENT: A NEW VISION FOR FUTURE U.S. ASSESSMENTS

Evaluation of the Process

Ongoing evaluation of assessment processes and products, as 

well as incorporating the lessons learned over time, is a specific 

objective of the USGCRP Strategic Plan.2 Evaluation efforts are 

considered integral to enabling learning and adaptive management of the assessment process, measuring the ability to meet 

both legally required objectives and strategic goals, maintaining institutional memory, and improving the assessment process and its contributions to scientific understanding as well as 

to society. Ongoing improvements in the assessment process 

also will support an iterative approach to decision-making in 

the context of rapid change.

Recommendations on Research Priorities

The GCRA requires regular evaluations of gaps in knowledge 

and assessments of uncertainties that require additional scientific input. A sustained assessment process provides for regular updates on science needs to the USGCRP’s annual research prioritization process, as well as to the triennial and decadal 

revisions to its research plan.
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30: SUSTAINED ASSESSMENT: A NEW VISION FOR FUTURE

U.S. ASSESSMENTS

References

1. GCRA, 1990: Global Change Research Act (Public Law 101-606, 

104 Stat. 3096-3104), signed on November 16, 1990. [Available 

online at http://www.gpo.gov/fdsys/pkg/STATUTE-104/pdf/

STATUTE-104-Pg3096.pdf]

2. USGCRP, 2012: The National Global Change Research Plan 

2012–2021: A Strategic Plan for the U.S. Global Change Research 

Program. 132 pp., The U.S. Global Change Research Program, 

Washington, D.C. [Available online at http://downloads.

globalchange.gov/strategic-plan/2012/usgcrp-strategic-plan-2012.

pdf]

3. Cash, D. W., and S. C. Moser, 2000: Linking global and local 

scales: Designing dynamic assessment and management processes. 

Global Environmental Change, 10, 109-120, doi:10.1016/S0959-

3780(00)00017-0. 

Clark, W. C., R. B. Mitchell, and D. W. Cash, 2006: Ch. 1: 

Evaluating the influence of global environmental assessments. 

Global Environmental Assessments: Information and Influence, R. B. 

Mitchell, W. C. Clark, D. W. Cash, and N. Dickson, Eds., The MIT 

Press, 1-26. 

4. Farrell, A., and J. Jäger, Eds., 2005: Assessments of Regional and Global 

Environmental Risks: Designing Processes for the Effective Use of Science in 

Decision-Making, 301 pp. [Available online at http://www.amazon.

com/Assessments-Regional-Global-Environmental-Risks/

dp/1933115041] 

Mitchell, R. B., W. C. Clark, D. W. Cash, and N. M. Dickson, Eds., 

2006: Global Environmental Assessments: Information and Influence. MIT 

Press, 352 pp. 

5. NRC, 2007: Analysis of Global Change Assessments: Lessons Learned.

National Research Council, Committee on Analysis of Global 

Change Assessments, Board on Atmospheric Sciences and Climate, 

Division on Earth and Life Studies. National Academies Press, 196 

pp. [Available online at http://www.nap.edu/catalog.php?record_

id=11868]

6. Buizer, J., P. Fleming, S. L. Hays, K. Dow, C. Field, D. Gustafson, 

A. Luers, and R. H. Moss, 2013: Preparing the Nation for Change: 

Building a Sustained National Climate Assessment. National 

Climate Assessment and Development Advisory Committee, 

Washington, D.C. [Available online at http://www.nesdis.noaa.

gov/NCADAC/pdf/NCA-SASRWG%20Report.pdf]

7. CCSP, 2008: Preliminary Review of Adaptation Options for Climatesensitive Ecosystems and Resources. A Report by the U.S. Climate Change 

Science Program and the Subcommittee on Global Change Research. J. S. 

Baron, B. Griffith, L. A. Joyce, P. Kareiva, B. D. Keller, M. A. 

Palmer, C. H. Peterson, J. M. Scott, (Authors), S. H. Julius, and 

J. M. West, Eds. U.S. Environmental Protection Agency, 873 pp. 

[Available online at http://downloads.globalchange.gov/sap/sap4-

4/sap4-4-final-report-all.pdf]

8. NRC, 2000: Ecological Indicators for the Nation. National Research 

Council, Commission on Geosciences, Environment, and 

Resources. The National Academies Press, 198 pp. [Available 

online at http://www.nap.edu/catalog.php?record_id=9720]

9. ——, 2010: Adapting to Impacts of Climate Change. America’s Climate 

Choices: Report of the Panel on Adapting to the Impacts of Climate Change.

National Research Council. The National Academies Press, 292 

pp. [Available online at http://www.nap.edu/catalog.php?record_

id=12783]

10. Moser, S. C., and L. Dilling, 2011: Ch.11: Communicating climate 

change: Closing the science-action gap. The Oxford Handbook of 

Climate Change and Society, J. S. Dryzek, R. B. Norgaard, and D. 

Schlosberg, Eds., Oxford University Press, 161-174. [Available 

online at http://www.climateaccess.org/sites/default/files/Moser_

Communicating%20Climate%20Change_0.pdf]

11. USGCRP, 2010: The National Climate Assessment NCA Report 

Series, Volume 1. Midwest Regional Workshop: February 22-

24, 2010 Chicago, Illinois, 35 pp., U.S. Global Change Research 

Program, Washington, D.C. [Available online at http://downloads.

globalchange.gov/nca/workshop-reports/midwest-regionalworkshop-report.pdf] 

——, 2010: The United States National Climate Assessment NCA 

Report Series, Volume 2. Strategic Planning Workshop. U.S. Global 

Change Research Program, Asheville, NC. [Available online at 

http://globalchange.gov/what-we-do/assessment] 

——, 2010: The United States National Climate Assessment 

NCA Report Series, Volume 4: Planning Regional and Sectoral 

Assessments for the National Climate Assessment. Planning Regional 

and Sectoral Assessments for the National Climate Assessment, Reston, VA, 

U.S. Geological Survey, U.S. Global Change Research Program, 55 

pp. [Available online at http://downloads.globalchange.gov/nca/

workshop-reports/regional-sectoral-workshop-report.pdf]
30: SUSTAINED ASSESSMENT: A NEW VISION FOR FUTURE U.S. ASSESSMENTS

References

725 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

12. ——, 2011: National Climate Assessment Strategy - Summary, 3 

pp., U.S. Global Change Research Program, Washington, D.C. 

[Available online at http://www.globalchange.gov/images/NCA/

nca-summary-strategy_5-20-11.pdf]

13. DOC, 2011: National Climate Assessment Development and 

Advisory Committee; Request for Nominations and Notice of 

Meeting. Federal Register, 76, 11427-11429. [Available online at 

http://www.gpo.gov/fdsys/pkg/FR-2011-03-02/pdf/2011-4562.

pdf]

14. Karl, T. R., J. T. Melillo, and T. C. Peterson, Eds., 2009: Global 

Climate Change Impacts in the United States. Cambridge University 

Press, 189 pp. [Available online at http://downloads.globalchange.

gov/usimpacts/pdfs/climate-impacts-report.pdf] 

NAST, 2000: Climate Change Impacts on the United States: The 

Potential Consequences of Climate Variability and Change, Report 

for the US Global Change Research Program, 163 pp., U.S. Global 

Climate Research Program, National Assessment Synthesis Team, 

Cambridge, UK. [Available online at http://library.globalchange.

gov/downloads/download.php?id=124]

15. NRC, 2009: Informing Decisions in a Changing Climate. National 

Research Council, Panel on Strategies and Methods for ClimateRelated Decision Support, Committee on the Human Dimensions 

of Global Change, Division of Behavioral and Social Sciences and 

Education. National Academies Press, 200 pp. [Available online at 

http://www.nap.edu/catalog.php?record_id=12626]

Photo Credits

Introduction to chapter; sky in top banner: ©Image Source/Corbis
726 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

30: SUSTAINED ASSESSMENT: A NEW VISION FOR FUTURE

U.S. ASSESSMENTS

Process for Developing Key Messages: 

Planning for the sustained assessment process, and for including 

a description of the process in a chapter of the third NCA report, 

began as soon as the report process was launched. Mechanisms 

for creating and implementing a sustained process were included 

as key discussion points in early NCA process workshops.11 Prior 

to the formation of the chapter author teams, the need for a sustained assessment was described in the NCA Strategy Summary.12

The amended charter for the National Climate Assessment and 

Development Advisory Committee (NCADAC) specifies that the 

NCADAC is “to provide advice and recommendations toward the 

development of an ongoing, sustainable national assessment of 

global change impacts and adaptation and mitigation strategies 

for the Nation.”13 To that end, the NCADAC formed a working 

group on sustained assessment, and the USGCRP Interagency National Climate Assessment Working Group (INCA) made this topic 

a priority in their regular meetings. The USGCRP also established 

“conduct sustained assessments” as one of four programmatic 

pillars in its recent Strategic Plan.2

The sustained assessment author team drew on a wide variety of 

source materials in framing the need for a sustained assessment 

process, including calls for sustained assessment in both previous 

National Climate Assessment reports14 and in several publications 

from the National Research Council5,9,15 that focused specifically 

on the National Climate Assessment. The author team also considered a rich literature on assessments in general (for example, Farrell and Jäger 2005 and Mitchell et al. 20064

). In developing the 

chapter describing the sustained assessment process, the author 

team first worked with the NCADAC, especially the initial NCADAC 

working group on sustained assessment, and the INCA to develop 

a vision for sustained assessment and a list of activities required 

to implement this vision. They then collected feedback from each 

of the chapters’ convening lead authors, agencies, chairs of other 

NCADAC working groups, and targeted stakeholders. Drawing on 

these comments and the knowledge bases cited above, the author 

team came to consensus on the objectives and categories of activities provided in the chapter through teleconference and email 

discussions. The NCADAC formed a new author team to produce 

a longer special report on the sustained assessment process. The 

report was completed in the late summer of 2013.6

SUPPLEMENTAL MATERIAL

TRACEABLE ACCOUNTS
727 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

REPORT APPENDIX1 DEVELOPMENT PROCESS

The National Climate Assessment (NCA) supports the U.S. 

Global Change Research Program (USGCRP) and its Strategic 

Plan1 in multiple ways. The Strategic Plan focuses on climate 

science that informs societal objectives; the USGCRP program 

and the NCA help build an information base to support climaterelated decisions, including decisions to reduce human contributions to future climate change, and to adapt to changes that 

are occurring now and are projected in the future. In order to 

facilitate the integration of federal science investments with 

academic, public, and private sector climate change research, 

the Third NCA process focused on building strong relationships 

with stakeholders and experts outside the government. Early 

in the process, the National Climate Assessment and Development Advisory Committee (NCADAC) and NCA Coordination 

Office developed a strategy to engage a broad range of the 

American public. Open participation, communication, and 

feedback have been integral to the preparation of this farreaching assessment.2

NCA Goal and Vision

As established by the NCADAC,3 the overarching goal of the 

NCA process is to enhance the ability of the United States to 

anticipate, mitigate, and adapt to changes in the global environment that are increasingly linked to human activities.

The vision is to advance an inclusive, broad-based, and sustained process for developing, assessing, and communicating 

scientific knowledge of the impacts, risks, vulnerabilities, and 

response options associated with a changing global climate, 

and to support informed decision-making across the United 

States.

Legislative Foundations

The NCA is conducted under the auspices of the Global Change 

Research Act (GCRA) of 1990.

4

The mandate for the U.S. Global 

Change Research Program as a whole is: “To provide for development and coordination of a comprehensive and integrated 

United States research program which will assist the Nation 

and the world to understand, assess, predict, and respond to 

human-induced and natural processes of global change.”

Section 106 of the GCRA requires a report to the President and 

the Congress every four years that integrates, evaluates, and 

interprets the findings of the USGCRP; analyzes the effects of 

global change on the natural environment, agriculture, energy 

production and use, land and water resources, transportation, 

human health and welfare, human social systems, and biological diversity; and analyzes current trends in global change, 

both human-induced and natural, and projects major trends 

for the subsequent 25 to 100 years.

Institutional Foundations

U.S. Global Change Research Program 

USGCRP is a federation of the research components of 13 

federal departments and agencies that supports the largest 

investment in climate and global change research in the world. 

USGCRP coordinates research activities across agencies and 

establishes joint funding priorities for research. USGCRP’s 

Strategic Plan, adopted in 2012, focuses on four major goals: 

advance science, inform decisions, conduct sustained assessments, and communicate and educate.1

 The USGCRP agencies 

maintain and develop observations, monitoring, data management, analysis, and modeling capabilities that support the nation’s response to global change. The agencies that comprise 

the USGCRP are:

U.S. Department of Agriculture

U.S. Department of Commerce

U.S. Department of Defense

U.S. Department of Energy

U.S. Department of Health & Human Services

U.S. Department of the Interior

U.S. Department of State

U.S. Department of Transportation

U.S. Environmental Protection Agency

National Aeronautics and Space Administration

National Science Foundation

The Smithsonian Institution

U.S. Agency for International Development
728 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

APPENDIX 1: REPORT DEVELOPMENT PROCESS

The Subcommittee on Global Change Research (SGCR) oversees USGCRP’s activities. SGCR operates under the direction 

of the National Science and Technology Council’s (NSTC) Committee on Environment, Natural Resources, and Sustainability 

(CENRS) and is overseen by the White House Office of Science 

and Technology Policy (OSTP). The SGCR coordinates interagency activities through the USGCRP National Coordination 

Office (NCO) and interagency working groups (IWGs).

National Climate Assessment (NCA) Components

The Interagency NCA Working Group (INCA) is comprised of 

representatives of the 13 government agencies listed above, 

plus additional agencies that have chosen to engage in supporting the NCA activities. INCA is responsible for coordinating, developing, and implementing interagency activities for 

the NCA, providing critical input to identify and support future 

NCA products, and developing interagency assessment capacity at the national and regional scales. Through INCA, the agencies have supported the development of the 30 chapters and 

the process to create the Third NCA report in a variety of ways.

The National Climate Assessment and Development Advisory Committee (NCADAC) is a 60-member federal advisory 

committee established by the Department of Commerce on 

behalf of USGCRP. Forty-four non-federal NCADAC members 

represent the public, private, and academic sectors; 16 nonvoting ex-officio members represent the USGCRP agencies, the 

Department of Homeland Security, the SGCR, and the White 

House Council on Environmental Quality. The NCADAC charter charges the group with developing the Third NCA report 

and with providing recommendations about how to sustain 

an ongoing assessment process. The NCADAC selected the 

authors of the individual chapters and coordinated many of 

the assessment activities leading to this report. This included 

NCADAC meetings and more than 20 NCADAC subcommittee 

working groups on specific assessment needs (for example, 

regional and sectoral integration, engagement and communication, indicators, and international linkages). An Executive 

Secretariat of 12 individuals (a subset of the full committee) helps to coordinate the activities of the full committee.

The NCA Coordination Office is a part of the USGCRP National 

Coordination Office in Washington, D.C. The office is supported 

and funded through an interagency agreement with the University Corporation for Atmospheric Research (UCAR). A team 

of UCAR staff and federal detailees (agency employees asOrganization of NCA components

Figure 1.
729 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

APPENDIX 1: REPORT DEVELOPMENT PROCESS

signed to the NCA Coordination Office) with expertise in planning, writing, and coordinating collaborative climate and environmental science and policy activities provides support for 

the development of the NCA report and sustained assessment.

The NCA Technical Support Unit (TSU) is funded by the National Oceanic and Atmospheric Administration (NOAA) and is 

located at NOAA’s National Climatic Data Center in Asheville, 

NC. The TSU staff provides multiple kinds of support to the 

NCA, including climate science research, data management, 

web design, graphic design, technical and scientific writing 

and editing, publication production, and meeting support.

The National Climate Assessment Network (NCAnet) consists 

of more than 100 partner organizations that work with the 

NCA Coordination Office, NCADAC, report authors, and USGCRP agencies to engage producers and users of assessment 

information.5 Partners extend the NCA process and products 

to a broad audience through the development of assessmentrelated capacities and products, such as collecting and synthesizing data or other technical and scientific inputs into the NCA, 

disseminating NCA report findings to a wide range of users, 

engaging producers and users of assessment information, supporting NCA events, and producing communications materials 

related to the NCA and its report findings.

Creating the Third NCA Report

Process Development 

The NCA Engagement Strategy provides a vision for participation, outreach, communication, and education processes that help 

make the NCA process and products accessible and useful to a wide variety of audiences. The overall goal of engagement is 

to create a more effective and successful NCA – improving the processes and products of the effort so that they are credible, 

salient, and legitimate and building the capacity of participants to engage in the creation and use of NCA products in decisionmaking.2

 The strategy describes a number of mechanisms through which scientific and technical experts, decision-makers, and 

members of the general public might learn about and participate in the NCA process.

As part of the assessment process, a series of 14 process workshops helped establish consistent assumptions and 

methodologies. The resulting reports provide a consistent foundation for the technical input teams and chapter authors.

The NCA Coordination Office organized listening sessions, symposia, and sessions at professional society meetings during the 

development of the NCA report and sustained assessment process. These sessions provided updates on the NCA process, 

solicited broad input from subject matter experts, and collected feedback on the approach, topics, and methodologies under 

consideration.

Figure 2. This graphic illustrates the activities and products that were developed during the Third NCA report development process.

Third National Climate Assessment Report Process
730 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

APPENDIX 1: REPORT DEVELOPMENT PROCESS

Technical Input Reports

A public Request for Information6 resulted in submission of 

more than 500 technical input documents authored by more 

than 800 individuals from academia, industry, and government, 

including 25 technical inputs7 sponsored by USGCRP agencies. 

These inputs included documents and data sets for review and 

consideration by the author teams that developed the NCA 

report. Technical input authors used a variety of mechanisms 

to engage stakeholders in the scoping, writing, and review of 

their documents, including workshops, web-based seminars, 

and public comment periods, among other methods.

In addition, the Technical Support Unit climate science team 

developed nine peer-reviewed regional climate scenario documents (one for each of the eight regions and one for the contiguous United States),8

 providing a scientific consensus view 

of historical climate trends and projections under the IPCC 

Special Report on Emissions Scenarios (SRES) A2 and B1 scenarios.9

 A separate interagency committee developed four 

peer-reviewed sea level rise scenarios.10 These scenarios were 

used by chapter authors as underpinnings for their impact assessments.

Third NCA Report Draft Development and Review

The NCADAC selected two to three convening lead authors 

and approximately six lead authors for each chapter, based on 

criteria that included expertise, experience, geography, and 

ensuring a variety of perspectives. They included authors from 

the public and private sectors, non-governmental organizations, and universities. Beginning in December 2011, each of 

the author teams met multiple times by phone, web, and in 

person to produce and refine drafts of their chapters. Traceable accounts developed for each chapter provide transparent 

information about the authors’ decision processes, scientific 

certainty, and their level of confidence related to the key findings of their respective chapters. All authors served in a volunteer capacity.

After reviewing the draft Third NCA report, the NCADAC released it for public review and comment on January 14, 2013.11

Concurrently, the NCA underwent an independent expert review by the National Research Council, a part of the National 

Academies. A three-month review period allowed individuals 

and groups to examine the draft and provide comments aimed 

at improvement. The comments were provided using a secure 

online comment system to ensure that all comments were captured and appropriately addressed.

Regional town hall meetings, conducted by the NCA Coordination Office (one per region, plus coasts) and by NCAnet partners (three additional meetings), brought together authors, 

NCADAC members, and members of the public to discuss the 

NCA process and encourage participants to submit comments 

on the draft report. Report authors, NCADAC members, NCA 

staff, and NCAnet partners organized, spoke at, and participated in sessions at professional society meetings, web-based 

seminars, community meetings, and other events similarly

aimed at providing an overview of the draft report and encouraging comments.12

By the time the public comment period closed on April 12, 

2013, the online comment system received 4,161 comments

from 644 government, non-profit, and commercial sector employees, educators, students, and the general public. Chapter 

author teams and the NCADAC amended the draft report in 

response to comments and prepared written responses to 

each comment received, and external review editors evaluated the adequacy of the responses to the comments on each 

chapter. As the result of a NCADAC consensus decision, the 

entire review process was “blind”, that is, NCADAC members 

and authors did not know the identity of commenters when 

responding to each comment. The public comments (including 

commenters’ identities) and the chapter authors’ responses to 

those comments were posted online with the final report.

The National Research Council provided a second review of the 

report, and the NCADAC considered this review in developing 

a final draft for submission to federal agencies for review in 

fall 2013.

NCA Final Report

Any adjustments to the NCADAC’s Fall 2013 draft as a result of 

the government review process were made with the authors’ 

approval, and the NCADAC approved the final form of the report in Spring 2014. Having been accepted and finalized following government review, the report is now provided as the 

assessment by the Federal Government of the United States, 

pursuant to the requirements of the Global Change Research 

Act. A number of products derived from the report support the 

outreach activities following the report release.

Engagement Activities

What follows is a sample of activities convened in support of 

the development of the Third NCA Report. A full list of activities is available online at http://assessment.globalchange.gov.

NCADAC Meetings: All meetings were open the public. The 

presentations, documents, and minutes for each NCADAC 

meeting are available online at http://www.nesdis.noaa.gov/

NCADAC/Meetings.html.

• April 4-6, 2011, Washington, DC http://www.nesdis.noaa.

gov/NCADAC/April_4_Meeting.html

• May 20, 2011, Teleconference

• August 16-18, 2011, Arlington, VA
731 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

APPENDIX 1: REPORT DEVELOPMENT PROCESS

• November 16-17, 2011, Boulder, CO

• April 10, 2012, Teleconference

• June 14-15, 2012, Washington, DC

• August 15, 2012, Teleconference

• September 27, 2012, Teleconference

• November 14-15, 2012, Silver Spring, MD

• January 11, 2013, Teleconference

• May 13, 2013, Teleconference

• July 9-10, 2013, Washington, DC

• November 18, 2013, Teleconference

• February 20-21, 2014, Washington, DC

• Spring 2014, Final approval of the Third NCA via teleconference

Process and Methodology Workshops: Reports from these 

workshops are available online at http://www.globalchange.

gov/what-we-do/assessment/nca-activities/workshop-andmeeting-reports.

• Midwest Regional Workshop, February 2010, Chicago, IL

• Strategic Planning Workshop, February 2010, Chicago, IL 

• Scoping the Product(s) and Work Plan for the Third National Assessment, June 2010, Washington, DC [no report 

available]

• Communications Scoping Meeting, July 2010, Washington, 

DC [no report available]

• International Scoping Meeting, August 2010, Washington, 

DC [no report available]

• Knowledge Management Workshop, September 2010, 

Reston, VA 

• Regional Sectoral Workshop, November 2010, Reston, VA 

• Ecological Indicators Workshop, November 2010, Washington, DC 

• Scenarios Workshop, December 2010, Arlington, VA 

• Climate Change Modeling and Downscaling Workshop, 

December, 2010, Arlington, VA 

• Valuation Techniques and Metrics Workshop, January 

2011, Arlington, VA 

• Vulnerability Assessments Workshop, January 2011, Atlanta, GA

• Physical Climate Indicators Workshop, March 2011, Washington, DC

• Societal Indicators Workshop, April 2011, Washington, DC

Agency-Sponsored Technical Input Development Workshops

• Monitoring Changes in Extreme Storm Statistics: State of 

Knowledge, July 2011, Asheville, NC

• Forestry Sector Stakeholder Workshop, July 2011, Atlanta, 

GA

• Land Use and Land Cover Stakeholder Workshop, November 1011, Salt Lake City, UT

• Energy Supply and Use Workshop, November 2011, Washington, DC

• Energy, Water, Land Planning Meeting, November 2011, 

Washington, DC

• Urban Infrastructure and Vulnerabilities Workshop, November 2011, Washington, DC

• Trends and Causes of Observed Changes in Heat Waves, 

Cold Waves, Floods, and Drought, Nov. 2011, Asheville, NC

• Trends in Extreme Winds, Waves, and Extratropical Storms 

along the Coasts, January 2012, Asheville, NC

• Ecosystems, Biodiversity, and Ecosystem Services Workshop, January 2012, Palo Alto, CA

• Water Sector Technical Input Workshop, January 2012, 

Washington, DC

• Coastal Zone Stakeholders Meeting, January 2012, 

Charleston, SC

• Climate Change and Health Workshop - Southeast, February 2012, Charleston, SC

• Rural Communities Workshop, Feb. 2012, Charleston SC

• Climate Change and Health Workshop - Northwest, February 2012, Seattle, WA

Listening Sessions

• Annual Meeting of the Association of American Geographers, April 2011, Seattle, WA

• American Water Resource Association Spring Specialty 

Conference, April 2011, Baltimore, MD

• International Symposium on Society and Resource Management, June 2011, Madison, WI

• Annual Soil and Water Conservation Society Conference, 

July 2011, Washington, DC

• Ecological Society of America Annual Meeting, August 

2011, Austin, TX

• American Meteorological Society Annual Meeting, January 2012, New Orleans, LA

Regional Town Hall Meetings

• Hawai‘i & Pacific Islands Town Hall, December 2012, Honolulu, HI

• Southwest Regional Town Hall, January 2013, San Diego, 

CA 

• Northeast Regional Town Hall, January 2013, Syracuse, NY 

• Great Plains Regional Town Hall, February 2013, Lincoln, 

NE 

• Alaska Regional Town Hall, February 2013, Anchorage, AK

• Midwest Regional Town Hall, February 2013, Ann Arbor, 

MI

• Southeast Regional Town Hall, February 2013, Tampa, FL 

• Northwest Regional Town Hall, March 2013, Portland, OR

• Oceans and Coasts Town Hall, April 2013, Washington, DC

NCAnet Partners Activities

The NCAnet Partners meet monthly (since January 2012) in 

Washington, DC; teleconference and web conference capabilities allow participants to join remotely. NCAnet Partners 

hosted more than 25 events around the country for the public 

and stakeholders throughout the NCA process. A list of partners, minutes from meetings, and a list of events and resulting 

products is available at http://ncanet.usgcrp.gov.
732 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

1. USGCRP, 2012: The National Global Change Research Plan 

2012–2021: A Strategic Plan for the U.S. Global Change Research 

Program. 132 pp., The U.S. Global Change Research Program, 

Washington, D.C. [Available online at http://downloads.

globalchange.gov/strategic-plan/2012/usgcrp-strategic-plan-2012.

pdf]

2. NCADAC, 2011: National Climate Assessment (NCA) Engagement 

Strategy, 27 pp., National Climate Assessment and Development 

Advisory Committee, Washington, DC. [Available online at 

http://www.globalchange.gov/images/NCA/nca-engagementstrategy_5-20-11.pdf]

3. ——, 2011: National Climate Assessment Strategy – Summary, 

3 pp., National Climate Assessment and Development Advisory 

Committee, Washington, DC. [Available online at http://www.

globalchange.gov/images/NCA/nca-summary-strategy_5-20-11.

pdf]

4. GCRA, 1990: Global Change Research Act (Public Law 101-606, 

104 Stat. 3096-3104), signed on November 16, 1990. [Available 

online at http://www.gpo.gov/fdsys/pkg/STATUTE-104/pdf/

STATUTE-104-Pg3096.pdf]

5. USGCRP: NCAnet: Building a network of networks to support the 

National Climate Assessment. [Available online at http://ncanet.

usgcrp.gov/]

6. DOC, 2011: Technical Inputs and Assessment Capacity on Topics 

Related to 2013 U.S. National Climate Assessment. Wednesday, 

July 13, 2011, 76, 41217-41219. [Available online at http://www.

gpo.gov/fdsys/pkg/FR-2011-07-13/html/2011-17379.htm]

7. USGCRP, cited 2013: National Climate Assessment: Available 

Technical Inputs. [Available online at http://www.globalchange.

gov/what-we-do/assessment/nca-activities/available-technicalinputs]

8. ——, cited 2013: Scenarios for Climate Assessment and Adaptation: 

Climate. [Available online at http://scenarios.globalchange.gov/

scenarios/climate]

9. IPCC, 2000: Special Report on Emissions Scenarios. A Special Report 

of Working Group III of the Intergovernmental Panel on Climate Change.

Cambridge University Press, 570 pp. [Available online at http://

www.ipcc.ch/ipccreports/sres/emission/index.php?idp=0]

10. USGCRP, cited 2013: Scenarios for Climate Assessment and 

Adaptation: Sea Level. [Available online at http://scenarios.

globalchange.gov/scenarios/sea-level]

11. NCADAC: Federal Advisory Committee Draft Climate Assessment. 

National Climate Assessment and Development Advisory 

Committee. [Available online at http://ncadac.globalchange.gov]

12. USGCRP: National Climate Assessment: Opportunities for 

Engagement. [Available online at http://www.globalchange.gov/

what-we-do/assessment/nca-activities]

APPENDIX 1: REPORT DEVELOPMENT PROCESS

References
733 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

INFORMATION QUALITY APPENDIX2 ASSURANCE PROCESS

Summary of Information Quality Assurance Process for the 

Third National Climate Assessment Report

Throughout the process of drafting this National Climate Assessment, guidance was provided to contributors, authors, 

federal advisory committee members, and staff regarding the 

requirements of the Information Quality Act (IQA). 

In September 2011, Preliminary Guidance on Information Quality Assurance in Preparing Technical Input for the National Climate Assessment (NCA)1

 was made available on the U.S. Global 

Change Research Program’s (USGCRP) website along with other 

information for those interested in submitting technical input 

to the NCA in response to the Request for Information posted 

in the Federal Register on July 13, 2011.2 This frequently asked 

questions-style document provided preliminary guidance regarding information quality for use by teams who submitted 

Expressions of Interest and Technical Inputs for use in the NCA.

In November 2011, the National Climate Assessment and Development Advisory Committee (NCADAC) approved the General Principles Used in the Development of Guidance for Assuring Information Quality in the National Climate Assessment.

3

The Principles were used by the NCADAC to draft guidance 

for all Convening Lead Authors (CLAs), Lead Authors, Review 

Editors, NCADAC, and Government Agencies and Reviewers to 

assure that information used in the NCA production was of appropriate quality relative to its intended use. 

Two tools were developed – a set of questions and a flowchart 

– to assist the authors and reviewers in determining whether 

and how to use potential source material in the NCA within the 

requirements of the IQA. These tools (collectively, Guidance 

on Information Quality Assurance to Chapter Authors of the 

National Climate Assessment: Question Tools) were approved 

by the NCADAC and introduced to the CLAs at workshops. They 

have been available on the USGCRP website since February 

2012.4 The Guidance requires consideration of the following 

criteria for each source of information used in the Third NCA 

Report:

• Utility: Is the particular source important to the topic of your 

chapter?

• Transparency and traceability: Is the source material identifiable and publicly available? 

• Objectivity: Why and how was the source material created? 

Is it accurate and unbiased?

• Information integrity and security: Will the source material 

remain reasonably protected and intact over time?
734 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

1. USGCRP, 2011: Frequently Asked Questions – Sept 2011; 

Preliminary Guidance on Information Quality Assurance in 

Preparing Technical Input for the National Climate Assessment, 

5 pp., U.S. Global Change Research Program. [Available online 

at http://globalchange.gov/images/NCA/nca-info-qualityassurance-faq.pdf]

2. DOC, 2011: Technical Inputs and Assessment Capacity on Topics 

Related to 2013 U.S. National Climate Assessment. Wednesday, 

July 13, 2011, 76, 41217-41219. [Available online at http://www.

gpo.gov/fdsys/pkg/FR-2011-07-13/html/2011-17379.htm]

3. USGCRP, 2011: General Principles; Used in the Development 

of Guidance for Assuring Information Quality in the National 

Climate Assessment, 2 pp., U.S. Global Change Research Program. 

[Available online at http://www.globalchange.gov/images/NCA/

Information-Quality-Principles-Draft_2011-11-16.pdf]

4. USGCRP, 2012: Guidance on Information Quality Assurance to 

Chapter Authors of the National Climate Assessment: Question 

Tools, 5 pp., U.S. Global Change Research Program. [Available 

online at http://downloads.usgcrp.gov/NCA/QuestionTools---2-21-12.pdf]

APPENDIX 2: INFORMATION QUALITY ASSURANCE PROCESS

References
735 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

INFORMATION DRAWN FROM THIS CHAPTER IS INCLUDED IN THE HIGHLIGHTS REPORT AND IS IDENTIFIED BY THIS ICON

Recommended Citation for Chapter

Walsh, J., D. Wuebbles, K. Hayhoe, J. Kossin, K. Kunkel, G. Stephens, P. Thorne, R. Vose, M. Wehner, J. Willis, D. Anderson, V. Kharin, T. Knutson, F. Landerer, T. Lenton, J. Kennedy, and R. Somerville, 2014: Appendix 3: Climate Science 

Supplement. Climate Change Impacts in the United States: The Third National Climate Assessment, J. M. Melillo, Terese 

(T.C.) Richmond, and G. W. Yohe, Eds., U.S. Global Change Research Program, 735-789. doi:10.7930/J0KS6PHH.

On the Web: http://nca2014.globalchange.gov/report/appendices/climate-science-supplement

First published May 2014. PDF revised October 2014. See errata (available at http://nca2014.globalchange.gov/downloads) for details.

Convening Lead Authors

John Walsh, University of Alaska Fairbanks

Donald Wuebbles, University of Illinois

Lead Authors

Katharine Hayhoe, Texas Tech University

James Kossin, NOAA National Climatic Data Center 

Kenneth Kunkel, CICS-NC, North Carolina State Univ., NOAA National Climatic Data Center 

Graeme Stephens, NASA Jet Propulsion Laboratory

Peter Thorne, Nansen Environmental and Remote Sensing Center

Russell Vose, NOAA National Climatic Data Center

Michael Wehner, Lawrence Berkeley National Laboratory

Josh Willis, NASA Jet Propulsion Laboratory

Contributing Authors

David Anderson, NOAA National Climatic Data Center 

Viatcheslav Kharin, Canadian Centre for Climate Modelling and Analysis, Environment Canada

Thomas Knutson, NOAA Geophysical Fluid Dynamics Laboratory

Felix Landerer, Jet Propulsion Laboratory

Tim Lenton, Exeter University

John Kennedy, UK Meteorological Office

Richard Somerville, Scripps Institution of Oceanography, Univ. of California, San Diego

Climate Change Impacts in the United States

APPENDIX 3 CLIMATE SCIENCE 

SUPPLEMENT
736 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

APPENDIX3 CLIMATE SCIENCE

Supplemental Messages

1. Although climate changes in the past have been caused by natural factors, human activities are 

now the dominant agents of change. Human activities are affecting climate through increasing 

atmospheric levels of heat-trapping gases and other substances, including particles.

2. Global trends in temperature and many other climate variables provide consistent evidence of 

a warming planet. These trends are based on a wide range of observations, analyzed by many 

independent research groups around the world.

3. Natural variability, including El Niño events and other recurring patterns of ocean-atmosphere 

 interactions, influences global and regional temperature and precipitation over timescales ranging 

 from months up to a decade or more.

4. Human-induced increases in atmospheric levels of heat-trapping gases are the main cause of 

 observed climate change over the past 50 years. The “fingerprints” of human-induced change also 

 have been identified in many other aspects of the climate system, including changes in ocean heat 

 content, precipitation, atmospheric moisture, and Arctic sea ice.

5. Past emissions of heat-trapping gases have already committed the world to a certain amount of 

 future climate change. How much more the climate will change depends on future emissions and the 

 sensitivity of the climate system to those emissions.

6. Different kinds of physical and statistical models are used to study aspects of past climate and

 develop projections of future change. No model is perfect, but many of them provide useful 

 information. By combining and averaging multiple models, many clear trends emerge.

7. Scientific understanding of observed temperature changes in the United States has greatly improved, 

 confirming that the U.S. is warming due to heat-trapping gas emissions, consistent with the climate 

 change observed globally.

8. Many other indicators of rising temperatures have been observed in the United States. These include 

 reduced lake ice, glacier retreat, earlier melting of snowpack, reduced lake levels, and a longer 

 growing season. These and other indicators are expected to continue to reflect higher temperatures.

9. Trends in some types of extreme weather events have been observed in recent decades, consistent 

 with rising temperatures. These include increases in heavy precipitation nationwide, especially in 

 the Midwest and Northeast; heat waves, especially in the West; and the intensity of Atlantic 

 hurricanes. These trends are expected to continue. Research on climate change’s effects on other 

 types of extreme events continues.

10. Drought and fire risk are increasing in many regions as temperatures and evaporation rates rise. The 

 greater the future warming, the more these risks will increase, potentially affecting the entire United 

 States.
737 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

APPENDIX 3: CLIMATE SCIENCE

11. Summer Arctic sea ice extent, volume, and thickness have declined rapidly, especially north of

 Alaska. Permafrost temperatures are rising and the overall amount of permafrost is shrinking. 

 Melting of land- and sea-based ice is expected to continue with further warming.

12. Sea level is already rising at the global scale and at individual locations along the U.S. coast.

 Future sea level rise depends on the amount of warming and ice melt around the world as well as 

 local processes like changes in ocean currents and local land subsidence or uplift.

This appendix provides further information and discussion on 

climate science beyond that presented in Ch. 2: Our Changing 

Climate. Like the chapter, the appendix focuses on the observations, model simulations, and other analyses that explain 

what is happening to climate at the national and global scales, 

why these changes are occurring, and how climate is projected 

to change throughout this century. In the appendix, however, 

more information is provided on attribution, spatial and temporal detail, and physical mechanisms than could be covered 

within the length constraints of the main chapter.

As noted in the main chapter, changes in climate, and the nature and causes of these changes, have been comprehensively 

discussed in a number of other reports, including the 2009 assessment: Global Climate Change Impacts in the United States1

and the global assessments produced by the Intergovernmental Panel on Climate Change (IPCC) and the U.S. National Academy of Sciences. This appendix provides an updated discussion 

of global change in the first few supplemental messages, followed by messages focusing on the changes having the greatest impacts (and potential impacts) on the United States. The 

projections described in this appendix are based, to the extent 

possible, on the CMIP5 model simulations. However, given the 

timing of this report relative to the evolution of the CMIP5 

archive, some projections are necessarily based on CMIP3 

simulations. (See Supplemental Message 5 for more on these 

simulations and related future scenarios).

Supplemental Message 1. 

Although climate changes in the past have been caused by natural factors, human activities 

are now the dominant agents of change. Human activities are affecting 

climate through increasing atmospheric levels of heat-trapping gases 

and other substances, including particles.

The Earth’s climate has long been known to change in response 

to natural external forcings. These include variations in the energy received from the sun, volcanic eruptions, and changes 

in the Earth’s orbit, which affects the distribution of sunlight 

across the world. The Earth’s climate is also affected by factors 

that are internal to the climate system, which are the result 

of complex interactions between the atmosphere, ocean, land 

surface, and living things (see Supplemental Message 3). These 

internal factors include natural modes of climate system variability, such as the El Niño/Southern Oscillation. 

Natural changes in external forcings and internal factors have 

been responsible for past climate changes. At the global scale, 

over multiple decades, the impact of external forcings on temperature far exceeds that of internal variability (which is less 

than 0.5°F).2 At the regional scale, and over shorter time periods, internal variability can be responsible for much larger 

changes in temperature and other aspects of climate. Today, 

however, the picture is very different. Although natural factors 

still affect climate, human activities are now the primary cause 

of the current warming: specifically, human activities that increase atmospheric levels of carbon dioxide (CO2

) and other 

heat-trapping gases and various particles that, depending on 

the type of particle, can have either a heating or cooling influence on the atmosphere.

The greenhouse effect is key to understanding how human 

activities affect the Earth’s climate. As the sun shines on the 

Earth, the Earth heats up. The Earth then re-radiates this heat 

back to space. Some gases, including water vapor (H2O), carbon dioxide (CO2

), ozone (O3), methane (CH4), and nitrous oxide 

(N2O), absorb some of the heat given off by the Earth’s surface 

and lower atmosphere. These heat-trapping gases then radiate 

energy back toward the surface, effectively trapping some of 

the heat inside the climate system. This greenhouse effect is a 

natural process, first recognized in 1824 by the French mathematician and physicist Joseph Fourier3

 and confirmed by British scientist John Tyndall in a series of experiments starting in 

1859.4 Without this natural greenhouse effect (but assuming 

the same albedo, or reflectivity, as today), the average surface 

temperature of the Earth would be about 60°F colder.

Today, however, the natural greenhouse effect is being artificially intensified by human activities. Burning fossil fuels (coal, 
738 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

APPENDIX 3: CLIMATE SCIENCE

Figure 1. Left: A stylized representation of the natural greenhouse effect. Most of the sun’s radiation reaches the Earth’s surface. 

Naturally occurring heat-trapping gases, including water vapor, carbon dioxide, methane, and nitrous oxide, do not absorb the 

short-wave energy from the sun but do absorb the long-wave energy re-radiated from the Earth, keeping the planet much warmer 

than it would be otherwise. Right: In this stylized representation of the human-intensified greenhouse effect, human activities, 

predominantly the burning of fossil fuels (coal, oil, and gas), are increasing levels of carbon dioxide and other heat-trapping gases, 

increasing the natural greenhouse effect and thus Earth’s temperature. (Figure source: modified from National Park Service5).

Human Influence on the Greenhouse Effect

Figure 2. This figure summarizes results of measurements taken from satellites of the amount of energy coming in to and going 

out of Earth’s climate system. It demonstrates that our scientific understanding of how the greenhouse effect operates is, in fact, 

accurate, based on real world measurements. (Figure source: modified from Stephens et al. 20126).

Earth’s Energy Balance
739 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

APPENDIX 3: CLIMATE SCIENCE

oil, and natural gas), clearing forests, and other human activities produce heat-trapping gases. These gases accumulate in 

the atmosphere, as natural removal processes are unable to 

keep pace with increasing emissions. Increasing atmospheric 

levels of CO2, CH4, and N2O (and other gases and some types of 

particles like soot) from human activities increase the amount 

of heat trapped inside the Earth system. This human-caused 

intensification of the greenhouse effect is 

the primary cause of observed warming in 

recent decades.

Carbon dioxide has been building up in the 

Earth’s atmosphere since the beginning of 

the industrial era in the mid-1700s. Emissions and atmospheric levels, or concentrations, of other important heat-trapping gases – including methane, nitrous oxide, and 

halocarbons – have also increased because 

of human activities. While the atmospheric 

concentrations of these gases are relatively 

small compared to those of molecular oxygen or nitrogen, their ability to trap heat 

is extremely strong. The human-induced 

increase in atmospheric levels of carbon dioxide and other heat-trapping gases is the 

main reason the planet has warmed over 

the past 50 years and has been an important factor in climate change over the past 

150 years or more.

Carbon dioxide levels in the atmosphere 

are currently increasing at a rate of 0.5% 

per year. Atmospheric levels measured 

at Mauna Loa in Hawai‘i and at other sites around the world 

reached 400 parts per million in 2013, higher than the Earth 

has experienced in over a million years. Globally, over the past 

several decades, about 78% of carbon dioxide emissions has 

come from burning fossil fuels, 20% from deforestation and 

other agricultural practices, and 2% from cement production. 

Some of the carbon dioxide emitted to the atmosphere is absorbed by the oceans, and some is absorbed by vegetation. 

Figure 3. Global carbon emissions from burning coal, oil, and gas and producing 

cement (1850-2009). These emissions account for about 80% of the total emissions 

of carbon from human activities, with land-use changes (like cutting down forests) 

accounting for the other 20% in recent decades (Data from Boden et al. 20127).

Carbon Emissions in the Industrial Age

Figure 4. Present-day atmospheric levels of carbon dioxide, methane, and nitrous oxide are notably higher than their 

pre-industrial averages of 280, 0.7, and 0.27 parts per million (ppm) by volume, respectively (left). Air sampling data 

from 1958 to 2013 show long-term increases due to human activities as well as short-term variations due to natural 

biogeochemical processes and seasonal vegetation growth (right). (Figure sources: (left) Forster et al. 2007;8 (right) 

Scripps Institution of Oceanography and NOAA Earth Systems Research Laboratory).

Heat-Trapping Gas Levels

2000 Years of Heat Trapping Gases CO2 1958–2013
740 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

APPENDIX 3: CLIMATE SCIENCE

About 45% of the carbon dioxide emitted by human activities 

in the last 50 years is now stored in the oceans and vegetation. 

The remainder has built up in the atmosphere, where carbon 

dioxide levels have increased by about 40% relative to preindustrial levels.

Methane levels in the atmosphere have increased due to human activities, including agriculture, with livestock producing 

methane in their digestive tracts, and rice farming producing it 

via bacteria that live in the flooded fields; mining coal, extraction and transport of natural gas, and other fossil fuel-related 

activities; and waste disposal including sewage and decomposing garbage in landfills. On average, about 55% to 65% of the 

emissions of atmospheric methane now come from human activities.14,15 Atmospheric concentrations of methane leveled off 

from 1999-2006 due to temporary decreases in both human 

and natural sources,14,15 but have been increasing again since 

then. Since preindustrial times, methane levels have increased 

by 250% to their current levels of 1.85 ppm.

Other greenhouse gases produced by human activities include nitrous oxide, halocarbons, and ozone.

Nitrous oxide levels are increasing, primarily as a result of fertilizer use and fossil fuel 

burning. The concentration of nitrous oxide has increased by about 20% relative to 

pre-industrial times.

Halocarbons are manufactured chemicals produced to serve specific purposes, 

from aerosol spray propellants to refrigerant coolants. One type of halocarbon, 

long-lived chlorofluorocarbons (CFCs), 

was used extensively in refrigeration, air 

conditioning, and for various manufacturing purposes. However, in addition to 

being powerful heat-trapping gases, they 

are also responsible for depleting stratospheric ozone. Atmospheric levels of CFCs 

are now decreasing due to actions taken 

by countries under the Montreal Protocol, 

an international agreement designed to 

protect the ozone layer. As emissions and 

atmospheric levels of halocarbons continue to decrease, their effect on climate 

will also shrink. However, some of the 

replacement compounds are hydrofluorocarbons (HFCs), which are potent heattrapping gases, and their concentrations 

are increasing.

Over 90% of the ozone in the atmosphere 

is in the stratosphere, where it protects 

the Earth from harmful levels of ultraviolet radiation from the sun. In the lower atmosphere, however, 

ozone is an air pollutant and also an important heat-trapping 

gas. Upper-atmosphere ozone levels have decreased because 

of human emissions of CFCs and other halocarbons. However, 

lower-atmosphere ozone levels have increased because of human activities, including transportation and manufacturing. 

These produce what are known as ozone precursors: air pollutants that react with sunlight and other chemicals to produce 

ozone. Since the late 1800s, average levels of ozone in the 

lower atmosphere have increased by more than 30%.16 Much 

higher increases have been observed in areas with high levels of air pollution, and smaller increases in remote locations 

where the air has remained relatively clean.

Human activities can also produce tiny atmospheric particles, 

including dust and soot. For example, coal burning produces 

sulfur gases that form particles in the atmosphere. These 

sulfur-containing particles reflect incoming sunlight away 

from the Earth, exerting a cooling influence on Earth’s surface. 

Figure 5. Air bubbles trapped in an Antarctic ice core extending back 800,000 

years document the atmosphere’s changing carbon dioxide concentration. Over 

long periods, natural factors have caused atmospheric CO2 concentrations to vary 

between about 170 to 300 parts per million (ppm). As a result of human activities 

since the Industrial Revolution, CO2 levels have increased to 400 ppm, higher than 

any time in at least the last one million years. By 2100, additional emissions from 

human activities are projected to increase CO2 levels to 420 ppm under a very low 

scenario, which would require immediate and sharp emissions reductions (RCP 

2.6), and 935 ppm under a higher scenario, which assumes continued increases in 

emissions (RCP 8.5). This figure shows the historical composite CO2 record based 

on measurements from the EPICA (European Project for Ice Coring in Antarctica) 

Dome C and Dronning Maud Land sites and from the Vostok station. Data from 

Lüthi et al. 20089 (664-800 thousand years [kyr] ago, Dome C site); Siegenthaler et 

al. 200510 (393-664 kyr ago, Dronning Maud Land); Pépin 2001, Petit et al. 1999, 

and Raynaud 200511 (22-393 kyr ago, Vostok); Monnin et al. 200112 (0-22 kyr ago, 

Dome C); and Meinshausen et al. 201113 (future projections from RCP 2.6 and 8.5).

Atmospheric Carbon Dioxide Levels
741 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

APPENDIX 3: CLIMATE SCIENCE

Another type of particle, composed mainly of soot, or black 

carbon, absorbs incoming sunlight and traps heat in the atmosphere, warming the Earth.

In addition to their direct effects, these particles can affect 

climate indirectly by changing the properties of clouds. Some 

encourage cloud formation because they are ideal surfaces 

on which water vapor can condense to form cloud droplets. 

Some can also increase the number, but decrease the average 

size of cloud droplets when there is not enough water vapor 

compared to the number of particles available, thus creating 

brighter clouds that reflect energy from the sun away from 

the Earth, resulting in an overall cooling effect. Particles that 

absorb energy encourage cloud droplets to evaporate by 

warming the atmosphere. Depending on their type, increasing 

amounts of particles can either offset or increase the warming 

caused by increasing levels of greenhouse gases.At the scale of 

the planet, the net effect of these particles is to offset between 

20% and 35% of the warming caused by heat-trapping gases.

The effects of all of these greenhouse gases and particles on 

the Earth’s climate depend in part on how long they remain 

in the atmosphere. Human-induced emissions of carbon dioxide have already altered atmospheric levels in ways that will 

persist for thousands of years. About one-third of the carbon 

dioxide emitted in any given year remains in the atmosphere 

100 years later. However, the impact of past human emissions 

of carbon dioxide on the global carbon cycle will endure for 

tens of thousands of years. Methane lasts for approximately a 

decade before it is removed through chemical reactions. Particles, on the other hand, remain in the atmosphere for only a 

few days to several weeks. This means that the effects of any 

human actions to reduce particle emissions can show results 

nearly immediately. It may take decades, however, before the 

results of human actions to reduce long-lived greenhouse gas 

emissions can be observed. Some recent studies17 examine 

various means for reducing near-term changes in climate, for 

example, by reducing emissions of short-lived gases like methane and particles like black carbon (soot). These approaches 

are being explored as ways to reduce the rate of short-term 

warming while more comprehensive approaches to reducing 

carbon dioxide emissions (and hence the rate of long-term 

warming) are being implemented.

In addition to emissions of greenhouse gases, air pollutants, 

and particles, human activities have also affected climate by 

changing the land surface. These changes include cutting and 

burning forests, replacing natural vegetation with agriculture 

or cities, and large-scale irrigation. These transformations of 

the land surface can alter how much heat is reflected or absorbed by the surface, causing local and even regional warming 

or cooling. Globally, the net effect of these changes has probably been a slight cooling influence over the past 100 years.

Considering all known natural and human drivers of climate 

since 1750, a strong net warming from long-lived greenhouse 

gases produced by human activities dominates the recent 

climate record. This warming has been partially offset by increases in atmospheric particles and their effects on clouds. 

Two important natural external drivers also influence climate: 

the sun and volcanic eruptions. Since 1750, these natural external drivers are estimated to have had a small net warming 

influence, one that is much smaller than the human influence. 

Natural internal climate variations, such as El Niño events in 

Figure 6. Different factors have exerted a warming influence 

(red bars) or a cooling influence (blue bars) on the planet. The 

warming or cooling influence of each factor is measured in 

terms of the change in radiative forcing in watts per square 

meter by 2005 relative to 1750. This figure includes all the 

major human-induced factors as well as the sun, the only 

major natural factor with a long-term effect on climate. The 

cooling effect of individual volcanoes is also natural, but is 

relatively short-lived and so is not included here. Aerosols 

refer to tiny particles, with their direct effects including, for 

example, the warming influence of black carbon (soot) and 

cooling influence of sulfate particles from coal burning. Indirect 

effects of aerosols include their effect on clouds. The net 

radiative influence from natural and human influences is a 

strong warming, predominantly from human activities. The 

thin lines on each bar show the range of uncertainty. (Figure 

source: adapted from Climate Change 2007: The Physical 

Science Basis. Working Group I Contribution to the Fourth 

Assessment Report of the Intergovernmental Panel on Climate 

Change, Figure 2.20 (A), Cambridge University Press15).

Relative Strengths of Warming 

and Cooling Influences
742 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

APPENDIX 3: CLIMATE SCIENCE

the Pacific Ocean, have also influenced regional and global climate. Several other modes of internal natural variability have 

been identified, and their effects on climate are superimposed 

on the effects of human activities, the sun, and volcanoes.

During the last three decades, direct observations indicate that 

the sun’s energy output has decreased slightly. The two major 

volcanic eruptions of the past 30 years have had short-term 

cooling effects on climate, lasting two to three years. Thus, 

natural factors cannot explain the warming of recent decades; 

in fact, their net effect on climate has been a slight cooling 

influence over this period. In addition, the changes occurring 

now are very rapid compared to the major changes in climate 

over at least the last several thousand years.

It is not only the direct effects from human emissions that affect climate. These direct effects also trigger a cascading set 

of feedbacks that cause indirect effects on climate – acting to 

increase or dampen an initial change. For example, water vapor is the single most important gas responsible for the natural 

greenhouse effect. Together, water vapor and clouds account 

for between 66% and 80% of the natural greenhouse effect.18

However, the amount of water vapor in the atmosphere depends on temperature; increasing temperatures increase the 

amount of water vapor. This means that the response of water 

vapor is an internal feedback, not an external forcing of the 

climate.

Observational evidence shows that, of all the external forcings, 

an increase in atmospheric CO2 concentration is the most important factor in increasing the heat-trapping capacity of the 

atmosphere. Carbon dioxide and other gases, such as methane 

and nitrous oxide, do not condense and fall out of the atmosphere, whereas water vapor does (for example, as rain or 

snow). Together, heat-trapping gases other than water vapor 

account for between 26% and 33% of the total greenhouse effect,18 but are responsible for most of the changes in climate 

over recent decades. This is a range, rather than a single number, because some of the absorption effects of water vapor 

overlap with those of the other important gases. Without the 

heat-trapping effects of carbon dioxide and the other non-water vapor greenhouse gases, climate simulations indicate that 

the greenhouse effect would not function, turning the Earth 

into a frozen ball of ice.19

The average conditions and the variability of the Earth’s climate 

are critical to all aspects of human and natural systems on the 

planet. Human society has become increasingly complex and 

dependent upon the climate system and its behavior. National 

and global infrastructures, economies, agriculture, and ecosystems are adapted to the present climate state, which from a 

geologic timescale perspective has been remarkably stable for 

the past several thousand years. Any significant perturbation, 

in either direction, would have substantial impacts upon both 

human society and the natural world. The magnitude of the 

human influence on climate and the rate of change raise concerns about the ability of ecosystems and human systems to 

successfully adapt to future changes.

Supplemental Message 2. 

Global trends in temperature and many other climate variables provide consistent evidence 

of a warming planet. These trends are based on a wide range of observations, analyzed by 

many independent research groups around the world.

There are many types of observations that can be used to detect changes in climate and determine what is causing these 

changes. Thermometer and other instrument-based surface 

weather records date back hundreds of years in some locations. Air temperatures are measured at fixed locations over 

land and with a mix of predominantly ship- and buoy-based 

measurements over the ocean. By 1850, a sufficiently extensive array of land-based observing stations and ship-borne observations had accumulated to begin tracking global average 

temperature. Measurements from weather balloons began in 

the early 1900s, and by 1958 were regularly taken around the 

world. Satellite records beginning in the 1970s provide additional perspectives, particularly for remote areas such as the 

Arctic that have limited ground-based observations. Satellites 

also provided new capabilities for mapping precipitation and 

upper air temperatures. Climate “proxies” – biological or physical records ranging from tree rings to ice cores that correlate 

with aspects of climate – provide further evidence of past climate that can stretch back hundreds of thousands of years.

These diverse datasets have been analyzed by scientists and 

engineers from research teams around the world in many different ways. The most high-profile indication of the changing 

climate is the surface temperature record, so it has received 

the most attention. Spatial coverage, equipment, methods of 

observation, and many other aspects of the measurement record have changed over time, so scientists identify and adjust 

for these changes. Independent research groups have looked 

at the surface temperature record for land21 and ocean22 as 

well as land and ocean combined.23,24 Each group takes a different approach, yet all agree that it is unequivocal that the 

planet is warming.

There has been widespread warming over the past century. 

Not every region has warmed at the same pace, however, 
743 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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and a few regions, such as the North Atlantic Ocean (Figure 

9) and some parts of the U.S. Southeast (Ch. 2: Our Changing 

Climate, Figure 2.7), have even experienced cooling over the 

last century as a whole, though they have warmed over recent 

decades. This is due to the stronger influence of internal variability over smaller geographic regions and shorter time scales, 

as mentioned in Supplemental Message 1 and discussed in 

more detail in Supplemental Message 3. Warming during the 

first half of the last century occurred mostly in the Northern 

Hemisphere. The last three decades have seen greater warming in response to accelerating increases in heat-trapping gas 

concentrations, particularly at high northern latitudes, and 

over land as compared to ocean.

Figure 8. Three different global surface temperature records all show increasing trends 

over the last century. The lines show annual differences in temperature relative to the 

1901-1960 average. Differences among data sets, due to choices in data selection, 

analysis, and averaging techniques, do not affect the conclusion that global surface 

temperatures are increasing. (Figure source: NOAA NCDC / CICS-NC).

Observed Change in Global Average Temperature

Figure 7. Changes in the mix and increasing diversity of technologies used to observe climate (IGY is the 

International Geophysical Year). (Figure source: adapted from Brönnimann et al. 200720).

Development of Observing Capabilities
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Even if the surface temperature had never been measured, scientists could still conclude with high confidence that the global 

temperature has been increasing because multiple lines of evidence all support this conclusion. Temperatures in the lower 

atmosphere and oceans have increased, as have sea level and 

near-surface humidity. Arctic sea ice, mountain glaciers, and 

Northern Hemisphere spring snow cover have all decreased. 

As with temperature, multiple research groups have analyzed 

each of these indicators and come to the same conclusion: all 

of these changes paint a consistent and compelling picture of 

a warming world.

Figure 9. Surface temperature trends for the period 1901-2012 (top) and 1979-2012 (bottom) from the National 

Climatic Data Center’s (NCDC) surface temperature product. The relatively coarse resolution of these maps does 

not capture the finer details associated with mountains, coastlines, and other small-scale effects. (Figure source: 

updated from Vose et al. 201224).

Temperature Trends: Past Century, Past 30+ Years
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Not all of the observed changes are directly related to temperature; some are related to the hydrological cycle (the way 

water moves cyclically among land, ocean, and atmosphere). 

Precipitation is perhaps the most societally relevant aspect of 

the hydrological cycle and has been observed over global land 

areas for over a century. However, spatial scales of precipitation are small (it can rain several inches in Washington, D.C., 

but not a drop in Baltimore) and this makes interpretation of 

the point-measurements difficult. Based upon a range of efforts to create global averages, it is likely that there has been 

little change in globally averaged precipitation since 1900. 

However, there are strong geographic trends including a likely 

increase in precipitation in Northern Hemisphere mid-latitude 

regions taken as a whole. In general, wet areas are getting wetFigure 10. Observed changes, as analyzed by many independent groups in different ways, of a range of climate indicators. All of 

these are in fact changing as expected in a warming world. Further details underpinning this diagram can be found at http://www.

ncdc.noaa.gov/bams-state-of-the-climate/. (Figure source: updated from Kennedy et al. 201025).

Indicators of Warming from Multiple Data Sets
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ter and dry areas are getting drier, consistent with an overall 

intensification of the hydrological cycle in response to global 

warming.

Analyses of past changes in climate during the period before instrumental records (referred to as paleoclimate) allow current 

changes in atmospheric composition, sea level, and climate 

(including extreme events), as well as future projections, to be 

placed in a broader perspective of past climate variability. A 

number of different reconstructions of the last 1,000 to 2,000 

years26,27 give a consistent picture of Northern Hemisphere 

temperatures, and in a few cases, global temperatures, over 

that time period. The analyses in the Northern Hemisphere indicate that the 1981 to 2010 period (including the last decade) 

was the warmest of at least the last 1,300 years and probably 

much longer.28,29 A reconstruction going back 11,300 years 

ago30 suggests that the last decade was warmer than at least 

72% of global temperatures since the end of the last ice age 

20,000 years ago. The observed warming of the last century 

has also apparently reversed a long-term cooling trend at midto high latitudes of the Northern Hemisphere throughout the 

last 2,000 years.

Other analyses of past climates going back millions of years indicate that past periods with high levels (400 ppm or greater) 

of CO2 were associated with temperatures much higher than 

today’s and with much higher sea levels.31

Figure 11. Global precipitation trends for the period 1901-2012 (top) and 1979-2012 

(bottom). (Figure source: NOAA NCDC / CICS-NC).

Precipitation Trends: Past Century, Past 30+ Years
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 Supplemental Message 3.

Natural variability, including El Niño events and other recurring patterns of ocean-atmosphere 

interactions, influences global and regional temperature and precipitation over 

timescales ranging from months up to a decade or more.

Natural variations internal to the Earth’s climate system can 

drive increases or decreases in global and regional temperatures, as well as affect precipitation and drought patterns 

around the world. Today, average temperature, precipitation, 

and other aspects of climate are determined by a combination 

of human-induced changes superimposed on natural variations in both internal and external factors such as the sun and 

volcanoes (see Supplemental Message 1). The relative magnitudes of the human and natural contributions to temperature 

and climate depend on both the time and spatial scales considered. The magnitude of the effect humans are having on global 

temperature specifically, and on climate in general, has been 

steadily increasing since the Industrial Revolution. At the global 

scale, the human influence on climate can be either masked or 

augmented by natural internal variations over timescales of a 

decade or so (for example, Tung and Zhou 201332). At regional 

and local scales, natural variations have an even larger effect. 

Over longer periods of time, however, the influence of internal 

natural variability on the Earth’s climate system is negligible; in 

other words, over periods longer than several decades, the net 

effect of natural variability tends to sum to zero.

There are many modes of natural variability within the climate 

system. Most of them involve cyclical exchanges of heat and 

energy between the ocean and atmosphere. They are manifested by recurring changes in sea surface temperatures, for 

example, or by surface pressure changes in the atmosphere. 

While many global climate models are able to simulate the spatial patterns of ocean and atmospheric variability associated 

with these modes, they are less able to capture the chaotic 

variability in the timescales of the different modes.33

The largest and most well-known mode of internal natural 

variability is the El Niño/Southern Oscillation or ENSO. This 

natural mode of variability was first identified as a warm 

current of ocean water off the coast of Peru, accompanied 

by a shift in pressure between two locations on either side of 

the Pacific Ocean. Although centered in the tropical Pacific, 

ENSO affects regional temperatures and precipitation around 

the world by heating or cooling the lower atmosphere in low 

latitudes, thereby altering pressure gradients aloft. These 

pressure gradients, in turn, drive the upper-level winds and 

the jet stream that dictates patterns of mid-latitude weather, 

as shown in Figure 13. In the United States, for example, the 

warm ENSO phase (commonly referred to as El Niño) is usually 

associated with heavy rainfall and flooding in California and 

the Southwest, but decreased precipitation in the Northwest.34

El Niño conditions also tend to suppress Atlantic hurricane 

formation by increasing the amount of wind shear in the region 

where hurricanes form.35 The cool ENSO phase (usually called 

Figure 12. Changes in the temperature of the Northern Hemisphere from surface 

observations (in red) and from proxies (in black; uncertainty range represented by 

shading) relative to 1961-1990 average temperature. These analyses suggest that current 

temperatures are higher than seen globally in at least the last 1700 years, and that the 

last decade (2001 to 2010) was the warmest decade on record. (Figure source: adapted 

from Mann et al. 200827).

1700 Years of Temperature Change from Proxy Data
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Figure 13. Typical January-March weather conditions and atmospheric circulation (jet streams shown by red and blue arrows) 

during La Niña and El Niño conditions. Cloud symbols show areas that are wetter than normal. During La Niña, winters tend 

to be unusually cold in eastern Alaska and western Canada, and dry throughout the southern United States. El Niño leads to 

unusually warm winter conditions in the northern U.S. and wetter than average conditions across the southern U.S. (Figure 

source: NOAA).

La Niña and El Niño Patterns

Figure 14. Trends in globally and annually averaged temperature when considering 

whether it was an El Niño year, a La Niña year, or a neutral year (no El Niño or 

La Niña event). The average global temperature is 0.4oF higher in El Niño years 

than in La Niña years. However, all trends show the same significant increase in 

temperature over the past 45 years. The years for the short-term cooling effect 

following the Mt. Pinatubo volcanic eruption are not included in the trends. (Figure 

source: adapted from John Nielsen-Gammon 2012.38 Data from NASA GISS 

temperature dataset39 and Climate Prediction Center Niño 3.4 index40).

Warming Trend and Effects of El Niño/La Niña

GISTEMP Land-Ocean Index
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La Niña) is associated with dry conditions in the Central Plains,36

as well as a more active Atlantic hurricane season. Although 

these and other conditions are typically associated with ENSO, 

no two ENSO events are exactly alike.

Natural modes of variability such as ENSO can also affect global 

temperatures. In general, El Niño years tend to be warmer than 

average and La Niña years, cooler. The strongest El Niño event 

recorded over the last hundred years occurred in 1998. Superimposed on the long-term increase in global temperatures due 

to human activities, this event caused record high global temperatures. After 1998, the El Niño event subsided, resulting in 

a slowdown in the temperature increase since 1998. Overall, 

however, years in which there are El Niño, La Niña, or neutral 

conditions all show similar long-term warming trends in global 

temperature (see Figure 14).

Natural modes of variability like ENSO are not necessarily stationary. For example, there appears to have been a shift in the 

pattern and timing of ENSO in the mid-1970s, with the location of the warm water pool shifting from the eastern to the 

central Pacific and the frequency of events increasing. Paleoclimate studies using tree rings show that ENSO activity over the 

last 100 years has been the highest in the last 500 years,37 and 

both paleoclimate and modeling studies suggest that global 

temperature increases may interact with natural variability in 

ways that are difficult to predict. Climate models can simulate 

the statistical behavior of these variations in temperature trends. For example, models can project whether some 

phenomena will increase or decrease in 

frequency, but cannot predict the exact 

timing of particular events far into the 

future.

There are other natural modes of variability in the climate system. For example, the North Atlantic Oscillation is 

frequently linked to variations in winter 

snowfall along the Atlantic seaboard. 

The Pacific Decadal Oscillation was first 

identified as a result of its effect on the 

Pacific salmon harvest. The influence of 

these and other natural variations on 

global temperatures is generally less 

than ENSO, but local influences may be 

large.

A combination of natural and human 

factors explains regional “warming 

holes” where temperatures actually 

decreased for several decades in the 

middle to late part of the last century 

at a few locations around the world. 

In the United States, for example, the 

Southeast and parts of the Great Plains and Midwest regions 

did not show much warming over that time period, though 

they have warmed in recent decades. Explanations include 

increased cloud cover and precipitation,41 increased small 

particles from coal burning, natural factors related to forest 

re-growth,42 decreased heat flux due to irrigation,43 and multidecade variability in North Atlantic and tropical Pacific sea surface temperatures.44,45 The importance of tropical Pacific and 

Atlantic sea surface temperatures on temperature and precipitation variability over the central U.S. has been particularly 

highlighted by many studies. Over the next few decades, as the 

multi-decadal tropical Pacific Ocean cycle continues its effect 

on sea surface temperatures, the U.S. Southeast could warm at 

a rate that is faster than the global average.45

At the global scale, natural variability will continue to modify 

the long-term trend in global temperature due to human activities, resulting in greater and lesser trends over relatively 

short time scales. Interactions among various components of 

the Earth’s climate system produce patterns of natural variability that can be chaotic, meaning that they are sensitive to the 

initial conditions of the climate system. Global climate models 

simulate natural variability with varying degrees of realism, but 

the timing of these random variations differs among models 

and cannot be expected to coincide with those of the actual 

climate system. Over climatological time periods, however, the 

net effect of natural internal variability on the global climate 

Figure 15. Observations of global mean surface air temperature show that although 

there can be short periods with little or even no significant upward trend (red trend lines 

in shaded areas), global temperature continues to rise unabated over long-term climate 

timescales (black trend line). The recent period, 1998-2012, is another example of a 

short-term pause embedded in the underlying warming trend. The differences between 

short-term trends and the underlying (long-term) trend are often associated with modes 

of natural variability such as El Niño and La Niña that redistribute heat between the 

ocean and atmosphere. (Data from NOAA NCDC).

Long-Term Warming and Short-Term Variation
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tends to average to zero. For example, there can be warmer 

years due to El Niño (such as 1998) and cooler years due to La 

Niña (such as 2011), but over multiple decades the net effect 

of natural variability on uncertainty in global temperature and 

precipitation projections is small.

Averaging (or compositing) of projections from different models smooths out the randomly occurring natural variations in 

the different models, leaving a clear signal of the long-term externally forced changes in climate, not weather. In this report, 

all future projections are averaged over 20- to 30-year time 

periods.

Supplemental Message 4. 

Human-induced increases in atmospheric levels of heat-trapping gases are the main cause of 

observed climate change over the past 50 years. The “fingerprints” of human-induced change 

also have been identified in many other aspects of the climate system, including changes in 

ocean heat content, precipitation, atmospheric moisture, and Arctic sea ice.

Determining the causes of climate changes is a field of research 

known as “detection and attribution.” Detection involves identifying a climate trend or event (for instance, long-term surface 

air temperature trends, or a particularly extreme heat wave) 

that is strikingly outside the norm of natural variations in the 

climate system. Similar to conducting forensic analysis on evidence from a crime scene, attribution involves considering the 

possible causes of an observed event or change, and identifying which factor(s) are responsible.

Detection and attribution studies use statistical analyses to 

identify the causes of observed changes in temperature, precipitation, and other aspects of climate. They do this by trying 

to match the complex “fingerprint” of the observed climate 

system behavior to a set of simulated changes in climate that 

would be caused by different forcings.46 Most approaches consider not only global but also regional patterns of changes over 

time.

Climate simulations are used to test hypotheses regarding the 

causes of observed changes. First, simulations that include 

changes in both natural and human forcings that may cause 

climate changes, such as changes in energy from the sun and 

increases in heat-trapping gases, are used to characterize what 

Figure 16. Simplified image of the methodology that goes into detection and attribution of climate changes. The natural factors 

considered usually include changes in the sun’s output and volcanic eruptions, as well as natural modes of variability such as El 

Niño and La Niña. Human factors include the emissions of heat-trapping gases and particles as well as clearing of forests and other 

land-use changes. (Figure source: NOAA NCDC / CICS-NC).

Detection and Attribution as Forensics
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effect those factors would have had working together. Then, 

simulations with no changes in external forcings, only changes 

due to natural variability, are used to characterize what would 

be expected from normal internal variations in the climate. The 

results of these simulations are compared to observations to 

see which provides the best match for what has really occurred.

Detection and attribution studies have been applied to study a 

broad range of changes in the climate system as well as a number of specific extreme events that have occurred in recent 

years. These studies have found that human influences are the 

only explanation for the observed changes in climate over the 

last half-century. Such changes include increases in surface 

temperatures,46,47 changes in atmospheric vertical temperature profiles,48 increases in ocean heat content,49 increasing atmospheric humidity,50 increases in intensity of precipitation51

and in runoff,52 indirectly estimated through changes in ocean 

salinity,53 shifts in atmospheric circulation,54 and changes in a 

host of other indices.46 Taken together these paint a coherent 

picture of a planet whose climate is changing primarily as a result of human activities.

Detection and attribution of specific events is more challenging than for long-term trends as there are less data, or 

evidence, available from which to draw conclusions. Attribution of extreme events is especially scientifically challenging.56

Many extreme weather and climate events observed to date 

are within the range of what could have occurred naturally, but 

the probability, or odds, of some of these very rare events occurring57 has been significantly altered by human influences on 

the climate system. For example, studies have concluded that 

there is a detectable human influence in recent heat waves 

in Europe,58 Russia,59 and Texas60 as well as flooding events in 

England and Wales,61 the timing and magnitude of snowmelt 

and resulting streamflow in some western U.S. states,62,63 and 

some specific events around the globe during 2011.64

Figure 17. Figure shows examples of the many aspects of the climate system in which changes have 

been formally attributed to human emissions of heat-trapping gases and particles by studies published 

in peer-reviewed science literature. For example, observed changes in surface air temperature at 

both the global and continental levels, particularly over the past 50 years or so, cannot be explained 

without including the effects of human activities. While there are undoubtedly many natural factors 

that have affected climate in the past and continue to do so today, human activities are the dominant 

contributor to recently observed climate changes. (Figure source: NOAA NCDC).

Human Influences Apparent in Many Aspects of the Changing Climate
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Figure 18. Changes in surface air temperature at the continental and global scales can only be explained by 

the influence of human activities on climate. The black line depicts the annually averaged observed changes. 

The blue shading shows climate model simulations that include the effects of natural (solar and volcanic) forcing 

only. The orange shading shows climate model simulations that include the effects of both natural and human 

contributions. These analyses demonstrate that the observed changes, both globally and on a continent-bycontinent basis, are caused by the influence of human activities on climate. (Figure source: updated from 

Jones et al. 201355).

Only Human Influence Can Explain Recent Warming
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Supplemental Message 5. 

Past emissions of heat-trapping gases have already committed the world to a certain 

amount of future climate change. How much more the climate will change depends on future 

emissions and the sensitivity of the climate system to those emissions.

A certain amount of climate change is already inevitable due to 

the build-up of CO2 in the atmosphere from human activities, 

most of it since the Industrial Revolution. A decrease in temperature would only be expected if there was an unexpected 

decrease in natural forcings, such as a reduction in the power 

of the sun. The Earth’s climate system, particularly the ocean, 

tends to lag behind changes in atmospheric composition by decades, and even centuries, due to the large heat capacity of the 

oceans and other factors. Even if all emissions of the relevant 

gases and particles from human activity suddenly stopped, a 

temperature increase of 0.5°F still would occur over the next 

few decades,65 and the human-induced changes in the global 

carbon cycle would persist for thousands of years.66

Global emissions of CO2 and other heat-trapping gases continue to rise. How much climate will change over this century and 

beyond depends primarily on: 1) human activities and resulting 

emissions, and 2) how sensitive the climate is to those changes 

(that is, the response of global temperature to a change in 

radiative forcing caused by human emissions). Uncertainties 

in how the economy will evolve, what types of energy will be 

used, or what our cities, buildings, or cars will look like in the 

future all limit scientists’ ability to predict the future changes 

in climate. Scientists can, however, develop scenarios – plausible projections of what might happen, under a given set of assumptions. These scenarios describe possible futures in terms 

of population, energy sources, technology, heat-trapping gas 

emissions, atmospheric levels of carbon dioxide, and/or global 

temperature change.

Over the next few decades, the greater part of the range (or 

uncertainty) in projected global and regional change is the result of natural variability and scientific limitations in our ability 

to model and understand the Earth’s climate system (natural 

variability is discussed in Supplemental Message 3 and scientific or model uncertainty in Supplemental Message 6). By the 

second half of the century, however, scenario uncertainty (that 

is, uncertainty about what will be the level of emissions from 

human activities) becomes increasingly dominant in determining the magnitude and patterns of future change, particularly 

for temperature-related aspects.67 Even though natural variability will continue to occur, most of the difference between 

present and future climates will be determined by choices that 

society makes today and over the next few decades. The further out in time we look, the greater the influence of human 

choices on the magnitude of future change.

For temperature, it is clear that increasing emissions from human activities will drive consistent increases in global and most 

regional temperatures and that these rising temperatures will 

increase with the magnitude of future emissions (see Figure 

19 and Ch. 2: Our Changing Climate, Figures 2.8 and 2.9). Uncertainty in projected temperature change is generally smaller 

than uncertainty in projected changes in precipitation or other 

aspects of climate.

Future climate change also depends on “climate sensitivity,” 

generally summarized as the response of global temperature 

to a doubling of CO2 levels in the atmosphere relative to preindustrial levels of 280 parts per million. If the only impact of 

increasing atmospheric CO2 levels were to amplify the natural 

greenhouse effect (as CO2 levels increase, more of the Earth’s 

heat is absorbed by the atmosphere before it can escape to 

space, as discussed in Supplemental Message 1), it would be 

relatively easy to calculate the change in global temperature 

that would result from a given increase in CO2 levels. However, 

a series of feedbacks within the Earth’s climate system acts to 

amplify or diminish an initial change, adding some uncertainty 

to the precise climate sensitivity. Some important feedbacks 

include:

•	 Clouds – Will warming increase or decrease 

cloudiness? Will the changes be to lower-altitude 

clouds that primarily reflect the sun’s energy, or 

higher clouds that trap even more heat within the 

Earth system?

•	 Albedo (reflectivity) – How quickly will bright white 

reflective surfaces, such as snow and ice that reflect 

most of the sun’s energy, melt and be replaced by 

a dark ocean or land area that absorbs most of the 

sun’s energy? How will vegetation changes caused by 

climate change alter surface reflectivity?

•	 Carbon dioxide absorption by the ocean and the 

biosphere – Will the rate of uptake increase in the 

future, helping to remove human emissions from the 

atmosphere? Or will it decrease, causing emissions to 

build up even faster than they are now?

Feedbacks are particularly important in the Arctic, where rising temperatures melt ice and snow, exposing relatively dark 

land and ocean, which absorb more of the sun’s energy, heating the region even further. Rising temperatures also thaw 

permafrost, releasing carbon dioxide and methane trapped 

in the previously frozen ground into the atmosphere, where 

they further amplify the greenhouse effect (see Supplemental 

Message 1). Both of these feedbacks act to further amplify the 
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initial warming due to human emissions of carbon dioxide and 

other heat-trapping gases.

Together, these and other feedbacks determine the long-term 

response of the Earth’s temperature to an increase in carbon 

dioxide and other emissions from human activities. Past observations, including both recent measurements and studies 

that look at climate changes in the distant past, cannot tell us 

precisely how sensitive the climate system will be to increasing 

emissions of heat-trapping gases if we are starting from today’s conditions. They can tell us, however, that the net effect 

of these feedbacks will be to increase, not diminish, the direct 

warming effect. In other words, the climate system will warm 

by more than would be expected from the greenhouse effect 

alone.

Quantifying the effect of these feedbacks on global and regional climate is the subject of ongoing data collection and 

active research. As noted above, one measure used to study 

these effects is the “equilibrium climate sensitivity,” which is 

an estimate of the temperature change that would result, once 

the climate had reached an equilibrium state, as a result of 

doubling the CO2 concentration from pre-industrial levels. The 

equilibrium climate sensitivity has long been estimated to be in 

the range of 2.7°F to 8.1°F. The 2007 IPCC Fourth Assessment 

Report15 refined this range based on more recent evidence to 

conclude that the value is likely to be in the range 3.6°F to 8.1°F, 

with a most probable value of about 5.4°F, based upon multiple observational and modeling constraints, and that it is very 

unlikely to be less than 2.7°F. Climate sensitivities determined 

from a variety of evidence agree well with this range, including 

analyses of past paleoclimate changes.68,69 This is substantially 

greater than the increase in temperature from just the direct 

radiative effects of the CO2 increase (around 2°F).

Some recent studies (such as Fasullo and Trenberth 201270) 

have suggested that climate sensitivities are at the higher end 

of this range, while others have suggested values at the lower 

end of the range.71,72 Some recent studies have even suggested 

that the climate sensitivity may be less than 2.7°F based on 

analyses of recent temperature trends.72 However, analyses 

based on recent temperature trends are subject to significant 

uncertainties in the treatment of natural variability,69 the effects of volcanic eruptions,73 and the effects of recent accelerated penetration of heat to the deep ocean.74

The equilibrium climate sensitivity is sometimes confused with 

the “transient climate response,” defined as the temperature 

change for a 1% per year CO2 increase, and calculated using the 

difference between the start of the experiment and a 20-year 

period centered on the time of CO2 doubling. This value is generally smaller than the equilibrium climate sensitivity because 

of the slow rate at which heat transfers between the oceans 

and the atmosphere due to transient heat uptake of the ocean.

The transient climate response is better constrained than the 

equilibrium climate sensitivity.15 It is very likely larger than 

1.8°F and very unlikely to be greater than 5.4°F. This transient 

response includes feedbacks that respond to global temperature change over timescales of years to decades. These “fast” 

feedbacks include increases in atmospheric water vapor, reduction of ice and snow, warming of the ocean surface, and 

changes in cloud characteristics. The entire response of the climate system will not be fully seen until the deep ocean comes 

into balance with the atmosphere, a process that can take 

thousands of years.

Combining the uncertainty due to climate sensitivity with the 

uncertainty due to human activities produces a range of future temperature changes that overlap over the first half of 

this century, but begins to separate over the second half of the 

century as emissions and atmospheric CO2 levels diverge.
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Figure 19. Two families of scenarios are commonly used for future climate projections: the 2000 Special Report on Emission 

Scenarios (SRES, left) and the 2010 Representative Concentration Pathways (RCP, right). The SRES scenarios are named by 

family (A1, A2, B1, and B2), where each family is designed around a set of consistent assumptions: for example, a world that is more 

integrated or more divided. In contrast, the RCP scenarios are simply numbered according to the change in radiative forcing (from 

+2.6 to +8.5 watts per square meter) that results by 2100. This figure compares SRES and RCP annual carbon emissions (top), 

carbon dioxide equivalent levels in the atmosphere (middle), and temperature change that would result from the central estimate 

(lines) and the likely range (shaded areas) of climate sensitivity (bottom). At the top end of the range, the older SRES scenarios are 

slightly higher. Comparing carbon dioxide concentrations and global temperature change between the SRES and RCP scenarios, 

SRES A1fI is similar to RCP 8.5; SRES A1B to RCP 6.0 and SRES B1 to RCP 4.5. The RCP 2.6 scenario is much lower than any 

SRES scenario because it includes the option of using policies to achieve net negative carbon dioxide emissions before end of 

century, while SRES scenarios do not. (Data from CMIP3 and CMIP5).

Emissions, Concentrations, and Temperature Projections
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Figure 20. Projected change in surface air temperature at the end of this century (2071-2099) relative to the end of the last century 

(1970-1999). The older generation of models (CMIP3) and SRES emissions scenarios are on the left side; the new models (CMIP5) 

and scenarios are on the right side. The scenarios are described under Supplemental Message 5 and in Figure 19. Differences 

between the old and new projections are mostly a result of the differences in the scenarios of the emission of heat-trapping gases 

rather than the increased complexity of the new models. None of the new scenarios are exactly the same as the old ones, although 

at the end of the century SRES B1 and RCP 4.5 are roughly comparable, as are SRES A1B and RCP 6.0. (Figure source: NOAA 

NCDC / CICS-NC).

Projected Annually-Averaged Temperature Change
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Figure 21. Projected changes in wintertime precipitation at the end of this century (2071-2099) relative to the average for 1970-1999. 

The older generation of models (CMIP3) and emissions scenarios are on the left side; the new models (CMIP5) and scenarios are 

on the right side. Hatched areas indicate that the projected changes are significant and consistent among models. White areas 

indicate that the changes are not projected to be larger than could be expected from natural variability. In both sets of projections, 

the northern parts of the U.S. (and Alaska) become wetter. Increases in both the amount of precipitation change and the confidence 

in the projections go up as the projected temperature rises. In the farthest northern parts of the U.S., much of the additional winter 

precipitation will still fall as snow. This is not likely to be the case farther south. (Figure source: NOAA NCDC / CICS-NC).

Projected Wintertime Precipitation Changes
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Figure 22. Projected changes in summertime precipitation toward the end of this century (2071-2099) relative to the average for 

1970-1999. The older generation of models (CMIP3) and emissions scenarios are on the left side; the new models (CMIP5) and 

scenarios are on the right side. Hatched areas indicate that the projected changes are significant and consistent among models. 

White areas indicate confidence that the changes are not projected to be larger than could be expected from natural variability.

In most of the contiguous U.S., decreases in summer precipitation are projected, but not with as much confidence as the winter 

increases. When interpreting maps of temperature and precipitation projections, readers are advised to pay less attention to small 

details and greater attention to the large-scale patterns of change. (Figure source: NOAA NCDC / CICS-NC).

Projected Summertime Precipitation Changes
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Figure 23. Historical emissions of carbon from fossil fuel (coal, oil, and gas) combustion and 

land-use change (such as deforestation) have increased over time. The growth rate was nearly 

three times greater during the 2000s as compared to the 1990s. This figure compares the 

observed historical (black dots) and projected future SRES (orange dashed lines) and RCP (blue 

solid lines) carbon emissions from 1970 to 2030. (Data from Boden et al. 201175 plus preliminary 

values for 2009 and 2010 based on BP statistics and U.S. Geological Survey cement data).

Carbon Emissions: Historical and Projected
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Supplemental Message 6. 

Different kinds of physical and statistical models are used to study aspects of past climate 

and develop projections of future change. No model is perfect, but many of them provide 

useful information. By combining and averaging multiple models, many clear trends emerge.

Climate scientists use a wide range of observational and computational tools to understand the complexity of the Earth’s 

climate system and to study how that system responds to external forces, including the effect of humans on climate. Observational tools are described in Supplemental Message 2.

Computational tools include models that simulate different 

parts of the climate system. The most sophisticated computational tools used by climate scientists are global climate models (previously referred to as “general circulation models”), or 

GCMs. Global climate models are mathematical models that 

simulate the physics, chemistry, and, increasingly, the biology 

that influence the climate system. GCMs are built on fundamental equations of physics that include the conservation of 

energy, mass, and momentum, and how these are exchanged 

among different parts of the climate system. Using these fundamental relationships, the models generate many important 

features that are evident in the Earth’s climate system: the jet 

stream that circles the globe 30,000 feet above the Earth’s surface; the Gulf Stream and other ocean currents that transport 

heat from the tropics to the poles; and even, when the models 

can be run at a fine enough spatial resolution to capture these 

features, hurricanes in the Atlantic and typhoons in the Pacific.

GCMs and other physical models are subject to two main types 

of uncertainty. First, because scientific understanding of the 

climate system is not complete, a model may not include an 

important process. This could be because that process is not 

yet recognized, or because it is known but is not yet understood well enough to be modeled accurately. For example, the 

models do not currently include adequate treatments of dynamical mechanisms that are important to melting ice sheets. 

The existence of these mechanisms is known, but they are 

not yet well enough understood to simulate accurately at the 

global scale. Also, observations of climate change in the distant 

past suggest there might be “tipping points,” or mechanisms 

of abrupt changes in climate change, such as shifts in ocean 

circulation, that are not adequately understood.76 These are 

discussed further in Appendix 4: FAQ T.

Second, many processes occur at finer temporal and spatial 

(time and space) scales than models can resolve. Models instead must approximate what these processes would look like 

at the spatial scale that the model can resolve using empirical equations, or parameterizations, based on a combination 

of observations and scientific understanding. Examples of 

important processes that must be parameterized in climate 

models include turbulent mixing, radiational heating/cooling, 

and small-scale physical processes such as cloud formation and 

precipitation, chemical reactions, and exchanges between the 

biosphere and atmosphere. For example, these models cannot represent every raindrop. However, they can simulate the 

total amount of rain that would fall over a large area the size 

of a grid cell in the model. These approximations are usually 

derived from a limited set of observations and/or higher resolution modeling and may not hold true for every location or 

under all possible conditions.

GCMs are constantly being enhanced as scientific understanding of climate improves and as computational power increases. 

For example, in 1990, the average model divided up the world 

into grid cells measuring more than 300 miles per side. Today, 

most models divide the world up into grid cells of about 60 to 

100 miles per side, and some of the most recent models are 

able to run short simulations with grid cells of only 15 miles 

per side. Supercomputer capabilities are the primary limitation 

on grid cell size. Newer models also incorporate more of the 

physical processes and components that make up the Earth’s 

climate system. The very first global climate models were 

designed to simulate only the circulation of the atmosphere. 

Over time, the ocean, clouds, land surface, ice, snow, and other 

features were added one by one. Most of these features were 

new modules that were developed by experts in those fields 

and then added into an existing GCM framework. Today, there 

are more than 35 GCMs created and maintained by more than 

20 modeling groups around the world. Some of the newest 

models are known as Earth System Models, or ESMs, which 

include all the previous components of a typical GCM but also 

incorporate modules that represent additional aspects of the 

climate system, including agriculture, vegetation, and the carbon cycle.

Some models are more successful than others at reproducing 

observed climate and trends over the past century,77 or the 

large-scale dynamical features responsible for creating the 

average climate conditions over a certain region (such as the 

Arctic78 or the Caribbean79). Evaluation of models’ success 

often depends on the variable or metric being considered in the 

analysis, with some models performing better than others for 

certain regions or variables.80 However, all future simulations 

agree that both global and regional temperatures will increase 

over this century in response to increasing emissions of heattrapping gases from human activities.15

Differences among model simulations over several years to 

several decades arise from natural variability (as discussed in 

Supplemental Message 3) as well as from different ways models characterize various small-scale processes. Averaging simu-
761 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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lations from multiple models removes the effects of randomly 

occurring natural variations. The timing of natural variations 

is largely unpredictable beyond several seasons (although 

such predictability is an active research area). For this reason, 

model simulations are generally averaged (as the last stage in 

any analysis) to make it easier to discern the impact of external 

forcing (both human and natural). The effect of averaging on 

the systematic errors depends on the extent to which models 

have similar errors or offsetting errors.

Despite their increasing resolution, most GCMs cannot simulate fine-scale changes at the regional to local scale. For that 

reason, downscaling is often used to translate GCM projections into the high-resolution information required as input 

to impact analyses. There are two types of models commonly 

used for downscaling: dynamical and statistical.

Dynamical downscaling models are often referred to as regional climate models since they include many of the same 

physical processes that make up a global climate model, but 

simulate these processes at higher resolution and over a relatively small area, such as the Northwest or Southeast United 

States. At their boundaries, regional climate models use output from GCMs to simulate what is going on in the rest of the 

world. Regional climate models are computationally intensive, 

but provide a broad range of output variables including atmospheric circulation, winds, cloudiness, and humidity at spatial 

scales ranging from about 6 to 30 miles per grid cell. They are 

also subject to the same types of uncertainty as a global model, such as not fully resolving physical processes that occur at 

even smaller scales. Regional climate models have additional 

uncertainty related to how often their boundary conditions 

are updated and where they are defined. These uncertainties 

can have a large impact on the precipitation simulated by the 

models at the local to regional scale. Currently, a limited set of 

regional climate model simulations based on one future scenario and output from five CMIP3 GCMs is available from the 

North American Regional Climate Change Assessment Program 

(these are the “NARCCAP” models used in some sections of 

this report). These simulations are useful for examining certain 

impacts over North America. However, they do not encompass 

the full range of uncertainty in future projections due to both 

human activities and climate sensitivity described in Supplemental Message 5.

Statistical downscaling models use observed relationships 

between large-scale weather features and local climate to 

translate future projections down to the scale of observations. 

Statistical models are generally very effective at removing errors in historical simulated values, leading to a good match between the average (multi-decadal) statistics of observed and 

statistically downscaled climate at the spatial scale and over 

the historical period of the observational data used to train 

the statistical model. However, statistical models are based 

on the key assumption that the relationship between largescale weather systems and local climate will remain constant 

over time. This assumption may be valid for lesser amounts of 

change, but could lead to errors, particularly in precipitation 

extremes, with larger amounts of climate change.81 Statistical 

models are generally flexible and less computationally demanding than regional climate models. A number of databases 

provide statistically downscaled projections for a continuous 

period from 1960 to 2100 using many global models and a 

range of higher and lower future scenarios (for example, the 

U.S. Geological Survey database described by Maurer et al. 

200782).83,84 Statistical downscaling models are best suited for 

analyses that require a range of future projections that reflect 

the uncertainty in emissions scenarios and climate sensitivity, 

at the scale of observations that may already be used for planning purposes.

Ideally, climate impact studies could use both statistical and 

dynamical downscaling methods. Regional climate models can 

directly simulate the response of regional climate processes to 

global change, while statistical models can better remove any 

biases in simulations relative to observations. However, rarely 

(if ever) are the resources available to take this approach. Instead, most assessments tend to rely on one or the other type 

of downscaling, where the choice is based on the needs of the 

assessment. If the study is more of a sensitivity analysis, where 

using one or two future simulations is not a limitation, or if it 

requires many climate variables as input, then regional climate 

modeling may be more appropriate. If the study needs to resolve the full range of projected changes under multiple models and scenarios or is more constrained by practical resources, 

then statistical downscaling may be more appropriate. However, even within statistical downscaling, selecting an appropriate method for any given study depends on the questions 

being asked. The variety of techniques ranges from a simple 

“delta” (change or difference) approach (subtracting historical 

simulated values from future values, and adding the resulting 

delta to historical observations, as used in the first national climate assessment85) to complex clustering and neural network 

techniques that rival dynamical downscaling in their demand 

for computational resources and high-frequency model output 

(for example, Kostopoulou and Jones 200786; Vrac et al. 200781). 

The delta approach is adequate for studies that are only interested in changes in seasonal or annual average temperature. 

More complex methods must be used for studies that require 

information on how climate change may affect the frequency 

or timing of precipitation and climate extremes.
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Figure 24. Some of the many processes often included in models of the Earth’s climate system. (Figure source: Karl 

and Trenberth 200387).

Modeling the Climate System
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Figure 25. Top: Illustration of the eastern North American 

topography in a resolution of 68 x 68 miles (110 x 110 km). 

Bottom: Illustration of the eastern North American topography 

in a resolution of 19 x 19 miles (30 x 30 km).

Increasing Model Resolution

Figure 26. The development of climate models 

over the last 35 years showing how the different 

components were coupled into comprehensive 

climate models over time. In each aspect (for 

example, the atmosphere, which comprises a wide 

range of atmospheric processes) the complexity 

and range of processes has increased over time 

(illustrated by growing cylinders). Note that during 

the same time the horizontal and vertical resolution 

has increased considerably. (Figure source: 

adapted from Cubasch et al. 201388).

Increasing Climate Model Components

Intergovernmental Panel on Climate Change Reports

FAR 1990

SAR 1995

TAR 2001

AR4 2007

AR5 2013
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Supplemental Message 7. 

Scientific understanding of observed temperature changes in the United States has greatly 

improved, confirming that the U.S. is warming due to heat-trapping gas emissions,

 consistent with the climate change observed globally. 

There have been substantial recent advances in our understanding of the continental U.S. temperature records. Numerous studies have looked at many different aspects of the record.28,89,90,91,92,93 These studies have increased confidence that 

the U.S. is warming, and refined estimates of how much.

Historical temperature data are available for thousands of 

weather stations. However, for a variety of practical and often 

unavoidable reasons, there have been frequent changes to individual stations and to the network as a whole. Two changes 

are particularly important. The first is a widespread change in 

the time at which observers read their thermometers. Second, 

most stations now use electronic instruments rather than traditional glass thermometers.

Extensive work has been done to document the effect of these 

changes on historical temperatures. For example, the change 

from afternoon to morning observations resulted in systematically lower temperatures for both maximum and minimum, artificially cooling the U.S. temperature record by about 0.5°F.93,94

The change in instrumentation was equally important but 

more complex. New electronic instruments generally recorded 

higher minimum temperatures, yielding an artificial warming 

of about 0.25°F, and lower maximum temperatures, resulting 

in an artificial cooling of about 0.5°F. This has been confirmed 

by extended period side-by-side instrument comparisons.95

Confounding this, as noted by a recent citizen science effort, 

the new instruments were often placed nearer buildings or 

other man-made structures.96 Analyses of the changes in siting 

indicate that this had a much smaller effect than the change in 

instrumentation across the network as a whole.89,91,93

Extensive work has been done to develop statistical adjustments that carefully remove these and other non-climate 

elements that affect the data. To confirm the efficacy of the 

adjustments, several sensitivity assessments have been undertaken. These include:

•	 a comparison with the U.S. Climate Reference 

Network;91,97

•	 analyses to evaluate biases and uncertainties;93

•	 comparisons to a range of state-of-the-art 

meteorological data analyses;92 and

•	 in-depth analyses of the potential impacts of 

urbanization.

90

These assessments agree that the corrected data do not overestimate the rate of warming. Rather, because the average 

effect of these issues was to reduce recorded temperatures, 

adjusting for these issues tends to reveal a larger long-term 

warming trend. The impact is much larger for maximum temperature as compared to minimum temperature because the 

adjustments account for two distinct artificial cooling signals: 

the change in observation time and the change in instrumentation. The impact is smaller for minimum temperature because 

the artificial signals roughly offset one another (the change in 

observation time cooling the record, the change in instrumentation warming the record). Even without these adjustments, 

however, both maximum and minimum temperature records 

show increases over the past century.

Geographically, maximum temperature has increased in most 

areas except in parts of the western Midwest, northeastern 

Great Plains, and the Southeast regions. Minimum temperature exhibits the same pattern of change with a slightly greater 

area of increases. The causes of these slight differences between maximum and minimum temperature are a subject of 

ongoing research.98 In general, the uncorrected data exhibit 

more extreme trends as well as larger spatial variability; in 

other words, the adjustments have a smoothing effect.

The corrected temperature record also confirms that U.S. average temperature is increasing in all four seasons. The heat that 

occurred during the Dust Bowl era is prominent in the summer 

record. The warmest summer on record was 1936, closely followed by 2012. However, twelve of the last fourteen summers 

have been above average. Temperatures during the other seasons have also generally been above average in recent years.
765 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Figure 27. Geographic distribution of linear trends in the U.S. Historical Climatology Network for the period 1895-2011. 

(Figure source: updated from Menne et al. 200991).

Trends in Maximum and Minimum Temperatures
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Figure 28. Continental U.S. seasonal temperatures (relative to the 1901-1960 average) for winter, spring, summer, and fall all show 

evidence of increasing trends. Dashed lines show the linear trends. Stronger trends are seen in winter and spring as compared to 

summer and fall. (Figure source: updated from Kunkel et al. 201399). 

U.S. Seasonal Temperatures
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Supplemental Message 8. 

Many other indicators of rising temperatures have been observed in the United States. These 

include reduced lake ice, glacier retreat, earlier melting of snowpack, reduced lake levels,

 and a longer growing season. These and other indicators are expected to 

continue to reflect higher temperatures.

While surface air temperature is the most widely cited measure of climate change, other aspects of climate that are affected by temperature are often more directly relevant to both 

human society and the natural environment. Examples include 

shorter duration of ice on lakes and rivers, reduced glacier extent, earlier melting of snowpack, reduced lake levels due to 

increased evaporation, lengthening of the growing season, and 

changes in plant hardiness zones. Changes in these and many 

other variables are consistent with the recent warming over 

much of the United States. Taken as a whole, these changes 

provide compelling evidence that increasing temperatures are 

affecting both ecosystems and human society.

Striking decreases in the coverage of ice on the Great Lakes 

have occurred over the last few decades (see Ch 2: Our Changing Climate, Key Message 11). The annual average ice cover 

area for the Great Lakes, which typically shows large year-toyear variability, has sharply declined over the last 30+ years.100

Based on records covering the winters of 1972-1973 through 

2010-2011, 12 of the 19 winters prior to 1991-1992 had annual average ice cover greater than 20% of the total lake area 

while 15 of the 20 winters since 1991-1992 have had less than 

20% of the total lake area covered with ice. This 

includes the three lowest ice extent winters of 

1997-1998, 2001-2002, and 2005-2006. A reduction in ice leading to more open water in winter 

raises concerns about possible increases in lake 

effect snowfall, although future trends will also 

depend on the difference between local air and 

water temperatures.

Smaller lakes in other parts of the country show 

similar changes. For example, the total duration of 

ice cover on Lake Mendota in Madison, Wisconsin, has decreased from about 120 days in the late 

1800s to less than 100 days in most years since 

1990.101 Average dates of spring ice disappearance 

on Minnesota lakes show a trend toward earlier 

melting over the past 60 years or so. These changes affect the recreational and commercial activities of the surrounding communities.

A long-term record of the ice-in date (the first 

date in winter when ice coverage closes the lake 

to navigation) on Lake Champlain in Vermont 

shows that the lake now freezes approximately 

two weeks later than in the early 1800s and over a 

week later than 100 years ago.102 Later ice-in dates 

are an indication of higher lake temperatures, as it takes longer 

for the warmer water to freeze in winter. Prior to 1950, the 

absence of winter ice cover on Lake Champlain was rare, occurring just three times in the 1800s and four times between 

1900 and 1950. By contrast, it remained ice-free during 42% 

of the winters between 1951 and 1990, and since 1991, Lake 

Champlain has remained ice-free during 64% of the winters. 

One- to two-week advances of ice breakup dates and similar 

length delays of freeze-up dates are also typical of lakes and 

rivers in Canada, Scandinavia, and northern Asia.15

While shorter durations of lake ice enhance navigational opportunities during winter, decreasing water levels in the Great 

Lakes present risks to navigation, especially during the summer. Water levels on Lakes Superior, Michigan, and Ontario 

have been below their long-term (1918-2008) averages for 

much of the past decade.103 The summer drought of 2012 

left Lakes Michigan and Ontario approximately one foot below their long-term averages. As noted in the second national 

climate assessment,1 projected water level reductions for this 

century in the Great Lakes range from less than a foot under 

lower emissions scenarios to between 1 and 2 feet under highFigure 29. The duration, or number of days, of ice cover on Lake Mendota, 

Wisconsin, has decreased over time. The 10 longest ice seasons are marked 

by blue circles, and the 11 shortest ice seasons are marked by red circles. 

Seven of the 10 shortest ice cover seasons have occurred since 1980. (Figure 

source: Kunkel et al. 2013107).

Ice Cover on Lake Mendota
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er emissions scenarios, with the smallest changes projected 

for Lake Superior and the largest change projected for Lakes 

Michigan and Huron.83 A notable feature is the large range 

(several feet) of water level projections among models.104

More recent studies have indicated that earlier approaches 

to computing evapotranspiration estimates from temperature 

may have overestimated evaporation losses.105 Accounting for 

land-atmosphere feedbacks may further reduce the estimates 

of lake level declines.106 These recent studies, along with the 

large spread in models, indicate that projections of Great Lakes 

water levels represent evolving research and are still subject to 

considerable uncertainty.

In the U.S. Southwest, indications of a changing climate over 

the last five decades include decreases in mountain snowpack,108 earlier dates of snowmelt runoff,

109,110 earlier onset of 

spring (as indicated by shifts in the timing of plant blooms and 

spring snowmelt-runoff pulses),111 general shifts in western 

hydroclimatic seasons,

112 and trends toward more precipitation falling as rain instead of snow over the West.113 The ratio 

of precipitation falling as rain rather than snow, the amount 

of water in snowpack, and the timing of peak stream flow on 

snowmelt-fed rivers all changed as expected with warming 

over the past dozen years, relative to the last century baselines.62

Changing temperatures affect vegetation through lengthening 

of the frost-free season and the corresponding growing season, and changing locations of plant tolerance thresholds. The 

U.S. average frost-free season length (defined as the number 

of days between the last and first occurrences of 32°F in spring 

and autumn, respectively) increased by about two weeks during the last century.114 The increase was much greater in the 

western than in the eastern United States. Consistent with the 

recent observed trends in frost-free season length, the largest 

projected changes in growing season length are in the mountainous regions of the western United States, while smaller 

changes are projected for the Midwest, Northeast, and Southeast. Related plant and animal changes include a northward 

shift in the typical locations of bird species115 and a shift since 

the 1980s toward earlier first-leaf dates for lilac and honeysuckle.116

Plant hardiness zones are determined primarily by the extremes of winter cold.117 Maps of plant hardiness have guided 

the selection of plants for both ornamental and agricultural 

purposes, and these zones are changing as climate warms. 

Plant hardiness zones for the U.S. have recently been updated 

using the new climate normals (1981-2010), and these zones 

show a northward shift by up to 100 miles relative to the zones 

based on the older (1971-2000) normals. Even greater northward shifts, as much as 200 miles, are projected over the next 

30 years as warming increases. Projected shifts are largest in 

the major agricultural regions of the central United States.

Evidence of a warming climate across the U.S. is based on a 

host of indicators: hydrology, ecology, and physical climate. 

Most of these are changing in ways consistent with increasing 

temperatures, and are expected to continue to change in the 

future as a result of ongoing increases in human-induced heattrapping gas emissions.

Figure 30. At many locations in the western U.S., the timing 

of streamflow in rivers fed by snowpack is shifting to earlier 

in the year. Red dots indicate stream gauge locations where 

half of the annual flow is now arriving anywhere from 5 to 20 

days earlier each year for 2001-2010, relative to the 1951-

2000 average. Blue dots indicate locations where the annual 

flow is now arriving later. Crosses indicate locations where 

observed changes are not statistically different from the past 

century baseline at 90% confidence levels, diamonds indicate 

gauges where the timing difference was significantly different 

at 90% confidence, and dots indicate gauges where timing 

was different at 95% confidence level. (Updated from Stewart 

et al. 2005110).

Streamflow from Snowmelt

 Coming Earlier in the Year
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Supplemental Message 9. 

Trends in some types of extreme weather events have been observed in recent decades, 

consistent with rising temperatures. These include increases in heavy precipitation 

nationwide, especially in the Midwest and Northeast; heat waves, especially in the West; and 

the intensity of Atlantic hurricanes. These trends are expected to continue. Research on 

climate change’s effects on other types of extreme events continues.

High impact, large-scale extreme events are complex phenomena involving various factors that can vcreate a “perfect 

storm.” Such extreme weather occurs naturally. However, the 

influence of human activities on global climate is altering the 

frequency and/or severity of many of these events.

Observations show that heavy downpours have already increased nationally. Regional and global models project increases in extreme precipitation for every U.S. region.118 Precipitation events tend to be limited by available moisture. For 

the heaviest, most rare events, there is strong evidence from 

observations119 and models118,120 that higher temperatures and 

the resulting moister atmosphere are the main cause of these 

observed and projected increases. Other factors that may also 

have an influence on observed U.S. changes in extreme precipitation are land-use changes (for example, changes in irrigation121,122) and a shift in the number of El Niño events versus La 

Niña events.

Climate change can also alter the characteristics of the atmosphere in ways that affect weather patterns and storms. In the 

mid-latitudes, where most of the continental U.S. is located, 

there is an increasing trend in extreme precipitation in the 

vicinity of fronts associated with mid-latitude storms (also 

referred to as extra-tropical [outside the tropics] cyclones123). 

There is also a northward shift in storms over the U.S.124 that 

are often associated with extreme precipitation. This shift is 

consistent with projections of a warming world.125 No change in 

mid-latitude storm intensity or frequency has been detected.

In the tropics, the most important types of storms are tropical cyclones, referred to as hurricanes when they occur in the 

Atlantic Ocean. Over the 40 years of satellite monitoring, there 

has been a shift toward stronger hurricanes in the Atlantic, 

with fewer Category 1 and 2 hurricanes and more Category 4 

and 5 hurricanes. There has been no significant trend in the 

global number of tropical cyclones126 nor has any trend been 

identified in the number of U.S. landfalling hurricanes.1 Two 

Figure 31. The map on the left shows the change in Plant Hardiness Zones calculated from those based on the 1971-2000 climate 

to those based on the 1981-2010 climate. Even greater changes are projected over the next 30 years (right). (Figure source: NOAA).

Shifts in Plant Hardiness Zones
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studies have found an upward trend in the number of extreme 

precipitation events associated with tropical cyclones,127 but 

significant uncertainties remain.122 A change in the number of 

Atlantic hurricanes has been identified, but interpreting its significance is complicated both by multi-decadal natural variability and the reliability of the pre-satellite historical record.128

The global satellite record shows a shift toward stronger tropical cyclones,126,129 but does not provide definitive evidence of 

a long-term trend. Nonetheless, there is a growing consensus 

based on scientific understanding and very-high-resolution 

atmospheric modeling that the strongest tropical cyclones, including Atlantic hurricanes, will become stronger in a warmer 

world.130

The number of heat waves has been increasing in recent years. 

On a decadal basis, the decade of 2001-2010 had the second 

highest number since 1901 (first is the 1930s). This trend has 

continued in 2011 and 2012, with the number of intense heat 

waves being almost triple the long-term average. Regionally, the Northwest, Southwest, and Alaska had their highest 

number of heat waves in the 2000s, while the 1930s were the 

highest in the other regions (note that the Alaskan time series 

begins in the 1950s). For the number of intense cold waves, the 

national-average value was highest in the 1980s and lowest in 

the 2000s. The lack of cold waves in the 2000s was prevalent 

throughout the contiguous U.S. and Alaska. Climate model 

simulations indicate that the recent trends toward increasing 

frequency of heat waves and decreasing frequency of cold 

waves will continue in the future.

The data on the number and intensity of severe thunderstorm 

phenomena (including tornadoes, thunderstorm winds, and 

hail) are not of sufficient quality to determine whether there 

have been historical trends.119 This scarcity of high-quality 

data, combined with the fact that these phenomena are too 

small to be directly represented in climate models,131 makes 

it difficult to project how these storms might change in the 

future.

Figure 32. Heavy downpours are increasing nationally, with especially large increases in the Midwest and Northeast.99 Despite 

considerable decadal-scale natural variability, indices such as this one based on 2-day precipitation totals exceeding a threshold 

for a 1-in-5-year occurrence exhibit a greater than normal occurrence of extreme events since 1991 in all U.S. regions except 

Alaska and Hawai‘i. Each bar represents that decade’s average, while the far right bar in each graph represents the average for 

the 12-year period of 2001-2012. Analysis is based on 726 long-term, quality-controlled station records. This figure is a regional 

expansion of the national index in Figure 2.16 of Chapter 2. (Figure source: updated from Kunkel et al. 201399).

Extreme Precipitation
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Supplemental Message 10. 

Drought and fire risk are increasing in many regions as temperatures and evaporation rates 

rise. The greater the future warming, the more these risks will increase, 

potentially affecting the entire United States.

As temperatures rise, evaporation 

rates increase, which (all else remaining equal) would be expected to lead to 

increased drying.131 The Palmer Drought 

Severity Index (PDSI),132 a widely used 

indicator of dryness that incorporates 

both precipitation and temperaturebased evaporation estimates, does 

not show any trend for the U.S. as a 

whole over the past century.133 However, drought intensity and frequency 

have been increasing over much of the 

western United States, especially during 

the last four decades. In the Southeast, 

western Great Lakes, and southern 

Great Plains, droughts have increased 

during the last 40 years, but do not 

show an increase when examined over 

longer periods encompassing the entire 

last century. In the Southwest, drought 

has been widespread since 2000; the 

average value of the PDSI during the 

2000s indicated the most severe average drought conditions of any decade. 

The severity of recent drought in the 

Southwest reflects both the decade’s 

low precipitation and high temperatures.

Seasonal and multi-year droughts affect wildfire severity.134

For example, persistent drought conditions in the Southwest, 

combined with wildfire suppression and land management 

practices,135 have contributed to wildfires of unprecedented 

size since 2000. Five western states (Arizona, Colorado, Utah, 

California, and New Mexico) have experienced their largest 

fires on record at least once since 2000. Much of the increase 

in fires larger than 500 acres occurred in the western United 

States, and the area burned in the Southwest increased more 

than 300% relative to the area burned during the 1970s and 

early 1980s.136

Droughts on a duration and scale that affect agriculture are 

projected to increase in frequency and severity in this century 

due to higher temperatures. Projections of the Palmer Drought 

Severity Index at the end of this century indicate that the normal state for most of the nation will be what is considered 

moderate to severe drought today.137,138 The PDSI is used by 

several states for monitoring drought and for triggering certain 

actions.139 It is also one component of the U.S. Drought Monitor.140 The closely related Palmer Hydrological Index is the most 

Figure 33. The area of the western U.S. in moderately to extremely dry conditions 

during summer (June-July-August) varies greatly from year to year but shows a longterm increasing trend from 1900 to 2012. (Data from NOAA NCDC State of the Climate 

Drought analysis).

Percent of West in Summer Drought

Figure 34. Although the average number of wildfires per year 

(black line) has decreased over time, the total area burned by 

wildfires (orange bars) in the continental U.S. (primarily in the 

western states) has nearly doubled since 2000 relative to the 

long-term 1960-1999 average (data shown are for 1960-2011).

(Data from the National Interagency Fire Center).

Changing Forest Fires in the U.S.
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important component of NOAA’s Objective Long-term Drought 

Indicator Blend,141 which is used by the U.S. Department of Agriculture to identify counties that are eligible to participate in 

certain Federal Government drought relief programs. The U.S. 

Drought Monitor is used by some states for similar purposes. 

Despite its widespread usage, the PDSI may be overly sensitive to future temperature increases.142 As temperatures 

increase during this century, these PDSI-based monitoring 

tools may over-estimate the intensity of 

drought during anomalous warm periods, 

so statutory adjustments to these tools may 

be warranted. However, the projection of increased drought risk is reinforced by a direct 

examination of future soil moisture content 

projections, which reveals substantial drying 

in most areas of the western U.S (Ch. 2: Our 

Changing Climate, Key Message 3).

Provided the wood and ground litter has 

dried out, the area of forest burned in many 

mid-latitude areas, including the western 

United States, may increase substantially 

as temperature and evapotranspiration increase, exacerbating drought.143 Under even 

relatively modest amounts of warming, significant increases in area burned are projected in the Sierra Nevada, southern Cascades, 

and coastal California; in the mountains 

of Arizona and New Mexico; on the Colorado Plateau; and in the Rocky Mountains.144

Other studies, examining a broad range of 

climate change and development scenarios, 

find increases in the chance of large fires for 

much of northern California’s forests.145

Long periods of consecutive days with little 

or no precipitation also can lead to drought. 

The average annual maximum number of 

consecutive dry days are projected to increase for the higher emissions scenarios 

in areas that are already prone to little 

precipitation by mid-century and increase thereafter (Ch. 2: 

Our Changing Climate, Key Message 5). Much of the western 

and southwestern U.S. is projected to experience statistically 

significant increases in the annual maximum number of consecutive dry days, on average up to 10 days above presentday values for parts of the contiguous U.S. by the end of this 

century under high emissions scenarios. Hence, some years are 

projected to experience substantially longer dry seasons.

Figure 35. The fractional areal extent of the contiguous U.S. and Mexico in 

extreme drought according to projections of the Palmer Drought Severity Index 

under an intermediate emissions scenario (SRES A1B, in between the B1 and 

A2 scenarios used elsewhere in this report) (Supplemental Message 5 and Ch. 

2: Our Changing Climate, Key Message 3). The Palmer Drought Severity Index 

is the most widely used measure of drought, although it is more sensitive to 

temperature than other drought indices and may over-estimate the magnitude 

of drought increases. The red line is based on observed temperature and 

precipitation. The blue line is from the average of 19 different climate models. 

The gray lines in the background are individual results from over 70 different 

simulations from these models. These results suggest an increasing probability 

of agricultural drought over this century throughout most of the U.S. (Figure 

source: Wehner et al. 2011138).

Extreme Drought in the U.S. and Mexico, Past and Future
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Supplemental Message 11. 

Summer Arctic sea ice extent, volume, and thickness have declined rapidly, especially north 

of Alaska. Permafrost temperatures are rising and the overall amount of permafrost is 

shrinking. Melting of land- and sea-based ice is expected to continue with further warming.

Increasing temperatures and associated impacts are apparent throughout the Arctic, including Alaska. Sea ice coverage 

and thickness, permafrost on land, mountain glaciers, and the 

Greenland Ice Sheet all show changes consistent with higher 

temperatures.

The most dramatic decreases in summer sea ice have occurred 

along the northern coastline of Alaska and Russia. Since the 

satellite record began in 1979, September (summer minimum) 

sea ice extent has declined by 13% per decade in the Beaufort Sea and 32% per decade in the Chukchi Sea,146 leaving the 

Chukchi nearly ice-free in the past few Septembers. Longerterm records based on climate proxies suggest that pan-Arctic 

ice extent in summer is the lowest it has been in at least the 

past 1,450 years.147 Winter ice extent has declined less than 

summer ice extent (see Ch. 2: Our Changing Climate, Key Message 11), indicative of a trend toward seasonal-only (as opposed to year-round) ice cover, which is relatively thin and vulnerable to melt in the summer. Recent work has indicated that 

the loss of summer sea ice may be affecting the atmospheric 

circulation in autumn and early winter. For example, there are 

indications that a weakening of subpolar westerly winds during 

autumn is an atmospheric response to a warming of the lower 

troposphere of the Arctic.148 Extreme summer ice retreat also 

appears to be increasing the persistence of associated mid-latitude weather patterns, which may lead to an increased probFigure 36. Change in the number of consecutive dry days (days receiving less than 

0.04 inches (1 mm) of precipitation) at the end of this century (2070-2099) relative 

to the end of last century (1971-2000) under the higher scenario, RCP 8.5. Stippling 

indicates areas where changes are consistent among at least 80% of the 25 models 

used in this analysis. (Supplemental Message 5 and Ch. 2: Our Changing Climate, 

Key Message 3). (Figure source: NOAA NCDC / CICS-NC).

Change in Maximum Number of Consecutive Dry Days
774 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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ability of extreme weather events that result from prolonged 

conditions, such as drought, flooding, cold spells, and heat 

waves.149 However, the combination of interannual variability 

and the small sample of years with extreme ice retreat make 

it difficult to identify a geographically consistent atmospheric 

response pattern in the middle latitudes. 

On land, changes in permafrost provide compelling indicators 

of a warming climate, as they tend to reflect long-term average 

changes in climate. Borehole measurements are particularly 

useful, as they provide information from levels below about 

10-meter depth where the seasonal cycle becomes negligible. 

Increases in borehole temperatures over the past several 

decades are apparent at various locations, including Alaska, 

northern Canada, Greenland, and northern Russia. The increases are about 3.6°F at the two stations in northern Alaska 

(Deadhorse and West Dock). In northern Alaska and northern 

Siberia, where permafrost is cold and deep, thaw of the entire 

permafrost layer is not imminent. However, in the large areas 

of discontinuous permafrost of Russia, Alaska, and Canada, 

average annual temperatures are sufficiently close to freezing 

that permafrost thaw is a risk within this century. Thawing of 

permafrost can release methane into the atmosphere, amplifying warming (see Supplemental Message 5), as well as potentially causing infrastructure and environmental damages.

There is evidence that the active layer (the near-surface layer 

of seasonal thaw, typically up to three feet deep) may be thickening in many areas of permafrost, including in northern Russia 

and Canada.152 Permafrost thaw in coastal areas increases the 

vulnerability of coastlines to erosion by ocean waves, which in 

turn are exacerbated by the loss of sea ice from coastal areas 

affected by storms.

Increased melt is reducing both the mass and areal extent of 

glaciers over much of the Northern Hemisphere. Over the past 

decade, the contribution to sea level rise from glaciers and 

small ice caps (excluding Greenland) has been comparable to 

the contributions from the Greenland Ice Sheet.153

Projections of future mass loss by glaciers and small ice caps 

indicate a continuation of current trends, although these projections are based only on the changes in temperature and 

precipitation projected by global climate models; they do not 

include the effects of dynamical changes (for example, glacier 

movement). While there is a wide range among the projections 

derived from different global climate models, the models are 

consistent in indicating that the effects of melting will outweigh 

the effects of increases in snowfall. The regions from which the 

contributions to sea level rise are projected to be largest are 

the Canadian Arctic, Alaska, and the Russian Arctic.151

Figure 37. The spatial extent of Arctic sea ice cover in September has decreased 

substantially in the past two decades, as shown in this pair of satellite images depicting 

sea ice concentrations. The reduction of September sea ice extent from 1992 (left) to 

2012 (right) has been nearly 50%, or about 1.2 million square miles (3 million square 

kilometers), which is nearly one-third the area of the contiguous United States. (Figure 

source: University of Illinois, The Cryosphere Today;150 Data from the National Snow 

and Ice Data Center).

Arctic Sea Ice Decline
775 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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Figure 38. Ground temperatures at depths between 33 and 66 feet 

(10 and 20 meters) for boreholes across the circumpolar northern 

permafrost regions. Lower panel shows locations of measurement 

sites in colors corresponding to lines in upper panel (Figure source: 

AMAP 2011151).

Permafrost Temperatures Rising
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On the left is a photograph of Muir Glacier in Alaska taken on August 13, 1941; on the right, a photograph 

taken from the same vantage point on August 31, 2004. Total glacial mass has declined sharply around the 

globe, adding to sea level rise. (Left photo by glaciologist William O, Field; right photo by geologist Bruce F. 

Molnia of the United State Geological Survey.)

Figure 39. Inputs of freshwater to the ocean from mountain glaciers, small ice caps, and the Greenland Ice Sheet 

have increased dramatically in the past two decades. The size of the circles in the figure is proportional to the 

five-year average freshwater contributions to the ocean from melting of land-based ice. The coloring indicates the 

relative contributions from the Greenland Ice Sheet (brown) and mountain glaciers from the Greenland periphery 

(orange), Iceland-Scandinavia-Svalbard (dark blue), the Canadian Arctic (yellow), southern Alaska (light blue), and 

the Russian Arctic (medium blue). The largest contributions from mountain glaciers have been from the Canadian 

Arctic and southern Alaska. Note that contributions from mass changes of the Greenland Ice Sheet are not available 

prior to the mid-1990s, but they are assumed to have been small during this earlier period because annual snow 

accumulation was in approximate balance with annual meltwater discharge. (Figure source: AMAP 2011151).

Melting of Arctic Land-based Ice
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APPENDIX 3: CLIMATE SCIENCE

Supplemental Message 12. 

Sea level is already rising at the global scale and at individual locations along the U.S. coast. 

Future sea level rise depends on the amount of warming and ice melt around the world as 

well as local processes like changes in ocean currents and local land subsidence or uplift.

The rising global average sea level is one of the hallmarks of 

a warming planet. It will also be one of the major impacts of 

human-caused global warming on both human society and the 

natural environment.

Global sea level is increasing as a result of two different processes. First, the oceans absorb more than 90% of the excess 

heat trapped by human interference with the climate system, 

and this warms the oceans.155 Like mercury in a thermometer, 

the warmer ocean water expands, contributing to global sea 

level rise. Second, the warmer climate also causes melting of 

glaciers and ice sheets. This meltwater eventually runs off into 

the ocean and contributes to sea level rise as well. A recent 

synthesis of surface and satellite measurements of the ice 

sheets shows that the rate at which the Greenland and Antarctic ice sheets contribute to sea level rise has been increasing rapidly and has averaged 0.02 inches (plus or minus 0.008) 

per year since 1992, with Greenland’s contribution being more 

than double that of Antarctica.156 In addition, local sea level 

change can differ from the global average sea level rise due 

to changes in ocean currents, local land movement, and even 

changes in the gravitational pull of the ice sheets and changes 

in Earth’s rotation.

There is high confidence that global sea level will continue to 

rise over this century and beyond and that most coastlines 

will see higher water levels. The rates of sea level rise along 

individual coastlines are difficult to predict, as they can vary 

depending on the region. For example, globally averaged sea 

level has risen steadily by about 2.4 inches over the past two 

decades. But during that time, many regions have seen much 

more rapid rise while some have experienced falling sea levels. 

These complicated patterns are caused by changes in ocean 

currents and movement of heat within the oceans. Many of 

these patterns are due in part to natural, cyclic changes in the 

oceans. On the West Coast of the United States, sea level has 

fallen slightly since the early 1990s. Recent work suggests that 

a natural cycle known as the Pacific Decadal Oscillation has 

counteracted most or all of the global sea level signal there. 

This means that in coming decades the West Coast is likely 

to see faster than average sea level rise as this natural cycle 

changes phase.157

Along any given coastline, determining the rate of sea level rise 

is complicated by the fact that the land may be rising or sinking. Along the Gulf Coast, for example, local geological factors 

including extraction of oil, natural gas, and water from underFigure 40. Projections of contributions to sea level rise by 2100 for 

seven regions that include all Arctic glaciers. Projections are based on 

temperature and precipitation simulated by ten different global climate 

models from CMIP3. For each region, the estimates are shown in different 

colors corresponding to the ten different models. (Figure source: adapted 

from Radić and Hock 2011154).

Melting Glaciers Lead to Sea Level Rise
778 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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ground reservoirs are causing the land to sink, which could 

increase the effect of global sea level rise by several inches by 

the end of this century.158 In some other locations, coastlines 

are rising as they continue to rebound from glaciation during 

the last glacial maximum. Predicting the future of any single 

coastline requires intimate knowledge of the local geology as 

well as the processes that cause sea levels to change at both 

the local and global scale.

Greenland and Antarctica hold enough ice to raise global sea 

levels by more than 200 feet if they were to melt completely. 

While this is very unlikely over at least the next few centuries, 

studies suggest that meltwater from ice sheets could contribute anywhere from several inches to 4.5 feet to global sea levels by the end of this century.159 Because their behavior in a 

warming climate is still very difficult to predict, these two ice 

sheets are the biggest wildcards for potential sea level rise in 

the coming decades. What is certain is that these ice sheets 

are already responding to the warming of the oceans and the 

atmosphere. Satellites that measure small changes in the gravitational pull of these two regions have proven that both Greenland and Antarctica are currently losing ice and contributing to 

global sea level rise.160

In the United States, an estimated 5 million people currently 

live within 4 feet of current high tide lines, which places them 

at increasing risk of flooding in the coming decades.161 Although 

sea level rise is often thought of as causing a slow inundation, 

the most immediate impacts of sea level rise are increases 

in high tides and storm surges. A recent assessment of flood 

risks in the United States found that the odds of experiencing a 

“100-year flood” are on track to double by 2030.

Figure 41. The patterns of sea level rise between 1993 and 2012 as measured by satellites. 

The complicated patterns are a reminder that sea levels do not rise uniformly.

162 (Figure source: 

University of Colorado, Sea Level Research Group).

Sea Level Rise, 1993-2012
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Figure 42. Rate of local ice sheet mass loss (in inches of water-equivalent-height per year) from Greenland (left) 

and Antarctica (right) from 2003 to 2012. The GRACE (Gravity Recovery and Climate Experiment) satellites 

measure changes in the pull of gravity over these two regions. As they lose ice to the oceans, the gravitational 

pull of Greenland and Antarctica is reduced. Analyses of GRACE data have now proven that both of the major 

ice sheets are currently contributing to global sea level rise due to ice loss. Over the periods plotted here, 

Greenland lost enough ice to raise sea level at a rate of 0.028 inches per year (0.72 mm/yr), and Antarctica 

lost ice at a rate that caused 0.0091 inches of sea level rise per year (0.24 mm/yr). (Figure source: NASA Jet 

Propulsion Laboratory, (left) updated from Velicogna and Wahr 2013;163 (right) updated from Ivins et al. 2013164).

Ice Loss from Greenland and Antarctica
780 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

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References
INFORMATION DRAWN FROM THIS CHAPTER IS INCLUDED IN THE HIGHLIGHTS REPORT AND IS IDENTIFIED BY THIS ICON

Recommended Citation for Chapter

Walsh, J., D. Wuebbles, K. Hayhoe, J. Kossin, K. Kunkel, G. Stephens, P. Thorne, R. Vose, M. Wehner, J. Willis, D. Anderson, 

V. Kharin, T. Knutson, F. Landerer, T. Lenton, J. Kennedy, and R. Somerville, 2014: Appendix 4: Frequently Asked Questions. 

Climate Change Impacts in the United States: The Third National Climate Assessment, J. M. Melillo, Terese (T.C.) Richmond, 

and G. W. Yohe, Eds., U.S. Global Change Research Program, 790-820. doi:10.7930/J0G15XS3.

On the Web: http://nca2014.globalchange.gov/report/appendices/faqs

Convening Lead Authors

John Walsh, University of Alaska Fairbanks

Donald Wuebbles, University of Illinois

Lead Authors

Katharine Hayhoe, Texas Tech University

James Kossin, NOAA National Climatic Data Center 

Kenneth Kunkel, CICS-NC, North Carolina State Univ., NOAA National Climatic Data Center 

Graeme Stephens, NASA Jet Propulsion Laboratory

Peter Thorne, Nansen Environmental and Remote Sensing Center

Russell Vose, NOAA National Climatic Data Center

Michael Wehner, Lawrence Berkeley National Laboratory

Josh Willis, NASA Jet Propulsion Laboratory

Contributing Authors

David Anderson, NOAA National Climatic Data Center 

Viatcheslav Kharin, Canadian Centre for Climate Modelling and Analysis, Environment Canada

Thomas Knutson, NOAA Geophysical Fluid Dynamics Laboratory

Felix Landerer, NASA Jet Propulsion Laboratory

Tim Lenton, Exeter University

John Kennedy, UK Meteorological Office

Richard Somerville, Scripps Institution of Oceanography, Univ. of California, San Diego

790

Climate Change Impacts in the United States

APPENDIX 4

FREQUENTLY ASKED QUESTIONS
791 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

FREQUENTLY 

APPENDIX4 ASKED QUESTIONS

A. How can we predict what climate will be like in 100 years if we can’t even predict the weather next week?

B. Is the climate changing? How do we know?

C. Climate is always changing. How is recent change different than in the past?

D. Is the globally averaged surface temperature still increasing? Isn’t there recent evidence that it is actually 

cooling?

E. Is it getting warmer at the same rate everywhere? Will the warming continue?

F. How long have scientists been investigating human influences on climate?

G. How can the small proportion of carbon dioxide in the atmosphere have such a large effect on our climate?

H. Could the sun or other natural factors explain the observed warming of the past 50 years?

I. How do we know that human activities are the primary cause of recent climate change?

J. What is and is not debated among climate scientists about climate change?

K. Is the global surface temperature record good enough to determine whether climate is changing?

L. Is Antarctica gaining or losing ice? What about Greenland?

M. Weren’t there predictions of global cooling in the 1970s?

N. How is climate projected to change in the future?

O. Does climate change affect severe weather?

P. How are the oceans affected by climate change?

Q. What is ocean acidification?

R. How reliable are the computer models of the Earth’s climate?

S. What are the key uncertainties about climate change?

T. Are there tipping points in the climate system?

U. How is climate change affecting society?

V. Are there benefits to warming?

W. Are some people more vulnerable than others?

X. Are there ways to reduce climate change?

Y. Are there advantages to acting sooner rather than later?

Z. Can we reverse global warming?

This section answers some frequently asked questions about 

climate change. The questions addressed range from those 

purely related to the science of climate change to those that 

extend to some of the issues being faced in consideration of 

mitigation and adaptation measures. The author team selected these questions based on those often asked in presentations to the public. The answers are based on peer-reviewed 

science and assessments and have been confirmed by multiple 

analyses.
792 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

APPENDIX 4: FREQUENTLY ASKED QUESTIONS

A. How can we predict what climate will be like in 100 years 

if we can’t even predict the weather next week?

Predicting how climate will change in future decades is a different scientific issue from predicting weather a few weeks from 

now. Weather is short term and chaotic, largely determined by whatever atmospheric system is moving through at the time, 

and thus it is increasingly difficult to predict day-to-day changes beyond about two weeks into the future. Climate, on the 

other hand, is a long-term statistical average of weather and is determined by larger-scale forces, such as the level of heattrapping gases in the atmosphere and the energy coming from the sun. Thus it is actually easier to project how climate will 

change in the future. By analogy, while it is impossible to predict the age of death of any individual, the average age of death 

of an American can be calculated. In this case, weather is like the individual, while climate is like the average. To extend this 

analogy into the realm of climate change, we can also calculate the life expectancy of the average American who smokes. We 

can predict that on average, a smoker will not live as long as a non-smoker. Similarly, we can project what the climate will be 

like if we emit less heat-trapping gas, and what it will be like if we emit more.

Weather is the day-to-day variations in temperature, precipitation, and other aspects of the atmosphere around us. Weather 

prediction using state-of-the-art computer models can be very 

accurate for a few days to more than a week in advance. Because weather forecasts are based on the initial conditions of 

the atmosphere and ocean at the time the prediction is made, 

accuracy decays over time. After about two weeks, the effects 

of small errors in defining these initial conditions grow so large 

that meteorologists can no longer discern what the weather 

will be like on any specific day or place.

Climate is long-term average weather – the statistics of weather over long time scales, typically of 30 years or more. Climate 

is primarily the result of the effects of local geography, such as 

distance from the equator, distance from the ocean, and local 

topography and elevation, combined with larger scale climate 

factors that can change over time. These include the amount of 

energy from the sun and the composition of the atmosphere, 

including the amount of greenhouse gases and tiny particles 

suspended in the atmosphere. Knowing all these factors enables scientists to quantify the climate at a given place and 

time. Climate change occurs when these large-scale climate 

factors change over time. 

Using our understanding of the physics of how the atmosphere 

works, we can estimate how climate will change in the future 

– in response to human activities, which are now changing 

Earth’s atmospheric composition faster than at any time in 

at least the last 800,000 years. It is also possible to estimate 

changes in the statistics of certain types of weather events, 

such as heat waves or heavy precipitation events, especially 

when we know what is causing them to change.

We know how climate has changed in the recent past, and often we know why those changes have occurred. For example, 

the increase in global temperature, or global warming, that has 

occurred over the last 150 years can only be explained if we 

include the impact of increasing levels of heat-trapping gases 

in the atmosphere caused by human activities. The present 

generation of climate models can successfully reproduce the 

past warming and therefore provide an essential tool to peer 

into the future.

The role of human activities in driving recent change is discussed in FAQ I. (In the context of a changing climate, the term 

“human activities” is used throughout these frequently asked 

questions to refer specifically to activities, such as extracting 

and burning fossil fuels, deforestation, agriculture, waste treatment, and so on, that produce heat-trapping gases like carbon 

dioxide, methane, and nitrous oxide and/or emissions of black 

carbon, sulfate, and other particles.) Other human activities, 

like changes in land use, can also alter climate, especially on 

local or regional scales, such as that which occurs with urban 

heat islands.

Figure 1. Climate change refers to the changes in average 

weather conditions that persist for an extended period of 

time, over multiple decades or even longer. Year-to-year and 

even decade-to-decade conditions do not necessarily tell us 

much about long-term changes in climate. One cold year, or 

even a few cold years in a row, does not contradict a longterm warming trend, even as one hot year does not prove it. 

(Figure source: adapted from Kunkel et al. 20131). 

U.S. Annual Average Temperature
793 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

APPENDIX 4: FREQUENTLY ASKED QUESTIONS

B. Is the climate changing? How do we know?

Yes. The world has warmed over the last 150 years, and that warming has triggered many other changes to the Earth’s climate. 

Evidence for a changing climate abounds, from the top of the atmosphere to the depths of the oceans. Changes in surface, 

atmospheric, and oceanic temperatures; melting glaciers, snow cover, and sea ice; rising sea level; and increase in atmospheric 

water vapor have been documented by hundreds of studies conducted by thousands of scientists around the world. Rainfall 

patterns and storms are changing and the occurrence of droughts is shifting.

Documenting climate change often begins with global average 

temperatures recorded near Earth’s surface, where people 

live. But these temperatures, recorded by weather stations, 

are only one indicator of climate change. Additional evidence 

for a warming world comes from a wide range of consistent 

measurements of the Earth’s climate system. It is the sum total 

of these indicators that lead to the conclusion that warming of 

our planet is unequivocal.

Evidence for a changing climate is not confined to the Earth’s 

surface. Measurements by weather balloons and satellites consistently show that the temperature of the troposphere – the 

lowest layer of the atmosphere – has increased. The temperature of the upper atmosphere, particularly the stratosphere, 

has cooled, consistent with expectations of changes due to 

increasing concentrations of CO2 and other greenhouse gases. 

The upper ocean has warmed, and more than 90% of the additional energy absorbed by the climate system since the 1960s 

has been stored in the oceans. As the oceans warm, seawater 

expands, causing sea level to rise.

As the troposphere warms, Arctic ice and glaciers melt, also 

causing sea level to rise. About 90% of the glaciers and landbased ice sheets worldwide are melting as the Earth warms, 

adding further to the sea level rise. Spring snow cover has 

decreased across the Northern Hemisphere since the 1950s. 

There have been substantial losses in sea ice in the Arctic 

Ocean, particularly at the end of summer when sea ice extent 

is at a minimum (see FAQ L for discussion of Antarctic sea ice).

Warmer air, on average, contains more water vapor. Globally, 

the amount of water vapor in the atmosphere has increased 

over the land and the oceans over the last half century. In turn, 

many parts of the planet have seen increases in heavy rainfall 

events. All of these indicators and all of the independent data 

sets for each indicator unequivocally point to the same conclusion: from the ocean depths to the top of the troposphere, the 

world has warmed and the climate has reacted to that warming.

Figure 2. These are just some of the many indicators measured globally over many decades that demonstrate that the Earth’s 

climate is warming. White arrows indicate increases, and black arrows show decreases. All the indicators expected to increase 

in a warming world are increasing, and all those expected to decrease in a warming world are decreasing. See Figure 3 for 

measurements showing these trends. (Figure source: NOAA NCDC; based on data updated from Kennedy et al. 20102).

Ten Indicators of a Warming World
794 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

APPENDIX 4: FREQUENTLY ASKED QUESTIONS

In summary, the evidence that climate is changing comes from 

a multitude of independent observations. The evidence that 

climate is changing because of human activity, as discussed in 

FAQ I and in more detail in Chapter 2: Our Changing Climate 

and Appendix 3: Climate Science Supplement, comes from observations, basic physics, and analyses from modeling studies.

Figure 3. This figure summarizes some of the many datasets documenting changes in the Earth’s climate, all of which are 

consistent with a warming planet. In all figures except the lower two in the right column, data are plotted relative to averages 

over the period 1960-1999 (Figure source: updated from Kennedy et al. 20102).

Indicators of Warming from Multiple Data Sets
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C. Climate is always changing. How is recent change different than in the past? 

The Earth has experienced many large climate changes in the past. However, current changes in climate are unusual for two 

reasons: first, many lines of evidence demonstrate that these changes are primarily the result of human activities (see Question I for more info); and second, these changes are occurring (and are projected to continue to occur) faster than many past 

changes in the Earth’s climate.

In the past, climate change was driven exclusively by natural 

factors: explosive volcanic eruptions that injected reflective 

particles into the upper atmosphere, changes in energy from 

the sun, periodic variations in the Earth’s orbit, natural cycles 

that transfer heat between the ocean and the atmosphere, and 

slowly changing natural variations in heat-trapping gases in the 

atmosphere. All of these natural factors, and their interactions 

with each other, have altered global average temperature over 

periods ranging from months to thousands of years. For example, past glacial periods were initiated by shifts in the Earth’s 

orbit, and then amplified by resulting decreases in atmospheric 

levels of carbon dioxide and subsequently by greater reflection of solar radiation by ice and snow as the Earth’s climate 

system responded to a cooler climate. Some periods in the 

distant past were even warmer than what is expected to occur 

from human-induced global warming. But these changes in the 

distant past generally occurred much more slowly than current 

changes.

Natural factors are still affecting the planet’s climate today. 

The difference is that, since the beginning of the Industrial 

Revolution, humans have been increasingly affecting global 

climate, to the point where we are now the primary cause of 

recent and projected future change.

Records from ice cores, tree rings, 

soil boreholes, and other forms of 

“natural thermometers,” or “proxy” 

climate data, show that recent climate change is unusually rapid compared to past changes. After a glacial 

maximum, the Earth typically warms 

by about 7°F to 13°F over thousands of years (with periods of rapid 

warming alternating with periods of 

slower warming, and even cooling, 

during that time). The observed rate 

of warming over the last 50 years 

is about eight times faster than the 

average rate of warming from a glacial maximum to a warm interglacial 

period.

Global temperatures over the last 

100 years are unusually high when 

compared to temperatures over 

the last several thousand years. Atmospheric carbon dioxide levels are 

currently higher than any time in at 

Figure 4. Global carbon emissions from burning coal, oil, and 

gas and from producing cement (1850-2009). These emissions 

account for about 80% of the total emissions of carbon from 

human activities, with land-use changes (like cutting down 

forests) accounting for the other 20% in recent decades. (Data 

from Boden et al. 20123).

Carbon Emissions in the Industrial Age

Figure 5. Changes in the temperature of the Northern Hemisphere from surface observations 

(in red) and from proxies (in black; uncertainty range represented by shading) relative to 

1961-1990 average temperature. These analyses suggest that current temperatures are 

higher than seen globally in at least the last 1700 years and that the last decade (2001 to 

2010) was the warmest decade on record. (Figure source: adapted from Mann et al. 20084).

1700 Years of Temperature Change from Proxy Data
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least the last 800,000 years. Paleoclimate studies indicate that 

temperature and atmospheric carbon dioxide levels have been 

higher in the distant past, millions of years ago, when the world 

was very different than it is today. But never before have such 

rapid, global-scale changes occurred during the history of human civilization.

Our societies have not been built to withstand the changes 

that are anticipated in the relatively near future, and thus are 

not prepared for the effects they are already experiencing: 

higher temperatures, sea level rise, and other climate change 

related impacts.

D. Is the globally averaged surface air temperature still increasing? 

Isn’t there recent evidence that it is actually cooling?

Global temperatures are still rising. Climate change is defined as a change in the average conditions over periods of 30 years or 

more (see FAQ A). On these time scales, global temperature continues to increase. Over shorter time scales, natural variability 

(due to the effects of El Niño and La Niña events in the Pacific Ocean, for example, or volcanic eruptions or changes in energy 

from the sun) can reduce the rate of warming or even create a temporary reduction in average surface air temperature. These 

short-term variations in no way negate the reality of long-term warming. The most recent decade was the warmest since 

instrumental record keeping began around 1880.

From 1970 to 2010, for example, global temperature trends 

taken at five-year intervals show both decreases and sharp 

increases. The five-year period from 2005 to 2010, for example, included a period in which the sun’s output was at 

a low point, oceans took up more than average amounts 

of heat, and a series of small volcanoes exerted a cooling 

influence by adding small particles to the atmosphere. 

These natural factors are thought to have contributed to a 

recent slowdown in the rate of increase in average surface 

air temperature caused by the buildup of human-induced 

greenhouse gases. But while there has been a slowdown in the 

rate of increase, temperatures are still increasing.

In addition, satellite and ocean observations indicate that 

most of the increased energy in the Earth’s climate system 

from the increasing levels of heat-trapping gases has gone 

into the oceans. These observations indicate that the Earthatmosphere climate system has continued to gain heat energy.

In the United States, there has been considerable decade-todecade variability superimposed on the long-term warming 

trend. In most seasons and regions, the 1930s were relatively 

warm and the 1960s/1970s relatively cool. The most recent 

decade of the 2000s was the warmest on record throughout 

the United States and globally.

Figure 6. Short-term trends in global temperature (blue lines 

show temperature trends at five-year intervals from 1970 to 

2010) can range from decreases to sharp increases. The 

evidence of climate change is based on long-term trends over 

20-30 years or more (red line). (Data from NOAA NCDC).

Short-term Variations Versus Long-term Trend

Figure 7. The last five decades have seen a progressive rise in 

Earth’s average surface temperature. Bars show the difference 

between each decade’s average temperature and the overall 

average for 1901 to 2000. The far right bar includes data for 

2001-2012. (Figure source: NOAA NCDC).

Global Temperature Change: Decade Averages
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E. Is it getting warmer at the same rate everywhere? Will the warming continue?

Temperatures are not increasing at the same rate everywhere, because temperature changes in a given location depend on 

many factors. However, average global temperatures are projected to continue increasing throughout the remainder of this 

century due to heat-trapping gas emissions from human activities.

The planet is warming overall (see FAQ I), but some locations 

could be cooling due to local factors. Temperature changes in 

a given location are a function of multiple factors, including 

global and local forces, and both human and natural influences. In some places, including the U.S. Southeast, temperatures 

actually declined over the last century as a whole (although 

they have risen in recent decades). Possible causes of the observed lack of warming in the Southeast during the 20th century include increased cloud cover and precipitation,5

 increases 

in the presence of fine particles called aerosols in the atmosphere (including those produced by burning fossil fuels and by 

natural sources), expanding forests in the Southeast over this 

period,6 decreases in the amount of heat conducted from land 

to the atmosphere as a result of increases in irrigation,7 and 

multi-decadal variability in sea surface temperatures in both 

the North Atlantic8 and the tropical Pacific9 Oceans. At smaller 

geographic scales, and during certain time intervals, the relative influence of natural variations in climate compared to the 

human contribution is larger than at the global scale. An observed decrease in temperature at an individual location does 

not negate the fact that, overall, the planet is warming.

In terms of impacts, “global warming” is probably not the most 

immediate thing most people would notice. A changing climate 

affects our lives in many more obvious ways, for example, by increasing the risk of severe weather events such as heat waves, 

heavy precipitation events, strong hurricanes, and many other 

aspects of climate discussed throughout this report.

For these reasons, many scientists prefer the term “climate 

change,” which connotes a much larger picture: broad changes 

in what are considered “normal” conditions. This term encompasses both increases and decreases in temperature, as well as 

shifts in precipitation, changing risk of certain types of severe 

weather events, and other features of the climate system. 

At the global scale, some future years will be cooler than the 

preceding year; some decades could even be cooler than the 

preceding decade (though that has not happened for more than 

six decades; see Figure 7). Brief periods of faster temperature 

increases and also temporary decreases in global temperature 

can be expected to continue into the future. Nonetheless, each 

successive decade in the last 30 years has been the warmest 

in the period of reliable instrumental records (going back to 

1850). Based on this historical record and plausible scenarios 

for future increases in heat-trapping gases, we expect that 

future global temperatures, averaged over climate timescales 

of 30 years or more, will be higher than preceding periods as 

a result of carbon dioxide and other heat-trapping gas emisFigure 8. Observed trend in temperature from 1900 to 2012; yellow 

to red indicates warming, while shades of blue indicate cooling. Gray 

indicates areas for which there are no data. There are substantial 

regional variations in trends across the planet, though the overall trend 

is warming. (Figure source: NOAA NCDC).

Temperature Trends, 1900-2012

Figure 9. Change in decadal-averaged annual 

temperature relative to the 1901-1960 average for 

the six National Climate Assessment regions in the 

contiguous United States. This figure shows how 

regional temperatures can be much more variable than 

global temperatures, going up and down from decade 

to decade; all regions, however, show warming over the 

last two decades or more. In the figure, 00s refers to 

the 12-year period of 2001-2012. (Figure source: NOAA 

NCDC / CICS-NC). 

 Decade-Scale Changes in Average 

Temperature for U.S. Regions
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sions from human activities. A portion of the carbon dioxide 

emissions from human activities will remain in the atmosphere 

for hundreds of years and continue to affect the global carbon cycle for thousands of years. Year-to-year projections of 

regional and local temperatures are more variable than global 

temperatures, and even at a particular location, future warming becomes increasingly likely over longer periods of time.1

F. How long have scientists been investigating human influences on climate?

The scientific basis for understanding how heat-trapping gases affect the Earth’s climate dates back to the French scientist 

Joseph Fourier, who established the existence of the natural greenhouse effect in 1824. The heat-trapping abilities of greenhouse gases were corroborated by Irish scientist John Tyndall with experiments beginning in 1859. Since then, scientists have 

developed more tools to refine their understanding of human influences on climate, from the invention of the thermometer, to 

the development of computerized climate models, to the launching of Earth observing satellites that, together, provide global 

data coverage.

The greenhouse effect is caused by heat-trapping gases, such 

as water vapor, carbon dioxide, and methane, in the Earth’s 

atmosphere. These gases are virtually transparent to the visible and ultraviolet wavelengths that comprise most of the 

sun’s energy, allowing nearly all of it to reach Earth’s surface. 

However, they are relatively opaque to the heat energy the 

Earth radiates back outward at infrared wavelengths. Other 

more abundant gases in the atmosphere like nitrogen and 

oxygen are largely transparent to the Earth’s infrared energy. 

Greenhouse gases trap some of the Earth’s energy inside the 

atmosphere and prevent it from escaping to space by absorbing and re-emitting that energy in all directions, rather than 

just upwards. Some of the trapped energy is re-radiated back 

down to the Earth’s surface. This natural trapping effect makes 

the average temperature of the Earth nearly 60°F warmer 

than what it would be otherwise. On other planets, like Venus, 

where there are much higher concentrations of heat-trapping 

gases in the atmosphere, the greenhouse effect has a much 

stronger influence on surface temperature, making conditions 

far too hot for life as we know it.

By the late 1800s, scientists were aware that burning coal, oil, 

or natural gas produced carbon dioxide, a key heat-trapping 

gas. They were also aware that methane, another heat-trapping gas, was released during coal mining and other human 

activities. And they knew that, since the Industrial Revolution, 

humans were producing increasing amounts of these gases. It 

was clear that humans were increasing the natural greenhouse 

effect and that this would warm the planet.

In 1890, Svante Arrhenius, a Swedish chemist, calculated the 

effect of increasing fossil fuel use on global temperature. This 

climate model, computed by hand, took two years to complete. 

Arrhenius’ results were remarkably similar to those produced 

by the most up-to-date global climate models today, although 

he did not anticipate that atmospheric levels of carbon dioxide 

would increase as quickly as they have.

In 1938, a British engineer, Guy Callendar, connected rising 

carbon dioxide levels to the observed increase in the Earth’s 

temperature that had occurred to date. In 1958, Charles David Keeling began to precisely measure atmospheric levels of 

carbon dioxide in the relatively unpolluted location of Mauna 

Loa on Hawai‘i. Today, those data provide a clear record of the 

effect of human activities on the chemical composition of the 

global atmosphere. Many more sources of data corroborate 

the work of these early pioneers in the field of climate science.

Figure 10. Scientists whose research was key to understanding the greenhouse effect and 

the impact of human activities on climate. 

 Early Scientists who Established the Scientific

 Basis for Climate Change
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G. How can the small proportion of carbon dioxide in the atmosphere

 have such a large effect on our climate? 

The reason heat-trapping gases like carbon dioxide, methane, and nitrous oxide have such a powerful influence on Earth’s 

climate is their potency: although they are transparent to visible and ultraviolet solar energy, allowing the sun’s energy to 

come in, they are very strong absorbers of the Earth’s infrared heat energy, blanketing the Earth and preventing some of the 

energy to escape to space.

Before the Industrial Revolution, natural levels of carbon dioxide in the atmosphere averaged around 280 parts per million (ppm), that is, 280 molecules of CO2

 per million molecules 

of air (which is mostly nitrogen and oxygen). In other words, 

carbon dioxide made up about 0.028% of the volume of the 

atmosphere. Methane and nitrous oxide, other heat-trapping 

gases, made up even less, about 700 parts per billion (ppb) and 

270 ppb, respectively. Over the last few centuries, emissions 

from human activities have increased carbon dioxide levels to 

about 400 ppm, or more than 3,000 billion tons – more than 

a 40% increase. Over the same time period, methane and nitrous oxide levels in the atmosphere have risen to around 1800 

ppb and 320 ppb, respectively.

As the concentrations in the atmosphere of these heat-trapping gases increase due to human activities, they are absorbing 

greater and greater amounts of infrared heat energy emitted 

from the Earth’s surface. As discussed in FAQ F, the gases then 

re-radiate some of this heat back to the surface, effectively 

trapping the heat inside the Earth’s climate system and warming the Earth’s surface.

These heat-trapping gases do not absorb energy equally across 

the infrared spectrum. Carbon dioxide absorption is very 

strong at certain wavelengths of infrared radiation, whereas 

water vapor absorbs more broadly across most of the spectrum. Water vapor is the most important naturally occurring 

heat-trapping greenhouse gas, but small increases in heat 

energy absorption by carbon dioxide and other heat-trapping 

gases trigger increases in water vapor that amplify the infrared 

trapping, leading to further warming. As a result, water vapor 

is considered a “feedback” rather than a direct forcing on climate.

Figure 11. (left) A stylized representation of the natural greenhouse effect. Most of the sun’s radiation reaches the Earth’s surface. 

Naturally occurring heat-trapping gases, including water vapor, carbon dioxide, methane, and nitrous oxide, do not absorb the shortwave energy from the sun but do absorb the long-wave energy re-radiated from the Earth, keeping the planet much warmer than it 

would be otherwise. (right) In this stylized representation of the human-intensified greenhouse effect, human activities, predominantly 

the burning of fossil fuels (coal, oil, and gas), are increasing levels of carbon dioxide and other heat-trapping gases, increasing the 

natural greenhouse effect and thus Earth’s temperature. (Figure source: modified from National Park Service10).

Human Influence on the Greenhouse Effect
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H. Could the sun or other natural factors explain the 

observed warming of the past 50 years? 

No. Since accurate satellite-based measurements of solar output began in 1978, the amount of the sun’s energy reaching 

Earth has slightly decreased, which should, on its own, result in slightly lower temperatures; but the Earth’s temperature has 

continued to rise. The sun can explain less than 10% of the increase in temperature since 1750, and none of the increase in 

temperature since 1960.

Patterns of vertical temperature change (from the Earth’s surface to the upper atmosphere) provide further evidence that 

the sun cannot be responsible for the observed changes in climate. An increase in solar output would warm the atmosphere 

consistently from top to bottom. Warming from increasing 

heat-trapping gases, on the other hand, should be concentrated in the lower atmosphere (troposphere), while the upper atmosphere (stratosphere) would cool. Satellite measurements 

and weather balloon records reveal that the troposphere has 

warmed, and the stratosphere has cooled. This observed pattern of vertical temperature change matches what we would 

expect from the increase in heat-trapping gases, not an increase in solar output.

Changes in the sun’s magnetic field are known to affect the 

intensity of cosmic rays reaching Earth’s atmosphere and there 

is some suggestion that this could affect cloud formation; 

however, observations indicate that the magnitude of this effect is much smaller than the effects from the human-related 

changes in heat-trapping gases and from particle emissions on 

clouds and the changes in climate.

Large explosive volcanic eruptions can cool climate for a few 

years after an eruption, if the eruption is powerful enough to 

send particles far up into the atmosphere. In the atmosphere, 

sulfur dioxide from volcanoes is converted into sulfuric acid 

particles that can scatter sunlight, cooling the Earth’s surface. 

Particles from exceptionally large eruptions like Mount Pinatubo in 1991 or Krakatoa in 1883 can reach all the way into the 

stratosphere, where they can stay for several years. Eventually, they fall back into the troposphere where they are rapidly 

removed by precipitation. Volcanoes also emit carbon dioxide, 

but this amount is less than 1% annually of the emissions occurring from human activities.

Thus, natural factors cannot explain recent warming. In fact, 

observed solar and volcanic activity would have tended to 

slightly cool the Earth, and other natural variations are too 

small to account for the amount of warming over the last 50 

years.

Figure 12. Changes in the global surface temperature (top) and the solar flux (bottom) since 

1900 (temperatures are relative to 1961-1990). The temperatures are based on thermometer 

observations of the Earth’s surface temperature, while the solar flux at the top of Earth’s 

atmosphere is based on satellite observations starting in 1978 and on proxy observations 

before then. (Figure source: NOAA NCDC / CICS-NC).

Measurements of Surface Temperature and Sun’s Energy
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I. How do we know that human activities are the 

primary cause of recent climate change? 

Many lines of evidence demonstrate that human activities are primarily responsible for recent climate changes. First, basic 

physics dictates that increasing the concentration of CO2 and other heat-trapping gases in the atmosphere will cause the 

climate to warm. Second, modeling studies show that when human influences are removed from the equation, climate would 

actually have cooled slightly over the past half century. And third, the pattern of warming through the layers of atmosphere 

demonstrates that human-induced heat-trapping gases are responsible, rather than some natural change.

Scientists are continually designing experiments to test whether observed climate changes are unusual and then to determine their causes. This field of study is known as “detection 

and attribution.” Detection involves looking for evidence of 

changes or trends. Attribution attempts to identify the causes 

of these changes from a line-up of “suspects” that include 

changes in energy from the sun, powerful volcanic eruptions 

– and today, human-induced emissions of heat-trapping gases.

Detection and attribution analyses have confirmed that recent 

changes cannot have been caused either by internal climate 

system variations or by solar and volcanic influences (see FAQs 

C and H). Human influences on the climate system – including heat-trapping gas emissions, atmospheric particulates, and 

land-use and land-cover change – are required to explain recent changes (see Figure 14).

Detection and attribution has been used to analyze the contribution of human influences to changes in global average 

conditions, in extreme events, and even in the change in risk of 

specific types of events, such as the 2003 European heat wave. 

Such analyses have found that it is virtually certain that observed changes in many aspects of the climate system are the 

result of influences of human activities. Scientific analyses also 

provide extensive evidence that the likelihood of some types 

of extreme events (such as heavy rains and heat waves) is now 

significantly higher due to human-induced climate change.

Figure 13. Figure shows examples of the many aspects of the climate system in which changes have 

been formally attributed to human emissions of heat-trapping gases and particles by studies published 

in peer-reviewed science literature. For example, observed changes in surface air temperature at 

both the global and continental levels, particularly over the past 50 years or so, cannot be explained 

without including the effects of human activities. While there are undoubtedly many natural factors 

that have affected climate in the past and continue to do so today, human activities are the dominant 

contributor to recently observed climate changes. (Figure source: NOAA NCDC).

Human Influences Apparent in Many Aspects of the Changing Climate
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Figure 14. Changes in surface air temperature at the continental and global scales can only be explained by the influence of human 

activities on climate. The black line depicts the annually averaged observed changes. The blue shading represents estimates from 

a broad range of climate simulations including solely natural (solar and volcanic) changes in forcing. The orange shading is from 

climate model simulations that include the effects of both natural and human contributions. These analyses demonstrate that the 

observed changes, both globally and on a continent-by-continent basis, are caused by the influence of human activities on climate.

(Figure source: updated from Jones et al. 201311). 

Only Human Influence Can Explain Recent Warming
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J. What is and is not debated among climate scientists about climate change? 

Multiple analyses of the peer-reviewed science literature have repeatedly shown that more than 97% of scientists in this field 

agree that the world is unequivocally warming and that human activity is the primary cause of the warming experienced over 

the past 50 years. Spirited debates on some details of climate science continue, but these fundamental conclusions are not in 

dispute.

The scientific method is built on scrutiny and debate among 

scientists. Scientists are rigorously trained to conduct experiments to test a question, or hypothesis, and submit their 

findings to the scrutiny of other experts in their field. Part of 

that scrutiny, known as “peer review,” includes independent 

scientists examining the data, analysis methods, and findings 

of a study that has been submitted for publication. This peer 

review process provides quality assurance for scientific results, 

ensuring that anything published in a scientific journal has been 

reviewed and approved by other independent experts in the 

field and that the authors of the original study have adequately 

responded to any criticisms or questions they received.

However, peer review is only the first step in the long process 

of acceptance of new ideas. After publication, other scientists 

will often undertake new studies that may support or reject 

the findings of the original study. Only after an exhaustive 

series of studies over many years, by many different research 

groups, are new ideas widely accepted.

Given that new scientific understanding emerges from this 

exhaustive process, the widespread 

agreement in the scientific community regarding the reality of climate 

change and the leading role of human activities in driving this change 

is striking. This consensus includes 

agreement on the fundamental scientific principles that underlie this 

phenomenon, as well as the weight 

of empirical evidence that has been 

accumulated over decades, and 

even centuries, of research (see 

FAQ F).

The conclusion that the world is 

warming, and that this is primarily due to human activity, is based 

on multiple lines of evidence, from 

basic physics to the patterns of 

change through the climate system 

(including the atmosphere, oceans, 

land, biosphere, and cryosphere). 

The warming of global climate 

and its causes are not matters of 

opinion; they are matters of scientific evidence, and that evidence 

is clear. Scientists do not “believe” in human-induced climate 

change; rather, the widespread agreement among scientists is 

based on the vast array of evidence that has accumulated over 

the last 200 years. When all of the evidence is considered, the 

conclusions are clear.

There is more work to be done to fully understand the many 

complex and interacting aspects of climate change, and important questions remain. Scientific debate continues on questions such as: Exactly how sensitive is the Earth’s climate to human emissions of heat-trapping gases? How will climate change 

affect clouds? How will climate change affect snowstorms in 

Chicago, tornadoes in Oklahoma, and droughts in California? 

How do particle and soot emissions affect clouds? How will climate change be affected by changes in clouds and the oceans? 

These detailed questions, and more, serve as healthy indicators that the scientific method is alive and well in the field of 

climate science. But the fact that climate is changing, that this 

is primarily in response to human activities, and that climate 

will continue to change in response to these activities, is not in 

dispute (see FAQ I).

Figure 15. The green band shows how global average temperature would have changed 

due to natural forces only, as simulated by climate models. The blue band shows model 

simulations of the effects of human and natural factors combined. The black line shows 

observed global average temperatures. As indicated by the green band, without human 

influences, temperature over the past century would actually have cooled slightly over 

recent decades. The match up of the blue band and the black line illustrate that only the 

inclusion of human factors can explain the recent warming. (Figure source: adapted from 

Huber and Knutti, 201212). 

Separating Human and Natural Influences on Climate
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K. Is the global surface temperature record good enough to determine 

whether climate is changing?

Yes. There have been a number of studies that have examined the U.S. and global temperature records in great detail. These 

have used a variety of methods to study the effects of changes in instruments, time of observations, station siting, and other 

potential sources of error. All studies reinforce high confidence in the reality of the observed upward trends in temperature.

Global surface temperatures are measured by weather stations over land and by ships and buoys over the ocean. These 

records extend back regionally for over 300 years in some locations and near-globally to the late 1800s.

Scientists have undertaken painstaking efforts to obtain, digitize, and collate these records. Because of the way these measurements have been taken, many of the records 

contain results that are skewed by, for example, 

a change of instrument or a station move. It is essential to carefully examine the data to identify and 

adjust for such effects before the data can be used 

to evaluate climate trends.

A number of different research teams have taken 

up this challenge. Some have spent decades carefully analyzing the data and continually reassessing 

their approaches and refining their records. These 

independently produced estimates are in very good 

agreement at both global and regional scales.

Scientists have also considered other influences 

that could contaminate temperature records. For 

example, many thermometers are located in urban 

areas that could have warmed over time due to the 

urban heat island effect (in which heat absorbed 

by buildings and asphalt makes cities warmer than 

the surrounding countryside). At least three different research teams have examined how this might 

affect U.S. temperature trends. All have found that 

this effect is adequately accounted for by the data corrections. 

At the global scale, if all of the urban stations are removed 

from the global temperature record, the evidence of warming over the past 50 years remains intact. Other studies have 

shown that the temperature trends of rural and urban areas in 

close proximity essentially match, even though the urban areas 

may have higher temperatures overall.

L. Is Antarctica gaining or losing ice? What about Greenland?

The ice sheets on both Greenland and Antarctica, the largest areas of land-based ice on the planet, are losing ice as the atmosphere and oceans warm. This ice loss is important both as evidence that the planet is warming, and because it contributes to 

rising sea levels.

One way that scientists are evaluating ice loss is by observing 

changes in the gravitational fields over Greenland and Antarctica. Fluctuations in the pull of gravity over these major ice 

sheets reflect the loss of ice over time. Over the last decade, 

the GRACE (Gravity Recovery and Climate Experiment) satellites have measured changes in the gravitational pull of the 

continents and revealed that, on the whole, both Greenland 

and Antarctica are losing ice. It is clear that these ice sheets 

are already losing mass as a result of human-induced climate 

change, and the evidence suggests that Greenland and Antarctica are likely to continue to lose ice mass for centuries. How 

rapidly the Greenland and Antarctic Ice Sheets will melt as 

warming continues represents the largest uncertainty in projections of future sea level rise.

Paleoclimate records show that the giant ice sheets of Greenland and Antarctica (as well as others, such as the Laurentide 

Ice Sheet that covered much of North America during the last 

glacial maximum) have expanded and contracted as the Earth 

cooled or warmed in the past. As temperature increases and 

precipitation patterns shift in response to human-induced climate change, scientists expect the ice sheets of Greenland and 

Figure 16. Three different global surface temperature records all show 

increasing trends over the last century. The lines show annual differences 

in temperature relative to the 1901-1960 average. Differences among data 

sets, due to choices in data selection, analysis, and averaging techniques, 

do not affect the conclusion that global surface temperatures are increasing. 

(Figure source: NOAA NCDC / CICS-NC). 

Observed Change in Global Average Temperature
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Antarctica to continue responding in a similar way. Over time 

horizons of hundreds to thousands of years, a general melting and reduction in the extent of both of these ice sheets is 

expected to occur in response to global warming. Over shorter 

time frames of years to decades, however, the response of 

these ice sheets is more complicated.

The Antarctic Ice Sheet is up to three miles deep and contains 

enough water to raise sea level about 200 feet. Because Antarctica is so cold, there is little melt of the ice sheet in the 

summer. However, the ice on the continent slowly flows down 

the mountains and through the valleys toward the ocean. 

Some parts of the ice sheet extend out into the ocean as “ice 

shelves.” Here, above-freezing ocean water speeds up the process called “calving” that breaks the ice into free floating icebergs. Melting and calving and the flow of ice into the oceans 

around Antarctica has accelerated in recent decades and is 

now contributing about 0.005 to 0.010 inches per year to sea 

level rise. It is possible that the West Antarctic Ice Sheet, which 

contains enough ice to raise global sea levels by 10 feet, could 

begin to lose ice much more quickly if ice shelves in the region 

begin to disintegrate at the edges.

Greenland contains only about one tenth as much ice as the 

Antarctic Ice Sheet, but if Greenland’s ice were to entirely 

melt, global sea level would rise 23 feet. Greenland is warmer 

than Antarctica, so unlike Antarctica, melting occurs over large 

parts of the surface of Greenland’s ice sheet each summer. 

Greenland’s melt area has increased over the past several decades. Satellite measurements indicate that the Greenland Ice 

Sheet is presently thinning at the edges (especially in the south) 

and slowly thickening in the interior, increasing the steepness 

of the ice sheet, which causes the ice to flow toward the ocean. 

Several of the major outlet glaciers that drain the Greenland 

Ice Sheet have sped up in the past decade. Recent scientific 

studies suggest that warming of the ocean at the edges of the 

outlet glaciers may contribute to this speed-up. Greenland’s 

ice loss has increased substantially in the past decade or two, 

and is now contributing 0.01 to 0.02 inches per year to sea level 

rise (about twice the rate of Antarctica’s mass loss). This increased rate of ice loss means that Greenland’s contribution 

to global sea level rise is now similar to the effect from smaller 

glaciers worldwide and from Antarctica.

M. Weren’t there predictions of global cooling in the 1970s?

No. An enduring myth about climate science is that in the 1970s the climate science community supposedly predicted “global 

cooling” and an “imminent” ice age. A review of the scientific literature shows that this was not the case. On the contrary, 

even then, discussions of human-related warming dominated scientific publications on climate and human influences.

Where did all the discussion about global cooling come from? 

First, temperature records from about 1940 to 1970 showed a 

slight global cooling trend, intensified by temporary increases 

in snow and ice cover across the Northern Hemisphere. Shortterm natural variations in the Earth’s climate (see FAQ A) and 

increasing emissions of sulfur and other particles from coalburning power plants, which reflect solar energy and have a 

net cooling effect on the Earth, likely contributed to cooler 

temperatures during that time period. Several unusually severe winters in Asia and parts of North America in the 1970s 

raised people’s concerns about cold weather. The popular 

press, including Time, Newsweek, and The New York Times, carried a number of articles about cooling at that time.

Second, climate scientists study both natural and humaninduced changes in climate. Over the last century, scientists 

have learned a great deal about what drives Earth’s ice ages. 

Scientific understanding of what are called the Milankovitch 

Figure 17. GRACE (Gravity Recovery and Climate Experiment) satellite measurements show that both Greenland and Antarctica 

are, on the whole, losing ice as the atmosphere and oceans warm. (Figure source: adapted from Wouters et al. 201313).

Ice Loss from the Two Polar Ice Sheets
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cycles (cyclical changes in the 

Earth’s orbit that can explain 

the onset and ending of ice 

ages) led a few scientists in 

the 1970s to suggest that the 

current warm interglacial period might be ending soon, 

plunging the Earth into a new 

ice age over the next few centuries. Scientists continue to 

study this issue today; the latest information suggests that, 

if the Earth’s climate were being controlled primarily by natural factors, the next cooling 

cycle would begin sometime 

in the next 1,500 years. However, humans have so altered 

the composition of the atmosphere that the next glaciation 

has now been delayed.

N. How is climate projected to change in the future? 

Climate is projected to continue to warm, with the amount of future warming ranging from another 3°F to another 12°F by 

2100, depending primarily on the level of emissions from human activities, principally the burning of fossil fuels. For precipitation, wet areas are generally projected to get wetter while dry areas get drier. More precipitation is expected to fall in heavy 

downpours. Natural variability will still play a role in year-to-year changes. 

Future climate cannot be “predicted” because human activities are currently the most important driver of climate change 

and we cannot predict what society will choose to do with regard to emissions. Rather, we can project the climate change 

that would result from a given set of assumptions, or future 

scenarios, regarding human activities (including changes in 

population, technology, economics, energy, and policy). Future 

changes also have some uncertainty due to natural variability, 

particularly over shorter time scales (see FAQ A) and limitations in scientific understanding of exactly how the climate 

system will respond to human activities (see FAQ S).

The relative importance of these three sources of uncertainty 

changes over time. Which type of uncertainty is most important also depends on what type of change is being projected: 

whether, for example, it is for average conditions or extremes, 

or for temperature or precipitation trends (see FAQ S).

Over the next few decades, global average temperature over 

30-year climate timescales is expected to continue to increase 

(see FAQ D), while natural variability still plays a significant role 

in year-to-year changes (see FAQ A). The amount of climate 

change expected over this time period is unlikely to be significantly altered by reducing current heat-trapping gas emissions 

alone or even by stabilizing atmospheric levels of carbon dioxide and other gases. This is because near-term warming will 

be caused primarily by emissions that have already occurred, 

due to the lag in the temperature response to changes in atmospheric composition. This lag is primarily the result of the 

very large heat storage capacity of the world’s oceans and the 

length of time required for that heat to be transferred to the 

deep ocean. At smaller geographical scales, temperatures are 

projected to increase in most regions in the next few decades, 

but a few regions could experience flat or even decreasing 

temperatures. Any climate change always represents the net 

effect of multiple global and local factors, both human-related 

and natural (see FAQ E).

Beyond the middle of this century, global and regional temperature changes will be determined primarily by the rate and 

amount of various emissions released by human activities, as 

well as by the response of the Earth’s climate system to those 

Figure 18. The number of papers classified as predicting, implying, or providing supporting 

evidence for future global cooling, warming, and neutral categories. Bars indicate number of 

articles published per year. Squares indicate cumulative number of articles published. For 

the period 1965 through 1979, the literature survey found seven papers suggesting further 

cooling, 20 neutral, and 44 warming. Even in the early years of the study of climate change, 

more science studies were discussing concerns about global warming than global cooling. 

(Figure source: Peterson et al. 200814).

Published Climate Change Research Papers
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APPENDIX 4: FREQUENTLY ASKED QUESTIONS

emissions. Efforts to rapidly and significantly 

reduce emissions of heat-trapping gases can 

still limit the global temperature increase to 

3.6°F (2°C) relative to the 1901-1960 time period. However, significantly greater temperature increases are expected if emissions follow 

higher scenarios associated with continuing 

growth in the use of fossil fuels; in that case, 

the increase in U.S. average air temperature is 

likely to exceed 11°F by the end of this century. 

This amount of temperature increase would reshape human societies in ways that are almost 

unthinkable to us today.

Precipitation patterns are also expected to continue to change throughout this century and 

beyond. In general, wet areas are projected to 

get wetter and dry areas, drier. In some areas, 

located in between wetter and drier areas, the 

total amount of precipitation falling over the 

course of a year is not expected to significantly 

change. Following the observed trends over 

recent decades, more precipitation is expected 

to fall as heavier precipitation events. In many 

mid-latitude regions, including the United 

States, there will be fewer days with precipitation but the wettest days will be wetter. Largescale shifts towards wetter or drier conditions and the projected increases in heavy precipitation are expected to be greater 

under higher emissions scenarios as compared to lower ones.

O. Does climate change affect severe weather?

Yes, climate change can and has altered the risk of certain types of extreme weather events. The harmful effects of severe 

weather raise concerns about how the risk of such events might be altered by climate change. An unusually warm month, a 

major flood or a drought, a series of intense rainstorms, an active tornado season, landfall of a major hurricane, a big snowstorm, or an unusually severe winter inevitably lead to questions about possible connections to climate change. 

For example, more extreme high temperatures and fewer 

extreme cold temperatures occur in a warmer climate (although extreme cold events can and do still occur – just less 

frequently). In the United States, more than twice as many high 

temperature records as compared to low temperature records 

were broken in the period of 2001-2012.

Also, in many areas, heavy rainfall events have already, and 

will continue to become more frequent and severe as climate 

continues to change. The intensity and rainfall rates of Atlantic 

hurricanes are projected to increase, with the strongest storms 

getting stronger. Recent research has shown how climate 

change can alter atmospheric circulation and weather patterns 

such as the jet stream, affecting the location, frequency, and 

duration of these and other extremes. While there have always 

been extreme events due to natural causes, scientific evidence 

indicates that the probability and severity of some types of 

events has increased due to climate change.

For other types of extreme weather events important to the 

United States, such as tornadoes and severe thunderstorms, 

more research is needed to understand how climate change 

will affect them. These events occur over much smaller scales, 

which makes observations and modeling more challenging. 

Projecting the future influence of climate change on these 

events can also be complicated by the fact that some of the 

risk factors for these events may increase with climate change, 

while others may decrease. 

Figure 19. Projected average annual temperature changes over the 

contiguous United States for multiple future scenarios relative to the 1901-

1960 average temperature. The dashed lines are results from the previous 

generation of climate models and scenarios, while solid lines show the most 

recent generation of climate model simulations and scenarios. Changes 

in temperature over the U.S. are expected to be higher than the change in 

global average temperatures (Figure 23). Differences in these projections 

are principally a result of differences in the scenarios. (Data from CMIP3, 

CMIP5, and NOAA NCDC). 

Observed and Projected U.S. Temperature Change
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P. How are the oceans affected by climate change?

The oceans cover more than two-thirds of the Earth’s surface and play a very important role in regulating the Earth’s climate 

and in climate change. Today, the world’s oceans absorb more than 90% of the heat trapped by increasing levels of carbon 

dioxide and other greenhouse gases in the atmosphere due to human activities. This extra energy warms the ocean, causing 

it to expand. This in turn causes sea level to rise. Of the global rise in sea level observed over the last 35 years, about 40% is 

due to this warming of the water. Most of the rest is due to the melting of glaciers and ice sheets. Ocean levels are projected 

to rise another 1 to 4 feet over this century, with the precise number largely depending on the amount of global temperature 

rise and polar ice sheet melt.

Observations from past climate combined with climate model 

projections of the future suggest that over the next 100 years 

the Atlantic Ocean’s overturning circulation (known as the 

“Ocean Conveyor Belt”) could slow down as a result of climate 

change. These ocean currents carry warm water northward 

across the equator in the Atlantic Ocean, warming the North 

Atlantic (and Europe) and cooling the South Atlantic. A slowdown of the Conveyor Belt would increase regional sea level 

rise along the east coast of the 

United States and change patterns of temperature in Europe 

and rainfall in Africa and the 

Americas, but would not lead to 

global cooling.

Warming ocean waters also 

affect marine ecosystems like 

coral reefs, which can be very 

sensitive to temperature changes. When water temperatures 

become too high, coral expel 

the algae (called zooxanthellae) 

which help nourish them and 

give them their vibrant color. 

This is known as coral bleaching. If the high temperatures 

persist, the coral die.

In addition to the warming, the 

acidity of seawater is increasing 

as a direct result of increasing 

atmospheric carbon dioxide 

(see FAQ Q). The oceans are 

now absorbing about a quarter 

of the carbon dioxide produced by human activities every year. 

The dissolved carbon dioxide reacts with seawater to form 

carbonic acid, which makes the water more acidic, making it 

more difficult for shellfish, corals, and other living things to 

grow their shells or skeletons. Both the increased acidity and 

higher temperature of the oceans are expected to negatively 

affect corals and other living things over the coming decades 

and beyond.

Figure 20. (Photo) Bleached brain coral; (Maps) The global extent and severity of mass 

coral bleaching have increased worldwide over the last decade. Red dots indicate severe 

bleaching. (Figure source: Marshall and Schuttenberg 2006;15 Photo credit: NOAA). 

Coral Bleaching
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APPENDIX 4: FREQUENTLY ASKED QUESTIONS

Q. What is ocean acidification?

As human-induced emissions of carbon dioxide build up in the atmosphere, excess carbon dioxide dissolves into the oceans, 

where it reacts with seawater to form carbonic acid, which makes ocean waters more acidic and corrosive. These changes to 

ocean chemistry can affect many living things, and possibly the entire food web.

Dissolved calcium and carbonate ions are the building blocks 

for the skeletons and shells of many living things in the oceans. 

Ocean acidification lowers the availability of carbonate ions in 

many parts of the ocean, affecting the ability of some marine 

life to produce and maintain their shells.

Since the beginning of the Industrial Revolution, the pH of surface ocean waters has fallen 

by 0.1 pH units, representing approximately a 

30% increase in acidity. The oceans will continue to absorb carbon dioxide produced by 

human activities and become even more acidic 

in the future. Projections of carbon dioxide levels indicate that by the end of this century the 

surface waters of the ocean could be as much 

as 150% more acidic, resulting in a pH that the 

oceans have not experienced for more than 20 

million years and effectively transforming marine life as we know it.

Ocean acidification is expected to affect ocean 

species to varying degrees. Some photosynthetic algae and seagrass species may benefit 

from higher CO2 conditions in the ocean, as 

they require CO2 to live, as do plants on land. On the other 

hand, studies have shown that a more acidic environment has 

dramatic negative effects on some calcifying species, including 

pteropods, oysters, clams, sea urchins, shallow water corals, 

deep sea corals, and calcareous plankton. When shelled species are at risk, the entire food web may also be at risk.

R. How reliable are the computer models of the Earth’s climate? 

Climate models are used to analyze past changes in the long-term averages and variations in temperature, precipitation, and 

other climate indicators, and to make projections of how these trends may change in the future. Today’s climate models do a 

good job at reproducing the broad features of the present climate and changes in climate, including the significant warming 

that has occurred over the last 50 years. Hence, climate models can be useful tools for testing the effects of changes in the 

factors that drive changes in climate, including heat-trapping gases, particulates from human and volcanic sources, and solar 

variability.

Scientists have amassed a vast body of knowledge regarding 

the physical world. Unlike many areas of science, however, 

scientists who study the Earth’s climate cannot build a “control Earth” and conduct experiments on this Earth in a lab. To 

experiment with the Earth, scientists instead use this accumulated knowledge to build climate models, or “virtual Earths.” In 

studying climate change, these virtual Earths serve as an important way to integrate different kinds of knowledge of how 

the climate system works. These models can be used to test 

scientific understanding of the response of the Earth’s climate 

to past changes (such as the transition from the last glacial 

maximum to our current warm interglacial period) as well as to 

develop projections of future changes (such as the response of 

the Earth’s climate to human activities).

Climate models are based on mathematical and physical equations representing the fundamental laws of nature and the 

many processes that affect the Earth’s climate system. When 

the atmosphere, land, and ocean are divided up into small 

grid cells and these equations are applied to each grid cell, 

the models can capture the evolving patterns of atmospheric 

pressures, winds, temperatures, and precipitation. Over longer 

timeframes, these models simulate wind patterns, high and 

low pressure systems, and other weather characteristics that 

make up climate.

Some important physical processes are represented by approximate relationships because the processes are not fully 

understood, or they are at a scale that a model cannot directly 

Figure 21. Pteropods, or “sea butterflies,” are sea creatures about the size of 

a small pea. Pteropods are eaten by organisms ranging in size from tiny krill to 

whales, and are an important source of food for North Pacific juvenile salmon. 

The photos above show what happens to a pteropod’s shell when it encounters 

seawater that is too acidic. The left panel shows a shell collected from a live 

pteropod from a region in the Southern Ocean where acidity is not too high. 

The shell on the right is from a pteropod collected in a region with higher acidity 

(Photo credits: (left) Bednaršek et al. 2012;16 (right) Nina Bednaršek). 

Ocean Acidification and the Food Web
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APPENDIX 4: FREQUENTLY ASKED QUESTIONS

represent. Examples include clouds, convection, and turbulent 

mixing of the atmosphere, for which important processes are 

much smaller than the resolution of current models. These 

approximations lead to uncertainties in model simulations of 

climate.

Climate models require enormous computing resources, especially to capture the geographical details of climate. Today’s 

most powerful supercomputers are enabling climate scientists 

to more thoroughly examine effects of climate change in ways 

that were impossible just five years ago. Over the next decade, 

computer speeds are predicted to increase another 100 fold 

or more, permitting even more details of the climate system 

to be explored.

S. What are the key uncertainties about climate change?

Available evidence gives scientists confidence that humans are having a significant effect on climate and will continue to do so 

over this century and beyond. In particular, continued use of fossil fuels and resulting emissions will significantly alter climate 

and lead to a much warmer world. Of course, it is impossible to predict the future with absolute certainty. The precise amount 

of future climate change that will occur over the rest of this century is uncertain for several reasons.

First, projections of future climate changes are usually based on 

scenarios (or sets of assumptions) regarding how future emissions may change as a result of population, energy, technology, 

and economics. Society may choose to reduce emissions or to 

continue to increase them. The differences in projected future 

climate under different scenarios are generally small for the 

next few decades. By the second half of the century, however, 

human choices, as reflected in these scenarios, become the 

key determinant of future climate change. And human choices 

are nearly impossible to predict.

A second source of uncertainty is natural variability, which affects climate over timescales from months to decades. These 

natural variations are largely unpredictable and are superimposed on the warming from increasing heat-trapping gases. 

Uncertainty in the sun’s future output is another source of 

variability that is independent of human actions. Estimates of 

past changes in solar variability over the last several millennia 

suggest that the magnitude of solar effects over this century 

are likely to be small compared to the magnitude of the climate 

change effects projected from human activities.

A third source of uncertainty involves limitations to our current scientific knowledge. The Earth’s climate system is complex, and continues to challenge scientists’ understanding of 

exactly how it may respond to human influences. ObservaFigure 22. The large-scale geographical patterns and approximate magnitude of the surface air 

temperature trend from 1980 to 2005 from observational data (left) is approximately captured by computer 

models of the climate system (right). The pattern from the computer models is an average based on 

43 different global climate models (CMIP5) used in the Intergovernmental Panel on Climate Change’s 

(IPCC) Fifth Assessment Report. The observations are a combination of both the human contribution 

to recent warming as well as the natural temperature variations. Averaging these model simulations 

suppresses the natural variations and thus shows mainly the human contribution, which is the reason that 

the smaller-scale details are different between the two maps. (Figure source: NOAA NCDC / CICS-NC).

Climate Models and Temperature Change
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APPENDIX 4: FREQUENTLY ASKED QUESTIONS

tions of the climate system have expanded substantially since 

the beginning of the satellite era, but are still limited. Climate 

models differ in the way they represent various processes (for 

example, cloud properties, ocean circulation, and turbulent 

mixing of air). As a result, different models produce slightly different projections of change, even when the models use the 

same scenarios. Scientists often use multiple models in order 

to represent this range of projected outcomes.

Finally, there is always the possibility that there are processes 

and feedbacks not yet being included in future projections. For 

example, as the Arctic warms, carbon trapped in permafrost 

may be released into the atmosphere, increasing the initial 

warming due to human emissions of heat-trapping gases (see 

FAQ T). 

However, for a given future scenario, the amount of future 

climate change can be specified within plausible bounds, determined not only from the differences in the “climate sensitivity” among models but also from information about climate 

changes in the past.

T. Are there tipping points in the climate system?

Most climate studies have considered only relatively gradual, continuous changes in the Earth’s climate system. However, 

there are a number of potential “tipping points” in the climate system – points where a threshold is crossed, resulting in a 

substantial change in the future state of the climate system, regionally and/or globally. 

Scientists have identified several aspects of the climate system 

that could pass a tipping point and/or change substantially under projected climate change (see Figure 24 for key examples). 

These tipping points have been identified based on observations of past abrupt climate changes, recent observations 

showing abrupt changes underway (for example, in the Arctic), 

process-based understanding of the dynamics of the climate 

system, and climate simulations showing tipping points in future projections. There is no clear scientific consensus at this 

time as to whether major tipping points, other than loss of the 

Arctic sea ice in summer, will be reached during this century.

Some tipping points are more imminent, and some would have 

larger impacts than others. For example, the rapid decline of 

Arctic sea ice exposes the darker ocean surface which absorbs 

increasing amounts of heats and reduces the amount of new 

seasonal ice formed. This drastic reduction in sea ice can tip the 

Arctic Ocean into a permanent, nearly ice-free state in summer 

(Ch.2: Our Changing Climate, Key Message 11). There is some 

Figure 23. Projected global average annual temperature changes for multiple future scenarios relative 

to the 1901-1960 average temperature. Each line represents a central estimate of global average 

temperature rise for a specific emissions pathway. Shading indicates the range (5th to 95th percentile) 

of results from a suite of climate models. The left panel shows results from the previous generation of 

climate models (CMIP3), and the right panel shows results from the most recent generation of climate 

models (CMIP5). Projections in 2099 for additional emissions pathways are indicated by the bars to the 

right of each panel. In all cases, temperatures are expected to rise, although the difference between 

lower and higher emissions pathways is substantial. (Data from CMIP3, CMIP5, and NOAA NCDC).

Emissions Levels Determine Temperature Rises
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APPENDIX 4: FREQUENTLY ASKED QUESTIONS

evidence that reductions in ice cover are already leading to 

changes in weather patterns affecting the U.S. and Europe.

Currently, the proximity, rate, and reversibility of tipping points 

are usually assessed through a mixture of climate modeling, 

literature review, and expert elicitation. However, there is a 

need for more research in this area. Climate scientists cannot 

predict when tipping points will be crossed because of uncertainties in the climate system and because we do not know 

what pathway future emissions will take. But an absence of 

certainty does not indicate an absence of risk. To use a medical 

analogy, just because your doctor cannot tell you the precise 

date and time that you will have a heart attack does not mean 

you should ignore medical advice to reduce your risk by taking preventative measures like exercising more, losing weight, 

and changing your diet. Medical science is imperfect, just like 

climate science, but it can provide very useful advice regarding the risks of our actions and choices – and the benefits of 

preventative measures.

U. How is climate change affecting society?

Multiple lines of evidence show that climate change is happening as a result of human activities. Climate change is altering 

the world around us, and these changes will become increasingly evident with each passing decade. Climate change is already 

leading to more intense rainfall events and other extreme weather patterns. It will lead to more droughts in some areas, more 

floods in others, and more frequent heat waves in many areas. Changing temperature and precipitation patterns, as well as 

increasing sea level, are important factors affecting various parts of the United States. For example, the risks associated with 

wildfires in the western U.S. are increasing, and coastal inundation is becoming a common occurrence in low-lying areas. 

Water supply availability is changing in many parts of the United States.

Many people are already being affected by the changes that 

are occurring, and more will be affected as these changes 

continue to unfold. To limit risks and maximize opportunities 

associated with the changes, it would be helpful for people to 

understand how climate change could affect them and what 

they can do to adapt, as well as what can be done to reduce 

future climate change by reducing global emissions. 

Figure 24. Stylized map of potential policy-relevant tipping elements in the Earth’s climate system overlain on population 

density. Question marks indicate systems whose status as tipping elements is particularly uncertain. (Figure source: 

adapted from Lenton et al. 200817)

Potential Tipping Points
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APPENDIX 4: FREQUENTLY ASKED QUESTIONS

Taking actions to reduce the emissions that cause climate 

change has costs. Not taking those actions has much greater 

costs.18

Climate change will affect ecosystems and human systems 

– such as agricultural, transportation, water resources, and 

health-related infrastructure – in ways we are only beginning 

to understand. Moreover, climate change interacts with other 

stressors, such as population increase, land-use change, and 

economic and political changes, in ways that we may not be 

able to anticipate, compounding the risks.

In general, the larger and faster the changes in climate, the 

more difficult it is for human and natural systems to adapt. 

The climate system has been relatively stable during the time 

that human civilizations have existed. Essentially, today’s built 

infrastructure has been developed based on the assumption 

that future climate will be like that of the past. This assumption 

is no longer valid.

Since climate change is already occurring, adaptation in some 

form is inevitable. The choice is between proactive adaptation 

(planning ahead to limit impacts) or reactive adaptation (where 

responses occur only after damages are already incurred). The 

America’s Climate Choices reports from the U.S. National Academy of Sciences discuss these issues in details. 

Figure 25. Climate change is likely to affect human society and the natural environment 

in many ways. The National Climate Assessment’s sectoral impacts chapters examine 

these impacts by category in detail. (Figure source: adapted from Phillipe Rekacewicz 

UNEP/GRID-Arendal 2012, “Vital Climate Graphics” collection19).

Potential Effects of Climate Change
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APPENDIX 4: FREQUENTLY ASKED QUESTIONS

V. Are there benefits to warming?

Some climate changes currently have beneficial effects for specific sectors or regions. For example, current benefits of warming include longer growing seasons for agriculture and longer ice-free periods for shipping on the Great Lakes. At the same 

time, however, longer growing seasons, along with higher temperatures and carbon dioxide, can increase pollen production, 

intensifying and lengthening the allergy season. Longer ice-free periods on the Great Lakes can result in more lake-effect 

snowfalls. 

Many analyses of this question have concluded that there will 

be more negative effects than positive ones. This is largely because our society and infrastructure have been built for the 

climate of the past, and any rapid change from that climate 

imposes difficulties and costs. For example, many major cities 

are located on the coasts where they are now vulnerable to sea 

level rise. And there has been rapid population growth in the 

U.S. Southwest, where increasing heat and drought threaten 

water supplies and cause increased wildfires. In addition, ecosystems that we rely on for our food and water are adapted to 

the cooler climate that our planet has experienced over recent 

centuries.

W. Are some people more vulnerable than others? 

People will be affected by climate change in various ways, but some groups are more vulnerable than others. For example, 

the poor, the very young, and some older people have less mobility and fewer resources to cope with extremely high temperatures, increased water scarcity, environmental degradation, and other impacts. People living in flood plains, coastal zones, 

and some urban areas are generally more vulnerable as well.

Children, primarily because of physiological and developmental factors, will disproportionately suffer from the effects of 

heat waves, air pollution, infectious illness, and trauma resulting from extreme weather events. The country’s older 

population also could be harmed more as the climate changes. 

Older people are at much higher risk of dying during extreme 

heat events. Pre-existing health conditions also make older 

adultssusceptible to cardiac and respiratory impacts of air pollution and to more severe consequences from infectious diseases. Limited mobility among older adults can also increase 

flood-related health risks. Limited resources and an already 

high burden of chronic health conditions, including heart disease, obesity, and diabetes, will place the poor at higher risk 

of health impacts from climate change than higher income 

groups. Potential increases in food cost and limited availability 

of some foods will exacerbate current dietary inequalities and 

have significant health ramifications for the poorer segments 

of our population. 
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APPENDIX 4: FREQUENTLY ASKED QUESTIONS

X. Are there ways to reduce climate change? 

The most direct way to significantly reduce the magnitude of future climate change is to reduce the emissions of heat-trapping 

gases. Emissions can be reduced in many ways, and increasing the efficiency of energy use is an important component of many 

potential strategies. For example, because about 28% of the energy used in the U.S. is used for transportation, developing and 

driving more efficient vehicles and changing to fuels that do not contribute significantly to heat-trapping gas emissions over 

their lifetimes would result in fewer emissions per mile driven. A large amount of energy in the U.S. is also used to heat and 

cool buildings, so changes in building design could dramatically reduce energy use. While there is no single silver bullet that 

will solve all the challenges posed by climate change, there are many options that can reduce our emissions and help prevent 

some of the potentially serious impacts of climate change. There will be some costs to these changes, but even very ambitious 

emissions reductions targets have relatively small costs over the decades it will take to implement them. 

Because impacts are already occurring and anticipated to increase, adaptation to the impacts of climate change will be 

required. Adaptation decisions range from being better prepared for extreme events such as floods and droughts, to identifying economic opportunities that come from investments 

in adaptation and mitigation strategies and technologies, to 

integrating considerations of new climate-related risks into 

city planning, public health and emergency preparedness, and 

ecosystem management.

Technological fixes such as “geoengineering” may be possible, 

but at least some such proposals would do nothing to slow 

ocean acidification, and would need to be done indefinitely. 

There are a wide variety of potential risks of geoengineering 

schemes, which are very poorly understood (see FAQ Z).

Figure 26. Reducing carbon emissions from a higher pathway (here, 

RCP 8.5) to a lower pathway (here, RCP 4.5) can be accomplished with a 

combination of many technologies and policies, illustrated here based on 

the “wedges” concept pioneered by Pacala and Socolow in 2004.

20 These 

wedges could include increasing the energy efficiency of appliances, 

vehicles, buildings, electronics, and electricity generation (orange 

wedges); reducing carbon emissions from fossil fuels by switching to lowercarbon fuels or capturing and storing carbon (blue wedges); and switching 

to renewable and non-carbon emitting sources of energy, including solar, 

wind, wave, biomass, tidal, and geothermal (green wedges). The shapes 

and sizes of the wedges shown here are illustrative only. (Data from Boden 

et al. 201221). 

Multiple Pathways for Reducing U.S. Emissions
816 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

APPENDIX 4: FREQUENTLY ASKED QUESTIONS

Y. Are there advantages to acting sooner rather than later?

The effects of current emissions of carbon dioxide and other heat-trapping gases on climate can take decades to fully manifest 

themselves. The resulting change in climate and the impacts of those changes can then persist for a long time. The longer 

these changes in climate continue, the greater the resulting impacts. It will become increasingly costly to adapt, and some 

systems will not be able to adapt if the change is too much or too fast. Thus it is not surprising that recent reports from the U.S. 

National Academy of Sciences, including America’s Climate Choices22 and America’s Energy Future,

23 have concluded that the 

environmental, economic, and humanitarian risks posed by climate change indicate a pressing need for substantial action to 

limit the magnitude of climate change and to prepare to adapt to its impacts. They also concluded that substantial reductions 

of heat-trapping gas emissions should be among the nation’s highest priorities.

The National Academy of Sciences and others have concluded 

that acting now will reduce the risks posed by climate change 

and the pressure to make larger, more rapid, and potentially 

more expensive reductions later. Actions taken to reduce 

vulnerability to climate change impacts can be considered as 

investments that can make sense economically, especially if 

they also offer protection against natural climate variations 

and extreme events. In addition, investment decisions made 

now about equipment and infrastructure can “lock in” emissions of heat-trapping gases for decades to come. Finally, while 

it may be possible to alter our responses to climate change, it 

is difficult or impossible to “undo” climate change once it has 

occurred.

Current efforts at local and state levels, and by the private 

sector, are important, but are insufficient to limit warming to 

the lower scenarios described throughout this report. Thus, 

numerous analyses have called for policies that establish coherent national and international goals and incentives, and 

that promote strong U.S. engagement in international-level response efforts. The National Academy of Sciences found that 

the inherent complexities and uncertainties of climate change 

will be best met by applying a risk management approach and 

by making efforts to significantly reduce heat-trapping gas 

emissions; prepare for adapting to impacts; invest in scientific 

research, technology development, and information systems; 

and facilitate engagement between scientific and technical experts and the many types of people making America’s climate 

choices.

Figure 27. This graph shows how earlier action to reduce U.S. emissions 

would be less difficult than delayed action. Two pathways show how a 

cumulative carbon emissions budget of 265 gigatons of CO2 could be 

maintained by 2050. By initiating reduced emissions efforts in 2010 (blue 

line), a 4% per year reduction would have been required; waiting until 2020 

to reduce emissions (red line) doubles the rate at which emissions must be 

reduced. (Figure source: Luers et al. 200724)

Two U.S. Emissions-Reduction Pathways
817 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

APPENDIX 4: FREQUENTLY ASKED QUESTIONS

Z. Can we reverse global warming?

While we can’t stop climate change in its tracks, we can limit it to less dangerous levels by reducing our emissions. Even if all 

human-related emissions of carbon dioxide and the other heat-trapping gases were to stop today, Earth’s temperature would 

continue to rise for a number of decades and then slowly begin to decline. However, focusing on short-lived types of emissions, 

such as methane and black carbon (soot), can reduce the rate of change in the near term. Because of the complex processes 

controlling carbon dioxide concentrations in the atmosphere, even after more than a thousand years, the global temperature 

would still be higher than it was in the pre-industrial period. As a result, without technological intervention, it will not be possible to totally reverse climate change. We do face a choice between a little more warming and lot more warming, however. 

The amount of future warming will depend on our future emissions.

In theory, it may be possible to reverse global warming through 

technological interventions called geoengineering. Three types 

of geoengineering approaches have been proposed to alter 

the climate system: 1) enhancing the natural processes that 

remove carbon dioxide from the atmosphere; 2) altering the 

amount of the sun’s energy that reaches the Earth (referred to 

Figure 28. To reduce the changes occurring in climate, we would need to stabilize 

atmospheric levels of carbon dioxide, not simply stabilize current emission levels of carbon 

dioxide. Just stabilizing emissions still leads to increasing amounts of carbon dioxide in 

the atmosphere, because emissions are greater than the sinks that remove it (blue lines). 

To stabilize levels of atmospheric carbon dioxide, emissions would need to be reduced 

significantly, on the order of 80% or more compared to the present day (green lines). 

The lower graph shows how carbon dioxide concentrations would be expected to evolve 

depending upon emissions for one illustrative case, but this applies for any chosen target. 

(Figure source: NRC 201125).

Emissions Reductions and Carbon Dioxide Concentrations
818 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

APPENDIX 4: FREQUENTLY ASKED QUESTIONS

as “solar radiation management”); and 3) direct capture and 

storage of CO2 from the atmosphere.

Various techniques for removal of carbon dioxide from the atmosphere have been proposed. At this time, however, there 

is no indication that any of them could be implemented on a 

large enough scale to have a significant effect. Investments in 

limiting emissions, combined with capturing and storing carbon, could possibly reverse the warming trend, but it remains 

to be seen if this is feasible.

Artificial injection of stratospheric particles and cloud brightening are two examples of “solar radiation management” 

techniques. The cooling effect that some types of particles 

have on the atmosphere has led to the proposal of an array 

of possible geoengineering projects, especially with the goal 

of offsetting the warming until more non-fossil fuel energy is 

put into place. However, the climate system is complex and 

experimenting without complete understanding could result 

in unintended and potentially dangerous side effects on our 

health, ecosystems, agricultural yields, and even the climate 

itself. Even if such engineering approaches were economically 

feasible, the potential impacts on the environment need to be 

better understood. One important consideration regarding solar radiation management is that ocean acidification would still 

continue even if warming could otherwise be reduced by reflecting light away from our atmosphere. Much more research 

is needed to see if such approaches could be environmentally 

feasible. In the meantime, there are significant concerns about 

ecological and other side effects of some of these technologies.
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APPENDIX 4: FREQUENTLY ASKED QUESTIONS

References

1. Kunkel, K. E., L. E. Stevens, S. E. Stevens, L. Sun, E. Janssen, D. 

Wuebbles, and J. G. Dobson, 2013: Regional Climate Trends and 

Scenarios for the U.S. National Climate Assessment: Part 9. Climate 

of the Contiguous United States. NOAA Technical Report NESDIS 

142-9. 85 pp., National Oceanic and Atmospheric Administration, 

National Environmental Satellite, Data, and Information Service, 

Washington, D.C. [Available online at http://www.nesdis.noaa.

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Climate_of_the_Contiguous_United_States.pdf]

2. Kennedy, J. J., P. W. Thorne, T. C. Peterson, R. A. Reudy, P. A. 

Stott, D. E. Parker, S. A. Good, H. A. Titchner, and K. M. Willett, 

2010: How do we know the world has warmed? [in “State of the 

Climate in 2009”]. Bulletin of the American Meteorological Society, 91,

S26-27, doi:10.1175/BAMS-91-7-StateoftheClimate. [Available 

online at http://journals.ametsoc.org/doi/abs/10.1175/BAMS-91-

7-StateoftheClimate]

3. Boden, T., G. Marland, and B. Andres, 2012: Global CO2 Emissions 

from Fossil-Fuel Burning, Cement Manufacture, and Gas Flaring: 1751-

2009. Carbon Dioxide Information Analysis Center, Oak Ridge 

National Laboratory. [Available online at http://cdiac.ornl.gov/

ftp/ndp030/global.1751_2009.ems]

4. Mann, M. E., Z. Zhang, M. K. Hughes, R. S. Bradley, S. K. Miller, 

S. Rutherford, and F. Ni, 2008: Proxy-based reconstructions of 

hemispheric and global surface temperature variations over the 

past two millennia. Proceedings of the National Academy of Sciences, 105,

13252-13257, doi:10.1073/pnas.0805721105. [Available online at 

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seasonal patterns in climate change, temperatures, and precipitation 

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CGCMS simulate the twentieth-century “warming hole” in the 

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9. Robinson, W. A., R. Reudy, and J. E. Hansen, 2002: General 

circulation model simulations of recent cooling in the east-central 

United States. Journal Of Geophysical Research, 107, ACL 4-1 - ACL 

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of observed historical near surface temperature variations to 

anthropogenic and natural causes using CMIP5 simulations. Journal 

of Geophysical Research, 118, 4001-4024, doi:10.1002/jgrd.50239. 

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12. Huber, M., and R. Knutti, 2012: Anthropogenic and natural 

warming inferred from changes in Earth’s energy balance. Nature 

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13. Wouters, B., J. L. Bamber, M. R. van den Broeke, J. T. M. Lenaerts, 

and I. Sasgen, 2013: Limits in detecting acceleration of ice sheet 

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14. Peterson, T. C., D. M. Anderson, S. J. Cohen, M. Cortez-Vázquez, 

R. J. Murnane, C. Parmesan, D. Phillips, R. S. Pulwarty, and J. M. 

R. Stone, 2008: Ch. 1: Why weather and climate extremes matter. 

Weather and Climate Extremes in a Changing Climate. Regions of Focus: 

North America, Hawaii, Caribbean, and U.S. Pacific Islands. A Report by 

the U.S. Climate Change Science Program and the Subcommittee on Global 

Change Research, T. R. Karl, G. A. Meehl, C. D. Miller, S. J. Hassol, 

A. M. Waple, and W. L. Murray, Eds., Department of Commerce, 

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15. Marshall, P., and H. Schuttenberg, 2006: A Reef Manager’s Guide to 

Coral Bleaching. Great Barrier Reef Marine Park Authority, IUCN 

Global Marine Programme, and U.S. National Oceanic and 

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16. Bednaršek, N., G. A. Tarling, D. C. E. Bakker, S. Fielding, E. M. 

Jones, H. J. Venables, P. Ward, A. Kuzirian, B. Lézé, R. A. Feely, 

and E. J. Murphy, 2012: Extensive dissolution of live pteropods 

in the Southern Ocean. Nature Geoscience, 5, 881-885, doi:10.1038/

ngeo1635. 

17. Lenton, T. M., H. Held, E. Kriegler, J. W. Hall, W. Lucht, S. 

Rahmstorf, and H. J. Schellnhuber, 2008: Tipping elements in the 

Earth’s climate system. Proceedings of the National Academy of Sciences, 

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http://www.pnas.org/content/105/6/1786.full.pdf+html]

18. Nordhaus, W., 2013: The Climate Casino: Risk, Uncertainty, and 

Economics for a Warming World. Yale University Press, 392 pp. 

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Programme. [Available online at http://www.grida.no/

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climate problem for the next 50 years with current technologies. 

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21. Boden, T., G. Marland, and B. Andres, 2012: Global CO2 Emissions 

from Fossil-Fuel Burning, Cement Manufacture, and Gas Flaring: 1751-

2009. Carbon Dioxide Information Analysis Center, Oak Ridge 

National Laboratory. [Available online at http://cdiac.ornl.gov/

ftp/ndp030/global.1751_2009.ems]

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24. Luers, A. L., M. D. Mastrandrea, K. Hayhoe, and P. C. Frumhoff 

2007: How to Avoid Dangerous Climate Change: A Target for U.S. 

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www.nap.edu/catalog.php?record_id=12877]

APPENDIX 4: FREQUENTLY ASKED QUESTIONS

References

820 CLIMATE CHANGE IMPACTS IN THE UNITED STATES
821 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

SCENARIOS 

APPENDIX5 AND MODELS

Scenarios

Scenarios provide ways to help understand what future conditions might be. Each scenario provides an example of what 

might happen under particular assumptions, and is neither 

a prediction nor a forecast. Instead, scenarios provide scientifically rigorous and consistent starting points for examining 

questions about an uncertain future and help us to visualize 

alternative futures in human terms. The military and businesses frequently use these powerful tools for future planning 

in high-stakes situations. Scenarios are used to help identify 

future vulnerabilities as well as to support decision-makers 

who are focused on limiting risk and maximizing opportunities. 

Three types of scenarios are used within this assessment to 

help frame the impact analyses in a consistent way: emissions 

scenarios (including population and land-use components); 

climate scenarios; and sea level rise scenarios. Each is briefly 

described below.

Emissions Scenarios

Emissions scenarios quantitatively illustrate how the release of 

different amounts of climate-altering gases and particles into 

the atmosphere will produce different future climate conditions. Such emissions result from human activities including 

fossil fuel energy production and use, agriculture, and other 

activities that change land use. These scenarios are developed 

using a wide range of assumptions about population growth, 

economic and technological development, and other factors. 

A wide range of assumptions is used because future trends depend on unpredictable human choices.

Perspectives on “plausible” emissions scenarios evolve over 

time. The Intergovernmental Panel on Climate Change (IPCC) 

has released three different sets of scenarios since 1990. In 

2000, the IPCC released a Special Report on Emission Scenarios1

 that provided a set of scenarios, known as the SRES, 

which described a wide range of socioeconomic futures and 

resulting emissions. Near the higher end of the range, the SRES 

A2 scenario represents a world with high population growth, 

low economic growth, relatively slow technology improvements and diffusion, and other factors that contribute to high 

emissions and lower adaptive capacity (for example, low per 

capita wealth). At the lower end of the range, the SRES B1 scenario represents a world with lower population growth, higher 

economic development, a shift to low-emitting efficient energy technologies that are diffused rapidly around the world 

through free trade, and other conditions that reduce the rate 

and magnitude of climate change as well as increase capacity 

for adaptation. The SRES A2 and B1 scenarios are the foundation scenarios used in this assessment to evaluate future impacts.

Recently, a new set of scenarios (Representative Concentration Pathways – RCPs) has been prepared and released by scientists who study emissions, climate, and potential impacts.2

This new set incorporates recent observations and research 

and includes a wider range of future conditions and emissions. 

Because climate model results are just now being released using the new scenarios, and there are few impact studies that 

employ them, the RCP climate scenarios are used sparingly in 

this assessment.

Scientists cannot predict which, if any, of the scenarios in either the SRES set or the RCP set is most likely because the future emissions pathway is a function of human choices. A wide 

range of societal decisions and policy choices will ultimately influence how the world’s emissions evolve, and ultimately, the 

composition of the atmosphere and the state of the climate 

system.

Climate Scenarios and Climate Models

Global models that simulate the Earth’s climate system are 

used, among other things, to evaluate the effects of human 

activities on climate. This assessment incorporates a new set of 

model simulations that have higher resolution and enhanced 

representation of Earth system physics, chemistry, and biology. These models use the new set of RCP emissions scenarios 

described above to project expected climate change given various assumptions about how human activities and associated 

emissions levels might change. 

The range of potential increases in global average temperature 

in the newest climate model simulations is wider than earlier 

simulations because a broader range of options for human behavior is considered. For example, the lowest of the new RCP 

scenarios assumes rapid emissions reductions that would limit 

the global temperature increase to about 3.7°F, a much lower 

level than in previous scenarios. The emissions trajectory in 

RCP 8.5 is similar to SRES A2 and RCP 4.5 is roughly comparable 

to SRES B1 (see Figure 1). These similarities between specific 

RCP and SRES scenarios make it possible to compare the results from different modeling efforts over time.
822 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

APPENDIX 5: SCENARIOS AND MODELS

Figure 1. Different amounts of heat-trapping gases released into the atmosphere by human activities produce 

different projected increases in Earth’s temperature. In the figure, each line represents a central estimate of global 

average temperature rise for a specific emissions pathway (relative to the 1901-1960 average). Shading indicates the 

range (5th to 95th percentile) of results from a suite of climate models. Projections in 2099 for additional emissions 

pathways are indicated by the bars to the right of each panel. In all cases, temperatures are expected to rise, although 

the difference between lower and higher emissions pathways is substantial. (Left) The panel shows the two main 

scenarios (SRES – Special Report on Emissions Scenarios) used in this report: A2 assumes continued increases 

in emissions throughout this century, and B1 assumes much slower increases in emissions beginning now and 

significant emissions reductions beginning around 2050, though not due explicitly to climate change policies. (Right)

The panel shows newer analyses, which are results from the most recent generation of climate models (CMIP5) 

using the most recent emissions pathways (RCPs – Representative Concentration Pathways). Some of these new 

projections explicitly consider climate policies that would result in emissions reductions, which the SRES set did not.8

The newest set includes both lower and higher pathways than did the previous set. The lowest emissions pathway 

shown here, RCP 2.6, assumes immediate and rapid reductions in emissions and would result in about 2.5°F of 

warming in this century. The highest pathway, RCP 8.5, roughly similar to a continuation of the current path of global 

emissions increases, is projected to lead to more than 8°F warming by 2100, with a high-end possibility of more than 

11°F. (Data from CMIP3, CMIP5, and NOAA NCDC).

Emissions Levels Determine Temperature Rises

Emissions scenarios

Two SRES global emissions scenarios were recommended for use 

by the authors of this report for impact studies. One is a higher emissions scenario (the A2 scenario from SRES) and the other is a lower 

emissions scenario (the B1 scenario from SRES). These two scenarios 

do not encompass the full range of possible futures: emissions could 

change less than those scenarios imply, or they could change even 

more. Recent carbon dioxide emissions have, in fact, been higher than 

in the A2 scenario. Whether this trend will continue is not possible to 

predict because it depends on societal choices.
823 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

APPENDIX 5: SCENARIOS AND MODELS

Sea Level Rise Scenarios

After at least two thousand years of little change, global sea 

level rose by roughly 8 inches over the last century, and satellite data provide evidence that the rate of rise over the past 

20 years has roughly doubled. In the United States, millions 

of people and many of the nation’s assets related to military 

readiness, energy, transportation, commerce, and ecosystems 

are located in areas at risk of increased coastal flooding because of sea level rise and associated storm surge.

Global sea level is rising and will continue to do so beyond the 

year 2100 as a result of increasing global temperatures. This 

occurs for two main reasons. First, when temperatures rise, 

ocean water heats up, causing it to expand. Second, when 

glaciers and ice sheets melt in response to hotter conditions, 

additional water flows into the oceans. Sea level is projected 

to rise an additional 1 to 4 feet in this century. Scientists are 

unable to narrow this range at present because the processes 

affecting the loss of ice mass from the large ice sheets are dynamic and still the subject of intense study.

Some impact assessments in this report use a set of sea level 

rise scenarios within this range, while others consider a wider 

range. Four scenarios (8 inches, 1 foot, 4 feet, and 6.6 feet of 

rise by 2100), along with explanations regarding how to use 

this information, are included in a guidance document on sea 

level rise that was provided to the National Climate Assessment (NCA) authors to use as the basis of impact assessments 

in coastal areas.3

Figure 2. Historical, observed, and possible future amounts of global sea level rise from 1800 to 

2100. Historical estimates4

 (based on sediment records and other proxies) are shown in red (pink 

band shows uncertainty range), tide gauge measurements in blue,5 and satellite observations in 

green.6 The future scenarios displayed here range from 8 inches to 6.6 feet in 2100.3 Sea level 

rise lower than 8 inches or higher than 6.6 feet is considered implausible by 2100. The orange 

line at right shows the currently projected range of sea level rise of 1 to 4 feet by 2100. The large 

range primarily reflects uncertainty about how ice sheets will respond to the warming ocean 

and atmosphere, and to changing winds and currents. (Figure source: Adapted from Parris et 

al. 2012,3 with contributions from NASA Jet Propulsion Laboratory).

Past and Projected Changes in Global Sea Level
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APPENDIX 5: SCENARIOS AND MODELS

Models and Sources of Uncertainty

There are multiple well-documented sources of uncertainty in 

climate model simulations. Some of these uncertainties can be 

reduced with improved models. Some may never be completely eliminated. The climate system is complex, including natural 

variability on a range of time scales, and this is one source of 

uncertainty in projecting future conditions. In addition, there 

are challenges with building models that accurately represent 

the physics of multiple interacting processes, with the scale 

and time frame of the available historical data, and with the 

ability of computer models to handle very large quantities of 

data. Thus, climate models are necessarily simplified representations of the real climate system.

One of the largest sources of uncertainty in projecting future 

conditions involves what decisions society will make about 

managing the emissions of greenhouse gases. By later this century, very different conditions would result from higher emissions scenarios (such as A2) than from lower ones (like B1).

Over the last decade, concerted efforts in climate modeling 

have focused on understanding and better quantifying the 

uncertainties inherent in model simulations of climate change 

and on improving model resolution and representations of 

physical and biological processes important to the climate system. It is very clear that progress is being made in the accuracy 

of models in representing the physics of the climate system at 

smaller scales. This is demonstrated, for example, by the ability 

of these models to replicate observed climate. 

To understand and better quantify uncertainty, multiple models generated by different modeling groups around the world 

are being used to identify common features in projections of 

climate change. The Third Coupled Model Intercomparison 

Project (CMIP3), and more recently CMIP5, established formalized structures that enable model evaluations against the 

climate record of the recent past. New elements of the CMIP5 

effort include a major focus on near-term, decade-length 

projections designed for regional climate change and on predictions from the new class of Earth system models that include coupled physical, chemical, and biogeochemical climate 

processes. CMIP3 findings are the foundation for most of the 

impact analyses included in this assessment. Newer information from CMIP5 was largely unavailable in time to serve as the 

foundation for this report and is primarily provided for comparison purposes. 

The breadth and depth of these analyses indicate that the 

modeling results in this report are robust. There is an important distinction to be made, however, between a “prediction” 

of what “will” happen and a “projection” of what future conditions are likely given a particular set of assumptions. All of the 

model results presented in this report are the latter: projections based on specified assumptions about emissions. The 

new regional projections provided in this report represent the 

state of the science in climate change modeling.7
825 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

APPENDIX 5: SCENARIOS AND MODELS

References

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Wuebbles, and J. G. Dobson, 2013: Regional Climate Trends and 

Scenarios for the U.S. National Climate Assessment: Part 9. Climate 

of the Contiguous United States. NOAA Technical Report NESDIS 

142-9. 85 pp., National Oceanic and Atmospheric Administration, 

National Environmental Satellite, Data, and Information Service, 

Washington, D.C. [Available online at http://www.nesdis.noaa.

gov/technical_reports/NOAA_NESDIS_Tech_Report_142-9-

Climate_of_the_Contiguous_United_States.pdf]

8. IPCC, 2007: Climate Change 2007: The Physical Science Basis. Contribution 

of Working Group I to the Fourth Assessment Report of the Intergovernmental 

Panel on Climate Change. S. Solomon, D. Qin, M. Manning, Z. Chen, 

M. Marquis, K. B. Averyt, M. Tignor, and H. L. Miller, Eds. 

Cambridge University Press, 996 pp. [Available online at http://

www.ipcc.ch/publications_and_data/publications_ipcc_fourth_

assessment_report_wg1_report_the_physical_science_basis.htm] 

Collins, M., R. Knutti, J. M. Arblaster, J.-L. Dufresne, T. Fichefet, 

F. P., X. Gao, W. J. Gutowski, T. Johns, G. Krinner, M. Shongwe, 

C. Tebaldi, A. J. Weaver, and M. Wehner, 2013: Ch. 12: Long-term 

climate change: Projections, commitments and irreversibility. 

Climate Change 2013: The Physical Science Basis. Contribution of Working 

Group I to the Fifth Assessment Report of the Intergovernmental Panel on 

Climate Change, T. F. Stocker, D. Qin, G.-K. Plattner, M. Tignor, 

S.K. Allen, J. Boschung, Nauels, Y. Xia, V. Bex, and P. M. Midgley, 

Eds., Cambridge University Press, 1029-1136. [Available online at 

http://www.climatechange2013.org/report/review-drafts/]
826 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

TOPICS FOR CONSIDERATION

APPENDIX6 IN FUTURE ASSESSMENTS

Although this report covers a broad range of topics related to 

understanding, assessing, and responding to global change as 

required by the Global Change Research Act,1 it is not possible 

to provide a comprehensive analysis of every topic in a single 

report. The following are important topics that could not be 

adequately covered in this report. In preparation for future 

synthesis reports, these are some topics that could be considered.

Economic Analyses

Documenting the costs of climate change impacts is extremely 

challenging because these impacts occur across multiple regions and sectors and over multiple time frames. The impacts 

include physical, ecological, and social components, and many 

are difficult to extract from underlying sources of vulnerability not caused by climate change. Also, while some types of 

extreme weather events are made more frequent and/or intense by climate change, it is rare that any event has a single 

cause. Since such events generally result from a combination 

of natural variability and climate change, it is difficult to assign 

a precise proportion of the costs associated with a particular 

event to climate change. Further, many impacts occur in ways 

that are difficult to translate into precise economic costs; for 

example, impacts to biodiversity, changes in quality of life, or 

social stresses are likely to be valued differently by different 

individuals and communities. Finally, it is challenging to assess the economic implications of rare events, which have low 

probability but high consequence – especially in cases where 

there is limited or non-existent data about the costs of such 

events in the past.

A number of studies have produced estimates of the economic 

damages expected from future climate change. However, 

there are currently no total economic damage estimates that 

are based on valuing and aggregating the various regional and 

sectoral impacts that are the focus of this assessment. Understanding these impacts in more detail could provide important 

input for adaptation and mitigation decisions. 

National Security

The implications of climate change for U.S. national security 

are significant, but they have not been analyzed in detail in this 

report because there are a number of recent unclassified U.S. 

Department of Defense (DoD) reports and reports of other 

groups that have rigorously addressed this topic. In 2010, the 

DoD released the Quadrennial Defense Review (QDR), for the 

first time acknowledging that climate change will play a “significant role in shaping the future security environment.”2

 Based 

on the QDR, the DoD is now incorporating and considering the 

consequences of climate change in its long-range strategic 

plans, including potential impacts to its facilities and missions. 

Other recent reports by the National Intelligence Council and 

the National Research Council (NRC) analyze the security implications of climate change.3

 The NRC found that “It is prudent to expect that over the course of a decade some climate 

events...will produce consequences that exceed the capacity 

of the affected societies or global systems to manage and that 

have global security implications serious enough to compel 

international response.” National security concerns are highly 

integrated with a variety of other economic, health, policy 

and resource management issues. The findings of the National 

Climate Assessment reports, as well as other environmental 

assessments, are influential in determining threats to national 

security. It will be useful in future reports to advance the state 

of knowledge of climate impacts in a manner that would improve the ability of the appropriate government institutions to 

determine how such impacts are integrated in complex ways 

with national security concerns and emergency preparedness.

Interactions between Adaptation and Mitigation Activities

An additional topic that requires further investigation is the 

state of knowledge of the intersections of adaptation and 

mitigation activities. Although adaptation, preparedness, and 

resilience are all related concepts, the emissions implications 

across the life of an adaptation project, including full assessment of the emissions associated with “supply chains” for 

manufactured goods and services, are difficult to assess for 

any project, and even more challenging on larger scales. In 

addition, there are options where mitigation and adaptation 

strategies have co-benefits and other combinations of strategies that can cause unintended negative consequences. For 

example, the water resource implications of increased production of biofuels are substantial in some regions of the United 

States, and may result in negative impacts on ecosystems, 

power production, or residential water supply (see Ch. 6: Agriculture; Ch. 10: Energy, Water, and Land; Ch. 27: Mitigation; 

and Ch. 28: Adaptation). It would be useful to explore these 

and related topics in more detail in future assessments.
827 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

APPENDIX 6: FUTURE ASSESSMENT TOPICS

References

1. GCRA, 1990: Global Change Research Act (Public Law 101-606, 

104 Stat. 3096-3104), signed on November 16, 1990. [Available 

online at http://www.gpo.gov/fdsys/pkg/STATUTE-104/pdf/

STATUTE-104-Pg3096.pdf]

2. DOD, 2010: Quadrennial Defense Review, 128 pp., U.S. 

Department of Defense. [Available online at http://www.defense.

gov/qdr/qdr%20as%20of%2029jan10%201600.pdf]

3. Fingar, T., 2008: National Intelligence Assessment on the National 

Security Implications of Global Climate Change to 2030, 21 pp., U.S. 

Office of the Director of National Intelligence. [Available online at 

http://www.fas.org/irp/congress/2008_hr/062508fingar.pdf] 

NRC, 2013: Climate and Social Stress: Implications for Security 

Analysis.National Research Council. The National Academies 

Press. [Available online at http://www.nap.edu/catalog.

php?record_id=14682]
828 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

ABBREVIATIONS AND ACRONYMS

AMAP – Arctic Monitoring and Assessment Programme

AR4 – IPCC Fourth Assessment Report

C2ES - Center for Climate and Energy Solutions

CDC – Centers for Disease Control and Prevention

CDP – Carbon Disclosure Project

CDR – Carbon Dioxide Removal

CEQ – White House Council on Environmental Quality

CCSP – Climate Change Science Program

CICS-NC – Cooperative Institute for Climate and Satellites, North Carolina

CMIP3 – Coupled Model Intercomparison Project, 3rd phase

CMIP5 – Coupled Model Intercomparison Project, 5th phase 

CO2 – Carbon Dioxide

DOC – U.S. Department of Commerce

DoD – U.S. Department of Defense

DOE – U.S. Department of Energy

DOI – U.S. Department of the Interior

DOT – U.S. Department of Transportation

EIA – Energy Information Administration

ENSO – El Niño-Southern Oscillation

EPA – U.S. Environmental Protection Agency

ERS – Economic Research Service

FAO – Food and Agriculture Organization

FEMA – Federal Emergency Management Agency

FIA – Forest Inventory Analysis

FWS – U.S. Fish and Wildlife Service

GAO – Government Accountability Office

GCIS – Global Change Information System

GCRA – Global Change Research Act

GHG – Greenhouse Gas

GPS – Global Positioning System

GRACE – Gravity Recovery and Climate Experiment

GWP – Global Warming Potential

ICCATF – Interagency Climate Change Adaptation Task Force

INCA – Interagency National Climate Assessment Working Group
829 CLIMATE CHANGE IMPACTS IN THE UNITED STATES

ABBREVIATIONS AND ACRONYMS

IPCC – Intergovernmental Panel on Climate Change

NASA – National Aeronautics and Space Administration

NCA – National Climate Assessment

NCADAC – National Climate Assessment and Development Advisory Committee

NCDC – National Climatic Data Center

NCEP – National Center for Environmental Prediction

NCO – National Coordination Office

NDMC – National Drought Mitigation Center

NESDIS – National Environmental Satellite, Data, and Information Service

NGO – Non-governmental Organizations

NIDIS – National Integrated Drought Information System

NOAA – National Oceanic and Atmospheric Administration

NPS – National Park Service

NRC – National Research Council

NRCS – Natural Resources Conservation Service

NSF – National Science Foundation

NSTC – National Science and Technology Council

NWS – National Weather Service

OMB – White House Office of Management and Budget

OSTP – White House Office of Science and Technology Policy

RCP – Representative Concentration Pathways

RGGI – Regional Greenhouse Gas Initiative

RISA – Regional Integrated Sciences and Assessments

SGCR – Subcommittee for Global Change Research

SOCCR – State of the Carbon Cycle Report

SRES – Special Report on Emissions Scenarios

SST – Sea Surface Temperature

TIR – Technical Input Report

TSU – Technical Support Unit

UCAR – University Corporation for Atmospheric Research

UNEP – United Nations Environment Programme

USACE – U.S. Army Corps of Engineers

USDA – U.S. Department of Agriculture

USFS – U.S. Forest Service

USGCRP – U.S. Global Change Research Program

USGS – U.S. Geological Survey

WGA – Western Governor’s Association

WSWC – Western States Water Council

U.S. Global Change Research Program

1717 Pennsylvania Avenue, NW • Suite 250 • Washington, DC 20006 USA

http://www.globalchange.gov

This report summarizes the science of climate change and the impacts 

of climate change on the United States, now and in the future. ©Timothy Hearsum/AgStock Images/Corbis ©Bill Miles/Mint Images/Corbis ©Steve Morgan/epa/Corbis

©Adam Hunger/Reuters/Corbis

U.S. National Climate Assessment