A massive crack in Antarctica’s fourth-biggest ice shelf has surged forward by at least 10 kilometres since early January. Scientists who have been monitoring the 175-kilometre rift in the Larsen C ice shelf say that it could reach the ocean within weeks or months, releasing an iceberg twice the size of Luxembourg into the Weddell Sea.


The plight of Larsen C is another sign that global warming is destabilizing ice along the eastern Antarctic Peninsula and raising sea levels. But scientists’ studies of the rift also illuminate how far glaciology has come since the collapse of the ice shelf’s northern siblings: Larsen A in 1995 and Larsen B in 2002, which occupied separate embayments further out along the peninsula.

“Larsen B was a turning poing in our understanding,” says Ala Khazendar, a geophysicist at NASA’s Jet Propulsion Laboratory in Pasadena, California. “It was the biggest collapse of its kind up to that point, and it served to demonstrate how ice shelves regulate the movement of ice from the interior of the ice sheet to the ocean.”

For decades beforehand, researchers had debated the extent to which ice shelves buttress glaciers on land—acting like corks that slow the land ice’s inevitable march to the sea. The late Bob Thomas, a NASA glaciologist who helped to popularize the idea, went so far as to uncork a bottle of wine and pour some out to demonstrate the effect during his talks.

Satellite data collected after Larsen B collapsed largely settled the debate. The speed at which glaciers connected to Larsen A and B flowed to the sea increased—by up to a factor of eight—after those ice shelves disintegrated, says Eric Rignot, a glaciologist at the University of California, Irvine. “Some of [the glaciers] have slowed down a little bit, but they are still flowing five times faster than before,” he notes.

Since Larsen B’s collapse, ice-sheet modellers have tweaked their simulations to better reflect the forces driving glacial flow and to help quantify this corking effect—bolstering confidence that limited observations from the Larsen ice shelves could be applied more broadly.

Researchers are now looking back to the history of Larsen A and B to understand what the future might hold for Larsen C, which covers 50,000 square kilometres with ice up to 350 metres thick. Many fear that the expanding crack is a sign that Larsen C has begun a long decline that will inevitably end in its total collapse. How soon that could come after the iceberg breaks off is an open question.

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