ANN ARBOR – An alarming crackup has begun at the foot of Antarctica’s vulnerable Thwaites Glacier, whose meltwater is already responsible for about 4 percent  of global sea level rise. An ice sheet the size of Florida, Thwaites ends its slide into the ocean as a floating ledge of ice 45 kilometers wide. But now, this ice shelf, riven by newly detected fissures on its surface and underside, is likely to break apart in the next 5 years or so, scientists reported today at a meeting of the American Geophysical Union.

The most dramatic sign of impending failure is a set of diagonal fractures that nearly span the entire shelf. Last month, satellites spotted accelerating movement of ice along the fractures, says Erin Pettit, a glaciologist at Oregon State University, Corvallis, who is part of a multiyear expedition studying the glacier. The shelf is a bit like a windshield with a series of slowly opening cracks, she says. “You’re like, I should get a new windshield. And one day, bang—there are a million other cracks there.”

Once the ice shelf shatters, large sections of the glacier now restrained by it are likely to speed up, says Ted Scambos, a glaciologist at the University of Colorado, Boulder, and a leader of the Thwaites expedition. In a worst case, this part of Thwaites could triple in speed, increasing the glacier’s contribution to global sea level in the short term to 5%, Pettit says.

Even more worrisome is the process that has weakened the ice shelf: incursions of warm ocean water beneath the shelf, which expedition scientists detected with a robotic submersible. Because Thwaites sits below sea level on ground that dips away from the coast, the warm water is likely to melt its way inland, beneath the glacier itself, freeing its underbelly from bedrock. A collapse of the entire glacier, which some researchers think is only centuries away, would raise global sea level by 65 centimeters. And because Thwaites occupies a deep basin into which neighboring glaciers would flow, its demise could eventually lead to the loss of the entire West Antarctic Ice Sheet, which locks up 3.3 meters of global sea level rise. “That would be a global change,” says Robert DeConto, a glaciologist at the University of Massachusetts, Amherst. “Our coastlines will look different from space.”

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