Shortwave cloud warming effect observed over highly reflective Greenland

Clouds significantly influence the Earth's energy budget, typically exerting a net global cooling effect by balancing 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. We find that the shortwave cloud warming effect on the Earth-atmosphere system is particularly prominent for optically thin clouds. Utilizing satellite-based observations over Greenland during the summer season from 2013 to 2022, we identify a positive shortwave cloud radiative effect when the ratio of surface albedo to top-of-atmosphere (TOA) reflectivity reaches a critical threshold, implying that cloud-induced warming can occur in any place when the surface is bright enough compared with TOA. The shortwave warming effect (with radiative effect up to 25 W/m²) over Greenland is primarily concentrated in the peripheries and southern margins-regions experiencing the most intense ice melt. From a global perspective, these warming clouds can contribute up to 0.36 W/m² to the summer Earth-atmosphere system in the Northern Hemisphere. These findings are critical for understanding the radiation budget in the polar regions, improving predictions of polar ice melt, and enhancing our comprehension of the Earth's energy budget.