A boat floats in Neko Harbour, Antarctica, in February 2018.Alexandre Meneghini / Reuters

One of the scariest scenarios for near-term, disastrous sea-level rise may be off the table for now, according to a new study previewed at a recent scientific conference.

Two years ago, the glaciologists Robert DeConto and David Pollard rocked their field with a paper arguing that several massive glaciers in Antarctica were much more unstable than previously thought. Those key glaciers—which include Thwaites Glacier and Pine Island Glacier, both in the frigid continent’s west—could increase global sea levels by more than three feet by 2100, the paper warned. Such a rise could destroy the homes of more than 150 million people worldwide.

They are now revisiting those results. In new work, conducted with three other prominent glaciologists, DeConto and Pollard have lowered some of their worst-case projections for the 21st century. Antarctica may only contribute about a foot of sea-level rise by 2100, they now say. This finding, reached after the team improved their own ice model, is much closer to projections made by other glaciologists.

It is a reassuring constraint placed on one of the most alarming scientific hypotheses advanced this decade. The press had described DeConto and Pollard’s original work as an “ice apocalypse” spawned by a “doomsday glacier.” Now their worst-case skyrocketing sea-level scenario seems extremely unlikely, at least within our own lifetimes.

Yet their work—and the work of other sea-level-rise scientists—still warns of potential catastrophe for our children and grandchildren. If every country meets its current commitment under the Paris Agreement, the Earth will warm about 2.7 degrees Celsius by the end of the century compared with its pre-industrial average. In their new research, DeConto and his colleagues say that there’s a tipping point, somewhere between 2 and 3 degrees Celsius of temperature rise, after which the West Antarctic Ice Sheet will slip into rapid and shattering collapse.

Their new research also raises the marginal risk of disaster. Officially, the Paris Agreement aims to keep global warming from exceeding 2 degrees Celsius, though many experts consider that goal fanciful. And even in that extremely optimistic scenario, West Antarctica still switches into unavoidable collapse about 10 percent of the time, according to the new research.

Their short-term revisions also barely change their long-term forecast of West Antarctic disintegration. If emissions keep rising, they warn that global sea level could rise by more than 26 feet by 2300.

These new results have not yet been peer-reviewed. DeConto, a professor at the University of Massachusetts at Amherst, presented them to other scientists last month at the fall meeting of the American Geophysical Union, the largest annual conference of Earth scientists in the world. He and his colleagues declined to comment for this story in keeping with an academic custom not to discuss new work with the press before its publication.

The new results inform one of the biggest outstanding questions—and most fervent debates—concerning how climate change will reshape our world: How much will the seas rise, and how fast will that upheaval occur? DeConto and several other American glaciologists—including Richard Alley, a professor at Penn State and a co-author of the new research—represent something like the vanguard of that discussion. They champion an idea called “marine ice-cliff instability,” or MICI, which maintains that West Antarctic glaciers will eventually crumble under their own weight. By the middle of next century, they warn, this mechanism could send ocean levels soaring at a rate of several feet per decade. For reference: Along the U.S. East Coast, the Atlantic Ocean has risen by only about a foot over the last 12 decades.

While “marine ice-cliff instability” might be clunky, the idea is cinematic. It holds that warm ocean waters will eventually chew away the floating ice shelves that gird Antarctic glaciers today. With these ice shelves gone, the glaciers will stand naked on the seafloor: towering, fragile cliffs of ice. Imagine a 300-foot-tall shard of sapphire rising from the ocean and stretching for miles in both directions, and you will have a sense of the awesome prospect of this new geography. You will also have a sense of its dangerous physics, because ice cannot support itself at such heights. As MICI kicks in, those sapphire walls will crack, buckle, and begin rapidly birthing hundred-foot splinters of frozen freshwater into the sea. And thus the oceans will rise.

Other researchers find this possible future somewhat fantastic. “We, as European modelers, are slightly more skeptical of the marine-cliff idea,” Frank Pattyn, a glaciologist at the Free University of Brussels, told me. “It has not been observed, not at such a scale.”

Yet even MICI’s skeptics agree: Our understanding of sea-level rise is rapidly growing more ominous. In its last major report, in 2014, the Intergovernmental Panel on Climate Change projected that oceans could rise two feet by 2100 if greenhouse-gas emissions continue on a worst-case trajectory. That number will almost certainly worsen in the IPCC’s next report, which is due in 2021, Pattyn said. “We are facing sea-level rise that is obviously going to be higher in the mean than what the IPCC’s ‘Fifth Assessment Report’ showed,” he said.

“Nobody’s debating that sea-level rise is happening. It’s back to how much, how fast,” Helen Amanda Fricker, a glaciologist at Scripps Institution of Oceanography, told me. Even the most optimistic scientists have recently increased their low-end estimates, she said. “It’s healthy to have this debate.”

There is only one place in the world where MICI is definitely happening: Jakobshavn Glacier, on the west coast of Greenland. (Locals call it Sermeq Kujalleq.) In the 19th century, Jakobshavn was a long river of ice that snaked out of its fjord to meet the surrounding, frozen bay. Now, the bay rarely freezes, and Jakobshavn has retreated miles back into its canyon, forming a tall, brittle cliff face that regularly births icebergs as tall as a house. (Some of those icebergs are so enormous that they get stuck leaving the fjord.) All that ice has to come from somewhere: These days, Jakobshavn empties ice from the center of Greenland twice as quickly as it did during the last century.

Last month, in a large hall at the same AGU conference, several hundred researchers gathered to see a set of presentations billed as a series of updates on new glacier and ice models. It was far closer to a proxy debate on the ice-cliff question. Several of the talks had “marine ice-cliff instability” in the title, and I had heard more than one group of glaciologists gossiping about it days in advance.

Alley, the Penn State glaciologist, addressed the sapphire-colored elephant in the room immediately after taking the dais. As he sees it, it’s just common sense that Antarctic glaciers will develop problematic ice cliffs. The Jakobshavn Glacier, only a few miles wide, has not significantly changed the rate of global sea-level rise. Pine Island Glacier in West Antarctica, on the other hand, is more than 30 miles wide. It holds enough ice to raise sea levels worldwide by about five feet. “What we’ve always relied on is that unzipping one fjord does not affect the global ocean,” Alley said. “What’s different is that here and here and here”—he pointed to glaciers in West Antarctica—“unzipping one fjord will matter a lot.”

In this scenario, he warned, “We will not have analogues … We are going to move outside the instrumental data that we use to calibrate our models.”

Then came the skeptics. Dan Martin, a computational scientist at Lawrence Berkeley National Lab, argued that his and his colleagues’ work showed that ice cliffs might simply be a product of running a computer model of ice physics at a too-low resolution. Eric Larour, a physicist at NASA, presented the possibility that the physics of the Earth itself might slightly counteract some rapid ice-cliff collapse. As the ice sitting on West Antarctica melts, the bedrock below it will bounce back up.

“When ice melts or thins, you can think that the Earth [below it] is going to rebound,” he said. That bedrock will rise, lifting the glacier partly out of the water. Such a mechanism could buy humanity some time, he said, giving us a “23 to 30 year delay” in the total collapse of West Antarctica. This effect might hold off the collapse of West Antarctica until 2250 or 2300, but then the ice sheet would disintegrate as fast as ever.

The meeting arrived at no clear conclusion. “It still doesn’t look good,” Brad Lipovsky, an Earth scientist at Harvard, told me. “That’s what I saw in the talks today. We’re still seeing that sea-level rise is going to be a major problem for coastal communities around the world.”

MICI remains a young idea, first proposed only six years ago. It need not be rejected simply because scientists haven’t arrived at hard conclusions yet, Fricker, the Scripps glaciologist, said. Marine ice-cliff instability remains a worrying possibility: a low-chance, high-danger tail risk of climate change. It’s just one of the many gambles that humanity is placing on its own future—and it’s not even the only mechanism that could cause West Antarctica to collapse. Researchers are also investigating another mechanism, “marine ice-sheet instability,” that could target some of the same fragile glaciers.

“It might not happen,” Fricker said. “But if there’s a chance that it could happen, then shouldn’t you involve that in your planning? If you’re hosting a picnic and it might rain, you don’t necessarily move the whole event, but you probably do make a Plan B. If you’re planning a city … you might as well keep this in the back of your mind.”

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