GLOBAL warming is likely to be less extreme than claimed, researchers said yesterday. The most likely temperature rise will be 1.9C (3.4F) compared with the 3.5C predicted by the Intergovernmental Panel on Climate Change. The Norwegian study says earlier predictions were based on rapid warming in the Nineties. But Oslo University’s department of geosciences included data since 2000 when temperature rises “levelled off nearly completely”.James Annan, a member of the Global Change Projection Research Programme at RIGC in Japan, comments on the Norwegian study and climate sensitivity at his blog:A sensitive matterhttp://julesandjames.blogspot.com/2013/02/a-sensitive-matter...So, sensitivity has been in the climate blogosphere a bit recently. Just a few days ago, that odd Norwegian press release got some people excited, but it's not clear what it really means....As I said to Andy Revkin (and he published on his blog), the additional decade of temperature data from 2000 onwards (even the AR4 estimates typically ignored the post-2000 years) can only work to reduce estimates of sensitivity, and that's before we even consider the reduction in estimates of negative aerosol forcing, and additional forcing from black carbon (the latter being very new, is not included in any calculations AIUI). It's increasingly difficult to reconcile a high climate sensitivity (say over 4C) with the observational evidence for the planetary energy balance over the industrial era....But the point stands, that the IPCC's sensitivity estimate cannot readily be reconciled with forcing estimates and observational data. All the recent literature that approaches the question from this angle comes up with similar answers, including the papers I mentioned above. By failing to meet this problem head-on, the IPCC authors now find themselves in a bit of a pickle. I expect them to brazen it out, on the grounds that they are the experts and are quite capable of squaring the circle before breakfast if need be. But in doing so, they risk being seen as not so much summarising scientific progress, but obstructing it.There's a nice example of this in Reto Knutti's comment featured by Revkin. While he starts out be agreeing that estimates based on the energy balance have to be coming down, he then goes on to argue that now (after a decade or more of generating and using them) he doesn't trust the calculations because these Bayesian estimates are all too sensitive to the prior choices. That seems to me to be precisely contradicted by all the available literature, which demonstrates that so long as absurd priors are avoided, the results are actually remarkably robust. Our own Climatic Change paper, Salvador Pueyo, Aldrin and the other papers above all use a wide range of different priors based on a range of different arguments but still arrive at very similar answers (at least, similar enough in the context of the hypothetical "long tail" for the pdf of climate sensitivity)! DB2
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