The Intergovernmental Panel on Climate Change (IPCC) was established in 1988 by the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) to assess climate change based on the latest science.
Through the IPCC, thousands of experts from around the world synthesize the most recent developments in climate science, adaptation, vulnerability, and mitigation every five to seven years. Governments request these reports through the intergovernmental process and the content is deliberately policy-relevant, but steers clear of any policy-prescriptive statements. Government representatives work with experts to produce the "summary for policymakers" (SPM) that highlights the most critical developments in language accesible to the world's political leaders. Scholars, academics and students can dig into the chapters and supplementary materials for a thorough and deeper understanding of the evidence.
The IPCC has issued comprehensive assessments in 1990, 1996, 2001, 2007 and 2013, methodology reports, technical papers, and periodic special reports assessing specific impacts of climate change (the latest ones in the works: oceans and ice cover, land degradation, impacts of 1.5°C warming).
The fifth assessment report, AR5, is the most comprehensive synthesis to date. Experts from more than 80 countries contributed to this assessment, which represents six years of work. More than 830 lead authors and review editors drew on the work of over 1000 contributors. About 2,000 expert reviewers provided over 140,000 review comments.
AR5 assessed more extensively than prior assessments the socioeconomic impacts of climate change and the challenges for sustainable development. The inclusive process by which IPCC assessments are developed and accepted by its members ensures exceptional scientific credibility. For this reason, AR5 serves as the basis to inform domestic and international climate policies. Many countries draw upon the IPCC in their national climate assessments. Such as the November 2017 release of the first volume of the U.S. fourth National Climate Assessment (NCA4), also referred to as the Climate Science Special Report (CSSR).
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.” The goal of these assessments is to inform international policy and negotiations on climate-related issues.
The assessments and climate action
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 was stronger than ever.
The Fourth Assessment Report (AR4, 2007) emphasized that “the warming of the climate system is unequivocal” and that it is affecting ecosystems worldwide. And the Fifth Assessment Report (AR5, 2013/14) asserted that “[h]uman influence on the climate system is clear, and recent anthropogenic emissions of greenhouse gases are the highest in history.” These findings informed the climate negotiations resulting in the Paris Agreement of 2015, in which 197 countries committed to limiting global warming to below 2°C.
In tandem with the Paris agreement, the Conference of Parties (COP) invited the IPCC to provide a special assessment on the impacts of climate change when global temperature reaches 1.5°C above pre-industrial levels. The special report will be released in 2018 and will serve as basis to discuss adaptation options in the context of sustainable development and poverty eradication.
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 AR5, Working Group I summarized the physical science basis of climate change. Working Group II addressed 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 assessed options for limiting heat-trapping emissions, evaluated methods for removing them from the atmosphere, and examined other means of slowing the warming trend, as well as related economic issues.
A separate Task Force on National Greenhouse Gas Inventories oversees the compilation of global warming emissions and removals 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.
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 direct financial support for their contributions to the IPCC 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 working group must have at least one co-chair from a non-OECD country.
In general, the appointed authors are widely recognized experts who represent a broad range of subject areas 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 .
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 perspectives. 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. Scientists are also encouraged to submit comments—AR5 WG1 alone generated 54,677 review comments. 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. Government representatives, however, do participate in the line-by-line review and revision of the much shorter summary for policymakers, or SPM, for each technical report. The working group’s lead authors write the SPM, technical experts review it first, and then government representatives provide a second round of reviews before the working group’s plenary session accepts it. 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 many other government representatives and experts in the session would ensure the language does not veer away from the evidence. 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 a single peer-reviewed publication in a scientific journal. 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. 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—about 2,000 for the entire AR5—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, 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 government review stage. Each government is entitled to organize any type of review process it deems appropriate. For example, in the past, the U.S. government sought comments from agencies, scientific experts, and the general public (through a notice in the Federal Register) as the starting point for its comments.
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
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.
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, scientists with controversial interpretations of the empirical evidence 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.
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 across the energy sector such as fossil fuels and renewables (e.g. the Electric Power Research Institute and ExxonMobil). Environmental examples have included representatives from NGOs from around the world (e.g. The International Indigenous People’s Forum on Climate Change, Environmental Defense, the Natural Resources Defense Council).
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. 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.”
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 assessment reports are scientific in nature, their 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. However, one always has the option to look into the chapters to see where the evolving science is heading and explore the references cited. The full assessment is a multi-level document for a wide array of audiences ranging from planners investing in protecting their communities to political leaders.
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.”
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.