I was a lead author of two chapters in AR2's Working Group 3 report (1995), convening lead author in the Special Report on Regional Impacts published in 2001 (Working Group 2), contributing author of one chapter in AR3's Working Group 1 report (2001), lead author of one chapter in AR3's Working Group 2 report (2001), and contributing author of one chapter in AR4's Working Group 2 report (2007).
Over the years, the IPCC has changed from a scientific institution that tries to be policy relevant to a political institution that pretends to be scientific. I regret that. There are already more than enough climate activists, while there are too few solid and neutral bodies that make down-to-earth and well-founded statements about climate change and climate policy.
Working Groups 2 and 3 of the AR4 violated all IPCC procedures. The conclusions are partly scientifically unfounded, and even partly copied from the environmental movement. The AR4 was substantially changed after the final review, also in parts that had already been accepted by the referees. Valid comments were ignored.
As a result, AR4 contains crude errors, only some of which are public knowledge. These errors can be found in the chapters, the technical summaries, the summaries for policy makers, and the synthesis report. The errors are not random. Working Group 2 systematically portrays climate change as a bigger problem than is scientifically acceptable. Working Group 3 systematically portrays climate policy as easier and cheaper than can be responsibly concluded based on academic research.
Of course Tol starts out by saying the temperature is going up, this is a man-made effect, and something should be done about it. There are the passwords that must be uttered to enter respectable society--otherwise you're out on the moors with King Lear and Mad Tom. But: he is convinced the major IPCC document is substantially bullshit.