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. The Earth's equilibrium climate sensitivity (ECS) to a doubling of atmospheric CO2, along with the transient climate response (TCR) andgreenhouse gas emissions pathways, determines the amount of future warming. Coupled climate models have in the past been important tools to estimateand understand ECS. ECS estimated from Coupled Model Intercomparison Project Phase 5 (CMIP5) models lies between 2.0 and 4.7 K (mean of3.2 K), whereas in the latest CMIP6 the spread has increased to 1.8–5.5 K (mean of 3.7 K), with 5 out of 25 models exceeding5 K. It is thus pertinent to understand the causes underlying this shift. Here we compare the CMIP5 and CMIP6 model ensembles and finda systematic shift between CMIP eras to be unexplained as a process of random sampling from modeled forcing and feedback distributions. Instead,shortwave feedbacks shift towards more positive values, in particular over the Southern Ocean, driving the shift towards larger ECS values in manyof the models. These results suggest that changes in model treatment of mixed-phase cloud processes and changes to Antarctic sea ice representationare likely causes of the shift towards larger ECS. Somewhat surprisingly, CMIP6 models exhibit less historical warming than CMIP5 models, despite anincrease in TCR between CMIP eras (mean TCR increased from 1.7 to 1.9 K). The evolution of the warming suggests, however, that several ofthe CMIP6 models apply too strong aerosol cooling, resulting in too weak mid-20th century warming compared to the instrumentalrecord.

On the climate sensitivity and historical warming evolution in recent coupled model ensembles