Cracks are still spreading where that massive Antarctic iceberg broke free

Cracks continue to spread on the Antarctic ice shelf where a trillion-ton iceberg roughly the size of Delaware broke free in July, scientists say. If these new cracks manage to shake loose key, stabilizing parts of the ice shelf, it’s possible that the ice shelf could collapse. That could speed the flow of Antarctic ice to the sea and cause sea levels to rise.

On July 12th, 2017, a deep fissure cleaved a 2,500-square-mile iceberg off of the Larsen-C ice shelf in Antarctica. Satellite images show the iceberg has drifted about three miles since it broke free, and smaller ice chunks have crumbled from both the ice shelf and the iceberg. Now, scientists Anna Hogg at the University of Leeds and Hilmar Gudmundsson from the British Antarctic Survey want to know what losing such a massive chunk of ice will mean for the rest of the Larsen-C ice shelf. So they’re keeping watch via satellite, spelling out their next steps in an article published today in the journal Nature Climate Change.

The Copernicus Sentinel-1 satellite snapped this picture of the A68 iceberg on July 30th, 2017.

The Copernicus Sentinel-1 satellite snapped this picture of the A68 iceberg on July 30th, 2017.
Photo: A. Fleming, British Antarctic Survey.

Ice shelves form when a land-based ice sheet dips into very cold ocean waters. Glaciers flowing off the landmass keep adding to the ice shelf, pushing it away from land until the ice wedges up against an underwater bump or ridge. That anchors the floating ice shelf, and turns it into a kind of cork that prevents the ice behind it from flowing into the ocean, Hogg told The Verge.

Sometimes, parts of the ice shelf break off to form icebergs — a process known as calving that’s a perfectly normal part of an ice sheet’s life cycle. Icebergs don’t contribute to sea level rise because they’re already floating on the ocean’s surface. But occasionally, when an iceberg calves, it might loosen the ice shelf’s connection with its anchor — essentially popping that frozen cork and letting the ice flow more freely from land into the sea. That’s what boosts sea levels, Hogg says. “That will be the thing that changes this from a natural event that’s occurring all the time into something that has real implications for people living in other parts of the globe.”

Scientists don’t know yet whether this massive iceberg was the cork for the Larsen-C ice shelf — and it’s too early to say that the iceberg’s calving is cause for concern. “One iceberg coming off an ice shelf doesn’t mean that ice shelf is about to collapse,” says Helen Fricker, a geophysicist at the Scripps Institution of Oceanography. “But what are the effects of the calving event on the ice shelf that’s left behind? That’s a really important thing to look at,” Fricker says.

That’s why Hogg and her colleagues plan to take satellite images of the ice sheet that capture ripples and creases in the ice’s surface. By measuring how quickly these crevasses move towards the water’s edge, the scientists can calculate how quickly the ice is flowing. “So, if you were to animate it, it literally does look like a river flowing — but we’re just doing it over a longer period of time,” Hogg says. If the ice sped up after the iceberg broke away, that could mean the massive iceberg had been important for anchoring the ice shelf.

But, Hogg adds, it’s also possible that the cork is still in place — but the cracks spreading along the edge of the ice shelf could yet dislodge it. “Although this one iceberg has calved off, the story might not be over,” she says. “So we’ll be constantly looking at ice speeds over the next months to years to really appreciate the picture that’s constantly evolving on Larsen-C.”