When the United States launched its Hexagon KH-9 surveillance satellites in the 1970s, the hope was to collect crucial intelligence on Soviet activities as they surveyed over 877 million square miles of the Earth’s surface. But these eyes in the sky ended up capturing images of a different threat to the future of the planet: the accelerating loss of ice on the Himalayas aka the world’s “third pole.” This phenomenon threatens the water supply of millions of people.

It’s hard to imagine the Himalayan mountain range without the characteristic icy plains and snow capped peaks that make it so iconic. But the range’s glaciers bear signs of rapid decline. Using film images captured by the KH-9 surveillance satellite in the 1970s, alongside modern day satellite data, scientists at Columbia University’s Earth Institute estimate that Himalayan glaciers have lost twice as much ice in the 21st century as they did in the 20th. They published their findings Wednesday in Science Advances.

Joshua Maurer, the lead study author and a Ph.D. candidate at Columbia says that KH-9’s data helped paint a new, clear picture of how the iconic mountain range has changed since the Seventies.

“The declassified spy satellite imagery clarify the picture of ice loss by providing a longer timespan of observation, which allows for more robust estimates,” Maurer tells Inverse.

A view of the Himalayas on the border of Sikkim, India and eastern Nepal, captured December 20, 1975 by a KH-9 HEXAGON spy satellite. Joshua Maurer

What We Learned From KH-9’s Images

Before they became a tool to alert the world to rapid ice loss in the Himalayas, the Hexagon KH-9 satellites began their lives as spy satellites. The Hexagon satellite was equipped with two cameras (called KH-9) that could create panoramic images to reveal terrain and potentially Soviet defense activities.

"The declassified spy satellite imagery clarify the picture of ice loss…"

Between 1973 and 1986, they captured film images that were then ejected from the satellites and sent careening back into the Pacific Ocean, for collection and analysis by the National Reconnaissance Office.

Using those images, Maurer and Columbia professor Joerg Schaefer, Ph.D., constructed a 3D model that shows changes in the elevations of 650 major glaciers. Specifically, they looked closely at a 2,000-kilometer transect that runs from Spiti Lahaul (a district in the northeast of India) to Bhutan, which touches the Eastern edge of the Himalayas.

The Hexagon KH-9 satellite system. Wikipedia Commons

They estimate that 10 inches of ice per year were lost across the 650 major glaciers in that region beginning in 1975. But their model also shows that this rate has rapidly accelerated since 2000. In the 21st century, the glaciers lost 20 inches of ice annually. At lower elevations, that rate is even higher — around 16 feet of ice loss per year.

In their explanation for the declines, the authors note that temperature increases likely play a large role, in addition to changes in monsoon frequency in the Eastern regions. For the glaciers to decline that quickly, they estimate that regional temperature would have to have increased between 0.4°C and 1.4°C after 2000. Their analysis of local meteorological stations supported this hypothesis: On average, air temperatures in high mountain Asia were an average of about 1°C warmer between 2000 and 2016 compared to the Seventies.

That temperature-related glacier decline has also shown up across the rest of the Hindu-Kush Himalayan region, a far larger swath of the mountains than these researchers looked at specifically. In February, a report in the Hindu-Kush Himalaya Assessment noted that 36 percent of the region’s glaciers could be completely melted by 2100, even if we manage to halt global temperature increases to 1.5°C by then.

What Is the “Third Pole?”

In the paper, the authors often refer to the Himalayan glaciers as the “Third Pole.” The region has earned this title in recent years because it’s the largest collection of permanent ice and permafrost in the world outside of the North and South Poles.

Changri Nup Glacier, one of the hundreds studied by the researchers. Much of it is covered by rocky debris. The peak of Mt. Everest is in the background on the left. Joshua Maurer

Maurer estimates that the region has lost an average of 8 billion tons of water (enough to fill 3.2 million Olympic swimming pools) in recent years as the glaciers have wasted away. That water loss, they note, has consequences.

Ice sheet loss at the Poles, for instance has been projected to cause sea level rises that threaten coastal cities. But the melting of the third pole will likely threaten the water supply of the millions of people in East Asia, who derive their fresh water sources from glacier runoff into local rivers.

As the glaciers thin, over time, so may the water supply:

“Reduced meltwater contributions to rivers, which will change the timing and magnitude of water supply in heavily populated downstream regions in the coming decades,” Maurer says.

In that sense, these US spy satellites succeeded in their mission. They’ve tracked the enemy’s activities and provided valuable intel that could likely save millions of lives. But what we didn’t expect was that the 40 years later, the enemy would be ourselves.

Abstract: Himalayan glaciers supply meltwater to densely populated catchments in South Asia, and regional observations of glacier change over multiple decades are needed to understand climate drivers and assess resulting impacts on glacier-fed rivers. Here, we quantify changes in ice thickness during the intervals 1975–2000 and 2000–2016 across the Himalayas, using a set of digital elevation models derived from cold war–era spy satellite film and modern stereo satellite imagery. We observe consistent ice loss along the entire 2000-km transect for both intervals and find a doubling of the average loss rate during 2000–2016 [−0.43 ± 0.14 m w.e. year−1 (meters of water equivalent per year)] compared to 1975–2000 (−0.22 ± 0.13 m w.e. year−1 ). The similar magnitude and acceleration of ice loss across the Himalayas suggests a regionally coherent climate forcing, consistent with atmospheric warming and associated energy fluxes as the dominant drivers of glacier change.