I was just going to briefly mention a new paper by Stevens, Sherwood, Bony and Webb in which they present [p]rospects for narrowing bounds on Earth’s equilibrium climate sensitivity. The basic argument is similar to what has – at times – been discussed here; consider what would be required for the Equilibrium Climate Sensitivity (ECS) to be, for example, small. If those conditions are not satisified, then one can eliminate that region of parameter space. Similarly for large ECS values.
For example, an would require:
- cooling of climate associated with anthropogenic aerosols would have to have been modest, and/or a historical “pattern effect” would have to be less important than indicated by models
- tropical sea-surface temperatures during the last glacial maximum (LGM, 21 kya) would need to have been at the warm end of the expected range, and/or a “pattern effect” for the LGM would have to be more important than current models predict
- climate feedbacks would have to have been much larger in past hot climates than they are at present, or else climate forcing at those times has been significantly underestimated; and
- cloud feedbacks from warming would have to be negative.
Given that all of these are quite unlikely suggests that an ECS smaller than 1.5K is unlikely. A similar argument can be made for an ECS greater than 4.5K. This is illustrated in the figure below.
The idea then is that you can combine the different lines of evidence, and use Bayesian inference, to determine a range for the ECS. I couldn’t quite tell if their analysis was intended to simply be illustrative, or not, but as the figure on the right shows, it suggests a 95% range of 1.6K – 4.1K and a median of 2.6K. This may not sound all that different from the IPCC’s 1.5K – 4.5K, but that is presented as a 66% range; the IPCC only regards less than 1K, and greater than 6K, as extremely/very unlikely. What’s presented in Stevens et al. (2016) also seems to rule out quite a bit of Nic Lewis’s range, based on energy balance estimates.
That’s all I was going to say. I think the basic idea is very sensible; when combining different estimates you should at least consider whether, or not, you’ve end up with a range that includes regions that are probably precluded due to physical arguments. You might argue that if some estimates include that portion of the range, then it should be physically plausible. That’s not really correct, since all estimates could be influenced by factors for which they can’t easily compensate (for example, energy balance methods don’t consider the full doubling of CO2 and can’t correct for possible influences from the pattern effect). Therefore using physical storylines to narrow the ECS bounds seems quite sensible.
Update: As Thorsten Mauritsen points out in the first comment, the second figure is probably just illustrative and shouldn’t be seen as a definitive analysis that has narrowed the range by using the physical storylines.