The presence of clouds in the sky at night is capable of enhancing Greenland ice sheet meltwater runoff by more than 30% each year, according to a study published in Nature Communications this week. The finding illustrates the need for accurate cloud representations in climate models in order to better predict future contributions of the Greenland ice sheet to global sea level rise.

Melting of the Greenland ice sheet has become a dominant contributor to recent global sea level rise. The extent of this melt is in part determined by the overlying cloud cover, which affects the amount of incoming and outgoing radiation. However, different cloud types - such as ice-only or liquid-bearing clouds - can have opposing effects on how much melt is generated, and both a lack of direct measurements and differences between models have so far limited our understanding of the precise contribution of different cloud types to melting.

Kristof Van Tricht and colleagues combine satellite remote sensing, ground-based observations and a regional climate model to quantify the impact of cloud cover on the balance of incoming and outgoing energy across the entire Greenland ice sheet surface. The authors show that both ice-only and liquid-bearing clouds warm the surface of the Greenland ice sheet by limiting the amount of outgoing radiation, but that the surface response is not as expected. Rather than increasing surface melt directly through warming during the day, the clouds’ warming effect is strongest at night, when, as opposed to a clear-skies scenario, their presence acts to limit the refreezing of water on the ice sheet surface and enhances annual meltwater runoff by more than 30%.

While this study highlights the strong sensitivity of the Greenland ice sheet surface energy balance to cloud cover, it does not take into account possible warming feedback effects between the snowpack present on the surface of the ice sheet and the atmosphere above, which could complicate the cloud effect.