isolated villages, blocked roads, delayed planes ... The flakes fell in abundance in recent days have caught the imagination. The snows of yesteryear were back! The climate machine, which was believed to crank through the foolishness of men, had recovered his ancestral way. Nature was ultimately stronger. You could hear that little noise forgotten in our cities: the crunch of snow underfoot.
The 7 cm of snow measured the 23 February at Paris-Montsouris and Orly, 5 cm in Saint-Brieuc, 10 cm in Calvados, 15 cm in the Channel, or even 20 cm in Bocognano (Corsica), are yet few thing compared to 40 cm of white powder fell in 1946 in Paris, 85 cm 1954 in Perpignan, 70 cm 1956 in Ramatuelle, 60 cm 1969 in Belfort, 54 cm 1971 in Saint-Etienne, 38 cm 1985 in Nice , 50 1986 cm in Langres, or 22 1993 cm in Carcassonne. More recently, in January 2003, 15 cm crystals were deposited on the Finistère, Aquitaine, Provence and Corsica.
The recent snowfall "is not exceptional," said Pierre Bessemoulin, director of climatology at Météo France. "Going back in the annals until the post-war period, we find fifteen snow episodes remarkable for their intensity and duration," he recalls.
The 8 days when snow was manifested in Paris between the 1er January and February 20 2005 are far from established record 24 days in the same period in 1963. It's the same for Rennes (3 days against 10 in 1985), Lille (12 against 26 in 1963), Strasbourg (15 against 30 in 1952 and 1965), Lyon (7 against 25 in 1953) or Bordeaux (4 against 9 in 1956 and 1987).
"The interannual variability of snow cover is very high," observes Pierre Etchevers, director of the Grenoble Center for the Study of Snow (CEN). The latter has a continuous series of measurements taken from 1960 at the Col de Porte, 1 320 meters above sea level, in the Chartreuse massif. It reveals an alternation of winters strongly or on the contrary weakly snowy, whose succession seems purely random.
It emerges however in an overall downward trend. In forty years, the snow depth at the pass door, measured over the last ten days of February decreased by more than a third, from 1,5 m within 1 meter.
By turning models of evolution of the snowpack according to the meteorological parameters, Grenoble researchers have been able to reconstruct the snow cover of the alpine massifs since the end of the 1950 years. "In the Northern Alps, snow levels remained stationary until the end of the 1990 years, then a marked decrease appears," says Pierre Etchevers. In the Southern Alps, the most marked decrease dates from the 1960 years, then from the 1980 years. "
This scarcity of white gold is clearly correlated with rising temperatures over the same period rose to 1 3 0C on alpine terrain. At the pass door, the average winter temperature is so high in 2 0C in forty years.
What will it be in the coming decades? He announces global warming the disappearance of winter snows? To find out, the researchers resumed their models and have applied to 34 massifs of the Alps and Pyrenees, assuming an increase of 2 0C the air temperature. Their calculations provide two different behaviors of the snowpack by altitude. Above a line between 2 000 and 2 500 m, the warming effect is low in winter, but spring melt would be earlier and faster.
Medium mountain, however, the hot shot would have a significant impact. To 1 500 m, the white season would be shortened by at least a month and the snow would melt to a trickle.