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

Abstract

The recent publication by Meteo-France of the results of the 30 years’ (1961-90) radio measurements of the temperature at Lyon Airport, and the functioning, since 1994, of the meteorological station at the Aiguille du Midi at a height of 3,842 m on Mont Blanc, gave the possibility to restart the studies already undertaken in 1975 on the relationship between the height of the 0°C isotherm and the variations of the Mont Blanc glaciers. Comparing the average monthly temperatures in Chamonix 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 definition of the elevations of the 0°C isotherm during the summer months, which are very close to those calculated through the temperature radio measurements. Applying the gradients calculated in the period 1994-1998 to the data of the Chamonix station since 1934, I obtained, for each 5-year period, the variations in the height reached in the summer by the 0°C isotherm. 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 occurred since 1939. In this way, I could establish that the expansion phases of 1939-1945 and 1954-1985 were preceded and partially accompanied by periods in which the 0°C isotherm in June and September was reaching 3,400 m, and in July was slightly above 3,600 m. During the same period the snowline fell even below 2,700 m. In this situation, on the Italian Mont Blanc glaciers, the glaciers accumulation basin reaches an area corresponding to more than 62% of the glaciers’ surface. During the glaciers’ retreat between 1946 and 1955, and then again since 1985, the average summer height of the 0°C isotherm was above 3,500 m. The snowline withdrew to about 2,900 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 represents merely 40% of the glacier. This substantially accounts for the current phase of pronounced linear and volumetric contraction of the Mont Blanc glaciers.