I thought this article by Steve McIntyre was an interesting window on the IPCC process. Frequent readers of this site know that I believe that feedbacks in the climate are the key issue of anthropogenic global warming, and their magnitude and sign separate mild, nearly unnoticeable warming from catastrophe. McIntyre points out that the IPCC fourth assessment spent all of 1 paragraph in hundreds of pages on the really critical issue:
As we’ve discussed before (and is well known), clouds are the greatest source of uncertainty in climate sensitivity. Low-level (“boundary layer”) tropical clouds have been shown to be the largest source of inter-model difference among GCMs. Clouds have been known to be problematic for GCMs since at least the Charney Report in 1979. Given the importance of the topic for GCMs, one would have thought that AR4 would have devoted at least a chapter to the single of issue of clouds, with perhaps one-third of that chapter devoted to the apparently thorny issue of boundary layer tropical clouds.
This is what an engineering study would do – identify the most critical areas of uncertainty and closely examine all the issues related to the critical uncertainty. Unfortunately, that’s not how IPCC does things. Instead, clouds are treated in one subsection of chapter 8 and boundary layer clouds in one paragraph.
It turns out that this one paragraph was lifted almost intact from the work of the lead author of this section of the report. The “almost” is interesting, though, because every single change made was to eliminate or tone down any conclusion that cloud feedback might actually offset greenhouse warming. He has a nearly line by line comparison, which is really fascinating. One sample:
Bony et al 2006 had stated that the “empirical” Klein and Hartmann (1993) correlation “leads” to a substantial increase in low cloud cover, which resulted in a “strong negative” cloud feedback. Again IPCC watered this down: “leads to” became a “suggestion” that it “might be” associated with a “negative cloud feedback” – the term “strong” being dropped by IPCC.
Remember this is in the context of a report that generally stripped out any words that implied doubt or lack of certainty on the warming side.