Impacts of the 2003 and 2015 summer heatwaves on permafrost-affected rock-walls in the Mont Blanc massif.

Rockfall is one of the main geomorphological processes that affects the evolution and stability of rock-walls. At high elevations, rockfall is largely climate-driven, very probably because of the warming of rock-wall permafrost. So with the ongoing global warming that drives the degradation of permafrost, the related hazards for people and infrastructure could continue to increase. The heatwave of summer 2015, which affected Western Europe from the end of June to August, had a serious impact on the stability of high-altitude rock-walls, including those in the Mont Blanc massif. A network of observers allowed us to survey the frequency and intensity of rock-wall morphodynamics in 2015, and to verify its relationship with permafrost. These observations were compared with those of the 2003 summer heatwave, identified and quantified by remote sensing. A comparison between the two years shows a fairly similar rockfall pattern in respect of total volumes and high frequencies (about 160 rockfalls >100m) but the total volume for 2003 is higher than the 2015 one (about 300,000m and 170,000m respectively). In both cases, rockfalls were numerous but with a low magnitude and occurred in permafrost-affected areas. This suggests a sudden and remarkable deepening of the active layer during these two summers, rather than a longer-term warming of the permafrost body.