A new study relying on declassified spy satellite images finds that the “Third Pole” is shrinking twice as fast as it did toward the end of the last century—an ominous sign for the more than 1 billion people who live along the rivers fed by those glaciers.

The big picture: The paper, published today in Science Advances, is one of the most comprehensive looks to date at the Himalayas’ vanishing rivers of ice, tracking the shifting shape of more than 650 glaciers across four decades. Researchers at Columbia University and the University of Utah used newly accessible images taken by US spy satellites between 1975 and 2000 that capture a 2,000-kilometer-wide stretch of the mountains spanning India, China, Nepal, and Bhutan. They converted those into 3D models, and compared them against modern satellite images of the same areas from the last two decades.

Thawing fast: The researchers found that from 2000 to 2016, glaciers in the region lost the equivalent of more than a vertical foot and half of ice each year—a marked uptick compared with the earlier portion of the time frame studied. The team also concluded that temperatures that began rising sharply across the region in the mid-1990s are the main driver of the melting (other factors, like shifts in precipitation and soot from burning fossil fuels and biomass, may play a role too).

The latest warning: The study is the sharpest warning yet that the fate of the Himalayan glaciers is becoming more precarious by the day—as is that of the people, farms, hydroelectric plants, and other businesses that depend upon them. A landmark report published in February concluded that 15% of the glaciers across the Hindu Kush region have already disappeared, and that two-thirds could vanish by the end of the century if the world continues to pump out greenhouse gases at current levels.

Asia’s water tower: Himalayan glaciers are the largest stockpile of ice outside the North and South Pole, and the wellspring of 10 major Asian waterways. Glacier and snow melt provide between 20% and 80% of the runoff to the upper reaches of the Ganges, Indus, Yangtze, and Yellow rivers, according to an earlier report by China Water Risk.

The impact of the accelerating melting is complex, but it is ultimately devastating for a broad swath of Asia. As I explained in an earlier piece on India’s increasingly perilous water challenges: “Initially the increased runoff will swell rivers, raising the risks of downstream flooding but sending Indians more water. That trend is likely to shift into reverse in the second half of the century, however, shrinking the flow to around 1.9 billion people who live along those rivers. The Ganges basin alone supports 600 million people, provides 12% of the country’s surface water, and accounts for 33% of GDP.”