The mountain section whose collapse triggered fatal floods in Nepal and China on August 26 had experienced unprecedented heat for the time of year, meteorological data analyzed by a researcher shows.
Average temperatures on the mountain glacier at an altitude of 5,200 meters (17,060 feet) were estimated at around five degrees Celsius between August 21 and 26.
This is exceptionally high.
They had not exceeded 4.0C over the same period in more than 55 years of data recording, although no direct link to the collapse has so far been established.
This series of highly localized temperatures was reconstructed by Robert Rohde, chief scientist at the independent Berkeley Earth organization in California.
At the site of the collapse, the average temperature recorded over that six-day period has risen by about 1.8C since the mid-20th century.
Without the additional heat brought by climate change, a comparable temperature pattern "would probably occur only once in a few thousand years", Rohde calculated for AFP.
The collapse came during the Langtang region's fourth-hottest summer on record, according to AFP's analysis of data from Europe's Copernicus observatory dating back to 1970.
- Tipping point -
"These hot conditions can also explain permafrost thaw and combined slope and glacier collapse," said French climatologist Valerie Masson-Delmotte.
Permafrost acts as ice cement, holding rock faces together. Its degradation makes them more unstable.
Satellite images also showed significant snowmelt in the days before the disaster, providing "a substantial source of liquid water that can infiltrate fractures in the mountain and facilitate landslides", said Etienne Berthier, a glaciologist at France's CNRS research agency.
Experts remain cautious, however, about the precise role played by the heatwave.
"It's not as straightforward as 'high temperature equals collapse,'" Kristen Cook, a geomorphologist at the Grenoble Alpes University in France, told AFP.
"If the temperature acted as a trigger, it seems that it needed to wait until the rock was right at the tipping point of collapse," she added.
Researchers are now examining the small movements observed in the mountain in the years before the collapse and their interaction with changes in temperature.
"Proving the rock near the fracture plane was significantly weakened as a result of meltwater and/or permafrost thaw" would be a "smoking gun" of climate change, according to Rhode of Berkeley Earth.
His temperature analysis used data from the Copernicus program's ERA5 climate model and a network of 14 high-altitude weather stations located between nine and 140 kilometers (six and 87 miles) from the collapsed glacier.
