The loss of a floating ice shelf at the front of Thwaites glacier in Antarctica won’t significantly speed up the glacier’s flow into the sea, according to a modelling study. This contradicts earlier predictions that the imminent break-up of the ice shelf would lead to a quicker rise in sea level, although scientists still fear this “doomsday glacier” will eventually collapse.
Formed where glaciers slide into the ocean, ice shelves often butt up against “pinning points” like bays, islands or underwater ridges, holding up the flow of the glaciers behind them. Antarctica’s ice shelves have  – or about 6 trillion tonnes of ice – in the past 25 years, and some glaciers appear to be accelerating into the ocean.
Of particular concern is the vulnerable Thwaites glacier, which is roughly the size of Britain and could raise sea levels more than 65 centimetres if it melts completely. The grounding line, where the glacier detaches from the sea floor and begins to float, has retreated 14 kilometres since the late 1990s, as more of the glacier has pushed into the sea.
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The ice shelf at the front of the glacier has become progressively smaller and weaker over the past few decades. In a , at the University of Colorado Boulder and his colleagues warned that within the next five years, it could shatter like a broken car windshield and increase Thwaites’s contribution to sea level rise by up to 25 per cent.
Now, at Northumbria University, UK, and his colleagues have used computer models to estimate how much buttressing the ice shelf provides to the glacier. They found that the total disintegration of the ice shelf wouldn’t significantly accelerate the flow of Thwaites glacier and would only lead to an extra 1 to 2 millimetres of sea level rise in the next 50 years.
“You really can get rid of the whole ice shelf overnight and the ice upstream will not notice,” says Gudmundsson.
The ice shelf probably did buttress the glacier in the past, but may have lost contact with underwater pinning points, he says. While Thwaites ice shelf sticks out into open ocean, nearby ice shelves are braced against a peninsula and the sides of a bay, so the glaciers behind them would speed up if they disappeared.
Scambos says the predictions in the 2021 study were based on the previous break-up of other ice shelves. But seaward acceleration registered by GPS stations on Thwaites ice shelf hasn’t been matched by the glacier, which is being slowed by rough terrain, he says.
“The area just upstream of the present grounding line is not that slippery,” says Scambos. “So you can remove the doorstop, but the door’s not ready to swing shut.”
at the British Antarctic Survey says the results show that the lack of buttressing is driving the retreat of Thwaites’s grounding line, rather than melting by ocean water. A published in February that was led by Davis, which sent a torpedo-shaped robot below Thwaites ice shelf, found that the overall melt rate there is slower than previously thought, although rapid melting is occurring in some underwater crevasses.
“It’s not a self-perpetuating retreat that has no stopping point,” he says. “Certainly that’s good news.”
While the grounding line is receding, Thwaites isn’t yet exhibiting runaway retreat, says Gundmundsson. Many scientists expect this will eventually happen because it lies in a basin that is largely below sea level.
“We believe there’s still time [to save the glacier],” he says. “But push it further upstream and it will enter unstable retreat. And that would have true global consequences.”
Geophysical Research Letters
