Tsunami of dirt: What caused Nepal’s deadly glacial flood?

Account subscription benefits alongside Premium Stories, Editorials, Opinions and more. Unlock these with Subscription

More than 150 people are dead and hundreds more missing after an enormous wave of water, ice, mud and rock cascaded more than 170 kilometres down the Bhote Koshi River valley in Nepal. The valley is the main link between Kathmandu in Nepal and Lhasa in Tibet. At least 34 Australians are missing. There was little warning.

Footage of the debris-laden flash flood is shocking. It is, unfortunately, not surprising.

As an expert in glaciers, I have tracked their accelerating retreat around the world as they melt – and the disasters that can follow. Early analysis suggests the abrupt collapse of a glacier triggered the disaster.

As climate change intensifies, glaciers worldwide are melting – even in high regions such as the Himalayas . The Himalayas are especially vulnerable, because the steep slopes of many of these very tall mountains are made of permafrost – rock and soil frozen for centuries. As temperatures rise, these slopes are beginning to melt or destabilise. If an earthquake or slope failure occurs, it can trigger a sudden and devastating debris flow.

It's too early to say this was the exact cause. But we know that the great ice stores of the Himalayas – known as the world's " third pole " because it contains the largest amount of snow and ice after the Arctic and Antarctic – are no longer safe.

What happened in Nepal is likely due to an enormous ice-and-rock landslide sweeping down a mountain pass and gathering up ice, water, rock, trees and dirt.

The landslide was so large it was originally thought to be an earthquake, as the seismic energy registered as a 4.4 magnitude quake on seismographs.

By the time it reached Gyirong Port – a China-Nepal border crossing – the wall of debris resembled a tsunami of dirt. Early estimates suggest the wave may have been 30 metres high.

Sometimes, disasters like these can be caused when glacial meltwater trapped in a lake by a landslide breaks free. One of these events – known as a glacial lake outburst flood – hit Nepal in 2024.

But early analysis suggests this isn't the case here. Satellite imagery suggests the landslide may have carved away large parts of a glacier, triggering its collapse.

We don't know yet what caused the glacier to collapse. But glaciers worldwide are broadly in retreat – and flood and debris flow disasters can follow.

A smaller flood hit the same district in Nepal last year, killing nine people. Also last year, a melting glacier in Switzerland triggered a huge debris flow that destroyed the village of Blatten. In 2023, a landslide and subsequent flood in my home country of Georgia killed 33 people at a mountain resort.

The high-elevation glaciers of the Himalayas can respond differently to climate change than glaciers at lower elevations or in relatively warmer regions. Thawing and degrading permafrost can also contribute by making steep Himalayan slopes less stable.