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A day before a wall of ice, rock and muddy water tore through Himalayan valleys along the Nepal-China border, climate researcher Dipesh Chapagain was discussing with a colleague how devastating such a disaster could be.
For the 40-year-old researcher, who grew up in the hills of eastern Nepal and now studies mountain hazards at the United Nations University in Bonn, Germany, the subject was both professional and deeply personal.
He has watched the risks facing the Himalayan region change within his own lifetime.
“When I was a kid, it was not like that,” he said. “Or when my parents were kids or my grandparents, that was not the major challenge they faced.”
Then, at 8:37am on Wednesday, a mountainside high in the Himalayas gave way.
The collapse was so powerful that seismic instruments initially detected it as a magnitude-5.2 earthquake. Bedrock beneath a glacier failed, sending a huge mass of ice and rock roughly 1,200 metres towards the valley floor.
Water, sediment and boulders then surged into rivers below, sweeping through towns and villages along one of Nepal’s main trade routes with China. At one monitoring station, the water level rose by as much as nine metres within half an hour.
By Friday, more than 500 people had reportedly died across Nepal and Tibet, while more than 1,500 remained missing, including hundreds of foreign nationals.
Chapagain’s immediate family was safe, but the disaster deeply affected him.
“It’s always hard to get these devastating messages from back home,” he said. “It’s getting more frequent and more intense.”
The catastrophe offers a stark illustration of a trend climate scientists have warned about for years: the frozen landscape of the Himalayas is changing rapidly, exposing millions of people to hazards that are becoming more difficult to predict and potentially more destructive.
Experts are still reconstructing the precise chain of events behind Wednesday’s disaster. Satellite imagery suggests that bedrock beneath a glacier collapsed, sending a huge mass of ice and rock into the Lhende Khola, a tributary of the Bhote Koshi River, and triggering the devastating cascade downstream.
The mechanism appears different from the glacial lake outburst floods studied by Chapagain and his colleagues at the Global Mountain Safeguard Research programme, or GLOMOS, which is part of UNU’s Institute for Environment and Human Security.
Their recent research examined 493 glacial lake outburst floods recorded from the earliest available data through 2024. It found that such events have become roughly five times more frequent since 1950, increasing from an average of about seven per decade to 34.
These floods generally occur when meltwater accumulates in lakes around retreating glaciers. The water may be held back by unstable ice, loose rock or sediment. When these natural dams fail, huge volumes of water can be released almost instantly.
Wednesday’s disaster appears to have followed a different sequence.
“This time, the intermediate step was skipped,” Chapagain said.
Instead of a glacial lake bursting, ice and rock collapsed directly from the mountainside, generating a powerful torrent below. Researchers are investigating whether the debris temporarily blocked the river before the obstruction eventually failed, although Chapagain stressed that this sequence has not yet been confirmed.
For scientists, the distinction is important. Downstream, however, the consequences can look remarkably similar: enormous quantities of water, ice and debris suddenly rushing into narrow valleys with little warning.
It remains unclear whether climate change directly caused Wednesday’s collapse. But global warming is rapidly transforming the high-altitude environment in which such disasters occur.
As glaciers retreat and permafrost thaws, rocks once supported or bound together by frozen ground and ice can become unstable. Increasing meltwater can also weaken mountain slopes, potentially creating conditions for landslides and other hazards.
The consequences extend far beyond isolated mountain communities. The Hindu Kush Himalaya supplies water to 10 major river basins on which around 2.1 billion people depend for drinking water, agriculture, energy and livelihoods.
But the mountains are not the only thing changing. Human settlement patterns are changing too.
Chapagain said families in many of Nepal’s hilly areas traditionally built homes higher on slopes while cultivating rice and other crops on flatter land near rivers. His own family’s home garden was on a mountaintop, while its rice paddies were located lower down near the river.
Over time, roads, businesses, hotels and hydropower projects have increasingly concentrated in river valleys, where construction is easier and economic opportunities are greater. Some higher settlements have also experienced drying water sources, encouraging people to move towards lower areas, he said.
The result is a dangerous convergence. As warming transforms the mountains above, more people and infrastructure are being concentrated along the very valleys through which floodwaters, rocks and debris can descend.
The valley devastated by Wednesday’s disaster illustrates that collision. The Bhote Koshi-Trishuli corridor is a major trade route towards Kathmandu and is lined with hydropower plants, bridges, roads, hotels and settlements.
The floods damaged power facilities and cut transport and communications links across the region.
The danger was not entirely unexpected. A glacial lake outburst flood struck the same river system last year, killing nine people and leaving 24 others missing.
For Chapagain, Wednesday’s catastrophe also highlighted another critical weakness: the limited amount of warning communities may receive before a disaster originating high in the mountains reaches them.
Nepalese authorities, he said, learned of the flood only as it was approaching the border. Alerts were subsequently passed downstream through text messages, social media, telephone calls and alarms.
Reducing the risk, he argues, must begin much higher in the mountains.
That means continuous monitoring of glaciers and unstable slopes, satellite observation, weather and river gauges, and early-warning systems capable of giving communities enough time to evacuate.
Because rivers and glaciers cross national borders, China, Nepal and countries farther downstream also need to exchange data rapidly, he said.
There is another fundamental challenge. Even if greenhouse gas emissions were sharply reduced, Chapagain said, glaciers would continue to lose mass for decades because of warming already locked into the climate system.
How much ice is ultimately lost, however, remains within humanity’s control. What happens in the immediate future is far less certain.
That makes adaptation not a response to some distant climate threat, but an urgent necessity for communities living beneath the Himalayas. For those communities, that future arrived on Wednesday. Source: UN News