I wrote a post recently about the new Kummer & Dessler paper on Equilibrium Climate Sensitivity (ECS). It was based on the earlier work of Shindell et al. (2014). The basic premise is that more land mass in the northern hemisphere (NH) means that it should be more sensitivity to changes in radiative forcing that the southern hemisphere (SH). However, aerosol, ozone and land-use forcings are also bigger in the NH than the SH (and have a cooling influence). This inhomogeneity in these forcings means that the change in temperature, ΔT, for a given globally averaged change in radiative forcing, ΔF, will be smaller than if all the forcings were homogeneous.
However, these inhomogeneities should reduce with time (as the GHG forcing increases and the aerosol – and other – forcings decrease). Therefore, if you use an energy budget method to estimate the transient climate sensitivity (TCR), you will underestimate the TCR unless you compensate for this inhomogeneity. Shindell et al. (2014) suggest using a correction factor, E, in front of the aerosol, ozone and land-use forcings, as shown below
My understanding of the Kummer & Dessler paper, is that they essentially apply the same correction to the ECS equation (I know they actually use an integral form, but I think it’s the same basic equation). So, to determine the ECS, you can use
where Q is the system heat uptake rate – the energy imbalance. However, Troy Masters has a blog post that seems to suggests that this efficacy factor shouldn’t be applied to the ECS calculation. I also had a discussion about this with Fred Moolton on my earlier post. My feeling is that Troy has a point, but I haven’t quite worked out what he’s done in his analysis.
To try and understand this, I modified my two-box model so that it considered the two hemispheres independently. I then set it up so that one was more sensitive than the other and modified the forcings so that there was an inhomogeneous component. It was fairly straightforward to show that this inhomogeneity would reduce the change in temperature compared to if the forcings were homogeneous – and, hence, illustrating that simple energy budget estimates would underestimate the TCR.
However, when I considered the ECS, the simple method didn’t underestimate the ECS. What seemed to be happening is that the reduction in ΔT produces a smaller Planck response (negative feedback) which means that the energy imbalance, Q, is greater when the forcings are inhomogeneous than when they’re not. This seems to compensate for the reduced ΔT and suggests that one shouldn’t apply the efficacy factor when estimating the ECS using an energy budget method.
Let me make a few extra comments, though. Kummer & Dessler is written by people who are clearly experts in this field. It is, therefore, quite possible that I’ve misunderstood something about this situation. Secondly, it’s not completely obvious that it really matters. Even without the efficacy factor, Kummer & Dessler get an ECS of 2.3K with a range from 1.6 – 4.1K. This is already quite consistent with other estimates. If they were to include the temperature correction suggested by Cowtan & Way, it would increase slightly, making it even more consistent. It’s also quite likely that these energy budget estimates don’t capture some non-linearities which would mean that they may well under-estimate the ECS anyway. As I think is suggested in this comment by James Annan to a Nic Lewis post, the linear assumption in the energy budget estimate means we should probably regard this as en effective climate sensitivity, rather than a true representation of the equilibrium climate sensitivity.
So, essentially, I’m confused about Kummer & Dessler and I’ve written this post mainly to, hopefully, illustrate an element of skepticism. I could also, of course, be illustrating my ignorance, but that’s a risk one should be willing to take 🙂
Update : As I mention in this comment, I think I’ve resolved my confusion. I had assumed that the reason for the different sensitivities in the two hemispheres was simply because of different heat contents. This – I think – is incorrect. There may be some effect from this, but it’s also because of different feedback responses. If one considers this, then the efficacy factor should apply also to the ECS, as well as to the TCR.