The IPCC: Who Are They and Why Do Their Climate Reports Matter?

Overview
The Intergovernmental Panel on Climate Change (IPCC) was established in 1988 by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) in recognition of the problem of global warming. Through the IPCC, climate experts from around the world synthesize the most recent climate science findings every five to seven years and present their report to the world’s political leaders. The IPCC has issued comprehensive assessments in 1990, 1996, and 2001; its Fourth Assessment Report (AR4) is scheduled for release in 2007.

AR4 will be the most comprehensive synthesis of climate change science to date. Experts from more than 130 countries are contributing to this assessment, which represents six years of work. More than 450 lead authors have received input from more than 800 contributing authors, and an additional 2,500 experts reviewed the draft documents.

AR4 will comprise three sections, or working groups, that deal with the scientific basis of global warming (Working Group I), its consequences (Working Group II), and options for slowing the trend (Working Group III). The IPCC will release summaries of the three working group documents over the course of 2007, culminating in the publication of the final “synthesis report” at the end of the year.

The inclusive process by which IPCC assessments are developed and accepted by its members ensures exceptional scientific credibility. As such, AR4 has the potential to play a key role in informing decision makers as they shape climate policies over the next several years.


Release Schedule for the Fourth Assessment Report (AR4)

Approximate Date	Report Section(s)	Description
February 2, 2007	Working Group I summary for policymakers (SPM) only	Summary of the current science on global warming
April 6, 2007	Working Group II SPM only	Summary of the expected consequences of global warming
May 4, 2007	Working Group III SPM only	Summary of options for slowing down global warming
May 2007	Working Groups I, II, and III technical reports	Each working group's full assessment, including SPM and technical summary (published by Cambridge University Press)
November 16, 2007	Synthesis report	Overview of the IPCC assessment in its entirety (with frequently asked questions), plus SPMs and technical summaries from each working group

IPCC History and Mission
The Intergovernmental Panel on Climate Change (IPCC) was established in 1988 under the auspices of the United Nations Environment Programme and the World Meteorological Organization for the purpose of assessing “the scientific, technical and socioeconomic information relevant for the understanding of the risk of human-induced climate change. It does not carry out new research nor does it monitor climate-related data. It bases its assessment mainly on published and peer reviewed scientific technical literature.” [1] The goal of these assessments is to inform international policy and negotiations on climate-related issues.

The First Three Assessments
The First Assessment Report (FAR) of the IPCC (1990), as well as a supplemental report prepared in 1992, supported the establishment of the United Nations Framework Convention on Climate Change (UNFCCC) at the United Nations Conference on Environment and Development (UNCED, commonly known as “The Earth Summit”) held in Rio de Janeiro, Brazil, in 1992. The UNFCCC treaty, which the United States has signed, serves as the foundation of international political efforts to combat global warming.

The IPCC’s reports were also influential at the first Conference of the Parties (COP) to the Climate Convention, held in Berlin, Germany, in 1995. Attendees produced the so-called Berlin Mandate, setting out the terms for a negotiation process that would produce binding commitments by industrial countries to reduce their heat-trapping emissions after the year 2000.

The significantly strengthened Second Assessment Report (SAR, 1996), along with additional special materials on the implications of various potential emission limitations and regional consequences, provided key input to the negotiations that led to the adoption of the Kyoto Protocol to the UNFCCC in 1997. The Kyoto Protocol is an international agreement that establishes binding targets for reducing the heat-trapping emissions of developed countries. After the SAR was published, a number of technical papers and special reports have been prepared on the impact of aircraft, land use, technology, and changing emission levels on global warming.

The Third Assessment Report (TAR, 2001) concluded that temperature increases over the twenty-first century could be significantly larger than previously thought, and that the evidence for human influence on climate change is stronger than ever. The level of acceptance of this assessment within the extensive community of IPCC scientists is quite remarkable. It is fair to say that the TAR represented a thorough, carefully explained view of the state of climate change science in 2001.

IPCC Structure
Historically, the IPCC has been organized into three working groups, a variety of task forces or special committees, and a small secretariat in Geneva. The topics assigned to the working groups have evolved somewhat over time. For the AR4, Working Group I has been charged with summarizing the physical science basis of climate change. Working Group II has been charged with addressing the vulnerability of human and natural systems to climate change (i.e., the negative and positive consequences of global warming) and options for adapting to the changes. Working Group III has been charged with assessing options for limiting heat-trapping emissions and other means of slowing the warming trend, as well as related economic issues. A separate Task Force on National Greenhouse Gas Inventories is overseeing the compilation of global warming emissions by country.

Each of these working groups has two co-chairs—one from a developed country and one from a developing country. An additional set of governmental representatives (frequently scientists) have been nominated by their countries to serve on the bureau of each working group. Together, the two co-chairs and the bureau members function as an executive committee, while the team of scientists drafting individual chapters of each working group’s assessment is sometimes referred to as the scientific core. Coordinating the efforts of each working group is a small technical support unit (TSU) that provides both technical and administrative support to the bureau and the scientific core.

AR4 Products
Each working group is charged with publishing an in-depth technical report, a technical summary, and a short summary for policymakers (SPM). In addition, the major findings and conclusions from all three reports provide the basis for a final “synthesis report.”

Each working group holds a plenary session (i.e., a meeting of the group’s full membership) to resolve final questions about its assessment and reach final approval of every word in its SPM. The entire IPCC will meet in May 2007 to approve the contributions of the three working groups, and again in November 2007 to approve the synthesis report. The resulting documents will be made public at the conclusion of each of these meetings.

Release Schedule for the Fourth Assessment Report (AR4)

Authors, Contributors, and Reviewers
The technical support units, co-chairs, and bureaus of each working group together assemble a list of proposed authors for its assessment, but the lead authors are selected by the entire working group. Governments and non-governmental organizations around the world are invited to nominate potential authors.

A government nomination does not imply that the scientist’s views are endorsed by that government, or that the scientist is expected to represent his or her government’s view. It may mean that a government has provided a scientist with financial support, but many scientists receive no financial support at all and others are merely reimbursed for travel expenses. Experts from developing nations who have received no financial support from their government are supported through the IPCC trust fund.

From these nominations, the full working group membership confirms 5 to 10 lead and coordinating lead authors, as well as two review editors, for each chapter of its assessment; every chapter must have at least one lead author from a developing country. In general, the appointed scientists are widely recognized experts who represent a broad range of expertise and opinion; they may come from academia, research facilities, industry, government, and non-government organizations (NGOs). A complete list of the lead authors is available at the IPCC website (www.ipcc.ch).

Lead authors and coordinating lead authors prepare a first draft of their chapter over a period of several months, reviewing and synthesizing peer-reviewed scientific literature. Lead authors also consult with expert scientists in the field, inviting those with needed expertise to serve as contributing authors. The chapter teams hold several author meetings to clarify the issues and reach agreement on the text’s scope, balance, and conclusions. Contributing authors help write specific sections, contribute specific data, or represent particular viewpoints. Though lead authors typically solicit such contributions, scientists are also encouraged, both individually and by their countries, to become contributing authors by submitting relevant material directly to the working group’s chairs.

The resulting first draft of a chapter then undergoes two rounds of scientific review and revision (described more fully below) before being finalized. Many authors attest that this review process ranks among the most extensive for any scientific document. For comparison, a paper published in a peer-reviewed science journal is typically reviewed by only two or three experts.

The revised chapters are then combined into a technical report by the technical support units and circulated to governments and NGOs accredited by the IPCC before being considered and “accepted” at the working group’s plenary session. Acceptance in this context means that government representatives to the IPCC agree that the documents present an objective, comprehensive, and balanced scientific review of the subject matter. Government representatives are not permitted to edit these book-length reports; in the end, it is the authors who bear the sole responsibility for the content of their chapters.

However, government representatives do participate in the line-by-line review and revision of the much shorter summary for policymakers, or SPM, for each technical report. The SPM is written by the working group’s lead authors, reviewed in two stages by technical experts, and finally by government representatives before being accepted at the working group’s plenary session. Each SPM is released separately over the course of several months.

Government representatives may certainly try to influence the SPM wording in ways that support their negotiating positions, but the overriding goal of this process (and a key challenge) is to ensure that the SPM adequately and appropriately represents the underlying technical report prepared by the scientific community. Therefore, all of the lead authors and at least several contributing authors are expected to attend their working group’s plenary session so they can render interpretations, suggest clarifications, and ensure scientific integrity. Differing views are welcomed as long as there is empirical evidence or plausible reasons to support them.

The Peer Review Process
The IPCC’s technical reports derive their credibility principally from an extensive, transparent, and iterative peer review process that, as mentioned above, is considered far more exhaustive than that associated with scientific journals. This is due to the number of reviewers, the breadth of their disciplinary backgrounds and scientific perspectives, and the inclusion of independent “review editors” who certify that all comments have been fairly considered and appropriately resolved by the authors. For example, see [2].

To be as inclusive and open as possible, a balanced review effectively begins with the choice of lead authors. By intentionally including authors who represent the full range of expert opinion, many areas of disagreement can be worked out in discussions among the authors rather than waiting until the document is sent out for review.

The first round of review is conducted by a large number of expert reviewers—more than 2,500 for the entire AR4—who include scientists, industry representatives, and NGO experts with a wide range of perspectives. Lead authors are required to consider all comments and incorporate those with scientific merit—a process overseen by review editors (two per chapter) who have expertise in the specific topic covered by a given chapter. All review comments are archived together with the authors’ responses and/or resulting actions, and are available upon request.

If major differences emerge, lead authors are encouraged to organize a meeting with both the contributing authors and review editors to discuss and resolve the differences. The goal is not to reach a potentially “watered-down” compromise that conceals scientific uncertainties or real differences in expert opinion, but to produce a report of the highest scientific integrity, reflecting the state of our understanding fairly and adequately.

The revised draft is then sent back to the expert reviewers and also to government representatives for the so-called government review stage. Each government is entitled to organize any type of review process it deems appropriate. The U.S. government, for example, seeks comments from agencies, scientific experts, and the general public (through a notice in the Federal Register) as the starting point for its comments. Again, lead authors prepare revisions in response to scientifically valid comments, and encourage reviewers and other experts to resolve any remaining major differences by communicating directly. The resulting document is then submitted to the working group’s plenary session for consideration and acceptance.

Representing a Range of Expert Opinions
As mentioned above, one critical strategy the IPCC uses to ensure the scientific credibility and political legitimacy of its reports is to represent the range of scientific opinion on climate change fairly. To this end, the IPCC provides several channels for input from experts along the entire spectrum of opinion, including global warming contrarians.

First, accredited NGOs from all sides of the issue are welcome as observers at the opening plenary session and some other sessions over the course of the report production cycle. In addition, well-known contrarians can and do become contributing authors by submitting material to lead authors, and play advisory roles for their governments by working with government representatives to revise and approve the final SPMs. (See [2].)

The presence of climate change experts from industry and environmental organizations in the assessment process also illustrates the IPCC’s desire to seek input from outside traditional research institutions. Industry examples have included representatives from the Electric Power Research Institute and ExxonMobil. Environmental examples have included representatives from Environmental Defense, the Natural Resources Defense Council, and others all over the world.

Climate contrarians frequently claim that the IPCC produces politically motivated reports that show only one side of the issues. Given the many stages at which experts from across the political and scientific spectrum are included in the process, however, this is a difficult position to defend. [3]

Furthermore, according to IPCC principles, lead authors are “required to record views in the text which are scientifically or technically valid, even if they cannot be reconciled with a consensus view.” [4]

Consensus Building within the IPCC
The word “consensus” is often invoked, and sometimes questioned, when speaking of IPCC reports. In fact, there are two arenas in which a consensus needs to be reached in the production of IPCC assessments; one is the meeting of the entire IPCC, in which unanimity is sought among government representatives. Even though such consensus is not required (countries are free to register their formal dissent), agreement has been reached on all documents and SPMs to date—a particularly impressive fact.

Consensus is also sought among the scientists writing each chapter of the technical reports. Because it would be clearly unrealistic to aim for unanimous agreement on every aspect of the report, the goal is to have all of the working group’s authors agree that each side of the scientific debate has been represented fairly.

The Role of Governments
Although AR4 is a scientific report, its purpose is to inform international political negotiations on climate issues. Therefore, governments—as the key stakeholders in these negotiations—play an essential role in the report’s production. Government representatives propose authors and contributors, participate in the review process, and help reach a consensus on the report’s major findings. This can result (especially in the SPMs) in language that is sometimes weaker than it otherwise might be.

But it also means that governments cannot easily criticize or dismiss a report that they themselves have helped shape and approved during political negotiations. As Sir John Houghton, co-chair of TAR Working Group I, once put it: “Any move to reduce political involvement in the IPCC would weaken the panel and deprive it of its political clout. . . . If governments were not involved, then the documents would be treated like any old scientific report. They would end up on the shelf or in the waste bin.” [5]

It is important, however, to reiterate a fundamental point about IPCC assessments: although governments are involved in the process and support it financially, science ultimately predominates. The chapters that underpin all the documents are written by and under the control of scientists, and scientists ensure that all the documents are both consistent with the findings of each chapter and scientifically credible in their own right.

CONCLUSION
The inclusive process by which IPCC assessments are developed and accepted by its members results in reports of exceptional scientific credibility. As such, AR4 (as proved to be the case with the three previous IPCC assessments) has the potential to be extremely influential in the formation of climate policy over the next several years.

REFERENCES
1. IPCC website. About IPCC. Online at www.ipcc.ch/about/about.htm
2. Edwards, P., and S. Schneider. 1997. Climate change: Broad consensus or “scientific cleansing”? Ecofables/Ecoscience 1:3–9.
3. Masood, E. 1996. Head of climate group rejects claims of political influence. Nature 381:455.
4. IPCC website. Principles & procedures. Online at www.ipcc.ch/about/procd.htm.
5. Alfsen, K., and T. Skodvin. 1998. The Intergovernmental Panel on Climate Change (IPCC) and scientific consensus, Policy Note 1998: 3. Center for International Climate and Environmental Research, University of Oslo.

Findings of the IPCC Fourth Assessment Report: Climate Change Science

After assessing decades of climate data recorded everywhere from the depths of the oceans to tens of miles above Earth's surface, leading scientists from around the world have reported major advances in our understanding of climate change. Released in February 2007—six years after the prior assessment by the Intergovernmental Panel on Climate Change (IPCC)—the IPCC Fourth Assessment Report’s Working Group I Summary for Policymakers synthesizes current scientific understanding of global warming and projects future climate change using the most comprehensive set of well-established global climate models.[1]

The Working Group I contribution is the first of three that comprise the full IPCC Fourth Assessment Report, which includes the input of more than 1,200 authors and 2,500 scientific expert reviewers from more than 130 countries. In subsequent reports, Working Group II evaluates “Impacts, Adaptation and Vulnerability” and Working Group III evaluates “Mitigation of Climate Change.”[2] What the IPCC Means by “Likely” When the IPCC ascribes a likelihood to a scientific finding, the term used reflects a specific range of certainty as defined by the chart below. Click here to see a larger version of the image. Human Responsibility for Climate Change The report finds that it is “very likely” that emissions of heat-trapping gases from human activities have caused “most of the observed increase in globally averaged temperatures since the mid-20th century.” Evidence that human activities are the major cause of recent climate change is even stronger than in prior assessments.[3] Warming Is Unequivocal The report concludes that it is “unequivocal” that Earth’s climate is warming, “as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice, and rising global mean sea level.” The report also confirms that the current atmospheric concentration of carbon dioxide and methane, two important heat-trapping gases, “exceeds by far the natural range over the last 650,000 years.” Since the dawn of the industrial era, concentrations of both gases have increased at a rate that is “very likely to have been unprecedented in more than 10,000 years.” Additional IPCC Findings on Recent Climate Change Rising Temperatures • Eleven of the last 12 years rank among the 12 hottest years on record (since 1850, when sufficient worldwide temperature measurements began). • Over the last 50 years, “cold days, cold nights, and frost have become less frequent, while hot days, hot nights, and heat waves have become more frequent.” Global and Continental Temperature Change Click here to see a larger version of the image. The black line represents observed surface temperature changes for the globe and each continent (based on temperatures recorded by measuring stations around the world). The blue band represents how the climate would have evolved over the past century in response to natural factors only (according to 19 computer simulations derived from five different climate models); the brown band represents how the climate would have changed in response to both human and natural factors (according to 58 computer simulations derived from 14 different climate models). The overlap of the brown band and black line suggests that human activity very likely caused most of the observed increase since the mid-20th century. Temperature change is plotted relative to the corresponding average for the 1901 to 1950 time period. Source: Climate Change 2007: The Physical Science Basis—Summary for Policymakers. Increasingly Severe Weather (storms, precipitation, drought) • The intensity of tropical cyclones (hurricanes) in the North Atlantic has increased over the past 30 years, which correlates with increases in tropical sea surface temperatures. • Storms with heavy precipitation have increased in frequency over most land areas. Between 1900 and 2005, long-term trends show significantly increased precipitation in eastern parts of North and South America, northern Europe, and northern and central Asia. • Between 1900 and 2005, the Sahel (the boundary zone between the Sahara desert and more fertile regions of Africa to the south), the Mediterranean, southern Africa, and parts of southern Asia have become drier, adding stress to water resources in these regions. • Droughts have become longer and more intense, and have affected larger areas since the 1970s, especially in the tropics and subtropics. Melting and Thawing • Since 1900 the Northern Hemisphere has lost seven percent of the maximum area covered by seasonally frozen ground. • Mountain glaciers and snow cover have declined worldwide. • Satellite data since 1978 show that the extent of Arctic sea ice during the summer has shrunk by more than 20 percent. Rising Sea Levels • Since 1961, the world’s oceans have been absorbing more than 80 percent of the heat added to the climate, causing ocean water to expand and contributing to rising sea levels. Between 1993 and 2003 ocean expansion was the largest contributor to sealevel rise. • Melting glaciers and losses from the Greenland and Antarctic ice sheets have also contributed to recent sealevel rise. Refined Projections of Climate Change Projected climate change for the second half of this century depends on the level of future heat-trapping emissions. The IPCC based its projections on six emission scenarios, running each one through sophisticated climate simulation programs. The lowest temperatures currently projected for the end of this century represent the lowest scenario the IPCC chose to evaluate—the “B1” scenario (see table below), which assumes a midcentury peak in global population, a rapid change toward a service and information economy, and a shift toward clean and resource-efficient technologies. The highest temperatures projected for the end of this century represent the highest scenario the IPCC chose to evaluate—the “A1FI” scenario, which assumes a mid-century peak in global population, rapid economic growth, and “fossil intensive” energy production and consumption. The IPCC’s prior assessment in 2001 used many more emission scenarios, so projections of temperature changes, sea level rise, etc. in that report are not directly comparable with those in the new assessment. Nevertheless, both assessments have shown that the degree of climate change in the decades ahead strongly depends on the emission scenario. The IPCC’s findings are therefore crucial to informing climate policy. Even if we act today to reduce our emissions from cars, power plants, land use, and other sources, we will see some degree of continued warming because past emissions will stay in the atmosphere for decades or more. If we take no action to reduce emissions, the IPCC concludes that there will be twice as much warming over the next two decades than if we had stabilized heat-trapping gases and other climate relevant pollutants in the atmosphere at their year 2000 levels. Additional IPCC Findings on Future Climate Change Rising Temperatures[4] • The full range of projected temperature increase is 2 to 11.5 degrees Fahrenheit (1.1 to 6.4 degrees Celsius) by the end of the century. Note that the upper end of the range is higher than the prior IPCC assessment, mainly because of increased understanding that “warming tends to reduce land and ocean uptake of atmospheric carbon dioxide, increasing the fraction of [carbon dioxide] emissions that remains in the atmosphere.” • The best estimate range of projected temperature increase, which extends from the midpoint of the lowest emission scenario to the midpoint of the highest, is 3.1 to 7.2 degrees Fahrenheit (1.8 to 4.0 degrees Celsius) by the end of the century. • “Warming is expected to be greatest over land and at most high northern latitudes, and least over the Southern [formerly Antarctic] Ocean and parts of the North Atlantic Ocean.” Increasingly Severe Weather (storms, precipitation, drought) • Tropical cyclones (hurricanes and typhoons) are likely to become more intense, with higher peak wind speeds and heavier precipitation associated with warmer tropical seas. • Increases in the amount of high latitude precipitation are very likely, while decreases are likely in most subtropical land regions (e.g., Egypt). • Extreme heat, heat waves, and heavy precipitation are very likely to continue becoming more frequent. Melting Ice • Sea ice is projected to shrink in both the Arctic and Antarctic under all model simulations. Some projections show that by the latter part of the century, late-summer Arctic sea ice will disappear almost entirely. Changes in the Ocean • The IPCC uses the term meridional overturning circulation (MOC), which is also known as thermohaline circulation, to refer to ocean circulation driven by differences in water density due to heat (thermo) and salt (haline) content. The MOC is an important mechanism for bringing heat to polar regions. If it slows down this heat transfer would slow down. The IPCC states that it is very likely that the Atlantic Ocean MOC will be 25 percent slower on average by 2100 (with a range from 0 to 50 percent). Nevertheless, Atlantic regional temperatures are projected to rise overall due to more significant warming from increases in heat-trapping emissions. • “Increasing atmospheric carbon dioxide concentrations will lead to increasing acidification of the ocean,” with negative repercussions for all shell-forming species and their ecosystems.[5] Projected Globally Averaged Surface Warming and Sea-Level Rise at the End of the 21st Century Temperature Change (°F at 2090 relative to 1980-1999) Sea-Level Rise (inches at 2090-2099 relative to 1980-1999) Case Best Estimate Likely Range Model-based range excluding future rapid dynamical changes in ice flow Constant Year 2000 concentrations 1.1 0.5 - 1.6 NA B1 scenario 3.2 2.0 - 5.2 7.1 - 15.0 A1T scenario 4.3 2.5 - 6.8 7.9 - 17.7 B2 scenario 4.3 2.5 - 6.8 7.9 - 16.9 A1B scenario 5.0 3.1 - 7.9 8.3 - 18.9 A2 scenario 6.1 3.6 - 9.7 9.1 - 20.1 A1Fl scenario 7.2 4.3 - 11.5 10.2 - 23.2 Key: Relative temperature change in °C is equal to °F divided by 1.8. One inch is equal to 2.54 cm. For example, The B2 Scenario has a best estimate temperature change of 2.4°C and a sea-level rise of 20 to 43 cm by 2090-2099. Sea-level Rise Compared with its prior assessment, the IPCC has used improved statistical methods for calculating several factors that contribute to global sea-level rise. These factors include: • ocean expansion resulting from increased water temperatures; • meltwater runoff from mountain glaciers around the world; and • meltwater runoff and calving (breaking off ) of ice from the Greenland and Antarctic ice sheets. The models used by the IPCC project that by the end of this century, the global average sea-level will rise between 7 and 23 inches (0.18 and 0.59 meters) above the 1980–1999 average. As with temperatures described above, this range is not directly comparable to the prior IPCC assessment because of the smaller number of emission scenarios evaluated and improved statistical methods. That being said, the midpoint of the scenarios used in both assessments differed by only 10 percent between the prior assessment and the current one. Also, if the observed contributions from the Greenland and Antarctic ice sheets between 1992 and 2003, the IPCC states, “were to grow linearly with global average temperature change,” the upper ranges of sea-level rise would increase by 3.9 to 7.9 inches (0.1 to 0.2 meters). In other words, in this example, the upper range for sea-level rise would be 31 inches (0.79 meters). Due to ongoing scientific uncertainty, the IPCC notes that the following factors are not fully reflected in its current sealevel rise models: • Carbon dioxide uptake. Evidence suggests that warming tends to reduce land and ocean uptake of atmospheric carbon dioxide, increasing the portion of carbon dioxide emissions that remain in the atmosphere. This would result in further warming and cause additional sea-level rise. • Ice sheet instability. Recent observations show that meltwater can run down cracks in the ice and lubricate the bottom of ice sheets, resulting in faster ice flow and increased movement of large ice chunks into the ocean. This process, and others related to ice flow dynamics, directly contributes to sea-level rise. While calling attention to these processes, which could result in a significantly higher global sea-level than that projected in its new report, the IPCC is careful to alert policy makers to the limits of our current ability to quantify these mechanisms: “Larger values cannot be excluded, but understanding of these effects is too limited to assess their likelihood or provide a best estimate or an upper bound for sea-level rise.” Some models do suggest that sustained warming between 2 and 7 degrees Fahrenheit above today’s global average temperature would initiate irreversible melting of the Greenland ice sheet— which could ultimately contribute about 23 feet to sea-level rise. This threshold is similar to the IPCC’s best estimate range for temperature increase by the end of this century. The risk for crossing this threshold could occur within our generation, while the consequences would be felt by future generations. Endnotes 1. Whenever practical, the exact language from the Summary for Policymakers is used throughout this document. To enhance clarity, slight modifications were made that maintain the intended meaning of the report. The Summary for Policymakers, released February 2, 2007, was the first contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (the Working Group I technical report is titled Climate Change 2007: The Physical Science Basis). Available at www.ipcc.ch. 2. For more background on IPCC history and process, visit www.ucsusa.org/global_warming/science/the-ipcc.html. 3. The Third Assessment Report (TAR 2001) concluded that “most of the observed warming over the last 50 years is likely to have been due to the increase in greenhouse gas concentration.” 4. The IPCC now displays greater confidence in the so-called equilibrium climate sensitivity test, which estimates the global average surface warming following a sustained doubling of carbon dioxide concentrations. Under this test it is likely that temperatures would increase between 3.6 and 8.1 degrees Fahrenheit (2.0 to 4.5 degrees Celsius) by the end of the century, with a best estimate of about 5.4 degrees Fahrenheit (3.0 degrees Celsius). 5. Projected “reductions in average global surface ocean pH are between 0.14 and 0.35 units over the 21st century, adding to the present decrease of 0.1 units since pre-industrial times.” This summary, drafted by B. Ekwurzel of the Union of Concerned Scientists (UCS) benefited from helpful reviews by T.Stocker (University of Bern), R.C.J. Somerville (Scripps Institution of Oceanography), S.J. Hassol (Climate Science Communicator), and P.C. Frumhoff (UCS).

Findings of the IPCC Fourth Assessment Report: Climate Change Impacts

The second policy-relevant report summary published in 2007 by the Intergovernmental Panel on Climate Change (IPCC) describes the impacts of global warming on society and the natural environment, as well as some of the available options for adapting. Released in April 2007—six years after the prior assessment by the IPCC—the Working Group II Summary for Policymakers is titled Climate Change 2007: Impacts, Adaptation and Vulnerability.[1]

The other components include the Working Group I report summary The Physical Science Basis and the Working Group III report summary Mitigation of Climate Change. This report is one component of the full IPCC Fourth Assessment Report, which includes the input of more than 1,200 authors and 2,500 scientific expert reviewers from more than 130 countries.[2]

Highlights from Working Group I The IPCC reported in February 2007 that it is “very likely” (>90 percent) that heat-trapping emissions from human activities have caused “most of the observed increase in globally averaged temperatures since the mid-20th century.” The IPCC based its projections of further climate change on six scenarios that assume various increases in concentrations of heat-trapping gases in the atmosphere. The IPCC projects temperature increases by the end of the century in the range of ~2°F (1.1°C) to ~11.5°F (6.4°C).[3]

Effects of Warming Are Apparent Worldwide
Human-induced warming over recent decades is already affecting many physical and biological processes on every continent. Nearly 90 percent of the 29,000 observational data series examined revealed changes consistent with the expected response to global warming, and the observed physical and biological responses have been greatest in the regions that warmed the most. This conclusion is further supported by recent advances in studies that compare observed changes with simulations that explicitly separate natural and human-induced factors affecting climate. This enables researchers to calculate how much of an observed change is attributable to human-induced warming. Challenges remain over the influence of other factors such as local pollution, invasive species, and land-use change.

The IPCC expects additional substantial effects on human society and natural environments around the world. One reason is that ~1 degree Fahrenheit (°F), or 0.6 degree Celsius (°C), of additional warming is already unavoidable due to past emissions. Further emissions would cause additional warming and thus additional impacts. The IPCC summary often does not explicitly predict the magnitude and timing of these consequences because they depend on the amount and rate of warming and, in some cases, on society’s ability to adapt.

IPCC Terminology Projections described in the text of this document are marked with asterisks that reflect the degree of confidence assigned to that conclusion by the IPCC: • Very high confidence*** = At least a 9 out of 10 chance • High confidence** = About an 8 out of 10 chance • Medium confidence* = About a 5 out of 10 chance Likelihood of occurrence: • Very likely = Greater than 90% probability • Likely = Greater than 66% probability

Changes in Water Resources
Hundreds of millions of people face water shortages that will worsen as temperatures rise.** Most at risk are current drought-affected regions, areas with heavily used water resources, and areas that get their water from glaciers and snowpack (including the western United States).

The land area affected by drought is expected to increase and water resources in affected areas could decline as much as 30 percent by midcentury. U.S. crops that are already near the upper end of their temperature tolerance range or depend on heavily used water resources could suffer with further warming.**

More than one-sixth of the world’s population currently lives near rivers that derive their water from glaciers and snow cover; these communities can expect to see their water resources decline over this century.** The IPCC expects many Latin American glaciers to disappear entirely over the next couple of decades,** and water resource competition to increase in western North America when decreased snowpack in the mountains reduces summer river flow.*** Many rivers that derive their water from melting glaciers or snow will have earlier peak runoff in spring and an overall increase in runoff, at least in the short term.** Such a temporary increase in water flow would not always be welcome; for example, melting glaciers in the Himalayas would increase flooding and rockslide risks, while flash flood risks could increase in Northern, Central, and Eastern Europe.

Impacts of Warming on North America

Click here to see more impacts of warming on North America (pdf). Most of these impacts are already under way and are expected to increase with further warming.

Changes in Food Production
The IPCC expects food production to decline in low-latitude regions (near the equator), particularly in the seasonally dry tropics, as even small temperature increases decrease crop yields in these areas.

The IPCC projections show drought-prone areas of Africa to be particularly vulnerable to food shortages due to a reduction in the land area suitable for agriculture; some rain-fed crop yields could decline as much as 50 percent by 2020.** The likely degradation of African coral reefs and mangroves would have negative consequences for fisheries. Rising lake temperatures in Africa combined with overfishing may also decrease fish supplies. Several developing countries in Asia face a continued very high risk of food shortages from a combination of projected declines in crop production, rapid population growth, and urbanization.

Under local average temperature increases of ~2 to ~5°F (1 to 3°C), regions such as Northern Europe, North America, New Zealand, and parts of Latin America could benefit from increased growing season length, more precipitation, and/or less frost, depending on the crop. However, these regions can also expect more flooding, and if local average temperatures rise beyond this range, crop yields could decline in some of these areas.* Note that these higher-latitude regions warm at a faster rate than the global average.

The populations most vulnerable to climate change-induced food shortages are those that depend on climate-sensitive food and water supplies and also lack the economic resources and government support to plan for or recover from extreme events such as floods or prolonged droughts.

Species in Peril
Up to 30 percent of plant and animal species could face extinction if the global average temperature rises more than ~3 to ~5°F (1.5 to 2.5°C)[4] relative to the 1980–1999 period.* Many projections suggest the low end of this temperature range could be reached by mid-century.

Many species have already shifted their ranges to higher latitudes (toward the poles) and higher elevations over the past several decades. Spring has been arriving earlier during this time, influencing the timing of bird and fish migration, egg laying, leaf unfolding, and spring planting for agriculture and forestry in the high northern latitudes.*** Satellite records since the early 1980s confirm that increased temperatures have produced longer growing seasons.**

Scientists expect the magnitude of these changes to increase along with temperatures over this century. Many species and ecosystems may not be able to adapt as the effects of global warming and its associated disturbances (including floods, drought, wildfire, and insects) are compounded by other stresses such as pollution and resource exploitation.** Polar and alpine species are especially vulnerable to the effects of climate change, as their unique habitats could shrink due to warming.

Acidification of the oceans due to increasing atmospheric carbon dioxide negatively affects marine shell-forming organisms such as corals and some plankton (and the species that depend upon them). Warming ocean waters represent another threat to corals; if they are unable to adapt to projected sea surface temperature increases of ~2 to ~5°F (1 to 3°C), corals could lose the algae that nourish and give them color, and many would die.*** Scientists expect coral reefs and man-groves in Africa to be degraded to the point that fisheries and tourism suffer.

Some areas, such as the national parks of Australia and New Zealand and many parts of tropical Latin America are likely to experience a significant loss of biodiversity. The Great Barrier Reef could experience such a loss by 2020.* By mid-century, tropical forests in the eastern Amazon Basin could be gradually replaced by less species-rich savanna because of rising temperatures and decreasing soil moisture.**

Escalating Hazards for Coasts
Flooding caused by sea-level rise is expected to affect millions of additional people every year by the end of this century, with small islands and the crowded delta regions around large Asian rivers (such as the Ganges-Brahmaputra) facing the highest risk.

Sea-level rise exposes coasts to higher risks of flooding and erosion, which would be exacerbated by growing population, increased human infrastructure within flood-prone areas, and human activities that increase erosion or local subsidence.*** Regions especially at risk are low-lying areas of North America, Latin America, Africa, populous coastal cities of Europe, crowded delta regions of Asia that face flood risks from both large rivers and ocean storms, and many small islands whose very existence is threatened by rising seas. In North America, current preparedness for rising seas, more frequent severe weather, and higher storm surges is low.

The Greenland and West Antarctic ice sheets face substantial melting if the global average temperature rises more than ~2 to ~7°F (1 to 4°C) relative to the period 1990–2000—eventually contributing to an additional sea-level rise of ~13 to ~20 feet (4 to 6 meters) or more.* This would result in the inundation of low-lying coastal areas, including parts of many major cities.

More Extreme Weather
The IPCC expects extreme weather and weather-related events to become more frequent and/or intense, with serious consequences for human health and well-being.

Scientists expect heat waves, droughts, wildfires, floods, severe storms, and dust transported between continents to cause locally severe economic damage and substantial social and cultural disruption. The IPCC projects an extended fire season for North America as well as increased threats from pests and disease (which could significantly enlarge the area burned in a fire).*** Moreover, because fires release the carbon stored in trees, an increase in wildfires would further worsen global warming.

Increases in the frequency of droughts and floods would negatively affect local food production, and communities in mountain regions would face an increased risk of floods caused by melting glaciers. In addition, the risk of flood-induced illness and death from diarrheal diseases could rise in South and Southeast Asia. A region’s vulnerability to such extreme events depends both on how much the climate changes and whether or not nations develop effective responses to potential threats.

Threats to Human Health
The IPCC expects heat waves, floods, storms, fires, and droughts related to global warming to contribute to increased rates of death, disease, and injuries for millions around the world.

U.S. cities that currently experience heat waves can expect increases in the number, intensity, and duration of heat waves over the course of the century. Scientists project an increase in the incidence of cardio-respiratory diseases caused by the higher concentrations of ground-level ozone (smog) that may accompany higher air temperatures. Some infectious diseases, such as those carried by insects and rodents, may also become more common in regions where those diseases are not currently prevalent.

Developing countries, many of which are already under stress, could experience increases in the incidence of diarrheal diseases and malnutrition and consequent disorders, affecting child growth and development. The populations most vulnerable to harsh living conditions in any nation—the elderly, children, and poor—may be unable to cope with further climate change.

The Warming We Can and Can’t Avoid
We need adaptation strategies to cope with those consequences of global warming that are already unavoidable due to past emissions. Many additional consequences can be avoided, minimized, or delayed by reducing our emissions.

Many of the unavoidable near-term consequences of global warming can be addressed through adaptation strategies such as building levees and restoring wetlands to protect coasts, altering farm practices to grow crops that can survive higher temperatures, building infrastructure that can withstand extreme weather, and implementing public health programs to help people in cities survive brutal heat waves. However, the options for successful adaptation diminish and the associated costs increase as temperatures continue to rise. One of the IPCC’s fundamental conclusions is that both adaptation and mitigation (in the form of emission reductions) are required to cope with climate change over the short and long term. These two broad categories of policy options are complementary tools in a policy portfolio designed with the ultimate goal of reducing climate risks as much as possible.

Some near-term adaptation strategies may increase local vulnerability in the long run (for example, where coastal levees encourage development in vulnerable regions, or where population grows in response to temporarily increased water resources from glacier-fed rivers that will ultimately disappear). In contrast, development can be planned in a way that reduces our vulnerability to climate change such as not building in flood plains. However, even responsible development could be overwhelmed by the impacts of climate change unless we also slow the rate of global warming.

Options for curbing climate change by reducing our heat-trapping emissions are explored in the IPCC Working Group III report to be published in May 2007.

Endnotes
1. Whenever practical, the exact language from the Working Group II Summary for Policymakers is used throughout this document. To enhance clarity, slight modifications were made that maintain the intended meaning of the report. The full text of the Summary for Policymakers is available at www.ipcc.ch.

2. Click here for more background on IPCC history and process.

3. The “~” indicates where temperatures have been rounded to whole numbers after being converted from Celsius to Fahrenheit. The range is partly a function of the uncertainty in how sensitive the global climate is to increasing levels of heattrapping emissions, and partly a function of societal decisions affecting the timing and amount of future emissions. Source: IPCC Working Group I, Climate Change 2007: The Physical Science Basis—Summary for Policymakers. Available at www.ipcc.ch.

4. Increases in global average temperature represent the increase above the global average during the period 1980–1999. The 1980–1999 average is 0.5°C above the 1850–1899 average, and the average today is around 0.8°C above the 1850–1899 average.


This summary, drafted by B. Ekwurzel of the Union of Concerned Scientists (UCS) and S.J. Hassol (climate science communicator), benefited from helpful reviews by V.R. Burkett (U.S. Geological Survey), K.L. Ebi (EES, L.L.C.), P.C. Frumhoff (UCS), M. Mastrandrea (Stanford University), M. Oppenheimer (Princeton University), P. Romero-Lankao (National Center for Atmospheric Research), S.W. Running (University of Montana), and G. Yohe (Wesleyan University). Funded in part by The Pew Charitable Trusts. The information contained herein is the sole responsibility of UCS.