Intergovernmental Panel on Climate Change


The Intergovernmental Panel on Climate Change is an intergovernmental body of the United Nations that is dedicated to providing the world with objective, scientific information relevant to understanding the scientific basis of the
risk of human-induced climate change, its natural, political, and economic impacts and risks, and possible response options.
The IPCC was established in 1988 by the World Meteorological Organization and the United Nations Environment Programme and was later endorsed by the United Nations General Assembly. Membership is open to all members of the WMO and UN.
The IPCC produces reports that contribute to the work of the United Nations Framework Convention on Climate Change, the main international treaty on climate change. The objective of the UNFCCC is to "stabilize greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system". The IPCC's Fifth Assessment Report was a critical scientific input into the UNFCCC's Paris Agreement in 2015.
IPCC reports cover the "scientific, technical and socio-economic information relevant to understanding the scientific basis of risk of human-induced climate change, its potential impacts and options for adaptation and mitigation." The IPCC does not carry out original research, nor does it monitor climate or related phenomena itself. Rather, it assesses published literature, including peer-reviewed and non-peer-reviewed sources. However, the IPCC can be said to stimulate research in climate science. Chapters of IPCC reports often close with sections on limitations and knowledge or research gaps, and the announcement of an IPCC special report can catalyse research activity in that area.
Thousands of scientists and other experts contribute on a voluntary basis to writing and reviewing reports, which are then reviewed by governments. IPCC reports contain a "Summary for Policymakers", which is subject to line-by-line approval by delegates from all participating governments. Typically, this involves the governments of more than 120 countries.
The IPCC provides an internationally accepted authority on climate change, producing reports that have the agreement of leading climate scientists and consensus from participating governments. The 2007 Nobel Peace Prize was shared between the IPCC and Al Gore.
Following the election of a new Bureau in 2015, the IPCC embarked on its sixth assessment cycle. Besides the Sixth Assessment Report, to be completed in 2022, the IPCC released the Special Report on Global Warming of 1.5 °C in October 2018, released an update to its 2006 Guidelines for National Greenhouse Gas Inventories—the 2019 Refinement—in May 2019, and delivered two further special reports in 2019: the Special Report on Climate Change and Land, published online on 7 August, and the Special Report on the Ocean and Cryosphere in a Changing Climate, released on 25 September 2019. This makes the sixth assessment cycle the most ambitious in the IPCC's 30-year history. The IPCC also decided to prepare a special report on cities and climate change in the seventh assessment cycle and held a in March 2018 to stimulate research in this area.

Origins and aims

The IPCC developed from an international scientific body, the Advisory Group on Greenhouse Gases set up in 1985 by the International Council of Scientific Unions, the United Nations Environment Programme, and the World Meteorological Organization to provide recommendations based on current research. This small group of scientists lacked the resources to cover the increasingly complex interdisciplinary nature of climate science. The United States Environmental Protection Agency and State Department wanted an international convention to agree restrictions on greenhouse gases, and the conservative Reagan Administration was concerned about unrestrained influence from independent scientists or from United Nations bodies including UNEP and the WMO. The U.S. government was the main force in forming the IPCC as an autonomous intergovernmental body in which scientists took part both as experts on the science and as official representatives of their governments, to produce reports which had the firm backing of all the leading scientists worldwide researching the topic, and which then had to gain consensus agreement from every one of the participating governments. In this way, it was formed as a hybrid between a scientific body and an intergovernmental political organisation.
The United Nations formally endorsed the creation of the IPCC in 1988. Some of the reasons the UN stated in its resolution include
The IPCC was tasked with reviewing peer-reviewed scientific literature and other relevant publications to provide information on the state of knowledge about climate change.

Organization

The IPCC does not conduct its own original research. It produces comprehensive assessments, reports on special topics, and methodologies. The assessments build on previous reports, highlighting the latest knowledge. For example, the wording of the reports from the first to the fifth assessment reflects the growing evidence for a changing climate caused by human activity.
The IPCC has adopted and published "Principles Governing IPCC Work", which states that the IPCC will assess:
This document also states that IPCC will do this work by assessing "on a comprehensive, objective, open and transparent basis the scientific, technical and socio-economic information relevant to understanding the scientific basis" of these topics. The Principles also state that "IPCC reports should be neutral with respect to policy, although they may need to deal objectively with scientific, technical and socio-economic factors relevant to the application of particular policies."
Korean economist Hoesung Lee has been the chair of the IPCC since 8 October 2015, with the election of the new IPCC Bureau.
Before this election, the IPCC was led by Vice-Chair Ismail El Gizouli, who was designated acting Chair after the resignation of Rajendra K. Pachauri in February 2015. The previous chairs were Rajendra K. Pachauri, elected in May 2002; Robert Watson in 1997; and Bert Bolin in 1988. The chair is assisted by an elected bureau including vice-chairs and working group co-chairs, and by a secretariat.
The Panel itself is composed of representatives appointed by governments. Participation of delegates with appropriate expertise is encouraged. Plenary sessions of the IPCC and IPCC Working Groups are held at the level of government representatives. Non-Governmental and Intergovernmental Organizations admitted as may also attend. Sessions of the Panel, IPCC Bureau, workshops, expert and lead authors meetings are by invitation only. About 500 people from 130 countries attended the 48th Session of the Panel in Incheon, Republic of Korea, in October 2018, including 290 government officials and 60 representatives of observer organizations. The opening ceremonies of sessions of the Panel and of Lead Author Meetings are open to media, but otherwise IPCC meetings are closed.
There are several major groups:
The IPCC receives funding through the IPCC Trust Fund, established in 1989 by the United Nations Environment Programme and the World Meteorological Organization, Costs of the Secretary and of housing the secretariat are provided by the WMO, while UNEP meets the cost of the Depute Secretary. Annual cash contributions to the Trust Fund are made by the WMO, by UNEP, and by IPCC Members. Payments and their size are voluntary. The Panel is responsible for considering and adopting by consensus the annual budget. The organization is required to comply with the Financial Regulations and Rules of the WMO.

Assessment reports

The IPCC has published five comprehensive assessment reports reviewing the latest climate science, as well as a number of special reports on particular topics. These reports are prepared by teams of relevant researchers selected by the Bureau from government nominations. Expert reviewers from a wide range of governments, IPCC observer organizations and other organizations are invited at different
stages to comment on various aspects of the drafts.
The IPCC published its First Assessment Report in 1990, a supplementary report in 1992, a Second Assessment Report in 1995, a Third Assessment Report in 2001, a Fourth Assessment Report in 2007 and a Fifth Assessment Report in 2014. The IPCC is currently preparing the Sixth Assessment Report, which will be completed in 2022.
Each assessment report is in three volumes, corresponding to Working Groups I, II, and III. It is completed by a synthesis report that integrates the working group contributions and any special reports produced in that assessment cycle.

Scope and preparation of the reports

The IPCC does not carry out research nor does it monitor climate related data. Lead authors of IPCC reports assess the available information about climate change based on published sources. According to IPCC guidelines, authors should give priority to peer-reviewed sources. Authors may refer to non-peer-reviewed sources, provided that they are of sufficient quality. Examples of non-peer-reviewed sources include model results, reports from government agencies and non-governmental organizations, and industry journals. Each subsequent IPCC report notes areas where the science has improved since the previous report and also notes areas where further research is required.
There are generally three stages in the review process:
Review comments are in an open archive for at least five years.
There are several types of endorsement which documents receive:
The Panel is responsible for the IPCC and its endorsement of Reports allows it to ensure they meet IPCC standards.
There have been a range of commentaries on the IPCC's procedures, examples of which are discussed later in the article. Some of these comments have been supportive, while others have been critical. Some commentators have suggested changes to the IPCC's procedures.

Authors

Each chapter has a number of authors who are responsible for writing and editing the material. A chapter typically has two "coordinating lead authors", ten to fifteen "lead authors", and a somewhat larger number of "contributing authors". The coordinating lead authors are responsible for assembling the contributions of the other authors, ensuring that they meet stylistic and formatting requirements, and reporting to the Working Group chairs. Lead authors are responsible for writing sections of chapters. Contributing authors prepare text, graphs or data for inclusion by the lead authors.
Authors for the IPCC reports are chosen from a list of researchers prepared by governments and participating organisations, and by the Working Group/Task Force Bureaux, as well as other experts known through their published work. The choice of authors aims for a range of views, expertise and geographical representation, ensuring representation of experts from developing and developed countries and countries with economies in transition.

First assessment report

The IPCC First Assessment Report was completed in 1990, and served as the basis of the UNFCCC.
The executive summary of the WG I Summary for Policymakers report says they are certain that emissions resulting from human activities are substantially increasing the atmospheric concentrations of the greenhouse gases, resulting on average in an additional warming of the Earth's surface. They calculate with confidence that CO2 has been responsible for over half the enhanced greenhouse effect. They predict that under a "business as usual" scenario, global mean temperature will increase by about 0.3 °C per decade during the century. They judge that global mean surface air temperature has increased by 0.3 to 0.6 °C over the last 100 years, broadly consistent with prediction of climate models, but also of the same magnitude as natural climate variability. The unequivocal detection of the enhanced greenhouse effect is not likely for a decade or more.

Supplementary report of 1992

The 1992 supplementary report was an update, requested in the context of the negotiations on the UNFCCC at the Earth Summit in Rio de Janeiro in 1992.
The major conclusion was that research since 1990 did "not affect our fundamental understanding of the science of the greenhouse effect and either confirm or do not justify alteration of the major conclusions of the first IPCC scientific assessment". It noted that transient simulations, which had been very preliminary in the FAR, were now improved, but did not include aerosol or ozone changes.

Second assessment report

Climate Change 1995, the IPCC Second Assessment Report, was finished in 1996. It is split into four parts:
Each of the last three parts was completed by a separate Working Group, and each has a Summary for Policymakers that represents a consensus of national representatives. The SPM of the WG I report contains headings:
  1. Greenhouse gas concentrations have continued to increase
  2. Anthropogenic aerosols tend to produce negative radiative forcings
  3. Climate has changed over the past century.
  4. The balance of evidence suggests a discernible human influence on global climate
  5. Climate is expected to continue to change in the future
  6. There are still many uncertainties

    Third assessment report

The Third Assessment Report was completed in 2001 and consists of four reports, three of them from its Working Groups:
A number of the TAR's conclusions are given quantitative estimates of how probable it is that they are correct, e.g., greater than 66% probability of being correct. These are "Bayesian" probabilities, which are based on an expert assessment of all the available evidence.
"Robust findings" of the TAR Synthesis Report include:
In 2001, 16 national science academies issued a joint statement on climate change.
The joint statement was made by the Australian Academy of Science, the Royal Flemish Academy of Belgium for Science and the Arts, the Brazilian Academy of Sciences, the Royal Society of Canada, the Caribbean Academy of Sciences, the Chinese Academy of Sciences, the French Academy of Sciences, the German Academy of Natural Scientists Leopoldina, the Indian National Science Academy, the Indonesian Academy of Sciences, the Royal Irish Academy, Accademia Nazionale dei Lincei, the Academy of Sciences Malaysia, the Academy Council of the Royal Society of New Zealand, the Royal Swedish Academy of Sciences, and the Royal Society.
The statement, also published as an editorial in the journal Science, stated "we support the conclusion that it is at least 90% certain that temperatures will continue to rise, with average global surface temperature projected to increase by between 1.4 and 5.8 °C above 1990 levels by 2100".
The TAR has also been endorsed by the Canadian Foundation for Climate and Atmospheric Sciences, Canadian Meteorological and Oceanographic Society, and European Geosciences Union.
In 2001, the US National Research Council produced a report that assessed Working Group I's contribution to the TAR. US NRC "generally agrees" with the WGI assessment, and describes the full WGI report as an "admirable summary of research activities in climate science".
IPCC author Richard Lindzen has made a number of criticisms of the TAR. Among his criticisms, Lindzen has stated that the WGI Summary for Policymakers does not faithfully summarize the full WGI report. For example, Lindzen states that the SPM understates the uncertainty associated with climate models. John Houghton, who was a co-chair of TAR WGI, has responded to Lindzen's criticisms of the SPM. Houghton has stressed that the SPM is agreed upon by delegates from many of the world's governments, and that any changes to the SPM must be supported by scientific evidence.
IPCC author Kevin Trenberth has also commented on the WGI SPM. Trenberth has stated that during the drafting of the WGI SPM, some government delegations attempted to "blunt, and perhaps obfuscate, the messages in the report". However, Trenberth concludes that the SPM is a "reasonably balanced summary".
US NRC concluded that the WGI SPM and Technical Summary are "consistent" with the full WGI report. US NRC stated:
the full report is adequately summarized in the Technical Summary. The full WGI report and its Technical Summary are not specifically directed at policy. The Summary for Policymakers reflects less emphasis on communicating the basis for uncertainty and a stronger emphasis on areas of major concern associated with human-induced climate change. This change in emphasis appears to be the result of a summary process in which scientists work with policy makers on the document. Written responses from U.S. coordinating and lead scientific authors to the committee indicate, however, that no changes were made without the consent of the convening lead authors and most changes that did occur lacked significant impact.

Fourth assessment report

The Fourth Assessment Report was published in 2007. Like previous assessment reports, it consists of four reports:
People from over 130 countries contributed to the IPCC Fourth Assessment Report, which took 6 years to produce. Contributors to AR4 included more than 2500 scientific expert reviewers, more than 800 contributing authors, and more than 450 lead authors.
"Robust findings" of the Synthesis report include:
Global warming projections from AR4 are shown below. The projections apply to the end of the 21st century, relative to temperatures at the end of the 20th century. Add 0.7 °C to projections to make them relative to pre-industrial levels instead of 1980–99. "Likely" range
B11.81.1 – 2.9A1T2.41.4 – 3.8B22.41.4 – 3.8A1B2.81.7 – 4.4A23.42.0 – 5.4A1FI4.02.4 – 6.4
"Likely" means greater than 66% probability of being correct, based on expert judgement.

Response to AR4

Several science academies have referred to and/or reiterated some of the conclusions of AR4. These include:
The Netherlands Environmental Assessment Agency has carried out two reviews of AR4. These reviews are generally supportive of AR4's conclusions. PBL make some recommendations to improve the IPCC process. A literature assessment by the US National Research Council concludes:
Climate change is occurring, is caused largely by human activities, and poses significant risks for—and in many cases is already affecting—a broad range of human and natural systems . This conclusion is based on a substantial array of scientific evidence, including recent work, and is consistent with the conclusions of recent assessments by the U.S. Global Change Research Program , the Intergovernmental Panel on Climate Change’s Fourth Assessment Report , and other assessments of the state of scientific knowledge on climate change.

Some errors have been found in the IPCC AR4 Working Group II report. Two errors include the melting of Himalayan glaciers, and Dutch land area that is below sea level.

Fifth assessment report

The IPCC's Fifth Assessment Report was completed in 2014. AR5 followed the same general format as of AR4, with three Working Group reports and a Synthesis report. The Working Group I report was published in September 2013.
Conclusions of AR5 are summarized below:
;Working Group I:
;Working Group II:
; Working Group III:
Projections in AR5 are based on "Representative Concentration Pathways". The RCPs are consistent with a wide range of possible changes in future anthropogenic greenhouse gas emissions. Projected changes in global mean surface temperature and sea level are given in the main RCP article.

Special reports

In addition to climate assessment reports, the IPCC publishes Special Reports on specific topics. The preparation and approval process for all IPCC Special Reports follows the same procedures as for IPCC Assessment Reports. In the year 2011 two IPCC Special Report were finalized, the Special Report on Renewable Energy Sources and Climate Change Mitigation and the Special Report on Managing Risks of Extreme Events and Disasters to Advance Climate Change Adaptation. Both Special Reports were requested by governments.

Special Report on Emissions Scenarios (SRES)

The Special Report on Emissions Scenarios is a report by the IPCC which was published in 2000. The SRES contains "scenarios" of future changes in emissions of greenhouse gases and sulfur dioxide. One of the uses of the SRES scenarios is to project future changes in climate, e.g., changes in global mean temperature. The SRES scenarios were used in the IPCC's Third and Fourth Assessment Reports.
The SRES scenarios are "baseline" scenarios, which means that they do not take into account any current or future measures to limit greenhouse gas emissions. SRES emissions projections are broadly comparable in range to the baseline projections that have been developed by the scientific community.

Comments on the SRES

There have been a number of comments on the SRES. Parson et al. stated that the SRES represented "a substantial advance from prior scenarios". At the same time, there have been criticisms of the SRES.
The most prominently publicized criticism of SRES focused on the fact that all but one of the participating models compared gross domestic product across regions using market exchange rates, instead of the more correct purchasing-power parity approach. This criticism is discussed in the main SRES article.

Special report on renewable energy sources and climate change mitigation (SRREN)

This assesses existing literature on renewable energy commercialisation for the mitigation of climate change. It was published in 2012 and covers the six most important renewable energy technologies in a transition, as well as their integration into present and future energy systems. It also takes into consideration the environmental and social consequences associated with these technologies, the cost and strategies to overcome technical as well as non-technical obstacles to their application and diffusion. The full report in PDF form is found
More than 130 authors from all over the world contributed to the preparation of IPCC Special Report on Renewable Energy Sources and Climate Change Mitigation on a voluntary basis – not to mention more than 100 scientists, who served as contributing authors.

Special Report on managing the risks of extreme events and disasters to advance climate change adaptation (SREX)

The was published in 2012. It assesses the effect that climate change has on the threat of natural disasters and how nations can better manage an expected change in the frequency of occurrence and intensity of severe weather patterns. It aims to become a resource for decision-makers to prepare more effectively for managing the risks of these events. A potentially important area for consideration is also the detection of trends in extreme events and the attribution of these trends to human influence. The full report, 594 pages in length, may be found in PDF form.
More than 80 authors, 19 review editors, and more than 100 contributing authors from all over the world contributed to the preparation of SREX.

Special Report on Global Warming of 1.5 °C (SR15)

When the Paris Agreement was adopted, the UNFCCC invited the Intergovernmental Panel on Climate Change to write a special report on "How can humanity prevent the global temperature rise more than 1.5 degrees above pre-industrial level". The completed report, Special Report on Global Warming of 1.5 °C, was released on 8 October 2018. Its full title is "Global Warming of 1.5 °C, an IPCC special report on the impacts of global warming of 1.5 °C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty".
The finished report summarizes the findings of scientists, showing that maintaining a temperature rise to below 1.5 °C remains possible, but only through "rapid and far-reaching transitions in energy, land, urban and infrastructure..., and industrial systems". Meeting the Paris target of is possible but would require "deep emissions reductions", "rapid", "far-reaching and unprecedented changes in all aspects of society". In order to achieve the 1.5 °C target, CO2 emissions must decline by 45% by 2030, reaching net zero by around 2050. Deep reductions in non-CO2 emissions will also be required to limit warming to 1.5 °C. Under the pledges of the countries entering the Paris Accord, a sharp rise of 3.1 to 3.7 °C is still expected to occur by 2100. Holding this rise to 1.5 °C avoids the worst effects of a rise by even 2 °C. However, a warming of even 1.5 degrees will still result in large-scale drought, famine, heat stress, species die-off, loss of entire ecosystems, and loss of habitable land, throwing more than 100 Million into poverty. Effects will be most drastic in arid regions including the Middle East and the Sahel in Africa, where fresh water will remain in some areas following a 1.5 °C rise in temperatures but are expected to dry up completely if the rise reaches 2 °C.

Special Report on climate change and land (SRCCL)

The final draft of the "Special Report on climate change and land" —with the full title, "Special Report on climate change, desertification, land degradation, sustainable land management, food security, and greenhouse gas fluxes in terrestrial ecosystems" was published online on 7 August 2019. The SRCCL consists of seven chapters, Chapter 1: Framing and Context, Chapter 2: Land-Climate Interactions, Chapter 3: Desertification, Chapter 4: Land Degradation, Chapter 5: Food Security, Chapter 5 Supplementary Material, Chapter 6: Interlinkages between desertification, land degradation, food security and GHG fluxes: Synergies, trade-offs and Integrated Response Options, and Chapter 7: Risk management and decision making in relation to sustainable development.

Special Report on the Ocean and Cryosphere in a Changing Climate (SROCC)

The "Special Report on the Ocean and Cryosphere in a Changing Climate" was approved on 25 September 2019 in Monaco. Among other findings, the report concluded that sea level rises could be up to two feet higher by the year 2100, even if efforts to reduce greenhouse gas emissions and to limit global warming are successful; coastal cities across the world could see so-called "storm of the century" at least once a year.

Methodology reports

Within IPCC the National Greenhouse Gas Inventory Program develops methodologies to estimate emissions of greenhouse gases. This has been undertaken since 1991 by the IPCC WGI in close collaboration with the Organisation for Economic Co-operation and Development and the International Energy Agency.
The objectives of the National Greenhouse Gas Inventory Program are:
The 1996 Guidelines for National Greenhouse Gas Investories provide the methodological basis for the estimation of national greenhouse gas emissions inventories. Over time these guidelines have been completed with good practice reports: Good Practice Guidance and Uncertainty Management in National Greenhouse Gas Inventories and Good Practice Guidance for Land Use, Land-Use Change and Forestry.
The 1996 guidelines and the two good practice reports are to be used by parties to the UNFCCC and to the Kyoto Protocol in their annual submissions of national greenhouse gas inventories.

2006 IPCC Guidelines for National Greenhouse Gas Inventories

The 2006 IPCC Guidelines for National Greenhouse Gas Inventories is the latest version of these emission estimation methodologies, including a large number of default emission factors. Although the IPCC prepared this new version of the guidelines on request of the parties to the UNFCCC, the methods have not yet been officially accepted for use in national greenhouse gas emissions reporting under the UNFCCC and the Kyoto Protocol.

Activities

The IPCC concentrates its activities on the tasks allotted to it by the relevant WMO Executive Council and UNEP Governing Council resolutions and decisions as well as on actions in support of the UNFCCC process. While the preparation of the assessment reports is a major IPCC function, it also supports other activities, such as the Data Distribution Centre and the National Greenhouse Gas Inventories Programme, required under the UNFCCC. This involves publishing default emission factors, which are factors used to derive emissions estimates based on the levels of fuel consumption, industrial production and so on.
The IPCC also often answers inquiries from the UNFCCC Subsidiary Body for Scientific and Technological Advice.

Nobel Peace Prize

In December 2007, the IPCC was awarded the Nobel Peace Prize "for their efforts to build up and disseminate greater knowledge about man-made climate change, and to lay the foundations for the measures that are needed to counteract such change". The award is shared with Former U.S. Vice-President Al Gore for his work on climate change and the documentary An Inconvenient Truth.

Criticisms

There is widespread support for the IPCC in the scientific community, which is reflected in publications by other scientific bodies and experts. However, criticisms of the IPCC have been made.
Since 2010 the IPCC has come under yet unparalleled public and political scrutiny. The global IPCC consensus approach has been challenged internally and externally, for example, during the 2009 Climatic Research Unit email controversy. It is contested by some as an information monopoly with results for both the quality and the impact of the IPCC work as such.

Projected date of melting of Himalayan glaciers

A paragraph in the 2007 Working Group II report, chapter 10 included a projection that Himalayan glaciers could disappear by 2035
This projection was not included in the final summary for policymakers. The IPCC has since acknowledged that the date is incorrect, while reaffirming that the conclusion in the final summary was robust. They expressed regret for "the poor application of well-established IPCC procedures in this instance". The date of 2035 has been correctly quoted by the IPCC from the WWF report, which has misquoted its own source, an ICSI report "Variations of Snow and Ice in the past and at present on a Global and Regional Scale".
Rajendra K. Pachauri responded in an interview with Science.

Overstatement of effects

Former IPCC chairman Robert Watson said, regarding the Himalayan glaciers estimation, "The mistakes all appear to have gone in the direction of making it seem like climate change is more serious by overstating the impact. That is worrying. The IPCC needs to look at this trend in the errors and ask why it happened". Martin Parry, a climate expert who had been co-chair of the IPCC working group II, said that "What began with a single unfortunate error over Himalayan glaciers has become a clamour without substance" and the IPCC had investigated the other alleged mistakes, which were "generally unfounded and also marginal to the assessment".

Emphasis of the "hockey stick" graph

The third assessment report prominently featured a graph labeled "Millennial Northern Hemisphere temperature reconstruction" based on a 1999 paper by Michael E. Mann, Raymond S. Bradley and Malcolm K. Hughes, which has been referred to as the "hockey stick graph". This graph extended the similar graph in :File:IPCC 1996 SAR Figure 3.20.png|Figure 3.20 from the IPCC Second Assessment Report of 1995, and differed from a schematic in the first assessment report that lacked temperature units, but appeared to depict larger global temperature variations over the past 1000 years, and higher temperatures during the Medieval Warm Period than the mid 20th century. The schematic was not an actual plot of data, and was based on a diagram of temperatures in central England, with temperatures increased on the basis of documentary evidence of Medieval vineyards in England. Even with this increase, the maximum it showed for the Medieval Warm Period did not reach temperatures recorded in central England in 2007. The MBH99 finding was supported by cited reconstructions by,, and, using differing data and methods. The Jones et al. and Briffa reconstructions were overlaid with the MBH99 reconstruction in Figure 2.21 of the IPCC report.
These studies were widely presented as demonstrating that the current warming period is exceptional in comparison to temperatures between 1000 and 1900, and the MBH99 based graph featured in publicity. Even at the draft stage, this finding was disputed by contrarians: in May 2000 Fred Singer's Science and Environmental Policy Project held a press event on Capitol Hill, Washington, D.C., featuring comments on the graph Wibjörn Karlén and Singer argued against the graph at a United States Senate Committee on Commerce, Science and Transportation hearing on 18 July 2000. Contrarian John Lawrence Daly featured a modified version of the IPCC 1990 schematic, which he mis-identified as appearing in the IPCC 1995 report, and argued that "Overturning its own previous view in the 1995 report, the IPCC presented the 'Hockey Stick' as the new orthodoxy with hardly an apology or explanation for the abrupt U-turn since its 1995 report". Criticism of the MBH99 reconstruction in a review paper, which was quickly discredited in the Soon and Baliunas controversy, was picked up by the Bush administration, and a Senate speech by US Republican senator James Inhofe alleged that "manmade global warming is the greatest hoax ever perpetrated on the American people". The data and methodology used to produce the "hockey stick graph" was criticized in papers by Stephen McIntyre and Ross McKitrick, and in turn the criticisms in these papers were examined by other studies and comprehensively refuted by, which showed errors in the methods used by McIntyre and McKitrick.
On 23 June 2005, Rep. Joe Barton, chairman of the House Committee on Energy and Commerce wrote joint letters with Ed Whitfield, chairman of the Subcommittee on Oversight and Investigations demanding full records on climate research, as well as personal information about their finances and careers, from Mann, Bradley and Hughes. Sherwood Boehlert, chairman of the House Science Committee, said this was a "misguided and illegitimate investigation" apparently aimed at intimidating scientists, and at his request the U.S. National Academy of Sciences arranged for its National Research Council to set up a special investigation. The National Research Council's report agreed that there were some statistical failings, but these had little effect on the graph, which was generally correct. In a 2006 letter to Nature, Mann, Bradley, and Hughes pointed out that their original article had said that "more widespread high-resolution data are needed before more confident conclusions can be reached" and that the uncertainties were "the point of the article".
The IPCC Fourth Assessment Report published in 2007 featured a graph showing 12 proxy based temperature reconstructions, including the three highlighted in the 2001 Third Assessment Report ; as before, and had both been calibrated by newer studies. In addition, analysis of the Medieval Warm Period cited reconstructions by and. Ten of these 14 reconstructions covered 1,000 years or longer. Most reconstructions shared some data series, particularly tree ring data, but newer reconstructions used additional data and covered a wider area, using a variety of statistical methods. The section discussed the divergence problem affecting certain tree ring data.

Conservative nature of IPCC reports

Some critics have contended that the IPCC reports tend to be by consistently underestimating the pace and impacts of global warming, and report only the "lowest common denominator" findings.
On the eve of the publication of IPCC's Fourth Assessment Report in 2007 another study was published suggesting that temperatures and sea levels have been rising at or above the maximum rates proposed during IPCC's 2001 Third Assessment Report. The study compared IPCC 2001 projections on temperature and sea level change with observations. Over the six years studied, the actual temperature rise was near the top end of the range given by IPCC's 2001 projection, and the actual sea level rise was above the top of the range of the IPCC projection.
Another example of scientific research which suggests that previous estimates by the IPCC, far from overstating dangers and risks, have actually understated them is a study on projected rises in sea levels. When the researchers' analysis was "applied to the possible scenarios outlined by the Intergovernmental Panel on Climate Change, the researchers found that in 2100 sea levels would be 0.5–1.4 m above 1990 levels. These values are much greater than the 9–88 cm as projected by the IPCC itself in its Third Assessment Report, published in 2001". This may have been due, in part, to the expanding human understanding of climate.
Greg Holland from the National Center for Atmospheric Research, who reviewed a multi-meter sea level rise study by Jim Hansen, noted “There is no doubt that the sea level rise, within the IPCC, is a very conservative number, so the truth lies somewhere between IPCC and Jim.”
In reporting criticism by some scientists that IPCC's then-impending January 2007 report understates certain risks, particularly sea level rises, an AP story quoted Stefan Rahmstorf, professor of physics and oceanography at Potsdam University as saying "In a way, it is one of the strengths of the IPCC to be very conservative and cautious and not overstate any climate change risk".
In his December 2006 book, , and in an interview on Fox News on 31 January 2007, energy expert Joseph Romm noted that the IPCC Fourth Assessment Report is already out of date and omits recent observations and factors contributing to global warming, such as the release of greenhouse gases from thawing tundra.
Political influence on the IPCC has been documented by the release of a memo by ExxonMobil to the Bush administration, and its effects on the IPCC's leadership. The memo led to strong Bush administration lobbying, evidently at the behest of ExxonMobil, to oust Robert Watson, a climate scientist, from the IPCC chairmanship, and to have him replaced by Pachauri, who was seen at the time as more mild-mannered and industry-friendly.

IPCC processes

, a long-time participant in the IPCC and coordinating lead author of the Fifth Assessment Report conceded in Science Magazine's State of the Planet 2008–2009 some limitations of the IPCC consensus approach and asks for concurring, smaller assessments of special problems instead of the large scale approach as in the previous IPCC assessment reports. It has become more important to provide a broader exploration of uncertainties. Others see as well mixed blessings of the drive for consensus within the IPCC process and ask to include dissenting or minority positions or to improve statements about uncertainties.
The IPCC process on climate change and its efficiency and success has been compared with dealings with other environmental challenges. In case of the Ozone depletion, global regulation based on the Montreal Protocol has been successful. In case of Climate Change, the Kyoto Protocol failed. The Ozone case was used to assess the efficiency of the IPCC process.
The lockstep situation of the IPCC is having built a broad science consensus while states and governments still follow different, if not opposing goals. The underlying linear model of policy-making of the more knowledge we have, the better the political response will be is being doubted.
According to Sheldon Ungar's comparison with global warming, the actors in the ozone depletion case had a better understanding of scientific ignorance and uncertainties. The ozone case communicated to lay persons "with easy-to-understand bridging metaphors derived from the popular culture" and related to "immediate risks with everyday relevance", while the public opinion on climate change sees no imminent danger. The stepwise mitigation of the ozone layer challenge was based as well on successfully reducing regional burden sharing conflicts. In case of the IPCC conclusions and the failure of the Kyoto Protocol, varying regional cost-benefit analysis and burden-sharing conflicts with regard to the distribution of emission reductions remain an unsolved problem. In the UK, a report for a House of Lords committee asked to urge the IPCC to involve better assessments of costs and benefits of climate change, but the Stern Review, ordered by the UK government, made a stronger argument in favor to combat human-made climate change.

Outdatedness of reports

Since the IPCC does not carry out its own research, it operates on the basis of scientific papers and independently documented results from other scientific bodies, and its schedule for producing reports requires a deadline for submissions prior to the report's final release. In principle, this means that any significant new evidence or events that change our understanding of climate science between this deadline and publication of an IPCC report cannot be included. In an area of science where our scientific understanding is rapidly changing, this has been raised as a serious shortcoming in a body which is widely regarded as the ultimate authority on the science. However, there has generally been a steady evolution of key findings and levels of scientific confidence from one assessment report to the next.
The submission deadlines for the Fourth Assessment Report differed for the reports of each Working Group. Deadlines for the Working Group I report were adjusted during the drafting and review process in order to ensure that reviewers had access to unpublished material being cited by the authors. The final deadline for cited publications was 24 July 2006. The final WG I report was released on 30 April 2007 and the final AR4 Synthesis Report was released on 17 November 2007.Rajendra Pachauri, the IPCC chair, admitted at the launch of this report that since the IPCC began work on it, scientists have recorded "much stronger trends in climate change", like the unforeseen dramatic melting of polar ice in the summer of 2007, and added, "that means you better start with intervention much earlier".

Burden on participating scientists

Scientists who participate in the IPCC assessment process do so without any compensation other than the normal salaries they receive from their home institutions. The process is labor-intensive, diverting time and resources from participating scientists' research programs. Concerns have been raised that the large uncompensated time commitment and disruption to their own research may discourage qualified scientists from participating.

Lack of error correction after publication

In May 2010, Pachauri noted that the IPCC currently had no process for responding to errors or flaws once it issued a report. The problem, according to Pachauri, was that once a report was issued the panels of scientists producing the reports were disbanded.

Proposed organizational overhaul

In February 2010, in response to controversies regarding claims in the Fourth Assessment Report, five climate scientists – all contributing or lead IPCC report authors – wrote in the journal Nature calling for changes to the IPCC. They suggested a range of new organizational options, from tightening the selection of lead authors and contributors, to dumping it in favor of a small permanent body, or even turning the whole climate science assessment process into a moderated "living" Wikipedia-IPCC. Other recommendations included that the panel employ a full-time staff and remove government oversight from its processes to avoid political interference.

Reframing of scientific research

The 2018 report What Lies Beneath by the Breakthrough - National Centre for Climate Restoration, with contributions from Kevin Anderson, James Hansen, Michael E. Mann, Michael Oppenheimer, Naomi Oreskes, Stefan Rahmstorf, Eric Rignot, Hans Joachim Schellnhuber, Kevin Trenberth, and others, urges the IPCC, the wider UNFCCC negotiations, and national policy makers to change their approach. The authors note, "We urgently require a reframing of scientific research within an existential risk-management framework."

InterAcademy Council review

In March 2010, at the invitation of the United Nations secretary-general and the chair of the IPCC, the InterAcademy Council was asked to review the IPCC's processes for developing its reports. The IAC panel, chaired by Harold Tafler Shapiro, convened on 14 May 2010 and released its report on 1 September 2010.
The IAC found that, "The IPCC assessment process has been successful overall". The panel, however, made seven formal recommendations for improving the IPCC's assessment process, including:
  1. establish an executive committee;
  2. elect an executive director whose term would only last for one assessment;
  3. encourage review editors to ensure that all reviewer comments are adequately considered and genuine controversies are adequately reflected in the assessment reports;
  4. adopt a better process for responding to reviewer comments;
  5. working groups should use a qualitative level-of-understanding scale in the Summary for Policy Makers and Technical Summary;
  6. "Quantitative probabilities should be used to describe the probability of well-defined outcomes only when there is sufficient evidence"; and
  7. implement a communications plan that emphasizes transparency and establish guidelines for who can speak on behalf of the organization.
The panel also advised that the IPCC avoid appearing to advocate specific policies in response to its scientific conclusions. Commenting on the IAC report, Nature News noted that "The proposals were met with a largely favourable response from climate researchers who are eager to move on after the media scandals and credibility challenges that have rocked the United Nations body during the past nine months".

Archiving

Papers and electronic files of certain working groups of the IPCC, including reviews and comments on drafts of their Assessment Reports, are archived at the in the Harvard Library.

Endorsements of the IPCC

Various scientific bodies have issued official statements endorsing and concurring with the findings of the IPCC.