Isolated villages, blocked roads, delayed planes ... The flakes that have fallen in abundance these last days have struck the spirits. The snows of yesteryear were back! The climatic machine, which was thought to be broken by the madness of men, had resumed its ancestral course. Nature was finally the strongest. It was heard at this little noise forgotten in our cities: the squealing of snow under the soles.
The 7 cm of snow measured on the 23 February in Paris-Montsouris and Orly, 5 cm in Saint-Brieuc, 10 cm in the Calvados, 15 cm in the Channel, or even 20 cm in Bocognano (Corsica), are however few compared to 40 cm of white powder fallen 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 home, in January 2003, 15 cm of crystals had settled on Finistere, Aquitaine, Provence and Corsica.
The recent snowfall "is not exceptional," says Pierre Bessemoulin, director of climatology at Météo France. "Going back in the annals until the post-war, there are about fifteen snowy episodes remarkable by their intensity and duration," he recalls.
The 8 days when the snow appeared in Paris between the January 1er and the 20 February 2005 are far from the record of 24 days established over 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 the snow cover is very high," observes Pierre Etchevers, director of the Snow Study Center (CEN) in Grenoble. This one has a continuous series of measurements carried out since 1960 at the Col de Porte, 1 320 meters of altitude, in the solid mass of Chartreuse. It reveals an alternation of winters strongly or on the contrary weakly snow-covered, whose succession seems purely random.
However, there is an overall downward trend. In forty years, the snow depth at the Porte pass, measured over the last ten days of February, has decreased by more than a third, from 1,5 m to less than 1 meters.
By rotating models of evolution of the snowpack according to meteorological parameters, the 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, the snow level remained stationary until the end of the 1990 years, then a marked decrease appears, describes Pierre Etchevers. In the Southern Alps, the most marked decrease dates from the 1960 years, then the 1980 years. "
This rarefaction of the 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 Porte pass, the average winter temperature has risen by 2 0C in forty years.
What will happen in the coming decades? Does global warming announce the disappearance of winter snows? To find out, researchers resumed their models and applied them to 34 massive Alps and Pyrenees, assuming a rise in 2 0C of air temperature. Their calculations predict two different behaviors of the snowpack according to the altitude. Above a line between 2 000 and 2 500 m, the effect of warming would be low in winter, but spring melt would be earlier and faster.
In mid-mountain, however, the heat stroke would have a significant impact. Towards 1 500 m, the white season would be shortened by at least a month and the layer of snow would melt away.