I’ve finally had a chance to read the new Kummer & Dessler paper The impact of forcing efficacy on the equilibrium climate sensitivity (ECS). It’s an analysis of climate sensitivity that uses energy budget estimates and also considers (as suggested by Drew Shindell – and others) that aerosol, ozone and land use forcings are inhomogeneous. I discussed Drew Shindell’s work in an earlier post and the basic idea is that the northern hemisphere (NH) should – because of all the land mass – warm faster than the southern hemisphere (SH). However, the aerosols and ozone are not well-mixed and have a larger effect in the NH than in the SH. This means that the global temperature response is smaller than it would be if these forcings were homogeneous (well-mixed).
One can correct for this by adding some factor, E, to these inhomogeneous forcings, e.g.,
The equation above is for the transient climate response (TCR), while Kummer & Dessler compute the ECS, but one can apply the same correction to estimates of the ECS. To get the ECS, Kummer & Dessler use an integral form of the following equation
where N is the energy imbalance, F is the change in radiative forcing, and dT is the change in surface temperature. They then solve for λ, which has units of Wm-2K-1. Inverting λ and multiplying by 3.7 Wm-2 gives the equilibrium temperature after a doubling of CO2.
Their results are shown in the table below. Assuming there’s no inhomogeneity in the forcings (E = 1) gives an ECS of 2.3K with a range from 1.6K to 4.1K. Assuming an efficacy factor of 1.33 gives an ECS of 3.0K, and an efficacy factor of 1.5 gives an ECS of 3.5K.
I should add, however, that Troy Masters did an analysis that suggested that the inhomogeneity correction may not be as large as suggested by Shindell. I haven’t quite worked out the significance of what Troy has illustrated, but as I see it, it’s fairly clear that E has to be greater than 1. It seems that it can’t be bigger than 1.5. What Troy’s done may well suggest that E is closer to 1 than to 1.5. However, even with E = 1, Kummer & Dessler get an ECS of 2.3K (range 1.6K – 4.1K). So, even if the inhomogeneity is only a 10% effect, it’s still acting to adjust the energy budget estimates back towards being quite similar to estimates using other methods (paleo, GCMs). Also, I think the Kummer & Dessler E = 1 estimate doesn’t include the Cowtan & Way temperature correction, which would increase the ECS further.
So, essentially it seems that as we consider these energy budget estimates in more detail we’re finding that the various corrections are acting to adjust these estimates so that they become more consistent with other estimates. This is good scientifically (it’s useful to have multiple methods that give similar results) but not necessary comforting. We certainly shouldn’t be pleased that the original energy budget estimates are likely to be underestimating climate sensitivity. We also shouldn’t – as some seem to do – argue that we should be optimistic and hope that these lower estimates are correct. Wishful thinking isn’t going to have any effect on reality.
Anyway, I’ll finish with a short video in which Andrew Dessler explains their recent paper.