When the white ice turns into a bluish mirror of the mountains.
Since the end of the Little Ice Age in the middle of the 19th century, glaciers have melted under the effect of global warming.
In the Swiss Alps, nearly 1,200 new glacial lakes have appeared in less than two hundred years, according to a study by researchers at the Swiss Federal Institute of Water Science and Technology (Eawag), University of Zurich and the Federal Office for the Environment, published on July 19.
"This research confirms the glacial retreat for several decades, which has accelerated over the past twenty years", summarizes Antoine Rabatel, researcher in glaciology at the Institute of Environmental Geosciences (IGE) at the University of Grenoble - Alpes.
Between 2006 and 2016, eighteen new lakes formed on average each year.
At the same time, those that already existed have grown.
The aquatic surface in the Swiss Alps has thus gained more than 150,000 m2 per year over the same decade.
"Visible proof of climate change in the Alps", note the authors of the inventory.
No worries for scientists
However, of the 1,200 lakes, nearly a quarter have shrunk or completely disappeared since 1850. “The glacier, seemingly motionless, is in fact still in motion. As it slides, it erodes the rock, creating pits where the glacier, as it melts, fills with water. But since the glacier supplies the lake with water, it also carries sediment, which can, in a few years or decades, reduce the surface area or fill the lake, ”explains Antoine Rabatel. Thus, on the Mer de Glace, in France, two small glacial lakes were filled by the deposit of sediment. He also recalls that a receding glacier does not necessarily create an expanse of water: this requires rather flat terrain, like a valley.
The appearance of these new lakes is not of concern to scientists.
“Most Swiss glacial lakes are very small, they cover less than one hectare and their water volume does not present a very significant danger for the populations.
But you have to watch them, they can continue to grow on the surface, ”said the glaciologist.
Read alsoGlobal warming: what if the glaciers disappeared?
An opinion shared by his colleague Christian Vincent, CNRS research engineer at IGE.
“We also have many examples in France of the emergence of new glacial lakes in the Mont-Blanc, Vanoise and Ecrins massifs.
Most do not represent viable threats, they are far from homes or infrastructure.
But some lakes present risks for the populations;
their level and volume are monitored regularly.
Those on the glacier front are unstable, because they are closed by dams of pebbles (moraines) carried by the glaciers.
Prevent natural risks
Another hazard can occur when the water from the glacier flowing towards the lake does not find an outlet, and causes its level to rise. To prevent the risk of sudden emptying, water can be pumped, as was the case for the Arsines glacier lake in the Ecrins in 1986 or the Rochemelon lake in the Haute Maurienne in 2004-2005. More surprisingly, threatening pockets of water can also form inside glaciers. As a precaution, that of the Tête Rousse glacier, in the Mont-Blanc massif, representing 60,000 cubic m, was emptied in the early 2010s.
Listing the lakes forming at glacial level can therefore help prevent natural risks, especially in these times of climate change. The Swiss are not the only ones to have understood this. In France, the Ministry of Ecological and Inclusive Transition has been carrying out an action plan for the past two years to prevent risks of glacial and periglacial origin. An inventory of the glacial lakes of the French Alps must be delivered at the end of 2021-beginning of 2022.
