Meltwater ponding has an underestimated radiative effect on the surface of the Greenland Ice Sheet.

Ponding of meltwater on the surface of the Greenland Ice Sheet has the potential to reduce ice sheet albedo and amplify mass loss. However, this process remains poorly constrained and is absent from models that project ice sheet mass balance. Here we demonstrate that meltwater ponding considerably increases the amount of energy available for melting the Greenland Ice Sheet. We first use satellite-derived products to show that meltwater ponding has a significant impact on spatial albedo patterns, particularly in the lower percolation zone. We then use drone imagery to demonstrate that, in the upper ablation zone, there are thousands of narrow streams and small pools (<100 m²) that collectively account for >50% of the total meltwater area. These small meltwater features are not resolved by surface water maps derived from medium-resolution satellite imagery, signifying that the radiative effect of meltwater ponding is three to four times stronger than predicted by satellite-based approaches. Our findings therefore place lower bounds on the radiative effect of meltwater ponding that could be used to advocate for the inclusion of this process into models that forecast Greenland Ice Sheet's contribution to sea-level rise.