An ice shelf is cracking in Antarctica, but not for the reason you think

A group of scientists is gathering this week in the U.K. to discuss a slab of ice that's cracking in Antarctica. The crack could soon split off a frozen chunk the size of Delaware.

One glacier scientist, Heidi Sevestre, spent six weeks last year living on that giant slab of ice off the Antarctic Peninsula.

"It's like being on a different planet," says Sevestre, a glaciologist with the University of St Andrews in Scotland. She and her colleagues would get really excited whenever they saw a bird pass overhead because it was the only other sign of life around.

"Everything is gigantic, everything is white," she says. And everything seemed so frozen and still. But it wasn't.

"When you're camping on the ice shelf, you have no idea that you're on something that is floating and moving," she says.

The ice shelf is in constant motion: rising with the tides, splitting off icebergs at its edges, and growing again as inland glaciers feed it.

The ice shelf Sevestre was studying is called Larsen C, and it now has a massive 90-mile crack running through it.

"The big rift is slicing the ice shelf from top to bottom," Sevestre says. It's now a third of a mile deep, and as wide across as 25 highway lanes.

But this is not just another sad climate change story. It's more complicated.

"A lot of things are going on deep inside the ice," says Adrian Luckman, a glaciologist at Swansea University in the U.K. He's also leading a project to track changes in the ice shelf.

Luckman says climate change is certainly influencing this region. Larsen C used to have two neighbors to the north, Larsen A and Larsen B. As the air and water warmed, those ice shelves started melting and then splintered into shards in 1995 and 2002.

But the crack in Larsen C seems to have happened on its own, for different reasons.

"This is probably not directly attributable to any warming in the region, although of course the warming won't have helped," says Luckman. "It's probably just simply a natural event that's just been waiting around to happen."

Larsen C has a bunch of cracks. All ice shelves do. This particular crack has been around since at least the 1960s. The unusual part is that in 2014, this crack — and only this crack — started growing in spurts. Why?

"Well, that is a little bit of a mystery and that's why it drew itself to our attention," says Luckman.

It left other cracks in the dust about 50 miles ago. Now, scientists are crunching satellite and radar data to figure out how.

"And that knowledge will be useful in helping us to understand other ice shelves and how they might respond to rifts coming into them," says Luckman.

One puzzling aspect is how it managed to plow through areas of softer ice, called suture zones, that bind the ice from neighboring glaciers into one giant sheet.

"There's something different about that ice that slows it down or causes it to hang up for some period of time," says Dan McGrath, a glaciologist at Colorado State University. But, starting in 2014, that soft ice did very little to slow down this rift.

"We need to get to the bottom of understanding what changed that allowed this rift to progress as it has, and will other rifts follow suit," says McGrath, who spent four field seasons camped out on the Larsen C ice shelf. (At one point, bad storms kept him inside his tent for more than a week. "Yeah, you're peeing in a bottle," he says. "There were moments during those seven days that I questioned whether I should have studied tropical reef ecology.")

Scientists are split on how important this crack is for the stability of the whole ice shelf.

"Just because this iceberg calves off, the ice shelf isn't just going to collapse and disappear overnight," says McGrath.

Some say if this giant section breaks off, it won't make a difference. Others think it could eventually cause the whole shelf to fall apart.

"I am cautiously worried," says Ala Khazendar, a geophysicist at NASA's Jet Propulsion Laboratory. "Ice shelves are very important. They are the gates of Antarctica in a way, and the gatekeepers of Antarctica."

The ice shelves are already floating, so if they fall apart it does not immediately affect sea levels. It's what they hold back — water from all the inland glaciers — that could be problematic.

Khazendar says there are two possible scenarios. One, the iceberg will break off, he says, "and nothing spectacular will happen for many, many years." The glaciers will bulk it up with ice until it's back to its former look. Or, two, this iceberg is just the first of many irreversible losses for Larsen C, which, in combination with enough warm summers, will be weakened and shatter like the previous Larsens.

"We shall see if that big calving leads to a collapse of the ice shelf. At the moment, this is still a big question mark," says Heidi Sevestre.

According to pessimistic estimates, if the ice shelf completely disintegrated and if all the water packed in those glaciers made their way to the sea, it could significantly raise global sea levels.

"It is quite a large impact, indeed," says Sevestre.

The 30-or-so ice shelf experts gathered in the U.K. this week aren't sure whether this more serious chain reaction will happen, but they are confident, at least, that the Delaware-sized chunk will come off. The crack only has about 10 miles left to go. Copyright 2019 NPR. To see more, visit https://www.npr.org.