Some extra water likely came from Greenland, whose ice currently contains over 20 feet (6 metres) of potential sea level rise.
That’s why researchers also suspect a collapse of the most vulnerable part of Antarctica, the West Antarctic ice sheet.
“There’s no way to get tens of meters of sea level rise without getting tens of meters of sea level rise from Antarctica,” said Rob DeConto, an Antarctic expert at the University of Massachusetts.
Some researchers, including DeConto, think they have found a key process – called marine ice cliff collapse – that can release a lot of sea level rise from West Antarctica in a hurry.
Its glaciers sit up against the ocean in all directions, and toward the center of the ice sheet, the seafloor slopes rapidly downward, even as the surface of the ice sheet itself grows much thicker, as much as two miles thick in total.
So if the gateway glaciers start to move backward – particularly a glacier named Thwaites, by far the largest of them – the ocean would quickly have access to much thicker ice.
The idea is that during the Eemian, this whole area was not a block of ice at all, but an unnamed sea.
In this configuration, warm water can cause a glacier to move backward and downhill, exposing ever thicker ice to the ocean – and thicker ice flows outward faster.
Marine ice sheet instability is probably underway already in West Antarctica, but in the model, it wasn’t enough.
That’s because Jakobshavn no longer has an ice shelf, a floating extension that used to grow out over the ocean at the front of the glacier and stabilize it.
Most of the glacier’s ice is under the water, but more than 100 meters (330 feet) extend above it – and for DeConto and Pollard, that’s the problem.
If Thwaites someday loses its own ice shelf and exposes a vertical front to the ocean, you would have ice cliffs hundreds of meters above the surface of the water.
And when they added this computation, it not only recreated Eemian sea level rise, it greatly increased their projection of how much ice Antarctica could yield in this century – more than three feet.
Since there are other drivers of sea level rise, like Greenland, this meant that we could see as much as six feet in total in this century, roughly double prior projections.
“What we pointed out was, if the kind of calving that we see in Greenland today were to start turning on in analogous settings in Antarctica, then Antarctica has way thicker ice, it’s a way bigger ice sheet, the consequences would be potentially really monumental for sea level rise,” DeConto said.
“We cannot recreate six meters of sea level rise early in the Eemian without accounting for some brittle fracture in the ice sheet model,” said DeConto.
Using a statistical technique to examine the results, Edwards and her collaborators find that the toppling of ice cliffs is not necessary to reproduce past warm periods after all.
It is a new science, she said, and without more modeling it’s unclear how ice cliffs will ultimately affect sea level rise.
The new critique, DeConto said, implies that “these processes aren’t important for future sea level rise.
Richard Alley, a well known glaciologist at Penn State University who has published with DeConto and Pollard, wrote in an email that “cliff retreat is not some strange and unexpected physical process; it is happening now in some places, has happened in the past, and is expected wherever sufficiently high temperatures occur in ocean or air around ice flowing into the ocean.”
And that makes a key difference, said Ted Scambos, an Antarctic researcher who is leading the US side of an international multimillion dollar mission to study Thwaites Glacier, and who is a senior researcher at the National Snow and Ice Data Center in Colorado.
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