The devastating flash flood that has ripped through the border communities of Nepal and Tibet has heightened fears that the climate crisis is destabilising the geology of mountain and polar regions, threatening millions of people.
Satellite imagery suggests a glacier collapse high in the Himalayas was the primary cause of the deadly torrent of mud, water and ice that tore along the Bhotekoshi River on Wednesday. It swept away buildings and roads and left at least ۳۶۰ dead and ۱,۴۰۰ missing.
A magnitude ۴.۴ earthquake was initially blamed for the collapse. However, it is now thought to have been caused by the huge wall of ice that crashed down on to the north slope of Langtang Lirung, a ۷,۰۰۰-metre (۲۳,۰۰۰ft) mountain, the impact of which registered a magnitude of ۵.۲.
“Additional analysis of long-period seismic waves indicates that the seismic energy was instead generated by a glacial collapse and debris flow,” the US Geological Survey said.
The ice-rock avalanche pushed boulders and other debris into the Lhende Khola River, a tributary of the Bhotekoshi River. Water levels in rivers downstream rose by between ۷ and ۹ metres within ۳۰ minutes, according to the International Centre for Integrated Mountain Development, an intergovernmental scientific institution.
Scientists are still examining the precise mechanics of the Himalayan disaster, but many observe that glaciers and permafrost around the world are being weakened by global heating, caused by the burning of gas, oil and coal.
The detachment of a chunk of the glacier wall came amid unusual heat that was likely to have melted the ice that bound the splintered bedrock together.
Two days before the collapse, the ground temperature hit its highest point in the last two years, according to Dr Hamish Pritchard, a glaciologist with the British Antarctic Survey, whose team had sensors ۶ miles (۱۰km) away from the site.
“These high temperatures would have weakened the snowpack, filled crevasses with water and thawed the bonds between ice and rock that hold these glaciers in place,” he told the Science Media Centre.
A series of intense heatwaves has hit the region since early this year and affected countries around the world. Another factor here was the monsoon season, which saturated the soil and swelled rivers.
Disasters like this were foretold. In ۲۰۲۳ the UN’s top scientific advisory body warned that previously ice-bound landmasses – from permafrost tundra to mountain glaciers – would become increasingly unstable as temperatures rose.
“Every increment of warming will multiply and intensify future hazards from cryosphere regions,” observed the Intergovernmental Panel on Climate Change in its sixth synthesis report. “Floods, landslides and freshwater shortages from glacier retreat and snow loss pose a serious threat to mountain regions across the world.”
Average global temperatures are nudging closer to ۱.۵C above preindustrial levels. Even faster warming is occurring in alpine and polar regions because the loss of snow and ice is removing the thermal insulation of the land.
Dr Richard Waller, a senior lecturer in physical geography at Keele University, said: “The progressive loss of snow and glacier ice results in the surface being less reflective, so it absorbs more solar energy and heats up. In combination with atmospheric warming, this is leading to the melting of mountain permafrost.
“Think of the high mountains like this as shattered bedrock glued together by ice-filled joints. As the permafrost and the ice-filled joints melt, then there’s the potential for these types of catastrophic failure.”
The thaw is altering the shape, colour and solidity of the landscape, creating or expanding glacial meltwater lakes and, in the long term, threatening the origin of many rivers and the drinking supplies of populations downstream. In many parts of Asia, hundreds of millions of people depend on the rivers such as the Ganges and the Brahmaputra that descend from the glaciers.
Scientists have high confidence that glacier retreat is caused by human-driven global heating. This has caused glacier collapses from the Alps to the Andes. In the Langtang catchment area in Nepal, glacier loss rates have increased more than fourfold since ۱۹۶۴, a study has shown.
This has contributed to a loss of life on multiple occasions. A smaller flood hit the Bhotekoshi River in July ۲۰۲۵ and in ۲۰۱۵, the Langtang Lirung mountain was hit by an earthquake-induced avalanche that destroyed Langtang village.
As emergency services look for bodies and humanitarian relief groups mobilise for survivors, others are asking where next. Waller said any high mountain areas with glaciers and permafrost could be vulnerable, including the European Alps, Caucasus, Himalayas, Andes and beyond.
As glacier weakening accelerates, Pritchard called for greater disaster preparedness. “More avalanches and more floods are inevitable, particularly as lakes grow in front of the retreating glaciers,” he said. “Early warning systems are urgently needed to protect the people living in these valleys.”
