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...

Abstract. Region-wide averaging of Himalayan glacier mass change has masked any catchment or glacier-scale variability in glacier recession; thus the role of a number of glaciological processes in glacier wastage remains poorly understood. In this study, we quantify mass loss rates over the period 2000–2015 for 32 glaciers across the Everest region and assess how future ice loss is likely to differ depending on glacier hypsometry. The mean mass balance of all 32 glaciers in our sample was −0.52 ± 0.22 m water equivalent (w.e.) a−1. The mean mass balance of nine lacustrine-terminating glaciers (−0.70 ± 0.26 m w.e. a−1) was 32 % more negative than land-terminating, debris-covered glaciers (−0.53 ± 0.21 m w.e. a−1). The mass balance of lacustrine-terminating glaciers is highly variable (−0.45 ± 0.13 to −0.91 ± 0.22 m w.e. a−1), perhaps reflecting glacial lakes at different stages of development. To assess the importance of hypsometry on glacier response to future temperature increases, we calculated current (Dudh Koshi – 0.41, Tama Koshi – 0.43, Pumqu – 0.37) and prospective future glacier accumulation area Ratios (AARs). IPCC AR5 RCP 4.5 warming (0.9–2.3 °C by 2100) could reduce AARs to 0.29 or 0.08 in the Tama Koshi catchment, 0.27 or 0.17 in the Dudh Koshi catchment and 0.29 or 0.18 in the Pumqu catchment. Our results suggest that glacial lake expansion across the Himalayas could expedite ice mass loss and the prediction of future contributions of glacial meltwater to river flow will be complicated by spatially variable glacier responses to climate change.

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Debris-covered glaciers are ubiquitous in the Himalaya, and supraglacial debris significantly alters how glaciers respond to climate forcing. Estimating debris thickness at the glacier scale, however, remains a challenge. This study inverts a subdebris melt model to estimate debris thickness for three glaciers in the Everest region from digital elevation model difference-derived elevation change. Flux divergences are estimated from ice thickness and surface velocity data. Monte Carlo simulations are used to incorporate the uncertainties associated with debris properties, flux divergence, and elevation change. On Ngozumpa Glacier, surface lowering data from 2010 to 2012 and 2012 to 2014 are used to calibrate and validate the method, respectively. The debris thickness estimates are consistent with existing in situ measurements. The method performs well over both actively flowing and stagnant parts of the glacier and is able to accurately estimate thicker debris (>0.5 m). Uncertainties associated with the thermal conductivity and elevation change contribute the most to uncertainties of the debris thickness estimates. The surface lowering associated with ice cliffs and supraglacial ponds was found to significantly reduce debris thickness, especially for thicker debris. The method is also applied to Khumbu and Imja-Lhotse Shar Glaciers to highlight its potential for regional application.Plain Language Summary Debris-covered glaciers are ubiquitous in the Himalaya, and this debris significantly alters the evolution of these glaciers. Estimating the thickness of debris on these glaciers, however, remains a challenge. This study develops a novel method for estimating the debris thickness on three glaciers in the Everest region of Nepal based on digital elevation models, surface velocity data, ice thickness estimates, and a debris-covered glacier energy balance model. The method was calibrated and validated on Ngozumpa Glacier, one of the largest debris-covered glaciers in Nepal, and was found to accurately estimate debris thickness. Specifically, this method was able to estimate thick debris (>0.5 m), which has been a major limitation of previous studies. This is important because thick debris significantly reduces glacier melt rates by insulating the underlying ice. This study creates a step-change in our ability to model the past, present, and future evolution of debris-covered glaciers.