Back in July, a massive avalanche thundered through a valley in the Tibet Plateau, sweeping 70 million tons of glacier ice onto the floor below and killing nine yak herders and hundreds of animals in its way. Scientists were puzzled, because the area is relatively flat and glacier collapse is unprecedented in western Tibet. Researchers analyzing the catastrophe have now published findings that suggest meltwater at the base of the glacier lubricated the ice on its path of destruction, meltwater that probably wouldn't have been there if not for rising temperatures in the region.

The initial avalanche spread a 30 m (98 ft) thick pile of debris across an area of 10 sq km (4 sq mi), making it one of the largest ice avalanches ever recorded. Only an ice avalanche at Russia's Kolka Glacier was comparable in size, which killed more than 100 people back in 2002.

"This is new territory scientifically," Andreas Kääb, a glaciologist at the University of Oslo, told NASA's Earth Observatory following the event. "It is unknown why an entire glacier tongue would shear off like this. We would not have thought this was even possible before Kolka happened."

The precise measurements of the avalanche's mass were calculated using satellite data and GPS, but how quickly the ice made its way down to the valley floor (according to eye witness accounts) seemed to belie the topography of the region.

Satellite images show the before (left) and just after (right) of the Aru Glacier collapse that occurred on July 17, 2016 in western Tibet(Credit: Ohio State University)

This lead scientists to suspect that meltwater had a part to play, essentially lubricating the ice and hastening its progress down the mountain. And now with some clever computer modeling, the researchers were able to recreate the avalanche in a virtual sense, and a series of simulations have led them to this same explanation: The only factor that created an avalanche in the replicated models was the presence of meltwater at the foot of the glacier.

"We still don't know exactly where the meltwater came from, but given that the average temperature at the nearest weather station has risen by about 1.5 °Celsius (2.7 °F) over the last 50 years, it makes sense that snow and ice are melting and the resulting water is seeping down beneath the glacier," says Lonnie Thompson, a glaciologist at Ohio State University.

Part of the reason scientists were concerned about this huge avalanche was because it was soon followed by another, in the same mountain range in September. The second avalanche is not known to have killed anyone, and researchers are still investigating how it was triggered. But two in quick succession in a region where glacial collapse is unheard of is an ominous sign, if not on a global level at least on a local one.

"It is probable that the recent and rapid warming in this region increased the internal temperature of the ice, which, coupled with increased precipitation, created very unstable conditions," the researchers write. "If true, then other glaciers in the region may be experiencing similar conditions, therefore making this region extremely dangerous."

The research was published in the Journal of Glaciology.

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