Scientists working at the universities of Fribourg and Lausanne have developed a model for the way Greenland’s meltwater drains into the sea. Their study, published in the journal Nature Communications, shows that the heating up of the planet is causing the formation of layers of ice under surface snow, which for now is preventing some of the upper layers’ meltwater from reaching the sea.

“The situation is bad but it is getting worse a little less quickly than we thought,” said Andrew Tedstone, researcher at the Faculty of Geosciences and Environment at the University of Lausanne (UNIL). Mr. Tedstone was talking about his latest discovery, made in part while he was a research associate at the University of Fribourg.

Andrew Tedstone studies the melting of the ice sheet in Greenland, a region that is warming around twice as rapidly as the rest of the world. The stakes couldn’t be higher. Currently it is estimated that between the calving (breaking off) of icebergs and the runoff of meltwater, sea level is rising one millimeter per year. If the Greenland ice sheet were to melt completely, the world’s oceans would be up to seven meters higher than they are today. Between now and 2100, it is estimated that this icemelt will contribute to a rise of world sea levels from 4 to 30 cm.

To understand the phenomena going on there and predict how glacial ice sheets will change, Andrew Tedstone and his team use satellite images and construct physical models of what is called thermal conduction, i.e., heat transfer, as well as take onsite measurements. The latest study looks at the way melting snow in the summer leads to the formation of impermeable, several meters thick, ice “slabs” deep below the surface. These slabs play a dual role. On the one hand, because of their low-permeability, they force meltwater to flow horizontally at the surface. On the other, they allow a certain quantity of that water to refreeze. As Andrew  Tedstone explained it, “We already knew that snow falling during the winter and compacting on the surface [called névé, a young granular type of snow] acts like a sponge when ice begins melting again. The water seeps in, then refreezes, forming layers of ice slabs beneath the snow. We thought, though, that these slabs, because of their impermeability, were accelerating the flow of meltwater to the sea. The latest study nevertheless shows that most of the surface meltwater in areas where ice slabs have formed can in fact refreeze before reaching the sea. “According to our analyses, the slabs made it possible for 56 gigatons of meltwater to refreeze between 2017 and 2022,” the researcher added. “By way of comparison, the Swiss Alps in their entirety contain around 43 Gt of glacier ice. So they are both refreezing points and emerging runoff zones.”

Horst Machguth, a coauthor of the study and researcher at the University of Fribourg, offered this conclusion, “Make no mistake, the Greenland ice sheet continues to melt and that phenomenon is contributing to rising sea levels in a highly problematic way. In the future, with the warm temperatures that are expected, the existing slabs will no longer be able to refreeze the meltwater.” Nevertheless, these new analyses will enable scientists to improve estimations of the phenomenon and better predict the effects of global warming on this unique and very specific region of the world.

The research results are published in Nature Communications; the research itself was funded by the  European Research Council (ERC).

Tedstone, A., Machguth, H., Clerx, N. et al. Concurrent superimposed ice formation and meltwater runoff on Greenland’s ice slabs. Nat Commun 16, 4494 (2025). https://doi.org/10.1038/s41467-025-59237-9