I’ve regularly written about the Equilibrium Climate Sensitivity (ECS), in particular estimates by Nic Lewis and why they are probably a bit too low. For balance I should probably now mention a new paper called Observational constraints on mixed-phase clouds imply higher climate sensitivity by Tan, Storelvmo & Zelinka.
You can probably get the key result from the description, which finishes with
Tan et al. used satellite observations to constrain the radiative impact of mixed phase clouds. They conclude that ECS could be between 5.0° and 5.3°C—higher than suggested by most global climate models.
The key point seems to be that they considered clouds feedbacks and suggest that cloud feedbacks might be much more positive than we currently think. They
show that the ECS can be up to 1.3°C higher in simulations where mixed-phase clouds consisting of ice crystals and supercooled liquid droplets are constrained by global satellite observations.
So, what’s the basic issue. As James Annan points out, an ECS of 5.3C is pretty hard to reconcile with our understanding of past climate changes. Bear in mind that the ECS is the equilibrium response considering fast feedbacks only, and so the equilibrium system sensitivity (ESS) – which includes slow feedbacks – could be higher. However, this study is considering clouds, which are fast feedbacks.
Also, even if energy balance estimates might be on the low side, they’re still a reasonable way to get a ballpark figure. If they’re wrong by a factor of two or more, it either means that natural variability is masking a lot of attributable warming, or the response is highly non-linear. Both are possible, but it seems unlikely that the difference can be quite this large.
A number of fairly high-profile scientists also seem rather skeptical; Kevin Trenberth in the Guardian article and Gavin Schmidt on Twitter and in Chris Mooney’s article. It’s clear that clouds are one of the major uncertainties with respect to climate sensitivity. However, just as some of the energy balance models seem to produce results that appear a bit too low, this study seems to be suggesting results that are somewhat on the high side. Also, promoting it as they have is – I would argue – somewhat sub-optimal. It’s one thing to present controversial results, but claiming that
global climate models have significantly underestimated how much the Earth’s surface temperature will rise if greenhouse gas emissions continue to increase as expected
is much stronger than is justified.