No abstract

No abstract

On 7 Feb 2021, a catastrophic mass flow descended the Ronti Gad, Rishiganga, and Dhauliganga valleys in Chamoli, Uttarakhand, India, causing widespread devastation and severely damaging two hydropower projects. Over 200 people were killed or are missing. Our analysis of satellite imagery, seismic records, numerical model results, and eyewitness videos reveals that ~27x106 m3 of rock and glacier ice collapsed from the steep north face of Ronti Peak. The rock and ice avalanche rapidly transformed into an extraordinarily large and mobile debris flow that transported boulders >20 m in diameter, and scoured the valley walls up to 220 m above the valley floor. The intersection of the hazard cascade with downvalley infrastructure resulted in a disaster, which highlights key questions about adequate monitoring and sustainable development in the Himalaya as well as other remote, high-mountain environments.

Heterogeneous glacier mass loss has occurred across High Mountain Asia on a multi-decadal timescale. Contrasting climatic settings influence glacier behaviour at the regional scale, but high intra-regional variability in mass loss rates points to factors capable of amplifying glacier recession in addition to climatic change along the Himalaya. Here we examine the influence of surface debris cover and glacial lakes on glacier mass loss across the Himalaya since the 1970s. We find no substantial difference in the mass loss of debris-covered and clean-ice glaciers over our study period, but substantially more negative (−0.13 to −0.29 m w.e.a −1) mass balances for lake-terminating glaciers, in comparison to land-terminating glaciers, with the largest differences occurring after 2000. Despite representing a minor portion of the total glacier population (~10%), the recession of lake-terminating glaciers accounted for up to 32% of mass loss in different sub-regions. The continued expansion of established glacial lakes, and the preconditioning of land-terminating glaciers for new lake development increases the likelihood of enhanced ice mass loss from the region in coming decades; a scenario not currently considered in regional ice mass loss projections. Glacier mass loss has occurred across large parts of High Mountain Asia over at least the last four decades 1-4 , although substantial spatial variability has been documented in the magnitude of glacier mass loss in the region. Glaciers in the Karakoram, Kunlun Shan and eastern Pamir have maintained mass balance to the present day 3,5-7 , whereas glaciers located in the Himalaya, in the Tien Shan and Nyainqentanghla have experienced substantial mass loss in recent decades 2,6. The disparity in regional mass loss rates has been attributed to the diminished sensitivity to warming of glaciers in the Karakoram, Kunlun Shan and eastern Pamir due to their accumulation of snowfall in winter months, rather than during the summer monsoon along the Himalaya 8. However, large intra-regional variability in glacier mass loss is evident along the Himalayan arc 6,9 , which suggests factors exist that are capable of exacerbating glacier recession in addition to climatic change here. Glaciers situated in the Himalaya commonly have extensive debris cover 10 , and an increasing number terminate into a glacial lake 11. A continuous debris mantle thicker than a few centimetres dampens sub-debris ablation rates 12. Modelling studies have shown how debris cover enables the persistence of greater glacier area in comparison with clean-ice in a changing climate 13,14. However, comparable thinning rates have been observed for clean-ice and debris-covered glaciers at similar elevations 15-17 at several locations in the Himalaya. Glacial lakes amplify ice loss from their host glaciers through mechanical calving and subaqueous melt 18,19. There are currently more than 700 proglacial lakes in the Himalaya 11,20 , which are all capable of directly influencing the behaviour of their host glacier. Progla...

Glaciers in the Karakoram exhibit irregular behavior. Terminus fluctuations of individual glaciers lack consistency and, unlike other parts of the Himalaya, total ice mass remained stable or slightly increased since the 1970s. These seeming anomalies are addressed through a comprehensive mapping of surge-type glaciers and surge-related impacts, based on satellite images (Landsat and ASTER), ground observations, and archival material since the 1840s. Some 221 surge-type and surge-like glaciers are identified in six main classes. Their basins cover 7,734 ± 271 km2 or ~43% of the total Karakoram glacierised area. Active phases range from some months to over 15 years. Surge intervals are identified for 27 glaciers with two or more surges, including 9 not previously reported. Mini-surges and kinematic waves are documented and surface diagnostic features indicative of surging. Surge cycle timing, intervals and mass transfers are unique to each glacier and largely out-of-phase with climate. A broad class of surge-modified ice introduces indirect and post-surge effects that further complicate tracking of climate responses. Mass balance in surge-type and surge-modified glaciers differs from conventional, climate-sensitive profiles. New approaches are required to account for such differing responses of individual glaciers, and effectively project the fate of Karakoram ice during a warming climate.