The IPCC claims that it has thousands of scientists and almost as many reviewers of the scientists' work to produce their reports. There are two problems, however. In the scientific world I move in, “review” means that your work is scrutinised by several independent, anonymous reviewers chosen by the editor.
However, when I entered the IPCC world, the reviewers were there at the worktable, criticising our drafts, and finally meeting with all us co-ordinators and many of the IPCC functionaries in a draftfest.
The product was not reviewed in the accepted sense of the word — there was no independence of review, and the reviewers were anything but anonymous. The result is not scientific.