Cold War-era spy satellite images are helping researchers get their first consistent look at how glaciers across the Himalayas are changing and what future water supplies might look like for millions of people who rely on their seasonal melt.

During the 1970s and '80s, the US "Hexagon" program launched a series of satellites that photographed various sites around the world that were of interest to intelligence agencies.  The satellites dropped the rolls of film, which were then collected in mid-air by military planes.  The photos were declassified in 2011, and are being digitized by the US Geological Survey (USGS) for scientists to use.  Some of these where images of the top of the world - the Himalayan Range. 

"What we are trying to do is to quantify by exactly how much are the glaciers retreating, how much ice are they losing and at what rate," said Josh Maurer from Columbia University in New York, who presented the research at the American Geophysical Union meeting in San Francisco.  "So we've used the images to extract 3D models of the terrain back in the 1970s."

Maurer and his co-authors are comparing the old photos to modern imagery produced by NASA and the Japanese space agency JAXA.  "We can see the height of the glacier ice in 1973, and we take those elevation models, and can take the difference between those and the modern day elevation models, and we can work out how the ice volume is changing over time," he said.

The researchers have been able to determine that the Himalayas are losing 0.17 to 0.25 of a meter of water every year as the earth gets warmer because of man-made climate change.  Roughly 20 percent of the world's population relies on the Himalayan glaciers' seasonal meltwater, in addition to monsoonal rain and snowfall, for drinking water, to grow crops, and to produce energy.

"As the glaciers shrink and retreat, the amount of run-off they provide to these streams will increase in the short term as they melt, but over the next 100 years it will decrease more and more, and that’s going to have a negative impact on water resources."