Ice floats near the coast of West Antarctica seen from a window of a NASA Operation IceBridge airplane on October 27, 2016. Meltwater flows risk hastening the collapse Antarctica's ice shelves—but in some cases, meltwater drainage could help keep ice shelves stable.
A surprisingly vast network of waterways cuts across Antarctica’s ice shelves, the floating tongues of ice emanating from the continent’s coastlines.
These seasonal flows of meltwater, a part of Antarctica’s natural water cycle, have been known for decades to crisscross the continent. Now, scientists have systematically catalogued them—revealing them to be more extensive than many scientists had thought.
In some cases, these systems achieve a scale that’s hard to fathom. East Antarctica’s Amery Ice Shelf, for one, bears streams that transport meltwater up to 75 miles, feeding ponds on the ice shelf’s surface that can get more than 50 miles long. The surface of its largest pond can grow by more than 400 NFL football fields in a single day, thanks to this drainage.
The findings, published on Wednesday in Nature, add vital information about Antarctic meltwater, which had been modeled as simply pooling where it melted. But it’s too early to say whether these meltwater systems, numbering nearly 700 in all, help or hurt Antarctica’s ice shelves on the balance—a high-stakes question, given the shelves’ potential role in sea level rise. (See what the world would look like if all the ice melted.)
The breakup of an ice shelf doesn’t affect sea level rise directly: By definition, an ice shelf is already floating in water. But University of Massachusetts-Amherst climatologist Rob DeConto notes that some ice shelves act as buttresses, impeding the seaward flow of the ice sheets on land behind them. Losing these ice shelves then accelerates the flow of ice from land to water, effectively turning on a spigot that raises sea levels.
“It’s kind of like letting the bouncer go at the door and letting floods [of people] into a concert or bar,” adds study coauthor Robin Bell, a glaciologist at Columbia University’s Lamont-Doherty Earth Observatory. “They’re sort of the gatekeepers: take them away, and more ice gets into the ocean.”
Meltwater can pose a threat to ice shelves’ stability, by saddling them with weight and widening their internal crevasses. In the days before the Larsen B Ice Shelf’s sudden 2002 collapse, for instance, ponds of strain-inducing meltwater covered its surface. Another patch of the same ice shelf, Larsen C, could give way within weeks or months.
In one of their two newly published studies, Bell and her coauthor Jonathan Kingslake caution that large-scale drainage could intensify the threat that meltwater poses, since it lets meltwater move around more effectively—especially if climate change keeps apace.
“That’s potentially important, because the amount of meltwater that forms in any one place isn’t just a function of the amount of melting; it also comes down to the fact that water’s moved in from the side over long distances,” says Kingslake, who is also a glaciologist at the Lamont-Doherty Earth Observatory.
On the other hand, Bell and Kingslake’s second study suggests that these river networks could be keeping at least one ice shelf more stable, by efficiently draining meltwater off the ice shelf’s surface.
Bell’s careful study of the Nansen Ice Shelf, a 695-square-mile tongue of ice that juts into Antarctica’s Ross Sea, shows that the ice shelf has been draining its meltwater into the ocean for at least the last century. The branching channels eventually merge, dumping meltwater into the ocean via a 425-foot-wide waterfall at the ice shelf’s edge.
On the low end, Bell says that this river on ice can move as much water as the United States’s Potomac River.
Terra (Still) Incognita
Bell adds that this sort of continent-wide survey was only possible thanks to decades’ worth of data, from satellite imagery to photos taken by military aircraft. For her analysis of the Nansen Ice Shelf, Bell even relied on century-old journals from the Northern Party, a contingent of Sir Robert Scott’s ill-fated Terra Nova expedition that didn’t venture to the South Pole.
“They took lots of beautiful measurements, but then they got stuck and had to spend the winter in a cave—that’s all [people] remember about them,” Bell says. “Being able to take these people’s science and give them credit for what they did… makes me very happy.”
But Bell and Kingslake, along with outside experts, emphasize that much still remains unknown about Antarctica, a forbidding place for scientists and scientific instruments alike.
“We are in a situation where we have an ice sheet that has the potential to add something like 180 feet to sea level, and we don’t know the topography of the sea floor underneath—and we don’t know the thickness of the ice,” says Helen Fricker, a glaciologist at the Scripps Institution of Oceanography who has studied the Amery Ice Shelf’s meltwater streams. “To get there and map the whole thing is a monumental task.
“We’re trying to understand this huge continent, but we’ve only got a handful of tools… and we’re doing the best we can,” she adds. “We’re trying to make a meal for fifty people with a butter knife.”