Obvious in high mountains, the link between rising temperatures and ground movements becomes difficult to establish at lower altitudes.
Roads diverted and access cut off due to landslides, mudslides or landslides… These events that experts group into the family of “gravitational phenomena” – rocks or other sediments falling, by virtue of the laws of gravity – are the daily lot of mountain dwellers. In the Aspe Valley, in the Pyrénées-Atlantiques, part of the RN 134 towards Spain collapsed during the night of Friday 6 to Saturday 7 September, swept away by a powerful mudslide. Earlier in the summer, other disasters shone the spotlight on these events, which are as unpredictable as they are destructive: that of the hamlet of La Bérarde (Savoie), swept away on the night of 21 June by a torrent of mud and rocks; the torrential rains that left at least 12 dead or missing in the Swiss Alps at the end of June; the collapse of a mountain slope on 25 July on a road in the Vercors.
These repeated disasters, which occurred after violent episodes of rain in mountainous areas, raise the question of the involvement of human-caused climate change, which causes extreme weather episodes across the globe. A suspect who is usually singled out, but who is not always directly responsible.
““We must be wary of generalizations,” insists Serge Taboulot, president of the Institute of Major Risks (IRMa) in Grenoble (Isère). “As far as high mountains are concerned, there is no comparison. The risk is increasing very clearly, everywhere in the world, in connection with climate change which is causing the permafrost to melt.” This frozen land, also called “permafrost”, had previously held the rock in place. But as temperatures rose, the thaw caused rock falls and other landslides that forced climbers to abandon many routes every summer, as they had become too dangerous.
“Torrential lavas, which are flows mixing rocks and water, also tend to intensify in the Alps with the increase in climatic extremes”abounds Ludovic Ravanel, geomorphologist at the CNRS and mountain guide. He explains that “thawing of the permafrost makes this loose material available” which rolls down the slopes in case of precipitation. The disaster that occurred in the hamlet of La Bérarde resulted in “probably from aa conjunction of several phenomena in cascade”, whose origin, in the heights of the Ecrins massif, “implicates climate change, but it is not yet clear to what extent,” estimates Serge Taboulot.
Because The link between climate change and landslides becomes increasingly fragile as we go down in altitude, which rules out de facto thawing permafrost as a trigger. “For other gravitational phenomena, other factors will take precedence, such as rock fatigue”, explains Ludovic Ravanel, specialist in the evolution of mid and high mountain environments in the face of climate changes.
Thus, the landslide of summer 2023 in Maurienne (Savoie), which is still paralyzing the railway line that connects France to Italy, is not attributed to climate change. Nor is the fall of 20 million tons of rocks on a road in Vercors in July. Rather than the effect of temperature or any weather event, Serge Taboulot points out in the latter case “the well-known fragility of the cliff in question”even though the area had just been hit by heavy rains.
Less studied than earthquakes or floods, landslides remain a little-known natural risk. To uncover their secrets, Jean-Philippe Malet, a geomorphologist at the CNRS, is working on developing a database on these disasters. “Ground movements are multifactorial events, he notes, but often one of those factors is water.” In this case, the rain. “It could be a relatively light rain, such as light rains over the long term which will saturate the soil and trigger ground movement. Where, conversely, a mega-storm, or a typhoon, with an enormous quantity of water falling in a few hours, list the researcher. It can also be the alternation with periods of drought, etc.”
However, rising temperatures caused by our greenhouse gas emissions are leading to more extreme rainfall and an increase in rainfall variability (alternations between droughts and floods). However, “We cannot say today that global warming is responsible for an increase in the number of gravitational phenomena,” specifies Jean-Philippe Malet, and this is due to a lack of sufficient data. Ludovic Ravanel recalls that “The hypothesis that changing precipitation systems can destabilize terrain remains poorly demonstrated.”
On the other hand, these new extremes are at least indirectly involved in these events. “When it comes to extreme precipitation events, clearly climate change makes them possible,” points out Serge Taboulot. “Almost all the time, the“Major disasters are linked to intensities and total quantities of precipitation that were unimaginable outside the context of climate change,” he notes. Thus, according to the group of experts who evaluate the link between extreme weather events and climate, the World Weather Attribution, Events such as Typhoon Gaemi, which caused 45 landslides in July in the Philippine islands alone, or the monsoon rains that ravaged the Indian state of Kerala would not have happened if human activities had not caused greenhouse gas emissions to explode.
To cite an example closer to home, the engineer mentions Storm Alex, which devastated the Tinée and Vésubie valleys in 2020. When, he says, “the unimaginable has happened.” Faced with uncertainties regarding the future role of extreme weather on an ecosystem as fragile and powerful as the mountain, Serge Taboulot notes that “We are not ready, especially because what we call ‘fifty-year and centennial precipitation’ is still based on the criteria of the last century. However, they occur every ten years in the Alps, or even every five years. Today’s centennial precipitation, in our current climate, quickly becomes a catastrophe.”
Since the 19th century, the average temperature of the Earth has warmed by 1.1°C. Scientists have established with certainty that this increase is due to human activities, consumers of fossil fuels (coal, oil and gas). This warming, unprecedented in its speed, threatens the future of our societies and biodiversity. But solutions – renewable energies, moderation, reduction of meat consumption – exist. Discover our answers to your questions on the climate crisis.