Comparison of APRP cloud feedbacks to CO2 level rise on the summer Arctic climate across the Eocene Deep-Time Model Intercomparison Project ensemble

The Approximate Partial Radiative Perturbation (APRP) method is a powerful tool for investigating the effects of changes in cloud characteristics, driven by increased CO₂ levels, on planetary albedo. The northern polar region is particularly sensitive to climate change. However, the summer temperature rise over the Arctic Ocean is relatively mild, and the mechanisms that suppress temperature increases are not fully understood.

We apply the APRP method to an ensemble of models participating in the Eocene Deep-Time Model Intercomparison Project (DeepMIP) and compare the effects of summer cloud feedback changes in the polar region to CO₂ level increases from 1× pre-industrial (PI) level to 3/4× PI for both Eocene and modern conditions across the ensemble.

Our results reveal a wide range of results, both in magnitude and in sign (warming/cooling) of radiative changes, between models and even within the same models across different timeslices. Changes in cloud scattering are the primary contributors to the inter-model spread of cumulative APRP cloud effects. This spread is further amplified by differences in the sign of APRP cloud absorption effects.

In contrast, the models provide relatively consistent results for APRP cloud fraction effects, with most simulating modest positive feedback from cloud fraction changes due to CO₂ increases. Nevertheless, the cumulative APRP cloud effects are minor compared to the net ocean-atmosphere energy flux changes over an ice-free Arctic Ocean. These fluxes might play a dominant role in inhibiting summer temperature increases in the polar region under elevated CO₂ levels.