Polythermal structure of a Himalayan debris-covered glacier revealed by borehole thermometry

Abstract

Runoff from high-elevation debris-covered glaciers represents a crucial water supply for millions of people in the Hindu Kush-Himalaya region, where peak water has already passed in places. Knowledge of glacier thermal regime is essential for predicting dynamic and geometric responses to mass balance change and determining subsurface drainage pathways, which ultimately influence proglacial discharge and hence downstream water availability. Yet, deep internal ice temperatures of these glaciers are unknown, making projections of their future response to climate change highly uncertain. Here, we show that the lower part of the ablation area of Khumbu Glacier, a high-elevation debris-covered glacier in Nepal, may contain ~56% temperate ice, with much of the colder shallow ice near to the melting-point temperature (within 0.8 °C). From boreholes drilled in the glacier’s ablation area, we measured a minimum ice temperature of −3.3 °C, and even the coldest ice we measured was 2 °C warmer than the mean annual air temperature. Our results indicate that high-elevation Himalayan glaciers are vulnerable to even minor atmospheric warming.

Going to Extremes: Installing the World’s Highest Weather Stations on Mount Everest

“…However, to what extent any melt would contribute to runoff (rather than refreezing at some point before leaving the glacier) remains to be determined. The anomalously warm borehole ice temperatures observed well above the mean annual air temperature on the lower Khumbu Glacier by Miles et al (2018) are certainly consistent with significant latent heat release from refreezing of meltwater in the Western Cwm. The high insolation that could enable considerable melt to occur at Camp II may also trigger melting at the South Col, with our simulation generating around 60 mm of meltwater despite a peak daily mean air temperature of −10°C (on 30 July), and not a single hourly mean air temperature above the melting point (maximum was −2.6°C on 7 July).…”

The influence of water percolation through crevasses on the thermal regime of a Himalayan mountain glacier

“…The thermal regime of a mountain glacier controls its hydrology, flow rheology and basal conditions affecting glacier dynamics, which in turn affect its behaviour in response to climate change. It influences erosion rates (Bennett and Glasser, 2011), potential glacier hazards (Faillettaz et al, 2011;Gilbert et al, 2015) and water resources in the glaciated catchments (Miller et al, 2012). It is thus essential to understand the processes causing and maintaining temperate basal conditions as well as the mechanisms leading to changes in the thermal regime of glaciers.…”

“…Very little is known about the thermal regime of the Himalayan glaciers due to the harsh conditions and logistical difficulties which make direct observations challenging in remote, high-altitude areas. Borehole temperature measurements, such as carried out on Khumbu, Yala and Gyabrag glaciers in the Himalayas (Miles et al, 2018;Mae, 1976;Watanabe et al, 1984;Liu et al, 2009), provide direct observations of the glaciers' thermal condition. However, the small number of boreholes gives only very limited information about the spatial distribution of the ice temperatures within the glacier.…”

The State of Remote Sensing Capabilities of Cascading Hazards Over High Mountain Asia

“…The formation and growth of new glacial lakes similarly introduces new hazard processes. The thawing of permafrost and transition of "polar" or cold-based glaciers (completely frozen) to polythermal, and polythermal to temperate glaciers (temperatures at or above the melting point at the bed) is a logical consequence of increased warming in HMA (Miles K. E. et al, 2018) and may contribute to a new propensity for glacier surges or ice avalanches-some of immense scales .…”