As I was promised recently, our (non-)uniform prior paper has officially appeared in dead tree format in the January issue of Climatic Change. Abstract below, and you can find it on my web site. It was waiting for a commentary which doesn't seem to have happened. I'm not sure how I should feel about that: one the one hand, it's a relief that someone doesn't get a free shot at criticising it (not that I know what they were going to say), on the other, perhaps this is the one thing worse than being talked about. I'll be interested to see if the appearance of the paper version will attract more interest (citations), given that it was officially published on-line back in 2009 anyway. It seems that many (most? all?) people have accepted the argument, without explicitly referring to it. Eg Sokolov et al in their 2009 paper only used an expert prior, referring to their previous use of a uniform prior as merely a sensitivity analysis. However, in that previous work, it was actually the uniform prior case that was presented as the main result - and which was featured in the AR4 by the IPCC, who ignored my explicit request that the expert prior results should at least be mentioned therein. I realise our paper was not published when Sokolov et al was written, so I'm not criticising them for not citing us. It will be interesting to see how the IPCC authors manage this particular conundrum this time around.
The equilibrium climate response to anthropogenic forcing has long been one of the dominant, and therefore most intensively studied, uncertainties in predicting future climate change. As a result, many probabilistic estimates of the climate sensitivity (S) have been presented. In recent years, most of them have assigned significant probability to extremely high sensitivity, such as P(S gt 6C) gt 5%. In this paper, we investigate some of the assumptions underlying these estimates. We show that the popular choice of a uniform prior has unacceptable properties and cannot be reasonably considered to generate meaningful and usable results. When instead reasonable assumptions are made, much greater confidence in a moderate value for S is easily justified, with an upper 95% probability limit for S easily shown to lie close to 4°C, and certainly well below 6°C. These results also impact strongly on projected economic losses due to climate change.