ABSTRACT
Surface meltwater can be retained in an ice sheet if it infiltrates the firn and refreezes. This is an important mass balance process for the Greenland Ice Sheet, reducing meltwater runoff and associated sea-level rise. The processes of meltwater infiltration and refreezing are not fully understood, however, and remain difficult to monitor remotely. We deployed vertical arrays of thermistors and time-domain reflectometry (TDR) probes to 4-m depth in the firn to continuously monitor meltwater infiltration and refreezing processes at DYE-2, Greenland. The observations provide a detailed picture of the coupled thermal and hydrological evolution of the firn through the 2016 melt season, including estimates of firn water content. The thaw and wetting fronts reached a maximum depth of 1.8 m, with meltwater infiltration concentrated in four main pulses of melting and subsurface warming that reached progressively deeper into the firn. The observations were used to constrain a coupled model of firn thermodynamics and hydrology, which was then run over the period 1950-2020, driven by meteorological forcing from GC-Net station data and ERA5 climate reanalyses. Model results suggest that decadal-scale firn evolution at DYE-2 is strongly influenced by extreme melt seasons such as those of 1968, 2012, and 2019, when meltwater infiltration reached depths of 6-7 m. Extreme melt years drive increases in firn temperature, ice content, and density, reducing firn meltwater retention capacity. Such processes are likely to govern future meltwater retention as the percolation zone extends to higher elevations in Greenland in the coming decades.
ABSTRACT
In recent decades, meltwater runoff has accelerated to become the dominant mechanism for mass loss in the Greenland ice sheet1-3. In Greenland's high-elevation interior, porous snow and firn accumulate; these can absorb surface meltwater and inhibit runoff4, but this buffering effect is limited if enough water refreezes near the surface to restrict percolation5,6. However, the influence of refreezing on runoff from Greenland remains largely unquantified. Here we use firn cores, radar observations and regional climate models to show that recent increases in meltwater have resulted in the formation of metres-thick, low-permeability 'ice slabs' that have expanded the Greenland ice sheet's total runoff area by 26 ± 3 per cent since 2001. Although runoff from the top of ice slabs has added less than one millimetre to global sea-level rise so far, this contribution will grow substantially as ice slabs expand inland in a warming climate. Runoff over ice slabs is set to contribute 7 to 33 millimetres and 17 to 74 millimetres to global sea-level rise by 2100 under moderate- and high-emissions scenarios, respectively-approximately double the estimated runoff from Greenland's high-elevation interior, as predicted by surface mass balance models without ice slabs. Ice slabs will have an important role in enhancing surface meltwater feedback processes, fundamentally altering the ice sheet's present and future hydrology.
