Richard Tol has been connected with the IPCC for a long time. He has been a lead author at least since 1995. He now says:
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.
He says that the reason for this is that the lead authors are now selected on political connections rather than scientific merit:
The most important problem of the IPCC is the nomination and selection of authors and bureau members. This is not based on academic quality (as it should be) but rather on political colour. The IPCC member states are represented by their environment departments. This responsibility should be transferred to their research departments or their academies.
Parts of the report are almost completely dominated by the environmental movement. Working Group 2 is the one that estimates how much climate change will affect us, and Working Group three is the group that focuses on how to deal with climate change:
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.
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.