The main legitimate criticism of the IPCC report is that it errs on the side of conservatism and underestimation of probable impacts. Science reporting of its nature is conservative, unlike political commentary. For example, one of the lead authors, Andrew Lacis from NASA, has been quoted by Andy Revkin (a skeptic) in the NY Times as saying:
There is a great deal of irony in this basically nonsensical stuff, some of which I find rather amusing. The global warming denier blogs, where this issue first came up, seem to think that I was being critical of the I.P.C.C. report in the same way as seen from their perspective, and, as a result, I have received e-mails from the denier crowd hailing my remarks and commending me for speaking up on this important topic.
Little do they realize that the basic thrust of my criticism of the I.P.C.C. draft was really to register a clear complaint that I.P.C.C. was being too wishy-washy and was not presenting its case for anthropogenic impact being the principal driver of global warming as clearly and forcefully as they could, and should...
...Had I been asked to write this chapter (which I wasnt), I would describe understanding and attributing of climate change as simply a problem in physics, which it actually is. I would have started the Executive Summary with: Human-induced warming of the climate system is established fact.
The IPCC report was written by senior scientists, who volunteered their time and expertise (unpaid) over several years and there are no errors in the science report AFAIK. The single error of the IPCC was in a background document, not a report of the state of the science, nor the policy documents or exec summaries, which are the ones most people read and rely upon (and which was why it probably took a while for an IPCC author to pick it up and report it).
Underestimation also arises because the IPCC did not allow for findings re the degree of likelihood of further sea level rises and other changes from observations in the Antarctic, because they were not available at the time it was written. These and later findings will be picked up in the next report. Recent observations of CO2 emissions, methane from tundra and glaciers in the Antarctic, suggest we are currently tracking more closely to a worse case scenario than the IPCC's business as usual scenario. This again is based on measurements since the IPCC 2007 report was finalised so are not errors, but demonstrate the conservate approach of the IPCC. Updates from the Arctic aren't looking that great, either.
There has been a lot of rubbish in the media claiming 'errors' and giving the wrong impression that the IPCC report has exaggerated the risks. This is not so, quite the contrary. There was only one error of future projection, not multiple errors of projection. No errors of fact (current situation or science) have been found AFAIK.
