It won’t take much to cause the entire West Antarctic Ice Sheet to collapse—and once it starts, it won’t stop. In the last year, a slew of papers has highlighted the vulnerability of the ice sheet covering the western half of the continent, suggesting that its downfall is inevitable—and probably already underway. Now, a new model shows just how this juggernaut could unfold. A relatively small amount of melting over a few decades, the authors say, will inexorably lead to the destabilization of the entire ice sheet and the rise of global sea levels by as much as 3 meters.

Previous models have examined the onset of the collapse in detail. In 2014, two papers, one in Science and one in Geophysical Review Letters, noted that the Thwaites Glacier, which some scientists call the “weak underbelly” of the West Antarctic Ice Sheet, has retreated dramatically over the past 2 decades. Most Antarctic researchers chalk this up to warm seawater melting the floating ice shelves at their bases; seawater temperatures there have risen since the 1970s, in part because of global temperature increases. Right now, an underwater ledge is helping anchor the glacier in place. But when the glacier retreats past that bulwark, it will collapse into the ocean; then seawater will intrude and melt channels into the ice sheet, setting the juggernaut in motion.

Scientists agree that this is going to happen, says Eric Rignot of the University of California, Irvine, lead author of the Geophysical Review Letters paper. “The real central question is the time scale.”

But most models have focused on short-term timescales, decades or a few centuries at most, says Anders Levermann, a climate scientist at the Potsdam Institute for Climate Impact Research in Germany and co-author of the new paper. He and climate scientist Johannes Feldmann, also of the Potsdam climate center, wanted to examine how the destabilization would progress in the longer term, over hundreds to thousands of years. “The big question was how far [the instability] would reach inland,” Levermann says.

To study this, they ran computer simulations focusing on the dynamic forces that would act on the ice over time, from frozen inland ice to fast-flowing ice streams to floating ice shelves. They used the model first to simulate existing, observed subsurface melting within the Amundsen Sea, a region of West Antarctica that includes two vulnerable glaciers, Thwaites and the Pine Island Glacier. The model simulated current observations of enhanced, rapid melting until it recreated the current positions of the glaciers. Then they turned down the heat: They returned the model’s ocean and atmosphere conditions to those existing in the later 20th century, rather than the current 21st century conditions that have been causing rapid melting. “We wanted to show [how] it unfolds without us pushing it anymore,” Levermann says.

What they found was that local destabilization of the Amundsen Sea region of West Antarctica ultimately causes the entire ice sheet to fall into the ocean over several centuries to several thousands of years, gradually adding 3 meters to global sea levels, they report online today in the Proceedings of the National Academy of Sciences. The model shows that “there’s no holding back,” Levermann says: Just a few decades of melting leads to “thousands of years of ice motion.” More than 150 million people globally live within just 1 meter of the sea; in the United States, a sea level rise of 3 meters would inundate many of the East Coast’s largest cities, including New York and Miami.

Video credit: Matthias Mengel, PIK

The model simulates melting at the base of the Amundsen Sea ice shelves at current rates over several decades. Once these shelves destabilize, the study suggests, the entire West Antarctic Ice Sheet becomes vulnerable, falling into the sea in the ensuing centuries to millennia.

One of the most startling results, he adds, was the knock-on effects of the melting. In an earlier study, the team found that the neighboring Filchner-Ronne and Ross ice shelves would not collapse on their own; the seafloor topography would keep them anchored in place and prevent the destabilizing inward rush of seawater. But when the Amundsen Sea region is destabilized, the model showed, the entering seawater was able to erode those ice shelves from the inside out.

“This paper does confirm what we hypothesized, that knocking out the Pine Island Glacier and Thwaites takes down the rest of the West Antarctic Ice Sheet,” says Ian Joughin, a glaciologist at the University of Washington, Seattle, who co-authored last year’s Science paper. He adds, however, that the model’s weakness is its resolution; it shows the destabilization of the glaciers occurring roughly 60 years from now, whereas present observations suggest that collapse is already underway. As a result, Joughin says, the model’s time scale for the collapse is probably too long. “It’s more likely measured in centuries rather than millennia.”

Indeed, “the jury is still out” on whether Feldmann and Levermann’s study got the time scale right, Rignot says. The long-term evolution of an ice sheet “is a very complex modeling problem. Some of the variables controlling the models are not all that well known,” he adds, including forces such as winds, ocean circulation, and how icebergs calve. “There is not a model out there that is getting it right, because they all have caveats. I think the discussion is ongoing, and is only going to be more interesting with time.”