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An enormous chunk of the Larsen C ice shelf on the Antarctic Peninsula now looks doomed to calve into the Weddell Sea, possibly within weeks.

Its fate is being sealed by a sudden change of direction in a 200-kilometre ice crack. Until last week, it had been running parallel to the Weddell Sea, but it has now turned seaward, satellite images have revealed.

The rift grew 17 kilometres between 25 and 31 May, having been stationary since January, and is now just .

“Now that so little ice remains joining the iceberg to the ice shelf, we expect propagation to be quicker, but we really cannot know for sure how long it will take,” says of Swansea University, UK, and head of , which monitors the ice shelf. “It could be any time, maybe within weeks, or possibly months.”

“Assuming the propagation speed doesn’t diminish, the shorter distance that the crack needs to grow will presumably bring forward the detachment date,” says of the British Antarctic Survey in Cambridge.

Top 10

Covering 5000 square kilometres, the future berg is in the top 10 largest ever recorded, says Luckman. When it finally breaks free, it will take with it 10 per cent of the Larsen C ice shelf. Two other northerly ice shelves on the peninsula – Larsen A and B – have already broken up, with Larsen B disintegrating in 2002.

Luckman says the sudden acceleration of the fissure occurred after it unexpectedly broke through a zone of resistant “suture ice” to reach colder, weaker ice. “The rift propagates quickly through the cold, brittle ice and slowly through zones of suture ice,” he says.

Because the ice is already afloat it won’t affect sea level when it calves. But Larsen C holds back glaciers from the Antarctic Peninsula ice sheet, so losing the ice shelf altogether could also accelerate glacier loss.

“What’s happening at Larsen C may be a useful lesson as to what may eventually happen to the much larger and more critical ice shelves elsewhere in Antarctica,” Luckman says.