A day before a wall of ice, rock and muddy water tore through Himalayan valleys along the Nepal-China border, Dipesh Chapagain was discussing with a colleague just how devastating such a disaster could be.
For the 40-year-old climate 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 region change within his own lifetime.
“When I was kid, it was not like that,” he told me. “Or when my parents were kids or my grandparents, that was not the major challenge [they faced].”
© UNU-EHS/Austin Gonzales
Dipesh Chapagain, on the campus of United Nations University in Bonn, Germany. .
A glacier fails, a mountain collapses
Then, on Wednesday morning at 8:37, a mountainside high in the Himalayas gave way.
The collapse was so vast that seismic instruments registered it as a magnitude-5.2 event, initially mistaking it for an earthquake. Bedrock beneath a glacier failed, carrying a huge mass of ice and rock roughly 1,200 meters toward the valley floor.
Water, sediment and boulders surged into the rivers below, sweeping through towns and villages along one of Nepal’s most important trade routes with China. At one monitoring station, the water rose as much as nine meters in half an hour.
By Friday, more than 500 people had been reported dead across Nepal and Tibet and more than 1,500 remained missing, including hundreds of foreign nationals.
Dr. Chapagain’s immediate family was spared. Still, the news took a toll on 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 is a brutal illustration of a problem climate scientists have been warning about for years. The frozen architecture of the Himalayas is changing, exposing millions of people to hazards that are becoming harder to predict and, in some cases, more destructive.
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Satellite imagery shows Timure, Nepal, before and after the catastrophic flood.
How a mountain collapse became a deadly flood
Experts are still piecing together the sequence of events that led to Wednesday’s disaster. Analysis of satellite imagery suggests that bedrock beneath a glacier gave way, sending a huge mass of ice and rock into the Lhende Khola, a tributary of the Bhotekoshi River, and triggering the destructive cascade that followed.
The mechanics differ from those of the glacial lake outburst floods that Dr. Chapagain and his colleagues at the Global Mountain Safeguard Research programme, known as GLOMOS, study across the Hindu Kush Himalaya. GLOMOS is part of UNU’s Institute for Environment and Human Security.
Their recent research analyzed 493 such floods from the earliest available records through 2024. It found that these events have become roughly five times more frequent since 1950, rising from an average of about seven per decade to 34.
Such floods typically occur when meltwater collects in lakes around retreating glaciers, often held back by unstable ice or loose rock and sediment. When those natural dams fail, enormous volumes of water can be released almost instantaneously.
Wednesday’s disaster appears to have followed a different chain of events.
“This time, the intermediate step was skipped,” Dr. Chapagain said.
Rather than a glacial lake bursting, rock and ice collapsed from the mountainside, generating a torrent below. Researchers are investigating whether the debris temporarily dammed the river before the obstruction gave way, a sequence Dr. Chapagain cautioned had not yet been confirmed.
For scientists, the distinction matters. Downstream, the result can look frighteningly similar: a vast volume of water, ice and debris released with little warning into narrow, inhabited valleys.
It is unclear whether climate change caused Wednesday’s collapse. But global warming is rapidly transforming the high-altitude environment in which such disasters occur. As glaciers melt and permafrost thaws, rock once supported or bound by ice can become unstable, while increased meltwater can further weaken mountain slopes.
The consequences extend far beyond mountain villages. The Hindu Kush Himalaya feeds 10 major river basins on which some 2.1 billion people depend for water, food, energy and livelihoods.
© UNICEF
Mud covers Rasuwa District in central-northern Nepal following devastating flash floods.
As people move into harm’s way
But the mountains are not the only thing changing. So is the geography of human life below them.
When Dr. Chapagain was a child, he said, families in many of Nepal’s hilly areas traditionally built their homes higher on the slopes while cultivating rice and other crops on flatter land closer to rivers. His own family’s home garden was on a mountaintop. Its rice paddies lay below, 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. Water sources in some higher settlements have also been drying up, Dr. Chapagain said, encouraging people to move downhill.
That has created a dangerous convergence: as warming alters the mountains above, more people and infrastructure are gathering in the corridors through which water and debris descend.
The valley devastated by Wednesday’s flood embodies that collision. The Bhote Koshi-Trishuli corridor carries a major trade route toward Kathmandu and is lined with hydropower plants, bridges, roads, hotels and settlements. The floods damaged power facilities and severed transport and communications links across the region.
The danger was already apparent. A glacial lake outburst struck the same river system last year, killing nine people and leaving 24 missing.
© UNICEF/Laxmi-Prasad-Ngakhusi
Buildings in Bagmati Province, Nepal, remain covered in mud after a devastating flash flood.
When minutes matter
For Dr. Chapagain, Wednesday’s catastrophe also exposed a critical weakness. How little warning communities may receive before a disaster born high in the mountains reaches them.
Nepalese authorities, he said, learned of the flood only as it was approaching the border. Alerts then went downstream through text messages, social media, telephone calls and alarms.
Reducing the danger, he argues, begins higher up: continuous monitoring of glaciers and unstable slopes, satellite observation, weather and river gauges, and early-warning systems capable of buying communities enough time to evacuate. Because rivers and glaciers cross national frontiers, China, Nepal and countries farther downstream must also exchange data quickly.
There is a deeper problem. Even if greenhouse gas emissions fell sharply, Dr. Chapagain said, glaciers would continue losing mass for decades because of warming already locked into the climate system. How much is eventually lost remains within humanity’s control. What happens in the near term, however, is far less so.
That makes adaptation, in his view, no longer preparation for a distant future.
For communities beneath the Himalayas, that future arrived Wednesday.
