Climate Change: Science and Policy Implications

Climate Change: Science and Policy Implications
Updated May 2, 2007
Jane A. Leggett
Specialist in Energy and Environmental Policy
Resources, Science, and Industry Division

Climate Change: Science and Policy Implications
Almost all scientists agree that the Earth’s climate is changing, having warmedoo
by 0.6 to 0.9 Celsius (1.1 to 1.6 Fahrenheit) since the Industrial Revolution.
Science indicates that the Earth’s global average temperature is now approaching, or
possibly has passed, the warmest experienced since human civilizations began to
flourish about 12,000 years ago. During the 20th Century, some areas became wetter
while others experienced more drought. Most climate scientists conclude that
humans have induced a large part of the climate change since the 1970s. Although
natural forces such as solar irradiance and volcanoes contribute to variability,
scientists cannot explain the climate changes of the past few decades without
fossil fuel use, land clearing, and industrial and agricultural emissions. Over the past

150 years, measured carbon dioxide concentrations have risen by more than one-

third, from about 280 parts per million (ppm) to about 380 ppm. The United States
contributes almost one-fifth of net global greenhouse gas emissions. Some impacts
of climate change are expected to be beneficial (e.g., increased agricultural
productivity in some regions), whereas others are expected to be adverse (e.g.,
drought in some regions, rising sea levels in some coastal areas).
Forecasting future climate conditions is challenging, and some major processes
remain poorly understood. However, methods are improving to characterize the
risks. Scientists have found it is very likely that rising greenhouse gas
concentrations, if they continue unabated, will raise the global average temperature
above natural variability by at least 1.5o Celsius (2.7o Fahrenheit) during the 21st
Century (above 1990 temperatures), with a small likelihood that the temperature rise
may exceed 5oC (9oF). The projections thought most likely by many climate
modelers are for a greenhouse gas-induced temperature rise of approximately 2.5 to
3.5oC (4.5 to 6.3oF) by 2100. However, the magnitude, rapidity, and details of the
change are likely to remain unclear for some time. Future climate change may
advance smoothly or sporadically, with some regions experiencing more fluctuations
in temperature, precipitation, and frequency or intensity of extreme events than
others. Some scientists emphasize potential beneficial effects of climate change, or
count on the ability of humans to adapt their behaviors and technologies to manage
climate change in the future; other scientists argue that the benefits of climate change
may be limited, even accounting for probable adaptation and its costs, and that there
are risks of abrupt, surprising change with accompanying dislocations.
The continuing scientific process has resulted in a better understanding of
climate change and generally confirms the broad conclusions made in previous
decades by the preponderance of scientists: that human activities emit greenhouse
gases that influence the climate, with potentially serious effects. Details have been
revised or refined, but the basic conclusion of the risks persists. The principal
questions remaining for the majority of scientists concern not whether greenhouse
gases will result in climate change, but the magnitude, speed, geographic details, and
likelihood of surprises, and the appropriate timing and options involved in addressing
the human components of climate change.

Human Activities that Influence Climate Change....................22
Methods To Compare Human and Natural Causes...................30
Attribution of Climate Change in the 20th Century...................32
Projections of Future Human-Driven Climate Change....................33
Implications of Climate Change for the Federal Government...........42
Appendix A: Natural Forces that Influence Climate......................47
Release of Methane Clathrates from Ocean Beds....................49
List of Figures
Figure 1. Global Temperature Change Since the Industrial Revolution........8
Figure 2. Trends in Average Annual Temperature, 1901-2005...............9
Figure 3. Changes in Frequency of Extreme Precipitation.................11
Figure 4. Sectoral Shares of Global GHG Emissions in 2000...............24
Figure 5. CO2, Methane and Nitrous Oxide Concentrations over 400,000 Years
Figure 6. Estimated Effects of Different Forcings on Global Temperature

List of Tables
Table 1. History of U.S. Expenditures for Climate Change Science...........2
Table 2. Trends in U.S. Temperature and Precipitation Change from

1902 to 2005, by Climatic Region................................14

Climate Change: Science and
Policy Implications
CLIMATIC EFFECTS OF POLLUTION: Carbon dioxide is being added to the
or even national efforts, could occur. Possibilities of bringing about
countervailing changes by deliberately modifying other processes that affect
climate may then be very important.
President’s Science Advisory Panel, 19651
For more than a century, scientists have known that adding carbon dioxide and
certain other gases to the atmosphere could warm the Earth, as expressed in the quote
above. During the past decade, mounting scientific evidence and public debate have
generated interest in the U.S. Congress to understand climate change, and potentially
to address related emissions of carbon dioxide, methane, and additional gases
generated by human activities. These human-related emissions have accumulated in
the atmosphere, raising concentrations dramatically. For example, carbon dioxide
emissions associated mostly with fossil fuel combustion and land clearing have
increased atmospheric concentrations by one-third since the Industrial Revolution,
from about 280 parts per million (ppm) in 1850 to 380 ppm today.2
Investment in science and technological research has been the cornerstone of the
federal strategy since the 1960s. Great strides have been made to collect observations
of relevant Earth processes; to develop a variety of models to analyze and forecast
atmospheric, ocean, land, and related economic and energy systems; and to
understand the potential impacts of climate change on humans and ecosystems.

1 President’s Science Advisory Panel, Restoring the Quality of Our Environment: Report of
the Environmental Pollution Panel, Report of The Environmental Pollution Panel,
President’s Science Advisory Committee, The White House (November 1965), p. 9.
2 Neftel, A., H. Friedli, E. Moor, H. Lötscher, H. Oeschger, U. Siegenthaler, and B. Stauffer,
1994, Historical CO2 record from the Siple Station ice core, in Trends: A Compendium of
Laboratory, U.S. Department of Energy (Oak Ridge: TN). Also, World Data Centre for
Greenhouse Gases (WDCGG), WMO Greenhouse Gas Bulletin: The State of Greenhouse
Gases in the Atmosphere Using Global Observations through 2005 (Geneva, 2006), at
[http://gaw.kishou.go.j p/wdcgg.html ].

Research has been conducted by universities, governmental agencies, research
institutions, and the private sector.
A history of climate change science funding is provided in Table 1.3 Most U.S.
funding has gone to researchers through the National Science Foundation (NSF), the
National Aeronautics and Space Administration (NASA), and the Department of4
Energy (DOE), as well as smaller amounts through other U.S. agencies and the
governments of other nations. Research collaboration across countries, gaining
efficiencies and insights across boundaries, has proceeded through the International
Biosphere/Geosphere Program, the World Meteorological Organization (WMO), and
others. The products — data, models, and analyses — have been peer-reviewed,
repeated, reproduced or contradicted, debated, and updated. Dozens of assessments
of various aspects of climate change science have been conducted by public multi-
disciplinary bodies, including the National Academy of Science (NAS), the U.S.
Congress’s former Office of Technology Assessment, and the Intergovernmental
Panel on Climate Change (IPCC).
Table 1. History of U.S. Expenditures for
Climate Change Science
($ millions)
Fiscal YearActual $Constant (2005) $
1989 134 209
1990 659 975
1991 954 1,355
1992 1,110 1,531
1993 1,326 1,775
1994 1,444 1,885
1995 1,760 2,234
1996 1,654 2,039
1997 1,656 1,995
1998 1,677 1,989
1999 1,657 1,925
2000 1,687 1,896
2001 1,728 1,886
2002 1,667 1,792
2003 1,766 1,857
2004 1,977 2,023
2005 1,865 1,865
2006 Estimate1,7091,674
2007 Request1,7151,643
Source: CCSP Annual Report to Congress, [
default.htm#funding], accessed Jan. 18, 2007.

3 The science funding presented in Table 1 is a subset of total U.S. expenditures on climate
change. For more information, see CRS Report RL33817, Climate Change: Federal
Expenditures, by Jane A. Leggett.
4 Funding also has been made available through the predecessors of these organizations.

The IPCC is the preeminent international body charged with periodically
assessing technical knowledge of climate change. It draws on thousands of scientists
with expertise on all aspects of climate change. The Fourth Assessment Report
(AR4) of the IPCC assessment is being issued as several reports in 2007. A summary
of selected key findings of the science assessment5 is provided below in
“Intergovernmental Panel on Climate Change — Climate Change 2007: The Physical
Science Basis.” Many governments use the IPCC assessments as one input to their
policy deliberations. The U.S. government has been a primary supporter of the IPCC
assessment, by sponsoring much of the research and monitoring that underpins the
assessment, as well as many experts representing a broad range of views on specific
topics, and by coordinating both external expert and governmental peer reviews of
the reports.
The continuing scientific process has resulted in an understanding of climate
change that is increasingly robust and has withstood a multitude of challenges; in
most respects, the evolving science confirms the broad conclusions made in previous
decades by the preponderance of scientists. Many details and complexities, however,
remain nebulous. Although most scientists are confident in their current
understanding of climate change, they are less certain about the future magnitude,
timing, and geographic details.
Climate Change Science Is a Process
The evolution of scientific understanding of climate change is an example of the
process of science. It is iterative, beginning with statements of hypotheses,
followed by testing and observations, scrutiny by other scientists, reproduction or
repudiation of results, and revisions to the state of knowledge. The practice of
critique and revision is desirable to support continuous improvement of
knowledge; some comments provide insights while others may be refuted.
Research may decrease certainty, or enhance it. The objective, like most science,
is to be able to produce ever more reliable predictions.
Such has been the case following the discovery described by Joseph Fourier in
1827 that Earth’s atmosphere acts like the glass of a greenhouse, letting in solar
energy, then trapping some of its heat, hence influencing the temperature of the
Earth’s surface. Related findings have been tested, critiqued, refuted, revised and
improved over almost two centuries. As a result of the scientific process, most
scientists are confident in their current understanding of climate change and the
role of human activities, including emissions of so-called greenhouse gases.
Ongoing research, debate, and refinement have yielded broadly accepted
agreement on climate change science.

5 Intergovernmental Panel on Climate Change Working Group I, Climate Change 2007: The
Physical Basis (Cambridge, UK: Cambridge University Press, 2007).

Intergovernmental Panel on Climate Change
Climate Change 2007: The Physical Science Basis
its fourth assessment of the science of climate change, updated with research
reported over the previous six years. Selected major findings from this report
include the following:
!“Global atmospheric concentrations of carbon dioxide, methane
and nitrous oxide have increased markedly as a result of human
activities since 1750 and now far exceed pre-industrial valued
determined from ice cores spanning many thousands of years.”
!“The atmospheric concentration of carbon dioxide in 2005
exceeds by far the natural range over the past 650,000 years (180
to 300 ppm) as determined by ice cores.”
!“The primary source of the increased atmospheric concentration
of carbon dioxide since the pre-industrial period results from
fossil fuel use, with land use change providing another significant
but smaller contribution. Annual fossil fuel carbon dioxide
emissions increased from an average of 6.4 [6.0 to 6.8] GtC per
year in the 1990s, to 7.2 [6.9 to 7.5] Gtv per year in 2000-2005 ....
Carbon dioxide emissions associated with land-use change are
estimated to be 1.6 GtC [0.5 to 2.7] per year over the 1990s,
although these estimates have a large uncertainty.”
!“Changes in solar irradiance since 1750 are estimated to cause a
radiative forcing ... which is less than half th estimate given in the
TAR [Third Assessment Report, 2001, of the IPCC].”
!“Warming of the climate system is unequivocal.” The updated
linear trend from 1906 to 2005 is 0.74oC [1.3oF].
!“Urban heat island effects are real but local, and have a negligible
influence (less than 0.006oC [0.01oF] per decade over land and
zero over the oceans).”
!“New data ... now show that losses from the ice sheets of
Greenland and Antarctica have very likely contributed to sea level
rise over 1993 to 2003.”
!“At continental, regional and ocean basin scales, numerous long-
term changes in climate have been observed. These include
changes in Arctic temperatures and ice, widespread changes in
precipitation amounts, ocean salinity, wind patterns and aspects
of extreme weather including droughts, heavy precipitation, heat
waves and the intensity of tropical cyclones.... Some aspects of
climate have not been observed to change.”

!“Paleoclimate information supports the interpretation that the
1300 years. The last time the polar regions were significantly
warmer than present for an extended period (about 125,000 years
ago), reductions in polar ice volume led to 4 to 6 meters [13 to 20
feet] of sea level rise.”
!“Most of the observed increase in globally averaged temperatures
increase in anthropogenic greenhouse gas concentrations.”
!“Difficulties remain in reliably simulating and attributing
observed temperature changes at smaller [than continental]
!“For the next two decades at warming of about 0.2oC [0.36oF] per
decade is projected for a range of SRES emission scenarios. Even
if the concentrations of all greenhouse gases and aerosols had

0.1oC [0.2oF] would be expected.”

!“The best estimate for the low [SRES greenhouse gas emission]
scenario (B1) is 1.8oC (likely range is 1.1oC to 2.9oC) [3.2oF
(likely range is 2.0oF to 5.2oF)], and the best estimate for the high
scenario (A1F1) is 4.0oC (likely range is 2.4oC to 6.4oC) [5.2oF
(likely range is 4.3oF to 9.5oF)].”
!“Warming is expected to be greatest over land and at most high
northern latitudes, and least over the Southern Ocean and parts of
the North Atlantic ocean.”
!“It is very likely that hot extremes, heat waves, and heavy
precipitation events will continue to become more frequent.”
!“Increases in the amount of precipitation are very likely in high-
latitudes, while decreases are likely in most subtropical land
regions (by as much as about 20% in the A1B scenario in 2100,...
continuing observed patterns in recent trends.”
!“Anthropogenic warming and sea level rise would continue for
centuries due to the timescales associated with climate processes
and feedbacks, even if greenhouse gas concentrations were to be
!“Based on current understanding of climate carbon cycle
feedback, model studies suggest that to stabilise at 450 ppm
carbon dioxide, could require that cumulative emissions over the
21st century be reduced from an average of approximately 670
[630 to 710] GtC to approximately 490 [375 to 600] GtC.”

Climate change has become a highly debated issue in which the U.S. Congress
has maintained an active and continuing interest. Congress has provided tens of
billions of dollars for research and additional programs (almost $5 billion in
FY2006). It established the Global Change Research Program (USGCRP) in the
Global Change Research Act of 1990 (P.L.101-606), aimed at understanding and
responding to global change, and requiring a scientific assessment for the President
and Congress at least every four years, as well as annual reports on activities and
budget. The Senate in 1992 ratified the Framework Convention on Climate Change,
which contains commitments from the United States and other parties to improve the
science and cooperate internationally on it, as well as to communicate about climate
change to the public.
The first and only national assessment was released by the executive branch in
2000.6 Our Changing Planet, an annual report to Congress, outlines climate-related
research progress and strategies.7 Various committees have held hearings to oversee
existing programs and to consider further options. There has been general consensus
regarding the benefits of additional scientific and technological research; however,
long-standing debate exists about whether and how to attempt to achieve mitigation
or adaptation through federal legislative action. As additional action is considered,
an understanding of climate change science may help inform consideration of relative
priorities, the scope of action, and timing, among other issues.
This report presents an overview of the science of climate change and its
potential impacts. It provides highlights on the impacts of climate change itself; it
does not address the mitigation of climate change (e.g., by controlling greenhouse gas
emissions) nor the economic issues associated with it. This report is organized into
four topics:
!Climate change and impacts that have been observed.
!Forces that are understood to be causing recent climate change.
!Projections of future climate change and impacts.
!Implications of climate change science for policy.
This report represents a snapshot of the science of climate change, which will
continue to evolve. It will be updated periodically to reflect new peer-reviewed
evidence and the developing state of scientific understanding.

6 U.S. Global Change Research Program, U.S. National Assessment of the Potential
of climate change and mechanisms for adaptation (Washington, 2000), available at
[http://www.usgc] .
7 U.S. Climate Change Science Program Office, Our Changing Planet: The U.S. Climate
Change Science Program for Fiscal Year 2007: A Report by the Climate Change Science
Program and The Subcommittee on Global Change Research, A Supplement to the
President’s Fiscal Year 2007 Budget (Washington, 2006), at [
usgcrp/Library/ocp2007/default.htm] .

Changes Observed in the Earth’s Climate
Scientists agree that the Earth’s climate is changing. The average global surfaceo
temperature has increased since the Industrial Revolution, by about 0.6 to 0.9
Celsius (1.1 to 1.6o Fahrenheit) from 1880 to today. The U.S. temperature has risen
at roughly twice the global average rate since the 1970s. The global climate of the
past few decades is likely approaching, or has already passed, the warmest since the8th
rise of human civilizations around 12,000 years ago. Precipitation in the 20
Century has increased by about 2%, more in the high northern latitudes, while drying
has occurred in parts of the tropics, especially in the Sahel and southern Africa.
Extreme precipitation events have increased in most of the few locations where data
are adequate for analysis.
The following sections cover observed climate change. The initial section
describes changes in the climate measured around the globe, first in temperature, then
precipitation, then extreme events. The subsequent section describes climate changes
specific to the United States. The last section on observed climate change identifies
findings from the study of climate centuries to millenia ago that have relevance to
consideration of recent and potential future climate change.
Global Climate Changes
Global Temperature. The average temperature of the Earth’s surface, the
global mean temperature (GMT), has increased about 0.6 to 0.9oC (about 1.1 to

1.6oF) from 1880 to 2004 (Figure 1).9 Warming occurs over land10 and sea11th

surfaces. The warming during the 20 century, however, was not smooth. Global
warming occurred from around 1910 to 1945, followed by a period of slightly
declining or stable temperatures into the 1970s. Since 1979, warming has returned
to about twice the rate of the 20th Century average, at about 0.18oC per decade

8 Obasi, Godwin O.P., “Climate Change — Reality, Expectation, and the Role of WMO and
National Meteorological and Hydrological Services,” lecture presented at the Sixth
Technical Conference on Management for Development of Meterorological Services in
Africa (Abuja, Federal Republic of Nigeria), November 6, 2000. Also, Hansen, J., et al.,
“Global temperature change,” PNAS, 103 (2006), pp. 14288-14293.
9 Lower value provided by National Oceanic and Atmospheric Administration/National
Climate Data Center (NOAA/NCDC), Climate of 2005 — Annual Report, NASA Goddard
Institute for Space Studies (2006). The upper value comes from. Intergovernmental Panel
on Climate Change Working Group I, Climate Change 2007: The Physical Basis
(Cambridge, UK: Cambridge University Press, 2007).
10 Adjustments to the measurements to account for urban heat islands, relocation of
observation stations, changes in measurement techniques, etc. are described by
NOAA/NCDC at [], which
also provides detailed technical references.
11 T.M. Smith and R.W. Reynolds, “A global merged land air and sea surface temperature
reconstruction based on historical observations (1880-1997),” J. Clim., vol. 18 (2005), pp.
2021-2036. The fact that warming occurs in sea surface temperatures is evidence that the
observed global warming is not an artifact of placement of weather stations and

(0.32oF/decade). A panel of the National Academy of Science has reconciled earlier
disagreement in confirming that warming has occurred also in the mid-troposphere,12
though less than at the surface.13
Globally, 2005 was the warmest in nearly 130 years of direct measurements;
2006 was the sixth warmest year on record. The 10 warmest years on record have
occurred since 1994.
Figure 1. Global Temperature Change Since the Industrial Revolution

Jan-Dec Global Surface Mean Temp Anomalies
NCDC/NESDIS/NOAA (Smith and Reynolds, 2005)
0.51.0Land and Ocean
0. 0 0. 0o C
1880 1900 1920 19 40 1960 1980 2000
Source: National Oceanic and Atmospheric Administration, National Climatic Data Center.
Note: Temperature anomalies are the differences between each year’s temperature and the climate
normal for 1961 to 1990, represented by the horizontal line at 0.0, which is from the University of East
Anglia’s Climate Research Unit.
12 The troposphere is the portion of the atmosphere that extends from the Earth’s surface up
to the stratosphere, about 17 kilometers high near the Equator and about 7 kilometers high
near the Poles.
13 National Research Council, Reconciling Observations of Global Temperature Change,
National Academy Press (Washington, 2000):
In the opinion of the panel, the warming trend in global-mean surface
temperature observations during the past 20 years is undoubtedly real and is
substantially greater than the average rate of warming during the twentieth
invalidates the conclusion that surface temperature has been rising.... [T]he
troposphere actually may have warmed much less rapidly than the surface from
1979 into the late 1990s, due both to natural causes (e.g., the sequence of
volcanic eruptions that occurred within this particular 20-year period) and human
activities (e.g., the cooling of the upper part of the troposphere resulting from
ozone depletion in the stratosphere). (p.2)

As Figure 2 shows, surface temperatures have increased nearly everywhere,
except for cooling in parts of the North Atlantic. Warming has been greatest in the14
northern high latitudes, such as Alaska, Canada, Northern Europe, and Russia.
Although warming over most land areas has occurred year-round, the increases have
generally been greater in the Northern Hemisphere during winter and spring.
Figure 2. Trends in Average Annual Temperature, 1901-2005

Source: NOAA/NCDC, Global Historical Climatology Network (GHCN)-Extended Reconstructed
Sea Surface Temperature (ERSST) data set.
Notes: Cells for which the temperature trend is statistically significant are marked with a + sign if
positive, and - if negative. Data for cells without a + or - mark do not exhibit a statistically significantth
trend for the 20 century. Significant cooling has occurred only in the northern Atlantic surface, the
southeast United States, and the southwest of South America.
14 Temperatures higher than today occurred from about 1925 to 1965, in the northern high
latitudes (60"N and above). Thomas L. Delworth and T.R. Knutson, “Simulation of Early
20th Century Global Warming,” Science, 287, no. 2246 (2000). The rapid Arctic warming
concentrated around 1940 was not widespread across all latitudes, whereas warming since
the 1980s can be seen at all latitudes.

Strong evidence of global warming since 1955 comes from measurements of
heat content of the world’s upper oceans, overall by 0.04oC since 1955.15 Because
oceans store about 84% of the heat on Earth,16 this small warming is considered a
strong signal of long-term change. Additional evidence of global warming comes
from the detection of increasing continental temperatures (measured by boreholes
into rock below the surface), by about 0.02oC, during the past five decades.17 Both
ocean and continental warming corroborate the elevated surface air temperatures.
Global Precipitation. Humans and ecosystems are affected by many aspects
of climate, including precipitation, which has increased over the past century. This
observed increase is consistent with scientific understanding that as warming
temperatures increase evaporation, precipitation will increase to maintain balance in
the water cycle. Over land, precipitation has increased by about 2% since 1900, but
the patterns are highly variable across time and in different locations. Only a few
regions show significant changes (Figure 3). Most of the United States and other
high latitudes, except eastern Russia, have seen greater wetness, whereas
precipitation has decreased in the sub-tropics, such as the Sahel in Africa.
Climate Extremes. Few patterns of change have emerged globally in the
frequency or intensity of most types of extreme events, according to NOAA’s
National Climatic Data Center (NCDC).18 No trend in global thunderstorm
frequencies has been identified. Though there has been a clear trend toward less
frequent extremely cold winter temperatures in some locations, there is no trend in
the frequency of extremely high temperatures. Researchers at NOAA’s National
Center for Atmospheric Research (NCAR) have found that “[w]idespread drying
eastern Australia. Rising global temperatures appear to be a major factor.”19 Drought
area increased more than 50%, mostly due to conditions in the Sahel and southern
Africa over the past few decades. Researchers have found that great floods20th
worldwide increased significantly during the 20 Century, especially in the latter half
of the period. The frequency of floods exceeding the 200-year flood levels also

15 Sydney Levitus et al., “Warming of the World Ocean, 1955-2003,” Geophysical Research
Letters, 32, no. 02604 (2005). “[A] mean temperature change of 0.01"C of the world ocean"
would correspond roughly to a mean temperature change of 10C of the global atmosphere
if all the heat associated with this ocean anomaly was instantaneously transferred from the
ocean to the atmosphere.” See also Sydney Levitus et al., “Anthropogenic Warming of
Earth’s Climate System,” Science, 292, no. 5515 (April 13, 2006), pp. 267-270.
16 Levitus et al. 2005. About 84% of the Sun’s energy absorbed by the Earth since the 1950s
has been stored in the oceans.
17 Ibid.
18 See [].
19 Dai, A., K. E. Trenberth, and T. Qian, “A global data set of Palmer Drought Severity
Index for 1870-2002: Relationship with soil moisture and effects of surface warming,”
Hydrometeorology, 5 (2004), pp. 1117-1130. See also [
/climind/pdsi.html] and [].
20 Great floods are defined as exceeding the levels of floods that would occur on average
once in every 100 years — the 100-year flood — in basins larger than 200,000 km2.

increased significantly, while the frequency of floods having return periods shorter
than 100 years did not increase significantly.21 In most regions, insufficient data
remain a challenge for assessing trends in climate variability, because of the
infrequency of events (by definition) and their spatial variability. In Figure 3,
countries that are not shaded do not have sufficient data to analyze rates of heavy
precipitation. Figure 3 shows that most often, in regions that have robust
precipitation data, extreme precipitation has increased; in a few regions, such as the
Sahel and East Africa, extreme precipitation has decreased.
Figure 3. Changes in Frequency of Extreme
P r e c i pita tion

Source: D.R. Easterling et al., “Observed Variability and Trends in Extreme Climate Events: A Brief
Review, Bulletin of the American Meteorological Society, 81 (March 2000), pp. 417-425. Available
at []. Figure provided by NOAA
National Climate Data Center.
Notes: Signs on the map indicate regions where significant changes in heavy precipitation have
occurred during the past decades, where sufficient data are available to analyze the trend. Plus signs
indicate positive trends in heavy precipitation; negative signs indicate decreasing trends in heavy
precipitation. Locations that are not shaded do not have sufficient data to analyze trends.
21 P.C.D. Milly et al., “Increasing risk of great floods in a changing climate,” Nature, 415
(January 31, 2002), pp. 514-517.

Contentious debate continues regarding trends in hurricane or cyclone frequency
and intensity. For about 85% of the world’s oceans, data are inadequate to detect
long-term changes.22 Only in the extra-tropical23 Atlantic basin has research
established a positive relationship between sea surface temperatures and increased
number and severity of hurricanes or cyclones.24 Since the mid-1980s, satellite data
reveal a distinct increase in tropical cyclone activity associated with higher eastern
Atlantic sea surface temperatures, as well as other factors.25 Much longer series of
high-quality observations and improved understanding of tropical cyclones are
needed to provide definitive attribution of changes in hurricane activity to natural
variability, greenhouse gas (GHG) forcing, or other processes. A November 2006
meeting of international experts on tropical cyclones, convened by the World
Meteorological Organization, concluded that “[d]espite the diversity of research
opinions on this issue, it is agreed that if there has been a recent increase in tropical
cyclone activity that is largely anthropogenic in origin, then humanity is faced with
a substantial and unanticipated threat.”26
Climate Changes Observed in the United States
At the global scale, average annual temperature and precipitation have increased
over the past century, especially since the 1970s. At a regional scale, such as for the
United States, significant climate changes have also been measured.
In the United States, both temperatures and precipitation have increased during
the 20th Century (Table 2), but with important regional variations.27 The mean
temperature for the contiguous United States has increased by 0.6oC (1.1oF) sinceoo

1901 (0.06C or 0.1F per decade), generally following the global oscillations. From

22 Landsea, C.W., G.D. Bell, W.M. Gray, S.B. Goldenberg, “The extremely active 1995
Atlantic hurricane season: Environmental conditions and verification of seasonal forecasts,”
Mon. Wea. Rev., 126 (1998), pp. 1174-1193; Landsea, C. W., “2005: Hurricanes and global
warming, Nature, 438, doi:10.1038/nature04477; Landsea,C.W., B.A.Harper, K.Hoarau, and
J.A.Knaff,Can we detect trends in extreme tropical cyclones? Science, 313 (2006), pp. 452-454.
23 Extra-tropical means outside of the tropics.
24 World Meteorological Organization, “Statement on Tropical Cyclones and Climate
Change” Report of the International Workshop on Tropical Cyclones” IWTC-6, November


25 Kerry Emanuel, “Increasing destructiveness of tropical cyclones over the past 30 years,”
Nature, 436 (August 4, 2005); P.J. Webster et al., “Changes in Tropical Cyclone Number,
Duration, and Intensity in a Warming Environment,” Science, 309, no. 5742 (September 16,
2005), pp. 1844-1846; Stanley B. Goldenberg et al., “The Recent Increase in Atlantic
Hurricane Activity: Causes and Implications,” Science, 293, no. 5529 (July 20, 2001), pp.


26 World Meteorological Organization. Ibid.
27 NOAA/National Climate Data Center, NOAA/NCDC. 2006, Global Surface Temperature
Anomalies, []
(accessed January 19, 2007). Described in Smith, T. M., and R. W. Reynolds, “A global
merged land air and sea surface temperature reconstruction based on historical observations
(1880-1997),” J. Clim., 18 (2005), pp. 2021-2036.

1979 to 2003, the rate of warming in the United States, at 0.33oC (0.6oF) per decade,

has been about twice the rate of the global average. Two regional exceptions to this
overall warming were (1) cooler winters and springs in the Southeast from 1901 to
1978, which reversed to warming after 1979 (except in Florida), and (2) generally
stable or cooler summer and autumn months in the South and central United States.
The warmest year for the United States in the 20th Century was 1998 (with
temperatures boosted by El Nino28), followed by 1934; 2006 was the third-warmest
year in the U.S. record, about 1.1oC (2oF) above the 20th Century average.29
U.S. average precipitation has increased 6.1% since 1895, but with more
variability and a less distinct pattern than temperature (Table 2). The increase in
precipitation was particularly pronounced in the central, south, and east north-central
climatic regions. The Northwest has had large and increasing inter-annual cycles of
precipitation since the 1970s, associated with ENSO (El Nino-Southern Oscillation)
events. In contrast, the Southwest and Hawaii have had decreases in precipitation,
although the trends are not statistically significant because of high inter-annualth
variability. The 20 Century was also marked by strong and extensive droughts in

1931 to 1938 and 1951 to 1956 in the United States.30

28 The El Nino-Southern Oscillation (El Nino, or ENSO) is an irregularly occurring climate
event (typically lasting one to two years every two to seven years) associated with above-
normal sea surface temperatures in the central tropical Pacific Ocean, as well as the Atlantic
and Indian Oceans; it affects weather across the globe. Conversely, La Nina is associated
with lower than normal sea surface temperatures.
29 NOAA/National Climate Data Center, NOAA/NCDC, 2006, Global Surface Temperature
Anomalies, []
merged land air and sea surface temperature reconstruction based on historical observations
30 However, the droughts in the 1930s and 1950s were neither more intense nor as sustained
as droughts that are apparent in the geologic record of the past 1,000 years. Evidence
suggests that two droughts lasting more than 20 years in much of the United States occurredthth
in the late 13 and 16 Centuries, and that some droughts in the Sierra Nevada may have
lasted more than 100 years before 1350 and 1110 A.D. Peter B. deMenocal, “Cultural
Responses to Climate Change During the Late Holocene,” Science, 292 (April 27, 2001),
pp. 667-674.

Table 2. Trends in U.S. Temperature and Precipitation Change
from 1902 to 2005, by Climatic Region
Climatic RegionTemperature(degrees F)Precipitation (%)
Northeast 1 .69 7 .31
So utheast -0.04 2.96
Central 0 .16 7 .91
So uth 0 .04 11.08
East North Central1.6011.55
West North Central1.702.96
So ut hwe s t 1 . 6 3 1 . 4 7
West 2.07 8.96
Northwest 1 .70 5 .45
Alaska 3.31 6.08
Hawaii 1.18 -9 .25
U.S. Climatic Regions

Source: Data and figure provided by NOAA/National Climate Data Center, at
[ oa/ncdc.html ].
Notes: The U.S. map is also available in color at [
science/recentpsc_precipanom.html]; the shading of each grid cell shows the trend of annual
average precipitation for that cell during the period 1895 to 2003. The trends, both in the
table and the map, are determined by statistical regression analysis.

The frequency of extreme precipitation events in the United States has
increased since the 1920s and 1930s, although frequencies in the late 1800s and early
1900s were about as high as in the 1980s and 1990s.31 The number of events “much
above normal” has increased by 20% since 1910.32 Extreme precipitation events
lasting from one to seven days increased at a rate of about 3% per decade from 1931
to 1996.33 In the Southeast, very heavy precipitation not associated with hurricanes
has increased by about 2.6% per decade on average in the 20th Century.34 Although
such increases in precipitation intensity and duration tend to increase the risk of
flooding, some land uses and investments in flood management actively work to
reduce such risks. Tornado frequency since 1955 has not changed much, although
the record is complicated by reporting uncertainties.35
Climate Lessons from the Distant Past
Climate change since the Industrial Revolution has been measured both
globally and across the United States, as increases in temperature and changes in
precipitation. Whether this constitutes natural variability, and how to interpret the
observed change, can be informed by putting it in the context of climate changes that
have occurred over the past thousand, ten thousand and hundreds of thousands of
A number of lessons can be gleaned from paleoclimatology, the study of past
climates, by piecing together extensive and disparate sets of proxy indicators, such
as ice cores, tree rings, fossil records, and the chemical composition of shells. First,
modern human civilizations have developed and thrived in the stable and relatively
temperate climate of the past 12,000 years, a period called the Holocene period.
Global average temperatures have varied fairly narrowly during the past 10,000 years,
within about 2oC, with the maxima around the current global mean temperature and
the minimum temperatures occurring in the 16th and 17th centuries, during the Little
Ice Age, about 1oC cooler than current temperatures. According to NOAA, no
evidence demonstrates that global annual temperatures at any time during the
Holocene were warmer than today.36

31 Kenneth E. Kunkel, “North American Trends in Extreme Precipitation,” Natural Hazards,

29, no. 2 (June 2003), pp. 291-305.

32 Karl, T. R., and R. W. Knight, “Secular Trends of Precipitation Amount, Frequency, and
Intensity in the United States,” Bulletin of the American Meteorological Society, vol. 79, no.

2 (1998), pp. 231-242.

33 Kunkel, Kenneth E., D.R. Easterling, Kelly Redmond, and Kenneth Hubbard, “Temporal
Variations of Extreme Precipitation Events in the United States: 1895-2000,” Geophysical
Research Letters, 30, no. 17 (September 2003), pp. 1900-1903.
34 Groisman, P. Y., R. W. Knight, T. R. Karl, D. R. Easterling, B. Sun, and J. H. Lawrimore,
“Contemporary Changes of the Hydrological Cycle over the Contiguous United States:
Trends Derived from In Situ Measurements,” Journal of Hydrometeorology, 5 (2004), pp.


35 NOAA/NCDC at [].
36 See []. About 6,000 years

Second, some climate shifts can be rapid, occurring on time scales of only
years to decades. Paleoclimatological records show abrupt shifts that have included
changes in hurricane frequency, flooding, drying of lakes, and several mega-droughts
lasting decades to centuries. Recently, evidence shows that the main warming events
ending the last ice age (about 15,000 years ago) took place in less than a decade.
Regionally, the shift was rapid and extreme, with Greenland’s temperature rising in
one step of around 8oC in a decade or less.37
This revelation that the Earth’s climate can change abruptly, with triggers,
amplifiers, and bounds that are not well understood, has precipitated grave concern
among many scientists. A number of studies have found that major ecological
restructuring has accompanied major climatic shifts in the past. Although human
societies have proven adaptable to moderate inter-annual variability and smooth
change, research in several regions indicates that significant structural, and
sometimes catastrophic, reorganizations of regional civilizations (e.g., the Mayas in
the 9th Century, African civilizations)38 have been triggered by past significant
climate changes.

36 (...continued)
ago, during the so-called Holocene Optimum, summers (only) were warmer in the Northern
Hemisphere (only) due to a shift in the Earth’s orbit. The astronomical forcing promptingth
this isolated warmth has not been present during the 20 Century. A panel of the National
Academy of Sciences concluded with high confidence that
20th century than during any comparable period during the preceding four
centuries.... Less confidence can be placed in large-scale surface temperature


(National Research Council, Surface Temperature Reconstructions for the Last 2,000 Years,
Washington: National Academies Press, 2006, p. 3).
With caution regarding the challenges and uncertainties associated with reconstructions of
climates in the distant past, several researchers using independent methods (but often the
same proxy data sets) have concluded that the Earth’s temperature since around 1990
appears to be higher than in at least 2,000 years, above the natural variability present in theth
record. Further, the climate record shows that the warming over the 20 Century has
occurred at a rate that is unprecedented for at least the last 1,000 years.
37 National Research Council, Abrupt Climate Change: Inevitable Surprises (Washington:
National Academies Press, 2002).
38 Lonnie G. Thompson et al., “Abrupt tropical climate change: Past and present,” PNAS,

103, no. 28 (July 11, 2006), pp. 10536-10543; R.B. Alley et al., “Abrupt Climate Change,”

Science, 299 (March 28, 2003), pp. 2005-2010; deMenocal, Peter B., “Cultural Responses
to Climate Change During the Late Holocene,” Science, 292 (2001), pp. 667-674.

Observed Impacts
Scientific research has revealed a number of changes in the Earth’s climate
system in the past century. This section addresses changes observed in human and
ecological systems that may be associated with, and perhaps caused by, the observed
climate changes.
Impacts of climate change over the past few decades are visible on human
activities and ecosystems. There is a likely bias that favors the reporting of detected
changes, rather than the reporting of no changes. Nonetheless, observations confirm
that most biological and physical systems studied are responding to warming andth
other climate changes over the 20 Century in ways scientists would expect, but there
have also been some surprises.
The northern boundary of successful corn production in the United States has
migrated north by 100 miles over the past three decades, according to an official of
the DuPont Corporation.39 One study explained roughly 25% of corn and 32% of
soybean yield trends in certain counties in the Midwest and Northern Great Plains by
observed temperature trends from 1982 to 1998.40 Some salmon fisheries in parts of
Alaska have seen record catches, benefitting from warmer temperatures, while in
western Alaska, the Pacific Northwest and Canada, salmon stocks have decreased,
having passed the upper limits of their temperature tolerance, according to the Alaska
Regional Assessment Group for the U.S. Global Change Research Program.41
In the U.S. West, decreasing trends in mountain snowpack and earlier42
snowmelt have altered the timing of stream flows. This has significant implications
for flood control, irrigation and summer drying of vegetation.
Studies continue to conclude that higher temperatures increase the risks of
heat-related illnesses and deaths, that these vary by location, and that the risks are
elevated in some regions, older and younger age categories, and impoverished

39 William Neibur, Vice President, DuPont Crop Genetics Research and Development,
personal communication (October 31, 2006).
40 David B. Lobell and Gregory P. Asner, “Climate and Management Contributions to
Recent Trends in U.S. Agricultural Yields,” Science, 299 (February 14, 2003), p. 1032.
41 Center for Global Change and Arctic System Research, The Potential Consequences of
Climate Variability and Change: Current Stresses and Future Climate Impacts on Key
Economic Sectors (December 1999), available at [
42 McCabe, Gregory, and Martyn Clark, “Trends and Variability in Snowmelt Runoff in the
Western United States,” Journal of Hydrometeorology, 6 (2005), pp. 476-482; Mote, P.W.
2003 “Trends in Snow Water Equivalent in the Pacific Northwest and their Climatic
Causes,” Geophysical Research Letters, vol. 30, no. 12, p. 1601; Hamlet, Alan F.,
Mote, and D.P. Lettenmaier, “Implications of changing 20 century precipitation variability
for water management in the western U.S.” (2007) (forthcoming).

populations.43 In Europe during the summer of 2003, as many as 52,000 people died
prematurely in the most severe heat wave in at least 500 years. One study concluded
it was very likely that human-driven climate change had more than doubled the risk
of occurrence of such an extreme heat event.44 Significant preparedness and
emergency response systems in a number of cities have substantially lowered the
local risks of mortality during heat waves over the past two decades.
Significant impacts of the warming climate are reported in ecological systems
on every continent. Of more than 1,600 species analyzed by two researchers, more
than half show changes in their phenologies, the timing of their life events (such as
egg-laying or blossoming dates), or their distributions (where they are found),
systematically and dominantly in the direction expected from regional climate
changes.45 Through the 1990s, oceanic plankton productivity has varied with sea
surface temperatures, with warming significantly lowering productivity.46 This raises
concerns among scientists because plankton are a major food source for many marine
Coral bleaching, triggered by some heat episodes, has become increasingly
widespread in many reef regions, including Hawaii, the Caribbean, and Australia’s
Great Barrier Reef. In addition, elevated concentrations of carbon dioxide in the
atmosphere are absorbed by, and are acidifying, the world’s oceans, posing risks to
shell-forming organisms and marine food chains.47
Warming and drying in southeast Alaska and the western United States from
the late 1980s to the present have resulted in pest outbreaks and fires, destroying
property, increasing fire management costs and loss of life, reducing economic forest

43 Kristie L. Ebi et al., “Climate Change and Human Health Impacts in the United States: An
Update on the Results of the U.S. National Assessment,” Environmental Health
Perspectives, 114, no. 9 (September 2006), pp. 1318-1324.
44 Peter A. Stott, D.A. Stone, and Myles R. Allen, “Human contribution to the European
heatwave of 2003,” Nature, 432 (December 2, 2004), pp. 610-614.
45 Camille Parmesan and Gary Yohe, “A globally coherent fingerprint of climate change
impacts across natural systems,” Nature, 421 (2003), pp. 37-42;
46 Michael J. Behrenfeld et al., “Climate-driven trends in contemporary ocean productivity,”
Nature, 444 (December 7, 2006), pp. 752-756.
47 Kleypas, J.A., R.A. Feely, V.J. Fabry, C. Langdon, C.L. Sabine, L.L. Robbins, et al.
Future Research, a report of a workshop held April 18-20 2006, St. Petersburg FL,
[]; Orr, James C., Victoria J.
Fabry, Olivier Aumont, Laurent Bopp, Scott C. Doney, Richard A. Feely, et al.
“Anthropogenic Ocean Acidification over the Twenty-First Century and its Impact on
Calcifying Organisms,” Nature, 437 (September 29, 2005), pp. 681-686; Park, Geun-Ha,
Kitack Lee, Pavel Tishchenko, Dong-Ha Min, Mark J. Warner, Lynne D. Talley, et al.
“Large accumulation of anthropogenic CO2 in the East (Japan) Sea and its significant impact
on carbonate chemistry,” Global Biogeochemical Cycles, 20, no. GB4013 (November 22,


production, and emitting severe air pollution with consequent health impacts.48
Record high temperatures and droughts have also triggered extensive fires in
Argentina, Greece, South Africa, and other locations. Some species, populations,
and individuals have shown benefits from warming conditions, whereas others that
are more reliant on cool habitats have been adversely affected or lost
Mountain glaciers have contracted worldwide over at least the past 200 years,
with evidence that the rate of melting or flow has accelerated in recent decades,
including in Argentina, Bhutan, Canada, India, Kyrgystan, Nepal, Switzerland,
Tanzania, Uganda, Venezuela, and the United States. The glacial ice sheets of
Greenland are melting overall, with an apparent acceleration in the period 2003 to
2005.49 In western Antarctica, accelerated ice melting and loss contrasts with
accumulating ice in East Antarctica because of increased precipitation.50 With loss
of buttressing sea ice, glacial flows have sped up in the past decade in parts of
western Antarctica.51 Record low Arctic sea ice extent in 2005, at 5.6 million square
kilometers, was 20% less than the 1970 to 2000 median.52 Polar bears, which rely
on sea ice to access food, have experienced a significant decrease in cub survival
rates from 2001 to 2006 in the Beaufort Sea area.53 Rising absolute sea levels

48 Breshears, David D., Neil S. Cobb, Paul M. Rich, Kevin P. Price, Craig D. Allen, Randy
G. Balice, et al. “Regional vegetation die-off in response to global-change-type drought,”
PNAS (October 2005); Center for Global Change and Arctic System Research, A Report of
the Alaska Regional Assessment Group: Current Stresses and Future Climate Impacts on
Key Economic Sectors, Fairbanks, Alaska, 1999. Westerling, A.L., H.G. Hidalgo, D.R.
Cayan, and T.W. Swetnam, “Warming and Earlier Spring Increase Western U.S. Forest
Wildfire Activity,” Science, 313 (August 18, 2006), pp. 940-943.
49 S.B. Luthcke et al., “Recent Greenland Ice Mass Loss by Drainage System from Satellite
Gravity Observations,” Science, 314, no. 5803 (November 24, 2006), pp. 1286-1289;
Luthcke et al., at []; W.
Krabill et al., Geophys. Res. Lett., 31, L24402 (2004); H. J. Zwally et al., J. Glaciol., 51,

509 (2005). R. Thomas et al., Geophys. Res. Lett., 33, L10503 (2006); E. Rignot, P.

Kanagaratnam, Science, 311, 986 (2006); O. M. Johannessen, K. Khvorostovsky, M. W.
Miles, L. P. Bobylev, Science, 310 (2005), p. 1013.
50 Robert Bindschadler, “The environment and evolution of the West Antarctic ice sheet:
setting the stage,” Philosophical transactions, Series A, Mathematical, physical, and
engineering sciences, 364, no. 1844 (July 15, 2006), pp. 1583-605; D J Wingham et al.,
“Mass balance of the Antarctic ice sheet,” Philosophical transactions, Series A,
Mathematical, physical, and engineering sciences, 364, no. 1844 (July 15, 2006) pp.


51 Anny Cazenave, “How Fast Are the Ice Sheets Melting?” Science, 314, no. 5803
(November 24, 2006), pp. 1250-1252.
52 See [].
53 Eric V. Regehr, Steven C. Armstrup, and Ian Stirling, Polar Bear Status in the Southern
Beaufort Sea, U.S. Geological Survey (Reston, VA, 2006).

(unaffected by vertical land movement) is attributable to expansion of the oceans’
waters as they warm, and inflow of water from melting glaciers.54
Likely Causes of Global Climate Change
The evidence is strong that the Earth’s climate is changing; the forces thought
to be driving observed climate changes are discussed below, including the evidence
that human activities, particularly greenhouse gas emissions, have contributed a large
influence on top of ongoing natural variability.
The Earth’s climate is driven by the energy balance of the Sun’s radiation
coming into and leaving the Earth’s atmosphere. The more active the Sun, the closer
the Earth to the Sun, or the greater the ability for the Sun’s energy to penetrate the
atmosphere and be absorbed by the Earth, the greater will be the warming tendency
on Earth. On the other hand, the less active the Sun, the farther the Earth is from the
Sun, the more the Earth’s atmosphere or surface reflect the radiation back out to
space, the greater the cooling tendency. The tilt of the Earth’s axis in its orbit around
the Sun, making one or the other hemisphere closer to the Sun most of the year,
drives the heating and cooling of the seasons outside of the tropics. Scientists
understand well the fundamental drivers of the Earth’s climate through geologic time;
for example, how the pattern of an irregular orbit around the Sun has led to regular
climate swings in and out of ice ages, or how massive volcanic eruptions can spew
particles into the atmosphere that block incoming radiation and cause temporary
Until human populations grew to large numbers — from a population of
about 5 million around 10,000 years ago to over 6 billion today — the global climate
was almost certainly not influenced by human activities. However, human clearing
of land, use of fossil fuels for energy, and other activities have greatly changed the
surface of the Earth and the composition of the atmosphere, leading almost certainly
to changes the Earth’s climate through the past 150 years. During this period, the
population grew from about 1.3 billion in 1850 to about 6.5 billion today,55
associated with land clearing, increasing affluence, a switch from wood to fossil
fuels, and industrialization — all increasing greenhouse gas emissions and
atmospheric concentrations. The IPCC science assessment concluded in 2007 that

54 Anthony A. Arend et al., “Rapid Wastage of Alaska Glaciers and Their Contribution to
Rising Sea Level,” Science, 297, no. 5580 (July 19, 2002) pp. 382-386.
55 See [].
56 Intergovernmental Panel on Climate Change Working Group I, Climate Change 2007: The
Physical Basis (Cambridge, UK: Cambridge University Press, 2007), [http://ipcc-].

The Concept of Radiative and Other Forcing of the Earth’s Climate
To compare the contributions of different agents to the balance of incoming and
outgoing energy, scientists use the concept of radiative forcing, which quantifies
the direct or indirect effect an agent has on global mean temperature. This concept
has proved successful in helping to predict global temperatures. A shortcut for
radiative forcing that is easier to compute and considered broadly reliable and,
hence, is often used to compare greenhouse gases is Global Warming Potential
(GWP). GWP is an index of how much a greenhouse gas may, by its potency and
quantity, contribute to global warming over a period of time, typically 25, 75, or
100 years. Non-radiative forcing is an as-yet-unquantified concept of the effect
on the Earth’s energy balance that does not directly and immediately involve
radiation, such as the effects of an increase in evaporation resulting from
agricultural irrigation.
A 2005 panel of the National Academy of Science (NAS) concluded that the
concept of radiative forcing is too limited to express contributions of different
agents to regional climates, variability, or aspects of the climate system other than
mean global temperature. For example, there is no measure of the influence of
agents on precipitation, winds, or other important aspects of climate that may
changedifferently than temperature. The NAS panel concluded that broader
concepts are needed to more fully describe the influences of different agents on
multiple aspects of climate. (National Research Council, Radiative Forcing of
Climate Change: Expanding the Concept and Addressing Uncertainties,
Washington: National Academies Press, 2005.)
The remainder of this section summarizes scientific knowledge of how human
activities influence climate change; the human components are described because
these are most readily addressed by public policies. In addition, greenhouse gases are
particularly implicated in recent climate change and are the target of numerous
programs and proposals intended to stabilize climate change. Natural forcings, over
which humans generally have limited control, are discussed in Appendix A.
Methods to compare the relative roles of human and natural forcings and conclusions
that attribute a large part of observed warming to human activities are discussed at
the end of this section.
The terms radiative forcing,57 non-radiative forcing, and GWP (see “The
Concept of Radiative and Other Forcing of the Earth’s Climate” above), will be used
in the following sections to explain and compare the contributions of different agents
to observed and future climate change. A forcing may lead to a change in climate.
In response to the climate change, other components of the Earth system may also
adjust, resulting in feedbacks to the climate that can either amplify (positive
feedbacks) or dampen (negative feedbacks) the initial change in climate. A number
of researchers have concluded from observing natural forcings and variability that

57 National Research Council, Radiative Forcing of Climate Change: Expanding the
Concept and Addressing Uncertainties (Washington: National Academies Press, 2005).

large climate changes can be triggered by very small changes in forcings because of
feedbacks that amplify the initial change.58
Human Activities that Influence Climate Change
Virtually all climate scientists agree that human activities have changed the
Earth’s climate, particularly since the Industrial Revolution.59 Consumption of fossil
fuels and clearing of land, as well as industrial and agricultural production release so-
called greenhouse gases (GHG). Other human-related influences on climate include
air pollution, such as tropospheric ozone and aerosols (tiny particles), land use
change, paving and urban development, and airplane emissions. The different ways
in which humans are affecting climate change are discussed in the following sections.
Greenhouse Gases. Greenhouse gas concentrations in the Earth’s
atmosphere have increased dramatically since the Industrial Revolution, with carbon60
dioxide growing from about 280 ppm in 1850 to about 380 ppm today. The
presence of greenhouse gases is critical to trapping the Sun’s energy and warming the
planet to habitable temperatures. Human activities, such as use of fossil fuels,
production of crops and livestock, and manufacture of various products, now emit
certain gases in sufficient quantities to have raised concentrations higher than they
have been for hundreds of thousands of years; the elevated concentrations are
changing the balance of solar radiation in and out of the Earth’s atmosphere and,
consequently, altering the Earth’s climate.
Greenhouse gases (GHG) in the atmosphere allow the Sun’s short wave-
length radiation to pass through to the Earth’s surface, but once the radiation is
absorbed by the Earth and re-emitted as longer wave-length radiation, GHG trap the
heat in the atmosphere. The best-understood greenhouse gases include carbon
dioxide (CO2), methane (CH4), nitrous oxide (N2O), and certain fluorinated
compounds, including chlorofluorocarbons (CFC), hydrochlorofluorocarbons
(HCFC), hydrofluorocarbons (HFC),61 perchlorofluorocarbons (PFC), and sulfur

58 R.B. Alley et al., “Abrupt Climate Change,” Science, 299 (March 28, 2003), pp.


59 National Research Council, Radiative Forcing of Climate Change: Expanding the
Concept and Addressing Uncertainties (National Academies Press, 2005); S. Fred Singer,
“Human Contribution to Climate Change Remains Questionable,” EOS Transactions, 80
(April 20, 1999), pp. 183-187.
Stauffer, 1994, Historical CO2 record from the Siple Station ice core, in Trends: A
Ridge National Laboratory, U.S. Department of Energy (Oak Ridge, TN). Also, World Data
Centre for Greenhouse Gases (WDCGG), WMO Greenhouse Gas Bulletin: The State of
Greenhouse Gases in the Atmosphere Using Global Observations through 2005 (Geneva,

2006), at [].

61 The production of CFC and HCF and additional substances is regulated by the EPA in
compliance with the Montreal Protocol to Protect the Stratospheric Ozone Layer, its London
Amendment, and other subsidiary international treaties. Because they are covered by the

hexaflouride (SF6).62 These greenhouse gases remain in the atmosphere for decades
to thousands of years and are generally well-mixed around the globe; hence, their
warming effects are largely global. (See “The Concept of Radiative and Other
Forcing of the Earth’s Climate” above). Moreover, the long atmospheric residence
time and the cumulative effects of gases have important implications for possible
policy responses. (See “Time Lags in the Climate System” below). Because these
GHG affect radiative balance of the Earth in similar ways, they can be compared
using measures of radiative forcing or Global Warming Potentials (GWP),63 the
latter being an easier but imperfect approximation.
The following human-related sources of the principal greenhouse gas
emissions have been identified:
!Carbon dioxide (CO2): combustion of fossil fuels, solid waste,
wood, and wood products; cement manufacture. Human activities
can also enhance or reduce removals of CO2 from the atmosphere by
vegetation and soils (e.g., via reforestation or deforestation).
!Methane: coal mining, natural gas handling, trash decomposition in
landfills, and digestion by livestock. Significant natural sources
include wetlands and termite mounds.

61 (...continued)
Montreal Protocol and subsidiary agreements, they are not covered by the Kyoto Protocol,
nor by many proposals for reductions of GHG emissions. However, there may be
opportunities to reduce their emissions further.
62 Water vapor is the most important greenhouse gas but is only indirectly affected by
human activities, as discussed in Appendix A. Additional pollutant emissions indirectly
affect climate change largely on the local to regional scale, including carbon monoxide
(CO), nitrogen oxides (NOx) and non-methane volatile organic compounds (NMVOC), and
particulate matter or aerosols. NOx and NMVOC, as well as methane (CH4), contribute to
ozone pollution (smog) in the troposphere, which is a greenhouse gas. Aerosols, which are
extremely small particles or liquid droplets, such as those produced by emissions of SO2 or
elemental carbon, can also strongly affect the absorption or reflection of radiation in the
atmosphere. Substances that deplete the stratospheric ozone layer, such as
chlororfluorocarbons (CFC), also indirectly affect the climate, because the loss of
stratospheric ozone causes local cooling and changes the patterns of temperatures and
atmospheric circulation. These radiatively important pollutants are controlled, to varying
degrees, by regulations in many countries (including the United States under the Clean Air
63 GWPs are a useful but imperfect shortcut for radiative forcing. They are calculated using
the potency of the radiative effect of one unit of a GHG times its potency, integrated over
the atmospheric lifetime of that GHG. GWPs require selecting a time period (typically 25,

75, or 100 years) over which the effects are taken into account. In other words, using 25-

year GWPs gives greater emphasis to the forcings that are potent but short-lived in the
atmosphere (e.g., methane), having greater effect on short-term global warming; the 100-
year GWPs give greater emphasis to the gases that last in the atmosphere for decades to a
hundred years (e.g., carbon dioxide), having greater effect on century-scale global warming.

!Nitrous oxide (N2O): nitrogen fertilizers, certain industrial
manufacturing, and combustion of solid waste and fossil fuels.
!Chlorofluorocarbons (CFC), hydrochlorofluorocarbons (HCFC),
hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), sulfur
hexafluoride (SF6): commercial, industrial, and household products.
The share of emissions coming from each sector varies greatly by gas. Figure
4 reflects the fact that agricultural production contributes very little to carbon dioxide
emissions (aside from land clearing), but is about 62% of nitrous oxide emissions
globally, mainly from fertilizer use.
Figure 4. Sectoral Shares of Global GHG Emissions in 2000

Was te
4%Electricity & Agriculture
He a t13%
Proce ss e s
Land Use Manufacturing & Construction
Change & 10%
Fo re s t ry
International Transportation
Bunker Fuels12%
2%Other Fuel Fugitive
Co mbus tio nEmis s io ns
Source: Data extracted from Climate Analysis Indicators Tool (CAIT) version 4.0 (Washington:
World Resources Institute, 2007), available at [].
Notes: The GHG emissions included in this data set are CO2, CH4, N2O, HFC, HCFC, and SF6, from
human-related sources only. Other greenhouse gases, such as tropospheric ozone, are not emitted
directly and so cannot be tallied as emissions. In addition, the CFC, HCFC, and other pollutants
limited by the Convention to Protect the Stratospheric Ozone are not included in this count; their
influence is still significant but declining under control programs. Nor does this figure include
emissions of aerosols, including sulfates, black carbon, and organic carbon, which may have strong
temporary and regional effects that cannot be quantified comparably with the long-lived gases
represented in this figure.

For the year 2000,64 CO2 constitutes approximately 72% of the human
contribution to GHG emissions; CH4 is about 18% and N2O is about 9%. There is
considerable uncertainty regarding some of the historical estimates, especially prior
to the 1950s.
Although most of the GHG occur naturally to some degree, the human-driven
emissions of GHG are increasing above the rate of their natural removals from the
atmosphere. Scientists are certain that GHG emissions from human activities have
increased GHG concentrations in the atmosphere to levels unprecedented for
hundreds of thousands, possibly even millions, of years. Over the past 150 years,
CO2 concentrations have increased globally by more than one-third, from about 280
ppm to current levels of about 380 ppm (Figure 5).65 Methane has increased by
about 150%, although the rate of increase has declined over the past decades, down
to essentially no growth (varying slightly) in recent years. N2O concentrations have
increased by 16% since the Industrial Revolution. (For data sourcing, see Figure 5.)

64 Data source: Emission Database for Global Atmospheric Research version 3.2, Fast Track

2000 Project, using 100-year GWPs.

65 For reference, the UN Framework Convention on Climate Change (UNFCCC), an
international treaty signed by the United States and ratified by the Congress in 1992,
establishes an objective of “stabilization of greenhouse gas concentrations in the atmosphere
(Art 2 ) (1) Although science can help to identify the degree of “interference” and
implications of climate changes at different concentration levels or degrees of temperature
change, most scientists agree that the determination of “dangerous” is a political decision,
not one that can be objectively decided by scientists. A number of proposals of stakeholders
in the United States and other countries most often aim to stabilize carbon dioxide
concentrations in the atmosphere at levels of 450, 550, or 650 ppm. Some scientists suggest
that current levels already have exceeded the “dangerous” threshold.

2, Methane and Nitrous Oxide Concentrations over 400,000 Years Ago to 2004

Source: Data accessed through the Carbon Dioxide Information Analysis Center (CDIAC), with full66
citations in footnote.
66 This figures uses data sets from numerous individual studies of Barnola, J.-M., D.
Raynaud, C. Lorius, and N.I. Barkov (2003), Historical CO2 record from the Vostok ice
core. Blunier, T. and E.J. Brook (2001), “Timing of millennial-scale climate change in
Antarctica and Greenland during the last glacial period,” Science, 291 (January 5, 2001), pp.
109-112. Chamard, P., L. Ciattaglia, A. di Sarra, and F. Monteleone (2001), Atmospheric
CO2 record from flask measurements at Lampedusa Island; James W. Elkins, James H.
Butler, Thayne M. Thompson, Geoffrey S. Dutton, Stephen A. Montzka, Bradley D. Hall,
Halocarbons and Other Atmosphere Trace Species Group (HATS) /CMDL/NOAA; D.M.
(1998) Historical CO2 records from the Law Dome DE08, DE08-2, and DSS ice cores;

Tropospheric Ozone. Ozone is another greenhouse gas, but it is not
emitted directly by humans. Although it occurs naturally, tropospheric ozone is
elevated by polluting emissions, such as nitrogen oxides from fuel combustion or
volatile organic compound (VOC) emissions from fuel leakage, solvent evaporation,
etc. Tropospheric ozone concentrations, both background levels and episodes of high
concentrations, have been increased, perhaps 50%, by polluting emissions since the67
Industrial Revolution. Ozone forms and dissipates quickly, so its concentrations
are unevenly distributed in time and space; hence it is difficult to compare the forcing
of troposphere ozone with other GHG through Global Warming Potentials.
Tropospheric ozone pollution drifting into the Arctic region may be responsible for68
one-third to one-half of the warming observed in its springs and summers. In many
countries, ozone concentrations are controlled by regulations that limit air pollutant
emissions, such as the Clean Air Act in the United States.
Sulfur and Carbon Aerosols. Aerosols are tiny particles suspended in
the air; some are there from natural sources, such as volcanoes and forest fires,
whereas others result from human pollution, such as emissions from powerplants or
vehicles. The principal aerosols of concern to climate change are sulfates, black
carbon, and organic carbon. Aerosols can scatter or absorb light, with cooling or
warming effects, respectively, depending on the size, color, composition, and other
characteristics of aerosols. Black carbon aerosols are thought primarily to warm the
atmosphere; organic carbon aerosols (emitted largely by forest fires) are thought to
have mostly a cooling effect.

66 (...continued)
Raynaud, and J.-M. Barnola (2002), “High resolution Holocene N2O ice core record and its
relationship with CH4 and CO2,” Glob. Biogeochemical. Cycles, volume 16, no. 1 (March

2002), 10.1029/2001GB001417; Hashita et al.; Keeling, C.D. and T.P. Whorf (2005),

Atmospheric CO2 records from sites in the SIO air sampling network; Neftel, A., H. Friedli,
E. Moor, H. Lötscher, H. Oeschger, U. Siegenthaler, and B. Stauffer (1994), Historical CO2
record from the Siple Station ice core; Petit, J.R. et al. (2001), Vostok Ice Core Data for
Series #2001-076, NOAA/NGDC Paleoclimatology Program (Boulder, CO); Sowers, T.,
R.B. Alley, and J. Jubenville (2003), “Ice Core Records of Atmospheric N2O Covering the
Last 106,000 Years,” Science, vol. 301, no. 5635, pp. 945-948 (August 15, 2003); Steele,
L. P., P. B. Krummel, and R. L. Langenfelds (2002), Atmospheric CO2 concentrations from
version); Thoning, K.W. and P.P. Tans (2000), Atmospheric CO2 records from sites in the
NOAA/CMDL continuous monitoring network. All in Trends: A Compendium of Data on
Global Change, Carbon Dioxide Information Analysis Center, Oak Ridge National
Laboratory, U.S. Department of Energy (Oak Ridge, TN), at [
trends/trends.htm] .
67 Larry W. Horowitz, “Past, present, and future concentrations of tropospheric ozone and
aerosols: Methodology, ozone evaluation, and sensitivity to aerosol wet removal,” Journal
of Geophysical Research, 111, no. D22211 (November 21, 2006).
68 Shindell, D.T., G.A. Schmidt, R.L. Miller, and D. Rind, “Northern Hemisphere winter
climate response to greenhouse gas, ozone, solar, and volcanic forcing,” J. Geophys. Res.,

106 (2001) pp. 7193-7210.

Sulfur aerosols (sulfates) scatter incoming solar radiation and have
consequent cooling influence on climate. This has been well known for decades but
only included in climate modeling since the early 1990s. Sulfate aerosols are a by-
product of sulfur emissions, largely from the burning of coal and oil, as well as some
industrial processes. Sulfur emissions and their aerosols have increased dramatically
over the past century.
Aerosol effects on temperature are both regional and short-lived (as particles
typically remain suspended in the atmosphere for days to weeks). Aerosol
concentrations in the atmosphere fluctuate greatly, are difficult to measure, and
consequently are uncertain by a factor of two or more. Aerosols are also understood
to affect precipitation patterns downwind of their emissions, although research is just
beginning to reveal the processes involved; they may influence monsoon water
cycles69 as well. Aerosols may having amplifying or dampening effects when
interacting with such factors as sea surface temperatures70 and snow cover.71 The role
of aerosols in driving various aspects of climate is one of the major uncertainties
being tackled by monitoring and research.
Emissions from Aviation. Emissions from fuel consumption by aircraft
and water vapor emissions in their exhaust both contribute to climate change in
special ways. First, these GHG are emitted at high altitudes, where few other GHG
are present, and therefore do not overlap other gases’ absorbing spectra, increasing
their relatively small contribution to global surface temperatures. They also affect
the vertical distribution of temperatures in the atmosphere. More complex, the
emissions of small particles and water vapor form ice crystals in aviation contrails
that can produce more clouds in the upper troposphere. These clouds can have a
cooling or warming effect depending on the characteristics of the ice crystals; most
scientists believe that the overall climate effect of contrails is a net warming. These
are not globally distributed and therefore have stronger regional than global effects.
Land Surface Changes. Although the Earth’s land surface changes
naturally, as part of ecosystem processes, humans have had a major impact on land
cover and land uses that, in turn, affect the climate system. At least one scientist has
provided evidence that human forest clearing and rice production, beginning roughly
8,000 and 5,000 years ago, respectively, may have significantly affected carbon
dioxide and methane concentrations, as could the carbon sequestration from forest
regrowth following abandonment of farms in Medieval times after the bubonic
plague. 72

69 Lau, K.-M., and K.-M. Kim, “Observational relationships between aerosol and Asian
monsoon rainfall, and circulation,” Geophysical Research Letters, 33 (2006) (L21810).
Letters, 32 no. 02604 (2005).
71 James Hansen and Larissa Nazarenko, “Soot climate forcing via snow and ice albedos,”
PNAS, 101, no. 2 (January 13, 2004), pp. 423-428.
72 William F. Ruddiman, “The Anthropogenic Greenhouse Era Began Thousands of Years
Ago,” Climatic Change, 61, no. 3 (December 2003), pp. 261-293.

Land Clearing. When humans clear land, as happened in the United States
from its early years and well into the 20th Century, CO2 is emitted mostly through
burning or decomposition, increasing CO2 concentrations in the atmosphere. As
vegetation regrows, it absorbs CO2 from the atmosphere, albeit more slowly than
emission occurred. Net deforestation is occurring globally, mainly in developing
countries. The United States, like many other countries, cleared land decades ago but
abandoned some of it with industrialization and migration to more fertile lands;
regrowth of forests on abandoned lands is, in net, estimated to be removing CO2 from
the atmosphere. In other words, U.S. forests overall are a net sink for carbon, not a
source at this time, on the order of 780 million metric tonnes of CO2 per year.73
Agriculture currently covers about one-third of the Earth’s land surface. Agriculture
can also alter the evaporation and transpiration of plants on land, and can alter local
to regional climates, and, via atmospheric circulation, possibly modify global climate
as well.
Land Cover Feedbacks. Land cover change also results from climate
change, and therefore can be a feedback within the climate system. On the one hand,
CO2 in the atmosphere is effectively a nutrient to plants, and this higher carbon
fertilization will tend to increase vegetation growth and remove more CO2 from the
atmosphere. Where precipitation increases, and, to a lesser degree, where currently
cool locations warm, vegetation is expected to increase, creating a negative feedback
to climate warming. To the degree that vines and other weedy plants thrive better in
higher CO2 and warmer temperatures than woody trees, the enhanced carbon uptake
may be short-lived. Moreover, warmer temperatures and greater moisture will tend
to speed up decomposition, and even potentially cause die-back at high levels,
generating a positive feedback to climate warming. In addition, trees and other
vegetation transpire water vapor (another GHG) into the atmosphere. Also, land
cover can alter the amount of dust raised by wind into the atmosphere.
Albedo. The reflectivity of the Earth’s surface is called albedo. Where the
Earth’s surface has low albedo (i.e., is not very reflective), the Sun’s radiation is
absorbed and warms the surface. Particles deposited on the snow/ice surface (e.g.,
from pollution) can darken the surface and increase melting. In places covered with
snow or ice, the surface has very high albedo; as the extent of snow and ice
diminishes with climate warming, the reflectivity decreases and creates a positive
feedback to climate, leading to more warming. Land cleared of its vegetation may
reflect light more than the dark leaves that previously shaded it, increasing reflection
of solar energy, and having a cooling impact. When snow is on the ground, the
removal of trees can have a particularly strong albedo cooling effect, but mostly in
winter or locations with permanent snow or ice. Land clearing tends to warm
temperatures near the equator and cool them at high latitudes.74

73 U.S. Environmental Protection Agency, The U.S. Inventory of Greenhouse Gas Emissions
and Sinks: Fast Facts (Washington , 2006).
74 N. Ramankutty, C. Delire, and P. Snyder, “Feedbacks between agriculture and climate:
An illustration of the potential unintended consequences of human land use activities,”
Global and Planetary Change, 54 (July 2006), pp. 79-93.

Methods To Compare Human and Natural Causes
Multiple factors simultaneously influence the Earth’s climate, and scientists
have developed a variety of methods that help determine which forcings are
contributing, and are likely most important, at any period. Several different lines of
evidence discovered in the past decade have led a large majority of scientists to
conclude that human-related greenhouse gas emissions have contributed substantially
to the increase in global mean temperature and other climate changes observed since
the 1970s, and probably over the past century. Additional factors also contribute,
including solar variability, volcanoes, and natural variability.
The simplest method of analyzing the role of greenhouse gases in climate
change is to compare CO2 concentrations in the atmosphere with surface
temperatures. Through the past million years or more, CO2 and CH4 concentrations
have been tightly correlated with global temperatures in the paleologic records.75
This correlation may be insufficient to discern the triggering cause of the changes,
but most scientists are confident that once warming has been initiated, there are
strong positive feedbacks to CO2 levels and again to climate warming, leading to a
strongly amplifying effect of the initial cause.76 This would explain instances of
timing mismatches, where rising CO2 concentrations lag behind rising temperatures,
evidenced in the paleoclimatological record. However, some scientists also raise
problems in the proxy records, pointing to time lags among the complex interactions
within the climate system, and to additional drivers of change that, at times, may
exceed the forcing of CO2 in the atmosphere.77
Another method scientists use to attribute climate change to various sources,
or to project future changes, relates to the concept of causative radiative forcing.
Figure 6 shows an estimation of the relative contributions of GHG and other agents
to radiative forcing from 1950 to 2000.78 The apparent strength of greenhouse gas
forcing has steadily grown and seems to dominate other known forcings in its effects
on warming the Earth’s climate. Volcanic emissions of stratospheric aerosols are
also major but episodic and short-lived drivers. Tropospheric aerosols, including
sulfates, black carbon, and organic carbon have had a much smaller but growing
cooling effect. From these data, the effect of solar variability on recent temperature
change is apparent, but small. Some scientists, however, contend that solar
variability has a larger role.

75 European Project for Ice Coring in Antarctica (EPICA), at [
esf_article.php?activity=1&article=85&domain=3]; Isabel P. Montanez et al., “CO2-Forced
Climate and Vegetation Instability During Late Paleozoic Deglaciation,” Science, 315, no.

5808 (January 5, 2007), pp. 87-91.

76 For example, Heiko Palike et al., “The Heartbeat of the Oligocene Climate System,”
Science, 314, no. 5807 (December 22, 2006), pp. 1894-1898.
77 Thomas J. Crowley and Robert A. Berner, “CO2 and Climate Change,” Science, 292, no.
5518 (May 4, 2001), pp. 870-872; Lee R. Kump, “Reducing uncertainty about carbon
dioxide as a climate driver,” Nature 418 (September 12, 2002), pp. 188-190.
78 J. Hansen et al., Journal of Geophysical Research, 107, D18, 4347, (2006).

A third method scientists have developed, especially since 2000, is the
concept of fingerprinting the patterns of radiative forcing and observed change, and
comparing them.79 Different forcing agents produce different patterns of climate
change over time and space, and even vertically in the atmosphere. For example,
volcanoes spew aerosols that persist only a few years in the atmosphere, creating a
temporary cooling effect over both land and oceans. Reductions in solar irradiance
can last decades and affect land temperatures more than oceans. These patterns are
very different from, for example, the influence of long-lived greenhouse gases that
are expected to exert long-lived, global influence.
The observed warming of the climate, described above in “Changes Observed
in the Earth’s Climate,” corresponds to expected greenhouse gas-induced patterns of
warming greater in winter than summer, more at night than daytime, and of generally
increasing precipitation, and more warming at high latitudes than low latitudes.
Another piece of evidence is the observed cooling, as expected, at 50 km and higher
in the atmosphere, which cannot be explained by seasonal or solar cycles. Also, the
increasing heat content of the oceans cannot be explained by urban heat islands or
other placement-related problems with measurement stations. Fingerprint methods
have proven critical in attributing the climate change over the past decades to
greenhouse gas emissions versus natural forcings.
Several studies have tried to explain historical climate change by running
computer models — mathematical simplifications of how scientists understand
climate processes to work — with different combinations of forcing agents. (See
“Use of Models for Climate Change Analysis” below.) The results from one are
presented in Figure 6. Scientists have repeatedly found that they cannot reproduce
the warming of the last half century with natural forcings alone, but can generate
warming patterns similar to observed climate changes when including greenhouse
gases and aerosols. These studies constitute one of the important lines of evidence
that lead scientists to conclude that recent climate change has been caused in large
part by greenhouse gases.

79 D.T. Shindell et al., “Volcanic and solar forcing of climate change during the preindustrial
era,” Journal of Climate, 16 (2003), pp. 4094-4107.

Figure 6. Estimated Effects of Different Forcings on Global
Temperature Since 1880

Source: J. Hansen et al., JGR, 107, D18, 4347, 200.
Attribution of Climate Change in the 20th Century
In conclusion, a variety of natural and human-related forces have influencedth
observed climate change throughout the 20 Century, including greenhouse gases
accumulating in the atmosphere, other air pollutants, land use change, solar
variability, and volcanoes. The relative importance of each of these factors is not
well quantified.80 Nonetheless, relying on a variety of tests, a preponderance ofth
scientists have concluded that the observed climate change over the 20 Century
cannot be explained without including the effects of rising concentrations of
greenhouse gases. A panel of the National Academy of Science, at the request of
President George W. Bush, reviewed the established research in 2001 and concluded
human activities, but we cannot rule out that some significant part of these81
changes is also a reflection of natural variability.
80 Thomas L. Delworth and T.R. Knutson, “Simulation of Early 20th Century Global
Warming,” Science, 287, no. 5461 (March 24, 2000), pp. 2246-2250.
81 National Research Council, Climate Change Science: An Analysis of Some Key Questions
(Washington: National Academies Press, 2001), p. 1.

The attribution to greenhouse gas forcing of significant climate change since
the 1970s has been strengthened since the NRC 2001 findings by a number of
additional studies, including matching of the spatial and temporal patterns of
greenhouse gas forcing with observed ocean heat distribution.82 The IPCC science
assessment concluded in 2007 that “most of the observed increase in globally
increase in anthropogenic greenhouse gas concentrations.”83
Projections of Future Human-Driven
Climate Change
Much of the discussion of future climate change is based on projections
produced by computer models that represent as completely as possible the relevant
factors that are today understood to influence the climate (including the effects of
past climates). These models are incomplete, as scientific understanding of the
relevant factors and processes is continuously developing. However, climate models
have improved substantially over the past decade, and experts believe that many now
do a better job of representing the current and historical climates. It is disagreement
about the ability of these models to predict future climate change that drives much
of the current climate change debate. This section explains how projections of
climate are produced and provides the range of forecasts provided by the many
climate analysis institutions around the world.
Most studies indicate, and experts generally agree, that growth of greenhouse
gas forcing, if it continues unabated, will raise global average temperatures well
above natural variability. The 2007 IPCC scientific assessment concluded, “[F]or the
next two decades a warming of about 0.2oC [0.36oF] per decade is projected for a

82 Tim P. Barnett et al., “Penetration of Human-Induced Warming into the World’s
Oceans,” Science, 309, no. 5732 (July 8, 2005), pp. 284-287. See also B D Santer et al.,
“Contributions of anthropogenic and natural forcing to recent tropopause height changes,”
Science, 301, no. 5632 (July 25, 2003), p. 479. Gerald A. Meehl et al., “Combinations of
17 (October 1, 2004), pp. 3721-3728. Pet D.T. Shindell et al., “Volcanic and solar forcing
of climate change during the preindustrial era,” Journal of Climate, 16 (2003), pp.
4094-4107; S. Fred Singer, “Human Contribution to Climate Change Remains
Questionable,” EOS Transactions, 80 (April 20, 1999), pp. 183-187. A. Stott et al.,
“Observational Constraints on Past Attributable Warming and Predictions of Future Global
Warming,” Journal of Climate, 29 (July 1, 2006); Simon F.B. Tett et al., “Causes of
twentieth-century temperature change near the Earth’s surface,” Nature, 399 (June 10,

1999), pp. 569-572.

83 Intergovernmental Panel on Climate Change Working Group I, Climate Change 2007: The

range of [SRES]84 emission scenarios. Even if the concentrations of all greenhouse
about 0.1oC [0.2oF] would be expected.” It further found, “[T]he best estimate for
the low [SRES] greenhouse gas emission] scenario (B1) is 1.8oC (likely85 range is
1.1oC to 2.9oC) [3.2oF (likely range is 2.0oF to 5.2oF)], and the best estimate for the
high scenario (A1F1) is 4.0oC (likely range is 2.4oC to 6.4oC) [5.2oF (likely range is
4.3oF to 9.5oF)].” It will be many years to decades before the wide range of
uncertainty in global average temperature increases can be narrowed with confidence.
(See “Use of Models for Climate Analysis” below.)
Climate models generally predict more heat waves, droughts, and floods;
extreme cold episodes are predicted to decrease; the centers of continents are likely
to experience summer warming and dryness. Scientists expect precipitation will be
more intense when it occurs (therefore also increasing runoff and the risk of
flooding).86 But it will be substantially harder to establish the range of possible
changes in the hydrologic cycle — or even direction of change for some regions —
both because there are fewer historical observations on which to build scientific
understanding, and because the physical constraints are weaker. Scientists expect
atmospheric and ocean circulation are likely to change as well.
Studies have found that future climate change will not be evenly distributed
geographically or temporally: even if the global mean temperature were to change
very little, regional climate changes could be dramatic because of the uneven
distribution of forcings by different agents and the connectedness of regions within
the climate system. Although almost all regions are expected to experience warming,
some regions are projected to become wetter while others become drier.
Future climate change is not likely to proceed smoothly, as often depicted by
averaged model results, but to swing up and down around a rising average, as has
occurred in the past. This variability around a rising average may complicate the
detection and prediction of change. Though the ability of scientists to understand and
model changes is advancing, there will remain major uncertainties in the forecasting
of local and seasonal climate changes that accompany global warming. Climate
models are not yet adept at capturing extreme events or abrupt changes, and there is
significant potential of important climate “surprises” that models may not predict.
It is unclear how serious future changes may be, given the climate variability to
which humans and ecosystems are already adapted.

84 Special Report on Emission Scenarios of the IPCC (2000). This report estimates
greenhouse gas emissions and uptake with a variety of plausible, no-control-policyst
assumptions over the 21 century.
85 “Likely” means greater than 66% likelihood.
86 Kevin E. Trenberth et al., “The Changing Character of Precipitation,” Bulletin of the
American Meteorological Society (September 2003), p. 1205.

Use of Models for Climate Change Analysis
In deciding whether to take action to address climate change, and what actions may be
effective, decision-makers seek projections of what to expect in the future. Scientists
cannot rely only on analogies to past climates because the Earth’s current biological,
chemical, and geologic systems, and human activities, have no precedent. Scientists use
models, first, to understand the system they are studying, building from theory and
validating with experiments and observations of the past, in order to interpret the causes
of past variability and to use that understanding to forecast the future.
Models are simplified representations of systems. Almost all of the models used for
climate change analysis are mathematical, and they are developed using a wide variety
of disciplines, including physics, atmospheric chemistry, economics, engineering,
ecology, and others. Over time, and especially in the past decade, different disciplines
have joined expertise and tools to provide more integrated — and complete — models
for analyzing climate change. As important, while climate models a few decades ago
were built primarily on theoretical understanding, the recent expansion of monitoring,
computing capacity, and funding has allowed data assimilation, or the use of real-world
observations, to improve the models.
Rigorous comparisons of models help to validate their performance by reproducing
observations of today’s climate, although good performance on this test does not
guarantee reliable future projections of climate changes or their patterns. Models are
also tested in their ability to reproduce paleoclimatic events, and are compared in detail
to understand why models respond differently to forcings.
Since the 1990s, climate models perform significantly better in reproducing current and
historical climates, although models diverge in important ways in the patterns of climate
that they produce. Since 2000, important improvements have been made in modeling
changes in some regions, while large discrepancies exist for a few regions. Climate
models are generally less successful in reproducing observed precipitation than
temperature, perhaps because of its higher natural variability, and extreme events and
local climate predictions require a higher resolution than global climate models currently
offer. Also, different models produce different results. In long-term projections of
climate change, the differences between climate models for a given GHG emission
scenario can be larger than the differences produced by one model running the range of
future GHG emission scenarios. Furthermore, there is some scientific opinion that,
while research is critically important for improving scientific understanding of the
climate system and for possible future changes, research may well increase the range of
uncertainty, as new processes are uncovered or existing structures are tested and revised.
Methods and models are improving in their ability to characterize important
uncertainties and to support risk assessment and risk management decisions in spite of
the unknowns, just as in other sectors such as finance, security and medicine. The
application of risk assessment and management techniques to climate change decision-
making is nascent, but is providing useful insights for incorporating uncertainties into

Another complication in forecasting climate change is the importance of
feedbacks — both positive and negative. A National Research Council report
concluded that feedbacks in the climate system have, many times in the geologic
record, amplified small initial climate perturbations into major climate cycles with
global mean temperature swings of 5o to 6oC.87 The close linkage, over hundreds of
thousand of years, between past temperature swings and carbon dioxide and methane
concentrations in the atmosphere strongly suggests that temperature change can
trigger strong positive amplifications through the carbon cycle and water vapor
feedbacks. Current models include such feedbacks, but they are very uncertain. The
most important and uncertain feedbacks affecting future climate projections include
water vapor feedback, cloud feedbacks, vegetation feedbacks, and albedo.88
Thus, while most climate scientists conclude with high confidence that future
climate change, forced by greenhouse gases, land use change, and natural factors, is
probable, the magnitude, rapidity, and details of the changes are likely to remain
unclear for many years, or even decades. There is near unanimity among climate
model projections that (1) past human emissions have committed the climate to some
change over the next few decades,89 and GHG emissions emitted from now on will
begin to dominate global warming by mid-century, and (2) feedbacks to the carbon
cycle tend to be positive, amplifying initial warming by greenhouse gases. The latter
suggests also that climate change may reduce the effectiveness of carbon uptake by
oceans and vegetation, and that more warming would require proportionately greater
GHG emission reductions to stabilize the climate system.
Impacts of Projected Climate Change
Projected impacts of future climate change indicate that there will be winners
and losers among regions, sectors, and income groups. Some groups may benefit
from a certain amount of climate change, whereas others may suffer harm. Regions
that fare relatively well may be negatively affected by changes in other regions
through trade, security, and humanitarian demands and immigration pressures.
Future generations are likely to experience more change, but may also be wealthier
and hence better able to adapt, although not uniformly so. Many species may become
extinct, while others are likely to flourish. The local effects of climate change may
contribute more to decision-making than national or global aggregates.
In April 2007, the IPCC released its fourth assessment of the impacts of
climate change and vulnerability to these impacts. Selected key findings from that
report are provided in the box below.



88 Feedbacks relating to human economies and population distributions may be very
important as well. However, there are very few “integrated” models capable of exploring
the physical and economic systems together.
89 Wigley, T M L, “The Climate Change Commitment,” Science, 307, pp. 1766-1769.

IPCC Climate Change 2007: Selected Key Findings on Impacts,
Adaptation and Vulnerability
Evidence from all continents and most oceans show that many natural systems
are being affected by regional climate changes, such as —
!enlargement and increased numbers of glacial lakes, ground
instability in permafrost regions, rock avalanches and changes in
some Arctic and Antarctic ecosystems;
!in many glacier- and snow-fed rivers, increased run-off and earlier
peak flows;
!effects on thermal structure and water quality in lakes and rivers
showing warming;
!earlier timing of spring events, such as leaf-unfolding, bird
migration and egg-laying;
!earlier “greening” of vegetation and longer growing seasons;
!poleward and upward shifts in ranges in plant and animal species;
!in oceans and freshwater systems, changes in algae, plankton and
fish abundance;
!changes in ranges and timing of migrations of fish in rivers; and
!effects on human systems are difficult to discern due to adaptation
and non-climatic influences.
Impacts will depend on changes in temperature as well as precipitation, sea levels
and ocean circulation, and concentrations of carbon dioxide, as well as other
features of the climate. The ability to adapt to climate change, to reduce
vulnerability, is expected to be more constrained for low-income populations,
especially in developing countries.
By mid-century, average annual river runoff and water availability are projected
to increase by 10-40% at high latitudes and some wet tropical areas, while
tropics, some of which are already water-stressed. Drought extent, heavy
precipitation events, and flood risks are expected to increase.
The resilience of many ecosystems is likely to be exceeded by an unprecedented
wildfire, insects, ocean acidification), and other changes (e.g. land use change,
pollution, over-exploitation of resources).
Crop productivity is projected to increase at mid- to high latitudes for local mean

temperatures up to 1-3oC (1.8-5.4oF) and then decline beyond that in some
regions. Especially in seasonally dry and tropical regions, crop productivity is
projected to decrease for even small local temperature increases. Adaptations
allow yields to be maintains for modest warming.
Coasts are projected to be exposed to increasing risks due to sea level rise, coastal
erosion, human-induced pressures, more frequent bleaching of corals, loss of
wetlands, and increased flooding.
Climate change may affect the health status of millions of people through increases
in malnutrition, increased deaths, disease and injury due to heat waves, floods,
storms, fires and droughts; increased air pollution and altered distribution of some
infectious diseases.
For agriculture, most models project overall benefits over the next few decades,
largely due to increased fertilization by CO2 in the atmosphere, although negative
impacts might occur in some regions and for some sub-populations.90 As climate
change progresses — several models suggest turning points at 2 to 4oC warmer than
1990 — projected impacts on crop agriculture become negative in most regions
except for the high latitudes. Adverse impacts on Africa may be of particular
concern.91 Research to date on agricultural impacts cannot be considered conclusive.
Few studies of agriculture have incorporated the effects of climate variability,92 or the
spread of pests, crop diseases, and weedy plants that could be favored by warmer
temperatures and higher CO2 concentrations. Very little research has been applied
to other important food sources, such as fruits and vegetables, livestock production,
fisheries, and crops grown to produce oil, which constitute the fastest growing shares
of agriculture. Biotechnological products and cropping flexibility have only partially
been included in agricultural impact studies. Experts believe impacts will also
depend on migration of agricultural production to regions favored by climate change,
with implications for land values and shifts in labor forces.
Climate change and the fertilization of vegetation by higher levels of CO2 in
the atmosphere are projected to have both positive and negative effects on forests.
However, as species reach their higher temperature tolerances, stress and
susceptibility to disease, pests, and drought are likely, possibly resulting in die-offs
such as those currently being experienced by forests in parts of the western United
States and Canada.93 If forests and vegetation are able to migrate or expand in

90 Robert Mendelsohn, “Measuring Climate Impacts with Cross Sectional Analysis,”
Climatic Change (forthcoming).
91 Gunther Fischer, Mahendra Shah, and Harij van Velthuizen, “Climate Change and
Agricultural Vulnerability,” International Institute for Applied Systems Analysis (2002),
[]; M. Parry, C. Rosenzweig, and M.
Livermore, “Climate Change, global food supply and risk of hunger,” Phil. Trans. Royal.
Soc. B. 360 (2005), pp. 2125-2138.
92 Robert Mendelsohn, op.cit.
93 Pacific Northwest Research Station, Western Forests, Fire Risk, and Climate Change

conjunction with projected climate change, the composition of land cover would
likely be altered, with significant agricultural, economic, cultural, and ecological
One risk appearing in some climate model projections is the possibility of
dieback of the Amazon rainforest, resulting in a self-reinforcing cycle of greater
drying and further dieback. This could result in an amplification of greenhouse-gas
induced climate change, as well as ecological change.
Models show a wide range in the projected decrease of Arctic sea ice extent,
from very little to, as most models show, an ice-free Arctic in summers by the end
of the century or sooner.94 Arctic sea ice melting is consequential for the global
climate. It would have ecological effects on polar bears, seals, bird populations, and
marine life, as well as on humans, including native cultural and subsistence systems,
and might raise national security and sovereignty issues.
Recent research on the melting of ice sheets and accumulation of snow and
ice at higher elevations of ice sheets has reduced scientists’ confidence in related
projections and implications for sea level rise over several centuries. While most
global models project somewhat lower rises in sea levels with future warming, some
scientists assert that these results contradict recent evidence in some locations of
faster melting than predicted.95 Understanding of dynamics of ice sheets is weak and
a source of large uncertainty regarding future sea level rise.
Major declines of live coral cover for reef systems around the world are
expected by many scientists, because of combined effects of greater frequency of
high temperatures and to higher ocean acidity from elevated CO2 concentrations.
(See “Carbon Dioxide and Ocean Acidification” below.) To the degree that live
coral reef cover declines, losses up the related food chain could be expected, with

93 (...continued)
(2004), at []; Dale, V.H.; Joyce, L.A.;
change and forest disturbances” Bioscience, 51, pp. 723-734 (2001); see also Flannigan,
M.D.; Bergeron, Y.; Engelmark, O.; Wotton, B.M., “Future wildfire in circumboreal forests
in relation to global warming” J. Veg. Sci., 9 (1998), pp. 469-476.
94 X. Zhang and J.E. Walsh, “Toward a seasonally ice-covered Arctic Ocean: Scenarios from
the IPCC AR4 model simulations,” Journal of Climate 19 (2006), pp. 1730-1747; O M
Johannessen and M W Miles, “Arctic sea ice and climate change — will the ice disappear
in this century?” Science progress 83 ( Pt 3) (2000), pp. 209-22; Marika Holland, Cecilia
Geophysical Research Letters, 33, no. L23503 (2006).
95 Jonathan T. Overpeck et al., “Paleoclimatic Evidence for Future Ice-Sheet Instability and
Rapid Sea-Level Rise” Science, 311 (5768), pp. 1747-1750 (March 24, 2006); M. T.
McCulloch, T. Esat, Chem. Geol., 169, 107 (2000). J. H. Mercer, Nature, 271, 321 (1978);
W. G. Thompson, S. L. Goldstein, Science, 308, 401(2005).H. J. Zwally et al., Science, 297,

218 (2002); T. A. Scambos, J. A. Bohlander, C. A. Shuman, P. Skvarca, Geophys. Res. Lett.,

31, 10.1029/2004GL020670 (2004).

possible economic consequences for fisheries and human food security in parts of the
Models predict that the northern tier of the United States, Canada, and most
of Europe are likely to experience more days with heavy precipitation (above 0.4
inches) by the late 21st Century.96 Some areas would undoubtedly benefit from
increases in precipitation. More of this is likely to fall as rain rather than snow, and
snow is likely to melt earlier. For some areas and systems, these changes would be
positive. Many scientists conclude that it is likely that there will be some increase
in tropical cyclone intensity if the climate continues to warm.97
Projected climate change is expected to have additional major repercussions
for ecological systems. The specific reorganization of ecosystems, and effects on
particular populations, species, landscapes, and ecosystem services to humans are
beyond reliable prediction, given relatively little monitoring and research and the
rudimentary state of models for understanding these processes. The effects are
expected to be highly localized, though some will be widespread and be linked to
changes in other regions through food chains, nutrient flows, atmospheric and ocean
circulations, etc. Some populations and species are likely to flourish in a more
temperate and humid environment. Some will be able to adapt and/or migrate to stay
within a hospitable habitat. Others will be affected by obstacles or patchiness of
suitable pathways, or migration rates slower than the movement of the appropriate
biome, disruptions in food chains or other critical dependencies among species, and
increased competition. Many ecologists expect high rates of extinctions and loss of
biological diversity if climate change projections are accurate.
Human health may benefit or suffer with future climate change. It is
uncertain at what point increased heat stress impacts may outweigh the declining cold
stress benefits;98 that turning point will be sooner the faster temperatures rise and the
slower effective prevention and response adaptations are established. Other adverse
health impacts that may increase include incidence of food- and vector-borne
diseases.99 Warmer temperatures may increase air pollution, as well as boost growth
of fungi, mold, and other allergens.100 In terms of mortality, the potential expansion
of malaria is by far the most critical health impact studied, though its growth may be

96 Claudia Tebaldi, Katharine Hayhoe, Julie M. Arblaster, and Gerald A. Meehl “Going to
extreme events,” Climatic Change, December 2006.
97 World Meteorological Organization, Statement on Tropical Cyclones and Climate
Change, Geneva (2006).
98 Tol, R.S.J., “New Estimates of the Damage Costs of Climate Change, Part II: Dynamic
Estimates,” Environmental and Resource Economics, 21 (2) (2002), pp. 135-160.
99 Populations of rodents, mosquitoes and other vectors spread disease to humans.
100 Ebi, Kristie L., David M. Mills, Joel B. Smith, and Anne Grambsch, “Climate Change
National Assessment,” Environmental Health Perspectives, 114, no. 9 (September 2006),
pp. 1318-1324.

mitigated as incomes rise by the improvement of public health systems,
environmental modifications and pesticides, and possible vaccines.
Carbon Dioxide and Ocean Acidification
Climate change research has revealed that elevated concentrations of carbon dioxide
in the atmosphere are changing the chemistry of the oceans to a state not witnessed
for at least 55 million years, prompting concern by many scientists about potentially
large effects on the marine food chain. As the oceans absorb CO2 from the
atmosphere — a normal process in the carbon cycle that helps to lower CO2 in the
atmosphere and slow climate change — the dissolved CO2 makes the normally
alkaline seawater more acidic. One study has found that the oceans have become
about 30% less alkaline since pre-industrial times.101 This process is popularly
referred to as ocean acidification. The high dissolved CO2 decreases carbonate
ions, which are needed by shell-forming creatures to make calcium carbonate for
their shells.
Research on more than a dozen species of corals and plankton indicates that net
calcification (the process of shell-building) would decrease by -4% to -56% if CO2
atmospheric concentrations were to double from pre-industrial levels (around 550
ppm),102 forecast by some models to occur as early as mid-century. Above varying
thresholds, the decrease in growth turns to dissolving of their shells. Some species
that show small effects may indicate unusual adaptive responses that may not be
needed at lower CO2 concentrations. In some cases, resource managers may be able
to help protect species in some locations. However, the few forecasts available
suggest potentially large impacts on the ability of coral reefs to grow under future
conditions. Moreover, scientists believe the current acidification is essentially
irreversible on human-time scales, as the natural processes to raise alkalinity operate
far too slowly to reverse the current trend.
The main unknown in how rapidly ocean acidification will occur is the future
trajectory of CO2 concentrations in the atmosphere, which depends largely on future
human-related emissions of CO2, mostly from fossil-fuel burning.103 Concerns for
impacts are particularly great for areas where the form of carbon needed for shelling
building (aragonite) is already low (i.e., the high latitudes) or where the ecosystems
are dominated by shell-building organisms; in the case of the Southern Ocean
encircling Antarctica, both conditions occur.

101 James C. Orr et al., “Anthropogenic Ocean Acidification over the Twenty-First Century
and its Impact on Calcifying Organisms,” Nature, 437 (September 29, 2005), pp. 681-686.
102 J.A. Kleypas et al., “Impacts of Ocean Acidification on Coral Reefs and Other Marine
Calcifiers: A Guide for Future Research,” a report of a workshop held April 18-20, 2006,
in St. Petersburg, FL, at []
(accessed January 23, 2007).
103 Orr, op. cit.

Implications of Climate Change for the Federal Government
The prospects of a changing climate have a variety of implications for the
U.S. federal government. First, scientific attention and public questions about
climate change have increased pressure for research to provide answers, and for
elected officials to make decisions about whether and how to address climate change.
The U.S. government invests around $6 billion yearly on climate change research,104
voluntary programs, and financial incentives to advance low-GHG-emitting
technologies. Setting clear and realistic objectives for research and programs remain
a near-term challenge for federal programs, but such measures facilitate continuing
oversight of program performance and improvements. Of particular interest may be
questions about the rate at which science (also integrated with economics) can
narrow uncertainties about the magnitude, rate, geographic distribution, and other
characteristics of climate change, and the degree to which changes may be
predictable, hence facilitating effective and timely adaptation. These questions bear
importantly on the trade-off between acting sooner with imperfect information versus
delaying action in expectation of reducing uncertainties.
Second, the federal government manages many assets that are potentially
affected by climate change. For example, climate change could bring benefits or
threats to public lands, particularly to national parks and other physical and biological
assets valued for their high natural and cultural amenities. As climate change alters
grasslands, forests, fisheries, and other resources, their values will change, as may
appropriate management objectives and plans. Similarly, climate change may affect
the demand and supply of energy for the operations of government, with implications
for infrastructure planning, expenses, and supply choices.
The federal government may find it desirable to redefine objectives and
provide for institutional flexibility and adaptive management as climate, and the
resources that depend on specific conditions, change. For example, such ecological
impacts as competition among species may lead to conflicts among endangered
species105 or other resource management goals. Evaluations of objectives and
practices in light of potential future climate change may enhance future successes of
resource management.
Although some climate changes and their impacts may occur relatively
smoothly, ecosystems frequently exhibit abrupt changes in response to incremental
pressures. In terms of socioeconomic consequences, abrupt changes may constitute
emergencies or even disasters that require federal responses and possibly financial
and other resources. Preparedness for, and managing aftermaths of, hurricanes,
droughts, pest infestations, fires, epidemics, coastal erosion, and deterioration of

104 For more information on federal climate change budgets, see CRS Report RL33817,
Climate Change: Federal Expenditures, by Jane A. Leggett.
105 Consider, for example, competition between the Cape Sable seaside sparrow versus the
snail kite, both endangered species, as drought and rainfall in the Everglades region create
overlapping habitat and conflict. (Land Letter, ENDANGERED SPECIES: As habitats
shrink, conflicts between protections grow, November 16, 2006, at [
Landletter/2006/11/16/#1], extracted November 28, 2006.

permafrost are a few examples of the physical pressures climate change may bring.
Such changes can also create social dislocations and strife. The Dust Bowl of the
1930s, as an illustration of a regional environmental disaster, resulted in one of the
largest dislocations in U.S. history.
Beyond immediate responses, the federal government also frequently acts as
the insurer of the last resort; this role could expand if private insurance becomes less
available or more costly, or if people do not procure adequate insurance. The
dramatic increase in economic losses since 1980 resulting from extreme weather
events makes clear that economic exposures to extreme weather events, such as
droughts, floods, hurricanes, tornadoes, and others can run in the billions in the
United States, and much higher globally.
Many experts suggest that all levels of government may find it useful to
consider possible climate change implications when planning their long-lasting
infrastructure or other projects, including energy procurement, water supply and
flood control, investment in buildings and transportation systems, etc. This could
lessen the problem of obsolete, expensive, or stranded assets that could arise with
possible climate and policy changes.
Further, expected climate changes outside the borders of the United States
could have important implications for the U.S. federal government as well. For
example, climate change impacts in Mexico, and many other developing countries,
could be more adverse than in the United States, due partly to more severe projected
climate changes and partly to lower capacity to adapt. The differential between
United States and foreign direct impacts may increase pressure on migration and
terms of trade; it may also increase demands for disaster relief, development
assistance, and other types of interventions.
Implications for Policy
The federal government and other institutions have invested billions of
dollars in researching climate change over more than five decades. Human emissions
of greenhouse gases have increased exponentially since the Industrial Revolution,
leading to higher concentrations of carbon dioxide, methane, and other gases than
have existed for hundreds of thousands (maybe millions) of years. Measurements
demonstrate with increasing confidence that the Earth’s climate has been warming,
especially in the past few decades, and changing in other ways. Science cannot
explain the recent patterns of climate change without including the influence of
elevated greenhouse gas concentrations. Although many uncertainties remain
concerning the future magnitude, rate, and other details of climate change, a
preponderance of scientists conclude that greenhouse gases emitted by fossil fuel use
and other human activities are virtually certain to induce global average warming by
at least 1.8oC (3.2oF) over the 21st Century — or as high as 4oC (5.2oF), and possiblyoo
more than 6C (more than 9F). These scientists assert that some warming is virtually
inevitable, because of the accumulation of past emissions in elevated atmospheric
concentrations of GHG. The time lags inherent in the climate system are an important

aspect affecting the effectiveness of various policy options. (See “Time Lags in the
Climate System” below).
Time Lags in the Climate System
Timing of changes: There are long time lags between emissions of most
greenhouse gases, their accumulation as rising concentrations in the atmosphere,
and the induced climate change that may last from decades to millenia.
Atmospheric lifetimes different greatly among GHG — from minutes to tens of
thousands of years, with CO2 typically assumed to have half remaining in the
atmosphere after about 100 years. The long-lived gases continue to influence the
climate as long as they remain in the atmosphere. Thus, emissions of, say, CO2
today are expected to continue to affect climate for hundreds of years. Conversely,
slowing the growth of concentrations, or stabilizing them, would require
proportionately large cuts in emissions or enhancements of removals (e.g., through
uptake by trees). Dominant influence over the next 50 years is likely the
accumulation in the atmosphere of past GHG emissions; changes in climate would
continue for many decades to centuries after GHG concentrations cease to grow.
Several points made by climate experts are important:
!Most GHG emitted today will affect the climate system for
decades to hundreds of years from now.
!Reducing some gases with short atmospheric lifetimes may
achieve relatively quick effects on climate, while the effects of
reducing long-lived gases will have greatest effect over decades
to centuries (i.e., long-term climate change).
!Near term benefits may be small for avoiding climate impacts
through GHG reductions, though the benefits may be large when
aggregated over the atmospheric lifetime of the gas.
!The more concentrations increase, the greater the emission
reductions required to achieve any specified GHG concentration
!At some point, the more warming occurs, the less effective natural
removals may be, requiring proportionately more GHG reductions
to achieve any specified GHG concentration target.
These conclusions raise a number of questions that may help direct further
research or science to support public policy decisions:
!What do policy-makers need to know from climate science in order
to decide whether to address climate change further?

!Are research priorities aligned with needs, and are realistic
objectives and timetables established, to answer key questions of
prospective decision-makers?
!Are appropriate mechanisms in place for communication between
researchers and decision-makers of many types? Considering the
risks and possible consequences of decision options, how much
confidence in the science (or specific conclusions) do decision-
makers need in order to make choices? Do researchers know what
decision-makers need from them?
!What are realistic estimates from researchers of when critically
needed information can be provided at the desired level of
confidence? Are decision-makers’ expectations consistent with the
time necessary for rigorous research and assessment?
!Since all uncertainties cannot be eliminated, are appropriate, useful
risk decision tools and management regimes in place (or planned) to
facilitate use of the best scientific information for different types of
decisions (e.g., resource management, GHG control policies,
financial decisions)?
!Is there a process to incorporate learning over time, as well as legal
and institutional adjustments, to adapt to emerging scientific
!Do communication channels exist to ensure that relevant decision-
makers (in Congress, agencies, states, localities, businesses, and
households) have access to useful and reliable information to make
appropriate and timely decisions?
!Are effective cooperative mechanisms (inter-governmentally and
internationally) useful and in place to gain efficiencies and share
insights, and yield results that are compatible with national goals?
Current and emerging science also provides key information for policy-
makers that may help to resolve the details of specific decisions, such as:
!Is it timely to act now? Over what timeframe should actions be
!What are the options for action and their relative effectiveness,
including consideration of
— scope of greenhouse gases or other forcing agents to cover;
— benefits and costs of investing to learn more, or acting now, or
— mechanisms to monitor achievements and refine strategies, etc.?

!What is the appropriate balance between research, acting on current
knowledge, adapting to anticipated change, and informing decision-
makers (public and private) of choices and implications?
There are a number of bills before the 110th Congress proposing a diversity
of approaches to address climate change and greenhouse gas reductions. There also
likely will be more hearings, both legislative and oversight. Thus, assuring that the
best scientific underpinning is available for policy-related deliberations will remain
a priority.

Appendix A: Natural Forces that Influence Climate
The Earth’s climate is variable, and scientists understand, to varying degrees,
the forces that have driven climate changes of the past. Many factors contribute to
changes in climate, regionally and globally, and no single factor acts alone.106 The
principal natural factors determining climate through geologic time include the
Earth’s orbit around the Sun, solar activity, ocean variability, volcanoes, release of
methane clathrates from the ocean bed, and chaotic variability. Human activities
have grown to a scale especially over the past century that it is almost certain that
these human activities have contributed to the climate change observed in recent
decades. This appendix provides brief information on the principal identified natural
drivers of climate. The human activities affecting climate — the principal reason
demanding current policy attention — are described in the “Human Activities that
Influence Climate Change” section above.
Earth’s Orbit Around the Sun
The shape of the Earth’s orbit around the Sun is not perfectly round. The
Earth’s axis is tilted, and wobbles as it turns. These and other other orbital behaviors
are fairly well understood and predictable in the ways they influence the Earth’s
climate. Generally, when the Earth is closer to the Sun, the Earth receives more
incoming radiation and warms.
Solar Activity
Several aspects of solar activity have been suggested as contributors to
climate change, although they remain disputed: total solar irradiance, the ultraviolet
component of irradiance, cosmic rays, and earth/solar magnetism. Although the
validity and magnitude of these remain in dispute, cosmic rays and earth/solar
magnetism remain particularly complex and poorly understood.
The variability of the Sun’s total irradiance, or how much solar energy the
Sun emits toward the Earth, influences the Earth’s climate, although its significance
is disputed. On long time scales, solar variability could have a large influence, as on
the cold temperatures of the Little Ice Age,107 but quantification of the solar role in
climate variability over periods of centuries to millenia is poor. For example, while
some scientists have found evidence that the Sun’s activity may have been relatively
high during the Medieval Warm Period, other scientists have found no evidence that
estimated varibility of Sun’s total irradiance has been sufficient to drive climate
variations over the past 1,000 years. For example, one study of recent climate, using
a very simple model, found that solar variability may have accounted for
approximately 40% of climatic variability over the past 140 years, in addition to
internal (unexplained) variability, greenhouse gas forcing, and other geophysical

106 S. Fred Singer, “Human Contribution to Climate Change Remains Questionable,” EOS
107 J. Beer, W. Mende, and R. Stellmacher, “The role of the sun in climate forcing,”
Quaternary Science Reviews, 19 (2000), pp. 403-415.

factors.108 Satellite measurements from 1978 dramatically improved the
measurement of solar activity; while there is a clear correlation between solar
sunspots and an 11-year radiation cycle, some scientists conclude that variations in
solar output have been too small since 1978 to have significantly induced the
observed global warming of the past three decades.109
Solar output of ultraviolet light has been postulated as causing the Maunder
Minimum (around 1650 to 1710) of the Northern Hemisphere’s Little Ice Age, when
scientists believe that the Sun was relatively quiet and emitted less ultraviolet light.
This, in turn, may have reduced the ozone in the Stratosphere. Ozone is a greenhouse
gas that warms the Earth, so the reduction of ozone in the stratosphere due to less
ultraviolet light may have cooled the Northern Hemisphere on average by a few
tenths of a degree centigrade. Some scientists have found that estimated ultraviolet
variance since 1915 correlates poorly with global average temperature.110
Ocean Variability
At least one scientist111 has hypothesized that the natural dynamics of ocean
systems may be periodic and have an influence at least on regional or hemispheric
climates. Understanding of the oceans may also elucidate factors that can trigger
abrupt climate changes. For example, evidence exists that periods of increased
freshwater flow to the North Atlantic and Arctic Oceans from the Laurentide ice
sheet (over Canada) may be responsible for abrupt and significant climate events in
NW Europe that took place in the Late-glacial and early Holocene.
Volcanic Eruptions
The presence of certain aerosols, or tiny particles suspended in the
atmosphere, can reflect sunlight away from the Earth. Over geologic time, and in
certain recent eruptions, the aerosols jetted into the atmosphere have caused
significant cooling for one to several years after an eruption. About 71,000 years ago,
an eruption of Mt. Toga in present-day Indonesia thrust about 2,800 times as much
aerosol dust into the atmosphere as the Mt. St. Helens eruption of 1980, and may
have been sufficient to cause a six-year volcanic winter and instigate a 1,000-year ice

109 P. Foukal et al., “Variations in solar luminosity and their effect on the Earth’s climate,”
Nature, 443 (September 14, 2006), pp. 161-166.
Nature, 443 (September14, 2006), pp. 161-166.
111 W. S. Broecker, S. Sutherland, T.-H. Peng, Science 286, 1132 (1999); Ray Bradley,
“1000 Years of Climate Change,” Science 288, no. 5470 (May 26, 2000), pp. 1353-1355.

age.112 More recently, in 1992, the eruption of Mt. Pinatubo was sufficient to lower
global average temperatures significantly for a few years.113
Release of Methane Clathrates from Ocean Beds
Methane is a potent greenhouse gas. Methane clathrates (or methane
hydrates) are a form of ice with methane trapped in their cystalline structures that
exist at cold temperatures and high pressures on the Earth’s ocean floor and in Arctic
continental shelves (the latter of which may be very shallow or even above ground).
Some hypothesize that the sudden release of methane clathrates may have been
implicated in the Earth’s most severe extinction event, which occurred suddenly
about 252 million years ago, resulting in a 5oC temperature increase globally, and
with an estimated loss of about 96% of marine species and 70% of terrestrial
vertebrate species. Another rapid warming, thought by some to have been caused by
massive releases of methane clathrates or carbon dioxide, and accompanied by major
extinctions, occurred at the beginning of the Paleocene-Eocene Thermal Maximum
or the Initial Eocene Thermal Maximum. Sea surface temperatures rose between 5o
and 8oC, and in the high Arctic, sea surface temperatures rose to about 23oC/73oF.
(Today’s mean annual temperature at the North Pole is around -20oC/-4oF.) There
is corroborating evidence in a rapid color change of ocean sediments that normal
deposition of white calcite shells of ocean animals stopped for some 50 thousand
years, which may have occurred with accompanying ocean acidification.114 (See
“Carbon Dioxide and Ocean Acidification” above.)
Water Vapor
Water vapor exists naturally in the Earth’s atmosphere and is the most
important greenhouse gas (see “Greenhouse Gases” section), accounting for around
two-thirds of the 33oC of additional warming our planet receives because of the115
presence of its atmosphere. However, a change in water vapor content of the
atmosphere can have warming or cooling effects, depending on where it is in the
atmosphere, in latitude and altitude. Warmer temperatures globally will tend to
increase the water vapor in the atmosphere, a positive feedback that tends to amplify
the warming. To the degree that certain clouds increase, especially low clouds, with
increased water vapor, a negative feedback may result, reducing the warming. Much
of the warming predicted by climate models in response to GHG results from
amplification by increased water vapor in the atmosphere from the initial increase in
GHG-forced temperature. Feedbacks to clouds are among the least understood
processes and account for large differences among climate model results. Although

112 NOAA at [].
113 D Santer et al., “Contributions of anthropogenic and natural forcing to recent tropopause
height changes,” Science, 301, no. 5632 (July 25, 2003), p. 479.
114 Zachos, James C., Ursula Rohl, Stephen A. Schellenberg, Appy Sluijs, David A. Hodell,
Daniel C. Kelly, et al. “Rapid Acidification of the Ocean During the Paleocene-Eocene
Thermal Maximum,” Science, 308, no. 5728 (June 10, 2005), pp. 1611-1615.
115 Lee R. Kump, “Reducing uncertainty about carbon dioxide as a climate driver,” Nature,


significant scientific advances in understanding clouds have been made since the year
2000, it will be at least several years before these are fully incorporated into scientific
assessments, and no doubt considerable uncertainty will remain for decades.
Chaotic Variability
Climate scientists say that there is natural or “chaotic” variability in the
climate system, meaning that there is a certain amount of random or unexplained
behavior of the climate. To some degree, this natural variability reflects what science
has not identified, cannot explain, or does not find a regular statistical pattern to
describe. The presence of unexplained variability does not mean that scientists
cannot make meaningful and useful statements about the past or future; it means that
there is an amount of uncertainty, resulting from unidentified or poorly understood
factors or randomness, that will remain in forecasts of the future.