The introductory "Summary for Policymakers" -- the only portion usually quoted in the media -- is written not by scientists at all, but by politicians, and approved, word-by-word, by political representatives from member nations. By IPCC policy, the individual report chapters -- the only text actually written by scientists -- are edited to "ensure compliance" with the summary, which is typically published months before the actual report itself. Fascinating revelation.
That's been widely known, at least here on FR, for quite some time. They've been doing it that way since the IPCC first started issuing it reports.