Intergovernmental Panel on Climate Change

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.[28][29] According to IPCC guidelines, authors should give priority to peer-reviewed sources.[28] Authors may refer to non-peer-reviewed sources (the "grey literature"), provided that they are of sufficient quality.[28] Examples of non-peer-reviewed sources include model results, reports from government agencies and non-governmental organizations, and industry journals.[28] 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:[28]

• Expert review (6–8 weeks)
• Government/expert review
• Government review of:
  • Summaries for Policymakers
  • Overview Chapters
  • Synthesis Report

Review comments are in an open archive for at least five years.

There are several types of endorsement which documents receive:

• Approval. Material has been subjected to detailed, line by line discussion and agreement.
  • Working Group Summaries for Policymakers are approved by their Working Groups.
  • Synthesis Report Summary for Policymakers is approved by Panel.
• Adoption. Endorsed section by section (and not line by line).
  • Panel adopts Overview Chapters of Methodology Reports.
  • Panel adopts IPCC Synthesis Report.
• Acceptance. Not been subject to line by line discussion and agreement, but presents a comprehensive, objective, and balanced view of the subject matter.
  • Working Groups accept their reports.
  • Task Force Reports are accepted by the Panel.
  • Working Group Summaries for Policymakers are accepted by the Panel after group approval.

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 (see also IPCC Summary for Policymakers). Some of these comments have been supportive,[30] while others have been critical.[31] Some commentators have suggested changes to the IPCC's procedures.[32]

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.[33]

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.

The IPCC First Assessment Report (FAR) 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" (BAU) scenario, global mean temperature will increase by about 0.3 °C per decade during the [21st] 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.

The 1992 supplementary report was an update, requested in the context of the negotiations on the UNFCCC at the Earth Summit (United Nations Conference on Environment and Development) 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 (time-dependent) simulations, which had been very preliminary in the FAR, were now improved, but did not include aerosol or ozone changes.

Climate Change 1995, the IPCC Second Assessment Report (SAR), was finished in 1996. It is split into four parts:

• A synthesis to help interpret UNFCCC article 2.
• The Science of Climate Change (WG I)
• Impacts, Adaptations and Mitigation of Climate Change (WG II)
• Economic and Social Dimensions of Climate Change (WG III)

Each of the last three parts was completed by a separate Working Group (WG), and each has a Summary for Policymakers (SPM) 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 (air temperature has increased by between 0.3 and 0.6 °C since the late 19th century; this estimate has not significantly changed since the 1990 report).
4. The balance of evidence suggests a discernible human influence on global climate (considerable progress since the 1990 report in distinguishing between natural and anthropogenic influences on climate, because of: including aerosols; coupled models; pattern-based studies)
5. Climate is expected to continue to change in the future (increasing realism of simulations increases confidence; important uncertainties remain but are taken into account in the range of model projections)
6. There are still many uncertainties (estimates of future emissions and biogeochemical cycling; models; instrument data for model testing, assessment of variability, and detection studies)

The Third Assessment Report (TAR) was completed in 2001 and consists of four reports, three of them from its Working Groups:

• Working Group I: The Scientific Basis[34]
• Working Group II: Impacts, Adaptation and Vulnerability[35]
• Working Group III: Mitigation[36]
• Synthesis Report[37]

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.[38] These are "Bayesian" probabilities, which are based on an expert assessment of all the available evidence.[39][40]

"Robust findings" of the TAR Synthesis Report include:

• "Observations show Earth's surface is warming. Globally, 1990s very likely warmest decade in instrumental record".[41] Atmospheric concentrations of anthropogenic (i.e., human-emitted) greenhouse gases have increased substantially.[41]
• Since the mid-20th century, most of the observed warming is "likely" (greater than 66% probability, based on expert judgement)[38] due to human activities.[41]
• Projections based on the Special Report on Emissions Scenarios suggest warming over the 21st century at a more rapid rate than that experienced for at least the last 10,000 years.[41]
• "Projected climate change will have beneficial and adverse effects on both environmental and socio-economic systems, but the larger the changes and the rate of change in climate, the more the adverse effects predominate."[41]
• "Ecosystems and species are vulnerable to climate change and other stresses (as illustrated by observed impacts of recent regional temperature changes) and some will be irreversibly damaged or lost."[41]
• "Greenhouse gas emission reduction (mitigation) actions would lessen the pressures on natural and human systems from climate change."[41]
• "Adaptation [to the effects of climate change] has the potential to reduce adverse effects of climate change and can often produce immediate ancillary benefits, but will not prevent all damages."[41] An example of adaptation to climate change is building levees in response to sea level rise.[42]

In 2001, 16 national science academies issued a joint statement on climate change.[43] 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 (Italy), 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 (UK).[43] The statement, also published as an editorial in the journal Science, stated "we support the [TAR's] 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".[44] The TAR has also been endorsed by the Canadian Foundation for Climate and Atmospheric Sciences,[45] Canadian Meteorological and Oceanographic Society,[46] and European Geosciences Union[47] (refer to "Endorsements of the IPCC").

In 2001, the US National Research Council (US NRC)[48] produced a report that assessed Working Group I's (WGI) contribution to the TAR. US NRC (2001)[49] "generally agrees" with the WGI assessment, and describes the full WGI report as an "admirable summary of research activities in climate science".[50]

IPCC author Richard Lindzen has made a number of criticisms of the TAR.[51] Among his criticisms, Lindzen has stated that the WGI Summary for Policymakers (SPM) does not faithfully summarize the full WGI report.[51] For example, Lindzen states that the SPM understates the uncertainty associated with climate models.[51] John Houghton, who was a co-chair of TAR WGI,[52] has responded to Lindzen's criticisms of the SPM.[53] 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.[53]

IPCC author Kevin Trenberth has also commented on the WGI SPM.[54] 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".[54] However, Trenberth concludes that the SPM is a "reasonably balanced summary".[54]

US NRC (2001)[55] concluded that the WGI SPM and Technical Summary are "consistent" with the full WGI report. US NRC (2001)[50] stated:

[...] the full [WGI] 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 (a) no changes were made without the consent of the convening lead authors (this group represents a fraction of the lead and contributing authors) and (b) most changes that did occur lacked significant impact.

The Fourth Assessment Report (AR4) was published in 2007.[56] Like previous assessment reports, it consists of four reports:

• Working Group I: The Physical Science Basis
• Working Group II: Impacts, Adaptation and Vulnerability
• Working Group III: Mitigation
• Synthesis Report

People from over 130 countries contributed to the IPCC Fourth Assessment Report, which took 6 years to produce.[56] Contributors to AR4 included more than 2500 scientific expert reviewers, more than 800 contributing authors, and more than 450 lead authors.[56]

"Robust findings" of the Synthesis report include:[57]

• "Warming of the climate system is unequivocal, as is now evident from observations of increases in global average air and ocean temperatures, widespread melting of snow and ice and rising global average sea level".[58]
• Most of the global average warming over the past 50 years is "very likely" (greater than 90% probability, based on expert judgement)[59] due to human activities.[58]
• "Impacts [of climate change] will very likely increase due to increased frequencies and intensities of some extreme weather events".[58]
• "Anthropogenic warming and sea level rise would continue for centuries even if GHG emissions were to be reduced sufficiently for GHG concentrations to stabilise, due to the time scales associated with climate processes and feedbacks".[58] Stabilization of atmospheric greenhouse gas concentrations is discussed in climate change mitigation.
• "Some planned adaptation (of human activities) is occurring now; more extensive adaptation is required to reduce vulnerability to climate change".[58]
• "Unmitigated climate change would, in the long term, be likely to exceed the capacity of natural, managed and human systems to adapt".[58]
• "Many impacts [of climate change] can be reduced, delayed or avoided by mitigation".[58]

Global warming projections from AR4[58] are shown below. The projections apply to the end of the 21st century (2090–99), relative to temperatures at the end of the 20th century (1980–99). Add 0.7 °C to projections to make them relative to pre-industrial levels instead of 1980–99. (UK Royal Society, 2010, p=10.[60] Descriptions of the greenhouse gas emissions scenarios can be found in Special Report on Emissions Scenarios.

"Likely" means greater than 66% probability of being correct, based on expert judgement.[59]

Several science academies have referred to and/or reiterated some of the conclusions of AR4. These include:

• Joint-statements made in 2007,[61] 2008[62] and 2009[63] by the science academies of Brazil, China, India, Mexico, South Africa and the G8 nations (the "G8+5").
• Publications by the Australian Academy of Science.[64]
• A joint-statement made in 2007 by the Network of African Science Academies.[65]
• A statement made in 2010 by the Inter Academy Medical Panel[66] This statement has been signed by 43 scientific academies.[lower-alpha 1]

The Netherlands Environmental Assessment Agency (PBL, et al., 2009;[67] 2010)[68] has carried out two reviews of AR4. These reviews are generally supportive of AR4's conclusions.[69][70] PBL (2010)[70] make some recommendations to improve the IPCC process. A literature assessment by the US National Research Council (US NRC, 2010)[71] 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 [emphasis in original text]. [...] 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 (see later section), and Dutch land area that is below sea level.[72]

Average IPCC AR5 climate model projections for 2081–2100 relative to 1986–2005, under low and high emission scenarios

The IPCC's Fifth Assessment Report (AR5) was completed in 2014.[73] AR5 followed the same general format as of AR4, with three Working Group reports and a Synthesis report.[73] The Working Group I report (WG1) was published in September 2013.[73]

Conclusions of AR5 are summarized below:

Working Group I

• "Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia".[74]
• "Atmospheric concentrations of carbon dioxide, methane, and nitrous oxide have increased to levels unprecedented in at least the last 800,000 years".[75]
• Human influence on the climate system is clear.[76] It is extremely likely (95-100% probability)[77] that human influence was the dominant cause of global warming between 1951 and 2010.[76]

Working Group II

• "Increasing magnitudes of [global] warming increase the likelihood of severe, pervasive, and irreversible impacts"[78]
• "A first step towards adaptation to future climate change is reducing vulnerability and exposure to present climate variability"[79]
• "The overall risks of climate change impacts can be reduced by limiting the rate and magnitude of climate change"[78]

Working Group III

• Without new policies to mitigate climate change, projections suggest an increase in global mean temperature in 2100 of 3.7 to 4.8 °C, relative to pre-industrial levels (median values; the range is 2.5 to 7.8 °C including climate uncertainty).[80]
• "(T)he current trajectory of global annual and cumulative emissions of GHGs is not consistent with widely discussed goals of limiting global warming at 1.5 to 2 degrees Celsius above the pre-industrial level."[81] Pledges made as part of the Cancún Agreements are broadly consistent with cost-effective scenarios that give a "likely" chance (66-100% probability) of limiting global warming (in 2100) to below 3 °C, relative to pre-industrial levels.[82]

Projections in AR5 are based on "Representative Concentration Pathways" (RCPs).[83] 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.

The sixth assessment report is scheduled to be released in the first half of 2022.[84] Its three working groups are titled The Physical Science Basis; Impacts, Adaptation and Vulnerability and Mitigation of Climate Change. They are all currently scheduled for publication in the second half of 2021.

The Special Report on Emissions Scenarios (SRES) is a report by the IPCC which was published in 2000.[86] The SRES contains "scenarios" of future changes in emissions of greenhouse gases and sulfur dioxide.[87] 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[88] and Fourth Assessment Reports.[89]

The SRES scenarios are "baseline" (or "reference") scenarios, which means that they do not take into account any current or future measures to limit greenhouse gas (GHG) emissions (e.g., the Kyoto Protocol to the United Nations Framework Convention on Climate Change).[90] SRES emissions projections are broadly comparable in range to the baseline projections that have been developed by the scientific community.[91]

There have been a number of comments on the SRES. Parson et al. (2007)[92] stated that the SRES represented "a substantial advance from prior scenarios". At the same time, there have been criticisms of the SRES.[93]

The most prominently publicized criticism of SRES focused on the fact that all but one of the participating models compared gross domestic product (GDP) across regions using market exchange rates (MER), instead of the more correct purchasing-power parity (PPP) approach.[94] This criticism is discussed in the main SRES article.

This report 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 here

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 (SRREN) on a voluntary basis – not to mention more than 100 scientists, who served as contributing authors.[85][95]

The report 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 here 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.[85][96]

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".[97] The completed report, Special Report on Global Warming of 1.5 °C (SR15), 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".[97]

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".[97][98] Meeting the Paris target of 1.5 °C (2.7 °F) is possible but would require "deep emissions reductions", "rapid",[98] "far-reaching and unprecedented changes in all aspects of society".[99] In order to achieve the 1.5 °C target, CO2 emissions must decline by 45% (relative to 2010 levels) by 2030, reaching net zero by around 2050. Deep reductions in non-CO2 emissions (such as nitrous oxide and methane) 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.[100][101][102]

The final draft of the "Special Report on climate change and land" (SRCCL)—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.[103] 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.[104][105]

The "Special Report on the Ocean and Cryosphere in a Changing Climate" (SROCC) was approved on 25 September 2019 in Monaco.[106] 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[s] of the century" at least once a year.[107]

The 1996 Guidelines for National Greenhouse Gas Investories provide the methodological basis for the estimation of national greenhouse gas emissions inventories.[109] 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.

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.[110] 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.[111]

A paragraph in the 2007 Working Group II report ("Impacts, Adaptation and Vulnerability"), chapter 10 included a projection that Himalayan glaciers could disappear by 2035

Glaciers in the Himalaya are receding faster than in any other part of the world (see Table 10.9) and, if the present rate continues, the likelihood of them disappearing by the year 2035 and perhaps sooner is very high if the Earth keeps warming at the current rate. Its total area will likely shrink from the present 500,000 to 100,000 km² by the year 2035 (WWF, 2005).

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.[122]

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".[123] Martin Parry, a climate expert[124] 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".[125]

IPCC WG1 Co-chair Sir John T. Houghton showing the IPCC fig. 2.20 hockey stick graph at a climate conference in 2005

The original northern hemisphere hockey stick graph of Mann, Bradley & Hughes 1999, smoothed curve shown in blue with its uncertainty range in light blue, overlaid with green dots showing the 30-year global average of the PAGES 2k Consortium 2013 reconstruction. The red curve shows measured global mean temperature, according to HadCRUT4 data from 1850 to 2013.

Figure 3.20 on p. 175 of the IPCC Second Assessment Report.

Comparison of MBH99 40-year average from proxy records, as used in IPCC TAR 2001 (blue), with IPCC 1990 schematic Figure 7.1.c (red) [based on Lamb 1965 extrapolating from central England temperatures and other historical records]; central England temperatures to 2007 shown from Jones et al. 2009 (green dashed line).[126] Also shown, Moberg et al. 2005 low frequency signal (black).

The third assessment report (TAR) prominently featured[127] a graph labeled "Millennial Northern Hemisphere temperature reconstruction" based on a 1999 paper by Michael E. Mann, Raymond S. Bradley and Malcolm K. Hughes (MBH99), which has been referred to as the "hockey stick graph". This graph extended the similar graph in 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.[126] The MBH99 finding was supported by cited reconstructions by Jones et al. 1998, Pollack, Huang & Shen 1998, Crowley & Lowery 2000 and Briffa 2000, 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.[128]

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".[129] 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,[130] and in turn the criticisms in these papers were examined by other studies and comprehensively refuted by Wahl & Ammann 2007,[131] which showed errors in the methods used by McIntyre and McKitrick.[132]

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.[133] 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.[134] 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".[135]

The IPCC Fourth Assessment Report (AR4) published in 2007 featured a graph showing 12 proxy based temperature reconstructions, including the three highlighted in the 2001 Third Assessment Report (TAR); Mann, Bradley & Hughes 1999 as before, Jones et al. 1998 and Briffa 2000 had both been calibrated by newer studies. In addition, analysis of the Medieval Warm Period cited reconstructions by Crowley & Lowery 2000 (as cited in the TAR) and Osborn & Briffa 2006. 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.[136]

Some critics have contended that the IPCC reports tend to be conservative by consistently underestimating the pace and impacts of global warming,[137] and report only the "lowest common denominator" findings.[138]

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.[139][140]

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 (IPCC), the researchers found that in 2100 sea levels would be 0.5–1.4 m [50–140 cm] 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.[141][142]

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."[143]

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".[144]

In his December 2006 book, Hell and High Water: Global Warming, 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.[145]

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.[146][147]

Michael Oppenheimer, 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.[119] It has become more important to provide a broader exploration of uncertainties.[119] Others see as well mixed blessings of the drive for consensus within the IPCC process and ask to include dissenting or minority positions[148] or to improve statements about uncertainties.[149][150]

The IPCC process on climate change and its efficiency and success has been compared with dealings with other environmental challenges (compare Ozone depletion and global warming). 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.[151] The Ozone case was used to assess the efficiency of the IPCC process.[152] The lockstep situation of the IPCC is having built a broad science consensus while states and governments still follow different, if not opposing goals.[153] The underlying linear model of policy-making of the more knowledge we have, the better the political response will be is being doubted.[153][154]

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.[155] 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.[155] The stepwise mitigation of the ozone layer challenge was based as well on successfully reducing regional burden sharing conflicts.[152] 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.[151] 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,[156] but the Stern Review, ordered by the UK government, made a stronger argument in favor to combat human-made climate change.[157]

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.[158] 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 (AR4) 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.[159] 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,[160] and added, "that means you better start with intervention much earlier".[161]

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.[162] Concerns have been raised that the large uncompensated time commitment and disruption to their own research may discourage qualified scientists from participating.[163]

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.[164]

In February 2010, in response to controversies regarding claims in the Fourth Assessment Report,[165][166] 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.[167][168] Other recommendations included that the panel employ a full-time staff and remove government oversight from its processes to avoid political interference.[169]

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."[170]