Climate Change: Science Update 2007

Climate Change: Science Update 2007
Updated March 12, 2008
Jane A. Leggett
Specialist in Environmental and Energy Policy
Resources, Science, and Industry Division

Climate Change: Science Update 2007
In 2007, the fourth major assessment of technical information on climate change
by the Intergovernmental Panel on Climate Change (IPCC) was published in
November. The year also saw continued release of new scientific findings on various
aspects of climate change.
The IPCC “Fourth Assessment Report” (AR4) critically reviewed the research
on science, impacts, and mitigation strategies, and underscored large areas of
agreement on climate issues (as well as some important uncertainties and
disagreements). The IPCC concluded that the Earth’s climate unequivocally has
warmed over the past century, and that while natural factors, including changes in
solar irradiance and volcanoes, have played roles in the observed changes, “most ofth
the observed increase in globally averaged temperatures since the mid-20 century
is very likely due to the observed increase in anthropogenic greenhouse gas1
concentrations.” Additional research published in 2007 showed continuously rising
concentrations of greenhouse gases and temperatures, record loss of Arctic sea ice
in the summer, transit by sailboat through the legendary Northwest Passage through
the Arctic, and other markers of climate change. Additional research indicated
several ecological risks — including mortality of the eastern Pacific gray whale and
lower survival rates among young polar bears — linked to climate change.
Concerns about climate change are based both on observed changes to date and
projections of what is likely to occur in the future. The IPCC concluded that
greenhouse gas emissions and concentrations in the atmosphere could be expectedst
to grow through the 21 Century in the absence of concerted climate change
mitigation policies. For a wide range of plausible GHG scenarios to 2100, the IPCCoo
projected “best guess” increases in global average temperatures from 1.8C to 4.0C
(3.2oF to 7.2oF). Its range of all scenarios to 2100, incorporating a fuller range ofoooo
uncertainties, was 1.1C to 6.4C (2.0F to 11.5F). Associated with the projections
are impacts that may be beneficial in some locations and for some sectors with small
changes in globally averaged climate, but that would be adverse for others,
particularly in regions that are already warm and dry, and may become more so.
Adverse effects are expected to multiply with accumulating climate change. Sea
levels could rise between 7 and 23 inches by 2100, not including the effects of
possible accelerated melting of the Greenland or Antarctic ice sheets. The risks of
abrupt and irreversible changes in the climate system — some potentially
catastrophic — continue to grow as the atmosphere moves further from its state
over the past several thousand years.
This report summarizes highlights of new scientific research and assessments
released in 2007 related to global warming. For more extensive background on
climate change, see CRS Report RL33849, Climate Change: Science and Policy
Implications, by Jane A. Leggett.

1 Intergovernmental Panel on Climate Change Working Group I, Climate Change 2007: The
Physical Basis (Cambridge, UK: Cambridge University Press, 2007), p. 8 of the Summary
for Policymakers.

In troduction ..................................................1
Observed Warming and Additional Metrics of Climate Change..........2
Attribution of Observed Changes Mostly to Greenhouse Gases..........2
Observed Impacts of Climate Changes.............................4
Extent of Arctic Sea Ice at Lowest Recorded Levels...............4
Further Melting of the Greenland Ice Sheet......................6
Melting and Thickening of Ice in Antarctica.....................6
No Melting of Some Permanent Ice Fields......................7
Melting of Glaciers and Ice, and Contribution to Sea Level Rise.....7
Weakened Uptake of Carbon in the Southern Ocean..............7
Observed Ecological Impacts of Climate Change.................8
Without Further GHG Mitigation Policies, GHG Emissions Will Grow...9
Projections of Future Climate...................................10
Projections of Future Impacts...................................10
Appendix A. Summary for Policymakers of the Synthesis Report of the
Fourth Assessment Report of the Intergovernmental Panel on
Climate Change..............................................12
List of Figures
Figure 1. Record Low Sea Ice Extent in 2007...........................5

Climate Change: Science Update 2007
Attention to the risks of climate change continues to grow in the United States
and worldwide. The attention is stimulated by continuing advances in scientific and
economic understanding of the risks, and by debate over policy options to manage
those risks. The year 2007 yielded a number of important new research and
assessment products, a selected set of which are summarized in this update report.
A fuller explanation of the processes involved in climate change, along with
uncertainties and controversies, is provided in CRS Report RL33849, Climate
Change: Science and Policy Implications, by Jane A. Leggett.
Among the most important products of 2007 was release of the fourth
assessment report of the Intergovernmental Panel on Climate Change (IPCC),2 along
with a Synthesis Report of the three volumes on science, impacts and vulnerabilities,
and mitigation options. On November 16, 2007, government officials from most
countries — including the United States — agreed on a Summary for Policymakers
of the Synthesis Report of the IPCC Fourth Assessment Report. For the reader’s
convenience, key findings from this Summary for Policy Makers are provided in
Appendix A of this report.3 This updated report discusses many of the IPCC findings
relating to science and impacts, along with a number of additional important research
findings that were released in 2007.

2 The IPCC is organized under the auspices of the United Nations and engages participation
of more than 2000 scientists from around the world. According to its website, “The IPCC
was established to provide the decision-makers and others interested in climate change with
an objective source of information about climate change. The IPCC does not conduct any
research nor does it monitor climate related data or parameters. Its role is to assess on a
comprehensive, objective, open and transparent basis the latest scientific, technical and
socio-economic literature produced worldwide relevant to the understanding of the risk of
human-induced climate change, its observed and projected impacts and options for
adaptation and mitigation. IPCC reports should be neutral with respect to policy, although
they need to deal objectively with policy relevant scientific, technical and socio-economic
factors. They should be of high scientific and technical standards, and aim to reflect a range
of views, expertise and wide geographical coverage” []
(extracted November 26, 2007). Previous assessment reports of the IPCC were published
in 1990, 1995, and 2001.
3 CRS has not independently evaluated and confirmed the IPCC conclusions.

Observed Warming and Additional Metrics of Climate Change
The Earth’s climate has warmed by 0.6o to 0.9o Celsius (1.1 to 1.6o Fahrenheit)4
since the Industrial Revolution. Precipitation has increased over the past century,
although some regions have become wetter and some have become drier, consistent
with scientists’ understanding of how heightened greenhouse gas concentrations
affect climate regionally. Observed increases in ocean temperatures, altered wind
patterns, extreme weather events, melting glaciers and sea ice, and timing of seasons
are also attributed in part to greenhouse gas forcing. The IPCC in 2007 declared that
“[w]arming of the climate system is unequivocal.... Observational evidence from all
continents and most oceans shows that many natural systems are being affected by
regional climate changes.”5
Although there is substantial natural variability in the climate system, a warming
trend continued through 2007.
The year 2007 the 10th warmest year for the contiguous U.S., since national
records began in 1895, according to preliminary data from NOAA’s National
Climatic Data Center in Asheville, N.C. The year was marked by exceptional
drought in the U.S. Southeast and the West, which helped fuel another extremely
active wildfire season. The year also brought outbreaks of cold air, and killer
heat waves and floods. Meanwhile, the global surface temperature for 2007 was
the fifth warmest since records began in 1880.... Including 2007, seven of the
eight warmest years on record have occurred since 2001 and the 10 warmest
years have all occurred since 1995. The global average surface temperature hasoo
risen between 0.6C and 0.7C since the start of the twentieth century, and the
rate of increase since 1976 has been approximately three times faster than the6
century-scale trend.
Attribution of Observed Changes
Mostly to Greenhouse Gases
The IPCC fourth assessment report concluded that “[m]ost of the observedth
increase in globally-averaged temperatures since the mid-20 century is very likely
due to the observed increase in anthropogenic7 GHG concentrations.”8 According to
the report, natural phenomena, such as volcanoes, solar variability and land cover
change, have undoubtedly influenced the observed climate change, but the dominant
driver of change since the 1970s is estimated to be the increase of greenhouse gases
(GHG) in the Earth’s atmosphere due to emissions from human-related activities.

4 IPCC, “Summary for Policymakers of the Synthesis Report of the IPCC Fourth Assessment
Report” (Intergovernmental Panel on Climate Change, 2007), at [
htm] (accessed November 27, 2007), p. 1.
5 Ibid, p. 1.
6 See [].
7 Human-caused.
8 IPCC, op.cit., p. 5.

Although the most potent greenhouse gas in the Earth’s atmosphere is water
vapor, it is understood not to be directly influenced at a large scale by human
activities, making carbon dioxide (CO2) the most important human-influenced GHG
globally. Other GHG are certain synthetic chlorinated and fluorinated chemicals,
methane, nitrous oxide, tropospheric ozone, and regional scale pollutants, such as
sulfates, and tiny carbon-containing particles called aerosols. In some regions and
over some periods, these latter air pollutants may dominate local climate changes.
One research study published in 2007 found that tropospheric ozone and aerosol
pollution (from other regions) may exert a stronger influence during non-summer
seasons on the Arctic climate than carbon dioxide and other long-lived GHG.9
Carbon dioxide (CO2) concentrations have grown from a pre-industrial
concentration of about 280 parts per million volume (ppm) to 379 ppm in 2005.10
The IPCC found that
[a]tmospheric concentrations of CO2 (379ppm) and methane (1774 parts per
billion - ppb) in 2005 exceed[ed] by far the natural range over the last 650,00011
years.” Global average CO2 concentrations reached 381 in 2006 and are very
likely to exceed the maximum again in 2007. The IPCC found that the increases
in CO2 concentrations since the Industrial Revolution are due primarily to human
use of fossil fuels, with land-use changes (primarily deforestation) making a
significant but smaller contribution. While over the past few decades, countries
have trended towards using cleaner, lower carbon fuels (such as natural gas
instead of coal), the IPCC noted that “the long-term trend of declining CO212
emissions per unit of energy supplied reversed after 2000.
Methane concentrations also grew from a pre-industrial value of about 715 ppb
to 1774 ppb in 2005.13 The rate of methane growth slowed and has been negative in
several years since about 1992 for a variety of reasons, including economic
restructuring, methane recovery for energy value, etc. Methane concentrations have
declined slightly since 2004.
The United States contributes almost one-fifth of net global greenhouse gas
emissions. China and the United States are now neck-and-neck as the largest emitters
of CO2. With China’s robust economic growth — dependent on industrialization
fueled largely by coal — China will become and remain the largest global emitter of
CO2 for the foreseeable future.14 Future greenhouse gas emissions will grow most

9 Drew Shindell, “Local and remote contributions to Arctic warming,” Geophysical
Research Letters 34, no. L14704 (2007).
10 IPCC Working Group I, Summary for Policymakers in Climate Change 2007: The
Physical Basis (Cambridge, UK: Cambridge University Press, 2007), p. 5, at [http://ipcc-].
11 IPCC, op.cit., p. 4.
12 Ibid.
13 Ibid.
14 While the U.S. emits less CO2 per unit of economic production than China (with “GHG

rapidly from developing economies, as they strive to eliminate poverty and raise
income levels towards those of the wealthier “Annex 1” countries.15 Future GHG
trajectories are widely uncertain, depending largely on the rate and composition of
economic growth, as well as technology and policy choices.
Observed Impacts of Climate Changes
The IPCC concluded in 2007 that
...discernible human influences extend beyond average temperature to other
aspects of climate. Human influences have:
!very likely contributed to sea level rise during the latter half of the 20th
!likely contributed to changes in wind patterns, affecting extra-tropical
storm tracks and temperature patterns
!likely increased temperatures of extreme hot nights, cold nights and cold
!more likely than not increased risk of heat waves, area affected by drought
since the 1970s and frequency of heavy precipitation events.
Anthropogenic warming over the last three decades has likely had a discernible
influence at the global scale on observed changes in many physical and16
biological systems.
Some highlights that emerged in 2007 of observed changes, understood to be the
result, at least in part, of human-induced climate change, are summarized below.
Extent of Arctic Sea Ice at Lowest Recorded Levels. Arctic sea ice
melted in 2007 to the smallest coverage since satellite measurements began in 1979
— perhaps 50% below sea ice extent of the 1950s.17 18 Average sea ice extent for
September 2007 was 4.28 million square kilometers (1.65 million square miles) —

23% below the previous record set in 2005 (see Figure 1). September 2007 was 39%

14 (...continued)
intensities” of about 562 versus 703 metric tonnes of CO2-equivalent per million dollars of
GDP), the United States emits about 24 tons of CO2-equivalent per person while China
emits only about 4 tons per person.
15 “Annex 1” of the United Nations Framework Convention on Climate Change (UNFCCC)
38 wealthier, industrialized countries, including the United States, Canada, Japan, the
European Union and its Members, Russia, Australia, New Zealand and others. Under the
principal of “common but differentiated commitments,” only the Annex 1 countries took on
aims in the UNFCCC to develop national plans to limit GHG emissions. See CRS Report
RL33826, Climate Change: The Kyoto Protocol, Bali Negotiations, and International
Actions, by Susan R. Fletcher and Larry Parker.
16 IPCC, op. cit., p. 6.
17 Using measurements from ship and aircraft before satellites became available.
18 See [], (accessed
November 27, 2007).

below the average extent of sea ice between 1979 and 2000. The rate of sea ice
decline since 1979, as measured in September 2007, is now approximately 10 percent
per decade, or 72,000 square kilometers (28,000 square miles) per year.
While natural climate variability explains part of the observed rapid sea ice loss,
it appears that scientists may have been underestimating the sensitivity of the ice
cover to the effects of GHG-induced warming. Updated estimates now project that
the Arctic Ocean could be ice-free in summer as early as 204019 or even 2030, if
recent accelerations in sea ice loss continue.20
Figure 1. Record Low Sea Ice Extent in 2007

Source: National Snow and Ice Data Center [].
Measurements by the National Aeronautics and Space Administration also
found a 23% loss in 2007 of the thicker “perennial” ice that last lasts more than one
year. A change in wind patterns starting around 2000 tripled the loss of perennial ice,
compared to the average from the 1970s through the 1990s. The loss rate jumped
higher again in 2007.
While extensive melting of Arctic sea ice is associated with GHG-induced
warming, some sea ice has exited the Arctic toward the Atlantic Ocean as well.
Simultaneously, Arctic currents seem to be reversing, returning to the pre-1990s
direction, in what appears to be a decadal pattern. According to NASA, “The results
19 Marika Holland, Cecilia M. Bitz, and Bruno Tremblay, “Future abrupt reductions in the
summer Arctic sea ice,” Geophysical Research Letters 33, no. L23503 (2006) [http://www.] (accessed December 22, 2006).
20 According to Mark Serreze, US National Snow and Ice Data Center, University of
Colorado, as quoted in David Adam, “Ice-free Arctic could be here in 23 years,” The
Guardian, September 5, 2007.

suggest not all the large changes seen in Arctic climate in recent years are a result of
long-term trends associated with global warming.”21
Sea ice controls key aspects of Arctic ecology, atmospheric circulation, polar
warming and other critical components of the Earth’s climate system. Moreover,
earlier seasonal melting of sea ice triggers a positive feedback that increases ocean
warming, further increasing sea ice melting, and so on.22 Some scientists have
expressed concern that recently observed sea ice loss may have passed a “threshold”
or a spiral of warming feedbacks.23
Further Melting of the Greenland Ice Sheet. U.S. satellite data revealed
that 2007 set a new record for melting across the Greenland ice sheet. The expanse24
of melting was twice the size of the United States. However, recent observations
— for example, the high melting rates in 2005 that startled many scientists — have
exposed greater variability and complexity in ice dynamics than previously
understood, as melting rates in 2006 returned closer to the average. Nonetheless, a
2007 reanalysis of surface melting of the Greenland ice sheet over the past 25 years
found that the rate of acceleration of melting was about double previous estimates.
The melting is closely correlated with summer temperatures. Between 1979 and

2005, the area of Greenland affected by melt on at least one day per year grew byo25

42%, while the mean temperature rose by 2.4C.

Melting and Thickening of Ice in Antarctica. Satellite observations
analyzed in 2007 indicate that the Antarctic ice sheet is losing mass overall; the
losses are mainly from the western Antarctic ice sheet. NASA satellites revealed that
snow is melting farther inland, at higher altitudes than before and, increasingly, on
the Ross Ice Shelf, which buffers land-based glaciers from the warmer ocean air.26
Some high elevation regions of the Antarctic ice sheet do not show a significant rate
of change or show less melting. Researchers identified a link between changes in
temperatures and the duration and area of melting in Antarctica, suggesting a
connection to global climate change. In another 2007 study, the British Antarctic
survey found that 300 glaciers studied increased their average flow rate by 12% from

1993 to 2003. This was attributed to thinning of the lower glaciers at the edge of the

21 See [].
22 Donald K. Perovich et al., “Increasing solar heating of the Arctic Ocean and adjacent seas,
1979-2005: Attribution and role in the ice-albedo feedback,” Geophysical Research Letters

34 (October 11, 2007).

23 See, for example, [


24 Marco Tedesco, “A New Record in 2007 for Melting in Greenland,” EOS Transactions

88 (September 1, 2007).

25 X. Fettweis, J.-P. van Ypersele, H. Gallée, F. Lefebre, and W. Lefebvre. “The 1979-2005
Greenland ice sheet melt extent from passive microwave data using an improved version of
the melt retrieval XPGR algorithm.” Geophysical Research Letters, 34, L05502,
doi:10.1029/2006GL028787 (2007).
26 NASA, “NASA Researchers Find Snowmelt in Antarctica Creeping Inland,” September

20, 2007, at [

html] (accessed November 30, 2007).

sea, allowing the glaciers above them to flow faster, similar to phenomena observed
in Greenland. The researchers tied local warming on the Antarctic Peninsula — some
of the fastest recent warming on Earth (nearly 3oC, or 4.4oF, over 50 years) — to
retreat of 87% of its glaciers and the observed increase in their flow rates.27
No Melting of Some Permanent Ice Fields. Not all glaciers and ice fields
are experiencing increased melting. For example, in Europe, while glaciers between

2,000 and 4,000 meters in altitude have lost an average of 1-1.5 kilometers of lengthth

through the 20 Century, others at high altitude — above 4,200 meters — have
changed very little in the same period. Some melting did occur, however, during the

2003 extreme heat wave.

Melting of Glaciers and Ice, and Contribution to Sea Level Rise. Of
the global melting of ice contributing to observed sea level rise, about 60% is
currently coming from relatively small land-based glaciers, and only about 28% is
coming from melting of the Greenland and Antarctic ice sheets. One report published
in 2007 concluded that the net amount of melting ice from glaciers and ice caps
flowing to the oceans each year is about 100 cubic kilometers — or about the volume
of Lake Erie.28 According to the report, the rate of melting has increased dramatically
and demonstrates the poor understanding researchers have regarding the dynamic
instability of snow and ice. Due in part to this poor understanding, the IPCC did not
include any potential acceleration of melting from Greenland and Antarctica when
it estimated potential sea level rise through the 21st Century of between 7 to 23
inches. With further warming, the acceleration of dynamic ice melt could raise the
estimates of sea-level rise by an additional 4 to 10 inches by 2100.29
Weakened Uptake of Carbon in the Southern Ocean. Research
published in 2007 concluded that net removal of CO2 from the atmosphere by the
Southern Ocean “sink” has weakened over the past 25 years. The researchers say this
is due to higher winds caused by the elevated levels of GHG in the atmosphere and
long-term depletion of stratospheric ozone. The greater storminess enhances mixing
and upwelling of ocean waters and increases the rate of release of CO2 to the
atmosphere more than the increase in photosynthetic removals due to higher30
atmospheric CO2 concentrations. This finding raises concern among some
scientists about whether human-related GHG emissions will continue to be removed
from the atmosphere by natural sinks at historic rates, which are assumed in many
projections of future GHG concentrations and climate.

27 H. D. Pritchard and D. G. Vaughan, “Widespread acceleration of tidewater glaciers on the
Antarctic Peninsula,” Journal of Geophysical Research 112 (June 6, 2007).
28 Mark F. Meier et al., “Glaciers Dominate Eustatic Sea-Level Rise in the 21st Century,”
Science (July 19, 2007).
29 Ibid.
30 Corinne Le Quere et al., “Saturation of the Southern Ocean CO2 Sink Due to Recent
Climate Change,” Science 316, no. 5832 (June 22, 2007).

Observed Ecological Impacts of Climate Change. A growing number
of studies are published each year investigating possible linkages between climate
change and ecological changes. Results from a few released in 2007 are highlighted
Observations have suggested that a recent increase in deaths of the eastern
Pacific gray whale — taken off the endangered species list since the mid-1990s —
to warmer than normal ocean temperatures, and “may represent first responses to
altered ecological conditions and reduced carrying capacity in the Bering Sea and
other habitats.”31 The increase in mortality is potentially due to a warming-linked
shrinkage of food supplies in the Bering Sea feeding grounds of these whales.
In many ecological systems, climate is a primary — but not the sole — factor
influencing the survival and behaviors of species. With the climate change
experienced in recent decades, land-use, climate change and other factors have been
associated with substantial range contractions, extinction of at least one species, and
numerous changes in the timing of animal and plant behavior. One study published
in 2007 found that, in western Europe, autumn 2006 and winter 2007 were extremely
likely to have been the warmest for more than 500 years. Plant responses to the
extreme warmth were visible, with some species having a second or extended32
flowering, and some species showed much earlier flowering following winter 2007.
Polar bears are among the species that depend on sea ice for hunting and must
fast during ice-free periods. The Western Hudson Bay of Canada has had ice-free
summer periods for many years and, although the local polar bear population had
previously appeared healthy, more recent observations have revealed lower survival33
rates among cubs and young bears. Similar patterns have now emerged in Southern
Hudson Bay and the Southern Beaufort Sea.34
Observations of several forest systems suggest that they are adapting to changes
in climate more effectively than some scientists had expected. More specifically,
NASA satellite imaging indicates that U.S. forests are adapting to the climate change
experienced to date, and that the overall productivity response to weather and
seasonal conditions has been closely linked to the number of different tree species in

31 S. Elizabeth Alter, Eric Rynes, and Stephen R. Palumbi, “DNA evidence for historic
population size and past ecosystem impacts of gray whales,” Proceedings of the National
Academy of Sciences (September 11, 2007).
32 Jürg Luterbacher et al., “Exceptional European warmth of autumn 2006 and winter 2007:
Historical context, the underlying dynamics, and its phenological impacts,” Geophysical
Research Letters 34 (June 19, 2007).
33 Regehr, Eric et al. “Survival and Population Size of Polar Bears in Western Hudson Bay
in Relation to Earlier Sea Ece Breakup,” Journal of Wildlife Management, v. 71, no. 8
(2007), pp. 2673-2683.
34 USGS, USGS Science to Inform U.S. Fish & Wildlife Service Decision Making on Polar
Bears: Executive Summary (Reston, VA, 2007), [

a forest area.35 In Brazil, the productivity of Amazon forests has been resilient in
spite of short but severe drought conditions in 2005, contrary to predictions of some
ecosystem models, although whether the resistance will be sustained under longer
drought — expected with climate change — is unknown.36, 37
Without Further GHG Mitigation Policies,
GHG Emissions Will Grow
The U.S. Climate Change Science Program (US CCSP) released its second
report in 2007, entitled “Scenarios of Greenhouse Gas Emissions and Atmospheric
Concentrations and Review of Integrated Scenario Development and Application.”38
This research produced new scenarios of future GHG emissions and concluded that
“In the reference scenarios,39 economic and energy growth, combined with continued
fossil fuel use, lead to changes in the Earth’s radiation balance that are three to four
times that already experienced since the beginning of the industrial age.”40 This
research also explored scenarios aimed at stabilizing the growth of GHG
concentrations in the atmosphere at four increasingly stringent levels: roughly 750
ppm, 650 ppm, 550 ppm, and 450 ppm (including multiple GHGs as CO2-
equivalents41). The analysis concluded, “The timing of GHG emissions reductions
varies substantially across the four radiative forcing stabilization levels. Under the
most stringent stabilization levels [450-550 ppm] emissions begin to decline
immediately or within a matter of decades. Under the less stringent stabilization
levels [750 ppm], CO2 emissions do not peak until late in the century or beyond, and42
they are 1½ to over 2½ times today’s levels in 2100.”
The results of the CCSP reference scenarios are similar to those of the 2000
Special Report on Emission Scenarios (SRES) of the IPCC, though the latter

35 NASA, “NASA Satellites Can See How Climate Change Affects Forests,”
[] (accessed
November 28, 2007).
36 Scott R. Saleska et al., “Amazon Forests Green-Up During 2005 Drought,” Science
(September 20, 2007).
37 Yadvinder Malhi et al., “Climate Change, Deforestation, and the Fate of the Amazon,”
Science (November 29, 2007).
38 See [].
39 “Reference scenarios” typically represent the researchers’ best estimates of future
trajectories without significant policy changes. They are frequently used, as in this project,
to compare with, estimate the impacts of, specific policy scenarios.
40 On p. 3 of the US CCSP report.
41 In order to compare and aggregate different greenhouse gases, various techniques have
been developed to index the effect each greenhouse gas to that of carbon dioxide, where the
effect of CO2 equals one. When the various gases are indexed and aggregated, their
combined quantity is described as the CO2-equivalent. In other words, the CO2-equivalent
quantity would have the same effect on, say, radiative forcing of the climate, as the same
quantity of CO2.
42 Ibid., p. 3.

explored a wider range of uncertainty in its reference projections. The SRES
projected global GHG emissions, without further climate change mitigation policies,
to increase by 25-90% (CO2-equivalent) from 2000 to 2030, with CO2-equivalent
concentrations growing in the atmosphere to 600-1550 ppm.
Projections of Future Climate
Scientists have found it very likely that rising greenhouse gas concentrations,
if they continue unabated, will increase 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).43 The projections thought most likely by many climate modelers are for
greenhouse gas-induced temperature rise of approximately 2.5o to 3.5oC (4.5 to 6.3oF)
by 2100. 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. Wet regions are expected to get more
precipitation and dry regions are expected to become drier. Floods, droughts, storms
and other extreme weather events are projected to increase, with impacts for
ecological and human systems.
Sea levels could rise by between 18 and 59 centimeters (between 7 and 23
inches) by 2100 due to expansion of oceans waters as they warm and additions of
meltwater (at current rates of melting) from land-based glaciers and ice caps. The
IPCC scientists were unable to include a quantitative estimate of the risks of
accelerated melting or possible collapse of the Greenland or Antarctic ice sheets due
to inadequacies of existing understanding of their dynamics.
Projections of Future Impacts
Some impacts of climate change are expected to be beneficial in some locations
with a few degrees of warming (e.g., increased agricultural productivity in some
regions, less need for space heating, opening of the Northwest Passage for shipping
and resource exploitation). Most impacts are expected to be adverse (e.g., lower
agricultural productivity in many regions, drought, rising sea levels, spread of disease
vectors, greater needs for cooling). Risks of abrupt, surprising climate changes, with
accompanying dislocations, are expected to increase as global average temperature
increases, and could push natural and socio-economic systems past key thresholds.
As the degree and distribution of climate changes continue, ranges of species are
likely to change. Climate change is highly likely to create substantial changes in
ecological systems and services in some locations, and may lead to ecological
surprises. The disappearance of some types of climate also raises risks of extinctions
of species, especially those with narrow geographic or climatic distributions, and

43 To put the magnitude of these potential increases in context, the current global, annual
mean temperature (GMT) of the Earth is approximately 14oC (57oF). The difference
between the current GMT and the low point of the last Ice Age, about 21,000 years ago, wasoo
roughly 7-8C (44-46F).

where existing communities disintegrate. Research published in 200744 projects that,
under the highest IPCC emissions scenario, 12% to 39% of the Earth’s land areas
may experience novel climates while 10% to 48% of land areas’ climates may
disappear by 2100 AD. In the lowest IPCC climate change scenarios, 4% to 20% of
land areas gain novel climates and 4% to 20% see existing climates disappear.
Because climate change will occur with different magnitudes and characteristics
in different regions, resulting dislocations and disparities across locations are
expected to increase pressure on international aid and migration, with possible
implications for political stability and security. Impacts may be alleviated with
investments in adaptation, although adaptation as a strategy is thought to be more
challenging and potentially less effective the more widespread, uncertain and severe
the climate changes.

44 John W. Williams, Stephen T. Jackson, and John E. Kutzbach, “Projected distributions
of novel and disappearing climates by 2100 AD,” Proceedings of the National Academy of
Sciences of the United States of America 104, no. 14 (April 3, 2007).

Appendix A. Summary for Policymakers
of the Synthesis Report of the Fourth Assessment
Report of the Intergovernmental Panel
on Climate Change
On November 16, 2007, government officials from most countries — including
the United States — agreed on a Summary for Policymakers of the Synthesis Report
of the IPCC Fourth Assessment Report. The Synthesis Report is derived from three
technical reports: “The Physical Science Basis” (February 2007); “Impacts,
Adaptation and Vulnerability” (April 2007); and “Mitigation of Climate Change”
(May 2007). It represents a consensus among government officials and researchers,
and will “constitute the core source of factual information about climate change
[upon which policymakers will] base their political action... in the coming years”
(IPCC Media Advisory, November 17, 2007). Key conclusions are excerpted (and
slightly reordered) below:
“Warming of the climate system is unequivocal....” (p. 1) “Observational
evidence from all continents and most oceans shows that many natural systems
are being affected by regional climate changes.” (p. 2)
“Global GHG emissions due to human activities have grown since pre-industrial
times.... Carbon dioxide (CO2) is the most important anthropogenic [greenhouse
gas] GHG. Its annual emissions grew by about 80% between 1970 and 2004. The
long-term trend of declining CO2 emissions per unit of energy supplied reversed
after 2000.” (p. 4)
“Atmospheric concentrations of CO2 (379ppm) and CH4 (1774 ppb) in 2005
exceed by far the natural range over the last 650,000 years. Global increases in
CO2 concentrations are due primarily to fossil fuel use, with land-use change
providing another significant but smaller contribution.” (p. 4)
“Most of the observed increase in globally-averaged temperatures since the mid-
20th century is very likely due to the observed increase in anthropogenic GHG
concentrations.” (p. 5)
“Even if the concentrations of all greenhouse gases and aerosols had been kepto
constant at year 2000 levels, a further warming of about 0.1C per decade would
be expected. Afterwards, temperature projections increasingly depend on specific
emission scenarios.” (p. 6)
“Anthropogenic warming could lead to some impacts that are abrupt or
irreversible, depending upon the rate and magnitude of the climate change.” (p.


“The uptake of anthropogenic carbon since 1750 has led to the ocean becoming
more acidic.... Increasing atmospheric CO2 concentrations lead to further
acidification.... [P]rogressive acidification of oceans is expected to have negative
impacts on marine shell-forming organisms (e.g. corals) and their dependent
species.” (p. 11)

“Sea level rise under warming is inevitable....The eventual contributions from
Greenland ice sheet loss could be several metres ... should warming in excess of

1.9-4.6/C above pre-industrial be sustained over many centuries.” (p. 21)

“Some systems, sectors and regions are likely to be especially affected by climate
change. Systems and sectors:
!particular ecosystems:
!terrestrial: tundra, boreal forest and mountain regions because of
sensitivity to warming; mediterranean-type ecosystems because of
reduction in rainfall; and tropical rainforests where precipitation declines
!coastal: mangroves and salt marshes, due to multiple stresses
!marine: coral reefs due to multiple stresses; the sea ice biome because of
sensitivity to warming
!water resources in some dry regions at mid-latitudes and in the dry tropics,
due to changes in rainfall and evapotranspiration, and in areas dependent
on snow and ice melt
!agriculture in low-latitudes , due to reduced water availability
!low-lying coastal systems, due to threat of sea level rise and increased risk
from extreme weather events
!human health in populations with low adaptive capacity.” (p. 11)
“[M]ore extensive adaptation than is currently occurring is required to reduce
vulnerability to climate change. There are barriers, limits and costs, which are
not fully understood.” (p. 14)
“[International cooperation] will help to reduce global costs for achieving a given
level of mitigation, or will improve environmental effectiveness. Efforts can
include ... emissions targets; sectoral, local, sub-national and regional actions;
RD&D programmes; adopting common policies; implementing development
oriented actions; or expanding financing instruments.” (p. 19)
“Decisions about macroeconomic and other non-climate policies can
significantly affect emissions, adaptive capacity and vulnerability.” (p. 19)
“Determining what constitutes ‘dangerous anthropogenic interference with the
climate system’ in relation to Article 2 of the UNFCCC involves value
judgements.” (p. 19)
“Limited and early analytical results from integrated analyses of the costs and
benefits of mitigation indicate that they are broadly comparable in magnitude,
but do not as yet permit an unambiguous determination of an emissions pathway
or stabilisation level where benefits exceed costs.” (p. 23)
“Many impacts can be reduced, delayed or avoided by mitigation.” (p. 20)
“There is high agreement and much evidence that all stabilisation levels assessed
can be achieved by deployment of a portfolio of technologies that are either
currently available or expected to be commercialised in coming decades,
assuming appropriate and effective incentives are in place....” (p. 22)
“An effective carbon-price signal could realise significant mitigation potential
in all sectors. Modelling studies show global carbon prices rising to 20-80
US$/tCO2-eq by 2030 are consistent with stabilisation at around 550 ppm CO2-eq

by 2100. For the same stabilisation level, induced technological change may
lower these price ranges to 5-65 US$/tCO2-eq in 2030.” (p. 18)
“All assessed stabilisation scenarios indicate that 60-80% of the reductions
would come from energy supply and use, and industrial processes, with energy
efficiency playing a key role in many scenarios. Including non-CO2 and CO2
land-use and forestry mitigation options provides greater flexibility and cost-
effectiveness. Low stabilisation levels require early investments and substantially
more rapid diffusion and commercialisation of advanced low emissions
technologies. Without substantial investment flows and effective technology
transfer, it may be difficult to achieve emission reduction at a significant scale.
Mobilizing financing of incremental costs of low-carbon technologies is
important.” (p. 22)
“The macro-economic costs of mitigation generally rise with the stringency of
the stabilisation target.” (p. 22)
“Impacts of climate change are very likely to impose net annual costs which will
increase over time as global temperatures increase. Peer-reviewed estimates of
the social cost of carbon in 2005 average US$12 per tonne of CO2, but the range
from 100 estimates is large (-$3 to $95/tCO2). This is due in large part to
differences in assumptions regarding climate sensitivity, response lags, the
treatment of risk and equity, economic and non-economic impacts, the inclusion
of potentially catastrophic losses, and discount rates. Aggregate estimates of
costs mask significant differences in impacts across sectors, regions and
populations and very likely underestimate damage costs because they cannot
include many non-quantifiable impacts.” (p. 23)
“Choices about the scale and timing of GHG mitigation involve balancing the
economic costs of more rapid emission reductions now against the corresponding
medium-term and long-term climate risks of delay.” (p. 23)