Anyone who is aware of what’s going on in climate science should have heard that the latest generation of climate models, known as CMIP6, seem to be suggesting a somewhat higher climate sensitivity than suggested by the previous CMIP5 models. The Equilibrium Climate Sensitivity (ECS) from CMIP5 models varied from 2.1K to 4.7K with a mean of 3.3K, while for the CMIP6 models it goes from 1.8K to 5.6K with a mean of 3.9K. In addition, 10 of the 27 CMIP6 GCMs having ECS values above 4.5K.
Thanks to a Twitter thread from Gavin Schmidt (which may have been the result of a tweet from Oliver Bothe) I’ve been made aware of a new paper from Mark Zelinka et al. that diagnoses the causes of higher climate sensitivity in CMIP6 models. According to their analysis, the main reason for the difference between the CMIP5 and CMIP6 ECS values is an enhanced SW low cloud feedback, mostly in the Southern extratropics (latitudes poleward of 30o). The idea being that there is a reduction in low level clouds in these regions which leads to an increase in the absorbed SW solar flux.The figure on the right illustrates the CMIP6 (orange) and CMIP5 (blue) SW low cloud feedback, and their difference (black). It clearly shows an increase poleward of 30S in the CMIP6 models, when compared to the CMIP5 models. As I understand it, this isn’t the only reason for the higher ECS values in the CMIP6 GCMs (they also suggest a sightly larger change in forcing due to a doubling of CO2) but it does seem to be the dominant factor.
Of course, the suggestion of an increased ECS in CMIP6 GCMs does not tell us that the ECS is indeed higher in the real world. The paper seems to suggest that the positive low cloud extratropical feedback is consistent with observations and theory. However, this doesn’t preclude that there could still be an error in some feedbacks that has yet to be established. The paper did, however, conclude with an interesting point. The enhanced low cloud SW feedback occurs in a region with efficient ocean heat uptake. If I understand this (which I may not) this might indicate that the efficient ocean heat uptake could be masking some surface warming while the climate is still changing (i.e., before reaching equilibrium).
I don’t know really know how to conclude this. It seems that there’s a growing understanding of why the CMIP6 GCMs suggest a higher ECS than the CMIP5 GCMs. This could reflect that the real world ECS is also higher than we had expected, but it’s still too early to really tell. I rather hope that it is not.
An emergent constraint on Transient Climate Response from simulated historical warming in CMIP6 models, by Femke Nijsse, Peter Cox, and Mark Williamson (2020).
Carbon-concentration and carbon-climate feedbacks in CMIP6 models, and their comparison to CMIP5 models, by Vivek Arora et al. (2020).