Oblique view of Himalayan landscape captured by a KH-9 HEXAGON satellite on December 20, 1975 on the border between eastern Nepal and Sikkim, India. There’s a 3D version available to explore as well.
Image: Josh Maurer/LDEO

During the Cold War, spy satellites were constantly circling the Earth. But while the intent of those satellites is generally to track human activities, the imagery they captured has also proved to be a trove of scientific data.

The latest use of Cold War imagery has revealed a huge acceleration in Himalayan ice loss. The results published in Science Advances on Wednesday show that glaciers in the Himalayas are disappearing twice as fast this century as they were during the Cold War, a finding that underscores just how quickly Earth’s Third Pole is changing.

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Today’s satellites use fancy sensors and digital imagery to capture the planet. In comparison, the Cold War-era KH-9 Hexagon series of American spy satellites seem downright quaint. The big lugs went up into orbit from 1973-1980 with 30-mile rolls of film. They would snap images of Earth and then return the film for processing by dropping a glorified bucket with a parachute back toward Earth where it would be snatched up by planes in what is the most elaborate dead drop in spying history.

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The images were initially used to track Soviet activities, but scientists at Columbia’s Lamont Doherty Earth Observatory and the University of Utah saw another handy use for them. The satellites captured numerous images of the Himalayas in the 1970s, which the research team compared to images captured by ASTER, an instrument used on NASA’s more modern Terra satellites. Using this mix of data, the scientists set out to compare the old school images with new, digital ones captured from 2000-2016 over a 1,240 mile (2,000 kilometer) transect of the world’s highest mountain range. The old satellite imagery was processed using techniques pioneered a few years ago to turn them into 3D models, which made them easier to compare to the modern data. All told, the study looked at 650 glaciers.

The comparison reveals that glaciers are disappearing in the 21st century at twice the rate they were in the late 20th century. The marked acceleration in ice loss is sharpest in lower elevation glaciers. Glacier thickness around 13,100 feet (4,000 meters), for example, declined about 4.5 feet (1.4 meters) in the 21st century compared to just 1.3 feet (0.4 meters) from 1975-2000.

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The reason for sharp uptick in ice loss isn’t too surprising. It’s getting hot. The study notes that the 21st century annual average temperature is a full 1.8 degrees Fahrenheit (1 degree Celsius) higher than it was for the period from 1975-2000. Other research also shows the Indian monsoon has been weaker in the central Himalayas, which in turn has reduced the amount of snow glaciers receive each year to replenish summer melt. Soot deposited on the glaciers from industrial activities in China and other parts of Asia could is yet another factor contributing to the loss. But given the uniform ice loss and rising temperatures, the study makes it clear that heat is likely the overriding factor contributing to the Himalayan meltdown.

“This is the clearest picture yet of how fast Himalayan glaciers are melting over this time interval, and why,” Joshua Maurer, a Ph.D. candidate at Columbia University who led the study, said in a statement.

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The disappearance of ice could affect water availability in valleys below—valleys which are home to hundreds of millions—that depend on meltwater for power generation and agriculture. The spy satellite data provide a key glimpse of how the region got to this point. And it’s also why research efforts in the Himalayas, including taking on-the-ground measurements, are so crucial to understand what comes next.