Images from Cold War spy satellites have revealed the dramatic extent of ice loss in the Himalayan glaciers.
Scientists compared photographs taken by a US reconnaissance programme with recent spacecraft observations and found that melting in the region has doubled over the last 40 years.
The study shows that since 2000, glaciers heights have been shrinking by an average of 0.5m per year.
The researchers say that climate change is the main cause.
"From this study, we really see the clearest picture yet of how Himalayan glaciers have changed," Joshua Maurer, from Columbia University's Lamont-Doherty Earth Observatory in New York, told BBC News.
The research is published in the journal Science Advances.
During the 1970s and 1980s, a US spy programme – codenamed Hexagon – launched 20 satellites into orbit to secretly photograph the Earth.
The covert images were taken on rolls of film that were then dropped by the satellites into the atmosphere to be collected mid-air by passing military planes.
The material was declassified in 2011, and has been digitised by the US Geological Survey for scientists to use.
Among the spy photos are the Himalayas – an area for which historical data is scarce.
By comparing these pictures with more recent satellite data from Nasa and the Japanese space agency (Jaxa), the researchers have been able to see how the region has changed.
The Columbia University team looked at 650 glaciers in the Himalayas spanning 2,000km.
The group found that between 1975 and 2000, an average of 4bn tonnes of ice was being lost each year.
But between 2000 and 2016, the glaciers melted approximately twice as fast – losing about 8bn tonnes of ice each year on average./BBC news