Step-change in supraglacial pond area on Tshojo Glacier, Bhutan, and potential downstream inundation patterns due to pond drainage events.

Climate change is causing Himalayan glaciers to shrink rapidly and natural hazards to increase, whilst downstream growth in population, infrastructure and agriculture is increasing vulnerability. Glacier shrinkage promotes the formation of glacial lakes, which act both as a resource and a hazard, via glacier lake outburst floods (GLOFs). GLOFs can rapidly generate large volumes of water and sediment, which threaten life, property, and infrastructure downstream, and Bhutan is one of the most vulnerable countries globally to these hazards. Almost all documented GLOF casualties in High Mountain Asia have been from ice-surface ponds or moraine dammed lakes, with supraglacial ponds often being responsible for recurrent GLOFs. Here we use remotely sensed imagery to quantify changes in supraglacial water storage on Tshojo Glacier, Bhutan, where previous supraglacial pond drainage events have necessitated downstream evacuation. Results showed that both total ponded area and its interannual variability doubled between 1987-2003 and 2007-2020, which was predominantly driven by large increases in the areas of the biggest ponds. These ponds drained regularly and have occupied the same location since at least 1967. Numerical modelling results show that pond outburst floods can reach between ~6 and 65 km downstream, impacting Lunana and Wolathang under the worst-case scenario.