The Intergovernmental Panel on Climate Change (IPCC) has just released the Working Group 1 (WG1) report of the fifth Assessment Report (AR5). A difference between what is presented here and what was presented in the fourth Assessment Report (AR4) is that the range for the Equilibrium Climate Sensitivity (ECS) is slightly larger (1.5 – 4.5oC, rather than 2 – 4.5oC) and there is no longer a best estimate. Warren Pearce has written a post on the Making Science Public blog called Just one number: has the IPCC changed its supply of evidence?
My personal view is that this difference is not that significant. The range is slightly larger and the reason why there is no best estimate is, apparently, because there is no agreement as to how to combine the results from the different methods so as to produce a reasonable best estimate. In particular, the more recent observationally constrained results differ from paleo-climatological and modelling estimates. Myself and others made comments along these lines. I, however, ended up in a rather odd exchange with Ben Pile in which he seemed to be suggesting that the IPCC report reflects the consensus view and therefore the lack of a best estimate meant no such estimate existed (he compared discussing it to discussing unicorns). He also implied that actually looking in the literature was no longer reasonable as the IPCC reports now provided the consensus view and that presenting anything that differed was tantamount to being a denier (I thought this would have been seen as a good thing, but maybe only certain people are allowed to question the IPCC).
So, I think Ben Pile is wrong about the role of the IPCC reports. They are not simply reports that indicate the consensus, they are synthesis reports. You may think that this is the same as consensus, but I think there is a subtlety. There is no requirement for everyone involved to agree with all the content of the IPCC reports. It is meant to reasonably reflect our current understanding with respect to climate science. The lack of a best estimate for climate sensitivity may be a prime example. It’s not that such a best estimate is not possible, it’s because there is no agreement as to how to synthesise the results from the different methods so as to produce one. This is not uncommon in science. What is not common is for the different methods to never converge. Just because no agreement can be reached today does not mean an agreement isn’t possible in future. In fact, it would be rather concerning if the different methods didn’t ultimately converge – that would tend to imply some fundamental problem with at least one of the methods.
Anyway, this post has got longer than I intended. What I was really intending to do was highlight Victor Venema’s recent review of the IPCC review. Victor has written a post reviewing what the IPCC report has said about the quality of the land surface temperature observations, Victor’s area of expertise. Victor has also linked to posts by other scientists who have reviewed the aspects of the report about which they have particular expertise. Victor also intends, I believe, to link to newer reviews as they are written. Not all reviews are positive (see Aslak Grinsted’s for example). So, despite Ben Pile’s assertion that questioning the IPCC report is tantamount to denial, some scientists seem comfortable doing so. Personally, I think that this is just healthy disagreements within the scientific community. Would be rather disturbing if everyone agreed with everything. That might make some think that there was a conspiracy or too much groupthink.