Technical Support Document for
Endangerment and Cause or
Contribute Findings for
Greenhouse Gases under
Section 202(a) of the
Clean Air Act
April 17, 2009
Climate Change Division, Office of Atmospheric Programs
U.S. Environmental Protection Agency
Washington, DC
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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
1. 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
IV
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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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Technical Support Document, U.S. Environmental Protection Agency
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 (CHO, 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 CFU 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 Ibs.
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 (1.3°F) (±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 she 11-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 overtime.
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 4°C 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)(l) 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.
l(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 CFLt 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
1 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: OHhttp://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: lHhttp://www.climatescience.gov/Library/sap/sap-guidelines-clarification-
aug2007.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.
l(b) Data and Scientific Findings Considered by EPA
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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
IPCC
IPCC
IPCC
CCSP
CCSP
CCSP
CCSP
CCSP
CCSP
CCSP
CCSP
CCSP
CCSP
CCSP
CCSP
CCSP
CCSP
CCSP
CCSP
NRC
NRC
NRC
NRC
EPA
EPA
AC I A
Short Title and Year of Publication
Working Group I: The Physical Science Basis (2007)
Working Group II: Impacts, Adaptation and Vulnerability (2007)
Working Group III: Mitigation of Climate Change (2007)
SAP 1.1: Temperature Trends in the Lower Atmosphere (2006)
SAP 1.2: Past Climate Variability and Change in the Arctic and at High Latitudes
(2009)
SAP 1.3: Re-analyses of Historical Climate Data (2008)
SAP 2.1: Scenarios of GHG Emissions and Atmospheric Concentrations (2007)
SAP 2.3: Aerosol Properties and their Impacts on Climate
SAP 2.4: Trends in Ozone-Depleting Substances (2008)
SAP 3.1: Climate Change Models (2008)
SAP 3.2: Climate Projections (2008)
SAP 3.3: Weather and Climate Extremes in a Changing Climate (2008)
SAP 3.4: Abrupt Climate Change (2008)
SAP 4.1: Coastal Sensitivity to Sea-Level Rise (2009)
SAP 4.2: Thresholds of Change in Ecosystems (2009)
SAP 4.3: Agriculture, Land Resources, Water Resources, and Biodiversity (2008)
SAP 4.5: Effects on Energy Production and Use (2007)
SAP 4.6: Analyses of the Effects of Global Change on Human Health (2008)
SAP 4.7: Impacts of Climate Change and Variability on Transportation Systems
(2008)
Climate Change Science: Analysis of Some Key Questions (2001)
Radiative Forcing of Climate Change (2005)
Surface Temperature Reconstructions for the Last 2,000 Years (2006)
Potential Impacts of Climate Change on U.S. Transportation (2008)
Impacts of Global Change on Regional U.S. Air Quality(2009)
Inventory of U.S. Greenhouse Gas Emissions and Sinks (2008)
Arctic Climate Impact Assessment (2004)
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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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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/uncertainryguidancenote.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
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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.
l(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)
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Part II
Emissions and Elevated Concentrations of Greenhouse Gases
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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 (CFLO, nitrous oxide (N2O),
hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6) resulting from
anthropogenic activities in the U.S.
Total emissions are presented in teragrams4 (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
C02 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).
1 teragram (Tg) = 1 million metric tons. 1 metric ton = 1,000 kg = 1.102 short tons = 2,205 Ibs.
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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_sub-
missions/application/x-zip-compressed/usa_2007_crf_l lapr.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.
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Figure 2.1: Total U.S. Greenhouse Gas Emissions: 1990-2006
8,000
7,000
6,000
Eq.5,000
3,000
2,000
1,000
0
6,148 6,106 6,192
6343 6,435 6,494 u'
1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006
Source: Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-2006 (EPA, 2008). Excludes land-
use change and forestry and international bunker fuels.
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).
Figure 2.2: U.S. GHG Emissions Allocated to Economic Sector.
2,500 ,
2,000 -
iff 1<50°-
8
P 1,000-
500-
0-
19
Electricity Generation
• ' Transportation
._-: — :nr "^
Agriculture
Residential
90 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006
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.
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Figure 2.3: Total GHG Emissions for the Year 2005 by Country and for
U.S. Section 202a Source Categories
U.S. Greenhouse Gas
Emissions from Source
Categories under Section
202(a) of the Clean Air Act
Source categories under
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
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).
Source: WRI Climate Analysis and Indicators Tool. Available at http://cait.wri.orq/.
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.
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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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 2008s.
• 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.
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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;
Forsteretal., 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 pre-
industrial 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 well-
understood but are clearly related to the
changes in the imbalance between CFi4
sources and sinks.
• The methane concentration grew 7 ppb
between 2006 and 2007, the first year-to-
year 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
10000
5000
Time (before 2005)
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.
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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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Part III
Global and U.S. Observed and Projected Effects from Elevated
Greenhouse Gas Concentrations
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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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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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Section 4
Radiative Forcing and Observed Climate Change
This section focuses primarily on the more significant effects associated with GHGs, which is their heat-
trapping 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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The other three GHGs reported by the U.S. Inventory—HFCs, PFCs and SF6-
forcing in 2005 of+0.017 (±0.002) W/m2 (Forster et al, 2007).
-have a total radiative
Figure 4.1: Global average radiative forcing (RF) estimates and ranges in 2005 for
anthropogenic GHG emissions and other factors
RF Terms
Long-lived
greenhouse gases
Ozone
Stratospheric water
vapour from CH4
Surface albedo
{Direct effect
Cloud albedo
effect
Linear contrails
Solar irradiance
Total net
anthropogenic
RF values (W m'g)
1.66 [1.49 to 1.83]
0.48 [0.43 to 0.53]
0.16 [0.14 to 0.18]
0.34 [0.31 to
-0.05 [-0.15 to 0.05]
0.35 [0.25 to 0.65]
0.07 [0.02 to 0.12]
-0.2 [-0.4 to 0.0]
0.1 [0.0 to 0.2]
-0.5 [-0.9 to -0.1]
-0.7 [-1.8 to -0.3]
0.01 [0.003 to 0.03]
0.12 [0.06 to 0.30]
1.6 [0.6 to 2.4]
Spatial scale
Global
Global
Continental
to global
Global
Local to
continental
Continental
to global
Continental
to global
Continental
Global
LOSU
High
High
Med
Low
Med
- Low
Med
- Low
Low
Low
Low
-2 -1 _ 0 1 2
Radiative Forcing (W m~2)
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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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 long-
term 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 etal, 2007):
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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
0.6
o 0.4
O5
O5
_L 0.2
0.0
CD
O)
O -0.2
o
§-0.4
O
J5 -0.6
-0.8
Annual global mean temperatures (black dots) with linear fits to the data.
14.6
14.4
3 m
14.2 s a
g |
CD r-J-
14.0
CD
CD 03
53 a
13.8 £• §
l«
13.6 -3&
o QL
13.4
13.2
1860 1880 1900 1920 1940 1960 1980 2000
Period Rate
• Annual mean
^ Smoothed series
I l 5-95% decadal error bars
Year?
25
50
100
150
°C per decade
0.177±0.052
0.128±0.026
0.074±0.018
0,045±0.012
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).
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Technical Support Document, U.S. Environmental Protection Agency
Figure 4.3: Reconstructions of (Northern Hemisphere average or
global average) surface temperature variations from six research
teams
0.6
--0.2
--0.4
-1.2
--0.6
--0.8
--1.0
-1.2
900
1900
Global Surface
Temperatures Over the
Last 2,000 Years
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.
Borehole temperatures (Huang et al. 2000) —Glacier lengths (Oerlemans 2005)
—Multiproxy (Mann and Jones 2003) —Multiproxy (Moberg et al. 2005)
Multiproxy (Hegerl et al. 2006) —Tree rings (Esper et al. 2002)
—Instrumental record (Jones et al. 2001)
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).
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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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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 1°F
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.
Northwest
Wesl
North Central Central
i East
I North Central
Northeast
Wesl
Soilhwwt
Alaska
Temperature change f F percenliry):
-3-2-1 0 1 23
Gray interval:-0,1 to0.1°F
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.
North west
West
North Central
East
North Central
Central
Northeast
West
Change In precipitation (% per century):
I I
-35-28-21-14 -7 0 7 14 21 28 35
Grav interval:-2 to 2%
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(1)) which is indicative of significant variability in regional
precipitation patterns overtime 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 yr1) 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 non-
18 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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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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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
Nino-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 1 l(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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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(1) 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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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 climate-
sensitive 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
likely10 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.
Global
Global Land
Global Ocean
IO.O
1.0
0.5
0.0
1.0
0.5
0.0
1900
1950
Year
2000 1900
1950
Year
2000 1900
1950
Year
2000
models using only
natural forcings
models using both natural
and anthropogenic forcings
observations
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
surface temperature with results simulated by climate models using
natural and anthropogenic forcings.
Not only has an
anthropogenic signal been
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
tropics. In the tropics, most
observational data sets
show more warming at the
surface than in the
troposphere, while almost
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).
1900
models using only
natural forcings
2000
models using both natural
and anthropogenic forcings
observations
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
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.
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 CHt emissions.
Figure 6.1 Observed, and Projected Global CO2 and CH4 Emissions for the IPCC SRES Scenarios
History A18 A1FI All Kt • it'
_ 30 j FossiI CO, Emissions
1900 1950 2000 2050 21002000 2050 21002000 2050 21002000 2050 21002000 2050 21002000 2050 2100
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
Box 6.1: IPCC Reference Case Emission Scenarios from the Special Report on Emission Scenarios
Al. The Al 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 Al scenario family develops into three groups that
describe alternative directions of technological change in the energy system. The three Al groups are distinguished by their
technological emphasis: fossil intensive (A1FI), non fossil energy sources (AIT), 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.
Bl. The Bl storyline and scenario family describes a convergent world with the same global population, that peaks in mid-
century and declines thereafter, as in the A 1 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 Bl and Al
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, AIT, A2, Bl 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 (lAMs). 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 lAMs 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
GtC).24
(GtCO2) (or 770 GtC) to approximately 9,322 GtCO2 (or 2,540
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).
| Figure 6.2: Projected Global Emissions of CO2 from Fossil Fuels
and Industrial Sources across CCSP Reference Scenarios
2020
2040
IGSM REF
2060
MERGE REF
2080
2100
MINICAM REF
Source: CCSP (2007b). Global emissions of CC>2 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.
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.
1 Gigatonne (Gt) = 1 billion metric tons.
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Figure 6.3 illustrates reference case emission projections for CO2, CH/t, 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
Grey area indicates
EMF21 range
2000 2020 2040 2060
2080 2100
2000 2020 2040 2060 2080
2100
50
40
20
£ 10
LU
CH4
50
Grey area indicates
EMF21 range
2000 2020 2040 2060 2080 2100
2000 2020 2040 2060
2080 2100
12
I"
O 8
| 6
I 2
Grey area indicates
EMF21 range
2000 2020 2040 2060 2080 2100
2000 2020 2040 2060 2080 2100
6
I5
o 4
I 3
£ 1
F-gases
Grey area indicates
EMF21 range
2000 2020 2040 2060 2080 2100
2000 2020
2040
2060 2080
2100
-AIM
EDGE
-GEMINI-E3
IMAGE
MESSAGE
POLES
-AMIGA
-EPPA
-GRAPE
IPAC
MiniCAM
-SGM
COMBAT
-FUND
-GTEM
MERGE
PACE
W1AGEM
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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Figure 6.4 Projected Global CH4 and N2O Emissions across three
CCSP Reference Scenarios
CH4 Emissions
2020 2040 2060 2080 2100 2000 2020 2040 2060 2080 2100
' IGSM REF
MERGE REF
MINICAM REF
For comparison, Figure 6.4
provides the global CFi4
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, CFi4
and N2O concentrations for
the SRES scenarios, and
Figure 6.6 shows the
associated radiative forcing
for these CO2 scenarios. In
general, reference
concentrations of CO2 and
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.
Source: CCSP (2007b). Global anthropogenic emissions of CH4 and N2O vary
widely among the reference scenarios. There is uncertainty in year 2000 ChU
emissions, with MIT's IGSM reference scenario ascribing more of the emissions to
human activity and less to natural sources. Differences in scenarios reflect, to a
large extent, different assumptions about whether current emissions rates will be
reduced significantly for other reasons, for example, whether higher natural gas
prices will stimulate capture of CH4for use as a fuel.
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Figure 6.5 Projected Global CO2, CH4 and N2O
Concentrations for the IPCC SRES Scenarios
1200
1000
g- 800
| 600
I
§ 400
o
200
0
2006
4000
2026
2046
2066
2086
3500
_a
£ 3000
o
I 2500
I
8 2000
^r
O
1500
1000
2006
2026
2046
2066
2086
500
g
ra
§ 400
8
o
CM
350
300
2006
2026
2046
2066
2086
— A1B --A1T
Scenarios
, ..A1FI —A2 —B1 B2
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).
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Figure 6.6 Projected Radiative Forcing from CO2 for the IPCC SRES Scenarios
History
A1B
Radiative Forcing
1900 1950 201
2050 21002000 2050 21002000 2050 21002000 2050 21002000 2050 21002000 2050 2100
Source: Meehl et al. (2007). Projected radiative forcing from CO2 for six illustrative SRES non-
mitigation 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):
"[Confidence 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
Bl 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 Bl (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
Bl 1.8°C(3.2°F) l.rCto2.90C(2.0°Fto5.2°F)
AIT 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, 200 la; Meehl
et al., 2007).
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Figure 6.8 Multi-Model Averages and Assessed Ranges for Surface Warming
O
6.0 -
5.0 -
4.0 -
A2
A1B
B1
Year 2000 Constant
Concentrations
20th century
©IPCC 2007:WG1-AR4
1900
2000
Year
2100
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,
200 Ib), 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 SPvES 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 more-
realistic 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
Alaska
Western North America
°C 4
1900 1950 2000 2050 2100 1900 1950 2000 2050 2100
Central North America
Eastern North America
°C 4
1900 1950 2000 2050 2100 1900
1950 2000
2050 2100
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.
Annual
December/January/February June/July/August
30"N
1ITN
180° 140°W 100°W 60°W 20°W180° 140°W 100°W 60°W 20°W180° 140°W 100°W 60°W 20°W
10 7 5 « 35 325 ! 11 1 OS 0 -09-1
Temperature Response (°C)
70«N
50-N
30°N
10»N
180°
140°W 100°W 60°W 20°W180° 14O°W 100°W 60°W 20°W180° 140°W 100"W 60°W
50 30 20 15 10 5 0 -5-10-15-20-30-50
Prec Response {%)
Source: Christensen et al. (2007). Top row: Annual mean, December-January-February, and June-July-
August temperature between 1980 to 1999 and 2080 to 2099, averaged over 21 models. Bottom row:
same as top, but for fractional change in precipitation.
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 A IB 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 A IB 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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Sea ice is projected to
shrink in both the Arctic
and the Antarctic under all
SRES scenarios. In some
projections, Arctic late-
summer 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.
Figure 6.11 Percent Snow Depth Changes in March
DS
100
50
25
10
5
1
-1
-5
-10
-25
-50
-100
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.
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, GA. 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 1°C (1.8°F) 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 Nino 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 model-
projected 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
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 Nino or the warm events, the warming of SSTs in the central and eastern equatorial Pacific. The
acronym, ENSO, is composed of El Nino-Southern Oscillation, where El Nino 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 QrU 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 60° S and 60° 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 ozone-
depleting 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 socio-
economic 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 32°C or 90°F) increases 4-
fold under the Bl 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 (AIB) 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 heat-
and 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 Vparahaemolyticus 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 half-
century."
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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 A IB (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, ozone-
related 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 PM2s 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, PM25 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 participate 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 1-3°C (~2-5°F), 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: 18-22°C for maize, 22-24°C for soybean, 15°C for wheat, 23-26°C for rice, 25°C for
sorghum, 25-26°C for cotton, 20-26°C for peanut, 23-24°C for dry bean, and 22-25°C 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 south-
eastern 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 1.2°C 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 1.2°C temperature rise will increase yield 2.5 percent in the Midwest where
temperatures during July, August, September average 22.5°C, but will decrease yield 3.5 percent in the
South, where mean temperature during July, August, and September averages 26.7°C. Likewise, in the
South, that same mean temperature will result in reduced rice, cotton, and peanut yields, which will de-
crease 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 re-
sponse 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, cool-
water species will be lost mainly from shallow lakes, and warm water species will thrive except in the
far south, where temperatures in shallow lakes will exceed survival thresholds.
Climate variability and change can also impact fisheries in coastal and estuarine waters, although non-
climatic factors, such as overfishing and habitat loss and degradation, are already responsible for reducing
fish stocks (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 3°C 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 mi2/year) has burned in wildfires, almost
twice the 1920-1980 average of about 13,000 km2/year (8,080 midyear);
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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 Nino-Southern Oscillation (ENSO)
positive phases and higher Palmer Drought Severity Indices.41 Major fire years tend to follow the
switching from El Nino to La Nina 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 etal, 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 .
Fusing 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.
ll(a) Water Supply and Snowpack
Surface Water and Snow pack
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 storm water 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.
ll(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.
1 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).
ll(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).
ll(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(1), 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 sea-
level 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 sea-
level 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, low-
lying areas, the most devastating impacts are likely to be associated with storm surge (Nicholls et al.,
2007). For example, the Maryland Geological Survey estimated that more than 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 Nino 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), "[industries, settlements and human society are accustomed
to variability in environmental conditions, and in many ways they have become resilient to it when it is a
part of their normal experience. Environmental changes that are more extreme or persistent than that
experience, however, can lead to vulnerabilities, especially if the changes are not foreseen and/or if
capacities for adaptation are limited."
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 1°C 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 (8-10.5°F; ~4.5-5.6°C), 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 1°C or 2°C 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 (5.5-8T (~3.1-4.4°C) increase in California) and precipitation decreases by 10 to
20%. However, future precipitation projections are quite uncertain so it is possible that precipitation 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)
(c) Forest area burned: Canada
(d) Relative sea level:
North American coasts
(b) Spring bud-burst dates
Aspen in Edmonton
Change in annual mean temperature (°C): 1955 to 2005
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-Pere, 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 °C 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 cool-
water (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
! 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 non-
climate 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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PartV
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 de-
stabilize 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 ecosystems65 (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).
iykJ^
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 South-
East 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-21 st 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 • By mid-century, increases in temperature and associated decreases in soil water are
America 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 • For human communities in the Arctic, impacts, particularly those resulting from changing
Regions 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 • Small islands, whether located in the tropics or higher latitudes, have characteristics which
Islands 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
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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
66Adgeretal. (2007), p. 720
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 Adgeretal. (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
75Adgeretal. (2007) p. 737
76Adgeretal. (2007), p. 719
77Adgeretal. (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-R-
08001, September.
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Technical Support Document - Appendix B
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.l describes the correspondence
between section 202(a) GHG emission source categories and IPCC source categories:
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Table TSD-B.l - Source categories included under section 202(a)
section 202(a) Source
Category
Passenger Cars
Light-Duty Trucks
Motorcycles
Buses
Medium/Heavy -Duty
Trucks
Cooling (all
transportation
sources)
IPCC Sector
Energy
Energy
Energy
Energy
Energy
Industrial
Processes
IPCC Source Category
lA3b (i) Cars
1 A3b (ii) Light-duty trucks
1 A3b (iv) Motorcycles
lA3b (iii) Heavy-duty trucks
and buses
lA3b (iii) Heavy-duty trucks
and buses
2F1 Refrigeration and Air
Conditioning Equipment
Greenhouse Gases
CO2, CH4, N2O
CO2, CH4, N2O
CO2, CH4, N2O
CO2, CH4, N2O
CO2, CH4, N2O
Hydrofluorocarbons
(HFCs)
Greenhouse gas emissions from aviation, pipelines, railways, and marine transport are included
in the IPCC Energy Sector under 1 A3 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
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Table TSD-B.2 presents historical emissions of all GHGs (CC>2, CH4, N2O, and HFCs) from section 202(a) source categories from
1990-2006, in carbon dioxide equivalent units (Tg CC^e).80 Passenger cars (39.1 percent), light-duty trucks (31.7 percent), and
medium/heavy-duty trucks (24.2 percent) emitted the largest shares of GHG emissions in 2006, followed by cooling (all transportation
sources) (4.2 percent), buses (0.7 percent) and motorcycles (0.1 percent). From 1990 to 2006, GHG emissions from section 202(a)
source categories grew by 35.2 percent due, in part, to increased demand for travel and the stagnation of fuel efficiency across the U.S.
vehicle fleet. Since the 1970s, the number of highway vehicles registered in the United States has increased faster than the overall
population, according to the Federal Highway Administration (FHWA).81 Likewise, the number of miles driven (up 40.6 percent from
1990 to 2006) and the gallons of gasoline consumed each year in the United States have increased steadily since the 1980s, according
to the FHWA and Energy Information Administration, respectively.82 These increases in motor vehicle usage are the result of a
confluence of factors including population growth, economic growth, urban sprawl, low fuel prices, and increasing popularity of sport
utility 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
Section 202(a) Sources
Passenger Cars
Light-Duty Trucks
Motorcycles
Buses
Medium/Heavy-Duty Trucks
Cooling (all trans. Sources)
Total
1990
656.9
336.2
1.8
8.3
228.8
0.0
1231.9
1995
633.9
428.6
1.8
9.1
272.4
18.6
1364.4
2000
670.3
489.5
1.9
10.9
342.9
52.6
1568.1
2001
673.1
492.7
1.7
10.0
342.0
57.2
1576.8
2002
686.5
502.6
1.7
9.7
356.2
61.1
1617.9
2003
664.4
536.2
1.7
10.5
352.6
64.4
1629.7
CO2e)
2004
660.7
556.9
1.8
14.7
365.4
67.8
1667.4
2005
677.3
516.3
1.6
11.8
393.2
69.7
1670.0
2006
651.2
528.2
1.9
12.1
402.5
69.5
1665.4
Between 1990 and 2006, GHG emissions from passenger cars decreased 0.9 percent, though there was some growth in GHG
emissions from 2000 to 2002, and again from 2004 to 2005. Emissions from light-duty trucks increased 57.1 percent from 1990-2006,
largely due to the increased use of sport-utility vehicles and other light-duty trucks. Meanwhile, GHG emissions from heavy-duty
trucks increased 75.9 percent, reflecting the increased volume of total freight movement and an increasing share transported by trucks.
In 1990 there were no hydrofluorocarbons (HFCs) used in vehicle cooling systems. HFCs were gradually introduced into motor
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 .
82 DOE (1993 through 2008) Transportation Energy Data Book. Office of Transportation Technologies, Center for Transportation Analysis, Energy Division,
Oak Ridge National Laboratory. ORNL-5198.
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vehicle air conditioning and refrigerating systems during the 1990s as chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons
(HCFCs) started to phase out of production as required under the Montreal Protocol and Title VI of the Clean Air Act.
Table TSD-B.3 presents GHG emissions from section 202(a) source categories alongside total U.S. emissions. The table also presents
emissions from the electricity generation and industrial sectors for comparison. In 1990, section 202(a) source categories emitted 20.0
percent of total U.S. emissions, behind the electricity generation sector (30.2 percent) and the industrial sector (23.8 percent). By
2006, section 202(a) source categories collectively were the second largest sector with 23.6 percent of total U.S. emissions, due both
to growth in vehicle emissions and a decline in emissions from industry.
Table TSD-B.3 - Sectoral comparison to total US greenhouse gas emissions (Tg CC^e)
U.S. Emissions
Section 202(a) GHG emissions
Share of U.S. (%)
Electricity Sector emissions
Share of U.S. (%)
Industrial Sector emissions
Share of U.S. (%)
Total US GHG emissions
1990
1231.9
20.0%
1859.1
30.2%
1460.3
23.8%
6148.3
1995
1364.4
21.0%
1989.7
30.6%
1478.0
22.8%
6494.0
2000
1568.1
22.3%
2328.9
33.1%
1432.9
20.4%
7032.6
2001
1576.8
22.8%
2290.9
33.1%
1384.3
20.0%
6921.3
2002
1617.9
23.2%
2300.4
33.0%
1384.9
19.8%
6981.2
2003
1629.7
23.3%
2329.4
33.3%
1375.5
19.7%
6998.2
2004
1667.4
23.6%
2363.4
33.4%
1388.9
19.6%
7078.0
2005
1670.0
23.4%
2430.0
34.1%
1354.3
19.0%
7129.9
2006
1665.4
23.6%
2377.8
33.7%
1371.5
19.4%
7054.2
Table TSD-B.4 compares total GHG emissions from section 202(a) source categories to all U.S. GHG emissions, global GHG
emissions from the transport sector (as defined by IPCC), and total global greenhouse gas emissions from all source categories, for the
year 2005.83 Section 202(a) GHG emissions are a significantly larger share of global transport greenhouse gas emissions (28.3
percent) than the corresponding share of all U.S. GHG emissions to the global total (18.4 percent), reflecting the relative size of the
transport sector in the U.S. compared to the global average. Section 202(a) GHG emissions were 4.3 percent of total global emissions
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 countries, and all sources. Global
estimates are 'gross' emissions estimates and do not include removals of greenhouse gas emissions from the atmosphere by terrestrial sinks (i.e., forests and other
biomass). Global data come from the World Resources Institute's Climate Analysis Indicators Tool, which contains national data submitted by Parties to the
UNFCCC, 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 CC^e)
All U.S. GHG emissions
Global transport GHG emissions
All global GHG emissions
2005
7,130
5,909
38,726
Sec 202(a) Share
23.4%
28.3%
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 CC>2 equivalent units. In 2006, CC>2
made up the largest share of emissions (93.9 percent), followed by HFCs (4.2 percent), N2O (1.8 percent) and CH4 (0.1 percent).
Since 1990, the share of HFCs has increased (from zero in 1990), whereas the share of the other gases have correspondingly
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
C02
CH4
N2O
HFCs
Total
Share of Sec
Share of Sec
Share of Sec
Share of Sec
GHGs
202(a) GHGs
202(a) GHGs
202(a) GHGs
202(a) GHGs
1990
1187.3
96.4%
4.2
0.34%
40.4
3.3%
0.0
0.0%
1231.9
1995
1291.9
94.7%
3.8
0.28%
50.1
3.7%
18.6
1.4%
1364.4
2000
1463.8
93.4%
2.9
0.18%
48.8
3.1%
52.6
3.4%
1568.1
2001
1470.5
93.3%
2.8
0.18%
46.4
2.9%
57.2
3.6%
1576.8
2002
1512.0
93.5%
2.4
0.15%
42.3
2.6%
61.1
3.8%
1617.9
2003
1524.2
93.5%
2.2
0.14%
38.9
2.4%
64.4
4.0%
1629.7
2004
1561.4
93.6%
2.1
0.13%
36.1
2.2%
67.8
4.1%
1667.4
2005
1565.6
93.8%
1.9
0.12%
32.7
2.0%
69.7
4.2%
1670.0
2006
1564.6
93.9%
1.8
0.11%
29.5
1.8%
69.5
4.2%
1665.4
(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 combustion process. During
combustion, the carbon (C) stored in the fuels is oxidized and emitted as CC>2 and smaller amounts of other carbon compounds,
including CH/t, carbon monoxide (CO), and non-methane volatile organic compounds (NMVOCs). These other C-containing non-
CC>2 gases are emitted as by-products of incomplete fuel combustion, but are, for the most part, eventually oxidized to CC>2 in the
atmosphere.
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As the dominant GHG emitted from motor vehicles and motor vehicle engines (93.9 percent of total emissions in 2006), CC>2 emission
trends in Table TSD-B.6 mirror those of the GHG emissions total. Carbon dioxide emissions grew by 31.8 percent between 1990 and
2006. Most of this growth occurred as a result of increased CC>2 emissions from light-duty trucks (60.5 percent) and medium/heavy-
duty trucks (76.1 percent). Emissions from passenger cars did not grow over the same time period.
Table TSD-B.6 - CC>2 emissions by section 202(a) source category (Tg
Sec. 202(a) Source Categories
Passenger Cars
Light-Duty Trucks
Motorcycles
Buses
Medium/Heavy-Duty Trucks
Cooling (all trans. Sources)
Total
1990
628.8
320.7
1.7
8.3
227.8
N/A
1187.3
1995
604.9
405.0
1.8
9.0
271.2
N/A
1291.9
2000
643.5
466.0
1.8
10.9
341.5
N/A
1463.8
2001
647.9
470.3
1.7
10.0
340.6
N/A
1470.5
2002
662.6
483.2
1.7
9.6
354.8
N/A
1512.0
2003
642.1
518.8
1.6
10.5
351.2
N/A
1524.2
2004
640.0
540.8
1.7
14.7
364.1
N/A
1561.4
2005
658.4
501.9
1.6
11.8
391.9
N/A
1565.6
2006
634.5
514.9
1.9
12.1
401.3
N/A
1564.6
Table TSD-B.7 presents CC>2 emissions from section 202(a) source categories alongside total U.S. CC>2 emissions. The table also
presents emissions from the electricity generation and industrial sectors for comparison. In 1990, section 202(a) source categories
emitted 23.4 percent of total U.S. CO2 emissions, behind the electricity generation sector (36.1 percent), and ahead of the industrial
sector (18.7 percent). By 2006, emissions from section 202(a) source categories increased to 26.2 percent of total U.S. CC>2 emissions.
Table TSD-B.7 - Sectoral comparison to total U.S. CC>2 emissions (Tg
U.S. CO2 Emissions
Section 202(a) CO2 emissions
Share of U.S. CO2(%)
Electricity Sector CO2
Share of U.S. CO2(%)
Industrial Sector CO2
Share of U.S. CO2(%)
Total U.S. CO2 emissions
1990
1187.3
23.4%
1829.7
36.1%
947.5
18.7%
5068.5
1995
1291.9
23.9%
1964.6
36.4%
977.3
18.1%
5394.2
2000
1463.8
24.6%
2310.8
38.9%
967.3
16.3%
5939.7
2001
1470.5
25.2%
2273.0
38.9%
943.2
16.1%
5846.2
C02)
2002
1512.0
25.6%
2283.0
38.6%
943.0
16.0%
5908.6
2003
1524.2
25.6%
2313.2
38.9%
948.4
15.9%
5952.7
2004
1561.4
25.9%
2346.2
38.9%
974.7
16.1%
6038.2
2005
1565.6
25.8%
2412.3
39.7%
965.3
15.9%
6074.3
2006
1564.6
26.2%
2360.3
39.4%
984.1
16.4%
5983.1
Table TSD-B.8 compares total CC>2 emissions from section 202(a) source categories to total U.S. emissions, global GHG emissions
from the transport sector (as defined by IPCC), and total global GHG emissions from all source categories, for the year 2005. Section
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202(a) CC>2 emissions are a significantly larger share of global transport greenhouse gas emissions (26.5 percent) than the
corresponding share of all U.S. CC>2 emissions to the global total (22.0 percent), reflecting the relative size of the transport sector in
the U.S. compared to the global average. Section 202(a) CC>2 emissions were 4.0 percent of total global greenhouse gas emissions in
2005.
Table TSD-B.8 - Comparison to U.S. and global greenhouse gas emissions (Tg CC^e)
Global Emissions
All US GHG emissions
All global CO2 emissions
Global transport GHG emissions
All global GHG emissions
2005
7,130
27,526
5,909
38,726
Sec 202(a)
22.0%
5.7%
26.5%
4.0%
CO2 Share
(b) Methane emissions from section 202(a) source categories
Methane emissions from motor vehicles are a function of the CFLt and hydrocarbon content of the motor fuel, the amount of
hydrocarbons passing uncombusted through the engine, and any post-combustion control of hydrocarbon emissions (such as catalytic
converters).
Table TSD-B.9 shows the trend in CH4 emissions from section 202(a) source categories since 1990, presented in carbon dioxide
equivalents. The combustion of gasoline in passenger cars and light-duty trucks was responsible for the majority (90.8 percent) of the
CH4 emitted from section 202(a) source categories. From 1990 to 2006, CH4 emissions decreased by 57.6 percent.
Table TSD-B.9 - CH4 emissions by section 202(a) source category (Tg CC^e)
202(a) Sources
Passenger Cars
Light-Duty Trucks
Motorcycles
Buses
Medium/Heavy-Duty Trucks
Cooling (all trans. Sources)
Total
1990
2.6
1.4
0.0
0.0
0.2
N/A
4.2
1995
2.1
1.4
0.0
0.0
0.2
N/A
3.8
2000
1.6
1.1
0.0
0.0
0.1
N/A
2.9
2001
1.5
1.1
0.0
0.0
0.1
N/A
2.8
2002
1.4
0.9
0.0
0.0
0.1
N/A
2.4
2003
1.3
0.8
0.0
0.0
0.1
N/A
2.2
2004
1.2
0.7
0.0
0.0
0.1
N/A
2.1
2005
1.1
0.7
0.0
0.0
0.1
N/A
1.9
2006
1.0
0.7
0.0
0.0
0.1
N/A
1.8
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Table TSD-B.10 presents CH4 emissions from section 202(a) source categories alongside total U.S. CH4 emissions. The table also
presents CH4 emissions from landfills and natural gas systems for comparison. In 2006, section 202(a) source categories emitted 0.3
percent of total U.S. CH4 emissions; landfills (22.6 percent) and natural gas systems (18.4 percent) represented a significantly larger
share. Overall, total U.S. CH4 emissions decreased by 8.4 percent (50.8 Tg CO2e) from 1990 to 2006, in part due to efforts to reduce
emissions at individual sources such as landfills and coal mines.
Table TSD-B.10 - Sectoral comparison to total U.S. CH/i emissions (Tg CC^e)
U.S. CH4 Emissions
Section 202(a) CH4 emissions
Share of U.S. CH4(%)
Landfill CH4 emissions
Share of U.S. CH4(%)
Natural Gas CH4 emissions
Share of U.S. CH4(%)
Total U.S. CH4 emissions
1990
4.2
0.70%
149.6
24.7%
124.7
20.6%
606.1
1995
3.8
0.64%
144.0
24.1%
128.1
21.4%
598.9
2000
2.9
0.50%
120.8
21.0%
126.5
22.0%
574.3
2001
2.8
0.50%
117.6
21.0%
125.3
22.4%
558.8
2002
2.4
0.43%
120.1
21.3%
124.9
22.2%
563.5
2003
2.2
0.40%
125.6
22.5%
123.3
22.1%
559.4
2004
2.1
0.38%
122.6
22.5%
114.0
20.9%
545.6
2005
1.9
0.36%
123.7
22.9%
102.5
19.0%
539.7
2006
1.8
0.32%
125.7
22.6%
102.4
18.4%
555.3
Table TSD-B.ll compares total CH4 emissions from section 202(a) source categories to U.S. GHG emissions, global GHG emissions
from the transport sector (as defined by IPCC), and to total global GHG emissions from all source categories, for the year 2005.
Section 202(a) CH4 emissions are a significantly smaller share of U.S., global transport, and global emissions in comparison to section
202(a) CC>2 emissions.
Table TSD-B.l 1 - Comparison to US and global greenhouse gas emissions (Tg CC^e)
Global Emissions
All U.S. GHG emissions
All global CH4 emissions
Global transport GHG emissions
All global GHG emissions
2005
7,130
6,408
5,909
38,726
Sec 202(a) CH4 Share
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 combustion. Nitrous oxide
emissions from motor vehicles and motor vehicle engines are closely related to fuel characteristics, air-fuel mixes, combustion
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temperatures, and the use of pollution control equipment. For example, some types of catalytic converters installed to reduce motor
vehicle 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, presented in carbon dioxide
equivalents. Section 202(a) emissions of N2O decreased by 26.9 percent from 1990 to 2006. Earlier generation control technologies
initially resulted in higher N2O emissions, causing a 24.2 percent increase in N2O emissions from motor vehicles between 1990 and
1995. Improvements in later-generation emission control technologies have reduced N2O output, resulting in a 41.1 percent decrease
in N2O emissions from 1995 to 2006. Overall, N2O emissions were predominantly from gasoline-fueled passenger cars (53.0 percent)
and 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
Passenger Cars
Light-Duty Trucks
Motorcycles
Buses
Medium/Heavy-Duty Trucks
Cooling (all trans. Sources)
Total
1990
25.4
14.1
0.0
0.0
0.8
N/a
40.4
1995
26.9
22.1
0.0
0.0
1.0
N/A
50.1
2000
25.2
22.4
0.0
0.0
1.2
N/A
48.8
2001
23.8
21.3
0.0
0.0
1.2
N/A
46.4
2002
22.5
18.5
0.0
0.0
1.3
N/A
42.3
2003
21.0
16.6
0.0
0.0
1.3
N/A
38.9
2004
19.5
15.3
0.0
0.0
1.3
N/A
36.1
2005
17.8
13.7
0.0
0.0
1.2
N/A
32.7
2006
15.6
12.7
0.0
0.0
1.1
N/A
29.5
Table TSD-B.13 presents N2O emissions from section 202(a) source categories alongside total U.S. N2O emissions. The table also
presents N2O emissions from agricultural soil management, and nitric acid production, for comparison. In 2006, section 202(a) source
categories emitted 8.0 percent of total U.S. N2O emissions, making it the second largest source category. By far the largest source
category in the U.S. is agricultural soil management, representing 72.0 percent of total N2O emissions in 2006. The third largest
source 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 CC^e)
US N2O Emissions
Section 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 (%)
Total U.S. N2O emissions
1990
40.4
10.5%
269.4
70.3%
17.0
4.4%
383.4
1995
50.1
72.7%
264.8
66. 9%
18.9
4.8%
395.6
2000
48.8
72.7%
262.1
67.9%
18.6
4.8%
385.9
2001
46.4
11.8%
277.0
70.5%
15.1
3.8%
392.9
2002
42.3
11.3%
262.0
69.7%
16.4
4.3%
376.1
2003
38.9
10.9%
247.3
69.4%
15.4
4.3%
356.6
2004
36.1
10.2%
246.9
69.8%
15.2
4.3%
353.5
2005
32.7
8.8%
265.2
77.7%
15.8
4.3%
370.1
2006
29.5
8.0%
265.0
72.0%
15.6
4.3%
367.9
Table TSD-B.14 compares total N2O emissions from section 202(a) source categories to U.S. GHG emissions, global GHG emissions
from the transport sector (as defined by IPCC), total global N2O emissions, and to total global GHG emissions from all source
categories, for the year 2005. Section 202(a) N2O emissions are just under 0.55 percent of global transport emissions and 0.08 percent
of all global GHG emissions.
Table TSD-B.14 - Comparison to U.S. and global greenhouse gas emissions (Tg CO2e)
All U.S. GHG emissions
All global N2O emissions
Global transport GHG emissions
All global GHG emissions
2005
7,130
3,286
5,909
38,726
Sec 202(a) N2O Share
0.46%
0.99%
0.55%
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 and HCFCs in section
202(a) cooling and refrigeration systems as they are being phased out under the Montreal Protocol and Title VI of the Clean Air Act.84
For example, HFC-134a has become a replacement for CFC-12 in mobile air conditioning systems. A number of HFC blends,
containing multiple compounds, have also been introduced. The emissions pathway can be complex, with HFCs being emitted to the
atmosphere during charging of cooling and refrigeration systems, during operation, and during decommissioning/disposal.
84
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, presented in carbon dioxide
equivalents. The estimates are presented as the total of all section 202(a) source categories together due to the fact that the U.S.
Inventory does not disaggregate data by vehicle mode. HFCs were not used in motor vehicles in 1990, but by 2006 emissions had
increased to 69.5 Tg CC^e. From 1995 to 2006, FIFC emission from section 202(a) source categories increased by 274%.
Table TSD-B.15 - HFC emissions by section 202(a) source category (Tg CC^e)
202 HFC Sources
Cooling & refrigerated transport
1990
0.000
1995
18.6
2000
52.6
2001
57.2
2002
61.1
2003
64.4
2004
67.8
2005
69.7
2006
69.5
Table TSD-B.16 presents FIFC emissions from section 202(a) source categories alongside total U.S. FIFC emissions. The table also
presents HFC emissions from HCFC-22 production, and all other end-use applications of substitutes for ozone depleting substances
(ODS substitutes), for comparison. In 2006, section 202(a) source categories emitted 55.8 percent of total U.S. HFC emissions,
making it the largest source category. Other applications of ODS substitutes (including foam blowing, fire protection, aerosol
propellants, solvents, and other applications) accounted for 41.2 percent. HCFC-22 chemical production results in by-product releases
of HFC-23, which accounted for 99 percent of HFC emissions in 1990, but declined by 2006 and now represents 11.0 percent.
Table TSD-B.16 - Sectoral comparison to total US HFC emissions (Tg
U.S. HFC Emissions
Section 202(a) HFC emissions
Share of US HFC (%)
HCFC-22 Production
Share of US HFC (%)
Other ODS Substitutes
Share of US HFC (%)
Total U.S. HFC emissions
1990
0.0
0.0%
36.4
98.6%
0.5
1.4%
36.9
1995
18.6
30.0%
33.0
53. 4%
10.3
16.6%
61.8
2000
52.6
52.5%
28.6
28.6%
18.9
18.9%
100.1
2001
57.2
58.4%
19.7
20.2%
21.0
21.4%
97.9
2002
61.1
57.5%
21.1
19.9%
24.1
22.7%
106.3
2003
64.4
61.7%
12.3
11.8%
27.7
26.6%
104.5
2004
67.8
58.2%
17.2
14.8%
31.5
27.0%
116.6
2005
69.7
57.4%
15.8
13.0%
35.9
2P.5%
121.4
2006
69.5
55.8%
13.8
11.1%
41.2
33.1%
124.5
Table TSD-B.17 compares total HFC emissions from section 202(a) source categories to U.S. GHG emissions, global GHG emissions
from the transport sector (as defined by IPCC), to total global HFC emissions, and to total global GHG emissions from all source
categories, for the year 2005. Section 202(a) HFC emissions are just over 1 percent of global transport emissions and 0.18 percent of
all 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 CC^e)
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 main sources of PFC
emissions in the U.S. are aluminum smelting and semiconductor manufacturing.
Similarly, sulfur hexafluoride (SFe) is not emitted from motor vehicles or motor vehicle engines in the U.S., although use of SFe for
tire inflation has been reported in other countries.85 The main sources of SF6 emissions in the U.S. are electrical transmission and
distribution systems, and primary magnesium smelting.
85 2006 IPCC Guidelines, Volume 3, Chapter 8.
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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.
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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, CUt 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 CH^ 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 time-
weighted 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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References for Appendix C
ACGIH (1994) 1994-1995 Threshold limit values for chemical substances and physical agents and biological
exposure indices. Cincinnati, OH: American Conference of Governmental Industrial Hygienists.
CATAMA (1953) Aviation Toxicology--an Introduction to the Subject and a Handbook of Data.
Committee on Aviation Toxicology, Aero Medical Association. The Blakiston Co.: New York, NY. pp.
6-9, 31-39, 52-55, 74-79, 110-115.
Dalgaard, J.B., G. Dencker, B. Fallentin, P. Hansen, B. Kaempe, J. Steensberger, Wilhardt, P. (1972) Fatal
poisoning and other health hazards connected with industrial fishing. Br. J. Ind. Med. 29: 307-316.
Hathaway GJ, Proctor NH, Hughes JP, and Fischman ML (1991) Proctor and Hughes' chemical hazards of the
workplace. 3rd ed. New York, NY: Van Nostrand Reinhold.
IPCC (2005) Special Report on Carbon Dioxide Capture and Storage. A Special Report of Working Group III of the
Intergovernmental Panel on Climate Change.
IPCC Technology and Economic Assessment Panel (2005) IPCC/TEAP Special Report on Safe-guarding the Ozone
Layer and the Global Climate System: Issues Related to Hydrofluorocarbons and Perfluorocarbons. Prepared
by Working Group I and III of the IPCC, and the TEAP. Edited by B. Metz, L. Kuijpers, S. Solomon, S.O.
Andersen, O. Davidson, J. Pons, D. de Jager, T. Kestin, M. Manning, and L.A. Meyers, 488 pp., Cambridge
University Press, Cambride, U.K., and New York, NY, USA.
Lambertsen, CJ. (1971) "Therapeutic Gases—Oxygen, Carbon Dioxide, and Helium." Drill's Pharmacology in
Medicine. Chapter 55, Ed. By J.R. DiPalma. McGraw-Hill Book Co.: New York, NY.
Meehl, G.A. et al. (2007) Global Climate Projections. In: Climate Change 2007: The Physical Science Basis.
Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate
Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller
(eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
Mine Safety and Health Administration (MSH A) (2007) The Danger of Oxygen Deficiency in Underground Coal
Mines. Department of Labor Program Information Bulletin NO. P07-05. Internet site located at
http://www.msha.gov/regs/complian/PIB/2007/pib07-05.asp
NIOSH (1997) NIOSH Pocket Guide to Chemical Hazards. U.S. Department of Health and Human Services, Public
Health Service, Centers for Disease Control and Prevention, Washington, D.C. NIOSH Publication no. 97-140;
NTIS no. PB-97177604. Internet site located at http://www.cdc.gov/niosh/npg/npgd0576.html
NIOSH (1994) International Chemical Safety Cards - Methane. U.S. Department of Health and Human Services,
Public Health Service, Centers for Disease Control and Prevention, Washington, D.C. Internet site located at
http://www.cdc.gov/niosh/ipcsneng/neng0291.html
NIOSH (1992) Recommendations for occupational safety and health: Compendium of policy documents and
statements. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for
Disease Control, National Institute for Occupational Safety and Health (NIOSH) Publication No. 92-100.
National Research Council (NRC) (2000) Emergency and Continuous Exposure Limits for Selected Airborne
Contaminants, Volume 1. National Academy Press, Washington, D.C.
National Research Council (NRC) (1996) Toxicity of Alternatives to Chlorofluorocarbons: HFC-134a and HCFC-
123. National Academy Press, Washington, D.C.
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OSHA (1989) Carbon Dioxide, Industrial Exposure and Control Technologies for OSHA Regulated Hazardous
Substances, Volume I of II, Substance A -1. Occupational Safety and Health Administration. Washington, DC:
U.S. Department of Labor. March.
Rom, WN (1992) Environmental and occupational medicine. 2nd ed. Boston, MA: Little, Brown and Company.
U.S. EPA (2000a) Carbon Dioxide as a Fire Suppressant: Examining the Risks, EPA 430-R-00-002, May.
U.S. EPA (2000b) Carbon Dioxide as a Fire Suppressant: Examining the Risks, Appendix B - Overview of Acute
Health Effects, EPA 430-R-00-002, May.
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