April 17, 2009 
Technical Support Document, U.S. Environmental Protection Agency 

Acknowledgments 

EPA authors and contributors: 

Benjamin DeAngelo, Jason Samenow, Jeremy Martinich, Doug Grano, Dina Kruger, Marcus Sarofim, 

William Perkins, Melissa Weitz, Leif Hockstad, William Irving, Lisa Hanle, Darrell Winner, David 

Chalmers, Chris Weaver, Susan Julius, Brooke Hemming, Sarah Garman, Rona Birnbaum, Paul 

Argyropoulos, Ann Wolverton, Al McGartland, Alan Carlin, John Davidson, Tim Benner, Carol Holmes, 

John Hannon, Jim Ketcham-Colwill, Andy Miller, Pamela Williams 

Federal expert reviewers 

Virginia Burkett, USGS; Phil DeCola; NASA (on detail to OSTP); William Emanuel, NASA; Anne 

Grambsch, EPA; Jerry Hatfield, USDA; Anthony Janetos; DOE Pacific Northwest National Laboratory; 

Linda Joyce, USDA Forest Service; Thomas Karl, NOAA; Michael McGeehin, CDC; Gavin Schmidt, 

NASA; Susan Solomon, NOAA; Thomas Wilbanks, DOE Oak Ridge National Laboratory 

Other contributors: 

Christine Teter, independent contractor for Stratus Consulting and Joel Smith, Stratus Consulting, assisted 

with editing and formatting. 

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Table of Contents 

Executive Summary..............................................................................................................ES-1 

I. Introduction 

1. Introduction and Background ..................................................................................................... 1

a. Scope and approach of this document ....................................................................................... 1 

b. Data and scientific findings considered by EPA ....................................................................... 3 

c. Roadmap for this document ...................................................................................................... 7 

II. Greenhouse Gas Emissions 

2. Greenhouse Gas Emissions and Atmospheric Concentrations ................................................. 9

a. U.S. and global greenhouse gas emissions................................................................................ 9 

b. Historic & current global greenhouse gas concentrations....................................................... 13 

III. Global and U.S. Observed and Projected Effects from Elevated Greenhouse Gas 

Concentrations 

3. Direct Effects of Elevated Greenhouse Gas Concentrations.................................................... 17

4. Radiative Forcing and Observed Climate Change ................................................................... 19

a. Radiative forcing due to greenhouse gases and other factors.................................................. 19 

b. Global changes in temperature ................................................................................................ 22

c. U.S. changes in temperature.................................................................................................... 27 

d. Global changes in precipitation............................................................................................... 28 

e. U.S. changes in precipitation................................................................................................... 29 

f. Global sea level rise ................................................................................................................ 30 

g. U.S. sea level rise .................................................................................................................... 31 

h. Global ocean acidification....................................................................................................... 31 

i. Global changes in physical and biological systems ................................................................ 31 

j. U.S. changes in physical and biological systems .................................................................... 34 

k. Global extreme events ............................................................................................................. 35 

l. U.S. extreme events................................................................................................................. 36 

5. Attribution of Observed Climate Change to Anthropogenic Greenhouse Gas Emissions at 

the Global and Continental Scale ............................................................................................... 39

a. Attribution of observed climate change to anthropogenic emissions...................................... 39 

b. Attribution of observed changes in physical & biological systems......................................... 43 

6. Projected Future Greenhouse Gas Concentrations and Climate Change .............................. 45

a. Global emission scenarios and associated changes in concentrations and radiative forcing... 45 

b. Projected changes in global temperature, precipitation patterns, sea level rise, and ocean 

acidification............................................................................................................................. 52 

c. Projected changes in U.S. temperature, precipitation patterns, sea level rise ......................... 57 

d. Cryosphere (Snow and Ice) projections, focusing on North America and the U.S ................. 60 

e. Extreme events projections, focusing on North America and the U.S. ................................... 61 

f. Abrupt climate change and high impact events....................................................................... 63 

g. Effects on/from stratospheric ozone........................................................................................ 66 

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h. Land use and land cover change......................................................................67 

IV. U.S. Observed and Projected Human Health and Welfare Effects from Climate Change 

7. Human Health .............................................................................................................................. 69

a. Temperature effects................................................................................................................. 69 

b. Extreme events ........................................................................................................................ 71 

c. Climate-sensitive diseases....................................................................................................... 72 

d. Aeroallergens........................................................................................................................... 73 

8. Air Quality.................................................................................................................................... 75

a. Tropospheric ozone ................................................................................................................. 75 

b. Particulate matter .................................................................................................................... 79 

c. Health Effects due to CO2-Induced Increases in Tropospheric O3 and Particulate Matter.....81 

9. Food Production and Agriculture .............................................................................................. 82

a. Crop yields and productivity ................................................................................................... 83 

b. Irrigation requirements ............................................................................................................ 85 

c. Climate variability and extreme events ................................................................................... 85 

d. Pests and weeds....................................................................................................................... 86 

e. Livestock ................................................................................................................................. 87 

f. Freshwater and marine fisheries.............................................................................................. 87

10. Forestry......................................................................................................................................... 89

a. Forest productivity ................................................................................................................. 89 

b. Wildfire and drought risk ........................................................................................................ 91 

c. Forest composition .................................................................................................................. 92 

d. Insects and diseases................................................................................................................. 93 

11. Water Resources .......................................................................................................................... 94

a. Water supply ........................................................................................................................... 94 

b. Water quality ........................................................................................................................... 97 

c. Extreme events ........................................................................................................................ 98 

d. Implications for water uses...................................................................................................... 99 

12. Sea Level Rise and Coastal Areas............................................................................................. 100

a. Vulnerable areas ................................................................................................................... 100 

b. Extreme events ...................................................................................................................... 103 

13. Energy, Infrastructure and Settlements .................................................................................. 105

a. Heating and cooling requirements......................................................................................... 105 

b. Energy production ................................................................................................................. 106 

c. Infrastructure and settlements ............................................................................................... 108 

14. Ecosystems and Wildlife............................................................................................................ 113

a. Ecosystems and species......................................................................................................... 113 

b. Ecosystem services................................................................................................................ 120 

c. Extreme events ...................................................................................................................... 120 

d. Implications for tribes ........................................................................................................... 120 

e. Implications for tourism...............................................................................121 

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V. International Observed and Projected Human Health and Welfare Effects from Climate 

Change 

15. International Impacts ................................................................................................................ 123

a. National security.................................................................................................................... 123 

b. Overview of international impacts ........................................................................................ 125 

References...............................................................................................................................129 

Appendix A on Adaptation.....................................................................................................140 

Appendix B on Greenhouse Gas Emissions from Section 202 Source Categories .........144 

Appendix C on Direct Effects of Greenhouse Gases on Human Health............................157 

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Executive Summary 

This document provides technical support for the endangerment analysis concerning greenhouse gas 

(GHG) emissions that may be addressed under the Clean Air Act. This document itself does not convey 

any judgment or conclusion regarding the question of whether GHGs may be reasonably anticipated to 

endanger public health or welfare, as this decision is ultimately left to the judgment of the Administrator. 

The conclusions here and the information throughout this document are primarily drawn from the 

assessment reports of the Intergovernmental Panel on Climate Change and the U.S. Climate Change 

Science Program. 

Observed Trends in Greenhouse Gas Emissions and Concentrations 

Greenhouse gases, once emitted, can remain in the atmosphere for decades to centuries, meaning 

that 1) their concentrations become well-mixed throughout the global atmosphere regardless of 

emission origin, and 2) their effects on climate are long lasting. The primary long-lived GHGs 

directly emitted by human activities include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), 

hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulfur hexafluoride (SF6). Greenhouse gases 

have a warming effect by trapping heat in the atmosphere that would otherwise escape to space. 

In 2006, U.S. GHG emissions were 7,054 teragrams1

 of CO2 equivalent2 (TgCO2eq). The dominant 

gas emitted is CO2, mostly from fossil fuel combustion. Methane is the second largest component of 

U.S. emissions, followed by N2O, and the fluorinated gases (HFCs, PFCs, and SF6). Electricity 

generation is the largest emitting sector (34% of total U.S. GHG emissions), followed by transportation 

(28%) and industry (19%). 

Transportation sources under section 202 of the Clean Air Act (passenger cars, light duty trucks, 

other trucks and buses, motorcycles, and cooling) emitted 1,665 TgCO2eq in 2006, representing 

almost 24% of total U.S. GHG emissions. 

U.S. transportation sources under section 202 made up 4.3% of total global GHG emissions in 2005, 

which, in addition to the U.S. as a whole, ranked only behind total GHG emissions from China, Russia 

and India but ahead of Japan, Brazil, Germany and the rest of the world’s countries. In 2005, total U.S. 

GHG emissions were responsible for 18% of global emissions, ranking only behind China, which was 

responsible for 19% of global GHG emissions. 

The global atmospheric CO2 concentration has increased about 38% from pre-industrial levels to 

2009, and almost all of the increase is due to anthropogenic emissions. The global atmospheric 

concentration of CH4 has increased by 149% since pre-industrial levels (through 2007); and the N2O 

concentration has increased 23% (through 2007). The observed concentration increase in these gases can 

also be attributed primarily to anthropogenic emissions. The industrial fluorinated gases, HFCs, PFCs, 

and SF6, have relatively low atmospheric concentrations but are increasing rapidly; these gases are almost 

entirely anthropogenic in origin. 

Historic data show that current atmospheric concentrations of the two most important directly 

emitted, long-lived GHGs (CO2 and CH4) are well above the natural range of atmospheric 

1

 One teragram (Tg) = 1 million metric tons. 1 metric ton = 1,000 kg = 1.102 short tons = 2,205 lbs. 2 Long-lived GHGs are compared and summed together on a CO2 equivalent basis by multiplying each gas by its 

Global Warming Potential (GWPs), as estimated by IPCC. In accordance with UNFCCC reporting procedures, the 

U.S. quantifies GHG emissions using the 100-year time frame values for GWPs established in the IPCC Second 

Assessment Report. 

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concentrations compared to the last 650,000 years. Atmospheric GHG concentrations have been 

increasing because anthropogenic emissions have been outpacing the rate at which GHGs are removed 

from the atmosphere by natural processes over timescales of decades to centuries. 

Observed Effects Associated with Global Elevated Concentrations of GHGs 

Current ambient air concentrations of CO2 and other GHGs remain well below published exposure 

thresholds for any direct adverse health effects, such as respiratory or toxic effects. 

The global average net effect of the increase in atmospheric GHG concentrations, plus other human 

activities (e.g., land use change and aerosol emissions), on the global energy balance since 1750 has 

been one of warming. This total net heating effect, referred to as forcing, is estimated to be +1.6 (+0.6 to 

+2.4) Watts per square meter (W/m2

), with much of the range surrounding this estimate due to 

uncertainties about the cooling and warming effects of aerosols. The combined radiative forcing due to 

the cumulative (i.e., 1750 to 2005) increase in atmospheric concentrations of CO2, CH4, and N2O is 

estimated to be +2.30 (+2.07 to +2.53) W/m2

. The rate of increase in positive radiative forcing due to 

these three GHGs during the industrial era is very likely to have been unprecedented in more than 10,000 

years. 

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. Global mean surface temperatures have risen by 0.74°C (1.3oF) (±0.18°C) over the 

last 100 years. Eight of the ten warmest years on record have occurred since 2001. 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. 

Most of the observed increase in global average temperatures since the mid-20th century is very 

likely due to the observed increase in anthropogenic GHG concentrations. Climate model 

simulations suggest natural forcing alone (e.g., changes in solar irradiance) cannot explain the observed 

warming. 

U.S. temperatures also warmed during the 20th and into the 21st century; temperatures are now 

approximately 0.7°C (1.3°F) warmer than at the start of the 20th century, with an increased rate of 

warming over the past 30 years. Both the IPCC and CCSP reports attributed recent North American 

warming to elevated GHG concentrations. In the CCSP (2008g) report the authors find that for North 

America, “more than half of this warming [for the period 1951-2006] is likely the result of human-caused 

greenhouse gas forcing of climate change.” 

Observations show that changes are occurring in the amount, intensity, frequency and type of 

precipitation. Over the contiguous U.S., total annual precipitation increased by 6.5% from 1901-2006. 

It is likely that there have been increases in the number of heavy precipitation events within many land 

regions, even in those where there has been a reduction in total precipitation amount, consistent with a 

warming climate. 

There is strong evidence that global sea level gradually rose in the 20th century and is currently 

rising at an increased rate. It is not clear whether the increasing rate of sea level rise is a reflection of 

short-term variability or an increase in the longer-term trend. Nearly all of the Atlantic Ocean shows sea 

level rise during the last 50 years with the rate of rise reaching a maximum (over 2 mm per year) in a 

band along the U.S. east coast running east-northeast. 

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Satellite data since 1978 show that annual average Arctic sea ice extent has shrunk by 2.7 ± 0.6% per 

decade, with larger decreases in summer of 7.4 ± 2.4% per decade. 

Widespread changes in extreme temperatures have been observed in the last 50 years across all 

world regions including the U.S. Cold days, cold nights, and frost have become less frequent, while hot 

days, hot nights, and heat waves have become more frequent. 

Observational evidence from all continents and most oceans shows that many natural systems are 

being affected by regional climate changes, particularly temperature increases. However, clearly 

attributing specific regional changes in climate to emissions of greenhouse gases from human activities is 

difficult, especially for precipitation. 

Ocean CO2 uptake has lowered the average ocean pH (increased acidity) level by approximately 0.1 

since 1750. Consequences for marine ecosystems may include reduced calcification by shell-forming 

organisms, and in the longer term, the dissolution of carbonate sediments. 

Observations show that climate change is currently affecting U.S. physical and biological systems in 

significant ways. The consistency of these observed changes in physical and biological systems and the 

observed significant warming likely cannot be explained entirely due to natural variability or other 

confounding non-climate factors 

Projections of Future Climate Change with Continued Increases in Elevated GHG Concentrations 

Most future scenarios that assume no explicit GHG mitigation actions (beyond those already 

enacted) project increasing global GHG emissions over the century, with climbing GHG 

concentrations. Carbon dioxide is expected to remain the dominant anthropogenic GHG over the course 

of the 21st century. The radiative forcing associated with the non-CO2 GHGs is still significant and 

increasing over time. 

Future warming over the course of the 21st century, even under scenarios of low emissions growth, 

is very likely to be greater than observed warming over the past century. According to climate 

model simulations summarized by the IPCC, through about 2030, the global warming rate is affected little 

by the choice of different future emission scenarios. By the end of the century, projected average global 

warming (compared to average temperature around 1990) varies significantly depending on emissions 

scenario and climate sensitivity assumptions, ranging from 1.8 to 4.0°C (3.2 to 7.2°F), with an uncertainty 

range of 1.1 to 6.4°C (2.0 to 11.5°F). 

All of the U.S. is very likely to warm during this century, and most areas of the U.S. are expected to 

warm by more than the global average. The largest warming is projected to occur in winter over 

northern parts of Alaska. 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, consistent with less 

warming over the oceans. 

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. 

Increases in the amount of precipitation are very likely in higher latitudes, while decreases are 

likely in most subtropical latitudes and the southwestern U.S., continuing observed patterns. The 

mid-continental area is expected to experience drying during summer, indicating a greater risk of drought. 

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Intensity of precipitation events is projected to increase in the U.S. and other regions of the world. 

More intense precipitation is expected to increase the risk of flooding and result in greater runoff and 

erosion that has the potential for adverse water quality effects. 

It is likely that hurricanes will become more intense, with stronger peak winds and more heavy 

precipitation associated with ongoing increases of tropical sea surface temperatures. Frequency changes 

in hurricanes are currently too uncertain for confident projections. 

By the end of the century, sea level is projected by IPCC to rise between 0.18 and 0.59 meters 

relative to around 1990 in the absence of increased dynamic ice sheet loss. Recent rapid changes at 

the edges of the Greenland and West Antarctic ice sheets show acceleration of flow and thinning. While 

understanding of these ice sheet processes is incomplete, their inclusion in models would likely lead to 

increased sea-level projections for the end of the 21st century. 

Sea ice extent is projected to shrink in the Arctic under all IPCC emission scenarios. 

Projected Risks and Impacts Associated with Future Climate Change 

Risk to society, ecosystems, and many natural Earth processes increase with increases in both the 

rate and magnitude of climate change. Climate warming may increase the possibility of large, 

abrupt regional or global climatic events (e.g., disintegration of the Greenland Ice Sheet or collapse 

of the West Antarctic Ice Sheet). The partial deglaciation of Greenland (and possibly West Antarctica) 

could be triggered by a sustained temperature increase of 1 to 4oC above 1990 levels. Such warming 

would cause a 4 to 6 meter rise in sea level, which would occur over a time period of centuries to 

millennia. 

CCSP reports that climate change has the potential to accentuate the disparities already evident in 

the American health care systems, as many of the expected health effects are likely to fall 

disproportionately on the poor, the elderly, the disabled, and the uninsured. 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. 

Severe heat waves are projected to intensify in magnitude and duration over the portions of the 

U.S. where these events already occur, with potential increases in mortality and morbidity, especially 

among the elderly, young and frail. 

The IPCC projects reduced human mortality from cold exposure through 2100. It is not clear 

whether reduced mortality from cold will be greater or less than increased heat-related mortality in the 

U.S. due to climate change. 

Increases in regional ozone pollution relative to ozone levels without climate change are expected 

due to higher temperatures and weaker circulation in U.S. and other world cities relative to air 

quality levels without climate change. Climate change is expected to increase regional ozone pollution, 

with associated risks in respiratory infection, aggravation of asthma, and premature death. In addition to 

human health effects, tropospheric ozone has significant adverse effects on crop yields, pasture and forest 

growth and species composition. The directional effect of climate change on ambient particulate matter 

levels remains uncertain. 

CCSP concluded that, with increased CO2 and temperature, the life cycle of grain and oilseed crops 

will likely progress more rapidly. But, as temperature rises, these crops will increasingly begin to 

experience failure, especially if climate variability increases and precipitation lessens or becomes 

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more variable. Furthermore, the marketable yield of many horticultural crops – e.g., tomatoes, onions, 

fruits – is very likely to be more sensitive to climate change than grain and oilseed crops. 

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. 

Cold-water 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. 

Climate change has very likely increased the size and number of forest fires, insect outbreaks, and 

tree mortality in the interior west, the Southwest, and Alaska, and will continue to do so. An 

increased frequency of disturbances like wildfire and insect outbreaks is at least as important to 

ecosystem function as incremental changes in temperature, precipitation, atmospheric CO2, nitrogen 

deposition, and ozone pollution. IPCC estimated that overall forest growth for North America as a whole 

will likely increase modestly (10-20%) as a result of extended growing seasons and elevated CO2 over the 

next century, but with important spatial and temporal variation. 

Coastal communities and habitats will be increasingly stressed by climate change impacts 

interacting with development and pollution. Sea level is rising along much of the U.S. 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. 

Climate change will likely further constrain already over-allocated water resources in some sections 

of the U.S., increasing competition among agricultural, municipal, industrial, and ecological uses. 

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

reliance on past conditions as the basis for current and future planning will no longer be appropriate, as 

climate change increasingly creates conditions well outside of historical observations. Rising 

temperatures will diminish snowpack and increase evaporation, affecting seasonal availability of water. 

In the Great Lakes and major river systems, lower levels are likely to exacerbate challenges relating to 

water quality, navigation, recreation, hydropower generation, water transfers, and bi-national 

relationships. Decreased water supply and lower water levels are likely to exacerbate challenges relating 

to aquatic navigation in the U.S. 

Higher water temperatures, increased precipitation intensity, and longer periods of low flows will 

exacerbate many forms of water pollution, potentially making attainment of water quality goals more 

difficult. As waters become warmer, the aquatic life they now support will be replaced by other species 

better adapted to warmer water. In the long term, warmer water and changing flow may result in 

deterioration of aquatic ecosystems. 

Ocean acidification is projected to continue, resulting in the reduced biological production of 

marine calcifiers, including corals. 

Climate change is likely to affect both U.S. energy use and energy production and physical and 

institutional infrastructures; and will likely interact with and possibly exacerbate ongoing 

environmental change and environmental pressures in settlements, particularly in Alaska where 

indigenous communities are facing major environmental and cultural impacts. The U.S. energy sector, 

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which relies heavily on water for hydropower and cooling capacity, may be adversely impacted by 

changes to water supply and quality in reservoirs and other water bodies. Water infrastructure, including 

drinking water and wastewater treatment plants, and sewer and stormwater management systems, will be 

at greater risk of flooding, sea level rise and storm surge, low flows, and other factors that could impair 

performance. 

Disturbances such as wildfires and insect outbreaks are increasing in the U.S. and are likely to 

intensify in a warmer future with drier soils and longer growing seasons. Although recent climate 

trends have increased vegetation growth, continuing increases in disturbances are likely to limit carbon 

storage, facilitate invasive species, and disrupt ecosystem services. 

Over the 21st century, changes in climate will cause species to shift north and to higher elevations 

and fundamentally rearrange U.S. 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. 

Within settlements experiencing climate change, certain parts of the population may be especially 

vulnerable; these include the poor, the elderly, those already in poor health, the disabled, those living 

alone, those with limited rights and power (such as recent immigrants with limited English skills), and/or 

indigenous populations dependent on one or a few resources. Thus, the potential impacts of climate 

change raise environmental justice issues. 

Climate change impacts in certain regions of the world may exacerbate problems that raise 

humanitarian, trade and national security issues for the U.S. The IPCC identifies the most vulnerable 

world regions as the Arctic, because of the effects of high rates of projected warming on natural systems; 

Africa, especially the sub-Saharan region, because of current low adaptive capacity as well as climate 

change; small islands, due to high exposure of population and infrastructure to risk of sea-level rise and 

increased storm surge; and Asian mega-deltas, such as the Ganges-Brahmaputra and the Zhujiang, due to 

large populations and high exposure to sea level rise, storm surge and river flooding. Climate change has 

been described as a potential threat multiplier with regard to national security issues. 

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Section 1 

Introduction and Background 

The purpose of this document is to provide scientific and technical information for an endangerment 

analysis regarding greenhouse gas (GHG) emissions from new motor vehicles and engines under section 

202(a) of the Clean Air Act. Section 202 (a)(1) of the Clean Air Act states that: 

the Administrator shall by regulation prescribe (and from time to time revise)...standards 

applicable to the emission of any air pollutant from any class or classes of new motor vehicles ..., 

which in his judgment cause, or contribute to, air pollution which may reasonably be anticipated 

to endanger public health or welfare. 

Thus, absent some other statutory provision, before EPA may issue standards addressing emissions of an 

air pollutant from new motor vehicles or new motor vehicle engines under section 202(a), the 

Administrator must make a so-called “endangerment finding.” That finding is a two step test. First, the 

Administrator must decide if, in her judgment, air pollution may reasonably be anticipated to endanger 

public health or welfare. Second, the Administrator must decide whether, in her judgment, emissions of 

any air pollutant from new motor vehicles or engines cause or contribute to this air pollution. If the 

Administrator answers both questions in the affirmative, EPA shall issue standards under section 202(a). 

This document itself does not convey any judgment or conclusion regarding the two steps of the 

endangerment finding, as these decisions are ultimately left to the judgment of the Administrator. 

1(a) Scope and Approach of this Document 

The primary GHGs that are directly emitted by human activities in general are those reported in EPA’s 

annual Inventory of U.S. Greenhouse Gas Emissions and Sinks and include carbon dioxide (CO2), 

methane (CH4), nitrous oxide (N2O), hydrofluorocarbons (HFCs), perfluorocarbons (PFCs) and sulfur 

hexafluoride (SF6). The primary effect of these gases is their influence on the climate system by trapping 

heat in the atmosphere that would otherwise escape to space. This heating effect (referred to as radiative 

forcing) is very likely to be the cause of most of the observed global warming over the last 50 years. 

Global warming and climate change can, in turn, affect health, society and the environment. There also 

are some cases where these gases have other non-climate effects. For example, elevated concentrations of 

CO2 can lead to ocean acidification and stimulate terrestrial plant growth, and CH4 emissions can 

contribute to background levels of tropospheric ozone, a criteria pollutant. These effects may in turn be 

influenced by climate change in certain cases. Carbon dioxide and other GHGs can also have direct 

health effects but at concentrations far in excess of current or projected future ambient concentrations. 

This document reviews a wide range of observed and projected vulnerabilities, risks and impacts due to 

the elevated levels of GHGs in the atmosphere and associated climate change. Any known or expected 

benefits of elevated atmospheric concentrations of GHGs or of climate change are documented as well 

(i.e., impacts can mean either positive or negative consequences). The extent to which observed climate 

change can be attributed to anthropogenic GHG emissions is assessed. The term “climate change” in this 

document generally refers to climate change induced by human activities, including activities that emit 

GHGs. Future projections of climate change, based primarily on future scenarios of anthropogenic GHG 

emissions, are shown for the global and national scale. 

The focus of the vulnerability, risk and impact assessment is primarily within the U.S. However, given 

the global nature of climate change, there is a brief review of potential international impacts. Greenhouse 

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gases, once emitted, become well mixed in the atmosphere, meaning U.S. emissions can affect not only 

the U.S. population and environment but other regions of the world as well; likewise, emissions in other 

countries can affect the U.S. Furthermore, impacts in other regions of the world may have consequences 

that transcend national boundaries and raise concerns for the U.S. 

The timeframe over which vulnerabilities, risks and impacts are considered is consistent with the 

timeframe over which GHGs, once emitted, have an effect on climate, which is decades to centuries for 

the primary GHGs of concern. Therefore, in addition to reviewing recent observations, this document 

generally considers the next several decades, the time period out to around 2100, and for certain impacts 

the time period beyond 2100. 

Adaptation to climate change is a key focus area of the climate change research community. This 

document, however, does not focus on adaptation because adaptation is essentially a response to any 

known and/or perceived risks due to climate change. Likewise, mitigation measures to reduce GHGs, 

which could also reduce long-term risks, are not addressed. The purpose of this document is to review the 

effects of climate change and not to assess any speculative policy or societal response to climate change. 

Adaptation will be mentioned to the extent that the impacts projections themselves contain some 

embedded assumptions about future adaptation.3 

3 A brief overview of adaptation is provided in Appendix A. 

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Box 1.1, Peer review, publication and approval processes for IPCC, CCSP and NRC reports 

Intergovernmental Panel on Climate Change 

The World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) 

established the Intergovernmental Panel on Climate Change (IPCC) in 1988. It bases its assessment mainly on 

peer reviewed and published scientific/technical literature. IPCC has established rules and procedures for 

producing its assessment reports. Report outlines are agreed to by government representatives in consultation 

with the IPCC bureau. Lead authors are nominated by governments and are selected by the respective IPCC 

Working Groups on the basis of their scientific credentials and with due consideration for broad geographic 

representation. For Working Group I there were 152 coordinating lead authors, and for Working Group II 48 

coordinating lead authors. Drafts prepared by the authors are subject to two rounds of review; the second round 

includes government review. For the IPCC Working Group I report, over 30,000 written comments were 

submitted by over 650 individual experts, governments and international organizations. For Working Group II 

there were 910 expert reviewers. Review Editors for each chapter are responsible for ensuring that all 

substantive government and expert review comments receive appropriate consideration. For transparency, IPCC 

documents how every comment is addressed. Each Summary for Policymakers is approved line-by-line, and the 

underlying chapters are then accepted, by government delegations in formal plenary sessions. Further 

information about IPCC’s principles and procedures can be found at: 0Hhttp://www.ipcc.ch/about/procd.htm. 

U.S. Climate Change Science Program 

The CCSP has identified 21 synthesis and assessment products (SAPs) that address the highest priorities for U.S. 

climate change research, observation and decision-support needs. As of January 16th, 2009, all 21 synthesis and 

assessment products had been completed. Different agencies have been designated the lead for different SAPs; 

EPA is the designated lead for three of the six SAPs addressing impacts and adaptation. For each SAP, there is 

first a prospectus that provides an outline, the proposed authors and process for completing the SAP; this goes 

through two stages of expert, interagency and public review. Authors produce a first draft which goes through 

expert review; a second draft is posted for public review. The designated lead agency ensures that the third draft 

complies with the Information Quality Act. Finally the SAP is submitted to the National Science and 

Technology Council (NSTC), a Cabinet-level council that coordinates science and technology research across 

the Federal government, for approval. Further information about the clearance and review procedures for the 

CCSP SAPs can be found at: 1Hhttp://www.climatescience.gov/Library/sap/sap-guidelines-clarificationaug2007.htm. 

National Research Council of the U.S. National Academy of Sciences 

The National Research Council (NRC) is part of the National Academies, which also comprise the National 

Academy of Sciences, National Academy of Engineering and Institute of Medicine. They are private, nonprofit 

institutions that provide science, technology and health policy advice under a congressional charter. The NRC 

has become the principal operating agency of both the National Academy of Sciences and the National Academy 

of Engineering in providing services to the government, the public and the scientific and engineering 

communities. Federal agencies are the primary financial sponsors of the Academies’ work. The Academies 

provide independent advice; the external sponsors have no control over the conduct of a study once the 

statement of task and budget are finalized. The NRC 2001 study, Climate Change Science: An Analysis of Some 

Key Questions, originated from a White House request. The NRC 2001 study, Global Air Quality: An 

Imperative for Long-Term Observational Strategies, was supported by EPA and NASA. The NRC 2004 study, 

Air Quality Management in the United States, was supported by EPA. The NRC 2005 study, Radiative Forcing 

of Climate Change: Expanding the Concept and Addressing Uncertainties, was in response to a CCSP request, 

and supported by NOAA. The NRC 2006 study, Surface Temperature Reconstructions for the Last 2,000 Years, 

was requested by the Science Committee of the U.S. House of Representatives. Each NRC report is authored by 

its own committee of experts, reviewed by outside experts, and approved by the Governing Board of the NRC. 

Uncertainties and confidence levels associated with the scientific conclusions and findings in this 

document are reported, to the extent that such information was provided in the original scientific reports 

upon which this document is based. 

1(b) Data and Scientific Findings Considered by EPA 

3
Technical Support Document, U.S. Environmental Protection Agency 

This document relies most heavily on existing, and in most cases very recent, synthesis reports of climate 

change science and potential impacts, which have gone through their own peer-review processes 

including review by the U.S. Government. The information in this document has been developed and 

prepared in a manner that is consistent with EPA's Guidelines for Ensuring and Maximizing the Quality, 

Objectivity, Utility and Integrity of Information Disseminated by the Environmental Protection Agency. 

In addition to its reliance on existing and primarily recent synthesis reports from the peer reviewed 

literature, it also underwent a technical review by 12 federal climate change experts, internal EPA review, 

and interagency review. 

These core reference (Table 1.1) documents include the 2007 Fourth Assessment Report of the 

Intergovernmental Panel on Climate Change (IPCC), Synthesis and Assessment Products of the U.S. 

Climate Change Science Program (CCSP), National Research Council (NRC) reports under the U.S. 

National Academy of Sciences (NAS), the EPA annual report on U.S. greenhouse gas emission 

inventories and the EPA assessment of the impacts of global change on regional U.S. air quality. 

Table 1.1, Core references relied upon most heavily in this document. 

Science body/author Short Title and Year of Publication 

IPCC Working Group I: The Physical Science Basis (2007) 

IPCC Working Group II: Impacts, Adaptation and Vulnerability (2007) 

IPCC Working Group III: Mitigation of Climate Change (2007) 

CCSP SAP 1.1: Temperature Trends in the Lower Atmosphere (2006) 

CCSP SAP 1.2: Past Climate Variability and Change in the Arctic and at High Latitudes 

(2009) 

CCSP SAP 1.3: Re-analyses of Historical Climate Data (2008) 

CCSP SAP 2.1: Scenarios of GHG Emissions and Atmospheric Concentrations (2007) 

CCSP SAP 2.3: Aerosol Properties and their Impacts on Climate 

CCSP SAP 2.4: Trends in Ozone-Depleting Substances (2008) 

CCSP SAP 3.1: Climate Change Models (2008) 

CCSP SAP 3.2: Climate Projections (2008) 

CCSP SAP 3.3: Weather and Climate Extremes in a Changing Climate (2008) 

CCSP SAP 3.4: Abrupt Climate Change (2008) 

CCSP SAP 4.1: Coastal Sensitivity to Sea-Level Rise (2009) 

CCSP SAP 4.2: Thresholds of Change in Ecosystems (2009) 

CCSP SAP 4.3: Agriculture, Land Resources, Water Resources, and Biodiversity (2008) 

CCSP SAP 4.5: Effects on Energy Production and Use (2007) 

CCSP SAP 4.6: Analyses of the Effects of Global Change on Human Health (2008) 

CCSP SAP 4.7: Impacts of Climate Change and Variability on Transportation Systems 

(2008) 

NRC Climate Change Science: Analysis of Some Key Questions (2001) 

NRC Radiative Forcing of Climate Change (2005) 

NRC Surface Temperature Reconstructions for the Last 2,000 Years (2006) 

NRC Potential Impacts of Climate Change on U.S. Transportation (2008) 

EPA Impacts of Global Change on Regional U.S. Air Quality(2009) 

EPA Inventory of U.S. Greenhouse Gas Emissions and Sinks (2008) 

ACIA 4 Arctic Climate Impact Assessment (2004) 
Technical Support Document, U.S. Environmental Protection Agency 

EPA is relying most heavily on these synthesis reports because they 1) are very recent and represent the 

current state of knowledge on climate change science, vulnerabilities and potential impacts; 2) have 

assessed numerous individual studies in order to draw general conclusions about the state of science; 3) 

have been reviewed and formally accepted by, commissioned by, or in some cases authored by, U.S. 

government agencies and individual government scientists and provide EPA with assurances that this 

material has been well vetted by both the climate change research community and by the U.S. 

government; and 4) in many cases, they reflect and convey the consensus conclusions of expert authors. 

Box 1.1 describes the peer review and publication approval processes of IPCC, CCSP and NRC reports. 

Peer review and transparency are key to each of these research organizations’ report development 

process. In compliance with the U.S. EPA’s information quality guidelines, this document relies on 

information that is objective, technically sound and vetted, and of high integrity. Box 1.2 describes the 

lexicon used by IPCC to communicate uncertainty and confidence levels associated with the most 

important IPCC findings; this document employs the same lexicon when referencing IPCC statements. 

The IPCC Fourth Assessment Report (AR4) consists of three volumes: Working Group I addresses the 

physical science of climate change; Working Group II addresses impacts, vulnerabilities and adaptation; 

and Working Group III addresses mitigation (i.e., emission reduction) measures. These IPCC 

assessments are generally global in scope but provide information at the country and regional level as 

well. The Working Group II volume contains individual chapters devoted to the key climate change 

impact sectors, which are also addressed in this document (e.g., human health, agriculture, water 

resources, etc.), as well as chapters devoted to key regions. This document relies heavily on the North 

America chapter of the IPCC Working Group II report, though this chapter may not provide as much 

regional detail within the U.S. as did the 2000 report, Climate Change Impacts on the United States: The 

Potential Consequences of Climate Variability and Change (NAST, 2000). 

Throughout this document, when these various assessments are referred to in general or as a whole, the 

full reports are cited. For example, a general reference to the CCSP report “Weather and Climate 

Extremes in a Changing Climate” is cited as “CCSP, 2008i” (tbe “i” differentiates the report from other 

CCSP reports published that same year). When specific findings or conclusions from these larger 

assessment reports are referenced, citations are given for the relevant individual chapter or section. For 

example, a finding from CCSP, 2008i chapter 5 “Observed Changes in Weather and Climate” by Kunkel 

et al. is cited as “Kunkel et al., 2008.” 

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Technical Support Document, U.S. Environmental Protection Agency 

Box 1.2: Communication of Uncertainty in the IPCC AR4 and U.S. CCSP SAPs 

IPCC AR4 Uncertainty Treatment 

A set of terms to describe uncertainties in current knowledge is common to all parts of the IPCC AR4 based on the 

Guidance Notes for Lead Authors of the IPCC Fourth Assessment Report on Addressing Uncertainties 

(http://www.ipcc.ch/activity/uncertaintyguidancenote.pdf), produced by the IPCC in July 2005. Any use of these 

terms in association with IPCC statements in this Endangerment document carries the same meaning as originally 

intended in the IPCC Fourth Assessment Report. 

Description of confidence 

On the basis of a comprehensive reading of the literature and their expert judgment, authors have assigned a 

confidence level to major statements on the basis of their assessment of current knowledge, as follows: 

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

High confidence About 8 out of 10 chance 

Medium confidence About 5 out of 10 chance 

Low confidence About 2 out of 10 chance 

Very low confidence Less than a 1 out of 10 chance 

Description of likelihood 

Likelihood refers to a probabilistic assessment of some well defined outcome having occurred or occurring in the 

future, and may be based on quantitative analysis or an elicitation of expert views. When authors evaluate the 

likelihood of certain outcomes, the associated meanings are: 

Virtually certain >99% probability of occurrence 

Very likely 90 to 99% probability 

Likely 66 to 90% probability 

About as likely as not 33 to 66% probability 

Unlikely 10 to 33% probability 

Very unlikely 1 to 10% probability 

Exceptionally unlikely <1% probability 

U.S. CCSP SAP Uncertainty Treatment 

In many of its SAPs, the U.S. CCSP uses the same or similar terminology to the IPCC to describe confidence and 

likelihood. However, there is some variability from report to report, so readers should refer to the individual SAPs 

for a full accounting of the respective uncertainty language. In this document, when referencing CCSP reports, EPA 

attempted to reflect the underlying CCSP reports’ terminology for communicating uncertainty. 

In some cases, this document references other reports and studies in addition to the core references of 

IPCC, CCSP, NRC, and, for greenhouse gas emissions, EPA. These references are primarily for major 

reports and studies produced by U.S. federal and state government agencies. This document also 

references data made available by other government agencies, such as NOAA and NASA. 

EPA just recently completed and published an assessment of the literature on the effect of climate change 

on air quality (EPA, 2009). Therefore, because EPA evaluated the literature itself in the preparation of 

that assessment, EPA does cite some individual studies it reviewed in its summary of this topic in Section 

8. Also, for Section 15a on the national security implications of climate change, this document cites a 

number of analyses and publications, from both inside and outside the government, since IPCC and CCSP 

assessments have not explicitly addressed these issues. 

EPA recognizes that scientific research is very active in many areas addressed in this document (e.g. 

aerosol effects on climate, climate feedbacks such as water vapor, and internal and external climate 

forcing mechanisms) as well as for some emerging issues (e.g. ocean acidification, and climate change 

6
Technical Support Document, U.S. Environmental Protection Agency 

effects on water quality). EPA recognizes the potential importance of new scientific research, but, except 

as described above, studies that have not yet been assessed by the scientific community are not included 

in this document. 

1(c) Roadmap for this Document 

The remainder of this document is structured as follows: 

• Part II, Section 2 describes sources of U.S. and global GHG emissions. How anthropogenic GHG 

emissions have contributed to changes in global atmospheric concentrations of GHGs is also 

described. 

• Part III, Sections 3 – 6 describe the effects of elevated concentrations of GHGs including any direct 

health and environmental effects; the heating or radiative forcing effects on the climate system; 

observed climate change (e.g., changes in temperature, precipitation and sea level rise) for the U.S. 

and for the globe; recent conclusions about the extent to which observed climate change can be 

attributed to the elevated levels of GHG concentrations; and summarize future projections of climate 

change—driven primarily by scenarios of anthropogenic GHG emissions—for the remainder of this 

century. 

• Part IV, Sections 7 – 14 review recent findings for the broad range of observed and projected 

vulnerabilities, risks and impacts for human health, society and the environment within the U.S. due 

to climate change. The specific sectors and systems include: 

o Human health (7) 

o Air Quality (8) 

o Food Production and Agriculture (9) 

o Forestry (10) 

o Water Resources (11) 

o Coastal Areas (12) 

o Energy, Infrastructure and Settlements (13) 

o Ecosystems and Wildlife (14) 

• Part V, Section 15 briefly addresses some key international impacts that may occur due to climate 

change, with a view towards how some of these impacts may in turn affect the U.S. 

o International Impacts (15) 

7
Technical Support Document, U.S. Environmental Protection Agency 

Part II 

Emissions and Elevated Concentrations of Greenhouse Gases 

8
Technical Support Document, U.S. Environmental Protection Agency 

Section 2 

Greenhouse Gas Emissions and Concentrations 

This section first describes current U.S. and global anthropogenic greenhouse gas (GHG) emissions. 

Future GHG emission scenarios are described in Part III, Section 6, however, these scenarios primarily 

focus on global emissions, rather than detailing individual U.S. sources. This section then focuses on 

historic and current global GHG atmospheric concentrations. 

2(a) U.S. and global greenhouse gas emissions 

To track the national trend in GHG emissions and carbon removals since 1990, EPA develops the official 

U.S. GHG inventory each year. In accordance with Article 4.1 of the United Nations Framework 

Convention on Climate Change (UNFCCC), the Inventory of U.S. Greenhouse Gas Emissions and Sinks 

includes emissions and removals of carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), 

hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6) resulting from 

anthropogenic activities in the U.S. 

Total emissions are presented in teragrams

4

 (Tg) of CO2 equivalent (TgCO2eq), consistent with 

Intergovernmental Panel on Climate Change (IPCC) inventory guidelines. To determine the CO2 

equivalency of different GHGs, in order to sum and compare different GHGs, emissions of each gas are 

multiplied by its global warming potential (GWP), a factor which relates it to CO2 in its ability to trap 

heat in the atmosphere over a certain timeframe. Box 2.1 provides more information about GWPs and the 

GWP values used in this section of the report. 

Box 2.1: Global Warming Potentials used in this document 

In accordance with UNFCCC reporting procedures, the U.S. quantifies GHG emissions using the 100-year time 

frame values for GWPs established in the IPCC Second Assessment Report (SAR) (IPCC, 1996). The GWP 

index is defined as the cumulative radiative forcing between the present and some chosen later time horizon (100 

years) caused by a unit mass of gas emitted now. All GWPs are expressed relative to a reference gas, CO2, 

which is assigned a GWP = 1. Estimation of the GWPs requires knowledge of the fate of the emitted gas and the 

radiative forcing due to the amount remaining in the atmosphere. To estimate the CO2 equivalency of a non-CO2 

GHG, the appropriate GWP of that gas is multiplied by the amount of the gas emitted. 

100-year GWPs 

 CO2 1

 CH4 21 

N2O 310 

 HFCs 140 to 6,300 (depending on type of HFC) 

 PFCs 6,500 to 9,200 (depending on type of PFC) 

 SF6 23,900 

The GWP for CH4 includes the direct effects and those indirect effects due to the production of tropospheric 

ozone and stratospheric water vapor. These GWP values have been updated twice, in the IPCC third (TAR 

2001) and fourth assessment reports (AR4 2007). 

4

 1 teragram (Tg) = 1 million metric tons. 1 metric ton = 1,000 kg = 1.102 short tons = 2,205 lbs. 

9 
Technical Support Document, U.S. Environmental Protection Agency 

The national inventory totals used in this report for the U.S. (and other countries) are gross emissions, 

which include GHG emissions from the electricity, industrial, commercial, residential and agriculture 

sectors. Emissions and sequestration occurring in the land-use, land-use change and forestry sector (e.g., 

forests, soil carbon etc.) are not included in gross national totals, but are reported under net emission 

totals (sources and sinks), according to international practice. In the U.S., this sector is a significant net 

sink, while in some developing countries it is a significant net source of emissions. 

Also excluded from emission totals in this report are bunker fuels (fuels used for international transport). 

According to UNFCCC reporting guidelines, emissions from the consumption of these fuels should be 

reported separately, and not included in national emission totals, as there exists no agreed upon 

international formula for allocation between countries. 

The most recent inventory was published in 2008 and includes annual data for the years 1990-2006. 

U.S. Greenhouse Gas Emissions 

In 2006, U.S. GHG emissions were 7,054.2 TgCO2eq (see Figure 2.1).5

 The dominant gas emitted is CO2, 

mostly from fossil fuel combustion (84.8%) (EPA, 2008). Weighted by GWP, CH4 is the second largest 

component of emissions, followed by N2O, and the high-GWP fluorinated gases (HFCs, PFCs, and SF6). 

Electricity generation (2377.8 TgCO2eq) is the largest emitting sector, followed by transportation (1969.5 

TgCO2eq) and industry (1371.5 TgCO2eq) (EPA, 2008) (Figure 2.2). Agriculture and the commercial and 

residential sectors emit 533.6 TgCO2eq, 394.6 TgCO2eq, and 344.8 TgCO2eq, respectively (EPA, 2008). 

Removals of carbon through land use, land-use change and forestry activities are not included in Figure 

2.2, but are significant; net sequestration is estimated to be 883.7 TgCO2eq in 2006, offsetting 12.5% of 

total emissions (EPA, 2008). 

5

 Per UNFCCC reporting requirements, the U.S. reports its annual emissions in gigagrams (Gg) with two significant 

digits (http://unfccc.int/files/national_reports/annex_i_ghg_inventories/national_inventories_submissions/application/x-zip-compressed/usa_2007_crf_11apr.zip). For ease of communication of the findings, the 

Inventory of U.S. Greenhouse Gas Emissions and Sinks report presents total emissions in Tg with one significant 

digit. 

10
Technical Support Document, U.S. Environmental Protection Agency 

U.S. emissions increased 

by 905.9 TgCO2eq, or 

14.7% between 1990 and 

2006 (see Figure 2.1) 

(EPA, 2008). 

Historically, changes in 

fossil fuel consumption 

have been the dominant 

factor affecting U.S. 

emission trends. The 

fundamental factors 

driving this trend include 

a generally growing 

domestic economy over 

the last 16 years, leading 

to overall growth in 

emissions from electricity 

generation (increase of 

30.7%) and transportation 

activities (increase of 

27.9%) (EPA, 2008). Over the same time period, industrial, residential, and commercial emissions 

decreased by 6.0%, 0.6%, and 0.6% respectively, while emissions increased in the agriculture (5.3%) 

sector (Figure 2.2) (EPA, 2008). 

11

Figure 2.1: Total U.S. Greenhouse Gas Emissions: 1990-2006 

8,000 

7,000 

6,000 

Eq.5,000 

Tg2 CO4,000 

3,000 

2,000 

1,000 

0 

Source: Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006 (EPA, 2008). Excludes landuse change and forestry and international bunker fuels. 

6,731 6,760 6,801 6,435 6,494 6,343 6,148 6,106 6,192 

7,054 7,130 6,981 6,998 6,839 7,033 6,921 7,078 

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 

6 

Figure 2.2: U.S. GHG Emissions Allocated to Economic Sector. 

0 

500 

1,000 

1,500 

2,000 

2,500 

1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 

Tg CO2 Eq. 

Transportation 

Industry 

Residential 

Agriculture 

Commercial 

Source: Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006 (EPA, 

2008). All GHGs. Excludes land use, land-use change and forestry and 

international bunker fuels. 

Electricity Generation 
Technical Support Document, U.S. Environmental Protection Agency 

U.S. Greenhouse Gas 

Emissions from Source 

Categories under Section 

202(a) of the Clean Air Act 

Source categories under 

Figure 2.3: Total GHG Emissions for the Year 2005 by Country and for 

U.S. Section 202a Source Categories 

Section 202(a) of the Act 

include passenger vehicles, 

light- and heavy-duty trucks, 

buses, motorcycles, and the 

cooling systems designed for 

passenger comfort, as well as 

auxiliary systems for 

refrigeration. 

In 2006, section 202(a) 

source categories collectively 

were the second largest 

GHG-emitting sector within 

the U.S. (behind the 

electricity generating sector), 

emitting 1,665 TgCO2eq and 

representing 24 percent of 

total U.S. GHG emissions. 

Between 1990 and 2006, total 

0 

1,000 

2,000 

3,000 

4,000 

5,000 

6,000 

7,000 

8,000 

China 

U.S.A. 

World Transport 

Russia

India 

U.S.Section 202(a) 

Japan

Brazil 

Germany 

Canada

U.K. 

Mexico 

Indonesia

Iran 

Tg

C

O

2

e

q. 

Source: WRI Climate Analysis and Indicators Tool. Available at http://cait.wri.org/. 

Excludes land use, land-use change and forestry, and international bunker fuels. 

More recent emissions data are available for the U.S. and other individual 

countries, but 2000 is the most recent year for which data for all countries and all 

gases are available. Data accessed February 20, 2009. 

GHG emissions from passenger cars decreased 0.9 percent, while emissions from light-duty trucks 

increased 57 percent, largely due to the increased use of sport-utility vehicles and other light-duty trucks. 

Globally in 2005, section 202(a) source category GHG emissions represented 31 percent of global 

transport GHG emissions and 4.3 percent of total global GHG emissions (Figure 2.3). The global 

transport sector was 14 percent of all global GHG emissions in 2005. If U.S. section 202(a) source 

category GHG emissions were ranked against total GHG emissions for entire countries, U.S. section 

202(a) emissions would rank behind only China, the U.S. as a whole, Russia and India, and would rank 

ahead of Japan, Brazil, Germany and every other country in the world. 

Further detail on these emissions can be found in Appendix B of this document. 

Global Greenhouse Gas Emissions 

Total global emissions are calculated by summing emissions of the six greenhouse gases, by country. The 

World Resources Institute compiles data from recognized national and international data sources in its 

Climate Analysis Indicators Tool (CAIT).6 Globally, total GHG emissions were 38,725.9 TgCO2eq in 

2005, the most recent year for which data are available for all countries and all GHGs (WRI, 2009).7 

This global total for the year 2005 represents an increase of about 26% from the 1990 global GHG 

emission total of 30,704.9 TgCO2eq (WRI, 2009). Excluding land use, land-use change, and forestry, 

U.S. emissions were 19% of the total year 2005 global emissions, (see Figure 2.3) (WRI, 2009). 

6

 Primary data sources referenced in CAIT include the U.S. Department of Energy’s Carbon Dioxide Information 

Analysis Center, the U.S. Environmental Protection Agency, the International Energy Agency and the National 

Institute for Public Health and the Environment, an internationally recognized source of non-CO2 data. 7

 Source: WRI Climate Analysis and Indicators Tool. Available at http://cait.wri.org/. 

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Technical Support Document, U.S. Environmental Protection Agency 

2(b) Historic and current global greenhouse gas concentrations 

Greenhouse gas concentrations in the atmosphere vary over very long time scales in response to natural 

influences such as geologic activity and temperature change associated with ice age cycles, but ice core 

data show nearly constant concentrations of CO2, CH4 and N2O over more than 10,000 years prior to the 

industrial revolution. However, since the industrial revolution, anthropogenic GHG emissions have 

resulted in substantial increases in the concentrations of GHGs in the atmosphere (IPCC, 2007d; NRC, 

2001). 

Carbon dioxide (CO2) 

Carbon dioxide concentrations have increased substantially from pre-industrial levels (Figure 2.4). The 

long-term trends in the CO2 concentrations are as follows (NOAA, 2009c; Forster et al., 2007): 

• The CO2 concentration has increased about 38% from a pre-industrial value of about 280 parts per 

million (ppm) to 386 ppm (which is about 0.039% of the atmosphere by volume) in 20088

. 

• The present atmospheric concentration of CO2 exceeds by far the natural range over the last 

650,000 years (180 to 300 ppm) as determined from ice cores (Jansen et al., 2007). 

• The annual CO2 concentration growth rate9

 has been larger since 2000 (2000-2008 average: 2.0 

ppm per year), than it has been since the beginning of continuous direct atmospheric 

measurements (1960–2005 average: 1.4 ppm per year) although there is year-to-year variability. 

Almost all of the increase in the CO2 concentration during the Industrial Era is due to anthropogenic 

emissions (Forster et al., 2007). Since the 1980s, about half of the anthropogenic emissions have been 

taken up by the terrestrial biosphere and the oceans, but observations demonstrate that these processes 

cannot remove all of the extra flux due to human activities. About half of the anthropogenic emissions 

have remained in the atmosphere (Forster et al., 2007). 

8

 The 2008 value is preliminary. 9

 The estimated uncertainty in the Mauna Loa annual mean growth rate is 0.11 ppm/yr. 

13
Technical Support Document, U.S. Environmental Protection Agency 

Methane (CH4) 

Methane concentrations have also risen 

substantially (Figure 2.4). The following trends 

in atmospheric methane have been observed 

according to NOAA’s report “State of the 

Climate in 2007” and IPCC (Horvitz, 2008; 

Forster et al., 2007): 

• The global atmospheric concentration of 

methane has increased from a pre-industrial 

value of about 715 parts per billion (ppb) to 

1732 ppb in the early 1990s, and was 1782 

ppb in 2007 – a 149% increase from preindustrial levels. 

• The atmospheric concentration of methane 

in 2007 exceeds by far the natural range of 

the last 650,000 years (320 to 790 ppb) as 

determined from ice cores (Jansen et al., 

2007). 

• Growth rates declined between the early 

1990s and mid-2000s. The reasons for the 

decrease in the atmospheric CH4 growth 

rate and the implications for future changes 

in its atmospheric burden are not wellunderstood but are clearly related to the 

changes in the imbalance between CH4 

sources and sinks. 

• The methane concentration grew 7 ppb 

between 2006 and 2007, the first year-toyear increase since 1998. The reasons for 

the increase in 2007 are not yet known, but 

analysis of carbon monoxide measurements 

suggests it is not from biomass burning. 

The observed increase in methane 

concentration is very likely due to 

anthropogenic activities, predominantly 

agriculture and fossil fuel use, but relative 

contributions from different source types are 

not well determined (Forster et al., 2007). 

Nitrous Oxide (N2O) 

The N2O concentration has increased 23% from 

its pre-industrial value of 262 ppb (Figure 

2.4)to 321 ppb in 2007 ( Horvitz, 2008). The 

concentration has increased linearly by about 

0.8 ppb yr

–1 over the past few decades and is 

due primarily to human activities, particularly 

Figure 2.4: Atmospheric concentrations of carbon 

dioxide, methane and nitrous oxide over the last 

10,000 years 

Source: IPCC (2007d). Atmospheric concentrations of 

carbon dioxide, methane and nitrous oxide over the last 

10,000 years (large panels) and since 1750 (inset panels). 

Measurements are shown from ice cores (symbols with 

different colors for different studies) and atmospheric 

samples (red lines). The corresponding radiative forcings 

(discussed in Section 2(e)) are shown on the right hand 

axes of the large panels. 

14
Technical Support Document, U.S. Environmental Protection Agency 

agriculture and associated land use change (Forster et al., 2007). Ice core data show that the present 

atmospheric concentration of N2O is higher than ever measured in the ice core record of the past 650,000 

years (Jansen et al., 2007). 

Fluorinated Gases 

Industrial fluorinated gases that serve as substitutes for CFCs (chlorofluorocarbons) and HCFCs 

(hydrochlorofluorocarbons), such as hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulphur 

hexafluoride (SF6), have relatively low atmospheric concentrations. Concentrations of many of these 

gases have increased by large factors (between 1.3 and 4.3) between 1998 and 2005. Their total radiative 

forcing in 2005 was +0.017 [±0.002] Wm-2 and is rapidly increasing by roughly 10% per year. These 

gases are almost entirely anthropogenic in origin (Forster et al., 2007). 

Ozone depleting substances covered by the Montreal Protocol 

Chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs) are GHGs that are entirely 

anthropogenic in origin. Emissions of these gases have decreased due to their phase-out under the 

Montreal Protocol, and the atmospheric concentrations of CFC-11 and CFC-113 are now decreasing due 

to natural removal processes (Forster et al., 2007). Ice core and in situ data confirm that industrial 

sources are the cause of observed atmospheric increases in CFCs and HCFCs (Forster et al., 2007). 

Ozone (O3) 

Tropospheric ozone is a short-lived greenhouse gas produced largely by chemical reactions of precursor 

species in the atmosphere and with large spatial and temporal variability. Changes in tropospheric ozone 

also occur due to changes in transport of ozone across the tropopause (Forster et al., 2007). Relative to 

the other greenhouse gases, there is less confidence in reproducing the changes in ozone associated with 

large changes in emissions or climate, and in the simulation of observed long-term trends in ozone 

concentrations over the 20th century (Forster et al., 2007). 

Aerosols (Sulfates, nitrates,organic and black carbon aerosols) 

Aerosols are small, short-lived particles present in the atmosphere with widely varying size, concentration 

and chemical composition, either directly emitted or formed in secondary reactions from emitted 

compounds. Aerosols are removed from the atmosphere primarily through cloud processing and wet 

deposition in precipitation, a mechanism that establishes average tropospheric aerosol atmospheric 

lifetimes at a week or less (CCSP, 2009a). On a global basis, aerosol mass derives predominantly from 

natural sources, mainly sea salt and dust. However, anthropogenic (manmade) aerosols, arising primarily 

from a variety of combustion sources, can dominate in and downwind of highly populated and 

industrialized regions, and in areas of intense agricultural burning (CCSP, 2009a). Aerosol optical 

density trends observed in the satellite and surface-based data records suggest that since the mid-1990s, 

the amount of anthropogenic aerosol has decreased over North America and Europe, but has increased 

over parts of east and south Asia; on average, the atmospheric concentration of low latitude smoke 

particles has increased, consistent with changes in emissions (CCSP, 2009a). Ice core data from 

Greenland and Northern Hemisphere mid-latitudes show a very likely rapid post-industrial era increase in 

sulfate concentrations above the preindustrial background, though in recent years, sulfur dioxide 

emissions have decreased globally and in many regions of the northern hemisphere. In general, the 

concentration, composition and distribution of aerosols in the paleoclimate record are not as well known 

as the long-lived greenhouse gases (Jansen et al., 2007). 

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Technical Support Document, U.S. Environmental Protection Agency 

Part III 

Global and U.S. Observed and Projected Effects from Elevated 

Greenhouse Gas Concentrations 

16
Technical Support Document, U.S. Environmental Protection Agency 

Section 3 

Direct Effects of Elevated Greenhouse Gas Concentrations 

Carbon dioxide and other GHGs can have direct effects that are independent of their radiative forcing on 

climate (the primary effect discussed throughout this document). Such effects are described below. 

Effects on Human Health 

Current and projected ambient greenhouse gas concentrations remain well below published thresholds for 

any direct adverse health effects, such as respiratory or toxic effects. The literature supporting this 

conclusion is described in Appendix C. 

Carbon Dioxide Fertilization 

Carbon dioxide can have stimulatory or fertilization effect on plant growth. There is debate and 

uncertainty about the sensitivity of crop yields to the direct effects of elevated CO2 levels. However, the 

IPCC (Easterling et al., 2007) concluded that elevated CO2 levels are expected to result in 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 CO2 through the FACE (Free Air 

CO2 Enrichment)10 experiments indicate that, at ambient CO2 concentrations of 550 ppm (approximately 

double the concentration from pre-industrial times) crop yields increase under unstressed conditions by 

10-25% for C3 crops, and by 0-10% for C4 crops (medium confidence)11. Crop model simulations under 

elevated CO2 are consistent with these ranges (high confidence) (Easterling et al., 2007). Globally and in 

the U.S., high temperatures and ozone exposure, however, can significantly limit the direct stimulatory 

CO2 response (see also Section 8 on Air Quality and Section 9 on Food Production and Agriculture). 

Elevated CO2 has raised an issue about forage quality for livestock. Elevated CO2 can increase the carbon 

to nitrogen ratio in forages and thus reduce the nutritional value of those grasses and thus affect animal 

weight and performance. The decline under elevated CO2 of C4 grasses, however, which are less 

nutritious than C3 grasses, may compensate for the reduced protein. Yet the opposite is expected under 

associated temperature increases (Hatfield et al., 2008). 

At much higher ambient CO2 concentrations, such as those areas exposed to natural CO2 outgassing due 

to volcanic activity, the main characteristic of long-term elevated CO2 zones at the surface is the lack of 

vegetation (IPCC, 2005). New CO2 releases into vegetated areas cause noticeable die-off. In those areas 

where significant impacts to vegetation have occurred, CO2 makes up about 20–95% of the soil gas, 

whereas normal soil gas usually contains about 0.2–4% CO2. Carbon dioxide concentrations above 5% 

may be dangerous for vegetation and as concentrations approach 20%, CO2 becomes phytotoxic. Carbon 

dioxide can cause death of plants through ‘root anoxia’, together with low oxygen concentration (IPCC, 

2005). 

As atmospheric CO2 increases, more CO2 is absorbed at the surface of oceans, estuaries, stream and lakes. 

Increases in the amount of dissolved CO2 and, for some species, bicarbonate ions (HCO3

-

) present in 

aquatic environments will lead to higher rates of photosynthesis in submerged aquatic vegetation, similar 

to the fertilization effects of CO2 enrichment on most terrestrial plants, if other limiting factors do not 

10 http://www.bnl.gov/face/ 11 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.

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Technical Support Document, U.S. Environmental Protection Agency 

offset the potential for enhanced productivity. A study cited in Nicholls et al. (2007) indicates algal 

growth may also respond positively to elevated dissolved inorganic carbon (DIC), though marine 

macroalgae do not appear to be limited by DIC levels. An increase in epiphytic or suspended algae would 

decrease light available to submerged aquatic vegetation and also increase the incidence of algal blooms 

that lower dissolved oxygen available to fish and shellfish (Nicholls et al., 2007). 

Ocean Acidification 

According to the IPCC (Fischlin et al., 2007) elevated CO2 concentrations are resulting in ocean 

acidification which may affect marine ecosystems (medium confidence). This is discussed further in 

sections 4h, 6b, and 14a. 

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Technical Support Document, U.S. Environmental Protection Agency 

Section 4 

Radiative Forcing and Observed Climate Change 

This section focuses primarily on the more significant effects associated with GHGs, which is their heattrapping ability (referred to as radiative forcing) that results in climate change. Observed climate change 

is reviewed, including changes in temperature, precipitation, and sea level rise, for the globe and the U.S. 

Observed changes in climate-sensitive physical and biological systems are also addressed, as well as 

observed trends in extreme events. Sections 7 to 15 provide more specific information on the sectoral 

implications of both the observed changes described here and the projected changes described in Section 

6. 

4(a) Radiative forcing due to greenhouse gases and other factors 

This section describes radiative forcing and the factors that contribute to it. Radiative forcing is a 

measure of the change that a factor causes in altering the balance of incoming (solar) and outgoing 

(infrared and reflected shortwave) energy in the Earth-atmosphere system, and thus shows the relative 

importance of different factors in terms of their contribution to climate change. Positive forcing means 

the factor causes a warming effect and negative forcing means the factor causes a cooling effect. 

Radiative forcing values presented here for GHGs and other factors come from the IPCC Fourth 

Assessment Report of Working Group I (2007). These radiative forcing values are the result of global 

changes in atmospheric concentrations of GHGs (see Section 2(c) above) and other factors, and are 

therefore not the result of U.S. transportation emissions in isolation. All values are for the year 2005 

relative to pre-industrial times in 1750, represent global averages, and are expressed in watts per square 

meter12 (W/m2

). 

IPCC (2007d) concluded that the understanding of anthropogenic warming and cooling influences on 

climate has improved since the TAR, leading to very high confidence13 that the global average net 

effect of human activities since 1750 has been one of warming, with a radiative forcing of +1.6 (+0.6 to 

+2.4) W/m2

. 

Greenhouse gases have a positive forcing because they absorb and reradiate in all directions outgoing, 

infrared radiation that would otherwise directly escape into space. The combined radiative forcing due to 

the cumulative (i.e., 1750 to 2005) increase 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) (see Figure 4.1). This positive radiative 

forcing, like the observed accumulation of these gases in the atmosphere, is primarily anthropogenic in 

origin. Furthermore, the IPCC (2007d) stated that the rate of increase in positive radiative forcing due to 

these three GHGs during the industrial era is “very likely to have been unprecedented in more than 10,000 

years.” 

The positive radiative forcing due to CO2 is the largest (+1.66 ± 0.17 W/m2

) (Figure 4.1), and has 

increased by 20% from 1995 to 2005, the largest change for any decade in at least the last 200 years. 

Methane is the second largest source of positive radiative forcing (+0.48 ± 0.05 W/m2

). Nitrous oxide has 

a positive radiative forcing of +0.16 (±0.02) W/m2

. 

12 Watts per square meter is the SI unit for radiative and other energy fluxes. 13 According to IPCC terminology, “very high confidence” conveys a 9 out of 10 chance of being correct. See Box 

1.3 on page 5 for a full description of IPCC’s uncertainty terms. 

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Technical Support Document, U.S. Environmental Protection Agency 

The other three GHGs reported by the U.S. Inventory—HFCs, PFCs and SF6—have a total radiative 

forcing in 2005 of +0.017 (±0.002) W/m2

 (Forster et al., 2007). 

Figure 4.1: Global average radiative forcing (RF) estimates and ranges in 2005 for 

anthropogenic GHG emissions and other factors 

Source: IPCC (2007d). 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 not included here are considered to have a very low LOSU. Volcanic 

aerosols contribute an additional natural forcing but 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. 

The ozone-depleting substances covered under the Montreal Protocol (chlorofluorocarbons (CFCs), 

hydrochlorofluorocarbons (HCFCs), and chlorocarbons) are also strong GHGs and, as a group, 

contributed +0.32 (±0.03) W/m2

 to anthropogenic radiative forcing in 2005. Their radiative forcing 

peaked in 2003 and is now beginning to decline (Forster et al., 2007). The radiative forcing due to the 

destruction of stratospheric ozone by these gases is estimated to be –0.05 ± 0.10 W/m2 with a medium 

level of scientific understanding (Solomon et al., 2007). 

In addition to the six main GHGs directly emitted by human activities, and the gases covered by the 

Montreal Protocol, there are additional anthropogenic and natural factors that contribute to both positive 

and negative forcing. 

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Technical Support Document, U.S. Environmental Protection Agency 

With regard to climate change, ozone affects the radiative budget of the atmosphere through its 

interaction with both shortwave and longwave radiation (Forster et al. 2007). Tropospheric ozone 

changes due to emissions of ozone-forming chemicals, or precursors (nitrogen oxides, carbon monoxide, 

and hydrocarbons including methane), contribute a positive forcing of +0.35 (+0.25 to +0.65) W/m2

. As 

described in CCSP (2008d), robust model simulations project climate change will also increase the 

radiative forcing from ozone by increasing stratosphere-troposphere exchange and hence ozone near the 

tropopause where it is most important radiatively. Unlike the GHGs mentioned above, tropospheric 

ozone is not as well-mixed in the global atmosphere because its atmospheric lifetime is on the order of 

weeks to months (versus decades to centuries for the well-mixed GHGs). Tropospheric ozone is a criteria 

air pollutant under the U.S. Clean Air Act. 

Emissions of ozone precursors and other substances also contribute to changes in levels of the reactive 

gas OH (the hydroxyl free radical). OH is a key chemical species that influences the lifetimes and thus 

radiative forcing values of CH4, HFCs, HCFCs and ozone; it also plays an important role in the formation 

of sulfate, nitrate and some organic aerosol species. (Forster et al. 2007). 

Anthropogenic emissions of aerosols contribute to both positive and negative radiative forcing. Aerosols 

are non-gaseous substances other than water or ice that are suspended in the atmosphere, and are either 

solid particles or liquid droplets. Most aerosols, such as sulfates (which are mainly the result of SO2 

emissions from fossil fuel burning), exert a negative forcing or cooling effect, as they reflect and scatter 

incoming solar radiation. Some aerosols, such as black carbon, cause a positive forcing by absorbing 

incoming solar radiation. IPCC (2007d) estimated that the net effect of all anthropogenic increases in 

aerosols (primarily sulfate, organic carbon, black carbon, nitrate and dust) produce 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)

14 forcing of –0.7 (–1.8 to –0.3) W/m2

. These forcings are now better understood 

than at the time of the IPCC Third Assessment Report (2001), but nevertheless remain the dominant 

uncertainty in radiative forcing (IPCC, 2007d). The direct radiative forcing of the individual aerosol 

species is less certain than the total direct aerosol radiative forcing. The estimates are: sulphate, -0.4 (+/- 

0.2) Wm-2; fossil fuel organic carbon, -0.05 (+/-0.05)Wm-2; fossil fuel black carbon, +0.2(+/-0.15) Wm-2; 

biomass burning, +0.03(+/-0.12) Wm-2; nitrate, -0.1(+/-0.1) Wm-2; and mineral dust, -0.1(+/-0.2) Wm-2. 

Including both fossil fuel and biomass burning sources, the total black carbon aerosol forcing is estimated 

to be 0.34 (0.09 to .59) W/m2

. In addition, black carbon can cause another positive radiative forcing 

effect (+0.1 (0.0 to +0.2) W/m2

) by decreasing the surface albedo of snow and ice with a low level of 

scientific understanding (Forster et al., 2007). A more recent estimate suggests a direct black carbon 

effect of 0.9 (0.4 to 1.2) W/m2, or more than half that of CO2 (Ramanathan and Carmichael., 2008). The 

spatial distribution of aerosol forcing is very different from that of the well-mixed GHGs, and they do not 

exert simple compensating or additive effects on climate (CCSP, 2008d). The total forcing associated 

with anthropogenic aerosols is less certain than that for GHGs, due to the indirect effects of aerosols, 

including cloud formation and albedo change. 

The radiative forcing from increases in stratospheric water vapor due to oxidation of anthropogenic 

increases in CH4 is estimated to be +0.07 ± 0.05 W/m2

 (Solomon et al., 2007). The level of scientific 

14 In addition to directly reflecting solar radiation, aerosols cause an additional, indirect negative forcing effect by 

enhancing cloud albedo (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 no central estimates of these indirect forcing effects are estimated by IPCC. These aerosol indirect 

effects remain some of the biggest uncertainties of the climate forcing/feedback processes (CCSP 2009a). 

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Technical Support Document, U.S. Environmental Protection Agency 

understanding is low because the contribution of CH4 to the corresponding vertical structure of the water 

vapor change near the tropopause is uncertain. 

Changes in surface albedo due to human-induced land cover changes exert a forcing of –0.2 (–0.4 to 0.0) 

W/m2

. Changes in solar irradiance since 1750 are estimated to cause a radiative forcing of +0.12 (+0.06 

to +0.30) W/m2. This is less than half of the estimate given in IPCC’s Third Assessment Report (2001), 

with a low level of scientific understanding (Solomon et al., 2007). Uncertainties remain large because 

of the lack of direct observations and incomplete understanding of solar variability mechanisms over long 

time scales. Empirical associations have been reported between solar-modulated cosmic ray ionization of 

the atmosphere and global average low-level clouds cover but evidence for a systematic indirect solar 

effect remains ambiguous. The lack of a proven physical mechanism and the plausibility of other causal 

factors make the association between galactic cosmic ray-induced changes in aerosol and cloud formation 

controversial (Solomon et al., 2007). 

Although water vapor is the most abundant naturally occurring greenhouse gas, direct emissions of water 

vapor due to human activities make a negligible contribution to radiative forcing (hence its absence in 

Figure 3.1). However, as temperatures increase, tropospheric water vapor concentrations increase 

representing a key positive feedback (enhances warming) 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). 

4(b) Global Changes in Temperature 

Multiple lines of evidence lead to the robust 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.” 

Air temperature is a main property of climate and the most easily measured, directly observable, and 

geographically consistent indicator of climate change. The extent to which observed changes in global 

and continental temperature, and other climate factors, can be attributed to anthropogenic emissions of 

GHGs is addressed in Section 5. 

Global Surface Temperatures 

Surface temperature is calculated by processing data from thousands of world-wide observation sites on 

land and sea. Parts of the globe have no data, although data coverage has improved with time. The longterm mean temperatures are calculated by interpolating within areas with no measurements using the 

collected data available. Biases may exist in surface temperatures due to changes in station exposure and 

instrumentation over land, or changes in measurement techniques by ships and buoys in the ocean. It is 

likely that these biases are largely random and therefore cancel out over large regions such as the globe or 

tropics (Wigley et al., 2006). Likewise, urban heat island effects are real but local, and have not biased 

the large-scale trends (Trenberth et al., 2007). 

The following trends in global surface temperatures have been observed, according to the IPCC 

(Trenberth et al., 2007): 

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Technical Support Document, U.S. Environmental Protection Agency 

• Global mean surface temperatures have risen by 0.74°C ±0.18°C when estimated by a linear trend 

over the last 100 years (1906–2005) as shown by the magenta line in Figure 4.2. The warmest years 

in the instrumental record of global surface temperatures are 1998 and 2005, with 1998 ranking first 

in one estimate, but with 2005 slightly higher in the other two estimates. 2002 to 2004 are the 3rd, 4th 

and 5th warmest years in the series since 1850. Eleven of the last 12 years (1995 to 2006) – the 

exception being 1996 – rank among the 12 warmest years on record since 1850. Temperatures in 

2006 were similar to the average of the past 5 years. 

• The warming has not been steady, as shown in Figure 4.2. Two periods of warming stand out: an 

increase of 0.35°C occurred from the 1910s to the 1940s and then a warming of about 0.55°C from 

the 1970s up to the end of 2006. The remainder of the past 150 years has included short periods of 

both cooling and warming. The rate of warming over the last 50 years is almost double that over the 

last 100 years (0.13°C ± 0.03°C vs. 0.07°C ± 0.02°C per decade). 

Figure 4.2: Annual global mean temperatures (black dots) with linear fits to the data. 

Source: Solomon et al., (2007). 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 (magenta) 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°C ± 0.19°C (1.37°F ± 0.34°F). 

• Land regions have warmed at a faster rate than the oceans. Warming has occurred in both land and 

oceans and in both sea surface temperature (SST) and nighttime marine air temperature over the 

oceans. However, for the globe as a whole, surface air temperatures over land have risen at about 

double the ocean rate after 1979 (more than 0.27°C per decade vs. 0.13°C per decade), with the 

greatest warming during winter (December to February) and spring (March to May) in the Northern 

Hemisphere. Recent warming is strongly evident at all latitudes in SSTs over each of the oceans. 

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Technical Support Document, U.S. Environmental Protection Agency 

• Average Arctic temperatures increased at almost twice the global average rate in the past 100 years. 

Arctic temperatures have high decadal variability, and a warm period was also observed from 1925 to 

1945. 

• Between 1901 and 2005, warming is statistically significant over most of the world’s surface with the 

exception of an area south of Greenland and three smaller regions over the southeastern U.S. and 

parts of Bolivia and the Congo basin. The lack of significant warming at about 20% of the locations, 

and the enhanced warming in other places, is likely to be a result of changes in atmospheric 

circulation. Warming is strongest 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. 

• Since 1979, warming has been strongest over western North America, northern Europe and China in 

winter, Europe and northern and eastern Asia in spring, Europe and North Africa in summer and 

northern North America, Greenland and eastern Asia in autumn. 

Box 4.1: Updated Global Surface Temperature Trends through 2008 

The global surface temperature trend analysis in IPCC (2007a) includes data through 2005. Three 

additional years of data have become available since then (2006-2008). According to NOAA’s 2008 

State of the Climate Report (NOAA, 2009a): 

• Eight of the ten warmest years on record have occurred since 2001. 

• The year 2008 tied with 2001 as the eighth warmest year on record for the Earth. The combined 

global land and ocean surface temperature from January-December was 0.88 degree F (0.49 

degree C) above the 20th Century average of 57.0 degrees F (13.9 degrees C). 

• Since 1880, the annual combined global land and ocean surface temperature has increased at a 

rate of 0.09 degree F (0.05 degree C) / decade. This rate has increased to 0.29 degree F (0.16 

degree C) / decade over the past 30 years. 

Data analyzed by NASA through 2008 show similar trends (NASA, 2009). It found 2008 was the ninth 

warmest year in the period of instrumental measurements, which extends back to 1880. In both analyses 

1998 and 2005 remain the two warmest years on record. 

Global Upper Air Temperatures 

Temperature measurements have also been made above the Earth's surface over the past 50 to 60 years 

using radiosondes (balloon-borne instruments) and for the past 28 years using satellites. These 

measurements support the analysis of trends and variability in the troposphere (surface to 10-16 km) and 

stratosphere (10-50 km above the earth's surface). 

The U.S. Climate Change Science Program prepared a report which assessed temperature changes in the 

atmosphere, differences in these changes at various levels in the atmosphere, and our understanding of the 

causes of these changes and differences. It concluded (Wigley et al., 2006): “...the most recent versions 

of all available data sets show that both the surface and troposphere have warmed, while the stratosphere 

has cooled. These changes are in accord with our understanding of the effects of radiative forcing agents 

and with the results from model simulations.” 

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Technical Support Document, U.S. Environmental Protection Agency 

The IPCC (Trenberth et al., 2007) re-affirmed the major conclusions of this CCSP report finding: 

• New analyses of radiosondes and satellite measurements of lower- and mid-tropospheric temperature 

show warming rates that are similar to those of the surface temperature record and are consistent 

within their respective uncertainties. 

• The satellite tropospheric temperature record is broadly consistent with surface temperature trends. 

The range (due to different data sets) of global surface warming since 1979 is 0.16°C to 0.18°C per 

decade compared to 0.12°C to 0.19°C per decade for estimates of tropospheric temperatures 

measured by satellite. 

• Lower-tropospheric temperatures measured by radiosondes have slightly greater warming rates than 

those at the surface over the period 1958 to 2005. The radiosonde record is markedly less spatially 

complete than the surface record and increasing evidence suggests that it is very likely that a number 

of records have a cooling bias, especially in the tropics. 

Lower stratospheric temperatures have cooled since 1979. Estimates from adjusted radiosondes, satellites 

and re-analyses are in qualitative agreement, suggesting a lower-stratospheric cooling of between 0.3°C 

and 0.6°C per decade since 1979. 

The global upper air temperature trend analysis in IPCC (2007a) described above includes data through 

2005. Three additional years of data have become available since then (2006-2008). The addition of 

these three years does not significantly alter the above trends. For example, in NOAA (2009) the satellite 

tropospheric temperature trend computed through 2008 ranges from + 0.11°C to + 0.15°C per decade 

compared to the estimate of + 0.12°C to + 0.19°C per decade given in IPCC (2007a). 

25
Source: NRC (2006b) Reconstructions of (Northern Hemisphere average or global 

average) surface temperature variations from six research teams (in different color 

shades) along with the instrumental record of global average surface temperature 

(in black). Each curve illustrates a somewhat different history of temperature 

changes, with a range of uncertainties that tend to increase backward in time (as 

indicated by the shading). 

Technical Support Document, U.S. Environmental Protection Agency 

Global Surface 

Temperatures Over the 

Last 2,000 Years 

Figure 4.3: Reconstructions of (Northern Hemisphere average or 

global average) surface temperature variations from six research 

teams 

Instrumental surface 

temperature records only 

began in the late 19th 

century, when a 

sufficiently large global 

network of measurements 

was in place to reliably 

compute global mean 

temperatures. To estimate 

temperatures further back 

in time, scientists analyze 

proxy evidence from 

sources such as tree rings, 

corals, ocean and lake 

sediments, cave deposits, 

ice cores, boreholes, 

glaciers, and documentary 

evidence. A longer 

temperature record can 

help place the 20th century 

warming into a historical 

context. 

The National Research 

Council conducted a study 

to describe and assess the state of scientific efforts to reconstruct surface temperature records for the Earth 

over approximately the last 2,000 years and the implications of these efforts for our understanding of 

global climate change. It found (NRC, 2006b): 

• Large-scale surface temperature reconstructions, as illustrated in Figure 4.3, yield a generally 

consistent picture of temperature trends during the preceding millennium, including 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. 

• It can be said with a high level of confidence 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 4.2 supports this 

conclusion. 

• 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. 

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Technical Support Document, U.S. Environmental Protection Agency 

• 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. 

Considering this study and additional research, the IPCC (2007d) concluded: “Paleoclimatic information 

supports the interpretation that the warmth of the last half century is unusual in at least the previous 1,300 

years.” 

4(c) U.S. changes in temperatures 

Like global mean temperatures, U.S. temperatures also warmed during the 20th and into the 21st century. 

According to NOAA (2009b) and data from NOAA15: 

• U.S. average annual temperatures are now approximately 1.25°F (0.69°C) warmer than at the start of 

the 20th century, with an increased rate of warming over the past 30 years. The rate of warming for 

the entire period of record (1895-2008) is 0.13°F/decade while the rate of warming increased to 

0.58°F/decade (0.32°C/decade) for the period from 1979-2008. 

• 2005-2007 were exceptionally warm years (among the top 10 warmest on record), while 2008 was 

slightly warmer than average (the 39th warmest year on record), 0.2°F (0.1°C) above the 20th century 

(1901-2000) mean. 

• The last ten 5-year periods (2004-2008, 2003-2007, 2002-2006, 2001-2005, 2000-2004, 1999-2003, 

1998-2002, 1997-2001, 1996-2000, and 1995-1999), were the warmest 5-year periods (i.e. pentads) in 

the 114 years of national records, demonstrating the anomalous warmth of the last 15 years. 

15 Data obtained from NOAA’s National Climatic Data Center. Data may be downloaded from: 

http://www.epa.gov/climatechange/anpr/data.html (see file: AnnualUSTemp_through 2008.xls) 

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Technical Support Document, U.S. Environmental Protection Agency 

Regional data analyzed from NOAA’s 

National Climatic Data Center U.S. 

Historical Climate Network16 

(USHCN), as illustrated in Figure 4.4, 

indicate warming has occurred 

throughout most of the U.S., with all 

but three of the eleven climate regions 

showing an increase of more than 1oF 

since 1901 through 2006 (NOAA, 

2007). As shown in Figure 4.4, the 

greatest temperature increase occurred 

in Alaska (3.3°F per century). The 

Southeast shows essentially no trend 

over the entire period, but has warmed 

since 1979. 

Including all of North America in its 

assessment of regional temperatures, 

the IPCC (Field et al., 2007) stated: 

• For the period 1955-2005, the 

greatest warming occurred in 

Alaska and north-western Canada, 

with substantial warming in the 

continental interior and modest 

warming in the south-eastern U.S. 

and eastern Canada. 

• Spring and winter show the 

greatest changes in temperature and 

daily minimum (night-time) 

temperatures have warmed more 

than daily maximum (daytime) 

temperatures. 

Figure 4.4: Map of the United States, depicting regional 

U.S. temperature trends for the period 1901 to 2006. 

Red shades indicate warming over the period and blue shades 

indicate cooling over the period. 

4(d) Global changes in precipitation 

A consequence of rising temperature is increased evaporation, provided that adequate surface moisture is 

available (e.g., over the oceans and other moist surfaces). The average atmospheric water vapor content 

has increased since at least the 1980s over land and ocean as well as in the upper troposphere (IPCC, 

2007d). When evaporation increases, more water vapor is available for precipitation producing weather 

systems leading to precipitation increases in some areas. Conversely, enhanced evaporation and 

evapotranspiration from warming accelerates land surface drying and increases the potential incidence 

and severity of droughts in other areas. 

16 Data for map obtained from NOAA’s National Climatic Data Center. Data may be downloaded from: 

http://www.epa.gov/climatechange/anpr/data.html (files are: CONUS mean temp anomaly 2.5X3.5_through 

2006.xls and AK-HI mean temp anomaly_through 2006.xls) 

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Observations show that changes are occurring in the amount, intensity, frequency and type of 

precipitation. According to the IPCC (Trenberth et al., 2007): 

• Long-term trends from 1900 to 2005 have been observed in precipitation amount over many large 

regions. Significantly increased precipitation has been observed in eastern parts of North and South 

America, northern Europe and northern and central Asia. Drying has been observed in the Sahel, the 

Mediterranean, southern Africa and parts of southern Asia. Precipitation is highly variable spatially 

and temporally, and data are limited in some regions. 

• More intense and longer droughts have been observed over wider areas since the 1970s, particularly 

in the tropics and subtropics. 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 (SSTs), 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) within many land regions, 

even in those where there has been a 

reduction in total precipitation 

amount, consistent with a warming 

climate and observed significant 

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 (1961–1990) 

temperatures were close to 0°C. 

4(e) U.S. changes in precipitation 

Data analyzed from NOAA’s National 

Climatic Data Center U.S. Historical 

Climate Network (USHCN)17 show that 

over the contiguous U.S., total annual 

precipitation increased at an average rate 

of 6.5% per century from 1901-2006. As 

shown in Figure 4.5, the greatest increases 

in precipitation were in the East North 

Central climate region (11.2% per 

century) and the South (10.5%). 

Precipitation in the Northeast increased by 

Figure 4.5: Map of the United States, depicting 

precipitation trends for the contiguous U.S. 1901-2006, 

Hawaii 1905-2006 and Alaska 1918-2006. 

Green shades indicate a trend towards wetter conditions over the 

period, and brown shades indicate a trend towards dryer conditions. 

No data are available for areas shaded in white. 

17 Data for maps obtained from NOAA’s National Climatic Data Center. Data may be downloaded from: 

http://www.epa.gov/climatechange/anpr/data.html (files are: CONUS precip anomaly 2.5X3.5_through 2006.xls and 

AK-HI precip anomaly_through 2006.xls) 

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8.3%, in the Southeast by 1.9%, the Central U.S. by 8.1%, the West North Central by 1.9%, the 

Southwest by 1.3%, the West by 9.1%, and the Northwest by 6.0%. 

Outside the contiguous U.S., Alaska experienced a precipitation increase of about 5.9% per century (since 

records began in 1918) and Hawaii experienced a decrease of 7.2% per century (since records begin in 

1905). 

Despite the overall national trend towards wetter conditions, a severe drought has affected the southwest 

U.S. from 1999 through 2008 (see Section 4(l)) which is indicative of significant variability in regional 

precipitation patterns over time and space. 

4(f) Global sea level rise 

There is strong evidence that global sea level gradually rose in the 20th century and is currently rising at 

an increased rate, after a period of little change between AD 0 and AD 1900 (IPCC, 2007a). 

According to Bindoff et al. (2007), there is high confidence that the rate of sea level rise increased 

between the mid-19th and mid-20th centuries. The average rate of sea level rise measured by tide gauges 

from 1961 to 2003 was 1.8 ± 0.5 mm per year (Bindoff et al., 2007). The global average rate of sea level 

rise measured by satellite altimetry during 1993 to 2003 was 3.1 ±0.7 mm per year (Bindoff et al., 2007). 

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 (Bindoff et al., 2007). 

The total 20th century sea level rise is estimated to be 0.17 ± 0.05 m (Bindoff et al., 2007). 

Two major processes lead to changes in global mean sea level on decadal and longer time scales: i) 

thermal expansion, and ii) the exchange of water between oceans and other reservoirs (glaciers and ice 

caps, ice sheets, and other land water reservoirs). It is believed that on average, over the period from 

1961 to 2003, thermal expansion contributed about one-quarter of the observed sea level rise, while 

melting of land ice accounted for less than half; the full magnitude of the observed sea level rise was not 

satisfactorily explained by the available data sets (Bindoff et al., 2007). During this period, global ocean 

temperature rose by 0.10°C from the surface to a depth of 700 m, contributing an average of 0.4 ± 0.1 mm 

per year to sea level rise (Bindoff et al., 2007). The contribution from ice was approximately 0.7 ± 0.5 

mm per year (Lemke et al., 2007). 

For recent years (1993-2003) for which the observing system is much better, 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. Thermal expansion contributed about 1.6 ± 0.5 mm per year, reflecting a 

high rate of warming for the period relative to 1961 to 2003 (Bindoff et al., 2007). The total contribution 

from melting ice to sea level change between 1993 and 2003 ranged from 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, from increasing surface melt on the Greenland Ice Sheet and from faster flow of parts of the 

Greenland and Antarctic Ice Sheets (Lemke et al., 2007). 

Thermal expansion and exchanges of water between oceans and other reservoirs cause changes in the 

global mean as well as geographically non-uniform sea level change. Other factors influence changes at 

the regional scale, including changes in ocean circulation or atmospheric pressure, and geologic processes 

(Bindoff et al., 2007). Satellite measurements (for the period 1993-2003) provide unambiguous evidence 

of regional variability of sea level change (Bindoff et al., 2007). In some regions, rates of rise have been 

as much as several times the global mean, while sea level is falling in other regions. According to IPCC 

(Bindoff et al., 2007), 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 

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with the rate of rise reaching a maximum (over 2 mm yr

–1) 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. 

4(g) U.S. sea level rise 

Sea level18 has been rising 0.08-0.12 inches per year (2.0-3.0 mm per year) along most of the U.S. 

Atlantic and Gulf coasts. The rate of sea level rise varies from about 0.36 inches per year (10 mm per 

year) along the Louisiana Coast (due to land sinking), to a drop of 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, 2006). According to the CCSP 

(2009b), in the mid-Atlantic region from New York to North Carolina, tide-gauge observations indicate 

that relative sea-level rise (the combination of global sea-level rise and land subsidence) rates were higher 

than the global mean and generally ranged between 2.4 and 4.4 millimeters per year, or about 0.3 meters 

(1 foot) over the twentieth century. 

Rosenzweig et al. (2007) document studies that find 75% of the shoreline removed from the influence of 

spits, tidal inlets and engineering structures is eroding along the U.S. East Coast probably due to sea level 

rise. They also cite 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. 

4(h) Global ocean acidification 

Ocean waters can absorb large amounts of CO2 from the atmosphere, because the gas is weakly acidic and 

the minerals dissolved in the ocean have over geologic time created a slightly alkaline ocean, with surface 

pH ranging from 7.9 to 8.25. The amount of carbon contained in the oceans has increased due to the 

elevated atmospheric pressure of CO2 from anthropogenic emissions (Denman et al., 2007). IPCC 

estimates that the total inorganic carbon content of the oceans increased by 118 ± 19 GtC between 1750 

and 1994 and continues to increase (Bindoff et al., 2007). This absorptive capacity of the oceans has 

resulted in atmospheric CO2 concentrations substantially lower than they otherwise would be. Since the 

beginning of the industrial revolution, sea surface pH has dropped by about 0.1 pH units, corresponding 

to a 30% increase in the concentration of hydrogen ions (Denman et al., 2007). 

Ocean acidification is causing a series of cascading changes to the chemistry of ocean water, including a 

decrease in the saturation state of calcium carbonate. Marine calcifiers, such as corals, are dependent 

upon this mineral to form shells, skeletons, and other protective structures. Reduced availability of 

calcium carbonate can slow or even halt calcification rates amongst these organisms (Fischlin et al., 

2007). The availability of carbonate is also important because it controls the maximum amount of CO2 

that the ocean is able to absorb (Bindoff et al. 2007, p. 406). More information regarding ocean 

acidification projections and impacts to marine ecosystems can be found in sections 6(b) and 14(a), 

respectively. It is important to note that ocean acidification is a direct consequence of fossil fuel CO2 

emissions, which are also the main driver of the anticipated climate change (Denman et al., 2007). 

4(i) Global changes in physical and biological systems 

Physical and biological systems on all continents and in most oceans are already being affected by recent 

climate changes, particularly regional temperature increases (very high confidence) (Rosenzweig. et al., 

2007). Climatic effects on human systems, although more difficult to discern due to adaptation and non18 U.S. sea level data obtained from the Permanent Service for Mean Sea Level < http://www.pol.ac.uk/psmsl/> of 

the Proudman Oceanographic Laboratory. 

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Technical Support Document, U.S. Environmental Protection Agency 

climatic drivers, are emerging (medium confidence) (Rosenzweig. et al., 2007). The majority of evidence 

comes from mid- and high latitudes in the Northern Hemisphere, while documentation of observed 

changes in tropical regions and the Southern Hemisphere is sparse (Rosenzweig. et al., 2007). Hence, the 

findings presented in this section apply generally to the globe but most directly to Europe and North 

America (including the U.S.) where these observational studies were conducted. The extent to which 

observed changes discussed here can be attributed to anthropogenic GHG emissions is discussed in 

Section 5. 

Cryosphere (Snow and Ice) 

Observations of the cryosphere (the “frozen” component of the climate system) have revealed changes in 

sea ice, glaciers and snow cover, freezing and thawing, and permafrost. According to IPCC, the 

following physical changes have been observed: 

Satellite data since 1978 show that annual average Arctic sea ice extent has shrunk by 2.7 ± 0.6 % per 

decade, with larger decreases in summer of 7.4 ± 2.4% per decade. The latest data from NASA indicate 

Arctic sea ice set a record low in September 2007, 38 percent below the 1979-2007 average (NASA 

Goddard Space Flight Center, 2007). The extent of the sea ice loss between 1979 and 2007 can be seen in 

Figure 4.6. In September 2008, Arctic sea ice reached its second lowest extent on record (NASA 

Goddard Space Flight Center, 2008). The size and speed of recent summer sea ice loss is highly 

anomalous relative to the previous few thousands of years (Polyak et al., 2009). 

Figure 4.6 Arctic Sea Ice Concentrations Comparisons 

1979 2007 

These two images, constructed from satellite data, compare arctic sea ice concentrations in September of 1979 and 

2007 (Images courtesy of NASA). 

• Antarctic sea ice extent shows no statistically significant average trends according to IPCC (2007d). 

However, the U.S. National and Snow and Ice Data Center states that Antarctic sea ice underwent a 

slight increase from 1979 to 2007 (NSIDC, 2009). 

• 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 (Lemke et al., 2007). 

• Mountain glaciers and snow cover have declined on average in both hemispheres. Northern 

hemisphere snow cover observed by satellite over the 1966 to 2005 period decreased 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. The strongest mass losses of mountain glaciers (per unit area) have been 

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Technical Support Document, U.S. Environmental Protection Agency 

observed in Patagonia, Alaska and the northwestern U.S. and southwest Canada. Because of the 

corresponding large areas, the biggest contributions to sea level rise came from Alaska, the Arctic and 

the Asian high mountains (Lemke et al., 2007). 

• Freeze-up date for river and lake ice has occurred later at a rate of 5.8 ± 1.6 days per century, 

averaged over available data for the Northern Hemisphere spanning the past 150 years. Breakup date 

has occurred earlier at a rate of 6.5 ± 1.2 days per century (Lemke et al., 2007). 

• Temperatures at the top of the permafrost layer have generally increased since the 1980s in the Arctic 

(by up to 3°C). The permafrost base has been thawing at a rate ranging up to 0.04 m per year in 

Alaska since 1992 and 0.02 m per year on the Tibetan Plateau 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% (Lemke et al., 2007). 

There are additional effects related to changes in the cryosphere. Melting of highly reflective snow and 

ice reveals darker land and ocean surfaces, creating a positive feedback that increases absorption of the 

sun’s heat and further warms the planet. Increases in glacial melt and river runoff add more freshwater to 

the ocean, raising global sea level. 

Hydrosphere 

The term “hydrosphere” refers to the component of the climate system comprising liquid surface and 

subterranean water, such as rivers, lakes, and underground water. Several changes in these features have 

been observed, as summarized by the IPCC (Rosenzweig et al., 2007): 

• Documented trends in severe droughts and heavy rains show that hydrological conditions are 

becoming more intense in some regions. Globally, very dry areas (Palmer Drought Severity Index— 

PDSI—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. Very wet areas (PDSI greater than or equal 

to +3.0) declined by about 5% since the 1970s, with precipitation as the major contributing factor 

during the early 1980s and temperature more important thereafter. The areas of increasing wetness 

include the Northern Hemisphere high latitudes and equatorial regions. 

• Climate change signals related to increasing runoff and stream flow have been observed over the last 

century in many regions, particularly in basins fed by glaciers, permafrost, and snowmelt. Evidence 

includes increases in average runoff of Arctic rivers in Eurasia, which has been at least partly 

correlated with climate warming, and earlier spring snowmelt and increase in winter base flow in 

North America and Eurasia due to enhanced seasonal snow melt associated with climate warming. 

• Freshwater lakes and rivers are experiencing increased water temperatures and changes in water 

chemistry. Surface and deep lake-waters are warming, with advances and lengthening of periods of 

thermal stability in some cases associated with physical and chemical changes such as increases in 

salinity and suspended solids, and a decrease in nutrient content. Lake formation and subsequent 

disappearance in permafrost have been reported in the Arctic. 

Changes in river discharge as well as in droughts and heavy rains in some regions indicate that 

hydrological conditions have become more intense but 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, but studies have been unable to separate the effects of variations in temperature 

and precipitation from the effects of human interventions such as groundwater management (Rosenzweig 

et al., 2007). 

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Biosphere 

According to IPCC, terrestrial ecosystems and marine and freshwater systems show that recent warming 

is strongly affecting natural biological systems (very high confidence) (Rosenzweig et al., 2007): 

• The overwhelming majority of studies of regional climate effects on terrestrial species reveal 

consistent responses to warming trends, including poleward and elevational range shifts of flora and 

fauna. Changes in abundance of certain species, including limited evidence of a few local 

disappearances, and changes in community composition over the last few decades have been 

attributed to climate change. 

• Responses of terrestrial species to warming across the northern hemisphere are well documented by 

changes in the timing of growth stages, especially the earlier onset of spring events, migration, and 

lengthening of the growing season. Changes in phenology (the timing of annual phenomena of 

animal and plant life) include clear temperature-driven extension of the growing season by up to 2 

weeks in the second half of the 20th century in mid- and high northern latitudes, mainly due to an 

earlier spring, but partly due also to a later autumn. Egg-laying dates have advanced in many bird 

species, and many small mammals have been found to come out of hibernation and to breed earlier in 

the spring now than they did a few decades ago. 

• Many observed changes in phenology and distribution of marine species have been associated with 

rising water temperatures, as well as other climate-driven changes in salinity, oxygen levels, and 

circulation. For example, plankton has moved poleward by 10° latitude over a period of 4 decades in 

the North Atlantic. While there is increasing evidence for climate change impacts on coral reefs, 

discerning the impacts of climate-related stresses from other stresses (e.g. over fishing and pollution) 

is difficult. Warming of lakes and rivers is affecting abundance and productivity, community 

composition, phenology, distribution and migration of freshwater species (high confidence). 

4(j) U.S. changes in physical and biological systems 

Many of the global changes in physical and biological systems mentioned in 4(h) broadly apply to the 

U.S. Some U.S.-specific changes in these systems cited in IPCC’s Fourth Assessment Report are 

described in this subsection, as well as in section 11(a) for physical systems related to water resources and 

section 14(a) related to biological systems. Of all the observed changes to physical systems assessed by 

IPCC (Rosenzweig et al., 2007) for North America (totaling 355), 94% of them were consistent with 

changes one would expect with average warming. Similar consistency was found between observed 

biological system changes and warming for North America (see discussion below under biosphere). 

Furthermore, a CCSP (2008e) assessment reported that climate changes are very likely already affecting 

U.S. water resources, agriculture, land resources, and biodiversity as a result of climate variability and 

change. It found, “The number and frequency of forest fires and insect outbreaks are increasing in the 

interior West, the Southwest, and Alaska. Precipitation, streamflow, and stream temperatures are 

increasing in most of the continental United States. The western United States is experiencing reduced 

snowpack and earlier peaks in spring runoff. The growth of many crops and weeds is being stimulated. 

Migration of plant and animal species is changing the composition and structure of arid, polar, aquatic, 

coastal, and other ecosystems” (Backlund et al., 2008a) 

Additional findings from this CCSP assessment along with results presented in IPCC’s Fourth 

Assessment Report are described below. 

Cryosphere (Snow and Ice) 

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In North America, from 1915 to 2004, snow covered area increased in November, December and January 

due to increases in precipitation. However, snow cover decreased during the latter half of the 20th 

century, especially during the spring over western North America (Lemke et al., 2007). Shifts towards 

earlier melt by about eight days since the mid-1960s were also observed in northern Alaska (Lemke et al., 

2007). Consistent with these findings, Lettenmaier et al. (2008) note a trend toward reduced mountain 

snowpack, and earlier spring snowmelt runoff peaks across much of the western U.S. 

The IPCC (Lemke et al., 2007) cites a study documenting glacier mass balance loss in the northwest U.S. 

and Alaska, with losses especially rapid in Alaska after the mid-1990s. Rosenzweig et al. (2007) refer to 

a study documenting evidence of present crustal uplift in response to recent glacier melting in Alaska. 

Hydrosphere 

Lettenmaier et al. (2008) document increases in U.S. streamflow during the second half of the 20th 

century consistent with increases in precipitation described in 4(e). 

Rosenzweig et al. (2007) indicate surface water temperatures have warmed by 0.2 to 2°C in lakes and 

rivers in North America since the 1960s. They also discuss 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. 

Biosphere 

The IPCC (Rosenzweig et al., 2007) assessed a multitude of studies that find changes in terrestrial 

ecosystems and marine and freshwater systems in North America. Of 455 biological observations 

assessed from these studies, 92% were consistent with the changes expected due to average warming. 

Backlund et al. (2008a) find: 

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

productivity (NPP) in the higher latitudes of North America. Over the last 19 years, global 

satellite data indicate an earlier onset of spring across the temperate latitudes by 10 to 14 days. 

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

change, nearly 60 percent of the 1598 species studied exhibited shifts in their distributions and/or 

phenologies over the 20- and 140-year time frame. 

• Subtropical and tropical corals in shallow waters have already suffered major bleaching events 

that are clearly driven by increases in sea surface temperatures. 

In addition, Ryan et al. (2008) note, “Climate change has very likely increased the size and number of 

forest fires, insect outbreaks, and tree mortality in the interior west, the Southwest, and Alaska.” 

4(k) Global extreme events 

Climate is defined not simply as average temperature and precipitation but also by the type, frequency 

and intensity of extreme events. The IPCC documents observed changes in climate extremes related to 

temperature, precipitation, tropical cyclones, and sea level. The changes described apply generally to all 

parts of the globe, including the U.S., although there are some regional and local exceptions due to 

patterns of natural climate variability. Current observations are summarized here, projected trends are 

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Technical Support Document, U.S. Environmental Protection Agency 

covered in Section 6, and the sectoral impacts of these changes are covered as relevant in Sections 7 to 

15. 

Temperature 

Widespread changes in extreme temperatures have been observed in the last 50 years. Cold days, cold 

nights, and frost have 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 

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 

(lowest 10%, based on 1961–1990) nights, which have become rarer over the 1951 to 2003 period. Warm 

(highest 10%) nights have become more frequent. 

Precipitation and Storms 

It is likely that there have been increases in the number of heavy precipitation events19 within many land 

regions, even in those where there has been a reduction in total precipitation amount, consistent with a 

warming climate and observed significant increasing amounts of water vapor in the atmosphere. 

Increases have also been reported for rarer precipitation events (1 in 50 year return period), but only a few 

regions have sufficient data to assess such trends reliably (Trenberth et al., 2007). 

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). 

The CCSP (2008i) report on extreme events, in its section on tropical cyclones (i.e. tropical storms and 

hurricanes), states that there have been spatially inhomogeneous increases in the power dissipation index, 

a measure of potential tropical cyclone destructiveness, over the last few decades (Kunkel et al., 2008). 

However, there remain reliability issues with historical data. Kunkel et al. (2008) refer to a study that was 

not able to corroborate the presence of upward intensity trends over the last two decades in ocean basins 

other than the North Atlantic. The report cautions that quantifying tropical cyclone variability is limited, 

sometimes seriously, by a large suite of problems with the historical record of tropical cyclone activity. 

Correspondingly, there is no clear trend in the annual numbers of tropical cyclones (IPCC, 2007d). 

High Sea Level 

Apart from non-climatic events such as tsunamis, extreme sea levels occur mainly in the form of storm 

surges generated by tropical or extra-tropical cyclones. There is evidence for an increase in extreme high 

sea level since 1975 based upon an analysis of 99th percentiles of hourly sea level at 141 stations over the 

globe (Bindoff et al., 2007). 

4(l) U.S. extreme events 

Many of the global changes in extreme events mentioned in 4(j) broadly apply to the U.S. Additionally, 

the U.S. CCSP (2008i) published a report that focused on changing climate extremes in the U.S. and 

North America. It concluded (Karl et al., 2008 in CCSP, 2008i): 

Many extremes and their associated impacts are now changing. For example, in recent decades most 

of North America has been experiencing more unusually hot days and nights, fewer unusually cold 

19 Heavy precipitation events refer to those in the 95th percentile of precipitation events. 

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days and nights, and fewer frost days. Heavy downpours have become more frequent and intense. 

Droughts are becoming more severe in some regions, though there are no clear trends for North 

America as a whole. The power and frequency of Atlantic hurricanes have increased substantially in 

recent decades, though North American mainland land-falling hurricanes do not appear to have 

increased over the past century. Outside the tropics, storm tracks are shifting northward and the 

strongest storms are becoming even stronger. 

Many of these changes were also assessed in IPCC’s Fourth Assessment Report and are described in this 

subsection. 

Temperature 

The IPCC (Trenberth et al., 2007) cites North America regional studies that all show patterns of changes 

in temperature extremes consistent with a general warming. Karl et al. (2008) find the number of heat 

waves (extended periods of extremely hot weather) has been increasing over the past 50 years but note the 

heat waves of the 1930s remain the most severe in the U.S. historical record. 

Extreme cold has been decreasing. Trenberth et al., 2007 cite a study finding intense warming of the 

lowest daily minimum temperatures over western and central North America. Trenberth et al. (2007) 

caution the observed changes of the tails of the temperature distributions are often more complicated than 

a simple shift of the entire distribution would suggest. Karl et al. (2008) note, “The last 10 years [1998­

2007] have seen fewer severe cold snaps than for any other 10-year period in the historical record, which 

dates back to 1895.” Karl et al. also indicate a decrease in frost days and a lengthening of the frost-free 

season over the past century. 

Precipitation and Storms 

In the contiguous U.S., Trenberth et al. (2007) cite studies finding statistically significant increases in 

heavy precipitation (the heaviest 5%) and very heavy precipitation (the heaviest 1%) of 14 and 20%, 

respectively. Much of this increase occurred during the last three decades of the 20th century and is most 

apparent over the eastern parts of the country. There is also evidence from Europe and the U.S. that the 

relative increase in precipitation extremes is larger than the increase in mean precipitation (Trenberth et 

al., 2007). 

Lettenmeir et al. (2008) find, “With respect to drought, consistent with streamflow and precipitation 

observations, most of the continental United States experienced reductions in drought severity and 

duration over the 20th century. However, there is some indication of increased drought severity and 

duration in the western and southwestern United States....” 

Diminishing snow pack and subsequent reductions in soil moisture appear to be factors in recent drought 

conditions in the western U.S. (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 multidecadal fluctuations (Trenberth et al., 2007). 

Karl et al. (2008) indicate a northward shift in the tracks of strong low-pressure systems (also known as 

mid-latitude storms and/or extratropical cyclones) in both the North Atlantic and North Pacific over the 

past fifty years with increases in storm intensity noted in the Pacific (data inconclusive in the Atlantic). 

Correspondingly, they also find northward shift in snow storm occurrence, which is also consistent with 

the warming temperatures and a decrease in snow cover extent over the U.S. 

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Technical Support Document, U.S. Environmental Protection Agency 

IPCC (2007d) reports there is observational evidence for an increase in intense tropical cyclone (i.e. 

tropical storms and/or hurricanes) activity in the North Atlantic (where cyclones develop that affect the 

U.S. East and Gulf Coasts) since about 1970, correlated with increases of tropical sea surface 

temperatures. Similarly, Kunkel et al. (2008) conclude (for the North Atlantic): “There have been 

fluctuations in the number of tropical storms and hurricanes from decade to decade, and data uncertainty 

is larger in the early part of the record compared to the satellite era beginning in 1965. Even taking these 

factors into account, 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.” 

High Sea Level 

Studies of the longest records of extremes in sea level are restricted to a small number of locations. 

Consistent with global changes, U.S.-based studies document increases in extreme sea level closely 

following the rise in mean sea level (Bindoff et al., 2007). 

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Section 5 

Attribution of Observed Climate Change to Anthropogenic Greenhouse Gas 

Emissions at the Global and Continental Scale 

This section addresses the extent to which observed climate change at the global and continental or 

national scale (described in Section 4) can be attributed to global anthropogenic emissions of GHGs. 

Section 2 describes the share of the U.S. transportation sector to U.S. and global anthropogenic emissions 

of GHGs, and the resultant share of U.S. transportation emissions to global increases in atmospheric 

concentrations of GHGs. 

Evidence of the effect of anthropogenic GHG emissions on the climate system, as well as climatesensitive systems and sectors, has increased over the last 15 years or so and even since the previous IPCC 

assessment published in 2001. The evidence in the recent IPCC Fourth Assessment Report (2007) is 

based on analyses 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. 

5(a) Attribution of observed climate change to anthropogenic emissions 

Computer-based climate models are the primary tools used for simulating the likely patterns of response 

of the climate system to different forcing mechanisms (both natural and anthropogenic). 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 (IPCC, 2007a). For additional 

discussion on the strengths and limitations of models, see Section 6(b). Attribution studies evaluate 

whether observed changes are consistent with quantitative responses to different forcings (from 

greenhouse gases, aerosols and natural forcings such as changes solar intensity) represented in well-tested 

models, and are not consistent with alternative physically plausible explanations. 

Studies to detect climate change and attribute its causes using patterns of observed temperature change 

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 

IPCC statements on the linkage between greenhouse gases and temperatures have strengthened since its 

early assessments (Solomon et al., 2007). 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, it 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, 2001b). The conclusion in IPCC’s 2007 Fourth Assessment Report (2007b) is 

the strongest yet: 

Most of the observed increase in global average temperatures since the mid-20th century is very 

likely20 due to the observed increase in anthropogenic greenhouse gas concentrations. 

20 According to IPCC terminology, “very likely” conveys a 90 to 99% probability of occurrence. See Box 1.3 on 

page 5 for a full description of IPCC’s uncertainty terms. 

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The increased confidence in the greenhouse gas contribution to the observed warming results from 

(Hegerl. et al., 2007): 

• an expanded and improved range of observations allowing attribution of warming to be more fully 

addressed jointly with other changes in the climate system 

• 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 

anthropogenic and natural forcings 

• multi-model ensembles that increase confidence in attribution results by providing an improved 

representation of model uncertainty 

Climate model simulations run by the IPCC, shown in Figure 5.1, suggest natural forcings alone cannot 

explain the observed warming (for the globe, the global land and global ocean). The observed warming 

can only be reproduced with models that contain both natural and anthropogenic forcings. 

Figure 5.1: Comparison of observed global-scale changes in surface temperature with 

results simulated by climate models using natural and anthropogenic forcings. 

Source: IPCC (2007d). Decadal averages of observations are shown for the period 1906 to 2005 (black 

line) plotted against the centre of the decade and relative to the corresponding average for 1901–1950. 

Lines are dashed where spatial coverage is less than 50%. Blue shaded bands show the 5–95% range 

for 19 simulations from five climate models using only the natural forcings due to solar activity and 

volcanoes. Red shaded bands show the 5–95% range for 58 simulations from 14 climate models using 

both natural and anthropogenic forcings. 

Additional evidence documented in the IPCC report supports its statement linking warming to increasing 

concentrations of greenhouse gases (Hegerl et al., 2007): 

• Warming of the climate system has been detected in changes of surface and atmospheric temperatures 

in the upper several hundred meters of the ocean, and in contributions to sea level rise. Attribution 

studies have established anthropogenic contributions to all of these changes. 

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• Analyses of paleoclimate data have increased confidence in the role of external influences on climate. 

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. 

The IPCC states that it is very unlikely 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 (Hegerl et al., 2007). It also states 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 (Hegerl et al., 2007). Solomon et al. 

(2007) indicate the sum of solar and volcanic forcing in the past half century would likely have produced 

cooling, not warming. 

Figure 5.2: Comparison of observed North American changes in 

Not only has an surface temperature with results simulated by climate models using 

anthropogenic signal been natural and anthropogenic forcings. 

detected for the surface 

temperatures, but evidence 

has also accumulated of an 

anthropogenic influence 

through the vertical profile 

of the atmosphere. 

Fingerprint studies21 have 

identified greenhouse gas 

and sulfate aerosol signals 

in observed surface 

temperature records, a 

stratospheric ozone 

depletion signal in 

stratospheric temperatures, 

and the combined effects 

of these forcing agents in 

the vertical structure of 

atmospheric temperature 

changes (Karl et al., 2006). 

However, an important 

inconsistency may have 

been identified in the Source: Hegerl et al. (2007). Decadal averages of observations are shown for the 

period 1906 to 2005 (black line) plotted against the centre of the decade and 

relative to the corresponding average for 1901–1950. Lines are dashed where 

tropics. In the tropics, most 

observational data sets spatial coverage is less than 50%. Blue shaded bands show the 5–95% range for show more warming at the 19 simulations from five climate models using only the natural forcings due to solar 

surface than in the activity and volcanoes. Red shaded bands show the 5–95% range for 58 

troposphere, while almost simulations from 14 climate models using both natural and anthropogenic forcings. 

all model simulations have 

larger warming aloft than at the surface. A possible explanation for this inconsistency is error in the 

observations, but the issue is still under investigation (Karl et al., 2006). 

21 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. 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 5.1 and 5.2). 

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The substantial anthropogenic contribution to surface temperature increases likely applies to every 

continent except Antarctica (which has insufficient observational coverage to make an assessment) since 

the middle of the 20th century (Hegerl. et al., 2007). Figure 5.2 indicates North America’s observed 

temperatures over the last century can only be reproduced using model simulations containing both 

natural and anthropogenic forcings. In the CCSP (2008g) report “Reanalysis of Historical Climate Data 

for Key Atmospheric Features: Implications for Attribution of Causes of Observed Change”, Dole et al. 

(2008) find that for North America “more than half of this warming [for the period 1951-2006] is likely 

the result of human-caused greenhouse gas forcing of climate change.” 

Difficulties remain in attributing temperature changes on smaller than continental scales and over time 

scales of less than 50 years. Attribution at these scales, with limited exceptions, has not yet been 

established (Hegerl et al., 2007). 

Temperature extremes have also likely been influenced by anthropogenic forcing. 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, such as the 2003 

European heat wave (Hegerl et al., 2007). Karl et al. (2008) conclude the increase in human-induced 

emissions of GHGs is estimated to have substantially increased the risk of a very hot year in the U.S., 

such as that experienced in 2006. They add that other aspects of observed increases in temperature 

extremes, such as changes in warm nights and frost days, have been linked to human influences. 

Additional Climate Variables 

There is evidence of anthropogenic influence in other parts of the climate system. The IPCC noted the 

following examples (Hegerl et al., 2007): 

• Trends over recent decades in the Northern and Southern Annular Modes22, which correspond to sea 

level pressure reductions over the poles, are likely related in part to human activity, affecting 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. Models including 

both greenhouse gas and stratospheric ozone changes simulate a realistic trend in the Southern 

Annular Mode, leading to a detectable human influence on global sea level pressure patterns. 

It is more likely than not that anthropogenic influence has contributed to increases in the frequency of the 

most intense tropical cyclones. Gutowski et al. (2008), as cited in the CCSP (2008i) report, likewise find 

evidence suggesting a human contribution to recent hurricane activity. However, they caution that a 

confident assessment of human influence on hurricanes will require further studies using models and 

observations, with emphasis on distinguishing natural from human-induced changes in hurricane activity 

through their influence on factors such as historical sea surface temperatures, wind shear, and atmospheric 

vertical stability. 

• The latitudinal pattern of change in land precipitation and observed increases in heavy precipitation 

over the 20th century appear to be consistent with the anticipated response to anthropogenic forcing. 

22 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 (see footnote 36). The Southern Annular Mode occurs in the 

Southern Hemisphere. 

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Attributing changes in precipitation to anthropogenic forcing at continental or smaller scales is more 

challenging. Clark et al. (2008) state that recent drought in the Southwest is consistent with projections of 

increasing subtropical aridity and recent trends in increasing precipitation intensity are also consistent 

with projected trends. However, Clark et al. caution that there is considerable natural variability in the 

hydroclimate in the Southwest and conclude that: “There is no clear evidence to date of human-induced 

global climate change on North American precipitation amounts.” Similarly, Karl et al. (2008) find heavy 

precipitation events averaged over North America have increased over the past 50 years at a rate higher 

than total precipitation increased, consistent with the observed increases in atmospheric water vapor, 

which have been associated with human-induced increases in greenhouse gases. 

5(b) Attribution of observed changes in physical and biological systems 

In addition to attributing the observed changes in average global- and continental-scale temperature and 

other climate variables to anthropogenic GHG forcing, a similar attribution can be made between 

anthropogenic GHG forcing and observed changes in physical systems (e.g., melting glaciers) and 

biological systems and species (e.g., geographic shift of species) which are shown to change as a result of 

recent warming. 

This section includes the observed changes in physical and biological systems in North America and in 

other parts of the world. 

The IPCC (2007b) concluded that, “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 states that, “A global assessment of data since 1970 has shown it is 

likely that anthropogenic warming has had a discernible influence on many physical and biological 

systems.” As detailed above in Section 5(a), recent warming of the last 50 years is very likely the result 

of the accumulation of anthropogenic GHGs in the atmosphere. 

Climate variability and non-climate drivers (e.g., land-use change, habitat fragmentation) need to be 

considered in order to make robust conclusions about the role of anthropogenic climate change in 

affecting biological and physical systems. IPCC (Rosenzweig et al. 2007) reviewed a number of joint 

attribution studies that linked responses in some physical and biological systems directly to anthropogenic 

climate change using climate, process, and statistical models. The conclusion of these studies is 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 (Rosenzweig et al. 2007).” 

The physical systems undergoing significant change include the cryosphere (snow and ice systems), 

hydrological systems, water resources, coastal zones and the oceans. These effects (reported with high 

confidence by IPCC (Rosenzweig et al. 2007) include 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. 

Regarding biological systems, the IPCC (Rosenzweig et al. 2007) reports 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, the 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. 

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Human system responses to climate change are more difficult to identify and isolate due to the larger role 

that non-climate factors play (e.g., management practices in agriculture and forestry, and adaptation 

responses to protect human health against adverse climatic conditions) (Rosenzweig et al., 2007). 

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Section 6 

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 likely23 be larger than those observed during the 20th century.” This section describes future 

GHG 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. Scenarios are 

story lines regarding possible futures. These storylines are designed to be internally consistent in their 

assumptions regarding population and economic growth, implementation of policies, technology change 

and adoption, and other factors that will influence emissions. Scenarios are not projections of the future, 

but are used to illustrate how the future might look if a given set of events occurred and policies 

implemented. All future GHG emission scenarios described in this section assume no new explicit GHG 

mitigation policies -- neither in the U.S. nor in other countries -- beyond those which were already 

enacted at the time the scenarios were developed. Future risks and impacts associated with the climate 

change projections are addressed in Part IV for domestic impacts and Part V for international impacts. 

6(a) Global emission scenarios and associated changes in concentrations and radiative forcing 

Greenhouse Gas Emissions 

As described in Section 4(a), a number of different GHGs and other factors, including aerosols, cause 

radiative forcing changes and thus contribute to climate change. This section discusses the range of 

published global reference (or baseline) future emission projections for which no explicit GHG mitigation 

policies beyond those currently enacted are assumed. 

The IPCC’s most recent future climate change projections from the Fourth Assessment Report (2007) 

(discussed in Section 6(b) below) are based on the GHG emission scenarios from the IPCC Special 

Report on Emission Scenarios (IPCC, 2000). Box 6.1 provides background information on the different 

SRES emission scenarios. The SRES developed a range of long-term (out to the year 2100) global 

reference scenarios for the major GHGs directly emitted by human activities and for some aerosols. The 

IPCC SRES scenarios do not explicitly account for implementation of the Kyoto Protocol. Figure 6.1 

presents the global IPCC SRES projections for the two most significant anthropogenic GHGs: CO2 

emissions primarily from the burning of fossil fuels, and CH4 emissions. 

Figure 6.1 Observed, and Projected Global CO2 and CH4 Emissions for the IPCC SRES Scenarios 

23 According to IPCC terminology, “very likely” conveys a 90 to 99% probability of occurrence. See Box 1.3 on 

page 5 for a full description of IPCC’s uncertainty terms. 

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Source: Meehl et al. (2007). Projected fossil CO2 and CH4 emissions for six illustrative SRES non-mitigation 

emission scenarios. Historical emissions (black lines) are shown for fossil and industrial CO2, and for CH4. 

Box 6.1: 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, that peaks in midcentury 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 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 towards 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. 

Box 6.2: CCSP (2007b) Reference Case Emission Scenarios from Synthesis and Assessment Product 2.1 

The scenarios in this report were developed using three integrated assessment models (IAMs). These models integrate 

socioeconomic and technological determinants of the emissions of GHGs with models of the natural science of Earth system 

response, including the atmosphere, oceans, and terrestrial biosphere. The three IAMs used are: 

• The Integrated Global Systems Model (IGSM) of the Massachusetts Institute of Technology’s Joint Program on 

the Science and Policy of Global Change. 

• The Model for Evaluating the Regional and Global Effects (MERGE) of GHG reduction policies developed 

jointly at Stanford University and the Electric Power Research Institute. 

• 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 2008-12 first period of the Kyoto Protocol and the U.S. goal of reducing GHG emissions per unit of 

its gross domestic product by 18% by 2012. The resulting reference cases are not predictions or best-judgment 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 GHG 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. 

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The main drivers of emissions are population, economic growth, technological change, and land-use 

activities including deforestation. 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 IPCC (2000). The range of GHG emissions in the scenarios widen over time to 

reflect uncertainties in the underlying drivers. Similar future GHG emissions can result from different 

socio-economic developments. The IPCC (2000) SRES report did not assign probabilities or likelihood 

to the scenarios, as it was stated that there is no single most likely, central, or best-guess scenario, either 

with respect to SRES scenarios or to the underlying scenario literature. This is why IPCC (2000) has 

recommended using a range of SRES scenarios with a variety of underlying assumptions for use in 

analysis. 

Despite the range in future emission scenarios, the majority of all reference-case scenarios project an 

increase of GHG emissions across the century, and show that CO2 remains the dominant GHG over the 

course of the 21st century. Total cumulative (1990 to 2100) CO2 emissions across the SRES scenarios 

range from 2,826 gigatonnes of CO2 (GtCO2) (or 770 GtC) to approximately 9,322 GtCO2 (or 2,540 

GtC).24 

Since the IPCC SRES (2000), 

new scenarios in the literature 

have emerged. The emission 

scenario range from the recent 

literature is similar to the range 

from the IPCC SRES. The 

IPCC (2007c) reported that 

baseline annual emission 

scenarios published since 

SRES are comparable in range 

to those presented in the SRES 

scenarios (25 to 135 GtCO2eq. 

per year in 2100). Studies 

since SRES used lower values 

for some drivers for emissions, 

notably population projections. 

However, for those studies 

incorporating these new 

population projections, 

changes in other drivers, such 

as economic growth, resulted 

in little change in overall 

emission levels (IPCC, 2007c). 

For comparison, Figure 6.2 provides global projections of CO2 emissions from the burning of fossil fuels 

and industrial sources from the three reference-case scenarios developed by the U.S. Climate Change 

Science Program (CCSP, 2007b). Box 6.2 provides background information on the reference case 

scenarios developed by the CCSP. The CCSP scenarios, because they were developed more recently than 

the IPCC SRES scenarios, do account for the implementation of the Kyoto Protocol for participating 

countries, but no explicit GHG 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. 

24 1 Gigatonne (Gt) = 1 billion metric tons. 

Figure 6.2: Projected Global Emissions of CO2 from Fossil Fuels 

and Industrial Sources across CCSP Reference Scenarios 

Source: CCSP (2007b). 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 GtC/yr 

24.0 GtC/yr. 

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Figure 6.3 illustrates reference case emission projections for CO2, CH4, N2O, and the fluorinated gases in 

aggregate (HFCs, PFCs, and SF6 or “F-gases”). The emissions projections in Figure 6.3 are from the 21st 

Study of Stanford University’s Energy Modeling Forum on multigas mitigation as referenced by Fisher et 

al. (2007). Eighteen models participated in the EMF-21 study and the emissions ranges in Figure 6.3 are 

representative of the literature. The broad ranges of EMF-21 emissions projections in Figure 6.3, 

especially for N2O and the F-gases, illustrate the uncertainties in projecting these future emissions, which 

is generally consistent with the range found in SRES. 

Modeling groups have developed a multiplicity of projections for the concentrations of aerosol species. 

Within the IPCC process, all the SRES scenarios specified sulfate emissions. The inclusion, magnitude 

and temporal evolution of other forcing agents such as nitrates and carbonaceous aerosols were left to the 

discretion of the individual modeling groups. There are still large uncertainties associated with current 

inventories of BC and OC, the ad hoc scaling methods used to produce future emissions, and considerable 

variation among estimates of the optical properties of carbonaceous aerosols. Given these uncertainties, 

future projections of forcing by BC and OC are quite dependent on the model and emissions assumptions 

(Meehl et al., 2009). Similarly, CCSP (2008d) concluded that one of the most important uncertainties in 

characterizing the potential climate impact of aerosols is the projection of their future emissions. 

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Figure 6.3: EMF-21 and IPCC Global Emissions Projections for CO2, CH4, N2O, and the 

Fluorinated Gases 

Source: CCSP (2007b). 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). 

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For comparison, Figure 6.4 Figure 6.4 Projected Global CH4 and N2O Emissions across three 

provides the global CH4 CCSP Reference Scenarios 

and N2O projections from 

the three U.S. CCSP 

reference-case scenarios 

(CCSP, 2007b). 

Future Concentration and 

Radiative Forcing 

Changes 

Figure 6.5 shows the latest 

IPCC projected increases 

in atmospheric CO2, CH4 

and N2O concentrations for 

the SRES scenarios, and Source: CCSP (2007b). Global anthropogenic emissions of CH4 and N2O vary 

Figure 6.6 shows the 

associated radiative forcing 

for these CO2 scenarios. In 

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 

general, reference reduced significantly for other reasons, for example, whether higher natural gas 

concentrations of CO2 and prices will stimulate capture of CH4 for use as a fuel. 

other GHGs are projected to increase. Concentrations of long-lived gases increase even for those 

scenarios where annual emissions toward the end of the century are assumed to be lower than current 

annual emissions. The CCSP scenarios show a similar picture of how atmospheric concentrations of the 

main GHGs and total radiative forcing change over time. 

Carbon dioxide 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 GHGs is still significant and growing over time. 

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Figure 6.5 Projected Global CO2, CH4 and N2O 

Concentrations for the IPCC SRES Scenarios 

Source: Meehl et al. (2007). 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 circulations models 

(AOGCMs). 
Technical Support Document, U.S. Environmental Protection Agency 

Figure 6.6 Projected Radiative Forcing from CO2 for the IPCC SRES Scenarios 

Source: Meehl et al. (2007). Projected radiative forcing from CO2 for six illustrative SRES nonmitigation emission scenarios as produced by a simple climate model tuned to 19 AOGCMs. 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. 

6(b) Projected changes in global temperature, precipitation patterns, sea level rise, and ocean 

acidification 

Using the emissions scenarios described in Section 6(a), computer models project future changes in 

temperature, precipitation and sea level at global and regional scales. According to the IPCC (Meehl at 

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.”25 

Confidence decreases in changes projected by global models at smaller spatial scales. Many important 

small-scale processes cannot be represented explicitly in models, and so must be included in approximate 

form as they interact with larger-scale features (Randall et al., 2007). Some of the most challenging 

aspects of understanding and projecting regional climate changes relate to possible changes in the 

circulation of the atmosphere and oceans, and their patterns of variability (Christensen et al., 2007). 

Nonetheless, the IPCC (2007d) concluded that recent advances in regional-scale modeling lead to higher 

confidence in projected patterns of warming and other regional-scale features, including changes in wind 

patterns, precipitation and some aspects of extremes and of ice. 

The CCSP (2008c) report “Climate Models: An Assessment of Strengths and Limitations” finds that 

models “have been steadily improving over the past several decades”, “show many consistent features in 

their simulations and projections for the future”, and “are able to simulate the recorded 20th Century 

global mean temperature in a plausible way.” However, it cautions that projections of precipitation in 

25 A number of climate models are developed and run at academic institutions and government supported research 

laboratories in the U.S. and other countries. The IPCC helps coordinate modeling efforts to facilitate comparisons 

across models, and synthesizes results published by several modeling teams. 

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some cases remain “problematic” (especially at the regional scale) and that “uncertainties in the climatic 

effects of manmade aerosols (liquid and solid particles suspended in the atmosphere) constitute a major 

stumbling block” in certain modeling experiments. It adds “uncertainties related to clouds increase the 

difficulty in simulating the climatic effects of aerosols, since these aerosols are known to interact with 

clouds and potentially can change cloud radiative properties and cloud cover.” 

Global Temperature 

The latest IPCC assessment uses a larger number of simulations available from a broader range of models 

to project future climate relative to earlier assessments (IPCC, 2007d). All of the simulations performed 

by the IPCC project warming, for the full range of emissions scenarios. 

For the next two decades, a warming of about 0.2°C (0.36°F) per decade is projected for a range of SRES 

emission scenarios (IPCC, 2007d). Even if the concentrations of all greenhouse gases and aerosols had 

been kept constant at year 2000 levels (see the “Year 2000 Constant Concentrations” scenario in Figure 

6.8), a further warming of about 0.1°C (0.18°F) per decade would be expected because of the time it takes 

for the climate system, particularly the oceans, to reach equilibrium (with year 2000 greenhouse gas 

levels). Through about 2030, the warming rate is mostly insensitive to choices between the A2, A1B, or 

B1 scenarios, and is consistent with that observed for the past few decades. Possible future variations in 

natural forcings (e.g., a large volcanic eruption) could change these values somewhat (Meehl et al., 2007). 

Large changes in emissions of short-lived gases could also have a near-term effect on temperatures, 

especially on the regional scale (CCSP, 2008d). 

According to IPCC (see Figure 6.8), by mid-century (2046–2065), the choice of scenario becomes more 

important for the magnitude of the projected warming, with average values of 1.3°C (2.3°F), 1.8°C 

(3.2°F) and 1.7°C (3.1°F) from the models for scenarios B1 (low emissions growth), A1B (medium 

emissions growth) and A2 (high emissions growth), respectively (Meehl et al., 2007). About a third of 

that warming is projected to be due to climate change that is already committed (as shown in the “Year 

2000 Constant Concentrations” scenario). By the 2090-2099 period (relative to the 1980-1999 range), 

projected warming varies significantly by emissions scenario. The full suite of SRES scenarios (given 

below) provides a warming range of 1.8°C to 4.0°C (3.2°F to 7.2°F) with an uncertainty range of 1.1°C 

to 6.4°C (2.0°F to 11.5°F). The multi-model average warming and associated uncertainty ranges for the 

2090-2099 period (relative to 1980-1999) for each scenario, as illustrated in Figure 6.8 are: 

Scenario Average Global Warming by End of Uncertainty Range 

Century Relative to ~1990 

B1 1.8°C (3.2°F) 1.1°C to 2.9°C (2.0°F to 5.2°F) 

A1T 2.4°C (4.3°F) 1.4°C to 3.8°C (2.5°F to 5.7°F) 

B2 2.4°C (4.3°F) 1.4°C to 3.8°C (2.5°F to 5.7°F) 

A1B 2.8°C (5.0°F) 1.7°C to 4.4°C (3.1°F to 7.9°F) 

A2 3.4°C (6.1°F) 2.0°C to 5.4°C (3.6°F to 9.7°F) 

A1FI +4.0°C (+7.2°F) 2.4°C to 6.4°C (4.3°F to 11.5°F) 

The wide range of uncertainty in these estimates reflects the different assumptions about future 

concentrations 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; Meehl 

et al., 2007). 

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Figure 6.8 Multi-Model Averages and Assessed Ranges for Surface Warming 

Source: IPCC (2007d). Solid lines are multi-model global averages of surface warming (relative to 1980– 

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 the AOGCMs in the left part 

of the figure, as well as results from a hierarchy of independent models and observational constraints. 

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Box 6.3: Climate Sensitivity 

The sensitivity of the climate system to a forcing is commonly expressed in terms of the global mean temperature 

change that would be expected after a time sufficiently long for both the atmosphere and ocean to come to 

equilibrium with the change in climate forcing (NRC, 2001). Since IPCC’s Third Assessment Report (IPCC, 

2001b), the levels of scientific understanding and confidence in quantitative estimates of equilibrium climate 

sensitivity have increased substantially (Meehl et al. 2007). 

Solomon et al. (2007) indicate there is increased confidence of key processes that are important to climate 

sensitivity due to improved comparisons of models to one another and to observations. Water vapor changes 

dominate the feedbacks affecting climate sensitivity and are now better understood. Observational and model 

evidence support a combined water vapor-lapse rate (the rate at which air temperature decreases with altitude) 

feedback that corresponds to about a 50% amplification of global mean warming. Cloud feedbacks remain the 

largest source of uncertainty. 

Basing their assessment on a combination of several independent lines of evidence, including observed climate 

change and the strength of known feedbacks simulated in general circulation models, the authors concluded that 

the global mean equilibrium warming for doubling CO2 (a concentration of approximately 540 ppm), or 

‘equilibrium climate sensitivity’, very likely greater than 1.5°C (2.7°F) and likely to lie in the range 2°C to 4.5°C 

(3.6°F to 8.1°F), with a most likely value of about 3°C (5.4°F). For fundamental physical reasons, as well as data 

limitations, the IPCC states a climate sensitivity higher than 4.5°C cannot be ruled out, but that agreement for these 

values with observations and proxy data is generally worse compared to values in the 2-4.5°C range (Meehl et al., 

2007). 

IPCC Climate Sensitivity Probabilities 

Less than 1.5°C 10% or less probability 

Less than 2.0°C 5-17% probability 

2°C to 4.5°C 66-90% probability 

Greater than 4.5°C 5-17% probability 

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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, consistent with observations (see Section 4b) during the latter part of 

the 20th century (Meehl et al., 2007). 

Global Precipitation 

Models simulate that global mean precipitation increases with global warming (Meehl et al., 2007). 

However, there are substantial spatial and seasonal variations. Increases in the amount of precipitation are 

very likely in high latitudes, while decreases are likely in most subtropical land regions, continuing 

observed patterns in recent trends in observations. According to Solomon et al. (2007): 

• In the northern hemisphere, a robust pattern of increased subpolar and decreased subtropical 

precipitation dominates the projected precipitation pattern for the 21st century over North 

America and Europe, while subtropical drying is less evident over Asia. 

• In the southern hemisphere, there are few land areas in the zone of projected supolar moistening 

during the 21st century, with the subtropical drying more prominent. 

• Projections of the precipitation over tropical land regions are more uncertain than those at higher 

latitudes. 

Global Sea Level Rise 

By the end of the century (2090-2099), sea level is projected by IPCC (2007d) to rise between 0.18 and 

0.59 meters relative to the base period (1980-1999). 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 very likely exceeds the 1961 to 2003 average rate (1.8 

± 0.5 mm per year). 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 contributes 70 to 75% of the central estimate for the rise in 

sea level for all scenarios (Meehl et al., 2007). Glaciers, ice caps and the Greenland Ice Sheet are also 

projected to add to sea level. The IPCC projects a range of sea level rise contributions from all glaciers, 

ice caps, and ice sheets between 0.04 meters to 0.23 meters, not including the possibility of rapid 

dynamical changes. The Antarctic ice sheet is estimated to be a negative contributor to sea level rise over 

the next century under these assumptions (Meehl et al., 2007). 

General Circulation Models indicate that the Antarctic Ice Sheet will receive increased snowfall without 

experiencing substantial surface melting, thus gaining mass and reducing sea level rise according to IPCC 

(Meehl et al., 2007). However, Meehl et al. (2007) note 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 projections above. For 

example, if ice discharge from these processes were to increase in proportion to global average surface 

temperature change, it would add 0.1 to 0.2 m to the upper bound of sea level rise by 2090 to 2099. 

Dynamical processes related to ice flow not included in current models but suggested by recent 

observations could increase the vulnerability of the ice sheets to warming, increasing future sea level rise. 

In the CCSP (2008a) report on abrupt climate change, Clark et al. (2008) find, “Recent rapid changes at 

the edges of the Greenland and West Antarctic ice sheets show acceleration of flow and thinning, with the 

velocity of some glaciers increasing more than twofold.” They add, “Inclusion of these processes in 

models will likely lead to sea-level projections for the end of the 21st century that substantially exceed the 

projections presented in the IPCC AR4 report.” 

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The CCSP (2009b) sea level rise report notes, “It has been suggested by Rahmstorf (2007) and other 

climate scientists that a global sea-level rise of 1 m (3 ft) is plausible within this century if increased 

melting of ice sheets in Greenland and Antarctica is added to the factors included in the IPCC estimates. 

Therefore, thoughtful precaution suggests that a global sea-level rise of 1m to the year 2100 should be 

considered for future planning and policy discussions.” 

The CCSP (2008c) report on the strengths and limitations of models notes that models of glacial ice are 

“in their infancy” and that “recent evidence for rapid variations in this glacial outflow indicates that morerealistic glacial models are needed to estimate the evolution of future sea level.” 

Sea level rise during the 21st century is projected by IPCC to have substantial geographic variability due 

to factors that influence changes at the regional scale, including changes in ocean circulation or 

atmospheric pressure, and geologic processes (Meehl et al., 2007). The patterns in different models are 

not generally similar in detail, but have some common features, including smaller than average sea level 

rise in the Southern Ocean, larger than average sea level rise in the Arctic, and a narrow band of 

pronounced sea level rise stretching across the southern Atlantic and Indian Oceans. 

Global Ocean Acidification 

The oceans have absorbed, and will continue to absorb, carbon dioxide emissions associated with 

anthropogenic activities. Surface ocean pH has decreased by 0.1 units due to oceanic absorption of CO2, 

and it is predicted to decrease by an additional 0.3-0.4 units by 2100 (Fischlin et al., 2007). This 

projected rate of decline may lead to ocean pH levels within a few centuries that have not been observed 

for a few hundred million years (Denman et al., 2007). Acidification is affecting calcium carbonate 

saturation in ocean waters, and is thereby reducing calcification rates of organisms who rely on the 

minerals for development. Future acidification is projected to result in under-saturated ocean waters (see 

Box 14.1 for information on the effects of this undersaturation). Polar and sub-polar surface waters and 

the Southern Ocean will be aragonite (a form of calcium carbonate) under-saturated by 2100, and Arctic 

waters will be similarly threatened (Fischlin et al., 2007). According to a model experiment using a 

‘business as usual’ emission scenario (IPCC -IS92a), bio-calcification will be reduced by 2100, in 

particular within the Southern Ocean, and by 2050 for aragonite-producing organisms (Denman et al., 

2007). 

6(c) Projected changes in U.S. temperature, precipitation patterns, and sea level rise 

IPCC’s Fourth Assessment Report includes projections for changes in temperature, precipitation and sea 

level rise for North America -- which can be generalized for the U.S – as well as some U.S. specific 

information. These projections are summarized in this section. 

U.S. Temperatures 

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

Figures in 6.9 and 6.10, and warm more than the global mean warming in most areas (Christensen et al., 

2007). For scenario A1B (moderate emissions growth), the largest warming through 2100 is projected to 

occur in winter over northern parts of Alaska, reaching 7-10°C (13-18°F) in the northernmost parts as 

shown in Figure 6.10, 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, consistent with less warming over the oceans. The average warming in 

the U.S. through 2100 is projected by nearly all the models used in the IPCC assessment to exceed 2°C 

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(3.6°F) for all scenarios (see Figure 6.9), with 5 out of 21 models projecting average warming in excess of 

4°C (7.2°F) for the A1B (mid-range) emissions scenario. 

The CCSP (2008e) report “The Effects of Climate Change on Agriculture, Land Resources, Water 

Resources, and Biodiversity” provides shorter-term temperature projections for the U.S. for the year 

2030. It projects a warming of approximately 1°C in the southeastern U.S., to more than 2°C in Alaska 

and northern Canada, with other parts of North America having intermediate values (Backlund et al., 

2008b). 

Figure 6.9: Temperature anomalies with respect to 1901 to 1950 for four North American 

land regions 

Source: Christensen et al. (2007). 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. 

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Figure 6.10: Projected temperature and precipitation changes over North America from 

the MMD-A1B simulations. 

Source: Christensen et al. (2007). Top row: Annual mean, December-January-February, and June-JulyAugust temperature between 1980 to 1999 and 2080 to 2099, averaged over 21 models. Bottom row: 

same as top, but for fractional change in precipitation. 

U.S. Precipitation 

A widespread increase in annual precipitation is projected by IPCC over most of the North American 

continent except the south and south-western part of the U.S. and over Mexico largely consistent with 

trends in recent decades (as described in Section 4) (Christensen et al., 2007). The largest increases are 

projected over northern North America (i.e. Canada and Alaska) associated with a poleward shift in storm 

tracks where precipitation increases by the largest amount in autumn and by the largest fraction in winter 

as shown in Figure 6.10. 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 southwest U.S. and 

northern Mexico. Accordingly, some models project drying in the southwest U.S., with more than 90% 

of the models projecting drying in northern and particularly western Mexico. On the windward slopes of 

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the mountains in the west, precipitation increases are likely to be enhanced due to orographic lifting26. In 

the northeastern U.S., annual mean precipitation is very likely to increase. 

U.S. Sea Level Rise 

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 (uplift) or 

increase (subsidence) the relative sea level rise at any site (Nicholls et al., 2007). 

Impact of Short-lived Species on U.S. Temperature and Precipitation 

Modeling results suggest that changes in short-lived species (mainly sulfates and black carbon) may 

significantly influence 21st century climate. A GFDL simulation of SRES scenario A1B shows that 

changes in short lived species could be responsible for up to 40% of the continental U.S. summertime 

warming projected to occur in this scenario by 2100 along with a statistically significant decrease in 

precipitation, mainly due to a combination of domestic sulfate emissions reductions and increases in 

Asian black carbon emissions (CCSP, 2008d). However, the CCSP study concludes that “we could not 

find a consensus in this report on the duration, magnitude, or even sign (warming or cooling) of the 

climate change due to future levels of the short-lived gases and particles” due to uncertainties about 

different pollution control storylines. 

6(d) Cryosphere (Snow and Ice) projections, focusing on North America and the U.S. 

Snow season length and snow depth are very likely to decrease in most of North America as illustrated in 

Figure 6.11, except in the northernmost part of Canada where maximum snow depth is likely to increase 

(Christensen et al., 2007). Widespread increases in thaw depth are projected over most permafrost regions 

globally (IPCC, 2007d). 

Lettenmaier et al. (2008) find where shifts to earlier snowmelt peaks and reduced summer and fall low 

flows have already been detected, continuing shifts in this direction are very likely. 

Meehl et al (2007) conclude that as the climate warms, glaciers will lose mass owing to dominance of 

summer melting over winter precipitation increases, contributing to sea level rise. 

26 Orographic lifting is defined as the ascent of air from a lower elevation to a higher elevation as it moves over 

rising terrain 

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Figure 6.11 Percent Snow Depth Changes in March 

Source: Christensen et al. (2007). Percent snow depth changes in March (only 

calculated where climatological snow amounts exceed 5mm of water equivalent), 

as projected by the Canadian Regional Climate Model (CCRM), driven by the 

Canadian General Circulation Model (CGCM), for 2041 to 2070 under SRES A2 

compared to 1961 to 1990. 

Sea ice is projected to 

shrink in both the Arctic 

and the Antarctic under all 

SRES scenarios. In some 

projections, Arctic latesummer sea ice disappears 

almost entirely by the latter 

part of the 21st century 

(IPCC, 2007d). Taking 

into account recent late 

summer sea ice loss, 

Polyak et al (2009) 

indicate that the Arctic 

Ocean may become 

seasonally ice free as 

early as 2040. 

6(e) Extreme events, 

focusing on North 

America and the U.S. 

Models suggest that 

human-induced climate 

change is expected to alter 

the prevalence and severity 

of many extreme events 

such as heat waves, cold 

waves, storms, floods, and 

droughts. This section describes CCSP (2008i) and IPCC’s projections for extreme events focusing on 

North America and the U.S. Sections 7-14 summarize some of the sectoral impacts of extreme events for 

the U.S. 

Temperature 

According to the IPCC, it is very likely that heat waves globally will become more intense, more 

frequent, and longer lasting in a future warm climate, whereas cold episodes are projected to decrease 

significantly. (Meehl, G.A. et al., 2007). Meehl et al., 2007 report on a study finding that the pattern of 

future changes in heat waves, with greatest intensity increases over western Europe, the Mediterranean 

and the southeast and western U.S., is related in part to circulation changes resulting from an increase in 

greenhouse gases. 

The IPCC cites a number of studies that project changes in temperature extremes in the U.S. (Christensen 

et al., 2007). One finds that the frequency and the magnitude of extreme temperature events changes 

dramatically under a high-end emission scenario (SRES A2), with increases in extreme hot events and 

decreases in extreme cold events. Another examines changes in temperature extremes in their 

simulations centered on California and finds increases in extreme temperature events, prolonged hot 

spells and increased diurnal temperature range. A third finds increases in diurnal temperature range in six 

sub-regions of the western U.S. in summer. 

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Karl et al. (2008) find for a mid-range scenario (A1B) of future GHG emissions, a day so hot that it is 

currently experienced only once every 20 years would occur every three years by the middle of the 

century over much of the continental U.S. and by the end of the century, it would occur every other year 

or more. 

Some implications for human health resulting from these projected changes in temperature extremes are 

discussed in Section 7(b). 

Precipitation and Storms 

Intensity of precipitation events is projected to increase globally, particularly in tropical and high latitude 

areas that experience increases in mean precipitation (Meehl et al., 2007). Even in areas where mean 

precipitation decreases (most subtropical and mid-latitude regions), precipitation intensity is projected to 

increase but there would be longer periods between rainfall events. Meehl et al. (2007) note that 

increases in heavy precipitation events have been linked to increases in flooding. 

The IPCC projects a tendency for drying in mid-continental areas during summer, indicating a greater risk 

of droughts in those regions (Meehl et al., 2007). 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 according to a study 

assessed in Meehl et al. (2007). Karl et al. (2008) indicate that for a mid-range emission scenario (A1B), 

daily precipitation so heavy that it now occurs only once every 20 years is projected to occur 

approximately every eight years by the end of this century over much of Eastern North America. 

Future projections in Gutowski et al. (2008) indicate strong mid-latitude (or extratropical) storms will be 

more frequent though the overall number of storms may decrease. 

Several regional studies in the IPCC project changes in precipitation extremes in parts of the U.S ranging 

from a decrease in heavy precipitation in California to an increase during winter in the northern Rockies, 

Cascades and Sierra Nevada mountain ranges (Christensen et al., 2007). For the contiguous U.S., a study 

in Christensen et al. (2007) finds widespread increases in extreme precipitation events under SRES A2 

(high emissions growth). 

Based on a range of models, it is likely that tropical cyclones (tropical storms and hurricanes) will become 

more intense, with stronger peak winds and more heavy precipitation associated with ongoing increases 

of tropical sea surface temperatures (IPCC, 2007d). Karl et al. (2008) analyze model simulations and find 

that for each 1oC (1.8oF) increase in tropical sea surface temperatures, core rainfall rates will increase by 

6-18% and the surface wind speeds of the strongest hurricanes will increase by about 1-8%. Storm surge 

levels are likely to increase due to projected sea level rise, but note the degree of projected increase has 

not been adequately studied. 

Karl et al. (2008) indicate projections in frequency changes in tropical cyclones are currently too 

uncertain for confident projections. 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 (Meehl et al., 2000). A 

number of modeling studies have also projected a general tendency for more intense but fewer storms 

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). 

Possible implications of extreme precipitation events in the U.S. for health are described in Chapter 7, for 

food production and agriculture in Section 9, for water resources in Section 11, for coastal areas in 

Section 12, and for ecosystems and wildlife in Section 14. 

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6(f) Abrupt Climate Change and High Impact Events 

CCSP (2008a), in its report on abrupt climate change, defines it as a “large-scale change in the climate 

system that takes place over a few decades or less, persists (or is anticipated to persist) for at least a few 

decades, and causes substantial disruptions in human and natural systems.” 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). Potential abrupt climate 

change implications in the U.S. are not discussed in the sections 7 through 14 (the U.S. sectoral impacts) 

because they cannot be predicted with confidence, particularly for specific regions. This section therefore 

focuses on the general risks of abrupt climate change globally, with some discussion of potential regional 

implications where information is available. 

According to the National Research Council (2002): “Technically, an abrupt climate change occurs when 

the climate system is forced to cross some threshold, triggering a transition to a new state at a rate 

determined by the climate system itself and faster than the cause.” Crossing systemic thresholds may lead 

to large and widespread consequences (Schneider et al., 2007). The triggers for abrupt climate change 

can be forces that are external and/or internal to the climate system including (NRC, 2002): 

• changes in the Earth’s orbit27 

• a brightening or dimming of the sun 

• melting or surging ice sheets 

• strengthening or weakening of ocean currents 

• 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 warmings (8 to 16°C within decades over Greenland) and coolings occurred repeatedly 

over the North Atlantic region (Jansen et al., 2007). These warmings 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 unlikely that they were associated with large changes in global mean surface temperature. 

The National Research Council concluded that anthropogenic forcing may increase the risk of abrupt 

climate change (NRC, 2002): 

“...greenhouse warming and other human alterations of the Earth system may increase the possibility of 

large, abrupt, and unwelcome regional or global climatic events. The abrupt changes of the past are not 

fully explained yet, and climate models typically underestimate the size, speed, and extent of those 

changes. Hence, 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 dynamical interactions in the atmosphere-ice-ocean 

system, or from the crossing of a threshold from slow external forcing (as described above) (Meehl et al., 

27 According to the National Research Council (2002), changes in the 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 into and warming 

out of ice ages (NRC, 2002). 

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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)28 and the North Atlantic 

Oscillation/Arctic Oscillation (NAO/OA). 29 

ENSO has important linkages to patterns of tropical sea surface temperatures, which historically have 

been strongly tied to drought, including “megadroughts” that likely occurred between 900-1600 A.D. over 

large regions of the southwestern U.S. and Great Plains (Clark et al., 2008). The possibility of severe 

drought as an abrupt change resulting from changes in sea surface temperatures in a warming world is 

assessed by Clark et al. (2008). They find that under greenhouse warming scenarios, the cause of modelprojected sub-tropical drying is an overall widespread warming of the ocean and atmosphere, in contrast 

to the causes of historic droughts (linked specifically to sea surface temperature). But they note models 

may not correctly represent the ENSO patterns of tropical SST change that could create impacts on global 

hydroclimate (e.g., drought) in addition to those caused by overall warming. The current model results do 

show drying over the southwestern U.S., potentially increasing the likelihood of severe and persistent 

drought there in the future. Clark et al. (2008) note this drying has already begun (see also section 4k) but 

caution it is not clear if the present drying is outside the range of natural variability and linked to 

anthropogenic causes. 

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 Meehl et al. 

(2007), the probability of an abrupt change in (or shutdown of) the MOC is low: “It is very unlikely that 

the MOC will undergo a large abrupt transition during the 21st century. Even further into the future, 

Clark et al. (2008) note “it is unlikely that the Atlantic MOC will collapse beyond the end of the 21st 

century because of global warming, although the possibility cannot be entirely excluded.” While models 

project a slowdown in the MOC over the 21st century and beyond, it is so gradual that 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 GHGs (Clark et al., 2008). Clark et al. (2008) do caution that 

while a collapse of the MOC is unlikely, the potential consequences of this event could be severe if it 

were to happen. Potential impacts include a southward shift of the tropical rainfall belts, additional sea 

level rise around the North Atlantic, and disruptions to marine ecosystems. 

The rapid disintegration of the Greenland Ice Sheet (GIS), which would raise sea levels 7 meters, is 

another commonly discussed abrupt change. Clark et al. (2008) report that observations demonstrate that 

it is extremely likely that the Greenland Ice Sheet is losing mass and that this loss has very likely been 

accelerating since the mid-1990s. In the CCSP (2009b) report “Past Climate Variability and Change in the 

Arctic and at High Latitudes”, Alley et al. (2009) find a threshold for ice-sheet removal from sustained 

summertime warming of 5°C, with a range of uncertainties from 2° to 7°C. Meehl et al. (2007) in IPCC 

28 ENSO describes the full range of the Southern Oscillation (see-saw of atmospheric mass or pressure between the 

Pacific and Indo-Australian areas) that includes both sea surface temperature (SST) increases as well as SST 

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 SSTs 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. 29 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 much 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 phenomenon. 

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suggest the complete melting of the GIS would only require sustained warming in the range of 1.9°C to 

4.6°C (relative to the pre-industrial temperatures) but suggest it would take many hundreds of years to 

complete. 

A collapse of the West Antarctic Ice Sheet (WAIS), which would raise seas 5-6 meters, 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 melting at the bottom by 

a warmer ocean, might contribute to a potential destabilization of the WAIS. 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). Clark et al. (2008) indicate that while ice is thickening over some higher 

elevation regions of Antarctica, substantial ice losses from West Antarctica and the Antarctic Peninsula 

are very likely occurring and that Antarctica is losing ice on the whole. Ice sheet models are only 

beginning to capture the small-scale dynamical 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 IPCC to project sea level rise. These models suggest Antarctica will 

gain mass due to increasing snowfall, (although recent studies find no significant continent-wide trends in 

snow accumulation over the past several decades; Lemke et al., 2007), reducing sea level rise. 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 WAIS or its 

contribution to sea level rise (Meehl et al., 2007). 

Considering the Greenland and West Antarctic ice sheets together, Schneider et al. (2007) find 

paleoclimatic evidence suggests that Greenland and possibly the WAIS contributed to a sea-level rise of 

4-6 meters during the last interglacial, when polar temperatures were 3-5 degrees C warmer, and the 

global mean was not notably warmer, than at present. Accordingly, they conclude with medium 

confidence that at least partial deglaciation of the Greenland ice sheet, and possibly the WAIS, would 

occur over a period of time ranging from centuries to millennia for a global average temperature increase 

of 1-4 degrees Celsius (relative to 1990-2000), causing a contribution to sea-level rise of 4-6 meters or 

more. 

Another potential abrupt change of concern assessed by CCSP (2008a) is the catastrophic release of 

methane from clathrate hydrates in the sea floor and to a lesser extent in permafrost soils. Clark et al. 

(2008) find the following: 

• The size of the hydrate reservoir is uncertain, perhaps by up to a factor of 10 making judgments 

about risk difficult to assess. 

• Although there are a number of suggestions in the literature about the possibility of a dramatic 

abrupt release of methane to the atmosphere, modeling and isotopic fingerprinting of ice-core 

methane do not support such a release to the atmosphere over the last 100,000 years or in the near 

future. 

Clark et al (2008) conclude: 

“While the risk of catastrophic release of methane to the atmosphere in the next century appears very 

unlikely, it is very likely that climate change will accelerate the pace of persistent emissions from 

both hydrate sources and wetlands. Current models suggest that wetland emissions could double in 

the next century. However, since these models do not realistically represent all the processes thought 

to be relevant to future northern high-latitude CH4 emissions, much larger (or smaller) increases 

cannot be discounted. Acceleration of persistent release from hydrate reservoirs is likely, but its 

magnitude is difficult to estimate.” 

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6(g) Effects on/from Stratospheric Ozone 

Substances that deplete stratospheric ozone, which protects the Earth’s surface from much of the sun’s 

biologically harmful ultraviolet radiation, are regulated under Title VI of the Clean Air Act. According to 

the World Meteorological Organization (2007), climate change that results from changing greenhouse gas 

concentrations will affect the evolution of the ozone layer through changes in chemical transport, 

atmospheric composition, and temperature. In turn, changes in the stratospheric ozone can have 

implications for the weather and climate of the troposphere. The coupled interactions between the 

changing climate and ozone layer are complex and scientific understanding is incomplete (WMO, 2007). 

Specific information on climate change effects on/from stratospheric ozone in the U.S. has not been 

assessed. Except where indicated, the findings in this section apply generally to the globe with a focus on 

polar regions. 

Effects of Elevated Greenhouse Gas Concentrations on Stratospheric Ozone 

The World Meteorological Organization’s (WMO) 2006 Scientific Assessment of Ozone Depletion 

(2007) concluded that future concentrations of stratospheric ozone are sensitive to future levels of the 

well-mixed greenhouse gases. According to the WMO (2007): 

• Future increases of 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 methane emission (from warmer and wetter soils) is expected to enhance ozone 

production in the lower stratosphere. 

• An increase in nitrous oxide emissions is expected to reduce ozone in the middle and high 

stratosphere. 

Two-dimensional models that include coupling between all of these well-mixed greenhouse gases and 

temperature project that ozone levels between 60o S and 60o N will return to 1980 values up to 15 years 

earlier than in models that are uncoupled (Bodeker et al., 2007). 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., 2007). 

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 cited in Baldwin et al. (2007) suggest increasing stratospheric water vapor would delay 

ozone layer recovery. Increases in stratospheric water vapor could also increase spring-time 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., 2007). Changes in the 

forcing and propagation of planetary waves30 in the polar winter are a major source of uncertainty for 

predicting future levels of Arctic ozone loss (Baldwin et al., 2007). 

30 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. 

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The CCSP (2008h) report “Trends in Emissions of Ozone-Depleting Substances, Ozone Layer Recovery, 

and Implications for Ultraviolet Radiation Exposure” includes results from two-dimensional chemistry 

transport models and three-dimensional climate chemistry models estimating the recovery of the ozone 

layer under a greenhouse gas scenario. It finds: 

• From 60°N to 60°S, global ozone is expected to return to its 1980 value up to 15 years earlier 

than the halogen recovery date because of stratospheric cooling and changes in circulation 

associated with GHG emissions. Global ozone abundances are expected to be 2 percent above 

the 1980 values by 2100 with values at mid-latitudes as much as 5 percent higher. 

• Model simulations show that the ozone amount in the Antarctic will reach the 1980 values 10 to 

20 years earlier (i.e. from 2040 to 2060) than the 2060 to 2070 time frame of when the ozonedepleting substances reach their 1980 levels in polar regions. 

• Most climate chemistry models show Arctic ozone values by 2050 larger than the 1980 values, 

with the recovery date between 2020 and 2040. 

Climate Change Effects from Stratospheric Ozone 

The WMO (2007) found changes to 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 mid-latitude westerly winds. The 

mechanism for this response is not well-understood. 

Modeling experiments (that simulate observed changes in stratospheric ozone and combined stratospheric 

ozone depletion and GHG 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 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 (Baldwin et al., 2007). 

6(h) Land-Use and Land Cover Change 

Changes in land surface (vegetation, soils, water) resulting from human activities can significantly affect 

local climate through shifts in radiation, cloudiness, surface roughness, and surface temperature. 

Solomon et al. (2007) find the impacts of land use change on climate are expected to be locally significant 

in some regions, but are small at the global scale in comparison with greenhouse warming. Similarly, the 

release of heat from anthropogenic energy production can be significant over urban areas, but is not 

significant globally (Solomon et al., 2007). 

The CCSP report (2008e) on the effects of climate change on agriculture, land resources, water resources, 

and biodiversity in the U.S. concludes 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. 

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Part IV 

U.S. Observed and Projected Human Health and Welfare Effects from 

Climate Change 

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Section 7 

Human Health 

Warm temperatures and extreme weather already cause and contribute to adverse human health outcomes 

through heat-related mortality and morbidity, storm-related fatalities and injuries, and disease. In the 

absence of effective adaptation, these effects are likely to increase with climate change. Depending on 

progress in health care and access, infrastructure, and technology, climate change could increase the risk 

of heat wave deaths, respiratory illness through exposure to aeroallergens and ozone (discussed in Section 

8), and certain diseases (CCSP, 2008b; Confalonieri et al, 2007). Studies in temperate areas (which 

would include large portions of the U.S.) have shown that climate change is projected to bring some 

benefits, such as fewer deaths from cold exposure. The balance of positive and negative health impacts as 

a result of climate change will vary from one location to another, and will alter over time as climate 

change continues (CCSP, 2008b). 

In its Third Assessment Report (TAR), the IPCC produced a number of key findings summarizing the 

likely climate change health effects in North America. These effects, which were reaffirmed in the IPCC 

Fourth Assessment Report (Field et al., 2007), include: 

• Increased deaths, injuries, infectious diseases, and stress-related disorders and other adverse effects 

associated with social disruption and migration from more frequent extreme weather. 

• Increased frequency and severity of heat waves leading to more illness and death, particularly among 

the young, elderly and frail. 

• Expanded ranges of vector-borne and tick-borne diseases in North America but with moderating 

influence by public health measures and other factors. 

The more recent CCSP (2008b) report on human health stated as one of its conclusions that, “The United 

States is certainly capable of adapting to the collective impacts of climate change. However, there will 

still be certain individuals and locations where the adaptive capacity is less and these individuals and their 

communities will be disproportionally impacted by climate change.” 

There are few studies which address the interactive effects of multiple climate change impacts or of 

interactions between climate change health impacts and other kinds of local, regional, and global socioeconomic 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 (Field et al., 2007). 

Vulnerability is the summation of all the factors of risk and resilience that determine whether individuals 

experience adverse health impacts. Specific subpopulations may experience heightened vulnerability for 

climate-related health effects. Climate change is very likely to accentuate the disparities already evident 

in the American health care systems as many of the expected health effects are likely to fall 

disproportionately on the poor, the elderly, the disabled, and the uninsured (Ebi et al., 2008). 

This section describes the literature on the impacts of climate change on human health in four areas: 

temperature effects, extreme events, climate sensitive diseases, and aeroallergens. The health impacts 

resulting from climate change effects on air quality are discussed in Section 8. 

7(a) Temperature Effects 

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According to the IPCC (2007d), it is very likely31 that there were warmer and fewer cold days and nights 

and warmer and more frequent hot days over most land areas during the late 20th century (see section 

4(b)). It is virtually certain that these trends will continue during the 21st century (see Section 6(b)). As a 

result of the projected warming, the IPCC projects increases in heat-related mortality and morbidity 

globally (IPCC, 2007b). The projected warming is expected to result in fewer deaths due to reduced 

exposure to the cold. It is not clear whether reduced mortality from cold will be greater or less than 

increased heat-related mortality in the U.S. due to climate change (Gamble et al., 2008). 

Increased heat exposure 

Heatwaves are associated with marked short-term increases in mortality (Confalonieri et al, 2007). Hot 

temperatures have also been associated with increased morbidity. A study cited in Field et al. (2007) 

indicates increased hospital admissions for cardiovascular disease and emergency room visits have been 

documented in parts of North America during heat events. The populations most vulnerable to hot 

temperatures are older adults, the chronically sick, the very young, city-dwellers, those taking 

medications that disrupt thermoregulation, the mentally ill, those lacking access to air conditioning, those 

working or playing outdoors, and the socially isolated (Ebi et al., 2008; IPCC, 2007b). 

Ebi et al. (2008) report statistics from the Centers for Disease Control and Prevention (CDC, 2005a) that 

indicate exposure to excessive natural heat caused 4,780 deaths during the period 1979 to 2002 in the 

U.S., and that an additional 1,203 deaths had hyperthermia reported as a contributing factor. They state 

these numbers are underestimates of the total mortality associated with heat waves because the person 

filling out the death certificate may not always list heat as a cause. 

Given projections for climate warming, heat-related morbidity and mortality are projected to increase 

globally (including in the U.S.) compared to a future with no climate change (Confalonieri et al, 2007). 

Heat exposures vary widely, and current studies do not quantify the years of life lost due to high 

temperatures. Estimates of heat-related mortality attributable to climate change are reduced but not 

eliminated when assumptions about acclimatization and adaptation are included in models. Confalionieri 

et al. (2007) cite a series of studies that suggests populations in the U.S. became less sensitive to high 

temperatures over the period 1964 to 1988, in part, due to these factors. On the other hand, growing 

numbers of older adults will increase the size of the population at risk because of a decreased ability to 

thermo-regulate is a normal part of the aging process (Confalonieri et al, 2007). In addition, according to 

a study in Confalonieri et al. (2007), almost all the growth in population in the next 50 years is expected 

to occur in cities where temperatures tend to be higher due to the urban heat island32 effect increasing the 

total number of people at risk of adverse health outcomes from heat. In other words, non-climatic factors 

related to demographics will have a significant influence on future heat-related mortality. 

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). Severe heat waves are projected to intensify in magnitude and duration over the 

portions of the U.S. where these events already occur (high confidence). The IPCC documents the 

following U.S. regional scenario projections of increases in heat and/or heat-related effects (Confalonieri 

et al, 2007; Field et al., 2007): 

31 According to IPCC terminology, “very likely” conveys a 90 to 99% probability of occurrence. See Box 1.3 on

page 5 for a full description of IPCC’s uncertainty terms. 32 A heat island refers to urban air and surface temperatures that are higher than nearby rural areas. Many U.S. cities 

and suburbs have air temperatures up to 10°F (5.6°C) warmer than the surrounding natural land cover. 

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• By the 2080s, in Los Angeles, the number of heat wave days (at or above 32oC or 90oF) increases 4­

fold under the B1 emissions scenario (low growth) and 6-8-fold under A1FI emissions scenario (high 

growth). Annual number of heat-related deaths in Los Angeles increases from about 165 in the 1990s 

to 319 to 1,182 for a range of emissions scenarios. 

• Chicago is projected to experience 25% more frequent heat waves annually by the period spanning 

2080-2099 for a business-as-usual (A1B) emissions scenario. 

Reduced cold exposure 

Cold waves continue to pose health risks in northern latitudes in temperature regions where very low 

temperatures can be reached in a few hours and extend over long periods (Confalonieri et al, 2007). 

Accidental cold exposure occurs mainly outdoors, among socially deprived people (alcoholics, the 

homeless), workers, and the elderly in temperate and cold climates but cold waves also affect health in 

warmer climates (Confalonieri et al, 2007). Living in cold environments in polar regions is associated 

with a range of chronic conditions in the non-indigenous population with acute risk from frostbite and 

hypothermia (Confalonieri et al, 2007). In countries with populations well-adapted to cold conditions, 

cold waves can still cause substantial increases in mortality if electricity or heating systems fail 

(Confalonieri et al, 2007). 

Ebi et al. (2008) cite a study reporting that from 1979 to 2002, an average of 689 reported deaths per year 

(range 417-1,021) in the U.S., totaling 16,555 over the period, were attributed to exposure to excessive 

cold temperatures. 

The IPCC projects reduced human mortality from cold exposure through 2100 (Confalonieri et al, 2007). 

It is not clear whether reduced mortality from cold will be greater or less than increased heat-related 

mortality in the U.S. due to climate change. Projections of cold-related deaths, and the potential for 

decreasing their numbers due to warmer winters, can be overestimated unless they take into account the 

effects of season and influenza, which is not strongly associated with monthly winter temperature (Ebi et 

al., 2008; Confalonieri et al, 2007). 

Aggregated changes in heat and cold exposure 

The IPCC (2007) does not explicitly assess studies since the TAR which analyze changes in both heatand cold-related mortality in the U.S. in the observed climate or for different future climate scenarios. 

However, a study cited in Confalonieri et al. (2007) find existing mortality patterns in U.S. cities are 

relatively insensitive to temperature variability. In a future warming climate, this study projects some 

mortality could be reduced with a winter-dominant warming and mortality increase with pronounced 

summer warming. Given the paucity of recent literature on the subject and the challenges in estimating 

and projecting weather-related mortality, IPCC concludes additional research is needed to understand 

how the balance of heat-and cold-related deaths might change globally under different climate scenarios 

(Confalonieri et al, 2007). 

7(b) Extreme Events 

In addition to the direct effects of temperature on heat and cold-related mortality, projected trends in 

climate change-related exposures of importance to human health will increase the number of people 

(globally, including in the U.S.) suffering from disease and injury due to floods, storms, droughts and 

fires (high confidence) (Confalonieri et al, 2007). 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) (Ebi et al., 2008). 

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Floods and storms 

The IPCC projects a very likely increase in heavy precipitation event frequency over most areas as 

described in Sections 6(b and c). Increases in the frequency of heavy precipitation events are associated 

with increased risk of deaths and injuries 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 (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). 

The IPCC (2007d) also projects likely increases in intense tropical cyclone activity as described in 

Section 6(b). Increases in tropical cyclone intensity are linked to increases in the risk of deaths, injuries, 

water and foodborne 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 IPCC), is the major 

killer in coastal storms where there are large numbers of deaths (Confalonieri et al, 2007). 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. In 2005, Hurricane Katrina claimed 

over 1800 lives in the vicinity of the low-lying U.S. Gulf Coast and lower income groups were 

disproportionately affected (Graumann et al., 2005; Nicholls et al., 2007; Confalonieri et al., 2007). While 

Katrina was a Category 3 hurricane and its path was forecast well in advance, there was a secondary 

failure of the levee system. This illustrates that multiple factors contribute to making a disaster and that 

adaptation measures may not fully avert adverse consequences (Ebi et al., 2008). Additional information 

about U.S. vulnerability to the potential for more intense tropical cyclones can be found in Section 12(b). 

Droughts 

Areas affected by droughts are likely to increase according to IPCC (2007d) as noted in Section 6(e). The 

health impacts associated with drought tend to most affect semi-arid and arid regions, poor areas and 

populations, and areas with human-induced water scarcity; hence many of these effects are likely to be 

experienced in developing countries and not directly in the U.S. Information about the effects of 

increasing drought on U.S. agriculture can be found in section 9(c). 

Wildfires 

In some regions, changes in the mean and variability of temperature and precipitation are projected to 

increase the size and severity of fire events, including in parts of the U.S. (Easterling et al., 2007). 

Wildfires can increase eye and respiratory illnesses and injuries, including burns and smoke inhalation 

(Ebi, et al., 2008). A study cited in Confalonieri et al. (2007) indicates large fires are also accompanied by 

an increased number of patients seeking emergency services for inhalation of smoke and ash. The IPCC 

(Field et al., 2007) noted a number of observed changes in U.S. wildfire size and frequency. Additional 

information on the effects of forest fires can be found in Sections 8(b) and 10(b). 

7(c) Climate-sensitive diseases 

The IPCC (2007b) notes that many human diseases are sensitive to weather. The incidence of airborne 

infectious diseases (e.g., coccidioidomycosis) varies seasonally and annually, due partly to climate 

variations such as drought, which is projected to increase in the southwestern U.S. (Field et al., 2007; 

Karl et al., 2008). 

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Waterborne disease outbreaks are distinctly seasonal (which suggests potential underlying environmental 

or weather control), clustered in particular watersheds, and associated with heavy precipitation. The risk 

of infectious disease following flooding in high-income countries is generally low, although increases in 

respiratory and diarrheal diseases have been reported after floods (Confalonieri et al, 2007). For example, 

after Hurricanes Katrina and Rita in 2005, contamination of water supplies with fecal bacteria led to many 

cases of diarrheal illness and some deaths (Ebi et al., 2008; CDC, 2005b; Confalonieri et al, 2007). 

Foodborne diseases show some relationship with temperature (e.g., increased temperatures have been 

associated with increased cases of Salmonellosis) (Confalonieri et al, 2007). Vibrio spp. infections from 

shellfish consumption may also be influenced by temperature (Confalonieri et al, 2007). For example, 

Confalonieri et al. (2007) cited a study documenting a 2004 outbreak of V parahaemolyticus linked to 

atypically high temperatures in Alaskan coastal waters. 

According to the CCSP (2008b) report, 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. While the U.S. has 

successful programs to protect water quality under the Safe Drinking Water Act and the Clean Water Act, 

some contamination pathways and routes of exposure do not fall under regulatory programs (e.g., dermal 

absorption from floodwaters, swimming in lakes and ponds with elevated pathogen levels, etc.). 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 (CCSP 2008b). 

The sensitivity of many zoonotic33 diseases to climate fluctuations is also highlighted by the IPCC (Field 

et al., 2007). A study in Field et al. (2007) linked above average temperatures in the U.S. during the 

summers of 2002-2004 to the greatest transmissions of West Nile virus. Saint Louis encephalitis has a 

tendency to appear during hot, dry La Nina years according to a study cited in Field et al. (2007). 

Associations between temperature and precipitation and tick-borne Lyme disease are also noted by IPCC 

(Field et al., 2007). A study cited in Field et al. (2007) found that the northern range limit of Ixodes 

scapularis, the tick that carries Lyme disease, could shift north by 200 km by the 2020s and 1000 km by 

the 2080s. 

Although large portions of the U.S. may be at potential risk for diseases such as malaria based on the 

distribution of competent disease vectors, locally acquired cases have been virtually eliminated, in part 

due to effective public health interventions, including vector and disease control activities. (Ebi et al., 

2008; Confalonieri et al, 2007). 

The IPCC concludes that human health risks from climate change will be strongly modulated by changes 

in health care, infrastructure, technology, and accessibility to health care (Field et al., 2007). The aging of 

the population and patterns of immigration and/or emigration will also strongly influence risks (Field et 

al., 2007). 

7(d) Aeroallergens 

Climate change, including changes in CO2 concentrations, could impact the production, distribution, 

dispersion and allergenicity of aeroallergens and the growth and distribution of weeds, grasses and trees 

that produce them (McMichael. et al., 2001; Confalonieri et al., 2007). These changes in aeroallergens 

33A zoonotic disease is any infectious disease that is able to be transmitted from an animal or nonhuman species to 

humans. The natural reservoir is a nonhuman reservoir. 

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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 impact aeroallergens and subsequently the prevalence of allergenic illnesses in the U.S. In addition, 

there are numerous other factors that affect aeroallergen levels and the prevalence of associated allergenic 

illnesses, such as changes in land use, air pollution, and adaptive responses, many of which are difficult to 

assess (Ebi et al., 2008). 

It has generally been observed that the presence of elevated CO2 concentrations and temperatures 

stimulates plants to increase photosynthesis, biomass, water use efficiency, and reproductive effort. The 

IPCC concluded that pollens are likely to increase with elevated temperature and CO2 (Field et al., 2007). 

Laboratory studies cited by Field et al. (2007) stimulated ragweed-pollen production by over 50% using a 

doubling of CO2. A U.S.-based field study referenced by Field et al. (2007) 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. 

The IPCC (Confalonieri et al, 2007) noted that climate change has caused an earlier onset of the spring 

pollen season in North America 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 of the world (Confalonieri et al, 2007). 

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Section 8 

Air Quality 

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 direct and indirect 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. In summarizing the impact of climate 

change on ozone and particulate matter (PM), the IPCC (Denman et al., 2007) states that “future climate 

change may cause significant air quality degradation by changing the dispersion rate of pollutants, the 

chemical environment for ozone and PM generation and the strength of emissions from the biosphere, 

fires and dust.” 

This section describes how climate change may alter ambient concentrations of ozone and PM with 

associated impacts on public health and welfare in the U.S. 

8(a) Tropospheric Ozone 

According to the IPCC (Denman et al., 2007), climate change is expected to lead to increases in regional 

ozone pollution in the U.S. and other countries. Ozone impacts on pubic health and welfare are described 

in EPA’s Air Quality Criteria Document for Ozone (EPA, 2006). Breathing ozone at sufficient 

concentrations can reduce lung function, thereby aggravating asthma or other respiratory conditions. 

Ozone exposure at sufficient concentrations has been associated with increases in respiratory infection 

susceptibility, medicine use by asthmatics, emergency department visits and hospital admissions. Ozone 

exposure may contribute to premature death, especially in susceptible populations. In contrast to human 

health effects, which are associated with short-term exposures, the most significant ozone-induced plant 

effects (e.g., biomass loss, 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. 

Tropospheric ozone is both naturally occurring and, as the primary constituent of urban smog, a 

secondary pollutant formed through photochemical reactions involving nitrogen oxides (NOx) and 

volatile organic compounds (VOCs) in the presence of sunlight. As described below, climate change can 

affect ozone by modifying (1) emissions of precursors, (2) atmospheric chemistry, and (3) transport and 

removal (Denman et al., 2007). There is now consistent evidence from models and observations that 

21st-century climate change will worsen summertime surface ozone in polluted regions of North America 

compared to a future with no climate change (Jacob and Winner, 2009). 

The IPCC (Denman et al., 2007) states that, for all world regions, “climate change affects the sources of 

ozone precursors through physical response (lightning), biological response (soils, vegetation, and 

biomass burning) and human response (energy generation, land use, and agriculture).” Nitrogen oxides 

emissions due to lightning are expected to increase in a warmer climate (Denman et al., 2007). 

Additionally, studies using general circulation models concur that influx of ozone from the stratosphere to 

the troposphere could increase due to large-scale atmospheric circulation shifts (i.e., the Brewer-Dobson 

circulation) in response to climate warming (Denman et al., 2007). The sensitivity of microbial activity in 

soils to temperature also points toward a substantial increase in the nitric oxide emissions (Brasseur et al., 

2006). As described below, biogenic VOC emissions increase with increasing temperature. 

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Climate induced changes of biogenic VOC emissions alone may be regionally substantial and cause 

significant increases in ozone concentrations (Hauglustaine et al., 2005; Hogrefe et al., 2004; European 

Commission, 2003). Sensitivity simulations for the 2050s, relative to the 1990s suggest under the A2 

(high-end) climate scenario that increased biogenic emissions alone add 1–3 parts per billion (ppb) to 

summertime average daily maximum 8-hour ozone concentrations 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 (for the 2090-2100 timeframe, relative to 1990-2000). 

Consistent with this, for nearly all simulations in the EPA Interim Assessment (2009), climate change is 

associated with increases in biogenic VOC emissions over most of the U.S., with especially pronounced 

increases in the Southeast. These biogenic emissions increases do not necessarily correspond with ozone 

concentration increases, however. The report suggests that the response of ozone to changes in biogenic 

emissions depends on how isoprene chemistry is represented in the models—models that recycle isoprene 

nitrates back to NOx will tend to simulate significant O3 concentration increases in regions with biogenic 

emissions increases, while models that do not recycle isoprene nitrates will tend to simulate small 

changes, or even O3 decreases. 

Climate change impacts on temperature could affect ozone chemistry significantly (Denman et al., 2007). 

A number of studies in the U.S. 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 (1) temperature-dependent biogenic VOC emissions, 

as mentioned above, (2) thermal decomposition of peroxyacetylnitrate (PAN), which acts as a reservoir 

for NOx, as described immediately below, and (3) association of high temperatures with regional 

stagnation, also discussed below (Denman et al., 2007). 

The EPA Interim Assessment (2009), however, reports that considering a single meteorological variable, 

such as temperature, may not provide a sufficient basis for determining future ozone risks due to climate 

change in every region. This is consistent with the potential for different competing effects in different 

regions. The modeling studies found some regions of the country where simulated increases in cloud 

cover, and hence decreases in the amount of sunlight reaching the surface, partially counteracted the 

effects of warming temperatures on ozone concentrations in these regions, to go along with the many 

regions where the effects of temperature and cloud cover reinforced each other in producing O3 increases. 

Climate change is projected to increase surface layer ozone concentrations in both urban and polluted 

rural environments due to decomposition of PAN at higher temperatures (Sillman and Samson, 1995; 

Liao and Seinfeld, 2006). Warming enhances decomposition of PAN, releasing NOx, an important ozone 

precursor (Stevenson et al., 2005). Model simulations (using the high-end A2 emissions scenario) with 

higher temperatures for the year 2100 showed that enhanced PAN thermal decomposition caused this 

species to decrease by up to 50% over source regions and ozone net production to increase (Hauglustaine 

et al., 2005). 

Atmospheric circulation can be expected to change in a warming climate and, thus, modify pollutant 

transport and removal. The CCSP (2008b) reports that stagnant air masses related to climate change are 

likely to degrade air quality in some densely populated areas. More frequent occurrences 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 (2007d) 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 halfcentury.” 

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The IPCC (Denman et al., 2007) cites a study for the eastern U.S. 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 activity decreased by approximately 10– 

20% in a future simulation (for 2050, under the mid-range IPCC A1B scenario), in general agreement 

with a number of observational studies over the northern midlatitudes and North America. Northeast U.S. 

summer pollution episodes are projected in this study to increase in severity and duration; pollutant 

concentrations in episodes increase 5-10% and episode durations increase from 2 to 3-4 days. Analysis of 

historical data supports both the trend in decreasing frequency of ventilation and the increase in summer 

pollution episodes (Leibensperger et al., 2008). 

Regarding the role water vapor plays in tropospheric ozone formation, the IPCC (Denman et al., 2007) 

reports 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 

states 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) reports that surface ozone concentration may increase 

with a warmer climate. For the continental U.S., the CCSP (2008b) report states that the northern 

latitudes are likely to experience the largest increases in average temperatures and they will also bear the 

brunt of increases in ground-level ozone and other airborne pollutants. 

Modeling studies discussed in EPA’s Interim Assessment (2009) show that simulated climate change 

causes increases in summertime O3 concentrations over substantial regions of the country, though this 

was not uniform, and some areas showed little change or decreases. For those regions that showed 

climate-induced increases, the increase in Maximum Daily 8-hour Average O3 concentration, a key metric 

for regulating U.S. air quality, was in the range of 2-8 ppb, averaged over the summer season. The 

increases were substantially greater than this during the peak pollution episodes that tend to occur over a 

number of days each summer. While the results from the different research groups agreed on the above 

points, their modeling systems did not always simulate the same regional patterns of climate-induced O3 

changes across the U.S. Certain regions show greater agreement than others: for example, there is more 

agreement on climate-induced increases for the eastern half of the country than for the West. Parts of the 

Southeast also show strong disagreements across the modeling groups. Where climate-change-induced 

increases in ozone do occur, damaging effects on ecosystems, agriculture, and health are expected to be 

especially pronounced, due to increases in the frequency of extreme pollution events. 

The EPA Interim Assessment (2009) suggests that climate change effects on ozone grow continuously 

over time, with evidence for significant increases emerging as early as the 2020s. The EPA Interim 

Assessment (2009) and the IPCC (Field et al., 2007; Wilbanks et al., 2007) cite a study which evaluates 

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 A2 (high-end) scenario. 

Studies reviewed in EPA’s Interim Assessment (2009) and Jacob and Winner (2009) indicate the largest 

increases in ozone concentrations due to climate change occur during peak pollution events. Mickley et 

al. (2004) find that climate change projected to occur under the A1B (mid-range) scenario results in 

significant changes that occur at the high end of the pollutant concentration distribution (episodes) in the 

Midwest and Northeast between 2000 and 2050 given constant levels of criteria pollutant emissions. 

Using the A2 (high-end) emissions scenario, Hogrefe et al. (2004) find that while regional climate change 

in the eastern U.S. causes the summer average daily maximum 8-hour ozone concentrations to increase by 

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2.7, 4.2, and 5.0 ppb in 2020s, 2050s, and 2080s (compared to 1990s), regional climate changes causes 

the fourth highest summertime daily maximum 8-hour ozone concentrations to increase by 5.0, 6.4, and 

8.2 ppb for the 2020s, 2050s, and 2080s, respectively (compared to 1990s) (Hogrefe et al., 2004). The 

CCSP (2008b) also reports climate change is projected to have a much greater impact on extreme values 

and to shift the distribution of ozone concentrations towards higher values, with larger relative increases 

in future decades. In addition, simulations reviewed in the EPA Interim Assessment (2009) showed that, 

for parts of the country with a defined summertime O3 season, climate change expanded its duration into 

the fall and spring. These findings raise particular health concerns. 

The IPCC (Field et al., 2007) states that, “warming and climate extremes are likely to increase respiratory 

illness, including exposure to pollen and ozone.” And the IPCC further states that “severe heat waves, 

characterized by stagnant, warm air masses and consecutive nights with high minimum temperatures will 

intensify in magnitude and duration over the portions of the U.S. and Canada, where they already occur 

(high confidence) (Field et al., 2007).” Further, as described in CCSP (2008b), there is some evidence that 

combined effects of heat stress and air pollution may be greater than simple additive effects and historical 

data show relationships between mortality and temperature extremes. 

Holding population, dose-response characteristics, and pollution prevention measures constant, ozonerelated deaths from climate change in the New York City metropolitan area are projected to increase by 

approximately 4.5% from the 1990s to the 2050s (under the high-end IPCC A2 scenario) (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). For A2 scenario in the 2050s, Bell et al. (2007) report that the projected effects of 

climate change on ozone in 50 eastern U.S. cities increased the number of summer days exceeding the 8­

hour U.S. EPA standard by 68%. On average across the 50 cities, the summertime daily 8-h maximum 

increased 4.4 ppb. Elevated ozone levels correspond to approximately a 0.11% to 0.27% increase in daily 

total mortality. The largest ozone increases are estimated to occur in cities with present-day high 

pollution. 

As noted in CCSP (2008b), the influence of climate change on air quality will play out against a backdrop 

of ongoing regulatory control of both ozone and particulate matter (PM) that will shift the baseline 

concentrations of these two important pollutants. The range of plausible short-lived emissions projections 

is very large. For example, emission projections used in CCSP (2008d) and in the Fourth Assessment 

Report of the IPCC differ on whether black carbon particle and nitrogen oxides emissions trends continue 

to increase or decrease. Improvements in our ability to project social, economic and technological 

developments affecting future emissions are needed. However, most studies to date that have examined 

potential future climate change impacts on air quality isolate the climate effect by holding precursor air 

pollutant emissions constant over time. 

The IPCC reports (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.” 

The IPCC (Denman et al., 2007) also states that “there are major discrepancies with observed long-term 

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, elevated levels of tropospheric ozone have significant adverse effects 

on crop yields in the U.S. and other world regions, pasture and forest growth and species composition 

(Easterling et al., 2007). Furthermore, the effects of air pollution on plant function may indirectly affect 

carbon storage; recent research showed that tropospheric ozone resulted in significantly less enhancement 

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of carbon sequestration rates under elevated CO2, due to negative effects of ozone on biomass 

productivity and changes in litter chemistry (Easterling et al., 2007). 

8(b) Particulate Matter 

Particulate matter (PM) impacts on public health and welfare are described in EPA’s Air Quality Criteria 

Document for Particulate Matter (EPA, 2004). 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 PM and significant health problems, including: aggravated asthma; 

chronic bronchitis; reduced lung 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. 

The overall directional impact of climate change on PM levels in the U.S. remains uncertain (National 

Assessment, 2008), as too few data yet exist for PM to draw firm conclusions about the direction or 

magnitude of climate impacts (CCSP, 2008b). However, preliminary results of modeling analyses 

reported in the EPA Interim Assessment (2009) are listed below. These analyses show a range of 

increases and decreases in PM concentrations in different regions and for different component chemical 

species in the same region: 

1. Precipitation is a more important primary meteorological driver of PM than of O3, due to its role 

in removing PM from the atmosphere (wet deposition). Precipitation, however, is particularly 

difficult to model and shows greater disagreement across simulations than other variables. 

2. Aerosol chemical processes, especially those concerning the formation of organic aerosols, are 

not fully understood and therefore not well characterized in current regional air quality models. 

3. Preliminary simulation results suggest that, globally, PM generally decreases as a result of 

simulated climate change, due to increased atmospheric humidity and increased precipitation. 

4. Regionally, simulated 2050 climate change produces both increases and decreases in PM (on the 

order of a few percent), depending on region. For the U.S., the largest simulated increases are 

found in the Midwest and Northeast. 

5. This PM response reflects the combined climate change responses of the individual species that 

make up PM (e.g., sulfate, nitrate, ammonium, black carbon, organic carbon, etc.). Depending on 

the region, these individual responses can be in competing directions. 

6. Increase in wildfire frequency associated with a warmer climate has the potential to increase PM 

levels in certain regions. 

Further, Jacob and Winner (2009) summarize the current state of knowledge as: 

“The response of PM to climate change is more complicated than that for ozone because of the diversity 

of PM components, compensating effects, and general uncertainty in GCM projections of the future 

hydrological cycle. Observations show little useful correlation of PM with climate variables to guide 

inferences of the effect of climate change. Rising temperature is expected to have a mild negative effect 

on PM due to volatilization of semi-volatile components (nitrate, organic), partly compensated by 

increasing sulfate production. Increasing stagnation should cause PM to increase. Precipitation frequency, 

which largely determines PM loss, is expected to increase globally but to decrease in southern North 

America and southern Europe. PM is highly sensitive to mixing depths but there is no consensus among 

models on how these will respond to climate change... Increases in wildfires driven by climate change 

could significantly increase PM concentrations beyond the direct effect of changes in meteorological 

variables.” 

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PM and PM 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; thus, human induced 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 

climate-induced changes in plant species composition and biomass distributions. Denman et al. (2007) 

cite a study in which biogenic emissions rates are predicted to increase, on average across world regions, 

by 10% per 1°C increase in surface temperature. 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 forest 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; 

Confalonieri et al., 2007). The IPCC (Field et al., 2007) reported with very high confidence that in North 

America disturbances like wildfire are increasing and are likely to intensify in a warmer future with drier 

soils and longer growing seasons. 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 compared to a future with no climate change (CCSP, 2008b). 

Pollutants from forest fires can affect air quality for thousands of kilometers (Confalonieri et al., 2007). 

A study cited in Field et al. (2007) found that, in the last three decades, the wildfire season in the western 

U.S. has increased by 78 days, and burn durations of large fires have increased from 7.5 to 37.1 days, in 

response to a spring-summer warming of 0.87°C. It also found earlier spring snowmelt has led to longer 

growing seasons and drought, especially at higher elevations, where the increase in wildfire activity has 

been greatest. Analysis by the State of California suggests that large wildfires could become up to 55% 

more frequent in some areas toward the end of the century due to continued global warming (California 

Climate Change Center, 2006). 

PM chemistry is affected by changes in temperature brought about by climate change as follows. 

Temperature is one of the most important meteorological variables influencing air quality in urban 

atmospheres because it directly 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 (increases particulate concentrations) 

and increased equilibrium vapor pressures for semi-volatile particulate compounds (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. 

Denman et al., 2007 note that there has been less work on the sensitivity of aerosols to meteorological 

conditions. It cites a study that produces regional model simulations for Southern California on 

September 25, 1996 projecting decreases in 24-hour average PM2.5 concentrations with increasing 

temperatures for inland portions of the South Coast air basin, and projecting increases for coastal regions. 

In CCSP (2008b), using the NYCHP integrated model, PM2.5 concentrations are projected to increase 

with climate change, with the effects differing by component species, with sulfates and primary PM 

increasing markedly and with organic and nitrated components decreasing, mainly due to movement of 

these volatile species from the particulate to the gaseous phase. 

The transport and removal of PM is highly sensitive to winds and precipitation. Removal of PM from the 

atmosphere occurs mainly by wet deposition (NAS, 2005). 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 and Seinfeld, 2006). 

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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 

hydrological cycle (NAS, 2005). For models to simulate accurately the seasonally varying pattern of 

precipitation, they must correctly simulate a number of processes (e.g., evapotranspiration, condensation, 

transport) that are difficult to evaluate at a global scale (Randall et al., 2007). 

In 1997 (62 FR 38680), EPA concluded that particulate matter produces adverse effects on visibility, and 

that visibility impairment is experienced (though not necessarily attributed to climate change) throughout 

the U.S., in multi-state regions, urban areas, and remote Federal Class I areas34. Visibility impairment 

depends strongly on ambient relative humidity (NARSTO, 2004). Although surface specific humidity 

globally has generally increased after 1976 in close association with higher temperatures over both land 

and ocean, observations suggest that relative humidity has remained about the same overall, from the 

surface throughout the troposphere (Trenberth et al., 2007). Nevertheless, increases in PM due to 

increases in wildfires induced by climate change might increase visibility impairment. 

8(c) Health Effects due to CO2-Induced Increases in Tropospheric O3 and Particulate Matter 

In addition to the analyses described above of climate change impacts on air quality, one study 

specifically examined the more direct effect of CO2 on air pollution mortality. As described in the CCSP 

(2008b) report, using a coupled climate-air pollution three-dimensional model, a study compared the 

health effects of pre-industrial vs. present-day atmospheric concentrations of CO2. The results suggest 

that increasing concentrations of CO2 increased tropospheric ozone and PM2.5, which increased mortality 

by about 1.1% per degree temperature increase over the baseline rate; the study estimated that about 40% 

of the increase was due to ozone and the rest to particulate matter. The estimated mortality increase was 

higher in locations with poorer air quality. 

34 The Clean Air Act defines mandatory Federal Class I areas as certain national parks (over 6,000 acres), wilderness 

areas (over 5,000 acres), national memorial parks (over 5,000 acres), and international parks that were in existence 

as of August 7, 1977. 

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Section 9 

Food Production and Agriculture 

Food production and the agricultural sector within the U.S. are sensitive to short-term climate variability 

and long-term climate change. This section addresses how observed and projected climate change may 

affect U.S. food production and agriculture. Food production and agriculture here include crop yields and 

production, livestock production (e.g., milk and meat), freshwater fisheries, and key climate-sensitive 

issues for this sector including drought risk and pests and weeds. 

In addition to changes in average temperatures and precipitation patterns, this section also addresses how 

U.S. food production and agriculture may be affected directly by elevated CO2 levels, as well as the 

frequency and severity of extreme events, such as droughts and storms. Climate change-induced effects 

on tropospheric ozone levels and their impacts on agriculture are discussed briefly in Section 8 on Air 

Quality. 

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 CCSP report on U.S. agriculture (Backlund et al., 2008a) made the following general conclusions for 

the U.S.: 

• With increased CO2 and temperature, the life cycle of grain and oilseed crops will likely progress 

more rapidly. But, as temperature rises, these crops will increasingly begin to experience failure, 

especially if climate variability increases and precipitation lessens or becomes more variable. 

• The marketable yield of many horticultural crops – e.g., tomatoes, onions, fruits – is very likely to 

be more sensitive to climate change than grain and oilseed crops. 

• Climate change is likely to lead to a northern migration of weeds. Many weeds respond more 

positively to increasing CO2 than most cash crops, particularly C3 “invasive” weeds35. Recent 

research also suggests that glyphosate, the most widely used herbicide in the U.S., loses its 

efficacy on weeds grown at the increased CO2 levels likely in the coming decades. 

• 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. 

• Projected increases in temperature and a lengthening of the 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. 

35 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. 

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

Establishment of perennial herbaceous species is reducing soil water availability early in the 

growing season. Shifts in plant productivity and type 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). 

The IPCC (2007b) made the following general conclusion about food production and agriculture for 

North America: 

• Moderate climate change in the early decades of the century is projected to increase aggregate 

yields of rainfed 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 depend on 

highly utilized water resources [high confidence].

36 

9(a) Crop yields and productivity 

• Observational evidence shows that, over the last century, aggregate yields of major U.S. crops 

have been increasing (USDA, 2007; Field et al., 2007), with significant regional and temporal 

variation. Multiple factors contribute to these long term trends, including seed technology, use of 

fertilizers, management practices, and climate change (e.g., lengthening of the growing season). 

For projected climate change effects, the IPCC summary conclusion of net beneficial effects in the early 

decades in the U.S. under moderate climate change, with significant regional variation, is supported by a 

number of recent assessments for most major crops, and is consistent with the previous IPCC Third 

Assessment (2001) conclusion.37 Moderate climate change for temperate regions such as the U.S. is 

described as local increases in temperature of 1-3oC (~2-5oF), which may occur within the next few 

decades or past mid-century depending on scenario (see Section 6 for temperature projections). Increased 

average warming leads to an extended growing season, especially for northern regions of the U.S. 

Further warming, however, is projected to have increasingly negative impacts in all regions (meaning 

both temperate, including the U.S., and tropical regions of the world) (Easterling et al. 2007). 

The CCSP report on agriculture (Hatfield et al., 2008) provides further crop-specific detail about optimum 

temperatures in order to assess the effects of future climate change. Crops are characterized by an upper 

failure-point temperature at which pollination and grain-set processes fail. Considering these aspects, 

Hatfield et al., (2008) detail the following optimum mean temperatures for grain yields of the major 

agronomic crops: 18-22oC for maize, 22-24oC for soybean, 15oC for wheat, 23-26oC for rice, 25oC for 

sorghum, 25-26oC for cotton, 20-26oC for peanut, 23-24oC for dry bean, and 22-25oC for tomato. 

36 According to IPCC terminology, “high confidence” conveys an 8 out of 10 chance of being correct. See Box 1.3 

on page 5 for a full description of IPCC’s uncertainty terms. 37 The North America chapter from the IPCC Third Assessment Report (Cohen et al. 2007) concluded: “Food 

production is projected to benefit from a warmer climate, but there probably will be strong regional effects, with 

some areas in North America suffering significant loss of comparative advantage to other regions (high 

confidence).” 

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Given the variable responses of different crops to temperature (and other climatic) changes, and the fact 

that different areas of the country specialize in different crops and have different regional climates, the 

variable future climate change effects among regions and crops are important to consider. The southeastern U.S. may be more vulnerable to increases in average temperature than more northern regions due 

to pre-existing temperatures that are already relatively high. Likewise, certain crops that are currently 

near climate thresholds (e.g., wine grapes in California) are likely to experience decreases in yields, 

quality, or both, even under moderate climate change scenarios (Field et al., 2007). 

Without the benefit of CO2, the anticipated 1.2oC rise in temperature over the next 30 years (a baseline 

assumption assumed in the CCSP (Hatfield et al., 2008) report) is projected to decrease maize, wheat, 

sorghum, and dry bean yields by 4.0, 6.7, 9.4, and 8.6 percent, respectively, in their major production 

regions. For soybean, the 1.2oC temperature rise will increase yield 2.5 percent in the Midwest where 

temperatures during July, August, September average 22.5oC, but will decrease yield 3.5 percent in the 

South, where mean temperature during July, August, and September averages 26.7oC. Likewise, in the 

South, that same mean temperature will result in reduced rice, cotton, and peanut yields, which will decrease 12.0, 5.7, and 5.4 percent, respectively (Hatfield et al., 2008). An anticipated CO2 increase from 

380 to 440 ppm will increase maize and sorghum yield by only 1 percent, whereas the listed C3 crops will 

increase yield by 6.1 to 7.4 percent, except for cotton, which shows a 9.2 percent increase (Hatfield et al., 

2008). 

Changes in precipitation patterns will play a large role in determining the net impacts of climate change at 

the national and sub-national scales, where uncertainties about precipitation changes remain very large. 

The IPCC (Field et al., 2007) reviewed integrated assessment modeling studies exploring the interacting 

impacts of climate and economic factors on agriculture, water resources, and biome boundaries in the 

U.S. and concluded 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, 

as well as urban and ecological uses. The critical importance of specific agro-climatic events, such as last 

frost, also introduces uncertainty in future projections (Field et al., 2007). 

There is still uncertainty about the sensitivity of crop yields in the U.S. and other world regions to the 

direct effects of elevated CO2 levels. 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 CO2 through the FACE (Free Air CO2 Enrichment)38 experiments indicate that, at 

ambient CO2 concentrations of 550 ppm (approximately double the concentration from pre-industrial 

times), crop yields increase under unstressed conditions by 10-25% for C3 crops, and by 0-10% 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 CO2 response. 

Hatfield et al. (2008) provides further detail about individual crop species responses to elevated CO2 

concentrations and the interactive effects with other climate change factors. Overall, the benefits of CO2 

rise over the next 30 years are projected to mostly offset the negative effects of temperature for most C3 

crops except rice and bean, while the C4 crop yields are reduced by rising temperature because they have 

little response to the CO2 rise (Hatfield et al., 2008). Thus, according to Hatfield et al. (2008), the 30-year 

outlook for U.S. crop production is relatively neutral. However, the outlook for U.S. crop production over 

the next 100 years would not be as optimistic, if temperature continues to rise along with climbing CO2 

concentrations, because the C3 response to rising CO2 is reaching a saturating plateau, while the negative 

temperature effects will become progressively more severe (Hatfield et al., 2008). 

38 http://www.bnl.gov/face/ 

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There are continual changes in the genetic resources of crop varieties and horticultural crops that will 

provide increases in yield due to increased resistance to water and pest stresses. These need to be 

considered in any future assessments of the climatic impacts; however, the genetic modifications have not 

altered the basic temperature response or CO2 response of the biological system (Hatfield et al., 2008). 

Although horticultural crops (fruits, vegetables and nuts) account for more than 40 percent of total crop 

market value in the U.S. (2002 Census of Agriculture), there is relatively little information on their response to CO2, and few reliable crop simulation models for use in climate change assessments compared 

to that which is available for major grain and oilseed crops (Hatfield et al., 2008). The marketable yield of 

many horticultural crops is likely to be more sensitive to climate change than grain and oilseed crops 

because even short-term, minor environmental stresses can negatively affect visual and flavor quality 

(Hatfield et al., 2008). 

9(b) Irrigation requirements 

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. The 

increase in irrigation demand due to climate change is expected in the majority of world regions including 

the U.S. due to decreased rainfall in certain regions and/or increased evaporation arising from increased 

temperatures. Longer growing seasons may contribute to the increased irrigation demands as well. 

Hatfield et al. (2008) describe in the CCSP report studies that examined changes in irrigation requires for 

the U.S. under climate change scenarios. For corn, a study cited in Hatfield et al. (2008) calculated that 

by 2030 irrigation requirements will change from -1 (Lower Colorado Basin) to +451 percent (Lower 

Mississippi Basin), because of rainfall variation. Given the variation in the sizes and baseline irrigation 

requirements of U.S. basins, a representative figure for the overall U.S. increase in irrigation requirements 

is 64 percent if stomatal effects are ignored, or 35 percent if they are included. Similar calculations were 

made for alfalfa, for which overall irrigation requirements are predicted to increase 50 and 29 percent in 

the next 30 years in the cases of ignoring and including stomatal effects, respectively. These increases are 

more likely due to the decrease in rainfall during the growing season and the reduction in soil water 

availability. 

9(c) Climate variability and extreme events 

Weather events are a major factor in annual crop yield variation. The projected impacts of climate change 

often consider changes in average temperature and precipitation patterns alone, while not reflecting the 

potential for altered variability in events such as droughts and floods. The potential for these events to 

change in frequency and magnitude introduces a key uncertainty regarding future projections of changes 

in agricultural and food production due to 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 direct effects of elevated CO2, 

moderate temperature increases over the near term, and longer growing seasons. 

Drought events are already a frequent occurrence, especially in the western U.S. Vulnerability to 

extended drought is, according to 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. Though droughts occur more 

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frequently and intensely in the western part of the U.S., the east is not immune from droughts and 

attendant reductions in water supply, changes in water quality and ecosystem function, and challenges in 

allocation (Field et al., 2007). 

Average annual precipitation is projected to decrease in the southwestern U.S. but increase over the rest 

of North America (Christensen et al., 2007). 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. Increased runoff due to intense 

precipitation on crop fields and animal agriculture operations may result in an increased contribution of 

sediments, nutrients, pathogens, and pesticides in surface waters (Kundzewicz et al., 2007). 

One economic consequence of excessive rainfall is delayed spring planting, which jeopardizes profits for 

farmers paid a premium for early season production of high value horticultural crops such as melon, 

sweet corn, and tomatoes (Hatfield et al., 2008). Field flooding during the growing season causes crop 

losses associated with anoxia, increases susceptibility to root diseases, increases soil compaction (due to 

use of heavy farm equipment on wet soils), and causes more runoff and leaching of nutrients and 

agricultural chemicals into groundwater and surface water (Hatfield et al., 2008). 

9(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 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 U.S. have led to earlier insect spring activity and proliferation of some species (Easterling, et al., 

2007). 

The growth of many crops and weeds is being stimulated (Backlund et al., 2008a). Weeds generally 

respond more positively to increasing CO2 than most cash crops, particularly C3 invasive weeds; and 

while there are many weed species that have the C4 photosynthetic pathway and therefore show a smaller 

response to atmospheric CO2 relative to C3 crops, in most agronomic situations, crops are in competition 

with both C3 and C4 weeds (Backlund et al.,, 2008a). The IPCC (Easterling et al., 2007) concluded, with 

high confidence, that climate variability and change modify the risks of fires, and pest and pathogen 

outbreaks, with negative consequences for food, fiber and forestry across all world regions. 

Climate change is likely to lead to a northern migration of weeds (Backlund et al, 2008a). Recent research 

also suggests that glyphosate, the most widely used herbicide in the U.S., loses its efficacy on weeds 

grown at the increased CO2 levels likely in the coming decades (Backlund et al., 2008a). 

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 the spatial and temporal distribution of 

diseases (Backlund et al., 2008a). 

Most studies, however, 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 direct stimulatory effect of elevated CO2 on crop yields. Research on 

the combined effects of elevated CO2 and climate change on pests, weeds and disease is still insufficient 

for U.S. and world agriculture (Easterling et al., 2007). 

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9(e) Livestock 

Hatfield et al. (2008) describe how temperature changes and environmental stresses can result in declines 

in physical activity and an associated decline in eating and grazing activity (for ruminants and other 

herbivores), or elicit a panting or shivering response, which increases maintenance requirements of the 

animal and contributes to decreases in animal productivity. 

Climate change has the potential to influence livestock productivity in a number of ways. Elevated 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; and climate 

change may affect the spread of animal diseases. The IPCC has generated a number of new conclusions 

in this area compared to the Third Assessment Report in 2001. These conclusions (Easterling et al., 

2007), along with those from the more recent CCSP report (Hatfield et al., 2008) include: 

• 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 a changing climate; such protection is more frequently available for non-ruminants 

(e.g., swine and poultry). 

• Based on expected vegetation changes and known environmental effects on forage protein, 

carbohydrate, and fiber contents, both positive and negative changes in forage quality are possible 

as a result of atmospheric and climatic change. Elevated 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 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 needs 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. 

9(f) Freshwater and marine fisheries 

Freshwater 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, land-use change). 

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;39 

• Salmonid species have been affected by warming in U.S. streams; 

39 Arctic includes large regions of Alaska, and the Alaskan indigenous population makes up largest indigenous 

population of the Arctic (see ACIA, 2004). 

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• Success of adult spawning and survival of fry brook trout is 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 rise within species-specific tolerance ranges. 

Regarding the impacts of future climate change, IPCC concluded, with high confidence for North 

America, that cold-water 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 will likely 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 U.S., cold-water species will likely disappear from all but the deeper lakes, coolwater 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 impact fisheries in coastal and estuarine waters, although nonclimatic factors, such as overfishing and habitat loss and degradation, are already responsible for reducing 

fish stocks (Nichols 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 (Nichols et al., 2007). Increased storm intensity, temperature and 

salt water intrusion in coastal water bodies can also adversely impact coastal fisheries production. 

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Section 10 

Forestry 

This section addresses how climate change may affect forestry, including timber yields, wildfires and 

drought risk, forest composition, and pests in the U.S. 

. 

The CCSP report addressing forestry and land resources (Ryan et al., 2008) made the following general 

conclusions for the U.S.: 

• Climate change has very likely increased the size and number of forest fires, insect outbreaks, and 

tree mortality in the interior west, the Southwest, and Alaska, and will continue to do so. An increased 

frequency of disturbance is at least as important to ecosystem function as incremental changes in 

temperature, precipitation, atmospheric CO2, nitrogen deposition, and ozone pollution. Disturbances 

partially or completely change forest ecosystem structure and species composition, cause short-term 

productivity and carbon storage loss, allow better opportunities for invasive alien species to become 

established, and command more public and management attention and resources. 

For North America, the IPCC (Field et al., 2007) concluded: 

• Overall forest growth in North America will likely increase modestly (10-20%) as a result of 

extended growing seasons and elevated CO2 over the next century, but with important spatial and 

temporal variation (medium confidence).40 

• 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). Although recent climate 

trends have increased vegetation growth, continuing increases in disturbances are likely to limit 

carbon storage, facilitate invasive species, and disrupt ecosystem services. Over the 21st century, 

pressure for species to shift north and to higher elevations will 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. 

10(a) Forest Productivity 

Forestry productivity is known to be sensitive to changes in climate variables (e.g. temperature, radiation, 

precipitation, water vapor pressure in the air, and wind speed), as these affect a number of physical, 

chemical, and biological processes in forest systems (Easterling, et al., 2007). However, as noted in a 

CCSP report addressing the forest sector (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. 

For the U.S. as a whole, forest growth and productivity have been observed to change, in part due to 

observed climate change. Nitrogen deposition and warmer temperatures have very likely increased forest 

growth where water is not limiting (Ryan et al., 2008). The IPCC (Field et al., 2007 and references 

therein) outlines a number of studies demonstrating the observed connection between changes in U.S. 

forest growth and changes in climate variables: 

40 According to IPCC terminology, “medium confidence” conveys a 5 out of 10 chance of being correct. See Box 

1.3 on page 5 for a full description of IPCC’s uncertainty terms. 

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• Forest growth appears to be slowly accelerating (less than 1% per decade) in regions where tree 

growth has historically been limited by low temperatures and short growing seasons; 

• The length of the vegetation growing season has increased an average of 2 days per decade since 

1950 in the conterminous U.S., with most of the increase resulting from earlier spring warming; 

• Growth is slowing in areas subject to drought; 

• On dry south-facing slopes in Alaska, growth of white spruce has decreased over the last 90 years, 

due to increased drought stress; 

• In semi-arid forests of the south-western U.S., growth rates have decreased since 1895, correlated 

with drought from warming temperatures; 

• Mountain forests are increasingly encroached upon from adjacent lowlands, while simultaneously 

losing high altitude habitats due to warming (Fischlin et al., 2007). 

• In Colorado, aspen have advanced into the more cold-tolerant spruce-fir forests over the past 100 

years; 

• A combination of warmer temperatures and insect infestations has resulted in economically 

significant losses of forest resource base to spruce bark beetle in Alaska. 

Forest productivity gains may result through: (i) the direct stimulatory CO2 fertilization effect (although 

the magnitude of this effect remains uncertain over the long term and can be curtailed by other changing 

factors); (ii) warming in cold climates, given concomitant precipitation increases to compensate for 

possibly increasing water vapor pressure deficits; and (iii) precipitation increases under water limited 

conditions (Fischlin et al., 2007). 

New studies suggest that direct CO2 effects on tree growth may be lower than previously assumed 

(Easterling et al., 2007). Additionally, the initial increase in growth increments may be limited by 

competition, disturbance, air pollutants (primarily tropospheric ozone), nutrient limitations, ecological 

processes, and other factors, and the response is site- and species-specific (Easterling et al., 2007). 

Similarly, Ryan et al. (2008) stated that, where nutrients are not limiting, rising CO2 increases 

photosynthesis and wood production (with younger stands responding most strongly), but that on infertile 

soils the extra carbon from increased photosynthesis will be quickly respired. 

The general findings from a number of recent syntheses using data from the three American and one 

European CO2-enrichment FACE study sites show that North American forests will absorb more CO2 and 

might retain more carbon as atmospheric CO2 increases. The increase in the rate of carbon sequestration 

will be highest (mostly in wood) on nutrient-rich soils with no water limitation and will decrease with 

decreasing fertility and water supply. Several yet unresolved questions prevent a definitive assessment of 

the effect of elevated CO2 on other components of the carbon cycle in forest ecosystems (Ryan et al., 

2008). 

As with crop yields, ozone pollution will modify the effects of elevated CO2 and any changes in 

temperature and precipitation, but these multiple interactions are difficult to predict because they have 

been poorly studied (Ryan et al., 2008). Nitrogen deposition also plays a role. Nitrogen deposition and 

warmer temperatures have very likely increased forest growth where water is not limiting and will 

continue to do so in the near future. Nitrogen deposition has likely increased forest growth rates over 

large areas, and interacts positively to enhance the forest growth response to increasing CO2. These 

effects are expected to continue in the future as N deposition and rising CO2 continue. 

For the projected temperature increases over the next few decades, most studies support the conclusion 

that a modest warming of a few degrees Celsius will lead to greater tree growth in the U.S. There are 

many causes for this enhancement including direct physiological CO2 effects, a longer growing season, 

and potentially greater mineralization of soil nutrients. Because different species may respond somewhat 

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differently to warming, the competitive balance of species in forests may change. Trees will probably 

become established in formerly colder habitats (more northerly, higher altitude) than at present (Ryan et 

al., 2008). 

Productivity gains in one area can occur simultaneously with productivity losses in other areas. For a 

widespread species like lodgepole pine, a 3oC 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). 

Climate change is expected to increase California timber production by the 2020s because of stimulated 

growth in the standing forest. In the long run (up to 2100), these productivity gains were offset by 

reductions in productive area for softwoods growth. Risks of losses from Southern pine beetle likely 

depend on the seasonality of warming, with winter and spring warming leading to the greatest damage 

(Easterling et al., 2007 and references therein). 

According to studies reviewed by IPCC (Field et al., 2007), the effects of climate change, in the absence 

of dramatic increases in disturbance, on the potential for commercial harvest in the 2040s ranged from 

mixed for a low emissions scenario to positive for a high emissions scenario. The tendency for North 

American producers to suffer losses increases if climate change is accompanied by increased disturbance, 

with simulated losses averaging US$1-2 billion/year over the 21st century according to a 2005 study 

referenced in Field et al. (2007). 

U.S. forestry, in addition to experiencing direct climate change effects, may be indirectly affected by 

changing forest productivity in different regions of the world. Easterling et al. (2007) cite a study 

showing 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. 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. 

Climate change will also substantially impact other non-timber goods and services, such as seeds, nuts, 

hunting, resins, plants used in pharmaceutical and botanical medicine, and in the cosmetics industry; these 

impacts will vary significantly across world regions (Easterling et al., 2007). 

10(b) Wildfire and Drought Risk 

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. More 

prevalent forest fire disturbances have recently been observed in the U.S. and other world regions 

(Fischlin, et al., 2007). Wildfires and droughts, among other extreme events (e.g., hurricanes) that can 

cause forest damage, pose the largest threats over time to forest ecosystems. 

Several lines of evidence suggest that large, stand-replacing wildfires will likely increase in frequency 

over the next several decades because of climate warming (Ryan et al., 2008). General climate warming 

encourages wildfires by extending the summer period that dries fuels, promoting easier ignition and faster 

spread (Field et al., 2007). 

The IPCC (Field et al., 2007 and references therein) 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

/year (13,700 mi

2

/year) has burned in wildfires, almost 

twice the 1920-1980 average of about 13,000 km2/year (8,080 mi

2

/year); 

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• The forested area burned in the western U.S. from 1987-2003 is 6.7 times the area burned from 1970­

1986; 

• 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 U.S. has increased by 78 days, and burn 

durations of fires greater than 1,000 ha (2,470 acres) have increased from 7.5 to 37.1 days, in 

response to a spring/summer warming of 0.87°C (1.4°F); 

• Earlier spring snowmelt has led to longer growing seasons and drought, especially at higher 

elevations, where the increase in wildfire activity has been greatest; 

• In the south-western U.S., fire activity is correlated with El Niño-Southern Oscillation (ENSO) 

positive phases and higher Palmer Drought Severity Indices.41 Major fire years tend to follow the 

switching from El Niño to La Niña conditions due to buildup of material during wet years followed 

by desiccation during a dry year, whereas small fires are strongly associated directly with previous 

year drought. Furthermore, increased temperature in the future will likely extend fire seasons 

throughout the western United States, with more fires occurring earlier and later than is currently 

typical, and will increase the total area burned in some regions. 

Though fires and extreme events are not well represented in models, current climate modeling studies 

suggest that increased temperatures and longer growing seasons will elevate fire risk in connection with 

increased aridity. Some research identifies the possibility of a 10% increase in the seasonal severity of 

fire hazard over much of the U.S. under climate change (Easterling, et al., 2007). For Arctic regions, 

forest fires are expected to increase in frequency and intensity (ACIA, 2004). In California, the risk of 

increased wildfires as a result of climate change has been identified as a significant issue (California 

Energy Commission, 2006). 

10(c) Forest Composition 

Climate change and associated changes in disturbance regimes will cause shifts in the distributions of tree 

species and alter forest species composition. 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 sub-Arctic boundary, the tree-line has moved about 6 mi (10 km) 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 growing season is increasing by 3 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 species 

41 The Palmer Drought Severity Index is used by NOAA and uses a formula that includes temperature and rainfall to 

determine dryness. It is most effective in determining long-term drought. Positive PDSI indicates wet conditions, 

negative PDSI indicates dry conditions. 

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migration. Bioclimate modeling based on outputs from five general circulation models suggests increases 

in tree species richness in the Northwest and decreases in the Southwest on long time scales (millennia). 

Over the next century, however, even positive long-term species richness may lead to short-term 

decreases because species that are intolerant of local conditions may disappear relatively quickly while 

migration of new species into the area may be quite slow (Field, et al., 2007; Currie 2001). The Arctic 

Climate Impact Assessment (2004) also concluded that vegetation zones are projected to migrate 

northward, with forests encroaching on tundra and tundra encroaching on polar deserts. Limitations in 

amount and quality of soils are likely to hinder these poleward shifts. No experiments have assessed the 

effect of changes in precipitation on forest tree species composition (Ryan et al., 2008). 

10(d) Insects and Diseases 

Insects and diseases are a natural part of forested ecosystems and outbreaks often have complex causes. 

The effects of insects and diseases can vary from defoliation and retarded growth, to timber damage, to 

massive forest diebacks. Insect life cycles can be a factor in pest outbreaks; and insect lifecycles are 

sensitive to climate change. Many northern insects have a two-year life-cycle, and warmer winter 

temperatures allow a larger fraction of overwintering larvae to survive. Recently, spruce budworm in 

Alaska has completed its life cycle in one year, rather than the previously observed duration of two years 

(Field et al., 2007). Recent warming trends in the U.S. have led to earlier spring activity of insects and 

proliferation of some species, such as the mountain pine beetle (Easterling et al., 2007). During the 

1990s, Alaska’s Kenai Peninsula experienced an outbreak of spruce bark beetle over 16,000 km2 with 10­

20% tree mortality (Anisimov et al., 2007). Also following recent warming in Alaska, spruce budworm 

has reproduced farther north reaching problematic numbers (Anisimov et al., 2007). Climate change may 

indirectly affect insect outbreaks by affecting the overall health and productivity of trees. For example, 

susceptibility of trees to insects is increased when multi-year droughts degrade the trees’ ability to 

generate defensive chemicals (Field, et al., 2007). Warmer temperatures have already enhanced the 

opportunities for insect spread across the landscape in the U.S. and other world regions (Easterling et al., 

2007). 

The IPCC (Easterling et al., 2007) stated that modeling of future climate change impacts on insect and 

pathogen outbreaks remains limited. Nevertheless, the IPCC (Field et al., 2007) states 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. 

The CCSP report (Ryan et al., 2008) states that the ranges of the mountain pine beetle and southern pine 

beetle are projected to expand northward as a result of average temperature increases. Increased 

probability of spruce beetle outbreak as well as increase in climate suitability for mountain pine beetle 

attack in high-elevation ecosystems has also been projected in response to warming (Ryan et al., 2008). 

Climate change can shift the current boundaries of insects and pathogens and modify tree physiology and 

tree defense. An increase in climate extremes may also promote plant disease and pest outbreaks 

(Easterling et al., 2007). 

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Section 11 

Water Resources 

This section covers climate change effects on U.S. water supply, water quality, extreme events affecting 

water resources, and water uses. Information about observed trends as well as projected impacts is 

provided. 

The vulnerability of freshwater resources in the U.S. to climate change varies from region to region. 

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

reliance on past conditions as the basis for current and future planning will no longer be appropriate, as 

climate change increasingly creates conditions well outside of historical observations (Lettenmaier et al., 

2008). Examples of large U.S. water bodies where climate change raises a concern include the Great 

Lakes, Chesapeake Bay, Gulf of Mexico, and the Columbia River Basin. 

For North America, IPCC (Field et al., 2007) concluded: 

• Climate change will constrain North America’s over-allocated water resources, increasing 

competition among agricultural, municipal, industrial, and ecological uses (very high confidence)42 . 

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, lower levels 

are likely to exacerbate challenges relating to water quality, navigation, recreation, hydropower 

generation, water transfers, and bi-national relationships. 

11(a) Water Supply and Snowpack 

Surface Water and Snowpack 

The semi-humid conditions of the eastern U.S. transition to drier conditions in the west that are 

interrupted by the Rocky Mountains. The driest climates, however, exist in the Intermountain West and 

Southwest, becoming more humid as one proceeds west and north to more humid conditions on the 

upslope areas of the Cascade and coastal mountain ranges, especially in the Pacific Northwest 

(Lettenmaier et al., 2008). 

IPCC reviewed a number of studies showing trends in U.S. precipitation patterns, surface water supply, 

and snowpack, and how climate change may be contributing to some of these trends (Field et al., 2007): 

• Annual precipitation has increased throughout most of North America. 

• Streamflow in the eastern U.S. has increased 25% in the last 60 years, but has decreased by about 2% 

per decade in the central Rocky Mountain region over the last century. 

• Since 1950, stream discharge in both the Colorado and Columbia river basins has decreased. 

• In regions with winter snow, warming has shifted the magnitude and timing of hydrologic events. 

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 U.S. from 1949-2004. 

• Spring and summer snow cover has also decreased in the U.S. West. 

42 According to IPCC terminology, “very high confidence” conveys a 9 out of 10 chance of being correct. See Box 

1.3 on page 5 for a full description of IPCC’s uncertainty terms. 

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• Break-up of river and lake ice across North America advanced by 0.2 – 12.9 days over the last 100 

years. 

In the Arctic, precipitation has increased by about 8% on average over the past century. Much of the 

increase has fallen as rain, with the largest increases occurring in autumn and winter. Later freeze-up and 

earlier break-up of river and lake ice have combined to reduce the ice season by one to three weeks in 

some areas. Glaciers throughout North America are melting, and the particularly rapid retreat of Alaskan 

glaciers represents about half of the estimated loss of glacial mass worldwide (ACIA, 2004). Permafrost 

plays a large role in the hydrology of lakes and ponds. The spatial pattern of lake disappearance strongly 

suggests that permafrost thawing is driving the changes. These changes to Arctic precipitation, ice extent, 

and glacial abundance will affect key regional bio-physical systems, act as climatic feedbacks (primarily 

by changing surface albedo), and have socio-economic impacts (high confidence) (Anisimov et al., 2007). 

In regions including the Colorado River, Columbia River, and Ogallala Aquifer, surface and/or 

groundwater resources are intensively used and subject to competition from agricultural, municipal, 

industrial, and ecological needs. This increases the potential vulnerability to future changes in timing and 

availability of water (Field et al., 2007). 

Projections for the western mountains of the U.S. suggest that warming, and changes in the form, timing, 

and amount of precipitation will very likely (high confidence) lead to earlier melting and significant 

reductions in snowpack by the middle of the 21st century (Lettenmaier et al., 2008; 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 U.S. 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, 2006). 

Globally, current water management practices are very likely to be inadequate to reduce the negative 

impacts of climate change on water supply reliability, flood risk, and aquatic ecosystems (very high 

confidence) (Kundzewicz et al., 2007). Less reliable supplies of water are likely to create challenges for 

managing urban water systems as well as for industries that depend on large volumes of water. U.S. 

water managers currently anticipate local, regional, or state-wide water shortages over the next ten years. 

Threats to reliable supply are complicated by high population growth rates in western states where many 

resources are at or approaching full utilization. In Eastern North America, daily precipitation so heavy 

that it now occurs only once every 20 years is projected to occur approximately every eight years by the 

end of this century, under a mid-range emission scenario (CCSP, 2008i). Potential increases in heavy 

precipitation, with expanding impervious surfaces, could increase urban flood risks and create additional 

design challenges and costs for stormwater management (Field et al., 2007). The IPCC (Field et al., 2007 

and references therein) reviewed several regional-level studies on climate change impacts to U.S. water 

management which showed: 

• 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 (IPCC: high confidence). Atmosphere-lake interactions contribute to the 

uncertainty in assessing these impacts though. 

• 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 Nevadas and Colorado River 

basin. 

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• 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. 

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 10-20% by the 2070s, with greatest declines in 

spring (ACIA, 2004 and reference therein). 

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 lens43, which can result in prolonged drought impacts. 

Many islands in the Caribbean (which include 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). 

Groundwater 

Groundwater systems generally respond more slowly to climate change than surface water systems. 

Limited data on existing supplies of groundwater makes it difficult to understand and measure climate 

effects. In general, groundwater levels correlate most strongly with precipitation, but temperature 

becomes more important for shallow aquifers, especially during warm periods. In semi-arid and arid 

areas, groundwater resources are particularly vulnerable because precipitation and streamflow are 

concentrated over a few months, year-to-year variability is high, and deep groundwater wells or reservoirs 

generally do not exist (Kundzewicz et al., 2007). 

With climate change, availability of groundwater is likely to be influenced by changes in withdrawals 

(reflecting development, demand, and availability of other sources) and recharge (determined by 

temperature, timing, and amount of precipitation, and surface water interactions) (medium confidence). 

In general, simulated aquifer levels respond to changes in temperature, precipitation, and the level of 

withdrawal. According to 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). 

Projections suggest that efforts to offset declining surface water availability by increasing groundwater 

withdrawals will be hampered by decreases in groundwater recharge in some water-stressed regions, such 

as the southwest US. Vulnerability in these areas is also often exacerbated by the rapid increase of 

population and water demand (high confidence) (Kundzewicz et al., 2007). Projections for the Ogallala 

aquifer region suggest 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 and 

reference therein). 

43 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. 

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In addition, 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. For a discussion of these 

impacts, please see Section 12. 

11(b) Water Quality 

The IPCC concluded with high confidence that higher water temperatures, increased precipitation 

intensity, and longer periods of low flows exacerbate many forms of water pollution and can impact 

ecosystems, human health, and water system reliability and operating costs (Kundzewicz et al., 2007). A 

CCSP (2008e) report also acknowledges that water quality is sensitive to both increased water 

temperatures and changes in precipitation. 

Pollutants of concern in this case include sediment, nutrients, organic matter, pathogens, pesticides, salt, 

and thermal pollution (Kundzewicz et al., 2007). IPCC (Kundzewicz et al., 2007) reviewed several 

studies discussing the impacts of climate change on water quality that showed: 

• In lakes and reservoirs, climate change effects are primarily caused by water temperature variations. 

These variations can be caused by climate change or indirectly through increases in thermal pollution 

as a result of higher demand for cooling water in the energy sector. This affects, for the U.S. and all 

world regions, dissolved oxygen regimes, redox potentials44, lake stratification, mixing rates, and the 

development of aquatic biota, as they all depend on water temperature. Increasing water temperature 

affects the self-purification capacity of rivers by reducing the amount of dissolved oxygen available 

for biodegradation. 

• Water pollution problems are exacerbated during low flow conditions where small water quantities 

result in less dilution and greater concentrations of pollutants. 

• Heavy precipitation frequencies in the U.S. were at a minimum in the 1920s and 1930s, and 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 but these events also provide the pulse flow needed for some ecosystems. 

North American simulations of future surface and bottom water temperatures of lakes, reservoirs, rivers, 

and estuaries consistently increase, with summer surface temperatures exceeding 30°C in midwestern and 

southern lakes and reservoirs. 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 temperature and variations in runoff are likely to produce adverse changes in water quality 

affecting human health, ecosystems, and water uses. Elevated surface water temperatures will promote 

algal blooms and increases in bacteria and fungi levels. Increases in water temperature can also make 

some contaminants, such as ammonia (EPA, 1999), more toxic for some species and foster the growth of 

microbial pathogens in sources of drinking water. Warmer waters also transfer volatile and semi-volatile 

compounds (ammonia, mercury, PCBs, dioxins, pesticides) from surface water bodies to the atmosphere 

more rapidly (Kundzewicz et al., 2007). Although this transfer will improve water quality, this may have 

implications for air quality. 

44 Redox potential is defined as the tendency of a chemical species to acquire electrons and therefore be reduced. 

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Lowering of the water levels in rivers and lakes can lead to re-suspension of bottom sediments and 

liberating compounds, with negative effects on water supplies (Field et al., 2007 and references therein). 

These impacts may lead to a bad odor and taste in chlorinated drinking water and greater occurrence of 

toxins. More intense rainfall will lead to increases in suspended solids (turbidity) and pollutant levels in 

water bodies due to soil erosion (Kundzewicz et al., 2007). Moreover, even with enhanced phosphorus 

removal in wastewater treatment plants, algal growth in water bodies may increase with warming over the 

long term. Increasing nutrient and sediment loads due to more intense runoff events will negatively affect 

water quality, requiring additional treatment to render it suitable for drinking water. 

Climate change is likely to make it more difficult to achieve existing water quality goals for sediment 

(IPCC: high confidence) because hydrologic changes affect many geomorphic processes including soil 

erosion, slope stability, channel erosion, and sediment transport (Field et al., 2007). IPCC reviewed a 

number of region-specific studies on U.S. water quality and projected that: 

• 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 and reference therein). 

• Decreases in snowcover and increases in winter rain on bare soil will likely lengthen the erosion 

season and enhance erosion intensity. This will increase the potential for sediment related water 

quality impacts in agricultural areas without appropriate soil management techniques (Field et al., 

2007 and reference therein). All studies on soil erosion suggest that increased rainfall amounts and 

intensities will lead to greater rates of erosion, within the U.S. 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 Cornbelt) may not provide sufficient erosion protection against future 

intense precipitation and associated runoff (Field et al., 2007). 

11(c) Extreme Events 

There are a number of climatic and non-climatic drivers influencing flood and drought impacts. Whether 

risks are realized depends on several factors. Floods can be caused by 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 dikes, dams, and reservoirs, etc.) (Kundzewicz et al., 2007). 

Precipitation intensity will increase across the U.S., but particularly at mid and high latitudes where mean 

precipitation also increases. This will affect the risk of flash flooding and urban flooding (Kundzewicz 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. In general, projected changes in precipitation extremes are larger than changes in mean 

precipitation (Field et al., 2007). 

The socio-economic impacts of droughts arise from the interaction between climate, natural conditions, 

and human factors such as changes in land use. In dry areas, excessive water withdrawals from surface 

and groundwater sources can exacerbate the impacts of drought (Kundzewicz et al., 2007). Although 

drought has been more frequent and intense in the western part of the U.S., the East is also vulnerable to 

droughts and attendant reductions in water supply, changes in water quality and ecosystem function, and 

challenges in allocation (Field et al., 2007). 

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In addition to the effects on water supply, extreme events, such as floods and droughts, will likely reduce 

water quality. Increased erosion and runoff rates during flood events will wash pollutants (e.g., organic 

matter, fertilizers, pesticides, heavy metals) from soils into water bodies, with subsequent impacts to 

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). 

11(d) Implications for Water Uses 

There are many competing water uses in the U.S. that will be adversely impacted by climate change 

impacts to water supply and quality. The IPCC reviewed a number of studies describing the impacts of 

climate change on water uses in the U.S. which showed: 

• Decreased water supply and lower water levels are likely to exacerbate challenges relating to 

navigation in the U.S. (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 North Sea 

Route is projected to increase from the current 20-30 days per year to 90-100 days by 2080 (ACIA, 

2004 and references therein). 

• Climate change impacts to water supply and quality will affect agricultural practices, including the 

increase of irrigation demand in dry regions and the aggravation of non-point source water pollution 

problems in areas susceptible to intense rainfall events and flooding (Field et al., 2007). For more 

information on climate change impacts to agriculture, please see Section 9. 

• The U.S. energy sector, which relies heavily on water for generation (hydropower) and cooling 

capacity, will be adversely impacted by changes to water supply and quality in reservoirs and other 

water bodies (Wilbanks et al., 2007). For more information on climate change impacts to the energy 

sector, please see Section 13. 

• Climate-induced environmental changes (e.g., loss of glaciers, reduced river discharge in some 

regions, reduced snow fall in winter) will affect park tourism, winter sport activities, inland water 

sports (e.g., fishing, rafting, boating), and other recreational uses dependent upon precipitation (Field 

et al., 2007). While the North American tourism industry acknowledges the important influence of 

climate, its impacts have not been analyzed comprehensively. 

• Ecological uses of water could be adversely impacted by climate change. Temperature increases and 

changed precipitation patterns alter flow and flow timing. These changes will threaten aquatic 

ecosystems (Kundzewicz et. al., 2007). For more information, on climate change impacts on 

ecosystems and wildlife, please see Section 14. 

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Section 12 

Sea Level Rise and Coastal Areas 

This section of the document discusses areas in the U.S. vulnerable to sea level rise, associated 

interactions with coastal development, important coastal processes, observed and projected impacts, and 

how climate change effects on extreme events will impact coastal areas. Information on the observed and 

projected rates of sea level rise due to climate change can be found in Sections 4(g) and 6(c), respectively. 

Information on ocean acidification is discussed in Sections 4(l), 6(b), and 14(a). 

The IPCC (Field et al., 2007) concluded the following when considering how climate change may affect 

sea level rise and coastal areas in North America: 

• Coastal communities and habitats will be increasingly stressed by climate change impacts interacting 

with development and pollution (very high confidence).45 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. 

12(a) Vulnerable Areas 

Interaction with Coastal Zone Development 

Coastal population growth in deltas, barrier islands, and estuaries has led to widespread conversion of 

natural coastal landscapes to agriculture and aquaculture as well as industrial and residential uses. 

According to NOAA (Crossett et al., 2007), approximately 153 million people (53% of the total 

population) lived in the 673 US coastal counties46 in 2003. This represents an increase of 33 million 

people since 1980, and by 2008, the number was projected to rise to 160 million. This population growth, 

the rising value of coastal property, and the projected increases in storm intensity have increased the 

vulnerability of coastal areas to climate variability and future climate change (IPCC, 2007b). 

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 sealevel rise, increases in sea surface temperature, and possible increases in extreme weather events (Mimura 

et al., 2007). 

Although climate change is impacting coastal systems, non-climate human impacts have been more 

damaging over the past century. The major non-climate impacts for the U.S. and other world regions 

include drainage of coastal wetlands, resource extraction47, deforestation, introductions of invasive 

species, shoreline protection, and the discharge of sewage, fertilizers, and contaminants into coastal 

45 According to IPCC terminology, “very high confidence” conveys a 9 out of 10 chance of being correct. See Box 

1.3 on page 5 for a full description of IPCC’s uncertainty terms. 46 “Coastal county” is generally defined in NOAA reports as a county in which at least 15 percent of its total land

area is located within a coastal watershed. 47 Resource extraction activities in coastal areas include sand/coral mining, hydrocarbon production, and commercial 

and recreational fishing.

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waters (Nicholls et al., 2007). The cumulative effect of these non-climate, anthropogenic impacts 

increases the vulnerability of coastal systems to climate-related stressors. 

Coastal Processes 

Climate change and sea-level rise affect sediment transport in complex ways. Erosion and ecosystem loss 

is affecting many parts of the US coastline, but it remains unclear to what extent these losses result from 

climate change instead of land loss associated with relative sea-level rise due to subsidence and other 

human drivers (Nicholls et al., 2007). 

Coastal wetland loss is also being observed in the U.S. 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). For more information regarding climate change impacts to coral reefs, see Section 14. 

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 accretes48, 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 U.S. 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). 

Observed Changes 

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. Over the past century in the U.S., more than 50% 

of the original salt marsh habitat has been lost. In Mississippi and Texas, over half of the shorelines 

eroded at average rates of 2.6 to 3.1 m/yr since the 1970s, while 90% of the Louisiana shoreline eroded at 

a rate of 12.0 m/yr (Nicholls et al., 2007 and references therein). 

In the Great Lakes where sea level rise is not a concern, both extremely high and low water levels 

resulting from changes to the hydrological 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. 

48 The term ‘vertical accretion’ is defined as the accumulation of sediments and other materials in a wetland habitat 

that results in build-up of the land in a vertical direction. 

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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. 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 U.S. 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). 

If sea-level rise occurs over the next century at a rate consistent with the higher range of the 2007 IPCC 

scenarios (i.e., 50-60 cm rise in sea level by 2100), it is about as likely as not that some barrier island 

coasts in the mid-Atlantic region will cross a geomorphic threshold and experience significant changes. 

Such changes include more rapid landward migration or barrier island segmentation (Gutierrez et al., 

2009). 

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 reference therein). 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). It is virtually certain that those tidal wetlands already 

experiencing submergence by sea-level rise and associated high rates of loss will continue to lose area in 

the future due to both accelerated rates sea-level rise as well as changes in other environmental and 

climate drivers (Cahoon et al., 2009). Salt-marsh biodiversity is likely to decrease in north-eastern 

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 sealevel rise through vertical accretion. 

Climate change is likely to have a strong impact on saltwater intrusion into coastal sources of 

groundwater in the U.S. 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. 

Reduced groundwater recharge associated with decreases in precipitation and increased 

evapotranspiration49 will exacerbate sea level rise effects 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 

49 Evapotranspiration is defined as the total amount of evaporation from surface water bodies (e.g., lakes, rivers, 

reservoirs), soil, and plant transpiration. In this context, warmer temperatures brought on by climate change will 

drive greater levels of evapotranspiration. 

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mangrove ecosystems can withstand a range of salinities on a short term basis; however, they are unlikely 

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, storm surge attenuation), the loss of these areas 

could be significant (Kundzewicz et al., 2007). 

The vulnerable nature of coastal indigenous communities to climate change arises from their geographical 

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). Sea ice extent in the Arctic Ocean is expected to continue to decrease and may even disappear 

entirely during summer months in the coming decades. This reduction of sea ice increases extreme 

coastal erosion in Arctic Alaska, due to the increased exposure of the coastline to strong wave action 

(CCSP, 2008i). These effects, along with sea level rise, will accelerate the already high coastal erosion 

rates in permafrost-rich areas of Alaska’s coastline, thereby forcing the issue of relocation for threatened 

settlements. It has been estimated that relocating the village of Kivalina, AK to a nearby site would cost 

US$54 million (Anisimov et al., 2007). 

For small islands, some studies suggest that sea-level rise could reduce island size, particularly in the 

Pacific, 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. Island infrastructure tends to predominate in coastal locations. 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 socio-economic 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). 

12(b) Extreme Events 

Although increases in mean sea level over the 21st century and beyond will inundate unprotected, lowlying 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 20 acres of the State’s 

land was lost on the western shore of Chesapeake Bay in the wake of Tropical Storm Isabel, causing 

approximately $84,000,000 in damages to shoreline structures (NRC, 2006a and references therein). 

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). Higher sea level 

provides an elevated base for storm surges to build upon and diminishes the rate at which low-lying areas 

drain, thereby increasing the risk of flooding from rainstorms (CCSP, 2009b). 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 19-68 years, on average, by the 2050s, and to 4-60 years by the 2080s (Wilbanks et al., 

2007; and references therein). 

Additionally, some major urban centers in the U.S. are situated in low-lying flood plains. For example, 

areas of New Orleans and its vicinity are 1.5-3 meters below sea level. Considering the rate of subsidence 

and using a mid-range estimate of 480 millimeters sea-level rise by 2100, it is projected that this region 

could be 2.5 to 4.0 meters 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 meters without waves) could be six to seven 

meters above areas that were heavily populated in 2004 (Field et al., 2007 and references therein). 

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The IPCC discusses a number of other extreme event scenarios and observations with implications for 

coastal areas of the US (See also section 6(f) for a discussion of abrupt changes and sea level rise): 

• Very large sea-level rises that would result from widespread deglaciation of Greenland and West 

Antarctic ice sheets imply major changes in coastlines and ecosystems, and inundation of low-lying 

areas, with greatest effects in river deltas. Relocating populations, economic activity, and 

infrastructure would be costly and challenging (IPCC, 2007b). 

• Under El Niño conditions, high water levels combined with changes in winter storms along the 

Pacific coast have produced severe coastal flooding and storm impacts. In San Francisco, 140 years 

of tide-gauge data suggest an increase in severe winter storms since 1950 and some studies have 

detected accelerated coastal erosion (Field et al., 2007). 

• Recent winters with less ice in the Great Lakes and Gulf of St. Lawrence have increased coastal 

exposure to damage from winter storms (Field et al., 2007). 

• Recent severe tropical and extra-tropical storms demonstrate that North American urban centers with 

assumed high adaptive capacity remain vulnerable to extreme events (Field et al., 2007). 

Demand for waterfront property and building land in the U.S. continues to grow, increasing the value of 

property at risk. Of the $19 trillion value of all insured residential and commercial property in the US 

states exposed to North Atlantic hurricanes, $7.2 trillion (41%) is located in coastal counties50. 

According to a study referenced in Field et al. (2007), this economic value includes 79% of the property 

in Florida, 63% of property in New York, and 61% of the property in Connecticut. The devastating 

effects of hurricanes Ivan in 2004 and 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). 

50 “Coastal county” is generally defined in NOAA reports as a county in which at least 15 percent of its total land 

area is located within a coastal watershed. 

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Section 13 

Energy, Infrastructure and Settlements 

According to IPCC (Wilbanks et al., 2007), “[i]ndustries, 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.” 

Climate change is likely51 to affect U.S. energy use and energy production; physical infrastructures; 

institutional infrastructures; and will likely interact with and possibly exacerbate ongoing environmental 

change and environmental pressures in settlements (Wilbanks et al., 2007), particularly in Alaska where 

indigenous communities are facing major environmental and cultural impacts on their historic lifestyles 

(ACIA, 2004). The research evidence is relatively clear that climate warming will mean reductions in 

total U.S. heating requirements and increases in total cooling requirements for buildings. These changes 

will vary by region and by season, but they will affect household and business energy costs and their 

demands on energy supply institutions. In general, the changes imply increased demands for electricity, 

which supplies virtually all cooling energy services but only some heating services. Other effects on 

energy consumption are less clear (CCSP, 2007a). 

13(a) Heating and Cooling Requirements 

With climate warming, less heating is required for industrial, commercial, and residential buildings in the 

U.S., but more cooling is required, with changes varying by region and by season. Net energy demand at 

a national scale will be influenced by the structure of the energy supply. The main source of energy for 

cooling is electricity, while coal, oil, gas, biomass, and electricity are used for space heating. Regions 

with substantial requirements for both cooling and heating could find that net annual electricity demands 

increase while demands for other heating energy sources decline. Critical factors for the U.S. are the 

relative efficiency of space cooling in summer compared to space heating in winter, and the relative 

distribution of populations in colder northern or warmer southern regions. Seasonal variation in total 

demand is also important. In some cases, due to infrastructure limitations, peak demand could go beyond 

the maximum capacity of the electricity transmission system (Wilbanks et al., 2007). 

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). Generally 

speaking, the net effects of climate change in the U.S. on total energy demand are projected to amount to 

between perhaps a 5% increase and decrease in demand per 1oC in warming in buildings. Existing studies 

do not agree on whether there would be a net increase or decrease in energy consumption with changed 

climate because a variety of methodologies have been used (CCSP, 2007a). 

In California, if temperatures rise according to a high scenario range (8-10.5oF; ~4.5-5.6oC), annual 

electricity demand for air conditioning could increase by as much as 20% by the end of the century 

(assuming population remains unchanged and limited implementation of efficiency measures) (California 

51 According to IPCC terminology, “likely” conveys a 66 to 90% probability of occurrence. See Box 1.3 on page 5 

for a full description of IPCC’s uncertainty terms. 

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Energy Commission, 2006)52. In Alaska, there will be savings on heating costs: modeling has predicted a 

15% decline in the demand for heating energy in the populated parts of the Arctic and sub-Arctic and up 

to one month decrease in the duration of a period when heating is needed (Anisimov et al., 2007). 

Overall, both net delivered energy and net primary energy consumption increase or decrease only a few 

percent with a 1oC or 2oC warming; however, there is a robust result that, in the absence of an energy 

efficiency policy directed at space cooling, climate change would cause a significant increase in the 

demand for electricity in the U.S., which would require the building of additional electricity generation 

capacity (and probably transmission facilities) worth many billions of dollars (CCSP, 2007a). 

Beyond the general changes described above, general temperature increases can mean changes in energy 

consumption in key climate-sensitive sectors of the economy, such as transportation, construction, 

agriculture, and others. Furthermore, there may be increases in energy used to supply other resources for 

climate-sensitive processes, such as pumping water for irrigated agriculture and municipal uses (CENR, 

2008). 

13(b) Energy Production 

Climate change could affect U.S. energy production and supply (a) if extreme weather events become 

more intense, (b) where regions dependent on water supplies for hydropower and/or thermal power plant 

cooling face reductions or increases in water supplies, (c) where changed conditions affect facility siting 

decisions, and (d) where climatic conditions change (positively or negatively) for biomass, wind power, 

or solar energy production (Wilbanks et al., 2007; CCSP 2007a). 

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 U.S. has some vulnerability to climate variability. Renewable 

energy sources tend to be more sensitive to climate variables; but fossil energy production can also be 

adversely effected by air and water temperatures, and the thermoelectric cooling process that is critical to 

maintaining high electrical generation efficiencies also applies to nuclear energy. In addition, extreme 

weather events have adverse effects on energy production, distribution, and fuel transportation (CCSP, 

2007a). 

Fossil and Nuclear Energy 

Climate change impacts on U.S. electricity generation at fossil and nuclear power plants are likely to be 

similar. 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 are vulnerable to 

fluctuations in water supply. Regional scale changes would likely 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. Plant designs are flexible and new technologies for water reuse, heat rejection, and use of 

alternative water sources are being developed; but, at present, some impact—significant on a local level— 

can be foreseen (CCSP, 2007a). 

Renewable Energy 

52 Temperature projections for the State of California are based on IPCC global emission scenarios as discussed in 

Section 6(a). 

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Because renewable energy depends directly on ambient natural resources such as hydrological resources, 

wind patterns and intensity, and solar radiation, it is likely to be more sensitive to climate variability than 

fossil or nuclear energy systems that rely on geological stores. Renewable energy systems in the U.S. are 

also vulnerable to damage from extreme weather events (CCSP, 2007a). 

Hydropower generation is sensitive to the amount, timing, and geographical pattern of precipitation as 

well as temperature (rain or snow, timing of melting). Reduced stream flows are expected to jeopardize 

hydropower production in some areas of the U.S., whereas greater stream flows, depending on their 

timing, might be beneficial (Wilbanks et al., 2007). In California, where hydropower now comprises 

about 15% of in-state energy production, diminished snow melt 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 (5.5-8oF (~3.1-4.4oC) increase in California) and precipitation decreases by 10 to 

20%. However, future precipitation projections are quite uncertain so it is possible that precipitation may 

increase and expand hydropower generation (California Energy Commission, 2006). 

North American wind and solar resources are about as likely as not to increase (medium confidence). 

Studies to date project wind resources that are either unchanged by climate change, or reduced by 0-40%. 

Future changes in cloudiness could slightly increase the potential for solar energy in North America south 

of 60°N, but one study projected that increased cloudiness will likely decrease the output of photovoltaics 

by 0-20% (Field et al., 2007). 

Bioenergy potential is climate-sensitive through direct impacts 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 on regional impacts under current climate change 

modeling pose a significant challenge in evaluating medium to long-term impacts on renewable energy 

production (CCSP, 2007a). 

Energy Supply and Transmission 

Extreme weather events can threaten coastal energy infrastructures and electricity transmission and 

distribution infrastructures in the U.S. and other world regions (Wilbanks et al., 2007). Hurricanes in 

particular can have severe impacts on energy infrastructure. In 2004, hurricane Ivan destroyed seven Gulf 

of Mexico oil drilling platforms and damaged 102 pipelines, while hurricanes Katrina and Rita in 2005 

destroyed more than 100 platforms and damaged 558 pipelines (CCSP, 2007a). Though it is not possible 

to attribute the occurrence of any singular hurricane to climate change, projections of climate change 

suggest that extreme weather events are very likely to become more intense. If so, then the impacts of 

Katrina may be a possible indicator of the kinds of impacts that could manifest as a result of climate 

change (CCSP, 2007a). 

In addition to the direct effects on operating facilities themselves, U.S. networks for transport, electric 

transmission, and delivery would be susceptible to changes due to climate change in stream flow, annual 

and seasonal precipitation patterns, storm severity, and even temperature increases (e.g., pipelines 

handling supercritical fluids may be impacted by greater heat loads) (CCSP, 2007a). 

U.S. rail transportation lines, which transport approximately 2/3 of the coal to the nation’s power plants 

(CCSP, 2007a), often closely follow riverbeds. More severe rainstorms can lead to flooding of rivers 

which 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 

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production, any significant disruption to the transportation network has serious implications for the 

overall reliability of the grid as a whole. (CCSP, 2007a) 

In the Arctic, soil subsidence caused by the melting of permafrost is a risk to gas and oil pipelines, 

electrical transmission towers, and natural gas processing plants (Wilbanks et al., 2007). Along the 

Beaufort Sea in Alaska, climate impacts on oil and gas development in the region are likely to result in 

both financial benefits and costs in the future. For example, offshore oil exploration and production are 

likely 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). 

13(c) Infrastructure and Settlements 

Climate change vulnerabilities of industry, settlement and society are mainly related to extreme weather 

events rather than to gradual climate change. The significance of gradual climate change, e.g., increases 

in the mean temperature, lies mainly in changes in the intensity and frequency of extreme events, 

although gradual changes can also be associated with thresholds beyond which impacts become 

significant, such as in the capacities of infrastructures (Field et al., 2007). Such climate-related thresholds 

for human settlements in the U.S. are currently not well understood (Wilbanks et al., 2008). 

Extreme weather events could threaten U.S. coastal energy infrastructure and electricity transmission and 

distribution infrastructures. Moreover, soil subsidence caused by the melting of permafrost in the Arctic 

region is a risk to gas and oil pipelines, and electrical transmission towers. Vulnerabilities of industry, 

infrastructures, settlements and society to climate change are generally greater in certain high-risk 

locations, particularly coastal and riverine areas, and areas whose economies are closely linked with 

climate-sensitive resources, such as agricultural and forest product industries, water demands and tourism; 

these vulnerabilities tend to be localized but are often large and growing (high confidence) (Wilbanks et 

al., 2007). Additionally, infrastructures are often connected, meaning that an impact on one can also 

affect others. For example, an interruption in energy supply can increase heat stress for vulnerable 

populations (Wilbanks et al., 2008). 

A few studies have projected increasing vulnerability of U.S. infrastructure to extreme weather related to 

climate warming unless adaptation is effective (high confidence). Examples include the New York 

Metropolitan Region, the mid-Atlantic Region, and the urban transportation network of the Boston 

metropolitan area (Wilbanks et al., 2007). In Alaska, examples where infrastructure is projected to be at 

“moderate to high hazard” in the mid-21st century include Shishmaref, Nome, Barrow, the Dalton 

Highway, and the Alaska Railroad (Field et al., 2007). Where extreme weather events become more 

intense and/or more frequent with climate change, the economic and social costs of those events will 

increase (high confidence) (Wilbanks et al., 2007). 

Buildings and Construction 

In some Arctic areas, interactions between climate warming and inadequate engineering are causing 

problems. The weight of buildings on permafrost is an important factor; while many heavy, multi-story 

buildings of northern Russia have suffered structural failures, the lighter-weight buildings of North 

America have had fewer such problems as permafrost has warmed. Continuous repair and maintenance is 

also required for building on permafrost, a lesson learned because many of the buildings that failed were 

not properly maintained. The problems now being experienced in Russia may be expected to occur 

elsewhere in the Arctic if buildings are not designed and maintained to accommodate future warming 

(ACIA, 2004). 

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The cost of rehabilitating community infrastructure damaged by thawing permafrost could be significant. 

Even buildings designed specifically for permafrost environments may be subject to severe damage if 

design criteria are exceeded. The impervious nature of ice-rich permafrost has been relied on for 

contaminant holding facilities, and thawing such areas could result in severe contamination of 

hydrological resources and large cleanup costs, even for relatively small spills (Anisimov et al., 2007). 

The construction season in the northern U.S. likely will lengthen with warming. In permafrost areas in 

Alaska, increasing depth of the “active layer” or loss of permafrost can lead to substantial decreases in 

soil strength. Construction methods are likely to require changes in areas currently underlain by 

permafrost, potentially increasing construction and maintenance cost (high confidence) (Field et al., 

2007). 

Transportation 

In a 2008 report entitled, “Potential Impacts of Climate Change on U.S. Transportation,” the National 

Research Council (NRC) issued the following finding: 

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 (NRC, 2008). 

NRC states that transportation infrastructure was designed for typical weather patterns, reflecting local 

climate and incorporating assumptions about a reasonable range of temperatures and precipitation levels 

(NRC, 2008). An increase in the frequency, intensity, or duration of heat spells in the U.S. and other 

world regions could cause railroad tracks to buckle, and affect roads through softening and traffic-related 

rutting. Warmer or less snowy winters will likely reduce delays, improve ground and air transportation 

reliability, and decrease the need for winter road maintenance. More intense winter storms could, 

however, increase risk for traveler safety and require increased snow removal. Continuation of the 

declining fog trend in at least some parts of North America should benefit transport (Field et al., 2007). 

Warming will likely affect infrastructure for surface transport at high northern latitudes, such as Alaska. 

Permafrost degradation reduces surface bearing capacity and potentially triggers landslides. While the 

season for transport by barge is likely to be extended, the season for ice roads will likely be compressed. 

Other types of roads are likely to incur costly improvements in design and construction (Field et al., 

2007). 

Similarly, NRC found the following: 

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 (NRC, 2008). 

Because of warming, the number of days per year in which travel on the tundra is allowed under Alaska 

Department of Natural Resources standards has dropped from over 200 to about 100 in the past 30 years, 

resulting in a 50% reduction in days that oil and gas exploration and extraction can occur (ACIA, 2004). 

Forestry is another industry that requires frozen ground and rivers. Higher temperatures mean thinner ice 

on rivers and a longer period during which the ground is thawed. This leads to a shortened period during 

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which timber can be moved from forests to sawmills, and increasing problems associated with 

transporting wood (ACIA, 2004). 

Lakes and river ice have historically provided major, winter transportation routes and connections to 

smaller settlements in the Arctic. Reductions in ice thickness will reduce the load-bearing capacity, and 

shortening of the ice-season will shorten period of access. Where an open-water network is viable, it will 

be sensible to increase reliance on water transport. In land locked locations, construction of all-weather 

roads may be the only viable option, with implications for significantly increased costs. Similar issues 

will impact the use of sea ice roads primarily used to access offshore facilities (Anisimov et al., 2007). 

Loss of summer sea ice will bring an increasingly navigable Northwest Passage. Increased marine 

navigation and longer summers will improve conditions for tourism and travel associated with research 

(Anisimov et al., 2007). Along with rising water temperatures, however, increased shipping will also 

multiply the risk of marine pests and pollution (Anisimov et al., 2007). 

Negative impacts on transportation very likely will include coastal and riverine flooding and landslides. 

Although offset to some degree by fewer ice threats to navigation, reduced water depth in the Great Lakes 

would lead to “light loading” and adverse economic impacts (Field et al., 2007). 

Of all the possible impacts on transportation, the greatest in terms of cost is that of flooding. The costs of 

delays and lost trips would be relatively small compared with damage to the infrastructure and to other 

property (Wilbanks et al., 2007). 

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 will likely intensify many climate impacts, 

particularly in relation to storm damage (CCSP, 2008f). 

Since much of the land in the Gulf Coast is sinking, 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. The CCSP report found that relative sea level rise in the study area is very 

likely to increase by at least 0.3 m (1 ft) across the region and possibly as much as 2 m (6 to 7 ft) 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 (2 to 4 ft) 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 (2007) findings, which assume no major changes in ice-sheet dynamics (CCSP, 

2008f). 

Twenty-seven percent of the major roads, 9 percent of the rail lines, and 72 percent of the ports in the 

region are at or below 122 cm (4 ft) in elevation, although portions of the infrastructure are guarded by 

protective structures such as levees and dikes. These protective structures could mitigate some impacts, 

but considerable land area is still at risk to permanent flooding from rising tides, sinking land, and erosion 

during storms. 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 (CCSP, 2008f). 

A great deal of the Gulf Coast study area’s infrastructure is subject to temporary flooding associated with 

storm surge. More than half (64 percent of interstates; 57 percent 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 

based on the study of a 5.5- and 7.0-m (18- and 23-ft) storm surge (CCSP, 2008f). 

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Aviation may also be affected. 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. Some of these risks are expected to be offset by improvements in technology 

and information systems (CENR, 2008). 

Settlements 

According to 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 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)”. 

Effects of climate change on human settlements in the U.S. are very likely to vary considerably according 

to location-specific vulnerabilities, with the most vulnerable areas likely to include: Alaska, flood-risk 

coastal zones and river basins, arid areas with associated water scarcity and areas where the economic 

base is climate sensitive (CCSP, 2007a). 

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, and 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). 

Communities in risk-prone U.S. regions have reason to be particularly concerned about any potential 

increase in severe 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. Vulnerabilities may be especially great 

for rapidly-growing and/or larger metropolitan areas, where the potential magnitude of both impacts and 

coping requirements are likely to be very large. On the other hand, such regions have greater opportunity 

to put more adaptable infrastructure in place and make decisions that limit vulnerability (CCSP, 2007a). 

Climate change has the potential not only to affect U.S. communities directly but also through 

undermining their economic bases. In particular, some regional economies are dependent on sectors 

highly sensitive to changes in climate: agriculture, forestry, water resources, or tourism. 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 safety. As sources of 

stress grow and combine, the resilience of social and political structures are expected to suffer, especially 

in locales with relatively limited social and political capital (CCSP, 2007a). Additionally, as noted in 

Wilbanks et al. (2008), “Human settlements are the foci for many economic, social, and governmental 

processes, and historical experience has shown that catastrophes in cities can have significant economic, 

financial, and political effects much more broadly.” 

Within settlements experiencing climate change, certain parts of the population may be especially 

vulnerable. These include the poor, the elderly, those already in poor health, the disabled, those living 

alone, those with limited rights and power (such as recent immigrants with limited English skills), and/or 

indigenous populations dependent on one or a few resources. Environmental justice issues are clearly 

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raised through examples such as warmer temperatures in urban areas having a more direct impact on 

those without air-conditioning (Wilbanks et al., 2008). 

Finally, growth and development is generally moving toward areas more likely to be vulnerable to the 

effects of climate change. For example, approximately half of the U.S. population, 160 million people, 

will live in one of 673 coastal counties by 2008. Coastal residents – particularly those on gently-sloping 

coasts – should be concerned about sea level rise in the longer term, especially if these areas are subject to 

severe storms and storm surges and/or if their regions are showing gradual land subsidence. Areas that 

have been classified as highly vulnerable to climate change (based on measures of physical vulnerability 

and adaptive capacity) include counties lying along the east and west coasts and Great Lakes, with 

medium vulnerability counties mostly inland in the southeast, southwest, and northeast (CCSP, 2007a). 

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Section 14 

Ecosystems and Wildlife 

This section of the document covers: 1) ecosystem and species-level impacts due to climate change and 

elevated CO2 levels, 2) implications for ecosystem services, 3) how climate change effects on extreme 

event frequency and intensity may impact ecosystems, and 4) impacts to tourism and recreation. 

For North America, the IPCC (Field et al., 2007; Fischlin et al., 200753) concluded: 

• 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).

54 Although recent 

climate trends have increased vegetation growth, continuing increases in disturbances are likely to 

limit carbon storage, facilitate invasive species, and disrupt ecosystem services. 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. 

14(a) Ecosystems and Species 

Ecosystems, plants, and animals are sensitive to climate variability and always have been. Three clearly 

observable connections between climate and terrestrial ecosystems are the seasonal timing of life-cycle 

events (referred to as phenology), responses of plant growth or primary production, and the biogeographic 

distribution of species (see Figure 14.1). 

IPCC reviewed a number of studies describing observations of climate change effects on plant species: 

(Field, et al., 2007 and references therein): 

• Between 1981 and 2000, global daily satellite data indicate earlier onset of spring “greenness” by 10­

14 days, particularly across temperate latitudes of the Northern Hemisphere. Field studies conducted 

in the same areas confirm these satellite observations. 

o Leaves are expanding earlier (e.g., apple and grape plants--2 days/decade at 72 sites in 

Northeastern U.S.). 

o Flowering plants are blooming earlier (e.g., lilac - 1.8 days/decade earlier from 1959-1993, at 800 

sites across North America, honeysuckle -- 3.8 days/decade earlier in the western U.S.). 

• The timing of autumn leaf senescence55 across the continental US, which is controlled by a 

combination of temperature, photoperiod and water deficits, shows weaker trends. 

IPCC also discussed several studies showing how North American animals are responding to climate 

change, with effects on phenology, migration, reproduction, dormancy, and geographic range (Field, et 

al., 2007 and references therein): 

53 Fischlin et al., 2007 citation refers to Chapter 4, “Ecosystems, Their Properties, Goods, and Services” in IPCC’s 

2007 Fourth Assessment Report, Working Group II. 54 According to IPCC terminology, “very high confidence” conveys a 9 out of 10 chance of being correct. See Box 

1.3 on page 5 for a full description of IPCC’s uncertainty terms. 55 The term ‘senescence’ is defined as the last stage of leaf development that includes changes in pigment

expression, cell death, and eventual leaf drop. 

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• Warmer springs have led to earlier nesting for 28 migrating bird species on the East coast of the U.S. 

and to earlier egg laying for Mexican jays and tree swallows. 

• Several frog species now initiate breeding calls 10-13 days earlier than a century ago in the Upstate 

New York region. 

• In lowland California, 16 of 23 butterfly species advanced the date of first spring flights an average 

24 days over 31 years. 

• Reduced water depth, related to recent warming, in Oregon lakes has increased exposure of toad eggs 

to UV-B, leading to increased mortality from a fungal parasite. 

• The 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. 

Changes in phenology vary between species, and the life-cycles of plants, prey animals, and predators 

may shift out of sync, causing species to become decoupled from their resource requirements. For 

example, the decline of long-distance migratory birds in the United States may originate in mistiming of 

breeding and food abundance due to differences in phenological shifts in response to climate change 

(Scott et al., 2008). 

Many North American species, like the Edith’s checkerspot butterfly, have shifted their ranges, typically 

to the north or to higher elevations (Field, et al., 2007). Migrating to higher elevations with more suitable 

temperatures can be an effective strategy for species if habitat connectivity56 exists and other biotic and 

abiotic conditions are appropriate. However, many organisms cannot shift their ranges fast enough to 

keep up with the current pace of climate change (Fischlin et al., 2007). In addition, species that require 

higher-elevation habitat (e.g., alpine pikas), or assemblages for which no substrate may exist at higher 

latitudes (e.g., coral reefs), often have nowhere to migrate (Fischlin et al., 2007). Cold- and cool-water 

fisheries, especially salmonids, have been declining as warmer/drier conditions reduce their habitat (Field 

et al., 2007). 

The direct effects of elevated CO2 concentrations and climate change to marine ecosystems include ocean 

warming, increased thermal stratification, reduced upwelling, sea level rise, increased wave height and 

frequency, loss of sea ice, and decreases in the pH and carbonate ion concentration of the surface oceans 

(see Box 14.1). 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). Additional compounding effects, such as higher seawater 

temperatures leading to bleaching events, or higher seawater temperatures and nutrients leading to 

increased risk of diseases in marine biota will make these ecosystems even more vulnerable to changes in 

ocean chemistry along the U.S. and other world regions (Fischlin, et al., 2007). Subtropical and tropical 

coral reefs in shallow waters have already suffered major bleaching events that are clearly driven by 

increases in sea surface temperatures (Janetos et al., 2008). The effects of various other stressors, 

particularly human impacts such as overfishing, pollution, and the introduction of invasive species, appear 

to be exacerbating the thermal stresses on reef systems and, at least on a local scale, exceeding the 

thresholds beyond which coral is replaced by other organisms (Nicholls, et al., 2007). 

In the Bering Sea along the Alaskan coast, rising air and sea water temperatures have caused reductions in 

sea-ice cover and primary productivity in benthic ecosystems57 (Anisimov et al., 2007). A change from 

Arctic to sub-Arctic conditions is happening with a northward movement of the pelagic-dominated 

56 Connectivity is defined as the degree to which a habitat is physically linked with other suitable areas for a 

particular species. 57 Benthic is defined as the deepest environment of a water body which usually includes the seabed or lake floor.

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marine ecosystem that was previously confined to the Southeastern Bering Sea (Anisimov, et al., 2007). 

Climate-related impacts observed in the Bering Sea include 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). 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). See Box 14.1 for more information on 

potential climate change impacts to polar bears. 

Ecosystem Level Projections 

For terrestrial ecosystems across all world regions, the IPCC concluded that substantial changes in 

structure and functioning of terrestrial ecosystems are very likely to occur with a global warming greater 

than 2 to 3°C above pre-industrial levels (high confidence) (Fischlin, et al., 2007). 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). Figure 14.1 

shows the observed NPP trend in North America between 1981 and 1988. 

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Figure 14.1: North American Observations (Field et al., 2007) 

Observed trends in some biophysical and socio-economic indicators. Background: change in annual mean 

temperature from 1955 to 2005. Insets: (a) trend in April 1 SWE across western North America from 1925­

2002, with a linear fit from 1950-2002, (b) Spring bud-burst dates for trembling aspen in Edmonton since 

1900, (c) anomaly in 5-year mean area burned annually in wildfires in Canada since 1930, plus observed 

mean summer air temperature anomaly, weighted for fire areas, relative to 1920-1999, (d) relative sea level 

rise from 1850-2000 for Churchill, MB, Pointe-au-Père, QB, New York, NY, and Galveston, TX, (e) hurricane 

energy (PDI), economic damages, and deaths from Atlantic hurricanes since 1900, and, (f) trend North 

American NPP (Net Primary Productivity) from 1981 to 1998. The ten studies upon which the data of this 

figure is based are summarized and referenced in Field et al. (2007). 

At high latitudes, several models project longer growing seasons and increased net primary productivity 

(NPP) as a result of forest expansion into tundra ecosystems. In the mid latitudes, simulated changes in 

NPP are variable, depending on whether there is sufficient enhancement of precipitation to offset 

increased evapotranspiration in a warmer climate. By the end of the 21st century, ecosystems in the 

northeast and southeast U.S. are projected to become carbon sources, while the western U.S. remains a 

carbon sink (Field, et al., 2007). 

The areal extent of drought-limited ecosystems is projected to increase 11% per oC warming in the 

continental U.S. Climate change and direct human land-use pressures are both likely to have adverse 

impacts on desert ecosystems and species. Increases in plant productivity resulting from the direct effects 

of rising atmospheric CO2 concentrations 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 

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ecosystems and evergreen conifer forests into mixed deciduous forests (Fischlin, et al., 2007). Climate 

models suggest a warmer, drier future climate for the Prairie Pothole Region, which would result in a 

reduction in, or elimination of, wetlands that provide waterfowl breeding habitat (CCSP, 2009d). These 

types of regional impacts are indicative of the kinds of changes that can be expected across large parts of 

the country. 

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). 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, IPCC concluded that on a 

global scale (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). 

• Endemic species58 richness is highest where regional palaeoclimatic 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 (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. 

In North America, climate change impacts on inland aquatic ecosystems will range from the direct effects 

of increased temperature and CO2 concentration to indirect effects associated with alterations in 

hydrological systems resulting from changes to precipitation regimes and melting glaciers and snow pack 

(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 will likely lead to changes in 

community composition and nutrient flow in ponds (Fischlin et al., 2007). Many warm-water and coolwater (freshwater) fish species will shift their ranges northward and to higher altitudes. In the continental 

U.S., cold-water 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 (Field et al., 2007). See also section 9(f) for 

a discussion of climate change impacts to freshwater and marine fish populations. 

Bioclimate modeling based on output from five general circulation models (GCMs) suggests that on the 

long (millennial) timescale there may be decreases of bird and mammal species richness in warmer, low 

elevation areas, but increases in cold high elevation zones, and increases of reptile species richness in all 

areas. Over the next century, however, even positive long-term species richness may lead to short term 

decreases because species that are intolerant of local conditions may disappear relatively quickly while 

58 Endemic species are unique to their location or region and are not found anywhere else on Earth. 

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migration of new species into the area may be quite slow (Field et al., 2007; Currie, 2001). Changes in 

plant species composition in response to climate change can increase ecosystem vulnerability to other 

disturbances, including fire and biological invasion. There are other possible, and even probable, impacts 

and changes in biodiversity-related relationships (e.g., disruption of the interactions between pollinators, 

such as bees, and flowering plants), for which we do not yet have a substantial observational database 

(Janetos et al., 2008). 

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 IPCC, climate change (very high confidence) and ocean acidification (see Box 14.1) due to 

the direct 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). Average pH for the ocean surface is projected to decrease by up to 0.3-0.4 units by 2100 (Fischlin 

et al., 2007). These impacts could result in potentially severe ecological changes to tropical and 

coldwater marine ecosystems where carbonate-based phytoplankton and corals are the foundation for the 

trophic system (Schneider et al., 2007). The IPCC concluded that it is very likely that a projected future 

sea surface temperature increase of 1 to 3°C will result in more frequent bleaching events and widespread 

mortality, if there is not thermal adaptation or acclimatization by corals and their algal symbionts 

(Nicholls et al., 2007). 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 (Nicholls et 

al., 2007). 

Box 14.1: Ocean Acidification Effects on Marine Calcifiers 

Elevated atmospheric concentrations of greenhouse gases impact the health of marine calcifiers by changing the 

physical and chemical properties of the oceans. Calcifiers play important roles in marine ecosystems by serving as 

the base of food chains, providing substrate, and helping to regulate biogeochemical cycles (Fischlin et al., 2007). 

Ocean acidification lowers the saturation of calcium carbonate (CaCO3) in sea water, making it more difficult for 

marine calcifiers to build shells and skeletons (Fischlin et al., 2007). The IPCC (Denman et al., 2007) made the 

following statements regarding ocean acidification: 

• The biological production of corals, as well as calcifying photoplankton and zooplankton within the water 

column, may be inhibited or slowed down as a result of ocean acidification; 

• Cold-water corals are likely to show large reductions in geographic range this century. 

• The dissolution of CaCO3 at the ocean floor will be enhanced, making it difficult for benthic calcifiers to 

develop protective structures; 

• Acidification can influence the marine food web at higher trophic levels. 

The impacts of elevated CO2 concentrations on oceanic chemistry will likely be greater at higher latitudes (Fischlin 

et al., 2007). Polar and sub-polar surface waters and the Southern Ocean are projected to be aragonite (a form of 

CaCO3) under-saturated by 2100, and Arctic waters will be similarly threatened (Denman et al., 2007). These 

impacts will likely threaten ecosystem dynamics in these areas where marine calcifiers play dominant roles in the 

food web and in carbon cycling (Fischlin et al., 2007). 

The overall reaction of marine biological carbon cycling and ecosystems to a warm and high-CO2 world is not yet 

well understood. In addition, the response of marine biota to ocean acidification is not yet clear, both for the 

physiology of individual organisms and for ecosystem functioning as a whole (Denman et al., 2007). 

For the Arctic, IPCC (Anisimov et al., 2007 and references therein) concluded that: 

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• Decreases in the abundance of keystone species59 are expected to be the primary factor in causing 

ecological cascades60 and other changes to 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 – see Box 14.2) and protection from predators (e.g., snow rabbit 

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, sub-arctic species have been unable to live at higher latitudes because of harsh conditions. 

Climate change induced warming will increase the rate at which sub-arctic 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 shown 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. 

• Loss of sea ice will impact species, such as harp seals, which are dependent on it for survival. 

• Climate warming is likely to increase the incidence of pests, parasites, and diseases such as musk ox 

lung worm and abomasal nematodes of reindeer. 

Box 14.2: Polar bears (adapted from Box 4.3 in Fischlin, et al., 2007) 

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. 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 5 to 7 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. 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, Southern Beaufort Seas- ~1,500 individuals in 

2006) are vulnerable to large-scale dramatic seasonal fluctuations in ice movements because of the associated 

decreases in abundance and access to prey and increases in the energetic costs of 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; small whales – for protection 

from predators) face similar consequences, not only in the Arctic but also in the Antarctic (Fischlin, et al., 2007). 

In 2005, the World Conservation Union’s (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 May 2008, the U.S. Fish and Wildlife Service listed the polar bear as a threatened species under the 

Endangered Species Act. This decision was based on scientific evidence showing that sea ice loss threatens, and will 

likely continue to threaten, polar bear habitat (FWS, 2008). 

59 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. 60 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. 

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14(b) Ecosystem Services 

Ecosystems provide many goods and services that are of vital importance for biosphere function and 

provide the basis for the delivery of tangible benefits to humans. These services include: maintenance of 

biodiversity, nutrient regulation, shoreline protection, food and habitat provisioning, sediment control, 

carbon sequestration, regulation of the water cycle and water quality, protection of human health, and the 

production of raw materials (Fischlin et al., 2007). Climate change is projected to have an increasing 

effect on the provisioning of ecosystem services in the U.S. Increasing temperatures and shifting 

precipitation patterns, along with the direct 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 (Millennium Ecosystem Assessment-Synthesis, 2005). 

Many U.S. ecosystems and the services they provide are already threatened by natural and anthropogenic 

non-climate stressors. Climate-related effects on ecosystems services will amplify the effects of nonclimate stressors. Multiple U.S. industries, such as timber, fisheries, travel, tourism, and agriculture that 

are already threatened could face substantially greater impacts with concurrent effects on financial 

markets (Ryan et al., 2008; Field et al., 2007). 

14(c) Extreme Events 

Many significant impacts of climate change on U.S. ecosystems and wildlife may emerge through 

changes in the intensity and the frequency of extreme weather events. Extreme events, such as 

hurricanes, can cause mass mortality in wildlife populations and contribute significantly to alterations in 

species distribution and abundance following the disturbance. 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. Droughts play an important role in forest dynamics as 

well, causing pulses of tree mortality in the North American woodlands. Greater intensity and frequency 

of extreme events may alter disturbance regimes in North American coastal ecosystems leading to 

changes in diversity and ecosystem functioning (Field et al., 2007; Fischlin et al., 2007). Species 

inhabiting saltmarshes, mangroves, and coral reefs are likely to be particularly vulnerable to these effects 

(Fischlin et al., 2007). Higher temperatures, increased drought, and more intense thunderstorms will very 

likely increase erosion and promote invasion of exotic grass species in arid lands (Ryan et al., 2008). 

14(d) Implications for Tribes 

North American indigenous communities whose health, economic well-being, and cultural traditions 

depend upon the natural environment will likely be affected by the degradation of ecosystem goods and 

services associated with climate change (Field et al., 2007). Among the most climate-sensitive North 

American communities are those of indigenous populations dependent on one or a few natural resources. 

About 1.2 million (60%) of U.S. tribal members live on or near reservations, and many pursue lifestyles 

with a mix of traditional subsistence activities and wage labor (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, and 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. These livelihoods are already being threatened by multiple climate-related 

factors, including reduced or displaced populations of marine mammals, caribou, seabirds, and other 

wildlife, losses of forest resources due to insect damage, and reduced/thinner sea ice, making hunting 

more difficult and dangerous (ACIA, 2004). 

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14(e) Implications for Tourism 

The U.S. ranks among the top ten nations for international tourism receipts (US $112 billion), having 

domestic tourism and outdoor recreation markets that are several times larger than most other countries. 

Nature-based tourism is a major market segment in North America, with over 900 million visitor-days in 

national/provincial/state parks in 2001. 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 U.S. and parts of the 

Great Lakes. The ten-day closure and clean-up following Hurricane Georges (September 1998) resulted 

in tourism revenue losses of approximately US $32 million in the Florida Keys. While the North 

American tourism industry acknowledges the important influence of climate, its impacts have not been 

analyzed comprehensively (Field et al., 2007 and references therein). 

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Part V 

International Observed and Projected Human Health and Welfare 

Effects from Climate Change

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Section 15 

International Impacts 

The primary focus of this document is on the observed and potential future impacts associated with 

elevated GHG concentrations and associated climate change within the U.S. However, EPA has 

considered the global nature of climate change in at least two ways for purposes of this document. 

First, GHGs, once emitted, remain in the atmosphere for decades to centuries, and thus become, for all 

practical purposes, uniformly mixed in the atmosphere, meaning that U.S. emissions have climatic effects 

not only in the U.S. but in all parts of the world. Likewise, GHG emissions from other countries can 

influence the climate of the U.S., and therefore affect human health, society and the natural environment 

within the U.S. All observed and potential future climate change impacts within the U.S. reviewed in this 

document consider climate change driven by global anthropogenic GHG emissions. 

Second, despite widely discussed metrics such as global average temperature, climate change will 

manifest itself very differently in different parts of the world, where regional changes in temperatures and 

precipitation patterns, for example, can deviate significantly from changes in the global average. This 

regional variation in climate change, coupled with the fact that countries are in very different positions 

with respect to their vulnerability and adaptive capacity, means that the impacts of climate change will be 

experienced very differently in different parts of the world. In general, the relatively poor nations may 

experience the most severe impacts, due to their heavier reliance on climate-sensitive sectors such as 

agriculture and tourism, and due to their lack of resources for increasing resilience and adaptive capacity 

to climate change (see Parry et al., 2007). In addition to the fact that U.S. GHG emissions may contribute 

to these impacts (see Section 2 for a comparison of U.S. total and transportation emissions to other 

countries’ emissions), climate change impacts in certain regions of the world may exacerbate problems 

that raise humanitarian and national security issues for the U.S. 

15(a) National Security 

A number of analyses and publications, both inside and outside the government, have focused on the 

potential U.S. national security implications of climate change. 

A public report prepared for the Department of Defense (Schwartz and Randall, 2003) examined what the 

effects on U.S. national security might be from an abrupt climate change scenario.61 The authors 

concluded that the resultant climatic conditions could lead to resource constraints and potentially destabilize the global geo-political environment, with resultant national security concerns for the U.S. 

The Arctic Climate Impact Assessment (2004) raised security issues, stating that as Arctic sea ice 

declines, historically closed sea passages will open, thus raising questions regarding sovereignty over 

shipping routes and ocean resources. In IPCC (Anisimov, 2007), a study shows projections suggesting 

that by 2050, the Northern Sea Route will have 125 days per year with less than 75% sea-ice cover, which 

represents favorable conditions for navigation by ice-strengthened cargo ships. This may have 

implications for trade and tourism as well. 

61 The abrupt climate change used for the study was the unlikely, but plausible, collapse of the thermohaline 

circulation in the Atlantic, modeled after an event that occurred 8,200 years ago. 

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The Center for Naval Analyses (CNA) Corporation, a non-profit national security analysis institution, 

issued a report entitled National Security and the Threat of Climate Change (2007), in which a dozen 

retired generals and admirals were briefed by climate change scientists and business leaders over the 

course of many months and then tasked with providing their own views and recommendations about the 

linkage between climate change and national security over the next 30 to 40 years. Among their 

conclusions was that climate change acts as a “threat multiplier” for instability in some of the most 

volatile regions of the world. “Projected climate change will seriously exacerbate already marginal living 

standards in many Asian, African, and Middle Eastern nations, causing widespread political instability 

and the likelihood of failed states,” said the authors. Regarding the potential impact of climate change on 

military systems, infrastructure and operations, the report stated that climate change will stress the U.S. 

military by affecting weapons systems and platforms, bases, and military operations. A U.S. Navy (2001) 

study was cited which states that an ice-free Arctic will require an increased scope for naval operations. 

Given these concerns, one of the recommendations of the CNA (2007) report was for the Department of 

Defense to conduct an assessment of the impact on U.S. military installations worldwide of rising sea 

levels, extreme weather events, and other possible climate change impacts over the next 30 to 40 years. 

The U.S. Congress has expressed national security concerns due to climate change by requesting that the 

defense and intelligence communities examine these linkages. H.R. 4986, passed in January 2008, 

requires the Department of Defense to consider the effect of climate change on its facilities, capabilities 

and missions. Specific directives in the bill include that future national security strategies and national 

defense strategies must include guidance for military planners to assess the risks of projected climate 

change on current and future armed forces missions, as well as update defense plans based upon these 

assessments (H.R. 4986, 2008). 

In June 2008 testimony before the House, Dr. Thomas Fingar, Deputy Director of National Intelligence 

for Analysis, laid out a national intelligence statement on the U.S. national security implications from 

climate change projected out to 2030. Using a broad definition for national security,

62 the assessment 

found that: 

“[G]lobal climate change will have wide-ranging implications for U.S. national security interests 

over the next 20 years...We judge that the most significant impact for the United States will be 

indirect and result from climate-driven effects on many other countries and their potential to 

seriously affect U.S. national security interests. We assess that climate change alone is unlikely 

to trigger state failure in any state out to 2030, but the impacts will worsen existing problems— 

such as poverty, social tensions, environmental degradation, ineffectual leadership, and weak 

political institutions. Climate change could threaten domestic stability in some states, potentially 

contributing to intra- or, less likely, interstate conflict, particularly over access to increasingly 

scarce water resources.” (Fingar, 2008) 

Building on that work, the National Intelligence Council in November 2008, in its publication Global 

Trends 2025: A Transformed World, discussed climate change impacts prominently. The report posed a 

scenario named “October Surprise,” which discussed the economic and sociopolitical ramifications of an 

extreme flooding event linked to global climate change in New York City in 2020 (NIC, 2008). 

62 This definition considered if the effects would directly impact the U.S. homeland, a U.S. economic partner, or a 

U.S. ally. Additionally, the potential for humanitarian disaster was focused on as well as if an effect would result in 

degrading or enhancing an element of national power. For more information, see Fingar, 2008. 

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15(b) Overview of International Impacts 

The IPCC Working Group II volume of the Fourth Assessment Report reviews the potential impacts in 

different regions of the world. The IPCC (Parry et al., 2007) identifies as the most vulnerable regions: 

• The Arctic, because of high rates of projected warming on natural systems; 

• Africa, especially the sub-Saharan region, because of current low adaptive capacity as well as 

climate change; 

• Small islands, due to high exposure of population and infrastructure to risk of sea-level rise and 

increased storm surge; 

• Asian mega deltas, such as the Ganges-Brahmaputra and the Zhujiang, due to large populations 

and high exposure to sea level rise, storm surge and river flooding. 

Table 15.1 summarizes the vulnerabilities and projected impacts for different regions of the world, as 

identified by IPCC (2007b). And the paragraphs that follow provide some additional detail for key 

sectoral impacts that have received attention by the research community. There is currently a lack of 

information about how these potential impacts in other regions of the world may influence international 

trade and migration patterns, which in turn could raise concerns for the U.S. 

On a global basis, according to IPCC, “projected climate change-related exposures are likely to affect the 

health status of millions of people, particularly those with low adaptive capacity,” through several factors 

including “the increased frequency of cardio-respiratory diseases due to higher concentrations of ground 

level ozone related to climate change (IPCC, 2007b).” More specifically, “cities that currently experience 

heat waves are expected to be further challenged by an increased number, intensity and duration of heat 

waves during the course of the century, with potential for adverse health impacts.” 

Mosquito-borne diseases which are sensitive to climate change, such as dengue and malaria are of great 

importance globally. Studies cited in Confalonieri et al. (2007) have reported associations between 

spatial, temporal, or spatiotemporal patterns dengue and climate, although these are not entirely 

consistent. Similarly, the spatial distribution, intensity of transmission, and seasonality of malaria is 

observed to be influenced by climate in sub-Saharan Africa (Confalonieri et al., 2007). In other world 

regions (e.g., South America, continental regions of the Russian Federation) there is no clear evidence 

that malaria has been affected by climate change (Confalonieri et al., 2007). Changes in reporting, 

surveillance, disease control measures, population changes and other factors such as land use change must 

to be taken into account when attempting to attribute changes in human diseases to climate change 

(Confalonieri et al., 2007). 

Food production is expected to be much more vulnerable to climate change in poorer regions of the world 

compared to food production in the U.S. and other high, northern latitude regions. The IPCC (2007b) 

stated with medium confidence63 that, at lower latitudes, especially seasonally dry and tropical regions, 

crop productivity is projected to decrease for even small local temperature increases (1-2°C; ~2-3.5°F), 

which would increase risk of hunger. Furthermore, increases in the frequency of droughts and floods are 

projected to affect local production negatively, especially in subsistence sectors at low latitudes. Drought 

conditions, flooding, and pest outbreaks are some of the current stressors to food security that may be 

influenced by future climate change. Sub-Saharan Africa is currently highly vulnerable to food insecurity 

(Easterling et al., 2007). A study cited by Easterling et al. (2007) projected increases in carbon storage 

on croplands globally under climate change up to 2100, but found that ozone damage to crops could 

significantly offset these gains. 

63 According to IPCC terminology, “medium confidence” conveys a 5 out of 10 chance of being correct. See Box 

1.3 on page 5 for a full description of IPCC’s uncertainty terms. 

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Regarding global forest production, the IPCC (Easterling et al., 2007) concluded that forestry production 

is estimated to change modestly with climate change in the short and medium term (medium confidence). 

The projected change in global forest products output ranges from a modest increase to a slight decrease, 

with significant variations regionally. There is projected to be a production shift from low latitude 

regions in the short-term, to high latitude regions in the long-term. Projected changes in the frequency 

and severity of extreme climate events have significant consequences for forestry production in addition 

to impacts of projected mean climate (high confidence) (Easterling et al., 2007). Climate variability and 

change also modify the risks of fires, and pest and pathogen outbreaks, with negative consequences for 

forestry (high confidence) (Easterling et al., 2007). 

The IPCC made the following conclusions when considering how climate change may affect water 

resources across all world regions: 

• 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) (Kundzewicz et al., 2007). 

• All regions show an overall net negative impact of climate change on water resources and freshwater 

ecosystems (high confidence). 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 negative effects due 

to increased precipitation variability and seasonal runoff shifts on water supply, water quality, and 

flood risk (high confidence) (Kundzewicz et al., 2007). 

• Climate change affects the function and operation of existing water infrastructure as well as water 

management practices. Adverse effects of climate change on freshwater systems aggravate the 

impacts of other stresses, such as population growth, changing economic activity, land use change, 

and urbanization. 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 changes are likely. Current water management practices are very likely to be 

inadequate to reduce negative impacts of climate change on water supply reliability, flood risk, 

health, energy, and aquatic ecosystems (very high confidence) (Kundzewicz et al., 2007). 

• In polar regions, components of the terrestrial cryosphere and hydrology are increasingly being 

affected by climate change. Changes to cryospheric processes64 are also modifying seasonal runoff 

(very high confidence) (Anisimov et al., 2007). 

The IPCC (Nicholls et al., 2007) identified that coasts are experiencing the adverse consequences of 

hazards related to climate and sea level (very high confidence). They are highly vulnerable to extreme 

events, such as storms which impose substantial costs on coastal societies. Through the 20th century, 

global rise of sea level contributed to increased coastal inundation, erosion, and ecosystem losses, but 

with considerable local and regional variation due to other factors (Nicholls et al., 2007). 

The IPCC (Fischlin et al., 2007) recently made the following conclusions when considering how climate 

change may affect ecosystems across all world regions: 

• During the course of this century the resilience of many ecosystems is likely to be exceeded by an 

unprecedented combination of changes in climate and in other global change drivers (especially land 

use, pollution, and overexploitation), if greenhouse gas emissions and other changes continue at or 

above current rates (high confidence). The elevated CO2 levels and associated climatic changes will 

64 Cryospheric processes are defined to include the annual freezing and melting of snow cover, ice sheets, lake and 

river ice, permafrost, and sea ice. 

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alter ecosystem structure, reduce biodiversity, perturb functioning of most ecosystems, and 

compromise the services they currently provide (high confidence). Present and future land use change 

and associated landscape fragmentation are very likely to impede species’ migrations and geographic 

range shifts in response to changes in climate (very high confidence). 

• Ecosystems and species are very likely to show a wide range of vulnerabilities to climate change, 

depending on the extent to which climate change alters conditions that could cross critical, 

ecosystem-specific thresholds (very high confidence). The most vulnerable ecosystems include coral 

reefs, the sea ice biome and other high latitude ecosystems (e.g. boreal forests), mountain ecosystems 

and Mediterranean-climate ecosystems

65 (high confidence). Least vulnerable ecosystems include 

savannas and species–poor deserts, but this assessment is especially subject to uncertainty relating to 

the CO2 fertilization effect and disturbance regimes such as fire (low confidence). 

Table 15.1: Examples of key regional impacts as identified by IPCC (2007b)

* 

Africa • New studies confirm that Africa is one of the most vulnerable continents to climate 

variability and change because of multiple stresses and low adaptive capacity. Some 

adaptation to current climate variability is taking place; however, this may be insufficient for 

future changes in climate. 

• By 2020, between 75 million and 250 million people are projected to be exposed to 

increased water stress due to climate change. If coupled with increased demand, this will 

adversely affect livelihoods and exacerbate water-related problems. 

• Agricultural production, including access to food, in many countries and regions is projected 

to be severely compromised by climate variability and change. The area suitable for 

agriculture, the length of growing seasons and yield potential, particularly along the margins 

of semi-arid and arid areas, are expected to decrease. This would further adversely affect 

food security and exacerbate malnutrition in the continent. In some countries, yields from 

rain-fed agriculture could be reduced by up to 50% by 2020. 

Asia • Glacier melt in the Himalayas is projected to increase flooding, and rock avalanches from 

destabilized slopes, and to affect water resources within the next two to three decades. 

This will be followed by decreased river flows as the glaciers recede. 

• Freshwater availability in Central, South, East and South-East Asia, particularly in large 

river basins, is projected to decrease due to climate change which, along with population 

growth and increasing demand arising from higher standards of living, could adversely 

affect more than a billion people by the 2050s. 

• Coastal areas, especially heavily-populated mega delta regions in South, East and SouthEast Asia, will be at greatest risk due to increased flooding from the sea and, in some mega 

deltas, flooding from the rivers. 

• It is projected that crop yields could increase up to 20% in East and South-East Asia while 

they could decrease up to 30% in Central and South Asia by the mid-21st century. The risk 

of hunger is projected to remain very high in several developing countries. 

• Endemic morbidity and mortality due to diarrhea disease primarily associated with floods 

and droughts are expected to rise in East, South and South-East Asia due to projected 

changes in the hydrological cycle associated with global warming. Increases in coastal 

water temperature would exacerbate the abundance and/or toxicity of cholera in South 

Asia. 

65 Mediterranean climate ecosystems feature subtropical climate with dry summers. Despite the name, these 

ecosystems exist in the US along the coasts of central and southern California. 

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Latin 

America 

• By mid-century, increases in temperature and associated decreases in soil water are 

projected to lead to gradual replacement of tropical forest by savanna in eastern Amazonia. 

Semi-arid vegetation will tend to be replaced by arid-land vegetation. There is a risk of 

significant biodiversity loss through species extinction in many areas of tropical Latin 

America. 

• In drier areas, climate change is expected to lead to salinization and desertification of 

agricultural land. Productivity of some important crops is projected to decrease and 

livestock productivity to decline, with adverse consequences for food security. In temperate 

zones soybean yields are projected to increase. 

• Sea-level rise is projected to cause increased risk of flooding in low-lying areas. Increases 

in sea surface temperature due to climate change are projected to have adverse effects on 

Mesoamerican coral reefs, and cause shifts in the location of south-east Pacific fish stocks. 

• Changes in precipitation patterns and the disappearance of glaciers are projected to 

significantly affect water availability for human consumption, agriculture and energy 

generation. 

Polar 

Regions 

• For human communities in the Arctic, impacts, particularly those resulting from changing 

snow and ice conditions, are projected to be mixed. Detrimental impacts would include 

those on infrastructure and traditional indigenous ways of life. 

• Beneficial impacts would include reduced heating costs and more navigable northern sea 

routes. 

Small 

Islands 

• Small islands, whether located in the tropics or higher latitudes, have characteristics which 

make them especially vulnerable to the effects of climate change, sea-level rise and 

extreme events. 

• Deterioration in coastal conditions, for example through erosion of beaches and coral 

bleaching, is expected to affect local resources, e.g., fisheries, and reduce the value of 

these destinations for tourism. 

• Sea-level rise is expected to exacerbate inundation, storm surge, erosion and other coastal 

hazards, thus threatening vital infrastructure, settlements and facilities that support the 

livelihood of island communities. 

• Climate change is projected by mid-century to reduce water resources in many small 

islands, e.g., in the Caribbean and Pacific, to the point where they become insufficient to 

meet demand during low-rainfall periods. 

* With the exception of some very high confidence statements for small islands, all other IPCC conclusions within the 

table are of either high or medium confidence. 

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Appendix A: Brief Overview of Adaptation 

Adaptation to climate change is the adjustment in the behavior or nature of a system to the effects of 

climate change. In the process of developing information to support the Administrator’s decision 

regarding whether elevated combined greenhouse gas (GHG) concentrations endanger public health or 

welfare, various questions were raised about the relevance of adaptation. As noted in the Introduction, 

this document does not focus on adaptation because it (like GHG mitigation) is essentially a response to 

any known and/or perceived risks due to climate change. Although adaptation was not considered 

explicitly in the document, it does note where the underlying references already take into account certain 

assumptions about adaptation when projecting future risks and impacts due to climate change. This 

appendix provides a brief review of the state of knowledge pertaining to adaptation. 

What is Adaptation? 

As defined in the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (2007), 

Adaptation to climate change takes place through adjustments to reduce vulnerability or enhance 

resilience in response to observed or expected changes in climate and associated extreme weather 

events. Adaptation occurs in physical, ecological and human systems. It involves changes in social 

and environmental processes, perceptions of climate risk, practices and functions to reduce potential 

damages or to realise new opportunities. 

Adaptations vary according to the system in which they occur, who undertakes them, the climatic stimuli 

that prompts them, and their timing, functions, forms and effects. Adaptation can be of two broad types: 

• Reactive or autonomous adaptation is the process by which species and ecosystems respond to 

changed conditions. An example is the northward migration of a species in response to 

increasing temperature. 

• Anticipatory adaptation is planned and implemented before impacts of climate change are 

observed. An example is the construction of dikes in response to (and to prepare for) expected 

sea level rise. 

Summary of the Scientific Literature on Adaptation 

1. There is experience with adapting to weather, climate variability and the current and projected 

impacts of climate change. 

o There is a long record of practices to adapt to the impacts of weather as well as natural 

climate variability. These practices include proactive steps like water storage and crop and 

livelihood diversification, as well as reactive or ex-post steps like emergency response, 

disaster recovery and migration.66 

o The IPCC (2007) states – with very high confidence67 – that “Adaptation to climate change is 

already taking place, but on a limited basis.”68 

66 Adger et al. (2007), p. 720 

67 A set of terms to describe uncertainties in current knowledge was used throughout IPCC’s Fourth Assessment 

Report. On the basis of a comprehensive reading of the literature and their expert judgment, IPCC authors assigned 

a confidence level to major statements on the basis of their assessment of current knowledge, as follows: 

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o A wide array of adaptation options is available, ranging from purely technological (i.e., sea 

walls), through behavioral changes (i.e., altered food and recreational choices), to managerial 

(e.g. altered farm practices), and to policy (e.g. planning regulations).69 

o Some programs have developed strategic plans for responding to climate change. An 

example of such a plan is EPA’s National Water Program Strategy: Response to Climate 

Change (EPA, 2008). 

2. Although adaptation options are known, available and used in some places, there are 

significant barriers to their adoption. 

o The IPCC states with very high confidence that “there are substantial limits and barriers to 

adaptation.” These include formidable environmental, economic, informational, social, 

attitudinal and behavioral barriers to the implementation of adaptation that are not fully 

understood.70 The IPCC also states that there are significant knowledge gaps for adaptation 

as well as impediments to flows of knowledge and information relevant to adaptation 

decisions.71 

3. Current scientific information does not provide sufficient information to assess how effective 

current and future adaptation options will be at reducing vulnerability to the impacts of climate 

change. The fact that a country has a high capacity to adapt to climate change does not mean 

that its actions will be effective at reducing vulnerability. 

o While many technologies and adaptation strategies are known and developed in some 

countries, the available scientific literature does not indicate how effective various options are 

at fully reducing risks, particularly at higher levels of warming and related impacts, and for 

vulnerable groups.72 

o High adaptive capacity does not necessarily translate into actions that reduce vulnerability. 

For example, despite a high capacity to adapt to heat stress through relatively inexpensive 

adaptations, residents in urban areas in some parts of the world, including European cities, 

continue to experience high levels of mortality.

73 To minimize the risks of heat stress 

domestically, EPA (2006) has worked collaboratively with other government agencies to 

provide guidance to municipalities on steps they can take to reduce heat-related morbidity 

and mortality.

74 

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

High confidence About 8 out of 10 chance 

Medium confidence About 5 out of 10 chance 

Low confidence About 2 out of 10 chance 

Very low confidence Less than a 1 out of 10 chance 

68Adger et al. (2007), p. 720 69 ibid 70 ibid 71 Adger et al. (2007), p. 719 72 ibid 73 ibid 74 Excessive Heat Events Guidebook (2006) 

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o Further research is needed to monitor progress on adaptation, and to assess the direct as well 

as ancillary effects of adaptation measures.75 

4. For any country – even one with high adaptive capacity -- it is particularly difficult to reduce 

vulnerability for all segments of the population. The most vulnerable and difficult to reach 

populations are the elderly, children and the poor. 

o The IPCC states with very high confidence that “adaptive capacity is uneven across and 

within societies.” There are individuals and groups within all societies that have insufficient 

capacity to adapt to climate change.76 

5. More adaptation will be required to reduce vulnerability to climate change. 77 Additional 

adaptation can potentially reduce, but is never expected to completely eliminate vulnerability to 

current and future climate change. 

o According to the IPCC, “adaptation alone is not expected to cope with all the projected 

effects of climate change, and especially not over the long term as most impacts increase in 

magnitude.” 78 

6. A portfolio of adaptation and mitigation measures can diminish the risks associated with 

climate change. 

o Even the most stringent mitigation efforts cannot avoid further impacts of climate change in 

the next few decades, which makes adaptation essential, particularly in addressing near-term 

impacts. Unmitigated climate change would, in the long term, be likely to exceed the 

capacity of natural, managed and human systems to adapt.79 

75 Adger et al. (2007) p. 737 76 Adger et al. (2007), p. 719 77Adger et al. (2007), p. 719 78 IPCC (2007), p. 19 79 IPCC (2007), p. 20 

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References for Adaptation Appendix 

Adger, W.N., S. Agrawala, M.M.Q. Mirza, C. Conde, K. O’Brien, J. Pulhin, R. Pulwarty, B. Smit and K. Takahashi, 

(2007) Assessment of adaptation practices, options, constraints and capacity. Climate Change 2007: Impacts, 

Adaptation and Vulnerability. Contribution of Working Group II to the Fourth Assessment Report of the 

Intergovernmental Panel on Climate Change, M.L. Parry, O.F. Canziani, J.P. Palutikof, P.J. van der Linden and 

C.E. Hanson, Eds., Cambridge University Press, Cambridge, UK, 717-743. 

Excessive Heat Events Guidebook. Report from the United States Environmental Protection Agency. 52 pages. June 

2006. EPA 430-B-06-005. 

IPCC (2007) Summary for Policymakers. In: Climate Change 2007: Impacts, Adaptation and Vulnerability. 

Contribution of Working Group II to the Fourth Assessment Report of the Intergovernmental Panel on Climate 

Change [M.L. Parry, O.F. Canziani, J.P. Palutikof, P.J. van der Linden and C.E. Hanson (eds.)]. Cambridge 

University Press, Cambridge, United Kingdom and New York, NY, USA. 

U.S. EPA (2008) National Water Program Strategy: Response to Climate Change. Office of Water. EPA 800-R08001, September. 

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Greenhouse Gas Emissions from Section 202(a) Source Categories 

This Annex provides greenhouse gas (GHG) emissions information from Clean Air Act section 

202(a) source categories. It includes an overview of the respective source categories with a 

description of how the emissions data from the Inventory of U.S. Greenhouse Gas Emissions and 

Sinks map to these source categories. Then, relevant emissions data are presented and 

comparisons are made between U.S. GHG emissions from section 202(a) source categories and 

domestic and global emissions data. To inform the Administrator’s assessment, the following 

types of comparisons for both the collective and individual emissions of GHGs from section 

202(a) source categories are provided: 

• as a share of total global aggregate emissions of the six GHGs; 

• as a share of total U.S. aggregate emissions of the six GHGs; and 

• as a share of the total global transportation emissions of the six GHGs. 

In addition, for each individual GHG, the following comparisons were also calculated: 

• as a share of total U.S. section 202(a) GHG emissions; 

• as a share of U.S. emissions of that individual GHG, including comparisons to the 

magnitude of emissions of that GHG from non-transport related source categories; 

• as a share of global emissions of that individual GHG; 

• as a share of global transport GHG emissions; and 

• as a share of all global GHG emissions. 

(A) Overview of section 202(a) source categories 

To inform the Administrator’s cause or contribute finding, EPA analyzed historical GHG 

emissions data for motor vehicles and motor vehicle engines in the United States from 1990 to 

2006 (the most recent year for which official EPA estimates are available). The motor vehicles 

and motor vehicle engines addressed include: 

• Passenger cars 

• Light-duty trucks 

• Motorcycles 

• Buses 

• Medium/heavy-duty trucks 

• Cooling (all transportation sources) 

The source of the emissions data is the Inventory of U.S. Greenhouse Gas Emissions and Sinks: 

1990-2006 (USEPA #430-R-08-002). The U.S. Inventory is organized around the source 

classification scheme put forth by the Intergovernmental Panel on Climate Change, in which 

emissions from motor vehicles and motor vehicle engines are reported within two different 

sectors: Energy, and Industrial Processes. Table TSD-B.1 describes the correspondence 

between section 202(a) GHG emission source categories and IPCC source categories: 

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Table TSD-B.1 – Source categories included under section 202(a) 

section 202(a) Source 

Category 

IPCC Sector IPCC Source Category Greenhouse Gases 

Passenger Cars Energy 1A3b (i) Cars CO2, CH4, N2O 

Light-Duty Trucks Energy 1A3b (ii) Light-duty trucks CO2, CH4, N2O 

Motorcycles Energy 1A3b (iv) Motorcycles CO2, CH4, N2O 

Buses Energy 1A3b (iii) Heavy-duty trucks 

and buses 

CO2, CH4, N2O 

Medium/Heavy-Duty 

Trucks 

Energy 1A3b (iii) Heavy-duty trucks 

and buses 

CO2, CH4, N2O 

Cooling (all 

transportation 

sources) 

Industrial 

Processes 

2F1 Refrigeration and Air 

Conditioning Equipment 

Hydrofluorocarbons 

(HFCs) 

Greenhouse gas emissions from aviation, pipelines, railways, and marine transport are included 

in the IPCC Energy Sector under 1A3 but are not included within section 202(a). 

(B) Greenhouse gas emissions from section 202(a) source categories 

(1) Total, combined GHG emissions from section 202(a) source categories 

145
Table TSD-B.2 presents historical emissions of all GHGs (CO2, CH4, N2O, and HFCs) from section 202(a) s

1990-2006, in carbon dioxide equivalent units (Tg CO2e).80 Passenger cars (39.1 percent), light-duty truckmedium/heavy-duty trucks (24.2 percent) emitted the largest shares of GHG emissions in 2006, followed by coosources) (4.2 percent), buses (0.7 percent) and motorcycles (0.1 percent). From 1990 to 2006, GHG emissionsource categories grew by 35.2 percent due, in part, to increased demand for travel and the stagnation of fuel effivehicle fleet. Since the 1970s, the number of highway vehicles registered in the United States has increased population, according to the Federal Highway Administration (FHWA).81 Likewise, the number of miles driven 1990 to 2006) and the gallons of gasoline consumed each year in the United States have increased steadily since to the FHWA and Energy Information Administration, respectively.82 These increases in motor vehicle usaconfluence of factors including population growth, economic growth, urban sprawl, low fuel prices, and increasiutility vehicles and other light-duty trucks that tend to have lower fuel efficiency. Table TSD-B.2 – Total greenhouse gas emissions by section 202(a) source category (Tg CO2e) 

Section 202(a) Sources 1990 1995 2000 2001 2002 2003 2004 2005 200Passenger Cars 656.9 633.9 670.3 673.1 686.5 664.4 660.7 Light-Duty Trucks 336.2 428.6 489.5 492.7 502.6 536.2 556.9 Motorcycles 1.8 1.8 1.9 1.7 1.7 1.7 1.8 Buses 8.3 9.1 10.9 10.0 9.7 10.5 14.7 Medium/Heavy-Duty Trucks 228.8 272.4 342.9 342.0 356.2 352.6 365.4 Cooling (all trans. Sources) 0.0 18.6 52.6 57.2 61.1 64.4 67.8 

677.3 516.3 

1.6 

11.8 

393.2 

69.7 

651528

1

12

402

69

Total 1231.9 1364.4 1568.1 1576.8 1617.9 1629.7 1667.4 1670.0 1665

Between 1990 and 2006, GHG emissions from passenger cars decreased 0.9 percent, though there was semissions from 2000 to 2002, and again from 2004 to 2005. Emissions from light-duty trucks increased 57.1 perlargely due to the increased use of sport-utility vehicles and other light-duty trucks. Meanwhile, GHG emisstrucks increased 75.9 percent, reflecting the increased volume of total freight movement and an increasing share In 1990 there were no hydrofluorocarbons (HFCs) used in vehicle cooling systems. HFCs were gradually 80 A Tg is one teragram, or one million metric tons. 81 FHWA (1996 through 2008) Highway Statistics. Federal Highway Administration, U.S. Department of

Transportation, Washington, DC. Report FHWA-PL-96-023-annual. Available online at <http://www.fhwa.dot.gov/policy/ohpi/hss/hss

82 DOE (1993 through 2008) Transportation Energy Data Book. Office of Transportation Technologies, Center for Transportation Anal

Oak Ridge National Laboratory. ORNL-5198. 

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vehicle air conditioning and refrigerating systems during the 1990s as chlorofluorocarbons (CFCs) and hyd(HCFCs) started to phase out of production as required under the Montreal Protocol and Title VI of the Clean AiTable TSD-B.3 presents GHG emissions from section 202(a) source categories alongside total U.S. emissions. Temissions from the electricity generation and industrial sectors for comparison. In 1990, section 202(a) source cpercent of total U.S. emissions, behind the electricity generation sector (30.2 percent) and the industrial secto2006, section 202(a) source categories collectively were the second largest sector with 23.6 percent of total U.Sto growth in vehicle emissions and a decline in emissions from industry. Table TSD-B.3 – Sectoral comparison to total US greenhouse gas emissions (Tg CO2e) 

U.S. Emissions 1990 1995 2000 2001 2002 2003 2004 2005 200Section 202(a) GHG emissions 1231.9 1364.4 1568.1 1576.8 1617.9 1629.7

Share of U.S. (%) 20.0% 21.0% 22.3% 22.8% 23.2% 23.3% 

Electricity Sector emissions 1859.1 1989.7 2328.9 2290.9 2300.4 2329.4 

Share of U.S. (%) 30.2% 30.6% 33.1% 33.1% 33.0% 33.3% 

Industrial Sector emissions 1460.3 1478.0 1432.9 1384.3 1384.9 1375.5 

Share of U.S. (%) 23.8% 22.8% 20.4% 20.0% 19.8% 19.7% 

 1667.4 

23.6% 

2363.4 

33.4% 

1388.9 

19.6% 

1670.0 

23.4% 

2430.0 

34.1% 

1354.3 

19.0% 

1665

23.6% 

2377

33.7% 

1371

19.4% 

Total US GHG emissions 6148.3 6494.0 7032.6 6921.3 6981.2 6998.2 7078.0 7129.9 7054

Table TSD-B.4 compares total GHG emissions from section 202(a) source categories to all U.S. GHG ememissions from the transport sector (as defined by IPCC), and total global greenhouse gas emissions from all souyear 2005.83 Section 202(a) GHG emissions are a significantly larger share of global transport greenhouse percent) than the corresponding share of all U.S. GHG emissions to the global total (18.4 percent), reflecting ttransport sector in the U.S. compared to the global average. Section 202(a) GHG emissions were 4.3 percent of in 2005. The global transport sector was 15.3 percent of all global emissions in 2005. 83 The year 2005 is the most recent year for which comprehensive greenhouse gas emissions data are available for all gases, all countrie

estimates are ‘gross’ emissions estimates and do not include removals of greenhouse gas emissions from the atmosphere by terrestrial sbiomass). Global data come from the World Resources Institute’s Climate Analysis Indicators Tool, which contains national data submUNFCCC, and other independent and peer-reviewed datasets (e.g., International Energy Agency). 

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Table TSD-B.4 – Comparison to global greenhouse gas emissions (Tg CO2e) 

2005 Sec 202(a) Share 

All U.S. GHG emissions 7,130 23.4% Global transport GHG emissions 5,909 28.3% All global GHG emissions 38,726 4.3% 

(2) Individual GHG emissions from section 202(a) source categories Table TSD-B.5 presents total GHG emissions from section 202(a) source categories by gas, in CO2 equivalenmade up the largest share of emissions (93.9 percent), followed by HFCs (4.2 percent), N2O (1.8 percent) aSince 1990, the share of HFCs has increased (from zero in 1990), whereas the share of the other gases decreased. Methane and N2O emissions have decreased in absolute terms since 1990. Table TSD-B.5 - Greenhouse gas emissions from section 202(a) source categories by gas (Tg CO2e) 

Section 202(a) Sources 1990 1995 2000 2001 2002 2003 2004 2005 200CO2 1187.3 1291.9 1463.8 1470.5 1512.0 1524.2 1561.4 1565.6 1564

Share of Sec 202(a) GHGs 

CH4 

96.4% 

4.2 

94.7% 

3.8 

93.4% 

2.9 

93.3% 

2.8 

93.5% 

2.4 

93.5% 

2.2 

93.6% 

2.1 

93.8% 

1.9 

93.9% 

1

Share of Sec 202(a) GHGs 

N2O 

0.34% 

40.4 

0.28% 

50.1 

0.18% 

48.8 

0.18% 

46.4 

0.15% 

42.3 

0.14% 

38.9 

0.13% 

36.1 

0.12% 

32.7 

0.11% 

29

Share of Sec 202(a) GHGs 

HFCs 

3.3% 

0.0 

3.7% 

18.6 

3.1% 

52.6 

2.9% 

57.2 

2.6%

61.1 

 2.4% 

64.4 

2.2% 

67.8 

2.0% 

69.7 

1.8% 

69

Share of Sec 202(a) GHGs 0.0% 1.4% 3.4% 3.6% 3.8% 4.0% 4.1% 4.2% 4.2% 

Total GHGs 1231.9 1364.4 1568.1 1576.8 1617.9 1629.7 1667.4 1670.0 1665

(a) Carbon dioxide emissions from section 202(a) source categories Carbon dioxide is emitted from motor vehicles and motor vehicle engines during the fossil fuel combustcombustion, the carbon (C) stored in the fuels is oxidized and emitted as CO2 and smaller amounts of otheincluding CH4, carbon monoxide (CO), and non-methane volatile organic compounds (NMVOCs). These othCO2 gases are emitted as by-products of incomplete fuel combustion, but are, for the most part, eventually oatmosphere. 

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As the dominant GHG emitted from motor vehicles and motor vehicle engines (93.9 percent of total emissions in trends in Table TSD-B.6 mirror those of the GHG emissions total. Carbon dioxide emissions grew by 31.8 perc2006. Most of this growth occurred as a result of increased CO2 emissions from light-duty trucks (60.5 percenduty trucks (76.1 percent). Emissions from passenger cars did not grow over the same time period. Table TSD-B.6 – CO2 emissions by section 202(a) source category (Tg CO2) 

Sec. 202(a) Source Categories 1990 1995 2000 2001 2002 2003 2004 2005 200Passenger Cars 628.8 604.9 643.5 647.9 662.6 642.1 640.0 658.4 634Light-Duty Trucks 320.7 405.0 466.0 470.3 483.2 518.8 540.8 501.9 514Motorcycles 1.7 1.8 1.8 1.7 1.7 1.6 1.7 1.6 1Buses 8.3 9.0 10.9 10.0 9.6 10.5 14.7 11.8 12Medium/Heavy-Duty Trucks 227.8 271.2 341.5 340.6 354.8 351.2 364.1 391.9 401Cooling (all trans. Sources) N/A N/A N/A N/A N/A N/A N/A N/A N/Total 1187.3 1291.9 1463.8 1470.5 1512.0 1524.2 1561.4 1565.6 1564

Table TSD-B.7 presents CO2 emissions from section 202(a) source categories alongside total U.S. CO2 emispresents emissions from the electricity generation and industrial sectors for comparison. In 1990, section 20emitted 23.4 percent of total U.S. CO2 emissions, behind the electricity generation sector (36.1 percent), and asector (18.7 percent). By 2006, emissions from section 202(a) source categories increased to 26.2 percent of tota

Table TSD-B.7 - Sectoral comparison to total U.S. CO2 emissions (Tg CO2) 

U.S. CO2 Emissions 1990 1995 2000 2001 2002 2003 2004 2005 200Section 202(a) CO2 emissions 1187.3 1291.9 1463.8 1470.5 1512.0

Share of U.S. CO2 (%) 23.4% 23.9% 24.6% 25.2% 25.6% 

Electricity Sector CO2 1829.7 1964.6 2310.8 2273.0 2283.0

Share of U.S. CO2 (%) 36.1% 36.4% 38.9% 38.9% 38.6% 

Industrial Sector CO2 947.5 977.3 967.3 943.2 943.0 

Share of U.S. CO2 (%) 18.7% 18.1% 16.3% 16.1% 16.0% 

 1524.2

25.6% 

 2313.2

38.9% 

948.4 

15.9% 

 1561.4 

25.9% 

 2346.2 

38.9% 

974.7 

16.1% 

1565.6 

25.8% 

2412.3 

39.7% 

965.3 

15.9% 

1564

26.2

2360

39.4

984

16.4

Total U.S. CO2 emissions 5068.5 5394.2 5939.7 5846.2 5908.6 5952.7 6038.2 6074.3 5983

Table TSD-B.8 compares total CO2 emissions from section 202(a) source categories to total U.S. emissions, gfrom the transport sector (as defined by IPCC), and total global GHG emissions from all source categories, for t

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202(a) CO2 emissions are a significantly larger share of global transport greenhouse gas emissions (26.corresponding share of all U.S. CO2 emissions to the global total (22.0 percent), reflecting the relative size of the U.S. compared to the global average. Section 202(a) CO2 emissions were 4.0 percent of total global greenh2005. Table TSD-B.8 – Comparison to U.S. and global greenhouse gas emissions (Tg CO2e) 

Global Emissions 2005 Sec 202(a) CO2 Share All US GHG emissions All global CO2 emissions Global transport GHG emissions All global GHG emissions 

7,130 

27,526 

5,909 

38,726 

22.0% 

5.7% 

26.5% 

4.0% 

(b) Methane emissions from section 202(a) source categories Methane emissions from motor vehicles are a function of the CH4 and hydrocarbon content of the motor hydrocarbons passing uncombusted through the engine, and any post-combustion control of hydrocarbon emissconverters). Table TSD-B.9 shows the trend in CH4 emissions from section 202(a) source categories since 1990, presenequivalents. The combustion of gasoline in passenger cars and light-duty trucks was responsible for the majoritCH4 emitted from section 202(a) source categories. From 1990 to 2006, CH4 emissions decreased by 57.6 perceTable TSD-B.9 – CH4 emissions by section 202(a) source category (Tg CO2e) 

202(a) Sources 1990 1995 2000 2001 2002 2003 2004 2005 200Passenger Cars 2.6 2.1 1.6 1.5 1.4 Light-Duty Trucks 1.4 1.4 1.1 1.1 0.9 Motorcycles 0.0 0.0 0.0 0.0 0.0 Buses 0.0 0.0 0.0 0.0 0.0 Medium/Heavy-Duty Trucks 0.2 0.2 0.1 0.1 0.1 Cooling (all trans. Sources) N/A N/A N/A N/A N/A 

1.3 0.8 0.0 0.0 0.1 

N/A 

1.2 0.7 0.0 0.0 0.1 

N/A 

1.1 0.7 0.0 0.0 0.1 

N/A 

10000

N/

Total 4.2 3.8 2.9 2.8 2.4 2.2 2.1 1.9 1

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Table TSD-B.10 presents CH4 emissions from section 202(a) source categories alongside total U.S. CH4 emispresents CH4 emissions from landfills and natural gas systems for comparison. In 2006, section 202(a) source percent of total U.S. CH4 emissions; landfills (22.6 percent) and natural gas systems (18.4 percent) represented share. Overall, total U.S. CH4 emissions decreased by 8.4 percent (50.8 Tg CO2e) from 1990 to 2006, in part demissions at individual sources such as landfills and coal mines. Table TSD-B.10 - Sectoral comparison to total U.S. CH4 emissions (Tg CO2e) 

U.S. CH4 Emissions 1990 1995 2000 2001 2002 2003 2004 2005 200Section 202(a) CH4 emissions 4.2 3.8 2.9 2.8 2.4 

Share of U.S. CH4 (%) 0.70% 0.64% 0.50% 0.50% 0.43% 

Landfill CH4 emissions 149.6 144.0 120.8 117.6 120.1 

Share of U.S. CH4 (%) 24.7% 24.1% 21.0% 21.0% 21.3% 

Natural Gas CH4 emissions 124.7 128.1 126.5 125.3 124.9 

Share of U.S. CH4 (%) 20.6% 21.4% 22.0% 22.4% 22.2% 

2.2 

0.40% 

125.6 

22.5% 

123.3 

22.1% 

2.1 

0.38% 

122.6 

22.5% 

114.0 

20.9% 

1.9 

0.36% 

123.7 

22.9% 

102.5 

19.0% 

1

0.32

125

22.6

102

18.4

Total U.S. CH4 emissions 606.1 598.9 574.3 558.8 563.5 559.4 545.6 539.7 555

Table TSD-B.11 compares total CH4 emissions from section 202(a) source categories to U.S. GHG emissions, gfrom the transport sector (as defined by IPCC), and to total global GHG emissions from all source categoriSection 202(a) CH4 emissions are a significantly smaller share of U.S., global transport, and global emissions in 202(a) CO2 emissions. Table TSD-B.11 – Comparison to US and global greenhouse gas emissions (Tg CO2e) 

Global Emissions 2005 Sec 202(a) CH4 Share All U.S. GHG emissions All global CH4 emissions Global transport GHG emissions All global GHG emissions 

7,130 

6,408 

5,909 

38,726 

0.03% 

0.03% 

0.03% 

0.005% 

(c) Nitrous oxide emissions from section 202(a) source categories Nitrous oxide (N2O) is a product of the reaction that occurs between nitrogen and oxygen during fuel combemissions from motor vehicles and motor vehicle engines are closely related to fuel characteristics, air-fu

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temperatures, and the use of pollution control equipment. For example, some types of catalytic converters insvehicle NOX, CO, and hydrocarbon emissions can promote the formation of N2O. Table TSD-B.12 shows the trend in N2O emissions from section 202(a) source categories since 1990, presenequivalents. Section 202(a) emissions of N2O decreased by 26.9 percent from 1990 to 2006. Earlier generatioinitially resulted in higher N2O emissions, causing a 24.2 percent increase in N2O emissions from motor vehic1995. Improvements in later-generation emission control technologies have reduced N2O output, resulting in a in N2O emissions from 1995 to 2006. Overall, N2O emissions were predominantly from gasoline-fueled passengand light-duty trucks (42.8 percent) in 2006. Table TSD-B.12 – N2O emissions by section 202(a) source category (Tg CO2e) 

202(a) Sources 1990 1995 2000 2001 2002 2003 2004 2005 200Passenger Cars 25.4 26.9 25.2 23.8 22.5 Light-Duty Trucks 

14.1 22.1 22.4 21.3 18.5 

Motorcycles 0.0 0.0 0.0 0.0 0.0 Buses 0.0 0.0 0.0 0.0 0.0 Medium/Heavy-Duty Trucks 0.8 1.0 1.2 1.2 1.3 Cooling (all trans. Sources) N/a N/A N/A N/A N/A 

21.0 16.6 

0.0 0.0 1.3 

N/A 

19.5 15.3 

0.0 0.0 1.3 

N/A 

17.8 13.7 

0.0 0.0 1.2 

N/A 

1512

001

N/

Total 40.4 50.1 48.8 46.4 42.3 38.9 36.1 32.7 29

Table TSD-B.13 presents N2O emissions from section 202(a) source categories alongside total U.S. N2O emispresents N2O emissions from agricultural soil management, and nitric acid production, for comparison. In 2006, categories emitted 8.0 percent of total U.S. N2O emissions, making it the second largest source category. By category in the U.S. is agricultural soil management, representing 72.0 percent of total N2O emissions in 20source in 2006 was nitric acid production (4.3 percent). 

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Table TSD-B.13 - Sectoral comparison to total U.S. N2O emissions (Tg CO2e) 

US N2O Emissions 1990 1995 2000 2001 2002 2003 2004 2005 200Section 202(a) N2O emissions 

Share of U.S. N2O (%) 

Agricultural Soil N2O emissions 

Share of U.S. N2O (%) 

Nitric Acid N2O emissions 

Share of U.S. N2O (%) 

40.4 

10.5% 

269.4 

70.3% 

17.0 

4.4% 

50.1 

12.7% 

264.8 

66.9% 

18.9 

4.8% 

48.8 

12.7%

262.1 

67.9%

18.6 

4.8%

46.4 

 11.8% 

277.0 

 70.5% 

15.1 

 3.8% 

42.3 

11.3%

262.0 

69.7%

16.4 

4.3%

38.9 

 10.9%

247.3 

 69.4%

15.4 

 4.3%

36.1 

 10.2% 

246.9 

 69.8% 

15.2 

 4.3% 

32.7 

8.8% 

265.2 

71.7% 

15.8 

4.3% 

29

8.0

265

72.0

15

4.3

Total U.S. N2O emissions 383.4 395.6 385.9 392.9 376.1 356.6 353.5 370.1 367

Table TSD-B.14 compares total N2O emissions from section 202(a) source categories to U.S. GHG emissions, gfrom the transport sector (as defined by IPCC), total global N2O emissions, and to total global GHG emiscategories, for the year 2005. Section 202(a) N2O emissions are just under 0.55 percent of global transport emisof all global GHG emissions. Table TSD-B.14 – Comparison to U.S. and global greenhouse gas emissions (Tg CO2e) 

2005 Sec 202(a) N2O Share 

All U.S. GHG emissions 7,130 0.46% All global N2O emissions 3,286 0.99% Global transport GHG emissions 5,909 0.55% All global GHG emissions 38,726 0.08% 

(d) HFC emissions from section 202(a) source categories HFCs (a term which encompasses a group of eleven related compounds) are progressively replacing CFCs a202(a) cooling and refrigeration systems as they are being phased out under the Montreal Protocol and Title VI oFor example, HFC-134a has become a replacement for CFC-12 in mobile air conditioning systems. A nucontaining multiple compounds, have also been introduced. The emissions pathway can be complex, with HFCatmosphere during charging of cooling and refrigeration systems, during operation, and during decommissioning84 2006 IPCC Guidelines, Volume 3, Chapter 7. Page 43. 

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Table TSD-B.15 shows the trend in HFC emissions from section 202(a) source categories since 1990, presenequivalents. The estimates are presented as the total of all section 202(a) source categories together due to Inventory does not disaggregate data by vehicle mode. HFCs were not used in motor vehicles in 1990, but bincreased to 69.5 Tg CO2e. From 1995 to 2006, HFC emission from section 202(a) source categories increased bTable TSD-B.15 – HFC emissions by section 202(a) source category (Tg CO2e) 

202 HFC Sources 1990 1995 2000 2001 2002 2003 2004 2005 200Cooling & refrigerated transport 0.000 18.6 52.6 57.2 61.1 64.4 67.8 69.7 69

Table TSD-B.16 presents HFC emissions from section 202(a) source categories alongside total U.S. HFC emispresents HFC emissions from HCFC-22 production, and all other end-use applications of substitutes for ozone (ODS substitutes), for comparison. In 2006, section 202(a) source categories emitted 55.8 percent of total making it the largest source category. Other applications of ODS substitutes (including foam blowing, fipropellants, solvents, and other applications) accounted for 41.2 percent. HCFC-22 chemical production results of HFC-23, which accounted for 99 percent of HFC emissions in 1990, but declined by 2006 and now represents Table TSD-B.16 - Sectoral comparison to total US HFC emissions (Tg CO2e) 

U.S. HFC Emissions 1990 1995 2000 2001 2002 2003 2004 2005 200Section 202(a) HFC emissions 

Share of US HFC (%) 

HCFC-22 Production 

Share of US HFC (%) 

Other ODS Substitutes 

Share of US HFC (%) 

0.0 

0.0% 

36.4 

98.6% 

0.5 

1.4% 

18.6 

30.0% 

33.0 

53.4% 

10.3 

16.6% 

52.6 

52.5%

28.6 

28.6%

18.9 

18.9%

57.2 

 58.4% 

19.7 

 20.2% 

21.0 

 21.4% 

61.1 

57.5%

21.1 

19.9%

24.1 

22.7%

64.4 

 61.7%

12.3 

 11.8%

27.7 

 26.6%

67.8 

 58.2% 

17.2 

 14.8% 

31.5 

 27.0% 

69.7 

57.4% 

15.8 

13.0% 

35.9 

29.5% 

69

55.8

13

11.1

41

33.1

Total U.S. HFC emissions 36.9 61.8 100.1 97.9 106.3 104.5 116.6 121.4 124

Table TSD-B.17 compares total HFC emissions from section 202(a) source categories to U.S. GHG emissions, gfrom the transport sector (as defined by IPCC), to total global HFC emissions, and to total global GHG emiscategories, for the year 2005. Section 202(a) HFC emissions are just over 1 percent of global transport emissioall global GHG emissions, but actually make up 18.3 percent of global HFC emissions. 

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Table TSD-B.17 – Comparison to US and global greenhouse gas emissions (Tg CO2e) 

2005 Sec 202(a) HFC Share 

All U.S. GHG emissions 7,130 0.98% All global HFC emissions 381 18.3% Global transport GHG emissions 5,909 1.18% All global GHG emissions 38,726 0.18% 

(e) PFC and SF6 emissions Perfluorocarbons (PFCs) are not emitted from motor vehicles or motor vehicle engines in the U.S. The emissions in the U.S. are aluminum smelting and semiconductor manufacturing. Similarly, sulfur hexafluoride (SF6) is not emitted from motor vehicles or motor vehicle engines in the U.S., altire inflation has been reported in other countries.85 The main sources of SF6 emissions in the U.S. are electdistribution systems, and primary magnesium smelting. 85 2006 IPCC Guidelines, Volume 3, Chapter 8. 

155
References for Appendix B 

U.S. EPA (2008) Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990 – 2006. EPA-430-R-08-005, 

Washington, DC. 

WRI (2009) Climate Analysis Indicators Tool (CAIT) Version 6.0. Available at http://cait.wri.org/. Accessed 

February 20, 2009. 

156 
Appendix C: Direct Effects of Ambient GHG Concentrations on Human Health 

Greenhouse gases, at both current and projected atmospheric concentrations, are not expected to pose 

exposure risks on human respiratory systems (i.e. breathing/inhalation). The literature supporting this 

conclusion is described below. 

Carbon dioxide (CO2) 

The direct effects of high CO2 concentrations on human health were assessed in the EPA (2000) report, 

Carbon Dioxide as a Fire Suppressant: Examining the Risks, and have also been reviewed by the IPCC 

(2005) Special Report on Carbon Dioxide Capture and Storage. At concentrations above about 2%, CO2 

has a strong effect on respiratory physiology and at concentrations above 7–10%, it can cause 

unconsciousness and death (IPCC, 2005). Exposure studies have not revealed any adverse health effect of 

chronic exposure to concentrations below 1%. At concentrations greater than 17 percent, loss of 

controlled and purposeful activity, unconsciousness, convulsions, coma, and death occur within 1 minute 

of initial inhalation of CO2 (OSHA, 1989; CCOHS, 1990; Dalgaard et al., 1972; CATAMA, 1953; 

Lambertsen, 1971). But CO2 is a physiologically active gas and is a normal component of blood gases 

(EPA, 2000). Acute CO2 exposure of up to 1 percent and 1.5 percent by volume is tolerated quite 

comfortably (EPA, 2000b). 

The ambient concentration of CO2 in the atmosphere is presently about 0.039 percent by volume (or 386 

ppm). Projected increases in CO2 concentrations from anthropogenic emissions range from 41 to 158 

percent above 2005 levels (of about 380ppm) or 535 to 983 parts per million (ppm) by 2100 (Meehl et al., 

2007) (see Section 5). Such increases would result in atmospheric CO2 concentrations of 0.054 to 0.098 

percent by volume in 2100, which is well below published thresholds for adverse health effects. 

Methane (CH4) 

Methane is flammable or explosive at concentrations of 5% to 15% by volume (50,000 to 150,000 ppm) 

of air (NIOSH, 1994; NRC, 2000). At high enough concentrations, CH4 is also a simple asphyxiant, 

capable of displacing enough oxygen to cause death by suffocation. Threshold limit values are not 

specified because the limiting factor is the available oxygen (NRC, 2000). Atmospheres with oxygen 

concentrations below 19.5 percent can have adverse physiological effects, and atmospheres with less than 

16 percent oxygen can become life threatening (MSHA, 2007). Methane displaces oxygen to 18% in air 

when present at 14% (140,000 ppm). 

When oxygen is readily available, CH4 has little toxic effect (NRC, 2000). In assessing emergency 

exposure limits for CH4, the NRC (2000) determined that an exposure limit that presents an explosion 

hazard cannot be recommended, even if it is well below a concentration that would produce toxicity. As 

such, it recommended an exposure limit of 5000 ppm for methane (NRC, 2000). The National Institute 

for Occupational Health Safety (NIOSH, 1994) established a threshold limit value (TLV) for methane at 

1,000 ppm. 

The current atmospheric concentration of CH4 is 1.78 ppm. The projected CH4 concentration in 2100 

ranges from 1.46 ppm to 3.39 ppm by 2100, well below any recommended exposure limits (Meehl et al., 

2007). 

Nitrous Oxide (N2O) 

Nitrous oxide is an asphyxiant at high concentrations. At lower concentrations, exposure causes central 

nervous system, cardiovascular, hepatic (pertaining to the liver), hematopoietic (pertaining to the 

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formation of blood or blood cells), and reproductive effects in humans (Hathaway et al., 1991). At a 

concentration of 50 to 67 percent (500,000 to 670,000 ppm) nitrous oxide is used to induce anesthesia in 

humans (Rom, 1992). 

NIOSH has established a recommended exposure limit (REL) for nitrous oxide of 25 ppm as a timeweighted average (TWA) for the duration of the exposure (NIOSH, 1992). The American Conference of 

Governmental Industrial Hygienists (ACGIH) has assigned nitrous oxide a TLV of 50 ppm as a TWA for 

a normal 8-hour workday and a 40-hour workweek (ACGIH, 1994). 

The NIOSH limit is based on the risk of reproductive system effects and decreases in audiovisual 

performance (NIOSH, 1992). The ACGIH limit is based on the risk of reproductive, hematological 

(related to the study of the nature, function, and diseases of the blood and of blood-forming organs), and 

nervous system effects (ACGIH 1994) 

The current atmospheric concentration of nitrous oxide is 0.32 ppm. The projected nitrous oxide 

concentration in 2100 ranges from 0.36 to 0.46 ppm in 2100, well below any exposure limits (Meehl et 

al., 2007). 

Fluorinated Gases (HFCs, PFCs, SF6) 

Most fluorinated gases emitted from anthropogenic activities are released in very small quantities relative 

to established thresholds for adverse health outcomes from exposure. The health effects of exposure to 

one illustrative HFC gas, one illustrative HCFC gas and SF6 are given in the context of their current 

atmospheric concentration. Chlorofluorocarbons are not included in this discussion given their phaseout 

under the Montreal Protocol. 

The NRC (1996) recommended a 1-hr emergency exposure guidance level (EEGL) of 4,000 ppm for 

HFC-134a. This recommendation was based on a no-observed-adverse-effect level of 40,000 ppm in 

cardiac-sensitization tests of male beagles (NRC, 1996). It recommended 24-hr EEGL of 1,000 ppm 

based on the fetotoxicity effects (slight retardation of skeletal ossification) observed in rats exposed to 

HFC-134a. Finally, it recommended a 90-day CEGL of 900 ppm based on a 2-year chronic toxicity study 

conducted in male rats exposed to HFC-134a at different concentrations for 6 hours/day, 5 days/week. 

The atmospheric concentration of HFC 134a in 2003 was in the range of 26 to 31 parts per trillion 

according to IPCC/TEAP (2005), many orders of magnitude below EEGLs. 

For HCFC-123, the end points of pharmacological or adverse effects considered for establishing an EEGL 

are cardiac sensitization, anesthesia or CNS-related effects, malignant hyperthermia, and hepatotoxicity. 

According to the NRC (1996), concentration required to produce cardiac sensitization in 50% of the 

animals) for HCFC-123 was determined in dog studies to be 1.9% (19,000 ppm) for a 5-minute exposure. 

The NRC recommended that 1,900 ppm (19,000 ppm divided by an uncertainty factor of 10 for 

interspecies variability) should be considered the human no-observed-effect level for a 1-min exposure to 

HCFC-123 on the basis of the dog cardiac-sensitization model. The concentration of HCFC-123 in 1996 

was 0.03 parts per trillion according to IPCC/TEAP (2005), many orders of magnitude below the 

established effect level. 

Sulfur hexafluoride (SF6) is a relatively non-toxic gas but an asphyxiant at high concentrations. The 

NIOSH, 1997) recommended exposure limit is 1,000 ppm. The SF6 concentration in 2003 was around 5 

parts per trillion according to IPCC/TEAP (2005), many orders of magnitude below the exposure limit. 

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