Intergovernmental Panel on Climate Change
The Intergovernmental Panel on Climate Change (IPCC) was established in 1988 by two United Nations organizations, the World Meteorological Organization (WMO) and the United Nations Environment Programme (UNEP) to assess the "risk of human-induced climate change". The Panel is open to all members of the WMO and UNEP.
Its reports are widely respected and cited and have been highly influential in forming national and international responses to climate change. At the same time, a few of the scientists whose work is summarized in these reports have accused the IPCC of bias.
The stated aims of the IPCC are threefold:
- assess scientific information on climate change
- assess the impacts of climate change
- formulate response strategies
- "The role of the IPCC is to assess on a comprehensive, objective, open and transparent basis 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 research nor does it monitor climate related data or other relevant parameters. It bases its assessment mainly on peer reviewed and published scientific/technical literature." 
The IPCC is led by government scientists, but also involves several hundred academic scientists and researchers. It synthesises the available information about climate change and global warming, has published four major reports reviewing the latest climate science, as well as more specialized reports.
The IPCC is currently (August 2004) in the process of preparing the fourth assessment report or AR4  . The working Group I report is due to be finalised during the first quarter of 2007, Working Group II and Working Group III reports in mid-2007. If it is decided to prepare one the AR4 Synthesis Report (SYR) would be finalised during the last quarter of 2007. Doumentataion on the scoping meetings for the AR4 are available  as are the outlines for the WG I report  and a provisional author list  .
Whilst the preparation of the assessment reports is the major IPCC function it also supports other activities, such as the National Greenhouse Gas Inventories Programme  and the Data Distribution Centre  .
As a summary of current peer reviewed and published science, each IPCC report notes areas where the science has improved since the previous report (except the first one, of course), and also notes areas where further research is required.
The IPCC published a first assessment report in 1990, a supplementary report in 1992, a second assessment report (SAR) in 1995, and a third assessment report (TAR) in 2001. Each of the assessment reports is in three volumes from the working groups I, II and III. Unqualified, "the IPCC report" is often used to mean the WG I report.
IPCC Third Assessment Report: Climate Change 2001
The most recent IPCC report is Climate Change 2001, the Third Assessment Report (TAR) WG I report . The lead editor was John Houghton.
The "headlines" from the summary for policymakers in the WG I report  were:
- An increasing body of observations gives a collective picture of a warming world and other changes in the climate system (The global average surface temperature has increased over the 20th century by about 0.6°C; Temperatures have risen during the past four decades in the lowest 8 kilometres of the atmosphere; Snow cover and ice extent have decreased)
- Emissions of greenhouse gases and aerosols due to human activities continue to alter the atmosphere in ways that are expected to affect the climate (Anthropogenic aerosols are short-lived and mostly produce negative radiative forcing; Natural factors have made small contributions to radiative forcing over the past century)
- Confidence in the ability of models to project future climate has increased (Complex physically-based climate models are required to provide detailed estimates of feedbacks and of regional features. Such models cannot yet simulate all aspects of climate (e.g., they still cannot account fully for the observed trend in the surface-troposphere temperature difference since 1979) and there are particular uncertainties associated with clouds and their interaction with radiation and aerosols. Nevertheless, confidence in the ability of these models to provide useful projections of future climate has improved due to their demonstrated performance on a range of space and time-scales  .
- There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities
- Human influences will continue to change atmospheric composition throughout the 21st century
- Global average temperature and sea level are projected to rise under all IPCC SRES scenarios
In its last report, IPCC stated that average surface temperature is projected to increase by 1.4 to 5.8 Celsius degrees over the period 1990 to 2100, and the sea level is projected to rise by 0.1 to 0.9 metres over the same period. The wide range in predictions is based upon several different scenarios that assume different levels of future CO2 emissions. Each scenario then has a range of possible outcomes associated with it. The most optimistic outcome assumes an aggressive campaign to reduce CO2 emssions, while the most pessimistic is a "business as usual" scenario. The more realistic scenarios fall in between.
IPCC predictions are based on the same models used to establish the importance of the different factors in global warming. These models need data about anthropogenic emissions of greenhouse gases and aerosols. These data are predicted from economic models based on 35 different scenarios. Scenarios go from pessimistic to optimistic, and predictions of global warming depend on the kind of scenario considered.
IPCC uses the best available predictions and their reports are under strong scientific scrutiny. The IPCC concedes that there is a need for better models and better scientific understanding of some climate phenomena, as well as the uncertainties involved. Critics assert that the available data is not sufficient to determine the real importance of greenhouse gases in climate change. Sensitivity of climate to greenhouse gases may be over-estimated or under-estimated estimated because of some flaws in the models and because the importance of some external factors may be misestimated. The predictions are based on scenarios, and the IPCC did not assign any probability to the 35 scenarios used.
Debate over Climate Change 2001
Castles and Henderson asserted that the IPCC has been using inflated economic growth rates, which result in increased emission estimates.  This was incorrect because IPCC growth and emissions rates were based upon several factors and not only GDP, as rebutted by Nebojsa Nakicenovic et al.
A few participants in IPCC Working Group I (Science) do not agree with the IPCC reports (of the 120 lead authors, 2 have complained  ). A particularly active critic, MIT physicist Richard Lindzen, expressed his unhappiness about those portions in the Executive Summary based on his contributions in May 2001 before the United States Senate Commerce Committee :
- "The summary does not reflect the full document... For example, I worked on Chapter 7, Physical Processes. This chapter dealt with the nature of the basic processes which determine the response of climate, and found numerous problems with model treatments – including those of clouds and water vapor. The chapter was summarized with the following sentence: 'Understanding of climate processes and their incorporation in climate models have improved, including water vapour, sea-ice dynamics, and ocean heat transport.'"
The "Summary for Policymakers" of the WG1 reports does include caveats on model treatments: Such models cannot yet simulate all aspects of climate (e.g., they still cannot account fully for the observed trend in the surface-troposphere temperature difference since 1979) and there are particular uncertainties associated with clouds and their interaction with radiation and aerosols. Nevertheless, confidence in the ability of these models to provide useful projections of future climate has improved due to their demonstrated performance on a range of space and time-scales.  .
These statements are in turn supported by the executive summary of chapter 8 of the report, which includes:
- Coupled models can provide credible simulations of both the present annual mean climate and the climatological seasonal cycle over broad continental scales for most variables of interest for climate change. Clouds and humidity remain sources of significant uncertainty but there have been incremental improvements in simulations of these quantities.
- Confidence in the ability of models to project future climates is increased by the ability of several models to reproduce the warming trend in 20th century surface air temperature when driven by radiative forcing due to increasing greenhouse gases and sulphate aerosols. However, only idealised scenarios of only sulphate aerosols have been used.
IPCC Second Assessment Report: Climate Change 1995
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, and each has a Summary for Policymakers (SfP) that represents a consensus of national representatives. The SfP of the WG I report contains headings:
- Greenhouse gas concentrations have continued to increase
- Anthropogenic aerosols tend to produce negative radiative forcings
- Climate has changed over the past century (air temperature has increased by between 0.3 and 0.6 oC since the late 19th century; this estimate has not significantly changed since the 1990 report).
- 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)
- 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)
- There are still many uncertainties (estimates of future emissions and biogeochemical cycling; models; instrument data for model testing, assessment of variability, and detection studies)
Debate over Climate Change 1995
Most scientists involved in climate research believe that the IPCC reports accurately summarise the state of knowledge. A few scientists have objected and made public comments to that effect, in fact so few that no-one can find any suitable quote to add in here.
The report formed the basis of negotiations over the Kyoto Protocol.
A December 20, 1995, Reuters report quoted British scientist Keith Shine, one of IPCC's lead authors, discussing the Policymakers' Summary. He said: "We produce a draft, and then the policymakers go through it line by line and change the way it is presented.... It's peculiar that they have the final say in what goes into a scientists' report." It is not clear, in this case, whether Shine was complaining that the report had been changed to be more skeptical, or less, or something else entirely.
Dr. Frederick Seitz, president emeritus of Rockefeller University and past president of the National Academy of Sciences, has publicly denounced the IPCC report, writing "I have never witnessed a more disturbing corruption of the peer-review process than the events that led to this IPCC report". He opposed it in the Leipzig Declaration of his Science and Environmental Policy Project.
In turn, Seitz's comments were vigourously opposed by the presidents of the American Meteorological Society and University Corporation for Atmospheric Research, who wrote about a systematic effort by some individuals to undermine and discredit the scientific process that has led many scientists working on understanding climate to conclude that there is a very real possibility that humans are modifying Earth's climate on a global scale. Rather than carrying out a legitimate scientific debate... they are waging in the public media a vocal campaign against scientific results with which they disagree  .
IPCC supplementary report, 1992
The 1992 supplementary report was an update, requested in the context of the negotiations on the Framework Convention on Climate Change 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.
IPCC First Assessment Report: 1990
The IPCC first assessment report was completed in 1990, and served as the basis of the United Nations Framework Convention on Climate Change (UNFCCC).
The executive summary of the policymakers summary of the WG I report includes:
- We are certain of the following: there is a natural greenhouse effect...; emissions resulting from human activities are substantially increasing the atmospheric concentrations of the greenhouse gases: CO2, methane, CFCs and nitrous oxide. These increases will enhance the greenhouse effect, resulting on average in an additional warming of the Earths surface. The main greenhouse gas, water vapour, will increase in response to global warming and further enhance it.
- We calculate with confidence that: ...CO2 has been responsible for over half the enhanced greenhouse effect; long-lived gases would require immeadiate reductions in emissions from human activities of over 60% to stabilise their concentrations at today's levels...
- Based on current models, we predict: under [BAU] increase of global mean temperature during the [21st] century of about 0.3 oC per decase (with an uncertainty range of 0.2 to 0.5 oC per decade); this is greater than that seen over the past 10,000 years; under other ... scenarios which assume progressively increasing levels of controls, rates of increase in global mean temperature of about 0.2 oC [to] about 0.1 oC per decade.
- There are many uncertainties in our predictions particularly with regard to the timing, magnitude and regional patterns of climate change, due to our incomplete understanding of: sources and sinks of GHGs; clouds; oceans; polar ice sheets.
- Our judgement is that: global mean surface air temperature has increased by 0.3 to 0.6 oC over the last 100 years...; The size of this warming is broadly consistent with predicion of climate models, but it is also of the same magnitude as natural climate variability. Thus the observed increase could be largely due to this natural variability; alternatively this variability and other human factors could have offset a still larger human-induced greenhouse warming. The unequivocal detection of the enhanced greenhouse effect is not likely for a decade or more.
Notice that the report did *not* attribute the observed warming to human influence.
In 1991, the SEPP surveyed IPCC contributors and researchers, along with a comparison group of global warming skeptics who had not contributed. 
The IPCC web site
- IPCC organisation
- Second Assessment Synthesis Report (pdf)
- Second Assessment Summary for Policymakers from the Working Group 1 (Science) Report.
- Third Assessment Report
- The IPCC Controversy - from the SEPP
- climate change - What is the IPCC by Jean-Marc Jancovici