Changes in the firn structure of the Greenland Ice Sheet caused by recent warming

Abstract. Atmospheric warming over the Greenland Ice Sheet during the last two decades has increased the amount of surface meltwater production, resulting in the migration of melt and percolation regimes to higher altitudes and an increase in the amount of solid ice from refrozen meltwater found in the firn above the equilibrium line. Here we present observations of near-surface (0–20 m) firn conditions in western Greenland obtained from campaigns between 1998 and 2014. We find a sharp increase in firn ice content in the form of thick widespread layers in the percolation zone, which decreases the capacity of the firn to store meltwater. The estimated total annual ice content retained in the firn in areas with positive surface mass balance west of the ice divide in Greenland reached a maximum of 74 ± 25 Gt in 2012, compared to the 1958–1999 average of 13 ± 2 Gt, while the percolation zone area more than doubled between 2003 and 2012. Increased melt and column densification resulted in surface lowering averaging −0.80 ± 0.39 m yr−1 between 1800 and 2800 m in the accumulation zone of western Greenland. Since 2007, annual melt and refreezing rates in the percolation zone at elevations below 2100 m surpass the annual snowfall from the previous year, implying that mass gain in the region is now in the form of refrozen meltwater. If current melt trends over high elevation regions continue, subsequent changes in firn structure will have implications for the hydrology of the ice sheet and related abrupt seasonal densification could become increasingly significant for altimetry-derived ice sheet mass balance estimates.