Reversed Surface-Mass-Balance Gradients on Himalayan Debris-Covered Glaciers Inferred from Remote Sensing

Rosie R Bisset,Matthias Huss,Robert G Bingham,Amaury Dehecq,Noel Gourmelen,Daniel N Goldberg

doi:10.3390/rs12101563

Abstract

Meltwater from the glaciers in High Mountain Asia plays a critical role in water availability and food security in central and southern Asia. However, observations of glacier ablation and accumulation rates are limited in spatial and temporal scale due to the challenges that are associated with fieldwork at the remote, high-altitude settings of these glaciers. Here, using a remote-sensing-based mass-continuity approach, we compute regional-scale surface mass balance of glaciers in five key regions across High Mountain Asia. After accounting for the role of ice flow, we find distinctively different altitudinal surface-mass-balance gradients between heavily debris-covered and relatively debris-free areas. In the region surrounding Mount Everest, where debris coverage is the most extensive, our results show a reversed mean surface-mass-balance gradient of −0.21 ± 0.18 m w.e. a−1 (100 m)−1 on the low-elevation portions of glaciers, switching to a positive mean gradient of 1.21 ± 0.41 m w.e. a−1 (100 m)−1 above an average elevation of 5520 ± 50 m. Meanwhile, in West Nepal, where the debris coverage is minimal, we find a continuously positive mean gradient of 1.18 ± 0.40 m w.e. a−1 (100 m)−1. Equilibrium line altitude estimates, which are derived from our surface-mass-balance gradients, display a strong regional gradient, increasing from northwest (4490 ± 140 m) to southeast (5690 ± 130 m). Overall, our findings emphasise the importance of separating signals of surface mass balance and ice dynamics, in order to constrain better their contribution towards the ice thinning that is being observed across High Mountain Asia.

Highlights

The glaciers in High Mountain Asia collectively form the largest glaciated area outside the polar regions, covering an estimated ~118,264 km2 [1]
Many of the glaciers in High Mountain Asia are characterised by supraglacial debris cover of varying thickness and extent, which plays an important role in modifying SMB through its impact on glacier ablation rates
We have presented an approach for producing spatially distributed estimates of glacial surface mass balance from remote-sensing observations, based on the principle of mass continuity

Summary

Introduction

The glaciers in High Mountain Asia collectively form the largest glaciated area outside the polar regions, covering an estimated ~118,264 km2 [1]. Quantifying and improving our understanding of glacier surface-mass-balance distribution across High Mountain Asia is critical in the effective prediction and mitigation of these impacts. Many of the glaciers in High Mountain Asia are characterised by supraglacial debris cover of varying thickness and extent, which plays an important role in modifying SMB through its impact on glacier ablation rates. Field-based studies have shown that a thin layer of supraglacial debris, less than a critical thickness of ~3–8 cm, enhances glacier melt rates (e.g., [6]) through a reduction in the ice-surface albedo, as first demonstrated experimentally [7], and more recently constrained from surface energy balance modelling (e.g., [8,9]). The ablation rates on heavily debris-covered glaciers are extremely difficult to measure, due to the challenges that are associated with drilling stakes through the debris layer, as well as the large heterogeneity of local ablation rates (e.g., [13,18])

Methods

Results

Conclusion