There's a reasonable chance that Antarctica's two most vulnerable ice shelves — the ends of massive glaciers that float over the ocean — will succumb to Earth's warming climate and eventually collapse into the sea.
These particular shelves, known as Larsen C and George VI, are perched on the Antarctic Peninsula — the finger that runs up towards South America. Glacial scientists have now gauged how much oceans would rise if the ice shelves fail, and the news isn't good.
The research, published Thursday in the journal The Cryosphere, shows that taken together, the glaciers' overall contribution to rising seas wouldn't be enormous — adding some 10 millimeters (under half an inch) by the end of the century.
But their collapse is a harbinger of what's to come: The Antarctic coasts are covered in ice shelves, and if they begin to collapse, rivers of ice, or glaciers, will be unleashed into the ocean, boosting sea levels even more.
The Antarctic Peninsula.
The shelves act as formidable "plugs," holding back thick masses of Antarctic ice from flowing into the water.
"These are just the two ice shelves considered to have the greatest risk of collapse at present," Nicholas Barrand, a glaciologist and study coauthor, said over email. "There are many, many more ice shelves in Antarctica, and many of them are much, much larger."
In 2002, an ice shelf the size of Rhode Island, Larsen B (just north of Larsen C), crumbled into the ocean and broke apart.
Larsen B breaking up into the ocean in 2002.
But Antarctica's ice sheet melting events are still mostly unprecedented in human history, so they're quite difficult to project, Josh Willis, a NASA oceanographer, said in an interview.
Barrand's projections — based upon how quickly ice moves and other geologic conditions — gives scientists a good idea of what will happen to the ice being held back by these two shelves, if they were to break up.
However, the looming question that still evades climate scientists is when will they collapse?
"How fast are these shelves going to break up — and can we predict that?" asks Willis. "When should we be worried? We don't have a good answer to that."
As Barrand noted, Antarctica has much larger ice sheets plugged behind its ice shelves farther south.
Some of the shelves, in western Antarctica, are believed to be particularly susceptible to warming oceans, as relatively warmer water is eating away at the ice shelves floating over the ocean, from the bottom.
One of these, the Pine Island glacier, made quite a media splash last year when it shed a Delaware-sized iceberg in July 2017.
"Pine Island is poised to really run away," said Willis. "Those are really kind of the big scary ones — but there are others, too."
The West Antarctic ice sheet contains enough frozen water to raise sea levels by some four feet alone, according to NASA.
A Manhattan-sized Pine Island iceberg breaking off from Antarctica in January 2017, before the even bigger event later that year.
But Antarctica, said Barrand, certainly isn't the only source of ice flowing into the sea.
When the incremental inputs of Larsen C, George VI, and Pine Island — should they let loose — are considered along with thawing Greenland and the melting of ice on mountains around the world, it adds up to a considerable amount of sea level rise.
Antarctica's potential to boost sea levels is significant, said Barrand, "but when added to other sources of freshwater to the global oceans, even more significant."