Comparing data obtained by Cold War-era spy satellites with images from modern stereo satellites, scientists have shown that Himalayan glaciers have lost more than a quarter of their ice mass since 1975, with melting occurring twice as fast after the turn of the century as average temperatures rose.
In the 1970s, at the height of the Cold War, the U.S. had deployed spy satellites that orbited the globe and took thousands of photographs, using a telescopic camera system, for reconnaissance purposes. Film recovery capsules would be ejected from the KH-9 Hexagon military satellites and parachuted back to Earth over the Pacific Ocean.
More than four decades later, scientists are using those same images to show the devastating impact of a warming earth on the Himalayan glaciers. The overlapping images, each covering 30,000 square kilometres with a ground resolution of six to nine metres, have been pieced together to form digital elevation models of the Himalayas of that era.
In an article published in the Science Advances journal on Wednesday, J.M. Maurer and co-authors analysed four decades of ice loss for 650 of the largest glaciers across a 2,000 km transect across the Himalayas.
“Our observed annual mass losses suggest that of the total ice mass present in 1975, about 87% remained in 2000 and 72% remained in 2016,” the study’s authors wrote. “We find similar mass loss rates across subregions and a doubling of the average rate of loss during 2000–2016 relative to the 1975–2000 interval,” they added.
The study goes on to assert that rising temperatures are responsible for the accelerating loss.
“This is consistent with the available multidecade weather station records scattered throughout HMA [High Mountain Asia, which includes all mountain ranges surrounding the Tibetan Plateau] which indicate quasi-steady mean annual air temperatures through the 1960s to the 1980s with a prominent warming trend beginning in the mid-1990s and continuing into the 21st century,” the authors wrote, noting an average increase of 1° celsius since 2000.
