How soon will the seas rise?

From the article:

But in 2016, a bombshell study in Nature concluded that crumbling ice cliffs could trigger a runaway process of retreat, dramatically hastening the timeline. The Intergovernmental Panel on Climate Change (IPCC) took notice, establishing a sobering new worst-case scenario: By 2100, meltwater from Antarctica, Greenland and mountain glaciers combined with the thermal expansion of seawater could raise global sea levels by over 2 meters (opens a new tab). And that would only be the beginning. If greenhouse gas emissions continue unabated, seas would rise a staggering 15 meters by 2300.

However, not all scientists are convinced by the runaway scenario. Thus, a tension has emerged over how long we have until West Antarctica’s huge glaciers vanish. If their retreat unfolds over centuries, humanity may have time to adapt. But if rapid destabilization begins in the coming decades through the controversial runaway process, the consequences could outpace our ability to respond. Scientists warn that major population centers — New York City, New Orleans, Miami and Houston — may not be ready.

“We’ve definitely not ruled this out,” said Karen Alley (opens a new tab), a glaciologist at the University of Manitoba whose research supports the possibility of the runaway process. “But I’m not ready to say it’s going to happen soon. I’m also not going to say it can’t happen, either.”

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In a 2021 update that incorporated additional factors into the simulations, DeConto and colleagues revised that estimate sharply downward, projecting less than 40 centimeters of sea-level rise by the century’s end under high-emission scenarios. Yet even as the numbers have shifted, DeConto remains convinced of the MICI concept. “It’s founded on super basic physical and glaciological principles that are pretty undeniable,” he said.

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“Yes, ice breaks off if you expose tall cliffs, but you have two stabilizing factors,” said Mathieu Morlighem, a glaciologist at Dartmouth College who led a 2024 study that identified these factors. First, as newly exposed glacier cliffs topple, the ice behind stretches and thins. As this happens, rapidly, “your ice cliff is going to be less of a tall cliff,” Morlighem said. Second, the flowing glacier brings more ice forward to replace what breaks off, slowing the cliff’s inland retreat and making a chain reaction of cliff toppling less likely.

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Another study challenging the MICI scenario noted that breaking ice also tends to form a mélange, a dense, jumbled slurry of icebergs and sea ice. This frozen slurry can act as a retaining wall, at least temporarily stabilizing the cliffs against collapse.

The bedrock beneath the ice might also be a key player. “The solid Earth is having much bigger impacts on our understanding of sea-level change than we ever expected,” said Frederick Richards, a geodynamicist at Imperial College London. Scientists have long recognized that when glaciers melt, the land rebounds like a mattress relieved of weight. But this rebound has been mostly dismissed as too sluggish to matter for several centuries. Now, high-precision GPS and other geophysical data reveal rebound occurring over decades, even years.

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While the conversation has centered on how the sea will lap away at the ice shelves, some scientists are increasingly concerned about what’s happening up top, as warming air melts the ice sheet’s surface. Nicholas Golledge, a glaciologist at Victoria University of Wellington, sees West Antarctica today as transitioning to the status of Greenland: Most of Greenland’s marine-vulnerable ice has already vanished, and surface melt dominates. That process, Golledge believes, may soon play a bigger role in Antarctica than most models assume.

Pooling meltwater, for example, contributed to the Larsen B collapse. As the water trickles into crevasses, it lubricates the bedrock and sediments below, making everything more slippery. The Columbia University glaciologist Jonny Kingslake says these processes are oversimplified or omitted in numerical simulations. “If you ignore hydrology change, you are underestimating retreat,” he said.

I think this could be summarized as “nobody knows but it might be very bad.” Predictions are based on mathematical models, but the models have large uncertainties and aren’t a crystal ball. They will likely be revised again.