“The glaciers are still being pushed out of balance,” Eric Rignot, a senior scientist at NASA and an author of the paper, told me. “Even though the ice sheet has [sometimes] been extremely cold and had low surface melt in the last year, the glaciers are still speeding up, and the ice sheet is still losing mass.”

The paper casts the transformation of the Greenland Ice Sheet as one of the profound geological shifts of our time. Scientists measure the mass of ice sheets in “gigatons”—each unit equal to 1 billion metric tons, or roughly the same amount of water that New York or Los Angeles uses in a year. Greenland, according to the study, has lost 4,976 gigatons of water since 1972. That’s enough water to fill 16 trillion bathtubs or 1.3 quadrillion gallon jugs. That much water weighs about 11 quadrillion pounds. (A quadrillion is 1 with 15 zeros after it.)

More worryingly, the paper finds that Greenland lost about half of that ice—roughly 2,200 gigatons—in the years between 2010 and 2018. The ice sheet has also failed to gain mass in any year since 1998.

This melting isn’t happening at a steady pace, in other words. Greenland’s demise seems to be accelerating. Think of Greenland as a huge inland ice sea, surrounded by faster-moving ice rivers (which are glaciers) that empty the sea and carry ice to the ocean. The paper finds that those rivers are speeding up, carrying ice out of the island’s core nearly twice as fast now as they did in the 1990s or 2000s.

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That’s an alarming result, because it means glaciers might now be shrinking Greenland from the bottom faster than hot weather can melt it from the top. And researchers believe that bottom-melting glaciers are less stable and more prone to rapid collapse. “If there’s a risk of rapid sea-level rise in the future, it will be associated with glaciers speeding up, and not anything happening at the surface,” Rignot said.

The paper’s findings are stirring in part because they go much further back in time. “A lot of the publications [about Greenland’s mass] start in 2000 or 2002, some go back to 1992, but this is the first time we go back another 20 years,” Rignot said. Historically, most studies of Greenland combine data from radar flybys, GPS beacons, and laser or gravity-sensing satellites. But there’s not enough data from before 1992 to be useful, so that’s when estimates usually stop.

Rignot and his colleagues helped hit upon a new resource. The U.S. Geological Survey’s Landsat satellites have circled the planet nonstop since 1972, imaging every speck of land on Earth every 16 days. This archive—which is a kind of Earth-science version of taking a photo of yourself every day for years—includes hundreds of images of Greenland. Rignot and his team taught a computer how to read those pictures of its icy surface, zooming in especially on the dozens of glaciers that connect the interior ice sheet to the sea.