Tolerable Warming over the Southern Ocean: Towards a Diminishing Negative Cloud Phase Feedback

Several climate models preliminarily report unprecedented high equilibrium climate sensitivities well above 5 K in the ongoing Coupled Model Intercomparison Project Phase 6 (CMIP6). The uncertainty in model estimates of the climate response is dominated by cloud feedbacks. As liquid clouds are optically thicker than ice clouds with similar properties, an initial tropospheric warming leading to a higher liquid water content in mixed-phase clouds would result in a cooling due to enhanced reflection of shortwave radiation (negative feedback). This negative feedback is strongest in the Southern Ocean region, with its extensive cloud cover that is situated in the mixed-phase temperature range (-40 to 0 °C) for most of the year. Thus, it can be hypothesised that a tropospheric warming over the Southern Ocean exceeding a certain threshold would exhaust the negative cloud phase feedback as the mixed-phase regime diminishes. Vertical profiles of liquid and ice water content derived from observations with the Cloud Profiling Radar on the CloudSat satellite and the Cloud-Aerosol Lidar with Orthogonal Polarization on CALIPSO are analysed in combination with temperature profiles from atmospheric reanalyses for the Southern Ocean region (40-70° S) between 2006 and 2017. These observations are compared to simulations with the Community Earth System Model Version 2 (CESM2), which recently reported an equilibrium climate sensitivity of 5.3 K. From these profiles and assuming no shift in cloud altitude, the threshold temperature increase at which practically no more phase transitions can take place can be deduced. Surpassing this tolerable warming could eventually result in a transition to an extremely sensitive climate regime.