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

Abstract

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 (Benn Abstract Sustained 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 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.Plain Language Summary Glaciers in the Himalaya are shrinking rapidly in response to ongoing climate change. Many of these glaciers are covered with thick layers of rock debris that insulate the ice surface from atmospheric warming. Recent observations suggest that, contrary to expectations, debris-covered glaciers are losing mass at similar rates to clean-ice glaciers. We explore the processes driving the rapid loss of ice from a debris-covered glacier using a glacier model with a novel representation of sub-debris melt. Our model shows that the rapid ice loss from Khumbu Glacier, Nepal, since 1984 cannot be explained solely by accounting for variations in debris thickness and the presence of ice cliffs and ponds across the glacier surface. Instead, the glacier has passed a dynamic tipping point i...

Be‐10 Dating of Ice‐Marginal Moraines in the Khumbu Valley, Nepal, Central Himalaya, Reveals the Response of Monsoon‐Influenced Glaciers to Holocene Climate Change

“…The glacier response times were shorter than the intervals of 0.6–2.0 kyr between phases of moraine construction (Table 2). The dynamic response time of Khumbu Glacier to the change in mean annual air temperature between the LIA and the present day was quantified using numerical modeling as 200–300 years (Rowan et al., 2021) and for Lobuche Glacier this will be shorter due to its relatively small size and steep slope (Oerlemans, 1989). Therefore, we suggest that the difference in the number of glacial stages represented at Khumbu and Lobuche Glaciers is the result of the superimposition of sediment that has buried some older moraine crests.…”

“…The equilibrium line altitude of debris-covered glaciers is difficult to infer from their geometry, as heavily debris mantled termini persist at lower elevations than would be possible for climatically equivalent clean-ice glaciers, but is estimated from numerical modeling as being above 6,000 m a.s.l. (Rowan et al, 2021). (Finkel et al, 2003;Richards, 2000;Richards et al, 2000;Rowan, 2017).…”

“…Extensive lateral and terminal moraines surround the glacier and merge with the debris‐covered terminus. The present‐day glacier is mantled with rock debris up to several meters thick near the terminus that generally thins up‐glacier (Rowan et al., 2021). Changri Nup Glacier and Changri Shar Glacier are former tributaries; satellite imagery indicates that they were confluent with Khumbu Glacier until at least 1984 CE.…”

Thinning and surface mass balance patterns of two neighboring debris-covered glaciers in southeastern Tibetan Plateau

“…They revealed that the tongue area showed a considerable dh (-2.81 m) during an ablation period and that its dynamic state is very weak, similar to what we observe for 23K Glacier. Other studies concerning the mass balance of Himalayan debris-covered glaciers also report on glaciers which possess a high dh rate (~-0.9-1.8 m a -1 ) and weaker dynamic state (Vincent et al, 2016;Nuimura et al, 2017;Brun et al, 2018;Rowan et al, 2021), and which possess dh patterns which are consistent with that of 23K Glacier. In summary, the dh pattern for 23K Glacier appears to conform to that of other Himalayan debris-covered glaciers, whilst that of 24K Glacier is exceptional is perhaps more anomalous.…”

Projected 21st-Century Glacial Lake Evolution in High Mountain Asia

“…Although the employed parameters are a good compromise for large-scale applications (Maussion et al, 2019), they may lead to overestimated ice volume (Farinotti et al, 2019a). Debris cover on glaciers, which considerably influences glacier mass balance (Rounce et al, 2021), is often investigated at a local scale (Anderson and Anderson, 2018;Rowan et al, 2021) and has only recently been incorporated into a global glacier model (GloGEM, Compagno et al, 2021). Because debris cover can both accelerate and reduce ice loss, we cannot comment at this point on the extent to which debris cover will influence glacial lake formation in HMA in the 21st century.…”