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New research involving Lancaster Environment Centre researchers has revealed that slush - or water-saturated snow and firn - is a far more widespread feature of the Greenland Ice Sheet than previously recognised.

The study, authored by Lancaster PhD student Emily Glen, supervised by Dr Amber Leeson, Dr Katie Miles, Professor Mal McMillan, and Alison Banwell (external, CIRES, Boulder), found that slush was the largest mapped surface meltwater feature on the ice sheet, covering an area up to nine times greater than the combined area of mapped supraglacial lakes, channels, and water-filled crevasses. Across nine years of satellite observations, slush covered an average of around 48,100 square kilometres, equivalent to 2.8 per cent of the Greenland Ice Sheet.

The findings highlight an overlooked component of Greenland’s surface hydrological system that could play an increasingly important role in icesheet melting as the climate warms.

The Greenland Ice Sheet has experienced increasing surface melt in recent decades, contributing significantly to its overall loss of ice. Rising temperatures have increased surface runoff, while extreme melt events are becoming an important feature of Greenland’s changing climate.

Previous research into surface meltwater has largely focused on features such as lakes and channels, which are relatively straightforward to identify using satellite imagery. Slush, however, has received much less attention despite its potential importance for how meltwater moves through and across the ice sheet.

Slush occurs when meltwater is unable to drain downwards through snow and firn (partially compacted snow), causing these layers to become saturated with water. This can happen when meltwater fills the available space within the firn, or when an impermeable layer of ice prevents further drainage.

Slush can allow meltwater to move more readily across the surface. When it refreezes, it can also form layers of ice within the snowpack, reducing the amount of space available to store future meltwater and potentially increasing runoff.

In their paper, ‘, the researchers produced the first ice-sheet-wide dataset of Greenland’s slush covering almost a decade (2016 to 2024), at a resolution of 100 metres. The team used imagery from the European Space Agency’s Sentinel-2 satellites alongside a machine-learning technique known as a random forest classifier to identify and map slush across the ice sheet.

The results show that the extent of slush varies considerably from year to year. In 2018, a relatively cool, low-melt year, slush covered around 18,700 square kilometres. In 2019, this increased to approximately 88,500 square kilometres, almost five times as much.

Slush was most extensive in southwestern and northern Greenland, while it was more persistent in northern and northeastern regions. During years with particularly high levels of melting, including 2019 and 2023, slush also extended further inland and to higher elevations. In 2018, the upper elevation limit of mapped slush was around 1,420 metres above sea level, compared with approximately 1,630 metres in 2019.

The study also highlights the potential impact of slush on Greenland’s melting. Because slush is darker than fresh snow, it absorbs more sunlight, increasing the amount of energy available to melt the ice surface. As a first-order upper-bound estimate, the researchers calculated that the additional energy absorbed could be equivalent to an average of 12.4 gigatonnes of ice melt per year if all of that additional energy were converted into melt.

The research demonstrates that slush has been an overlooked part of Greenland’s meltwater system. Its large and highly variable extent means that leaving slush out of models of the ice sheet could result in an incomplete picture of how meltwater is produced, stored, and transported. As Greenland continues to warm, the researchers say that explicitly accounting for slush alongside lakes, channels, and other meltwater features will be important for improving our understanding of how the ice sheet responds to climate change. This new nine-year dataset provides a foundation for monitoring slush and improving models of future icesheet change. Dr Amber Leeson, who was Emily's lead supervisor, said:

“Emily’s work has given us, for the first time, a way to see just how widespread and dynamic this previously difficult-to-map part of Greenland’s hydrological system is. That is important because slush may influence both the amount of energy absorbed at the ice-sheet surface and the way meltwater is stored and routed. Being able to observe it across the whole of Greenland and from year to year gives us a new opportunity to represent these processes more realistically in models of future ice-sheet change.”