Three million years ago, the atmosphere’s carbon dioxide levels was not so different of that of today, but sea levels were dozens of meters whore. Looking back this far offers a foreboding glimpse into the future, as satellite records show that melting Antarctic ice sheets are also on track to increase this period’s oceans. The puzzle for scientists is that the climate models they create don’t seem to match what they see with their own eyes.
“A lot of people have been scratching their heads trying to figure out what’s missing from our ice sheet models,” said Alex Bradley, an ice dynamics researcher at the British Antarctic Survey, part of the UK’s Natural Environment Research Council.
This week, two new papers in the journal Nature added to the growing pile of evidence that scientists’ models do not capture a complete picture of Antarctica’s rapid decline. One study, Published Thursday, found that more than twice as much meltwater can weigh on the surface of ice shelves, extensions of glaciers that float on the ocean, than scientists previously thought. The study published on Tuesday identified a new potential tipping point: Where the land-anchored ice meets the sea, warm ice creeps underneath and melts away from the belly.
From above and below, Antarctica’s vault of ice, holding back nearly 60 meters of potential sea level rise, looks more threatened than ever. But none of the dynamics detailed by these recent studies are used in climate models—potentially leading to an underestimation of how high seas may rise in the coming decades.
The tipping point identified by Bradley and his colleagues focuses on one of the most fragile areas of the Antarctic ice sheet: the ground line. Here the ice flows from the land and begins to float on the sea. As the oceans warm, they melt a gap between the ice and the land and push back the ground line.
“So the ice slides on top of a rock that acts as a brake on the flow of the ice,” Bradley said. “And if you start removing some of that brake, then the ice essentially flows faster.” The water erodes a cavity under the ice, inviting more water to penetrate even further. And as the melt begins to penetrate kilometers beneath the ice, the tipping point emerges: a self-perpetuating cycle of increasingly rapid melting.
“This process is actually much more sensitive than we previously understood,” Bradley said. He thinks this may be the “missing piece” that has prevented climate models from seeing the amount of melting scientists have observed.
Melting is not the only way seawater works under a glacier. For years, scientists have known that the pulsating tides causing the shelf to lift up and down like a lever, pumping miles of water beneath the surface. While both the tides and melting warmth work together against all Antarctic glaciers, some are more vulnerable to one than the other. According to Bradley, Thwaites Glacier, the so-called “Doomsday” glacier that guards the edge of the West Antarctic Ice Sheet, could retreat faster as a result of this tidal pump than of the melting mechanism that his paper identified.
In addition to guarding the ground line, the ice shelves attached to these glaciers act as plugs that hold back the ice sheet’s spillage. But the risks to these bastions are not entirely captured by models either.
“When you remove that plug, it just allows all the ice held back by it to flow into the ocean faster,” says Rebecca Dell, a researcher at the University of Cambridge’s Scott Polar Research Institute. “The ice flow speed effectively increases when the ice shelf collapses.”
One of the forces that could cause such a collapse is meltwater, which Dell is studying. As the sun hits the glacier surface, the heat melts the ice into ponds. These pools weigh on the ice layer and trickle through cracks to the pressure of the water split it open. Much of this melt—the shallow drag that hasn’t yet formed a deep pool—is harder to detect simply by looking at satellite imagery; it can be confused with clouds or shadows.
Dell and her colleagues’ study mapped meltwater across 57 Antarctic ice shelves by applying a machine learning method to existing satellite records that can sniff out detectable wavelengths of light. They found that including this melt in their models meant that at least 2.5 times more meltwater could pool on ice sheets than previously accounted for. And because the meltwater isn’t as white as ice, it reflects less of the sun’s energy and absorbs more of its heat — putting ice shelves even more at risk than scientists realized.
While there are few key questions to be answered — such as how much water it takes to break ice — Dell said recent research, including studies like hers and Bradley’s, has brought scientists closer to fully modeling Antarctica’s ice dynamics and a remove some uncertainty from predictions of sea level rise.
“Some scientists look at the melting below the ice shelf because of the ocean, I look at the melting on top because of the atmosphere,” said Dell, who sees modeling Antarctica’s changes as a big puzzle. “We just have to put all those components together.”
