i) IPCC was not founded to simply aggregate climate science findings but to collect evidence for man-made climate change. This one-eyed look at the results over-emphasized of course the human influence. The knowledge of the natural changes came second hand which explains the confusion surrounding the current Hiatus.
ii) WG1 is doing an excellent job in describing the science and should be read by anyone interested in the subject. The science presented is correct but do not cover all scientific findings and views (see above)
iii) WG2 and WG3 are based on the assumption that human induced climate change is real and indeed needs action. I would call this the political part of the IPCC-report since several policies are suggested which to me is outside the scope of science. Many of the contributors are affiliated with WWF, Greenpeace etc. which to me make this part of the IPCC-report less credible. And, among those climate scientists I know they disregard these parts altogether sticking to the WG1.
iv) The SPM (summary for policymakers) is written in parallell without looking at the WG1 draft (Yes its true) meaning that the content in the rest of the report cannot deviate too much from SPM. However, anyone with the slightest scientific training will note the difference between WG1 and SPM. I´d say that some of the conclusions and statements in the final SPM find no support in the WG1-part! (The latest report, AR5, actually downplayed the climate threat by for instance lowering the TCR value by some 30%. TCR = Transient Climate Response ; The value that determines the temperature outcome in the near future)
The IPCC is probably very aware of this. The WG1 part is vast and who has the time to read through it all (well I did) and then also make complaints? What are the chances that anyone would care? One can follow the process on how the text is processed on the IPCC homepage. Most remarks are dismissed by the lead author (=consensus Gate keepers) even though the remarks have serious peer-reviewed references backing up their arguments. Especially so, if the argument doesn't support the AGW theory. (And, yes, these remarks do exist!). That is how you create consensus....
Some voices from scholars on the IPCC and their methods:
1. Dr Robert Balling: "The IPCC notes that "No significant acceleration in the rate of sea level rise during the 20th century has been detected." This did not appear in the IPCC Summary for Policymakers.
2. Dr John Christy: "Little known to the public is the fact that most of the scientists involved with the IPCC do not agree that global warming is occurring. Its findings have been consistently misrepresented and/or politicized with each succeeding report."
3. Dr Judith Curry: "I'm not going to just spout off and endorse the IPCC because I don't have confidence in the process."
4. Dr Robert Davis: "Global temperatures have not been changing as state of the art climate models predicted they would. Not a single mention of satellite temperature observations appears in the IPCC Summary for Policymakers."
5. Dr Willem de Lange: "In 1996 the IPCC listed me as one of approximately 3000 "scientists" who agreed that there was a discernible human influence on climate. I didn't. There is no evidence to support the hypothesis that runaway catastrophic climate change is due to human activities."
6. Dr Peter Dietze: "Using a flawed eddy diffusion model, the IPCC has grossly underestimated the future oceanic carbon dioxide uptake."
7. Dr John Everett: "It is time for a reality check. The oceans and coastal zones have been far warmer and colder than is projected in the present scenarios of climate change. I have reviewed the IPCC and more recent scientific literature and believe that there is not a problem with increased acidification, even up to the unlikely levels in the most-used IPCC scenarios."
8. Dr Eigil Friis-Christensen: "The IPCC refused to consider the sun's effect on the Earth's climate as a topic worthy of investigation. The IPCC conceived its task only as investigating potential human causes of climate change."
9. Dr Vincent Gray: "The [IPCC] climate change statement is an orchestrated litany of lies."
10. Dr Mike Hulme: "Claims such as '2500 of the world's leading scientists have reached a consensus that human activities are having a significant influence on the climate' are disingenuous … The actual number of scientists who backed that claim was only a few dozen."
11. Dr Georg Kaser: "This number [of receding glaciers reported by the IPCC] is not just a little bit wrong, it is far out by any order of magnitude … It is so wrong that it is not even worth discussing."
12. Dr Aynsley Kellow: "I'm not holding my breath for criticism to be taken on board, which underscores a fault in the whole peer review process for the IPCC: there is no chance of a chapter [of the IPCC report] ever being rejected for publication, no matter how flawed it might be."
13. Dr Madhav Khandekar: "I have carefully analysed adverse impacts of climate change as projected by the IPCC and have discounted these claims as exaggerated and lacking any supporting evidence."
14. Dr Hans Labohm: "The alarmist passages in the IPCC Summary for Policymakers have been skewed through an elaborate and sophisticated process of spin-doctoring."
15. Dr Andrew Lacis: "There is no scientific merit to be found in the Executive Summary [of the IPCC report]. The presentation sounds like something put together by Greenpeace activists and their legal department."
16. Dr Chris Landsea: "I cannot in good faith continue to contribute to a [IPCC] process that I view as both being motivated by pre-conceived agendas and being scientifically unsound."
17. Dr Richard Lindzen: "The IPCC process is driven by politics rather than science. It uses summaries to misrepresent what scientists say and exploits public ignorance."
18. Dr Philip Lloyd: "I am doing a detailed assessment of the IPCC reports and the Summaries for Policy Makers, identifying the way in which the Summaries have distorted the science. I have found examples of a summary saying precisely the opposite of what the scientists said."
19. Dr Martin Manning: "Some government delegates influencing the IPCC Summary for Policymakers misrepresent or contradict the lead authors."
20. Dr Patrick Michaels: "The rates of warming, on multiple time scales, have now invalidated the suite of IPCC climate models. No, the science is not settled."
21. Dr Johannes Oerlemans: "The IPCC has become too political. Many scientists have not been able to resist the siren call of fame, research funding and meetings in exotic places that awaits them if they are willing to compromise scientific principles and integrity in support of the man-made global-warming doctrine."
22. Dr Roger Pielke: "All of my comments were ignored without even a rebuttal. At that point, I concluded that the IPCC Reports were actually intended to be advocacy documents designed to produce particular policy actions, but not a true and honest assessment of the understanding of the climate system."
23. Dr Tom Segalstad: "The IPCC global warming model is not supported by the scientific data."
24. Dr Fred Singer: "Isn't it remarkable that the Policymakers Summary of the IPCC report avoids mentioning the satellite data altogether, or even the existence of satellites — probably because the data show a slight cooling over the last 18 years, in direct contradiction of the calculations from climate models?"
25. Dr Richard Tol: "The IPCC attracted more people with political rather than academic motives. In AR4, green activists held key positions in the IPCC and they succeeded in excluding or neutralising opposite voices."
26. Dr David Wojick: "The public is not well served by this constant drumbeat of alarms fed by computer models manipulated by advocates."