By using more than 25,000 satellite images, Andrew Tedstone and Horst Machguth, two researchers from the University of Fribourg, were able to observe that the runoff of water from the melting of the Greenland ice sheet has increased significantly in recent years. This phenomenon, caused by global warming, is contributing to rising sea levels.

The Greenland ice sheet is a huge mass of Arctic ice. If it were to melt completely, it would raise the level of the oceans by 7 metres. Since the Arctic has been warming over the last decades, scientists have been able to determine an increase in the amount of ice lost from the ice sheet which enters the ocean. Recently, the Greenland Ice Sheet has contributed to a rise of 1 mm in sea level each year.

The increasingly impermeable nature of the ice sheet
A large part of the ice sheet is covered by a firn, in other words, old snow that turns into ice by gradually compacting. The top few metres of this compacted snow functions like a sponge. When its surface melts, the water flows into the pores below. There the water refreezes, forming thin layers of ice inside the sponge, and so does not flow away towards the sea. However, with the recent increase in episodes of melting, the amount of water percolating to the interior of the firn has increased, causing a significant expansion of these layers of ice which are now several metres thick, making it more difficult for water to seep through the pores of the firn and thus preventing it from refreezing. This raises the question: where does the meltwater go now?

Increased surface runoff
Two researchers from the University of Fribourg, Andrew Tedstone and Horst Machguth, have analysed more than 25,000 satellite images to track the formation of the rivers on the surface of the ice sheet each summer. In their new study, published in Nature Climate Change, they established that the area drained by the surface rivers increased by 29% between 1985 and 2020, an increase that is equivalent to the total surface area of Switzerland. «The extensive melting and refreezing have transformed the upper layers of snow and firn into thick, near-impermeable ice, which encourages runoff from higher areas of the ice sheet, even in  cooler summers,» explains Andrew Tedstone. «Water that used to refreeze where it was, is now forced to flow away, forming vast rivers that eventually reach the sea.» The researchers estimate that 5-10% of the recent loss of ice from the ice sheet are due exclusively to runoff from higher areas of the ice sheet.

More melting to come
As a result of global warming, these higher areas of the ice sheet will experience even more significant melting. This is likely to lead to further expansion of the ice layers, resulting in the formation of new surface rivers. So, there will be ever larger areas of the ice sheet beginning to drain meltwater towards the sea.

«These satellite observations reveal the extent to which the process of water refreezing in the firn is affecting sea levels,» concludes Horst Machguth, who leads the University of Fribourg's project on Greenland’s increased runoff, funded by the European Research Council. «Together with our field work on the ice sheet, these results are essential for refining the models of ice sheet runoff which we use to gauge future sea level rise».

> Publication in Nature Climate Change lesen