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. There is an Aldrin et al paper, published some time ago - which gave a decent constraint on climate sensitivity, though nothing particularly surprising or interesting IMO. We thought we had sorted out the sensitivity kerfuffle several years ago, but it seems that the rest of the world still hasn't yet caught up. 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 But the Norwegian press release seems to refer to as yet unpublished research, and some of the claims seem a bit hard to credit. So we will have to wait for more details before drawing any more solid conclusions.

Before then, there was the minor blogstorm (at least in some quarters) surrounding Nic Lewis' criticism of the IPCC's stubborn adherence to their old estimate of climate sensitivity. This, of course, being despite the additional evidence which I've just mentioned above.

When I looked at the IPCC drafts, I didn't actually notice the substantial change in estimated aerosol uncertainty that Nic focussed on. With limited time and energy to wade through several hundred pages of draft material, I mostly looked for how and where they had (or had not, but perhaps should have) referred to my work, to make sure it was fairly and accurately represented. I was pretty unimpressed with some parts of first draft, actually, and made a number suggestions. Of course in line with the IPCC conditions, I'm not going to say what was or was not in any draft. According to IPCC policy, my comments will all be available in the fullness of time, but I have also criticised this delayed release so in the spirit of openness here is one comment I made about their discussion of sensitivity in Chapter 12 (p55 in the first order draft):

It seems very odd to portray our work as an outlier here. Sokolov et al 2009, Urban and Keller 2010, Olson et al (in press JGR) have also recently presented similar results (and there may be more as yet unpublished, eg Aldrin at the INI meeting back in 2010). Such "observationally constrained pdfs" were all the rage a few years ago and featured heavily in the last IPCC report, there is no clear explanation for your sudden dismissal of them in favour of what seems to be a small private opinion poll. A more balanced presentation could be: "Annan and Hargreaves (2011a) criticize the use of uniform priors and argue that sensitivities above 4.5°C are extremely unlikely (less than 5%). Similar results have been obtained by a number of other researchers [add citations from the above]."

Note for the avoidance of any doubt I am not quoting directly from the unquotable IPCC draft, but only repeating my own comment on it. However, those who have read the second draft of Chapter 12 will realise why I previously said I thought the report was improved :-) Of course there is no guarantee as to what will remain in the final report, which for all the talk of extensive reviews, is not even seen by the proletariat, let alone opened to their comments, prior to its final publication. The paper I refer to as a "small private opinion poll" is of course the Zickfeld et al PNAS paper. The list of pollees in the Zickfeld paper are largely the self-same people responsible for the largely bogus analyses that I've criticised over recent years, and which even if they were valid then, are certainly outdated now. Interestingly, one of them stated quite openly in a meeting I attended a few years ago that he deliberately lied in these sort of elicitation exercises (i.e. exaggerating the probability of high sensitivity) in order to help motivate political action. Of course, there may be others who lie in the other direction, which is why it seems bizarre that the IPCC appeared to rely so heavily on this paper to justify their choice, rather than relying on published quantitative analyses of observational data. Since the IPCC can no longer defend their old analyses in any meaningful manner, it seems they have to resort to an unsupported "this is what we think, because we asked our pals". It's essentially the Lindzen strategy in reverse: having firmly wedded themselves to their politically convenient long tail of high values, their response to new evidence is little more than sticking their fingers in their ears and singing "la la la I can't hear you".

Of course, this still leaves open the question of what the new evidence actually does mean for climate sensitivity. I have mentioned above several analyses that are fairly up to date. I have some doubts about Nic Lewis' analysis, as I think some of his choices are dubious and will have acted to underestimate the true sensitivity somewhat. For example, his choice of ocean heat uptake is based on taking a short term trend over a period in which the observed warming is markedly lower than the longer-term multidecadal value. I don't think this is necessarily a deliberate cherry-pick, any more than previous analyses running up to the year 2000 were (the last decade is a natural enough choice to have made) but it does have unfortunate consequences. Irrespective of what one thinks about aerosol forcing, it would be hard to argue that the rate of net forcing increase and/or over-all radiative imbalance has actually dropped markedly in recent years, so any change in net heat uptake can only be reasonably attributed to a bit of natural variability or observational uncertainty. Lewis has also adjusted the aerosol forcing according to his opinion of which values are preferred - concidentally, he comes down on the side of an answer that gives a lower sensitivity. His results might be more reasonable if he had at least explored the sensitivity of his result to the assumptions made. Using the last 30y of ocean heat data and simply adopting the official IPCC forcing values rather than his modified versions (since after all, his main point is to criticise the lack of coherence in the IPCC report itself) would add credibility to his analysis. A still better approach would be to use a model capable of representing the transient change, and fitting it to the entire time series of the various relevant observations. Which is what people like Aldrin et al have done, of course, and which is why I think their results are superior.

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)! It looks rather like the IPCC authors have invented this meme as some sort of talismanic mantra to defend themselves against having to actually deal with the recent literature.