Yes I think you are in fact right that plausible priors do seem to exclude ECS above 5 degrees.
You pick out a major problem in drawing conclusions about ECS—the IPCC does not explain how they arrive at their pdf of ECS and the estimate seems to be produced somewhat subjectively from various current estimates from instrumental and paleoclimatic data and from their own expert judgement as to what weight to give to different studies. I think this means that they give some weight to pdfs with a very fat tail, which seems to be wrong, given their use of uniform priors. This might mean that their tail estimate is too high
Intuitively I’m inclined to agree that the probability of very high or low climate sensitivities is overestimated due to the existence of a few separate lines of evidence that give us similar estimates, and because some studies have used inappropriate priors.
But I’ve heard climate science experts say it’s harder to “nail down” the upper end of the ECS range, IIUC because of the multiplicative nature of positive feedbacks. A simple blackbody model of Earth with no feedbacks says that doubling CO2 would give us about 1.1°C of warming (IIRC) but there are several feedbacks in which a temperature increase causes a larger temperature increase: water vapour, ice albedo, permafrost melt (not technically included in ECS, but worth considering along with the effect of destabilizing shallow clathrates, if any), cloud feedback (thought to be small), a potential increase in drought leading to higher albedo, changes to the oceanic depth-temperature gradient / changes to ocean currents (which reminds me, global warming could ultimately cause cooling in Europe which implies that if the ECS is X, the typical warming would be above X outside Europe, and as I have noted elsewhere in this discussion, the land will warm more than the ocean surface at equilibrium).
When you stack the PDF of all the feedbacks together, the tail of the distribution gets uncomfortably long. (I didn’t read much of Annan & Hargreaves so if their analysis specifically addresses the “stacking” of feedbacks, let me know.) [Overall, there have been new papers suggesting we can constrain ECS below 4° and others saying we can’t, so I think we need to give the dust some time to settle—while still looking for tractable things we can do in this area.]
Note that the historical data on the ECS doesn’t help much to constrain the upper end of the temperature range because ECS is likely not independent from initial temperature; we’ll be reaching temperature zones Earth hasn’t had for many millions of years, and we don’t have very solid data going back beyond 800,000 years.
Yes I think you are in fact right that plausible priors do seem to exclude ECS above 5 degrees.
You pick out a major problem in drawing conclusions about ECS—the IPCC does not explain how they arrive at their pdf of ECS and the estimate seems to be produced somewhat subjectively from various current estimates from instrumental and paleoclimatic data and from their own expert judgement as to what weight to give to different studies. I think this means that they give some weight to pdfs with a very fat tail, which seems to be wrong, given their use of uniform priors. This might mean that their tail estimate is too high
Intuitively I’m inclined to agree that the probability of very high or low climate sensitivities is overestimated due to the existence of a few separate lines of evidence that give us similar estimates, and because some studies have used inappropriate priors.
But I’ve heard climate science experts say it’s harder to “nail down” the upper end of the ECS range, IIUC because of the multiplicative nature of positive feedbacks. A simple blackbody model of Earth with no feedbacks says that doubling CO2 would give us about 1.1°C of warming (IIRC) but there are several feedbacks in which a temperature increase causes a larger temperature increase: water vapour, ice albedo, permafrost melt (not technically included in ECS, but worth considering along with the effect of destabilizing shallow clathrates, if any), cloud feedback (thought to be small), a potential increase in drought leading to higher albedo, changes to the oceanic depth-temperature gradient / changes to ocean currents (which reminds me, global warming could ultimately cause cooling in Europe which implies that if the ECS is X, the typical warming would be above X outside Europe, and as I have noted elsewhere in this discussion, the land will warm more than the ocean surface at equilibrium).
When you stack the PDF of all the feedbacks together, the tail of the distribution gets uncomfortably long. (I didn’t read much of Annan & Hargreaves so if their analysis specifically addresses the “stacking” of feedbacks, let me know.) [Overall, there have been new papers suggesting we can constrain ECS below 4° and others saying we can’t, so I think we need to give the dust some time to settle—while still looking for tractable things we can do in this area.]
Note that the historical data on the ECS doesn’t help much to constrain the upper end of the temperature range because ECS is likely not independent from initial temperature; we’ll be reaching temperature zones Earth hasn’t had for many millions of years, and we don’t have very solid data going back beyond 800,000 years.