Over a matter of days in late September, Stef Lhermitte watched via satellite as a new, massive crack formed along the edge of Antarctica's Pine Island Glacier.

Just last year that glacier shed a Manhattan-sized slab of ice. But that particular iceberg was relatively small.

Lhermitte, a geoscientist specializing in remote sensing at the Netherlands' Delft University of Technology, expects this latest rift, when it eventually breaks, to produce an iceberg roughly 30 kilometers wide by 10 kilometers across (19 miles by 6 miles).

That would be Pine Island's sixth-largest calving event since 2001 — producing an iceberg five times the size of Manhattan.

A new 30 km long rift appeared across Pine Island Glacier since September indicating the upcoming calving of a ~300km² iceberg [1/n] pic.twitter.com/Dnh3YMKYIs — Stef Lhermitte (@StefLhermitte) October 4, 2018

"It is impossible to forecast, but I would expect it to calve somewhere this Antarctic summer [U.S. winter], but it is difficult to further fine-tune it," Lhermitte said over email.

This calving event wouldn't be record-breaking, nor an immediate red alert. But it unquestionably perpetuates a troubling trend.

Like most of West Antarctica's ice shelves — the ends of glaciers floating over the ocean — Pine Island is retreating inland and thinning at an accelerated pace, said Lhermitte.

These ice shelves are hugely important; they hold back immense masses of Antarctic ice from flowing into the ocean, just like a plug or cork.

And this is a cork you don't want to remove.

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"They’re like the cork in a bottle," Josh Willis, a NASA oceanographer who researches glaciers from aboard aircraft, said in an interview. "If you break off a shelf, they [glaciers] can speed up very quickly."

Pine Island is already breaking off more ice than it can replenish.

This means the ice plug is retreating back to land, ultimately becoming more vulnerable to weakening, or collapse. This would likely let loose rivers of ice into the sea, which would eventually mean yards — not feet — of sea level rise.

Accelerating ice loss in Antarctica. Image: IMBIE/PLANETARY VISIONS

"In West Antarctica there have been a bunch of famous ice shelves that have collapsed completely — and the glaciers upstream accelerated," said Willis, noting events like the dramatic Larson B ice shelf collapse.

But glaciers like Pine Island hold back much more Antarctic ice, so scientists like Lhermitte are watching closely. He receives satellite images of these glaciers, sometimes multiple times each day, so he can observe any changes — like new massive cracks forming over the ice.

It's important to note, however, that these Antarctic glaciers regularly shed ice into the sea.

"They’re breaking off all the time," said Willis.

But Pine Island appears to be shedding ice more quickly than it has in the past.

"In the past it has been every 6-10 years, but in recent years the calving events seem to be more frequent (2015, 2017, potentially 2018-2019)," noted Lhermitte.

"What really matters over time is whether the ice breaks off faster than it advances," added Willis. "The short answer is, yes, it's retreated quite a ways."

Pine Island Glacier: the movie. From Oct 2014 to recent calving in 108 #sentinel1 images @CopernicusEU @ESA_EO pic.twitter.com/oYKvelCKPd — Stef Lhermitte (@StefLhermitte) October 13, 2017

Getting consumed from beneath

The plight of the mighty West Antarctic glaciers is largely caused by relatively warm ocean waters eating away at the floating ice shelves, from beneath.

"Oceanic melting (from the bottom) plays an important role in this process," said Lhermitte, but also notes that's it's quite important where the glacier is able to "ground" itself on the seafloor after these calving events.

If this 19-mile long crack should rupture, Pine Island's grounding line — where the ice meets the seafloor — will may move further back. This puts the shelf at further peril: Eventually, it will run out of real-estate.

Relatively warmer waters eating away at ice shelves. Image: Giphy

But just how fast this will happen is the million, or trillion, dollar question.

"We really don’t know for sure how fast they’re going to collapse," said Willis.

As for the looming break-off of another considerable chunk of ice from Pine Island, it's certainly meaningful — but it's not yet an alarm bell.

"Not every one is a red alert," said Willis. "But trust me, one of them will be, and I'll be sure to let you know."