A few years ago I posted a video by Andrew Dessler that was discussing whether or not Equilibrium Climate Sensitivity could be less than 3oC. The bottom line was that the best estimate for ECS is about 3oC. Given that we’re quite confident about water vapour feedback, lapse rate feedback, and ice albedo feedback, the main way in which ECS could be much lower than this (say < 2oC) is if there were a strongly negative cloud feedback. Cloud feedbacks are probably the feedbacks about which there is the greatest uncertainty, and so this is not necessarily impossible.
More recently, however, I highlighted a TED talk by Kate Marvel that discussed her work on clouds. The basic conclusion was that the observations are pointing towards clouds acting to intensify the warming – they’re a positive feedback. In fact, Kate Marvel indicates that there is no observational evidence that clouds will substantially slow down global warming.
The reason I’m writing this is because there is a new Nature Commentary called Clearing Clouds of Uncertainty by Mark Zelinka, David Randall, Mark Webb and Steven Klein. Their commentary is really a summary of our recent understanding and – as illustrated by the figure on the right – they conclude that the evidence is converging on the cloud feedback likely being positive. The circles indicate the multi-model average feedback, and the coloured lines show the across model standard deviation. The thin grey lines extend to the model extrema. Essentially, the total cloud feedback is probably positive and has a likely range from about 0.2 Wm-2K-1 to about 0.7 Wm-2K-1.
The implication of this – as Andrew Dessler highlighted – is that it is unlikely that the ECS can be less than 2oC. We have a pretty good understanding of the other feedback processes (water vapour, lapse rate, and ice albedo) and the cloud feedback being positive strongly implies an ECS > 2oC. Some energy balance estimates of climate sensitivity suggest that the ECS is more likely less than 2oC, than above 2oC and – as I think I may have suggested before – I do think that those who promotes these results should put some effort into explaining how this is possible.
If water vaour, lapse rate and ice albedo, by themselves, suggest an ECS > 2oC and if cloud feedbacks probably amplify this, then how can the ECS be less than 2oC? My view is that this is simply because these energy balance estimates are a bit too simple and don’t necessarily capture all the relevant processes. I would, however, be happy to hear some kind of physically motivated argument for ECS < 2oC.