This situation is straining ecosystems and could limit the use of this water in Switzerland’s nuclear and hydropower industries.
For a long time, meltwater from snow and glaciers has limited the warming of the Swiss rivers, allowing them to maintain a relatively low temperature throughout the year. But summer temperature spikes, especially since the turn of the century, are threatening to upset this delicate balance. In 2018, Swiss conservationists had to move fish populations to higher-altitude streams to prevent certain death from overheating, water shortage and disease.
In a study published in the journal Hydrology and Earth System Sciences, researchers at EPFL’s Laboratory of Cryospheric Sciences (CRYOS), the Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), UNIL, UNIBE and Crealp set out to investigate what was happening. The team analyzed river temperature and discharge trends across Switzerland using two data sets – post-1979 records from 33 measurement sites, and post-1999 records from 52 sites. They observed that river waters had warmed by an average of 0.33°C per decade since 1980, and by 0.37°C per decade in the last 20 years.
“We were surprised to find that Swiss rivers are warming at 95% of the rate of the surrounding air,” says Adrien Michel, a doctoral assistant at the CRYOS lab and the paper’s lead author. “The conventional wisdom was that the melting of snow and glaciers and the fact that this water then flowed into lakes were counteracting the effect of warmer air on the Swiss plateau. That’s no longer the case.”
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