It’s currently the AGU Fall Meeting and there was a session on Climate sensitivity and feedbacks. It included talks by Kate Marvel and Kyle Armour, and I noticed that the Convener, Andrew Dessler, had tweeted
This leads to one of the biggest “skeptical” talking points: “Observational estimates of ECS are much lower than models” or “Models are too sensitive to CO2”. In the session today, it’s clear that the scientific community has beaten the shit out of this problem. 2/
— Andrew Dessler (@AndrewDessler) December 14, 2017
This is something I’ve written about a number of times and it does now seem that there is mostly agreement that the observationally-based estimates are not really indicative of a lower equilibrium climate sensitivity (ECS). There are a number of reasons (that I try to elaborate in some of my earlier posts) but as Kate Marvel says in her summary it’s essentially that
You might think we could estimate this from observations: we’ve emitted carbon dioxide, and the temperature has risen. But the future may differ from the past, and there’s reason to think that the warming we’ve experienced so far is different from the warming to come.
An additional factor can also be the method, in particular the choice of prior. James Annan has a recent post in which he uses a different prior to that used in some of the other analyses. It produces a 5-95% range of 1.2oC – 4.8oC, which seems mostly consistent with the IPCC range of 1.5oC – 4.5oC (which is – I think – more properly a 17-83% range). The best estimate is 2.1oC, which is maybe still a bit lower than other estimates, but still seems reasonable.
Maybe I’ll finish this post by also mentioning the recent Royal Society report which also says
value below 2oC for the lower end of the likely range of equilibrium climate sensitivity now seems less plausible.