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United States spy satellites that secretly kept watch over the Himalayas during the Cold War are helping researchers piece together the most detailed view yet of the region’s accelerating ice loss.

The U.S. military used a telescopic camera system to take thousands of photographs worldwide as part of its surveillance of the Soviet Union, China and other countries, dating as far back as the 1970s.

Now declassified, that satellite data is helping researchers track the melting of the Himalayan glaciers, and it is revealing changes across the region that scientists say are consistent with global warming. The results could help communities in Nepal, India and neighboring countries who rely on those glaciers prepare for future flooding and changes in their water resources as the climate changes.

In a new study, scientists analyzed satellite images of 650 glaciers as the ice changed over four decades and found that, on average, the Himalayan glaciers melted twice as fast between 2000 and 2016 as they did during the previous quarter century, from 1975 to 2000.

“This is the clearest picture yet of how fast Himalayan glaciers are melting over this time interval, and why,” said Josh Maurer, the study’s lead author and a Ph.D. candidate in Columbia University’s Earth and Environmental Sciences program.

The melting was driven by increases in air temperature, according to the study, published Wednesday in Science Advances. Since the melting was consistent across the region, the researchers couldn’t attribute it to more variable, localized effects such as black carbon, which darkens ice so it absorbs more energy, or precipitation.

“That doesn’t rule out impact by these other sources,” said study co-author Summer Rupper, an associate professor of geography at the University of Utah. But the evidence suggests warming temperatures are driving the extensive melting—and both are picking up speed.

“As warming accelerates, the loss of glacier ice—that rate accelerates, essentially, in lockstep with what the climate system is doing,” Rupper said.

Being able to track the changes over time helps researchers parse out trends connected with global warming. “You remove some of that year-to-year variability and you get what the actual long-term change looks like,” she said.

Watching the Himalaya Change—in 3D

Researchers have mostly looked at changes in the amount of land the glaciers cover, Rupper said. “But that doesn’t actually tell us how much ice or water is being lost,” she said.

To gauge the change in glacier volume, the scientists created a method to “essentially do what our eyes do naturally,” using overlapping images to construct a three-dimensional image of the terrain.

That image construction, done manually, is time-consuming. So Maurer, a former student of Rupper’s, developed an automated way to consistently process the data and provide the big picture changes over time.

Maurer said his experiences traveling to research remote areas in the Himalayas as an undergraduate helped him understand the importance of using satellite imagery to study difficult-to-access places like mountain regions. He worked for about two years developing the computer code used to automate the satellite image analysis.

The findings can help to calibrate and validate other computer models to project what the future might look like under different climate scenarios, he said. That could help policymakers and societies who depend on Himalayan glaciers to sustain their communities.

Glacier Melt Creates New Risks Downstream

The satellite data spans India, China, Nepal and Bhutan. Each of those countries, in addition to Afghanistan, Bangladesh, Myanmar and Pakistan, are members of the International Center for Integrated Mountain Development (ICIMOD), a regional partnership that has been closely studying the changes now underway in the mountain glaciers.

ICIMOD released a report earlier this year stating that, in a best-case scenario, Himalayan glaciers will lose more than one-third of their mass by the end of the century—and if greenhouse gas emissions continue at current rates, two-thirds could be gone.

That melting could have devastating consequences for communities in and near the Himalayan mountains, many of whom already face poverty and malnutrition. Glacier melt adds to those vulnerabilities, threatening floods, landslides and the loss of their water supply.

“With glacier melt, the impact is biggest for mountain communities who are directly dependent on glaciers,” said ICIMOD Chair David Molden, a development specialist and civil engineer.

Communities that rely on Himalayan glaciers for water already see the effects of shrinking ice, Molden said. As canals that channel glacier water to communities run dry, they have to dig new ones, higher in the mountains. “You can look at the mountain and kind of see these stripes of canals hitting the glacier,” Molden said.

Prediction Models Can Help Villages Prepare

The researchers plan to continue working with years of satellite data to compare changes in other regions, too. “We’d like to use this imagery for glacier studies in other areas such as Patagonia or the Arctic,” Maurer said.

They hope their findings can create a framework that will help communities adapt to potential changes and hardships as temperatures continue to warm.

“What does that change actually mean in terms of downstream water resources?” Rupper said. For these communities, “we can give some sense of projections. We don’t know exactly what the climate systems will do.”

Molden said the tracking of changes in the Himalayan region could be important in helping to persuade policymakers to plan for the effects of climate change.

“It’s strengthening the case that climate change is coming more rapidly than we expect,” Molden said.