At last, the IPCC AR4 is out - at least, most of it is (there is still supplementary material to come). Since I was unhappy with some of what was written in the previous (2nd) draft of Chapter 9, I looked at that first.
At first glance, I'm pleased to see that it has been significantly improved. The drafts were never meant to be a polished final version, and indeed were only released on condition that they were kept private (although the 2nd draft can easily be found on the web). So I'll restrict my comments to what they have agreed on for the final version itself.
Section 9.6 "Observational Constraints on Climate Sensitivity", contains the following:
The defence the IPCC authors provide for the use of the uniform distribution is that it "enables comparison of constraints obtained from the data in different approaches". Of course this is not the same thing as generating a pdf which credibly represents the opinion of an intelligent researcher, but they don't actually go so far as to explicitly state this rather embarassing fact (which leads inescapably to the conclusion that these "pdfs" cannot be considered policy-relevant and used in decision support, eg economic analyses such as the Stern report etc). Most of the results they quote are based on uniform priors, but they hardly had a choice since this approach dominates the recent literature.
The section chapter also makes extensive reference to the "multiple constraints" argument (a significant feature of Hegerl et al's Nature paper, as well as our GRL paper), which is great. As I said more than a year ago, our calculation was rather simplistic and anyone who doesn't like it is welcome to generate their own answer, taking account of the arguments we have presented. Interestingly, I'm still waiting...
So in summary it might not be exactly what I would have written myself, but it's clearly a step in the right direction and it seems like the IPCC comment/review system has had some effect. We'll have to wait a little while longer to see what else they wrote about Bayesian estimation in the Appendix, since this is still not published. Whether this means Frame and Allen will now have the sense to slink away and pretend the whole sorry mess about uniform priors never really happened, remains to be seen.
At first glance, I'm pleased to see that it has been significantly improved. The drafts were never meant to be a polished final version, and indeed were only released on condition that they were kept private (although the 2nd draft can easily be found on the web). So I'll restrict my comments to what they have agreed on for the final version itself.
Section 9.6 "Observational Constraints on Climate Sensitivity", contains the following:
"Note that uniform prior distributions for ECS [equilibrium climate sensitivity], which only require an expert assessment of possible range, generally assign a higher prior belief to high sensitivity than, for example, non-uniform prior distributions that depend more heavily on expert assessments (e.g., Forest et al., 2006)."Many people may think this statement is too trivial to be worth making much of, but when I made essentially the same point about a uniform prior implying high prior belief in high sensitivity, Allen and Frame dismissed it as "just a rhetorical flourish". This statement from the IPCC also appears to directly contradict much of the peer-reviewed literature, which claims that uniform priors represent ignorance. It is encouraging to see that it is now the consensus of 2,500 climate scientists that this is not the case :-) Another significant aspect is the comment that even uniform priors "require an expert assessment of possible range", which at least takes a baby step towards acknowledging our point that the choice of upper bound can have a dramatic influence on the result. As far as I know, this critical detail (which undermines the whole rationale for uniform priors) does not appear anywhere in the peer-reviewed literature, although one reviewer did single it out as a particularly interesting point in one of our submissions. It could also conceivably be called trivial were it not for the fact that so many people have apparently been oblivious to it (or else deliberately deceptive in failing to mention it) for several years.
The defence the IPCC authors provide for the use of the uniform distribution is that it "enables comparison of constraints obtained from the data in different approaches". Of course this is not the same thing as generating a pdf which credibly represents the opinion of an intelligent researcher, but they don't actually go so far as to explicitly state this rather embarassing fact (which leads inescapably to the conclusion that these "pdfs" cannot be considered policy-relevant and used in decision support, eg economic analyses such as the Stern report etc). Most of the results they quote are based on uniform priors, but they hardly had a choice since this approach dominates the recent literature.
The section chapter also makes extensive reference to the "multiple constraints" argument (a significant feature of Hegerl et al's Nature paper, as well as our GRL paper), which is great. As I said more than a year ago, our calculation was rather simplistic and anyone who doesn't like it is welcome to generate their own answer, taking account of the arguments we have presented. Interestingly, I'm still waiting...
So in summary it might not be exactly what I would have written myself, but it's clearly a step in the right direction and it seems like the IPCC comment/review system has had some effect. We'll have to wait a little while longer to see what else they wrote about Bayesian estimation in the Appendix, since this is still not published. Whether this means Frame and Allen will now have the sense to slink away and pretend the whole sorry mess about uniform priors never really happened, remains to be seen.