The presence of clouds lowers climate sensitivity in the MPI-ESM1.2 climate model

Clouds affect the sensitivity of the climate system by changing their distribution, height and optical properties under climate change. Additionally, clouds have a masking effect on CO2 forcing and can affect other feedback mechanisms such as the surface albedo feedback. To shed light on the overall effect of clouds, we compute how much the equilibrium climate sensitivity to a doubling of CO2 (ECS) changes when clouds are made transparent to radiation in an Earth system model (MPI-ESM1.2). Practically, to stabilise the model climate at near pre-industrial temperatures the solar constant was reduced by 8.8 percent. Our experiments suggest that clouds play a stabilising role in the model, with a clear-sky ECS of 4.34 K, which is higher than the corresponding full-sky ECS of 2.80 K. Detailed partial radiative perturbation diagnostics show that clouds strengthen the lapse rate and water vapour feedbacks and dampen the albedo feedback.