It is now established that the melting of mountain glaciers and ice caps, on the surface, is accelerating under the effect of global warming. But did you know that, in parallel, a network of rivers and lakes unfolds under the thick ice layers, helping to destabilize them and considerably increase the risk of natural disasters?
As in June 2024, when the hamlet of the Bérarde was devastated by sludge and debris flows carrying everything in its path. The torrential flood was the result of intense precipitation, but also the rupture of a pocket of sub-Glacial water from the neighboring glacier of good stone.
To anticipate such disasters, it is necessary to be interested in the interactions between the flow mechanics of the glaciers and the hydrological network which they conceal. And this for the glaciers of today, in terms of poles and in the mountains, as for the vestiges of past glaciations.
Water under the glaciers, a natural lubricant
Like a gigantic treadmill, the glaciers continuously transfer ice from the interior to the periphery at speeds between a few tens of meters and a few kilometers per year. This transfer derives mainly from gravity: ice is a very viscous liquid which flows under the effect of its weight and the slope.
But under the ice, other mechanisms capable of accelerating this displacement can be activated, especially when the cast iron generated on the surface by warming air infiltrates the network of crevices. In doing so, she “lubricates” the interface between ice and the ground.
This water, under pressure due to the mass of ice that rests above, decreases friction. Everything then happens as if the glaciers made “ventriglisse” on the bed of rocks and underlying sediment.
Millions of cubic kilometers of ice therefore see their transit speed accelerating: it is multiplied by two or three. This phenomenon is at the origin of the slimming of glacial caps and the increase in the production of icebergs when ice languages end their races in the ocean.
The future of Greenland, Antarctica and, more broadly, the extent of the increase in sea level thus depend on the interactions between ice, water and soil within a labyrinthine network of rivers and sub-Glacial lakes.
Water pocket pockets
On its way, water will encounter depressions and cavities in which it will accumulate to form lakes nestled in the rock or in the ice. The acceleration of water production, under the effect of the melting of surface ice cream due to climate change, is thus at the origin of an increase in the number and volume of sub-Glacial water tanks.
When they reach their too full, the pressure is such that the water can lift the glacier or break the ice dams. What cause the brutal emptying of millions of cubic meters of water, like the siphon of a bathtub that would be removed at once.
This emptying is not without consequence: it is at the origin of the transient acceleration of the ice flow (linked to the “ventriglisse” effect evoked above), but also of ice avalanches (dislocation and collapse of glaciers) and glacial debacles that can trigger phenomena of torrential lava.
These events are the culmination of a long and invisible process during which the sub-Glacial hydrological network, which evolves silently, ends up going beyond its point of balance.
Sudden and unpredictable changes
This balance point was crossed on July 12, 1892 when a lake located under the Glacier de Tête-Rousse, in the Mont-Blanc massif, dumped some 200,000 m3 Water in the Saint-Gervais valley, carrying nearly a million cubic meters-that is to fill 400 Olympic pools! – sediment, blocks and uprooted trees on its route.
From the torrential lava, well known to the mountaineers, then swept over Saint-Gervais leaving little chance of the inhabitants of the village: 175 have disappeared in the disaster. The story was repeated in June 2024 with the partial disappearance of the hamlet of Bérarde, in the Massif des Écrins.
The number of risk-at-risk lakes has increased considerably in recent years not only in France, but also in all mountain ranges. All of the lakes that reside under, above or in front of the glaciers-and which are, therefore, at risk of pouring out under the effect of the melting of ice-threatens nearly 15 million people on a global scale. At least 4,000 people have already died following such emptying.
It remains to specifically locate the risk. Barely a thousand sub-Glacial lakes are currently identified worldwide, a very probably underestimated number. There is therefore an urgency to predict and prevent the glacial hazards linked to the storage of cast iron water. To do this, it is necessary to detect the presence of these lakes, to decipher their evolution and to anticipate the phenomena of emptying in the difficult context of an undergoing hydrological network in full mutation.
Countries with adequate technical means can carry out surveillance campaigns and even undertake costly pumping/drainage operations of these water tanks to temporarily reduce the risk of rupture. But in the current climate context, the multiplication of lakes could complicate the task of the local authorities.
A difficult to access underglacial world
It must be said that these are difficult to access environments. Sub-Glacial environments are distinguished by their hostile, unpredictable and inaccessible-except for some experienced glaciospeleologists).
Geophysical methods using in particular the propagation of radio waves in and under the ice make it possible to achieve ultrasounds of the glacier, but they remain too punctual, limited in time and space, to understand its true complexity.
How then to predict the spatial and temporal evolution of such an environment lost in several tens, hundreds or even thousands of meters of ice? We know that there is sometimes gigantic pressure there – which can be similar to that which one meets several hundred meters underwater – and that there is a sandwich of rocks, sediments, water and ice whose interactions evolve at the rate of the daily and seasonal variations of the production of cast iron water.
Is this under-Glacial environment transforming under the effect of climate change? Is a dangerous rocking point near? To answer this question, being interested in the glacial history of our planet is precious.
Last Ice Age Lessons
Better understanding on a large scale the operation of the hydrological network requires taking an interest in ice giants, such as Antarctica or Greenland. Under the ice, eroded cast iron, transports, deposits sediments, ice deforms them when it compresses them and shears, shaping a unique landscape. Nowadays, it is hidden under a thick coat of ice, but now disappeared glacial caps offer us privileged access to the vestiges of this environment.
There are large expanses – tens of millions of square kilometers across North America, Scandinavia and Siberia – in an exceptional state of preservation, composed of a labyrinth of valleys and sedimentary mounds shaped by the action of ice and water.
By analyzing the incredible diversity of dimensions and forms wooded by this chaotic sedimentary relief, paleoglaciologists strive to bring together the different parts of the puzzle to rebuild the sub-Glacial mechanisms which controlled the dynamics of these caps now disappeared.
Field campaigns, statistical analysis of the relief, the treatment of satellite image banks – with the precious help of artificial intelligence – and modeling are the modern paleoglaciologist toolbox. Ultimately, it should make it possible to improve digital models which describe the global (past, current and future) functioning of ice caps. The challenge is to more precisely integrate the interactions between ice, water and sediments in times of climate change.
The improvement of these models could make it possible to better predict any non -linear effects – these famous “rocking points” so difficult to model – in the behavior of glaciers.
If we consider these non -linear effects or not, forecasts to increase the sea level by the end of the century can vary by more than a meter apart. Going to find elements of response in the glacial archives that hide in the landscapes of the planet would therefore be precious to better know what to expect.
The future of our glaciers and ice caps may be written, at least in part, in the remains of their ancestors.
