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Supplements of Geografia Fisica e Dinamica Quaternaria
Volume V - 2001


Proceedings of the 8th Italian Glaciological Meeting


Le oscillazioni della quota dell’isoterma 0 °C e le variazioni dei ghiacciai del Monte Bianco – The elevation of the isotherm 0 °C and the changes in the glaciers of Mont Blanc

Pages 29-39


The recent publication by Meteo-France of the results of the 30 years’ (1961-90) radio m,easurements of the temp eratu re at Lyon Airport , and the func tioning, since 1994, of the meteorological station at the Aiguille du Mid i at a height of 3842 m. on Mont Blanc , gave the possibility to restart the studies already undert aken in 1975 on the relationship between the height of the O°C Isotherm and the variations of the Mont Blanc glaciers. Compa ring the average monthl y temp eratur es in Chamo nix and Aiguille du Midi , a mountain that peaks over the town, it was possible to calculate the temperature lapse rate between the two stations. This allowed the definit ion of the elevations of the 0° C isoth erm during the summer months, which are very dose to those calculated throu gh the temperature radio measurement s. App lying the gradient s calculated in the per iod 1994-1998 to the data of the Chamonix station since 1934, I obtained, for each 5-year per iod , the va iati~ns in the height reached in the summer by the O°C isothe rm. The snowline is set on average 700 m below such height. I then correlated the data of each five-year period with the four glacial phases that occ urred since 1939. In this way, I could establish th at the expansion ph ases of 1939-1945 and 1954-1985 were preceded and parti ally accompanied by period s in which the 0 °C isothe rm in June and Septembe r was reaching 3400 m, and in Jul y was slightly above 3600 m. During the same period the snowline fell even below the 2700 m. In this situ ation, on the Italian Mont Blanc glaciers, the glaciers accumul ation ba sin reach es an area corresponding to mor e than 62% of the glaciers surface. During the glaciers ret reat between 1946 and 1955, and then again since 1985, the average summer height of the O°C isoth erm was above 3500 m. Th e snowline withdrew to abo ut 2900 m. Consequently, on the Italian side of Mont Blanc, the glaciers accumulation basin decreased to an area of less than 50% of the glaciers surface. Since 1991 such zone rep resents merely 40% of the glacier. Th is substa nt ially accounts for the current ph ase of pro nounce d linear and volumetric cont raction of the Mont Blanc glaciers.

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