A model estimate of Clear-Sky Equilibrium Climate Sensitivity

Investigating the impact of clouds on Earth’s response to increased 𝐶𝑂2 concentrations is the primary objective of this study. Employing the MPI-ESM1.2 Earth System Model, we simulate a world where clouds are rendered transparent to radiation, compensating for the induced warming by reducing the solar constant by 8.8%. By comparing this Clear-Sky system with the regular Full-Sky climate simulation, we aim to quantify the effects of clouds through a subtraction process. To estimate the feedback and extrapolate the forcing, we employ the Gregory regression method. Surprisingly, the simulated Clear-Sky world exhibits a higher Equilibrium Climate Sensitivity of 4.34𝐾 which, compared to the Full-Sky value of 2.80𝐾, suggests an overall stabilizing role of clouds. By means of the PRP technique we analyze individual feedback mechanisms. We find that the radiative impact of clouds intensifies the response to warming primarily through negative contributions to the feedback parameter from albedo and lapse-rate feedback. However, the observed changes in the former are smaller than anticipated, and primarily due to abnormal sea-ice formation in the Southern Ocean within the Clear-Sky simulation. The modification in the lapse-rate feedback is dominated by differences in tropical regions between the two experiments, where the Full-Sky simulation exhibits a larger amount of high convective clouds. Singularly, our findings indicate that the total forcing resulting from doubling the 𝐶𝑂2 concentration is not significantly masked by clouds. This is attributed to the counterbalancing effect of the direct 𝐶𝑂2 forcing dampening, the reduction in negative albedo fast adjustment due to Southern Ocean sea-ice growth, and a positive clouds fast adjustment. Overall, this research contributes to a comprehensive understanding of how clouds influence Earth’s response to rising 𝐶𝑂2 levels, highlighting their complex interplay with other feedback mechanisms and climate sensitivity.