Why do Asian glaciers melt more slowly than others? Glaciologists are working on this phenomenon, focusing in particular on the many “black glaciers” in the region, covered in moraine debris. They reveal some of their specific characteristics and vulnerabilities.
Asia’s black glaciers defy the ice melting phenomenon. These Himalayan giants are melting at a far slower rate than the world average. “However, some areas on their surface are melting particularly quickly”, shares glaciologist Patrick Wagnon. “Why? Because of the ice cliffs and small lakes, specific structures found on part of their surface, which trap solar radiation. (1)”
Glaciers partly covered in moraine debris are referred to as black glaciers. The rock fragments lining these glaciers, of a size ranging from that of a speck of dust to that of a car, accumulate on the surface into a layer which increases in thickness as the ice tongue flows downwards. The stones come from rock falls and landslides, which is why the steepest ranges feature more black glaciers. They account for 10% to 15% of glaciated areas in Asia, up to 25% in the south-eastern part of the Himalayas, compared with just a few % in the Alps. This detrital cover has a major influence on their dynamics.
This dark layer, somewhat counter-intuitively, contributes to slowing down the melting of ice. “In fact, as long as its thickness does not exceed a few centimetres, the debris cover accelerates the melting by enhancing the absorption of solar radiation, as its albedo?Reflectivity of a surface is lower than that of the ice or snow”, explains the specialist. “But beyond this critical thickness, the insulating effect of debris, the thickness of which can reach 3 or 4 metres, outweighs the albedo effect. In this case, the glacier is protected against radiation and melts more slowly than if it were not covered in debris.” Nevertheless, black glaciers show signs of vulnerability…
Increased melting areas
“Asia’s black glaciers have a very heterogeneous, chaotic surface, with many small lakes and evenly distributed banks which can reach heights of several metres or tens of metres, referred to as ice cliffs, free of debris because of their steep slopes”, points out glaciologist Fanny Brun, award winner of the 2018 France L’Oréal-UNESCO programme for Women and Science. “Dark or dirty, these specific areas are characterised by their low reflectivity; they absorb more solar radiation, warm up and transfer the heat to the glacier”.
Based on observations of the Changri Nup glacier in Nepal, using a drone and satellites, from 2015 to 2017, scientists managed to establish a digital map of the annual melt volume, thereby quantifying losses linked to ice cliffs. “They contribute 25% of the total melt even though they only account for 7% of the surface area”, explains the young researcher. “With a melt capacity three to four times higher than debris-covered ice, ice cliffs, along with surface lakes, are the Achilles heel of Asia’s black glaciers.” A previous study(2) determined that lakes’ contribution to the melt is approximately fourteen times higher than that of other areas of the back glacier.
Research will now focus on verifying that these solar energy entry points also have an effect on black glaciers from other parts of the world.
Find out more about black glaciers and the PRESHINE programme
In this video, Patrick Wagnon presents the assumptions which would explain the “anomaly of the Karakoram and Kunlun”. In contrast with others, the glaciers in this region located between Pakistan and China are generally stable, and are even gaining a little mass.
F. Brun, E. Berthier, P. Wagnon, A. Kääb and D. Treichlert, A spatially resolved estimate of High Mountain Asia glacier mass balances, 2000-2016 ; Nature Geoscience ; 7 août 2017
Patrick Wagnon and Thibaut Vergoz, a photographer, talk about the prowess needed to fly a drone at an altitude of more than 5,000 metres, and to capture this unique experience in photographs. The objective was to supplement satellite images and field measurements by flying over and mapping out the Changri Nup site, one of Asia’s black glaciers, studied as part of the PRESHINE interdisciplinary research programme (IRD/CNRS).
1. F. Brun, P. Wagnon, E. Berthier, J.M. Shea, W. W. Immerzeel , P. D. A. Kraaijenbrink , C. Vincent , C. Reverchon, D. Shrestha, Y. Arnaud, Ice cliff contribution to the tongue-wide ablation of Changri Nup Glacier, Nepal, Central Himalaya, Cryosphere, 29 mars 2018.
2. E. S. Miles, I. Willis, P. Buri, J. F. Steiner, N. S. Arnold & F. Pellicciotti, Surface pond energy absorption across four Himalayan glaciers accounts for 1/8 of total catchment ice loss, Geophysical Research Letters, 17 septembre 2017.