Optical images reveal the role of high temperatures in triggering the 2021 Chamoli landslide

Abstract. The 2021 Chamoli ice-rock landslide formed a landslide-flood hazard chain by claiming 200 lives and destroying two hydroelectric power plants. The reason for this landslide has been elusive due to difficulty in retrieving reliable deformation time series before the landslide. Here we proposed a histogram-based method to reconcile deformation inconsistencies measured in different optical sensors. We find the Chamoli ice-rock deformed >80 m from 2013 to 2021 with exceptionally high deformations in summers of 2017/18, which were related to high summer temperatures. Final collapse in February 2021 is also related to high temperature. Rising temperatures weakened shear strength of the ice on the sliding plane triggering the Chamoli landslide to move. With climate warming, more similar, hard to predict ice-rock landslides in deglaciating high mountains are inevitable, posing new challenges to local communities and beyond. Optical remote sensing images provide an indispensable data in deciphering early precursors of similar hazards.