Atmospheric Blocking Drives Recent Albedo Change Across the Western Greenland Ice Sheet Percolation Zone

Greenland Ice Sheet (GrIS) albedo has decreased over recent decades, contributing to enhanced surface melt and mass loss. However, it remains unclear whether GrIS darkening is due to snow grain size increases, higher concentrations of light-absorbing impurities (LAIs), or a combination. Here, we assess albedo controls in the western GrIS percolation zone using in situ albedo, LAI, and grain size measurements. We find a significant correlation between albedo and snow grain size (p < 0.01), but not with LAIs. Modeling corroborates that LAI concentrations are too low to significantly reduce albedo, but larger grain sizes could reduce albedo by at least similar to 3%. Strong atmospheric blocking increases grain sizes and reduces albedo through increased surface temperature, fewer storms, and higher incoming shortwave radiation. These findings clarify the mechanisms by which anomalously strong blocking contributed to recent GrIS albedo decline and mass loss, highlighting the importance of improving projections of future blocking. Plain Language Summary Satellite measurements and computer models have shown that the reflectivity (or albedo) of the Greenland Ice Sheet has decreased in recent decades, causing increased melt and sea level rise. It is unknown whether this albedo decline is due to increased impurities in the snow, larger snow grain sizes, or both. Field measurements show that the amount of impurities is too small to affect albedo in our field area. However, larger snow grain sizes could lower albedo enough to cause the observed trend. We demonstrate how a recent increase in the frequency of atmospheric high pressure systems over Greenland increases grain sizes via several mechanisms and contributes to Greenland's observed albedo decline and faster melt.