Delaware-sized iceberg breaks off Antarctic ice shelf

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COMPLETELY CRACKED Images from the European Space Agency’s Sentinel-1 satellites confirm the crack of Larsen C has reached the Weddell Sea, allowing an iceberg weighing 1 trillion metric tons to calve.

With a final rip, an iceberg roughly the size of Delaware has broken off Antarctica’s Larsen C ice shelf. Anticipated for weeks, the fracture is one of the largest calving events ever recorded.

On July 12, satellite images confirmed a nearly 5,800-square-kilometer, 1-trillion-metric-ton chunk of ice, equivalent to 12 percent of Larsen C’s total area, split from the ice shelf. “[We] have been surprised how long it took for the rift to break through the final few kilometers of ice,” Adrian Luckman, a glaciologist at Swansea University in Wales, said in a blogpost for Project MIDAS, which has been tracking the effects of a warming climate on the ice shelf. Now the focus will shift to the stability of the remaining ice shelf and the fate of the giant iceberg.

Scientists had been monitoring Larsen C since 2014, when they noticed that a crack in the ice shelf had grown roughly 20 kilometers in less than nine months (SN: 7/25/15, p. 8). After a relative lull in 2015, the crack grew another 40 kilometers in 2016 and then 10 more in the first half of January 2017, bringing its total length to 175 kilometers. At that point, its tip was 20 kilometers from the Weddell Sea.

The crack grew another 17 kilometers between May 25 and May 31 — at times traveling parallel to the edge and ultimately putting it within 13 kilometers of the ice front. Then, in late June, the outer part of the ice shelf picked up speed, putting new pressure on the crack and the entire shelf. “The remaining ice is strained near to breaking point,” Luckman tweeted June 30. “It won’t be long now.”

Yet the vigil lasted nearly two more weeks. By July 6, the crack had come within 5 kilometers of the edge of the ice. Then, sometime between July 10 and July 12, it finally reached the water, allowing the huge hunk of ice to splinter off into the sea.

The ice loss dramatically alters the landscape of Larsen C, Luckman notes. “Maps will need to be redrawn.” And that could be the least of the trouble ahead, says Adam Booth, a geophysicist at the University of Leeds in England also with Project MIDAS. “The calving event is significant because it is likely a precursor to something much bigger, potentially the collapse of the whole Larsen C ice shelf,” Booth says. The same thing happened to the neighboring Larsen B ice shelf in 2002, after it calved a Rhode Island-sized iceberg (SN: 3/30/02, p. 197).

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