Impact of climate warming on avalanche activity in French Alps and increase of proportion of wet snow avalanches

摘要

The cross-analysis of two well documented databases, EPA snow avalanche chronicles and the reanalysis of snow mantle, allows quantifying the changes in avalanche activity as a response to climate warming. In the French Alps and even if the signal is affected by a significant variability, it is shown that, on average over the past thirty years, the run-out altitudes increased and the frequency and the proportion of powder avalanches decreased. In Chamonix valley a substantial increase of the proportion of wet snow avalanches since the seventies has been shown, concomitant to the local climate warming. The reanalysis of a large number of avalanches that occurred in Chamonix valley between 1958 and 2010 allowed drawing up significant correlations between the basal friction coefficient and the snow properties. On average, the friction coefficient increases with snow temperature. This trend is coherent with the run-out altitude increase observed over the French Alps since 1980. However the friction coefficient drops to small values when snow water content exceeds a certain threshold, hence very wet snow avalanches have large extensions. In addition, because their density and height are high and the velocity low, the pressure they develop is higher than expected notably in the run-out zone. The expected consequences on the avalanche hazard of these proved trends is presented together with the projections concerning amount and quality of snow for the next century as well as expected altitudinal and seasonal shifts in avalanche activity.