In my post about Nic Lewis’s latest estimates, I suggested that if his prior produces a posterior distribution that indicates that the ECS is more likely less than 2oC, than above 2oC, maybe he should consider a more physically motivated prior than what he currently prefers. I also suggested in the comments that
One might argue that if Nic Lewis is going to continue publishing papers suggesting a non-negligible chance of an ECS close to – or below – 1K, then he will have to provide some kind of physical motivation for such a scenario.
Of course, I didn’t define non-negligible, but some of Nic’s results suggest an ~ 17% chance of the ECS lying below 1oC.
To be clear, I think Nic’s work is interesting and that this work makes a positive contribution to our understanding of climate sensitivity. My basic point, though, is that if you are investigating some kind of physical system, and your analysis produces results that are somewhat inconsistent with our physical understanding of that system, then you – or someone else – will eventually have to provide some kind of physically-motivated justification for those results. Nic did respond to this point, by saying
OK. The fact that my study’s results show a material amount of probability at ECS levels of below 1 K, whilst some other evidence suggests that is unlikely, seems irrelevant to me. The normal scientific methods involves individual studies reporting estimates based on their own data and analysis. …..
As for paleoclimate studies, some of them show significant probability at low ECS levels. Eg, Hargeaves et al (2012) GRL had a similar 5% uncertainty bound to that in my paper.
I think I know what Nic is getting at in the first part of his response; his work stands by itself, so he’s not obliged to explain this inconsistency. In a sense, this is true, but it doesn’t make the inconsistency irrelevant. At some point we will want to understand why different methods give somewhat different results.The latter part of Nic’s comment is more interesting, though. It refers to Hargreaves et al. (2012) – Can the Last Glacial Maximum constrain climate sensitivity. The result they present is in the figure to the right. Essentially they present a 5 – 95% range of 1.0 – 4.2oC, and a median of 2.5oC. I guess a 5% probability of an ECS below 1oC is non-negligible, but it’s quite a bit smaller than 17%, and their result still suggests that the ECS is more likely above 2oC, than below. There’s also a more recent paper (Using palaeo-climate comparisons to constrain future projections in CMIP5, by Schmidt et al. 2014) that seems to update this slightly. Their result produces a 90% range of 1.7 – 4.9oC and a median of 3.3oC; somewhat higher than that in Hargreaves et al. (2012). [Edit : As Paul S points out in the comments, this is the prior. The result is a 90% range of 1.5 – 4.7oC and a median of 3oC.]
So, it’s a little odd that in response to my point that the physical evidence suggests that the ECS is more likely above 2oC than below 2oC, and probably not below 1oC, Nic refers to a paper that essentially suggests exactly this. I will add, though, that both Hargreaves et al. (2012) and Schmidt et al. (2014) might be biased because the models don’t consider atmospheric dust or vegetation changes. Specifically, Hargreaves et al. (2012) say
The PMIP2 experimental protocol for the LGM omits forcing due to atmospheric dust [Claquin et al., 2003] and vegetation changes [Crucifix and Hewitt, 2005], but while these are poorly constrained, they are likely to be net cooling influences. …. We can attempt to account for this (at least approximately) by increasing the modelled tropical cooling results of the PMIP2 models by a factor of 1/0.85. When we do this, the median Bayesian posterior estimate is reduced to 2.0oC with a 5–95% range of 0.2–4.oC, and the regression-based estimate falls to 2.0oC with a range of 0.8–3.6oC.
So, atmospheric dust/vegetation changes could reduce the ECS range and might reduce the median estimate to 2oC. Of course, the more recent Schmidt et al. (2014) paper suggests a higher median and slightly higher range than Hargreaves et al. (2012), so this effect may not be quite as significant as Hargreaves et al. (2012) suggest. To be honest, I’m not entirely sure I get this, but I think it’s because the vegetation changes and atmospheric dust are feedbacks (not forcings) and so they influence the temperature, without changing the forcings.
Anyway, my basic point was that the physical evidence appears to suggest that the ECS is more likely above 2oC, than below 2oC. Most paleo estimates appear to suggest the same. If Nic Lewis is going to continue presenting results that suggest it is more likely below 2oC than above, then I think it would be in his interest to try and present a physically motivated argument in support of this. As Dikran Marsupial (who happens to be a statistician) says
the hierarchy is physics > statistics >= chimps pulling numbers from a bucket
I’ll end with that 😀