Scale of glacier loss in Himalayas previously hidden, say scientists studying accelerated melting
- Despite using satellite data, studies of lake-terminating glaciers in the ‘third pole’ overlooked 2.7 gigatonne loss, says paper
- Findings important for understanding the impact of water resources and flooding in the region, says Chinese researcher
The loss of mass in glaciers terminating into lakes in the greater Himalayas region has been significantly underestimated, according to a new study.
Glaciers are an important water resource but they are melting at an accelerating rate because of climate change.
The study by an international team, including researchers from the Chinese Academy of Sciences, found that the loss of around 2.7 gigatonnes (Gt) of glaciers terminating at lakes in the Himalayas was missed in calculations made between 2000 and 2020.
That weight is equivalent to 570 million elephants, or more than 1,000 times the total number of elephants living in the world, according to the authors. Their study was published in the peer-reviewed journal Nature Geoscience earlier this month.
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The Himalayas, also known as the third pole, has the most glaciers outside the Arctic and Antarctica. The region is the source of 10 major rivers in Asia and delivers water to almost 2 billion people.
The Himalayas host more than 5,000 glacial lakes, many of which are “proglacial” lakes formed by moraines – material such as soil and rock left behind a glacier – or ice dams.
Compared to glaciers that terminate on land, lake-terminating glaciers retreat faster and have a higher rate of glacier mass loss, according to the study. Glacial retreat refers to a glacier shrinking over time because of a variety of factors, including rising temperatures, changes in precipitation patterns and other environmental factors.
Lake-terminating glaciers only account for 10 to 15 per cent of the total glacier area but they contributed to more than 30 per cent of the overall loss of glacier mass between 2000 and 2015, the study said.
However, the traditional method scientists use to estimate glacier mass loss only reflects changes to the glacier and water surface, and cannot measure underwater glacier loss when the ice is replaced by water because the glacial lakes expand.
“Many previous studies assessing the mass changes of glaciers for a large region or the globe used optical satellite data,” said Tobias Bolch, the study’s corresponding author and a professor at Graz University of Technology in Austria.
“The data used can only measure the surface of a body – for example, the glacier or water surface – but not the subsurface.
“However, for lake-terminating glaciers, the retreating ice is replaced by water as the lake area is increasing towards the glaciers … satellite images can only provide the information about the elevation of the water surface, and all the ice which is now replaced by water is not considered.”
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Bolch said the study was of particular significance because the results of previous studies of mass glacier loss were used to calibrate models to project the future.
“And therefore these models also underestimate the future mass loss, so it is very likely that glaciers will shrink even faster in the future than predicted.”
The authors used satellite images to produce boundaries of proglacial lakes in the greater Himalayas at intervals between 1990 and 2020. Then they measured the depth of 16 proglacial lakes using an uncrewed surface vessel between 2018 and 2021.
They estimated the volume of proglacial lakes using a formula relating the area of the lake to its volume.
The authors found that in the two decades to 2020, proglacial lakes in the region increased by 47 per cent in number, 33 per cent in area and 42 per cent in volume.
They said an estimated glacier mass loss of around 2.7 Gt – or 6.5 per cent of the loss – was underestimated.
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The most significant underestimation, accounting for 10 per cent, occurred in the central Himalayas. Lake Galong Co, with an area of 5 sq km (1.93 square miles), was the site of a 65 per cent underestimation.
Globally, between 2000 and 2020, the underestimated glacier mass loss of lake-terminating glaciers may be as high as 211.5 Gt, according to the authors.
The study had important implications for water availability in regions that were highly dependent on glacier meltwater, such as regions in Central Asia or Pakistan where the Indus River was the main source of water, Bolch said.
Study lead author Zhang Guoqing, a researcher with the Chinese Academy of Sciences, said the findings were important for understanding the impact of regional water resources and glacial lake outburst floods.
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“By accounting for the mass loss from lake-terminating glaciers, the researchers can more accurately assess the annual mass balance of these glaciers compared to land-terminating ones, thus further highlighting the accelerated threat of glacier mass loss across the greater Himalayas,” Zhang was quoted as saying on the website of the University of St Andrews, where Bolch previously worked.
Alexandre Bevington, a research hydrologist with the British Columbia ministry of forests, commented in the journal: “The authors present an exciting new breakthrough in the assessment of total glacier mass and glacier mass loss in the Himalaya, with implications for other regions.
“Overall, this work is useful, original, and a noteworthy achievement in terms of understanding glacier mass change and, more specifically, the previously unaccounted role of subaqueous ice loss,” said Bevington, who is not involved in the study.
