IPCC stands for Intergovernmental Panel on Climate Change. It is a scientific body set up by the World Meteorological Organization (WMO) and by the United Nations Environment Programme (UNEP) in 1988. The IPCC gives a summary of the latest state of scientific knowledge regarding climate change (working group 1), its impacts and adaptation (working group 2) and its mitigation (prevention; working group 3) typically every five to six years. These assessment reports are written by hundreds and reviewed by thousands of scientists. I am most familiar with the science of climate change, so this discussion pertains mainly to working group 1.
IPCC Assessment Reports
The reports basically give an overview of the recent scientific literature (i.e. published in peer reviewed journals and not those articles in your local newspaper). The IPCC does not do research itself, although the authors are all practicing scientists. All scientific literature covering the topic is assessed; so called “skeptical” journal articles just as well as those agreeing with the “mainstream”. It just so happens that there are far more journal articles agreeing with the mainstream than disagreeing. And disagreement is possible in many different shades of grey and in different directions: Some claim that climate change is less problematic, while others that it is more problematic than the mainstream scientific opinion.
IPCC Summary for Policymakers
The Summary for Policymakers (SPM) is the document that will be most widely read (mainly because of its more manageable volume) and is regarded as the most influential publicly and politically (scientifically much less so). While the main assessment reports are written solely by scientists, the Summary for Policymakers requires word by word approval from all government delegations. It must at the same time be consistent with the rest of the report. The “skeptical” governments come to the plenary approval meeting determined to insure that none of the statements in the SPM are overly confident or alarmist.
This procedure, plus the fact that scientists are professionally focused on uncertainty and often wary of overstatements, causes the reports, and especially the SPM, to be on the conservative side in their assessment of the risks of climate change. However, through a meticulous comparison with the underlying report, the SPM is also scientifically sound.
Criticism on the IPCC
The IPCC has been criticized (mainly by “skeptics”) to be a political (rather than a scientific) body and people have tried to discredit their reports as being biased. However, the IPCC office exists of a few handfuls of people doing mainly secretarial work, while the IPCC process and the actual assessments are undertaken by hundreds of scientists from the whole world. These are scientists by profession, voluntarily participating in the IPCC process. Thus, although the IPCC is a UN body, the actual work of assessing the scientific evidence is done by scientists active in the field. Therefore I do not agree with this accusation of bias.
Furthermore, since its mandate is to assess the scientific literature, it weighs the amount of evidence according to this literature. Minority theories which are considered highly uncertain and/or highly disputed are mentioned, but do not get equal weight as mainstream theories which have strong and for the most part undisputed evidence behind them. That is the whole point of such an assessment in my opinion: putting things in perspective, namely the perspective of the recent scientific literature.
In case you favor a certain theory (e.g. that the sun has caused the majority of the recent warming) and you find that it is underrepresented in the IPCC assessment report, you would be tempted to automatically conclude that the report is biased. Such “minority theories” are mentioned, but also put in the context of the heavy criticism that they have endured in the scientific arena (e.g. the fact that the solar output has not increased since the 1950’s, so it was not likely responsible for recent warming). A different conclusion would be that your favorite theory is not supported by the scientific evidence, and -as a result- not by the majority of scientists either. (Not just by the scientists having written the particular chapter, but by the majority of authors having published on the topic in recent years.) You can then conclude that all those scientists are wrong (and discard the evidence to the contrary), or you can conclude that they are likely right and change your own opinion about the subject. Needless to say, most people tend to react in the former rather than the latter manner. It probably feels great thinking that you’re smarter than all those scientists. If you genuinely believe that you are right, I suggest you write up your argumentation, submit it to Science or Nature, and wait for a Nobel Prize.
Not all scientific theories have equal merit, and it is one of the tasks of these assessment reports to clarify the relative merit of different theories. According to the evidence available (as published in the scientific literature), what is most likely the case? And what is less likely, but not impossible? Both need mentioning, but they should not be treated with equal footing. As an aside, this is a critical mistake that the popular media often make: they give the appearance of equal merit to different theories, whereas in the scientific discussion they often have vastly different strengths of evidence to support them. This so called “balanced reporting” gives a false picture of the scientific thinking about the subject.