Himalayan glaciers are melting twice as fast now as they were before the turn of the century, according to a new study that relied on recently declassified Cold War-era satellite imagery.
The study, which appeared in Science Advances on Wednesday, is the latest indication that climate change is eating the Himalayan glaciers, threatening water supplies for hundreds of millions of people downstream across South Asia.
"This is the clearest picture yet of how fast Himalayan glaciers are melting over this time interval, and why," said lead author Joshua Maurer, a doctoral candidate at Columbia University in New York.
Scientists combed 40 years of satellite observations spanning 2,000 kilometers (1,243 miles) across India, China, Nepal and Bhutan, and found that the glaciers have been losing the equivalent of a foot-and-a-half (45 centimeters) of ice each year since 2000.
Many of the 20th-century observations came from recently declassified US spy satellite imagery.
The figure is double the amount of melting that took place from 1975 to 2000.
Past research has found similar trends, but the latest work is bigger in its geographic and historic scope.
It concluded that rising temperatures are the biggest factor.
Though temperatures vary from place to place, average temperatures were one degree Celsius (1.8 degrees Fahrenheit) higher between 2000 to 2016 than they were between 1975 and 2000.
Other factors the researchers blamed were changes in rainfall, with reductions tending to reduce ice cover, and the burning of fossil fuels which lead to soot that lands on snowy glacier surfaces, absorbing sunlight and hastening melting.
"It shows how endangered (the Himalayas) are if climate change continues at the same pace in the coming decades," said Etienne Berthier, a glaciologist at France's Laboratory for Studies in Geophysics and Spatial Oceanography, who was not involved in the study.