Assessing the impact of climate change on snow avalanche activity in France over the last 60 winters using hierarchical Bayesian spatio-temporal change point models

Snow avalanches are mainly ruled by temperature fluctuations, heavy precipitations and wind regimes, so that climate change is likely to modify the frequency and magnitude of both ordinary and extreme events. However, reference scenarios and return periods for avalanche hazard management are always computed in the engineering practiced under the assumption of a stationary process. On a more phenomenological point of view, contrary to other phenomena such as tropical storms, snow avalanches are very rarely used as proxy indicators that point out signals of climate change. This study focuses on avalanche occurrences and runout altitudes in France over the last six decades. For both variables, a hierarchical spatio-temporal modelling framework is proposed to quantify the interannual fluctuations possibly resulting from climate change. First, the regional annual component is isolated from the total variability using a nonlinear analysis of variance. Second, the latent structured time trend is distinguished from the random noise with different time series shifting level sub-models. The hierarchical structure obtained takes into account the uncertainty related to the estimation of the annual component for the quantification of the time trend. Bayesian inference is performed using Markov Chain Monte Carlo simulations. Avalanche occurrences are studied in the northern French Alps. No systematic modifications in occurrence regime could be found over the last 60 years. This suggests that climate change has recently had little impact on the avalanching rhythm in France. Significant temporal patterns have though occurred. They consist in complex combination of abrupt changes and pseudo-periodic cycles of approximately 15 years. Avalanche runout altitudes are studied in the whole French territory. A change in runout altitude regime has occurred in France around 1976. Between 1946 and 1976, a decrease of 55 m has affected the mean runout altitude, but the probability of a high magnitude event has remained constant. After the change point, the mean runout altitude has regained its initial state, whereas the probability of a high magnitude avalanche has been divided by two. A retreat of avalanche is therefore engaged in France since nearly 30 years. This especially concerns high magnitude events, since the return period associated with an avalanche reaching its minimal altitude on a mean path has increased from 20 to 40 years over the last 30 years. Avalanche occurrences and runout altitudes are therefore differently influenced by changes in constraining climatic factors, so that the interest of a joint temporal modelling of the two phenomena would be limited. One possible explanation is that dry snow avalanches are progressively replaced by wet snow avalanches because of climate worming, thus keeping constant the number of events, but reducing their magnitude by modifying snow rheology. To confirm this statement, further research is needed to compare and explicitly correlate the obtained annual effects with climatic data such as precipitation and temperature series. This will allow improving our knowledge on climate change in the alpine space and its consequence on avalanche hazard.