Three things to know about the altitude of Mont-Blanc, which has lost more than three meters since 2010

It has regained its historic altitude. Mont-Blanc, the highest peak in Western Europe, was measured in mid-September at 4,807.81 meters, announced a team of surveyors on Wednesday, September 29. This is almost a meter less compared to the measurement carried out in 2017, and more than three meters compared to 2010, according to scientists. How to explain this “shrinkage”? Franceinfo takes stock.

The altitude of Mont-Blanc “oscillates continuously”

This altitude of 4,807.81 meters was recorded during a three-day mission led by the departmental chamber of surveyors of Haute-Savoie, in mid-September. Since 2001, its experts have visited the roof of Europe every two years to carry out regular surveys using sensors linked to satellites and “model the ice cap”. The objective is “to constitute and maintain a precise and reliable data bank. These can be used by experts and above all transmitted to future generations”, explained Wednesday two surveyors during a press point in Saint-Gervais-Les-Bains (Haute-Savoie).

This new measure ultimately corresponds to “the altitude that we found in our books [de géographie] at school” and which has been learned by heart by generations of French schoolchildren, Jean des Garets and Denis Borel joked again. A measure which itself dates back to the end of the 19th century.

But in reality, “since the dawn of time, the altitude of Mont-Blanc has fluctuated continuously”, they stressed. The last measurement made public, in 2017, reported an altitude of 4,808.72 meters, itself down compared to those of previous years. That of 2019, “exceptionally low”, at only 4,806.03 meters, had not been made public. The surveyors had in fact preferred “wait for the 2021 measure for more educational and scientific explanations”.

The highest altitude (4 810.90 meters) was recorded in 2010, but was very close to that of 2001, according to figures taken by BFMTV (4,810.40 meters). Overall, the figures show a decrease in the height of the top of a “mean” 13 centimeters per year since 2001, note the surveyors.

Variations linked to precipitation and wind

If these numbers vary from time to time, it is because the top is “covered with a layer of ‘eternal snow’ which functions like a huge snowdrift”. This ice and snow cap measures approximately 15 meters and “varies according to altitude winds and precipitation”. The stronger the precipitation and the weaker the wind, the more snow accumulates at altitude “, detailed Sciences and the Future in 2013.

The summit is thus higher at the end of the summer because the massif is a “dune complex” where the wind, more violent in winter, planes the snow more than in summer. “Perhaps the Mont-Blanc varies more in size during the same year than from year to year at the same period”, underlined the surveyor-expert Philippe Borrel, interviewed by the specialized magazine.

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For their part, the specialists carry out the measurement in “removing the last layer of fresh snow on the summit, about 40 to 50 centimeters”, for “measure the summit point of the ice”, explains Jean des Garets to France 3 Auvergne-Rhône-Alpes. The 2021 expedition benefited from very favorable weather conditions, which made it possible to stay three hours at the top and to “pick up in every nook and cranny” the summit cap. Enough to “reach a level of precision never achieved before”, welcomed the surveyors.

The top “rocky” culminates, for its part, “at 4,792 meters”, emphasize the scientists. It continues to rise from one to three millimeters per year, “due to the continuous tectonic uplift of the massif”, according to LCI.

The impact of climate change is difficult to assess

If the surveyors have established an average drop in the altitude of Mont-Blanc since 2001, Denis Borel believes that we should not “to draw a hasty conclusion on measurements which have been carried out only since the years 2001 with the precision which one shows you today”. “We measure, we observe (…). We are there as sentinels of the environment”, he added. It is now up to “climatologists, glaciologists and other scientists to exploit all the data collected and to advance all the hypotheses to explain this phenomenon”.

Difficult, at this stage, to link this “shrinkage” to the climate crisis. At this altitude, global warming does not affect the size of the summit which depends only on snowfall and wind “, assures Ludovic Ravanel, geomorphologist at the CNRS, interviewed by The Parisian. He admits, however, that the rise in temperatures could, in the long term, have an impact on the melting of the summit snow during the summer.

Scientists have already established that the Alpine Arc is particularly affected by the climate crisis. One of its most iconic glaciers, the Mer de Glace, has retreated about two kilometers since 1850. The glacier, which is in the Mont-Blanc massif, has also lost 120 meters in thickness over the course of the last century.

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