The joint driving effects of climate and weather changes caused the Chamoli glacier-rock avalanche in the high altitudes of the India Himalaya

Ice avalanches are one of the most devastating mountain hazards, and can pose a great risk to the security of the surrounding area. Although ice avalanches have been widely observed in mountainous regions around the world, only a few ice avalanche events have been studied comprehensively, due to the lack of available data. In this study, in response to the recent catastrophic rock-ice avalanche (7 February 2021) at Chamoli in the India Himalaya, we used high-resolution satellite images and found that this event was actually a glacier-rock landslide, where the collapse of the rock-ice body was caused by the sliding of the bedrock beneath the glacier, for which the source area and volume loss were about 2.89×10 5 m 2 and 2.46×10 7 m 3 , respectively, corresponding to an average elevation change of about -85 m. Furthermore, visual analysis of the dense time-series satellite images shows that the overall downward sliding of the collapsed rock-ice body initiated around the summer of 2017, and thereafter exhibited clear seasonality (mainly in summer). Meteorological analysis reveals a strong rainfall anomaly in the initiation period of the sliding and a remarkable winter warming anomaly in the 40 days before the collapse. Comparisons of multi-temporal digital elevation models (DEMs) further suggest that the glacier geometry in the collapsed areas was likely changing (i.e., accelerated surface thinning in the lower part of the glaciers and insignificant change in the upper part), which is consistent with the region-wide climate warming. Finally, by combining the above findings and a geomorphic analysis, we conclude that the rock-ice avalanche event was mainly caused by the joint effects of climate and weather changes acting on a steeply sloping and fracture-prone geological condition. The findings of this study provide new and valuable evidence for the study of slope/glacier instability at high altitudes. This study also highlights that, for the Himalaya and other high mountain ranges, there is an urgent need to identify the glaciers that have a high risk of ice avalanches.