Tracing the Moisture That Nourishes the World’s Highest Glacier

Tracing the Moisture That Nourishes the World’s Highest Glacier

Blue circle with white text reading "Shaping the Future of Science" and "AGU Fall Meeting"The Khumbu Glacier snakes down from the summit of Mount Everest into northeastern Nepal. The highest glacier in the world, it’s famous for being a treacherous rite of passage for Everest mountaineers. However, the Khumbu Glacier, along with others nearby, is also a key source of water for downstream communities in the Hindu Kush–Himalaya–Karakoram region.

Now, using newly installed weather stations on and around Mount Everest, scientists have determined that the Khumbu Glacier is largely nourished by moisture transported from the northern Bay of Bengal. Predicting whether this glacier will advance or retreat in the future might hinge on measurements of the Bay of Bengal’s sea surface temperature, scientists suggest, because warmer waters translate into a greater potential for evaporation.

A Far-Reaching Expedition

In 2019, 34 scientists and a cadre of support staff convened in Nepal for the National Geographic and Rolex Perpetual Planet Everest Expedition. Ten research teams—spanning biology, geology, glaciology, mapping, and meteorology—accomplished a number of firsts, including drilling the world’s highest ice core and placing the two highest-elevation weather stations. (The logistics of supporting such an expedition were myriad: More than 2,900 kilograms of equipment and supplies had to be transported, with everything traveling by modes ranging from helicopter to dzo.)

Baker Perry, a climate scientist at Appalachian State University in Boone, N.C., participated in the expedition. As colead of the meteorology team, he helped oversee the installation of five automatic weather stations on and around Mount Everest, including one just a few hundred meters below the mountain’s 8,850-meter summit.

Working on the upper reaches of Mount Everest was a challenge even for Perry, who has spent time above 5,500 meters in the Andes mountains. “It was a new challenge getting up that high,” he said. “It’s just a different landscape.”

Into the Void

But installing weather stations at high altitudes is extremely important, said Perry, because many glaciers are at elevation. If we want to better understand what controls a glacier’s growth and retreat, we need to get a handle on the atmospheric conditions it’s exposed to, he said. And right now, only a handful of weather stations are positioned above about 5,200 meters in the Himalayas. “There’s this incredible data void up there,” said Perry.

Perry and his colleagues have now analyzed data from several of the weather stations they installed. They extracted measurements like temperature, relative humidity, and wind speed and tabulated 171 precipitation events that occurred between 1 June 2019 and 31 May 2020 near the Khumbu Glacier. Precipitation can nourish glaciers by depositing snow that ultimately gets compacted into ice.

The researchers then used a computer model of how the atmosphere moves to trace each precipitation event backward in time and space. They found that more than 60% of the precipitation originated from the northern Bay of Bengal, with the remainder split roughly evenly between the Indo-Gangetic Plain and the Indus River Delta/Arabian Sea. That result is consistent with earlier results based on satellite data.

Watch the Temperature

These findings pinpoint the Bay of Bengal as an area to watch, said Perry. The amount of precipitation that ultimately falls in the accumulation zone of the Khumbu Glacier—a factor that dictates, in part, whether the glacier will advance or retreat—may be tied to the sea surface temperature in the Bay of Bengal, he said. “The warmer the sea surface temperatures there, the warmer the air, and the greater the potential for evaporation.”

These results were published last month in One Earth and will be presented at AGU’s Fall Meeting 2020.

Perry and his colleagues aren’t finished studying the Khumbu Glacier—they’re particularly keen to understand how it will be affected by climate change. One hint comes from measurements of the freezing-level height on the glacier, the elevation at which precipitation ceases falling as rain and instead falls as snow. Freezing-level heights on the Khumbu Glacier have consistently crept upward by about 7 meters per year since 2005, Perry and his collaborators have shown.

That’s a worrying trend, said Mauri Pelto, a glaciologist at Nichols College in Dudley, Mass., not involved in the research. A higher freezing-level height means that a glacier will receive a larger fraction of its precipitation in the form of rainfall, which will trigger melting, he said. “Snow adds to its mass, at least temporarily, whereas rain immediately generates melt.”

There’s another side effect of increased rainfall, Perry and his colleagues noted. By contributing an influx of water to lakes, it could potentially destabilize some of the many glacial lakes in the Hindu Kush–Himalaya–Karakoram region, said Perry. “This is a growing concern.”

—Katherine Kornei (@KatherineKornei), Science Writer

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