Response of debris-covered glaciers in the Mount Everest region to recent warming, and implications for outburst flood hazards

Abstract

In areas of high relief, many glaciers have extensive covers of supraglacial debris in their ablation zones, which alters both rates and spatial patterns of melting, with important consequences for glacier response to climate change. Wastage of debris-covered glaciers can be associated with the formation of large moraine-dammed lakes, posing risk of glacier lake outburst floods (GLOFs). In this paper, we use observations of glaciers in the Mount Everest region to present an integrated view of debris-covered glacier response to climate change, which helps provide a long-term perspective on evolving GLOF risks. In recent decades, debris-covered glaciers in the Everest region have been losing mass at a mean rate of 0.32 m yr¹, although in most cases there has been little or no change in terminus position. Mass loss occurs by 4 main processes: (1) melting of clean ice close to glacier ELAs; (2) melting beneath surface debris; (3) melting of ice cliffs and calving around the margins of supraglacial ponds; and (4) calving into deep proglacial lakes. Modelling of processes (1) and (2) shows that Everest-region glaciers typically have an inverted ablation gradient in their lower reaches, due to the effects of a down-glacier increase in debris thickness. Mass loss is therefore focused in the mid parts of glacier ablation zones, causing localised surface lowering and a reduction in downglacier surface gradient, which in turn reduce driving stress and glacier velocity, so the lower ablation zones of many glaciers are now stagnant. Model results also indicate that increased summer temperatures have raised the altitude of the rain-snow transition during the summer monsoon period, reducing snow accumulation and ice flux to lower elevations. As downwasting proceeds, formerly efficient supraglacial and englacial drainage networks are broken up, and supraglacial lakes form in hollows on the glacier surface. Ablation rates around supraglacial lakes are typically one or two orders of magnitude greater than sub-debris melt rates, so extensive lake formation accelerates overall rates of ice loss. Most supraglacial lakes are 'perched' above hydrological base level, and are susceptible to drainage if they become connected to the englacial drainage system. Speleological surveys of conduits show that large englacial voids can be created by drainage of warm lake waters along pre-existing weaknesses in the ice. Roof collapses can open these voids up to the surface, and commonly provide the nuclei of new lakes. Thus, by influencing both lake drainage and formation, englacial conduits exert a strong control on surface ablation rates. An important threshold is crossed when downwasting glacier surfaces intersect the hydrological base level of the glacier. Base-level lakes formed behind intact moraine dams can grow monotonically, and in some cases can pose serious GLOF hazards. Glacier termini can evolve in different ways in response to the same climatic forcing, so that potentially hazardous lakes will form in some situations but no...

Brief communication: Observations of a glacier outburst flood from Lhotse Glacier, Everest area, Nepal

“…Dam failure would require an englacial conduit to be temporarily blocked, which could occur if meltwater refroze in the conduits over the winter (Gulley et al, 2009a) or if passage closure processes caused an englacial conduit to close (Benn et al, 2012). The former blockage scenario seems more likely since these glacier outburst floods have occurred in back-to-back years and the refreezing of meltwater is an annual process.…”

“…The rapid drainage of stored lake water through hydraulically efficient pathways is another plausible triggering mechanism that commonly occurs for supraglacial ponds in the Everest region (Benn et al, 2012). Field observations of supraglacial ponds (Fig.…”

“…During the last half-century, debris-covered glaciers in the Everest region have experienced significant mass loss (e.g., Bolch et al, 2011), which has led to the development of glacial lakes and supraglacial ponds (Benn et al, 2012). Proglacial lakes may develop if the surface gradient of the glacier is gentle (< 2 • ), while steeper gradients (> 2 • ) will help drain these ponds (Quincey et al, 2007).…”

“…This causes supraglacial ponds to have large temporal and spatial vari-ations as they frequently drain and fill (Horodyskyj, 2015;Miles et al, 2016;Watson et al, 2016). This drainage can occur on the glacier's surface and/or subsurface (Benn et al, 2012).…”

“…For debris-covered glaciers, the drainage of supraglacial ponds commonly occurs through englacial conduits, which facilitate connections to areas of lower hydraulic potential (Gulley and Benn, 2007). These englacial conduits develop on debris-covered glaciers in the Himalaya through cut-and-closure mechanisms associated with meltwater streams, the exploitation of high-permeability areas that provide alternative pathways to the impermeable glacier ice, and through hydrofracturing processes (Gulley and Benn, 2007;Benn et al, 2009; Gulley et al, 2009a, b).During the last half-century, debris-covered glaciers in the Everest region have experienced significant mass loss (e.g., Bolch et al, 2011), which has led to the development of glacial lakes and supraglacial ponds (Benn et al, 2012). Proglacial lakes may develop if the surface gradient of the glacier is gentle (< 2 • ), while steeper gradients (> 2 • ) will help drain these ponds (Quincey et al, 2007).…”