Warm seawater is being squeezed past West Antarctic grounding lines melting even the inland ice

Think of an ice shelf as a cork that’s keeping the rest of the glacier, that ice sheet, from sliding into the ocean. The Florida-sized Thwaites Glacier, for instance, is known as the “Doomsday Glacier” for good reason: It’s attached to a seamount off the coast and is holding back ice that would raise global sea levels by two feet if it all melted. Last month, scientists reported that Thwaites’ ice shelf could crumble in three to five years.

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But current glacier melt models don’t account for a phenomenon called tidal pumping. Whenever the tide rises, it heaves Thwaites’ ice sheet upward, allowing relatively warm seawater to rush farther upstream underneath the glacier. That drives melting along its belly, making the ice sheet more prone to fracture. “It means that warm water that is at the bottom of the glacier can infiltrate up to several kilometers upstream,” says University of Houston physicist Pietro Milillo, who is studying Antarctic glaciers. “And all of a sudden you start realizing, ‘Wait a minute! The models that actually predict the future state of the glaciers do not have these kinds of phenomena. They basically have a grounding line that is fixed.’”

The measurements are dire. In 2017, Pope’s grounding line fell back over two miles in just three and a half months. Between 2016 and 2018, Smith logged a mile and a quarter retreat a year, while Kohler pulled back three quarters of a mile. And when that grounding line starts retreating, it initiates a cascade of catastrophes: The more of the glacier’s underside that’s exposed to seawater, the more melting. “Once you trigger a subtle retreat, they're going to just keep retreating and retreating, which means that they're going to keep speeding up,” says Milillo. “Speeding up the glacier acts like a chewing gum: The glacier thins, and by thinning also you have a speed-up, because while not in contact with the bed, there is less resistance to the flow. Which means the glacier [movement] will accelerate and in turn will inject more ice into the ocean.”

Last week, another paper from researchers at the Georgia Institute of Technology, CalTech, and Dartmouth College modeled how warm seawater is likely even squeezing past the grounding line, accelerating melting even further. Scientists previously thought that the grounding line acts as a kind of barrier to keep seawater from slipping underneath the ice sheet resting on the ground. But this new mathematical modeling suggests that if the ground is flat or “retrograde,” meaning it slopes deeper into the interior of the ice sheet—and both apply to these glaciers in West Antarctica—saltwater can indeed intrude past the grounding line. Like, way past.

Specifically, without factoring in this kind of melting, the model projected that Antarctica’s glaciers might contribute between 3.5 and 6.7 inches to sea level rise by the year 2100. But with intrusion-like melting, that doubles to 8.3 and 11 inches. If his team’s new paper is correct in showing that seawater is indeed pushing past the grounding line and causing melt upstream, Robel says, “then it's not crazy that these models could be producing much higher rates of sea level rise.” (It’s worth noting that even small changes in sea level are catastrophic, particularly in low-lying areas where a fraction of an inch goes a long way.)