The Role of Differential Ablation and Dynamic Detachment in Driving Accelerating Mass Loss From a Debris‐Covered Himalayan Glacier

Ann V Rowan,Owen King,David L Egholm,Duncan J Quincey,Evan S Miles,Bryn Hubbard,Josephine Hornsey,Katie E Miles

doi:10.1029/2020jf005761

Abstract

AbstractSustained mass loss from Himalayan glaciers is causing supraglacial debris to expand and thicken, with the expectation that thicker debris will suppress ablation and extend glacier longevity. However, debris‐covered glaciers are losing mass at similar rates to clean‐ice glaciers in High Mountain Asia. This rapid mass loss is attributed to the combined effects of; (a) low or reversed mass balance gradients across debris‐covered glacier tongues, (b) differential ablation processes that locally enhance ablation within the debris‐covered section of the glacier, for example, at ice cliffs and supraglacial ponds, and (c) a decrease in ice flux from the accumulation area in response to climatic warming. Adding meter‐scale spatial variations in supraglacial debris thickness to an ice‐flow model of Khumbu Glacier, Nepal, increased mass loss by 47% relative to simulations assuming a continuous debris layer over a 31‐year period (1984–2015 CE) but overestimated the reduction in ice flux. Therefore, we investigated if simulating the effects of dynamic detachment of the upper active glacier from the debris‐covered tongue would give a better representation of glacier behavior, as suggested by observations of change in glacier dynamics and structure indicating that this process occurred during the last 100 years. Observed glacier change was reproduced more reliably in simulations of the active, rather than entire, glacier extent, indicating that Khumbu Glacier has passed a dynamic tipping point by dynamically detaching from the heavily debris‐covered tongue that contains 20% of the former ice volume.

Highlights

Supraglacial rock debris is present on 7% of the global mountain glacier area, dramatically affecting the sensitivity of these glaciers to climate change (Herreid & Pellicciotti, 2020)
A paradox exists in the hypothesis that such high rates of mass loss from debris-covered glaciers is the result of enhanced climatic forcing at the glacier surface, as greater surface melting across the ablation area of a debris-covered glacier will increase the rate of englacial debris emergence, resulting in thickening of supraglacial debris and reduced ablation over decadal timescales
The simulations presented for the active and entire glacier assuming differential ablation beneath a discontinuous debris layer, that is, where debris thickness varies on a meter-scale and each model cell includes ablation “hotspots” such as ice cliffs and supraglacial ponds, resulted in values that better reproduced the present-day ice thickness and extent, debris thickness and net mass balance observed for the present-day glacier, and observations of recent glacier surface change, than those simulations assuming a continuous debris layer without ablation “hotspots” when a positively skewed distribution of supraglacial debris including ice cliffs and supraglacial ponds was used (h0 > 0.5 m) (Figures 3 and 4)

Summary

Introduction

Supraglacial rock debris is present on 7% of the global mountain glacier area, dramatically affecting the sensitivity of these glaciers to climate change (Herreid & Pellicciotti, 2020). Debris-covered ice represents 30% of the glacier mass in ablation areas in High Mountain Asia (Kraaijenbrink et al, 2017). Supraglacial debris in the Everest region is typically sufficiently thick to reduce ablation by insulating the underlying ice surface (Nicholson & Benn, 2013). As a result, these debris-covered glaciers have experienced lower sensitivity to atmospheric warming than would be expected for climatically equivalent clean-ice surfaces Low climatic sensitivity promoted the development of long, low-angle debris-covered tongues that have a greater longevity than climatically equivalent clean-ice glaciers (Rowan et al, 2015). The following mechanisms have been suggested as possible causes of the debris-cover anomaly (Pellicciotti et al, 2015): 1. Increasing climatic sensitivity of low-angle debris-covered tongues due to suppressed, or reversed, ablation gradients as the climate warms (Benn et al, 2012)

Results

Discussion

Conclusion