The catastrophic avalanche of rock and ice on Nepal’s Langtang Lirung mountain, near the country’s northern border, was not the product of a single trigger. It was the culmination of environmental and geological changes that developed over years and decades before the mountainside finally collapsed on 26 August at an altitude of 5,150 metres.
The disaster killed 1,300 people and, 22 days later, more than 5,000 people were still missing. A new report by World Weather Attribution (WWA), a group of scientists specialising in calculating the influence of climate change on extreme weather events, has now examined the chain of factors that helped create the conditions for the unprecedented Himalayan disaster.
Among the most significant was the retreat of the glaciers themselves. According to the report, warming has caused considerable glacier retreat, removing what the scientists describe as “stabilising ice” around the area where the break occurred. The glacier near the collapsed slope, in particular, had retreated more than 300 metres between 2010 and 2026.
Walter Immerzeel, a professor at Utrecht University and one of the authors of the WWA report, stresses that the collapse cannot be attributed to a single event. The factors, he says, “acted together over time” and accumulated across years and decades. Climate warming was one element. Another was a powerful earthquake that struck the region in April 2015.
The earthquake had a magnitude of 7.8 and caused widespread devastation across Nepal, triggering numerous landslides in the central part of the country. Scientists suspect that the earthquake may have weakened the mass of rock supporting the glacier, potentially “preconditioning the slope” for its eventual failure.
The significance of that combination became apparent when the mountainside collapsed. The break triggered an enormous flood of water, ice and debris that surged downstream, gathering additional material as it travelled along the river channel. The resulting torrent reached the Rasuwagadhi border crossing, 22 kilometres downstream, in just seven minutes, travelling at an average speed of 188 kilometres per hour.
Within the following 15 minutes, it had swept through the border town of Timure and the trading settlement of Syabrubesi. Pilgrims, border personnel and workers at a hydroelectric plant were caught in the disaster.
The scale and speed of the event led Immerzeel to describe it as “a disaster without precedent in the Himalaya”. Its mechanism, he notes, can be compared with the Chamoli disaster in India’s Uttarakhand, where a glacier also broke away in February 2021. But the amount of material involved in the Nepalese collapse was approximately five times greater, while the impact was also considerably larger.
The flood itself was not composed simply of water released from the glacier. The WWA analysis identifies several possible sources: melting glacier ice, water stored beneath the glacier and within permafrost, ice and water carried within the debris, and water already flowing in the river.
Heavy precipitation also helped build the conditions for the disaster. During October and November 2025, abundant snowfall was recorded in the area. As temperatures subsequently increased, that accumulated snow turned into water, adding to the volume available when the collapse occurred.
The role of temperature was particularly significant. July and August were “substantially warmer” than the average for the area, according to the study. But the longer-term warming trend is also visible in the physical transformation of the glaciers and the mountain slopes.
Glaciers throughout the region have thinned by approximately half a metre each year since 2000, Immerzeel says. The glacier close to the collapsed slope retreated more than 300 metres between 2010 and 2026. Its presence was particularly important because, as Immerzeel explains, “it basically supports the rock wall”.
Climate warming has also raised the freezing level in the high Himalaya. The WWA report says this exposes ice and permafrost deep within the bedrock to higher and more prolonged melting temperatures, weakening mountain slopes. Researchers estimate that the freezing threshold — the altitude at which temperatures reach 0 degrees Celsius — is rising by 100 metres every decade during the monsoon and post-monsoon seasons.
That shift does not mean temperatures remain above freezing throughout the year. Instead, the changing seasonal threshold increases the number of freeze-thaw cycles. Water enters fractures in the rock, becomes liquid and subsequently freezes again, creating stresses within those fractures.
The process provides another link between long-term warming and the physical stability of the Himalayan mountains. “There is no doubt that climate change is one of the factors that drove the disaster,” says Friederike Otto, professor of Climate Science at Imperial College London and co-founder of WWA.
For Otto, the connection has significance beyond the immediate scientific explanation of the collapse. It should, she argues, be “discussed in the context of loss and damage” associated with climate change.
That question has become increasingly important for countries vulnerable to climate impacts, many of which have historically contributed less to the problem. For years, such countries have pressed within the United Nations for a compensation mechanism requiring wealthier nations and the largest fossil-fuel users to provide support to countries suffering the consequences of warming.
In May, the UN General Assembly took a symbolic step in that direction by approving a resolution sponsored by Vanuatu, a small Pacific island republic. The resolution called for states that fail to meet their climate obligations to provide reparations to nations most severely affected by warming.
The resolution was supported by 141 countries despite active opposition from the United States and a group of petro-states led by Saudi Arabia. It was not legally binding, however, and the commitment would still need to be incorporated into climate treaties.
The Langtang Lirung disaster therefore sits at the intersection of forces operating on radically different timescales: a single catastrophic collapse, an earthquake more than a decade earlier, years of glacier retreat, changing freeze-thaw cycles and a warming climate. The WWA analysis does not reduce the disaster to one cause. Instead, it identifies how several conditions accumulated until a mountain slope could no longer withstand the forces acting upon it.
