Gavin Schmidt has a Realclimate post about a recent talk given by Steve Koonin. Somewhat bizarrely, Koonin has a response to Gavin’s post that he has posted on WUWT. Given that Koonin has no climate expertise, presumably he thinks that his status as a physics Professor gives him the credibility to speak about the topic. Bit odd that he would then post his response on a site that has none.
Like Gavin, I also watched Koonin’s talk. Gavin’s post covers most of the issues, but there were a couple of things that bugged me that I thought I would highlight here. At about 11 minutes he highlights that the increase in atmospheric CO2 has produced a change in radiative forcing of about 2.5 Wm-2 (correct). He then goes on to say that it’s a relatively small perturbation, less than 1%.
He’s made this claim before and there is already a beautiful response from Andy Lacis. The only way the perturbation can be less than 1% is if you compare it to some of the large surface fluxes. This may seem reasonable, but it’s not really. What’s more relevant is how much we’ve perturbed the natural greenhouse effect. Relative to having no atmosphere and with the same albedo, the natural greenhouse effect enhances surface temperatures by about 33K. We’ve already warmed by about 1K, most of which is probably due to our emissions. Hence, we’ve already perturbed the natural greenhouse effect by 3% and are heading towards perturbing it by 10%, or more. As any physicist should realise, perturbing a complex, non-linear system by more than 10% is unlikely to be insignificant.
At about 19 minutes he goes on about the absolute surface temperature from the models differing by as much as 3K, and suggests that – given that this is larger than the change in surface temperature – it should give us pause as to the model responses being right. This is a somewhat more complex/nuanced issue, but it’s not really a very good reason to question how the models respond to radiative perturbations. We don’t actually have a very accurate estimate for the absolute global surface temperature. As Koonin himself showed earlier, there are a number of quite large surface fluxes, the balance of which will set the equilibrium surface temperature. Since we can’t easily measure these surface fluxes, we can’t produce an accurate estimate for the absolute global surface temperature. A relatively small change in one of these surface fluxes, can change the resulting surface temperature by a few K.
What’s more relevant is whether or not we expect the response to radiative perturbations to depend very strongly on the base state. The answer is not really, and this is also largely what the models indicate. What we’re interested in is how we expect the system to change in response to our emission of greenhouse gases into the atmosphere. Given that we don’t expect this to depend very strongly on the base state, that the models don’t produce exactly the same base state doesn’t necessarily mean we should question the resulting responses. This is not to say that we should ignore this issue; it may be that the reason for the difference in the base state is that some of the models are incorrectly representing some physical processes. However, simply highlighting this difference and suggesting that it brings into question how the models respond to perturbations is a pretty weak argument, and you might expect a physicist to get the nuance.
Anyway, that’s my quick addition to the comments about Steve Koonin’s recent talk. In case it’s not clear why this is potentially interesting, Steve Koonin is one of the originators of the idea of some kind of red team exercise, in which a supposedly independent group challenge the basics of climate science. Given this presentation, I’m not convinced it would be worthwhile, other than in demonstrating that most of those associated with this red team idea don’t really understand the topic very well.