It has been said many times, but it is always worth pointing out again at the time of this new IPCC report just how flawed the IPCC process is and how little the IPCC summaries have to do with, you know, science.
The IPCC involves numerous experts in the preparation of its reports. However, chapter authors are frequently asked to summarize current controversies and disputes in which they themselves are professionally involved, which invites bias. Related to this is the problem that chapter authors may tend to favor their own published work by presenting it in a prominent or flattering light. Nonetheless the resulting reports tend to be reasonably comprehensive and informative. Some research that contradicts the hypothesis of greenhouse gas-induced warming is under-represented, and some controversies are treated in a one-sided way, but the reports still merit close attention.
A more compelling problem is that the Summary for Policymakers, attached to the IPCC Report, is produced, not by the scientific writers and reviewers, but by a process of negotiation among unnamed bureaucratic delegates from sponsoring governments. Their selection of material need not and may not reflect the priorities and intentions of the scientific community itself. Consequently it is useful to have independent experts read the underlying report and produce a summary of the most pertinent elements of the report.
Finally, while the IPCC enlists many expert reviewers, no indication is given as to whether they disagreed with some or all of the material they reviewed. In previous IPCC reports many expert reviewers have lodged serious objections only to find that, while their objections are ignored, they are acknowledged in the final document, giving the impression that they endorsed the views expressed therein.