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. How clouds change in a warmer climate remains one of the largestuncertainties for the equilibrium climate sensitivity (ECS). While a largespread in the cloud feedback arises from low-level clouds, it was recentlyshown that mixed-phase clouds are also important for ECS. If mixed-phaseclouds in the current climate contain too few supercooled cloud droplets, toomuch ice will change to liquid water in a warmer climate. As shown by Tan et al. ( 2016 ) , this overestimates the negative cloud-phase feedback andunderestimates ECS in the CAM global climate model (GCM). Here we usethe newest version of the ECHAM6-HAM2 GCM to investigate the importance ofmixed-phase and ice clouds for ECS. Although we also considerably underestimate the fraction of supercooledliquid water globally in the reference version of the ECHAM6-HAM2 GCM, we do notobtain increases in ECS in simulations with more supercooled liquid water inthe present-day climate, different from the findings by Tan et al. ( 2016 ) . Wehypothesize that it is not the global supercooled liquid water fraction thatmatters, but only how well low- and mid-level mixed-phase clouds with cloud-top temperatures in the mixed-phase temperature range between 0 and − 35  ∘ C that are not shielded by higher-lying ice clouds aresimulated. These occur most frequently in midlatitudes, in particular overthe Southern Ocean where they determine the amount of absorbed shortwaveradiation. In ECHAM6-HAM2 the amount of absorbed shortwave radiation over theSouthern Ocean is only significantly overestimated if all clouds below0  ∘ C consist exclusively of ice. Only in this simulation is ECSsignificantly smaller than in all other simulations and the cloud opticaldepth feedback is the dominant cloud feedback. In all othersimulations, the cloud optical depth feedback is weak and changes in cloudfeedbacks associated with cloud amount and cloud-top pressure dominate theoverall cloud feedback. However, apart from the simulation with only icebelow 0  ∘ C, differences in the overall cloud feedback are nottranslated into differences in ECS in our model. This insensitivity to thecloud feedback in our model is explained with compensating effects in theclear sky.

The importance of mixed-phase and ice clouds for climate sensitivity in the global aerosol–climate model ECHAM6-HAM2