Since I haven’t had much chance to write anything recently, I thought I would briefly advertise a couple of papers that may be of interest to my regular readers. One is by Clare Marie Flynn and Thorsten Mauritsen and is [o]n the Climate Sensitivity and Historical Warming Evolution in Recent Coupled Model Ensembles and compares the CMIP5 and CMIP6 models ensembles.
The CMIP6 ensemble suggests a shift towards a higher equilibrium climate sensitivity (ECS), when compared with the CMIP5 ensemble. The Flynn & Mauritsen paper illustrates that this can’t be due to chance, suggesting that the CMIP6 mean ECS is indeed highly unusual. Consistent with the paper I discussed here, they seem to find that the shift in ECS is mostly due to an increase in the shortwave cloud feedback, mostly in the Southern extratropics. Even though there is a shift to a higher ECS, they also find that none of the models with a Transient Climate Response (TCR) above 2.5oC matches the post-1970s warming. This seems broadly consistent with the results from the paper I discussed in this post.The other paper I thought I would highlight is Observational constraints on the effective climate sensitivity from the historical period, by Kasia Tokarska and colleagues. They make use of detection and attribution techniques to derive the surface air temperature and ocean warming that can be attributed directly to greenhouse gas increases. They then use this, together with an energy balance model, to infer the effective climate sensitivity (which they refer to as ). As shown in the figure on the right, they find a 5-95% range of 1.3oC – 3.1oC for the period 1862-2012, and 1.7oC – 4.6oC for the period 1955-2012.
For the two time periods, the median values are 2.0oC (1862-2012) and 2.8oC (1955-2012). However, they do highlight that [O]ur estimate of is lower than the documented ECS of some climate models (e.g. CMIP5 multi-model mean ECS of 3.22oC; Forster et al 2013), including that of some used in the analysis. However, it is well understood that time-dependent feedbacks might render lower than at equilibrium. This is because lower values for than at equilibrium can be explained by the effects of changing strength of the feedbacks at higher levels of warming.
That was all I was really going to say. Both papers are open access, so I’d encourage those who are interested to read them in more detail.