Isolated villages, blocked roads, delayed planes… The snowflakes that have fallen in abundance in recent days have struck everyone's mind. The snows of yesteryear were back! The climatic machine, which was believed to be broken by the madness of men, had resumed its ancestral course. Nature was ultimately the stronger. We could hear it with that little noise forgotten in our towns: the screeching of snow under the soles.
The 7 cm of snow measured on February 23 in Paris-Montsouris and Orly, 5 cm in Saint-Brieuc, 10 cm in Calvados, 15 cm in the Manche, or even 20 cm in Bocognano (Corsica), are however few compared to the 40 cm of white powder that fell in 1946 in Paris, 85 cm in 1954 in Perpignan, 70 cm in 1956 in Ramatuelle, 60 cm in 1969 in Belfort, 54 cm in 1971 in Saint-Etienne, 38 cm in 1985 in Nice , 50 cm in 1986 in Langres, or 22 cm in 1993 in Carcassonne. Closer to us, in January 2003, 15 cm of crystals were deposited in Finistère, Aquitaine, Provence and Corsica.
The recent snowfall "is not exceptional", underlines Pierre Bessemoulin, director of climatology at Météo France. "Going back in the annals until the post-war period, there are about fifteen snowy episodes remarkable for their intensity and duration," he recalls.
The 8 days when the snow appeared in Paris between January 1 and February 20, 2005 are far from the 24-day record established over the same period in 1963. The same goes 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 great,” observes Pierre Etchevers, director of the Grenoble Center for Snow Studies (CEN). This has a continuous series of measurements carried out since 1960 at the Col de Porte, at an altitude of 1 meters, in the Chartreuse massif. It reveals an alternation of strongly or on the contrary slightly snowy winters, the succession of which seems purely random.
However, there is an overall downward trend. In forty years, the depth of snow at the Col de Porte, measured over the last ten days of February, has decreased by more than a third, from 1,5 m to less than 1 meter.
By running models of the evolution of the snowpack as a function of meteorological parameters, Grenoble researchers have been able to reconstruct the snow cover of the Alpine massifs since the end of the 1950s. “In the Northern Alps, the level of snow cover is remained stationary until the end of the 1990s, then a marked decrease appears, describes Pierre Etchevers. In the Southern Alps, the most marked decrease dates from the 1960s, then the 1980s. "
This rarefaction of white gold is clearly correlated with the rise in temperatures which, over the same period, increased from 1 to 3 0C on the alpine reliefs. At the Col de Porte, the average winter temperature has risen by 2C in forty years.
What will happen in the coming decades? Does global warming herald the disappearance of winter snows? To find out, the researchers took their models and applied them to 34 massifs in the Alps and the Pyrenees, assuming a 2C rise in air temperature. Their calculations predict two different behaviors of the snowpack depending on the altitude. Above a line between 0 and 2 m, the effect of warming would be weak in winter, but the spring melt would be earlier and faster.
In middle mountains, on the other hand, the heat stroke would have a significant impact. Around 1 m, the white season would be shortened by at least a month and the layer of snow would melt like sorrow.