Earlier observations had shown a decade-long speed-up, thinning and retreat of glacier tongues extending into fjords. But the extreme surface melting observed in 2012 represents another type of mass loss altogether – run-off. To understand this phenomenon better, a paper in Environmental Research Letters (ERL) reviews recent advances in Greenland-ice-sheet hydrology.

Each year, the Greenland ice sheet is balanced by mass gains from snowfall, and losses roughly split between calving icebergs and run-off generated by melting on the ice-sheet surface. While net mass balance is positive, it has become increasingly less positive since the late 20th century, primarily driven by increased surface melting.

In recent years, scientific studies undertaken over a range of scales and using a multitude of geophysical exploration methods have revealed how run-off losses are determined by the ice-sheet surface mass balance interacting with a complex hydrological system consisting of supraglacial lakes and streams, water percolating into firn, and an englacial and subglacial hydrologic network of moulins, crevasses, cavities and channelled systems.

Better understanding of the three components of this hydrological system will be critical for constraining future sea-level-rise estimates. First, darkening of the ice-sheet surface lowers its albedo. This enhances surface melting and lowers albedo even further.

Second, the hydrological system itself and how it develops also appears to strongly influence the dynamics of the ice sheet. As the ice sheet accelerates, more mass is brought to lower elevations, where melting occurs at faster rates. However, new work suggests that the development of efficient hydrological drainage systems acts to reduce ice-sheet velocities such that high-melt years essentially self-regulate dynamic ice-mass loss.

Finally, recent discoveries of meltwater storage in firn as sub-surface aquifers and refrozen as ice layers in firn pore space suggest that meltwater losses can be buffered over decades. This means that the present enhanced melting may not translate to a rise in sea level in years to come.

Greenland ice-sheet hydrology is a rapidly developing subfield, straddling both cryospheric and hydrologic science, and is advancing thanks to increasingly sophisticated modelling and observational techniques – from small-scale field experiments to large-scale modelling and remote sensing analyses. All of these exploratory modes are needed to understand the fate of the Greenland ice sheet and its potential to raise global sea levels significantly in the 21st century.

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