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

Abstract

Abstract. Atmospheric warming over the Greenland Ice Sheet during the last 2 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 ice content from refrozen meltwater found in the firn above the superimposed ice zone. Here we present field and airborne radar observations of buried ice layers within the near-surface (0-20 m) firn 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 near-surface 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, modeled 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 retained after melt 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.

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

“…A key uncertainty in the mass budget is the degree of meltwater retention due to refreezing and capillary forces (e.g., Janssens and Huybrechts, 2000;Harper et al, 2012;Vernon et al, 2013;van As et al, 2016). 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.…”

On the recent contribution of the Greenland ice sheet to sea level change

“…These changes are highly significant considering that typical values for total firn air content in the dry snow zone range between 20 and 25 m . Recent research has shown that warm summers can generate thick ice layers that prevent meltwater from reaching the deeper pores, further reducing the meltwater buffering capacity (De la Peña et al, 2015;Machguth et al, 2016).…”

“…This can be achieved through statistical/dynamical downscaling in combination with targeted in situ observations. Examples of important processes that are poorly or not at all represented in current models are interactive snow/ice darkening by future enhanced dust/black carbon deposition or microbiological processes (Stibal et al, 2012), and sub-, supra-and englacial hydrology, including vertical and horizontal flow of meltwater in firn or over ice lenses (De la Peña et al, 2015;Machguth et al, 2016). Other emerging research topics of GrIS melt climate are the impact of atmospheric circulation changes on Greenland melt (Hanna et al, 2013a(Hanna et al, , 2014McLeod and Mote, 2016;Tedesco et al, 2013), the impact of rain on ice sheet motion (Doyle et al, 2015), the effect of liquid water clouds on the surface energy balance and melt (Bennartz et al, 2013;Van Tricht et al, 2016) and the increased role of turbulent heat exchange during strong melting episodes over the margins of the GrIS .…”

“…During 1991-2015, the refrozen fraction averaged 41 %, down from 44 % in 1961-1990, signalling a decrease in the retention efficiency of the GrIS firn layer. In RACMO2.3 this is mainly caused by a decrease in firn pore space , but in reality this effect is exacerbated by the formation of impenetrable ice lenses during warm summers, preventing the surface meltwater from reaching the deeper firn layers and using the full retention potential (De la Peña et al, 2015;Machguth et al, 2016). Figure 4 clearly demonstrates the large interannual variability of the major GrIS SMB components P tot , ME and RU.…”

Simulating ice layer formation under the presence of preferential flow in layered snowpacks

“…Water may flow laterally over ice layers or crusts, which reduces travel times in catchments and has a significant impact on catchment-scale hydrology; alternatively, preferential flow in snow may promote vertical percolation instead (Eiriksson et al, 2013). Recent studies have demonstrated that the increased melt on the Greenland Ice Sheet during the last few decades led to changes in the firn structure, particularly through the formation of ice layers by percolating water in sub-freezing snow (de la Peña et al, 2015). These ice layers can reach considerable vertical extents on the order of 1 m (Machguth et al, 2016) and may reduce the storage capacity of meltwater in the firn by making access to deeper firn layers more difficult.…”