Friday, November 04, 2011

Solution to the paradox of climate sensitivity

A lot of bloggable papers have suddenly appeared, so I will work through them over the next few days.

First, a quick comment about this interesting paper: "Solution to the paradox of climate sensitivity" by Salvador Pueyo. In it, he argues that we should use a log-uniform prior for estimating climate sensitivity. This is fundamentally an "Objective Bayes" approach, that "non-informative" can be interpreted in a unique way. I don't much like this point of view, but if one is going to take it, then it should at least be done properly, and he seems to have provided decent arguments in that direction. Readers may recall that IPCC authors have in the past claimed that a uniform distribution was the unique correct representation of ignorance, which formed one of the planks of their assessment of the literature in the AR4.

As we showed here, all this talk of a long tail basically vanishes when anything other than a uniform prior is used, so in that sense this new paper is broadly compatible with our existing results which were based on a subjective paradigm. However, I'm not sure how it would work with a more complex multivariate approach, as has been common in this sort of work (eg simultaneously considering the three major uncertainties of ocean heat uptake, aerosol forcing and sensitivity).

What the new IPCC authors will make of it all is anyone's guess. Perhaps we will find out in December some time, when the first draft is scheduled to be opened for comments.


Hank Roberts said...

thanks for the reminder about this post; in the summary, the author writes:

"Rather than advocating a direct use of this prior, I recommend to refine it with a limited use of expert elicitation or other methods. A sound prior is a key ingredient in the process to reach a consensus low-uncertainty estimate of climate sensitivity to inform climate policy. "

Has anyone ever gone further into this idea, either to improve on it or show it's not much help? (Now that the IPCC drafts are happening)

James Annan said...

Not that I'm aware of, though AIUI Dan Rowlands is/was working on Jeffreys' priors (see here). But there's nothing peer reviewed yet.

My confident prediction is that the "long tail" will vanish for any sensible alternative to a uniform prior.