The mountain section whose collapse triggered deadly floods in Nepal and China on August 26 experienced exceptionally high temperatures in the days before the disaster, according to meteorological data reconstructed and analysed by Robert Rohde, chief scientist at the independent Berkeley Earth organisation in California.
At an altitude of 5,200 metres, average temperatures at the glacier are estimated to have reached around 5 degrees Celsius between August 21 and 26. Such temperatures were exceptional for the time of year. Over more than 55 years of recorded data, temperatures at the site had not exceeded 4.0C during the same period.
Although no direct link between the heat and the collapse has yet been established, Rohde’s analysis indicates that temperatures during the six-day period were about 1.8C higher than those recorded at the site in the mid-20th century.
Rohde calculated that without the additional heat associated with climate change, a comparable temperature pattern “would probably occur only once in a few thousand years”. The finding points to the extraordinary nature of the conditions preceding the disaster, while stopping short of establishing that climate change directly caused the mountain collapse.
The collapse occurred during the fourth-hottest summer on record in the Langtang region, according to an analysis of data from Europe’s Copernicus observatory dating back to 1970.
The unusually warm conditions may have affected the stability of the mountain in several ways. French climatologist Valerie Masson-Delmotte said the heat could help explain permafrost thaw as well as the combined collapse of slopes and glaciers.
Permafrost can act as a form of ice cement, helping to hold rock faces together. When it degrades, the stability of those formations can be reduced, potentially increasing their vulnerability to collapse.
Satellite imagery provided another indication of unusual conditions before the disaster. The images showed significant snowmelt in the days preceding the collapse. Etienne Berthier, a glaciologist at France’s CNRS research agency, said the meltwater could have provided “a substantial source of liquid water that can infiltrate fractures in the mountain and facilitate landslides”.
Scientists remain cautious, however, about assigning a precise role to the heatwave. Kristen Cook, a geomorphologist at Grenoble Alpes University in France, stressed that the relationship between temperature and collapse is not necessarily straightforward.
“It’s not as straightforward as ’high temperature equals collapse,’” Cook said.
She added that if temperature had acted as a trigger, the mountain may already have needed to be close to a critical point of instability before the heat could contribute to its failure.
Researchers are therefore examining evidence from the years before the disaster, including small movements detected in the mountain and how those movements interacted with changing temperatures. Their aim is to determine whether the mountain had been progressively weakened before the final collapse.
For Rohde, establishing that link would provide much stronger evidence of climate change’s role. He said proving that the rock near the fracture plane had been significantly weakened by meltwater, permafrost thaw or both would constitute a “smoking gun” for climate change.
The temperature reconstruction was based on data from the Copernicus programme’s ERA5 climate model and information from 14 high-altitude weather stations situated between nine and 140 kilometres from the collapsed glacier.
The evidence therefore presents a picture of an exceptionally warm period, substantial snowmelt and a mountain system that may already have been approaching instability. Yet the precise chain of events remains under investigation.
The distinction is significant. The data indicate that the glacier experienced extraordinary heat immediately before the collapse, while the scientific evidence has not established that the heat alone caused it. Researchers are now seeking to determine whether rising temperatures, melting water and degrading permafrost helped push an already unstable mountain towards a fatal tipping point.
