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Ice cliffs are hot spots of melt for debris-covered glaciers, such as the Lirung Glacier in the Nepalese Himalayas.

Ice cliffs are hot spots of melt for debris-covered glaciers, such as the Lirung Glacier in the Nepalese Himalayas.

Eduardo Soteras

Dirty glaciers melt just as fast as clean ones

BERLIN—Glaciers covered with thick layers of rocks and boulders should be insulated from a warming world. But two new studies—presented here last month at the Third Pole Environment Workshop—find that dirty Himalayan glaciers are melting away just as quickly as clean ones. As a result, debris-covered glaciers might be disappearing faster than previously thought, threatening the long-term water supplies of more than a billion people.

Glaciers in the Himalaya Mountains cover an area of 50,000 square kilometers. Intense monsoons erode the still-growing mountains, creating thick blankets of debris that cover more than 10% of the total glacier area. “But we know very little about how much ice they are losing over a long period of time,” because the remoteness and rugged terrains have prevented long-term field measurements, says Harish Chandra Nainwal, a glaciologist at Hemwati Nandan Bahuguna Garhwal University in Srinagar, India, who is not involved in the studies.

So glaciologist Francesca Pellicciotti of the Swiss Federal Institute of Technology’s Institute of Environmental Engineering in Zurich, Switzerland; Tobias Bolch, a remote-sensing expert at the University of Zurich; and colleagues turned to declassified data from a U.S. spy satellite called Hexagon, which was used in the 1970s and 1980s at the height of the Cold War. The researchers compared the satellite’s measurements of glacier surface elevations in 1974 with those from the Shuttle Radar Topography Mission, which flew on board the space shuttle Endeavour in February 2000. In all, the researchers studied four debris-covered glaciers in Nepal’s Langtang River watershed.

Dirty glaciers, it turns out, melt just as fast as clean ones. The height of debris-covered glaciers thinned by 32 centimeters a year between 1974 and 1999, the team will report in an upcoming issue of the Journal of Glaciology. That’s similar to the average thinning rate of clean glaciers in the Himalayas. Yet the debris-covered glaciers have not retreated significantly since 1974—despite the fact that they are melting away. “So we have this dead ice body slowly wasting away for decades” with meters of rocks on top of it, without giving away any signs of doom and gloom, Bolch says. “This is quite different from clean glaciers, which tend to recede rapidly when losing ice,” he says.

The findings illustrate why changes in the length of dirty glaciers is a poor indicator of their health, Nainwal says. “People have been saying that there has been little retreat [with many Himalayan glaciers], so there is no problem.”

To understand how debris-covered glaciers were wasting away, another team of researchers—led by glacier hydrologist Walter Immerzeel of Utrecht University in the Netherlands—flew a sensor-ridden unmanned aerial vehicle across the tongue of the Lirung Glacier in Langtang before and after the monsoon season in May and October 2013, respectively—the first such mission in the Himalayas. The sensors allowed the researchers to map changes in surface elevations of the glaciers. The team found that the majority of the melting took place at ice cliffs and lakes on the glacier, up to 10 times faster than on other areas of the glacier. 

Ice cliffs, a unique feature of debris-covered glaciers, “are normally very dark and dirty and absorb a lot of solar radiation,” Pellicciotti says. More crucially, the ice cliffs also soak up heat emitted by hot rocks around them, she says. Heated by strong solar radiation at high elevations, the rocks can reach temperatures as hot as 40°C during the day. “The ice cliffs are literally surrounded by hot stoves,” she says. The researchers also spotted numerous lakes on the glacier, often next to an ice cliff, in May 2013. Water is darker and absorbs more solar radiation than the surrounding ice. “You can see the warm water cutting further into the ice cliff, possibly accelerating the glacial melt,” Immerzeel says.

Curiously, when the scientists went back 5 months later after the monsoon season, all the lakes were gone. They suspect that there must have been channels in the glacier that were frozen in May but were then opened by warm lake water and monsoonal rain. “This may have allowed the drainage to occur and caused more melt underneath the debris,” Immerzeel says. After feeding the field measurements into a computer model, the researchers found that a third of the total ice melt in the Langtang catchment originates from the parts of the glaciers that are covered by debris, which constitute only 27% of the glaciated area. The result “highlights the importance of debris-covered glaciers for streamflow,” which has hitherto been underappreciated, Pellicciotti says.

As the climate continues to warm, assessing the health of debris-covered glaciers across the Himalayas is more pressing than ever, researchers say. The retreat—or, worse, the total disappearance—of Himalayan glaciers would deprive about a billion people of their precious solid water reservoirs. Glacier melt is a key source of agricultural water, especially in spring before the arrival of the annual monsoon. Already several debris-covered glacier tongues in Langtang have been disconnected from their source regions. “They are kept alive solely by the redistribution of snow through avalanches,” Immerzeel says. His team plans to investigate how conduits in the glaciers develop and evolve throughout the year and how they interact with ice cliffs and supraglacial lakes by using radars that can penetrate through the debris.

Meanwhile, Nainwal’s team has drilled through debris more than 1 meter thick and planted about 60 bamboo stakes into the ice of the Satopanth Glacier in the western Himalayas, so it can monitor the changes in ice volumes in the coming decades. At 4000 to 5300 meters above sea level, “it’s a major engineering work,” Nainwal says. Such field investigations are essential for reducing the uncertainties of satellite analyses and for understanding the physical processes that drive the melting, Bolch says. “This will allow us to better predict the glacier status and water availability in the future.”