For almost a decade, climate scientists have been trying to get their heads around a particularly disastrous scenario for how West Antarctica’s gigantic ice sheet might break apart, bringing catastrophe to the world’s coasts.
It goes like this: Once enough of the ice sheet’s floating edges melt away, what remains are immense, sheer cliffs of ice facing the sea. These cliffs will be so tall and steep that they are unstable. Great chunks of ice start breaking away from them, exposing even taller, even more-unstable cliffs. Soon, these start crumbling too, and before long you have runaway collapse.
As all this ice tumbles into the ocean, and assuming that nations’ emissions of heat-trapping gases climb to extremely high levels, Antarctica could contribute more than a foot to worldwide sea-level rise before the end of the century.
This calamitous chain of events is still hypothetical, yet scientists have taken it seriously enough to include it as a “low-likelihood, high-impact” possibility in the United Nations’ latest assessment of future sea-level increase.
Now, though, a group of researchers has put forth evidence that the prospect may be more remote than previously thought. As humans burn fossil fuels and heat the planet, West Antarctica’s ice remains vulnerable to destruction in many forms. But this particular form, in which ice cliffs collapse one after the other, looks less likely, according to the scientists’ computer simulations.
“We’re not saying that we’re safe,” said Mathieu Morlighem, a professor of earth science at Dartmouth College who led the research. “The Antarctic ice sheet is going to disappear; this is going to happen. The question is how fast.”
The speed at which West Antarctica’s ice disintegrates matters hugely for human civilization and the environment. A slower breakdown gives seaside populations more time to mount defenses or move inland. It gives coastal ecosystems such as wetlands and mangroves more time to adapt.
Still, there’s a lot of uncertainty about how ice breaks apart under stress, Dr. Morlighem said. So it remains hard to say with high confidence how much time the world has to prepare for higher seas. “We still have a lot to do to reduce these uncertainties,” he said.
Dr. Morlighem and his colleagues’ results were published Wednesday in the journal Science Advances.
The West Antarctic ice sheet is one of several giant systems in nature that global warming might be pushing toward collapse. Scientists are working to understand the possibility that once warming exceeds certain levels, these systems might be tipped out of balance, triggering catastrophic changes that would be impossible to reverse for centuries, even millenniums.
In their simulations, Dr. Morlighem and his colleagues used information from recently developed modeling that describes how an ice cliff cracks into pieces. They focused on the Thwaites Glacier, a Florida-size river of ice that is among the fastest receding and least stable in Antarctica.
The researchers simulated two situations that they described as “worst-case scenarios” for Thwaites. First, they tested what would happen if the ice shelf — that is, the glacier’s floating edge — completely disappeared today. (In reality, the shelf will probably melt away gradually.) Then they tested what would happen if the shelf vanished in 2065. This would give the glacier several more decades to retreat inland, exposing ice cliffs that are higher than the ones currently near the water’s edge.
In both circumstances, they found that, once the shelf was removed, the glacier’s ice started flowing more quickly out to sea. This prevented the cliffs at the end of the glacier from becoming so tall as to trigger a runaway breakdown.
Several researchers who study Antarctica but weren’t involved in the new study said its findings were useful but unlikely to be the last word on the matter.
Scientists need to keep finding ways to ground their models in observations from the real world, said Rob DeConto, a professor of earth, geographic and climate sciences at the University of Massachusetts Amherst.
How strong or weak is West Antarctica’s ice by the time it flows from the continent’s interior to the ocean? How much damage does it sustain along the way? How much liquid water does it hold? How does that affect the way it breaks? “Those are all things that we need a better understanding of,” said Dr. DeConto, who was among the researchers who first proposed that ice-cliff instability could spell ruin for West Antarctica.
The problem — or one of them, at least — is that scientists don’t have enough observations of how ice behaves when it fractures, said Richard Alley, a professor of geosciences at Pennsylvania State University. Ice can collapse quickly, he said. If scientists don’t happen to be watching, they can easily miss it. “The data we are getting now are not sufficient,” Dr. Alley said.
Given how many of the world’s major cities sit on coasts, and given how much it could cost to protect them against higher seas, there’s a lot riding on scientists to get the projections right, Dr. Alley said.
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