Larsen C ice shelf and the Weddell Sea, covered with sea ice. (NASA)

This story has been updated.

In early 2002, scientists peering in on Antarctica from satellites saw something they found very alarming.

The Larsen B ice shelf, bigger than Rhode Island and 720 feet thick, underwent a sudden collapse in only 35 days (video here). It was following in the footsteps of the 1,500-square-kilometer Larsen A ice shelf, which collapsed in 1995, but Larsen B was more than two times larger.

The ice shelf, according to the National Snow and Ice Data Center, had “likely existed” for 12,000 years before it collapsed and ceased to be a feature of the Earth.

Larsen A and Larsen B left behind Larsen C, a dramatically bigger mass floating outward into icy waters off the Antarctic peninsula. It’s the biggest ice shelf in this region of the continent, encompassing an area “two and a half times the size of Wales and slightly smaller than Scotland,” according to the British Antarctic Survey. It’s also now the most north-lying — which, in this case, means in the direction of warmer South America — of the Antarctic peninsula’s great ice shelves that remain.

[The melting of Antarctica was already really bad. It just got worse]

And Larsen C, too, is starting to look vulnerable, suggests a newly published study by a team of researchers with the British Antarctic Survey, the U.S. Geological Survey and several U.S. universities. The research notes several mechanisms that “could pose an imminent risk” to the ice shelf.

A study earlier this year had already found a growing rift in the ice shelf that is “likely in the near future to generate the largest calving event since the 1980s,” as the paper put it. And researchers also know that the ice shelf has been sinking — in other words, becoming lower in the water. Because ice shelves float atop the water with the vast majority of their mass below the surface and only a small percentage above, such sinking suggests a loss of overall mass.

The sinking has been about a meter in the last 15 years, which may not sound like much for an ice shelf that rises about 50 meters out of the water and is some 350 meters thick. But it’s well within scientists’ ability to measure — and it worries them.

Now, in the new paper in the journal The Cryosphere, scientists report on the result of 15 years’ worth of aerial and ground-based radar surveys of the Larsen C ice shelf, seeking to understand reasons for its dip into the water. There are two principal possibilities: “compaction” — in which the ice atop the ice shelf loses some of the air that is contained within it, perhaps due to warmer temperatures and surface melting — and loss of mass from beneath, presumably due to warmer seas melting the ice shelf from below.

A double whammy for Larsen C

The upshot of the study is that both factors seem to be involved. “The ice shelf lost 4 meters of ice, and it lost 60 centimeters of air, and that caused the upper surface to go down by 1 meter,” explains Paul Holland of the British Antarctic Survey, the study’s lead author. “Effectively, of the lowering, about half was due to air loss, and the other half was due to ice loss.”

In other words, it appears that the warming of the Antarctic Peninsula — where a possible new record high temperature for the entire Antarctic continent of 63.5 degrees Fahrenheit was just set — may be contributing to ice melt at the surface of Larsen C. Meanwhile, warmer seas may also be taking a toll from beneath.

All of which, well, brings up bad memories. “We know with Larsen B, it did this lowering trick before it collapsed,” says Holland. “We also know that its snowpack was completely depleted of air before it collapsed.”

Still, Larsen C remains a truly gigantic ice shelf, and these processes alone — if they were the only things happening — would not necessarily lead to a collapse this century, the new study estimates. There’s just too much ice to lose. The study estimates that a collapse due to air loss and subsequent meltwater seeping through the ice shelf could take as many as 250 years to unfold, and one due to melting ice from below might take 170 to 500 years.

But these also may not be the only processes afoot. And as past research suggested in the wake of the 1995 Larsen A ice shelf collapse, “after retreat beyond a critical limit, ice shelves may disintegrate rapidly.”

As mentioned before, a large rift in the southern part of the Larsen C ice shelf has “propagated rapidly beyond a band of marine ice that has stabilised all such rifts during the observational era,” notes the new paper. The study found that this could threaten the stability of the ice shelf “within a few years.” It is one reason the new study says the ice shelf could face an “imminent risk.”

“In the southern end of Larsen C, there’s a big sort of rift, a crack that goes all the way through the ice shelf,” says Holland. “If this rift propagates further,” he continues, “and causes an iceberg calving, it’s possible this could cause a destabilization of the ice shelf. This is one of the mechanisms that could be one of the first signs of a collapse.”

A prior study reached a similar conclusion:

It seems inevitable that this rift will lead to a major calving event which will remove between 9 and 12 % of the ice shelf area and leave the ice front at its most retreated observed position. More significantly, our model shows that the remaining ice may be unstable. The Larsen C Ice Shelf may be following the example of its previous neighbour, Larsen B, which collapsed in 2002 following similar events.

Another potential vulnerability, meanwhile, involves a feature called Bawden Ice Rise, which is literally what it sounds like — a region on the Antarctic Peninsula where the ice rises, presumably because of a rock beneath it. Another “imminent risk” could occur from the “ungrounding” of the Larsen C ice shelf from the rise.

However, there is much uncertainty about each of these possibilities, so it is important to emphasize that the researchers are far from certain that Larsen C will collapse any time soon. Officially, the researchers who conducted the work now think that “a collapse could occur within a century, although maybe sooner and with little warning,” according to the British Antarctic Survey.

Faster sea level rise, but not massive

The good news is that unlike other Antarctic ice shelves that have been observed to be losing mass — such as in West Antarctica and the East Antarctic Totten glacier region — Larsen C does not hold back a very large amount of land-based ice that would, upon sliding into the ocean, contribute to sea level rise. Holland estimates that the contribution to sea level rise from that ice — currently hemmed in by the ice shelf — would be about 10 centimeters.

Still, he says, “if Larsen C were to collapse at some point in the next 100 years, you’d expect within the few decades after that collapse, a much faster rate of sea level rise than if it hadn’t collapsed.”

Sea level is already rising at a faster rate than it was in the last century, recent research suggests, due to more loss of polar ice from Greenland and Antarctica.

[Sea levels are rising at a faster clip as polar melt accelerates, new study shows]

“Even a little sea-level rise can matter to coastal people already on the edge,” adds Richard Alley, a glaciologist at Penn State University, in email comments on the new study. “And understanding this will help in understanding the larger, more globally important events that remain possible in our future.”

So in sum, scientists don’t really know when we might lose the Larsen C ice shelf, but they’re definitely concerned. And combined with other recent news regarding the ice shelves of West Antarctica and East Antarctica’s massive Totten Glacier, it’s enough to make you think — yet again — that ice giant at the bottom of the Earth is stirring.

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