Observations of pronounced Greenland ice sheet firn warming and implications for runoff production

Abstract

Field measurements of shallow borehole temperatures in firn across the northern Greenland ice sheet are collected during May 2013. Sites first measured in 1952-1955 are revisited, showing long-term trends in firn temperature. Results indicate a pattern of substantial firn warming (up to +5.7°C) at midlevel elevations (1400-2500 m) and little temperature change at high elevations (>2500 m). We find that latent heat transport into the firn due to meltwater percolation drives the observed warming. Modeling shows that heat is stored at depth for several years, and energy delivered from consecutive melt events accumulates in the firn. The observed warming is likely not yet in equilibrium with recent melt production rates but captures the progression of sites in the percolation facies toward net runoff production.

Heat sources within the Greenland Ice Sheet: dissipation, temperate paleo-firn and cryo-hydrologic warming

“…Extended areas of surface melt might lead to increasing firn temperatures, affecting large parts of the ice sheet. Such recent warming has already been detected in the study area (Polashenski et al, 2014).…”

“…Such conditions with perennial water within the firn have recently been found in the accumulation area in southern Greenland Harper et al, 2012). Currently, firn temperatures are again warming in the accumulation zone upstream of the study area (Polashenski et al, 2014), and will leave their imprint in the thermal state of the ice sheet. Warm paleo-firn only affects the upper ablation area, and explains only measured temperatures of borehole TD5, but is not sufficient to reproduce warm temperatures at depth at sites GULL and FOXX.…”

Liquid Water Flow and Retention on the Greenland Ice Sheet in the Regional Climate Model HIRHAM5: Local and Large-Scale Impacts

“…As the climate has warmed, the zone where melt and rainfall occurs over the snowpack has expanded to higher elevations in the last decade (Howat et al, 2013;de la Peña et al, 2015). Observations from Polashenski et al (2014) confirm that firn warming is a both long-term (>50 years) and widespread effect. Successive warm summers have also led to the formation of reduced permeability ice lens complexes that expand runoff into the accumulation area, e.g., at the KAN-U site at 1840 m a.s.l (above sea level) in 2012 (Machguth et al, 2016a).…”

“…Ice lenses appear to reduce percolation, acting as a barrier to "deep percolation, " i.e., percolation below the previous year's accumulation (Machguth et al, 2016a). Refreezing releases latent heat and acts to warm the snowpack, a phenomenon that has been observed in the Greenland firn over the last 50 years (Polashenski et al, 2014) and seen in modeled snow packs in regional climate simulations (e.g., van den Broeke et al, 2009). …”

“…The process by which water percolates in the snowpack was comprehensively described in a Darcian type flow model by Colbeck (1972). Melt percolation has been identified and observed across many glaciers, particularly in the Alps and the Arctic, where deep snow packs often exist overlying glacier ice (e.g., Müller, 1976;Braithwaite et al, 1994;Parry et al, 2007;Wright et al, 2007;Humphrey et al, 2012;Gascon et al, 2013;Polashenski et al, 2014). The meltwater penetration depth is controlled by the temperature and density of the snowpack.…”