The role of differential ablation in driving accelerated mass loss from debris-covered Himalayan glaciers: predicting the life expectancy of Khumbu Glacier, Nepal

Sustained glacier mass loss causes supraglacial debris layers to expand and thicken, with the expectation that thicker debris will suppress ablation and extend glacier life expectancy. However, regional satellite observations of glacier mass change indicate that debris-covered glaciers are shrinking at similar rates to clean-ice glaciers across High Mountain Asia. This greater-than-expected mass loss has been partly attributed to differential ablation processes that locally enhance mass loss within the debris-covered section of the glacier, for example at ice cliffs and supraglacial ponds, and to differential dynamics as ice flow responds to climate change. We used numerical modelling of the feedbacks between debris transport, ice flow and mass balance to test the hypothesis that differential ablation is responsible for the rapid decay of debris-covered Khumbu Glacier in the Everest region of Nepal. Parameterising mass balance to account for metre-scale variations in debris thickness increased net ice volume loss, accounting for 79% of the glacier surface elevation change observed between 1984-2015 CE. The model underestimated mass loss in the upper ablation area, where some or all of the remaining ice volume change is likely to result from dynamic detachment of the debris-covered tongue from the upper active glacier. Under a moderate future warming scenario (RCP4.5), Khumbu Glacier is projected to lose 46% of ice volume by 2100 CE and 83% by 2200 CE. Physical detachment of the debris-covered tongue from the upper active section of the glacier will occur before 2100 CE, reducing the volume of the active ice mass to only the area above the base on the Khumbu icefall. After physical detachment reduces the size of the glacier, ice flow slows, resulting in a dynamic shutdown that causes the death of this iconic glacier by 2170 CE.