At least 403 people, including 341 foreign nationals, are missing after flash floods swept through the Nepal-Tibet border region on Wednesday, with scientists attributing the disaster to a massive glacial collapse rather than an earthquake, according to officials and experts.
What happened in Nepal?
On Wednesday morning local time, a landslide of ice and rock triggered a torrent of mud, water, and debris that burst the banks of the Trishuli river, also known as the Bhotekoshi, which runs between China and Nepal. Homes, roads, bridges, and power plants were swept away, leaving devastation in the region.
Officials in Nepal reported 403 people missing, including 341 from foreign countries such as India, the US, the UK, Australia, South Korea, and Malaysia. In Tibet, China's state broadcaster said at least three people were killed and 265 were missing.
Experts at the Kathmandu-based International Centre for Integrated Mountain Development (ICIMOD) said water levels on the Trishuli river rose by as much as 9 meters in half an hour. The Lhende Khola river, a tributary of the Bhotekoshi, was most affected, they said.
What caused the flooding?
An avalanche from a glacier is believed to be the likely cause of the flash flooding, according to initial reports. Nepal's foreign minister, Shisir Khanal, told parliament on Wednesday that an earthquake in the area may have triggered a landslide, but the US Geological Survey (USGS) later clarified that no earthquake had occurred.
The USGS said the event was initially reported as a magnitude 4.4 earthquake, but further analysis showed the “seismic event was a glacial collapse and debris flow that caused subsequent catastrophic impacts downstream.” The glacial collapse registered as a magnitude 5.2 seismic event on the Richter scale, occurring near the Nepal-China border at 8.37am local time on Wednesday.
Assoc Prof Daniel Shugar, a geomorphologist at the University of Calgary in Canada, said initial satellite analysis showed a 0.2 sq km chunk of ice “collapsed about 1.2km vertically.” “What I can see in … satellite imagery from Planet Labs, the lower part of a glacier broke off at about 5,200 metres and crashed onto the valley floor about 1,200 metres below,” he said.
That was “likely enough to have converted a lot of potential to kinetic (heat) energy, melting the ice,” he wrote on Bluesky. Shugar also noted that satellite imagery showed a large amount of snow had melted in the 24 hours prior to the disaster, suggesting it was probably warm, and that construction could be a factor.
Climate change and glacier collapse
Prof Andrew Mackintosh, a glaciologist at Monash University in Melbourne, said it was unclear whether the collapse was isolated to the glacier or involved the whole mountainside. “When a glacier of that scale collapses, it generates a lot of water, and then there’s a cascading sequence of events … where the water and the sediment combine and bulks up and creates the flood,” he said.
An international group of glacier researchers said that while a glacial collapse was “certainly involved,” it was “unlikely to be the sole cause.” Mackintosh noted that “there have been so few of these documented that we’re not really able to make very clear statements about the specific relationship between glacier collapse and climate change yet.”
However, scientists have “very high confidence in the relationship between glacier retreat and climate change,” he added, noting that mountain permafrost is degrading under a warming climate, acting as a “glue that holds the rock and sometimes adjacent glaciers … together on very steep slopes.”
Regional risks and past events
The Himalayas – spanning Nepal, India, China, Bhutan, and Pakistan – are warming twice as fast as the global average, increasing risks of avalanches, landslides, and floods. The United Nations said in 2023 that Nepal’s mountains had lost close to one-third of their ice in just over 30 years due to global warming, and Hindu Kush Himalayan glaciers disappeared 65% faster between 2011 and 2020 than in the previous decade.
In 2021, a Himalayan glacier broke off and caused flash flooding in the Indian state of Uttarakhand, leaving 200 people dead or missing. Though not thought to be a cause of the current event, mountain areas are also at risk of “glacial lake outburst floods,” which occur when natural dam walls breach.
Assoc Prof Adrian McCallum, a glaciologist and polar engineer at the University of the Sunshine Coast in Australia, said: “With a warming climate, glaciers recede, ice becomes water, that water gets trapped.” More than 12,000 deaths have been attributed to such floods globally, and Nepal has more than 2,000 glacial lakes, of which 21 are considered potentially dangerous, according to the UN.
