A Report of the

National Assessment 

Synthesis Team

US Global Change 

Research Program

CLIMATE CHANGE IMPACTS 

ON THE UNITED STATES

The Potential Consequences of Climate Variability and Change 

Overview

Humanity's influence on the global climate

will grow in the coming century. Increasingly,

there will be significant climate-related

changes that will affect each one of us. 

We must begin now to consider our

responses, as the actions taken today will

affect the quality of life for us and future

generations.
This report was produced by the National Assessment Synthesis Team,an advisory

committee chartered under the Federal Advisory Committee Act to help the US

Global Change Research Program fulfill its mandate under the Global Change

Research Act of 1990. The National Science and Technology Council has forwarded

this report to the President and Congress for their consideration as required by the

Global Change Research Act.

Administrative support for the US Global Change Research Program is provided by

the University Corporation for Atmospheric Research,which is sponsored by the

National Science Foundation. Any opinions, findings and conclusions or recommendations expressed in this publication are those of the authors and do not necessarily reflect the views of the National Science Foundation or the University

Corporation for Atmospheric Research.

This report was prepared at the request of the US government and is therefore in

the public domain.However,some materials used in the report are copyrighted (as

noted in the section on Graphics and Photography Sources) and permission was

granted to the US government for their publication in this report. In all cases ,

credit must be given for copyrighted materials. For commercial uses that include

copyrighted materials,permission for reproduction must be sought from the copyright holders.

The recommended citation of this report is as follows: 

National Assessment Synthesis Team,

Climate Change Impacts on the United States:

The Potential Consequences of Climate Variability and Change,

US Global Change Research Program,Washington DC,2000
What is this Assessment?

The National Assessment of the Potential Consequences of Climate

Variability and Change is a landmark in the major ongoing effort to

understand what climate change means for the US. Climate science

is developing rapidly and scientists are increasingly able to project

some changes at the regional scale, identifying regional vulnerabilities, and assessing potential regional impacts. Science increasingly

indicates that the Earth’s climate has changed in the past and continues to change, and that even greater climate change is very likely

in the 21st century. This Assessment has begun a national process

of research, analysis, and dialogue about the coming changes in climate, their impacts, and what Americans can do to adapt to an

uncertain and continuously changing climate. This Assessment is

built on a solid foundation of science conducted as part of the

United States Global Change Research Program (USGCRP).

What is this document and who is the NAST?

This document is the Assessment Overview, written by the National

Assessment Synthesis Team (NAST). The NAST is a committee of

experts drawn from governments, universities, industry, and nongovernmental organizations. It has been responsible for broad oversight of the Assessment, with the Federal agencies of the USGCRP.

This Overview is based on a longer, referenced "Foundation" report,

written by the NAST in cooperation with independent regional and

sector assessment teams. These two national-level, peer-reviewed

documents synthesize results from studies conducted by regional

and sector teams, and from the broader scientific literature. 

Why was this Assessment undertaken?

The Assessment was called for by a 1990 law, and has been conducted under the USGCRP in response to a request from the

President’s Science Advisor. The NAST developed the

Assessment’s plan, which was then approved by the National

Science and Technology Council, the cabinet-level body of agencies

responsible for scientific research, including global change research,

in the US government.

ABOUT THIS DOCUMENT
National Assessment 

Synthesis Team Members

Jerry M.Melillo,Co-chair

Ecosystems Center

Marine Biological Laboratory

Anthony C. Janetos,Co-chair

World Resources Institute

Thomas R.Karl,Co-chair

NOAA National Climatic Data Center

Robert Corell (from January 2000)

American Meteorological Society and

Harvard University 

Eric J. Barron

Pennsylvania State University

Virginia Burkett

USGS National Wetlands Research Center

Thomas F. Cecich

Glaxo Wellcome Inc.

Katharine Jacobs

Arizona Department of Water Resources

Linda Joyce

USDA Forest Service

Barbara Miller

World Bank

M.Granger Morgan

Carnegie Mellon University

Edward A. Parson (until January 2000)

Harvard University 

Richard G. Richels

EPRI 

David S.Schimel

National Center for Atmospheric Research

Independent Review Board of the

President’s Committee of Advisers on

Science and Technology (PCAST)

Peter Raven,Co-chair

Missouri Botanical Garden and PCAST

Mario Molina,Co-chair

MIT and PCAST

Burton Richter

Stanford University

Linda Fisher

Monsanto

Kathryn Fuller

World Wildlife Fund

John Gibbons

National Academy of Engineering

Marcia McNutt

Monterey Bay Aquarium Research Institute

Sally Ride

University of California San Diego and PCAST

William Schlesinger

Duke University

James Gustave Speth

Yale University

Robert White

University Corporation for Atmospheric

Research,and Washington Advisory Group

Additional Lead Authors

Lynne Carter (National 

Assessment Coordination Office)

David Easterling (NOAA National 

Climatic Data Center)

Benjamin Felzer (National Center for 

Atmospheric Research) 

John Field (University of Washington)

Paul Grabhorn (Grabhorn Studio)

Susan Joy Hassol (Aspen Global 

Change Institute)

Michael MacCracken (National 

Assessment Coordination Office)

Joel Smith (Stratus Consulting) 

Melissa Taylor (National Assessment 

Coordination Office) 

Thomas Wilbanks (Oak Ridge National 

Laboratory)

Other Contributors

This report is also based on the work of hundreds of individuals and organizations participating in regional and sector activities across

the country without whose input,support,

and expertise,it would not have been possible.In addition,many reviewers provided

comments on drafts of the report.Additional

credits and acknowledgements can be found

in the appendix.
TABLE OF CONTENTS

Summary 6 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Our Changing Climate 12 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Tools for Assessing Climate Change Impacts 14 . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Looking at America’s Climate 20 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Ecosystems in the Future 24 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Our Changing Nation 30 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Regions 38 . . . . . . . . . . . . . . . . . . . . . . . . . . .

Northeast 40 . . . . . . . . . . . . . . . . . . . . . . .

Southeast 46 . . . . . . . . . . . . . . . . . . . . . . .

Midwest 52 . . . . . . . . . . . . . . . . . . . . . . . .

Great Plains 58 . . . . . . . . . . . . . . . . . . . . . .

West 64 . . . . . . . . . . . . . . . . . . . . . . . . . . .

Pacific Northwest 68 . . . . . . . . . . . . . . . . .

Alaska 74 . . . . . . . . . . . . . . . . . . . . . . . . . .

Islands 80 . . . . . . . . . . . . . . . . . . . . . . . . . .

Native Peoples and Homelands 84 . . . . . . .

Sectors 88 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Agriculture 90 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Water 96 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Human Health 102 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Coastal Areas and Marine Resources 108 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Forests 114 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Conclusions 120 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Research Pathways 124 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Appendix I: Biographical Sketches of NAST Members 136 . . . . . . . . . . . . . . . . . . .

Appendix II: Acknowledgements 142 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Glossary 154 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

0

10%

20%

30%

40%

50%

60%

70%

80%

1900 1910 1920 1930 1940 1950 1960 1970 1980 1990

0

20

40

60

80

100

120

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% of US Population in Urban Areas Total Rural Population % of US Population in Rural Areas Total Urban Population

(Snow Cover,

 Vegetation, Reflectivity,

 Topography and Land Use)

Precipitation Evaporation

Ocean Model Layers

Stratus Clouds Transition from

Solid to VaporOutgoing Heat Energy

Snow Cover Runoff

Evaporative

and Heat Energy Exchanges

Incoming Solar Energy

Atmosphere Cumulus Clouds Cirrus Clouds

Sea

Ice

Vertical Overturning

Precipitation & Evaporation Heat & Salinity Ocean Exchange Currents, Temperature, and Salinity

Atmospheric GCM

Winds

 Soil Moisture Land Surface Processes

Ocean

 Bottom

 Topography Realistic Ocean GCM

Geography

Stratus

 Clouds
What is the purpose of this

Assessment?

The Assessment’s purpose is to synthesize, evaluate, and report on what we

presently know about the potential consequences of climate variability and

change for the US in the 21st century. It

has sought to identify key climatic vulnerabilities of particular regions and sectors, in the context of other changes in

the nation’s environment, resources, and

economy. It has also sought to identify

potential measures to adapt to climate

variability and change. Finally, because

present knowledge is limited, the

Assessment has sought to identify the

highest priority uncertainties about

which we must know more to understand

climate impacts, vulnerabilities, and our

ability to adapt. 

How did the process involve

both stakeholders and scientists in this Assessment?

This first National Assessment involved

both stakeholders and scientific experts.

Stakeholders included, for example, public and private decision-makers, resource

and environmental managers, and the

general public. The stakeholders from

different regions and sectors began the

Assessment by articulating their concerns in a series of workshops about climate change impacts in the context of

the other major issues they face. In the

workshops and subsequent consultations, stakeholders identified priority

regional and sector concerns, mobilized

specialized expertise, identified potential

adaptation options, and provided useful

information for decision-makers. The

Assessment also involved many scientific experts using advanced methods,

models, and results. Further, it has stimulated new scientific research in many

areas and identified priority needs for

further research.

What is the breadth of this

Assessment?

Although global change embraces many

interrelated issues, this first National

Assessment has examined only climate

change and variability, with a primary

focus on specific regions and sectors.

In some cases, regional and sector

analyses intersect and complement each

other. For example, the Forest sector

and the Pacific Northwest have both provided insights into climate impacts on

Northwest forests. 

The regions cover the nation. Impacts

outside the US are considered only

briefly, with particular emphasis on

potential linkages to the US. Sector

teams examined Water, Agriculture,

Human Health, Forests, and Coastal

Areas and Marine Resources. This first

Assessment could not attempt to be

comprehensive: the choice of these five

sectors reflected an expectation that

they were likely to be both important and

particularly informative, and that relevant

data and analytic tools were available –

not a conclusion that they are the only

important domains of climate impact.

Among the sectors considered, there

was a continuum in the amount of information available to support the

Assessment, with some being at far earlier stages of development. Future

assessments should consider other

potentially important issues, such as

Energy, Transportation, Urban Areas,

and Wildlife.

Each regional and sector team is publishing a separate report of its own

analyses, some of which are still continuing. The Overview and Foundation

reports consequently represent a snapshot of our understanding at the present

time. 

After identifying potential

impacts of climate change,

what kinds of societal

responses does this report

explore?

Responses to climate change can be of

two broad types. One type involves

adaptation measures to reduce the

harms and risks, and maximize the benefits and opportunities, of climate change,

whatever its cause. The other type

involves mitigation measures to reduce

human contributions to climate change.

After identifying potential impacts, this

Assessment sought to identify potential

adaptation measures for each region and

sector studied. While this was an important first step, it was not possible at this

stage to evaluate the practicality, effectiveness, or costs of the potential adaptation measures. Both mitigation and

adaptation measures are necessary elements of a coherent and integrated

response to climate change. Mitigation

measures were not included in this

Assessment, but are being assessed in

other bodies such as the United Nations

Intergovernmental Panel on Climate

Change (IPCC). 

ABOUT THE ASSESSMENT PROCESS

2
Does the fact that this report

excludes mitigation mean

that nothing can be done to

reduce climate change?

No. An integrated climate policy will

combine mitigation and adaptation

measures as appropriate. If future world

emissions of greenhouse gases are

lower than currently projected, for whatever reason, including intentional mitigation, then the rate of climate change, the

associated impacts, and the cost and difficulty of adapting will all be reduced. If

emissions are higher than expected, then

the rate of change, the impacts, and the

difficulty of adapting will be increased.

But no matter how aggressively emissions are reduced, the world will still

experience at least a century of climate

change. This will happen because the

elevated concentrations of greenhouse

gases already in the atmosphere will

remain for many decades, and because

the climate system responds to changes

in human inputs only very slowly.

Consequently, even if the world takes

mitigation measures, we must still adapt

to a changing climate. Similarly, even if

we take adaptation measures, future

emissions will have to be curbed to stabilize climate. Neither type of response

can completely supplant the other.

How are computer models

used in this Assessment?

State-of-the-science climate models have

been used to generate climate change

scenarios. Computer models of ecological systems, hydrological systems, and

various socioeconomic systems have

also been used in the Assessment, to

study responses of these systems to the

scenarios generated by climate models.

What additional tools,

besides models, were used

to evaluate potential climate

change impacts?

In addition to models, the Assessment

has used two other ways to think about

potential future climate. First, the

Assessment has used historical climate

records to evaluate sensitivities of

regions and sectors to climate variability

and extremes that have occurred in the

20th century. Looking at real historical

climate events, their impacts, and how

people have adapted, gives valuable

insights into potential future impacts that

complement those provided by model

projections. In addition, the Assessment

has used sensitivity analyses, which ask

how, and how much, the climate would

have to change to bring major impacts

on particular regions or sectors. For

example, how much would temperature

have to increase in the South before

agricultural crops such as soybeans

would be negatively affected? What

would be the result for forest productivity of continued increases in temperature

and leveling off of the CO2fertilization

effect?

Has this report been peer

reviewed?

This Overview and the underlying

Foundation document have been extensively reviewed. More than 300 scientific

and technical experts have provided

detailed comments on part or all of the

report in two separate technical

reviews. The report was reviewed at

each stage for technical accuracy by

the agencies of the US Global Change

Research Program. The public also provided hundreds of helpful suggestions

for clarification and modification during

a 60-day public comment period. A

panel of distinguished experts convened

by the President's Committee of

Advisors on Science and Technology

has provided broad oversight, and monitored the authors response to all

reviews.

3
4

What are scenarios and why are they used?

Scenarios are plausible alternative futures – each an example of what might happen under particular

assumptions. Scenarios are not specific predictions or forecasts. Rather, scenarios provide a starting

point for examining questions about an uncertain future and can help us visualize alternative futures in

concrete and human terms. The military and industry frequently use these powerful tools for future planning in high-stakes situations. Using scenarios helps to identify vulnerabilities and plan for contingencies.

Why are climate scenarios used in this Assessment and how were they

developed?

Because we cannot predict many aspects of our nation's future climate, we have used scenarios to help

explore US vulnerability to climate change. Results from state-of-the-science climate models and data

from historical observations have been used to generate a variety of such scenarios. Projections of

changes in climate from the Hadley Centre in the United Kingdom and the Canadian Centre for Climate

Modeling and Analysis served as the primary resources for this Assessment. Results were also drawn

from models developed at the National Center for Atmospheric Research, NOAA's Geophysical Fluid

Dynamics Laboratory, and NASA's Goddard Institute for Space Studies.

For some aspects of climate, virtually all models, as well as other lines of evidence, agree on the types of

changes to be expected. For example, all climate models suggest that the climate is going to get warmer,

the heat index is going to rise, and precipitation is more likely to come in heavy and extreme events. This

consistency lends confidence to these results.

For some other aspects of climate, however, the model results differ. For example, some models, including the Canadian model, project more extensive and frequent drought in the US, while others, including the

Hadley model, do not. The Canadian model suggests a drier Southeast in the 21st century while the

Hadley model suggests a wetter one. In such cases, the scenarios provide two plausible but different alternatives. Such differences can help identify areas in which the models need improvement. 

Many of the maps in this document are derived from the two primary climate model scenarios. In most

cases, there are three maps: one shows average conditions based on actual observations from 1961-1990;

the other two are generated by the Hadley and Canadian model scenarios and reflect the model's projection of change from those average conditions.

What assumptions about emissions are in these two climate scenarios?

Because future trends in fossil fuel use and other human activities are uncertain, the Intergovernmental

Panel on Climate Change (IPCC) has developed a set of scenarios for how the 21st century may evolve.

These scenarios consider a wide range of possibilities for changes in population, economic growth, technological development, improvements in energy efficiency, and the like. The two primary climate scenarios

used in this Assessment are based on one mid-range emissions scenario for the future that assumes no

major changes in policies to limit greenhouse gas emissions. Some other important assumptions in this

scenario are that by the year 2100:

• world population will nearly double to about 11 billion people;

• the global economy will continue to grow at about the average rate it has been growing, 

reaching more than ten times its present size;

• increased use of fossil fuels will triple CO2emissions and raise sulfur dioxide emissions, 

resulting in an atmospheric CO2concentration of just over 700 parts per million; and

• total energy produced each year from non-fossil sources such as wind, solar, biomass, hydroelectric, 

and nuclear will increase to more than ten times its current amount, providing more than 40% of the 

world’s energy, rather than the current 10%.

ABOUT SCENARIOS AND UNCERTAINTY

Many of the maps

in this document

are derived from

the two primary climate model scenarios. In most

cases, there are

three maps: one

shows average

conditions based

on actual observations from 1961-

1990; the other two

are generated by

the Hadley and

Canadian model

scenarios and

reflect the model's

projection of

change from those

average conditions.
5

How is the likelihood of various impacts expressed?

To integrate a wide variety of information and differentiate more likely from less likely outcomes, the NAST

developed a common language to express the team's considered judgement about the likelihood of results.

The NAST developed its collective judgements through discussion and consideration of the supporting

information. Historical data, model projections, published scientific literature, and other available information all provided input to these deliberations, except where specifically stated that the result comes from a

particular model scenario. In developing these judgements, there were often several lines of supporting

evidence (e.g., drawn from observed trends, analytic studies, model simulations). Many of these judgements were based on broad scientific consensus as stated by well-recognized authorities including the

IPCC and the National Research Council. In many cases, groups outside the NAST reviewed the use of

terms to provide input from a broader set of experts in a particular field.

Language Used to Express Considered Judgement

The Assessment’s Emissions Scenario Falls in the 

Middle of the other IPCC Emissions Scenarios

0% 50% 100%

“LITTLE CHANCE”

OR

“VERY UNLIKELY”

“UNLIKELY”

OR

“SOME CHANCE”

“POSSIBLE”

“LIKELY”

OR

“PROBABLE”

“VERY LIKELY”

OR

“VERY PROBABLE”

Common Language

Likelihood

The graph shows a comparison

of the projections of total carbon dioxide emissions (in billions of metric tons of carbon,

GtC) and the human-induced

warming influence due to all

the greenhouse gases and sulfate aerosols for the emissions

scenarios prepared by the IPCC

in 1992 and 2000. As is apparent from the graph, both the

emissions scenario and the

human-induced warming influence assumed in this

Assessment lie near the midrange of the set of IPCC scenarios. Further detail can be

found in the Climate chapter in

the Foundation report.

Both the emissions

scenario and the

human-induced

warming influence

assumed in this

Assessment lie

near the mid-range

of the set of IPCC

scenarios.

8

7

6

5

4

3

2

1

0

600 1000

Cumulative Carbon Emissions during 21st Century (GtC)

IPCC 2000 Emissions Scenarios for Year 2100

IPCC 1992 Emissions Scenarios for Year 2100

Hadley Centre Model Emissions Scenario for Year 2100

Canadian Centre Model Emissions Scenario for Year 2100

1600 2000 2600
6
7

SUMMARY

CLIMATE CHANGE AND OUR NATION

ong-term observations confirm that our climate is now changing at a rapid rate.

Over the 20th century, the average annual US temperature has risen by almost

1°F (0.6°C) and precipitation has increased nationally by 5 to 10%,mostly due to

increases in heavy downpours. These trends are most apparent over the past few

decades. The science indicates that the warming in the 21st century will be significantly larger than in the 20th century. Scenarios examined in this Assessment,

which assume no major interventions to reduce continued growth of world greenhouse gas emissions,indicate that temperatures in the US will rise by about 5-9°F

(3-5°C) on average in the next 100 years,which is more than the projected global

increase. This rise is very likely to be associated with more extreme precipitation

and faster evaporation of water, leading to greater frequency of both very wet and

very dry conditions.

This Assessment reveals a number of national-level impacts of climate variability and

change including impacts to natural ecosystems and water resources. Natural

ecosystems appear to be the most vulnerable to the harmful effects of climate

change,as there is often little that can be done to help them adapt to the projected

speed and amount of change. Some ecosystems that are already constrained by climate,such as alpine meadows in the Rocky Mountains,are likely to face extreme

stress,and disappear entirely in some places. It is likely that other more widespread

ecosystems will also be vulnerable to climate change. One of the climate scenarios

used in this Assessment suggests the potential for the forests of the Southeast to

break up into a mosaic of forests,savannas, and grasslands. Climate scenarios suggest likely changes in the species composition of the Northeast forests,including

the loss of sugar maples. Major alterations to natural ecosystems due to climate

change could possibly have negative consequences for our economy, which

depends in part on the sustained bounty of our nation’s lands,waters,and native

plant and animal communities.

A unique contribution of this first US Assessment is that it combines national-scale

analysis with an examination of the potential impacts of climate change on different

regions of the US. For example,sea-level rise will very likely cause further loss of

coastal wetlands (ecosystems that provide vital nurseries and habitats for many fish

species) and put coastal communities at greater risk of storm surges,especially in

the Southeast. Reduction in snowpack will very likely alter the timing and amount

of water supplies,potentially exacerbating water shortages and conflicts,particularly throughout the western US. The melting of glaciers in the high-elevation West

and in Alaska represents the loss or diminishment of unique national treasures of

the American landscape. Large increases in the heat index (which combines temperature and humidity) and increases in the frequency of heat waves are very likely.

These changes will,at minimum,increase discomfort,particularly in cities. It is very

probable that continued thawing of permafrost and melting of sea ice in Alaska will

further damage forests,buildings,roads,and coastlines,and harm subsistence livelihoods. In various parts of the nation,cold-weather recreation such as skiing will

very likely be reduced,and air conditioning usage will very likely increase.

L

The findings in this report

are based on a synthesis

of historical data, model

projections, published scientific research, and other

available information,

except where specifically

noted.
8

Highly managed ecosystems appear more robust, and some potential benefits have been identified. Crop and forest productivity is likely to increase in

some areas for the next few decades due to increased carbon dioxide in the

atmosphere and an extended growing season. It is possible that some US

food exports could increase,depending on impacts in other food-growing

regions around the world. It is also possible that a rise in crop production

in fertile areas could cause prices to fall,benefiting consumers. Other benefits that are possible include extended seasons for construction and warm

weather recreation,reduced heating requirements,and reduced cold-weather mortality.

Climate variability and change will interact with other environmental stresses and socioeconomic changes. Air and water pollution,habitat fragmentation, wetland loss,coastal erosion,and reductions in fisheries are likely to be

compounded by climate-related stresses. An aging populace nationally, and

rapidly growing populations in cities,coastal areas,and across the South and

West are social factors that interact with and alter sensitivity to climate variability and change.

There are also very likely to be unanticipated impacts of climate change during the next century. Such "surprises" may stem from unforeseen changes in

the physical climate system,such as major alterations in ocean circulation,

cloud distribution,or storms;and unpredicted biological consequences of

these physical climate changes,such as massive dislocations of species or

pest outbreaks. In addition,unexpected social or economic change,including major shifts in wealth,technology, or political priorities,could affect our

ability to respond to climate change.

Greenhouse gas emissions lower than those assumed in this

Assessment would result in reduced impacts. The signatory

nations of the Framework Convention on Climate Change

are negotiating the path they will ultimately take. Even

with such reductions,however, the planet and the

nation are certain to experience more than a century

of climate change,due to the long lifetimes of

greenhouse gases already in the atmosphere and

the momentum of the climate system. Adapting

to a changed climate is consequently a necessary component of our response strategy.

The warming in the 21st century

will be significantly larger than in

the 20th century.

Natural ecosystems, which are

our life support system in many

important ways, appear to be the

most vulnerable to the harmful

effects of climate change...

Major alterations to natural

ecosystems due to climate

change could possibly have

negative consequences for our

economy, which depends in part

on the sustained bounty of our

nation’s lands, waters, and

native plant and animal communities. 
9

SUMMARY

CLIMATE CHANGE AND OUR NATION

daptation measures can,in many cases,reduce the magnitude of harmful

impacts,or take advantage of beneficial impacts. For example,in agriculture,

many farmers will probably be able to alter cropping and management practices.

Roads,bridges,buildings,and other long-lived infrastructure can be designed taking

projected climate change into account. Adaptations,however, can involve trade-offs,

and do involve costs. For example,the benefits of building sea walls to prevent sealevel rise from disrupting human coastal communities will need to be weighed

against the economic and ecological costs of seawall construction. The ecological

costs could be high as seawalls prevent the inland shifting of coastal wetlands in

response to sea-level rise, resulting in the loss of vital fish and bird habitat and other

wetland functions,such as protecting shorelines from damage due to storm surges.

Protecting against any increased risk of water-borne and insect-borne diseases will

require diligent maintenance of our public health system. Many adaptations,

notably those that seek to reduce other environmental stresses such as pollution

and habitat fragmentation,will have beneficial effects beyond those related to climate change.

Vulnerability in the US is linked to the fates of other nations,and we cannot evaluate national consequences due to climate variability and change without also considering the consequences of changes elswhere in the world. The US is linked to

other nations in many ways,and both our vulnerabilities and our potential responses will likely depend in part on impacts and responses in other nations. For example,conflicts or mass migrations resulting from resource limits,health,and environmental stresses in more vulnerable nations could possibly pose challenges for global

security and US policy. Effects of climate variability and change on US agriculture

will depend critically on changes in agricultural productivity elsewhere,which can

shift international patterns of food supply and demand. Climate-induced changes in

water resources available for power generation,transportation,cities,and agriculture are likely to raise potentially delicate diplomatic issues with both Canada and

Mexico.

This Assessment has identified many remaining uncertainties that limit our ability to

fully understand the spectrum of potential consequences of climate change for our

nation. To address these uncertainties,additional research is needed to improve

understanding of ecological and social processes that are sensitive to climate,application of climate scenarios and reconstructions of past climates to impacts studies,

and assessment strategies and methods. Results from these research efforts will

inform future assessments that will continue the process of building our understanding of humanity's impacts on climate,and climate's impacts on us.

The magnitude of climate

change impacts depends on

time period and geographic

scale. Short-term impacts differ from long-term impacts,

and regional and local level

impacts are much more pronounced than those at the

national level.

For the nation as a whole,

direct economic impacts are

likely to be modest, while in

some places, economic losses or gains are likely to be

large. For example, while

crop yields are likely to

increase at the national scale

over the next few decades,

large increases or decreases

in yields of specific crops in

particular places are likely.

Through time, climate change

will possibly affect the same

resource in opposite ways.

For example, forest productivity is likely to increase in the

short term, while over the

longer term, changes in

processes such as fire,

insects, drought, and disease

will possibly decrease forest

productivity.

A
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KEY FINDINGS

1. Increased warming 

Assuming continued growth in world greenhouse gas emissions, the primary climate models used in this

Assessment project that temperatures in the US will rise 5-9oF (3-5oC) on average in the next 100 years.

A wider range of outcomes is possible.

2. Differing regional impacts 

Climate change will vary widely across the US. Temperature increases will vary somewhat from one

region to the next. Heavy and extreme precipitation events are likely to become more frequent, yet some

regions will get drier. The potential impacts of climate change will also vary widely across the nation.

3. Vulnerable ecosystems 

Many ecosystems are highly vulnerable to the projected rate and magnitude of climate change. A few,

such as alpine meadows in the Rocky Mountains and some barrier islands, are likely to disappear entirely

in some areas. Others, such as forests of the Southeast, are likely to experience major species shifts or

break up into a mosaic of grasslands, woodlands, and forests. The goods and services lost through the

disappearance or fragmentation of certain ecosystems are likely to be costly or impossible to replace.

4. Widespread water concerns 

Water is an issue in every region, but the nature of the vulnerabilities varies. Drought is an important concern in every region. Floods and water quality are concerns in many regions. Snowpack changes are

especially important in the West, Pacific Northwest, and Alaska.

5. Secure food supply

At the national level, the agriculture sector is likely to be able to adapt to climate change. Overall, US

crop productivity is very likely to increase over the next few decades, but the gains will not be uniform

across the nation. Falling prices and competitive pressures are very likely to stress some farmers, while

benefiting consumers.

6. Near-term increase in forest growth

Forest productivity is likely to increase over the next several decades in some areas as trees respond to

higher carbon dioxide levels. Over the longer term, changes in larger-scale processes such as fire,

insects, droughts, and disease will possibly decrease forest productivity. In addition, climate change is

likely to cause long-term shifts in forest species, such as sugar maples moving north out of the US.

7. Increased damage in coastal and permafrost areas

Climate change and the resulting rise in sea level are likely to exacerbate threats to buildings, roads,

powerlines, and other infrastructure in climatically sensitive places. For example, infrastructure damage is

related to permafrost melting in Alaska, and to sea-level rise and storm surge in low-lying coastal areas.

8. Adaptation determines health outcomes 

A range of negative health impacts is possible from climate change, but adaptation is likely to help protect

much of the US population. Maintaining our nation's public health and community infrastructure, from

water treatment systems to emergency shelters, will be important for minimizing the impacts of waterborne diseases, heat stress, air pollution, extreme weather events, and diseases transmitted by insects,

ticks, and rodents.

9. Other stresses magnified by climate change

Climate change will very likely magnify the cumulative impacts of other stresses, such as air and water

pollution and habitat destruction due to human development patterns. For some systems, such as coral

reefs, the combined effects of climate change and other stresses are very likely to exceed a critical

threshold, bringing large, possibly irreversible impacts.

10. Uncertainties remain and surprises are expected 

Significant uncertainties remain in the science underlying regional climate changes and their impacts.

Further research would improve understanding and our ability to project societal and ecosystem impacts,

and provide the public with additional useful information about options for adaptation. However, it is likely

that some aspects and impacts of climate change will be totally unanticipated as complex systems

respond to ongoing climate change in unforeseeable ways.

Model Projected

US Temperatures
IMPACTS OF CLIMATE CHANGE

It is very likely that the US will get substantially warmer. Temperatures are

projected to rise more rapidly in the next one hundred years than in the last

10,000 years. It is also very likely that there will be more precipitation

overall, with more of it coming in heavy downpours. In spite of this, some

areas are likely to get drier as increased evaporation due to higher temperatures outpaces increased precipitation. Droughts and flash floods are likely to become more frequent and intense.

PERMAFROST AREAS

It is very probable that rising temperatures will cause

further permafrost

thawing,

damaging

roads,

buildings,

and

forests in

Alaska.

WATER SUPPLY

Reduced summer runoff,

increased winter runoff, and

increased demands are likely to

compound current stresses on

water supplies and flood management,especially in the western US.

ISLANDS

Sea-level rise and storm

surges will very likely threaten public health

and safety

and possibly reduce

the availability of

fresh water.

CORAL REEFS

Increased CO2and ocean

temperatures,especially combined with

other stresses,will possibly exacerbate

coral reef

bleaching

and die-off.

FRESHWATER ECOSYSTEMS

Increases in water temperature and

changes in seasonal patterns of runoff will

very likely disturb

fish habitat and

affect recreational uses of

lakes,streams,

and wetlands.

SPECIES DIVERSITY

While it is possible that some

species will adapt to changes in

climate by shifting their ranges,

human and geographic barriers,

and the presence of invasive

non-native species

will limit the

degree of adaptation that can

occur. Losses in

local biodiversity

are likely to accelerate towards the

end of the 21st century.

FORESTRY

Timber inventories are likely to

increase over the 21st centur y.

Hardwood productivity is likely to increase

more than

softwood

productivity

in some

regions,

including the

Southeast.

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