Greenland Ice Sheet surface melt amplified by snowline migration and bare ice exposure

Greenland Ice Sheetmass loss has recently increased because of enhanced surface melt and runoff. Since melt is critically modulated by surface albedo, understanding the processes and feedbacks that alter albedois a prerequisite for accurately forecasting mass loss. Using satellite imagery, we demonstrate the importance of Greenland’s seasonally fluctuating snowline, which reduces ice sheet albedoand enhances melt by exposing dark bare ice. From 2001 to 2017, this process drove 53% of net shortwave radiationvariability in the ablation zoneand amplified ice sheetmelt five times more than hydrological and biological processes that darken bare ice itself. In a warmer climate, snowline fluctuations will exert an even greater control on melt due to flatter ice sheettopography at higher elevations. Current climate models, however, inaccurately predict snowline elevations during high melt years, portending an unforeseen uncertainty in forecasts of Greenland’s runoff contribution to global sea level rise.