Abstract

Debris-covered glaciers in the central Himalaya have now experienced several decades of sustained ice loss, manifested predominantly in glacier surface lowering. In particular, glacier surfaces of low longitudinal gradient and low ice surface velocity have developed locally complex surface topographies and undergone profound changes in supraglacial hydrology. In this study we examine the development of complex ice surface topography across six debris-covered glaciers in the Everest region over the last four decades via a new metric of glacier surface relief applied to Digital Elevation Models (DEMs). We focus in on Khumbu Glacier, and use fine spatial and temporal resolution DEMs covering a period of 28 months to quantify the contemporary contribution of ice cliff and supraglacial pond expansion to overall mass loss from stagnant areas of ice. On the broader scale, we find three common long-term changes in glacier surface topography, (1) glacier-wide expansion of high relief topography in response to ice cliff and supraglacial pond network evolution, (2) up-glacier expansion of high local relief zones that may be caused by differential sub-debris melt beneath thin debris, and (3) increase in glacier surface relief proximal to glacier termini caused by supraglacial stream incision where linked proglacial-supraglacial hydrological networks exist. Overall, we contend that these topographic measurements will be important for understanding glacier surface water storage and also the energy balance of a debris-covered glacier surface, both of which could exacerbate future ice loss and downstream meltwater supply.

Highlights

  • The Himalaya contains the greatest terrestrial ice mass outside the Polar Regions, comprising more than 18,000 glaciers (RGI Consortium, 2017)

  • We developed a new surface metric that examines the local variability in glacier surface topography to quantify the impact of long-term ice loss on glacier surface morphometry in the region

  • The most spatially extensive changes that are evident in our data are, the 1) glacier-wide increase of high relief zones in association with ice cliff and supraglacial pond network expansion, 2) up-glacier expansion of glacier surface topography of high local relief that may be associated with differential melt beneath expanding but thin surface debris, and 3) increase in glacier surface relief proximal to the glacier terminus, possibly due to supraglacial meltwater channel incision where a linked proglacial-supraglacial hydrological network exists

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Summary

Introduction

The Himalaya contains the greatest terrestrial ice mass outside the Polar Regions, comprising more than 18,000 glaciers (RGI Consortium, 2017). There is much spatial variability in the current mass loss rate of Himalayan glaciers, with local climate (Mukherjee et al, 2018), glacier terminus type (Brun et al, 2019; King et al, 2019) and glacier surface debris cover (Brun et al, 2019; King et al, 2019) all influencing ice loss rates. The presence of debris cover complicates the response of a glacier to climate change (Rowan et al, 2015; Anderson and Anderson, 2016) because it influences the downward flux of energy that is available for melt at the glacier surface. Many debris-covered glaciers persist at relatively low altitudes no longer occupied by clean ice glaciers (Rowan et al, 2015)

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