Many GCMs participating in the Sixth Coupled Model Intercomparison Project (CMIP6) exhibit ECS values that exceed 5K, well outside the upper end of the IPCC AR5 ‘likely’ range (4.5K) due to extratropical SW cloud feedbacks that are more positive than those simulated in previous generation CMIP5 GCMs. The extratropical cloud feedback of 57 CMIP5 and CMIP6 GCMs are examined in the Southern Ocean. Cloud source and sink processes are found to be an important determinant of simulated cloud feedback. GCMs differ dramatically in their representation of these processes. Cloud feedback is predicted based on sensitivity of liquid cloud to temperature and moisture convergence and the sensitivity of radiation to cloud. Competition between these sensitivities is discussed in the context of an idealized atmosphere-only simulation. Constraints based on reanalysis combined with the World Climate Research Programme (WCRP) assessment framework are used to refine the possible range of climate sensitivity that can be reconciled with observations of the present day climate.