Hansen’s 10C

A recent comment asked about James Hansen’s recent paper in which it is claimed that equilibrium global warming for today’s GHG level is 10°C. I’ve finally had a chance to look at it and I think I understand what is being suggested.

One suggestion in the paper is that the GHG forcing today is already 4 Wm^-2, which is equivalent to a doubling of atmospheric CO₂. It also argues that the aerosol forcing is large enough that this implies an equilibrium climate sensitivity (ECS) of 4^oC. This is somewhat higher than the typical best estimate of 3^oC, but within the likely range (2^oC – 4.5^oC).

The paper then considers the glacial cycles and argues that an ECS of 4^oC is consistent with the glacial cycles, given global temperature changes of 6^oC and total changes of radiative forcing (ice sheets plus GHGs) of 6 Wm^-2.

The paper then argues that if the GHG forcing is 2.5Wm^-2, this implies an equilibrium response to the GHG forcing of 2.4 Wm^-2/^oC, giving an equilibrium temperature change to a 4 Wm^-2 GHG forcing of 10^oC.

As I understand it, the problem with this is explained in this Realclimate post. The glacial cycles are driven by Milankovitch cycles (orbital variations) that don’t, by themselves, have a big impact on global temperatures, but can have large regional impacts that influence ice sheets. Changes to the ice sheets can influence temperatures, which can then change atmospheric GHG concentrations, which then also influence temperatures, and then ice sheets, etc.

Hence, these are coupled in a complex way. If you want to estimate the ECS, you can combine the longer-term changes (GHG concentrations, ice sheets, etc) into a single forcing and you get an ECS of around 3^oC. To determine the response to the GHGs alone would require separating the influence of the orbital variations and GHGs on the ice sheets, which isn’t straightforward.

It will also depend on the climate state; if there are large ice sheets the response will probably be larger than when the ice sheets are much smaller, as they are today. So, I don’t think it’s correct to claim that equilibrium global warming for today’s GHG level is 10°C. There are a few other comments I could make about the paper, but since I’m trying to keep these posts short, I’ll stop there.