The Pine Island glacier was already scary. The 160-mile-long river of ice is known as “the weak underbelly” of West Antarctica. It contributes more to sea-level rise than any other glacier on the continent and ranks among the fastest melting glaciers in the world.

Unlike other Antarctic glaciers, Pine Island is not sheltered from the warming ocean by a vast expanse of sea ice. The only thing preventing it from flowing directly into the Amundsen Sea embayment is a shelf of floating ice that sticks out from the glacier’s edge. This shelf is like a cork in a bottle, pressing against the stable sides of the bay to contain the tremendous pressure at its back.

But the ice shelf is tearing itself apart. It has lost one-fifth of its mass in the last five years, shedding icebergs the size of cities. Rifts have opened up in the center of the shelf, potentially adding to the instability.

If this disintegration continues, “the whole shelf could potentially fall apart in the next few years, which is greatly faster than what we expected,” said Ian Joughin, a glaciologist at the University of Washington’s Applied Physics Laboratory who co-wrote the new report.

The loss of the ice shelf would accelerate Pine Island’s decline even further. The faster it flows, the more ice it spits into the ocean, raising sea levels. The glacier already adds a sixth of a millimeter to sea level rise each year; but losing the ice shelf could double or triple that rate, Joughin said. Pine Island contains roughly 180 trillion tons of ice — enough to cause 1.6 feet of sea level rise.

Previously, scientists had focused on the slow but steady thinning of the ice shelf as warm ocean waters seep underneath it. This melting makes ice shelves more vulnerable to collapse during the Antarctic summer, when high temperatures cause melting on the surface. But, since temperatures in West Antarctica are rarely more than a few degrees above freezing, that process was expected to take centuries to unfold.

What’s happening now is much faster and less predictable, Joughin said. It appears that the rapid slide of the glacier is creating fractures in the ice shelf, which leads to more pieces breaking off, or “calving.” Computer simulations and mathematical models support the idea that this process is responsible for the glacier’s speed up.

NASA scientists flew over one of Pine Island’s newly formed bergs in 2018. Even from 1,500 feet, the Seattle-sized fragment took up the researcher’s entire field of vision. “It was spectacular and inspiring and humbling at the same time,” Brooke Medley, deputy project scientist for Operation Ice Bridge, said in a blog post from the agency.

Just two years later, more bergs broke off along the ice shelf’s “shear margin,” where it attaches to thicker ice along the sides of the bay.