Scientific Assessment of the Effects of 

Global Change on the United States 

A Report of the Committee on Environment and Natural Resources 

National Science and Technology Council 

May 2008 
Scientific Assessment of the Effects of Global Change on the United States ii

Committee on the Environment and Natural Resources 

National Science and Technology Council 

George Gray 

Environmental Protection Agency 

Conrad Lautenbacher 

National Oceanic and Atmospheric 

Administration 

Sharon Hays 

Office of Science and Technology Policy 

Michael H. Freilich 

National Aeronautics and Space 

Administration 

James Connaughton 

Council on Environmental Quality 

Robert E. Foster 

Department of Defense 

Mark Myers 

U.S. Geological Survey 

Leonard P. Hirsch 

Smithsonian Institution 

Bridgette K. Ellis 

Tennessee Valley Authority 

Gale Buchanan

U.S. Department of Agriculture 

Linda Lawson

Department of Transportation 

Kathie L. Olsen 

National Science Foundation 

Samuel H. Wilson 

Department of Health & Human Services 

James Bates 

Office of Management and Budget 

Bruce Davis 

Department of Homeland Security 

Claudia McMurray 

Department of State 

Samuel P. Williamson 

Office of the Federal Coordinator for 

Meteorology 

Raymond L. Orbach 

Department of Energy 

Joel Kupersmith 

Veterans Health Administration 
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Scientific Assessment of the Effects of Global Change on the United States iii
Description of Expert Review 

This scientific assessment was reviewed in draft form by individuals chosen for their breadth of 

perspective and technical expertise. The purpose of the independent review was to provide 

candid and critical comments that helped ensure the scientific soundness of the published report. 

The review comments remain confidential to protect the integrity of the deliberative process. The 

following individuals are thanked for their review of this report: 

Eric Barron, University of Texas, Austin 

Sally Benson, Stanford University 

Leon Clarke, Pacific Northwest National Laboratory 

Randall Crane, University of California, Los Angeles 

Andrew Dessler, Texas A&M University 

William Emanuel, University of Virginia, Charlottesville 

Marvin Geller, Stony Brook University 

Jennifer Hayes, U.S. Forest Service 

Gabrielle Hegerl, University of Edinburgh 

Jeremy Hess, Emory University 

Katherine Jacobs, Arizona Water Institute 

Ashby Johnson, Houston-Galveston Area Council 

Jay Lawrimore, National Oceanic and Atmospheric Administration 

Thomas Lovejoy, Heinz Center 

Robert Webb, National Oceanic and Atmospheric Administration 

Warren Wiscombe, National Aeronautics and Space Administration 

Responsibility for the final content of this report rests entirely with the Committee on 

Environment and Natural Resources. 

This report was prepared in accordance with Section 515 of the Treasury and General 

Government Appropriations Act for Fiscal Year 2001 (Public Law 106-554) and the Information 

Quality Act guidelines issued by the Department of Commerce and NOAA pursuant to Section 

515. For purposes of compliance with Section 515, this report is an “interpreted product” as that 

term is used in NOAA guidelines and is classified as “highly influential.” This document does 

not express any regulatory policies of the United States or any of its agencies or provide 

recommendations for regulatory action. 

Scientific Assessment of the Effects of Global Change on the United States iv
Scientific Assessment of the Effects of Global Change on the United States v

Table of Contents 

COMMITTEE ON THE ENVIRONMENT AND NATURAL RESOURCES NATIONAL 

SCIENCE AND TECHNOLOGY COUNCIL II

TABLE OF CONTENTS V

SECTION I. EXECUTIVE SUMMARY 1

I.1 Introduction 1

I.2 Causes of Climate Change 1

I.3 Trends and Projections of Physical Changes 3

I.3.a Temperature 3

I.3.b Precipitation, runoff, and drought 4

I.3.c Ice and snow 5

I.3.d Sea level 6

I.3.e Atlantic hurricanes 7

I.3.f Abrupt climate change 7

I.3.g Stratospheric ozone 8

I.4 Effects of Global Change on the Natural Environment and Human Systems 8

I.4.a Biological diversity, ecosystem composition, and the natural environment 8

I.4.b Agriculture and food production 11

I.4.c Water resources 12

I.4.d Social systems and settlements 13

I.4.e Human health 14

I.4.f Energy production, use, and distribution 16

I.4.g Transportation 17

SECTION II: INTRODUCTION 18

II.1. Objectives 18

II.2. Relationship to Other Assessments and Primary Scientific Literature 19

II.3. Characterization of Uncertainty 20

II.3.a Description of confidence 20

II.3.b Description of likelihood 20

II.4. Report Structure 21

SECTION III: EVALUATION OF OVERALL PROGRESS BY CCSP 22

III.1 Program Inputs 24

III.1.a Surface-based observing networks 24

III.1.b Satellite observations 25
III.1.c Field campaigns 27

III.1.d Data management 28

III.1.e Modeling 30

III.2. Program Outputs 31

III.2.a Goal 1: 31

III.2.b Goal 2: 33

III.2.c Goal 3: 35

III.2.d Goal 4: 38

III.2.e Goal 5: 41

SECTION IV: TRENDS AND PROJECTIONS OF GLOBAL ENVIRONMENTAL 

CHANGE 46

IV.1. Radiative Forcing and Observed Climate Change 46

IV.1.a Radiative forcing due to greenhouse gases and other factors 46

Carbon dioxide, methane, and nitrous oxide 47

Fluorocarbons, chlorocarbons, and sulfur hexafluoride 48

Ozone 48

Water vapor 49

Aerosols 49

Land cover changes, linear contrails, and solar irradiance 50

IV.1.b Observed temperature changes 50

Temperature trends 50

Temperature patterns 54

Temperature extremes 56

Surface temperature over the last 2,000 years 57

IV.1.c Observed precipitation changes 59

Precipitation trends and patterns 59

Precipitation extremes and storms 62

IV.1.d Observed changes in other physical systems 63

Cryosphere (snow and ice) 64

Hydrosphere 66

Sea level 67

IV.2. Attribution of Observed Climate Change to Human Activities at the Global and Continental Scale 70

IV.2.a Attribution of observed climate change to anthropogenic emissions and land use / land cover 70

Temperature 71

Additional climate variables 74

IV.2.b Attribution of observed changes in physical and biological systems 75

IV.3. Projected Future Greenhouse Gas Concentrations and Climate Change 77

IV.3.a Global emission scenarios and associated changes in concentrations and radiative forcing 77

Greenhouse gas emissions 77

Future concentration and radiative forcing changes 83

IV.3.b Projected changes in physical components of the climate system 85

Temperature projections 86

Precipitation projections 90

Sea level rise projections 91

Cryosphere (snow and ice) projections 92

Projections of extreme events 93

Abrupt climate change projections 95

IV.3.c Land cover change as an environmental driver 97

IV.3.d Effects on and from stratospheric ozone 98

Effects of elevated greenhouse gas concentrations on stratospheric ozone 99

Scientific Assessment of the Effects of Global Change on the United States vi
Effects of stratospheric ozone changes on climate 100

SECTION V: ANALYSIS OF THE EFFECTS OF GLOBAL CHANGE ON THE 

NATURAL ENVIRONMENT AND HUMAN SYSTEMS 102

V.1 Biological Diversity, Ecosystem Composition, and the Natural Environment 103

V.1.a Ecosystem distribution and phenology 104

Ecosystem distribution 104

Effects on phenology 105

Projected impacts 108

V.1.b Ecosystem services 112

Projected impacts 112

V.1.c Extreme events 113

Projected impacts 115

V.1.d Sea level rise 115

Projected impacts 117

V.1.e Vulnerable ecosystems 118

Coastal and marine ecosystems 118

Arctic sea ice ecosystem 119

Temperate montane ecosystems 120

V.1.f Adaptation options 121

V.2 Agriculture and Food Production 122

V.2.a Crop yields and productivity 123

Increasing temperatures 124

Increasing CO2 125

V.2.b Water use and irrigation requirements 125

V.2.c Climate variability and extreme events 126

V.2.d Pests and weeds 128

V.2.e Projections for agriculture production 128

V.2.f Livestock 130

V.2.g Freshwater and marine fisheries 132

V.3 Land Resources 134

V.3.a Forests 135

V.3.b Grasslands and shrublands 136

V.3.c Productivity 137

Forest productivity 137

Grassland and shrubland productivity 138

V.3.d Disturbance 139

Wildland fires 139

Insects and pathogens 141

Storm events 143

V.3.e Species composition 143

Forest composition 143

Grassland and shrubland composition 144

V.4 Water Resources 145

V.4.a Water supply and precipitation 146

Surface water and precipitation 146

Projected impacts 148

V.4.b Groundwater 149

V.4.c Water quality 149

V.4.d Extreme events 152

V.4.e Implications for water users 154

Scientific Assessment of the Effects of Global Change on the United States vii
V.5 Social Systems and Settlements 156

V.5.a U.S. population trends 157

V.5.b Vulnerable communities 157

V.5.c Sea level rise 160

Projected impacts 162

V.5.d Extreme events 164

Floods and storms 164

Wildland fire 166

Drought 167

V.6 Human Health 167

V.6.a Temperature effects 170

Increased heat exposure 170

Reduced cold exposure 172

V.6.b Extreme events 173

Floods and storms 173

Wildfires 174

Droughts 175

V.6.c Climate-sensitive infectious diseases 175

Water quality 176

V.6.d Aeroallergens 177

V.6.e Air quality 178

Tropospheric ozone 178

Particulate matter 181

V.7 Energy Production, Use, and Distribution 183

V.7.a Energy use 184

Heating 184

Cooling 185

Combined heating and cooling 186

V.7.b Energy production and distribution 187

Fossil and nuclear energy 190

Renewable energy 191

Extreme events 193

V.8 Transportation 194

V.8.a Study region 195

V.8.b Transportation infrastructure 196

V.8.c Trends in climate and coastal change in the Gulf Coast 197

V.8.d Impacts of changing climate on Gulf Coast transportation 198

V.8.e Impacts by transportation mode 200

Roads 200

Freight and passenger rail 203

Marine 205

Aviation 207

Pipelines 208

REFERENCES 211

Scientific Assessment of the Effects of Global Change on the United States viii
I. Executive Summary 1

 

Section I. Executive Summary 

I.1 Introduction 

The climate is changing, and these changes are affecting the world around us. In order to deal 

with the changes that are taking place now and to prepare for those that are likely to happen in 

the future, decisionmakers need information about global change and its effects on the Nation 

and the world we live in. 

This national scientific assessment integrates, evaluates, and interprets the findings of the U.S. 

Climate Change Science Program (CCSP) and draws from and synthesizes findings from 

previous assessments of the science, including reports and products by the Intergovernmental 

Panel on Climate Change (IPCC). It analyzes current trends in global change, both natural and 

human-induced, and it projects major trends for the future. It analyzes the effects of these 

changes on the natural environment, agriculture, water resources, social systems, energy 

production and use, transportation, and human health. It is intended to help inform discussion of 

the relevant issues by decisionmakers, stakeholders, and the public. As such, this report 

addresses the requirements for assessment in the Global Change Research Act of 1990.1

This assessment addresses not only climate change, but also other change in the global 

environment—including water resources, oceans, atmospheric chemistry, land productivity, and 

ecological systems—that may alter the capacity of Earth to sustain life. This broader set of 

changes is referred to as ‘global change,’ as defined in the Global Change Research Act. 

Over the past several years, our understanding of climate variability and change and our ability 

to estimate their future effects has improved significantly. The conclusions in this assessment 

build on the vast body of observations, modeling, decision support, and other types of activities 

conducted under the auspices of CCSP and from previous assessments of the science, including 

reports and products by the IPCC, CCSP, and others. This assessment and the underlying 

assessments have been subjected to and improved through rigorous peer reviews. 

I.2 Causes of Climate Change 

Our understanding of climate change continues to grow, enabling scientists to draw increasingly 

certain conclusions about its causes and impacts. For example, in their most recent assessment 

of climate change science, the IPCC concluded that it is unequivocal that the average 

temperature of Earth’s surface has warmed recently and it is very likely (greater than 90% 

probability)2 that most of this global warming is due to increased concentrations of humangenerated greenhouse gases. Several lines of evidence, including those outlined in the following 

sections, point to a strong human influence on climate. Although these individual lines of 

evidence vary in their degrees of certainty, when considered together they provide a compelling 

and scientifically sound explanation of the changes to Earth’s climate—including changes in 

1

 15 USC Chapter 56A 

2

 Definitions for terms used in statements of confidence and likelihood can be found in Section II.3: Characterization 

of Uncertainty. 
surface temperature, ice extent, and sea level rise—observed at global and continental scales 

over the past few decades. [IV.1 and IV.2]3

Several different types of gases in the atmosphere warm the planet by trapping energy that 

would otherwise be emitted to space. These ‘greenhouse gases’ include water vapor, carbon 

dioxide, methane, ozone, nitrous oxide, and several fluorine- and chlorine-containing gases. Of 

the greenhouse gases that are increasing in atmospheric concentration as a direct result of 

human activities, carbon dioxide is contributing most to the recent warming. The globally 

averaged concentration of carbon dioxide in the atmosphere has increased from about 280 

parts per million (ppm) in the 18th century to 383 ppm in 2007. Emissions of carbon dioxide 

from fossil fuel use and from the effects of land use change are the primary sources of this 

increase. The current atmospheric concentration of carbon dioxide greatly exceeds the natural 

range of the last 650,000 years (180 to 300 ppm) as determined from ice cores. Indeed, the 

atmospheric levels of all major greenhouse gases have also increased significantly in the past 

century. [IV.1.a] 

An increasing body of scientific research supports the conclusion that, while greenhouse gases 

are but one of many factors that affect climate, they are very likely the single largest cause of 

the recent warming. Other factors that affect climate may contribute either a warming or 

cooling influence. For example, some types of tiny particles in the air introduce a cooling 

influence. These particles, known as aerosols, include sulfate, organic carbon, nitrate, and 

mineral dust. Black carbon aerosols introduce a warming influence. Deposition of black carbon 

on snow and ice also contributes a warming influence on the climate by decreasing surface 

reflectivity that would otherwise deflect more solar energy back into space. Aerosols have 

been observed to increase cloudiness and cloud reflectivity, both of which have a cooling 

influence. Other changes to surface reflectivity, as well as variations in solar irradiance, can 

have either warming or cooling influences. [IV.1.a] 

Studies that rigorously quantify the effect of different external influences on observed changes 

(attribution studies) conclude that most of the recent global warming is very likely due to humangenerated increases in greenhouse gas concentrations. A large number of climate model 

simulations show that natural factors alone cannot explain the observed warming in the second 

half of the 20th century of Earth’s land masses and oceans, or that of the North American 

continent. On the other hand, simulations that include human factors are able to reproduce 

important large-scale features of the recent changes. Several studies indicate that natural factors 

played an important role in the warming of the first half of the 20th century. Attribution studies 

show that it is likely (greater than 66% probability) that there has been a substantial human 

contribution to the surface temperature increase in North America. 

According to the IPCC, model simulations of regional (sub-continental-scale) climate have 

improved, and a limited number of regional climate features are consistently captured in multiple 

climate model projections. Regional changes in the United States that are consistently projected 

in climate models are discussed throughout the report, and include some significant regional 

changes in temperature and precipitation. There are limits to how effectively climate change and 

 

3

 The report section(s) denoted in square brackets are where information supporting a particular statement can be 

found. 

I. Executive Summary 2
its impacts can be projected on small scales, such as at the scale of a single city or county, a 

single state, or even a group of states, especially over the next decade or two. Climate changes on 

small scales are difficult to discern, in part, because climate variability averaged over small 

regions is greater than the variability averaged over large regions. For these and other reasons, 

attribution of the drivers of long-term temperature changes on time scales of less than 50 years 

and at regional scales (e.g., county, state, or multiple states, as opposed to continental), with 

limited exceptions, has not yet been established. [IV.2.] 

In addition to average temperatures, recent work shows that human activities have also likely

influenced extremes in temperature. Many indicators of climate extremes—including the annual 

numbers of frost days, warm and cold days, and warm and cold nights—show changes that are 

consistent with warming. For example, there is evidence that human-induced warming may4

have substantially increased the risk of extremely warm summer conditions in some regions. 

Discernible human influences extend to additional aspects of climate, including the recent 

decreases in Arctic sea ice extent, patterns of sea level pressure and winds, and the global-scale 

pattern of land precipitation. [IV.2.a] 

According to CCSP Synthesis and Assessment Product (SAP) 3.3, it is very likely that the 

human-induced increase in greenhouse gases has contributed to the increase in sea surface 

temperatures in the hurricane formation regions. There is a strong statistical connection between 

tropical Atlantic sea surface temperatures and Atlantic hurricane activity as measured by an 

index that accounts for storm intensity, frequency, and duration on decadal timescales over the 

past 50 years. This evidence suggests a substantial human contribution to recent hurricane 

activity. However, a confident assessment of human influence on hurricanes will require further 

studies using models and observations. [IV.2.a] 

I.3 Trends and Projections of Physical Changes 

This section describes observed historical trends and model-based projections of future changes 

to physical and chemical components of the environment. 

 

I.3.a Temperature 

The climate system is warming, as is now evident from direct observations of increases in global 

average air and ocean temperatures and inferences from widespread melting of snow and ice, 

rising global average sea level, and other indicators. As determined by the IPCC, the globally 

averaged temperature rise over the last 100 years (1906–2005) is 1.33 ± 0.32 °F when estimated 

by a linear trend. The rate of global warming over the last 50 years (0.23 ± 0.05 °F per decade) is 

almost double that for the past 100 years (0.13 ± 0.04 °F per decade). In addition, as assessed by 

the IPCC, it is very likely that average Northern Hemisphere temperatures during the second half 

of the 20th century were warmer than any other 50-year period in the last 500 years, and it is 

likely that this 50-year period was the warmest Northern Hemisphere period in the last 1,300 

years. Land regions have warmed faster than the oceans—about double the ocean rate after 1979 

 

4

 Non-italicized terms and phrases such as “may,” “are expected,” and “are projected” are used to indicate the 

possibility of the occurrence of an event or phenomenon without assignment of a formal level of likelihood. 

I. Executive Summary 3
(more than 0.49 °F per decade). The greatest warming is at high northern latitudes during spring 

and winter. [IV.1.b] 

Like global average temperatures, U.S. average temperatures also increased during the 20th and 

into the 21st century, according to federal statistics. The last decade is the warmest in more than 

a century of direct observations in the United States. Average temperatures for the contiguous 

United States have risen at a rate near 0.6 °F per decade in the past few decades. But warming is 

not distributed evenly across space or time. [IV.1.b] 

The number of U.S. heat waves has been increasing since 1950, though it should be noted that 

the heat waves associated with the severe drought of the 1930s remain the most severe in the 

U.S. historical record. There have also been fewer unusually cold days during the last few 

decades, and the last 10 years have seen fewer severe cold waves than for any other 10-year 

period in the historical record, which dates back to 1895. [IV.1.b]

The IPCC concluded that continued greenhouse gas emissions at or above current rates are 

expected to cause further warming and to induce many changes during the 21st century that will 

very likely be larger than those of the 20th century. For the next 20 years, a globally averaged 

warming of about 0.3 to 0.4 °F per decade is projected for a range of scenarios of greenhouse gas 

emissions5

. Through about 2030, there is little difference in the warming rate projected using a 

variety of emissions scenarios. Possible future variations in natural factors, such as a large 

volcanic eruption, could introduce variations to this long-term warming projection. Even if 

atmospheric greenhouse gas levels remained constant, the globally averaged warming would 

continue to be nearly 0.2 °F per decade during the next two decades because of the time it takes 

for the climate system, particularly the oceans, to reach equilibrium. [IV.3.b] 

By the mid-21st century, the effect of the choice of emission scenario becomes more important 

in terms of the magnitude of the projected warming, with model projections of increases in 

globally averaged temperature of approximately 2 to 3 °F for several of the IPCC scenarios. 

According to the IPCC, all of North America is very likely to warm during this century, and to 

warm more than the global average increase in most areas. Nearly all the models assessed by the 

IPCC project that the average warming in the United States will exceed 3.6 °F, with 5 out of 21 

models projecting that average warming will exceed 7.2 °F by the end of the century. The largest 

warming in the United States is projected to occur in winter over northern parts of Alaska. In 

regions near the coasts, the projected warming during the 21st century is expected to be less than 

the national average. According to CCSP SAP 3.3, abnormally hot days and nights and heat 

waves are very likely to become more frequent, and cold days and cold nights are very likely to 

become much less frequent over North America. [IV.3.b]

I.3.b Precipitation, runoff, and drought 

 

5

 A scenario is a coherent, internally consistent and plausible description of a possible future state of the world. It is 

not a forecast; rather, each scenario is one alternative image of how the future can unfold. The scenarios cited in this 

document do not include greenhouse gas emission reduction initiatives. 

I. Executive Summary 4
According to historical records, the total annual precipitation over the contiguous United States 

has increased at an average rate of 6% per century from 1901 to 2005, with significant variability 

over time and by region. The greatest increases in precipitation were in the northern Midwest and 

the South. The contiguous United States has had statistically significant increases in heavy 

precipitation, primarily during the last three decades of the 20th century and over the eastern 

parts of the country. [IV.1.c] 

In keeping with the overall precipitation trends, most of the United States has experienced 

decreases in drought severity and duration during the second half of the 20th century. However, 

a severe drought has affected the southwestern United States from 1999 through 2007. The 

southeastern United States has also recently experienced severe drought. On a longer time scale, 

reconstructions of droughts using tree rings and geological evidence show that much more severe 

droughts have occurred over the last 2,000 years than those that have been observed in the 

instrumental record, notably, the Dust Bowl drought of the 1930s and extensive drought in the 

1950s. [IV.1.d] 

Streamflow in the eastern United States has increased 25% in the last 60 years. However, it has 

decreased by about 2% per decade in the central Rocky Mountain region over the past century. 

The annual peak of streamflow in snowmelt-dominated western mountains is now generally 

occurring at least a week earlier than in the middle of the 20th century. Winter stream flow is 

increasing in basins with seasonal snow cover. The fraction of annual precipitation falling as rain 

(rather than snow) increased in the last half century. [IV.1.d] 

Most climate models project an increase in winter precipitation in the northern tier of states and a 

decrease in portions of the Southwest during the 21st century. Summer precipitation is projected 

to decrease in the Northwest of the contiguous United States and increase in Alaska; it is 

uncertain whether summer precipitation will increase or decrease over large portions of the 

interior United States. In northern regions of North America, the magnitude of precipitation 

increase is projected to be greatest in autumn, whereas winter precipitation is projected to 

increase by the largest fraction relative to its present amount. A majority of climate models 

generally show winter increases in northern regions and summer decreases in western and 

southern regions. [IV.3.b] 

In the 21st century, precipitation over North America is projected to be less frequent but more 

intense. This increase in storminess is projected to be accompanied by greater extreme wave 

heights along the coasts. [IV.3.b] 

I.3.c Ice and snow 

Observations indicate that annual average Arctic sea ice extent decreased by 2.7 ± 0.6% per 

decade between 1978 and 2005. Larger decreases were observed in summer (7.4 ± 2.4% per 

decade). In 2007, Arctic sea ice extent was approximately 23% below the previous all-time 

minimum observed in 2005. In addition to the decreased extent, the average sea ice thickness in 

the central Arctic very likely decreased by up to approximately 3 feet from 1987 to 1997, 

according to the Arctic Climate Impacts Assessment and the IPCC. Along the Alaskan coast, 

reductions in the thickness and spatial extent of sea ice are creating more open water, allowing 

I. Executive Summary 5
winds to generate stronger waves, which increase shoreline erosion. Antarctic sea ice extent 

shows no statistically significant average trend. [IV.1.d] 

The snow-covered area of North America increased in the November to January season from 

1915 to 2004 due to increases in precipitation. However, spring snow cover in mountainous 

regions of the western United States generally decreased during the latter half of the 20th 

century. The IPCC determined that this latter trend is very likely due to long-term warming, with 

potential influence from decadal-scale natural variability. In Alaska, where the warming has been 

particularly pronounced, the permafrost base has been thawing at a rate of up to 1.6 inches per 

year since 1992. [IV.1.d] 

The date that rivers and lakes freeze over has become later (average rate of 5.8 ± 1.6 days per 

century) and the ice breakup date has happened earlier (average rate of 6.5 ± 1.2 days per 

century), according to an analysis of 150 years of available data for the Northern Hemisphere. In 

addition to these changes in seasonal ice and snow, glaciers have been losing mass in the 

northwestern United States and Alaska, with losses especially rapid in Alaska after the mid1990s. [IV.1.d] 

Snow cover is projected to continue to decrease as the climate warms. According to the IPCC, 

results from multiple model simulations indicate that an Arctic Ocean free of summer ice is 

likely by the end of the century, with some models suggesting that this could occur as soon as 

2040. Glaciers and terrestrial ice sheets are projected to continue to lose mass as increases in 

summertime melting outweigh increases in wintertime precipitation. This will contribute to sea 

level rise. Widespread increases in thaw depth are projected over most permafrost regions. 

[IV.3.b] 

I.3.d Sea level 

There is strong evidence that global average sea level gradually rose during the 20th century, 

after a period of little change between A.D. 0 and A.D. 1900, and is currently rising at an 

increased rate. The global average rate of sea level rise from 1993 to 2003 was 0.12 ± 0.03 

inches per year, significantly higher than the 20th century average rate of 0.067 ± 0.02 inches per 

year. Two major processes lead to changes in global mean sea level on decadal and longer time 

scales, and each currently account for about half of the observed sea level rise: expansion of the 

ocean volume due to warming, and the exchange of water between the oceans and land reservoirs 

of water, including glaciers and land ice sheets. [IV.1.d] 

U.S. sea level data from at least as far back as the early 20th century show that along most of the 

U.S. Atlantic and Gulf Coasts, sea level has been rising 0.8 to 1.2 inches per decade. The rate of 

relative sea level rise varies from a few inches per decade along the Louisiana Coast (due to 

sinking land) to a drop of a few inches per decade in parts of Alaska (due to rising land). 

[IV.1.d] 

Along with increases in global ocean temperatures, the IPCC projects that global sea level will 

rise between 7 and 23 inches by the end of the century (2090–2099) relative to the base period 

(1980–1999). According to the IPCC, the average rate of sea level rise during the 21st century is

I. Executive Summary 6
very likely to exceed the 1961–2003 average rate. Storm surge levels are expected to increase due 

to projected sea level rise. Combined with non-tropical storms, rising sea level extends the zone 

of impact from storm surge and waves farther inland, and will likely result in increasingly greater 

coastal erosion and damage according to the IPCC. Potential accelerations in ice flow of the kind 

recently observed in some Greenland outlet glaciers and West Antarctic ice streams could 

substantially increase the contribution from the ice sheets to sea level, a possibility not reflected 

in the aforementioned projections. Understanding of these processes is limited and there is no 

consensus on their magnitude and thus on the upper bound of sea level rise rates. [IV.3.b] 

I.3.e Atlantic hurricanes 

As recognized in recent assessments, detection of long-term trends in tropical cyclone activity is 

complicated by multi-decadal variability and the quality of the tropical cyclone records prior to 

routine satellite observations that began in about 1970. Even taking these factors into account, 

CCSP SAP 3.3 concluded that it is likely that the annual numbers of tropical storms, hurricanes, 

and major hurricanes in the North Atlantic have increased over the past 100 years, a time in 

which Atlantic sea surface temperatures also increased. Additionally, there is evidence for an 

increase in extreme wave height characteristics over the past couple of decades, associated with 

more frequent and more intense hurricanes. [IV.1.c] 

It is likely that hurricane rainfall and wind speeds will increase in response to human-caused 

warming, according to CCSP SAP 3.3. There is less confidence in the projected changes in the 

number of tropical cyclones. The apparent increase in the proportion of very intense storms since 

1970 in some regions is much larger than simulated by current models for that period, 

highlighting the uncertainty associated with this issue. [IV.3.b] 

Trends in other extreme weather events that occur at small spatial scales—such as tornadoes, 

hail, lightning, and dust storms—cannot be determined at the present time due to insufficient 

evidence. [IV.3.b] 

I.3.f Abrupt climate change 

An abrupt climate change occurs when the climate system crosses a threshold, which triggers a 

transition into a new state that may have large and widespread consequences. Over at least the 

last 100,000 years, abrupt regional warming (up to 29 °F within decades over Greenland) and 

cooling events occurred repeatedly over the North Atlantic region. Greenhouse warming and 

other human alterations of the Earth system may increase the possibility of abrupt climate 

change. One such possible change is a rapid decrease in the rate of flow of the major deep ocean 

circulation pattern in the North Atlantic that affects North American and European climate. The 

IPCC reported that an abrupt slowdown of this circulation during the 21st century is very unlikely

(less than 10% probability), but longer-term changes cannot currently be assessed with 

confidence. Other possible abrupt changes are rapid disintegrations of the Greenland Ice Sheet 

and the West Antarctic Ice Sheet, both of which could raise sea level by several feet. Although 

models suggest the complete melting of the Greenland Ice Sheet (leading to at least 20 feet of sea 

level rise) would only require sustained warming in the range of 3.4 to 8.3 °F (relative to preindustrial temperatures), it is expected to be a slow process that would take many centuries to 

I. Executive Summary 7
complete. There is presently no consensus on the long-term future of the West Antarctic Ice 

Sheet or its contribution to sea level rise. [IV.3.b] 

I.3.g Stratospheric ozone 

Ozone at very high altitudes in the atmosphere plays an important role in shielding life on Earth 

from harmful ultraviolet radiation. Springtime polar ozone depletion continues to be severe when 

winter temperatures in the polar stratosphere (above approximately 6 miles) are particularly cold. 

According to the World Meteorological Organization, the average concentration of atmospheric 

ozone outside of polar regions is no longer declining, as it was in the 1990s. Measurements from 

some stations in relatively unpolluted locations indicate that ultraviolet radiation levels have 

been decreasing since the late 1990s, in accordance with observed ozone increases. However, 

ultraviolet radiation levels are still increasing at some Northern Hemisphere stations as a 

consequence of long-term changes in other factors such as clouds and atmospheric particulates 

that also affect ultraviolet radiation. [IV.1.a] 

According to the World Meteorological Organization, it is unlikely that total ozone averaged 

over the region 60° S to 60° N will decrease significantly below the low values of the 1990s, 

because the abundances of ozone-depleting substances have peaked and are in decline. The 

current best estimate is that ozone between 60° S and 60° N will return to pre-1980 levels around 

the middle of the 21st century. Model simulations suggest that changes in climate, specifically 

the cooling of the stratosphere associated with increases in the abundance of carbon dioxide, may 

hasten the return of global column ozone to pre-1980 values by up to 15 years. [IV.3.d] 

I.4 Effects of Global Change on the Natural Environment and Human 

Systems 

According to CCSP SAP 4.3, it is very likely that temperature increases, increasing carbon 

dioxide levels, and altered patterns of precipitation are already affecting U.S. water resources, 

agriculture, land resources, biodiversity, and human health, among other things. SAP 4.3 also 

concluded that it is very likely that climate change will continue to have significant effects on 

these resources over the next few decades and beyond. [V.] 

I.4.a Biological diversity, ecosystem composition, and the natural environment 

Ecosystems provide society with a number of goods (e.g., food, fiber, fuel, pharmaceutical 

products) and services (e.g., cycling of water and nutrients, regulation of weather and climate, 

removal of waste products, recreational and spiritual opportunities), and are essential to human 

health and well-being. Biodiversity (i.e., the variety of all forms of life, from genes to species to 

ecosystems) is a fundamental building block of many of the services that ecosystems provide. It 

is intrinsically important both because of its contribution to the functioning of ecosystems and 

because it is difficult or impossible to recover or replace once it has eroded. The effects of 

climate change on U.S. ecosystems include changes in the timing and length of the growing 

season, primary production, and species distributions. According to CCSP SAP 4.3, it is very 

likely that climate change will increase in importance as a driver for changes in biodiversity over 

the next several decades, although for most ecosystems it is not currently the largest driver of 

I. Executive Summary 8
change. Other elements of global change (e.g., land management and use and nitrogen 

deposition) will continue to play a significant role in determining the future fate of ecosystems. 

Adaptive responses will as well. [V.1] 

Some general findings from CCSP SAP 4.3, the IPCC, and other assessments, are provided 

below: 

• The consistency of observed significant changes in physical and biological systems and 

observed significant warming across the globe likely cannot be explained entirely by natural 

variability or other confounding non-climate factors. [V.1] 

• There has been a significant lengthening of the growing season and increase in net primary 

productivity in the higher latitudes of North America. Over the last 19 years, global satellite 

data indicate that the onset of spring is taking place 10 to 14 days earlier across the temperate 

latitudes. Net primary productivity is projected to increase at high latitudes due to extended 

growing seasons and carbon dioxide fertilization. Projections for temperate latitudes are 

unclear due to uncertainty in whether precipitation increases will be great enough to offset 

evapotranspiration increases. [V.1.a] 

• In an analysis of 866 peer-reviewed papers exploring the ecological consequences of climate 

change worldwide, nearly 60% of the 1,598 species studied exhibited shifts in their 

distributions and/or timing of their annual cycles that correspond to recent large-scale climate 

change patterns. [V.1] 

• The resilience of many ecosystems is likely to be exceeded this century by an unprecedented 

combination of climate change, associated disturbances (e.g., flooding, drought, wildfire, 

insects, ocean acidification), and other global change drivers (e.g., land use change, 

pollution). [V.1.a] 

• In North America, warming has generally resulted in and is expected to continue to result in 

shifts of species ranges poleward and to higher altitudes. However, species that require 

higher-elevation habitat, such as alpine ecosystems, may have nowhere to migrate. [V.1.a] 

• Over the course of this century, net carbon uptake by terrestrial ecosystems is projected to 

peak before mid-century and then weaken or even reverse, thus amplifying climate change. 

[V.1.a] 

• Changes in temperature and precipitation will very likely decrease the cover of vegetation 

that protects the ground surface from wind and water erosion. [V.1.e] 

• Many effects of climate change on U.S. ecosystems and wildlife may emerge most strongly 

through potential changes in the intensity and frequency of extreme events such as hurricanes 

and disturbances such as wildfires. In the near term, fire effects are expected to generally 

exceed direct climate effects on ecosystems. [V.1.c] 

• While there will always be uncertainties associated with the future extent of climate change, 

the response of ecosystems to climate impacts, and the effects of management, it is both 

possible and essential for management practices to help protect climate-sensitive ecosystems. 

[V.1.f] 

Some regional and biome-specific findings from CCSP SAP 4.3 and the IPCC are provided 

below: 

I. Executive Summary 9
• The rapid rates of warming that have been seen in the Arctic in recent decades (and are 

projected for at least the next century) are dramatically reducing the snow and ice covers that 

provide denning and foraging habitat for polar bears and other ice-dependent species. [V.1.e] 

• The Alaskan tree line is expected to move northward and to higher elevations. Forests will 

replace significant amounts of existing tundra, and tundra vegetation will move into polar 

deserts. These changes are projected to increase carbon uptake, which would tend to offset 

warming effects. However, the reduced reflectivity associated with the vegetation land cover 

is expected to outweigh this, causing further warming. [V.1.e] 

• Where adequate water is available, nitrogen deposition and warmer temperatures have very 

likely increased forest growth and will continue to do so in the near future. However, it is 

difficult to separate the role of climate from other factors. Rising carbon dioxide levels will 

very likely increase photosynthesis in forests, but this increase will likely only enhance wood 

production in young forests on fertile soils. [V.1.e] 

• In the last three decades, the wildfire season in the western United States has lengthened and 

burn durations have increased. Climate change has also very likely increased the size and 

number of insect outbreaks and tree mortality that help to fuel wildfires in the interior West, 

the Southwest, and Alaska. These trends are very likely to continue. [V.1.c] 

• Many plants and animals in arid ecosystems are near their physiological limits for tolerating 

temperature and water stress and even slight changes in stress will have significant 

consequences. Climate change in arid regions is very likely to be detrimental to river and 

riparian ecosystems, increase erosion, and promote invasion of exotic grass species in arid 

lands. Climate change in arid regions is also likely to create physical conditions conducive to 

wildfire. In arid regions where ecosystems have not co-evolved with a fire cycle, the loss of 

iconic megaflora, such as saguaro cacti and Joshua trees, is very likely. [V.1.a] 

• On small oceanic islands with cloud forests or high-elevation ecosystems, such as the 

Hawaiian Islands, human-induced climate change, land use changes, and invasive species are 

likely to have synergistic effects that drive several species (e.g., some endemic birds) to 

extinction. [V.1.c] 

• Erosion and ecosystem loss is affecting many parts of the U.S. coastline, but it is unclear to 

what extent these losses result from temperature and precipitation changes, sea level rise, and 

other human drivers. Coastal wetland loss is occurring where these ecosystems are squeezed 

between natural and artificial landward boundaries and rising sea levels. To date, more than 

50% of the original salt marsh habitat in the United States has been lost. Approximately 20% 

of the remaining coastal wetlands in the U.S. mid-Atlantic region are potentially at risk of 

inundation between 2000 and 2100. Salt marsh biodiversity is projected to decrease in 

northeastern marshes through expansion of non-native species such as Spartina alterniflora. 

Erosion of barrier islands has increased the height of waves that reach the shorelines of 

coastal bays. [V.1.c] 

• The increasing carbon dioxide level in the atmosphere has made the oceans more acidic. This 

acidification is expected to have negative impacts on marine shell-forming organisms and 

consequently large portions of the marine food chain. Corals in many tropical regions are 

experiencing substantial mortality from increasing water temperatures and increasing storm 

intensity in some regions, on top of a host of other ongoing challenges from development and 

tourism, fishing, and pollution. Increasing ocean acidification is expected to exacerbate these 

effects. [V.1.a] 

I. Executive Summary 10
• Land-based ecosystems in the northeastern and southeastern United States will likely become 

carbon sources, while the western United States will likely remain a carbon sink. [V.1.a]

I.4.b Agriculture and food production 

The many U.S. crops and livestock (valued at about $200 billion in 2002) are strongly affected 

by weather and climate factors (such as temperature, precipitation, carbon dioxide 

concentrations, and water availability). Vulnerability of this sector to climate change is a 

function of many interacting factors, including pre-existing climatic and soil conditions, changes 

in pest competition, water availability, and the sector’s capacity to cope and adapt through 

management practices, seed and cultivar technology, and changes in economic competition 

among regions [V.2]. The following findings are based on CCSP SAP 4.3 and the IPCC: 

• With increased carbon dioxide levels and temperature, the lifecycle of grain and oilseed 

crops will likely progress more rapidly. But, as temperatures rise, these crops will 

increasingly begin to fail, especially if climate variability increases and precipitation lessens 

or becomes more variable. [V.2] 

• The marketable yield of many horticultural crops (e.g., tomatoes, onions, and fruits) is very 

likely to be more sensitive to climate change than grain and oilseed crops. [V.2.c] 

• Climate change is likely to lead to northward migration of weed species. Increasing carbon 

dioxide levels are likely to help many weeds, particularly some types of invasive weeds, more 

than most cash crops. Recent research also suggests that glyphosate, the most widely used 

herbicide in the United States, loses its efficacy on weeds grown at the increased carbon 

dioxide levels that are projected for the coming decades. [V.2.d] 

• Disease pressure on crops and domestic animals will likely increase with earlier springs and 

warmer winters, which will allow proliferation and higher survival rates of pathogens and 

parasites. Regional variation in warming and changes in rainfall will also affect spatial and 

temporal distribution of disease. [V.2.e] 

• Projected increases in temperature and a longer growing season will likely extend forage 

production into late fall and early spring, thereby decreasing need for winter season forage 

reserves. However, these benefits will very likely be affected by regional variations in water 

availability. [V.2.f] 

• Climate-change-induced shifts in plant species are already underway in rangelands. 

Establishment of perennial herbaceous species is reducing the availability of soil moisture 

early in the growing season. [V.3.e] 

• Shifts in the productivity and type of plants will likely also have significant impact on 

livestock operations. Higher temperatures will very likely reduce livestock production during 

the summer season, but these losses will very likely be partially offset by warmer 

temperatures during the winter season. For ruminants, current management systems generally 

do not provide shelter to buffer the adverse effects of changing climate. Such protection is 

more frequently available for non-ruminants (e.g., swine and poultry). [V.2.f] 

• Cold freshwater fisheries are likely to be negatively affected. Warm freshwater fisheries will 

generally benefit. The results for cool-water fisheries will be mixed, with gains in the 

northern and losses in the southern portions of ranges. Effects of increasing temperature on 

marine fisheries are already occurring, with rapid poleward shifts in some regions. These 

shifts are expected to continue in the future. [V.2.g] 

I. Executive Summary 11
I.4.c Water resources 

Plants, animals, natural and managed ecosystems, and human settlements are susceptible to 

variations in the storage, fluxes, and quality of water, all of which are sensitive to climate 

change. The effects of climate on the Nation’s water storage capabilities and hydrologic 

functions will have significant implications for water management and planning as natural 

processes become more variable. Other factors affecting water resources include water pollution, 

damming of rivers, wetland drainage, reduced stream flow, and lowering of the groundwater 

table (e.g., due to irrigation). Although climate-related changes have been small compared to 

these other pressures to date, climate change is expected to result in increasing effects in the 

future. [V.4] 

Although U.S. water management practices are generally quite advanced, particularly in the 

West, the reliance on past conditions as the foundation for current and future planning and 

practice will no longer be tenable as climate change and variability increasingly create conditions 

that are well outside of historical parameters, eroding predictability. The findings below, 

reported by the IPCC, CCSP SAP 4.3, and other sources, are based on the ongoing and projected 

water cycle changes described earlier. [V.4] 

• In some mountain areas, earlier snowmelt peaks result in reduced low flows in the summer 

and fall. Continuing shifts in this direction are very likely and may substantially affect the 

performance of reservoir systems through changes in the seasonality of streamflow. [V.4.a] 

• Water quality is sensitive to increased water temperatures, changes in precipitation, and other 

climate-related factors. However, most water quality changes observed so far across the 

continental United States are likely attributable to causes other than climate change. Higher 

temperatures and nutrient loads will tend to reduce the oxygen content of water with potential 

negative impacts on aquatic organisms. Increases in intense rain events will tend to result in 

the introduction of more sediment, nutrients, pathogens, and toxics into water bodies from 

non-point sources. The intrusion of saline water into groundwater supplies is likely to 

adversely affect water quality in coastal regions around the United States. These water 

quality changes could impose enormous costs on water treatment infrastructure. [V.4.c] 

• Stream temperatures are likely to increase as the climate warms and are very likely to have 

effects on aquatic ecosystems and water quality. Changes in temperature will be most evident 

during low flow periods, when they are of greatest concern. [V.4.d] 

• Projections suggest that efforts to offset the declines in available surface water by increasing 

withdrawal of groundwater will be hampered by decreases in groundwater recharge in some 

water-stressed regions, such as the southwestern United States. [V.4.b] 

• Less reliable supplies of water are expected to create challenges for managing urban water 

systems as well as for industries that depend on large volumes of water. Across North 

America, vulnerability to extended drought is increasing as population growth and economic 

development create more demands from agricultural, municipal, and industrial uses, resulting 

in frequent over-allocation of water resources. Examples of vulnerable U.S. regions include: 

the heavily-used water systems of the West that rely on capturing snowmelt runoff, such as 

the Columbia and Colorado River systems; portions of California; the New York area, as a 

I. Executive Summary 12
consequence of greater water supply variability; and many islands such as the U.S. territories 

of Puerto Rico and U.S. Virgin Islands. [V.4.e] 

• Trends toward more efficient water use are likely to continue in the coming decades. 

Pressures for reallocation of water will be greatest in areas of highest population growth, 

such as the Southwest. Declining per capita (and, in some cases, total) water consumption 

will help mitigate the impacts of climate change on water resources. [V.4.d] 

I.4.d Social systems and settlements 

Human systems include social, economic, and institutional structures and processes. These 

systems are influenced by multiple factors and stresses (e.g., access to financial resources, 

urbanization, and shifts in demographics). U.S. settlements will feel the effects of climate change 

as they interact with these other factors. Climate change effects could push stressed systems 

beyond sustainable thresholds. Climate sensitivity varies across settlements and industrial 

sectors. While it may appear that industrialized countries like the United States are well equipped 

to cope with gradual climate change at a national level, at a local level there may be substantial 

variability in climate effects and capacities to adapt. On the other hand, some U.S. settlements 

may find opportunities in climate change. [V.5]

According to CCSP SAP 4.6, some of the key aspects of human settlements that are affected by 

climate change include: human health; water and other urban infrastructures; energy 

requirements; urban metabolism (e.g., the welfare and activities of urban communities); 

economic competitiveness, opportunities, and risks; and social and political structures. Several of 

these challenges are discussed in other sections of the Executive Summary (and the body of the 

report). 

Globally, the most vulnerable industries, settlements, and societies are generally those in coastal 

and river flood plains, those whose economies are closely linked with climate-sensitive 

resources, and those in areas prone to extreme weather events, especially in places that are being 

rapidly urbanized. Poor communities can be especially vulnerable, particularly those 

concentrated in high-risk areas. According to the IPCC, the most vulnerable areas in the United 

States are likely to be Alaska (e.g., indigenous communities dependent on hunting climatesensitive species), coastal and river basin locations that are susceptible to flooding, arid areas 

where water scarcity is a pressing issue, and areas whose economic bases are climate-sensitive. It 

is possible that regions exposed to risks from climate change will see movement of population 

and economic activity to other locations. One reason is public perceptions of risk, but a more 

powerful driving force may be the availability of insurance. [V.5] 

Some of the key findings from CCSP SAP 4.6 and the IPCC related to settlements are outlined 

below. 

• Population growth is generally shifting toward areas (e.g., coastal regions) more likely to be 

vulnerable to the effects of climate change. Demand for waterfront property and land for 

building in the United States continues to grow, increasing the value of property at risk. 

[V.5.b] 

I. Executive Summary 13
• Coastal population increases together with likely increases in hurricane rainfall and wind 

speeds and greater storm surge due to sea level rise will continue to increase coastal 

vulnerabilities in the Southeast and Gulf Coast. Urban centers that were once assumed to 

have a high adaptive capacity remain vulnerable to extreme events such as hurricanes. 

[V.5.b] 

• The degradation of coastal ecosystems, especially wetlands and coral reefs, can have serious 

implications for the well-being of societies that depend on them. The costs of adaptation for 

vulnerable coasts are generally much less than the costs of not acting. [V.5.c] 

• For small islands, particularly in the Pacific, some studies suggest that sea level rise could 

reduce island size, raising concerns for parts of Hawaii and other U.S. territories. [V.5.c] 

• Climate and extreme events can have substantial effects on local economies. For example, 

tourism could be affected by drought-influenced water levels in rivers and reservoirs, cleanup 

following multiple storm outbreaks, and changes in the length of the tourist season (e.g., ski 

season and beach season). [V.5.d] 

• As discussed elsewhere, wildfires have increased in extent and severity in recent years and 

are very likely to intensify in a warmer future. At the same time, the population has been 

expanding into fire-prone areas, increasing society’s vulnerability to wildfire. [V.5.d]

I.4.e Human health 

Climate variability and change can affect health directly and indirectly. The heat stress 

associated with a warmer environment can directly affect the body. Climate change can also 

make it possible for animal-, water-, and food-borne diseases to spread or emerge in areas where 

they had been limited or had not existed, or it can make it possible for such diseases to disappear 

by making areas less hospitable to the disease carrier or pathogen. Climate can also affect the 

incidence of diseases associated with air pollutants and aeroallergens. 

Climate impacts on health are complex and will be influenced by multiple factors, including 

demographics; population and regional vulnerabilities; the social, economic, and cultural 

context; availability of resources and technological options; built and natural environments; 

public health infrastructure; and the availability and quality of health and social services. CCSP 

SAP 4.6 concluded that climate change is very likely to accentuate the disparities already evident 

in the American health care system. As discussed in CCSP SAP 4.6 and the IPCC, the extent to 

which communities are prepared and have the capacity to adapt will determine the severity of the 

following climate change impacts. Conclusions from those reports are discussed below. [V.6] 

Temperature-related conclusions include the following: 

• It is very likely that heat-related morbidity and mortality will increase over the coming 

decades, however net changes in mortality are difficult to estimate because, in part, much 

depends on complexities in the relationship between mortality and global change. High 

temperatures tend to exacerbate chronic health conditions. An increased frequency and 

severity of heat waves is expected, leading to more illness and death, particularly among the 

young, elderly, frail, and poor. [V.6] 

• In many cases, the urban heat island effect may increase heat-related mortality. High 

temperatures and high air pollution can interact to result in additional health impacts. [V.6] 

• Climate change is projected to lead to fewer deaths from cold exposure. [V.6.a] 

I. Executive Summary 14
Conclusions related to animal-, food-, and water-borne diseases include the following: 

• Climate change is likely to increase the risk and geographic spread of vector-borne infectious 

diseases, including Lyme disease and West Nile virus. [V.6.c] 

• There will likely be an increase in the spread of several food and water-borne pathogens 

among susceptible populations depending on the pathogens’ survival, persistence, habitat 

range and transmission under changing climate and environmental conditions. However, 

major human epidemics of these diseases in the United States are unlikely if the public health 

infrastructure is maintained and improved as needed. [V.6 and V.6.c] 

• Federal and state laws and regulatory programs protect much of the U.S. population from 

waterborne disease. However, if climate variability increases, current and future deficiencies 

in watershed protection, infrastructure, and storm drainage systems will tend to increase the 

risk of contamination events. [V.6.c] 

Conclusions related to air quality include the following: 

• As the climate becomes warmer and more variable, air quality is likely to be affected. [V.6.e] 

• Climate change can be expected to influence the concentration and distribution of air 

pollutants through a variety of processes, including the modification of biogenic emissions, 

the change of chemical reaction rates, wash-out of pollutants by precipitation, and 

modification of weather patterns that influence pollutant buildup. [V.6.e] 

• In studies holding pollution emissions constant, climate change was found to lead to 

increases in regional ground-level ozone pollution in the United States and other countries. It 

is well-documented that breathing air containing ozone can reduce lung function, increase 

susceptibility to respiratory infection, and contribute to premature death in people with heart 

and lung disease. [V.6.e] (The health effects of stratospheric ozone depletion on the United 

States have not been assessed here.) 

• Climate change and changes in carbon dioxide concentration could increase the production 

and allergenicity of airborne allergens and affect the growth and distribution of weeds, 

grasses, and trees that produce them, which may increase the incidence of allergic rhinitis. 

[V.6.e] 

• Uncertainties in climate models make the direction and degree of change in air quality and 

aeroallergens somewhat speculative. [V.6.d] 

Conclusions related to extreme events include the following: 

• Increases in extreme weather (e.g., storms, flooding) and accompanying events (e.g., wildfire 

resulting from prolonged drought) may lead to increases in deaths, injuries, infectious 

diseases, interruptions of medical care for chronic disease treatment, and stress-related 

disorders and other adverse effects associated with social disruption and migration. [V.6.d] 

• Extreme climate events may also have substantial mental health impacts (e.g., post-traumatic 

stress disorder and depression). [V.6] 

• High-density populations in low-lying coastal regions, such as the U.S. Gulf of Mexico, 

experience a high health burden from weather disasters, particularly among lower income 

groups. [V.6] 

• Wildfires pose significant direct health threats. They can also have substantial effects through 

increased eye and respiratory illnesses due to fire-related air pollution and mental health 

impacts from evacuations, lost property, and damage to resources. Wildfires, with their 

I. Executive Summary 15
associated decrements to air quality and pulmonary effects, are likely to increase in 

frequency, severity, distribution, and duration in the Southeast, the Intermountain West and 

the West. [V.6 and V.6.b] 

• Morbidity and mortality due to an event will tend to increase with the intensity and duration 

of the event, but will tend to decrease with advance warning and preparation. [V.6.a] 

I.4.f Energy production, use, and distribution 

To date, most discussions on energy and climate change have focused on mitigating human 

effects on climate. However, along with this role as a driver of climate change, the energy sector 

will be subject to the effects of climate change. As discussed in the IPCC and CCSP SAP 4.5, 

climate change is likely to affect the use and production of energy in the United States. It is likely

to affect physical and institutional infrastructures and is likely to interact with and possibly 

exacerbate ongoing environmental change and environmental and population-related pressures in 

settlements. Concerns about climate change impacts could change perceptions and valuations of 

energy technology alternatives. Responses to climate change may lead to the following changes 

in energy demand according to the IPCC and CCSP SAP 4.5: [V.7] 

• decreases in the amount of energy consumed in residential, commercial, and industrial 

buildings for space heating and water heating; [V.7.a] 

• increases in energy used in residential, commercial, and industrial buildings for space 

cooling; [V.7.a] 

• increases in energy consumed for residential and commercial refrigeration and industrial 

process cooling (e.g., in thermal power plants or steel mills); [V.7.a] 

• increases in peak demand for electricity in most regions of the country, except in the Pacific 

Northwest; [V.7.a] 

• increases in energy used to supply other resources for climate-sensitive processes, such as 

pumping water for irrigated agriculture and municipal uses; [V.7.a] 

• changes in the balance of energy use among delivery forms and fuel types, as between 

electricity used for air conditioning and natural gas used for heating; [V.7] and 

• changes in energy consumption in key climate-sensitive sectors of the economy, such as 

transportation, construction, agriculture, and others. [V.7] 

According to CCSP SAP 4.5, in the absence of energy efficiency measures directed at space 

cooling, climate change is expected to cause a significant increase in the demand for electricity 

in the United States, which would require the building of additional electricity production 

facilities (and probably transmission facilities) at an estimated cost of many billions of dollars. 

[V.7.a] 

Climate change could affect production, supply, and transmission of energy in the United States 

in the following ways according to the IPCC and CCSP SAP 4.5: 

• direct impacts from increased intensity of extreme weather events, [V.7.b] 

• reduced water supplies in regions dependent on water resources for hydropower and/or 

thermal power plant cooling, [V.7.b] 

• facility siting decisions affected by changing conditions, [V.7.b] and 

I. Executive Summary 16
• positive or negative impacts on production of biomass, wind power, or solar energy where 

climate conditions change. [V.7.b] 

Significant uncertainty exists about the potential impacts of climate change on energy production 

and distribution, in part because the timing and magnitude of climate impacts are uncertain. 

Nonetheless, every existing source of energy in the United States has some vulnerability to 

climate variability. Although effects on the existing infrastructure might be categorized as 

modest, local and industry-specific impacts could be large, especially in areas that may be prone 

to disproportional warming (e.g., Alaska) or weather disruptions (e.g., the Gulf Coast and Gulf of 

Mexico). [V.7.b] 

I.4.g Transportation 

Increasing global temperatures, rising sea levels, and changing weather patterns pose significant 

challenges to our country’s transportation venues including: roadways, railways, transit systems, 

marine transportation systems, airports, and pipeline systems. The U.S. transportation network is 

vital to the Nation’s economy, safety, and quality of life. The following findings, derived from 

the IPCC and CCSP SAP 4.7 (focused on Gulf Coast transportation impacts), relate to the 

relationship between climate change and transportation in North America: [V.8] 

• Warmer or less snowy winters are likely to reduce delays, improve ground and air 

transportation reliability, and decrease the need for winter road maintenance. However, more 

intense winter storms could increase risks for traveler safety and require increased localized 

snow removal. [V.8] 

• Increasing frequency, intensity, or duration of heat spells could cause railroad tracks to 

buckle or kink and could affect roads through softening and traffic-related rutting. [V.8] 

• Coastal and riverine flooding and landslides are very likely to cause negative impacts on 

roads, rails, and ports. The crucial connectivity of the transportation system means that the 

services of the network can be threatened even if small segments are wiped out. [V.8] 

• Transportation infrastructure is likely to be particularly affected in more northerly latitudes. 

Permafrost degradation reduces surface load-bearing capacity and potentially triggers 

landslides. While the season for transport by barge is likely to be extended, the season for ice 

roads that utilize waterways is likely to be compressed. [V.8] 

• Climate change may worsen the vulnerability of Gulf Coast and eastern transportation 

systems to hurricanes and tropical storms as warming seas give rise to more energetic storms 

and as sea level rises. [V.8.d] 

• An increase in the frequency of extreme precipitation events may contribute to increased 

accident rates; more frequent short-term flooding and bridge scour, as well as more culvert 

washouts; exceeding the capacity of storm drain infrastructure; and more frequent landslides, 

requiring increased maintenance. [V.8] 

• Increases in precipitation and the frequency of severe weather events could negatively affect 

aviation. Higher temperatures affect aircraft performance and increase the necessary runway 

lengths. [V.8] 

• Some of these risks are expected to be offset by improvements in technology and information 

systems. [V.8] 

I. Executive Summary 17
II. Introduction 18

Section II: Introduction 

II.1. Objectives 

Earth’s climate and other key aspects of the natural environment are changing, and these changes 

are significantly affecting society and nature. The U.S. Climate Change Science Program 

(CCSP) and its international partners are making a concerted scientific effort to better understand 

the characteristics and causes of these changes, their past and future course, and their 

implications. By communicating the findings of these efforts, CCSP is making important strides 

toward its overarching vision: 

A nation and the global community empowered with the science-based knowledge to manage the 

risks and opportunities of change in the climate and related environmental systems. 

The specific objectives of this assessment are to integrate, evaluate, and interpret the findings of 

CCSP to support informed discussion about the relevant issues by decisionmakers, stakeholders, 

the media, and the public. This assessment reviews a wide range of observed and projected 

vulnerabilities, risks, and impacts associated with global change, focusing particularly on the 

effects of climate change. It also discusses the effects of changing ozone concentrations; 

ecosystem shifts; and changes in air quality, land and water use, and ocean chemistry. This 

assessment focuses primarily on the United States. However, given the global nature of the 

issues, it also discusses international impacts. 

Sidebar: 

Global Change: Changes in the global environment (including alterations in climate, land productivity, 

oceans or other water resources, atmospheric chemistry, and ecological systems) that may alter the 

capacity of the Earth to sustain life (from the Global Change Research Act of 1990). 

Climate Change: A statistically significant variation in the mean state of the climate and/or in its variability, 

persisting for an extended period (typically decades or longer), due to either natural or human causes 

(adapted from the CCSP Strategic Plan).

This report is pursuant to the Global Change Research Act of 1990,6

 which requires the U.S. 

Global Change Research Program7

 to “combine and interpret information readily usable by 

policymakers attempting to formulate effective strategies for preventing, mitigating, and 

adapting to the effects of global change.” The specific assessment requirement is as follows: 

Scientific Assessment 

On a periodic basis (not less frequently than every 4 years), the Council, through the Committee,8

shall prepare and submit to the President and the Congress an assessment which – 

 

6

 15 USC Chapter 56A 

7

 The U.S. Global Change Research Program is incorporated within CCSP. 8

 The “Council” refers to the Federal Coordinating Council on Science, Engineering, and Technology, which has 

been supplanted by the National Science and Technology Council. The “Committee” refers to the Committee on 

Earth and Environmental Sciences, which has been supplanted by the Committee on Environment and Natural 

Resources under the council. 
(1) integrates, evaluates, and interprets the findings of the Program and discusses the scientific 

uncertainties associated with such findings; 

(2) 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 

(3) analyzes current trends in global change, both human-induced and natural, and projects major 

trends for the subsequent 25 to 100 years.9

This scientific assessment addresses each of the three components of this requirement. 

The findings contained in this assessment build upon the vast body of observations, process 

studies, modeling, and other types of basic and applied research conducted under the auspices of 

CCSP. In so doing, it integrates, evaluates, and interprets findings of the program, as called for in 

the first paragraph of the assessment requirement of the Global Change Research Act of 1990. 

The following sections of the introduction describe the manner in which uncertainty is 

characterized in this assessment, its relationship to other assessments, and how the assessment is 

structured. 

II.2. Relationship to Other Assessments and Primary Scientific 

Literature 

This scientific assessment integrates, evaluates, and interprets the findings of CCSP and other 

scientific investigations of global change. It analyzes the trends and effects of global 

environmental changes, with a particular focus on climate change. This scientific assessment 

primarily draws from and synthesizes findings from previous assessments of the state of the 

science. All of these underlying assessments involved leading experts in the relevant disciplines 

who analyzed the existing body of primary, peer-reviewed scientific literature, and observations. 

These underlying assessments were all subjected to and improved through rigorous peer reviews 

of their own. 

In some cases, this assessment cites the primary literature, particularly when the underlying 

assessment derived a particular conclusion from a single or small number of studies. In cases 

where the authors of the underlying assessments drew from a large number of studies or where 

they exercised their expert judgment on a particular issue, the assessment itself is cited. 

This scientific assessment draws significantly from and extends the findings of the 

Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Reports that were 

released in 2007.10 Within the IPCC reports, this assessment draws most heavily from the North 

America chapter of the IPCC Working Group II report, as well as several chapters from the IPCC 

Working Group I report. The IPCC reports, which represent the international scientific consensus 

on climate change, are written and reviewed by hundreds of the world’s leading environmental 

and social scientists studying climate change. The United States was a leading contributor to the 

IPCC: 

• U.S. scientists actively participated, serving as authors and reviewers; 

 

9

 15 USC Chapter 56A, Subchapter 1, Section 2936 

10 See <www.ipcc.ch>. 

II. Introduction 19
• CCSP sponsored key science that underpinned the IPCC reports; and 

• the United States hosted the Technical Support Unit that facilitated production of the IPCC 

report on the physical science of climate change. 

This scientific assessment also incorporates findings from several CCSP synthesis and 

assessment products that integrate and analyze CCSP results related to specific sectors, 

disciplines, and cross-cutting issues.11 The CCSP synthesis and assessment products focus on 

issues of importance, with a particular emphasis on North America. 

Other key assessments used in this report include the United Nations Environment Programme’s 

Scientific Assessment of Ozone Depletion: 2006 (WMO, 2007), the Arctic Climate Impact 

Assessment (ACIA, 2005), and several reports from the National Research Council. Nearly all of 

the major assessment reports cited in this document were released within the past few years. The 

National Assessment of the Potential Consequences of Climate Variability and Change (NAST, 

2001) is also cited and served as a foundation for a significant portion of the research carried out 

under CCSP that is drawn from in the assessments cited in this document. 

II.3. Characterization of Uncertainty 

In characterizing uncertainty, this assessment uses an approach similar to the one used in the 

IPCC Fourth Assessment Report,12 which was also generally used in the CCSP synthesis and 

assessment reports that provided input to this assessment. The terms used to describe the levels 

of confidence and likelihood associated with individual findings in the IPCC Fourth Assessment 

Report and CCSP synthesis and assessment products have been adopted in this assessment. 

II.3.a Description of confidence 

On the basis of a comprehensive reading of the literature, their analysis of current knowledge, 

and their expert judgment, the authors of the underlying assessments have assigned confidence 

levels to major statements. These statements of confidence (printed in italics) use the following 

definitions: 

Very high confidence At least a 9 out of 10 chance of being correct 

High confidence About an 8 out of 10 chance 

Medium confidence About a 5 out of 10 chance 

Low confidence About a 2 out of 10 chance 

Very low confidence Less than a 1 out of 10 chance 

II.3.b Description of likelihood 

Likelihood refers to an assessment of the probability, based on expert judgement or quantitative 

analysis (when available), that some well-defined outcome has occurred or will occur in the 

future. The descriptions of likelihood (printed in italics) use the following definitions: 

 

11 See <www.climatescience.gov>. 

12 This approach is described in Guidance Notes for Lead Authors of the IPCC Fourth Assessment Report on 

Addressing Uncertainties, which can be found at <www.ipcc.ch/pdf/supporting-material/uncertainty-guidancenote.pdf>. 

II. Introduction 20
Virtually certain greater than 99% probability of occurrence 

Very likely 90 to 99% probability 

Likely 66 to 90% probability 

More likely than not greater than 50% probability 

About as likely as not 33 to 66% probability 

Unlikely 10 to 33% probability 

Very unlikely 1 to 10% probability 

Exceptionally unlikely less than 1% probability 

Non-italicized terms and phrases such as “may,” “are expected,” and “are projected” are used to 

indicate the possibility of the occurrence of an event or phenomenon without assignment of a 

formal level of likelihood. 

II.4. Report Structure 

The body of the report is divided into three main sections: 

 Evaluation of Overall Progress by CCSP. This section provides illustrations of the 

program’s integrated efforts toward its major objectives and interprets some of the major 

advances that have been made. 

 Trends and Projections of Global Environmental Change. This section describes key 

observations, causes, and future projections of global change, both nationally and 

globally. This section provides much of the underpinning for the subsequent section. 

 Analysis of the Effects of Global Change on the Natural Environment and Human 

Systems. This section describes the effects of climate and other environmental drivers on 

key U.S. assets and sectors, including natural and managed ecosystems, water resources, 

human health, welfare, settlements, energy supply and demand, and transportation. 

II. Introduction 21
III. Evaluation of Overall Progress by the CCSP 22

Section III: Evaluation of Overall Progress by CCSP 

The Climate Change Science Program (CCSP) articulated five overarching goals in its strategic 

plan (CCSP, 2003): 

1. Improve knowledge of the Earth’s past and present climate and environment, including 

its natural variability, and improve understanding of the causes of observed variability 

and change. 

2. Improve quantification of the forces bringing about changes in the Earth’s climate and 

related systems. 

3. Reduce uncertainty in projections of how the Earth’s climate and related systems may 

change in the future. 

4. Understand the sensitivity and adaptability of different natural and managed ecosystems 

and human systems to climate and related global changes. 

5. Explore the uses and identify the limits of evolving knowledge to manage risks and 

opportunities related to climate variability and change. 

This section reports key progress toward these goals since the completion of CCSP’s Strategic 

Plan in 2003. In providing an assessment that includes consideration of the accomplishments of 

CCSP over the last several years, it is important to differentiate between different kinds of 

accomplishments. A conceptual framework for addressing the program’s accomplishments and 

developing performance metrics by which its progress could be evaluated was provided by the 

National Research Council (NRC) in its report, Thinking Strategically: The Appropriate Use of 

Metrics for the Climate Change Science Program (NRC, 2005b). In this report, the NRC 

identified five different types of metrics: process metrics, input metrics, output metrics, outcome 

metrics, and impact metrics. 

The progress that CCSP has made over the past several years comes from a variety of efforts. In 

implementing its program, CCSP uses four approaches of scientific research, observations, 

decision support, and communications (CCSP, 2003). In particular, as new observational 

capabilities become available, they can be thought of as providing an input metric (e.g., launch 

of a new satellite) that leads to outputs (e.g., large data sets that are made available to the 

scientific research community through a data archive). Scientists use the observations (together 

with relevant models of varying degrees of complexity) to enhance the state of scientific 

knowledge, and thereby create outcomes that can be evaluated using a variety of metrics. 

Similarly, complex modeling and assimilation systems that are developed to describe prior 

and/or predict future Earth system evolution, support assessments, and produce consistent data 

sets that can be used widely by the scientific community may be thought of as inputs, while the 

results from them can be considered as outputs, with corresponding metrics associated with them 

as their capabilities and availability increase. 

The following are among the output metrics identified by the NRC (2005b): 

1. The research has engendered significant new avenues of discovery. 
2. The program has led to the identification of uncertainties, increased understanding of 

uncertainties, or reduced uncertainties that support decision-making or facilitate the 

advance of other areas of science. 

3. The program has yielded improved understanding, such as (a) more consistent and 

reliable predictions or forecasts, (b) increased confidence in our ability to simulate and 

predict climate change and variability, and (c) broadly accepted conclusions about key 

issues or relationships. 

Impact metrics would be applicable once the scientific results can be used for policymaking, 

resource management, and decision support. Indeed, the NRC has defined three classes of impact 

metrics (NRC, 2005b): 

1. The results of the program have informed policy and improved decision making. 

2. The program has benefited society in terms of enhancing economic vitality, promoting 

environmental stewardship, protecting life and property, and reducing vulnerability to the 

impacts of climate change. 

3. Public understanding of climate issues has increased. 

A number of examples of progress that CCSP has made toward meeting these goals are outlined 

in this section. Many of these are best considered as outcome goals, as they represent the 

knowledge, understanding, and predictive capability that come from the program. Some, 

especially those associated with Goal 5, can be thought of as impact goals because of how that 

goal is defined. However, given the large investment made by CCSP agencies in observational 

and modeling capability, and the very significant increase in capability that has become available 

since the publication of the CCSP Strategic Plan in 2003, it is important that the input and output 

metrics most closely associated with these advances also be addressed. Thus, this section begins 

with a number of CCSP accomplishments that can best be thought of as input metrics and 

concludes with a number of cross-cutting accomplishments that are primarily output metrics. 

Program outcomes and impacts have been assessed in a number of NRC reports and elsewhere. 

CCSP provides key large-scale observational efforts that underpin our Nation’s climate research, 

including satellite observations, surface-based measurement networks, and field campaigns. 

However, many of the observing systems used by CCSP were developed primarily for purposes 

other than climate research and are not included in CCSP’s budget. During the approximately 

five years since the release of the strategic plan, several important advances in observational 

capability have come to fruition through enhanced deployment of surface-based networks, 

launch and/or releases of global data sets from satellites, and the conduct of field campaigns that 

integrate surface-based, airborne, and satellite-based measurements. The data from these 

investments are made broadly available to scientific and applications communities and they have 

had an appreciable impact on climate research in the United States. 

This section briefly summarizes some of these accomplishments. These accomplishments, which 

are drawn largely from the program’s annual reports (Our Changing Planet, OCP) for FY 2004 

through FY 2009, are not intended to comprehensively describe the program’s inputs and 

outputs. This section is not a comprehensive assessment of the program’s capability to deliver 

the knowledge necessary to address the key scientific and societal issues related to the effects of 

global change. Rather, this section is intended to provide illustrative examples of the broad and 

integrated range of CCSP’s activities. 

III. Evaluation of Overall Progress by the CCSP 23
III.1 Program Inputs 

III.1.a Surface-based observing networks 

Ocean Monitoring: CCSP agencies have been contributing to a significant growth in worldwide 

ocean in situ monitoring over the past several years. An initial ocean observing system for 

climate has reached 59% completion towards its planned completion in 2013, with the United 

States currently supporting nearly 50% of the ocean-based observing platforms [OCP 200913]. A 

network of tropical moored buoys has been extended into the Indian Ocean, with the number of 

sites increasing from 6 [OCP 2007] to 8 [OCP 2008] to a planned total of 12 by the end of 2008 

[OCP 2009]. The Argo network of profiling arrays is now globally complete, with some 3,000 

instruments deployed around the world [OCP 2009], with the United States responsible for 

approximately half of them [OCP 2008]. 

Aerosol and Radiation Monitoring: The Aerosol Robotic Network (AERONET) continues to 

expand, with some 230 automated sites now instrumented with sun photometers around the 

world. Sites added most recently include the Tibetan plateau and the Ganges floodplain of India. 

Scientific advances implemented recently into the network include more accurate measures of 

particle size and improved determination of aerosol light absorption [OCP 2009]. Threedimensional information about aerosol distributions is being obtained from the Micro-Pulse 

Lidar Network (MPL Net), for which 13 stations are now operating around the world, with 5 

more to be put into place in the near future. Most MPL Net stations are co-located with Aerosol 

Robotic Network stations so that the column data obtained with passive techniques can be intercompared with profile data obtained with active approaches. Data products produced by MPL 

Net include aerosol optical depth, absorption, and size distribution; aerosol and cloud heights; 

and planetary boundary layer structure and evolution [OCP 2009]. The Baseline Surface 

Radiation Network has been expanded to 38 sites and has data records up to 15 years long at its 

original 6 stations [OCP 2008]. 

Atmospheric Radiation Measurement (ARM) Program: The ARM program has deployed a 

mobile facility that can be temporarily relocated around the world to provide the same type of 

comprehensive surface-based measurements of atmospheric radiation that are made at the ARM 

permanent sites. The mobile facility was first deployed in Point Reyes, California, as part of a 

joint study by the Department of Energy and the Department of Defense [OCP 2007], and was 

then deployed to Niamey, Niger, in 2006 [OCP 2008]. During this deployment, the mobile 

facility was able to make detailed observations of a large Saharan dust storm that took place over 

North Africa in March 2006 [OCP 2008]. In 2007 it was deployed to the Black Forest region of 

Germany, and a deployment to China is planned for 2008 [OCP 2009]. The results from the 

mobile facility can be compared to those obtained from the ARM fixed sites in the Arctic, 

tropics, and mid-latitudes. 

AmeriFlux: Based on systematic eddy-covariance measurements, AmeriFlux Network sites 

provide data on exchange of carbon dioxide (CO2), water vapor, and energy between air and 

 

13 The date of the Our Changing Planet reports refers to the fiscal year for which they were prepared, not the year in 

which they were published. 

III. Evaluation of Overall Progress by the CCSP 24
plant canopies for a range of different types of ecosystems. Time averages of these continuous 

measurements are expressed as net ecosystem exchange (NEE) for any selected time period, and 

the annual CO2 NEE, for example, is often termed net ecosystem production (NEP), or the 

quantity of carbon gained or lost by the system. Some AmeriFlux data products also include 

measures of respiration, which when combined with NEE enable estimates of gross primary 

production (GPP). Both NEP and GPP are important for calculating terrestrial carbon budgets 

and for carbon cycle analysis and modeling. AmeriFlux data products also include a number of 

corollary biometric and micrometeorological measurements that are used to understand carbon 

cycle processes. (See <public.ornl.gov/ameriflux/>.) 

Ground-Water Climate Response Network: The federal government, in cooperation with 

hundreds of federal, state, and local agencies, collects nationally consistent information about the 

Nation’s groundwater resources and helps define and manage those resources. The U.S. 

Geological Survey (USGS) maintains a network of wells to monitor the effects of droughts and 

other climate variability on groundwater levels. The Ground-Water Climate Response Network 

consists of a national network of about 140 wells monitored as part of the USGS Ground-Water 

Resources Program, supplemented by more than 200 wells monitored as part of the Cooperative 

Water Program that meet the same network criteria. 

Soil Climate Analysis Network (SCAN) and Snowpack Telemetry (SNOTEL): SCAN is an in situ

real-time observation network. The information from SCAN aids in data assimilation for drought 

assessments; flood response; integrated pest management; land productivity in relation to soil 

moisture and temperature changes; help in predicting shifts in wetlands, crops, and other 

ecosystems; and would serve as ground-truth for satellite soil moisture information. SCAN data 

are being examined for use in modeling carbon fluxes. At each SCAN site, minimum 

measurements include precipitation, relative humidity, wind speed, solar radiation, and 

barometric pressure above ground, and soil moisture and soil temperature at several depths 

below the surface. SNOTEL site measurements include snow depth and moisture content. (See 

<www.wcc.nrcs.usda.gov>.) 

U.S. Historical Climatology Network (USHCN): The USHCN is a high-quality data set of daily 

and monthly records of basic meteorological variables from over 1,000 observing stations across 

the 48 contiguous United States. Daily data include observations of maximum and minimum 

temperature, precipitation amount, snowfall amount, and snow depth from 1,062 stations; 

monthly data consist of monthly averaged maximum, minimum, and mean temperature and total 

monthly precipitation from 1,221 stations. Most of these stations are U.S. Cooperative Observing 

Network stations located generally in rural locations, while some are National Weather Service 

First Order stations that are often located in more urbanized environments. The USHCN has 

been developed over the years to assist in the detection of regional climate change and to analyze 

U.S. climate. 

III.1.b Satellite observations 

GRACE: The Gravity Recovery and Climate Experiment consists of two satellites flying in 

formation, from which spatial and temporal variability in Earth’s gravitational field can be 

measured and used to determine information about the distribution of mass in the Earth. Not only 

III. Evaluation of Overall Progress by the CCSP 25
can GRACE provide information about the relatively time-independent variability of the 

gravitational field associated with Earth’s solid matter, but it can provide information about 

shorter-term variability associated with changes in the distribution of water (both frozen and 

liquid). This latter variability can then be used to provide information about distributions of 

groundwater, mass of ice sheets, and ocean bottom pressure (from which deep ocean currents can 

be evaluated) [OCP 2007]. The ability of GRACE to make usefully accurate surface-water 

storage estimates over seasonal time scales demonstrates its potential for applied use, and the 

availability of comparisons between observed and modeled water storage estimates provides 

increased confidence in the global annual patterns and seasonal cycle of water storage. The 

satellite observations have shown some low-latitude biases in the models, however, and the 

availability of the satellite data should contribute to future improvement of models of water 

storage [OCP 2008]. 

ICESat: The Ice, Cloud, and Land Elevation Satellite was launched in January 2003. While it 

measures surface elevations of ice and land, vertical distributions of clouds and aerosols, and 

vegetation canopy heights, all with unprecedented accuracy, its primary emphasis has been on 

acquiring time series of ice-sheet elevation changes that can be used to determine the mass 

balance of ice sheets, most notably those in Greenland and Antarctica. Additional ice-related 

capabilities have included the ability to characterize detailed topographic features of ice sheets, 

ice shelves, and ice streams and the ability to gather new observations of sea-ice thickness 

distribution [OCP 2007]. 

SORCE: The Solar Radiation and Climate Experiment satellite was launched in 2003. This 

satellite contains four instruments measuring not only the total solar irradiance (TSI), but also 

spectrally resolved solar irradiance (SSI) over a very broad range of wavelengths. The SORCE 

satellite overlapped other U.S. satellites measuring TSI and SSI, allowing for improved 

capability for determining trends, which can be very challenging in the absence of overlap. This 

is due to the challenge to metrology that is associated with space-based solar irradiance 

measurement, for which the variation in TSI over a solar cycle is of the order of only 0.3% 

(peak-to-peak). Besides supporting long-term trend observations, SORCE has contributed to 

studies of shorter-term solar variability. In 2004, SORCE was able to document both changes in 

solar luminosity associated with the transit of Venus across the Sun and the largest solar X-ray 

flare ever recorded [OCP 2006]. The four-plus years of SORCE observations are providing 

previously unavailable information concerning the full wavelength dependence of solar 

irradiance over a significant fraction of the solar cycle, including at visible and infrared 

wavelengths where such data were previously only available over very brief periods (e.g., the 

length of a Space Shuttle mission) [OCP 2008]. Continuity of TSI measurements is planned with 

the Glory satellite to be launched in 2009. Additional TSI and SSI measurements are being 

considered as part of the plan for re-manifestation of the climate sensors that were lost from the 

National Polar-Orbiting Operational Environmental Satellite System as part of the NunnMcCurdy certification process completed in 2006. 

Aura: The Aura spacecraft, designed to study the trace constituent and aerosol composition of 

the troposphere and stratosphere, was launched on 15 July 2004. Aura contains four instruments 

whose measurement goals include ozone depletion, global air quality, and the radiative forcing 

of climate. It is an international mission, with instruments or components contributed by the 

III. Evaluation of Overall Progress by the CCSP 26
United Kingdom, the Netherlands, and Finland. While most of Aura’s instruments provide 

markedly enhanced capabilities over those flown previously, Aura can also provide continuity in 

measurements of ozone column and profile, aerosol distributions, and trace constituent profiles 

that were obtained from other platforms, most notably the Upper Atmosphere Research Satellite, 

which took data from 1991 through 2005, and the Total Ozone Mapping Spectrometer series of 

satellites [OCP 2006]. The Aura satellite flies as part of an international constellation (‘A-Train’) 

of polar-orbiting spacecraft whose equatorial crossing times are all within approximately 15 

minutes of each other in the early afternoon (~1:30 p.m. at the locality that they fly over). The 

near simultaneity of the measurements from the different spacecraft and instruments will allow 

for synergistic studies that would not be possible without the coordinated flights. Aura results are 

affecting a number of areas, including studies of the relationship between upper-tropospheric 

water vapor and ice that demonstrate the existence of a positive feedback associated with cloudinduced transport of moisture to the upper troposphere [OCP 2007] and those of how ozone, 

water vapor, and ozone-depleting trace gases are transported between the troposphere and 

stratosphere, which will allow for improvements in the models used to represent the response of 

the ozone layer to future concentrations of greenhouse gases [OCP 2008]. 

COSMIC: The Constellation Observing System for Meteorology, Ionosphere, and Climate 

consists of six satellites containing radio-occultation instruments based on the Global Positioning 

System technique for determining temperature and moisture profiles of the atmosphere. The 

system was launched on 14 April 2006. It is a joint U.S.–Taiwanese effort, and represents the 

first focused effort to use the radio-occultation technique to obtain a spatially distributed set of 

profiles (~2500 every 24 hours) that can be used for both climate studies and weather 

forecasting. The high vertical resolution of the radio-occultation technique and the absolute 

nature of the measurement will make the COSMIC data useful for climate studies [OCP 2007]. 

CloudSat and CALIPSO: The Cloud Satellite and Cloud-Aerosol Lidar and Infrared Pathfinder 

Satellite Observation satellites were launched in June 2006 to use radar and lidar observations, 

respectively, for determination of the three-dimensional distribution of clouds and aerosols in 

Earth’s atmosphere. These satellites were launched into the A-train constellation (see Aura 

section above), and the active remote-sensing techniques used on these spacecraft complement 

the passive techniques used on the other members of the constellation. Both CloudSat and 

CALIPSO represent partnerships—CloudSat with the Canadian Space Agency and the U.S. Air 

Force, and CALIPSO with the French space agency, CNES. Among the scientific issues being 

addressed by CloudSat and CALIPSO are the relationship between aerosols, clouds, and 

precipitation and the distribution of clouds over polar regions in winter [OCP 2009]. 

III.1.c Field campaigns 

TC4: CCSP scientists completed the Tropical Composition, Cloud, and Climate Coupling (TC4) 

experiment in Costa Rica during the summer of 2007. This field campaign, which included three 

large aircraft (ER-2, WB-57, DC-8) flying coordinated trajectories, focused on identifying and 

quantifying the chemical and dynamical processes that take place in Earth’s tropical tropopause 

layer. A particular emphasis was the study of the composition, formation, and radiative 

properties of clouds (cirrus and sub-visible cirrus) in this region. Besides the aircraft, the 

III. Evaluation of Overall Progress by the CCSP 27
campaign made use of data from the A-Train constellation of satellites, as well as three groundbased and balloon sonde stations [OCP 2009]. 

CLASIC: The Cloud and Land Surface Interaction Field Campaign was carried out in June 2007 

in the region surrounding the ARM network’s Southern Great Plains site in Oklahoma. Three 

‘supersites’ were heavily instrumented to obtain ground-based measurements linking observed 

carbon and moisture fluxes to atmospheric structure, and several research aircraft were used as 

well. This campaign was coordinated with the Mid-Continent Initiative study of the North 

American Carbon Program to help understand the influence of the land surface on atmospheric 

concentrations of aerosols, gases, and other constituents [OCP 2009]. 

ICARTT: The International Consortium for Atmospheric Research on Transport and 

Transformation was carried out in the summer of 2004 to study the factors involved in the 

intercontinental transport of pollution and its effects on radiation balance over North America 

and the North Atlantic. This campaign involved scientists from the United States and five other 

countries, working together with aircraft-, ship-, satellite-, and surface-based observations [OCP 

2006]. 

MILAGRO: The Megacity Initiative: Local and Global Research Observations was carried out in 

the spring of 2006, bringing together several planned field experiments that were addressing the 

impacts of large sources of air pollution in North America (most notably the Mexico City area) 

on the tropospheric trace constituent and aerosol distributions over a broader geographic region. 

This effort brought together measurement initiatives by CCSP agencies, and allowed for 

synergistic observations using surface-based, airborne, and satellite-based platforms. Results 

showed higher than expected formation of organic aerosols within the atmosphere, and should 

help improve future calculations of aerosol influences on climate [OCP 2006, 2007, 2008]. 

III.1.d Data management 

Giovanni: A new tool to facilitate the use of space-based remote-sensing data by eliminating a 

number of tedious steps that investigators would normally have to take before they could begin 

data analysis has been developed and implemented at the National Aeronautics and Space 

Administration’s (NASA) Goddard Space Flight Center. This tool is known as Giovanni 

(Goddard Earth Sciences Data and Information Services Center Interactive Online Visualization 

and Analysis Infrastructure), and will make it easier for scientists and other users to access data 

from NASA’s Earth Observing System. Among the steps previously required that will be 

eliminated through use of Giovanni are searching for appropriate data files, requesting them from 

a central archive, transferring the files to the user’s own computing system, and extracting the 

relevant data from potentially unfamiliar data formats. This online tool will allow scientists to 

spend a larger fraction of their time working with the actual data and thus better comprehend 

regional events and the interconnected nature of global environmental processes [OCP 2008]. 

(See <daac.gsfc.nasa.gov/techlab/giovanni/>.) 

Carbon Dioxide Information and Analysis Center (CDIAC): The Department of Energy (DOE) 

maintains CDIAC to provide comprehensive, long-term data management support, analysis, and 

information services in support of DOE climate change research, the global climate research 

III. Evaluation of Overall Progress by the CCSP 28
community, and the general public. CDIAC data support research on both the present-day carbon 

budget and temporal changes in carbon sources and sinks. Particular issues that can be addressed 

with CDIAC data include emission rates and atmospheric and oceanic concentrations of CO2, as 

well as information on fluxes of CO2 within the atmosphere and between the land surface and 

the atmosphere [OCP 2007]. In 2008, CDIAC will begin to offer CO2 measurements from buoys, 

research cruises, and volunteer observing ships, as well as syntheses of data from the Atlantic 

and Pacific regions [OCP 2009]. (See <cdiac.ornl.gov/>.) 

Global Change Master Directory (GCMD): The GCMD provides descriptive and spatial 

information about data sets relevant to global change research. It contains over 18,200 metadata 

descriptions of data sets coming from some 2,800 government agencies, research institutions, 

archives, and universities. Besides facilitating research on climate change, the GCMD contains 

data and metadata that support work in numerous Earth science disciplines, with a particular 

emphasis on supporting multidisciplinary research. Software upgrades are made periodically in 

response to user needs and to capitalize on new technology [OCP 2008]. (See 

<gcmd.nasa.gov/>.) 

Global Observing System Information Center (GOSIC): GOSIC is an operational data facility 

implemented through the National Oceanic and Atmospheric Administration’s (NOAA) National 

Climate Data Center on behalf of the global observing community. GOSIC provides information 

and facilitates easier access to data produced by the Global Climate Observing System, the 

Global Ocean Observing System, the Global Terrestrial Observing System, and other observing 

programs. While the data themselves may be highly distributed, the existence of a single entry 

point will facilitate the use of these data by a wide variety of users. GOSIC includes search 

capability across a broad range of international data centers to support full utilization of data sets 

produced by multiple nations [OCP 2009]. (See <www.gosic.org/>.) 

Earth Observing System Data and Information System (EOSDIS): EOSDIS provides convenient 

mechanisms for locating and accessing products from NASA’s Earth Observing System. Data 

may be obtained by users either electronically or on physical media. EOSDIS provides access to 

a very large set of data through the eight Distributed Active Archive Centers and several 

additional data sources. These centers process data products from instrument data, archive and 

distribute the resulting data, and provide a full range of user support. More than 2,100 distinct 

data products are archived and distributed through the Distributed Active Archive Centers and 

associated centers [OCP 2007]. (See <www.esdis.eosdis.nasa.gov/>.) 

Earth Science Research, Education, and Applications Solutions Network (REASoN): NASA’s 

REASoN program created 40 cooperative agreements that are designed to support the 

development of higher-level science products based on data from NASA’s satellite programs, to 

use these products to advance Earth system research, to develop and demonstrate new 

technologies for data management and distribution, and to contribute to interagency efforts to 

improve the maintenance and accessibility of data and information systems. The REASoN tasks 

were initiated in 2004 and most completed their work in 2007 [OCP 2007]. A follow-on 

program, Making Earth System Data Records for Use in Research Environments (MEaSUREs), 

was initiated in 2007. (See <reason-projects.gsfc.nasa.gov/>.) 

III. Evaluation of Overall Progress by the CCSP 29
Scientific Data Stewardship (SDS): The SDS is a paradigm for data management implemented at 

NOAA to use an integrated suite of functions for preserving and exploiting the full scientific 

value of environmental data. The functions include careful monitoring of system performance for 

long-term applications, generating authoritative long-term records from multiple observing 

platforms, assessing the state of Earth system components, and proper archiving of and timely 

access to data and metadata [OCP 2006]. 

National Data Centers: The Comprehensive Large-Array Data Stewardship System is NOAA’s 

data system for providing data from current and future polar-orbiting and geostationary 

operational satellite systems. The system will provide permanent, secure storage and safe, 

efficient access between data centers and customers. The system will be sized to handle a 

significant increase in data rates that will occur as the next generation of observing systems is 

implemented. Initial accomplishments include implementation of free Internet-based customer 

access to data from NOAA’s Geostationary Operational Environmental Satellite [OCP 2006]. 

(See <www.class.ngdc.noaa.gov/>.) 

III.1.e Modeling 

IPCC Model Analyses and Evaluation: Three U.S. groups carried out a number of model runs to 

support the development of the IPCC Fourth Assessment Report. These groups were those at the 

National Center for Atmospheric Research, the NOAA Geophysical Fluid Dynamics Laboratory, 

and the NASA Goddard Institute for Space Studies. All models are consistent in projecting 

global warming in the 21st century in response to anthropogenic forcing. They include improved 

representation of the El Niño–Southern Oscillation (ENSO) compared to previous models, 

although they do not fully represent many aspects of ENSO including its effects on climate in 

North America and elsewhere [OCP 2008]. Data from these model runs are available from the 

Program for Climate Model Diagnosis and Intercomparison (PCMDI; <www-pcmdi.llnl.gov/>) 

at the Lawrence Livermore National Laboratory. PCMDI, together with its university and 

government laboratory partners, is applying its collective expertise to support modeling studies 

initiated by the IPCC. It is providing facilities for the storage and distribution of terascale data 

sets from multiple coupled atmosphere–ocean general circulation model (GCM) simulations of 

present-day climate as well as climate changes resulting from large transient increases in carbon 

dioxide [OCP 2006]. 

North American Regional Climate Change Assessment Program (NARCCAP): NARCCAP is an 

effort funded by multiple agencies to meet the climate scenario needs of the United States and 

Canada. It aims to develop an ensemble of regional climate change scenarios for North America 

and develop and apply statistical methods to systematically investigate the uncertainties in future 

climate projections at the regional level. NARCCAP uses a multi-model approach that includes 

four global atmosphere–ocean coupled models to provide global climate change simulations; six 

regional climate models to downscale the global simulations; and two global atmosphere models 

in time-slice experiments to provide climate change scenarios at the same horizontal resolution 

as the regional climate models. These regional and global simulations contributed to the IPCC 

Fourth Assessment Report. The NARCCAP model archive is accessible from the Earth System 

Grid. (See <www.narccap.ucar.edu>.) 

III. Evaluation of Overall Progress by the CCSP 30
III.2. Program Outputs 

This section describes CCSP progress toward its overarching goals since 2003, when the 

program’s initial Strategic Plan was released. 

III.2.a Goal 1: Improve knowledge of the Earth’s past and present climate and 

environment, including its natural variability, and improve understanding of the causes of 

observed variability and change. 

CCSP research requires a solid foundation of observations, monitoring data, and analyses of 

these data to provide a better understanding of Earth system processes, to understand the 

magnitude and extent of climate variations, and to test and to improve models. Analyses of 

collected observations and monitoring data underpin all aspects of climate system study. In the 

past few years, key analyses of collected data have provided important insights into 

understanding the nature and variability of the Earth system. 

One of the most notable advances has been the increased certainty ascribed by the IPCC to the 

influence of humans on climate. The 2001 IPCC Working Group I report (IPCC, 2001) 

concluded that “...most of the observed warming over the last 50 years is likely to have been due 

to the increase in greenhouse gas concentrations.” By 2007, the IPCC (2007a) had strengthened 

that conclusion to “very likely.” This change was made in part on the basis of work carried out 

under the auspices of CCSP, including expanded observational records, improvements in the 

simulation of many aspects of climate, and a series of new attribution studies that evaluated 

whether observed changes are quantitatively consistent with the expected response to external 

forcings and inconsistent with alternative physically plausible explanations. 

Another example of a body of work with profound impact is the progress made in understanding 

temperature and moisture changes at continental scales, and in understanding the magnitude of 

climate change at high latitudes, in the Arctic and Antarctic regions of Earth. Analysis of 

temperature and moisture records together with satellite images and ground-based measurements 

for North America and Europe show that both continents are experiencing earlier transitions 

from winter to summer. Warmer spring temperatures are causing earlier spring green-up of 

vegetation, together with longer growing seasons overall. Observations in the western United 

States also indicate that the annual peak in spring river runoff is occurring earlier in the season 

and is supplying less water during the growing season (Mote et al., 2005). At high latitudes, 

satellite, airborne, and ground-based observations suggest that significant changes are occurring 

in the mass balance of the Greenland and Antarctic Ice Sheets. These changes are inferred to be 

caused by warming at high latitudes. New satellite-based observations of the polar regions 

indicate significant reductions in the volume of the Greenland Ice Sheet (Velicogna and Wahr, 

2005), declining Arctic sea ice cover, and loss of ice mass in Antarctica despite no measurable 

change in snowfall over the last 50 years (Velicogna and Wahr, 2006). Analyses of observations 

together with climate model simulations suggest that the pattern of high-latitude temperature 

change is more readily explained by human activity and natural climate forcing than by internal 

variability alone. 

III. Evaluation of Overall Progress by the CCSP 31
Another key temperature finding comes from the body of analytical work reported in the CCSP 

synthesis and assessment report, Temperature Trends in the Lower Atmosphere: Steps for 

Understanding and Reconciling Differences (Karl et al., 2006). Previously reported data showing 

discrepancies between the amounts of warming near the surface and higher in the atmosphere 

have been used to challenge the reliability of climate models and the reality of human-induced 

global warming. In these earlier observations, surface data showed substantial global-average 

warming, while early versions of satellite and radiosonde data showed little or no warming above 

the surface. Now that errors in the satellite and radiosonde data have been identified and 

corrected, the discrepancy no longer exists. New data sets have also been developed that do not 

show such discrepancies. For recent decades, all current atmospheric data sets now show globalaverage warming that is similar to the surface warming. A next step is to improve climatemodeling results: while these observations are consistent with the results from climate models at 

the global scale, discrepancies in the tropics remain to be resolved. In the meantime, these 

observations and modeling results have provided increased scientific confidence in our 

understanding of observed climatic changes and their causes. 

It is critical to observe and understand variations in the average state of particular climate 

parameters like temperature and precipitation, but it is as important to understand natural 

variability and changes in the frequency or intensity of extreme events like drought and 

unusually wet conditions. CCSP has made important progress in understanding the climate 

system’s natural variability. One of the most well known variability events is the El Niño 

phenomenon, which recurs on a time scale of approximately two to seven years and involves a 

warming of the eastern tropical Pacific in combination with changes in atmospheric circulation. 

Recent research links decadal changes in this pattern to droughts and wet conditions over North 

America and suggests that a portion of such decadal changes may be predictable (Seager et al., 

2005). Additional recent accomplishments on the topic of drought used the geologic record to 

reconstruct drought history over the past millennium, using proxy records derived from tree rings 

and sediment cores (Cook et al., 2004). This research indicates that recent U.S. droughts, while 

severe enough to cause substantial impacts to society, are relatively minor in comparison to 

naturally occurring droughts over the past thousand years. Interpreting changes in the 

characteristics of extreme events remains one of CCSP’s ongoing research frontiers. 

Additional work on atmospheric conditions provided an improved understanding of climate 

influences on ozone distribution. Using satellite measurements corroborated by surface 

measurements, an important study found increases in ozone in the Antarctic middle stratosphere 

during Southern Hemisphere summer (December). Model simulations showed that these 

increases were caused by the delayed transition from dynamic spring conditions to more stable 

summer conditions due to the spring ozone hole. The lengthening of less-stable spring dynamics 

forces the descent of ozone-rich air from higher levels of the atmosphere to the mid-stratosphere 

(about 30 km altitude). The same study also found that future greenhouse gas increases would 

produce similar ozone increases. Another study found that doubling of CO2 caused a 

strengthening of the atmospheric circulation responsible for the global distribution of ozone. The 

results of this study indicate that total ozone will increase at high latitudes of the Northern and 

Southern Hemispheres and decrease in the tropics. 

III. Evaluation of Overall Progress by the CCSP 32
Oceans, too, are experiencing change. Observations of global sea level increases are consistent 

with the declining volume of land ice as well as observations of ocean warming, which 

contributes to sea level rise by expanding ocean volume. Observations of the North Atlantic 

indicate a reduction in salinity (Curry and Mauritzen, 2005), which climate system models 

indicate may lead to a slowdown of the large-scale ocean circulation that transports heat to highlatitude regions (Stouffer et al., 2006a). 

Ocean heat storage is the largest component of Earth’s climate system for storing the energy 

imbalance between the sources and sinks of thermal energy, thus trends in this storage rate are 

important to quantify. Global-scale observations of ocean temperature indicate an overall pattern 

of warming (e.g., Levitus et al., 2001). Such trends are generally consistent with climate model 

projections that include greenhouse gas forcings (e.g., Barnett et al., 2005). However, 

observation-based estimates of ocean heat storage show significant regional interannual and 

decadal-scale variations. Recent studies indicate that the Southern Hemisphere warming may 

have been underestimated (Gille, 2008), and that correction of the systematic instrumentation 

errors results in diminished decadal variability and a larger, steadier long-term trend (Wijffels et 

al., 2008). 

III.2.b Goal 2: Improve quantification of the forces bringing about changes in the 

Earth’s climate and related systems. 

It is essential to understand the factors responsible for global environmental change in order to 

make long-term climate projections. These forcing factors include changing levels of greenhouse 

gases, land cover changes, airborne dust and other particles (aerosols), and solar variability. 

CCSP research gives considerable attention to identifying and quantifying the effects of these 

forcing factors, and to understanding the ways in which factors cause feedbacks among Earth 

systems. As in the previous goal, the following examples of progress toward CCSP Goal 2 result 

from the integrated focus of multiple CCSP research elements. 

Climate and the global carbon cycle are a tightly coupled system where changes in climate affect 

the transfer of atmospheric CO2 to the terrestrial biosphere and the ocean, and vice versa. An 

important conclusion of recent carbon cycle research is that future warming is likely to lead to a 

further decrease in the efficiencies of land and ocean in absorbing excess CO2 (i.e., a positive 

feedback) (Fung et al., 2005). This assessment is based on advances in U.S. and global carbon 

observations and improvements in carbon cycle models. Controlled experiments on carbon 

uptake and release in ecosystems are one means of improving our understanding of carbon cycle 

dynamics, which can contribute to corresponding carbon cycle model improvements. For 

example, Free-Air Carbon Dioxide Enrichment experiments, in which CO2 is purposely injected 

into the air around a small plot of land, have led to the conclusion that the mass of carbon in 

ecosystems initially tends to increase when exposed to increased levels of CO2 (Jastrow et al., 

2005; Norby et al., 2005). This increase may be limited by the availability of nutrients, although 

a comprehensive meta-analysis indicates that nitrogen supply generally keeps pace with plant 

demands in natural systems (Luo et al., 2006). Other controlled experiments in which ecosystems 

are purposely warmed generally indicate greater ecosystem CO2 release with higher 

temperatures. However, there are still significant uncertainties associated with the biospheric 

III. Evaluation of Overall Progress by the CCSP 33
response to climate change, particularly with respect to the complex and dynamic nature of 

ecosystems and their interactions with climate and the hydrologic cycle. 

CCSP has made significant advances in understanding the processes responsible for the 

production and destruction of other greenhouse gases, including methane and nitrous oxide. For 

example, recent analyses estimate that approximately 60% of all methane emissions from 

wetlands occur in the tropics (Melack et al., 2004). In polar regions, recent studies have 

elucidated processes by which carbon, currently trapped either as organic matter or methane 

hydrates in the permafrost (frozen soil), is released to the atmosphere (Zimov et al., 2006). 

Warming increases these releases and can create an amplifying feedback loop. Another example 

of progress on non-carbon greenhouse gases is the work that has improved understanding of 

interactions between climate variability and near-surface ozone, which is a health hazard. 

One of the largest uncertainties in projections of potential future climate change is the role of 

aerosols. Recent research has reduced some of this uncertainty, in part through efforts made 

possible by the Climate Change Research Initiative. In the first phase of preparing the synthesis 

and assessment report that deals with aerosol properties and their impacts on climate, a 

comprehensive paper has been published that reviews recent progress in characterizing aerosols 

and assessing the direct effect of aerosols on climate change (Yu et al., 2006). This work served 

as a major resource in the preparation of the IPCC Fourth Assessment Report. 

Another important recent advance is improved estimates of the amount of carbon being 

sequestered in North America and globally, and in particular, how the rate of carbon uptake is 

changing in all ecosystems. These estimates are made through the innovative combination of 

carbon cycle models and observations of carbon concentrations and isotopes (Fung et al., 2005). 

A key goal of the North American Carbon Program is to further improve estimates of carbon 

sources and sinks. Work of this nature is vital for assessing the efficacy of natural carbon uptake, 

as well as the potential for purposeful carbon capture in managed ecosystems. Observations of 

ocean carbon are important for addressing uncertainties associated with the global carbon 

budget. New global-scale ocean carbon analyses indicate increasing carbon concentrations in 

ocean water. In addition to confirming the oceans as a significant carbon sink, this information is 

also being used to estimate the increase in ocean acidity caused by increasing amounts of 

dissolved CO2 and the potentially deleterious consequences for marine ecosystems (Orr et al., 

2005). Recent measurements of carbon sedimentation along continental shelves have shown 

these regions to be responsible for a significant fraction of oceanic carbon uptake (Muller-Karger 

et al., 2005). 

Recent climate warming has been particularly intense in boreal and Arctic regions, leading to 

concern that increasing air temperature in these ecosystems may lead to an increase in the 

incidence of wildfires. Longer growing seasons lead to increased fire fuel loads, and increased 

temperatures can lead to drier conditions. In turn, wildfire provides rapid release of carbon and 

produces aerosols that affect atmospheric conditions. Beyond the emission of CO2 and other 

greenhouse gases, understanding the consequences of large-scale wildfires for climate is 

challenging due to the many additional ways in which they influence the lower atmosphere and 

surface of Earth. A recent study in Alaska found that there was intensification in the climate 

warming in the first year after a major fire but a slight decrease in the local climate warming 

III. Evaluation of Overall Progress by the CCSP 34
when averaged over the 80 years of the study. The long-term result, which was primarily due to 

plant regrowth increasing the summer reflectivity of the burned surface, appeared to be more 

significant than the greenhouse gases emitted by the fire (Randerson et al., 2006). The study 

results suggest that future increases in wildfire in some parts of the boreal zone of Alaska may 

have different feedbacks to global warming than previously thought. 

Scientists are also studying the possibility that increased permafrost thawing due to warming in 

Arctic regions could cause the release of substantial amounts of carbon long held in the frozen 

tundra. There appear to be two potential mechanisms for the carbon to reach the atmosphere: 

drainage of the carbon-rich river flow into the Arctic Ocean with subsequent emission, and direct 

respiration or recycling of the newly thawed carbon. Measurements made in the Yukon River 

Basin in northern Canada have shown that the latter process predominates (Guo and MacDonald, 

2006). These results have significance for understanding both the movement of carbon through 

Arctic landscapes, and the potential effects of that carbon on ecosystems and the atmosphere. 

CCSP’s interdisciplinary research on the carbon cycle has also produced a set of analyses using 

long-term observations of several young and mature forests. Results from this work show that 

forest carbon storage has been increasing in these ecosystems and is not in balance with the 

carbon lost by respiration and decay. This result is contrary to the contemporary concept of near 

balance of carbon sources and sinks in mature forests (Zhou et al., 2006). The gain in forest 

carbon is typical of findings from U.S.-based large-scale networks, as well as observations made 

in mature forests in China. Therefore, evidence is mounting that these sinks for atmospheric CO2

offer significant potential for modulating the rate of atmospheric CO2 increase (Urbanski et al., 

2007). 

In western states, large changes in land cover and land use have occurred over the past century, 

with rapidly expanding urbanization along the Pacific coast and extensive agricultural 

development inland. Researchers exploring the effects of urbanization and agriculture on 

regional climate have found that irrigated agriculture in California tended to lower average and 

maximum near-surface air temperatures, while conversion of natural vegetation to urban areas 

tended to increase near-surface air temperatures. The surface temperature changes and their 

associated effects on the atmosphere also caused changes in the regional airflow. Overall, it was 

found that conversion of natural vegetation to irrigated agriculture has likely had a larger effect 

on the climate of California than urban growth, but that increased conversion of irrigated land to 

urban/suburban development could alter this conclusion (Kueppers et al., 2007). 

III.2.c Goal 3: Reduce uncertainty in projections of how the Earth’s climate and related 

systems may change in the future. 

Uncertainty provides a measure of the reliability of forecasts of future climate and related 

Earth systems. Reducing uncertainty is crucial to providing decisionmakers with tools for 

assessing strategies for adaptation, mitigation, and other forms of risk reduction. However, the 

wording of the goal is incomplete—reducing uncertainty is only part of the story. Improving 

the projections themselves and understanding both the nature and implications of uncertainties 

are equally important, as noted in the NRC metrics report (NRC, 2005b). The thrust of this 

goal is, therefore, to improve projections, and to characterize their uncertainty, in order to 

III. Evaluation of Overall Progress by the CCSP 35
improve the utility of projections of how Earth’s climate and related systems may change in 

the future. CCSP has significantly advanced the ability to estimate future Earth system 

conditions at time scales ranging from months to centuries and at spatial scales ranging from 

regional to global. The primary tools for Earth system prediction and projection are computer 

models that reflect the best available knowledge of Earth system processes. Reducing 

uncertainty requires continual integration of observations and modeling across the full range of 

climate and Earth systems research. 

When recent model simulations of the climate of the past 100 years are compared to observations 

at coarse temporal and spatial scales, the results generally indicate improvements over previous 

generations of models, including the ability to represent weather systems, climate variability 

(e.g., monsoons and El Niño), ocean processes (e.g., the Gulf Stream), surface hydrology, and 

other Earth system processes, components, and dynamics (Collins et al., 2006; Schmidt et al., 

2006). One of the ways in which these models have advanced is through improvements in the 

representation of the processes responsible for key Earth system feedbacks such as those 

associated with water vapor, clouds, sea ice, and the carbon cycle (Delworth et al., 2006; 

Gnanadesikan et al., 2006; Wittenberg et al., 2006). 

For a model to produce a realistic climate projection, it must include realistic representations of 

physical processes such as cloudiness, precipitation, and solar energy. Understanding the 

influence and feedbacks of clouds on climate has proven to be a challenging task for scientists 

worldwide, but progress is being made. Recent innovative studies using newly developed, 

detailed models of cloud processes that are coupled with a global climate model provide results 

that are significantly more consistent with observations than traditional cloud modeling 

techniques. The incorporation of improved cloud representation in climate models is expected to 

reduce the uncertainty in predictions of the global and regional water cycle and surface climate. 

Energy from the Sun not reflected back to space provides the driving energy to Earth’s weather 

and climate systems. Clouds are a major component in the global reflectance of sunlight. Yearto-year variability in the global reflectance is dominated by the variability of cloudiness in the 

tropics. On the other hand, scientists have recently found little change in the year-to-year 

variability of reflectance at middle and high latitudes despite decreases in the highly reflective 

snow and sea ice cover. This result appears to be due to the compensating increase in cloud cover 

balancing the decreasing surface-level reflectance. Clouds continue to provide the largest source 

of uncertainty in model estimates of climate sensitivity, although a recent study finds evidence 

that, in most climate models used in the IPCC Fourth Assessment Report, clouds provide a 

positive feedback. 

The magnitude of future warming will be strongly influenced by the extent to which atmospheric 

water vapor concentration increases in response to an initial warming caused by increases in CO2

and other greenhouse gases. An accurate representation of this feedback in climate models is 

critical for making long-term climate projections. Recent innovative analyses have shown that 

water vapor increases in the upper atmosphere measured by satellites and balloon-borne sensors 

are generally consistent with state-of-the-art climate model simulations, lending credence to the 

ability of current models to represent the water vapor feedback (Cess, 2005; Soden et al., 2005). 

III. Evaluation of Overall Progress by the CCSP 36
Analyses of climate model simulations generated for the IPCC Fourth Assessment Report have 

identified several additional characteristics of climate change projections common to all of the 

models (Held and Soden, 2006). Examples of these robust model projections include strong 

sub-tropical drying, weakening of large-scale tropical atmospheric motions, and expansion of 

the poleward upper atmospheric wind pattern known as the Hadley circulation. In another 

study, several models were used to investigate the effects of the freshwater input from melting 

ice and glaciers on the currents in the North Atlantic (Stouffer et al., 2006a). These currents are 

important due to their large-scale transport of heat. The study concluded that, in response to 

expected levels of freshwater input in the northern North Atlantic, the average modeled largescale deep ocean current weakens by about 30% by the end of the century. All models simulate 

some weakening of this deep circulation, but no model simulates a complete shutdown of it. 

CCSP researchers also use the geological record to test and apply climate models, particularly in 

cases where that knowledge has a bearing on climate change processes relevant to current 

society. One such analysis involves the largest known extinction in Earth’s history, which took 

place approximately 250 million years ago at the Permian–Triassic boundary when 

approximately 95% of marine and 75% of terrestrial species were lost. In this study, a climate 

model simulation indicated that the elevated levels of CO2 during this period led to climatic 

conditions inhospitable to both marine and terrestrial life (Kiehl and Shields, 2005). It is 

hypothesized that a critical level of high-latitude warming was reached where the connection of 

oxygen-rich surface waters to the deep ocean was dramatically reduced—thus leading to a 

shutdown of marine biologic activity, which in turn led to increased atmospheric CO2 and 

accelerated warming. 

The historical record provides a broader set of observations to test and apply climate models to 

help reduce uncertainty in their projections of the future. A recent study used a simple model to 

attempt to reproduce paleoclimate reconstructions of Northern Hemisphere temperature over the 

past seven centuries in response to estimated solar, volcanic, and greenhouse gas forcing during 

this period (Hegerl et al., 2006). This study suggests that, for the current century, very high 

climate sensitivities predicted by some models for a doubling of atmospheric greenhouse gas 

concentrations are less likely than previously thought. 

The CCSP modeling strategy utilizes a multi-tiered approach in which new and improved Earth 

system sub-models (e.g., clouds, ecosystem dynamics, and sea ice) are developed and tested by 

individual researchers or small research teams. When significant improvements in these submodels arise, they are integrated as appropriate into high-end Earth system models. A result of 

these ongoing efforts is a set of U.S. models that expand beyond earlier atmosphere–ocean 

models to include relatively sophisticated representations of land surface hydrology, sea ice, 

ecosystems, and atmospheric chemistry. Several U.S. Earth system modeling centers have used 

variations of these models to produce ensembles of projections that are providing important new 

perspectives on potential future climate system change (Meehl et al., 2004a; Stouffer et al., 

2006b). These ensembles are also being used to characterize the intrinsic uncertainty associated 

with potential future climate change. 

A set of new high-resolution climate model simulations has been completed for North America 

that provides information at a scale finer than 100 km x 100 km (Han and Roads, 2004; Leung et 

III. Evaluation of Overall Progress by the CCSP 37
al., 2004; Mason, 2004; Wood et al., 2004).The ability of these regional-scale models to 

represent climate processes is being assessed.14 These regional and global simulations, based on 

models developed at U.S. institutions, contributed to the IPCC Fourth Assessment Report. 

Because Earth system models are extremely complex and benefit greatly from input and 

evaluation by multiple research teams, several new efforts have been initiated to enable sharing, 

testing, and improvement of these models by diverse groups of researchers (Meehl et al., 2004b, 

2005). Many of the recent model simulations referred to above are now widely available through 

a new capability for data archiving and dissemination developed by the PCMDI.15 Large strides 

have been made in creating climate model code according to a set of standards that facilitates 

exchange of sub-models (e.g., the Earth System Model Framework), which enables researchers 

to readily trace the source of differences between various models and between models and 

observations. The U.S. Climate Variability and Predictability Program (CLIVAR) is exploring a 

new approach for bringing together observers, theorists, and high-end modelers to improve key 

model deficiencies (USCLIVAR, 2002; Bretherton et al., 2004). This approach is attempting to 

significantly reduce the time lags that often exist between the observation of key climate 

processes and the integration of these processes into more comprehensive Earth system models. 

Several high-end Earth system modeling efforts in the United States, which involve many 

different, independent research teams, are using these types of new collaborative approaches and 

tools to evaluate, improve, and integrate model components. 

The projections made by CCSP research pertain not just to physical climate, but also to other 

components of the Earth system, including atmospheric chemistry. Continuing research has 

provided an estimate that the recovery of the Antarctic ozone hole will occur approximately 10 

to 20 years later than the previous estimate of 2050 (Newman et al., 2006). As a result of the 

Montreal Protocol and its amendments, the use of ozone-depleting substances has been greatly 

reduced. Improved understanding of atmospheric dynamics now gives 2001 as a better estimate 

of when the peak in ozone-depleting substances occurred in the Antarctic stratosphere. This date 

is later than had been estimated previously and results in a longer projected time scale for 

recovery back to pre-1980 (unperturbed) levels of these substances. 

III.2.d Goal 4: Understand the sensitivity and adaptability of different natural and 

managed ecosystems and human systems to climate and related global changes. 

Goals 4 and 5 are expected to be areas of growth for CCSP over the coming years, in keeping 

with the increased availability of results from Goal 1 through 3 as inputs. Progress is reported 

here for the past several years to the present. Significant advances have been made in 

understanding the potential impacts of climate change and in the improvement of methodologies 

as new information has become available. CCSP research typically uses many different sources 

of information, including analyses of paleoclimate data, direct monitoring and observations, 

process studies, and model-based projections. Increasingly, research also accounts for the 

dynamic nature of the response of human and natural systems to climate change. 

 

14 See <www.narccap.ucar.edu>. 

15 See <www-pcmdi.llnl.gov>. 

III. Evaluation of Overall Progress by the CCSP 38
Research to understand ecosystem and human system sensitivity and adaptability encompasses 

and integrates a wide range of potential impacts on societal needs such as water, health, and 

agriculture, as well as potential impacts on natural terrestrial and marine ecosystems. This 

integration is exemplified by the development of several of CCSP’s synthesis and assessment 

products (SAPs), particularly the seven products under Goal 4. One example is SAP 4.1, which 

combines census population data, topographic elevation, shore protection, and land use 

information to study the potential socioeconomic impacts of different scenarios of sea level rise. 

Other important research into the potential implications of sea level rise also points to the need to 

account for a wide variety of factors when assessing future impacts. For example, some measures 

to protect coastlines may carry negative side effects, such as the potential for wetlands loss when 

inland barriers are constructed, preventing the wetlands from migrating inland in response to 

rising sea level (Cahoon et al., 2006). 

Similarly, the adaptability of complex coastal ecosystems is becoming better understood. For 

example, studies in a Chesapeake Bay marsh ecosystem showed that rising sea level, increasing 

CO2, and high rainfall can interact and improve the growth of a relatively tall bulrush at the 

expense of a hay-like cordgrass that grows in thick mats (Erickson et al., 2007). The thick 

cordgrass mats trap sediment and organic material more effectively than do the bulrushes, and so 

bulrush-dominated marshes are less able to rise in elevation through the addition of sediments. 

Such changes in species composition, caused by interacting global change factors, can therefore 

influence the ability of coastal marshes to adapt and keep abreast of sea level rise. 

Carbon cycle scientific research and modeling are highly relevant to carbon management needs, 

as demonstrated by a recent study that estimated the spatial variability of net primary production 

and potential biomass accumulation over the conterminous United States (Potter et al., 2007). 

This study’s model-based predictions indicate a potential to remove carbon from the atmosphere 

at a rate of 0.3 Gt of carbon per year through afforestation of low-production crop and rangeland 

areas. This rate of carbon sequestration could offset about one-fifth of the annual fossil fuel 

emissions of carbon in the United States. 

Observational and modeling studies of terrestrial ecosystems indicate a wide variety of changes 

in which it appears that climate variations play a significant role. For example, recent evidence 

indicates a northward expansion of the ranges of many bird and butterfly species in the United 

States corresponding to warming in the region (Sekercioglu et al., 2004). Declines in Arctic sea 

ice, observed both in situ and by satellites, have been linked to increasing vulnerability of polar 

bear populations (ACIA, 2005). Satellite and in situ observations also indicate a trend toward 

earlier growth of spring vegetation (Angert et al., 2005). In addition to temperature and 

hydrologic changes, the increasing level of atmospheric CO2 is thought to play a role in changing 

ecosystem distributions and characteristics due to its fertilizing effect. Agricultural yield models 

account for this effect and project a range of agricultural impacts depending on the magnitude 

and nature of future climate change, crop types, and the types of adaptive measures that are 

adopted. Recent research indicates that different strategies may be required to manage insects, 

weeds, and diseases in agricultural systems (Ziska and Runion, 2006). 

Another example of the ecological consequences of climate change involving insects and 

affecting adaptability is the devastation of millions of acres of western U.S. and Canadian pines 

III. Evaluation of Overall Progress by the CCSP 39
by bark beetles during the warmth and drought of 2000 to 2004. Recent modeling and 

observations revealed that beetles invading the northernmost lodgepole pine trees are now only a 

few miles from previously pristine jack pine populations (Logan and Powell, 2007). This may 

create a direct pathway of invasion to valued pine forests in the eastern United States and 

Canada. 

CCSP’s integrated approach to understanding the sensitivity and adaptability of natural 

ecosystems to climate change has also been applied in remote, high-latitude regions. The West 

Antarctic Peninsula is experiencing some of the largest, most rapid warming on Earth, which is 

causing loss of sea ice and increased snow precipitation. In turn, these changes are having major 

contrasting impacts on the adaptability of different penguin species. For example, in the vicinity 

of Anvers Island near the West Antarctic Peninsula during the last three decades, populations 

shifted south, so that local abundance of the ice-dependent and snow-intolerant Adelie penguins 

decreased by 65% (currently about 5,000), while the abundance of Chinstraps and Gentoos 

increased by 2,730% and 4,600% (currently about 300 and 650), respectively (Ducklow et al., 

2007). Climate warming in the Canadian Arctic has caused significant declines in total cover and 

thickness of sea ice and progressively earlier ice breakup in some areas. These changes have had 

a substantial effect on polar bear populations, causing them to extend their normal fast for longer 

periods during the open-water season (Stirling and Parkinson, 2006). 

Components of CCSP research funded in part through the U.S. Joint Global Ocean Flux Study 

Program have explored ecosystem impacts in the open ocean resulting from climate variability 

and change as well as from changes in ocean chemistry and thermal structure. An example of a 

chemical impact is the chain of events causing the oceans to become more acidic due to chemical 

changes resulting from the absorption of increasing concentrations of atmospheric CO2 (Orr et 

al., 2005). Ocean warming tends to increase vertical stratification (layering) and thus slow the 

overturning of nutrient-rich deep-ocean waters (Schmittner, 2005). Recent model projections 

suggest that increased ocean acidification and increased layering of the upper ocean due to 

warming are likely to reduce plankton production. These model results are supported by satellite 

observations indicating significant changes in photosynthetic plankton concentrations, including 

declines in the North Atlantic and Pacific and increases in the Indian Ocean (Gregg et al., 2003). 

In addition to managed ecosystems, CCSP research has expanded understanding of the 

sensitivity and adaptability of a variety of other societal sectors. One of these is human health, 

which may be affected by changes in temperature and storm intensity, or through changes in 

distributions of insects that carry pathogens. An example of research in this area is the effects of 

climate change on heat waves. Observational and modeling work suggests that the probability of 

heat waves such as the one that occurred in Europe in 2003 has increased significantly, and that 

future warming may make heat waves of similar magnitude a normal summer occurrence within 

several decades (Meehl and Tebaldi, 2004). Recent research on the societal dimensions of 

climate variations has shown that physical climate analyses, such as the aforementioned study of 

heat waves, must be assessed within a complex fabric of other social and environmental factors 

(Poumadere et al., 2005). An example is the general increase in financial losses due to hurricanes 

over the past century, which is probably attributable more to expanding coastal development than 

to any changes in hurricane characteristics (Pielke et al., 2005). In regions such as central Africa, 

where the capacity to adapt to environmental variations is often relatively low, recent research 

III. Evaluation of Overall Progress by the CCSP 40
has shown strong correlations between year-to-year climate variations and malaria outbreaks 

(Thomson et al., 2006). 

These are a few examples of CCSP’s research examining the sensitivity and adaptability of 

human and natural systems to climate variability and change. It is clear from this work that 

climate variations can have both beneficial and adverse effects on environmental and 

socioeconomic systems. However, future projections indicate that the larger the magnitude and 

rate of climate change, the more likely it is that adverse effects will dominate (NRC, 2002). 

III.2.e Goal 5: Explore the uses and identify the limits of evolving knowledge to manage 

risks and opportunities related to climate variability and change. 

The Nation’s basic research on global environmental variability and change has led toward a 

number of important opportunities for applying that knowledge to help decisionmakers make 

good choices supported by sound science. Over the past several years, CCSP has taken three 

main approaches to explore and communicate the potential uses and limits of this knowledge: 1) 

developing scientific syntheses and assessments; 2) supporting and exploring adaptive 

management and planning capabilities; and 3) developing methods to support climate change 

policy inquiries. Some key examples of progress are provided below. 

The key focus of the program’s synthesis and assessment activities is its current suite of 21 

SAPs, which are intended to provide current evaluations of the science foundation for use in 

informing public debate, policy, and operational decisions and for defining and setting the future 

direction and priorities of the program. For example, SAP 1.1 (Karl et al., 2006) largely resolved 

a long-standing debate concerning an apparent discrepancy of global temperature trends at the 

surface and in the lower atmosphere; SAP 2.1a (Clarke et al., 2007) provided key policy-relevant 

insights regarding technology options for stabilizing greenhouse gas concentrations; and SAP 4.4 

(Julius et al., 2008) frames an important set of considerations for management of ecosystems on 

federally managed lands. These reports examine various aspects of climate change science and 

impacts on a national, regional, or sectoral basis as appropriate to each topic. A description of 

each product and its schedule for completion can be found at 

<www.climatescience.gov/Library/sap/sap-summary.php>. 

Another important focus for the program’s synthesis and assessment activities has been its 

involvement in the IPCC and other international assessment activities, including providing 

coordination and support to the IPCC Fourth Assessment Report. The IPCC’s major activity is to 

prepare at regular intervals these comprehensive assessments of policy-relevant scientific, 

technical, and socioeconomic information appropriate to the understanding of human-induced 

climate change, potential impacts of climate change, and options for mitigation and adaptation. 

Approximately 120 U.S. scientists are IPCC authors and 15 are review editors. The United States 

co-chairs and hosts IPCC Working Group I, which primarily addresses physical science aspects 

of climate change. The United States has also played significant roles in the World 

Meteorological Organization (WMO) / United Nations Environment Programme (UNEP) ozone 

assessments (e.g., WMO, 2007), the Arctic Climate Impact Assessment (ACIA, 2005), and the 

Millennium Ecosystem Assessment (MEA, 2005), among others. 

III. Evaluation of Overall Progress by the CCSP 41
The second of CCSP’s decision-support approaches is the exploration of adaptive management 

strategies. Activities under this approach develop and evaluate options for adjusting to variability 

and change in climate and other conditions through ‘learning by doing’ and integrating 

knowledge with practice. This area of work grows out of the insight that a key to assessment and 

decision support is close and ongoing interaction between users and producers of information. 

While this area is less well developed than some others in CCSP’s portfolio, it is a strong and 

integral component of the missions of several CCSP member agencies, and those agencies 

provide extensive stakeholder engagement and decision support for adaptation, mitigation, and 

management of risk to policymakers and land managers at national, regional, state, and local 

levels. 

As one example of the development of methods to support climate change policy inquiries, 

CCSP scientists developed and documented a ‘water supply stress index’ that calculates water 

shortage risks across the conterminous United States. The index is based on models and 

observations that integrate the effects of climate, land cover, and current water uses by 

municipalities and industries on water supply (Sun et al., 2006). The water supply stress index 

and the methods associated with it will be used by local and regional decisionmakers to quantify 

the likelihood of future water shortages under changing climate, water, and land uses for 

determining adaptation practices. Incorporation of the subsurface water table into regional 

climate models is important, since land cover changes produce significant effects on the water 

table and the hydrologic cycle. Shallow water tables can be either a sink or source of water to the 

surface soil, depending on the relative balance of infiltration versus evaporation (Fan et al., 

2007). Recent studies using detailed observations and regional climate models have found that 

the fraction of rainfall that either recharges groundwater or ends up as streamflow tends to 

decrease when the fraction of land devoted to agriculture increases. This result suggests that 

intensive agriculture can amplify surface water stresses, particularly during drought conditions 

(Jayawickreme and Hyndman, 2007). 

Another example of this work is an ongoing project that brings together researchers who study 

climate processes and their effects on the U.S. Southwest with individuals and organizations that 

need climate information to make informed decisions (Jacobs et al., 2005). Numerous tangible 

benefits from this project have helped a wide variety of decisionmakers, from state and local 

water planners to farmers to public health officials. For example, the project developed a suite of 

products that make predictions of water availability months in advance, allowing water managers 

to adjust reservoir levels accordingly to meet the competing demands for this scarce resource. 

Yet another example is the combined use of satellite-based observations of fires and moisture 

conditions together with seasonal climate forecasts to provide information to fire managers to 

help them make early and effective decisions about the resources they will need to cope with 

emerging fires and fire-season dangers. One way in which this information is communicated is 

through annual workshops targeted separately at eastern and western U.S. fire hazards, which 

bring together climate scientists and forecasters with fire managers to produce seasonal fire 

outlooks.16 There are many other examples of the exploratory use of seasonal-to-interannual 

climate information for decisionmaking both domestically and internationally. 

 

16 See <www.ispe.arizona.edu/climas/conferences/NSAW>. 

III. Evaluation of Overall Progress by the CCSP 42
In addition to CCSP’s work on adaptive management at seasonal-to-interannual time scales, the 

program is also developing valuable information for long-term (decades to centuries) adaptation 

issues. One example is the program’s analyses of ways in which agricultural practices might be 

adjusted to take advantage of rising CO2 levels and to cope with potentially warmer temperatures 

and decreased moisture availability (Boote et al., 2005). Recent work has shown that sufficient 

variability exists within some crop species to begin selecting for crop varieties that could 

maintain or increase yields in a future enhanced CO2 environment. 

An example of regional decision support is the work carried out by the Consortium for Atlantic 

Regional Assessment (CARA), which is providing data and tools to help decisionmakers 

understand how outcomes of their decisions could be affected by potential changes in both 

climate and land use. On an interactive, user-friendly Web site, CARA has organized data on 

climate (historical records and future projections from seven global climate models), land cover, 

and socioeconomic and environmental variables to help inform local and regional 

decisionmakers.17 The CARA tools and tutorials are designed to help decisionmakers understand 

the issues related to land use and climate change by gathering, organizing, and presenting 

information for evaluating alternative mitigation strategies. 

A workshop involving scientists and managers, co-led by several CCSP agencies under the 

auspices of the U.S. Coral Reef Task Force, resulted in the publication of A Reef Manager’s 

Guide to Coral Bleaching (Marshall and Schuttenberg, 2006). The combined research results 

among state/territorial, federal, academic, nongovernmental, and international scientists 

concluded that warming sea surface temperatures are a key factor in mass coral bleaching events. 

The guide provides managers with strategies to support the natural resilience of coral reefs in the 

face of climatic change. 

CCSP’s third decision-support approach is to help inform inquiries related to climate change 

policy, in part by using comparative analyses of climate change scenarios. One example is a 

collaborative effort between climate scientists and New York City water infrastructure planners 

that is using regional-scale hydrologic scenarios to inform the long-lasting investments that are 

being considered in the modernization of the city’s water supply system.18 Another example is 

the application of carbon cycle research to assess the potential feasibility, magnitude, and 

permanence of a variety of different carbon sequestration options (Sarmiento et al., 1999). An 

initial result from this line of work is the preliminary conclusion that the restoration of inland 

wetlands could be a particularly efficient means for sequestering carbon in North American 

prairie lands (Euliss et al., 2006). 

CCSP researchers have also developed new metrics for estimating greenhouse gas emissions and 

carbon sequestration in the agricultural and forestry sectors (Birdsey, 2006). These sectors can 

reduce atmospheric greenhouse gas concentrations by increasing carbon sequestration in biomass 

and soils, by reducing fossil fuel emissions through use of biomass fuels, and by substituting 

agricultural and forestry products that require less energy than other materials to produce. The 

 

17 See <www.cara.psu.edu>. 

18 See <www.ccsr.columbia.edu/cig/taskforce>. 

III. Evaluation of Overall Progress by the CCSP 43
DOE’s National Greenhouse Gas Registry is using the new metrics as the basis for reporting 

greenhouse gas information from the agricultural and forestry sectors.19 

Another important way in which CCSP is helping to inform climate change policy inquiries is 

through integrated assessment modeling, which considers the social and economic factors that 

may lead to climate change (e.g., greenhouse gas emissions) and the resultant effects of those 

activities on the Earth system and human welfare. These models are useful for considering the 

costs and effects of various policy options. One important result of this work suggests that 

reducing emissions of greenhouse gases other than CO2 could be an economically efficient first 

step in reducing the overall atmospheric burden of greenhouse gases (Hansen and Sato, 2004). 

Another important new set of analyses assesses various policy options while accounting for 

inherent scientific and economic uncertainty (Webster et al., 2003). 

These examples collectively provide a sketch of the progress CCSP has made toward its goals. 

Additional information on these items and others can be found in CCSP’s annual report to 

Congress, Our Changing Planet. While these accomplishments, taken together, represent 

significant progress in climate change science and constitute a substantial portfolio of work, 

many questions remain to be answered within each of CCSP’s strategic goals. The remainder of 

the report presents science findings, many of which originated from CCSP-sponsored work, 

organized by topic. 

 

19 See <www.eia.doe.gov/oiaf/1605/frntvrgg.html>. 

III. Evaluation of Overall Progress by the CCSP 44
III. Evaluation of Overall Progress by the CCSP 45
IV. Trends and Projections of Global Environmental Change 46

 

Section IV: Trends and Projections of Global Environmental 

Change 

IV.1. Radiative Forcing and Observed Climate Change 

Radiative forcing is a change caused by an environmental factor in the balance of solar 

radiation coming into and the infrared and reflected shortwave radiation going out of the 

atmosphere. As such, measures of radiative forcing—expressed in watts per square meter 

(W/m2

)—can be used to weigh the relative importance of various factors, such as greenhouse 

gases or land cover, in terms of their contribution to climate change. A positive forcing means 

the factor causes a warming effect, and a negative forcing means the factor causes a cooling 

effect. 

This section describes radiative forcing and the factors that contribute to it, as well as 

observations of climate change both globally and in the United States. This Scientific 

Assessment does not delineate specific sectoral contributions to increasing greenhouse gas 

levels, such as the contributions from the transportation sector described in OFCM (2002). 

The radiative forcing values presented here come from the Working Group I contribution to 

the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (IPCC, 

2007a) and show the change between the pre-industrial time of 1750 and the year 2005. These 

values represent global averages—they are the result of global changes in atmospheric 

concentrations of greenhouse gases and other factors, and are therefore not specific to the 

activities of any country in isolation. 

IV.1.a Radiative forcing due to greenhouse gases and other factors 

Greenhouse gases have a positive forcing (a warming effect) because they absorb and radiate 

outgoing infrared radiation that would otherwise escape directly into space. This radiation by 

greenhouse gases occurs in all directions, with a net effect of warming the air and surface 

below (in comparison to the absence of greenhouse gases). Water vapor is the most important 

naturally occurring greenhouse gas. However, direct anthropogenic emissions of water vapor 

correspond to less than 1% of natural sources and make a negligible contribution to radiative 

forcing. 

The level of scientific understanding of human influences on climate improved significantly 

between the IPCC’s Third Assessment Report (IPCC, 2001) and the Fourth Assessment Report 

(IPCC, 2007a). In 2007, the IPCC reported the global average net radiative forcing was +1.6 

(+0.6 to +2.4) W/m2

. The IPCC concluded with very high confidence (at least a 9 out of 10 

chance of being correct)20 that human activities have led to a net warming of the climate. 

20 Definitions for terms used in statements of confidence and likelihood can be found in Section II.3: 

Characterization of Uncertainty. 
Figure IV.1 shows the IPCC’s (2007d) globally averaged radiative forcing estimates for 

anthropogenic greenhouse gas emissions and other factors for 2005. 

Figure IV.1. Global average radiative forcing (RF) estimates and ranges in 2005 for anthropogenic carbon 

dioxide (CO2), methane (CH4), nitrous oxide (N2O) and other important factors, together with the typical 

geographical extent (spatial scale) of the forcing and the assessed level of scientific understanding (LOSU). 

The net anthropogenic radiative forcing and its range are also shown. These require summing asymmetric 

uncertainty estimates from the component terms and cannot be obtained by simple addition. Additional 

forcing factors about which there is a very low level of scientific understanding are not included here. 

Volcanic aerosols, which also contribute a natural forcing, are not included in this figure due to their 

episodic nature. The range for linear contrails does not include other possible effects of aviation on 

cloudiness. Source: IPCC (2007d). 

Carbon dioxide, methane, and nitrous oxide 

The greenhouse gases carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) persist in 

the atmosphere for decades to centuries to longer periods. As such, their emission has long-term 

impacts on climate. The combined radiative forcing due to the cumulative increase (from 1750 to 

2005) in atmospheric concentrations of CO2, CH4, and N2O is +2.30 W/m2

, with an uncertainty 

range of +2.07 to +2.53 W/m2

. The observed accumulation of these gases in the atmosphere is 

primarily from human activities, and as such, the positive radiative forcing from these gases is 

primarily anthropogenic in origin as well. The IPCC (2007d) stated that the rate of increase in 

IV. Trends and Projections of Global Environmental Change 47
positive radiative forcing due to these three greenhouse gases during the industrial era is very 

likely (greater than 90% probability)21 to have been unprecedented in more than 10,000 years. 

The largest positive radiative forcing is due to CO2 (+1.66 ± 0.17 W/m2

), and the trend for the 

radiative forcing from CO2 is increasing. The period from 1995 to 2005 showed a 20% increase 

in its radiative forcing—the largest change for any decade in the last 200 years. The IPCC 

concluded, “Emissions of CO2 from fossil fuel use and from the effects of land use change on 

plant and soil carbon are the primary sources of increased atmospheric CO2” (Solomon et al., 

2007). 

The next largest components of positive radiative forcing are CH4 (+0.48 ± 0.05 W/m2

) and N2O 

(+0.16 ± 0.02 W/m2

). Current atmospheric CH4 levels are almost triple that of the pre-industrial 

era. These levels are due to continuing anthropogenic emissions of CH4, which are greater than 

natural emissions (Solomon et al., 2007). N2O increases in the atmosphere since pre-industrial 

times are also primarily from human activities, in particular land use associated with agriculture. 

Fluorocarbons, chlorocarbons, and sulfur hexafluoride 

The ozone-depleting substances covered under the Montreal Protocol—chlorofluorocarbons 

(CFCs), hydrochlorofluorocarbons (HCFCs), and chlorocarbons—are also strong greenhouse 

gases. As a group, they contributed +0.32 ± 0.03 W/m2

 to anthropogenic radiative forcing in 

2005. In the two decades since the Montreal Protocol, the atmospheric concentrations of CFCs 

peaked and have since begun to decline. Increases in HCFCs have slowed notably in recent 

years. In response, the radiative forcing from CFCs and HCFCs peaked in 2003 and is now 

beginning to decline (Forster et al., 2007). 

These ozone-depleting substances have reduced global stratospheric ozone levels by about 4% 

from 1980 levels (IPCC, 2007d). The decreased levels of stratospheric ozone are estimated to 

have a slightly negative radiative forcing effect of -0.05 ± 0.10 W/m2. This estimate has a 

medium level of scientific understanding (Solomon et al., 2007). 

As a group, hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6) 

had a total radiative forcing in 2005 of +0.017 ± 0.002 W/m2 (Forster et al., 2007). While the 

current atmospheric concentrations of these gases—all anthropogenic compounds—are small, 

their concentrations are rapidly increasing (Solomon et al., 2007). 

Ozone 

Ozone (O3) in the troposphere (the atmosphere from Earth’s surface to 10–16 km altitude) has a 

positive radiative forcing, with an estimated value of +0.35 (+0.25 to +0.65) W/m2. Unlike the 

greenhouse gases mentioned above, tropospheric O3 is much shorter lived in the atmosphere. Its 

atmospheric lifetime is of the order of weeks to months (as opposed to decades to centuries for 

the well-mixed greenhouse gases) and is therefore not as well mixed in the global atmosphere. 

Tropospheric O3 is a secondary product of photochemistry in the atmosphere and changes in its 

 

21 Definitions for terms used in statements of confidence and likelihood can be found in Section II.3: 

Characterization of Uncertainty. 

IV. Trends and Projections of Global Environmental Change 48
abundance are driven by changes in emissions of its precursors, mainly nitrogen oxides, carbon 

monoxide, and hydrocarbons, which all have significant anthropogenic sources. 

According to the World Meteorological Organization (WMO, 2007), spring polar O3 depletion 

continues to be severe when winter temperatures in the polar stratosphere (above approximately 

10 km) are particularly cold. The average concentration of atmospheric O3 outside of polar 

regions is no longer declining, as it was in the 1990s. Measurements from some stations in 

unpolluted locations indicate that ultraviolet radiation levels have been decreasing since the late 

1990s, in accordance with observed ozone increases. However, ultraviolet radiation levels are 

still increasing at some Northern Hemisphere stations as a consequence of long-term changes in 

other factors such as clouds and atmospheric particulates that also affect ultraviolet radiation. 

Trends in tropospheric O3 vary in sign and magnitude depending on location. It is noted that low 

latitudes show significant upward trends in tropospheric O3 (Solomon et al., 2007). 

Water vapor 

Although water vapor is the most important naturally occurring greenhouse gas, direct 

anthropogenic emissions of water vapor correspond to less than 1% of natural sources and make 

a negligible contribution to radiative forcing. However, as temperatures increase, tropospheric 

water vapor concentrations increase, representing a key feedback but not a forcing of climate 

change (Solomon et al., 2007). Feedbacks are defined as processes in the climate system (such as 

a change in water vapor concentrations) that can either amplify or dampen the system’s initial 

response to radiative forcing changes (NRC, 2003). From this perspective, water vapor is 

excluded as a radiative forcing agent in Figure IV.1. The IPCC (Solomon et al., 2007) concluded 

that observations demonstrate increases in both lower-tropospheric and upper-tropospheric water 

vapor since at least the 1980s. 

Water vapor in the stratosphere (the atmosphere at 10 to 50 km above Earth’s surface) is 

increasing due to oxidation of CH4 and is estimated to have a positive radiative forcing of +0.07 

± 0.05 W/m2

 (Solomon et al., 2007). The concentration of any species, including water vapor, 

changes with altitude, often in relatively predictable ways, such that the concentration profile in 

relation to altitude will have a predictable ‘structure’ to it. The contribution of CH4 to the 

corresponding change in this vertical structure of the water vapor near the tropopause is 

uncertain and leads to a low overall level of scientific understanding for this radiative forcing 

component. The IPCC also noted that other potential human causes of stratospheric water vapor 

increases that could contribute to radiative forcing are poorly understood (Solomon et al., 2007). 

Aerosols 

Aerosols are non-gaseous substances, either solid particles or liquid droplets, suspended in the 

atmosphere. Some aerosols, for example sulfate aerosols that are mainly the result of sulfur 

dioxide (SO2) emissions, exert a negative forcing (cooling effect) by reflecting and scattering 

incoming solar radiation. Whereas, other types of aerosols, for example black carbon, absorb 

incoming solar radiation, leading to a positive forcing. As such, anthropogenic emissions of 

aerosols contribute to both positive and negative radiative forcing. These direct interactions of 

IV. Trends and Projections of Global Environmental Change 49
aerosols with radiation are listed under the aerosol direct effect radiative forcing term in Figure 

IV.1. 

In addition to directly reflecting solar radiation, aerosols cause an additional, indirect negative 

forcing effect by enhancing cloud albedo, which is a measure of reflectivity or brightness. This 

occurs because aerosols act as particles around which cloud droplets can form. An increase in the 

number of aerosol particles leads to a greater number of smaller cloud droplets, which leads to 

enhanced cloud albedo. Aerosols also influence cloud lifetime and precipitation, but these effects 

are considered part of the climate response and the IPCC made no central estimates of these 

indirect forcing effects. 

The uncertainties in the radiative forcing estimates associated with the direct and indirect effects 

of aerosols are better understood now than at the time of the IPCC Third Assessment Report 

(2001a). Nevertheless, they remain the dominant uncertainty in radiative forcing (IPCC, 2007d). 

In particular, significant changes in the estimates of the direct radiative forcing due to biomassburning, nitrate, and mineral dust aerosols have occurred since the Third Assessment Report 

(2001a). The IPCC (2007d) estimated that the net effect of all aerosols (primarily sulfate, organic 

carbon, black carbon, nitrate, and dust) is a cooling effect, with a total direct radiative forcing of 

-0.5 (-0.9 to -0.1) W/m2

 and an additional indirect cloud albedo (i.e., enhanced reflectivity) 

forcing of -0.7 (-1.8 to -0.3) W/m2. Black carbon aerosols cause yet another forcing effect by 

decreasing the surface albedo of snow and ice (+0.1 (0.0 to +0.2) W/m2

). 

Land cover changes, linear contrails, and solar irradiance 

Three last radiative forcing estimates are listed in Figure IV.1. Changes in surface albedo due to 

human-induced land cover changes, such as deforestation, exert a forcing of -0.2 (-0.4 to 0.0) 

W/m2. Persistent linear contrails from global aviation contribute a small radiative forcing of 

+0.01 (+0.003 to +0.03) W/m2

. Changes in solar irradiance since 1750 are estimated to have 

caused a radiative forcing of +0.12 (+0.06 to +0.30) W/m2. 

IV.1.b Observed temperature changes 

Observational evidence from all continents and most oceans shows that many natural systems are 

being affected by regional climate changes, particularly temperature increases (IPCC, 2007b). 

Temperature trends 

Numerous lines of evidence robustly lead to the conclusion that the climate system is warming. 

The IPCC (2007d) stated in its Fourth Assessment Report, “Warming of the climate system is 

unequivocal, as is now evident from observations of increases in global average air and ocean 

temperatures, widespread melting of snow and ice, and rising global average sea level.” Section 

IV.2 describes the extent to which changes in global and continental temperature and other 

climate factors have been observed. 

Of the measurable climate properties, air temperature is the most easily measured, directly 

observable, and geographically consistent indicator of climate change. Data from thousands of 

IV. Trends and Projections of Global Environmental Change 50
worldwide observation sites on land and sea are used to calculate surface temperature, with 

interpolation (approximation of the values based on discrete points) within areas of the globe 

where no observational data exists. Although biases may exist in surface temperatures (changes 

in station exposure and instrumentation over land, or changes in measurement techniques by 

ships and buoys in the ocean), these biases are likely (66 to 90% probability) to be largely 

random and, therefore, will cancel out over large regions such as the tropics or the entire globe 

(Karl et al., 2006). Likewise, the heat island effects of urban areas are real, but local. They have 

been accounted for in the analyses of temperature records and have not biased the large-scale 

trends (Trenberth et al., 2007). 

The IPCC (Trenberth et al., 2007) reported the following trends in global surface temperatures: 

• Global mean surface temperatures have risen, especially in the past three decades. The 

temperature rise over the last 100 years (1906–2005) is 0.74 ± 0.18 °C when estimated by a 

linear trend (purple line in Figure IV.2). The rate of warming over the last 50 years (0.13 ± 

0.03 °C per decade; orange line in Figure IV.2) is almost double that for the past 100 years 

(0.07 ± 0.02 °C per decade). These conclusions are based on analyses of multiple qualitycontrolled data sets of in situ temperature observations described in Trenberth et al. (2007). 

Figure IV.2. Annual global mean temperatures (black dots) with linear fits to the data. The left-hand axis 

shows temperature anomalies relative to the 1961 to 1990 average and the right-hand axis shows 

estimated actual temperatures, both in °C. Linear trends are shown for the last 25 (yellow), 50 (orange), 

100 (purple), and 150 years (red). The smooth blue curve shows decadal variations with the decadal 90% 

error range shown as a pale blue band about that line. The total temperature increase from the period 

1850 to 1899 to the period 2001 to 2005 is 0.76 ± 0.19 °C. Source: Solomon et al. (2007). 

IV. Trends and Projections of Global Environmental Change 51
• The warmest years in the instrumental record of global (ocean and land combined) surface 

temperatures on record (since the mid-19th century) have mainly occurred in the past 12 

years. Including 2007, 7 of the 8 warmest years on record have occurred since 2001 and the 

10 warmest years have all occurred since 1995. The warmest two years are 1998 and 2005 

(1998 ranks first in one estimate, and 2005 ranks slightly higher in the other two estimates). 

The years 2002 to 2004 are the third, fourth, and fifth warmest years since 1850. The year 

2007 tied for second warmest in the period of instrumental data, behind the record warmth of 

2005, according to the National Aeronautics and Space Administration (NASA GISS, 

2008a). The global land surface average temperature in 2007 was the warmest on record, 

according to both NASA and National Oceanic and Atmospheric Administration (NOAA) 

analyses. 

• The global average diurnal temperature range (the average range the temperature spans—

from high temperature to low temperature—over the course of a day) has stopped decreasing. 

Previously (1950s through 1980s), the diurnal temperature range had been decreasing at a 

rate of approximately 0.1 °C per decade, but the diurnal temperature range has not changed 

since the 1980s because daytime and nighttime temperatures have increased at approximately 

the same rate. 

Radiosonde (balloon-borne) instruments have been used over the past 50 to 60 years, and 

satellites have been used over the past 28 years to measure the temperature throughout the 

troposphere (from Earth’s surface to 10–16 km altitude) and in the stratosphere (10–50 km above 

Earth’s surface). The Climate Change Science Program (CCSP) SAP 1.1 (Karl et al., 2006) 

concluded that the most recent versions of all available data sets show that both the surface and 

troposphere have warmed, while the stratosphere has cooled, which is in accord with our 

understanding of the effects of radiative forcing. 

 

The IPCC conclusions (Trenberth et al., 2007) are consistent with the major conclusions of SAP 

1.1. The IPCC reported that new analyses of radiosondes and satellite measurements of global 

lower- and mid-tropospheric temperature show warming rates that are consistent within their 

respective uncertainties in comparison to one another and are similar to those of the surface 

temperature record. This largely resolved a discrepancy reported in the IPCC Third Assessment 

Report (2001a) concerning observations of greater warming at the surface than in the lower 

atmosphere. The range of global surface warming since 1979 is between 0.16 and 0.18 °C per 

decade, depending on the data set used. Estimates of tropospheric temperatures measured by 

satellite show a warming range of 0.12 to 0.19 °C per decade. The agreement between the 

surface warming rates and those measured by radiosondes for the lower troposphere is not as 

close as that for satellites; however, the radiosonde record is notably less spatially complete than 

the surface record, and increasing evidence suggests that it is very likely that a number of 

radiosonde records have a cooling bias, especially in the tropics. 

Globally averaged lower-stratospheric temperatures have cooled since 1979, which is one of the 

expected results from increasing atmospheric concentrations of greenhouse gases. Estimates 

from adjusted radiosondes, satellites, and reanalyses are in rough agreement, suggesting a lowerstratospheric cooling of between 0.3 and 0.6 °C per decade since 1979. 

IV. Trends and Projections of Global Environmental Change 52
Like global mean temperatures, U.S. temperatures also warmed during the 20th and into the 21st 

century. U.S. annual average temperature is now approximately 0.6 °C warmer than at the start 

of the 20th century, according to NOAA (2008a), with an increased rate of warming over the 

past 30 years. Temperatures in the period from the 1930s through the 1970s changed little both 

over the United States and globally. There was a general warming trend from about 1915 to the 

1930s. 

The NOAA data for the average annual temperature for the contiguous United States shows that 

1998 was the warmest year on record, with 2006 and 1934 as the next warmest (within 0.06 °C 

of the record set in 1998). The average U.S. temperatures in these years fall within the error bars 

of the analyses (see Figure IV.3). Due to differences in data sets, processing, and analysis, 

NASA found that 1934 was the warmest on record for the contiguous United States, followed by 

1998 and 1921. The fourth warmest year on record in its analysis was 2006. The warmth in 1934 

was associated with extreme dryness in the continental interior. 

 

The year 2007 was the 10th warmest year for the contiguous United States since national records 

began in 1895, according to preliminary data from the NOAA National Climatic Data Center 

(NOAA, 2008a). Additionally, the past 9 years have all been among the 25 warmest years on 

record for the contiguous United States, a streak that is unprecedented in the historical record, 

and the last nine 5-year periods22 were the warmest 5-year periods in the last 113 years of 

national records, illustrating the anomalous warmth of the last decade. 

Figure IV.3. Temperature averaged annually over the lower 48 states from 1895 to 2007. Source: 

NOAA/NCDC (NOAA, 2008b) 

 

22 The last nine 5-year periods referred to here are: 2003–2007, 2002–2006, 2001–2005, 2000–2004, 1999–2003, 

1998–2002, 1997–2001, 1996–2000, 1995–1999. 

IV. Trends and Projections of Global Environmental Change 53
U.S. temperature data analyzed by NASA (NASA GISS, 2008b) show similar trends over the 

past 100 years. 

Temperature patterns 

The rates of temperature change are not the same for different regions around the globe (see 

Figure IV.4). 

• Over the oceans, warming has occurred in both sea surface temperature and nighttime marine 

air temperature. The average global surface air temperature over the oceans has risen at a rate 

of 0.13 °C per decade since 1979. Recent warming is strongly evident at all latitudes in sea 

surface temperatures over each of the oceans (Trenberth et al., 2007). 

• Land regions have warmed faster than the oceans—about double the ocean rate after 1979 

(more than 0.27 °C per decade), with the greatest warming during winter (December to 

February) and spring (March to May) in the Northern Hemisphere (Trenberth et al., 2007). 

• The warming in the last 30 years is widespread over the globe and is greatest at higher 

northern latitudes. In the past 100 years, average Arctic temperatures increased at almost 

twice the global average rate. Arctic temperatures have high decadal variability, and a warm 

period was also observed from 1925 to 1945 (Trenberth et al., 2007). 

• For 1901 to 2005, the rate of warming is statistically significant over most of the world’s 

surface with several exceptions: an area south of Greenland, three smaller regions over the 

southeastern United States, and parts of Bolivia and the Congo basin. At about 20% of the 

locations where there is no statistically significant warming, it is likely that the lack of 

significant warming is driven by changes in atmospheric circulation (Karoly and Wu, 2005). 

• Enhanced warming relative to the global average has been observed in other locations, 

notably over the continental interiors of Asia and northwestern North America and over some 

mid-latitude ocean regions of the Southern Hemisphere as well as southeastern Brazil (see 

Figure IV.4). Since 1979, warming has been strongest over western North America, northern 

Europe, and China in winter; over Europe and northern and eastern Asia in spring; over 

Europe and North Africa in summer; and over northern North America, Greenland, and 

eastern Asia in autumn (Trenberth et al., 2007). 

IV. Trends and Projections of Global Environmental Change 54
Figure IV.4. Linear trend of annual temperatures for 1901 to 2005 (left; °C per century) and 1979 to 2005 (right; °C 

per decade). Areas in grey either have insufficient data to produce reliable trends or have slightly negative trends. 

The minimum number of years needed to calculate a trend value is 66 years for 1901 to 2005 and 18 years for 1979 

to 2005. An annual value is available if there are 10 valid monthly temperature anomaly values. The data set used 

was produced by NCDC from Smith and Reynolds (2005 in Trenberth et al., 2007). Trends significant at the 5% 

level are indicated by white + marks. Source: Trenberth et al. (2007). 

Regional data for North America confirm that warming has occurred throughout most of the 

United States. The U.S. Historical Climate Network of NOAA’s National Climatic Data Center 

found that for all but 3 of the 11 climate regions, the average temperature increased more than 

0.6 °C between 1901 and 2005 (NOAA, 2007b). An area of particularly pronounced warming is 

Alaska, where the temperature has increased at a rate of 1.8 °C per century. Although the 

Southeast experienced a very slight cooling trend over the entire period (-.02 °C per century), 

this region shows warming since 1979. Figure IV.5 shows a map of regional U.S. temperature 

trends for the period 1901 to 2005, using an analysis of data from the U.S. Historical Climate 

Network (NOAA Version 1). 

The IPCC (Field et al., 2007), describing temperature changes for all of North America in its 

assessment of regional temperatures, stated that for the period 1955 to 2005, the greatest 

warming occurred in Alaska and northwestern Canada, with substantial warming in the 

continental interior and modest warming in the southeastern United States and eastern Canada. 

Additionally, daily minimum (nighttime) temperatures in North America have warmed more 

than daily maximum (daytime) temperatures, and spring and winter show the greatest changes in 

temperature. 

IV. Trends and Projections of Global Environmental Change 55
Figure IV.5. U.S. trends of annually averaged temperature for the period 1901 to 2006 in °F per century. Data are 

from the Historical Climate Data Network. Source: NOAA/NCDC. 

Temperature extremes 

Climate is defined not only by average temperature and precipitation, but also by the type, 

frequency, and intensity of extreme events. Observed changes in climate extremes related to 

temperature, precipitation, tropical cyclones, and sea level apply generally to all parts of the 

globe, including the United States, although there are some regional and local exceptions due to 

patterns of natural climate variability (IPCC, 2007d). Substantial changes in the frequency of 

extreme events are expected from a shift in the mean temperature based on simple statistical 

reasoning (Solomon et al., 2007). Trends in events that occur at small spatial scales (such as 

tornadoes, hail, lightning, and dust storms) cannot be determined due to insufficient data 

(Solomon et al., 2007). 

Widespread changes in extreme temperatures have been observed in the last 50 years around the 

world, and these changes in temperature extremes are consistent with the general warming 

(Solomon et al., 2007). On a global basis, cold days, cold nights, and frost have generally 

become less frequent, while hot days, hot nights, and heat waves have become more frequent 

(IPCC, 2007d). A widespread reduction in the number of frost days in mid-latitude regions, an 

IV. Trends and Projections of Global Environmental Change 56
increase in the number of warm extremes, and a reduction in the number of daily cold extremes 

are observed in 70 to 75% of the land regions where data are available. The most marked 

changes are for cold nights (lowest 10%, based on 1961 to 1990), which have become rarer over 

the 1951 to 2003 period. Warm nights (highest 10%) have become more frequent. Additionally, 

beginning in the second half of the 20th century, heat waves have increased in duration 

(Solomon et al., 2007). 

North America regional studies consistently show patterns of changes in temperature extremes 

consistent with a general warming (Trenberth et al., 2007), including intense warming of the 

lowest daily minimum temperatures over western and central North America (Robeson, 2004 in 

Trenberth et al., 2007). The IPCC (Trenberth et al., 2007) cautions that although the observed 

changes of the high- and low-temperature tails of the distributions may be consistent with a 

simple shift of the entire temperature distribution, the reality is often more complicated. 

CCSP SAP 3.3 (Karl et al., 2008) concluded that in the United States there has been a shift 

towards a warmer climate with an increase in extreme high temperatures and a reduction in 

extreme low temperatures: 

• Since the record hot year of 1998, 6 of the last 10 years (1998–2007) have had annual 

average temperatures that fall in the hottest 10% of all years on record for the United States. 

The number of heat waves (extended periods of extremely hot weather) also has been 

increasing since 1950 (however, the heat waves of the 1930s remain the most severe in the 

U.S. historical record). 

• There have been fewer unusually cold days in the United States during the last few decades 

and the last 10 years have had fewer severe cold waves than any other 10-year period in the 

historical record, which dates back to 1895. There has been a decrease in frost days and a 

lengthening of the frost-free season over the past century. For the United States as a whole, 

the average length of the frost-free season over the 1895 to 2000 period increased by almost 

two weeks. 

Surface temperature over the last 2,000 years 

The observed warming in the instrumental temperature records (which only began in the late 

19th century, when the global network of measurements became large enough for reliable 

computation of global mean temperatures) can be put in a historical context by examining longer 

temperature records from sources such as tree rings, corals, ocean and lake sediments, cave 

deposits, ice cores, boreholes,23 glaciers, and documentary evidence. 

A National Research Council study (NRC, 2006c) assessing the state of scientific efforts to 

reconstruct surface temperature records for Earth over approximately the last 2,000 years 

concluded the following: 

 

23 Deep, narrow holes drilled vertically on land. 

IV. Trends and Projections of Global Environmental Change 57
• Overall, there is a generally consistent picture of temperature trends during the preceding 

millennium. In particular, relatively warm conditions centered around A.D. 1000 (identified 

by some as the ‘Medieval Warm Period’) and a relatively cold period (or ‘Little Ice Age’) 

centered around 1700 are evident. (Figure IV.6 shows the aggregate results from several 

large-scale surface temperature reconstructions.) 

• It can be said with a high level of confidence (8 out of 10 chance)24 that global mean surface 

temperature was higher during the last few decades of the 20th century than during any 

comparable period during the preceding four centuries. The observed warming in the 

instrumental record shown in Figure IV.2 supports this conclusion. The IPCC (Solomon et 

al., 2007) concluded that it is very likely that average Northern Hemisphere temperatures 

during the second half of the 20th century were warmer than any other 50-year period in the 

last 500 years. 

• Less confidence can be placed in large-scale surface temperature reconstructions for the 

period from A.D. 900 to 1600. Presently available proxy evidence indicates that temperatures 

at many, but not all, individual locations were higher during the past 25 years than during any 

period of comparable length since A.D. 900. The uncertainties associated with reconstructing 

hemispheric mean or global mean temperatures from these data increase substantially 

backward in time through this period and are not yet fully quantified. On the basis of the 

NRC’s conclusions and other sources of information, the IPCC (Solomon et al., 2007) 

concluded that average Northern Hemisphere temperatures during the second half of the 20th 

century were likely the warmest in at least the past 1,300 years. It also concluded that a 

substantial fraction of the decadal temperature variability of the seven centuries preceding 

1950 is very likely attributable to natural external forcing (Hegerl et al., 2007). 

• Very little confidence can be assigned to statements concerning the hemispheric mean or 

global mean surface temperature prior to about A.D. 900 because of sparse data coverage and 

because the uncertainties associated with proxy data and the methods used to analyze and 

combine them are larger than during more recent time periods. 

 

24 Definitions for terms used in statements of confidence and likelihood can be found in Section II.3: 

Characterization of Uncertainty. 

IV. Trends and Projections of Global Environmental Change 58
Reconstructions of Northern Hemisphere surface temperature from several research teams 

Figure IV.6. Record of average Northern Hemisphere temperature variation during the last 1,300 years based on 

reconstructions using multiple climate proxies (e.g., tree ring widths) from 10 different research teams (see IPCC 

(2007d) for a list of those teams). The darker the shading, the more overlap there is among the various research 

teams’ temperature reconstructions. Source: IPCC (2007d). 

IV.1.c Observed precipitation changes 

Precipitation trends and patterns 

As surface temperatures rise, the evaporation of water vapor increases from oceans and other 

moist surfaces. Increased evaporation is leading to higher concentrations of water vapor in the 

atmosphere. Increased atmospheric water vapor tends to produce weather systems that lead to 

increased precipitation in some areas. At the same time, increased evaporation and 

evapotranspiration from warming can lead to increased land surface drying and, therefore, 

increased potential incidence and severity of droughts in other areas. 

Observations show that changes are occurring in the amount, intensity, frequency, and type of 

precipitation. It is noted that precipitation is highly variable spatially and temporally. There is 

limited data available for large regions of the globe, such that robust long-term trends have not 

been established for certain regions. A global map of precipitation trends from 1901 to 2005 is 

provided in Figure IV.7. 

IV. Trends and Projections of Global Environmental Change 59
Figure IV.7. Trends in annual land precipitation amounts for 1901 to 2005 (percent per century). The percentage is 

based on the means for the 1961 to 1990 period. Areas in grey have insufficient data to produce reliable trends. 

Trends significant at the 5% level are indicated by black + marks. Source: Trenberth et al. (2007; data from 

NOAA/NCDC). 

Some of the conclusions from the IPCC concerning global precipitation changes are noted below 

(Trenberth et al., 2007): 

• Long-term trends from 1900 to 2005 have shown significant increases in precipitation 

amount over many large regions, in particular the eastern parts of North and South America, 

northern Europe, and northern and central Asia. 

• The same long-term trends (1900 to 2005) have shown drying in the Sahel, the 

Mediterranean, southern Africa, and parts of southern Asia. 

• More intense and longer droughts have been observed over wider areas since the 1970s, 

particularly in the tropics and subtropics. In general terms, drought is a “prolonged absence 

or marked deficiency of precipitation,” a “deficiency of precipitation that results in water 

shortage for some activity or for some group,” or a “period of abnormally dry weather 

sufficiently prolonged for the lack of precipitation to cause a serious hydrological imbalance” 

(Heim, 2002 in Trenberth et al., 2007). Increased drying linked with higher temperatures and 

decreased precipitation has contributed to changes in drought. The regions where droughts 

have occurred seem to be determined largely by changes in sea surface temperatures, 

IV. Trends and Projections of Global Environmental Change 60
especially in the tropics, through associated changes in the atmospheric circulation and 

precipitation. Decreased snowpack and snow cover have also been linked to droughts. 

• It is likely that there have been increases in the number of heavy precipitation events (e.g., 

95th percentile events) within many land regions, including regions where there has been a 

reduction in total precipitation amount. This is consistent with a warming climate and the 

observations of increasing amounts of water vapor in the atmosphere. 

• Rising temperatures have generally resulted in rain rather than snow in locations and seasons 

where climatological average temperatures for 1961 to 1990 were close to freezing (0 °C), 

for example the boreal high latitudes. 

Over the contiguous United States, annual precipitation totals have increased at an average rate 

of 6% per century from 1901 to 2005, according to an analysis of data from the NOAA National 

Climatic Data Center U.S. Historical Climate Network (Version 1; NOAA, n.d.). There has been 

significant variability in regional U.S. precipitation patterns over time and space. As shown in 

Figure IV.8, the greatest increases in precipitation were in the East North Central climate region 

(12% per century) and the South (11%). The smallest increases in precipitation were in the 

Southeast (3%), the West North Central (3%) and the Southwest (1%). 

Outside of the contiguous United States, Alaska experienced a precipitation increase of about 6% 

per century (since records began in 1918), and Hawaii experienced a decrease of just over 9% 

per century (since records begin in 1905). 

Despite the overall national trend towards wetter conditions, a severe drought has affected the 

southwestern United States from 1999 through 2007, and more recently the southeastern United 

States has experienced severe drought as well (NWS CPC, 2008). 

IV. Trends and Projections of Global Environmental Change 61
Figure IV.8. U.S. trends in annual precipitation totals for the period 1901 to 2006 in percent per century. Data are 

from the Historical Climate Data Network. Source: NOAA/NCDC (NOAA, n.d.). 

 

Precipitation extremes and storms 

Heavy precipitation events refer to those in the 95th percentile of precipitation events. The IPCC 

(Solomon et al., 2007) reports that it is likely that there have been increases in the number of 

heavy precipitation events within many land regions, even in those regions where there has been 

a reduction in total precipitation amount; this is consistent with a warming climate and observed 

increases in the amount of water vapor in the atmosphere, which have been significant. Increases 

have also been reported for rarer (1 in 50 year return period) precipitation events, but only a few 

regions have sufficient data to assess such trends reliably (Trenberth et al., 2007). 

Observations over the contiguous United States show statistically significant increases in heavy 

precipitation (the heaviest 5%) and very heavy precipitation (the heaviest 1%) of 14% and 20%, 

respectively, primarily during the last three decades of the 20th century (Kunkel et al., 2003 in 

Trenberth et al., 2007 and Groisman et al., 2004 in Trenberth et al., 2007). This increase is most 

apparent over the eastern parts of the country. Some evidence suggests that the relative increase 

in precipitation extremes is larger than the increase in mean precipitation (Trenberth et al., 2007). 

CCSP SAP 3.3 (Karl et al., 2008) concluded that very heavy precipitation (the heaviest 1%) in 

IV. Trends and Projections of Global Environmental Change 62
the continental United States increased by 20% over the past century while total precipitation 

increased by 7%. Additionally, the monsoon season is beginning about 10 days later than usual 

in Mexico and in general, for the summer monsoon in southwestern North America, there are 

fewer rain events, but the events are more intense (Karl et al., 2008). 

On a global basis, more intense and longer droughts have been observed over wider areas since 

the 1970s, particularly in the tropics and subtropics (IPCC, 2007d). Increased drying linked with 

higher temperatures and decreased precipitation has contributed to changes in drought (IPCC, 

2007d). 

In the western United States, diminishing snow pack and subsequent reductions in soil moisture 

appear to have contributed to recent drought conditions (Trenberth et al., 2007). This drought has 

also been attributed to changes in atmospheric circulation associated with warming of the 

western tropical Pacific and Indian Oceans as well as multi-decadal fluctuations (Hoerling and 

Kumar, 2003 and McCabe et al., 2004 in Trenberth et al., 2007). 

Detection of long-term trends in tropical cyclone activity is complicated by multi-decadal 

variability and the quality of the tropical cyclone records prior to the start of routine satellite 

observations in about 1970. Even taking this into account, the IPCC concluded that the 

observational evidence shows an increase of intense (Category 4 and 5) tropical cyclone activity 

in the North Atlantic since about 1970, which correlates with increases in tropical sea surface 

temperatures (Solomon et al., 2007). There is no clear global trend in the total numbers of 

tropical cyclones since 1970 (IPCC, 2007d). Globally, estimates of the potential destructiveness 

of hurricanes show a significant upward trend since the mid-1970s, with a trend toward longer 

lifetimes and greater storm intensity. Such trends are strongly correlated with tropical sea surface 

temperature (Trenberth et al., 2007). Additionally, there is evidence for an increase in extreme 

wave height characteristics over the past couple of decades, associated with more frequent and 

more intense hurricanes (Karl et al., 2008). 

A range of models with resolutions ranging from 100 to 20 km predict that future tropical 

cyclones (typhoons and hurricanes) will become more intense, with larger peak wind speeds and 

more heavy precipitation associated with ongoing increases in tropical sea surface temperatures. 

There is less confidence in the projections of tropical cyclone frequency. The apparent increase 

in the proportion of very intense storms since 1970 in some regions is much larger than 

simulated by current models for that period (Solomon et al., 2007). According to SAP 3.3 (Karl 

et al., 2008), rainfall and wind speeds associated with North Atlantic and North Pacific 

hurricanes are likely to increase in response to human-caused warming. Analyses of model 

simulations suggest that for each 1 °C (1.8 °F) increase in tropical sea surface temperatures, core 

rainfall rates will increase by 6 to 18% and the surface wind speeds of the strongest hurricanes 

will increase by about 1 to 8%. 

IV.1.d Observed changes in other physical systems 

Of all the observed changes to physical systems assessed by the IPCC (Rosenzweig et al., 2007) 

for North America (totaling 355), 94% were consistent with changes one would expect with 

average warming. This section covers changes to the cryosphere (snow and ice), the 

IV. Trends and Projections of Global Environmental Change 63
hydrosphere, and sea level. Changes to the biosphere are covered in Section V.1. The extent to 

which observed changes discussed here can be attributed to anthropogenic greenhouse gas 

emissions is discussed in Section IV.2.b. 

Cryosphere (snow and ice) 

The cryosphere is the ‘frozen’ component of the climate system, including sea ice, glaciers, snow 

cover, and permafrost. According to the IPCC (Lemke et al., 2007) and the Arctic Climate 

Impacts Assessment (ACIA, 2005), the following global-scale physical changes have been 

observed: 

• Overall, snow cover has decreased in most regions, especially in spring and summer. Satellite 

data from 1966 to the present show decreases in Northern Hemisphere snow cover in every 

month except November and December, with a stepwise drop of 5% in the annual mean in 

the late 1980s. In the Southern Hemisphere, the few long records or proxies mostly show 

either decreases or no changes in the past 40 years or more. 

• For North America, snow-covered area increased in November, December, and January from 

1915 to the present due to increases in precipitation. Over western North America, however, 

snow cover decreased during the latter half of the 20th century, especially during the spring 

(Groisman et al., 2004 in Lemke et al., 2007). In northern Alaska, observations show shifts 

toward earlier melt by about eight days since the mid-1960s (Stone et al., 2002 in Lemke et 

al., 2007). 

• Freeze-up and breakup dates for river and lake ice have high spatial variability and trends in 

these dates are generally toward later and earlier occurrences, respectively. However, the 

average of available data for the Northern Hemisphere spanning the past 150 years shows the 

freeze-up date for river and lake ice has occurred later at a rate of 5.8 ± 1.6 days per century 

and the breakup date has occurred earlier at a rate of 6.5 ± 1.2 days per century. 

• Annual average Arctic sea ice extent has decreased by 2.7 ± 0.6% per decade, based on 

satellite data since 1978. Larger decreases have been observed in summer (7.4 ± 2.4% per 

decade). Figure IV.9 shows the extent of the sea ice loss between 1979 and 2005. Since 2005, 

Arctic sea ice extent has declined even further. According to the National Snow and Ice Data 

Center the average sea ice extent for September 2007 was 4.28 million square kilometers, the 

lowest September on record, shattering the previous record for the month, set in 2005, by 23 

percent.25 In contrast, Antarctic sea ice extent shows no statistically significant average 

trends (IPCC, 2007d). 

 

• The average sea ice thickness in the central Arctic has very likely decreased by up to 1 m 

from 1987 to 1997, based upon submarine-derived data. Model-based reconstructions support 

this, suggesting an Arctic-wide reduction of 0.6 to 0.9 m over the same period. 

 

25 See <nsidc.org/news/press/2007_seaiceminimum/20070810_index.html>. 

IV. Trends and Projections of Global Environmental Change 64
• Mountain glaciers have declined on average in both hemispheres. The strongest mass losses 

of mountain glaciers (per unit area) have been observed in Patagonia, Alaska, the northwest 

United States, and southwest Canada. Because of the corresponding large areas, the biggest 

contributions to sea level rise came from glaciers in Alaska, the Arctic, and the Asian high 

mountains. 

• Since the 1980s, temperatures at the top of the Arctic permafrost layer have generally 

increased by up to 3 °C. In Alaska, the permafrost base has been thawing at a rate of up to 

0.04 m per year since 1992; on the Tibetan Plateau, the rate of permafrost base thawing has 

been 0.02 m per year since the 1960s. The maximum area covered by seasonally frozen 

ground has decreased by 7% in the Northern Hemisphere since 1900, with a decrease in 

spring of up to 15%. 

Comparison of Arctic sea ice minima 

1979 

2007 

Figure IV.9. These two images, constructed from 

satellite data, compare Arctic sea ice concentrations in 

September of 1979 and 2007. Source: NASA, updated 

from ACIA (2004). 

The Arctic Climate Impact Assessment (ACIA, 2004) found that the area of the Greenland Ice 

Sheet that experiences some melting increased about 16% from 1979 to 2002, with 2002 

experiencing the largest affected area in the record since 1979 (subsequent years since 2004 have 

seen even more extensive melting). 

There are additional effects related to changes in the cryosphere. First, melting of highly 

IV. Trends and Projections of Global Environmental Change 65
reflective snow and ice reveals darker land and ocean surfaces, increasing absorption of the 

Sun’s heat and further warming the planet. Additionally, increases in glacial melt and river 

runoff add freshwater to the ocean, raising global sea level. There is also the potential positive 

feedback (in which a result of climate warming in turn causes further climate warming) from the 

possible release of CH4 trapped within subsea permafrost. However, available observations do 

not permit an assessment of changes that might have occurred with respect to this (Lemke et al., 

2007). 

Hydrosphere 

The term ‘hydrosphere’ refers to the component of the climate system consisting of liquid 

surface and subterranean water, such as rivers, lakes, and underground water. The IPCC 

(Rosenzweig et al., 2007) summarized several observed changes in these features: 

• Documented trends in severe droughts and heavy rains show that hydrologic conditions are 

becoming more intense in some regions, with indications of intensified droughts in drier 

regions and evidence for areas of increasing wetness, in particular the Northern Hemisphere 

high latitudes and equatorial regions. Globally, very dry areas (areas with a Palmer Drought 

Severity Index (PDSI) rating of less than or equal to -3.0) have more than doubled since the 

1970s due to a combination of El Niño–Southern Oscillation events and surface warming 

(Dai et al., 2004 in Trenberth et al., 2007). Very wet areas (PDSI greater than or equal to 

+3.0) declined by about 5% since the 1970s. The major contributing factor in this decline 

was precipitation during the early 1980s with temperature more important thereafter. 

• Over the last century, increasing runoff and streamflow that can be related to climate change 

has been observed in many regions, particularly in basins fed by glaciers, permafrost, and 

snowmelt. Evidence shows that average runoff of Arctic rivers in Eurasia has increased, and 

this increasing runoff has been at least partly correlated with climate warming. There is also 

evidence of earlier spring snowmelt and increased winter base flow in North America and 

Eurasia due to enhanced seasonal snowmelt associated with climate warming. 

• In the Arctic, lakes forming and then disappearing in permafrost have been observed. 

Freshwater lakes and rivers are experiencing increased water temperatures and changes in 

water chemistry, including changes in water acidity and mineral concentrations. Surface and 

deep lake waters are warming. With this warming, periods of thermal stability are advancing 

and lengthening. In some cases, this is associated with physical and chemical changes, such 

as increases in salinity and suspended solids, and a decrease in nutrient content. 

Changes in river discharge, as well as in droughts and heavy rains, in some regions indicate that 

hydrologic conditions have become more intense. However, significant trends in floods and in 

evaporation and evapotranspiration have not been detected globally. Some local trends in 

reduced groundwater and lake levels have been reported. However, studies have been unable to 

separate the effects of variations in temperature and precipitation from the effects of human 

interventions, such as groundwater management, land use change, and reservoir construction 

(Rosenzweig et al., 2007). 

IV. Trends and Projections of Global Environmental Change 66
The IPCC (Rosenzweig et al., 2007) indicates surface water temperatures have warmed by 0.2 to 

2 °C in lakes and rivers in North America since the 1960s. Additionally, there is evidence for an 

earlier occurrence of spring peak river flows and an increase in winter base flow in basins with 

important seasonal snow cover in North America. 

CCSP SAP 4.3 notes that the current hydrologic observing system was not designed for purposes 

of detecting climate change or its effects on water resources and that many of the data are 

fragmented, poorly integrated, and in many cases unable to meet the predictive challenges of a 

rapidly changing climate (Lettenmaier et al., 2008). Despite this, CCSP (Lettenmaier et al., 

2008) does list a number of conclusions related to water resources in the United States: 

• During the second half of the 20th century, most of the United States experienced increases 

in precipitation and streamflow, due to a combination of decadal-scale climate variability and 

long-term change. 

• During the second half of the 20th century, most of the United States experienced decreases 

in drought severity and duration. However, there is some indication of increased drought 

severity and duration in the western and southwestern United States, which may have 

resulted from increased actual evaporation dominating the trend toward increased soil 

wetness. On a longer time scale, paleoclimatic reconstructions of droughts show that much 

more severe droughts have occurred over the last 2,000 years than those that have been 

observed in the instrumental record, notably, the Dust Bowl drought of the 1930s and 

extensive drought in the 1950s. 

• Across the western United States, there is a trend toward reduced mountain snowpack and 

earlier spring snowmelt runoff peaks, which is very likely due to long-term warming, with 

potential influence from decadal-scale variability (see discussion in Section V.4 about the 

impacts of these changes). 

• As the climate warms, stream temperatures are likely to increase, with effects on aquatic 

ecosystems. There is some evidence that temperatures have increased in some western U.S. 

streams, although a comprehensive analysis has yet to be conducted. Temperature changes 

will be most evident during low flow periods, when they are of greatest concern. 

Sea level 

There is strong evidence that global sea level gradually rose in the 20th century, after a period of 

little change between A.D. 0 and A.D. 1900, and is currently rising at an increased rate (IPCC, 

2007a). 

According to the IPCC (Bindoff et al., 2007): 

• There is high confidence that the rate of sea level rise increased between the mid-19th and 

mid-20th centuries. 

IV. Trends and Projections of Global Environmental Change 67
• The average rate of sea level rise measured by tide gauges from 1961 to 2003 was 1.8 ± 0.5 

mm per year. For the entire 20th century, the average rate was 1.7 ± 0.5 mm per year. 

• There is considerable decadal variability in the rate of global mean sea level rise. The global 

average rate of sea level rise measured by satellite altimetry during 1993 to 2003 was 3.1 ± 

0.7 mm per year, significantly higher than the 20th century average rate. Coastal tide gauge 

measurements confirm this observation. It is unclear whether the faster rate for 1993 to 2003 

is a reflection of short-term variability or an increase in the longer-term trend. 

Two major processes lead to changes in global mean sea level on decadal and longer time scales: 

1) thermal expansion and 2) the exchange of water between oceans and other reservoirs, 

including glaciers and ice caps, ice sheets, and other land water reservoirs. The IPCC (Bindoff et 

al., 2007) concluded: 

• Overall, for the period 1961 to 2003, the full magnitude of the observed sea level rise was not 

satisfactorily explained by the available data sets, where thermal expansion contributed about 

one-quarter of the observed sea level rise and melting of land ice accounted for less than half 

of the observed sea level rise. The observing system is much better for recent years (1993 to 

2003). In this period, thermal expansion and melting of land ice each account for about half 

of the observed sea level rise, although there is some uncertainty in the estimates. 

• On average, over the period from 1961 to 2003, global ocean temperature from the surface to 

a depth of 700 m rose by 0.10 °C, contributing an average of 0.4 ± 0.1 mm per year to sea 

level rise. For the period 1993 to 2003, thermal expansion contributed about 1.6 ± 0.5 mm 

per year, reflecting a higher rate of warming for this period relative to 1961 to 2003. 

• On average, over the period from 1961 to 2003, melting of land ice accounted for 

approximately 0.7 ± 0.5 mm per year of sea level rise (Lemke et al., 2007). The total 

contribution from melting ice to sea level change between 1993 and 2003 was 1.2 ± 0.4 mm 

per year. The rate increased over the 1993 to 2003 period, primarily due to increasing losses 

from mountain glaciers and ice caps, increasing surface melt on the Greenland Ice Sheet, and 

faster flow of parts of the Greenland and Antarctic Ice Sheets (Lemke et al., 2007). 

Sea level rise is highly non-uniform around the world. In some regions, rates of rise have been as 

much as several times the global mean, while sea level is falling in other regions. This nonuniformity is driven by thermal expansion and exchanges of water between oceans and other 

reservoirs, as well as changes in ocean circulation or atmospheric pressure, and geologic 

processes (Bindoff et al., 2007). According to the IPCC (Bindoff et al., 2007), satellite 

measurements for the period 1993 to 2003 provide unambiguous evidence of regional variability 

of sea level change. The largest sea level rise since 1992 has taken place in the western Pacific 

and eastern Indian Oceans, while nearly all of the Atlantic Ocean shows sea level rise during the 

past decade with the rate of rise reaching a maximum (over 2 mm per year) in a band running 

east–northeast from the U.S. East Coast. Sea level in the eastern Pacific and western Indian 

Oceans has been falling. 

IV. Trends and Projections of Global Environmental Change 68
U.S. sea level data obtained from the Permanent Service for Mean Sea Level of the Proudman 

Oceanographic Laboratory26 reach at least as far back as the early 20th century. These data show 

that along most of the U.S. Atlantic and Gulf Coasts, sea level has been rising 2.0 to 3.0 mm per 

year. The rate of sea level change varies from a rise of about 10 mm per year along the Louisiana 

Coast (due to land sinking) to a drop of tens of millimeters per decade (a few inches per decade) 

in parts of Alaska (because land is rising). Records from the coast of California indicate that sea 

levels have risen almost 18 cm during the past century (California Energy Commission, 2006b). 

Local sea levels can actually be falling in some cases (for example, the Pacific Northwest coast) 

if the land level is rising more than the sea level (Gamble et al., 2008). 

The IPCC (Rosenzweig et al., 2007) concluded that along the U.S. East Coast, 75% of the 

shoreline that is removed from the influence of spits, tidal inlets, and engineering structures is 

eroding, which is probably due to sea level rise. It also cites studies reporting losses in coastal 

wetlands observed in Louisiana, the mid-Atlantic region, and in parts of New England and New 

York, in spite of recent protective environmental regulations. 

CCSP SAP 4.1 (Titus et al., 2008) reported a number of effects of sea level rise relevant to the 

United States. Approximately one-sixth of the Nation’s land close to sea level is in the midAtlantic, and as such, the report focuses on the mid-Atlantic states: 

• Sea level rise is virtually certain (>99% probability) to cause some areas of dry land to 

become inundated. Approximately 900 to 2,100 km2

 (350 to 800 mi2) of dry land, half of 

which is in North Carolina, would be flooded during spring high tides if sea level rises 50 cm 

(20 in), assuming no shore protection measures are taken. 

• Nationally, it is very likely that erosion will increase in response to sea level rise, especially 

in sandy shore environments that comprise all of the mid-Atlantic coast. Within the midAtlantic region, it is virtually certain (>99% probability) that coastal headlands, spits, and 

barrier islands will also erode in response to future sea level rise. 

• It is virtually certain that the Nation’s tidal wetlands already experiencing submergence by 

sea level rise and associated high rates of loss (e.g., Mississippi River Delta in Louisiana, 

Blackwater River marshes in Maryland) will continue to lose area under the influence of 

future accelerated rates of sea level rise and changes in other climate and environmental 

drivers. 

• It is very unlikely (1 to 10% probability) that there will be a net increase in tidally influenced 

wetland area on a national scale over the next 100 years, given current wetland loss rates and 

the few occurrences of new tidal wetland expansion (e.g., Atchafalaya Delta in Louisiana). 

• Depending on local conditions, habitat may be lost or migrate inland in response to sea level 

rise. Loss of tidal marshes would seriously threaten coastal ecosystems, causing fish and 

birds to move or produce less offspring. Many estuarine beaches may also be lost, 

threatening species such as the terrapin and horseshoe crab. 

 

26 See <www.pol.ac.uk/psmsl/>. 

IV. Trends and Projections of Global Environmental Change 69
• There is evidence for an increase in incidence of extreme high sea level globally and in the 

United States since 1975 based upon an analysis of 99th percentiles of hourly sea level at 141 

stations around the world (Bindoff et al., 2007). A global analysis is not feasible over the 

entire 20th century because longer records back in time are limited in space and undersampled in time (Solomon et al., 2007). 

IV.2. Attribution of Observed Climate Change to Human Activities at 

the Global and Continental Scale 

The previous section (Section IV.1) describes observations presented primarily in the recent 

IPCC Fourth Assessment Report (2007d) of global- and continental-scale temperature increases, 

changes in other climate variables and physical and biological systems, and the radiative forcing 

caused by anthropogenic versus natural factors. This section addresses the extent to which 

observed climate change at the global and continental or national scale can be attributed to 

global anthropogenic emissions of greenhouse gases. 

Computer-based climate models are the primary tools used for simulating the probable patterns 

of climate system response to natural and anthropogenic forcings, such as greenhouse gases, 

aerosols, and solar intensity (see Section IV.1.a). Confidence in these models comes from their 

foundation in accepted physical principles and from their ability to reproduce observed features 

of current climate and past climate changes (Meehl et al., 2007) (see Section IV.3.b). Studies to 

attribute causes of climate change use well-tested models to evaluate whether observed changes 

are consistent with quantitative responses to different forcings and are not consistent with 

alternative physically plausible explanations. Attribution studies often use ‘fingerprint methods’ 

to rigorously test for the presence of a climatic response to greenhouse gas increases and other 

forcings, and to evaluate whether similarities between observed patterns and model-simulated 

fingerprints could have occurred by chance. These methods account for feedbacks enhancing or 

decreasing the response to individual external influences compared to model-simulated 

responses. 

Diffi culties remain in attributing temperature changes at smaller than continental scales and over 

time scales of less than 50 years. Attribution results at these scales have, with limited exceptions, 

not been established. Averaging over smaller regions reduces the natural variability less than 

does averaging over large regions, making it more diffi cult to distinguish between changes 

expected from external forcing and variability. In addition, temperature changes associated with 

some modes of variability are poorly simulated by models in some regions and seasons. 

Furthermore, the small-scale details of external forcing and the response simulated by models are 

less credible than large-scale features (IPCC, 2007a). 

IV.2.a Attribution of observed climate change to anthropogenic emissions and 

land use / land cover 

There is clear evidence that human-induced warming of the climate system is widespread 

(Hegerl et al., 2007). Studies to detect climate change and attribute its causes using patterns of 

IV. Trends and Projections of Global Environmental Change 70
observed changes in temperature show clear evidence of human influences on the climate system 

(Karl et al., 2006). Discernible human influences extend to additional aspects of climate, 

including ocean warming, continental-average temperatures, temperature extremes, and wind 

patterns (Hegerl et al., 2007). 

Temperature 

The IPCC has strengthened its statement on the linkage between greenhouse gases and 

temperatures with each assessment report over the past two decades. The IPCC’s First 

Assessment Report in 1990 contained little observational evidence of a detectable anthropogenic 

influence on climate (IPCC, 1990). In its Second Assessment Report in 1995, the IPCC stated the 

balance of evidence suggests a discernible human influence on the climate of the 20th century 

(IPCC, 1996). The Third Assessment Report in 2001 concluded that most of the observed 

warming over the last 50 years is likely to have been due to the increase in greenhouse gas 

concentrations (IPCC, 2001). The conclusion in the IPCC’s 2007 Fourth Assessment Report 

(2007d) is the strongest yet: 

Most of the observed increase in global average temperatures since the mid-20th century is very 

likely due to the observed increase in anthropogenic greenhouse gas concentrations. 

The IPCC (Hegerl et al., 2007) cites a number of factors for the increased confidence in the 

greenhouse gas contribution to the observed warming: 

• an expanded and improved range of observations, allowing attribution of warming to be more 

fully addressed jointly with other changes in the climate system; 

• analyses of paleoclimate data that have increased confidence in the role of external 

influences on climate; 

• improvements in the simulation of many aspects of present mean climate and its variability 

on seasonal to inter-decadal time scales; 

• more detailed representations of processes related to aerosol and other forcings in models; 

• simulations of 20th-century climate change that use many more models and much more 

complete inclusion of anthropogenic and natural forcings; and 

• multi-model ensembles that increase confidence in attribution results by providing an 

improved representation of model uncertainty. 

The results from climate model simulations evaluated by the IPCC (Figure IV.10) show that 

natural forcings alone cannot explain the observed warming for the globe, the global land, and 

global ocean and that the observed warming can only be reproduced with models that contain 

both natural and anthropogenic forcings. The IPCC (Hegerl et al., 2007) cites additional new 

evidence linking warming to increasing concentrations of greenhouse gases: 

IV. Trends and Projections of Global Environmental Change 71
• Attribution studies have established anthropogenic contributions to changes in surface and 

atmospheric temperatures (Section IV.1.b), changes in temperatures in the upper several 

hundred meters of the ocean, and changes in sea level pressure (Section IV.1.d). 

• Coupled climate models used to predict future climate have been used to reproduce key 

features of past climates using boundary conditions and radiative forcing for those periods. 

It is very unlikely, according to the IPCC (Hegerl et al., 2007), that the global pattern of warming 

observed during the past half century is due to only known natural external causes (solar activity 

and volcanoes) since the warming occurred in both the atmosphere and ocean and took place 

when natural external forcing factors would likely have produced cooling. The IPCC (Hegerl et 

al., 2007) also concluded that greenhouse gas forcing alone would likely have resulted in 

warming greater than observed if there had not been an offsetting cooling effect from aerosols 

and natural forcings during the past half century. 

Figure IV.10. Comparison of observed global-scale changes in surface temperature with results 

simulated by climate models using natural and anthropogenic forcings. Decadal averages of observations 

are shown for the period 1906 to 2005 (black line) plotted against the center of the decade and relative to 

the corresponding average for 1901 to 1950. Lines are dashed where spatial coverage is less than 50%. 

Blue shaded bands show the 5 to 95% range for 19 simulations from five climate models using only the 

natural forcings due to solar activity and volcanoes. Red shaded bands show the 5 to 95% range for 58 

simulations from 14 climate models using both natural and anthropogenic forcings. Source: IPCC 

(2007d). 

In addition to evidence from surface temperatures, evidence has also accumulated for an 

anthropogenic influence through the vertical profile of the atmosphere. Fingerprint studies,27

rather than only linear trend comparisons,28 have identified greenhouse gas and sulfate aerosol 

signals in observed surface temperature records, a stratospheric ozone depletion signal in 

 

27 Fingerprint studies use rigorous statistical methods to compare the patterns of observed temperature changes with 

model expectations and determine whether or not similarities could have occurred by chance. 

28 Linear trend comparisons are less powerful than fingerprint analyses for studying cause–effect relationships, but 

can highlight important differences and similarities between models and observations (as in Figures IV.10 and 

IV.11). 

IV. Trends and Projections of Global Environmental Change 72
stratospheric temperatures, and the combined effects of these forcing agents in the vertical 

structure of atmospheric temperature changes (Karl et al., 2006). There is a potentially important 

inconsistency in the tropics, where most observational data sets show more warming at the 

surface than in the troposphere. In contrast, almost all model simulations have larger warming 

aloft than at the surface. The issue is still under investigation and may be due to errors in the 

observations (Karl et al., 2006). 

It is likely that there has been a substantial anthropogenic contribution to surface temperature 

increases since the middle of the 20th century on every continent except Antarctica, which has 

insufficient observational coverage to make an assessment (Hegerl et al., 2007). Figure IV.11 

shows that the observed temperatures over the last century for North America can only be 

reproduced using model simulations containing both natural and anthropogenic forcings, and not 

just natural forcings. Attribution studies show that it is likely that there has been a substantial 

human contribution to the surface temperature increase in North America. Fingerprint attribution 

studies detect a significant anthropogenic contribution to North American temperature change 

that can be separated from the response to natural forcings and from internal climate variability. 

As noted above, attribution of temperature changes on scales smaller than continental and for 

time scales of less than 50 years, with limited exceptions, has not yet been established (Hegerl et 

al., 2007). 

IV. Trends and Projections of Global Environmental Change 73
Figure IV.11. Comparison of observed North American changes in surface 

temperature with results simulated by climate models using natural and 

anthropogenic forcings. Decadal averages of observations are shown for the 

period 1906 to 2005 (black line) plotted against the center of the decade and 

relative to the corresponding average for 1901 to 1950. Lines are dashed where 

spatial coverage is less than 50%. Blue shaded bands show the 5 to 95% range for 

19 simulations from five climate models using only the natural forcings due to 

solar activity and volcanoes. Red shaded bands show the 5 to 95% range for 58 

simulations from 14 climate models using both natural and anthropogenic 

forcings. Source: Hegerl et al. (2007). 

In addition to average temperatures, anthropogenic forcings have also likely influenced extremes 

in temperature (Hegerl et al., 2007). Many indicators of climate extremes, including the annual 

numbers of frost days, warm and cold days, and warm and cold nights, show changes that are 

consistent with warming (Hegerl et al., 2007). An anthropogenic influence has been detected in 

some of these indices, and there is evidence that anthropogenic forcing may have substantially 

increased the risk of extremely warm summer conditions regionally. The 2003 European heat 

wave is an example of such temperature extremes (Hegerl et al., 2007). 

Additional climate variables 

There is evidence of anthropogenic influence in other parts of the climate system. The IPCC 

noted the following global examples (Hegerl et al., 2007): 

• Recent decreases in Arctic sea ice extent (Section IV.1.d) are likely to have been contributed 

to by anthropogenic forcings. 

IV. Trends and Projections of Global Environmental Change 74
• Human activity is likely related to trends over recent decades in the Northern and Southern 

Annular Modes,29 which correspond to sea level pressure reductions over the poles. These 

modes affect storm tracks, winds, and temperature patterns in both hemispheres. Models 

reproduce the sign of the Northern Annular Mode trend, but the simulated response is smaller 

than observed. A realistic trend in the Southern Annular Mode is simulated by models that 

include both greenhouse gas and stratospheric ozone changes, suggesting this trend is due to 

human influence on global sea level pressure patterns. 

• The latitudinal pattern of change in land precipitation over the 20th century appears to be 

consistent with the anticipated response to anthropogenic forcing. The same is true for 

observed increases in heavy precipitation. 

• It is more likely than not (greater than 50% probability) that anthropogenic influence has 

contributed to increases in the frequency of the most intense tropical cyclones (i.e., category 

4 and 5). 

Regarding this latter issue, the CCSP synthesis and assessment product on climate extremes 

(Karl et al., 2008), concluded the following regarding hurricane activity specifically in the North 

Atlantic: 

It is very likely that the human-induced increase in greenhouse gases has contributed to the increase in sea 

surface temperatures in the hurricane formation regions. There is a strong statistical connection between 

tropical Atlantic sea surface temperatures and Atlantic hurricane activity as measured by the Power 

Dissipation Index (combines storm intensity, frequency, and duration) and particularly with frequency on 

decadal timescales over the past 50 years. This evidence suggests a substantial human contribution to 

recent hurricane activity. However, a confident assessment of human influence on hurricanes will require 

further studies using models and observations, with emphasis on distinguishing natural from humaninduced changes in hurricane activity through their influence on factors such as historical sea surface 

temperatures, wind shear, and atmospheric vertical stability. 

IV.2.b Attribution of observed changes in physical and biological systems 

As detailed in Section IV.2.a, warming in the last 50 years is very likely the result of the 

accumulation of anthropogenic greenhouse gases in the atmosphere. Observed changes in 

physical systems (e.g., melting glaciers) and biological systems and species (e.g., geographic 

shift of species) that are shown to change as a result of recent warming can be attributed to 

anthropogenic greenhouse gas forcing. This section includes the observed changes in physical 

and biological systems in North America and in other parts of the world, which the IPCC 

(2007b) concluded are discernibly influenced by recent warming: “Observational evidence from 

all continents and most oceans shows that many natural systems are being affected by regional 

climate changes, particularly temperature increases.” Furthermore, the IPCC (2007e) concluded 

that: “Anthropogenic warming over the last three decades has likely had a discernable influence 

at the global scale on observed changes in many physical and biological systems.” 

 

29 Annular modes are preferred patterns of change in atmospheric circulation corresponding to changes in the 

zonally averaged mid-latitude westerly winds. The Northern Annular Mode has a bias to the North Atlantic and has 

a large correlation with the North Atlantic Oscillation. The Southern Annular Mode occurs in the Southern 

Hemisphere. 

IV. Trends and Projections of Global Environmental Change 75
In order to make robust conclusions about the role of anthropogenic climate change in affecting 

biological and physical systems, climate variability and non-climate drivers (e.g., land use 

change and habitat fragmentation) all need to be considered. The IPCC (Rosenzweig et al., 2007) 

concluded from a number of joint attribution studies that the consistency of observed significant

changes in physical and biological systems and observed significant warming across the globe 

likely cannot be explained entirely due to natural variability or other confounding non-climate 

factors.

The physical systems undergoing significant change include the cryosphere (snow and ice 

systems), hydrologic systems, water resources, coastal zones, and the oceans. A number of 

effects are reported with high confidence by the IPCC (Rosenzweig et al., 2007): 

• ground instability in mountain and permafrost regions; 

• shorter travel season for vehicles over frozen roads in the Arctic; 

• enlargement and increase of glacial lakes in mountain regions and destabilization of moraines 

damming these lakes; 

• changes in Arctic flora and fauna including the sea ice biomes and predators higher in the 

food chain; 

• limitations on mountain sports in lower-elevation alpine areas; and 

• changes in indigenous livelihoods in the Arctic. 

CCSP SAP 4.3 (Backlund et al., 2008) discusses the role of climate change in determining the 

levels of snow water equivalent in the Pacific Northwest of North America. It concluded that 

observed snow water equivalent changes in the western United States for the period 1950 to 

2000 can be attributed to temperature rather than precipitation changes (Mote, 2003 and Hamlet 

et al., 2005 in Backlund et al., 2008). 

Regarding biological systems, the IPCC (Rosenzweig et al., 2007) concluded with very high 

confidence that the overwhelming majority of studies of regional climate effects on terrestrial 

species reveal trends consistent with warming, including: 

• poleward and elevational range shifts of flora and fauna; 

• earlier onset of spring events, migration, and lengthening of the growing season; 

• changes in abundance of certain species, including limited evidence of a few local 

disappearances; and 

• changes in community composition. 

See Section V for further description of these changes in the physical and biological systems.

Human system responses to climate change are more difficult to identify and isolate due to the 

larger role of non-climate factors, such as management practices in agriculture and forestry, and 

adaptation responses to protect human health against adverse climatic conditions.

CCSP SAP 4.3 (Backlund et al., 2008) concluded that, due to a lack of appropriate observations, 

it is currently not possible to understand the extent to which climate change is damaging or 

enhancing the goods and services that ecosystems provide or how additional climate change 

would affect the future delivery of such goods and services. It concluded: 

IV. Trends and Projections of Global Environmental Change 76
[I]n principle it is possible to evaluate both damages and benefits from climate change for any 

region and/or ecosystem, but such studies will need to be very carefully designed and 

implemented in order to yield defensible quantification. Until then, we will need to continue to 

rely on a combination of existing observations made for other purposes and on model output to 

construct such estimates (Backlund et al., 2008). 

IV.3. Projected Future Greenhouse Gas Concentrations and Climate 

Change 

According to the IPCC (2007d), “continued greenhouse gas emissions at or above current rates 

would cause further warming and induce many changes in the global climate system during the 

21st century that would very likely be larger than those observed during the 20th century.” This 

section describes future greenhouse gas emission scenarios, the associated changes in 

atmospheric concentrations and radiative forcing, and the resultant changes in temperature, 

precipitation, and sea level at global and U.S. scales. An important note is that all future 

greenhouse gas emission scenarios described in this section assume no new explicit greenhouse 

gas mitigation policies (neither in the United States nor in other countries) beyond those that 

were already enacted at the time the scenarios were developed. Future risks and impacts 

associated with the climate change projections are addressed in Part V. 

IV.3.a Global emission scenarios and associated changes in concentrations and 

radiative forcing 

Greenhouse gas emissions 

Section IV.1.a described a number of different greenhouse gases, aerosols, and other factors that 

cause radiative forcing changes and thus contribute to climate change. This section discusses the 

range of published global reference (or baseline) future emission projections, which are primarily 

drawn from the IPCC Special Report on Emission Scenarios (SRES) (IPCC, 2000). The SRES 

developed a range of long-term global reference scenarios for the major greenhouse gases 

emitted by human activities and for some aerosols out to the year 2100. The main drivers of 

emissions are population, economic growth, technological change, and land use activities 

including deforestation. 

The scenarios described in this report were developed and published elsewhere using integrated 

assessment models. These models integrate socioeconomic and technological determinants of the 

emissions of greenhouse gases with models of the natural science of Earth system response, 

including the atmosphere, oceans, and terrestrial biosphere. The detailed underlying assumptions 

(including final and primary energy by major fuel types) across all scenarios, and across all 

modeling teams that produced the scenarios, can be found in the SRES (IPCC, 2000). Box IV.1 

provides background information on the different SRES emission scenarios. The IPCC SRES 

scenarios assume no explicit greenhouse gas mitigation policies beyond those currently enacted 

and, and having been initiated prior to the inception of the Kyoto Protocal, do not explicitly 

IV. Trends and Projections of Global Environmental Change 77
account for it. None of the scenarios are viewed as more likely than the others, and they were not 

constructed using formal uncertainty analysis. 

Figure IV.12 presents the global SRES projections for the two most significant anthropogenic 

greenhouse gases: CO2 and CH4. The ranges of greenhouse gas emissions in the scenarios widen 

over time as a result of uncertainties in the underlying drivers, where CO2 emissions are 

primarily from the burning of fossil fuels and CH4 emissions are primarily from biogenic sources 

such as wetlands, ruminant animals, rice agriculture, and biomass burning. The IPCC (2000) 

SRES report did not assign probabilities or likelihood to the scenarios, and stated that there is no 

single most probable, central, or best-guess scenario, either with respect to the scenarios or to the 

underlying scenario literature. As a result, the IPCC (2000) recommended using the range of 

scenarios, with their variety of underlying assumptions, in analyses. 

Despite the range in future emission scenarios, the majority of all reference-case scenarios 

project an increase in greenhouse gas emissions across the century and show that CO2 remains 

the dominant greenhouse gas over the course of the 21st century. Total cumulative (1990 to 

2100) CO2 emissions across the SRES scenarios range from 2,826 gigatons of CO2 (GtCO2) (or 

770 GtC) to approximately 9,322 GtCO2 (or 2,540 GtC).30 The 90th percentile range in 2100 for 

annual CO2 emissions is 17 to 135 GtCO2, compared to about 34 GtCO2 emitted in 2000. 

Since the IPCC SRES (IPCC, 2000), new scenarios in the literature have emerged, but these 

generally have a range similar to the emission scenarios from the IPCC SRES. These newer 

scenarios have used lower values for some drivers of emissions, notably population projections. 

However, for those studies incorporating these new lower population projections, changes in 

other drivers, such as economic growth, offset these differences, resulting in little change in 

overall emission levels (IPCC, 2007c). 

Box IV.1. IPCC Reference Case Emission Scenarios from the Special Report on Emission Scenarios 

A1. The A1 storyline and scenario family describes a future world of very rapid economic growth, global 

population that peaks in mid-century and declines thereafter, and the rapid introduction of new and 

more efficient technologies. Major underlying themes are convergence among regions, capacity 

building, and increased cultural and social interactions, with a substantial reduction in regional 

differences in per capita income. The A1 scenario family develops into three groups that describe 

alternative directions of technological change in the energy system. The three A1 groups are 

distinguished by their technological emphasis: fossil intensive (A1FI), non-fossil energy sources (A1T), 

or a balance across all sources (A1B) (where balanced is defined as not relying too heavily on one 

particular energy source, on the assumption that similar improvement rates apply to all energy supply 

and end use technologies). 

A2. The A2 storyline and scenario family describes a very heterogeneous world. The underlying theme 

is self-reliance and preservation of local identities. Fertility patterns across regions converge very 

slowly, which results in continuously increasing population. Economic development is primarily 

regionally oriented and per capita economic growth and technological change more fragmented and 

slower than other storylines. 

B1. The B1 storyline and scenario family describes a convergent world with the same global 

population, which peaks in mid-century and declines thereafter, as in the A1 storyline, but with rapid 

change in economic structures toward a service and information economy, with reductions in material 

 

30 1 gigaton (Gt) = 1 billion metric tons. 

IV. Trends and Projections of Global Environmental Change 78
intensity and the introduction of clean and resource-efficient technologies. The emphasis is on global 

solutions to economic, social, and environmental sustainability, including improved equity, but without 

additional climate initiatives. 

B2. The B2 storyline and scenario family describes a world in which the emphasis is on local solutions 

to economic, social, and environmental sustainability. It is a world with continuously increasing global 

population, at a rate lower than A2, intermediate levels of economic development, and less rapid and 

more diverse technological change than in the B1 and A1 storylines. While the scenario is also oriented 

toward environmental protection and social equity, it focuses on local and regional levels. 

An illustrative scenario was chosen for each of the six scenario groups: A1B, A1FI, A1T, A2, B1, and 

B2. All should be considered equally sound. 

The SRES scenarios do not include additional climate initiatives, which means that no scenarios are 

included that explicitly assume implementation of the United Nations Framework Convention on 

Climate Change or the emissions targets of the Kyoto Protocol. 

Figure IV.12. Observed and projected global CO2 and CH4 emissions for the IPCC Special Report on Emission 

Scenarios. Projected fossil CO2 and CH4 emissions for six illustrative non-mitigation emission scenarios. Historical 

emissions (black lines) are shown for fossil and industrial CO2 and for CH4. Source: Meehl et al. (2007). 

CCSP developed several reference-case emission scenarios as part of SAP 2.1 (Clarke et al., 

2007). Global projections of CO2 emissions from the burning of fossil fuels and industrial 

sources from these three reference cases are shown in Figure IV.13 for comparison. Box IV.2 

provides background information on the reference case scenarios developed by CCSP. The 

CCSP scenarios, because they were developed more recently than the IPCC SRES scenarios, do 

account for the implementation of the Kyoto Protocol by participating countries along with the 

U.S. greenhouse gas intensity goal, but no explicit greenhouse gas mitigation policies beyond the 

Kyoto Protocol. The emissions in 2100 are approximately 88 GtCO2 (24 GtC). This level of 

emissions is above the post-SRES IPCC median of 60 GtCO2 (16 GtC), but well within the 90th 

percentile of the IPCC range. The recent Raupach et al. (2007) study suggests that the actual 

emissions trajectory since 2000 was close to the highest-emission scenario, A1FI. More 

importantly, the emissions growth rate since 2000 exceeded that for the A1FIscenario. 

IV. Trends and Projections of Global Environmental Change 79
Box IV.2. CCSP Reference Case Emission Scenarios from Synthesis and Assessment Product 

2.1 

The three integrated assessment models used in CCSP SAP 2.1 are: 

1. The Integrated Global Systems Model (IGSM) of the Massachusetts Institute of 

Technology’s (MIT) Joint Program on the Science and Policy of Global Change. 

2. The Model for Evaluating the Regional and Global Effects (MERGE) of greenhouse 

gas reduction policies developed jointly at Stanford University and the Electric Power 

Research Institute. 

3. The MiniCAM Model of the Joint Global Change Research Institute, a partnership 

between the Pacific Northwest National Laboratory and the University of Maryland. 

The MiniCAM model was also used to generate IPCC SRES scenarios. 

Each modeling group produced a reference scenario under the assumption that no climate 

policies are imposed beyond current commitments, namely the 2008–2012 first period of the 

Kyoto Protocol and the U.S. goal of reducing greenhouse gas emissions per unit of its gross 

domestic product by 18% by 2012. The resulting reference cases are not predictions or bestjudgment forecasts but scenarios designed to provide clearly defined points of departure for 

studying the implications of alternative stabilization goals. The modeling teams used model 

input assumptions they considered ‘meaningful and plausible’. The resulting scenarios provide 

insights into how the world might evolve without additional efforts to constrain greenhouse 

gas emissions, given various assumptions about principal drivers of these emissions such as 

population increase, economic growth, land and labor productivity growth, technological 

options, and resource endowments. 

IV. Trends and Projections of Global Environmental Change 80
Figure IV.13. Projected global emissions of CO2 from fossil fuels and 

industrial sources across CCSP reference scenarios. Global emissions of CO2

from fossil fuel combustion and other industrial sources, mainly cement 

production, increase over the century in all three reference scenarios. By 

2100, emissions reach 22.5 to 24.0 GtC per year. Source: Clarke et al. (2007). 

In addition to the emission scenarios described above, 18 models were involved in the 21st Study 

of Stanford University’s Energy Modeling Forum on multi-gas mitigation (EMF-21; see de la 

Chesnaye and Weyant, 2006 in Fisher et al., 2007) to produce a range of emission scenarios that 

are representative of the literature. Figure IV.14 illustrates reference-case emission projections 

for CO2, CH4, N2O, and the fluorinated gases in aggregate (HFCs, PFCs, and SF6 or ‘F-gases’) 

from EMF-21 and from the IPCC SRES (IPCC, 2000). The broad ranges of EMF-21 emissions 

projections in Figure IV.14, especially for N2O and the F-gases, illustrate the uncertainties in 

projecting these future emissions, which are generally consistent with the range found in the 

SRES. 

IV. Trends and Projections of Global Environmental Change 81
Figure IV.14. EMF-21 and IPCC global emissions projections for CO2, CH4, N2O, and the fluorinated 

gases. Development of baseline emissions in EMF-21 scenarios developed by a number of different 

modeling teams (left) and a comparison between EMF-21 and SRES scenarios (right) from de la 

Chesnaye and Weyant (2006); see also Van Vuuren et al. (2006). Source: Fisher et al. (2007). 

For comparison, Figure IV.15 provides the global CH4 and N2O projections from the three CCSP 

reference-case scenarios (CCSP, 2007). 

IV. Trends and Projections of Global Environmental Change 82
Figure IV.15. Projected global CH4 and N2O emissions across three CCSP 

reference scenarios. Global anthropogenic emissions of CH4 and N2O vary widely 

among the reference scenarios. There is uncertainty in year-2000 CH4 emissions, 

with MIT’s IGSM reference scenario ascribing more of the emissions to human 

activity and less to natural sources. Differences in scenarios reflect, to a large 

extent, different assumptions about whether current emissions rates will be 

reduced significantly for other reasons, for example, whether higher natural gas 

prices will stimulate capture of CH4 for use as a fuel. Source: Clarke et al. (2007). 

Future concentration and radiative forcing changes 

Figure IV.16 shows the latest IPCC projected increases in atmospheric CO2, CH4, and N2O 

concentrations for the SRES scenarios, and Figure IV.17 shows the associated radiative forcing 

for CO2 from these scenarios. In general, reference concentrations of CO2 and other greenhouse 

gases are projected to increase. This is true even for those scenarios where annual emissions near 

the end of the century are assumed to be lower than current annual emissions, due to the long 

atmospheric lifetimes of these gases. The CCSP scenarios show a similar picture of how 

atmospheric concentrations of the main greenhouse gases and total radiative forcing change over 

time. 

CO2 is projected to be the largest contributor to total radiative forcing in all periods and the 

radiative forcing associated with CO2 is projected to be the fastest growing. The radiative forcing 

associated with the non-CO2 greenhouse gases is still significant and growing over time. 

IV. Trends and Projections of Global Environmental Change 83
IV. Trends and Projections of Global Environmental Change 84
Figure IV.16. Projected global CO2, CH4, and N2O concentrations for the IPCC Special 

Report on Emission Scenarios. Projected fossil CO2, CH4, and N2O concentrations for six 

illustrative SRES non-mitigation emission scenarios as produced by a simple climate model 

tuned to 19 atmosphere–ocean general circulation models. Source: Meehl et al. (2007). 

Figure IV.17. Projected radiative forcing from CO2 for the IPCC Special Report on Emission Scenarios. 

Projected radiative forcing from CO2 for six illustrative SRES non-mitigation emission scenarios as 

produced by a simple climate model tuned to 19 atmosphere–ocean general circulation models. The lighter 

shaded areas depict the change in this uncertainty range if carbon cycle feedbacks are assumed to be lower 

or higher than in the medium setting. Source: Meehl et al. (2007). 

IV.3.b Projected changes in physical components of the climate system 

In order to project future changes in the climate system, including temperature, precipitation, and 

sea level at global and regional scales, academic institutions and government-supported research 

laboratories in the United States and other countries have developed a number of computer 

models that simulate the Earth system and that are based on the various emissions scenarios 

described in Section IV.3.a. The IPCC helps coordinate modeling efforts to facilitate 

comparisons across models, and synthesizes results published by several modeling teams. 

According to the IPCC (Meehl et al., 2007): 

[C]onfidence in models comes from their physical basis, and their skill in representing observed 

climate and past climate changes. Models have proven to be extremely important tools for 

simulating and understanding climate, and there is considerable confidence that they are able to 

provide credible quantitative estimates of future climate change, particularly at larger scales. 

Models continue to have significant limitations, such as in their representation of clouds, which 

lead to uncertainties in the magnitude and timing, as well as regional details, of predicted climate 

change. Nevertheless, over several decades of model development, they have consistently 

provided a robust and unambiguous picture of significant climate warming in response to 

increasing greenhouse gases. 

Confidence in changes projected by global models decreases at smaller spatial and temporal 

scales because many important small-scale processes, in particular clouds, cannot be represented 

explicitly in models, and so must be included in approximate form as they interact with largerscale features (Randall et al., 2007). On these scales, natural climate variability is relatively 

larger. In addition, uncertainties in local forcings and feedbacks also make it difficult to estimate 

the contribution of greenhouse gas increases to observed small-scale changes (IPCC, 2007d). 

Some of the most challenging aspects of understanding and projecting regional climate changes 

(e.g., at the scale of states and counties) relate to possible changes in the circulation of the 

IV. Trends and Projections of Global Environmental Change 85
atmosphere and oceans, and their patterns of variability (Christensen et al., 2007). Nonetheless, 

the IPCC (2007d) concluded that recent advances in regional-scale modeling have led to higher 

confidence in projected patterns of warming and other regional-scale features. 

Temperature projections 

This section describes the temperature projections from a suite of climate models included in the 

IPCC’s Fourth Assessment. Compared to earlier assessments, the latest IPCC assessment uses a 

larger number of simulations available from a broader range of models to project future climate 

(IPCC, 2007d). All of the simulations assessed by the IPCC during its Fourth Assessment project 

that the climate system will warm, for the full range of emissions scenarios. 

The IPCC found that increased confidence has been gained in short-term projections by 

comparing previous IPCC projections of climate change to recent observations. For the next two 

decades, a global average warming of about 0.2 °C per decade is projected for a range of SRES 

emission scenarios (IPCC, 2007d; see Figure IV.18). Committed warming from greenhouse 

gases already released into the environment accounts for a global average increase of about 0.1 

°C per decade during the next two decades because of the time it takes for the climate system, 

particularly the oceans, to reach equilibrium. This is shown in Figure IV.18 for the ‘Year 2000 

Constant Concentrations’ scenario that shows projected increases in temperature assuming that 

the concentrations of all greenhouse gases and aerosols are kept constant at year 2000 levels. 

Through about 2030, the warming rate is affected little by different scenario assumptions or 

different assumptions about climate sensitivity, and is consistent with that observed for the past 

few decades. It is noted that possible future variations in natural forcings (e.g., a large volcanic 

eruption) could change these values somewhat (Meehl et al., 2007). 

As shown in Figure IV.18, by mid-century (2046 to 2065), the choice of scenario becomes more 

important for the magnitude of the projected global average warming, with average values of 1.3, 

1.8, and 1.7 °C from the models for scenarios B1 (low emissions growth), A1B (medium 

emissions growth), and A2 (high emissions growth), respectively (Meehl et al., 2007). About 

one-third of that warming is projected to be due to climate change that is already committed to, 

again shown in the ‘Year 2000 Constant Concentrations’ scenario. 

IV. Trends and Projections of Global Environmental Change 86
Figure IV.18. Multi-model averages and assessed ranges for surface warming. Solid lines are multi-model 

global averages of surface warming (relative to 1980 to 1999) for the scenarios A2, A1B, and B1, shown as 

continuations of the 20th-century simulations. Shading denotes the ±1 standard deviation range of 

individual model annual averages. The orange line is for the experiment where concentrations were held 

constant at year 2000 values. The grey bars at right indicate the best estimate (solid line within each bar) 

and the likely range assessed for the six SRES marker scenarios. The assessment of the best estimate and 

likely ranges in the grey bars includes results from the atmosphere–ocean general circulation models shown 

in the left part of the figure, as well as results from a hierarchy of independent models and observational 

constraints. Source: IPCC (2007d). 

By the end of the century (2090 to 2099), projected global average surface warming varies 

significantly by emissions scenario. The full suite of SRES scenarios provide warming for 2090 

to 2099 relative to 1980 to 1999 with a range of 1.8 to 4.0 °C with an uncertainty range of 1.1 to 

6.4 °C. The multi-model average warming and associated uncertainty ranges for 2090 to 2099 

(relative to 1980 to 1999) for each scenario, as illustrated in Figure IV.18, are as follows: 

Table IV.3.b.1 

Scenario Projected Global Average Surface

Warming by End of Century Relative 

to ~1990

Uncertainty Range

°C °F °C °F 

B1 1.8 3.2 1.1 to 2.9 2.0 to 5.2 

A1T 2.4 4.3 1.4 to 3.8 2.5 to 5.7 

IV. Trends and Projections of Global Environmental Change 87
B2 2.4 4.3 1.4 to 3.8 2.5 to 5.7 

A1B 2.8 5.0 1.7 to 4.4 3.1 to 7.9 

A2 3.4 6.1 2.0 to 5.4 3.6 to 9.7 

A1FI 4.0 7.2 2.4 to 6.4 4.3 to 11.5 

The wide range in these estimates reflects the different assumptions about future anthropogenic 

emissions of greenhouse gases and aerosols in the various scenarios considered by the IPCC and 

the differing climate sensitivities of the various climate models used in the simulations (NRC, 

2001a). 

Geographical patterns of projected warming show greatest temperature increases over land 

(roughly twice the global average temperature increase) and at high northern latitudes, and less 

warming over the southern oceans and North Atlantic. These projections are consistent with 

trends in observations (see Section IV.1.b) during the latter part of the 20th century (Meehl et al., 

2007). According to the Arctic Climate Impact Assessment (ACIA, 2004), the Arctic is predicted 

to warm faster than the global average, with projected Arctic warming of 4 to 7 °C over the next 

100 years; this is consistent with the IPCC’s conclusions (Christensen et al., 2007). 

IV. Trends and Projections of Global Environmental Change 88
Temperature anomalies with respect to 1901 to 1950 for four North American land 

regions 

Figure IV.19. Temperature anomalies with respect to 1901 to 1950 for four North American land 

regions for 1906 to 2005 (black line) and as simulated (red envelope) by multi-model dataset (MMD) 

models incorporating known forcings, and as projected for 2001 to 2100 by MMD models for the A1B 

scenario (orange envelope). The bars at the end of the orange envelope represent the range of projected 

changes for 2091 to 2100 for the B1 scenario (blue), the A1B scenario (orange), and the A2 scenario 

(red). The black line is dashed where observations are present for less than 50% of the area in the decade 

concerned. Source: Christensen et al. (2007). 

IV. Trends and Projections of Global Environmental Change 89
Figure IV.20. Projected temperature and precipitation changes over North America from the MMDA1B simulations. Top row: annual mean, December-January-February, and June-July-August 

temperature between 1980 to 1999 and 2080 to 2099, averaged over 21 models. Bottom row: same as 

top, but for fractional change in precipitation. Source: Christensen et al. (2007). 

The IPCC found that all of North America is very likely not only to warm during this century (as 

shown in Figures IV.19 and IV.20), but to warm more than the global mean warming in most 

areas (Christensen et al., 2007). The average warming in the United States is projected by nearly 

all the models used in the IPCC assessment to exceed 2 °C, with 5 out of 21 models projecting 

average warming in excess of 4 °C. The largest warming is projected to occur in winter over 

northern parts of Alaska, reaching 10 °C for scenario A1B (moderate emissions scenario relative 

to other IPCC scenarios) in the northernmost parts (as shown in Figure IV.20), due to the 

positive feedback from a shorter season of snow cover. In western, central, and eastern regions of 

North America, the projected warming has less seasonal variation and is not as large, especially 

near the coast, which is consistent with less warming over the oceans. An increase in surface 

evaporation is expected to accompany the projected widespread increase in temperature. 

Precipitation projections 

IV. Trends and Projections of Global Environmental Change 90
Overall, future model projections show that global mean precipitation increases with the 

warming of the climate (Meehl et al., 2007), but with substantial spatial and seasonal variations. 

The IPCC concluded that increases in the amount of precipitation are very likely at high latitudes, 

while decreases are likely in most subtropical land regions, continuing observed patterns in 

recent trends in observations. 

A widespread increase in annual precipitation is projected by the IPCC over most of the North 

American continent except the southern and southwestern part of the United States and over 

Mexico. This is largely consistent with trends in recent decades (as described in Section IV.1.c) 

(Christensen et al., 2007). Figure IV.20 shows that the largest increases are projected over the 

northern tier of states (including Alaska) associated with a poleward shift in storm tracks, where 

the magnitude of precipitation increase is projected to be greatest in autumn, whereas winter 

precipitation is projected to increase by the largest fraction relative to its present amount. In 

western North America, modest changes in annual mean precipitation are projected, but the 

majority of models indicate an increase in winter and a decrease in summer. Models show 

greater consensus on winter increases to the north and on summer decreases to the south. These 

decreases are consistent with enhanced subsidence and flow of drier air masses in the 

southwestern United States and northern Mexico. Accordingly, some models project drying in 

the southwestern United States, and more than 90% of the models project drying in northern and 

particularly western Mexico. In the northeastern United States, annual mean precipitation is very 

likely to increase. It is uncertain whether summer precipitation will increase or decrease over 

large portions of the interior United States. On the windward slopes of the mountains in the west, 

precipitation increases are likely to be enhanced due to orographic lifting (ascent of air from a 

lower elevation to a higher elevation as it moves over rising terrain). 

It is likely that droughts will continue to be exacerbated by earlier and possibly lower spring 

snowmelt runoff in the mountainous West, which results in less water available in late summer 

(Karl et al., 2008). A recent analysis (Milly et al., 2005 in Karl et al., 2008) shows that several 

atmosphere–ocean general circulation models (AOGCMs) project greatly reduced annual water 

availability over the southwestern United States and northern Mexico in the future. 

Sea level rise projections 

Along with increases in global ocean temperatures, the IPCC (2007d) projects sea level rise of 

between 0.18 and 0.59 m by the end of the century (2090 to 2099) relative to the base period 

(1980 to 1999). These numbers represent the lowest and highest projections of the 5 to 95% 

ranges for all SRES scenarios considered collectively and include neither uncertainty in carbon 

cycle feedbacks nor rapid dynamical changes in ice sheet flow. In all scenarios, the average rate 

of sea level rise during the 21st century is very likely to exceed the 1961 to 2003 average rate 

(1.8 ± 0.5 mm per year). Again, there are committed effects, where even if greenhouse gas 

concentrations were to be stabilized, sea level rise would continue for centuries due to the time 

scales associated with climate processes and feedbacks (IPCC, 2007d). Thermal expansion of 

ocean water is the primary driver of sea level rise, contributing 70 to 75% of the central estimate 

for the rise in sea level for all scenarios (Meehl et al., 2007). 

IV. Trends and Projections of Global Environmental Change 91
Over the coming century, glaciers, ice caps, and the Greenland Ice Sheet are also projected to 

add to sea level. The Antarctic Ice Sheet, however, is projected by general circulation models to 

receive increased snowfall without experiencing substantial surface melting, thus gaining mass 

and reducing sea level rise, according to the IPCC (Meehl et al., 2007). There is potential 

uncertainty in these projections for the Antarctic Ice Sheet associated with dynamical processes 

related to ice flow that are not included in current models, but are suggested by recent 

observations. This could increase the vulnerability of the ice sheets to warming, increasing future 

sea level rise. Meehl et al. (2007) note that further accelerations in ice flow of the kind recently 

observed in some Greenland outlet glaciers and West Antarctic ice streams could substantially 

increase the contribution from the ice sheets, a possibility not reflected in the upper bound of 

global sea level projections. For example, if ice discharge from these processes were to increase 

in proportion to global average surface temperature change, it would add at least 0.1 to 0.2 m to 

the upper bound of sea level rise by 2090 to 2099. Understanding of these processes is limited 

and there is no consensus on their magnitude (Meehl et al., 2007; IPCC, 2007d). 

Sea level rise during the 21st century is projected by the IPCC to have substantial geographic 

variability due to factors discussed in Section IV.1.d that influence changes at the regional scale, 

including changes in ocean circulation or atmospheric pressure, and geologic processes (Meehl 

et al., 2007). The patterns simulated by different models are not generally similar in detail, but 

have some common features, including smaller than global average sea level rise in the Southern 

Ocean, larger than global average sea level rise in the Arctic, and a narrow band of pronounced 

sea level rise stretching across the southern Atlantic and Indian Oceans. 

For North American coasts, emissions scenario A1B shows sea level rise values close to the 

global mean, with slightly higher rates in eastern Canada and western Alaska, and stronger 

positive anomalies in the Arctic. The projected rate of sea level rise off the low-lying U.S. South 

Atlantic and Gulf Coasts is also higher than the global average. Vertical land motion from 

geologic processes may decrease (through uplift) or increase (through subsidence) the relative 

sea level rise at any site (Nicholls et al., 2007). 

Cryosphere (snow and ice) projections 

The IPCC (Meehl et al., 2007) concluded the following during the Fourth Assessment: 

• snow cover and sea ice extent are projected to decrease as the climate warms; 

• glaciers and ice caps are projected to lose mass as increases in summer melting outweigh 

increases in winter precipitation, which will contribute to sea level rise; and 

• widespread increases in thaw depth are projected over most permafrost regions globally. 

The Arctic Climate Impact Assessment (ACIA, 2004) concluded that snow cover will decline by 

10 to 20% by the 2070s and that the projected decreases in snow and ice cover are very likely to 

persist for centuries. 

Focusing on North America, snow season length and snow depth are very likely to decrease in 

most of North America, as illustrated in Figure IV.21. The exception is in the northernmost part 

of Canada, where maximum snow depth is likely to increase (Christensen et al., 2007). Sea ice is 

projected to shrink in both the Arctic and the Antarctic under all SRES scenarios (IPCC, 2007d). 

IV. Trends and Projections of Global Environmental Change 92
In some projections, Arctic late-summer sea ice disappears almost entirely by the latter part of 

the 21st century. 

Percent Snow Depth Changes in March 

Figure IV.21. Percent snow depth changes in March (only calculated where 

climatological snow amounts exceed 5 mm of water equivalent), as projected by 

the Canadian Regional Climate Model (Plummer et al., 2006 in Christensen et al., 

2007), driven by the Canadian General Circulation Model, for 2041 to 2070 under 

the SRES A2 scenario compared to 1961 to 1990. Source: Christensen et al. 

(2007). 

Projections of extreme events 

Models suggest that human-induced climate change will alter the prevalence and severity of 

many extreme events such as heat waves, cold waves, storms, floods, and droughts. Projections 

of global temperature from the IPCC (Meehl et al., 2007) show that it is very likely that heat 

waves will become more intense, more frequent, and longer lasting in a future warm climate, 

whereas cold episodes are projected to decrease significantly. Meehl and Tebaldi (2004, in 

Meehl et al., 2007) found that the pattern of future changes in heat waves—which shows the 

greatest intensity increases over western Europe, the Mediterranean, and the southeastern and 

western United States—is related in part to circulation changes resulting from an increase in 

greenhouse gases. 

CCSP SAP 3.3 (Karl et al., 2008) concluded for North America that abnormally hot days and 

nights and heat waves are very likely to become more frequent and that cold days and cold nights 

IV. Trends and Projections of Global Environmental Change 93
are very likely to become much less frequent31. Additionally, the number of days with frost is 

very likely to decrease. Focusing on North America and the United States, the IPCC (Christensen 

et al., 2007) cites a number of studies that project changes in temperature extremes in the United 

States. Diffenbaugh et al. (2005 in Christensen et al., 2007) find that the frequency and the 

magnitude of extreme temperature events change dramatically under a high-end emission 

scenario (SRES A2), with increases in extreme hot events and decreases in extreme cold events. 

Bell et al. (2004, in Christensen et al., 2007) examine changes in temperature extremes in their 

simulations centered on California and find increases in extreme temperature events, prolonged 

hot spells, and increased diurnal temperature range. Leung et al. (2004, in Christensen et al., 

2007) find increases in diurnal temperature range in six sub-regions of the western United States 

in summer. 

Projections of global precipitation extremes and droughts show that the intensity of precipitation 

events is projected to increase, particularly in tropical and high-latitude areas that experience 

increases in mean precipitation (Meehl et al., 2007). Even in areas where mean precipitation is 

projected to decrease (most subtropical regions), precipitation intensity is projected to increase. 

However, there would be longer periods between rainfall events. The IPCC (Meehl et al., 2007) 

also found that increases in heavy precipitation events have been linked to increases in flooding. 

The IPCC (Meehl et al., 2007) projected a tendency for drying in mid-continental areas during 

summer due to higher temperatures, indicating a greater risk of droughts in those regions. 

Extreme drought increases from 1% of present-day land area to 30% by the end of the century in 

the A2 (high emissions growth) scenario (Burke et al., 2006 in Meehl et al., 2007). 

Several regional studies in the IPCC project changes in precipitation extremes in parts of the 

United States. These changes range from a decrease in heavy precipitation in California (Bell et 

al., 2004 in Christensen et al., 2007) to an increase during winter in the northern Rockies, 

Cascades, and Sierra Nevada mountain ranges (Leung et al., 2004 in Christensen et al., 2007). 

For the contiguous United States, Diffenbaugh et al. (2005, in Christensen et al., 2007) found 

widespread increases in extreme precipitation events under SRES A2 (high emissions growth). 

CCSP SAP 3.3 (Karl et al., 2008) concluded for North America that, on average, precipitation is 

likely to be less frequent but more intense, and precipitation extremes are very likely to increase. 

For example, for a mid-range emission scenario, daily precipitation so heavy that it now occurs 

only once every 20 years is projected to occur approximately every 8 years by the end of this 

century over much of eastern North America (Karl et al., 2008). 

In its Fourth Assessment, the IPCC concluded that it is likely that future tropical cyclones 

(typhoons and hurricanes) will become more intense, with larger peak wind speeds and more 

heavy precipitation associated with ongoing increases in tropical sea surface temperatures (IPCC, 

2007d; Meehl et al., 2007). Some modeling studies have projected a decrease in the number of 

tropical cyclones globally due to increased stability of the tropical atmosphere in a warmer 

climate, characterized by fewer weak storms and greater numbers of intense storms. A number of 

modeling studies have also projected a general tendency for more intense but fewer storms 

 

31 SAP 3.3 (Karl et al., 2008) does not use discrete quantitative bounds to characterize the likelihood of events, in 

contrast to the IPCC (Meehl et al., 2007). The characterization of likelihood is, however, qualitatively similar 

between the two assessments. 

IV. Trends and Projections of Global Environmental Change 94
outside the tropics, with a tendency towards more extreme wind events and higher ocean waves 

in several regions associated with these deepened cyclones (Meehl et al., 2007). 

Some of the sectoral impacts of extreme events for the United States are summarized throughout 

section V of this report. 

Abrupt climate change projections 

Abrupt climate change refers to sudden (of the order of decades) large changes in some major 

component of the climate system, with rapid, widespread effects. Abrupt climate changes are an 

important consideration because, if triggered, they could occur so quickly and unexpectedly that 

human or natural systems would have difficulty adapting to them (NRC, 2002). This section 

focuses on the general risks of abrupt climate change globally, with some discussion of potential 

regional implications where information is available. Potential abrupt climate change 

implications for the United States are not discussed in the Section V because they cannot be 

predicted with confidence, particularly for specific regions. 

An abrupt climate change occurs when the climate system is forced to cross some threshold, 

which triggers a transition into a new state (NRC, 2002). Crossing systemic thresholds may lead 

to large and widespread consequences (Schneider et al., 2007). As detailed by the NRC (2002), 

the triggers for abrupt climate change can be forces that are external and/or internal to the 

climate system, including 

• changes in Earth’s orbit,32 

• a brightening or dimming of the Sun, 

• melting or surging ice sheets, 

• strengthening or weakening of ocean currents, and 

• emissions of climate-altering gases and particles into the atmosphere. 

More than one of these triggers can operate simultaneously, since all components of the climate 

system are linked. 

 

Scientific data show that abrupt changes in the climate at the regional scale have occurred 

throughout history and are characteristic of the Earth’s climate system (NRC, 2002). During the 

last glacial period, abrupt regional warming events (likely up to 16 °C within decades over 

Greenland) and cooling events occurred repeatedly over the North Atlantic region (Jansen et al., 

2007). These warming events likely had some large-scale effects such as major shifts in tropical 

rainfall patterns and redistribution of heat within the climate system, but it is improbable that 

they were associated with large changes in global mean surface temperature. 

The NRC (2002) concluded that anthropogenic forcing could increase the risk of abrupt climate 

change, making the following statements: 

 

32 According to the NRC (2002), changes in Earth’s orbit occur too slowly to be prime movers of abrupt change but 

might determine the timing of events. Abrupt climate changes of the past were especially prominent when orbital 

processes were forcing the climate to change during the cooling leading into and warming leading out of ice ages 

(NRC, 2002). 

IV. Trends and Projections of Global Environmental Change 95
• Greenhouse warming and other human alterations of the Earth system may increase the 

possibility of large, abrupt, and unwelcome regional or global climatic events. 

• Abrupt changes of the past are not fully explained yet, and climate models typically 

underestimate the size, speed, and extent of those changes. 

• Therefore, future abrupt changes cannot be predicted with confidence, and climate surprises 

are to be expected. 

Changes in weather patterns (sometimes referred to as weather regimes or natural modes) can 

result from abrupt changes that might occur spontaneously due to dynamic interactions in the 

atmosphere/ice–ocean system, or from crossing a threshold from slow external forcing (as 

described above) (Meehl et al., 2007). In a warming climate, changes in the frequency and 

amplitudes of these patterns might not only evolve rapidly, but also trigger other processes that 

lead to abrupt climate change (NRC, 2002). Examples of these patterns include the El Niño–

Southern Oscillation (ENSO)33 and the North Atlantic Oscillation/Arctic Oscillation 

(NAO/AO).34

Scientists have investigated the possibility of an abrupt slowdown or shutdown of the Atlantic 

meridional overturning circulation (MOC) triggered by greenhouse gas forcing. The MOC 

transfers large quantities of heat to the North Atlantic and Europe, so an abrupt change in the 

MOC could have important implications for the climate of this region (Meehl et al., 2007). 

However, according to the IPCC (Meehl et al., 2007), the probability of an abrupt change in (or 

shutdown of) the MOC during the 21st century is very unlikely. However, longer-term changes in 

the MOC cannot be assessed with confidence. The slowdown in the MOC projected by most 

models is gradual, so the resulting decrease in heat transport to the North Atlantic and Europe 

would not be large enough to reverse the warming that results from the increase in greenhouse 

gases. 

The rapid disintegration of the Greenland Ice Sheet, which would raise sea levels 7 m, is another 

commonly discussed abrupt change. Although models suggest the complete melting of the 

Greenland Ice Sheet would only require sustained warming in the range of 1.9 to 4.6 °C (relative 

to pre-industrial temperatures), it is expected to be a slow process that would take many 

hundreds of years to complete (Meehl et al., 2007). 

A collapse of the West Antarctic Ice Sheet, which would raise sea levels 5 to 6 m, has been 

discussed as a low-probability, high-impact response to global warming (NRC, 2002; Meehl et 

al., 2007). The weakening or collapse of ice shelves, caused by melting on the surface or by 

 

33 ENSO describes the full range of the Southern Oscillation (seesaw of atmospheric mass or pressure between the 

Pacific and Indo-Australian areas) that includes both sea surface temperature increases as well as sea surface 

temperature decreases when compared to a long-term average. It has sometimes been used by scientists to relate 

only to the broader view of El Niño or the warm events–the warming of sea surface temperatures in the central and 

eastern equatorial Pacific. The acronym, ENSO, is composed of El Niño–Southern Oscillation, where El Niño is the 

oceanic component and the Southern Oscillation is the atmospheric component of the phenomenon. 34 The North Atlantic Oscillation (NAO) is the dominant mode of winter climate variability in the North Atlantic 

region ranging from central North America to Europe and into Northern Asia. The NAO is a large-scale seesaw in 

atmospheric mass or pressure between the subtropical high and the polar low. Similarly, the Arctic Oscillation (AO) 

refers to opposing atmospheric pressure patterns in northern middle and high latitudes. The NAO and AO are 

different ways of describing the same general phenomenon. 

IV. Trends and Projections of Global Environmental Change 96
melting at the bottom by a warmer ocean, might contribute to a potential destabilization of the 

West Antarctic Ice Sheet. Recent satellite and in situ observations of ice streams behind 

disintegrating ice shelves highlight some rapid reactions of ice sheet systems (Lemke et al., 

2007). Ice sheet models are only beginning to capture the small-scale dynamic processes that 

involve complicated interactions with the glacier bed and the ocean at the perimeter of the ice 

sheet (Meehl et al., 2007). These processes are not represented in the models used by the IPCC to 

project sea level rise, which suggest Antarctica will gain mass due to the likelihood of increasing 

snowfall, reducing sea level rise. It is noted that recent studies find no significant continent-wide 

trends in accumulation over the past several decades (Lemke et al., 2007). But it is possible that 

acceleration of ice discharge could become dominant, causing a net positive contribution. Given 

these competing factors, there is presently no consensus on the long-term future of the West 

Antarctic Ice Sheet or its contribution to sea level rise (Meehl et al., 2007). 

IV.3.c Land cover change as an environmental driver 

The SAP 4.3 report from CCSP (Backlund et al., 2008) concluded that global climate change 

effects will be superimposed on and modify those resulting from land use and land cover patterns 

in ways that are as of yet uncertain. 

A literature review of the relationship between climate change and land use indicates that land 

use change has had a much greater effect on ecosystems than has climate change and that the 

vast majority of land use changes have little to do with climate or climate change (Dale, 1997 in 

Backlund et al., 2008). In the near term, climate fluctuation and change will influence 

ecosystems primarily through the impact of land use on ecosystems, and the response of 

ecosystems to land use (Backlund et al., 2008). Changes in land cover can make restoration 

prohibitively costly, and in some cases, impossible. This is particularly the case for native 

rangeland that is disturbed, whether intentionally through intensive agriculture or unintentionally 

through climate change. 

As discussed in SAP 4.3 (Backlund et al., 2008), land use changes are an important driver of 

changes in arid lands. Today’s arid lands reflect a legacy of historic land uses, and future land 

use practices will arguably have the greatest impact on arid land ecosystems in the next two to 

five decades. Grazing has traditionally been the most extensive land use in arid regions. 

However, land use has significantly shifted to exurban development and recreation in recent 

decades. Thus, in addition to grazing, the response of arid lands to climate change will be 

influenced by environmental pressures related to air pollution, nitrogen deposition, energy 

development, motorized off-road vehicles, feral pets, and horticultural invasive species. Many 

plants and animals in arid ecosystems are near their physiological limits for tolerating 

temperature and water stress, such that even slight changes in stress will have significant 

consequences. Projected climate changes will increase the sensitivity of arid lands to 

disturbances such as grazing and fire. Invasion of non-native grasses will increase fire frequency. 

In the near-term, fire effects will exceed climate effects on ecosystem structure and function. 

The current land use system in the United States requires high resource inputs, from the use of 

synthetic fertilizer on crops to the transport of crops to animal feeding operations. In addition to 

being inefficient with regard to fuel use, this system creates environmental problems ranging 

IV. Trends and Projections of Global Environmental Change 97
from erosion to degradation of water supplies due to high nutrient levels. SAP 4.3 (Backlund et 

al., 2008) discussed specific details related to the effects of land use changes: 

• Land cover and land use patterns are changing. For example, as exurban development 

spreads to previously undeveloped areas, U.S. forests are becoming increasingly fragmented, 

further raising fire risk and compounding the effects of summer drought, pests, and warmer 

winters. 

• It is unlikely that the hydrologic trends detected in the various studies reviewed above can be 

attributed, at least in large part, to land cover and land use change, but sufficient questions 

remain that it cannot be definitively ruled out. 

• There has been long-standing controversy in determining the relative contribution of climatic 

and anthropogenic factors as drivers of desertification. Local fence line contrasts argue for 

the importance of land use (e.g., changes in grazing and fire regimes). Vegetation change in 

areas with no known change in land use argues for climatic drivers. 

The Arctic Climate Impact Assessment (ACIA, 2004) also discusses projected changes in land 

cover, stating that as the climate warms, the tree line is expected to move northward and to 

higher elevations. In these projections, forests will replace significant amounts of existing tundra 

and tundra vegetation will move into polar deserts. These changes are likely to increase carbon 

uptake, which would partially offset warming effects. However, the reduced reflectivity 

associated with the vegetation land cover is likely to outweigh this, causing further warming. 

Ultimately, the ability to attribute effects to either climate change or land use changes is 

hampered by the lack of observations of land use changes. As noted by SAP 4.3 (Backlund et al., 

2008), there is no coordinated national network for monitoring changes in land resources 

associated with climate change, most disturbances (storms, insects, and diseases), and changes in 

land cover/land use. Thus, separating climate effects from other environmental stresses is 

difficult but in some cases feasible. For example, when detailed water budgets exist, the effects 

of land use, climate change, and consumptive use on water levels can be calculated. While 

climate effects can be difficult to quantify at small scales, sometimes regional effects can be 

separated. For example, regional trends in productivity, estimated using satellite methods, can 

often be assigned to regional trends in climate versus land use, although on any individual smallscale plot, climate effects may be primary or secondary. In other cases, scientific understanding 

is sufficiently robust that models in conjunction with observations can be used to estimate 

climate effects. Overall, SAP 4.3 (Backlund et al., 2008) recommended that more refined 

analysis and/or monitoring systems designed specifically for detecting climate change effects 

would provide more detailed and complete information and probably capture a range of more 

subtle impacts, which might, in turn, lead to early warning systems and more accurate forecasts 

of potential future changes. 

IV.3.d Effects on and from stratospheric ozone 

Stratospheric ozone protects Earth’s surface from much of the Sun’s biologically harmful 

ultraviolet radiation. According to the WMO (2007), climate change that results from changing 

IV. Trends and Projections of Global Environmental Change 98
greenhouse gas concentrations will affect the evolution of the ozone layer through changes in 

chemical transport, atmospheric composition, and temperature. In turn, changes in stratospheric 

ozone can affect the weather and climate of the troposphere. The coupled interactions between 

the changing climate and the ozone layer are complex, and scientific understanding is incomplete 

(WMO, 2007). Specific information on climate change effects on and from stratospheric ozone 

in the United States has not been assessed. Except where indicated, the findings in this section 

apply generally to the globe with a focus on polar regions. 

The 2006 Scientific Assessment of Ozone Depletion (WMO, 2007) concluded that it is unlikely

that total ozone averaged over the region 60° S to 60° N will decrease significantly below the 

low values of the 1990s, because the abundances of ozone-depleting substances have peaked and 

are in decline. The current best estimate is that ozone between 60° S and 60° N will return to pre1980 levels around the middle of the 21st century. 

Effects of elevated greenhouse gas concentrations on stratospheric ozone 

WMO (2007) concluded that future concentrations of stratospheric ozone are sensitive to future 

levels of the well-mixed greenhouse gases. WMO (2007) found that there are several competing 

effects: 

• Future increases in greenhouse gas concentrations, primarily CO2, will contribute to the 

average cooling in the stratosphere. Stratospheric cooling is expected to slow gas-phase 

ozone depletion reactions and increase ozone. 

• Enhanced CH4 emissions (from warmer and wetter soils) are expected to enhance ozone 

production in the lower stratosphere. 

• An increase in N2O emissions is expected to reduce ozone in the middle and upper 

stratosphere. 

Two-dimensional models that include coupling between all of these well-mixed greenhouse 

gases and temperature project that ozone levels between 60° S and 60° N will return to 1980 

values up to 15 years earlier than projected by models that are uncoupled (Bodeker et al., 2006). 

The impact of stratospheric cooling on ozone might be the opposite in polar regions, where 

cooling could cause increases in polar stratospheric clouds, which, given enough halogens, 

would increase ozone loss (Bodeker et al., 2006). 

Concentrations of stratospheric ozone are also sensitive to stratospheric water vapor 

concentrations, which may remain relatively constant or increase (Baldwin et al., 2007). 

Increases in water vapor would cause increases in hydrogen oxide (HOx) radicals, affecting 

ozone loss processes (Baldwin et al., 2007). Several studies (Dvortsov and Solomon, 2001 and 

Shindell, 2001 in Baldwin et al., 2007) suggest that increasing stratospheric water vapor would 

delay ozone layer recovery. Increases in stratospheric water vapor could also increase spring 

ozone depletion in the polar regions by raising the temperature threshold for the formation of 

polar stratospheric clouds (WMO, 2007). 

The possible effects of climate change on stratospheric ozone are further complicated by possible 

changes in climate dynamics. Climate change can affect temperatures, upper-level winds, and 

storm patterns, which, in turn, impact planetary waves that affect the stratosphere (Baldwin et al., 

IV. Trends and Projections of Global Environmental Change 99
2007). Changes in the forcing and propagation of planetary waves35 in the polar winter are a 

major source of uncertainty for predicting future levels of Arctic ozone loss (Austin et al., 2003 

in Baldwin et al., 2007). 

Effects of stratospheric ozone changes on climate 

 

The WMO (2007) found that changes in the temperature and circulation of the stratosphere affect 

climate and weather in the troposphere. The dominant tropospheric response, simulated in 

models and identified in analyses of observations, comprises changes in the strength of midlatitude westerly winds. The mechanism for this response is not well understood. 

Modeling experiments, which simulate observed changes in stratospheric ozone and combined 

stratospheric ozone depletion and greenhouse gas increases, also suggest that Antarctic ozone 

depletion, through its effects on the lower-stratospheric vortex, has contributed to the observed 

surface cooling over interior Antarctica and warming of the Antarctic Peninsula, particularly in 

summer (Baldwin et al., 2007). While the physics of these effects are not well understood, the 

simulated pattern of warming and cooling is a robust result seen in many different models, and is 

well supported by observational studies. 

As the ozone layer recovers, tropospheric changes that have occurred as a result of ozone 

depletion are expected to reverse. However, temperature changes due to increasing greenhouse 

gas concentrations may offset this reversal (Baldwin et al., 2007). 

 

35 A planetary wave is a large horizontal atmospheric undulation that is associated with the polar-front jet stream and 

separates cold, polar air from warm, tropical air. 

IV. Trends and Projections of Global Environmental Change 100
IV. Trends and Projections of Global Environmental Change 101
V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 102

 

Section V: Analysis of the Effects of Global Change on the 

Natural Environment and Human Systems 

In the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report, Field et 

al. (2007)1

 concluded: 

[The] United States and Canada will experience climate changes through direct effects of local 

changes (e.g., temperature, precipitation, and extreme weather events), as well as through indirect 

effects, transmitted among regions by interconnected economies and migrations of humans and 

other species. Variations in wealth and geography, however, lead to an uneven distribution of 

likely impacts, vulnerabilities and capacities to adapt [North America chapter of Working Group 

II]. 

Observed and potential effects of these global changes are discussed in the following sections: 

V.1 Biological diversity, ecosystems, and the natural environment; 

V.2 Agriculture; 

V.3 Forestry; 

V.4 Water resources; 

V.5 Social systems and settlements; 

V.6 Human health; 

V.7 Energy production, use, and distribution; and 

V.8 Transportation. 

This discussion draws heavily on the Climate Change Science Program (CCSP) Synthesis and 

Assessment Products (SAPs) 4.3, 4.4, 4.5, 4.6, and 4.7. These products included an extensive 

examination of the relevant scientific literature to examine the effects of climate change on U.S. 

agriculture, land and water resources, biodiversity, social systems, human health, and 

transportation. Other important source documents include the following chapters in the IPCC 

Working Group II volume of the Fourth Assessment Report: “Assessment of observed changes 

and responses in natural and managed systems” (Rosenzweig et al., 2007); “North America” 

(Field et al., 2007); “Ecosystems, their properties, goods and services” (Fischlin et al., 2007); 

“Food, fiber, and forest products” (Easterling et al., 2007); “Freshwater resources and their 

management” (Kundzewicz et al., 2007); “Coastal systems and low-lying areas” (Nicholls et al., 

2007); “Industry, settlement, and society” (Wilbanks et al., 2007a); “Human health” 

(Confalonieri et al., 2007); “Polar regions” (Anisimov et al., 2007); and “Small islands” (Mimura 

et al., 2007). 

Recent years have seen a substantial increase in scientific research on global change, yielding an 

improved understanding of emerging effects and areas of vulnerability (Field et al., 2007). 

Multiple stressors are contributing to these emerging changes. As discussed in the previous 

section, there is a robust scientific consensus that human-induced climate change is occurring. 

The Fourth Assessment Report concluded that “the understanding of anthropogenic warming and 

cooling influences on climate has improved since the Third Assessment Report, leading to very 

high confidence (at least a 9 out of 10 chance)36 that the global average net effect of human 

activities since 1750 has been one of warming” (IPCC, 2007a). According to the IPCC, 

36 Definitions for terms used in statements of confidence and likelihood can be found in Section II.3: 

Characterization of Uncertainty. 
terrestrial ecosystems and marine and freshwater systems show that recent warming is strongly 

affecting natural biological systems (very high confidence) (Rosenzweig et al., 2007). Other 

stressors that are also affecting ecosystems include changing patterns of land use and 

management, invasive species, air and water pollution, and disturbances such as wildland fires, 

floods, and outbreaks of plant pathogens and vegetation defoliators. These stressors may cause 

effects on their own, or they may interact with other stressors, resulting in nonlinear impacts 

(Field et al., 2007). 

V.1 Biological Diversity, Ecosystem Composition, and the Natural 

Environment 

Ecosystems provide a number of goods (e.g., food, fiber, fuel, pharmaceutical products) and 

services (e.g., cycling of water and nutrients, regulation of weather and climate, removal of 

waste products, sustaining biological diversity, providing recreational and spiritual opportunities) 

to society. Changing global conditions have implications for the health of terrestrial and aquatic 

ecosystems and the goods and services required by our growing population. The IPCC (Field et 

al., 2007) found that North American animals are responding to climate change, with effects 

observed on phenology, migration, reproduction, dormancy, and geographic range (Parmesan 

and Yohe, 2003; Root et al., 2003, 2005; Parmesan and Galbraith, 2004).

The potential impacts of climate change on ecosystems have long have been a concern to the 

scientific community (Peters and Lovejoy, 1992; IPCC, 1990). Substantial research has 

examined the effects of climate change on vegetation and wildlife, leading to the conclusion that 

the changing climate is already having real and demonstrable impacts on a variety of ecosystem 

types (Janetos et al., 2008). In their summary of findings, the IPCC (Working Group II) recently 

concluded, “Recent studies have allowed a broader and more confident assessment of the 

relationship between observed warming and impacts than was made in the Third Assessment.” 

That assessment concluded that “there is high confidence [about an 8 out of 10 chance37] that 

recent regional changes in temperature have had discernible impacts on many physical and 

biological systems.” In the Fourth Assessment Report, the IPCC concluded, “Observational 

evidence from all continents and most oceans shows that many natural systems are being 

affected by regional climate changes, particularly temperature increases” (IPCC, 2007b). CCSP 

SAP 4.3 (Backlund et al., 2007) concluded that it is very likely that climate changes—including 

temperature increases, increasing CO2 levels, and altered patterns of precipitation—are already 

affecting U.S. water resources, agriculture, land resources, and biodiversity. This report also 

concluded that it is very likely that climate change will increase in importance as a driver for 

changes in biodiversity over the next several decades, although for most ecosystems it is not 

currently the largest driver of change. 

Of the 455 biological observations assessed by the IPCC, 92% were consistent with the changes 

expected due to average warming (Rosenzweig et al., 2007). Indeed, an analysis of 866 peerreviewed papers exploring the ecological consequences of climate change found that nearly 60% 

 

37 Definitions for terms used in statements of confidence and likelihood can be found in Section II.3: 

Characterization of Uncertainty. 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 103
of the 1,598 species studied exhibited shifts in their distributions and/or phenologies over the 20- 

and 140-year timeframes (Parmesan and Yohe, 2003). In their review of the research on effects 

on North American ecosystems, Field et al. (2007) wrote, “Over the 21st century, changes in 

climate will cause species to shift north and to higher elevations and fundamentally rearrange 

North American ecosystems. Differential capacities for range shifts and constraints from 

development, habitat fragmentation, invasive species, and broken ecological connections will 

alter ecosystem structure, function, and services.” 

The following sections discuss specific conclusions regarding current findings and trends. 

Specifically, they address: 1) ecosystem distribution and phenology, 2) ecosystem services, 3) 

ecosystem effects from extreme events, 4) ecosystem effects from sea level rise, and 5) 

vulnerable ecosystems. 

V.1.a Ecosystem distribution and phenology

Ecosystem distribution 

As described in SAP 4.3 (Janetos et al., 2008), evidence from two meta-analyses (Parmesan and 

Yohe, 2003; Root et al., 2003) and a synthesis (Parmesan, 2006) suggests that recent climatic 

warming has significantly contributed to a long-term, large-scale alteration of animal and plant 

populations across a broad array of taxa (Root and Schneider, 2006; Root et al., 2003; Parmesan, 

2006). In North America, climate warming is expected to result in shifts of species ranges 

poleward and upward along elevational gradients (Parmesan, 2006). A key obstacle to detecting 

change is the inadequacy of the current set of observations and inventories. 

Many animals, including most mammals, have evolved powerful mechanisms to regulate their 

physiology. Therefore, they will primarily experience climate change effects through pathways 

involving their food source, habitat, and predators, rather than through direct effects of climate 

change on body temperature (Schneider and Root, 1996). As warming occurs, species may 

migrate to higher elevations where more suitable temperatures exist. This is possible where 

habitat connectivity (the degree to which a habitat is physically linked with other suitable areas 

for a particular species) exists and other biotic and abiotic conditions are appropriate. However, 

species that require higher-elevation habitat, such as alpine ecosystems, often have nowhere to 

migrate. 

To date, relatively few studies have been conducted at a scale that encompasses an entire 

species’ range. These studies have evaluated distributional shifts of amphibians (Pounds et al., 

1999, 2006), pikas (Beever et al., 2003), birds (Dunn and Winkler, 1999), and butterflies 

(Parmesan, 1996, 2006). The majority of studies infer range shifts from observations made at a 

smaller scale, either from a small portion of the species range or from changes in species 

abundance within local communities (Parmesan, 2006). 

The IPCC (Field et al., 2007) found: 

• “Many North American species have shifted their ranges, typically to the north or to higher 

elevations (Parmesan and Yohe, 2003). 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 104
• Edith’s checkerspot butterfly has become locally extinct in the southern, low-elevation 

portion of its western North American range but has extended its range 90 km north and 120 

m higher in elevation (Parmesan, 1996; Croier, 2003; Parmesan and Galbraith, 2004). 

• Red foxes have expanded northward in northern Canada, leading to retreat of competitively 

subordinate arctic foxes (Hersteinsson and Macdonald, 1992).” 

Janetos et al. (2008) review substantial research examining warming-induced shifts in 

distribution among butterflies. As climate changes, the distribution of butterflies in North 

America is shifting northward, including a contraction at the southern end of their historical 

range, and to higher elevations. In a synthesis of prior research, Parmesan (2006) found that 30 

to 75% of studied butterfly species had expanded northward, less than 20% had contracted 

southward, while the remainder was stable. 

The review (Janetos et al., 2008) also identified similar results that have been identified in 

European species. Sixty-three percent of a sample of 35 non-migratory butterflies had shifted 

their ranges to the north by 35 to 240 km during the 20th century while 3% had shifted to the 

south (Parmesan et al., 1999). In central Spain, Wilson et al. (2007) documented that the richness 

and composition of butterfly species between 1967 to 1973 and 2004 to 2005 shifted uphill by 

293 m. Ultimately, these shifts resulted in a net decline in species richness over approximately 

90% of the study region (Wilson et al., 2007). Over a 19-year study period, Franco et al. (2006) 

found climate change was a driver of local extinction for three species of butterflies in Britain. In 

addition, these results indicate that range boundaries shifted 70 to 100 km northward for Aricia 

artaxerxes and Erebia aethiops and 130 to 150 m uphill for E. epiphron in a region with 

estimated latitudinal and elevational temperature shifts of 88 km northward and 98 m uphill in 

the same time period. Additional research on 51 British butterfly species found those with 

northern and/or montane distributions have disappeared from low elevations while colonizing 

higher-elevation sites, but did not find evidence for a systematic shift northward across all 

species (Hill et al., 2002). Model results project 65 and 24% declines in range sizes for northern 

and southern species, respectively, for the period 2070 to 2099. 

Effects on phenology 

Phenology is the study of the times of recurring natural phenomena, for example, the date of 

emergence of leaves and flowers, the first flight of butterflies, and the first appearance of 

migratory birds. 

Substantial evidence indicates natural systems are being demonstrably affected by climate 

change. Impacts include changes in the timing of the onset, completion, and length of the 

growing season; phenology; primary production, and as discussed above, species distributions 

and diversity (Parmesan and Yohe, 2003). Field-based analyses of multiple species indicate 

average shifts in the arrival of key spring indicators of 2.3 days per decade across all species 

(Parmesan and Yohe, 2003) with shifts as great as 5.1 days per decade (Root et al., 2003). 

Growing season length and primary production shifts

Research reviewed by Janetos et al. (2008) suggests a significant lengthening of the growing 

season and higher net primary productivity in the higher latitudes of North America where 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 105
temperature increases are relatively high. Global satellite data since 1981 indicate an earlier 

onset of spring across the temperate latitudes by 10 to 14 days (Zhou at al., 2001; Lucht et al., 

2002) as well as an increase in summer photosynthetic activity (Zhou et al., 2001). Similarly, an 

analysis of climate variables in the higher latitudes in Europe indicates a lengthening of the 

growing season of 1.1 to 4.9 days per decade since 1951 (Menzel et al., 2003). Numerous field 

studies have documented consistent earlier leaf expansion (Beaubien and Freeland, 2000; Wolfe 

et al., 2005) and earlier flowering (Schwartz and Reiter, 2000; Cayan et al., 2001) across 

different species and ecosystem types. Some of the specific examples contained in the IPCC 

(Field et al., 2007) are: 

• earlier flowering in lilac by 1.8 days per decade from 1959 to 1993 at 800 North American 

study sites (Schwart and Reiter, 2000); 

• earlier flowering in honeysuckle by 3.8 days per decade in the western United States (Cayan 

et al., 2001); 

• earlier leaf expansion in apple and grape by 2 days per decade in 72 northeastern United 

States sites (Wolfe et al., 2005); and 

• earlier leaf expansion by 2.6 days per decade since 1900 in trembling aspen growing in the 

region around Edmonton, Alberta, Canada (Beaubien and Freeland, 2000). 

Research reviewed in Janetos et al. (2008) found that between 1982 and 1998, net primary 

production increased nearly 10% in the continental United States (Boisvenue and Running, 

2006). The largest documented increases were in croplands and grasslands of the central United 

States, as a result of improved water balance (Lobell et al., 2002; Nemani et al., 2002; Hicke and 

Lobell, 2004). In contrast, forest productivity increased by less than 1% per decade and is 

generally limited by low temperature and short growing seasons in the higher latitudes and 

elevations (Caspersen et al., 2000; McKenzie et al., 2001; Joos et al., 2002; Boisvenue and 

Running, 2006). However, productivity has decreased in forested regions subject to drought from 

climate warming since 1895 (McKenzie et al., 2001) and subalpine regions (e.g., Monson et al., 

2005; Sacks et al., 2007). Recently, widespread mortality over 12,000 km2

 of lower-elevation 

forest in the southwest United States demonstrated the impacts of increased temperature and the 

associated multiyear drought (Breshears et al., 2005) even as productivity at tree line had 

increased previously (Swetnam and Betancourt, 1998). In addition, in summarizing research on 

North American forests, the IPCC indicated forest productivity “can be influenced indirectly by 

climate through effects on disturbance, especially from wildfire, storms, insects and diseases” 

(Field et al., 2007). The IPCC also noted the large increases in area burned by wildfires over the 

last three decades.

Shifts in phenology

Regarding the effects of climate change on wildlife in North America, Field et al. (2007) made 

the following conclusions: 

Warmer springs have led to earlier nesting for 28 migrating bird species on the east coast of the 

U.S. (Butler, 2003) and to earlier egg laying for Mexican jays (Brown et al., 1999) and tree 

swallows (Dunn and Winkler, 1999). In northern Canada, red squirrels are breeding 18 days 

earlier than 10 years ago (Reale et al., 2003). Several frog species now initiate breeding calls 10 to 

13 days earlier than a century ago (Gibbs and Breisch, 2001). In lowland California, 70% of 23 

butterfly species advanced the date of first spring flights by an average 24 days over 31 years 

(Forister and Shapiro, 2003). 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 106
Specific effects are reviewed in greater detail in CCSP SAP 4.3, The Effects of Climate Change 

on Agriculture, Land Resources, Water Resources, and Biodiversity (Janetos et al., 2008). Key 

findings are presented below. 

Migratory birds (adapted from Janetos et al., 2008) 

For migratory birds, the timing of arrival at breeding territories and overwintering grounds is an 

important determinant of reproductive success, survivorship, and fitness. Climate variability on 

interannual and longer time scales can influence migration behavior by altering the timing of 

arrival and/or departure. Disruptions in the phenology of migration to summer and wintering 

areas have been noted for long-distance, continental migrations as well regional, local, or 

elevational migrations (Dunn and Winkler, 1999; Winkler et al., 2002; Butler, 2003; Cotton, 

2003). Reviewing arrival dates of short- and long-distance migrants, Jonzen et al. (2006) found 

that, while both exhibited changes in migration timing, long-distance migrants have advanced 

their spring arrival more than have short-distance migrants, based on data from 1980 to 2004. In 

a review of multiple data sets from the United Kingdom, Germany, Switzerland, and Denmark, 

Thorup et al. (2007) found advances in spring arrival across all insectivorous songbirds included 

in the study (with similar trends among studies and locations), while the timing of autumn 

departure was variable. 

While the advancement of spring migration phenology is one specific effect of climate change 

(Root et al., 2003), ultimately, the indirect effects may be more important in determining the 

long-term impacts on species persistence and diversity. Of particular importance is the potential 

for mismatches in the timing of migration, breeding, and peak food availability. There is no a 

priori reason to expect migrants and their respective food sources to shift their phenologies at the 

same rate. A differential shift can lead to mistimed reproduction and reduced population success 

(Stenseth and Mysterud, 2002; Visser et al., 2004, 2006; Visser and Both, 2005). 

Reviewing the relevant literature, Janetos et al. (2008) concluded the responses are likely to be 

highly complex depending on species-specific traits, characteristics of local microhabitats, and 

aspects of local microclimates. These points are illustrated by a long-term study of the migratory 

pied flycatcher (Ficedula hypoleuca), which exhibited an advance in laying date, hatching date, 

and clutch size in response to the advancing peak abundance of their food supply. However, the 

timing of food availability and subsequent impacts were not uniform across their range. 

Populations of the flycatcher have declined by about 90% over the past two decades in areas 

where the food for provisioning nestlings peaks early in the season but not in areas with a late 

food peak (Both, 2006). 

Butterflies (adapted from Janetos et al., 2008) 

The migration of butterflies in the spring is highly correlated with spring temperatures. Research 

has documented earlier arrivals in 16 of 23 species in central California (Forister and Shapiro, 

2003), 26 of 35 species in the United Kingdom (Roy and Sparks, 2000), and all studied species 

(17) in Spain (Stefanescu et al., 2004). 

Changes in timing of migrations and distributions are expected to present resource mismatches 

that will influence population success and the probability of extinction. To date, a few studies 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 107
have linked population extinctions directly to climate change (McLaughlin et al., 2002, Franco et 

al., 2006). Substantial research found that phenological asynchrony in butterfly–host interactions 

in California led to population extinctions of the checkerspot butterfly (E. editha) during extreme 

drought and low-snowpack years (Singer and Ehrlich, 1979; Ehrlich et al., 1980; Singer and 

Harter, 1996; Thomas et al., 1996). A modeling experiment for two populations of checkerspot 

butterfly suggested the decline was hastened by increasing variability in precipitation associated 

with climate change in a region that allowed no distributional shifts because of persistent habitat 

fragmentation (McLaughlin et al., 2002). These results are supported by additional model 

projections that suggest butterfly species could adapt to changing conditions by shifting their 

ranges in response to warming. However, limited habitat availability and connectivity may limit 

migration ability resulting in significant population declines (Hill et al., 2002). 

Amphibians (adapted from Janetos et al., 2008) 

There is evidence that amphibian breeding is occurring earlier in some regions, driven by 

increasing temperatures (Blaustein et al., 2001; Gibbs and Breisch, 2001; Beebee, 2002). 

However, responses are complex. Statistical tests (Blaustein et al., 2002) indicated that half of 

the 20 species examined by Beebee (1995), Reading (1998), Gibbs and Breisch (2001), and 

Blaustein et al. (2001) are breeding earlier. Even in the half not exhibiting statistically significant 

earlier breeding, they showed biologically important trends toward breeding earlier that, if 

continued, are expected to become statistically significant (Blaustein et al., 2002). These findings 

suggest the influence of climate on amphibian breeding patterns in many species. However, 

while some temperate-zone frog and toad populations show a trend toward breeding earlier, 

others do not. For example, Fowler’s toad (Bufo fowleri) a late breeder, has bred progressively 

later in spring over the past 15 years on the north shore of Lake Erie (Blaustein et al., 2001).

Projected impacts 

Specific conclusions from the IPCC (Fischlin et al., 2007) regarding global-scale changes 

included the following: 

• The resilience of many ecosystems is likely to be exceeded this century by an unprecedented 

combination of climate change, associated disturbances (e.g., flooding, drought, wildfire, 

insects, and ocean acidification), and other global change drivers (e.g., land use change, 

pollution, and over-exploitation of resources) (high confidence). 

• Over the course of this century, net carbon uptake by terrestrial ecosystems is likely to peak 

before mid-century and then weaken or even reverse, thus amplifying climate change (high 

confidence). 

• Approximately 20 to 30% of plant and animal species assessed so far are likely to be at 

increased risk of extinction if increases in global average temperature exceed 1.5 to 2.5 °C 

(medium confidence, about a 5 out of 10 chance38). 

• For increases in global average temperature exceeding 1.5 to 2.5 °C and accompanying 

atmospheric carbon dioxide (CO2) concentrations, major changes are projected in ecosystem 

structure and function, species’ ecological interactions, and species’ geographical ranges—

with predominantly negative consequences for biodiversity, and ecosystem goods and 

services, such as water and food supply (high confidence). 

 

38 Definitions for terms used in statements of confidence and likelihood can be found in Section II.3: 

Characterization of Uncertainty. 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 108
• The progressive acidification of oceans due to increasing atmospheric CO2 is expected to 

have negative impacts on marine shell-forming organisms (e.g., corals) and their dependent 

species (medium confidence). 

Specific to North American ecosystems, Field et al. (2007) drew the following conclusions: 

• Net primary productivity is projected to increase at high latitudes with mixed results at midlatitudes driven by whether precipitation increases are great enough to offset increased 

evapotranspiration demands (Bachelet et al., 2001; Berthelot et al., 2002; Gerber et al., 2004; 

Woodward and Lomas, 2004b).

• The areal extent of ecosystems limited by drought is projected to increase by 11% per degree 

Celsius of warming in the continental United States (Bachelet et al., 2001).

• Ecosystems in the northeastern and southeastern United States are likely to become carbon 

sources, while those in the western United States are likely to remain carbon sinks (Bachelet 

et al., 2004).

• Several simulations (Cox et al., 2000; Berthelot et al., 2002; Fung et al., 2005) indicate that, 

over the 21st century, warming will lengthen growing seasons. Despite some decreased sink 

strength, resulting from greater water limitations in western forests and higher respiration in 

the tropics, this projected lengthening of growing seasons would sustain forest carbon sinks 

in North America (medium confidence). 

• Overall forest growth in North America will likely increase modestly (10 to 20%) as a result 

of extended growing seasons and elevated CO2 over the next century (M. Morgan et al., 

2001), but with important spatial and temporal variations (medium confidence). 

• A 2 °C temperature increase in the Olympic Mountains (United States) would cause 

dominant tree species to shift upward in elevation by 300 to 600 m, causing temperate 

species to replace subalpine species over 300 to 500 years (Zolbrod and Peterson, 1999). As 

for widespread species, temperature increases would have variable effects throughout their 

range. For example, for lodgepole pine an increase of 3 °C would increase growth in the 

northern part of its range, decrease growth in the middle, and result in substantial mortality in 

its southern extent (Rehfeldt et al., 2001). 

• Bioclimatic modeling based on output from five general circulation models suggests that, 

over the next century, vertebrate and tree species richness will decrease in most parts of the 

conterminous United States, even though long-term trends (over millennia) ultimately favor 

increased richness in some taxa and locations (Currie, 2001). 

Population and community dynamics 

Ecosystems and species are influenced by the interaction of multiple factors including climate 

variables, land use and management, disturbance, and invasive species. Increases in plant 

productivity resulting from the effects of rising concentrations of atmospheric CO2 may partially 

offset these adverse effects. In California, temperature increases greater than 2 °C may lead to 

the conversion of shrubland into desert and grassland ecosystems and evergreen conifer forests 

into mixed deciduous forests (Fischlin et al., 2007). In SAP 4.3, Janetos et al. (2008) also 

conclude that projected increases in CO2 are expected to stimulate the growth of most plant 

species. Some invasive plants are expected to respond with greater growth rates than noninvasive plants. While some invasive plants may have higher growth rates and greater maximal 

photosynthetic rates relative to native plants under increased CO2, definitive evidence of a 

general benefit of CO2 enrichment to invasive plants over natives has not emerged. 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 109
The sea ice biome accounts for a large proportion of primary production in polar waters and 

supports a substantial food web. In the Northern Hemisphere, projections of ocean biological 

response to climate warming by 2050 show contraction of the highly productive marginal sea ice 

biome by 42% (Fischlin et al., 2007). Recent decreases in Arctic sea ice minima have been more 

rapid than in the past few decades.39 In the Bering Sea, primary productivity in surface waters is 

projected to increase, the ranges of some cold-water species will shift north, and ice-dwelling 

species will experience habitat loss (ACIA, 2004). 

Species-level projections 

After reviewing studies on the projected impacts of climate change on species, the IPCC made 

the following global-scale conclusions (Fischlin et al., 2007 and references therein): 

• Projected impacts on biodiversity are significant and of key relevance, since global losses in 

biodiversity are irreversible (very high confidence). 

• Richness of endemic species (those that are unique to their location or region and are not 

found anywhere else on Earth) is highest where regional paleoclimatic changes have been 

subtle, providing circumstantial evidence of their vulnerability to projected climate change 

(medium confidence). With global average temperature changes of 2 °C above pre-industrial 

levels, many terrestrial, freshwater, and marine species (particularly endemics across the 

globe) are at a far greater risk of extinction than in the geological past (medium confidence). 

• Approximately 20 to 30% of species (with a global uncertainty range from 10 to 40%, but 

varying among regional biota from as low as 1% to as high as 80%) will be at increasingly 

high risk of extinction by 2100. 

• Based on relationships between habitat area and biodiversity, on average 15 to 37% (ranging 

from 9% under the most optimistic assumptions to 52% under the most pessimistic) of plant 

and animal species in a global sample may be “committed to extinction” by 2050, although 

actual extinctions will be strongly influenced by human forces and could take centuries 

(Thomas et al., 2004)

In North America, climate change impacts on inland aquatic ecosystems will range from the 

effects of increased temperature and CO2 concentration on vegetation growth to the effects 

associated with alterations in hydrologic systems resulting from changes in precipitation regimes 

and melting glaciers and snowpack (Fischlin et al., 2007). For many freshwater animals, such as 

amphibians, migration to breeding ponds and the production of eggs is intimately tied to 

temperature and moisture availability. Asynchronous timing of breeding cycles and pond drying 

due to the lack of precipitation can lead to reproductive failure. Differential responses among 

species in arrival or persistence in ponds are expected to lead to changes in community 

composition and nutrient flow in ponds (Fischlin et al., 2007). 

Bioclimatic modeling based on output from five general circulation models suggests that, over 

the next century, vertebrate and tree species richness will decrease in most parts of the 

conterminous United States, even though long-term trends (millennial scale) ultimately favor 

increased richness in some taxa and locations (Field et al., 2007). Changes in the composition of 

 

39 See <www.nsidc.org>. 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 110
plant species in response to climate change can increase ecosystem vulnerability to other 

disturbances, including fire and biological invasion. 

On small oceanic islands with cloud forests or high-elevation ecosystems, such as the Hawaiian 

Islands, extreme elevation gradients exist, ranging from nearly tropical to alpine environments. 

In these ecosystems, anthropogenic climate change, land use changes, and biological invasions 

will work synergistically to drive several species (e.g., endemic birds) to extinction (Mimura et 

al., 2007). 

According to the IPCC, climate change (very high confidence) and ocean acidification due to the 

effects of elevated CO2 concentrations (medium confidence) will impair a wide range of 

planktonic and other marine organisms that use aragonite to make their shells or skeletons 

(Fischlin et al., 2007). These impacts could result in potentially severe ecological changes in 

tropical and coldwater marine ecosystems where carbonate-based phytoplankton and corals are 

the foundation for the trophic system (Schneider et al., 2007). Moreover, Nicholls et al. (2007) 

wrote that corals have low adaptive capacity to thermal stress. Sea surface temperature increases 

of 1 to 3 °C are projected to result in more frequent bleaching events and widespread mortality, if 

there is not thermal adaptation or acclimatization by corals and their algal symbionts. However, 

the ability of coral reef ecosystems to withstand the impacts of climate change will depend to a 

large degree on the extent of degradation from other anthropogenic pressures and the frequency 

of future bleaching events (Nicholls et al., 2007). 

For the Arctic, the IPCC (Anisimov et al., 2007 and references therein) made the following 

conclusions: 

• Decreases in the abundance of keystone species40 are expected to be the primary factor in 

causing ecological cascades41 and other changes in ecological dynamics. 

• Arctic animals are likely to be most vulnerable to warming-induced drying of small water 

bodies; changes in snow cover and freeze–thaw cycles that affect access to food (e.g., polar 

bear dependence on sea ice for seal hunting) and protection from predators (e.g., snow hare 

camouflage in snow); changes that affect the timing of behavior (e.g., migration and 

reproduction); and influx of new competitors, predators, parasites, and diseases. 

• In the past, subarctic species have been unable to live at higher latitudes because of harsh 

conditions. Warming induced by climate change will increase the rate at which subarctic 

species are able to establish. Some non-native species, such as the North American mink, 

will become invasive, while other species that have already colonized some Arctic areas are 

likely to expand into other regions. The spread of non-native, invasive plants will likely have 

adverse impacts on native plant species. For example, experimental warming and nutrient 

addition has showed that native mosses and lichens become less abundant when non-native 

plant biomass increases. 

• Bird migration routes and timing are likely to change as the availability of suitable habitat in 

the Arctic decreases. 

 

40 A keystone species is defined as a species that has a disproportionate effect on its environment relative to its 

abundance or total biomass. Typically, ecosystems experience dramatic changes with the removal of such a species. 41 Ecological cascades are defined as sequential chains of ecological effects, including starvation and death, 

beginning at the bottom levels of the food chain and ascending to higher levels, including apex predators. 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 111
• Loss of sea ice will affect species, such as harp seals, that depend on it for survival. 

• Climate warming is likely to increase the incidence of pests, parasites, and diseases such as 

musk ox lungworm and abomasal nematodes of reindeer. 

V.1.b Ecosystem services 

The IPCC (Fischlin et al., 2007) concluded that ecosystems provide many goods and services 

that are of vital importance for the functioning of the biosphere, and provide the basis for the 

delivery of tangible benefits to human society. The Millennium Ecosystem Assessment (MEA, 

2005b) defines these to include supporting, provisioning, regulating, and cultural services. The 

IPCC (Fischlin et al., 2007) described these as follows: 

i. Supporting services, such as primary and secondary production, and biodiversity, a resource that 

is increasingly recognized to sustain many of the goods and services that humans enjoy from 

ecosystems. These provide a basis for three higher-level categories of services. 

ii. Provisioning services, such as products (cf. Gitay et al., 2001), i.e., food (including game, roots, 

seeds, nuts and other fruit, spices, fodder), fibre (including wood, textiles) and medicinal and 

cosmetic products (including aromatic plants, pigments). 

iii. Regulating services, which are of paramount importance for human society such as (a) carbon 

sequestration, (b) climate and water regulation, (c) protection from natural hazards such as floods, 

avalanches or rock-fall, (d) water and air purification, and (e) disease and pest regulation. 

iv. Cultural services, which satisfy human appreciation of ecosystems and their components. 

Projected impacts 

Increasing temperatures and shifting precipitation patterns, along with the effects of elevated 

CO2 concentrations, sea level rise, and changes in climatic variability, will affect the quantity 

and quality of these services. By the end of the 21st century, climate change and its impacts may 

be the dominant driver of biodiversity loss and changes in ecosystem services globally (MEA, 

2005a). 

As noted in SAP 4.3, biological diversity provides a fundamental underpinning for these services 

(Janetos et al., 2008). While not specific to the United States, a major finding of the MEA from a 

global perspective was that 16 of 24 analyzed ecosystem services were being used in 

unsustainable ways. The MEA evaluated the relative magnitudes and importance of a number of 

different drivers of changes in ecosystems, and whether the importance of those drivers was 

likely to increase, decrease, or remain the same over the next several decades. The conclusion 

was that although climate change was not currently the most important driver of change in many 

ecosystems, it was one of the only drivers whose importance was expected to continue to 

increase in all ecosystems over the next several decades. 

As an example of an ecosystem service experiencing current changes, in SAP 4.3 Janetos et al. 

(2008) note the increasing recognition of, and recent declines in abundance, observed for some 

pollinators, particularly the introduced honey bee (Apis mellifera) (NRC, 2006b). The economic 

significance of pollination is underscored by the fact that about three-quarters of the world’s 

flowering plants depend on pollinators, and that almost one-third of the food that we consume 

results from their activity. The majority of pollinators are insects, whose distributions, 

phenology, and resources are all being affected by climate change (Inouye, 2007). Unfortunately, 

with the exception of honey bees and butterflies, few data are available on the abundance and 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 112
distribution of pollinators, so it has been difficult to assess their status and the changes that they 

may be undergoing (NRC, 2006b). 

V.1.c Extreme events 

Many effects of climate change on U.S. ecosystems and wildlife may emerge most strongly 

through changes in the intensity and the frequency of extreme events (Fischlin et al., 2007). In 

their review of the literature, Fischlin et al. (2007) identify multiple impacts of extreme events, 

such as hurricanes, tropical storms, and wildfires. These events can have substantial immediate 

effects, such as mass mortality in vegetation and wildlife populations, while also contributing 

significantly to alterations in species distribution, abundance, and fitness following the 

disturbance (Parmesan et al., 2000 in Fischlin et al., 2007). For example, the aftermath of a 

hurricane can cause coastal forest to die from storm surge-induced salt deposition, or wildlife 

may find it difficult to find food, thus lowering the chance of survival (Wiley and Wunderle, 

1994 in Fischlin et al., 2007). Droughts play an important role in forest dynamics as well, 

causing pulses of tree mortality (Breshears et al., 2005 in Fischlin et al., 2007). Heat waves can 

contribute to both short-term (e.g., vegetation desiccation, mortality, reduced primary 

production, and increased wildfire extent) and long-term effects (e.g., depletion of oxygen in 

aquatic systems and conversion of vegetation type), as evidenced by the 2003 European heat 

wave (e.g., Beniston, 2004; Schär et al., 2004; Gobron et al., 2005; Ciais et al., 2005 in Fischlin 

et al., 2007). Greater magnitude and frequency of extreme events will alter disturbance regimes 

in coastal ecosystems, leading to changes in diversity and ecosystem function (e.g., Bertness and 

Ewanchuk, 2002 in Fischlin et al., 2007). Species inhabiting salt marshes, mangroves, and coral 

reefs are likely to be particularly vulnerable to these effects (Fischlin et al., 2007). To date, more 

than half of the original salt marsh habitat in the United States has been lost (Kennish, 2001). 

Research reviewed by the IPCC (Field et al., 2007) indicated: 

• Extreme events can add to other stresses on ecological integrity (Scavia et al., 2002; Burkett 

et al., 2005), including shoreline development and nitrogen eutrophication (Bertness et al., 

2002).

• Many coastal ecosystems in North America are potentially exposed to storm-surge flooding 

(Titus and Richman, 2001; Titus, 2005); one area in which these extremes are evident is the 

San Francisco area, where 140 years of tide gauge data suggest an increase in severe winter 

storms since 1950 (Bromirski et al., 2003). 

• Impacts on coastal communities and ecosystems can be more severe when major storms 

occur in short succession, limiting the opportunity to rebuild natural resilience (Forbes et al., 

2004). 

• Recent winters with less ice in the Great Lakes and Gulf of St. Lawrence have increased 

coastal exposure to damage from winter storms (Forbes et al., 2002). 

Other potential extreme events influenced by a warming climate include wildfires, floods, 

drought, insects, and pathogens. Research summarized by the IPCC (Field et al., 2007) indicated: 

• Since 1980, an average of 22,000 km2

 per year has burned in U.S. wildfires, almost twice the 

1920 to 1980 average of 13,000 km2

 per year (Schoennagel et al., 2004). 

• The forested area burned in the western United States from 1987 to 2003 is 6.7 times the area 

burned from 1970 to 1986 (Westerling et al., 2006). 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 113
• In Canada, burned area has exceeded 60,000 km2

 per year three times since 1990, twice the 

long-term average (Stocks et al., 2002). 

• Wildfire-burned area in the North American boreal region increased from 6,500 km2 per year 

in the 1960s to 29,700 km2

 per year in the 1990s (Kasischke and Turetsky, 2006). 

• Human vulnerability to wildfires has also increased, with a rising population in the wildland–

urban interface. 

• A warming climate encourages wildfires through a longer summer period that dries fuels, 

promoting easier ignition and faster spread (Running, 2006). Westerling et al. (2006) found 

that in the last three decades, the wildfire season in the western United States has increased 

by 78 days, and burn durations of fires greater than 1,000 hectares in area have increased 

from 7.5 to 37.1 days, in response to a spring/summer warming of 0.87 °C. 

The IPCC (Field et al., 2007) noted that insects and diseases are a natural part of ecosystems; 

however, changing conditions are increasing the severity of their effects. Forests often 

experience insect epidemics that kill trees over large regions. The lifecycles of these insects are 

climate-sensitive (Williams and Liebhold, 2002). As found by the IPCC (Field et al., 2007), 

warming temperatures are contributing to changes in insect lifecycles and may be contributing to 

more widespread epidemics. Specifically, many northern insects have a two-year lifecycle and 

warmer winter temperatures have allowed a greater proportion of overwintering larvae to 

survive. For example: 

• “Recently, spruce budworm in Alaska has completed its lifecycle in one year, rather than the 

previous two (Volney and Fleming, 2000). 

• Mountain pine beetle has expanded its range in British Columbia into areas previously too 

cold (Carroll et al., 2004). 

• Insect outbreaks often have complex causes. Susceptibility of the trees to insects is increased 

when multiyear droughts degrade the trees’ ability to generate defensive chemicals (Logan et 

al., 2003). Recent dieback of aspen stands in Alberta was caused by light snowpacks and 

drought in the 1980s, triggering defoliation by tent caterpillars, followed by woodboring 

insects and fungal pathogens (Hogg et al., 2002).” 

Other key findings from SAP 4.3 (Janetos et al., 2008) included the following: 

• “Evidence is beginning to accumulate that links the spread of pathogens to a warming 

climate. For example, the chytrid fungus (Batrachochytrium dendrobatidis) is a pathogen that 

is rapidly spreading world-wide and decimating amphibian populations. A recent study by 

Pounds and colleagues (2006) showed that widespread amphibian extinction in the 

mountains of Costa Rica is positively linked to global climate change.” 

• To date, geographic range expansion of pathogens related to warming temperatures have 

been the most easily detected (Harvell et al., 2002), perhaps most readily for arthropod-borne 

infectious disease (Daszak et al., 2000). However, a recent literature review found additional 

evidence gathered through field and laboratory studies that support hypotheses that 

latitudinal shifts of vectors and diseases are occurring under warming temperatures. 

• Nonetheless, invasive plants in general may better tolerate a wider range of environmental 

conditions and may be more successful in a warming world because they can migrate and 

establish in new sites more rapidly than native plants and they are not usually limited by 

pollinators or seed dispersers (Vila et al., 2007). 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 114
• Finally, it is critical to recognize that other elements of climate change (e.g., nitrogen 

deposition and land conversion) will play a significant role in the success of invasive plants 

in the future, either alone or under elevated CO2 (Vila et. al., 2007). 

Projected impacts 

The IPCC highlights the following projections that are expected to have significant implications 

for ecosystems: 

• Some studies project widespread increases in extreme precipitation (Christensen et al., 2007), 

with greater risks of not only flooding from intense precipitation, but also droughts from 

greater temporal variability in precipitation. 

• In general, projected changes in precipitation extremes are larger than changes in mean 

precipitation (Meehl et al., 2007). 

• Future trends in hurricane frequency and intensity remain very uncertain. Climate models 

with sufficient resolution to depict some aspects of individual hurricanes tend to project some 

increases in both peak wind speeds and precipitation intensities (Meehl et al., 2007). The 

pattern is clearer for extra-tropical storms, which are likely to become more intense, but 

perhaps less frequent, leading to increased extreme wave heights in the mid-latitudes (Meehl 

et al., 2007). 

Impacts on ecosystem structure and function may be amplified by changes in extreme 

meteorological events and increased disturbance frequencies (Field et al., 2007). Ecosystem 

disturbances, caused either by humans or by natural events, accelerate both loss of native 

species and invasion of exotics (Sala et al., 2000). Changes in plant species composition in 

response to climate change can facilitate other disturbances, including fire (Smith et al., 

2000) and biological invasion (Zavaleta and Hulvey, 2004). In North America, disturbances 

like wildfire and insect outbreaks are increasing and are likely to intensify in a warmer future 

with drier soils and longer growing seasons (very high confidence) (Field et al., 2007). 

V.1.d Sea level rise 

The IPCC (Rosenzweig et al., 2007) concluded that many “coastal regions are already 

experiencing the effects of relative (local) sea-level rise, from a combination of climate-induced 

sea-level rise, geological and anthropogenic-induced land subsidence, and other local factors.” 

Overall, sea levels have been rising at a rate of 1.7 to 1.8 mm per year over the last century, with 

an increased rate of 3 mm per year over the last decade (e.g., Church et al., 2004; Bindoff et al., 

2007 in Rosenzweig et al., 2007). They also conclude that sea level rise may be contributing to 

coastal erosion across the eastern United States (e.g., Zhang et al., 2004 in Rosenzweig et al., 

2007). In addition, regional sea level rise has contributed to increased storm surge impacts along 

the North American eastern coast, although there has not been an increase in storm events 

(Zhang et al., 2000 in Rosenzweig et al., 2007). Due to low elevation and relative sea level rise, 

the U.S. Gulf Coast is particularly vulnerable to storm surges (Rosenzweig et al., 2007). These 

findings illustrate the interactive effects of climate impacts through rising sea levels and extreme 

storm events. Drawing on results like this, the IPCC concluded with high confidence that “coasts 

are experiencing the adverse consequences of hazards related to climate and sea level” (Parry et 

al., 2007). 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 115
For small islands, the coastline is long relative to island area. As a result, many resources and 

ecosystem services are threatened by a combination of human pressures and climate change 

effects including sea level rise, increases in sea surface temperature, and possible increases in 

extreme weather events (Mimura et al., 2007). 

Climate change and sea level rise affect sediment transport in complex ways, as described by the 

IPCC (Nicholls et al., 2007). Erosion and ecosystem loss is affecting many parts of the U.S. 

coastline, but it remains unclear to what extent these losses result from climate change as 

opposed to land loss associated with relative sea level rise due to subsidence and other human 

drivers (Nicholls et al., 2007). 

Coastal wetland loss is also occurring where these ecosystems are squeezed between natural and 

artificial landward boundaries and rising sea levels, a process known as ‘coastal squeeze’ (Field 

et al., 2007). The degradation of coastal ecosystems, especially wetlands and coral reefs, can 

have serious implications for the well-being of societies dependent on them for goods and 

services (Nicholls et al., 2007). 

Engineering structures, such as bulkheads, dams, levees, and water diversions, limit sediment 

supply to coastal areas. Wetlands are especially threatened by sea level rise when insufficient 

amounts of sediment from upland watersheds are deposited on them. If sea level rises slowly, the 

balance between sediment supply and morphological adjustment can be maintained if a salt 

marsh vertically accretes (builds up through accumulated sediments and other materials), or a 

lagoon infills, at the same rate. However, an acceleration in the rate of sea level rise may mean 

that coastal marshes and wetlands cannot keep up, particularly where the supply of sediment is 

limited (e.g., where coastal floodplains are inundated after natural levees or artificial 

embankments are overtopped) (Nicholls et al., 2007). 

Although open coasts have been the focus of research on erosion and shore stabilization 

technology, sheltered coastal areas in the United States are also vulnerable and suffer secondary 

effects from rising seas (NRC, 2006a). For example, barrier island erosion in Louisiana has 

increased the height of waves reaching the shorelines of coastal bays. This has enhanced erosion 

rates of beaches, tidal creeks, and adjacent wetlands. The impacts of accelerated sea level rise on 

gravel beaches have received less attention than sandy beaches. However, these systems are 

threatened by sea level rise, even under high wetland accretion rates. The persistence of gravel 

and cobble-boulder beaches will also be influenced by storms, tectonic events, and other factors 

that build and reshape these highly dynamic shorelines (Nicholls et al., 2007). 

According to the IPCC, most of the world’s sandy shorelines retreated during the past century 

and climate change-induced sea level rise is one underlying cause (Nicholls et al., 2007). To date 

in the United States, more than 50% of the original salt marsh habitat has already been lost 

(Kennish, 2001 in Field et al., 2007). In Mississippi and Texas, over half of the shorelines have 

eroded at average rates of 2.6 to 3.1 m per year since the 1970s, while 90% of the Louisiana 

shoreline has eroded at a rate of 12.0 m per year (Nicholls et al., 2007 and references therein). 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 116
In the Arctic, coastal stability is affected by factors common to all areas (i.e., shoreline exposure, 

relative sea level change, climate, and local geology), and by factors specific to the high latitudes 

(i.e., low temperatures, ground ice, and sea ice) (Anisimov et al., 2007). Adverse impacts have 

already been observed along Alaskan coasts and traditional knowledge points to widespread 

coastal change in Alaska. Rising temperatures in Alaska are reducing the thickness and spatial 

extent of sea ice. This creates more open water and allows for winds to generate stronger waves, 

which increase shoreline erosion. Sea level rise and thawing of coastal permafrost exacerbate this 

problem. Higher waves will create even greater potential for this kind of erosion damage (ACIA, 

2004). 

Projected impacts 

The U.S. coastline is long and diverse with a wide range of coastal characteristics (Shaw et al., 

1998; Dyke and Peltier, 2000; Zervas, 2001 in Field et al., 2007). Yet, relative sea level is rising 

in many areas (O’Reilly et al., 2005 in Field et al., 2007). Sea level rise changes the shape and 

location of coastlines by moving them landward along low-lying contours and exposing new 

areas to erosion (NRC, 2006a). Coasts subsiding due to natural or human-induced causes will 

experience larger relative rises in sea level. In some locations, such as deltas and coastal cities 

(e.g., the Mississippi Delta and surrounding cities), this effect can be significant (Nicholls et al., 

2007). Rapid development, including an additional 25 million people in the coastal United States 

over the next 25 years, will further reduce the resilience of coastal areas to rising sea levels 

(Field et al., 2007). Superimposed on the impacts of erosion and subsidence, the effects of rising 

sea level will exacerbate the loss of waterfront property and increase vulnerability to inundation 

hazards (Nicholls et al., 2007). 

Up to 21% of the remaining coastal wetlands in the U.S. mid-Atlantic region are potentially at 

risk of inundation between 2000 and 2100 (Field et al., 2007 and references therein). The IPCC 

(Field et al., 2007) concluded with high confidence that the rates of coastal wetland loss, in the 

Chesapeake Bay and elsewhere, will increase with accelerated sea level rise, in part due to 

‘coastal squeeze.’ Salt marsh biodiversity is likely to decrease in northeastern marshes through 

expansion of non-native species such as Spartina alterniflora. The IPCC (Field et al., 2007) 

projects that many U.S. salt marshes in less-developed areas can potentially keep pace with sea 

level rise through vertical accretion. 

Other conclusions drawn by the IPCC (Parry et al., 2007) included: 

• Coasts are very likely to be exposed to increasing risks in future decades due to many 

compounding climate change factors (e.g., sea level rise, increase in sea surface 

temperatures, increased tropical storm intensity, and ocean acidification) (very high 

confidence). 

• The impact of climate change on coasts is exacerbated by increasing human-induced 

pressures (very high confidence). 

• Adaptation costs for vulnerable coasts are much less than the costs of inaction (high 

confidence). 

• The unavoidability of sea level rise, even in the longer term, frequently conflicts with 

present-day human development patterns and trends (high confidence). 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 117
Specifically regarding North America coasts, the IPCC (Field et al., 2007) found: 

• Coastal communities and habitats will be increasingly stressed by climate change impacts 

interacting with development and pollution (very high confidence). Sea level is rising along 

much of the coast, and the rate of change will increase, exacerbating the impacts of 

progressive inundation, storm surge flooding, and shoreline erosion. 

• Storm impacts are likely to be more severe, especially along the Gulf and Atlantic Coasts. 

Salt marshes, other coastal habitats, and dependent species are threatened by sea level rise, 

fixed structures blocking landward migration, and changes in vegetation. Population growth 

and rising value of infrastructure in coastal areas increases vulnerability to climate variability 

and future climate change.

V.1.e Vulnerable ecosystems

Coastal and marine ecosystems 

Effects of elevated CO2 concentrations and climate change on marine ecosystems include ocean 

warming, increased thermal stratification, reduced upwelling, sea level rise, increased wave 

height and frequency, loss of sea ice, increased risk of diseases in marine biota, and decreases in 

the pH and carbonate ion concentration of the surface oceans. Warmer ocean temperatures will 

contribute to changes in upwelling dynamics and decreased primary production along the 

California Current. In 2005, the upwelling season was delayed by three months (it began in 

August rather than a normal start in April to May), leading to a significant reduction in plankton 

production. The recruitment success among fish, birds, and mammals dependent upon the 

plankton was reduced significantly. 

The United States has extensive coral reef ecosystems in both the Caribbean Sea and the Pacific 

Ocean. Coral reefs are very diverse ecosystems, home to a complex of species that support both 

local and global biodiversity and human societies. The sensitivity of coral reefs is increasingly 

recognized. With lower pH, aragonite (calcium carbonate) that is used by many organisms to 

make their shells or skeletons will decline or become under-saturated, affecting coral reefs and 

other marine calcifiers (e.g., pteropods—marine snails). Reef systems are increasingly vulnerable 

due to increasing thermal stress, interacting with the effects of various other stressors including 

increasing storm intensity, overfishing, pollution, and the introduction of invasive species 

leading to replacement of corals by other organisms in some locations (Fischlin et al., 2007; 

Nicholls et al., 2007). 

In SAP 4.3, Janetos et al. (2008) indicate that it has been estimated that coral reefs provide $30 

billion in annual ecosystem service value (Cesar et al., 2003). The effects of climate change in 

marine systems is highlighted by the 2006 listing of two species of corals in the Caribbean as 

Threatened under the Endangered Species Act (Federal Register, 2006). The major threats that 

motivated the listings of elkhorn (Acropora palmata) and staghorn (A. cervicornis) corals were 

disease, elevated sea surface temperatures, and hurricanes—all of which relate to climate change 

and its effects (Mann and Emanuel, 2006; Muller et al., 2007). 

The northern Bering Sea along the Alaskan coast supports some of the highest benthic faunal 

biomass densities in the world’s oceans (Janetos et al., 2008). Rising air and seawater 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 118
temperatures have caused reductions in sea ice cover and primary productivity in benthic (the 

deepest environment of a water body, usually the seabed or lake floor) ecosystems. These rising 

temperatures are leading to a northward shift in the boundary between Arctic (northern Bering 

Sea) and subarctic (southern Bering Sea) waters resulting in a pelagic-dominated marine 

ecosystem that was previously confined to the southeastern Bering Sea (Anisimov et al., 2007). 

Changing conditions have led to significant reductions in seabird and marine mammal 

populations, increases in pelagic fish, occurrences of previously rare algal blooms, abnormally 

high water temperatures, and smaller salmon runs in coastal rivers (ACIA, 2004; Grebmeier et 

al., 2006 in Anisimov et al., 2007). Plants and animals in polar regions are also vulnerable to 

attacks from pests and parasites that develop faster and are more prolific in warmer and moister 

conditions (Anisimov et al., 2007). 

Arctic sea ice ecosystem 

(adapted from Janetos et al., 2008) 

Changes in the Arctic are resulting in substantial shifts in habitat, especially for sea icedependent species. The sea ice, which provides habitat both below and above the ocean, has been 

in retreat for at least 30 years (Rothrock et al., 2003; Stroeve et al., 2005). Models project an 

Arctic Ocean free of summer sea ice by the end of this century (Overpeck et al., 2005), with 

increasing evidence suggesting this could happen by 2050 (Holland et al., 2006) and some 

models suggesting that it could happen as soon as 2040 (Holland et al., 2006). Ice loss to date is 

already causing measurable changes in polar bear and ringed seal populations and fitness 

(Stirling et al., 1999; Derocher et al., 2004; Ferguson et al., 2005). 

Sea ice seasonally covered as much as 15 million square kilometers of the Arctic Ocean before it 

began declining in the 1970s. For millennia, that ice has been integral to an ecosystem that 

provides for polar bears and the indigenous people. The ice also strongly influences the climate, 

oceanography, and biology of the Arctic Ocean and surrounding lands. Further, sea ice 

influences global climate in several ways, including via its high albedo and its role in 

atmospheric and oceanic circulation. Organisms that depend on sea ice ranging from ice algae to 

seals and polar bears will diminish in number or become extinct. Many changes have already 

been observed and are predicted to accelerate along with the rates of climate change. 

At the base of the sea ice ecosystem are epontic algae adapted to very low light levels. Blooms of 

those algae on the undersurface of the ice are the basis of a food web leading through 

zooplankton and fish to seals, whales, polar bears, and people. Sea ice also strongly influences 

winds and water temperature, both of which influence upwelling and other oceanographic 

phenomena whereby nutrient-rich water is brought up to depths at which there is sufficient 

sunlight for phytoplankton to make use of those nutrients. 

Among the more southerly and seasonally ice-covered seas, the Bering Sea produces the 

Nation’s largest commercial fish harvests as well as supporting subsistence economies of 

Alaskan Natives. Ultimately, the fish populations depend on plankton blooms regulated by the 

extent and location of the ice edge in spring. Naturally, many other organisms, such as seabirds, 

seals, walruses, and whales, depend on primary production, mainly in the form of those plankton 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 119
blooms. As Arctic sea ice continues to diminish, the location, timing, and species makeup of the 

blooms is changing in ways that appear to favor marked changes in community composition. 

There are an estimated 20,000 to 25,000 polar bears (Ursus maritimus) worldwide, mostly 

inhabiting the annual sea ice over the continental shelves and inter-island archipelagos of the 

circumpolar Arctic (IUCN Polar Bear Specialist Group, 2006). Polar bears are specialized 

predators that hunt ice-breeding seals and are therefore dependent on sea ice for survival. After 

emerging in spring from a five- to seven-month fast in nursing dens, females require immediate 

nourishment and thus depend on close proximity between land and sea ice before the sea ice 

breaks up. 

The IPCC noted that continuous access to sea ice allows bears to hunt throughout the year, but in 

areas where the sea ice melts completely each summer, they are forced to spend several months 

in tundra fasting on stored fat reserves until freeze-up (Fischlin et al., 2007). The two Alaskan 

populations (Chukchi Sea: ~2,000 individuals in 1993, and southern Beaufort Sea: ~1,500 

individuals in 2006) are vulnerable to large-scale, dramatic seasonal fluctuations in ice 

movements because of the associated decreases in abundance of and access to prey and increases 

in the amount of energy needed for hunting (FWS, 2007). The IPCC projects that with a 

warming of 2.8 °C above pre-industrial temperatures and associated declines in sea ice, polar 

bears will face a high risk of extinction. Other ice-dependent species (e.g., walruses for rest and 

small whales for protection from predators) face similar consequences, not only in the Arctic but 

also in the Antarctic (Fischlin et al., 2007). 

Recent modeling of reductions in sea ice cover and polar bear population dynamics predicted 

declines within the coming century that (though varied by population) totaled 66% of all polar 

bears (Amstrup et al., 2007 in Janetos et al., 2008). In 2005, the World Conservation Union 

(IUCN) Polar Bear Specialist Group concluded that the IUCN Red List classification for polar 

bears should be upgraded from ‘Least Concern’ to ‘Vulnerable’ based on the likelihood of an 

overall decline in the size of the total population of more than 30% within the next 35 to 50 years 

(Fischlin et al., 2007). In January 2007, the U.S. Fish and Wildlife Service determined that 

sufficient scientific evidence exists of a global threat to polar bears to warrant proposing it for 

listing as a threatened species under the Endangered Species Act (1973) (FWS, 2007). This 

decision was based in part on future risks to the species from climate change (Fischlin et al., 

2007). 

Temperate montane ecosystems 

(adapted from Janetos et al., 2008) 

In SAP 4.3, Janetos et al. (2008) wrote about the sensitivity of temperate montane ecosystems to 

global change. These ecosystems are characterized by cooler temperatures and often increased 

precipitation compared to surrounding lowlands. Consequently, much of that precipitation falls 

in the form of snow, which serves to insulate the ground from freezing air temperatures, stores 

water that is released as the snow melts during the following growing season, and triggers 

vertical migration by animal species that cannot survive in deep snow. With increasing 

temperatures, more precipitation is falling as rain (Johnson, 1998). Diaz et al. (2003) found that, 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 120
over the past three to five decades, all the major continental mountain chains exhibited upward 

shifts in the height of the freezing level. 

These environmental changes are also resulting in the disappearance of glaciers in most montane 

areas around the world. The changes in patterns and abundance of meltwater from these glaciers 

have significant implications for the one-sixth of the world’s population that is dependent upon 

glaciers and melting snowpack for water supplies (Barnett et al., 2005b). Plant and animal 

communities are also affected as glaciers recede, exposing new terrain for colonization in an 

ongoing process of succession (e.g., for spider communities, see Gobbi et al., 2006). One group 

of organisms whose reproductive phenology is closely tied to snowmelt is amphibians, for which 

this environmental cue is apparently more important than temperature (Corn, 2003). Hibernating 

and migratory species that reproduce at high altitudes during the summer are also being affected 

by the ongoing environmental changes. For example, marmots are emerging a few weeks earlier 

than they used to in the Colorado Rocky Mountains, and robins are arriving from wintering 

grounds weeks earlier in the same habitats (Inouye et al., 2000). Species such as deer, bighorn 

sheep, and elk, which move to lower altitudes for the winter, may also be affected by changing 

temporal patterns of snowpack formation and disappearance. 

Flowering phenology has been advancing in these habitats (Inouye and Wielgolaski, 2003) as 

well as in others at lower altitudes, mirroring what is occurring at higher latitudes (Wielgolaski 

and Inouye, 2003). There is a very strong correlation between the timing of snowmelt, which 

integrates snowpack depth and spring air temperatures, and the beginning of flowering by 

wildflowers in the Colorado Rocky Mountains (e.g., Inouye et al., 2002, 2003). The abundance 

of flowers can have effects on a variety of consumers, including pollinators (Inouye et al., 1991), 

herbivores, seed predators, and parasitoids, all of which are dependent on flowers, fruits, or 

seeds. An unexpected consequence of earlier snowmelt in the Rocky Mountains has been the 

increased frequency of frost damage to montane plants, including the loss of new growth on 

conifer trees, of fruits on some plants such as Erythronium grandiflorum (glacier lilies), and of 

flower buds of other wildflowers (e.g., Delphinium spp., Helianthella quinquenervis, etc.) 

(Inouye, 2008). Over time, this damage may lead to significant demographic consequences. 

V.1.f Adaptation options 

SAP 4.4 (Julius et al., 2008) concludes that “while there will always be uncertainties associated 

with the future path of climate change, the response of ecosystems to climate impacts, and the 

effects of management, it is both possible and essential for adaptation to proceed using the best 

available science.” The term ‘adaptation’ in this document refers to adjustments in human social 

systems (e.g., management) in response to climate stimuli and their effects. Ecosystem 

management always occurs in the context of desired ecosystem conditions or natural resource 

management goals. SAP 4.4 presents a series of analyses of adaptation options for several 

different types of federally managed ecosystems. On the basis of these analyses, the report 

concludes that: 

• Many existing best management practices for ‘traditional’ stressors of concern have the 

added benefit of ameliorating climate change exacerbations of those stressors. 

• Seven ‘adaptation approaches’ can be used for strategic adjustment of best management 

practices to maximize ecosystem resilience to climate change. Here, resilience refers to the 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 121
amount of change or disturbance that a system can absorb without undergoing a fundamental 

shift to a different set of processes and structures. The seven approaches are: 

o Protecting key ecosystem features, which involves focusing management 

protections on structural characteristics, organisms, or areas that represent 

important ‘underpinnings’ or ‘keystones’ of the overall system. 

o Reducing anthropogenic stresses, which is the approach of minimizing localized 

human stressors (e.g., pollution, fragmentation) that hinder the ability of species 

or ecosystems to withstand climatic events. 

o Representation, which refers to protecting a portfolio of variant forms of a 

species or ecosystem so that, regardless of the climatic changes that occur, there 

will be areas that survive and provide a source for recovery. 

o Replication, which centers on maintaining more than one example of each 

ecosystem or population within a management system, such that if one area is 

affected by a disturbance, replicates in another area provide insurance against 

extinction and a source for recolonization of affected areas. 

o Restoration, which is the practice of rehabilitating ecosystems that have been lost 

or compromised. 

o Refugia, which are areas that are less affected by climate change than other areas 

and can be used as sources of ‘seed’ for recovery or as destinations for climatesensitive migrants. 

o Relocation, which refers to human-facilitated transplantation of organisms from 

one location to another in order to bypass a barrier (e.g., urban area). 

• Levels of confidence in these adaptation approaches vary and are difficult to assess, yet are 

essential to consider in adaptation planning. 

• The success of adaptation strategies may depend on recognition of potential barriers to 

implementation and creation of opportunities for partnerships and leveraging. 

• The Nation’s adaptive capacity can be increased through expanded collaborations 

among ecosystem managers. 

• The Nation’s adaptive capacity can be increased through creative re-examination of program 

goals and authorities. 

• Establishing current baselines, identifying thresholds, and monitoring for changes will be 

essential elements of any adaptation approach. 

• Beyond ‘managing for resilience,’ the Nation’s capability to adapt will ultimately depend on 

our ability to be flexible in setting priorities and ‘managing for change.’ 

V.2 Agriculture and Food Production 

In SAP 4.3, Hatfield et al. (2008) write that the food and agriculture sector includes a wide range 

of plant and animal production systems. The United States Department of Agriculture (USDA) 

classifies 116 plant commodity groups as agricultural products, as well as four livestock 

groupings (beef cattle, dairy, poultry, and swine) and products derived from animal production 

(e.g., cheese and eggs). These diverse commodities are produced in a variety of climates, 

regions, and soils. However, regardless of where they are grown, crops and livestock are affected 

by temperature, precipitation, CO2, and water availability. Indeed, variability in yield from year 

to year is mostly (and strongly) related to weather effects during the growing season (Hatfield et 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 122
al., 2008). These variations also affect crops and livestock through their effects on insects, 

disease, and weeds. Not only does U.S. agriculture produce necessary products, it has substantial 

economic value ($200 billion in 2002). Just over half (52%) derives from livestock, with the rest 

generated by crops (21% from fruit and nuts, 20% from grain and oilseed, 2% from cotton, and 

5% from other commodity production). 

The agricultural sector within the United States is sensitive to both short-term climate variability 

and long-term climate change. Productivity is driven by the interaction of a variety of variables 

including temperature, radiation, precipitation, humidity, and wind speed (Easterling et al., 

2007). The latitudinal distribution of crops around the world is a function of climatic conditions 

including the total season precipitation and patterns of variability (Olesen and Bindi, 2002), as 

well as photoperiod (e.g., Leff et al., 2004). 

Vulnerability of the U.S. agricultural sector to climate change is a function of many interacting 

factors including pre-existing climatic and soil conditions, changes in pest competition, water 

availability, and the sector’s capacity to cope and adapt through management practices, seed and 

cultivar technology, and changes in economic competition among regions. The IPCC (Easterling 

et al., 2007) found that the growth, development, and yield of crops are dependent upon their 

responses to their climatic environment (Porter and Semenov, 2005). Particular crops are suited 

to a particular range of conditions, thus production is reduced and damage can occur when 

thresholds are exceeded, even for short periods in some cases (Wheeler et al., 2000; 

Wollenweber et al., 2003 in Easterling et al., 2007). 

This section addresses how observed and projected climate change may affect U.S. food 

production and agriculture, including crop yields, irrigation requirements, effects from extreme 

events, pests and weeds, livestock production (e.g., milk and meat), and fisheries. 

V.2.a Crop yields and productivity 

The productivity of most agricultural enterprises has increased dramatically over recent decades 

due to cumulative effects from technology, fertilizers, innovations in seed stocks and 

management techniques, and changing climate influences. Given the interaction of these various 

factors, it is difficult to identify the specific impact from any one factor on specific yield 

changes. The largest changes are probably due to technological innovations (Hatfield et al., 

2008). However, weather events are a major factor in annual crop yield variation. 

Reviewing crop yields in North America, the IPCC (Field et al., 2007) found the following: 

• Yields of major commodity crops in the United States have increased consistently over the 

last century, typically at rates of 1 to 2% per year (Troyer, 2004), with significant variations 

across regions and between years. 

• In the midwestern United States from 1970 to 2000, corn yield increased 58% and soybean 

yield increased 20%, with annual weather fluctuations resulting in year-to-year variability 

(Hicke and Lobell, 2004). 

• For twelve major crops in California, climate fluctuations over the last 20 years have not had 

large effects on yield, though they have been a positive factor for oranges and walnuts and a 

negative factor for avocados and cotton (Lobell et al., 2006). 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 123
Increasing temperatures 

(adapted from Hatfield et al., 2008) 

In SAP 4.3, Hatfield et al. (2008) found that crop species differ in their cardinal temperatures 

(critical temperature range) for lifecycle development. There is a base temperature at which 

growth commences and an optimum temperature when the plant develops as fast as possible. 

Generally, increasing temperatures accelerate progression of a crop through its lifecycle 

(phenological) phases up to a species-dependent optimum temperature beyond which 

development slows. 

Yield responses to temperature vary among species based on the specific temperature 

requirements. As temperatures increase, plants with a lower optimum temperature will exhibit 

significant decreases in yield before those with a higher optimum. Moreover, high temperatures 

will interact with other factors, such as low water availability, to further reduce productivity. 

Research reported in Hatfield et al. (2008) found variable reductions in maize yield. One study 

found a 17% reduction per 1 °C increase across the United States (although this study did not 

include effects of water availability) (Lobell and Asner, 2003). Another study found that the 

response of global maize production to both temperature and rainfall over the period 1961 to 

2002 was reduced 8.3% per 1°C warming (Lobell and Field, 2007). 

Soybean has cardinal temperatures that are somewhat lower than those of maize. Responses to 

increasing temperatures are regionally dependent. Yield may actually increase 2.5% with a 1.2 

°C rise in the upper Midwest, but would decrease 3.5% for 1.2 °C increase in the South (Boote et 

al., 1996, 1997). Lobell and Field (2007) reported a 1.3% decline in soybean yield per 1 °C 

increase in temperature, taken from global production against global average temperature during 

July to August, weighted by production area. 

For wheat, Lawlor and Mitchell (2000) found that a 1 °C rise would shorten the reproductive 

phase by 6%, grain filling duration by 5%, and would reduce grain yield and harvest index 

proportionately. Bender et al. (1999) analyzed spring wheat grown at nine sites in Europe and 

found a 6% decrease in yield per 1 °C temperature rise. Lobell and Field (2007) reported a 5.4% 

decrease in global mean wheat yield per 1 °C increase in temperature. In addition, wheat 

illustrates the nonlinear effects that may occur as temperatures increase. In the wheat-growing 

regions of the Great Plains, yield is estimated to decline 7% per 1 °C increase in air temperature 

between 18 and 21 °C, and about 4% per 1 °C increase in air temperature above 21 °C, not 

considering any reduction in photosynthesis or grain-set. 

The response of rice to temperature has been well studied (Baker and Allen, 1993a,b; Baker et 

al., 1995; Horie et al., 2000). Baker et al. (1995) summarized many of their experiments from 

sunlit controlled-environment chambers and concluded that the optimum mean temperature for 

grain formation and grain yield of rice is 25 °C. They found that grain yield is reduced about 

10% per 1 °C temperature increase above 25 °C, until reaching zero yield at 35 to 36 °C mean 

temperature (Baker and Allen, 1993a; Peng et al., 2004). Mean air temperature during the rice 

grain-filling phase in summer in the southern United States and many tropical regions is about 26 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 124
to 27 °C. These are above the 25 °C optimum, which illustrates that temperatures elevated above 

current ones are projected to reduce rice yields in the United States and tropical regions by about 

10% per 1 °C rise, or about 12% for a 1.2 °C rise. 

Reviewing the literature for North America, the IPCC (Field et al., 2007) found the following 

regarding increasing temperatures: 

• In the Corn and Wheat Belts of the United States, yields of corn and soybeans from 1982 to 

1998 were negatively affected by warm temperatures, decreasing 17% for each 1 °C of 

warm-temperature anomaly (Lobell and Asner, 2003). 

• In California, warmer nights have enhanced the production of high-quality wine grapes 

(Nemani et al., 2001), but additional warming may not result in similar increases. 

Increasing CO2

As with their responses to temperature, crops respond differently to increasing CO2

concentrations. The evidence for maize response to CO2 is sparse and questionable (Hatfield et 

al., 2008). On its own, the expected increment of CO2 increase over the next 30 years is 

anticipated to have a negligible effect (i.e., 1%) on maize production (Leakey et al., 2006). In 

contrast, based on the metadata summarized by Ainsworth et al. (2002), a doubling of 

atmospheric CO2 concentrations is expected to yield a 38% increase in soybean yield. In the 

midwestern United States, an atmospheric CO2 increase from 380 to 440 ppm is projected to 

increase yield by 7.4%. For wheat, a cool-season cereal, doubling atmospheric CO2

concentrations (350 to 700 ppm) increased grain yield by about 31%, averaged over many data 

sets (Amthor, 2001). For rice, doubling atmospheric CO2 concentrations (330 to 660 ppm) 

increased grain yield by about 30% (Horie et al., 2000). 

The certainty level of biomass and yield response of these C3 crops

42 to CO2 is likely to very 

likely, given the large number of experiments and the general agreement in response across the 

different C3 crops. 

V.2.b Water use and irrigation requirements 

(adapted from Hatfield et al., 2008) 

Projected trends have conflicting effects on likely water needs. Increasing temperatures and a 

lengthening of the growing season will contribute to increased water demand. However, 

increasing CO2 concentrations will contribute to reduced stomatal conductance (speed of water 

vapor evaporation from plant pores) and decreased demand (Ainsworth and Long, 2005; 

Ainsworth and Rogers, 2007). 

The net irrigation requirement is the difference between seasonal evapotranspiration for a wellwatered crop and the amounts of precipitation and soil water storage that are available during a 

growing season. Some researchers have attempted to estimate future changes in irrigation water 

requirements based on projected climate changes (including rainfall changes) from general 

 

42 C3 and C4 refer to different carbon fixation pathways in plants during photosynthesis. C3 is the most common 

pathway, and C3 crops (e.g., wheat, soybeans, and rice) are more responsive than C4 crops such as maize to increases 

in CO2. 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 125
circulation models and estimates of decreased stomatal conductance due to elevated atmospheric 

CO2 (e.g., Allen et al., 1991; Izaurralde et al., 2003). For corn, Izaurralde et al. (2003) calculated 

that by 2030, irrigation requirements will vary from a reduction of 1% in the lower Colorado 

Basin to an increase of 451% in the lower Mississippi Basin, because of rainfall variation. Given 

the variation in the sizes and baseline irrigation requirements of U.S. river basins, a 

representative figure for the overall U.S. increase in irrigation requirements is 64% if stomatal 

effects are ignored or 35% if they are included. 

Research suggests the impacts of climate change on irrigation water requirements may be large 

(Easterling et al., 2007). The IPCC considered this to be a new, robust finding since the Third 

Assessment Report in 2001. There is an expected increase in irrigation demand due to climate 

change in the majority of world regions including the United States due to decreased rainfall in 

certain regions and/or increased evapotranspiration arising from increased temperatures. In 

modeling studies of future climate change, additional irrigation is often assumed in order to 

counterbalance the potential adverse yield effects of significant temperature increases (Easterling 

et al., 2007). 

V.2.c Climate variability and extreme events 

The IPCC (Easterling et al., 2007) found that “short-term natural extremes, such as storms and 

floods, interannual and decadal climate variations, as well as large-scale circulation changes, 

such as the El Niño–Southern Oscillation (ENSO), all have important effects on crop, pasture 

and forest production (Tubiello, 2005).” Indeed, the authors cite recent research that indicates El 

Niño-like conditions result in higher probabilities of reduced farm incomes across most of 

Australia (O’Meagher, 2005 in Easterling et al., 2007). They also mention a recently recognized 

correlation between the winter North Atlantic Oscillation (NAO) and conditions that favor 

improved wheat quality in the United Kingdom (Atkinson et al., 2005 in Easterling et al., 2007) 

but result in drought conditions and reduced summer growth in grasslands (Kettlewell et al., 

2006 in Easterling et al., 2007). 

In addition to changes in average climatic variables, such as temperature and precipitation, it is 

important to examine the potential for altered variability in extreme events such as extended heat 

waves, droughts, and floods. The potential for these events to change in frequency and 

magnitude introduces a key uncertainty regarding the yield of U.S. agriculture even under 

modest climate change. On this issue, the IPCC (Easterling et al., 2007) drew the following 

conclusion: 

Recent studies indicate that climate change scenarios that include increased frequency of heat 

stress, droughts and flooding events reduce crop yields and livestock productivity beyond the 

impacts due to changes in mean variables alone, creating the possibility for surprises. Climate 

variability and change also modify the risks of fires, and pest and pathogen outbreaks, with 

negative consequences for food, fiber and forestry (high confidence). 

The adverse effects on crop yields due to droughts and other extreme events may offset the 

beneficial effects of elevated atmospheric CO2, moderate temperature increases over the near 

term, and longer growing seasons. 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 126
On this topic, the IPCC (Field et al., 2007) reported that during the past decade, agriculture in 

North America has been exposed to many severe weather events. They concluded: “More 

variable weather, coupled with out-migration from rural areas and economic stresses, has 

increased the vulnerability of the agricultural sector overall, raising concerns about its future 

capacity to cope with a more variable climate (Senate of Canada, 2003; Wheaton et al., 2005).”

Drought events are already a frequent occurrence, especially in the western United States. 

Vulnerability to extended drought is, according to the IPCC (Field et al., 2007), increasing across 

North America as population growth and economic development increase demands from 

agricultural, municipal, and industrial uses, resulting in frequent over-allocation of water 

resources. While often associated with the western United States, the eastern region has also 

experienced droughts and attendant reductions in water supply, changes in water quality and 

ecosystem function, and challenges in allocation (Field et al., 2007). 

Regarding future precipitation, projections suggest decreasing average annual precipitation in the 

southwestern United States but increases over the rest of North America (Christensen et al., 

2007). Increasing precipitation can also cause challenges, depending on its intensity, amount, and 

timing. Some studies project widespread increases in extreme precipitation (Christensen et al., 

2007), with greater risks of not only flooding from intense precipitation, but also droughts from 

greater temporal variability in precipitation. 

In SAP 4.3, Hatfield et al. (2008) examine specific impacts and make the following conclusions: 

• Historical data for many parts of the United States indicate an increase in the frequency of 

high-precipitation events (e.g., >5 cm in 48 hours), and this trend is projected to continue for 

many regions. 

• Excessive rainfall results in delayed spring planting, leading to a reduction in profits through 

a premium available for early season production of high-value horticultural crops such as 

melon, sweet corn, and tomatoes. 

• Flooding during the growing season can result in crop losses associated with anoxia and 

increased susceptibility to root diseases, while also resulting in additional impacts including 

soil compaction (due to use of heavy farm equipment on wet soils), and can lead to additional 

runoff and leaching of nutrients and chemicals into surface and groundwater. 

• Concentrating a greater proportion of rainfall in high precipitation events will increase the 

likelihood of water deficiencies at other times because of reductions in rainfall frequency 

(Hatfield and Prueger, 2004). 

• Storm events that deliver heavy rainfall are also often accompanied by strong wind gusts, 

increasing the potential for lodging of crops and reduced productivity. 

• Wetter conditions at harvest time may result in reduced quality of many crops.

• Regarding temperature variation, short-term, episodic temperature increases can also lead to 

substantial effects when occurring just prior to, or during, critical crop pollination phases. 

Crop sensitivity and ability to compensate during later, improved weather varies across 

crops. 

Easterling et al. (2007) include a brief description of agricultural losses sustained during the 

2003 European heat wave (temperatures up to 6 °C above means accompanied by 300 mm 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 127
precipitation deficits). There were substantial reductions in crop yield: 36% for maize in Italy’s 

Po valley, and 30% for maize, 25% for fruit, and 21% for wheat in France (Ciais et al., 2005). 

Uninsured economic losses were estimated at € 13 billion (Sénat, 2004). 

V.2.d Pests and weeds 

Pests and weeds can reduce crop yields, cause economic losses to farmers, and require 

management control options. How climate change (elevated atmospheric CO2, increased 

temperatures, altered precipitation patterns, and changes in the frequency and intensity of 

extreme events) may affect the prevalence of pests and weeds is an issue of concern for food 

production and the agricultural sector. Recent warming trends in the United States have led to 

earlier spring activity by insects and proliferation of some species (Easterling et al., 2007). 

Additionally, research suggests that increased climate extremes may promote plant disease and 

pest outbreaks (Alig et al., 2004; Gan, 2004). 

In particular, the IPCC review indicated that interactions between CO2, temperature, and 

precipitation will play an important role in determining plant damage from pests in future 

decades (Stacey and Fellows, 2002; Chen et al., 2004; Salinari et al., 2006; Zvereva and Kozlov, 

2006 in Easterling et al., 2007). However, to date, most studies continue to investigate pest 

damage as a separate function of either elevated ambient CO2 concentrations or temperature. 

Pests and weeds are additional factors that, for example, are often omitted when projecting the 

stimulatory effect of elevated CO2 on crop yields. Research on the combined effects of elevated 

atmospheric CO2 and climate change on pests, weeds, and disease is still insufficient for U.S. 

and world agriculture (Easterling et al., 2007). 

V.2.e Projections for agriculture production 

For North American agriculture, the IPCC (2007b) concluded with high confidence: “Moderate 

climate change [1–3 °C] in the early decades of the century is projected to increase aggregate 

yields of rain-fed agriculture by 5–20%, but with important variability among regions. Major 

challenges are projected for crops that are near the warm end of their suitable range or which 

depend on highly utilized water resources.” Field et al. (2007) further explained that crops 

currently near their climate thresholds, such as wine grapes in California, are likely to suffer 

decreases in yields, quality, or both, with even modest warming (medium confidence) (Hayhoe et 

al., 2004; White et al., 2006). As for regional variations, increased climate sensitivity is 

anticipated in the southeastern United States as well as in the U.S. Corn Belt (Carbone et al., 

2003), but not in the Great Plains (Mearns et al., 2003 in Field et al., 2007). Increased average 

warming leads to an extended growing season, especially for northern regions of the United 

States. 

However, for global agriculture, the IPCC concluded: “At lower latitudes, especially seasonally 

dry and tropical regions, crop productivity is projected to decrease for even small local 

temperature increases (1–2 °C), which would increase the risk of hunger. (medium confidence).” 

Further warming is projected to have increasingly negative impacts in all regions (Easterling et 

al., 2007). 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 128
Reviewing recent integrated assessment models that explore interacting impacts of climate and 

economic factors on agriculture, water resources, and biome boundaries in the conterminous 

United States (e.g., Edmonds and Rosenberg, 2005; Izaurralde et al., 2005; Smith et al., 2005) 

led Field et al. (2007) to conclude that scenarios with decreased precipitation create important 

challenges, restricting the availability of water for irrigation and at the same time increasing 

water demand for irrigated agriculture and urban and ecological uses. There are still considerable 

uncertainties regarding precipitation changes. In addition, Field et al. (2007) found that the 

critical importance of specific agro-climatic events (e.g., last frost) introduces uncertainty in 

future projections (Mearns et al., 2003). 

The IPCC (Easterling et al., 2007) concluded with high confidence: “Projected changes in the 

frequency and severity of extreme climate events will have more serious consequences for food 

and forestry production, and food insecurity, than will changes in projected means of 

temperature and precipitation.” The authors cited modeling studies that suggest increasing 

frequency of crop loss from extreme events may counteract increased crop yields from rising 

temperatures. 

There is still debate and uncertainty about the sensitivity of crop yields in the United States and 

other world regions to the direct effects of elevated atmospheric CO2 levels. However, the IPCC 

(Easterling et al., 2007) concluded that elevated CO2 levels are expected to contribute to small 

beneficial impacts on crop yields. The IPCC confirmed the general conclusions from its previous 

Third Assessment Report in 2001. Experimental research on crop responses to elevated 

atmospheric CO2 through the FACE (Free Air CO2 Enrichment)43 experiments indicate that, at 

ambient CO2 concentrations of 550 ppm (approximately double the concentration of preindustrial times), crop yields increase under unstressed conditions by 10 to 25% for C3 crops, and 

by 0 to 10% for C4 crops (medium confidence). Crop model simulations under elevated CO2 are 

consistent with these ranges (high confidence) (Easterling et al., 2007). High temperatures, water 

and nutrient availability, and ozone exposure, however, can significantly limit the direct 

stimulatory effects of CO2. 

Analysis in SAP 4.3 suggests the complexity of these results. Hatfield et al. (2008) examined the 

interaction between temperature, precipitation, and atmospheric CO2. Conclusions vary across 

crops and regions, from decreased yield in some cases to increases in other scenarios. 

Specifically, research reviewed in Hatfield et al. (2008) found that in many cases weeds respond 

more positively to increasing CO2 than cash crops, particularly C3 invasive weeds that reproduce 

by vegetative means (roots, stolons, etc.) (Ziska, 2003; Ziska and George, 2004). Indeed, in all 

weed–crop competition studies where the photosynthetic pathway is the same, weed growth is 

favored as CO2 increases (Ziska and Runion, 2006a). Moreover, increasing atmospheric CO2

levels may reduce the efficacy of glyphosate, the most widely used herbicide in the United States 

(Ziska et al., 1999). Increasing temperatures also increase the potential for northward expansion 

of weed species’ ranges (Patterson et al., 1999). 

SAP 4.3 also considered the effects of climate changes on the range of beneficial and harmful 

insects, microbes, and other organisms associated with agro-ecosystems. Temperature is the 

single most important factor affecting insect ecology, epidemiology, and distribution, while plant 

 

43 See <www.bnl.gov/face/>. 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 129
pathogens will be highly responsive to humidity and rainfall, as well as temperature (Coakley et 

al., 1999). To control the same insects, current trends demonstrate substantially greater 

applications of insecticide in warmer, more southern regions of the United States, compared to 

cooler, higher-latitude regions. For example, the frequency of pesticide sprays for control of 

lepidopteran insect pests in sweet corn currently ranges from 15 to 32 applications per year in 

Florida (Aerts et al., 1999), to 4 to 8 applications in Delaware (Whitney et al., 2000), and to 0 to 

5 applications per year in New York (Stivers, 1999). Warmer winters are projected to increase 

populations of insect species that are currently marginally overwintering in high latitude regions, 

such as flea beetles (Chaetocnema pulicaria), which act as a vector for bacterial Stewart’s Wilt 

(Erwinia sterwartii), an economically important corn pathogen (Harrington et al., 2001). 

Moreover, an overall increase in humidity and frequency of heavy rainfall events projected for 

many parts of the United States will tend to favor some leaf and root pathogens (Coakley et al., 

1999). However, these effects may be counterbalanced in some regions by an increase in shortto medium-term drought, which will decrease the duration of leaf wetness and reduce some 

forms of pathogen attack on leaves. 

Considering this range of factors, Easterling et al. (2007) concluded that the vulnerability of 

North American agriculture to climatic change is multi-dimensional and is determined by 

interactions among pre-existing conditions, stresses stemming from climate change (e.g., 

changes in pest competition, water availability), and the sector’s capacity to cope with multiple, 

interacting factors, including economic competition from other regions as well as advances in 

crop cultivars and farm management (Parson et al., 2003 in Field et al., 2007). Water access is 

the major factor limiting agriculture in southeast Arizona, but farmers in the region perceive that 

technologies and adaptations such as crop insurance have recently decreased vulnerability 

(Vasquez-Leon et al., 2002 in Field et al., 2007). Areas with marginal financial and resource 

endowments (e.g., the U.S. northern plains) are especially vulnerable to climate change (Antle et 

al., 2004 in Field et al., 2007). Unsustainable land use practices will tend to increase the 

vulnerability of agriculture in the U.S. Great Plains to climate change (Polsky and Easterling, 

2001 in Field et al., 2007). 

V.2.f Livestock 

Climate change has the potential to influence livestock productivity in a number of ways. 

Elevated atmospheric CO2 concentrations can affect forage quality. Thermal stress can directly 

affect the health of livestock animals. An increase in the frequency or magnitude of extreme 

events can lead to livestock loss. Furthermore, climate change may affect the spread of animal 

diseases. In the 2007 Fourth Assessment Report, the IPCC generated a number of new 

conclusions in this area compared to the Third Assessment Report in 2001. The following 

conclusions can be applied to United States and other livestock-producing regions (Easterling et 

al., 2007): 

• Elevated atmospheric CO2 can increase the carbon to nitrogen ratio in forages and thus 

reduce the nutritional value of those grasses, which in turn affects animal weight and 

performance. Under elevated CO2, a decrease of C4 grasses and an increase of C3 grasses 

(depending upon the plant species that remain) may occur, which could potentially reduce or 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 130
alter the nutritional quality of the forage grasses available to grazing livestock. However, the 

exact effects on both types of grasses and their nutritional quality still need to be determined. 

• Increased climate variability (including extremes in both heat and cold) and droughts may 

lead to livestock loss. The impact on animal productivity due to increased variability in 

weather patterns will likely be far greater than effects associated with the average change in 

climatic conditions. 

• Thermal stress reduces productivity and conception rates and is potentially life threatening to 

livestock. According to one study reviewed by the IPCC (Frank et al., 2001), the U.S. 

percentage decrease in swine, beef, and dairy milk production in 2050 averaged 1.2, 2.0, and 

2.2%, respectively, using one climate model, and 0.9, 0.7, and 2.1%, respectively, using a 

different climate model. 

• Increasing temperatures may enable the spread of animal diseases from low to mid-latitudes, 

resulting in new threats and potentially reduced health for livestock (White et al., 2003; 

Anon, 2006; van Wuijckhuise et al., 2006). 

In SAP 4.3, CCSP (Backlund et al., 2008) concluded that higher temperatures will very likely

reduce livestock production during the summer season, but these losses will very likely be 

partially offset by warmer temperatures during the winter season. The further examination of the 

potential for thermal stress in SAP 4.3 (Hatfield et al., 2008) included the following: 

• As environmental conditions result in core body temperature beyond normal diurnal 

boundaries, the animal must begin to conserve or dissipate heat to maintain homeostasis 

through behavioral, physiological, and metabolic thermoregulatory processes (Mader et al., 

1997; M. Davis et al., 2003). 

• These activities often result in declines in physical activity and an associated decline in 

eating and grazing activity as well as shifts in cardiac output, blood flow to extremities, and 

passage rate of digesta. 

• Production losses in domestic animals are largely attributed to altered feed intake and 

increases in maintenance requirements to sustain a constant body temperature. (Mader et al., 

2002; M. Davis et al., 2003; Mader and Davis, 2004). 

• Voluntary feed intake increases (after a one- to two-day decline) by as much as 30% under 

cold stress and decreases by as much as 50% almost immediately under heat stress (NRC, 

1987). 

• Temperatures beyond the ability of the animal to dissipate result in reduced performance 

(i.e., production and reproduction), health, and well-being if adverse conditions persist (Hahn 

et al., 1992; Mader, 2003). Such reductions can result in substantial economic losses. 

• Nighttime recovery is an essential element of survival when severe heat challenges occur 

(Hahn and Mader, 1997; Amundson et al., 2006). 

• Heat wave events have resulted in documented livestock losses (Hahn and Mader, 1997; 

Hahn et al., 1999, 2001). Animal losses are particularly high for rapid changes in temperature 

due to extreme events when the animals are unable to acclimate44 (High Plains Regional 

Climate Center, 2000). 

 

44 High Plains Regional Climate Center < http://www.hprcc.unl.edu/> 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 131
V.2.g Freshwater and marine fisheries 

Fisheries are sensitive to changes in temperature and water supply, which affect flows of rivers 

and streams, as well as lake levels. Climate change can interact with other factors that affect the 

health of fish and productivity of fisheries (e.g., habitat loss and land use change). The IPCC 

(Easterling et al., 2007) found the following: 

• Increased temperatures can lead to seasonal increases in fish growth but may also increase 

risks at the upper end of their thermal tolerance zone. The specific effects of interactions 

between increasing temperature and other global changes including declining pH are unclear. 

• Direct effects of increasing temperature on marine and freshwater systems are already 

occurring, with rapid poleward shifts in some regions. Future shifts are expected to continue. 

Local extinctions are occurring at the edges of current ranges, particularly for freshwater and 

anadromous species such as salmon and sturgeon (Friedland et al., 2003; Reynolds et al., 

2005). 

• Observed changes in primary production will affect fisheries as their effects are transferred 

through the food chain. While results vary regionally, evidence indicates reduced nutrient 

supply to the upper productive layer of the Pacific and Atlantic Oceans (McPhaden and 

Zhang, 2002; Curry and Mauritzen, 2005). 

• Climate change has been implicated in mass mortalities of a wide variety of aquatic species, 

but limited data exists to confirm the correlation generally (Harvell et al., 1999). However, 

research does confirm the poleward spread of two protozoan parasites from the Gulf of 

Mexico to Delaware Bay and north, resulting in mass mortalities of oysters (Hofmann et al., 

2001). 

• Climate change is expected to result in the largest economic impacts in the fisheries sector in 

central and northern Asia, the western Sahel, and coastal, tropical regions of South America 

(Allison et al., 2005) as well as some small and medium-sized island states (Aaheim and 

Sygna, 2000). 

• Regional climate variability can severely affect the productivity and species composition of 

major marine fisheries such as tuna (skipjack, yellowfin, and albacore) and Peruvian anchovy 

(Barber, 2001; Lehodey et al., 2003). Models need to be improved to account for decadal 

trends in addition to ENSO events as they will have a greater impact on the food web. 

In SAP 4.3, Janetos et al. (2008) further examined the impacts of the NAO, the Pacific Decadal 

Oscillation (PDO), and ENSO on marine ecosystems. 

• The NAO has been strongly positive since the 1980s, resulting in dramatic impacts on 

northeast Atlantic ecosystems. These impacts include increased flow of oceanic water into 

the English Channel and North Sea, a northward shift in the distribution of warm water 

zooplankton species (Beaugrand, 2004), and concomitant changes in dominance in fish 

communities from whiting (hake) to sprat (similar to a herring). Similar ecosystem shifts 

were observed in the Baltic Sea, where drastic changes in both zooplankton and fish 

communities have been observed (Kenny and Mollman, 2006). 

• In the North Pacific, the PDO refers to the east–west shifts in location and intensity of the 

Aleutian Low in winter (Mantua et al., 1997). Widespread ecological changes have been 

observed, including increased productivity of the Gulf of Alaska when the PDO is in positive 

phase, resulting in dramatic increases in salmon production (Mantua et al., 1997) and a 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 132
reversal of demersal fish community dominance from a community dominated by shrimps to 

one dominated by pollock (Anderson and Piatt, 1999). Associated changes in the California 

Current ecosystem include dramatic decreases in zooplankton (McGowan et al., 1998) and 

salmon (Pearcy, 1991) when the PDO changed to a positive phase in 1977. There is also 

evidence that the large oscillations in sardine and anchovy populations are associated with 

PDO shifts, such that during positive (warm) phases, sardine stocks are favored but during 

negative (cool) phases, anchovy stocks dominate (e.g., Chavez et al., 2003). 

• ENSO events negatively affect zooplankton and fish stocks, resulting in a collapse of 

anchovy stocks in offshore ecosystems of Peru. Losses of anchovies, which are harvested for 

fishmeal, affect global economies because fishmeal is an important component of chicken 

feed as well as high-protein supplements in aquaculture feed. In waters off the West Coast of 

the United States, plankton and fish stocks may collapse due to sudden warming of the 

waters (by 4 to 10 °C) as well as through poleward advection of tropical species into 

temperate zones. Many of the countries most affected by ENSO events are developing 

countries in South America and Africa, with economies that are largely dependent upon 

agricultural and fishery sectors as a major source of food supply, employment, and foreign 

exchange. 

The IPCC (Field et al., 2007 and references therein) reviewed a number of North American 

studies showing how freshwater fish are sensitive to, or are being affected by, observed changes 

in climate: 

• Cold- and cool-water fisheries, especially salmonids, have been declining as warmer/drier 

conditions reduce their habitat. The sea-run salmon stocks are in steep decline throughout 

much of North America. 

• Pacific salmon have been appearing in Arctic rivers.

• Salmonid species have been affected by warming in U.S. streams. 

• Success of adult spawning and survival of fry brook trout are closely linked to cold 

groundwater seeps, which provide preferred temperature refuges for lake-dwelling 

populations. Rates of fish egg development and mortality increase with temperature increases 

within species-specific tolerance ranges. 

In addition, Janetos et al. (2008) identified impacts in the Bering Sea, which is among the more 

southerly seasonally ice-covered seas. The Bering Sea produces the Nation’s largest commercial 

fish harvests and supports subsistence economies of Alaskan Natives. Ultimately, the fish 

populations depend on plankton blooms regulated by the extent and location of the ice edge in 

spring. As Arctic sea ice continues to diminish, the location, timing, and species makeup of the 

blooms is changing in ways that appear to promote marked changes in community composition. 

Specifically, the spring melt of sea ice in the Bering Sea has long favored the delivery of organic 

material to a benthic community of bivalve mollusks, crustaceans, and other organisms. Those 

benthic organisms, in turn, are important food for walruses, gray whales, bearded seals, eider 

ducks, and many fish species. The earlier ice melt resulting from a warming climate, however, 

leads to later phytoplankton blooms that are largely consumed by zooplankton near the sea 

surface, vastly decreasing the amount of organic material reaching the benthos. This is expected 

to result in a radically altered community of organisms favoring a different suite of upper-level 

consumers and altering the subsistence and commercial harvests of fish and other marine 

organisms. 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 133
Regarding the impacts of future climate change, the IPCC concluded, with high confidence for 

North America, that coldwater fisheries will likely be negatively affected, warm-water fisheries 

will generally benefit, and the results for cool-water fisheries will be mixed, with gains in the 

northern and losses in the southern portions of ranges (Field et al., 2007). A number of specific 

impacts by fish species and region in North America are projected (Field et al., 2007 and 

references therein): 

• Salmonids, which prefer cold water, are likely to experience the most negative impacts. 

• Arctic freshwaters are likely to be most affected, as they will experience the greatest 

warming. 

• Many warm-water and cool-water species will shift their ranges northward or to higher 

altitudes. 

• In the continental United States, coldwater species will likely disappear from all but the 

deeper lakes; cool-water species will be lost mainly from shallow lakes; and warm-water 

species will thrive, except in the far south, where temperatures in shallow lakes will exceed 

survival thresholds. 

Climate variability and change can also affect fisheries in coastal and estuarine waters, although 

non-climatic factors, such as overfishing and habitat loss and degradation, are already 

responsible for reducing fish stocks (Nicholls et al., 2007). Coral reefs, for example, are 

vulnerable to a range of stresses and for many reefs, thermal stress thresholds will be crossed, 

resulting in bleaching, with severe adverse consequences for reef-based fisheries (Nicholls et al., 

2007). Increased storm intensity, temperature, and saltwater intrusion in coastal water bodies can 

also adversely affect coastal fisheries production. 

V.3 Land Resources 

The Nation has a large and diverse land base that can be classified based on the type of 

vegetation supported. In the following sections, we discuss two major land types in the United 

States: forests, and grasslands and shrublands. 

In SAP 4.3, Ryan et al. (2008) found that climate strongly influences both forests and arid lands 

by shaping broad patterns of ecological communities, vegetation, and wildlife species, their 

productivity, and the ecosystem goods and services they provide. The interaction of vegetation 

and climate is a fundamental tenet of ecology. Substantial research has documented how 

vegetation has changed with climate over the past several thousand years, providing not only a 

historic description of vegetation but also a predictive ability to anticipate upcoming conditions 

by drawing on these historic trends. However, because current changes are occurring at an 

unprecedented rate, in many cases these previously identified relationships and trends may no 

longer adequately describe future conditions. For example, emerging changes have been 

identified in the physical climate, levels of CO2 in the atmosphere, nitrogen deposition, local 

areas of ozone pollution, species invasions, and disturbance patterns. These factors cause 

important changes themselves, but their interactions are difficult to predict. This is particularly 

so because these interactions represent novel combinations beyond society’s experience base. 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 134
Terrestrial ecosystems are dynamic. Disturbances (such as drought, storms, insect outbreaks, 

grazing, and fire) have a strong influence on ecological communities and landscapes. In many 

cases, the timing, magnitude, and frequency of disturbances are influenced by climate. 

Accordingly, a changing climate will lead to changes in disturbances and ultimately the 

composition, structure, and function of ecosystems (including productivity, water yield, erosion, 

and carbon storage) as well as susceptibility to future disturbance across U.S. land resources 

(Dale et al., 2001). 

Disturbance may reset and rejuvenate some ecosystems in some cases and cause enduring 

change in others. In SAP 4.3, Ryan et al. (2008) provided two examples. First, climate may favor 

the spread of invasive exotic grasses into arid lands where the native vegetation is too sparse to 

carry a fire. When these areas burn, they typically convert to non-native monocultures and the 

native vegetation is lost. In another example, drought may weaken trees and make them 

susceptible to insect attack and death—a pattern that recently occurred in the Southwest. In these 

forests, drought and insects converted large areas of mixed pinyon–juniper forests into juniper 

forests. However, fire is an integral component of many forest ecosystems, and many tree 

species (such as the lodgepole pine forests that burned in the Yellowstone fires of 1988) depend 

on fire for regeneration. Ryan et al. (2008) conclude that climate effects on disturbance are 

expected to shape terrestrial ecosystems as much as the direct effects of climate. 

V.3.a Forests 

Forests occur globally and, although forest types are quite diverse, they are all characterized by 

extensive tree cover. Regarding global forests, research reviewed in Easterling et al. (2007) 

indicates: 

• Globally, forest ecosystems are found on 41.6 million km2

, making up approximately 30% of 

all land. 

• Of forestland, 42% occurs in the tropics, 25% in temperate zones, and 33% in the boreal zone 

(e.g., Sabine et al., 2004). 

• Forests are among the most productive terrestrial ecosystems. 

• Currently about one-quarter of anthropogenic CO2 emitted globally comes from deforestation 

activities, primarily in tropical and subtropical regions (Houghton, 2003). 

• Notwithstanding, forests still sequester the largest fraction of terrestrial ecosystem carbon 

stocks, recently estimated at 1,640 petagrams of carbon (PgC),45 equivalent to about 220% of 

atmospheric carbon (Sabine et al., 2004). 

• Forests provide a number of goods (in addition to timber products) that are important for 

subsistence livelihoods (Gitay et al., 2001; Shvidenko et al., 2005). They also provide key 

ecosystem services including providing habitat for an increasing fraction of biodiversity 

(particularly in areas subject to land use pressures: Hassan et al., 2005; MEA, 2005a); carbon 

sequestration; climate regulation; soil and water protection or purification (>75% of the 

world’s usable freshwater supplies come from forested catchments: Shvidenko et al., 2005); 

and recreational, cultural, and spiritual benefits (Reid et al., 2005). 

 

45 1 Pg = 1,000,000,000,000,000 grams = 1 Gt. 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 135
In SAP 4.3, Ryan et al. (2008) described the U.S. forest resource in the following manner. 

Forests cover about 3 million square kilometers (740 million acres) of the United States, making 

up approximately one-third of the land cover. While occurring in every state, forests are most 

prevalent in the humid eastern United States, the West Coast, at higher elevations in the interior 

West and Southwest, and along riparian corridors in the plains states (Zhu and Evans, 1994). The 

U.S. forest resource is quite diverse. In the eastern United States, there are 1.54 million square 

kilometers (380 million acres) of forestland—most of which (83%) is privately owned and 74% 

of which is broadleaf forest. The 1.46 million square kilometers (360 million acres) of forestland 

in the western United States are split between public (57%) and private ownership and largely 

consist of coniferous forest types (78%) (USDA Forest Service and U.S. Geological Survey, 

2002). 

As with global forests, U.S. forests provide a range of goods and ecosystem services important to 

the well-being of the people of the United States, including raw material for wood and paper 

products as well as many non-consumptive values and uses. Many Americans are strongly 

attached to their forests (Ryan et al., 2008). While all forests have considerable economic value, 

many values are not easily quantified (Costanza et al., 1997; Daily et al., 2000; Krieger, 2001; 

MEA, 2005b). A changing climate will alter forests and their ability to continue to provide these 

goods and services at current levels. In addition, impacts increase as human population, 

recreation, and tourism increase in forested regions across the nation. 

V.3.b Grasslands and shrublands 

As defined in the State of the Nation’s Ecosystems report these lands include diverse ecosystem 

types including the sagebrush steppes of the northern Rockies, the prairies of the Midwest and 

the Great Plains, the deserts of the Southwest and the intermountain West, the Alaskan tundra 

and shrublands, and the scrublands of Florida (Heinz Center, 2002). These ecosystems cover 

more than 3.1 million square kilometers (770 million acres), making up more than a third of the 

U.S. land base. A majority are privately owned, particularly east of the Rocky Mountains. These 

ecosystems are very diverse; example vegetation types include annual grasslands and chaparral 

in California, sagebrush/bunchgrass in the Great Basin, hot-desert shrublands of New Mexico, 

plains grasslands of mid-America, the oak savanna of Texas, wet grasslands of Florida, and 

tundra of Alaska. 

Fischlin et al. (2007) found these systems are generally rich in grazing, browsing, and other 

fauna (especially but not only in Africa), and are strongly controlled by fire (Bond et al., 2005 

and/or grazing regimes (Scholes and Archer, 1997; Fuhlendorf et al., 2001). In many cases, 

disturbance regimes are human-managed (e.g., Sankaran, 2005), although fire regimes depend 

also on seasonality of ignition events and rainfall-dependent accumulation of flammable material 

(Brown et al., 2005). 

The IPCC (Fischlin et al., 2007) drew the following conclusions regarding these systems: 

• These systems appear more sensitive than previously thought to variability of, and changes 

in, major climate change drivers. 

• The CO2-fertilization impact and warming effect of rising atmospheric CO2 have contrasting 

effects on their dominant functional types (trees and C3 grasses may benefit from rising CO2

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 136
but not from warming; C4 grasses may benefit from warming, but not from CO2 fertilization). 

Uncertain, nonlinear, and rapid changes in ecosystem structure and carbon stocks are likely. 

• Carbon stocks are very likely to be strongly reduced under more frequent disturbance, 

especially by fire, and disturbance and drought impacts on vegetation cover may exert 

regional feedback effects. 

• On average, grasslands are likely to show reduced carbon sequestration (due to enhanced soil 

respiratory losses through warming, fire regime changes, and increased rainfall variability). 

However, possible regional gains in woody cover (through CO2 fertilization and increased 

plant carbon stocks) cannot be excluded. 

• Scientific predictive skill is currently limited by very few field-based, multi-factorial 

experiments, especially in tropical systems. 

• Projected range shifts of mammal species will be limited by fragmented habitats and human 

pressures, and declines in species richness are likely, especially in protected areas. 

• Because of the important control by disturbance, management options exist to develop 

adaptive strategies for carbon sequestration and species conservation goals. 

V.3.c Productivity 

Forest productivity 

Climate strongly influences forest productivity and species composition. Research reviewed in 

the North America IPCC chapter indicates that forest growth appears to have increased slightly 

in the previous decade (less than 1% per decade) in regions where growth has historically been 

limited by low temperatures and short growing seasons (Caspersen et al., 2000; McKenzie et al., 

2001; Joos et al., 2002; Boisvenue and Running, 2006 in Field et al., 2007). However, as noted 

by Ryan et al. (2008), it is difficult to separate the role of climate from other potentially 

influencing factors particularly because these interactions vary by location. Other potentially 

influential factors include increases in precipitation (observed in the Midwest and Lake States), 

increases in nitrogen deposition, temperature increases and a lengthened growing season in the 

northern United States, changing age structure of forests (greater percentage of forests in young 

age classes), and evolving management practices. 

Research reviewed by the IPCC (Field et al., 2007) indicates the vegetation growing season has 

increased by an average of 2 days per decade since 1950 in Canada and the conterminous United 

States, with most of the increase resulting from earlier spring warming (Bonsal et al., 2001; 

Easterling 2002; Bonsal and Prowse, 2003; Feng and Hu, 2004). While this allows a greater 

period of growth and, thus, potential to increase productivity, earlier warming can also contribute 

to dryer conditions and increased potential for disturbance, both of which may act to offset the 

increases. As above, the effects of these trends vary by region. Field et al. (2007) found that in 

temperature-limited areas, productivity had increased, while in areas subject to drought it had 

decreased. For example, height growth had increased since the 1970s in black spruce at the 

forest–tundra transition in eastern Canada (Gamache and Payette, 2004). On the other hand, 

radial growth of white spruce on dry south-facing slopes in Alaska has decreased over the last 90 

years (Barber et al., 2000), while growth rates in semi-arid forests of the southwestern United 

States have decreased since 1895, which is correlated with drought linked to warming 

temperatures (McKenzie et al., 2001). For a widespread species like lodgepole pine, a 3 °C 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 137
temperature increase would increase growth in the northern part of its range, decrease growth in 

the middle, and decimate southern forests (Field et al., 2007). 

Additional research presented by Field et al. (2007) provided evidence that these relationships 

can also occur within the same region based on topographical differences. Research in subalpine 

forests in the Pacific Northwest from 1895 to 1991 found that growth of subalpine fir and 

mountain hemlock was negatively correlated with spring snowpack depth and positively 

correlated with summer temperatures. This indicates temperature limitations on the growing 

season on high-elevation, north-facing slopes. At lower elevations, however, growth was 

negatively correlated with summer temperature, suggesting water limitations. (Peterson and 

Peterson, 2001; Peterson et al., 2002 in Field et al., 2007). There is also evidence of shifting 

species ranges. For example, aspen have advanced into the more cold-tolerant spruce–fir forests 

in Colorado over the past 100 years (Elliott and Baker, 2004 in Field et al., 2007) and lodgepole 

pine has advanced northward into areas previously dominated by the more cold-tolerant black 

spruce in the Yukon (Johnstone and Chapin, 2003 in Field et al., 2007). 

Overall, productivity gains in one area may be offset by losses elsewhere, both through direct 

effects of climate changes and through important secondary effects on frequency and intensity of 

natural disturbances such as fire, insect outbreaks, ice storms, and windstorms. For example, 

Easterling et al. (2007) cited research projecting short-term productivity increases in California 

forests, in the area available for productive softwood growth, through 2020 with reductions in the 

long run (up to 2100) (Mendelsohn, 2003). Recent years have also seen a substantial increase in 

the area affected by wildfires and insect outbreaks in U.S. forests. These disturbances are further 

discussed in the following section. Easterling et al. (2007) also noted the changing climate will 

have substantial impacts on non-timber goods (e.g., seeds, nuts, hunting, resins, and plants used 

in pharmaceutical and botanical medicine and in the cosmetics industry) and services (e.g., 

wildlife habitat and recreation opportunities) offered by forest ecosystems. 

Moreover, the commercial U.S. forest sector is expected to be affected by changing forest 

productivity in different regions of the world. Sohngen and Sedjo (2005) show two climate 

change scenarios where North American forests undergo more dieback in general than forests in 

other regions of the world, and where certain North American forest yields increase but less so 

compared to other regions (in Easterling et al., 2007). The implication is that forests in other 

parts of the world (including tropical forests with shorter rotations) could have a competitive 

advantage within the global forestry sector under a changing climate. 

Grassland and shrubland productivity 

Fischlin et al. (2007) found the ecosystem function and species composition of grasslands and 

savanna are likely to respond mainly to precipitation change and warming in temperate systems. 

In tropical systems, however, CO2 fertilization and emergent responses to herbivory and fire 

regime will also exert strong control. Specific conclusions included the following: 

• Rainfall change and variability is very likely to affect vegetation in tropical grassland and 

savanna systems with, for example, a reduction in cover and productivity simulated along an 

aridity gradient in southern African savanna in response to the observed drying trend of about 

8 mm per year since 1970 (Woodward and Lomas, 2004a). 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 138
• Changing amounts and variability of rainfall may also strongly control the responses of 

temperate grassland to future climate change (Novick et al., 2004; Zha et al., 2005). For 

example, a Canadian grassland fixed roughly five times as much carbon in a year with 30% 

higher rainfall, while a 15% rainfall reduction led to a net carbon loss (Flanagan et al., 2002). 

Similarly, Mongolian steppe grassland switched from carbon sink to carbon source in 

response to seasonal water stress, although carbon balance was neutral on an annual basis (Li 

et al., 2005). 

• Nonlinear responses to increasing rainfall variability may be expected, as ecosystem models 

of mixed C3/C4 grasslands show initially positive net primary productivity relationships with 

increasing rainfall variability, but greater variability ultimately reduces both net primary 

productivity and ecosystem stability even if the rainfall total is kept constant (Mitchell and 

Csillag, 2001). 

• Empirical results for C4 grasslands confirm a similar monotonic relationship between net 

primary productivity and rainfall variability (Nippert et al., 2006). 

• Increased rainfall variability was more significant than rainfall amount for tall-grass prairie 

productivity (Fay et al., 2000, 2002), with a 50% increase in dry-spell duration causing a 

10% reduction in net primary productivity (Fay et al., 2003) and a 13% reduction in soil 

respiration (Harper et al., 2005). 

• Elevated CO2 has important effects on production and soil water balance in most grassland 

types, mediated strongly by reduced stomatal conductance and resulting increases in soil 

water (Leakey et al., 2006) in many grassland types (Nelson et al., 2004; Niklaus and Körner, 

2004; Stock et al., 2005). 

• In short-grass prairie, elevated CO2 and 2.6 °C warming increased production by 26 to 47%, 

regardless of grass photosynthetic type (J. Morgan et al., 2001). In C4 tropical grassland, no 

relative increase in herbaceous C3 success occurred with a doubling of the ambient CO2

(Stock et al., 2005). Regional climate modeling indicates that CO2-fertilization effects on 

grasslands may scale up to affect regional climate (Eastman et al., 2001). 

V.3.d Disturbance 

In large measure, the composition, structure, function, and condition of our land resources have 

been shaped by their disturbance history (both human-induced and natural). Disturbances drive 

change at both small (e.g., local wind event resulting in minor windthrow) and large scales (e.g., 

the 2002 Biscuit fire covering approximately 2,000 square kilometers (500,000 acres) in southern 

Oregon and northern California). On an annual basis, more than 220,000 square kilometers (55 

million acres) of forestland in the United States are impacted by disturbance annually (Dale et 

al., 2001). 

Wildland fires 

While in some cases a changing climate may have positive impacts on the productivity of forest 

systems, changes in disturbance patterns are expected to have a substantial impact on overall 

gains or losses. According to studies reviewed by the IPCC (Field et al., 2007), in the absence of 

dramatic increases in disturbance, the effects of climate change on the potential for commercial 

forest harvest in the 2040s ranged from mixed for a low emissions scenario to positive for a high 

emissions scenario (Perez-Garcia et al., 2002). However, the tendency for North American 

producers to suffer losses increases if climate change is accompanied by increased disturbance, 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 139
with simulated losses averaging $1 to 2 billion per year over the 21st century (Sohngen and 

Sedjo, 2005). 

More prevalent fire activity has recently been observed in the United States and other world 

regions (Fischlin et al., 2007). Wildfires and droughts, among other extreme events (e.g., 

hurricanes), can result in short-term losses and long-term shifts in forest ecosystems. Wildland 

fires can also have direct economic implications due to large fire suppression costs (OFCM, 

2005). The frequency and severity of these extreme events are expected to be altered by climate 

change. 

In a review of fire activity in the western United States from 1974 to 2004, Westerling et al. 

(2006) found that both the frequency of large wildfires and fire season length increased 

substantially after 1985, and that these changes were correlated with advances in the timing of 

spring snowmelt and increases in spring and summer air temperatures. They concluded that 

earlier spring snowmelt contributed to greater wildfire frequency by extending the period during 

which ignitions could potentially occur and by reducing water availability to ecosystems in midsummer, thus enhancing drying of vegetation and surface fuels (Westerling et al., 2006). These 

trends in increased fire size correspond with the increased cost of fire suppression (Calkin et al., 

2005). 

Fire activity also has increased in recent decades in boreal forests across North America as more 

than twice as much area burned during the 1980s and 1990s than during the 1960s and 1970s. As 

in the western United States, a key predictor of burned area in boreal North America is air 

temperature, with warmer summer temperatures causing an increase in burned area on both 

interannual and decadal time scales (Gillett et al., 2004, Duffy et al., 2005, Flannigan et al., 2005 

in Field et al., 2007). In Alaska, for example, June air temperatures alone explained 

approximately 38% of the variance of the natural log of annual burned area during 1950 to 2003 

(Duffy et al., 2005). 

The IPCC (Field et al., 2007) noted a number of observed changes to U.S. wildfire size and 

frequency, often associating these changes with changes in average temperatures: 

• Since 1980, an average of about 22,000 km2 per year (13,700 mi2 per year) has burned in 

wildfires, almost twice the 1920 to 1980 average of about 13,000 km2 per year (8,080 mi2 per 

year) (Schoennagel et al., 2004). 

• The forested area burned in the western United States from 1987 to 2003 is 6.7 times the area 

burned from 1970 to 1986 (Westerling et al., 2006). 

• Human vulnerability to wildfires has increased, with a rising population in the wildland–

urban interface. 

• In the last three decades, the wildfire season in the western United States increased by 78 

days, and burn durations of fires greater than 1,000 hectares (2,470 acres) have increased 

from 7.5 to 37.1 days, in response to a spring/summer warming of 0.87 °C (Westerling et al., 

2006). 

• Earlier spring snowmelt has led to longer growing seasons and drought, especially at higher 

elevations, where the increase in wildfire activity has been greatest (Westerling et al., 2006). 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 140
• In the southwestern United States, fire activity is correlated with ENSO positive phases and 

higher Palmer Drought Severity Indices (Kitzberger et al., 2001; McKenzie et al., 2004).46

These findings led the IPCC to conclude (Field et al., 2007): “Disturbances such as wildfire and 

insect outbreaks are increasing and are likely to intensify in a warmer future with drier soils and 

longer growing seasons (very high confidence).” 

This conclusion is supported by findings reported in SAP 4.3. Ryan et al. (2008) found that 

several lines of evidence suggest that large, stand-replacing wildfires are expected to increase in 

frequency over the next several decades because of climate. Historical records unequivocally 

show that warmer and drier periods during the last millennium are associated with more frequent 

and severe wildfires in western forests. General circulation model (GCM) projections of future 

climate during 2010 to 2029 suggest that the number of low-humidity days (and high fire danger 

days) will increase across much of the western United States, allowing for more wildfire activity 

with the assumption that fuel densities and land management strategies remain the same 

(Flannigan et al., 2000; Brown et al., 2004). Flannigan et al. (2000) used two GCM simulations 

of future climate to calculate a seasonal severity rating related to fire intensity and difficulty of 

fire control. Depending on the GCM used, forest fire seasonal severity rating in the Southeast is 

projected to increase from 10 to 30%, and from 10 to 20% in the Northeast by 2060. Other biome 

models used with a variety of GCM climate projections simulate a larger increase in fire activity 

and biomass loss in the Southeast, sufficient to convert the southernmost closed-canopy 

Southeast forests to savannas (Bachelet et al., 2001). 

In SAP 4.3, Ryan et al. (2008) found that in grasslands, particularly arid lands, non-native plant 

invasions, promoted by enhanced nitrogen deposition (Fenn et al., 2003) and increased 

anthropogenic disturbance (Wisdom et al., 2005), will have a major impact on how arid land 

ecosystems respond to climate and climate change. Once established, non-native annual and 

perennial grasses can create a continuous load of fine fuels in areas where vegetation historically 

occurred in patches across the landscape. This results in a changed fire regime and predisposes 

arid lands to fires more frequent and intense than those with which they evolved. 

Not only will an increase in fire activity result in a greater area of disturbance, fire emissions 

across North America will have important consequences for climate forcing agents, air quality, 

and ecosystem services. More frequent fire will increase emissions of greenhouse gases and 

aerosols (Amiro et al., 2001 in Ryan et al., 2008) and increase deposition of black carbon 

aerosols on snow and sea ice (Flanner et al., 2007 in Ryan et al., 2008). Even though many 

forests will regrow and sequester the carbon released in the fire, forests burned in the next few 

decades can be sources of CO2 for decades and not recover the carbon lost for centuries (Kashian 

et al., 2006 in Ryan et al., 2008) 

Insects and pathogens 

Insects and diseases are a natural part of forested ecosystems; they are nearly always present at 

endemic levels within terrestrial ecosystems. However, when conditions are right, their 

 

46 The Palmer Drought Severity Index is used by the National Oceanic and Atmospheric Administration and uses a 

formula that includes temperature and rainfall to determine dryness. It is most effective in determining long-term 

drought. 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 141
populations increase dramatically to epidemic levels with a concomitant increase in the damage 

they produce. Their effects vary from defoliation and retarded growth, to reduction in timber 

quality, to massive forest diebacks. Insect lifecycles are sensitive to climate variables and can be 

a factor in determining when they increase to epidemic levels. 

Ryan et al. (2008) noted that insects and pathogens affect more area annually than any other 

disturbance event. Outbreaks are estimated to result in $1.5 billion annually in losses to U.S. 

forest ecosystems (Dale et al., 2001). Extensive reviews of the effects of climate change on 

insects and pathogens have reported many cases where climate change has affected and/or will 

affect forest insect species range and abundance (Ayres and Lombardero, 2000; Malmström and 

Raffa, 2000; Bale et al., 2002). Ryan et al. (2008) cite the following examples of major outbreaks 

in recent years: 

• Two mountain pine beetle outbreaks affected more than 10 million hectares (Mha) of forest 

in British Columbia (Taylor et al., 2006) and another 267,000 ha in Colorado (Colorado State 

Forest Service, 2007). 

• More than 1.5 Mha of forest was attacked by spruce beetle in southern Alaska and western 

Canada (Berg et al., 2006). 

• Greater than 1.2 Mha of pinyon pine mortality occurred because of extreme drought, coupled 

with an ips beetle outbreak in the Southwest (Breshears et al., 2005). 

• Ecologically important whitebark pine is being attacked by mountain pine beetle in the 

northern and central Rockies (Logan and Powell, 2001). For example, almost 70,000 ha, or 

17%, of whitebark pine forest in the Greater Yellowstone Ecosystem is infested by mountain 

pine beetle (Gibson, 2006). 

Climate plays a major role in driving, or at least influencing, infestations of these important 

forest insect species in the United States (e.g., Holsten et al., 1999; Logan et al., 2003; Carroll et 

al., 2004; Tran et al., in press in Ryan et al., 2008), and the evidence suggests these recent large 

outbreaks are influenced by observed increases in temperature. Specifically, temperature controls 

lifecycle development rates, influences synchronization of mass attacks required to overcome 

tree defenses, and determines winter mortality rates and suitable range (Hansen et al., 2001; 

Logan and Powell, 2001; Hansen and Bentz, 2003; Tran et al., in press in Ryan et al., 2008). In 

addition, warming trends in the United States have also resulted in a longer period of activity for 

insects and proliferation of some species, such as the mountain pine beetle (Easterling et al., 

2007). Changing climatic conditions may also affect insect outbreaks by affecting the overall 

health and productivity of trees. Specifically, drought stress, resulting from decreased 

precipitation and/or warming, reduces the ability of a tree to mount a defense against insect 

attack (Carroll et al., 2004; Breshears et al., 2005 in Ryan et al., 2008). 

Research reviewed by Ryan et al. (2008) suggests that warming temperatures are expected to 

result in an expansion of suitable range for mountain pine beetle (Logan and Powell, 2001) and 

southern pine beetle (Ungerer et al., 1999) and increase the probability of spruce beetle outbreak 

(Logan et al., 2003). Climate change also appears to be encouraging the expansion of non-native 

insects, including the hemlock woolly adelgid (Parker et al., 1999) and the gypsy moth (Logan et 

al., 2003). 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 142
Storm events 

While less is known about the effects of storms, it is expected that tree mortality resulting from 

storms would reduce future carbon sequestration. Ryan et al. (2008) cited recent analyses that 

demonstrated that a single Category 3 hurricane or severe ice storm could each transfer to the 

decomposable pool the equivalent of 10% of the carbon the United States sequesters annually, 

with subsequent reductions in sequestration caused by damage to forest stands (McNulty, 2002; 

H. McCarthy et al., 2006 in Ryan et al., 2008). For example, Hurricanes Rita and Katrina 

together damaged a total of 2,200 ha and 63 million cubic meters of timber volume (Stanturf et 

al., 2007). When decomposed over the next several years, this will release a total of 105 Tg 

carbon into the atmosphere, roughly equal to the annual net sink for U.S. forests (Chambers et 

al., 2007). 

The IPCC (Field et al., 2007) found with high confidence that, across North America, impacts of 

climate change on commercial forestry potential are likely to be sensitive to changes in 

disturbances from insects and diseases, as well as wildfires. 

V.3.e Species composition 

Forest composition 

Climate change and associated changes in disturbance regimes will result in shifts in the 

distributions of tree species and the composition of forest stands. With warming, forests will 

extend further north and to higher elevations. Over currently dry regions, increased precipitation 

may allow forests to displace grasslands and savannas. Changes in forest composition in turn can 

alter the frequencies, intensities, and impacts of disturbances such as fire, insect outbreaks, and 

disease. 

In Alaska and neighboring Arctic regions, there is strong evidence of recent vegetation 

composition change, as outlined by the IPCC (Anisimov et al., 2007 and references therein): 

• Aerial photographs show increased shrub abundance in 70% of 200 locations. 

• Along the Arctic to subarctic boundary, the tree line has moved about 10 km (6 mi) 

northwards, and 2% of Alaskan tundra on the Seward Peninsula has been displaced by forest 

in the past 50 years. 

• The pattern of northward and upward tree line advances is comparable with earlier Holocene 

changes. 

• Analyses of satellite images indicate that the length of the growing season is increasing by 

three days per decade in Alaska. 

Likely rates of migration northward and to higher elevations are uncertain and depend not only 

on climate change, but also on future land use patterns and habitat fragmentation, which can 

impede seed source and species migration. Moreover, given that many plants are long-lived, 

species are expected to continue to persist for some time in their previous ranges despite 

changing conditions. Bioclimatic modeling based on outputs from five GCMs suggests that, over 

the next century, tree species richness will decrease in most parts of the conterminous United 

States, even though long-term trends (millennia) ultimately favor increased richness in some 

locations (Field et al., 2007). The Arctic Climate Impact Assessment (ACIA, 2004) also 

concluded that vegetation zones are projected to migrate northward, with forests encroaching on 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 143
tundra and tundra encroaching on polar deserts. Limitations in amount and quality of soils are 

expected to hinder these poleward shifts. 

Grassland and shrubland composition 

Generally, these ecosystems have a long history of land management including grazing, fire 

suppression, and preferential management for particular species. These activities have a strong 

influence on the current species composition. For example, in some cases fire suppression has 

resulted in encroachment by trees from surrounding forests or upland slopes (e.g., juniper 

encroachment in the sagebrush steppe in the western United States). Research suggests 

increasing atmospheric CO2 levels may promote the conversion of savannas to greater tree 

dominance (Bond et al., 2003). 

In many regions, particularly arid lands of the United States, species composition is being 

markedly affected by invasions of non-native grasses. As noted above, such changes can have 

marked effects on the disturbance regime. Indeed, SAP 4.3 (Ryan et al., 2008) found that in arid 

lands of the United States, non-native grasses often act as ‘transformer species’ (Richardson et 

al., 2000; Grice, 2006) in that they change the character, condition, form, or nature of a natural 

ecosystem over substantial areas. Land use and climate markedly influence the probability, rate, 

and pattern of alien species invasion, and future change in each of these drivers will interact to 

strongly affect scenarios of plant invasion and ecosystem transformation (Sala et al., 2000; 

Walther et al., 2002; Hastings et al., 2005). Plant invasions are strongly influenced by seed 

dispersal and resource availability, but disturbance and abrupt climatic changes also play key 

roles (Clarke et al., 2005). Changes in ecosystem susceptibility to invasion by non-native plants 

may be expected with changes in climate (Ibarra et al., 1995; Mau-Crimmins et al., 2006), CO2

(Smith et al., 2000; Nagel et al., 2004), and nitrogen deposition (Fenn et al., 2003). Elevation 

gradients have also been shown to influence the spread of invasive species. For cheatgrass, a 

common exotic annual in the Great Basin, the rate of invasion is related to temperature at higher 

elevations and soil water availability at lower elevations. Increased variability in soil moisture 

and reductions in perennial herbaceous cover also increased susceptibility of low-elevation sites 

to cheatgrass invasion (Chambers et al., 2007). 

As noted in SAP 4.3 (Ryan et al., 2008), non-native plant invasions, promoted by enhanced 

nitrogen deposition (Fenn et al., 2003) and increased anthropogenic disturbance (Wisdom et al., 

2005), will have a major impact on how arid land ecosystems respond to climate and climate 

change. Once established, non-native annual and perennial grasses can generate massive, highcontinuity fine-fuel loads that predispose arid lands to fires more frequent and intense than those 

with which they evolved. The result is the potential for desert scrub, shrub–steppe, and desert 

grassland/savanna biotic communities to be quickly and radically transformed into monocultures 

of invasive grasses over large areas. This is already well underway in the cold desert region 

(Knapp, 1998) and is in its early stages in hot deserts (Williams and Baruch, 2000; Kupfer and 

Miller, 2005; Salo, 2005; Mau-Crimmins et al., 2006). By virtue of their profound impact on the 

fire regime and hydrology, invasive plants in arid lands are expected to exceed direct climate 

impacts on native vegetation where they gain dominance (Clarke et al., 2005). 

Chronic disturbance, such as grazing, will also affect rates of ecosystem change in response to 

changing climatic conditions because it reduces vegetation resistance to slow, long-term changes 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 144
in climate (Cole, 1985; Overpeck et al., 1990). Plant communities dominated by long-lived 

perennials may exhibit considerable biological inertia, and changes in community composition 

may lag behind significant changes in climate. 

V.4 Water Resources 

Water is vital for all life and is essential to the health and welfare of ecosystems as well as 

human and social systems. Historically, the availability of water has played an important role in 

shaping plant, animal, and human communities. Most of the Earth’s water is contained in the 

oceans; only 3% is freshwater. Of this 3%, a majority (68.7%) is frozen in ice caps and glaciers 

while the remainder is split between groundwater (approximately 30%) and surface water 

(approximately 0.3%). Given water’s importance, plant, animal, and human communities are all 

sensitive to variations in the availability, storage, fluxes, and quality of surface and groundwater. 

These, in turn, are sensitive to climate change. 

In SAP 4.3, Lettenmaier et al. (2008) found that water resource systems have been designed and 

operated to maintain a reliable supply despite the wide variability that can exist in water 

availability, primarily that of surface water, over days to months and years. However, these 

approaches are all based on a similar assumption known as stationarity, in which natural systems 

are assumed to fluctuate within a range of known conditions based on the historical record. 

However, as noted by Arnell (2002), Lettenmaier (2003), Milly et al. (2008), and others, in the 

era of climate change this assumption is no longer tenable. Kundzewicz et al. (2007) found that 

because water infrastructure (e.g., dikes and pipelines) has been designed for stationary climatic 

conditions, the global population is highly vulnerable to climate change impacts on freshwater 

resources. 

The IPCC (Kundzewicz et al., 2007) found that climate change is one of many factors exerting 

pressure on existing freshwater systems. Other factors include water pollution, damming of 

rivers, wetland drainage, reduction in streamflow, and lowering of the groundwater table (e.g., 

due to irrigation). The authors conclude that while climate-related changes have been small 

compared to these other pressures to date, climate change is expected to result in increasing 

effects in the future. Ultimately, each of these factors influences the availability of and access to 

freshwater. In this section, we review effects of global change on water supply, water quality, 

and extreme events, and explore the implications for water use. 

In regards to the hydrologic observing systems on which these sections are based, Lettenmaier et 

al. (2008) found that the current hydrologic observing system was not designed specifically for 

the purpose of detecting the effects of climate change on water resources. In many cases, the 

resulting data are unable to meet the predictive challenges of a rapidly changing climate. 

Overall, the IPCC (Kundzewicz et al., 2007) made the following conclusions: 

• The negative impacts of climate change on freshwater systems outweigh the benefits (high 

confidence). 

• All IPCC regions show an overall net negative impact of climate change on water resources 

and freshwater ecosystems (high confidence). 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 145
• Areas in which runoff is projected to decline are likely to face a reduction in the value of the 

services provided by water resources (very high confidence). 

• The beneficial impacts of increased annual runoff in other areas will be tempered by the 

negative effects of increased precipitation variability and seasonal runoff shifts on water 

supply, water quality, and flood risks (high confidence). 

• The impacts of climate change on freshwater systems and their management are mainly due 

to the observed and projected increases in temperature, sea level, and precipitation variability 

(very high confidence). 

• More than one-sixth of the world’s population lives in glacier- or snowmelt-fed river basins 

and will be affected by the seasonal shift in streamflow, an increase in the ratio of winter to 

annual flows, and possibly the reduction in low flows caused by decreased glacier extent or 

snow water storage (high confidence). 

V.4.a Water supply and precipitation 

Surface water and precipitation 

The primary driver of the land surface hydrologic system is precipitation. Current vulnerabilities 

of the system are strongly tied to precipitation variability. Observed trends in both are reviewed 

here. In the IPCC Fourth Assessment Report, Kundzewicz et al. (2007) found that climateinduced changes in river flows and lake and wetland levels depend on changes in the volume, 

timing, and intensity of precipitation and snowmelt, as well as the amount of precipitation falling 

as snow or rain (Chiew, 2007). Precipitation is expected to increase globally with important 

regional variations. Current observations show increased precipitation over land north of 30° N 

from 1901 to 2005 and decreases over land between 10° S and 30° N after the 1970s. Snow 

cover and glaciers are decreasing in most regions, particularly in spring. Permafrost is thawing 

between 0.02 m per year (Alaska) and 0.4 m per year (Tibetan Plateau). In many cases, 

precipitation may be more variable, with increases in one season followed by decreases in 

another. Intensified droughts have been observed in some drier regions since the 1970s. Changes 

in evapotranspiration driven by temperature, radiation, atmospheric humidity, increased 

atmospheric CO2 concentration, and wind speed can offset small increases in precipitation and 

intensify the effects of decreased precipitation on surface waters. 

As noted in Kundzewicz et al. (2007), effects vary across catchments depending on specific 

physical, hydrological, and geological characteristics. Current observations have identified 

highly variable streamflows globally, with increases in some basins and declines in others. 

Warming temperatures have resulted in earlier spring peak flows and increased winter base flows 

in North America and Eurasia. Catchments that are already stressed due to non-climatic drivers 

are highly vulnerable to additional impacts. In addition, vulnerability to precipitation variability 

is highest in semi-arid and arid low-income countries, where precipitation and streamflow are 

concentrated over a few months, and where year-to-year variations are high (Lenton, 2004). 

Without adequate storage infrastructure, these regions are highly vulnerable to current climate 

variability and expected increases in variability with future climate change. 

Human water use has resulted in reduced water levels in many lakes worldwide. In some cases, 

declining precipitation was also a significant cause (e.g., in the case of Lake Chad, where both 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 146
decreased precipitation and increased human water use account for the observed decrease in lake 

area since the 1960s) (Coe and Foley, 2001 in Kundzewicz et al., 2007). For the many lakes, 

rivers, and wetlands that have shrunk mainly due to human water use and drainage, climate 

change is expected to exacerbate the situation if it results in reduced net water availability 

(precipitation minus evapotranspiration). Observations have also identified warming lake 

temperatures and reduction in ice cover on lakes worldwide. 

Surface water availability and precipitation differ greatly across the United States. Generally, 

conditions become increasingly dry from east to west. However, conditions in the upslope areas 

of the Cascade and coastal mountain ranges, especially in the Pacific Northwest, are much more 

humid (Lettenmaier et al., 2008). The driest climates occur in the Intermountain West and the 

Southwest. Precipitation variability follows similar trends with less variability in the humid areas 

(eastern United States and Pacific Northwest) and the greatest variability in the arid and semiarid West (Lettenmaier et al., 2008). The IPCC (Kundzewicz et al., 2007) concluded with high 

confidence that semi-arid and arid areas are particularly exposed to the impacts of climate change 

on freshwater. 

Specific to the United States, the IPCC identified the following trends in surface water supply, 

precipitation patterns, and snowpack (Field et al., 2007): 

• Streamflow in the eastern United States has increased 25% in the last 60 years (Groisman et 

al., 2004), but over the last century has decreased by about 2% per decade in the central 

Rocky Mountain region (Rood et al., 2005). 

• Since 1950, stream discharge in both the Colorado and Columbia River Basins has 

decreased, while over the same time period annual evapotranspiration from the conterminous 

United States increased by 55 mm (Walter et al., 2004). 

• In regions with winter snow, warming has shifted the magnitude and timing of hydrologic 

events (Mote et al., 2005; Regonda et al., 2005; Stewart et al., 2005). The fraction of annual 

precipitation falling as rain (rather than snow) increased at 74% of the weather stations 

studied in the western mountains of the United States from 1949 to 2004 (Knowles et al., 

2006). 

• In Canada, warming from 1900 to 2003 led to a decrease in total precipitation as snowfall in 

the West and on the Prairies (Vincent and Mekis, 2006). 

• Spring and summer snow cover has decreased in the U.S. West (Groisman et al., 2004). April 

snow water equivalent has declined 15 to 30% since 1950 in the western mountains of North 

America, particularly at lower elevations and primarily due to warming rather than changes 

in precipitation (see Mote et al., 2003, 2005; Lemke et al., 2007). 

• Streamflow peaks in the snowmelt-dominated western mountains of the United States 

occurred one to four weeks earlier in 2002 than in 1948 (Stewart et al., 2005). 

• Breakup of river and lake ice across North America has advanced by 0.2 to 12.9 days over 

the last 100 years (Magnuson et al., 2000). 

As reported in Lettenmaier et al. (2008), Mauget (2003) analyzed data from 167 stream gauge 

stations over the period 1939 to 1998 and found increasing streamflows over time in the eastern 

United States with a more or less reverse pattern in the western United States, with an onset of 

dry conditions beginning in the 1980s. Similar to global observations, U.S. effects will vary from 

region to region. 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 147
As for the Arctic, research reviewed in the Arctic Climate Impact Assessment concluded that 

precipitation has increased by about 1% per decade over the past century although the results are 

highly variable spatially (ACIA, 2005). Much of the increase has fallen as rain, with the largest 

increases occurring in autumn and winter. The ice season has been reduced by one to three 

weeks in some areas from a combination of later freeze-up and earlier breakup of river and lake 

ice. Glaciers throughout North America are melting. Alaskan glaciers are melting particularly 

fast and represent about half of the estimated loss of glacial mass worldwide (ACIA, 2004). 

Permafrost plays an important role in providing an impermeable surface and forming lakes and 

ponds. The spatial pattern of lake disappearance strongly suggests that permafrost thawing is 

driving the changes. In the Arctic, river discharge to the ocean has increased during the past few 

decades, and peak flows in the spring are occurring earlier. These changes are projected to 

accelerate with future climate change. Snow cover extent in Alaska is projected to decrease by 

10 to 20% by the 2070s, with greatest declines in spring (ACIA, 2004). 

Projected impacts 

Reviewing the research on these trends, Lettenmaier et al. (2008) drew the following conclusions 

in SAP 4.3: 

• There is a trend toward reduced mountain snowpack and earlier spring snowmelt runoff 

peaks across much of the western United States. Evidence suggests this trend is very likely 

attributable, at least in part, to long-term warming, although some part may have been played 

by decadal-scale variability, including a shift in the PDO in the late 1970s. Where shifts to 

earlier snowmelt peaks and reduced summer and fall low flows have already been detected, 

continuing shifts in this direction are expected and may have substantial impacts on the 

performance of reservoir systems. 

• The most recent (IPCC Fourth Assessment Report) climate model simulations project 

increased runoff over the eastern United States, gradually transitioning to little change in the 

Missouri and lower Mississippi, to substantial decreases in annual runoff in the interior of the 

West (Colorado and Great Basin). The projected drying in the interior of the West is quite 

consistent among models. These changes are, very roughly, consistent with observed trends 

in the second half of the 20th century, which show increased streamflow over most of the 

United States, but sporadic decreases in the West. 

• Snowpacks in the mountainous headwaters regions of the western United States generally 

declined over the second half of the 20th century, especially at lower elevations and in 

locations where average winter temperatures are close to or above 0 °C. These trends toward 

reduced winter snow accumulation and earlier spring melt are also reflected in a tendency 

toward earlier runoff peaks in the spring, a shift that has not occurred in rainfall-dominated 

watersheds in the same region. 

• Climate model projections of increased temperatures and slight precipitation increases 

indicate that modest streamflow increases are expected in the East, but that larger (in 

absolute value) declines are expected in the West, where the balance between precipitation 

and evaporative demand changes will be dominated by increased evaporative demand. 

However, because of the uncertainty in climate model projections of precipitation change, 

future projections of streamflow are highly uncertain across most of the United States. One 

exception is watersheds that are dominated by spring and summer snowmelt, most of which 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 148
are in the western United States. In these cases, where shifts to earlier snowmelt peaks and 

reduced summer and fall low flows have already begun to be detected, continuing shifts in 

this direction are generally expected and may have substantial impacts on the performance of 

reservoir systems. 

V.4.b Groundwater 

In SAP 4.3, Lettenmaier et al. (2008) found that relatively few studies have assessed the 

sensitivity of groundwater systems to a changing climate. Reviewing the available literature 

(e.g., Vaccaro, 1992; Loaiciga et al., 2000; Hanson and Dettinger, 2005; Scibek and Allen, 2006; 

Gurdak et al., 2007), the authors concluded that the ability to predict the effects of climate and 

climate change on groundwater systems is nowhere near as advanced as for surface water 

systems. 

The available research suggests that groundwater systems generally respond more slowly to 

climate change than surface water systems. In general, groundwater levels correlate most 

strongly with precipitation, but temperature becomes more important for shallow aquifers, 

especially during warm periods. With climate change, availability of groundwater is expected to 

be influenced by changes in withdrawals (reflecting development, human and agricultural 

demand, and availability of other sources) and recharge (determined by temperature, timing, and 

amount of precipitation, and surface water interactions) (medium confidence) (Kundzewicz et al., 

2007). In general, simulated aquifer levels respond to changes in temperature, precipitation, and 

the level of withdrawal. 

According to the IPCC, base flows were found to decrease in scenarios that are drier or have 

higher pumping rates, and increase in wetter scenarios on average across world regions 

(Kundzewicz et al., 2007). The IPCC projects that efforts to offset declining surface water 

availability due to increasing precipitation variability will be hampered by the fact that 

groundwater recharge will decrease considerably in some already water-stressed regions (high 

confidence) where vulnerability is often exacerbated by the rapid increase in population and 

water demand (very high confidence). This is expected to be particularly acute in some waterstressed regions, such as the southwestern United States. Projections for the Ogallala aquifer 

region show that natural groundwater recharge decreases more than 20% in all simulations with 

different climate models and future warming scenarios of 2.5 °C or greater (Field et al., 2007). 

Groundwater resources can also be adversely impacted in coastal areas by saltwater intrusion 

from sea level rise. In addition, they conclude that sea level rise will extend areas of salinization 

of groundwater and estuaries, resulting in a decrease in freshwater availability for humans and 

ecosystems in coastal areas (very high confidence) (Kundzewicz et al., 2007). 

V.4.c Water quality 

(adapted largely from Lettenmaier et al., 2008) 

Water quality reflects the chemical inputs from the air and landscape and their biogeochemical 

transformation within the water (Murdoch et al., 2000). A warming climate may influence water 

quality through increased water temperatures and modification of regional patterns of 

precipitation. However, most water quality changes observed so far across the continental United 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 149
States are likely attributable to causes other than climate change (Lettenmaier et al., 2008). 

Attribution of changes in water quality to climate impacts is difficult as water quality is sensitive 

to other human activities, particularly land use practices that alter landscapes and modify the flux 

of water as well as its thermal and nutrient characteristics. 

Lettenmaier et al. (2008) found that two main factors that influence water quality are temperature 

and water quantity. Higher temperatures enhance rates of biogeochemical transformation and 

physiological processes of aquatic plants and animals. As temperatures increase, the ability of 

water to hold dissolved oxygen declines, with potential negative impacts on aquatic organisms. 

High nutrient loads can also contribute to anoxic conditions. Increased streamflow can dilute 

nutrient concentrations and thus diminish excessive biological production. However, higher 

flows can flush excess nutrients from sources of origin in a stream. The overall balance of these 

competing effects in a changing climate is not yet known. 

Most studies examining the responses of water quality over time have focused on nutrient 

loading. Ramstack et al. (2004) reconstructed water chemistry before European settlement for 55 

Minnesota lakes. They found that lakes in forested regions showed very little change in water 

quality since 1800. By contrast, about 30% of urban lakes and agricultural lakes showed 

significant increases in chloride (urban) or phosphorus (agricultural). These results indicate the 

strong influence of land use on water quality indicators. 

Recent historical assessments of changes in water quality due to temperature trends have largely 

focused on salmonid fishes in the western United States (e.g., Bartholow, 2005; Crozier and 

Zabel, 2006 in Lettenmaier et al., 2008). Increases in water temperature can influence salmon by 

negatively affecting different life stages. This research suggests that smaller snowpacks that 

reduce autumn flows and cause higher water temperatures are expected to reduce salmon 

survival. Petersen and Kitchell (2001) found that warmer water temperatures are also associated 

with an expected higher mortality rate of young salmon due to fish predators. 

Studies reviewed by the IPCC (Field et al., 2007; Kundzewicz et al., 2007) provide the following 

conclusions. In lakes and reservoirs, climate change effects are primarily caused by water 

temperature variations (caused by climate change or thermal pollution as a result of higher 

demand for cooling water in the energy sector) affecting dissolved oxygen regimes, redox 

potentials,47 lake stratification, mixing rates, and the development of aquatic biota. In addition, 

increasing water temperature reduces the self-purification capacity of rivers by decreasing the 

amount of dissolved oxygen available for biodegradation. Simulations of future North American 

surface and bottom water temperatures of lakes, reservoirs, rivers, and estuaries consistently 

show increases of 2 °C to 7 °C, with summer surface temperatures exceeding 30 °C in 

midwestern and southern lakes and reservoirs. The IPCC projects that warming is likely to 

extend and intensify summer thermal stratification in surface waters, further contributing to 

oxygen depletion (Field et al., 2007 and references therein). 

Higher water temperatures, increased precipitation intensity, and longer periods of low flows 

exacerbate many forms of water pollution and can affect ecosystems, human health, and water 

system reliability and operating costs. Pollutants of concern in this case include sediment, 

 

47 Redox potential is defined as the tendency of a chemical species to acquire electrons and therefore be reduced. 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 150
nutrients, organic matter, pathogens, pesticides, salt, and thermal pollution (Kundzewicz et al., 

2007). Elevated surface water temperatures will promote algal blooms and increases in bacteria 

and fungi levels. The frequency of heavy precipitation events in the United States have increased 

through the 1990s (Field et al., 2007). Increases in intense rain events result in the introduction 

of more sediment, nutrients, pathogens, and toxics into water bodies from non-point sources. 

Increasing nutrient and sediment loads (due to more intense runoff events) will negatively affect 

water quality, possibly rendering a source unusable unless special treatment is introduced. 

Intense rainfall will lead to increases in suspended solids (turbidity) and pollutant levels in water 

bodies due to soil erosion (Kundzewicz et al., 2007). Even with enhanced phosphorus removal in 

wastewater treatment plants, algal growth in water bodies may increase with warming over the 

long term. Conditions are exacerbated during low flow conditions where small water quantities 

result in less dilution and greater concentrations of pollutants. In addition, fluctuating levels of 

lakes and reservoirs can reduce water quality through resuspension of bottom sediments and 

liberating compounds (Field et al., 2007). 

The IPCC reviewed a number of region-specific studies on U.S. water quality and made the 

following projections (Field et al., 2007; Kundzewicz et al., 2007): 

• Changes in precipitation may increase nitrogen loads from rivers in the Chesapeake and 

Delaware Bay regions by up to 50% by 2030 (Kundzewicz et al., 2007). 

• Decreases in snow cover and increases in winter rain on bare soil will likely lengthen the 

erosion season and enhance erosion intensity. This will increase the potential for sedimentrelated water quality impacts in agricultural areas (Field et al., 2007). 

• Increased rainfall amounts and intensities will lead to greater rates of erosion within the 

United States and in other regions unless protection measures are taken (Kundzewicz et al., 

2007). Soil management practices (e.g., crop residue, no-till) in some regions (e.g., the Corn 

Belt) may not provide sufficient erosion protection against future intense precipitation and 

associated runoff (Field et al., 2007). 

• For the Midwest, in simulated low flows used to develop pollutant discharge limits (Total 

Maximum Daily Loads) flows decrease over 60% with a 25% decrease in mean precipitation, 

reaching up to 100% with the incorporation of irrigation demands (Eheart et al., 1999). 

• Restoration of beneficial uses (e.g., to address habitat loss, eutrophication, beach closures) 

under the Great Lakes Water Quality Agreement will likely be vulnerable to declines in water 

levels, warmer water temperatures, and more intense precipitation (Mortsch et al., 2003). 

• Based on simulations, phosphorus remediation targets for the Bay of Quinte (Lake Ontario) 

and the surrounding watershed could be compromised as 3 to 4 °C warmer water 

temperatures contribute to 77 to 98% increases in summer phosphorus concentrations in the 

bay (Nicholls, 1999), and as changes in precipitation, streamflow, and erosion lead to 

increases in average phosphorus concentrations in streams of 25 to 35% (Walker, 2001). 

The IPCC (Kundzewicz et al., 2007) concluded that, globally, higher water temperatures, 

increased precipitation intensity, and longer periods of low flows exacerbate many forms of 

water pollution, affecting ecosystems, human health, and water system reliability and operating 

costs (high confidence). For North America, they also concluded that climate change is likely to 

make it more difficult to achieve existing water quality goals for sediment (high confidence) 

(Field et al., 2007). 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 151
V.4.d Extreme events 

Higher temperatures increase the water-holding capacity of the atmosphere and encourage 

greater evaporation, resulting in conditions that favor increased climate variability, with more 

intense precipitation and more droughts (Trenberth et al., 2003; Milly et al., 2008). A changing 

climate can influence the occurrence of floods and droughts. Many climatic and non-climatic 

drivers influence flood and drought impacts, including intense and/or long-lasting precipitation 

events, rapid snowmelt, dam failure, or reduced conveyance due to ice jams or landslides. Flood 

magnitude and spatial extent depend on the intensity, volume, and time of precipitation, and the 

antecedent conditions of rivers and their drainage basins (e.g., presence of snow and ice, soil 

composition, level of human development, existence of prevention infrastructure) (Kundzewicz 

et al., 2007). 

Research reviewed by the IPCC (Kundzewicz et al., 2007) suggested that while research to date 

has not provided clear evidence for a climate-related trend in floods during the last decades, there 

is suggestive evidence that floods may have been affected by the observed increase in 

precipitation intensity and other observed climate changes (e.g., an increase in westerly weather 

patterns during winter over Europe) that lead to very rainy low-pressure systems that often 

trigger floods (Kron and Berz, 2007). Globally, the number of great inland flood catastrophes 

during the last 10 years (between 1996 and 2005) is twice as large, per decade, as between 1950 

and 1980, while economic losses have increased by a factor of five (Kron and Berz, 2007). The 

increase in flood damage is primarily driven by an increase in population and wealth in 

vulnerable areas and changes in land use. Floods have been the most reported natural disaster 

events in Africa, Asia, and Europe, and have affected more people across the globe (140 million 

per year on average) than all other natural disasters (WDR, 2003, 2004). In Bangladesh, three 

extreme floods have occurred in the last two decades, and in 1998, about 70% of the country’s 

area was inundated (Mirza, 2003; Clarke and King, 2004). In some river basins (e.g., the Elbe 

River Basin in Germany), increasing flood risk drives the strengthening of flood protection 

systems by structural means, with detrimental effects on riparian and aquatic ecosystems 

(Wechsung et al., 2005). 

Globally, increased intensity and variability of precipitation is projected to increase the risks of 

flooding and drought in many areas (high confidence). Many of these areas (e.g., Mediterranean 

Basin, western United States, southern Africa, and northeastern Brazil) will suffer a decrease in 

water resources due to climate change (very high confidence) (Kundzewicz et al., 2007). 

In the United States, precipitation intensity will increase across the country, particularly at 

middle and high latitudes where mean precipitation also increases. This will affect the risk of 

flash flooding and urban flooding in these regions (Kundzewicz et al., 2007). In general, 

projected changes in precipitation extremes are larger than changes in mean precipitation (Field 

et al., 2007). 

Some studies project widespread increases in extreme precipitation with greater risks of not only 

flooding from intense precipitation, but also droughts from greater temporal variability in 

precipitation (Christensen et al., 2007). Droughts affect rain-fed agricultural production as well 

as water supply for domestic, industrial, and agricultural purposes. Globally, some semi-arid and 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 152
sub-humid regions (e.g., Australia, western United States, southern Canada, and the Sahel) have 

suffered from more intense and multi-annual droughts, highlighting the vulnerability of these 

regions to the increased drought occurrence that is expected in the future due to climate change 

(Nicholson, 2005). 

Across North America, vulnerability to extended drought is increasing as population growth and 

economic development create more demands from agricultural, municipal, and industrial uses, 

resulting in frequent over-allocation of water resources (Alberta Environment, 2002; Morehouse 

et al., 2002; Postel and Richter, 2003; Pulwarty et al., 2005 in Field et al., 2007). While much of 

the United States has experienced increases in precipitation and streamflow, there is some 

evidence of long-term drying and increases in drought severity and duration in the West and 

Southwest. This is probably due to a combination of decadal-scale climate variability and longterm change (Lettenmaier et al., 2008). However, the eastern regions are also vulnerable to 

droughts and attendant reductions in water supply, changes in water quality and ecosystem 

function, and challenges in allocation (Dupigny-Giroux, 2001; Bonsal et al., 2004; Wheaton et 

al., 2005). 

Projections for the western mountains of the United States suggest that warming, and changes in 

the form, timing, and amount of precipitation will very likely lead to earlier melting and 

significant reductions in snowpack by the middle of the 21st century (high confidence; Field et 

al., 2007). In mountainous snowmelt-dominated watersheds, projections suggest advances in the 

timing of snowmelt runoff, increases in winter and early spring flows (raising flooding 

potential), and substantially decreased summer flows. Heavily utilized water systems of the 

western United States that rely on capturing snowmelt runoff, such as the Columbia River 

system, will be especially vulnerable (Field et al., 2007). Reduced snowpack has been identified 

as a major concern for the State of California (California Energy Commission, 2006b). 

In SAP 4.3, Lettenmaier et al. (2008) found that U.S. consumptive use of water per capita has 

declined over the last two decades, primarily as a result of various improvements in water use 

efficiency related both to legal mandates and to water pricing, as well as some changes in water 

laws that have facilitated reallocation of water, particularly in the western United States and 

during droughts. Trends toward increased water use efficiency seem likely to continue in the 

coming decades. Pressures for reallocation of water will be greatest in areas of highest 

population growth, such as the Southwest. Declining water consumption, if it continues, will 

help mitigate the impacts of climate change on water resources. 

There is evidence that much more severe droughts have occurred in North America prior to the 

instrumental record of roughly the last 100 years (Lettenmaier et al., 2008). For instance, 

Woodhouse and Overpeck (1998), using paleoclimatic indicators (primarily tree rings), find that 

many droughts over the last 2,000 years have eclipsed the major U.S. droughts of the 1930s and 

1950s, with much more severe droughts occurring as recently as the 1600s. Although the nature 

of future drought stress remains unclear, for those areas where climate models suggest drying, 

such as the Southwest (e.g., Seager et al., 2007), droughts that are more severe than those 

encountered in the instrumental record may become increasingly likely. 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 153
In addition to the effects on water supply, extreme events, such as floods and droughts, are 

expected to reduce water quality. Increased erosion and runoff rates during flood events will 

wash pollutants (e.g., organic matter, fertilizers, pesticides, and heavy metals) from soils into 

water bodies, subsequently affecting species and ecosystems. During drought events, the lack of 

precipitation and subsequent low flow conditions will impair water quality by reducing the 

amount of water available to dilute pollutants. These effects from floods and droughts will make 

it more difficult to achieve pollutant discharge limits and water quality goals (Kundzewicz et al., 

2007). 

V.4.e Implications for water users 

On global water use, the IPCC (Kundzewicz et al., 2007) drew the following conclusions: 

• Climate change affects the function and operation of existing water infrastructure as well as 

water management practices (very high confidence). 

• Adverse effects of climate on freshwater systems aggravate the impacts of other stresses, 

such as population growth, changing economic activity, land use change, and urbanization 

(very high confidence). 

• Globally, water demand will grow in the coming decades, primarily due to population growth 

and increased affluence. Regionally, large changes in irrigation water demand as a result of 

climate change are likely (high confidence). 

• Current water management practices are very likely to be inadequate to reduce the negative 

impacts of climate change on water supply reliability, flood risk, health, energy, and aquatic 

ecosystems (very high confidence). Improved incorporation of current climate variability into 

water-related management would make adaptation to future climate change easier (very high 

confidence). 

In the United States, many competing water uses will be adversely affected by climate change 

impacts on water supply and quality. The IPCC reviewed a number of studies describing the 

impacts of climate change on water uses in the United States that showed the following: 

• Decreased water supply and lower water levels are likely to exacerbate challenges 

relating to navigation in the United States (Field et al., 2007). Some studies have found 

that low flow conditions may restrict ship loading in shallow ports and harbors 

(Kundzewicz et al., 2007). However, navigational benefits from climate change exist as 

well. For example, the navigation season for the Northern Sea Route is projected to 

increase from the current 20 to 30 days per year to 90 to 100 days by 2080 (ACIA, 2004). 

• Climate change impacts on water supply and quality will affect agricultural practices, 

including the increase in irrigation demand in dry regions and the aggravation of nonpoint source water pollution (e.g., pollution from urban areas, roads, or agricultural 

fields) problems in areas susceptible to intense rainfall events and flooding (Field et al., 

2007). 

• The U.S. energy sector, which relies heavily on water for generation (hydropower) and 

cooling capacity, will be adversely impacted by changes in water supply and quality in 

reservoirs and other water bodies (Wilbanks et al., 2007a). 

Less reliable supplies of water are expected to create challenges for managing urban water 

systems as well as for industries that depend on large volumes of water. U.S. water managers 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 154
currently anticipate local, regional, or statewide water shortages over the next 10 years. Threats 

to reliable supply are complicated by high population growth rates in western states where many 

resources are at or approaching full utilization. The IPCC (Field et al., 2007) reviewed several 

regional-level studies of climate change impacts on U.S. water management that showed the 

following: 

• In the Great Lakes–St. Lawrence Basin, many, but not all, assessments project lower net 

basin supplies and lake water levels. Lower water levels are likely to influence many sectors, 

with multiple, interacting impacts. These impacts are projected with high confidence, in 

which atmosphere–lake interactions contribute to the uncertainty in assessment. 

• Urban water supply systems in North America often draw water from considerable distances, 

so climate impacts need not be local to affect cities. By the 2020s, 41% of the water supply to 

southern California is likely to be vulnerable due to snowpack loss in the Sierra Nevada 

mountains and the Colorado River Basin. 

• The New York area will likely experience greater water supply variability. New York City’s 

system can likely adapt to future changes, but the region’s smaller systems may be 

vulnerable, leading to a need for enhanced regional water distribution plans. 

Drawing on these studies, the IPCC concluded that climate change will constrain North 

America’s over-allocated water resources, increasing competition among agricultural, municipal, 

industrial, and ecological uses (very high confidence) (Field et al., 2007). Rising temperatures 

will diminish snowpack and increase evaporation, affecting seasonal availability of water. 

Higher demand from economic development, agriculture, and population growth will further 

limit surface and groundwater availability. In the Great Lakes and major river systems across the 

United States, lower water levels are likely to exacerbate challenges relating to water quality, 

navigation, recreation, hydropower generation, water transfers, and bi-national relationships. 

In the Arctic, river discharge to the ocean has increased during the past few decades, and peak 

flows in the spring are occurring earlier. These changes are projected to accelerate with future 

climate change. Snow cover extent in Alaska is projected to decrease by 10 to 20% by the 2070s, 

with greatest declines in spring (ACIA, 2004). 

The IPCC concluded with high confidence that under most climate change scenarios, water 

resources in small islands around the globe are likely to be seriously compromised (Mimura et 

al., 2007). Most small islands have a limited water supply, and water resources in these islands 

are especially vulnerable to future changes and distribution of rainfall. Reduced rainfall typically 

leads to decreased surface water supply and slower recharge rates of the freshwater lens,48 which 

can result in prolonged drought impacts. Many islands in the Caribbean (which include the U.S. 

territories of Puerto Rico and U.S. Virgin Islands) are likely to experience increased water stress 

as a result of climate change. Under all SRES scenarios, reduced rainfall in summer is projected 

for the Caribbean, making it unlikely that the demand for water resources will be met. Increased 

rainfall in winter is unlikely to compensate for these water deficits due to lack of storage capacity 

(Mimura et al., 2007). 

 

48 A freshwater lens is defined as a relatively thin layer of freshwater within island aquifer systems that floats on an 

underlying mass of denser seawater. Numerous factors control the shape and thickness of the lens, including the rate 

of recharge from precipitation, island geometry, and geologic features such as the permeability of soil layers. 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 155
V.5 Social Systems and Settlements 

In their review of industry, settlement, and society, the IPCC found (Wilbanks et al., 2007a) that 

human systems include social, economic, and institutional structures and processes. These 

systems are influenced by multiple factors and stresses (e.g., access to financial resources, 

urbanization, demographic shifts) and weather and climate are often viewed as secondary 

benefits (Ocampo and Martin, 2003; Thomas and Twyman, 2005). Effects from a changing 

climate will be felt in settlements through interactions with these other factors, easing or 

aggravating multiple stresses and, potentially, pushing stressed systems across a threshold of 

sustainability (Wilbanks, 2003b in Wilbanks et al., 2007a). On the other hand, some U.S. 

settlements may find opportunities in climate change. For example, extended growing seasons 

may allow for greater agricultural productivity in some areas, while warmer winters could reduce 

cold exposure in northern states. 

Climate sensitivity varies across settlements and industrial sectors. As noted by the IPCC, while 

it may appear that industrialized countries are well equipped to cope with gradual climate change 

at a national level, at a local level there may be substantial variability in climate effects and 

capacities to adapt (Environment Canada, 1997; Kates and Wilbanks, 2003; London Climate 

Change Partnership, 2004; O’Brien et al., 2004; Kirshen et al., 2006). According to Wilbanks et 

al. (2007a), “Industries, settlements and human society are accustomed to variability in 

environmental conditions, and in many ways they have become resilient to it when it is a part of 

their normal experience. Environmental changes that are more extreme or persistent than that 

experience, however, can lead to vulnerabilities, especially if the changes are not foreseen and/or 

if capacities for adaptation are limited.” Moreover, vulnerability is greater when changes occur 

rapidly, resulting in a limited ability to plan for or cope with impacts (Gamble et al., 2008). 

However, effects from a changing climate can be positive as well as negative. 

The IPCC identified three primary ways that climate and climate change affect human systems 

(Wilbanks et al., 2007a): 

• Climate provides a context for a variety of climate-sensitive human activities ranging from 

agriculture to tourism. For instance, rivers fed by rainfall enable irrigation and transportation 

and can enrich or damage landscapes. 

• Climate affects the cost of maintaining climate-controlled internal environments for human 

life and activity. Clearly, higher temperatures increase costs of cooling and reduce costs of 

heating. See additional discussion of energy effects in Section V.7. 

• Climate interacts with other types of stresses on human systems, in some cases reducing 

stresses but in other cases exacerbating them. For example, drought can contribute to rural–

urban migration, which, combined with population growth, increases stress on urban 

infrastructures and socioeconomic conditions. 

To date, relatively limited research has been conducted specifically on the effects of climate 

change on U.S. settlements. SAP 4.6 drew on the available research to begin to examine 

emerging findings and identify areas and issues that merit further discussion. Accordingly, this 

section draws heavily from their analysis. Findings from the IPCC are also illustrated in the 

sections below. 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 156
V.5.a U.S. population trends 

An overview of population trends and settlement patterns provides a framework to understand 

impacts of global change on social systems, settlements, and welfare. In SAP 4.6, Gamble et al. 

(2008) found that a changing climate, interacting with changes in land use and demographics, 

has the capacity to affect important human activities. This section begins by briefly exploring 

demographic trends expected to be affected by changes in temperature, precipitation, and 

extreme climate-related events. 

The U.S. Census Bureau (2000a,b, 2002, 2005, 2007) provides the following description of 

national population trends: 

• Since 1980, the U.S. population has grown by more than 40 million. 

• This growth has been unevenly distributed around the Nation. More than 500 U.S. counties 

lost population over this time frame (with a total reduction of more than 2 million people). In 

addition, a small number of counties (40) accounted for more than half the growth (from 

either migration or natural increase). 

• Over the next 25 years the United States is expected grow by more than 60 million people. 

This growth is also expected to be distributed unevenly across the country. Seven states are 

expected to account for more than two-thirds of this growth (Florida, Texas, California, 

Arizona, North Carolina, Georgia, and Washington). 

• The majority of the total growth is also expected to take place in large urban areas and 

coastal counties, while many rural and urban fringe counties are expected to experience rapid 

growth in percentage terms. 

• The U.S. population age 65 and over is expected to double in size within the next 25 years. 

By 2030, almost 1 out of 5 Americans—some 72 million people—will be 65 years or older. 

The age group 85 and older is now the fastest-growing segment of the U.S. population. 

The greatest population growth between 2000 and 2005 occurred in western and southern states. 

The quickest growing states were Nevada (20.8%), Arizona (15.8%), Florida (11.3%), Georgia 

(10.8%), and Utah (10.6%). The growth in western states is expected to continue. Indeed, the 11 

western states are expected to account for one-third of all U.S. population growth over the next 

25 years (U.S. Census Bureau, 2005). One of the most pronounced trends over the past few 

decades has been movement toward coastal areas. This trend is also expected to continue in the 

future. In the following sections we explore interactions between these demographic trends and 

changing climatic variables. 

V.5.b Vulnerable communities 

Vulnerability to climate changes varies across population segments and settlement areas. Some 

will be more sensitive to climate effects or ill equipped to undertake the necessary steps to adapt. 

This section addresses these issues from the community level, while Section V.6 emphasizes 

potential impacts on human health. 

Vulnerability to or opportunities from climate change are related to three factors (Clark et al., 

2000): 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 157
• Exposure: What effects will a place be exposed to (e.g., temperature or precipitation changes, 

changes in storm exposure and/or intensity, and changes in the sea level)? 

• Sensitivity: If primary climate changes were to occur, how sensitive are the activities and 

populations of a settlement to those changes? For instance, a city dependent substantially on 

a regional agricultural or forestry economy might be considered more sensitive than a city 

whose economy is based mainly on an industrial sector less sensitive to climate variations. 

• Adaptive capacity: If effects occur, what is the capability of a settlement to handle those 

impacts without disabling damages, perhaps even with new opportunities? 

Vulnerability to the impacts of climate change is expected to vary across regions and settlements. 

Indeed, in many cases, vulnerability will vary based on specific characteristics of the settlement 

(Wilbanks et al., 2008). For example, some settlements are particularly vulnerable because of 

their location on or near coasts subject to storms and sea level rise or in areas at risk of other 

extreme events (e.g., wildfires). Some are already stressed by other forces that might interact 

with climate change effects, such as rapid population growth, aging physical infrastructures, 

poverty, and social friction. Some are considerably dependent on ecosystem services (e.g., 

snowpack as a source for water resources) or linkage systems, such as bridges or electric power 

lines, which could be vulnerable to impacts of climate change. 

However, in SAP 4.6 Wilbanks et al. (2008) identify the following key concerns that transcend 

location: 

• Effects on health. Higher temperatures in urban areas are related to higher concentrations of 

ground-level ozone, which has the potential to cause respiratory problems if not controlled. 

There is also some evidence that combined effects of heat stress and air pollution may be 

greater than simple additive effects (Patz and Balbus, 2001). Moreover, historical data show 

relationships between mortalities and temperature extremes (Rozenzweig and Solecki, 2001). 

Other health concerns include changes in exposure to water- and food-borne diseases, vectorborne diseases, concentrations of plant species associated with allergies, and exposures to 

extreme weather events such as storms, floods, and fires (discussed further in Section V.6). 

• Effects on water and other urban infrastructures. Changes in precipitation patterns may lead 

to reductions in snowpack, river flows, groundwater levels, and in coastal areas to saline 

intrusion in rivers and groundwater, affecting water supply. In addition, warming may 

increase water demands (Kirshen, 2002; IPCC, 2007b). Moreover, storms, floods, and other 

severe weather events may affect other infrastructures, such as sanitation, transportation, 

supply lines for food and energy, and communication. Exposed structures such as bridges 

and electricity transmission networks are especially vulnerable. In many cases, 

infrastructures are interconnected—an impact on one can also affect other sectors and 

locations (Kirshen et al., forthcoming). For example, an interruption in energy supply in 

particular locations may result in increased heat stress for vulnerable populations who rely on 

electricity for cooling (Ruth et al., 2006). 

• Effects on energy requirements. Warming is virtually certain to increase energy demand in 

U.S. cities for cooling in buildings, while it reduces demands for heating in buildings (see 

section V.7). Demands for cooling during warm periods could jeopardize the reliability of 

service in some regions by exceeding the supply capacity, especially during periods of 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 158
unusually high temperatures. Higher temperatures also affect costs of living and business 

operation by increasing costs of climate control in buildings (Amato et al., 2005; Ruth and 

Lin, 2006; Kirshen et al., forthcoming). 

• Effects on the urban metabolism. An urban area is a living, complex mega-organism, 

associated with a host of inputs, transformations, and outputs: heat, energy, materials, and 

others (Decker et al., 2000). An example is the Urban Heat Index, which measures the degree 

to which built and paved areas are associated with higher temperatures than surrounding rural 

areas. Imbalances in the urban metabolism can aggravate climate change impacts, such as the 

role of the Urban Heat Index in the formation of smog in cities. 

• Effects on economic competitiveness, opportunities, and risks. Climate change has the 

potential not only to affect settlements directly, but also to affect them through impacts on 

other areas linked to their economies at regional, national, and international scales (Solecki 

and Rosenzweig, 2006). In addition, it can affect a settlement’s economic base if it is 

sensitive to climate, as in areas where settlements are based on agriculture, forestry, water 

resources, or tourism (IPCC, 2007b). Climate can also be a factor in an area’s comparative 

advantage for economic production and growth and can influence the costs of doing business 

(e.g., differences in costs for climate control for buildings). It is possible that regions exposed 

to risks from climate change will see movement of population and economic activity to other 

locations. One reason is public perceptions of risk, but a more powerful driving force may be 

the availability of insurance. The insurance sector is one of the most adaptable of all 

economic sectors, and its exposure to costs from severe storms and other extreme weather 

events is likely to lead it to withdraw (or to make much more expensive) private insurance 

coverage from areas vulnerable to climate change impacts (IPCC, 2007b). 

 

• Effects on social and political structures. Climate change can add to stress on social and 

political structures by increasing management and budget requirements for public services 

such as public health care, disaster risk reduction, and even public security. As sources of 

stress grow and combine, the resilience of social and political structures that are already 

somewhat unstable is expected to suffer, especially in areas with relatively limited resources 

(Sherbinin et al., 2006). Where climate change adds to stress levels in settlements, it is 

expected to be especially problematic for vulnerable parts of the population: the poor, the 

elderly, those already in poor health, the disabled, those living alone, those with limited 

rights and power (e.g., recent immigrants with limited English skills), and/or indigenous 

populations dependent on one or a few resources. As one example, warmer temperatures in 

urban summers have a more immediate impact on populations who live and work without air 

conditioning. 

Overall, climate change effects on human settlements in the United States are expected to occur 

as a result of interaction with other processes. Driving forces and stresses—technological, 

economic, and institutional—will have more impact on the sustainability of most settlements 

than climate change per se (Wilbanks et al., 2007a). 

According to the IPCC (2007b): 

The most vulnerable industries, settlements and societies are generally those in coastal and river 

flood plains, those whose economies are closely linked with climate-sensitive resources, and those 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 159
in areas prone to extreme weather events, especially where rapid urbanization is occurring (high 

confidence). Poor communities can be especially vulnerable, in particular those concentrated in 

high-risk areas. They tend to have more limited adaptive capacities, and are more dependent on 

climate-sensitive resources such as local water and food supplies (high confidence). 

In the United States, the most vulnerable areas are likely to be Alaska, coastal and river basin 

locations susceptible to flooding, arid areas where water scarcity is a pressing issue, and areas 

whose economic bases are climate-sensitive (Field et al., 2007). 

In Alaska and elsewhere in the Arctic, indigenous communities are facing major economic and 

cultural impacts. Many indigenous peoples depend on hunting polar bear, walrus, seals, and 

caribou; herding reindeer; fishing; and gathering, not only for food and to support the local 

economy, but also as the basis for cultural and social identity. Changes in species’ ranges and 

availability, access to these species, a perceived reduction in weather predictability, and travel 

safety in changing ice and weather conditions present serious challenges to human health and 

food security, and possibly even the survival of some cultures (ACIA, 2004). In addition, 

thawing ground is beginning to destabilize transportation, buildings, and other facilities, posing 

needs for rebuilding with ongoing warming adding to construction and maintenance costs. One 

recent estimate of the value of Alaska’s public infrastructure at risk from climate change by 2080 

set the value at tens of billions of today’s dollars. The largest estimated public costs were 

replacement of buildings, bridges, and other structures with long lifetimes (Larsen et al., 2007). 

As highlighted in Section V.5.a, population growth is generally shifting toward areas more likely 

to be vulnerable to the effects of climate change. For example, many rapidly growing places in 

the intermountain West rely heavily on winter snowpack to provide municipal water resources. 

Projections of decreased snowpack and earlier spring melting suggest lower stream flows in the 

future, particularly during the high-demand period of summer. Moreover, as discussed in the 

next section, coastal areas are particularly at risk to climate change effects. 

V.5.c Sea level rise 

Sea levels are rising and the IPCC concluded with high confi dence that the rate of sea level rise 

increased from the 19th to the 20th centuries (IPCC, 2007a). As described in section IV.1.d, the 

causes for observed sea-level rise over the past century include thermal expansion of seawater as 

it warms and changes in land ice (e.g., melting of glaciers and snow caps). Over the 20th 

century, sea level rose about 1.7 ± 0.5 mm/yr (Bindoff et al., 2007). For the period 1993 to 2003, 

the rate was nearly twice as fast, at 3.1 ± 0.7 mm/yr. Some of this recent increase may be due to 

the observed acceleration in the rate of Greenland ice melting over the past decade (Rignot, 

2006). However, there is considerably decadal variability in the tide gauge record so that it is 

unknown whether the higher rate in 1993 to 2003 is due to decadal variability or an increase in 

the longer-term trend. (Bindoff et al., 2007). 

Over this same time period, land use intensity in coastal areas increased dramatically, a trend that 

is expected to continue (Nicholls et al., 2007). Regarding the effects of this use, the IPCC 

concluded (Nicholls et al., 2007): 

• Coastal population growth in many of the world’s deltas, barrier islands, and estuaries has led 

to widespread conversion of natural coastal landscapes to agriculture, aquaculture, 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 160
silviculture, and industrial and residential uses (Valiela, 2006). 

• It has been estimated that 23% of the world’s population lives both within 100 km of the 

coast and below 100 m above sea level, and population densities in coastal regions are about 

three times higher than the global average (Small and Nicholls, 2003) 

• Migration of people to coastal regions is common in both developed and developing nations. 

Sixty percent of the world’s 39 metropolises with a population of over 5 million are located 

within 100 km of the coast, including 12 of the world’s 16 cities with populations greater 

than 10 million. 

Growth in coastal population has kept pace with population growth in other parts of the country, 

but given the small land area of the coasts, the density of coastal communities has been 

increasing (Crossett et al., 2004). Over 50% of the U.S. population now lives in the coastal zone, 

and coastal areas are projected to continue to increase in population, with associated increases in 

population density, over the next several decades. The overlay of this migration pattern with 

climate change projections has several implications. Perhaps the most obvious is the increased 

exposure of people and property to the effects of sea level rise and hurricanes (Kunkel et al., 

1999). With rapidly growing communities near coastlines, property damages would be expected 

to increase even without any changes in storm frequency or intensity (Changnon et al., 2003). 

The IPCC characterized some of the implications of this coastal population growth and 

associated human activities as follows (Nicholls et al., 2007): 

• Enlargement of natural coastal inlets and dredging of waterways for navigation, port 

facilities, and pipelines exacerbate saltwater intrusion into surface and groundwater. 

• Increasing shoreline retreat and risk of flooding of coastal cities in Thailand (Durongdej, 

2001; Saito, 2001), India (Mohanti, 2000), Vietnam (Thanh et al., 2004), and the United 

States (Scavia et al., 2002) have been attributed to degradation of coastal ecosystems by 

human activities, illustrating a widespread trend. 

• The major direct impacts include drainage of coastal wetlands, deforestation and reclamation, 

and discharge of sewage, fertilizers, and contaminants into coastal waters. Extractive 

activities include sand mining and hydrocarbon production, harvests of fisheries and other 

living resources, introductions of invasive species, and construction of seawalls and other 

structures. 

• Engineering structures (such as damming, channelization, and diversions of coastal 

waterways) harden the coast, change circulation patterns, and alter freshwater, sediment, and 

nutrient delivery. Natural systems are often altered, even by soft engineering solutions, such 

as beach nourishment and foredune construction (Nordstrom, 2000; Hamm and Stive, 2002). 

• Ecosystem services on the coast are often disrupted by human activities. For example, 

tropical and subtropical mangrove forests and temperate salt marshes provide goods and 

services (they accumulate and transform nutrients, attenuate waves and storms, bind 

sediments, and support rich ecological communities), which are reduced by large-scale 

ecosystem conversion for agriculture, industrial and urban development, and aquaculture. 

According to the IPCC (Scavia et al., 2002; Lotze et al., 2006), the impacts of these other human 

activities on the coastal zone have been more significant over the past century than effects that 

can be attributed to observed climate change. The cumulative effect of these non-climate, 

anthropogenic impacts increases the vulnerability of coastal systems to climate-related stressors. 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 161
For example, the degradation of natural coastal systems due to climate change, such as wetlands, 

beaches, and barrier islands, removes the natural defenses of coastal communities against 

extreme water levels during storms. Accordingly, Nicholls et al. (2007) found that many coastal 

cities are heavily dependent upon artificial coastal defenses (e.g., Tokyo, Shanghai, Hamburg, 

Rotterdam, and London). These urban systems are vulnerable to low-probability extreme events, 

with rising sea levels and increased extreme water levels exceeding prior defense standards and 

leading to systemic failures (domino effect). The IPCC found that the ports, roads, and railways 

along the U.S. Gulf and Atlantic Coasts are especially vulnerable to coastal flooding (Nicholls et 

al., 2007). 

In the Great Lakes where sea level rise is not a concern, both extremely high and low water 

levels resulting from changes in the hydrologic cycle have been damaging and disruptive to 

shoreline communities (Nicholls et al., 2007). High lake water levels increase storm surge 

flooding, accelerate shoreline erosion, and damage industrial and commercial infrastructure 

located on the shore. Conversely, low lake water levels can pose problems for navigation, expose 

intake/discharge pipes for electrical utilities and municipal water treatment plants, and cause 

unpleasant odors. 

The IPCC analyzed flood risk in 2080 from rising sea levels and storm surges under different 

climate scenarios (Nicholls et al., 2007). Under each scenario, the population at risk increases 

substantially. However, results show that upgrading coastline defenses can dramatically reduce 

projected impacts. 

Projected impacts 

Globally, the IPCC (Nicholls et al., 2007) made the following conclusions regarding sea level 

rise: 

• Coasts will be exposed to increasing risks, including coastal erosion, over coming decades 

due to climate change and sea level rise (very high confidence). 

• The impact of climate change on coasts is exacerbated by increasing human-induced 

pressures (very high confidence). 

• Adaptation for the coasts of developing countries will be more challenging than for coasts of 

developed countries, due to constraints on adaptive capacity (high confidence). 

• Adaptation costs for vulnerable coasts are much less than the costs of inaction (high 

confidence). 

• The unavoidability of sea level rise, even in the longer term, frequently conflicts with 

present-day human development patterns and trends (high confidence). 

The U.S. coastline is long and diverse with a wide range of coastal characteristics. Sea level rise 

changes the shape and location of coastlines by moving them landward along low-lying contours 

and exposing new areas to erosion (NRC, 2006a). Coasts subsiding due to natural or humaninduced causes will experience larger relative rises in sea level. In some locations, such as deltas 

and coastal cities (e.g., the Mississippi Delta and surrounding cities), this effect can be 

significant (Nicholls et al., 2007). Rapid development, including an additional 25 million people 

in the coastal United States over the next 25 years, will further reduce the resilience of coastal 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 162
areas to rising sea levels and increase the economic resources and infrastructure vulnerable to 

impacts (Field et al., 2007). Superimposed on the impacts of erosion and subsidence, the effects 

of rising sea level will exacerbate the loss of waterfront property and increase vulnerability to 

inundation hazards (Nicholls et al., 2007). 

Climate change is expected to have a strong impact on saltwater intrusion into coastal sources of 

groundwater in the United States and other world regions. Sea-level rise and high rates of water 

withdrawal, promote the intrusion of saline water into the groundwater supplies, which adversely 

affects water quality. In some locations, reduced groundwater recharge, associated with 

decreases in precipitation and increases in evapotranspiration, will exacerbate the effects of sea 

level rise on salinization rates (Kundzewicz et al., 2007). This effect could impose enormous 

costs on water treatment infrastructure (i.e., costs associated with relocating infrastructure or 

building desalinization capacity), especially in densely populated coastal areas. Saltwater 

intrusion is also projected to occur in freshwater bodies along the coast. Estuarine and mangrove 

ecosystems can withstand a range of salinities on a short-term basis. However, they are not 

expected to survive permanent exposure to high-salinity environments. Saltwater intrusion into 

freshwater rivers has already been linked with the decline of bald cypress forests in Louisiana 

and cabbage palm forests in Florida. Given that these ecosystems provide a variety of ecosystem 

services and goods (e.g., spawning habitat for fish, pollutant filtration, sediment control, and 

storm surge attenuation), the loss of these areas could be significant (Kundzewicz et al., 2007). 

Coastal indigenous communities are particularly vulnerable to impacts from sea level rise based 

on their geographic location, reliance on the local environment for aspects of everyday life such 

as diet and economy, and the current state of social, cultural, economic, and political change 

taking place in these regions (Anisimov et al., 2007). In northern areas of ice-rich permafrost, 

such as Alaska, coastal erosion rates are among the highest anywhere. These rates could be 

increased by rising sea levels, forcing the issue of relocation for threatened settlements. Adapting 

to these changes will be costly. For example, it has been estimated that relocating the village of 

Kivalina, Alaska (377 inhabitants in the 2000 census) to a nearby site would cost $54 million 

(Anisimov et al., 2007). 

For small islands, particularly in the Pacific, some studies suggest that sea level rise could result 

in a reduction of island size, raising concerns for Hawaii and other U.S. territories (Mimura et 

al., 2007). In some cases, accelerated coastal erosion may lead to island abandonment, as has 

been documented in the Chesapeake Bay. In the Caribbean and Pacific Islands, more than 50% 

of the population lives within 1.5 km of the shore. International airports, roads, capital cities, and 

other types of infrastructure are typically sited along the coasts of these islands as well. 

Therefore, the socioeconomic well-being of island communities will be threatened by 

inundation, storm surge, erosion, and other coastal hazards resulting from climate change (high 

confidence) (Mimura et al., 2007). 

Regarding the effects of climate change on sea level rise and coastal areas in North America, the 

IPCC (Field et al., 2007) concluded: 

• Rates of coastal wetland loss in the Chesapeake Bay and elsewhere will increase with 

accelerated sea level rise, in part due to ‘coastal squeeze’ (IPCC: high confidence). 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 163
• Coastal communities and habitats will be increasingly stressed by climate change impacts 

interacting with development and pollution (very high confidence). Sea level is rising along 

much of the coast, and the rate of change will increase in the future, exacerbating the impacts 

of progressive inundation, storm-surge flooding, and shoreline erosion. 

• Storm impacts are likely to be more severe, especially along the Gulf and Atlantic Coasts. 

Salt marshes, other coastal habitats, and dependent species are threatened by sea level rise, 

fixed structures blocking landward migration, and changes in vegetation. Population growth 

and rising value of infrastructure in coastal areas increases vulnerability to climate variability 

and future climate change. 

V.5.d Extreme events 

While individual extreme events cannot be directly linked to climate change, changing climate 

conditions are expected to result in increased incidence of extreme weather events. The 

combined effects of severe storms and sea level rise in coastal areas or increased risks of fire in 

drier arid areas are examples of how climate change may increase the magnitude of challenges 

already facing risk-prone communities. For example, the IPCC (Wilbanks et al., 2007a) 

estimated that, of the 131 million people affected by natural disasters in Asia in 2004, 97% were 

affected by weather-related disasters. Exposures in highly populated coastal and riverine areas 

and small island nations have been especially significant (ADRC et al., 2005). 

Extreme events can also have substantial secondary effects on local economies. The United 

States ranks among the top 10 nations for international tourism receipts ($112 billion), with 

domestic tourism and outdoor recreation markets that are several times larger than most other 

countries. Climate variability affects many segments of this growing economic sector. For 

example, wildfires in Colorado (2002) caused tens of millions of dollars in tourism losses by 

reducing visitation and destroying infrastructure. Similar economic losses during that same year 

were caused by drought-affected water levels in rivers and reservoirs in the western United 

States and parts of the Great Lakes. The 10-day closure and clean-up following Hurricane 

George (September 1998) resulted in tourism revenue losses of approximately $32 million in the 

Florida Keys (Field et al., 2007 and references therein). Impacts of climate change on other 

recreational activities such as skiing, fishing, and hunting are highlighted in Saunders et al. 

(2008). While the North American tourism industry acknowledges the important influence of 

climate, its impacts have not been analyzed comprehensively. 

Floods and storms 

Nicholls et al. (2007) concluded that, globally, coasts are experiencing the adverse consequences 

of hazards related to climate and sea level (very high confidence). Although increases in mean 

sea level over the 21st century and beyond will inundate unprotected, low-lying areas, the most 

devastating impacts are likely to be associated with storm surge (Nicholls et al., 2007). For 

example, the Maryland Geological Survey estimated that more than 8 ha (20 acres) of the state’s 

land was lost on the western shore of Chesapeake Bay in the wake of Tropical Storm Isabel in 

2003, causing approximately $84 million in damages to shoreline structures (NRC, 2006a). 

Coasts are highly vulnerable to extreme events, such as storms, which impose substantial costs 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 164
on coastal societies. Globally, about 120 million people are exposed to tropical cyclone hazards 

every year, and these storms killed 250,000 people from 1980 to 2000. 

The IPCC (2007d) projects likely increases in intense tropical cyclones. Increases in tropical 

cyclone intensity are linked to increases in the risk of deaths, injuries, water- and food-borne 

diseases, as well as post-traumatic stress disorders (IPCC, 2007b). Drowning by storm surge, 

heightened by rising sea levels and more intense storms (as projected by the IPCC), is the major 

killer in coastal storms when there are large numbers of deaths (Confalonieri et al., 2007). Highdensity populations in low-lying coastal regions such as the U.S. Gulf of Mexico experience a 

high health burden from weather disasters, particularly among lower income groups. 

In SAP 4.6, Wilbanks et al. (2008) provide the following description of vulnerability to storm 

activity in the southeast United States. 

Recent hurricanes striking the coast of the U.S. Southeast cannot be attributed clearly to climate 

change, but if climate change increases the intensity of storms as projected (IPCC, 2001; Emanuel, 

2005) that experience suggests a range of possible impacts. As an extreme case, consider the 

example of Hurricane Katrina (Wilbanks, 2007a). In 2005, the city of New Orleans had a 

population of about half a million, located on the delta of the Mississippi River along the U.S. 

Gulf Coast. Urban development throughout the 20th Century has significantly increased land use 

and settlement in areas vulnerable to flooding, and a number of studies had indicated growing 

vulnerabilities to storms and flooding. In late August 2005, Hurricane Katrina moved onto the 

Louisiana and Mississippi coast with a storm surge, supplemented by waves, reaching up to 8.5 m 

above sea level. In New Orleans, the surge reached around 5 m, overtopping and breaching 

sections of the city’s 4.5 m defenses, flooding 70 to 80% of New Orleans, with 55% of the city’s 

properties inundated by more than 1.2 m and maximum flood depths up to 6 m. 1,101 people died 

in Louisiana, nearly all related to flooding, concentrated among the poor and elderly. Across the 

whole region, there were 1.75 million private insurance claims, costing in excess of $40 billion 

(Hartwig, 2006), while total economic costs are projected to be significantly in excess of $100 

billion. Katrina also exhausted the federally backed National Flood Insurance Program (Hunter, 

2006), which had to borrow $20.8 billion from the Government to fund the Katrina residential 

flood claims. In New Orleans alone, while flooding of residential structures caused $8-$10 billion 

in losses, $3-6 billion was uninsured. 34,000-35,000 of the flooded homes carried no flood 

insurance, including many that were not in a designated flood risk zone (Hartwig, 2006). Six 

months after Katrina, it was estimated that the population of New Orleans was 155,000, with the 

number projected to rise to 272,000 by September 2008—56% of its pre-Katrina level (K. 

McCarthy et al., 2006). 

The recent severe tropical and extra-tropical storms, and resulting impacts, led the IPCC to 

conclude that North American urban centers with assumed high adaptive capacity remain 

vulnerable to extreme events (Field et al., 2007). 

The vulnerability of some major urban centers in the United States is exacerbated by the fact that 

they are situated in low-lying flood plains. For example, areas of New Orleans and its vicinity 

are 1.5 to 3 m below sea level. Considering the rate of subsidence and using a mid-range 

estimate of 480 mm sea level rise by 2100, it is projected that this region could be 2.5 to 4.0 m or 

more below mean sea level by 2100 (Field et al., 2007). In this scenario, a storm surge from a 

Category 3 hurricane (estimated at 3 to 4 m without waves) could be 6 to 7 m above areas that 

were heavily populated in 2004 (Field et al., 2007). 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 165
Superimposed on accelerated sea level rise, storm intensity, wave height, and storm surge 

projections suggest more severe coastal flooding and erosion hazards (Nicholls et al., 2007). In 

New York City and Long Island, flooding from a combination of sea level rise and storm surge 

could be several meters deep (Field et al., 2007). Projections suggest that the return period of a 

100-year flood event in this area might be reduced to 19 to 68 years, on average, by the 2050s, 

and to 4 to 60 years by the 2080s (Wilbanks et al., 2007a). 

In addition, the IPCC projects a very likely increase in heavy precipitation event frequency over 

most areas. Heavy precipitation events are associated with increased risk of floods as well as 

infectious, respiratory, and skin diseases (IPCC, 2007b). Floods are low-probability, high-impact 

events that can overwhelm physical infrastructure, human resilience, and social organization 

(Confalonieri et al., 2007). Flood health impacts include deaths, injuries, infectious diseases, 

intoxications, and mental health problems (Greenough et al., 2001; Ahern et al., 2005 in 

Confalonieri et al., 2007). Flooding may also lead to contamination of waters with dangerous 

chemicals, heavy metals, or other hazardous substances, from storage or from chemicals already 

in the environment (Confalonieri et al., 2007). As the risk of flooding increases with climate 

change, so does the importance of major drainage systems. For example, new design approaches 

which explicitly design roads to act as drains can radically reduce the duration of flooding 

(OFCM, 2004). 

Demand for waterfront property and land for building in the United States continues to grow, 

increasing the value of property at risk. Of the $19 trillion value of all insured residential and 

commercial property in the United States exposed to North Atlantic hurricanes, $7.2 trillion 

(41%) is located in coastal counties. This economic value includes 79% of the property in 

Florida, 63% of property in New York, and 61% of the property in Connecticut (AIR, 2002 in 

Field et al., 2007). The devastating effects of Hurricane Ivan in 2004 and Hurricanes Katrina, 

Rita, and Wilma in 2005 illustrate the vulnerability of North American infrastructure and urban 

systems that were not designed or not maintained to adequate safety margins. When protective 

systems fail, impacts can be widespread and multi-dimensional (Field et al., 2007). 

Wildland fire 

Wildland fires are discussed in greater detail in Section V.3.d. Disturbances such as forest fires, 

insect outbreaks, ice storms, and hurricanes can change forest productivity, carbon cycling, and 

species composition (Ryan et al., 2008). Forest fires and bushfires cause burns, smoke inhalation, 

and other injuries. Large fires are also accompanied by an increased number of patients seeking 

emergency services for inhalation of smoke and ash (Hoyt and Gerhart, 2004 in Confalonieri et 

al., 2007). 

Wildfires have increased in extent and severity in recent years. At the same time, population has 

been growing rapidly, expanding beyond urban areas to at-risk areas known as the wildland–

urban interface, where human settlements and development meet or intermix with wildland fuel 

(Hammer et al., 2007). These trends have resulted in increased vulnerability to both ecological 

and social impacts from increased fire activity, including financial losses from damaged 

property, lost revenue to local businesses, disruption of local social networks when residents are 

displaced, and a sense of devastation among citizens who have strong connections to the 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 166
surrounding landscape (Toman et al., 2008). In some regions, changes in the mean and 

variability of temperature and precipitation are projected to increase the frequency and severity 

of fire events, including parts of the United States (Easterling et al., 2007). 

Drought 

Observed changes in drought are discussed in Sections IV.1.c and IV.1.d and projections of 

moisture availability and drought are discussed in Section IV.3.b. Areas affected by droughts are

likely to increase, according to the IPCC (2007d). Specifically, it is likely that droughts will 

continue to be exacerbated by earlier and possibly lower spring snowmelt runoff in the 

mountainous West, which results in less water available in late summer (Karl et al., 2008). If 

total precipitation decreases or becomes more variable, extending the kinds of drought that have 

affected much of the interior West in recent years, water scarcity will be exacerbated, and 

increased water withdrawals from wells could affect aquifer levels and pumping costs (Wilbanks 

et al., 2008). Moreover, drying increases risks of fires, which have threatened urban areas in 

California and other western areas in recent years. Thus, projections of increased frequencies of 

drought combined with a greater fire risk put the increasing populations of desert Southwest 

cities at risk (Wilbanks et al., 2008). Health impacts associated with drought tend to mostly 

affect semi-arid and arid regions, poor areas and populations, and areas with human-induced 

water scarcity. Information about the effects of increasing drought on U.S. agriculture can be 

found in Section V.2.c. 

V.6 Human Health 

In SAP 4.6, Ebi et al. (2008) found that weather, climate variability, and climate change can 

affect health through the effects of environmental temperature on bodily functioning. Climate 

change can also have important secondary effects on heath. For example, climate change can 

alter or disrupt natural systems, making it possible for vector-, water- and food-borne diseases to 

spread or emerge in areas where they had been limited or not existed, or making it possible for 

such diseases to disappear by making areas less hospitable to the vector or pathogen (NRC, 

2001b). Climate also can affect the incidence of diseases associated with air pollutants and 

aeroallergens. Ultimately, climate impacts on health are complex and will be influenced by 

multiple factors, including demographics; population and regional vulnerabilities; the future 

social, economic, and cultural context; availability of resources and technological options; built 

and natural environments; public health infrastructure; and the availability and quality of health 

and social services. 

A comprehensive assessment of the potential impacts of climate variability and change on 

human health was published in 2000 as part of the First National Assessment of the Potential 

Impacts of Climate Variability and Change undertaken by the U.S. Global Change Research 

Program (NAST, 2001). These findings have been significantly updated in recent IPCC and 

CCSP reports. 

In its Fourth Assessment Report, the IPCC produced a number of key findings summarizing the 

potential health effects of climate change in North America. The IPCC (2007b) concluded that, 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 167
overall, it is expected that benefits will be outweighed by the negative health effects of rising 

temperatures worldwide, especially in developing countries. Gamble et al. (2008) concluded that 

there may be fewer cases of illness and death associated with climate change in the United States 

than in the developing world. Nevertheless, increased costs to human health and well being are 

anticipated for the United States. 

Key conclusions drawn primarily from the IPCC Fourth Assessment Report (Field et al., 2007) 

for North America include: 

• Increased deaths, injuries, infectious diseases, and stress-related disorders and other adverse 

effects associated with social disruption, migration, and loss of place from more frequent 

extreme weather. 

• Increased frequency and severity of heat waves leading to more illness and death, particularly 

among the young, elderly, frail, poor, and outdoor workers and athletes. 

• Expanded ranges of vector- and tick-borne diseases in North America but with modulation 

by public health measures and other factors. 

SAP 4.6 (Gamble et al., 2008) concluded that greater wealth and a more developed public health 

system and infrastructure (e.g., water treatment plants, sewers, and drinking water systems; 

roads, rails and bridges; flood control structures) will continue to enhance our capacity to 

respond to climate change. Similarly, governments’ capacities for disaster planning and 

emergency response are key assets that should allow the United States to adapt to many of the 

health effects associated with climate change. Other conclusions based in large part on SAP 4.6 

(Gamble et al., 2008) regarding the health effects projected for the United States are summarized 

below.

• It is very likely that heat-related morbidity and mortality will increase over the coming 

decades, however net changes in mortality are difficult to estimate because, in part, much 

depends on complexities in the relationship between mortality and global change. According 

to the U.S. Census, the U.S. population is aging; the percent of the population over age 65 is 

projected to be 13% by 2010 and 20% by 2030 (over 50 million people). Older adults, very 

young children, and persons with compromised immune systems are vulnerable to 

temperature extremes. This suggests that temperature-related morbidity and mortality are 

likely to increase. Similarly, heat-related mortality affects poor and minority populations 

disproportionately, in part due to lack of air conditioning. The concentration of poverty in 

inner city neighborhoods leads to disproportionate adverse effects associated with urban heat 

islands. However, few studies have attempted to link the epidemiological findings to climate 

scenarios for the United States, and studies that have done so have focused on the effects of 

changes in average temperature, with mixed results. 

• The impacts of higher temperatures in urban areas and likely associated increases in 

tropospheric ozone concentrations may contribute to or exacerbate cardiovascular and 

pulmonary illness if current regulatory standards are not attained. In addition, stagnant air 

masses related to climate change are likely to degrade air quality in some densely populated 

areas. It is important to recognize that the United States has a well-developed and successful 

national regulatory program for ozone, PM2.5, and other criteria pollutants. That is, the 

influence of climate change on air quality will play out against a backdrop of ongoing 

regulatory control that will shift the baseline concentrations of air pollutants. Studies to date 

have typically held air pollutant emissions constant over future decades (i.e., have examined 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 168
the sensitivity of ozone concentrations to climate change rather than projecting actual future 

ozone concentrations). Physical features of communities, including housing quality and green 

space, social programs that affect access to health care, aspects of population composition 

(level of education, racial/ethnic composition), and social and cultural factors are all likely to 

affect vulnerability to air quality. 

• Hurricanes, extreme precipitation resulting in floods, and wildfires also have the potential to 

affect public health through direct and indirect health risks. The health risks associated with 

such extreme events are likely to increase with the size of the population and the degree to 

which it is physically, mentally, or financially constrained in its ability to prepare for and 

respond to extreme weather events. For example, coastal evacuations prompted by imminent 

hurricane landfall are only moderately successful. Many of those who are advised to flee to 

higher ground stay behind in inadequate shelter. Surveys find that the public is either not 

aware of the appropriate preventive actions or incorrectly assesses the extent of their personal 

risk. 

• There will likely be an increase in the spread of several food- and water-borne pathogens 

among susceptible populations depending on the pathogens’ survival, persistence, habitat 

range and transmission under changing climate and environmental conditions. The primary 

climate-related factors that affect these pathogens include temperature, precipitation, extreme 

weather events, and shifts in their ecological regimes. Consistent with our understanding of 

climate change on human health, the impact of climate on food and water-borne pathogens 

will seldom be the only factor determining the burden of human injuries, illness, and death. 

• Health burdens related to climate change will vary by region. For the continental United 

States, the northern latitudes are likely to experience the largest increases in average 

temperatures; they will also bear the brunt of increases in ground-level ozone and other 

airborne pollutants. Because Midwestern and Northeastern cities are generally not as well 

adapted to the heat as Southern cities, their populations are likely to be disproportionately 

affected by heat related illnesses as heat waves increase in frequency, severity, and duration. 

The range of many vectors is likely to extend northward and to higher elevations. For some 

vectors, such as rodents associated with Hantavirus, ranges are likely to expand, as the 

precipitation patterns under a warmer climate enhance the vegetation that controls the rodent 

population. Forest fires with their associated decrements to air quality and pulmonary effects 

are likely to increase in frequency, severity, distribution, and duration in the Southeast, the 

Intermountain West and the West.

• Finally, climate change is very likely to accentuate the disparities already evident in the 

American health care system. Many of the expected health effects are likely to fall 

disproportionately on the poor, the elderly, the disabled, and the uninsured. The most 

important adaptation to ameliorate health effects from climate change is to support and 

maintain the United States’ public health infrastructure. 

Valid projections of health effects require solid data on dose–response relationships between 

climate variables and health outcomes. These data are often lacking for several outcomes of 

concern. Additional research and development are needed to ensure that surveillance systems 

account for and anticipate the potential effects of climate change (NRC, 2001b). Surveillance 

systems will be needed in locations where changes in weather and climate may foster the spread 

of climate-sensitive pathogens and vectors into new regions (NRC, 2001b). Understanding 

associations between disease patterns and environmental variables can be used to develop early 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 169
warning systems that warn of outbreaks before most cases have occurred. Increased 

understanding is needed of how to design these systems where there is limited knowledge of the 

interactions among climate, ecosystems, and infectious diseases (NAS, 2001c). To date, few 

studies address the interaction of multi-sector climate impacts or of interactions between climate 

change health impacts and other kinds of local, regional, and global changes (Field et al., 2007). 

For example, climate change impacts on human health in urban areas will be compounded by 

aging infrastructure, maladapted urban form and building stock, urban heat islands, air pollution, 

population growth, and an aging population. 

The remainder of this section describes the literature on the impacts of climate change on human 

health in four areas noted above: primary temperature effects, extreme events, climate-sensitive 

infectious diseases, and aeroallergens and air quality. 

V.6.a Temperature effects 

According to the IPCC (2007a), it is very likely that there were fewer cold days and nights and 

warmer and more frequent hot days over most land areas during the late 20th century. It is 

virtually certain that these trends will continue during the 21st century. 

As a result of the projected warming, the IPCC projects increases in heat-related mortality and 

morbidity globally (including in the United States) (Field et al., 2007; IPCC, 2007b). The 

projected warming is expected to result in fewer cold-related deaths due to reduced exposure to 

the cold. However, net changes in mortality are difficult to estimate because, in part, much 

depends on complexities in the relationship between mortality and the changes associated with 

global change. 

Increased heat exposure 

In addition to increases in average temperatures, heat waves have become more frequent (IPCC, 

2007a). Heat waves are associated with marked short-term increases in mortality (Confalonieri et 

al., 2007). India has experienced 18 heat waves since 1980, each resulting in multiple deaths 

ranging from an estimated 29 deaths in 2000 to more than 3,000 deaths in 2003 (Mohanty and 

Panda, 2003; Government of Andhra Pradesh, 2004 in Confalonieri et al., 2007). Heat waves 

have also resulted in substantial mortality in more industrialized nations. The 2003 European 

heat wave is estimated to have caused 35,000 deaths (e.g., Hemon and Jougla, 2004; MartinezNavarro et al., 2004; Johnson et al., 2005 in Confalonieri et al., 2007). 

Hot temperatures have also been associated with increased morbidity, although there is less 

information available (Ebi et al., 2008). While effects have varied across studies, Schwartz et al. 

(2004) documented increased hospital admissions for cardiovascular disease and emergency 

room visits in North America during heat events (Schwartz et al., 2004). 

In SAP 4.6, Ebi et al. (2008) identified the following heat-related impacts on U.S. health: 

• Exposure to excessive natural heat caused a reported 4,780 deaths during the period 1979 to 

2002, while hyperthermia was reported as a contributing factor in an additional 1,203 deaths 

(CDC, 2005a). 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 170
• Heat is expected to contribute to the exacerbation of chronic health conditions, such as 

cardiovascular, renal, respiratory, diabetes, and nervous system disorders. Several analyses 

have found associations between increased heat and increased all-cause mortality (MedinaRamon et al., 2006). 

• Vulnerability varies among populations. Particularly vulnerable groups include the elderly, 

the very young, city-dwellers, those with less education, people on medications such as 

diuretics, the socially isolated, the mentally ill, those lacking access to air conditioning, and 

outdoor laborers (e.g., McGeehin and Mirabelli, 2001; Diaz et al., 2002; Klinenberg, 2002). 

Among the most well-documented heat waves in the United States are those that occurred in 

1980 (St. Louis and Kansas City, Missouri), 1995 (Chicago, Illinois), and 1999 (Cincinnati, 

Ohio; Philadelphia, Pennsylvania; and Chicago, Illinois). In all these episodes, the highest 

death rates occurred in people over 65 years of age. 

• Less information exists on temperature-related morbidity, and those studies that have 

examined hospital admissions and temperature have not seen consistent effects, either by 

cause or by demonstrated coherence with mortality effects where both deaths and 

hospitalizations were examined simultaneously (Semenza et al., 1999; Kovats et al., 2004; 

Schwartz et al., 2004). 

• In many cases, the urban heat island effect may increase heat-related mortality. This effect 

may cause air temperatures in urban areas to increase by 1 to 6 °C over the surrounding 

suburban and rural areas, due to absorption of heat by dark paved surfaces and buildings; 

lack of vegetation and trees; heat emitted from buildings, vehicles, and air conditioners; and 

reduced airflow around buildings (Pinho and Orgaz, 2000; Vose et al., 2004; Xu and Chen, 

2004). However, in some regions, urban areas may not experience greater heat-related 

mortality than in rural areas (Sheridan and Dolney, 2003). 

• High temperatures and high air pollution can interact to result in additional health impacts. 

The extent of interaction varies by location (Goodman et al., 2004; Bates, 2005; Ren et al., 

2006). 

Reviewing the research and emerging projections to date, the IPCC projected that heat-related 

morbidity and mortality will increase globally (Confalonieri et al., 2007). Estimates of heatrelated mortality attributable to climate change are reduced but not eliminated when models 

include assumptions about acclimatization and adaptation (e.g., implementation of space 

cooling). Past research indicates that populations in the United States became less sensitive to 

high temperatures over the period 1964 to 1988 due, at least in part, to these factors (R. Davis et 

al., 2002, 2003, 2004 in Confalonieri et al., 2007). 

However, demographic trends suggest that the vulnerable population in the United States will 

increase in the coming years. Across North America, the population over the age of 65 (the 

segment of the population most at risk of dying from heat waves) will increase slowly to 2010, 

and then grow dramatically as the ‘baby boomers’ age (Field et al., 2007). Moreover, much of 

the population growth in the next 50 years is expected to occur in cities (Cohen, 2003 in 

Confalonieri et al., 2007), increasing the total population exposed to the urban heat island effect. 

The IPCC projects the following U.S. regional increases in heat and/or heat-related effects 

(Confalonieri et al., 2007; Field et al., 2007): 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 171
• Severe heat waves are projected to intensify in magnitude and duration over the portions of 

the United States where these events already occur (high confidence). 

• By the 2080s, in Los Angeles, the number of heat wave days (at or above 32 °C or 90 °F) 

increases four-fold under the B1 emissions scenario (low growth) and six- to eight-fold under 

the A1FI (high growth) emissions scenario (Hayhoe, 2004). The annual number of projected 

heat-related deaths in Los Angeles increases from about 165 in the 1990s to 319 to 1,182 in 

the 2080s for a range of emissions scenarios. 

• Chicago is projected to experience 25% more frequent heat waves annually for the period 

spanning 2080 to 2099 for a business-as-usual (A1B) emissions scenario (Meehl and Tebaldi, 

2004). 

Reduced cold exposure 

The IPCC found that exposure to cold conditions results in adverse health impacts particularly in 

northerly latitudes, where very low temperatures can be reached in a few hours and continue for 

extended periods (Confalonieri et al., 2007). Specific findings included the following: 

• Adverse impacts occur mainly outdoors, among socially deprived people (e.g., alcoholics and 

the homeless), workers, and the elderly in temperate and cold climates (Ranhoff, 2000). 

• Living in cold environments in polar regions is associated with a range of chronic conditions 

in the non-indigenous population (Sorogin et al, 1993) as well as with acute risk from 

frostbite and hypothermia (Hassi et al., 2005). 

• In general, population sensitivity is greater in temperate countries with mild winters, as 

populations are less well adapted to cold (Eurowinter Group, 1997; Healy, 2003). However, 

in countries with populations well adapted to cold conditions, cold waves can still cause 

substantial increases in mortality if electricity or heating systems fail. 

• Cold waves also affect health in warmer climates, such as in South East Asia (EM-DAT, 

2006). 

In SAP 4.6, Ebi et al. (2008) wrote about specific impacts of cold exposure in the United States. 

An average of 689 reported deaths annually (ranging from 417 to 1,021 per year) from 1979 to 

2002 were attributed to exposure to excessive natural cold (16,555 total deaths) (Fallico et al., 

2005). However, the mortality burden is probably underestimated because cold also contributes 

to deaths caused by respiratory and cardiovascular diseases. Because many factors contribute to 

winter mortality, it is highly uncertain how climate change could affect mortality. No projections 

have been published for the United States that incorporate critical factors, such as the influence 

of influenza outbreaks. 

Vulnerability is associated with the following factors: 

• Location of residence: living in Alaska, New Mexico, North Dakota, or Montana, or living in 

a state with a milder climate that experiences rapid temperature changes (North and South 

Carolina) or a western state with greater ranges in nighttime temperatures (e.g., Arizona) 

(Fallico et al., 2005); 

• Race: African Americans show higher vulnerability (Fallico et al., 2005); 

• Level of education: vulnerability is higher among those with lower education (O’Neill et al., 

2003); 

• Gender: females are more vulnerable (Wilkinson et al., 2004); 

• Pre-existing respiratory illness (Wilkinson et al., 2004); 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 172
• Living in nursing homes (Hajat et al., 2007); and 

• Other factors such as lack of protective clothing (Donaldson et al., 2001), income inequality, 

fuel poverty, and low residential thermal standards (Healy, 2003). 

While the IPCC found that health projections have improved in recent years, there is still a lack 

of information on the effects of thermal stress on mortality outside the industrialized countries 

and limited specific results are available by country (Confalonieri et al., 2007). Moreover, the 

authors provide a caution that projections of reduced cold-related deaths can be overestimated 

unless they take into account the effects of influenza and season (Armstrong et al., 2004 in 

Confalonieri et al., 2007). Ultimately, Confalonieri et al. (2007) conclude that additional research 

is needed to understand how the balance of heat-related and cold-related mortality could change 

under different socioeconomic scenarios and climate projections. 

While projections suggest the reductions in cold-related deaths will be greater than increases in 

heat-related deaths in the United Kingdom (Donaldson et al., 2001), limited research examines 

potential outcomes in the United States under different climate scenarios (Confalonieri et al., 

2007). 

V.6.b Extreme events 

In addition to the immediate effects of temperature on mortality and morbidity, a changing 

climate can also result in substantial secondary effects such as through extreme events. 

Vulnerability to weather disasters depends on the attributes of the people at risk (including where 

they live, age, income, education, and disability) and on broader social and environmental factors 

(level of disaster preparedness, health sector responses, and environmental degradation). 

As Ebi et al. (2008) concluded in SAP 4.6, the United States experiences a wide range of 

extreme weather events, including hurricanes, floods, tornadoes, blizzards, windstorms, and 

drought. Other extreme events, such as wildfires, are strongly influenced by meteorological 

conditions. Morbidity and mortality due to an event increase with the intensity and duration of 

the event, and can decrease with advance warning and preparation. Health also can be affected 

by extreme events through secondary mechanisms such as carbon monoxide poisonings from 

portable electric generator use for electricity (CDC, 2006) or increased incidence of 

gastroenteritis cases among hurricane evacuees (CDC, 2005a). Moreover, these events can lead 

to substantial mental health impacts (e.g., post-traumatic stress disorder and depression) 

(Middleton et al., 2002; Russoniello et al., 2002; Verger et al., 2003; North et al., 2004; Fried et 

al., 2005; Weisler et al., 2006). 

Climate effects on extreme events are discussed in greater detail in Section V.5.d above. Here the 

discussion is limited to additional information on health affects. 

Floods and storms 

Impacts of floods and storms include deaths, injuries, infectious diseases, intoxications, and 

mental health problems (Greenough et al., 2001; Ahern et al., 2005 in Confalonieri et al., 2007). 

Flooding may also lead to contamination of waters with dangerous chemicals, heavy metals, or 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 173
other hazardous substances, from storage or from chemicals already in the environment 

(Confalonieri et al., 2007). In addition, increases in tropical cyclone intensity are linked to 

increases in the risk of deaths, injuries, water- and food-borne diseases, and post-traumatic stress 

disorders (IPCC, 2007b). 

Major storms and floods can result in huge numbers of deaths. Confalonieri et al. (2007) cite the 

following examples: 

• In 2003, 130 million people were affected by floods in China (EM-DAT, 2006). 

• In 1999, 30,000 people died from storms followed by floods and landslides in Venezuela. 

• In 2000 and 2001, 1,813 people died in floods in Mozambique (IFRC, 2002; Guha-Sapir et 

al., 2004). 

High-density populations in low-lying coastal regions such as the U.S. Gulf of Mexico 

experience a high health burden from weather disasters, particularly among lower income 

groups. The 2005 hurricane season in the United States resulted in double the average number of 

lives lost over the previous 65 years. Overall, 2,002 lives were lost, and over 1,800 of those 

occurred during two storm events (Katrina and Rita) (NOAA, 2007a). 

Following the storm event, lasting impacts may result particularly when sanitation systems have 

been overwhelmed. Indeed, in Confalonieri et al. (2007) the IPCC noted that populations with 

poor sanitation infrastructure and high burdens of infectious disease often experience increased 

rates of diarrheal diseases after flood events. For example, increases in cholera (Sur et al., 2000; 

Gabastou et al., 2002), cryptosporidiosis (Katsumata et al., 1998), and typhoid fever (Vollaard et 

al., 2004) have been reported in middle- and low-income nations. The impacts can be substantial; 

the above-cited 2001 floods in Mozambique are estimated to have resulted in an additional 447 

deaths from increased incidence of diarrheal disease (Cairncross and Alvarinho, 2006). 

While the IPCC (Confalonieri et al., 2007) noted that risks of infectious disease following floods 

are generally low in high-income countries, important exceptions have occurred. Most notably, 

following Hurricanes Katrina and Rita, the contamination of the drinking water supply with fecal 

bacteria resulted in increased incidence of diarrheal illness and some deaths (CDC, 2005b). 

Wildfires 

Regarding wildfires, the immediate risk of mortality is largely a function of the population in the 

affected area and the speed and intensity with which the wildfire moves through those areas. 

Although mortalities still occur in the United States on an annual basis (ranging from 14 to 28 

from 1998 to 2002), a well-established warning and evacuation system exists and has resulted in 

substantial reductions. Wildfires can have substantial effects, including through increased eye 

and respiratory illnesses due to fire-related air pollution and mental health impacts from 

evacuations, lost property, and damage to resources. Large fires are also accompanied by an 

increased number of patients seeking emergency services for inhalation of smoke and ash (Hoyt 

and Gerhart, 2004). Further discussion of health effects from particulate emissions (including 

those from wildfires) is in Section V.6.e under ‘Particulate Matter.’ 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 174
Droughts 

The IPCC (Confalonieri et al., 2007) concluded that drought can lead to the following health 

effects: 

• deaths, malnutrition (under-nutrition, protein-energy malnutrition, and/or micronutrient 

deficiencies), infectious diseases, and respiratory diseases (Menne and Bertollini, 2000); 

• diminished dietary diversity and reduced overall food consumption with subsequent negative 

health impacts (e.g., Hari Kumar et al., 2005; Aziz et al., 1990); and 

• population movements, particularly rural to urban migration, which may result in 

displacement impacts from overcrowding and a lack of safe water, food, and shelter 

(Choudhury and Bhuiya, 1993; Menne and Bertollini, 2000; del Ninno and Lundberg, 2005).

V.6.c Climate-sensitive infectious diseases 

Ebi et al. (2008) observed that the distinct seasonal pattern exhibited by most vector-, water-, or 

food-borne and animal-associated diseases suggests that weather and/or climate influence their 

distribution and incidence throughout the United States. They concluded that evidence for the 

United States indicates that climate affects both the abundance of vectors and ticks and the 

distributions of vectors and ticks that can carry West Nile virus, Western Equine encephalitis, 

Eastern Equine encephalitis, Bluetongue virus, and Lyme disease—perhaps affecting disease 

risk, but sometimes in counter-intuitive ways that do not necessarily translate to increased 

disease incidence (Wegbreit and Reisen, 2000; Subak, 2003; McCabe and Bunnell, 2004; 

DeGaetano, 2005; Purse et al., 2005; Kunkel et al., 2006; Ostfeld et al., 2006; Shone et al., 

2006). Field et al. (2007) concluded that climate change is likely to increase the risk and 

geographic spread of vector-borne infectious diseases, including Lyme disease and West Nile 

virus. 

As for water- and food-borne diseases, Ebi et al. (2008) concluded that these cause significant 

morbidity in the United States. For example, there were 1,330 food-related disease outbreaks in 

2002 (Lynch et al., 2006). In 2004, there were 34 outbreaks from recreational water and 30 

outbreaks from drinking water (Dziuban et al., 2006; Liang et al., 2006). Water- and food-borne 

diseases primarily result in gastroenteritis. In 2003 and 2004, gastroenteritis was noted in 48 and 

68% of reported recreational and drinking water outbreaks, respectively (Dziuban et al., 2006; 

Liang et al., 2006). Most pathogens of concern for food- and waterborne exposure are enteric and 

transmitted by the fecal–oral route. Climate may influence the pathogen by influencing its 

growth, survival, persistence, transmission, or virulence or through interactions with land use 

practices and climate variability. Storm events and flooding may result in the contamination of 

food crops (especially produce such as leafy greens and tomatoes) with feces from nearby 

livestock or feral animals. 

Ebi et al. (2008) note that the incidence of many water- and food-borne diseases is associated 

with temperatures. Given these associations, Ebi et al. (2008) concluded that increasing 

temperatures may result in increased incidence of disease, including the following: 

• Salmonellosis: annual peaks in salmonellosis cases occur within one to six weeks of the 

highest reported ambient temperatures in North America (United States and Canada) (Fleury 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 175
et al., 2006; Naumova et al., 2007), Australia (D’Souza et al., 2004), and several countries 

across Europe (Kovats et al., 2004a). 

• Leptospirosis: the most widespread zoonotic disease in the world, re-emerging in the United 

States (Meites et al., 2004), is linked to warm temperatures. 

• Vibrio spp.: accounts for 20% of sporadic shellfish-related illnesses and over 95% of deaths 

(Lipp and Rose, 1997; Morris, 2003); infections are more frequently associated with warm 

temperatures (e.g., Janda et al., 1988; Lipp et al., 2002). 

• Protozoan parasites, particularly Cryptosporidium and Giardia: case reports peak in late 

summer and early fall, particularly among younger age groups, generally associated with 

recreational water use (Dietz and Roberts, 2000; Furness et al., 2000). 

• Naegleria fowleri: infections are relatively rare, but nearly always fatal (Lee et al., 2002); 

associated with naturally warm or thermally polluted bodies of water. 

Such associations with temperature were not readily identified for the prevalence or infection 

rates of Campylobacter spp. (Fleury et al., 2006; Naumova et al., 2007 in Ebi et al., 2008). 

Temperature associations were also not readily identified for epidemiologically significant 

viruses including enteroviruses, rotaviruses, hepatitis A virus, and norovirus. Viruses account for 

67% of food-borne disease, and the vast majority of these are due to norovirus (Mead et al., 

1999). Norovirus infections typically peak in the winter (Cook et al., 1990; Lynch et al., 2006 in 

Ebi et al., 2008). 

Research also suggests that climate variables, including regional variations and ENSO 

interactions, may be an important driver for influenza outbreaks (Ebi et al., 2008). However, 

available research has not identified general trends or mechanisms regarding the role of climate 

in infection (e.g., Ebi et al., 2001; Flahault et al., 2004; Viboud et al., 2004; Dushoff et al., 2005; 

Greene et al., 2006; Choi et al., 2006 in Ebi et al., 2008). No studies have been able to identify a 

clear role for temperature in viral infection patterns (Ebi et al., 2008). 

Ebi et al. (2008) concluded that further expansion of ranges of insect- and rodent-borne diseases 

are expected in North America. However, major human epidemics of these diseases in the United 

States are unlikely if the public health infrastructure is maintained and improved as needed. 

Water quality 

Research reviewed in SAP 4.6 (Ebi et al., 2008) identified an association between precipitation 

intensity and outbreaks of waterborne illness in U.S. drinking water. An analysis of waterborne 

outbreaks associated with drinking water in the United States between 1948 and 1994 found that 

51% of outbreaks occurred following a daily precipitation event in the 90th percentile while 68% 

occurred when precipitation levels reached the 80th percentile (Curriero et al., 2001). Thomas et 

al. (2006) found that risk of waterborne disease doubled when rainfall amounts surpassed the 

93rd percentile. The relationship between rainfall and outbreaks exists for both surface and 

groundwater, although the association is strongest for surface water (Rose et al., 2000). 

As noted above, floodwaters may increase the likelihood of contaminated drinking water and 

also lead to incidental exposure to contaminated flood waters. Severe flooding following 

Hurricane Floyd in 1999 resulted in twice the average rate of gastrointestinal illness in the 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 176
affected areas (Setzer and Domino, 2004). Following the 2001 floods in the Midwest, contact 

with floodwater was shown to increase the rate and risk of gastrointestinal illness, especially 

among children. Tap water, however, continued to meet all regulatory standards (Wade et al., 

2004). 

Ebi et al. (2008) concluded that federal and state laws and regulatory programs protect much of 

the U.S. population from waterborne disease. However, if climate variability increases, current 

and future deficiencies in watershed protection, infrastructure, and storm drainage systems will 

tend to increase the risk of contamination events. 

V.6.d Aeroallergens 

Exposure to allergens results in allergic illnesses in approximately 20% of the U.S. population 

(AAAAI, 1996–2006). Although there is substantial evidence suggesting a causal relationship 

between aeroallergens and allergic illnesses, it remains unclear which aeroallergens are most 

important for sensitization and subsequent disease development (Nielsen et al., 2002). Not only 

the type, but also the amount of aeroallergen to which an individual is exposed, influences the 

development of an allergic illness. 

Climate change, including changes in atmospheric CO2 concentrations, could affect the 

production, distribution, dispersion, and allergenicity of aeroallergens and the growth and 

distribution of weeds, grasses, and trees that produce them (Confalonieri et al., 2007). These 

changes in aeroallergens and subsequent human exposures could affect the prevalence and 

severity of allergy symptoms. However, the scientific literature does not provide definitive data 

or conclusions on how climate change might affect aeroallergens and subsequently the 

prevalence of allergic illnesses in the United States. In addition, there are numerous other factors 

that affect aeroallergen levels and the prevalence of associated allergic illnesses, such as changes 

in land use, air pollution, and adaptive responses, many of which are difficult to assess. 

The IPCC concluded that pollens are likely to increase with elevated temperature and 

atmospheric CO2 concentrations in North America (Field et al., 2007). Moreover, warming and 

climate extremes are likely to increase pollen and ozone levels, both of which have the potential 

to exacerbate symptoms in people with respiratory illness. Laboratory studies that used a 

doubling of atmospheric CO2 stimulated a greater than 50% increase in ragweed pollen 

production (Wayne et al., 2002). A U.S.-based field study, which used existing temperature/CO2

concentration differences between urban and rural areas as a proxy for climate change, found 

that ragweed grew faster, flowered earlier, and produced significantly greater aboveground 

biomass and ragweed pollen at urban locations than at rural locations (Ziska et al., 2003 in Field 

et al., 2007). 

The IPCC (Confalonieri et al., 2007) noted that climate change has caused an earlier onset of the 

spring pollen season in North America (D’Amato et al., 2002; Weber, 2002; Beggs, 2004) and 

that there is limited evidence that the length of the pollen season has increased for some species. 

However, it is unclear whether the allergenic content of these pollens has changed. The IPCC 

concluded that introductions of new invasive plant species with high allergenic pollen present 

important health risks, noting that ragweed (Ambrosia artemisiifolia) is spreading in several parts 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 177
of the world (Rybnicek and Jaeger, 2001; Huynen and Menne, 2003; Taramarcaz et al., 2005; 

Cecchi et al., 2006 in Confalonieri et al., 2007). 

V.6.e Air quality 

The IPCC (2007b) projects with virtual certainty “declining air quality in cities” due to “warmer 

and fewer cold days and nights and/or warmer/more frequent hot days and nights over most land 

areas” across all world regions. Furthermore, the IPCC reports with very high confidence that 

climate change impacts on human health in U.S. cities will be compounded by population growth 

and an aging population (Field et al., 2007). Surface air concentrations of air pollutants are 

highly sensitive to winds, temperature, humidity, and precipitation (Denman et al., 2007). 

Climate change can be expected to influence the concentration and distribution of air pollutants 

through a variety of processes, including the modification of biogenic emissions, the change in 

chemical reaction rates, wash-out of pollutants by precipitation, and modification of weather 

patterns that influence pollutant buildup. 

In summarizing the impact of climate change on ozone and particulate matter, the IPCC 

(Denman et al., 2007) found that “future climate change may cause significant air quality 

degradation by changing the dispersion rate of pollutants, the chemical environment for ozone 

and particulate matter generation and the strength of emissions from the biosphere, fires and 

dust.” The IPCC also noted that large uncertainties limit the ability to provide a simple 

quantitative description of the interactions between biogeochemical processes and climate 

change (Denman et al., 2007).

In SAP 4.6, Ebi et al. (2008) concluded that currently millions of Americans live in areas that do 

not meet the health-based National Ambient Air Quality Standards for ozone and fine particulate 

matter, both of which have the potential to be influenced by climate change. 

Tropospheric ozone 

Ground-level ozone is formed mainly by reactions that occur in polluted air in the presence of 

sunlight. The key precursors for formation include nitrogen oxides (emitted mainly by the 

burning of fuels) and volatile organic compounds (emitted both by burning of fuels and by 

evaporation from vegetation and stored fuels). Ground-level ozone formation increases with 

greater sunlight and higher temperatures. 

According to the IPCC (Denman et al., 2007), climate change is expected to lead to increases in 

regional ozone pollution in the United States and other countries. Breathing air containing ozone 

can reduce lung function, thereby aggravating asthma and other respiratory conditions. Ozone 

exposure has been associated with increases in respiratory infection susceptibility, medicine use 

by asthmatics, emergency department visits, and hospital admissions. Exposure may contribute 

to premature death in people with heart and lung disease (EPA, 2006). Vulnerability to ozone 

health effects is greater for persons who spend time, especially with physical exertion, outdoors 

during episode periods because this results in a higher cumulative dose to the lung. Thus, 

children, outdoor laborers, and athletes may be at greater risk than people who spend more time 

indoors and who are less active (Ebi et al., 2008). In contrast to human health effects, which are 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 178
associated with short-term exposures, the most significant ozone-induced plant effects (e.g., 

biomass loss and yield reductions) result from the accumulation of ozone exposures over the 

growing season, with differentially greater impact resulting from exposures to higher 

concentrations and/or longer durations (EPA, 2006). 

Tropospheric ozone is both naturally occurring and, as the primary constituent of urban smog, a 

secondary pollutant. As a secondary pollutant, it is formed through photochemical reactions 

involving nitrogen oxides (NOx) and volatile organic compounds (VOCs) in the presence of 

sunlight. Biomass burning, which is known to make a large contribution to ozone in the tropical 

troposphere (Thompson et al., 1996), is expected to increase in the tropics and at high latitudes 

with climate change (Price and Rind, 1994; Stocks et al., 1998; Williams et al., 2001; Brown et 

al., 2004). As described below, climate change can affect ozone by modifying 

• emissions of precursors, 

• atmospheric chemistry, and 

• transport and removal (Denman et al., 2007). 

The IPCC (Denman et al., 2007) concluded that, for all world regions, climate change affects the 

sources of ozone precursors in the following ways: 

• Physical response: NOx emissions due to lightning are expected to increase in a warmer 

climate. General circulation models concur that climate warming could increase the influx of 

ozone from the stratosphere to the troposphere due to large-scale atmospheric circulation 

shifts (i.e., the Brewer–Dobson circulation). 

• Biological response (soils, vegetation): biogenic VOC emissions increase with increasing 

temperature. Climate-induced changes in biogenic VOC emissions alone may be regionally 

substantial and cause significant increases in ozone concentrations (European Commission, 

2003; Hogrefe et al., 2004; Hauglustaine et al., 2005). Sensitivity simulations for the 2050s, 

relative to the 1990s (under the IPCC A2 scenario discussed in Section IV.3.a), indicate that 

increased biogenic emissions alone add one to three parts per billion (ppb) to summertime 

average daily maximum 8-hour ozone concentrations49 in the Midwest and along the Eastern 

Seaboard (Hogrefe et al., 2004). The IPCC (Meehl et al., 2007) reports that biogenic 

emissions are projected to increase by between 27 and 59%, contributing to a 30 to 50% 

increase in ozone formation over northern continental regions (under the IPCC A2 scenario 

for the 2090–2100 timeframe, relative to 1990–2000) (Denman et al., 2007). 

 

Climate change impacts on temperature and water vapor could affect ozone chemistry 

significantly (Denman et al., 2007). A number of studies in the United States have shown that 

summer daytime ozone concentrations correlate strongly with temperature. That is, ozone 

generally increases at higher temperatures. This correlation appears to reflect contributions of 

comparable magnitude from: 

• temperature-dependent biogenic VOC emissions (as mentioned above); 

• thermal decomposition of peroxyacetylnitrate (PAN), which acts as a reservoir for NOx, (as 

described immediately below); and 

• the association of high temperatures with regional stagnation (also discussed below) 

 

49 The daily maximum 8-hour ozone concentration is a measurement of air quality based on averaging the fourthhighest daily maximum ozone concentrations over an 8-hour period. The standard is set at 0.08 ppm. 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 179
(Denman et al., 2007). 

Climate change is projected to increase surface layer ozone concentrations in both urban and 

polluted rural environments (any world region of high emissions) due to decomposition of PAN 

at higher temperatures (Sillman and Samson, 1995; Liao et al., 2006). Warming enhances 

decomposition of PAN, releasing NOx, an important ozone precursor (Stevenson et al., 2005). 

Model simulations for the year 2100 show that enhanced PAN thermal decomposition leads to an 

increase in ozone net production (Hauglustaine et al., 2005). 

Atmospheric circulation can be expected to change in a warming climate and thus modify 

pollutant transport and removal. The more frequent occurrence of stagnant air events in urban or 

industrial areas could enhance the intensity of air pollution events, although the importance of 

these effects is not yet well quantified (Denman et al., 2007). The IPCC (2007a) concluded that 

“extra-tropical storm tracks are projected to move poleward, with consequent changes in wind, 

precipitation, and temperature patterns, continuing the broad pattern of observed trends over the 

last half-century.” 

The IPCC (Denman et al., 2007) cited the Mickley et al. (2004) study for the eastern United 

States, which found an increase in the severity and persistence of regional pollution episodes due 

to the reduced frequency of ventilation by storms tracking across Canada. This study found that 

surface cyclone (storm) activity decreased by approximately 10 to 20% in a future simulation 

(for 2050, under the IPCC A1B scenario), which is in general agreement with a number of 

observational studies over the northern mid-latitudes and North America (Zishka and Smith, 

1980; Agee, 1991; Key and Chan, 1999; McCabe et al., 2001). In this study, summer pollution 

episodes in the northeastern United States were projected to increase in severity and duration; 

pollutant concentrations in episodes increase by 5 to 10% and episode durations increase from 

two to three to four days. 

Regarding the role that water vapor plays in tropospheric ozone formation, the IPCC (Denman et 

al., 2007) reported that simulations for the 21st century indicate a decrease in the lifetime of 

tropospheric ozone due to increasing water vapor. The projected increase in water vapor both 

decelerates the chemical production and accelerates the chemical destruction of ozone (Meehl et 

al., 2007). Overall, the IPCC found that climate change is expected to decrease background 

tropospheric ozone due to higher water vapor and to increase regional and urban-scale ozone 

pollution due to higher temperatures and weaker air circulation (Denman et al., 2007; 

Confalonieri et al., 2007). 

For North America, the IPCC (Field et al., 2007) reported that surface ozone concentration may 

increase with a warmer climate. Several studies predict increases in ozone concentrations due to 

climate change in the near future (2020s and 2030s) as well as for 2050 and beyond. The IPCC 

(Field et al., 2007; Wilbanks et al., 2007a) cited Hogrefe et al. (2004) who evaluated the effects 

of climate change on regional ozone in 15 U.S. cities, finding that average summertime daily 8-

hour maximum ozone concentrations could increase by 2.7 ppb in the 2020s and by 4.2 ppb in 

the 2050s (under the IPCC A2 scenario). For the 2050s (under the IPCC A2 scenario), the IPCC 

(Field et al., 2007) cited Bell et al. (2007), who reported that the projected effects of climate 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 180
change on ozone in 50 eastern U.S. cities increases the number of summer days exceeding the 8-

hour EPA standard by 68% (Bell et al., 2007). 

Mickley et al. (2004 in Confalonieri et al., 2007) projected that between 2000 and 2050 

significant changes will occur at the high end of the pollutant concentration distribution 

(episodes) in the Midwest and Northeast. Bell and Ellis (2004) also found that the largest 

increases in ozone concentrations occur near peak values. While the summer average of daily 

maximum 8-hour ozone concentrations increases by 2.7, 4.2, and 5.0 ppb in the 2020s, 2050s, 

and 2080s (compared to the 1990s), respectively, the fourth-highest summertime daily maximum 

8-hour ozone concentrations (levels used to measure compliance with air quality standards for 

ozone) increase by 5.0, 6.4, and 8.2 ppb for the 2020s, 2050s, and 2080s, respectively when 

compared to the 1990s (Hogrefe et al., 2004). These findings raise particular health concerns. 

The IPCC (Field et al., 2007) found that “warming and climate extremes are likely to increase 

respiratory illness, including exposure to pollen and ozone.” Holding population, dose-response 

characteristics, and pollution prevention measures constant, ozone-related deaths from climate 

change in the United States are projected to increase by approximately 4.5% from the 1990s to 

the 2050s (under the IPCC A2 scenario) (Bell et al., 2007; Field et al., 2007). According to the 

IPCC (Field et al., 2007), the “large potential population exposed to outdoor air pollution 

translates this small relative risk into a substantial attributable health risk.” In New York City, 

health impacts could be further exacerbated by climate change interacting with urban heat island 

effects (Field et al., 2007). However, all of these models predict increasing ozone without 

considering expected emissions controls that would likely be required to ensure meeting the 

National Ambient Air Quality Standards. 

The IPCC reported (Denman et al., 2007) that “the current generation of tropospheric ozone 

models is generally successful in describing the principal features of the present-day global 

ozone distribution.” However, they also find “there are major discrepancies with observed longterm trends in ozone concentrations over the 20th century” and “resolving these discrepancies is 

needed to establish confidence in the models.”

In addition to human health effects, tropospheric ozone has significant adverse effects on crop 

yields in the United States and other world regions, pasture and forest growth, and species 

composition (EPA, 2006; Easterling et al., 2007). Furthermore, the effects of air pollution on 

plant function may affect carbon storage. Recent research showed that tropospheric ozone 

resulted in significantly less enhancement of carbon sequestration rates under elevated CO2

(Loya et al., 2003), due to negative effects of ozone on biomass productivity and changes in litter 

(organic ground cover) chemistry (Booker et al., 2005; Liu et al., 2005). 

Particulate matter 

Particulate matter is a far more complex pollutant than ozone. Particulate matter is a complex 

mixture of anthropogenic, biogenic, and natural materials suspended as aerosol particles in the 

atmosphere. When inhaled, the smallest of these particles can reach the deepest regions of the 

lungs. Scientific studies have found an association between exposure to particulate matter and 

significant health problems, including aggravated asthma, chronic bronchitis, reduced lung 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 181
function, irregular heartbeat, heart attack, and premature death in people with heart or lung 

disease. Particle pollution also is the main cause of visibility impairment in the Nation’s cities 

and national parks (EPA, 2004). 

The overall directional impact of climate change on particulate matter levels in the United States 

remains uncertain. The body of literature specifically addressing the potential effects of climate 

change on particulate matter is limited, but there is a substantial body of literature describing 

how ambient particulate matter responds to a wide range of meteorological conditions. On the 

basis of this information, broad conclusions can be drawn concerning the behavior of ambient 

particulate matter concentrations given a set of the meteorological changes anticipated in a 

warming climate. Those meteorological changes are expected to affect particulate matter 

concentrations by modifying 

• emissions of primary particulate matter and particulate matter precursor emissions, 

• aerosol photochemistry, and 

• transport and removal, as described below. 

Particulate Emissions 

Particulate matter and particulate matter precursor emissions are affected by climate change 

through physical response (wind-blown dust), biological response (forest fires and vegetation 

type/distribution), and human response (energy generation). Most natural aerosol sources are 

controlled by climatic parameters like wind, moisture, and temperature. Therefore, humaninduced climate change is expected to affect the natural aerosol burden. Biogenic organic 

material is both directly emitted into the atmosphere and produced by VOCs. All biogenic VOC 

emissions are highly sensitive to changes in temperature, and are also highly sensitive to climateinduced changes in plant species composition and biomass distributions. Biogenic emission rates 

are predicted to increase, on average, across world regions by 10% per 1 °C increase in surface 

temperature (Guenther et al., 1993; Denman et al., 2007). The response of biogenic secondary 

organic carbon aerosol production to a temperature change, however, could be considerably 

lower than the response of biogenic VOC emissions, since aerosol yields can decrease with 

increasing temperature (Denman et al., 2007). Particulate matter emissions from wildland fires 

can contribute to acute and chronic illnesses of the respiratory system, particularly in children, 

including pneumonia, upper respiratory diseases, asthma, and chronic obstructive pulmonary 

diseases (WHO, 2002; Bowman and Johnston, 2005; Moore et al., 2006 in Confalonieri et al., 

2007). 

Photochemistry 

Particulate matter chemistry is affected by changes in temperature brought about by climate 

change. Temperature is one of the most important meteorological variables influencing air 

quality in urban atmospheres because it affects gas and heterogeneous chemical reaction rates 

and gas-to-particle partitioning. The net effect that increased temperature has on airborne particle 

concentrations is a balance between increased production rates for secondary particulate matter 

(which increases particulate concentrations) and increased equilibrium vapor pressures for semivolatile particulate compounds (which decreases particulate concentrations). Increased 

temperatures may either increase or decrease the concentration of semi-volatile secondary 

reaction products, such as ammonium nitrate, depending on ambient conditions. Regions with 

relatively warm initial temperatures (>17 °C) may experience a reduction in particulate 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 182
ammonium nitrate concentrations as temperature increases, while regions with relatively cool 

initial temperatures (<17 °C) may experience minor reductions or even small increases in 

particulate ammonium nitrate concentrations as temperature increases. 

Transport and Removal 

The transport and removal of particulate matter is highly sensitive to winds and precipitation. 

Removal of particulate matter from the atmosphere occurs mainly by wet deposition (in which 

atmospheric pollutants mix with water vapor and fall to Earth as precipitation) (NRC, 2005a). 

Sulfate lifetime, for example, is estimated to be reduced from 4.7 days to 4.0 days as a result of 

increased wet deposition (Liao et al., 2006). Precipitation also affects soil moisture, with impacts 

on dust source strength and on stomatal opening/closure of plant leaves, hence affecting biogenic 

emissions (Denman et al., 2007). Precipitation has generally increased over land north of 30° N 

over the period 1900 to 2005 and it has become significantly wetter in eastern parts of North 

America (Trenberth et al., 2007). However, model parameterizations of wet deposition are highly 

uncertain and not fully realistic in their coupling to the hydrologic cycle (NRC, 2005a). For 

models to simulate accurately the seasonally varying pattern of precipitation, they must correctly 

simulate a number of processes (e.g., evapotranspiration, condensation, and transport) that are 

difficult to evaluate at a global scale (Randall et al., 2007). 

V.7 Energy Production, Use, and Distribution 

To date, most discussions on energy and climate change have focused on mitigation. However, 

“along with this role as a driver of climate change, the energy sector will be subject to effects of 

climate change” (Wilbanks et al., 2007b). Due to the pervasive role that energy plays in our 

society, even relatively small disruptions in energy supply can have major economic and social 

consequences. As discussed in earlier sections, a warming climate is expected to have certain 

effects in the United States, including rising average temperatures in most regions, changes in 

precipitation amounts and seasonal patterns in many regions, changes in the intensity and pattern 

of extreme weather events, and rising sea levels in coastal areas. These effects are expected to 

lead to changes in the production, consumption, and distribution of energy resources. For 

instance, average warming can be expected to increase energy requirements for cooling and 

reduce energy requirements for heating. Changes in precipitation could affect prospects for 

hydropower, positively or negatively. Increases in storm intensity could threaten further 

disruptions of the sort experienced in 2005 with Hurricane Katrina. Concerns about climate 

change impacts could change perceptions and valuations of energy technology alternatives. Any 

or all of these types of effects could have very real meaning for energy policies, decisions, and 

institutions in the United States, affecting discussions of courses of action and appropriate 

strategies for risk management (Wilbanks et al., 2007b). 

In SAP 4.5, CCSP (Scott and Huang, 2007; Wilbanks et al., 2007b) cited the following potential 

effects: 

• decreases in the amount of energy consumed in residential, commercial, and industrial 

buildings for space heating and water heating; 

• increases in energy used in residential, commercial, and industrial buildings for space 

cooling; 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 183
• increases in energy consumed for residential and commercial refrigeration and industrial 

process cooling (e.g., in thermal power plants or steel mills); 

• increases in energy used to supply other resources for climate-sensitive processes, such as 

pumping water for irrigated agriculture and municipal uses; 

• changes in the balance of energy use among delivery forms and fuel types, as between 

electricity used for air conditioning and natural gas used for heating; 

• changes in energy consumption in key climate-sensitive sectors of the economy, such as 

transportation, construction, agriculture, and others; 

• positive or negative effects on prospects for hydropower from changes in precipitation 

patterns and amounts; 

• further disruptions of the sort experienced in 2005 with Hurricanes Katrina and Rita; and 

• changing perceptions and valuations of energy technology alternatives from concerns about 

climate change impacts. 

Because of the emphasis on mitigation, limited information is available about climate change 

effects on the energy sector. SAP 4.5 is the first overview of vulnerability and adaptive response 

issues for the energy sector in the United States. This section relies heavily on this work. 

V.7.a Energy use 

(adapted from Scott and Huang, 2007) 

According to the IPCC, climate change is likely to affect U.S. energy use and energy production, 

physical infrastructures, and institutional infrastructures, and will likely interact with and 

possibly exacerbate ongoing environmental change and environmental pressures in settlements 

(Wilbanks et al., 2007a). As noted in SAP 4.5, overall research is relatively clear that climate 

warming will mean reductions in total U.S. heating requirements and increases in total cooling 

requirements for buildings (Wilbanks et al., 2007b). Changes will vary by region and season and 

will affect household and business energy demands and cost. In general, the changes imply 

increased demands for electricity, which supplies virtually all cooling energy services but only 

some heating services. Natural gas is the most common heating fuel, fuel oil is commonly used 

in the Northeast, while in parts of the country with relatively short, mild winters and/or 

inexpensive electricity, electricity is commonly used for heating. 

With climate warming, less heating is required for industrial, commercial, and residential 

buildings in the United States. A review of the relevant literature in SAP 4.5 (Scott and Huang, 

2007) found that projected reductions varied across studies depending on the amount of 

temperature change in the climate scenario, the calculated sensitivity of the building stock to 

warming, and the adjustments allowed in the building stock over time. 

Heating 

Studies on residential heating reductions have shown considerable variation. As reported in SAP 

4.5 (Scott and Huang, 2007): 

• Mansur et al. (2005) identified relatively modest reductions. When natural gas is available, 

the marginal impact of a 1 °C increase in January temperatures is predicted to reduce 

residential electricity consumption by 2.8% for electricity-only consumers and 2% for natural 

gas customers. 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 184
• Scott et al. (2005) projected about a 6 to 10% decrease in space heating per 1 °C increase by 

2020. 

• The decreases in Huang’s (2006) studies varied considerably by location and building 

vintage; the overall average was about 9.2% per 1 °C increase. 

• Regional level studies using building models found a sensitivity of about 6 to 10% per 1 °C 

in temperature change and only about 1% per 1 °C in studies using econometrics. This is 

possibly due in part to reactive increases in energy consumption (energy consumption ‘takebacks’) as heating energy costs decline with warmer weather as well as choice of region. 

• Two studies particularly highlighted the potential for regional variation as results differed 

substantially despite the involvement of many of the same researchers using very similar 

methodologies. Amato et al. (2005) projected about a 7 to 33% decline in space heating in 

the 2020s in Massachusetts, which has a long heating season, while Ruth and Lin (2006) 

projected only a 2 to 3% decline space heating energy during the same time frame in 

Maryland, which has a much milder heating season. 

Similar results are projected in the commercial sector (Scott and Huang, 2007): 

• With building equipment and shell efficiencies maintained at 1990 baseline levels and a 3.9 

°C temperature change, Belzer et al. (1996) predicted a decrease in annual space heating 

energy requirements of about 7.4 to 9.0% per 1 °C. 

• Mansur et al. (2005) projected that a 1 °C increase in January temperatures would reduce 

electricity consumption by about 3%, natural gas consumption by 3%, and fuel oil demand 

by 12%. 

• Huang (2006) showed that the impact of climate change on commercial building energy use 

varies greatly depending on climate and building type. For the entire U.S. commercial sector, 

the simulations showed a 9.2% decrease per 1 °C. 

• As with the residential results, regional analyses suggest more dramatic decreases in energy 

demand in colder regions than in warmer ones. However, the differences are smaller in the 

commercial sector because commercial building energy demands are dominated by internal 

loads such as lighting and equipment. In Loveland and Brown (1990), results vary from a 

10.1 to 12.5% decrease per 1 °C. Findings for Massachusetts vary from a 7 to 8% decline in 

2020 (Amato et al., 2005) and for Maryland a 2.7% decrease (Ruth and Lin, 2006). 

 

Cooling 

When considering demands for cooling, however, all studies reviewed in SAP 4.5 suggested 

increases in energy demand (primarily provided by electricity) in all regions with climate 

warming. Scott and Huang (2007) found that, in most cases, this effect is nonlinear with respect 

to temperature and humidity, such that the percentage impact increases more than 

proportionately with increases in temperature (Sailor, 2001). Overall, the commercial sector 

appears to be less sensitive to climate increases. Findings are summarized below: 

• National studies projected residential increases in electricity demand by approximately 5 to 

20% per 1 °C temperature increase (Rosenthal et al., 1995; Mansur et al., 2005; Scott et al., 

2005; Huang, 2006). For the commercial sector, the projected increases varied from 3.5 to 

15%. 

• Two of these studies looked specifically at the residential sector cooling demand (rather than 

all electricity). Scott et al. (2005) projected a 12 to 20% increase in cooling demand per 1 °C 

in 2020, while Huang (2006) projected a 22.4% increase per 1 °C. 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 185
• Loveland and Brown (1990) found cooling energy consumption increased by between 55.7% 

(for Fort Worth, which started from a relatively high base) and 146% (for Seattle, which 

started from a very low base) for a temperature increase of 3.7 to 4.7 °C. This implies about a 

17 to 31% increase in cooling energy consumption per 1 °C. 

• In mild, cooler climates, relatively small increases in temperature can have a large impact on 

air-conditioning energy use by reducing the potential for relying on natural ventilation or 

night cooling. For example, in California, Mendelsohn (2003) projected that total energy 

expenditures for electricity used for residential cooling would increase nonlinearly and that 

net overall energy expenditures would increase with warming in the range of 1.5 °C. Sailor 

(2001) projected that per 1 °C increases would be 0.5% in New York and 5.8% in Florida. 

• Amato et al. (2005) projected a 6.8% increase in Massachusetts by 2020 (increasing 10 to 

40% by 2030) while results in Ruth et al. (2006) for the more southerly state of Maryland 

appear to be quite sensitive to electricity prices, ranging from a 2.5% increase at high prices 

(about 8 cents per kWh, 1990 dollars) to a 24% increase if prices were low (about 6 cents per 

kWh, 1990 dollars). 

• State-level studies of commercial cooling generally follow national trends. Analyses 

performed with building energy models generally indicate a 10 to 15% electric energy 

increase for cooling per 1 °C. The econometric studies also show increases, although the 

percentage increases are much smaller as these models generally include the change in 

consumption of all electricity rather than just that used for space cooling. 

These results are compounded by potential increases in market penetration of air conditioning, 

particularly in areas where temperature increases are expected to be greatest. Sailor and Pavlova 

(2003) have projected that the extra market penetration of air conditioning more than doubled the 

energy use due to temperature alone. Other trends that can influence future demand include 

demographic shifts (e.g., population currently expanding in southern and western states), 

increases in the floor space per building occupant in both the residential and commercial sectors, 

and improvements in building efficiency and building shell performance (Scott and Huang, 

2007). 

Combined heating and cooling 

Many of these studies address both heating and cooling and attempt to come to a ‘bottom line’ 

net result for energy consumption. Results differ based on methods, time frame, scenario, and 

geography—with some showing slight annual increases in overall energy demand (e.g., Linder 

and Inglis, 1989; Mendelsohn, 2001; Mansur et al., 2005) and others slight reductions (e.g., 

Rosenthal et al., 1995; Scott et al., 2005). Results also differ by region. For example, Sailor’s 

state-level econometric analyses (Sailor and Muñoz, 1997; Sailor, 2001; Sailor and Pavlova, 

2003) projected a 5% increase per 1 °C warming in residential per capita electricity demand in 

Florida (a summer-peaking state dominated by air-conditioning demand) and a 3% decrease per 

1 °C warming in Washington State (which uses electricity extensively for heating and is a 

winter-peaking system). Similarly, at the individual city level, Loveland and Brown (1990) 

projected lower residential energy load in northern cities such as Chicago, Minneapolis, and 

Seattle and increased energy loads in southern cities such as Charleston, Fort Worth, and 

Knoxville—while office buildings showed an overall increase in energy loads in all six cities. 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 186
Another important concern for energy use is timing of demand. In SAP 4.5, Scott and Huang 

(2007) indicate that research to date suggest temperature increases would increase peak demand 

for electricity in most regions of the country, except in the Pacific Northwest. For example, a 

multiregional study of regional electricity demand found that although annual electricity 

consumption increased slightly (3.4–5.1%), peak electricity demand would increase between 8.6 

and 13.8%, and capacity requirements would increase 13.1 to 19.7%—costing tens of billions of 

dollars (Linder and Inglis, 1989). 

In SAP 4.5, Scott and Huang (2007) found that across all these studies, results suggested only a 

slight change (increase or decrease) in net energy demand per 1 °C. However, even minor 

changes could have substantial economic results. For example, in California small increases of 

energy use of 0.6 to 2.6% would signify increases of 1,741 GWh to 7,516 GWh, while increases 

of 0.34 to 1.51% or 2.57 to 2.99% in electricity demand correspond to increased peak demand by 

221 to 967 MW and 1,648 to 1,916 MW (Baxter and Calandri, 1992). To put these impacts in 

perspective, actual growth in non-coincident peak demand between 1990 and 2004 was 8,650 

MW for total end-use load and 9,375 MW for gross generation (California Energy Commission, 

2006a). 

Drawing on the range of results covered here, Scott and Huang (2007) conclude that across all 

findings there is a robust result that, in the absence of a energy efficiency measures directed at 

space cooling, climate change would cause a significant increase in the demand for electricity in 

the United States, which would require the building of additional electricity generation (and 

probably transmission facilities) worth many billions of dollars. 

These findings led the IPCC to the following conclusion for North America: 

Recent North American studies generally confirm earlier work showing a small net change 

(increase or decrease, depending on methods, scenarios, and location) in net demand for energy in 

buildings but a significant increase in demand for electricity for space cooling, with further 

increases caused by additional market penetration of air conditioning (high confidence). (Field et 

al., 2007) 

V.7.b Energy production and distribution 

(adapted from Bull et al., 2007) 

Climate change could affect U.S. energy production and supply in the following ways (Wilbanks 

et al., 2007a): 

• immediate impacts from increased intensity of extreme weather events; 

• reductions in water supplies in regions dependent on water resources for hydropower and/or 

thermal power plant cooling; 

• influencing facility siting decisions (based on changing conditions); and 

• positive or negative impacts for biomass, wind power, or solar energy production (where 

climatic conditions change). 

Significant uncertainty exists about the potential impacts of climate change on energy production 

and distribution, in part because the timing and magnitude of climate impacts are uncertain. 

Nonetheless, every existing source of energy in the United States has some vulnerability to 

climate variability. Bull et al. (2007) found that U.S. energy production is dominated by fossil 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 187
fuels: coal, petroleum, and natural gas. Renewable energy sources tend to be more sensitive to 

climate variables, but production from traditional fossil sources can also be adversely affected by 

air and water temperatures, while nuclear energy also requires a thermoelectric cooling process. 

Moreover, extreme weather events can affect all sources through adverse effects on production, 

distribution, and fuel transportation. 

In SAP 4.5, Bull et al. (2007) reviewed the limited research literature available on these impacts 

to date and concluded that while effects on the existing infrastructure might be categorized as 

modest, local and industry-specific impacts could be large, especially in areas that may be prone 

to disproportional warming (Alaska) or weather disruptions (Gulf Coast and Gulf of Mexico). 

Potential impacts are highlighted in Table V.7.1. 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 188
Table V.7.1. Mechanisms of Climate Impacts on Various Energy Supplies in the United States. 

Percentages are of total domestic consumption (T = water/air temperature, W = wind, H = humidity, P = 

precipitation, and E = extreme weather events). (Source: EIA, 2004) 

Energy Supplies Climate Impact Mechanisms 

Coal (22%) 

Cooling water quantity and quality (T), 

cooling efficiency (T, W, H), erosion in 

surface mining 

Natural Gas (23%) 

Cooling water quantity and quality (T), 

cooling efficiency (T, W, H), disruptions 

of off-shore extraction (E) 

Petroleum (40%) 

Cooling water quantity and quality, 

cooling efficiency (T, W, H), disruptions 

of off-shore extraction and transport (E) 

Fossil 

Fuels 

(86%) 

Liquefied Natural Gas (1%) Disruptions of import operations (E) 

Nuclear 

(8%) 

Cooling water quantity and quality (T), 

cooling efficiency (T, W, H) 

Hydropower 

Water availability and quality, 

temperature-related stresses, operational 

modification from extreme weather 

(floods/droughts), (T, E) 

 Biomass 

• Wood and forest products 

Possible short-term impacts from timber 

kills or long-term impacts from timber kills 

and changes in tree growth rates (T, P, H, 

E, CO2 levels) 

• Waste (municipal solid 

waste, landfill gas, etc.) n/a 

• Agricultural resources 

(including derived 

biofuels) 

Changes in food crop residue and 

dedicated energy crop growth rates (T, P, 

E, H, CO2 levels) 

Wind Wind resource changes (intensity and 

duration), damage from extreme weather 

Solar Insolation changes (clouds), damage from 

extreme weather 

Renewables

(6%) 

Geothermal Cooling efficiency for air-cooled 

geothermal (T) 

 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 189
Fossil and nuclear energy 

As illustrated in Table V.7.1, climate change is expected to affect fossil fuel and nuclear power 

in similar ways. In both cases, the most direct climate impacts are related to power plant cooling 

and water availability. As currently designed, power plants require significant amounts of water, 

and they will be vulnerable to fluctuations in water supply. Regional-scale changes would 

probably mean that some areas would see significant increases in water availability, while other 

regions would see significant decreases. In those areas seeing a decline, the impact on power 

plant availability or even siting of new capacity could be significant (Bull et al., 2007). While 

new plant designs are flexible and new technologies for water reuse, heat rejection, and use of 

alternative water sources are being developed, situations where the development of new power 

plants is being slowed down or halted due to inadequate cooling water are becoming more 

frequent throughout the United States (SNL, 2006 in Bull et al., 2007). 

In addition, Bull et al. (2007) found that average ambient temperatures can affect generation 

cycle efficiency, along with cooling water requirements in the electrical sector, and water 

requirements for energy production and refining. While these impacts may appear small based on 

the proportional reduction in system efficiency or reliability, the vast scale of the energy industry 

amplifies the impact. A net reduction in generation of 1% due to increased ambient temperature 

would represent a drop in supply of 25 billion kilowatt-hours (Maulbetsch and DiFilippo, 2006). 

In the Gulf of Mexico, which accounts for 20 to 30% of the total domestic oil and gas production 

in the United States, energy producers must adhere to guidelines regarding water temperature 

impacts as a result of the formation of related anoxic zones. Constraints on produced water 

discharges could increase costs and reduce production in the Gulf of Mexico region and 

elsewhere. 

Regarding the requirements for the production of energy from fossil fuels, Bull et al. (2007) 

wrote in SAP 4.5: 

An October 2005 report produced by the National Energy Technology Laboratory stated, in part, 

that the production of energy from fossil fuels (coal, oil, and natural gas) is inextricably linked to 

the availability of adequate and sustainable supplies of water. While providing the United States 

with a majority of its annual energy needs, fossil fuels also place a high demand on the Nation’s 

water resources in terms of both use and quality impacts (EIA, 2005). Thermoelectric generation is 

water intensive; on average, each kWh of electricity generated via the steam cycle requires 

approximately 25 gallons of water, a weighted average that captures total thermoelectric water 

withdrawals and generation for both once-through and recirculating cooling systems to produce. 

According to the United States Geological Survey (USGS), power plants rank only slightly behind 

irrigation in terms of freshwater withdrawals in the United States (USGS, 2004), although 

irrigation withdrawals tend to be more consumptive. Water is also required in the mining, 

processing, and transportation of coal to generate electricity all of which can have direct impacts 

on water quality. Surface and underground coal mining can result in acidic, metal-laden water that 

must be treated before it can be discharged to nearby rivers and streams. In addition, the USGS 

estimates that in 2000 the mining industry withdrew approximately 2 billion gallons per day of 

freshwater. Although not directly related to water quality, about 10% of total U.S. coal shipments 

were delivered by barge in 2003 (USGS, 2004). Consequently, low river flows can create 

shortfalls in coal inventories at power plants. 

Freshwater availability is also a critical limiting factor in economic development and 

sustainability, which directly impacts electric-power supply. A 2003 study conducted by the 

Government Accountability Office indicates that 36 States anticipate water shortages in the next 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 190
10 years under normal water conditions, and 46 States expect water shortages under drought 

conditions (GAO, 2003). Water supply and demand estimates by the Electric Power Research 

Institute (EPRI) for the years 1995 and 2025 also indicate a high likelihood of local and regional 

water shortages in the United States (EPRI, 2003). The area that is expected to face the most 

serious water constraints is the arid southwestern United States. 

Accordingly, the competition for access to water is expected to increase between power 

producers and other needs. 

Renewable energy 

Because renewable energy depends directly on ambient natural resources (such as hydrologic 

resources, wind patterns and intensity, and solar radiation), it is expected to be more sensitive to 

climate variability than fossil or nuclear energy systems that rely on geological stores. 

Renewable energy systems are also vulnerable to damage from extreme weather events. At the 

same time, renewable energy production is a primary means for mitigating the impacts of 

potential climate change by reducing energy-related greenhouse gas emissions (Bull et al., 2007). 

Hydropower 

Hydropower is the largest U.S. renewable source of electricity. Projects vary from those with 

large, multipurpose reservoirs to small run-of-river projects that have little or no active water 

storage. Hydropower generation is sensitive to the amount, timing, and geographical pattern of 

precipitation as well as temperature (rain or snow, timing of melting) and varies greatly from 

year to year. For example, the difference between the most recent high (2003) and low (2001) 

generation years is 59 billion kilowatt-hours, approximately equal to the total electricity from 

biomass sources and much more than the generation from all other non-hydropower renewables 

(EIA, 2006). The amount of water available for hydroelectric power varies greatly from year to 

year, depending upon weather patterns and local hydrology, as well as on competing water uses, 

such as flood control, water supply, recreation, and in-stream flow requirements (e.g., 

conveyance to downstream water rights, navigation, and protection of fish and wildlife). In 

addition to climate variability, the annual fluctuations in hydropower are also affected by 

multiple-use operational policies and regulatory compliance. 

Reduced stream flows are expected to jeopardize hydropower production in some areas of the 

United States, whereas greater streamflows, depending on their timing, might be beneficial 

(Wilbanks et al., 2007a). In California, where hydropower now comprises about 15% of in-state 

energy production, diminished snowmelt flowing through dams will decrease the potential for 

hydropower production by up to 30% if temperatures rise to the medium warming range by the 

end of the century (5.5–8 °F or ~3.1–4.4 °C increase in California) and precipitation decreases by 

10 to 20%. However, future precipitation projections are quite uncertain so it is possible that 

precipitation will increase and expand hydropower generation (California Energy Commission, 

2006b). 

Hydroelectric generation is highly sensitive to changes in precipitation and river discharge. In 

many cases, changes in streamflow are magnified as water flows through multiple power plants 

in a river basin. For example, Nash and Gleick (1993) estimated that the change in generation 

was three times the change in streamflow in the Colorado River. Climate impacts on hydropower 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 191
occur when either the total amount or the timing of runoff is altered, for example, when natural 

water storage in snowpack and glaciers is reduced under hotter climates (e.g., melting of glaciers 

in Alaska and the Rocky Mountains of the United States). Projections that climate change is 

expected to reduce snowpack and associated runoff in the West are a matter of particular 

concern. Significant changes are being detected now in the flow regimes of many western rivers 

(Dettinger, 2005) that are consistent with the predicted effects of global warming. 

Hydropower operations can also be affected through changes in water quality and reservoir 

dynamics. For example, warmer air temperatures and a more stagnant atmosphere cause more 

intense stratification of reservoirs behind dams and a depletion of dissolved oxygen in 

hypolimnetic waters50 (Meyer et al., 1999). Where hydropower dams have tailwaters supporting 

coldwater fisheries for trout or salmon, warming of reservoir releases may have unacceptable 

consequences and require changes in project operation that reduce power production. In addition, 

evaporation from the surface of reservoirs may lead to reduced water availability for hydropower 

or other uses. While the effects of climate change on evaporation rates is not straightforward, 

evidence suggests it will have the greatest impacts on large reservoirs with large surface area 

located in arid, sunny parts of the United States, such as Lake Mead on the lower Colorado River 

(Westenburg et al., 2006). 

The IPCC concluded (Field et al., 2007): 

• For a 2 to 3 ° C warming in the Columbia River Basin and British Columbia Hydro service 

areas, the hydroelectric supply under worst-case water conditions for winter peak demand 

will likely increase (high confidence). Bull et al. (2007) anticipated impacts with other 

values. Specifically, summer power generation will likely conflict with in-stream flow 

targets and salmon restoration goals established under the Endangered Species Act (Payne et 

al., 2004). 

• Colorado River hydropower yields will likely decrease significantly (medium confidence) 

(Christensen et al., 2004), as will Great Lakes hydropower (Moulton and Cuthbert, 2000; 

Lofgren et al., 2002; Mirza, 2004). 

Biomass 

Biomass energy is primarily used for industrial process heating, although use for transportation 

fuels and electricity generation is increasing. Current biomass sources include the following 

(EIA, 2005): 

• black liquor from the pulp and paper industry to provide process heat as well as to generate 

electricity (29% of U.S. bioenergy); 

• wood and wood waste from sources such as lumber mills for industrial power (more than 

19%); 

• combusted municipal solid waste and recovered landfill gas (about 16% of current U.S. 

biomass energy); and 

• ethanol fuel made predominantly from corn grown in the Midwest (10% of U.S. biomass 

energy production). 

 

50 Hypolimnetic water is the layer of water in a thermally stratified lake that lies below the thermocline, is 

noncirculating, and remains perpetually cold. 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 192
Direct impacts of climate change on biomass power derived from a waste stream may be limited 

unless there are significant changes in forest or agricultural productivity that is a source of the 

waste stream. Efforts to increase ethanol production maybe affected through direct effects on 

crop growth and availability of irrigation water. Warming and precipitation increases are 

expected to allow the bioenergy crop switchgrass, for instance, to compete effectively with 

traditional crops in the central U.S (Field et al., 2007). Renewable energy production is highly 

susceptible to localized and regional changes in the resource base. As a result, the greater 

uncertainties about regional impacts under current climate change modeling pose a significant 

challenge in evaluating medium- to long-term impacts on renewable energy production (Bull et 

al., 2007). 

Wind and Solar 

Wind power is the fastest growing renewable energy technology, with total generation increasing 

to 14 billion kilowatt-hours in 2005 (EIA, 2006). Climate effects can have a substantial influence 

on wind energy through variability in wind patterns. Currently, photovoltaic electricity is 

primarily used in off-grid locations and rooftop systems where state or local tax incentives and 

utility incentives are present. California and Arizona currently have the only existing utility-scale 

systems (EIA, 2005), with additional projects being developed in Colorado, Nevada, and 

Arizona. Solar power generation can be affected by changes in solar radiation (e.g., increases in 

cloud cover or aerosols). One international study predicts that a 2% decrease in global solar 

radiation will decrease solar cell output by 6% overall (Fidje and Martinsen, 2006). In addition, 

Pan et al. (2004) projected that increased cloudiness may decrease the potential output of 

photovoltaic electricity by 0 to 20% for the 2040s. However, Meehl et al. (2007) report that 

reductions in cloudiness could slightly increase the potential for solar energy in North America 

south of 60° N. 

 

The IPCC concluded that wind and solar resources are about as likely as not to increase (medium 

confidence) (Field et al., 2007). 

Extreme events 

Climate change may cause significant shifts in current weather patterns and increase the severity 

and possibly the frequency of major storms (NRC, 2002). Extreme weather events can threaten 

coastal energy infrastructures and electricity transmission and distribution infrastructures in the 

United States and other world regions (Wilbanks et al., 2007a). Impacts can range from localized 

events (e.g., railroad track distortions due to temperature extremes and blocked transportation 

corridors due to flooding) to larger-scale impacts (e.g., flooding from hurricanes). Hurricanes in 

particular can have severe impacts on energy infrastructure. Offshore production is particularly 

susceptible to extreme weather events. In 2004, Hurricane Ivan destroyed seven Gulf of Mexico 

oil-drilling platforms and damaged 102 pipelines. Hurricanes Katrina and Rita in 2005 destroyed 

more than 100 platforms and damaged 558 pipelines (Bull et al., 2007). Though it is not possible 

to attribute the occurrence of any singular storm event to climate change, projections of climate 

change suggest that extreme weather events are expected to increase (heavy precipitation events, 

very likely; intense tropical cyclone activity, likely (IPCC, 2007b). 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 193
The potential impacts of more severe weather are not limited to hurricane-prone areas. Rail 

transportation lines, which transport approximately two-thirds of the coal to the Nation’s power 

plants (EIA, 2002), often closely follow riverbeds, especially in the Appalachian region. More 

severe rainstorms can lead to flooding of rivers that then can wash out or degrade the nearby 

roadbeds. Flooding may also disrupt the operation of inland waterways, the second-most 

important method of transporting coal. With utilities carrying smaller stockpiles and projections 

showing a growing reliance on coal for a majority of the Nation’s electricity production, any 

significant disruption to the transportation network has serious implications for the overall 

reliability of the grid as a whole. 

Energy impacts of episodic events can linger for months or years, as illustrated by the continued 

loss of oil and gas production in the Gulf of Mexico (MMS, 2006a,b,c cited in Bull et al., 2007) 

eight months after the 2005 hurricanes. 

In the Arctic, soil subsidence caused by thawing permafrost is a risk to gas and oil pipelines, 

electrical transmission towers, nuclear power plants, and natural gas processing plants (Wilbanks 

et al., 2007a). Along the Beaufort Sea in Alaska, climate impacts on oil and gas development in 

the region are expected to result in both financial benefits and costs. For example, offshore oil 

exploration and production are expected to benefit from less extensive and thinner sea ice, 

although equipment will have to be designed to withstand increased wave forces and ice 

movement (ACIA, 2004). 

V.8 Transportation 

Increasing global temperatures, rising sea levels, and changing weather patterns pose significant 

challenges to our country’s transportation venues including: roadways, railways, transit systems, 

marine transportation systems, airports, and pipeline systems. The U.S. transportation network is 

vital to the Nation’s economy, safety, and quality of life. To date, relatively little research has 

evaluated the risks that climate change poses to the safety and resilience of this key sector. 

Drawing on the available research, the National Research Coucil (NRC, 2008) concluded the 

following in its recent report on the potential impacts of climate change on U.S. transportation: 

• “Climate change will affect transportation primarily through increases in several types of 

weather and climate extremes, such as very hot days; intense precipitation events; intense 

hurricanes; drought; and rising sea levels, coupled with storm surges and land subsidence. 

The impacts will vary by mode of transportation and region of the country, but they will be 

widespread and costly in both human and economic terms and will require significant 

changes in the planning, design, construction, operation, and maintenance of transportation 

systems.” 

• “Potentially, the greatest impact of climate change for North America’s transportation 

systems will be flooding of coastal roads, railways, transit systems, and runways because of 

global rising sea levels, coupled with storm surges and exacerbated in some locations by land 

subsidence.” 

The IPCC reported the following findings for North America (Field et al., 2007): 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 194
• Climate changes in winter are expected to result in a mix of impacts. Warmer or less snowy 

winters will likely reduce delays, improve ground and air transportation reliability, and 

decrease the need for winter road maintenance (Pisano et al., 2002). This will also lengthen 

the construction season. However, more intense winter storms could increase risks for 

traveler safety (Andrey and Mills, 2003) and require increased localized snow removal. 

• Increasing frequency, intensity, or duration of heat spells could cause railroad tracks to 

buckle or kink (Rosetti, 2002) and affect roads through softening and traffic-related rutting 

(Zimmerman, 2002). 

• Declining fog should benefit transportation in at least some parts of North America (Muraca 

et al., 2001; Hanesiak and Wang, 2005). 

• Negative impacts of climate change on transportation will very likely result from coastal and 

riverine flooding and landslides (Burkett, 2002). 

• Although offset to some degree by fewer ice threats to navigation, reduced water depth in the 

Great Lakes would lead to the need for ‘light loading’ and, hence, adverse economic impacts 

(du Vair et al., 2002; Quinn, 2002; Millerd, 2005). 

• Transportation infrastructure will likely be particularly affected in northerly latitudes (Nelson 

et al., 2002). Permafrost degradation reduces surface load-bearing capacity and potentially 

triggers landslides (Smith and Levasseur, 2002; Beaulac and Doré , 2005). While the season 

for transport by barge is likely to be extended, the season for ice roads will likely be 

compressed (Lonergan et al., 1993; Lemmen and Warren, 2004; Welch, 2006). Other types 

of roads are likely to require costly improvements in design and construction (Stiger, 2001; 

McBeath, 2003). 

• Some of these risks are expected to be offset by improvements in technology and information 

systems (Andrey and Mills, 2004). Impacts of warming on paved roads can be ameliorated 

with altered road design, construction, and management (including changes in the asphalt 

mix) and the timing of spring load restrictions (Clayton et al., 2005; Mills et al., 2006). 

Gulf Coast Case Study 

To explore climate effects on transportation systems and infrastructure, SAP 4.7 used a case 

study approach focused on a segment of the U.S. central Gulf Coast. Given the limited nature of 

available information on this topic, the authors selected a case study approach to contribute to the 

development of research methodologies for application in other locations, while generating 

useful information for local and regional decisionmakers. The Gulf Coast was selected as the 

study region because of the presence of a full range of transportation modes and infrastructure 

and its importance as a transportation hub for vital goods and services for the country as a whole. 

Potter et al. (2008) found that the methods used in the case study can be applied to any region; 

however, the modeled climate projections and the specific implications of these scenarios for 

transportation facilities are specific to the Gulf Coast study area. 

The following sections are excerpted from SAP 4.7. 

V.8.a Study region 

(adapted from Potter et al., 2008) 

In SAP 4.7, the authors describe the study area as follows: 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 195
• Includes 48 contiguous coastal counties in four states, running from Houston/Galveston, 

Texas, to Mobile, Alabama. 

• The region is home to almost 10 million people living in a range of urban and rural settings 

and contains critical transportation infrastructure that provides vital services to its constituent 

states and the Nation as a whole. 

• It is also highly vulnerable to sea level rise and storm impacts. 

• The coastal geography of the region is highly dynamic due to a unique combination of 

geomorphic, tectonic, marine, and atmospheric forcings that shape both the shoreline and 

interior landforms. 

• Due largely to its sedimentary history, much of the central Gulf Coast region is low-lying; 

the great majority of the study area lies below 30 m in elevation. 

• Because of this low relief, much of the region is prone to flooding during heavy rainfall 

events, hurricanes, and lesser tropical storms. 

• Land subsidence is a major factor in the region; specific rates of subsidence vary across the 

region, influenced both by the geomorphology of specific locations (e.g., sediments vary over 

time) as well as by human activities. 

• Most of the coastline is also highly vulnerable to erosion and wetland loss, particularly in 

association with tropical storms and frontal passages. An estimated 56,000 ha (217 mi2) of 

land were lost in Louisiana alone during Hurricane Katrina. 

• Many Gulf Coast barrier islands are retreating and diminishing in size. The Chandeleur 

Islands, which serve as a first line of defense from approaching hurricanes for the New 

Orleans region, lost roughly 85% of their surface area during Hurricane Katrina. 

• As barrier islands and mainland shorelines erode and submerge, onshore facilities in lowlying coastal areas become more susceptible to inundation and destruction. 

V.8.b Transportation infrastructure 

(adapted from Potter et al., 2008) 

The central Gulf Coast study area’s transportation infrastructure is a robust network of multiple 

modes—critical both to the movement of passengers and goods within the region and to national 

and international transport as well: 

• The region has 27,000 km (17,000 mi) of major highways—about 2% of the Nation’s major 

highways—that carry 83.5 billion vehicle miles of travel annually. The area is served by 13 

major transit agencies: over 136 providers offer a range of public transit services to Gulf 

Coast communities. 

• Roughly two-thirds of all U.S. oil imports are transported through this region, and pipelines 

traversing the region transport over 90% of domestic Outer Continental Shelf oil and gas. 

Approximately one-half of all the natural gas used in the United States passes through or by 

the Henry Hub gas distribution point in Louisiana. 

• The study area is home to the largest concentration of public and private freight handling 

ports in the United States, measured on a tonnage basis. These facilities handle a huge 

share—around 40%—of the Nation’s waterborne tonnage. Four of the top five tonnage ports 

in the United States are located in the region: South Louisiana, Houston, Beaumont, and New 

Orleans. The study area also has four major container ports. 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 196
• Overall, more than half of the tonnage (54%) moving through study area ports is petroleum 

and petroleum products. Additionally, New Orleans provides the ocean gateway for much of 

the U.S. interior’s agricultural production. 

• The region sits at the center of transcontinental trucking and rail routes and contains one of 

only four major points in the United States where railcars are exchanged between the 

dominant eastern and western railroads. 

• The study area also hosts the Nation’s leading and third-leading inland waterway systems 

(the Mississippi River and the Gulf Intracoastal) based on tonnage. The inland waterways 

traversing this region provide 20 states with access to the Gulf of Mexico. 

• The region hosts 61 publicly owned, public-use airports, including 11 commercial service 

facilities. Over 3.4 million aircraft takeoffs and landings take place at these airports annually, 

led by the major facilities at George Bush Intercontinental (IAH) (Houston), William P. 

Hobby (Houston), and Louis Armstrong New Orleans International. IAH also is the leading 

airport in the study area for cargo, ranking 17th in the Nation for cargo tonnage. 

Given the scale and strategic importance of the region’s transportation infrastructure, it is critical 

to consider the potential vulnerabilities in the network that may be presented by climate change. 

A better understanding of these risks will help inform transportation managers as they plan future 

investments. 

V.8.c Trends in climate and coastal change in the Gulf Coast 

(adapted from Savonis et al., 2008) 

The central Gulf Coast is particularly vulnerable to climate variability and change because of the 

frequency with which hurricanes strike, because much of its land is sinking relative to mean sea 

level, and because much of its natural protection—in the form of barrier islands and wetlands—

has been lost. While difficult to quantify, the loss of natural storm buffers is expected to intensify 

many of the climate impacts identified in this report, particularly in relation to storm damage. 

• Relative Sea Level Rise—Much of the land in the Gulf Coast is sinking due to compaction 

of the sediment. As a result, this area is facing much higher increases in relative sea level rise 

(the combination of local land surface movement and change in mean sea level) than most 

other parts of the U.S. coast. Based on the output of an ensemble of GCMs, run with a range 

of IPCC emissions scenarios, relative sea level in the study area is very likely to increase by 

at least 0.3 m across the region and possibly as much as 2 m in some parts of the study area 

over the next 50 to 100 years. The analysis of even a middle range of potential sea level rise 

of 0.3 to 0.9 m indicates that a vast portion of the Gulf Coast from Houston to Mobile may be 

inundated in the future. The projected rate of relative sea level rise for the region during the 

next 50 to 100 years is consistent with historical trends, region-specific analyses, and the 

IPCC Fourth Assessment Report (2007a) findings, which assume no major changes in ice 

sheet dynamics. Protective structures, such as levees and sea walls, could mitigate some of 

these impacts, but considerable land area is still at risk of permanent flooding from rising 

tides, sinking land, and erosion during storms. Subsidence alone could account for a large 

part of the change in land area through the middle of this century, depending on the portion 

of the coast that is considered. Sea level rise induced by the changing climate will 

substantially worsen the impacts of subsidence on the region. 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 197
• Storm Activity—The region is vulnerable today to transportation infrastructure damage 

during hurricanes and, given the potential for increases in the number of hurricanes 

designated as Category 3 and above, this vulnerability is expected to increase. This 

preliminary analysis did not quantitatively assess the impact of the loss of protective barrier 

islands and wetlands, which will only serve to make storm effects worse. It also did not 

consider the possible synergistic impacts of storm activity with rising sea levels. 

• Average Temperature Increase—All GCMs used by the IPCC in its Fourth Assessment 

Report (Christensen et al., 2007) indicate an increase in average annual Gulf Coast 

temperature through the end of this century. Based on GCM runs under three different IPCC 

emission scenarios (A1B, A2, and B1; discussed in Section IV.3.a), the average temperature 

in the Gulf Coast region appears likely to increase by at least 1.5 ± 1 °C during the next 50 

years, with the greatest increase in temperature occurring in the summer. 

• Temperature Extremes—Increases in average temperature will bring increases in extreme 

high temperature. Based on historical trends and model projections, it is expected that the 

number of days above 32.2 °C will increase significantly across the study area; this has 

implications for transportation operations and maintenance. The number of days above 32.2 

°C could increase by as much as 50% during the next 50 years. 

• Precipitation Change—Future changes in precipitation are much more difficult to model 

than temperature. Precipitation trends in the study area suggest increasing values with some 

regions in Mississippi and Alabama showing significant long-term trends. Yet while some 

GCM results indicate that average precipitation will increase in this region, others indicate a 

decline in average precipitation during the next 50 to 100 years. Because of this ambiguity, it 

is difficult to reach conclusions about what the future holds regarding change in mean 

precipitation. Even if average precipitation increases slightly, average annual runoff in the 

region could decline as temperature and evapotranspiration rates increase. 

• Extreme Rainfall Events—Average annual precipitation increased at most recording 

stations within the study area since 1919, and the literature indicates that a trend toward more 

rainfall and more frequent heavy downpours is expected. At this stage, climate-modeling 

capacity is insufficient to quantify effects on individual precipitation events, but the potential 

for temporary flooding in this region is clear. In an area where flooding already is a concern, 

this tendency could be exacerbated by extreme rainfall events. This impact will become 

increasingly important as relative sea level rises, putting more and more of the study area at 

risk. 

V.8.d Impacts of changing climate on Gulf Coast transportation 

(adapted from Savonis et al., 2008) 

Based on the trends in climate and coastal change, transportation infrastructures and the services 

that require them are vulnerable to future climate changes as well as other natural phenomena. 

While more study is needed to specify how vulnerable they are and what steps could be taken to 

reduce that vulnerability, it is clear that transportation planners in this region should not ignore 

these impacts. 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 198
• Inundation from Relative Sea Level Rise—While greater or lesser rises in relative sea level 

are possible, this study analyzed the effects of relative sea level rise of 0.6 and 1.2 m as 

realistic scenarios. Based on these levels, an untenable portion of the region’s road, rail, and 

port network is at risk of permanent flooding. 

Twenty-seven percent of the major roads, 9% of the rail lines, and 72% of the ports are at or 

below 122 cm in elevation, although portions of the infrastructure are guarded by protective 

structures such as levees and dikes. While flood protection measures will continue to be an 

important strategy, rising sea levels in areas with insufficient protection may be a major 

concern for transportation planners. Furthermore, the crucial connectivity of the intermodal 

system in the area means that the services of the network can be threatened even if small 

segments are inundated. 

While these impacts are very significant, they can be addressed and adaptive strategies 

developed if transportation agencies carefully consider them in their decisions. The 

effectiveness of such strategies will depend on the strategies selected and the magnitude of 

the problem because scenarios of lower emissions demonstrate lesser impacts. It may be that 

in some cases, the adaptive strategy may be wholly successful, while in others further steps 

may need to be taken 

• Flooding and Damage from Storm Activity—As the central Gulf Coast already is 

vulnerable to hurricanes, so is its transportation infrastructure. This study examined the 

potential for short-term flooding associated with a 5.5- and a 7.0-m storm surge. Based on 

these levels, a great deal of the study area’s infrastructure is subject to temporary flooding. 

More than half (64% of interstates; 57% of arterials) of the area’s major highways, almost 

half of the rail miles, 29 airports, and virtually all of the ports are subject to flooding. 

The nature and extent of the flooding depends on where a hurricane makes landfall and its 

specific characteristics. Hurricanes Katrina and Rita demonstrated that that this temporary 

flooding can extend for miles inland. 

This study did not examine in detail the potential for damage due to storm surge, wind 

speeds, debris, or other characteristics of hurricanes since this, too, greatly depends on where 

the hurricane strikes. Given the energy associated with hurricane storm surge, concern must 

be raised for any infrastructure in its direct path that is not designed to withstand the impact 

of a Category 3 hurricane or greater. 

Climate change appears to worsen the region’s vulnerability to hurricanes, as warming seas 

give rise to more energetic storms. The literature indicates that the intensity of major storms 

may increase by 5 to 20%. This indicates that Category 3 storms and higher may return more 

frequently to the central Gulf Coast and thus cause more disruptions of transportation 

services. 

The impacts of such storms need to be examined in greater detail; storms may cause even 

greater damage under future conditions not considered here. If the barrier islands and 

shorelines continue to be lost at historical rates and as relative sea level rises, the destructive 

potential of tropical storms is expected to increase. 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 199
• Effects of Temperature Increase—As the average temperature in the central Gulf Coast is 

expected to rise by 0.5 to 2.5 °C, the daily high temperatures, particularly in summer, and the 

number of days above 32.2 °C are also expected to increase. These combined effects will 

raise costs related to the construction, maintenance, and operations of transportation 

infrastructure and vehicles. Maintenance costs will increase for some types of infrastructure 

because they deteriorate more quickly at temperatures above 32 °C. An increase in daily high 

temperatures could increase the potential for rail buckling in certain types of track. 

Construction costs could increase because of restrictions on days above 32 °C, since work 

crews may be unable to be deployed during extreme heat events and concrete strength is 

affected by the temperature at which it sets. Increases in daily high temperatures would affect 

aircraft performance and runway length because runways need to be longer when daily 

temperatures are higher (all other things being equal). While potentially costly and 

burdensome, these impacts may be addressed by transportation agencies by absorbing the 

increased costs and increasing the level of maintenance for affected facilities. 

• Effects of Change in Average Precipitation—It is difficult to determine how transportation 

infrastructure and services might be affected by changes in average precipitation, since 

models project either a wetter or a drier climate in the southeastern United States. In either 

case, the changes in average rainfall are relatively slight, and the existing transportation 

network may be equipped to manage this. 

• Effects of Increased Extreme Precipitation Events—Of more concern is the potential for 

short-term flooding due to heavier downpours. Even if average precipitation declines, the 

intensity of those storms can lead to temporary flooding as culverts and other drainage 

systems are overloaded. Further, the Louisiana Department of Transportation and 

Development reports that prolonged flooding of one to five weeks can damage the pavement 

substructure and necessitate rehabilitation (Gaspard et al., 2007). The central Gulf Coast 

already is prone to temporary flooding, and transportation representatives struggle with the 

disruptions these events cause. As the climate changes, flooding will probably become more 

frequent and more disruptive as the intensity of these downpours is expected to increase. As 

relative sea level rises, it appears probable that even more infrastructure will be at risk 

because overall water levels already will be so much higher. While these impacts cannot be 

quantified at present, transportation representatives can monitor where flooding occurs and 

how the sea is rising as an early warning system about what facilities are at immediate risk 

and warrant high-priority attention. In a transportation system that already is under stress due 

to congestion, and with people and freight haulers increasingly dependent on just-in-time 

delivery, the economic, safety, and social ramifications of even temporary flooding may be 

significant. 

V.8.e Impacts by transportation mode 

(adapted from Kafalenos et al., 2008) 

Roads 

Similar to most of the Nation, roads make up the backbone of the transportation network in the 

Gulf Coast. Highways are the chief mode for transporting people across the region, and, 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 200
combined with rail, move freight throughout the region and to other parts of the United States. 

While temperature and precipitation changes have some implications for highway design and 

maintenance, the key impacts on the highway network result from relative sea level rise and 

storm surge. 

Temperature 

• Projected changes in average temperatures appear to have moderate implications for 

highways, while increases in extreme heat may be significant. 

• Maintenance and construction costs for roads and bridges are expected to increase as 

temperatures increase. Further, higher temperatures cause some pavement materials to 

degrade faster, requiring earlier replacement. Such costs are expected to grow as the number 

of days above 32 °C (90 °F)—projected to grow from the current average of 77 days to a 

range of 99 to 131 days over the next century—increases, as well as the projected maximum 

record temperatures in the region. 

• Currently, the designs of steel and concrete bridges and of pavements in the study area 

typically are based on a maximum design temperature of 46 to 53 °C. The increase in 

maximum record temperatures implied by the climate model projections are less than these 

values, although under the climate scenarios they would approach those values over the next 

century. It may be prudent for future designers of highway facilities to ensure that joints in 

steel and concrete bridge superstructures and concrete road surfaces can adequately 

accommodate thermal expansion resulting from these temperatures. 

Precipitation 

The analysis indicates little change in mean annual precipitation through either 2050 or 2100, but 

the range of possible futures includes both reductions and increases in seasonal precipitation. In 

either case, the analysis points to potential reductions in soil moisture and runoff as temperatures 

and the number of days between rainfall events increase. 

Scenarios of insignificant change or reduction in average precipitation, coupled with drier soils 

and less runoff, may result in the following: 

• decline of slides in slopes adjacent to highways; 

• less settling under pavements, with a decrease in cracking and undermining of pavement base 

courses; 

• uniform decreases in runoff could reduce scouring of bridge piers in rivers and streams; and 

• stresses on animal and plant populations brought about by higher temperatures and changes 

in rainfall patterns could make it more difficult and expensive to mitigate the impacts of 

highway development on the natural environment. 

Modest increases in average annual rainfall may not significantly affect pavement settling, 

bridge scour, and ecosystem impacts because of the effects of increasing temperature on 

evaporation rates. 

However, an increase in the frequency of extreme precipitation events may contribute to the 

following: 

• increased accident rates; 

• more frequent short-term flooding and bridge scour, as well as more culvert washouts; 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 201
• exceeding the capacity of storm-water management infrastructure; and 

• more frequent slides, requiring increased maintenance. 

However, some states, such as Louisiana, already address precipitation through pavement 

grooving and sloping, and thus may have adequate capacity to handle some increase in 

precipitation. 

Relative Sea Level Rise 

The effects of 61 and 122 cm (2 and 4 ft) changes were analyzed to assess the implications of 

relative sea level rise on highways. Currently, about 209 km (130 mi) or about 1% of major 

highways (interstates and arterials) in the study region are located on land that is at or below sea 

level. The presence or absence of protective structures was not considered in this baseline 

analysis, but would be an important factor in subsequent assessments. 

Risk from 61-cm relative sea level rise: 

• Twenty percent of the arterial miles and 19% of the interstate miles in the study area are at 

elevations below 61 cm (2 ft) and thus at risk from sea level rise unless elevated or protected 

by levees. 

• Most of the highways at risk are located in the Mississippi River Delta near New Orleans. 

• The most notable highways at risk are I-10 and U.S. 90, with 220 km (137 mi) and 235 km 

(146 mi), respectively, passing through areas that will be below sea level if sea levels rise by 

61 cm (2 ft). 

Risk from 122-cm relative sea level rise: 

• Twenty-eight percent of the arterial miles and 24% of the interstate miles are at elevations 

below 122 cm (4 ft). 

• The majority of the highways at risk from a 122-cm (4-ft) increase in relative sea level are 

similarly located in the Mississippi River Delta near New Orleans. 

• The most notable highways at risk remain I-10 and U.S. 90, with the number of kilometers 

increasing to 684 km (425 mi) and 628 km (390 mi) passing through areas below sea level, 

respectively. 

Many of the National Highway System (NHS) Intermodal Connectors, public roads that provide 

access to major transportation facilities, such as rail, ports, and airports, pass through low-lying 

areas concentrated in the Mississippi River Delta, where sea level rise is expected to have the 

most pervasive impact. Of the 1,041 km (647 mi) of Intermodal Connectors, 238 km (148 mi), or 

23%, are at risk from a 61-cm (2-ft) increase in relative sea levels; and a total of 444 km (276 

mi), or 43%, are at risk from a 122-cm (4-ft) increase. The following terminals are at risk: 

• for the 61-cm (2-ft) increase scenario—the New Orleans International Airport, Port 

Fourchon, most rail terminals in New Orleans, ferry terminals in New Orleans, and ferry 

terminals outside of the Mississippi River Delta in Galveston and Houston; and 

• for the 122-cm (4-ft) scenario—all those above, as well as port facilities in Lake Charles, 

Galveston, Pascagoula, and Gulfport. 

It is worth noting that the loss of use of a small individual segment of a given highway may 

make significant portions of that road network impassable. Further, even if a particular interstate 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 202
or arterial is passable, if the feeder roads are flooded, then the larger road becomes less usable. 

Storm Activity 

About half of the region’s arterial miles and about three-quarters of the intermodal connectors 

are vulnerable to a storm surge of 5.5 m (18 ft), and these proportions are even higher for a 7-m 

(23-ft) storm surge. 

Surge Wave Crests and Effects on Bridges 

The wave energy during storm surge events is greatest at the crest of the wave. The facilities 

most at risk are bridge decks and supports that are constructed at the wave height levels reached 

during a storm. The impact of the 2005 hurricanes vividly illustrates some of the factors involved 

in infrastructure vulnerability. While only a small percentage of the study area’s bridges are 

located at the shore and have bridge decks or structures at these heights, when storm waves meet 

those bridges, the effect is devastating; massive bridge spans were dislodged during Hurricane 

Katrina. Although these bridges are few in number compared to the over 8,000 bridges in the 

functionally classified system, over 24 bridges were hit by wave surges resulting from Hurricane 

Katrina and experienced serious damage. 

Surge Inundation 

A substantial portion of the highway system across the study area is vulnerable to surge 

inundation: 51% of all arterials and 56% of the interstates are in the 5.5-m (18-ft) surge risk 

areas. At the 7-m (23-ft) level, these percentages increase only slightly: 57% of all arterials and 

64% of the interstates are in 7-m (23-ft) surge risk areas. 

The risk from surge inundation for NHS Intermodal (IM) Connectors is even greater than that for 

all highways. Seventy-three percent of IM Connector miles are located in areas that would be 

inundated by a 5.5-m (18-ft) surge, and the proportion of IM connectors that is vulnerable at the 

7-m (23-ft) level is only slightly higher. 

While inundation from storm surges is a temporary event, during each period of inundation the 

highway is not passable, and after the surge dissipates, highways must be cleared of debris 

before they can function properly. Of particular concern is that a substantial portion of all of the 

major east–west highways in the study area, particularly I-10/I-12, are at risk to storm surge 

inundation in some areas, and during storm events and the recovery period from these events, all 

long-distance highway travel through the study area is expected to be disrupted. 

Wind 

Wind from storms may affect the highway signs, traffic signals, and lighting fixtures throughout 

the study area. The wind design speed for signs and supports in the study area is typically 160 to 

200 km per hour (100 to 125 mph). These designs should accommodate all but the most severe 

storm events. More significant safety and operational impacts are expected from debris blown 

onto roadways and from crashes precipitated by debris or severe winds. 

Freight and passenger rail 

Rail lines in the region play a key role in transporting freight, and a minor role in intercity 

passenger traffic. Rail connectivity is vital to the functioning of many, if not most, of the marine 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 203
freight facilities in the study area. Of the four main climate drivers examined in this study, storm 

surge could be the most significant for rail. 

Temperature 

Projected increases in average temperatures generally fall within the current standards for 

existing rail track and facilities. However, the increase in temperature extremes—very hot 

days—could increase the incidence of buckling or ‘sun kinks’ on all the rail tracks in the study 

area. This occurs when compressive forces in the rail, due to restrained expansion during hot 

weather, exceed the lateral stiffness of the track causing the track to become displaced laterally. 

 

Precipitation 

The primary impacts on rail infrastructure from precipitation are erosion of the track subgrade 

and rotting of wooden crossties. Erosion of the subgrade can wash away ballast and weaken the 

foundation, making the track unstable for passage of heavy locomotives and railcars. Without 

ballast, wood crossties would rot at a faster rate, leading to more buckling and unstable track. 

The precipitation projections do not indicate that design changes are warranted to prevent 

increased erosion or moisture damage to railroad track. The runoff projections point to even 

fewer problems with erosion over the next century than are present today, due to possibly less 

precipitation and slightly higher temperatures. However, if the frequency and/or the intensity of 

extreme rainfall events increase, it could lead to higher rates of erosion and railroad bridge scour, 

as well as higher safety risks and increased maintenance requirements. 

It should be noted that many existing facilities at low elevations are protected by levees and other 

physical structures, which provide some resistance to gradual changes in sea level and the 

impacts of storm surge. 

Relative Sea Level Rise 

The effects on rail lines and facilities of relative sea level of 61 and 122 cm (2 and 4 ft) over the 

next 50 to 100 years were analyzed. The obvious impacts for both of these sea level rise 

scenarios are water damage or complete submersion of existing rail track and facilities. 

Currently, about 80 km (50 mi) or about 2% of rail lines in the study region are located on land 

that is at or below sea level. 

Most of the rail lines in and around New Orleans are expected to be affected by relative sea level 

rise. The heavily traveled CSX line between Mobile and New Orleans, which was damaged 

during Hurricane Katrina, also is at risk, as are several area short lines. The following rail lines 

would be affected if relative sea level rises 61 cm (2 ft): 

• most rail lines in and around New Orleans; 

• BNSF line between Lafayette and New Orleans; 

• CN/IC line into New Orleans; 

• CSX line between Mobile and New Orleans; 

• CSX line north of Mobile; 

• Louisiana and Delta Railroad west of New Orleans; 

• portions of the MSE rail line in Mississippi; 

• the New Orleans and Gulf Coast Railway line between New Orleans and Myrtle; 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 204
• NS line into New Orleans; 

• portions of the Port Bienville Railroad; 

• segments of the UP line west of New Orleans; and 

• various segments of track around Lake Charles and Galveston. 

Further degradation of these lines is expected to occur should relative sea level increase by 122 

cm (4 ft), with additional problems on the KCS route into New Orleans, the NS line north of 

Mobile, and selected track segments around Beaumont and Houston. 

Storm Activity 

Hurricane Katrina provided a vivid example of the devastating impacts of severe storm events on 

the rail system in the Gulf Coast Study Area. Making landfall on 29 August 2005, Katrina caused 

damage to all of the major railroads in the region. BNSF, CN, KCS, and UP all suffered damage, 

mostly to yards in and around New Orleans. CSX track and bridges also were damaged. NS had 

nearly 8 km (5 mi) of track washed away from the Lake Pontchartrain Bridge, which is 9.3 km 

(5.8 mi) long. By 13 September 2005, most of these railroads had resumed operations into New 

Orleans, at least on a partial basis. 

One-third of the rail lines in the study region are vulnerable to a storm surge of 5.5 m (18 ft), and 

41% are vulnerable to a storm surge of 7.0 m (23 ft). These include the heavily traveled CSX line 

from New Orleans to Mobile, and the UP and BNSF lines from New Orleans to Houston. Cities 

at risk include Mobile, Gulfport, Biloxi, New Orleans, Baton Rouge, Lafayette, Lake Charles, 

Beaumont, Port Arthur, and Galveston. Facilities at less than 5.5 m (18 ft) of elevation have the 

highest risk of 5.5-m (18-ft) storm surge impacts. These include 43% of the rail facilities in the 

study region. An additional 11 facilities are between 5.5 and 7.0 m (18 and 23 ft) of elevation 

and are expected to be affected by a 7.0-m (23-ft) storm surge. 

Marine 

Marine facilities, including both freight and non-freight ports, marinas, and industry support 

facilities, are most vulnerable to storm surge and relative sea level rise. Marine facilities and 

waterways are vital to the region, and to the Nation as a whole. The study area is one of the 

Nation’s leading centers of marine activity. Much of the region’s economy is directly linked to 

waterborne commerce; and in turn, this waterborne commerce supports a substantial portion of 

the U.S. economy. 

Higher Temperatures 

Higher temperatures may affect port facilities in three key ways. First, higher temperatures will 

increase costs of terminal construction and maintenance, particularly of any paved surfaces that 

will deteriorate more quickly if the frequency of high temperatures increases. Second, higher 

temperatures will lead to higher energy consumption and costs for refrigerated warehouses or 

‘reefer slots’ (electrical plug-ins for containers with on-board cooling units). Third, higher 

temperatures would probably lead to increased stress on temperature-sensitive structures. 

Container-handling cranes, warehouses, and other marine terminal assets are made of metals. 

With increasing record temperatures and days over 32 °C (90 °F), it may be necessary to design 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 205
for higher maximum temperatures in replacement or new construction. On the other hand, most 

dock and wharf facilities are made of concrete and lumber, which are generally less sensitive to 

temperature fluctuations. It is possible that lock and dam structures could be affected, although 

this will require further investigation. 

Precipitation 

The prospect of more intense precipitation events could require the capacity of some stormwater retention and treatment facilities to be increased. The handling of storm water can be a 

significant expense for container terminals, auto terminals, and other terminals with large areas 

of impervious surface. Increasing environmental regulatory requirements also may add to costs 

of adapting storm-water handling infrastructure. 

Relative Sea Level Rise 

Of freight facilities in the study area, about 72% are vulnerable to a 122-cm (4-ft) rise in relative 

sea level. Of the 994 freight facilities in the U.S. Army Corps of Engineers database, 638 (64%) 

are in areas with elevations between 0 and 61 cm (2 ft) above sea level, and another 80 (8%) are 

in areas with elevations between 61 and 122 cm (2 and 4 ft). More than 75% of facilities are 

potentially vulnerable in Beaumont, Chocolate Bayou, Freeport, Galveston, New Orleans, 

Pascagoula, Plaquemines, Port Arthur, Port Bienville, and Texas City. Between 50 and 75% of 

facilities are potentially vulnerable in Gulfport, Houston, Lake Charles, Mobile, South Louisiana, 

and the Tennessee–Tombigbee Waterway. Only Baton Rouge, with 6% of facilities potentially at 

risk, appears to be well positioned to avoid impacts of sea level rise. 

A similar situation faces non-freight facilities. Seventy-three percent of study area marine nonfreight facilities are potentially vulnerable to a 122-cm (4-ft) increase in relative sea level. Of the 

810 non-freight facilities in the U.S. Army Corp of Engineers database, 547 (68%) are in areas 

with elevations between 0 and 61 cm (2 ft) above sea level, and another 47 (6%) are in areas with 

elevations between 61 and 122 cm (2 and 4 ft). More than 75% of facilities are potentially 

vulnerable in Beaumont, Chocolate Bayou, Freeport, Galveston, New Orleans, Pascagoula, 

Plaquemines, Port Arthur, the Tennessee–Tombigbee Waterway, and Texas City; between 50% 

and 75% of facilities are potentially vulnerable in Houston, Lake Charles, Mobile, and South 

Louisiana. Twenty-seven percent of Gulfport facilities and no Baton Rouge facilities are 

potentially at risk. 

It is important to note that many existing facilities at low elevations are protected by levees and 

other physical structures, which should provide resistance to gradual changes in sea levels. The 

specific effects of existing protections have not been considered in this study. For facilities that 

are not appropriately protected, either by elevation or by structures, rising water levels pose an 

increased risk of chronic flooding, leading, in the worst case, to permanent inundation of marine 

terminal facilities, either completely or in part, rendering them inoperable. 

Navigable depths are expected to increase in many harbors and navigation channels as a result of 

rising sea levels. This could lead to reduced dredging costs, but higher costs where rising water 

levels require changes to terminals. The functionality and/or protections of lock and dam 

structures controlling the inland waterway system also may be affected by relative sea level rise. 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 206
Storm Activity: Water and Wind Damage 

While the actual facilities that would be flooded depend on the particulars of a given storm—the 

landfall location, direction, tidal conditions, etc.—fully 99% of all study area facilities are 

vulnerable to temporary and permanent impacts resulting from a 7.0-m (23-ft) storm surge, while 

almost 98% are vulnerable to temporary and permanent impacts resulting from a 5.5-m (18-ft) 

storm surge. All facilities are vulnerable to wind impacts. Similar to sea level rise, storm surge 

impacts on highway and rail connections could affect the ability to utilize ports for transport of 

goods to and from affected ports. 

Aviation 

Severe weather events could have the greatest impacts on aviation. Ultimately, the impact on the 

operational aspects of aviation could potentially supersede the overall magnitude of combined 

effects on aviation due to other factors discussed below. 

Temperature 

Higher temperatures affect aircraft performance and the runway lengths that are required. 

Generally speaking, the higher the temperature, the longer the runway that is required. 

Temperature increases considered by this report would indicate a small increase in baseline 

runway length requirements, assuming other relevant factors are held constant. However, 

advances in engine technology and airframe materials are expected to offset the potential effects 

of the temperature increases analyzed in this report, so that current runway lengths are expected 

to be sufficient. 

Precipitation 

In general, airlines, airports, and aircraft operate more efficiently in dry weather conditions than 

wet. Weather is a critical influence on aircraft performance and the outcome of the flight 

operations while taking off, landing, and while aloft. Precipitation affects aircraft and airports in 

several ways, such as decreasing visibility, slowing air traffic by requiring greater separation 

between aircraft, and decreasing braking effectiveness. On the ground, effects include creating 

turbulence, increasing the risk of icing of wings, and affecting engine thrust. 

Precipitation effects depend on whether annual precipitation increases or decreases (results were 

inconclusive). Less precipitation is expected to have the following effects: 

• reduce aircraft and air traffic delays; 

• reduce periods of wet surfaces on runways, taxiways, and apron; 

• reduce the risk of wing icing in the winter months; 

• possibly increase convective weather (turbulence), as well as increase the number and 

severity of thunderstorms; and 

• increase water vapor in the atmosphere, particularly during the summer months, potentially 

resulting in increased haze and reduced visibility for pilots. 

Increased precipitation is expected to have the following effects: 

• periods of low visibility would increase; and 

• general aviation pilots would either learn to fly in Instrument Flight Rules (IFR) conditions 

by becoming ‘instrument rated’ or not fly during periods of reduced visibility and 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 207
precipitation. 

In either scenario, an increased intensity of individual rainfall events is expected. Increased 

extreme precipitation events would also affect commercial service aircraft operations. During 

severe thunderstorm activity, it is not unusual for an airline to cancel flights or, at a minimum, 

experience delays in operations. Navigation in heavy precipitation is possible and currently 

occurs on a daily basis in the national air system. However, precipitation almost always creates 

delays, particularly at the most congested airports. In addition, 8 of the 61 airports in the study 

area are located within 100-year floodplains. 

Relative Sea Level Rise 

Analysis indicates that three airports in the study area would be below mean sea level if relative 

sea level increases by 122 cm (4 ft). Each of these airports currently is protected by preventive 

infrastructure such as dikes and levees, which will need to be maintained. If feeder roads in the 

area are inundated, however, access to these airports may be disrupted. All three airports are 

located in Louisiana, and include New Orleans International (elevation: 122 cm or 4 ft), one of 

the study area’s large commercial service airports, South LaFourche (elevation: 30 cm or 1 ft), a 

very small general aviation facility, and New Orleans Naval Air Station Joint Reserve Base 

(elevation: 91 cm or 3 ft), a military airport. 

Storm Activity 

Both storm surge and hurricane force winds can damage airport facilities. A variety of airports in 

the region would be vulnerable to the impacts of storm surge of 5.5 and 7.0 m (18 and 23 ft), 

though this depends on the specific characteristics of each individual storm event, including 

landfall location, wind speed, direction, and tidal conditions. There are 22 airports with 

elevations in the 0- to 5.5-m (18-ft) category and an additional 7 airports in the 5.8-m to 7.0-m 

(19- to 23-ft) category. This list includes some major airports in the region, such as New Orleans 

International. In addition, the commercial service airport in Lake Charles, Louisiana would be 

vulnerable. 

Pipelines 

There is a combined total of 42,520 km (26,427 mi) of onshore liquid (oil and petroleum 

product) transmission and natural gas transmission pipelines in the Gulf Coast study area. This 

includes 22,913 km (14,241 miles) of onshore natural gas transmission pipelines and 19,607 km 

(12,186 miles) of onshore hazardous liquid pipelines (PHMSA, 2007). This region is essential to 

the distribution of the Nation’s energy supply through pipeline transportation, and historically 

the landside pipelines have been relatively secure from disruption by increased storm activity 

and intensity. A number of risks and vulnerabilities to climate-related impacts have been 

revealed, however, particularly for submerged or very low elevation pipelines. 

Some historical weather events have resulted in only minor impacts on pipelines, with the 

notable exceptions of Hurricanes Andrew’s, Ivan’s, Katrina’s, and Rita’s fairly extensive damage 

to underwater pipelines and flooded distribution lines in areas where houses were destroyed. 

Storm surge and high winds historically have not had much impact on pipelines—either onshore 

transmission lines or offshore pipelines—since they are strong structures; well stabilized and/or 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 208
buried underground. Yet offshore pipelines have been damaged in relatively large numbers on 

occasion, as during Hurricanes Andrew and Ivan. Temperature shifts resulting from climate 

scenario projections are not expected to have much effect on pipelines. Increases or decreases in 

precipitation—either long-term or in the frequency or extent of droughts or inundation—could 

affect soil structure. Sea level rise would probably have little direct effect, but could affect water 

tables, soil stability, and the vulnerability of pipelines to normal wave action as well as storm 

surge. 

Changes in soil structure, stability, and subsidence—whether undersea, landside, or in wetlands 

or transition elevations—could play an important role in pipeline-related risks. However, there is 

little information on this topic outside of earthquake risks. Recently, concerns have been raised 

about how wave action could affect the seabed, either by liquefying/destabilizing the sand or silt 

surface above a buried pipeline or by gradually eroding away seabed that had been covering the 

pipeline. It is unclear at present whether a changing climate might lead to conditions that 

exacerbate these effects and cause additional damage. 

Temperature 

The great majority of the transmission pipeline system is buried under at least 0.9 m (3 ft) of soil 

cover, both onshore and offshore. Pipelines typically carry products at significant temperature 

variations (natural gas is under pressure; whereas petroleum products are heated considerably 

above ambient temperatures). There is not expected to be any significant effect on pipelines due 

to direct effects from increased (or decreased) temperatures. 

Precipitation Changes 

Sustained periods of increases or decreases in precipitation, whether over months or the 

cumulative effect across years, can cause substantial soil changes due to drought or saturation. 

Changes in water tables may occur both from local climate changes as well as from global 

effects such as sea level rise. An increase in water table level or increased surface water runoff 

can cause erosion or slumping (collapse) of the soil surface, thereby leading to potential for 

pipeline exposure. In the lowland and marsh areas particularly associated with the coastal regions 

of Louisiana, the soil is being washed away due to storm activity. With the disappearance of the 

soil, the pipelines in these regions are losing cover. 

Detailed analysis of geology and pipeline-specific conditions are required to draw more precise 

conclusions regarding the potential for serious disruption of the transmission pipeline system by 

climate-related soil changes. Nonetheless, this is an area of concern, as a considerable and 

unpredictable portion of the pipeline system could be vulnerable to these climate change and sealevel-induced impacts. 

Another vulnerability is from expected short-term changes (such as torrents and floods), where 

significant changes in rate and energy of water flow are a result of increased precipitation. Risk 

analysis of the impacts of extreme events is required to determine appropriate adaptation or 

mitigation actions. 

Storm Activity 

Wave action during storms may affect pipelines. For offshore pipelines, in instances where 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 209
significant subsidence occurs and the pipeline segment is exposed, this section is exposed to 

wave action. High-energy waves may subject a pipeline to stress levels it was not designed to 

withstand, causing a fracture. An exposed offshore pipeline also could be vulnerable to lateral 

and vertical displacement, exposure to vessel traffic and fishing trawls, or rupture by currents. 

Storm damage will vary depending on the particulars of a given storm—the landfall location, 

direction of the storm, and tidal conditions. For example, Hurricanes Andrew and Ivan caused 

substantial damage to pipelines throughout the storm-front region. After Ivan, oil refineries had 

ample products to supply, but the pipelines could not deliver due to damages. In contrast, 

damage to pipelines from Katrina and Rita was relatively minor; most pipelines were ready to 

take product, but were hampered by the lack of available product due to refinery damage and/or 

power shortages. 

V. Analysis of the Effects of Global Change on the Natural Environment and Human Systems 210
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