## Saturday, May 30, 2009

### More on multiple constraints

Hank has been pestering me to comment on this paper (preprint on Klaus Keller's web page), which appeared not so long ago. It seems to have been aimed squarely at the "we can't hope to estimate climate sensitivity" hand-wringers, and as such you won't be too surprised to find that it has my full approval. The paper illustrates how observations of surface warming, and ocean heat content, can jointly be much more powerful in climate sensitivity than either observation is by itself. The basic reason for this is elegantly demonstrated in their Figure 2: although the surface warming by itself is compatible with high sensitivity, this requires a high ocean mixing to keep the warming rate down. Ocean heat content is also by itself compatible with high sensitivity, but only if ocean mixing is low and the warming is restricted to the upper levels, otherwise the ocean takes up too much heat in total. Taking account of both the surface air temperature and the ocean heat content is only possible if sensitivity is moderate.

I expect this point must be already implicit in much previous work (eg Chris Forest and Reto Knutti have used both surface and ocean observations together) but I don't suppose it has been so clearly illustrated in this way. Note that the authors are not actually presenting an accurate estimate, but rather presenting the argument that such an estimate is in principle possible. Two reasons for this are that (1) the ocean heat observations are currently rather poor, and (2) that we don't know the total forcing accurately enough due to aerosol uncertainties. So they are restricted to presenting the hypothetical possibility of such a result, and suggesting that research to tighten up these other uncertainties would be valuable. But even so, it serves to contradict stuff like that strange Roe and Baker paper which claimed (with Frame and Allen eulogising it) that we can't possibly hope to generate an accurate estimate for fundamental physical reasons. That simply isn't true.

There's another Urban and Keller paper I am less excited by, Probabilistic hindcasts and projections of the coupled climate, carbon cycle, and Atlantic meridional overturning circulation systems: A Bayesian fusion of century-scale observations with a simple model which I think was described as "in press" at the EGU. Unfortunately it's another of these start-with-a-uniform-prior-and-don't-use-much-information papers, which therefore ends up in a moderately "alarming" result. As with the previous paper, they present it as a sort of proof of concept, so I don't think they will mind me saying that I don't find their numerical results particularly credible. Among their 18(!) parameters, one of their most critically important parameters (hydrological sensitivity) seems to be just a guess that is completely unidentifiable from the observations used. They also estimate climate sensitivity, and have generated the typical long tail with high values that is inevitable when one starts (as they did) with a uniform prior and uses little data to constrain it. Their posterior estimate for sensitivity (shown somewhere in Figure 2) appears to have a mean of about 5C and a high probability of exceeding 7C, which IMO is ridiculous (and nowhere do they explain why they disagree so violently with the entire IPCC). To his credit, Nathan Urban did say he would not use uniform priors so readily in the future, but it's too late to change this one.