Effect of cloud feedbacks to CO2 level rise on the summer Arctic climate within the Eocene Deep-Time Model Intercomparison Project

Warmer-than-today and ice-free early Eocene Arctic region serves as a testbed for possible future climate changes in the northern polar region dominated by increasing CO₂ forcing and associated shrinking sea ice. It is essential for our understanding of recent climate changes to investigate short wave (SW) cloud effects on the SAT rise in the Arctic region in the high CO₂ worlds as well as to separate the temperature changes in cloud-free from all-sky conditions in the same region. Here we present the first results of the boreal summer SW cloud feedbacks to atmospheric CO₂ level rise from 1x to 3x pre-industrial level of 280 ppm across the ensemble of models participating in the Eocene Deep Time Model Intercomparison Project (DeepMIP). We use a relatively novel approximate partial radiative perturbation (APRP) method to decompose the cloud feedback into the contribution from changes in cloud fraction, absorption and scattering (including cloud albedo feedback) as well as to separate the radiative effects of cloud changes from surface albedo changes. Our first results show discrepancies between the models regarding the effects of clouds on surface air temperature changes. Most of the models show that the net effect of clouds has a relatively modest positive effect (warming) on surface temperature changes however one of the models shows a cooling effect. These differences are due to different effects of cloud fraction and scattering across the ensemble. Furthermore, all models show warming due to surface albedo changes and moderate warming due to atmospheric non-cloud effects. However, surface albedo changes show big discrepancies in magnitudes between the models that result from particularly big differences in the overcast conditions.