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In 2016, two glaciers in Tibet collapsed triggering huge avalanches that caused widespread devastation. Such a collapse had only been seen once before, so the events suggested that glaciers break apart more frequently than anybody realised.

of the University of Oslo, Norway, and his colleagues have combined eyewitness reports with remote-sensing images and other data to better understand how the glaciers came to fall apart. They have also spotted the telltale signs of impending collapse, knowledge that could be used to warn people nearby.

The first glacier to give way was in the Aru Range on the western Tibetan plateau. The researchers dubbed it “Aru-1”. On 17 July 2016, almost half of the glacier detached in one huge chunk and went sliding and crashing down the valley. The ensuing avalanche covered more than 8 kilometres and killed nine people, all animal herders.

Kääb’s team estimates that 68 million cubic metres of ice was released by the collapse of Aru-1.

Two months later, on 21 September, a second glacier called Aru-2 also fell apart just 2.6 km away. This time the glacier lost its bottom half in two distinct chunks, which totalled 83 million cubic metres.

Each event was terrifyingly fast, taking just 2 or 3 minutes. The ice moved at an average of 20 metres per second (72 kilometres per hour).

Ice avalanches

The only previous case of a glacier collapsing in such a dramatic way occurred in 2002 in the Russian Caucasus, when the lower portion of the Kolka glacier detached. About 100 million cubic metres of ice and mud slid down the valley at up to 300 km per hour, ultimately covering over 18 km and including ().

“I was actually there,” says Kääb, and he joined a team that investigated it. They concluded that a multitude of factors had combined to cause the disaster, and that it was unlikely to be repeated any time soon. “No reason to be worried that it could happen somewhere else.”

But the events in Tibet have changed Kääb’s mind. “From now on, we know that it can happen at other places,” he says. “It will be a very rare combination of time-dependent factors, but we cannot really exclude it.”

Kääb’s team analysed satellite photos of the glaciers taken in the months leading up to the collapses. They spotted telltale crevasses in the middle of the glacier, which had formed over that time period and were roughly where it sheared in two. They also checked seismometer data in the area to check there were no earthquakes in the area that might have set off the collapse.

Because there are so few known instances of glaciers falling apart like this, Kääb says we can’t yet know if such events are becoming more common. The world is warming as a result of our greenhouse gas emissions, and this has caused glaciers around the world to retreat. Intuitively, it would make sense if some disintegrated as a result, but Kääb says it is too early to tell.

“Some of the factors that we’re pretty sure were involved in the collapses… are changing through climate change,” he says. However, some factors like the type of bedrock are not, and they are just as important. “You cannot get a glacier to collapse just by turning on the heat.”

It is also impossible to tell which areas are more at risk. “They could happen in places it didn’t happen before and nobody was thinking it could happen,” says Kääb. He hopes people will come forward with stories of collapses, which may be known to local people but not to scientists.

The one good news is that there seems to be a warning sign, visible in satellite imagery. “We know now, if we see specifically a special type of crevasse, in our heads alarm bells go,” says Kääb.