Jun 9, 2017

A Crack in an Antarctic Ice Shelf Is 8 Miles From Creating an Iceberg the Size of Delaware

Antarctica
by
Jugal K. Patel
,
New York Times
Image: NASA blue marble imagery
Image: NASA blue marble imagery

A rapidly advancing crack in Antarctica’s fourth-largest ice shelf is getting close to a full break, according to scientists. It has accelerated this year in an area already threatened by warming temperatures, and is now only about eight miles from the edge of the ice shelf.

The crack in Larsen C is more than 120 miles long, and some parts of it are as wide as two miles.

Once the crack reaches all the way across the ice shelf, it will create one of the largest icebergs ever recorded, according to Project Midas, a research team from Swansea University and Aberystwyth University that has been monitoring the rift since 2014.

Because of the amount of stress the crack is placing on the remaining eight miles of the shelf, the team expects the break to happen soon.

The glaciologist Eric J. Rignot, a professor at the University of California, Irvine, and a senior scientist at the NASA Jet Propulsion Laboratory, says the rift’s recent turn toward the shelf is a strong sign of when the full break could occur.

“In my experience, when the rift takes a 90 degree turn like this is, it will happen in the next few weeks, no more than that,” he said.

The time-lapse image below shows the rift gradually widening from late 2014 to January of this year.

Ice shelves, which form through runoff from glaciers, float in water and provide structural support to the glaciers that rest on land. When an ice shelf collapses, the glaciers behind it can accelerate toward the ocean. Higher temperatures in the region are hastening the ice shelf’s retreat.

If it breaks at the crack, Larsen C will be at its smallest size ever recorded.

The ice front would also be left much closer to the ice shelf’s compressive arch, a line that scientists say is critical for structural support. If the front retreats past that line, the northernmost part of the shelf could collapse within months. It could also significantly change the landscape of the Antarctic peninsula.

“At that point in time, the glaciers will react,” Dr. Rignot said. “If the ice shelf breaks apart, it will remove a buttressing force on the glaciers that flow into it. The glaciers will feel less resistance to flow, effectively removing a cork in front of them.

The crack in Larsen C is a third of a mile deep, down to the floor of the ice shelf.

Scientists fear that two crucial
anchor points will be lost.

According to Dr. Rignot, the stability of the whole ice shelf is threatened.

“You have these two anchors on the side of Larsen C that play a critical role in holding the ice shelf where it is,” he said. “If the shelf is getting thinner, it will be more breakable, and it will lose contact with the ice rises.”

Ice rises are islands overridden by the ice shelf, allowing them to shoulder more of the weight of the shelf. Scientists have yet to determine the extent of thinning around the Bawden and Gipps ice rises, though Dr. Rignot noted that the Bawden ice rise was much more vulnerable.

“We’re not even sure how it’s hanging on there,” he said. “But if you take away Bawden, the whole shelf will feel it.”

The collapse of the Larsen C ice
shelf may not significantly affect
global sea level rise, but the collapse
of other vulnerable ice shelves will.

The Larsen A and B ice shelves — both much smaller than Larsen C — disintegrated in 1995 and 2002. Neither contributed significantly to global sea level rise, however, because they were already floating above water, and the glaciers behind them did not contain a substantial volume of ice.

According to Dr. Rignot, the collapse of Larsen C would add only a tiny amount of water to the global sea level. Of greater concern to scientists is how the collapse of ice shelves can affect the glaciers that flow behind them, because the melting of those glaciers can cause much greater increases in sea level. Scientists see the impending Larsen C collapse as a warning that far larger amounts of ice in West Antarctica could be vulnerable.

“I think of the Larsen C as a natural laboratory that almost lets us do an experiment on an ice shelf,” said Thomas P. Wagner, who runs NASA’s effort to study the polar regions. “When it breaks, what the heck happens next?"