Shortwave cloud warming effect observed over Greenland

Clouds play a pivotal role in regulating the Earth's energy budget, primarily by exerting a global net cooling effect through the competing effects of shortwave radiation shading and longwave radiation trapping. However, here we report a shortwave warming effect by clouds over Greenland, contrary to the conventional belief of a cooling effect. Utilizing satellite observations from the Greenland region during the summers from 2013 to 2022, we identify a positive shortwave cloud radiative forcing when the ratio of surface albedo to top-of-atmosphere (TOA) reflectivity surpasses 1.42, implying that cloud induced warming can occur in any place when the surface is bright enough compared with TOA. Moreover, we find that the shortwave cloud warming effect on the Earth-atmosphere system is particularly prominent for optically thin clouds. These findings are crucial for understanding the radiation budget over polar regions and improving the prediction of polar ice melting.