The melting of the West Antarctic ice sheet isn’t necessarily permanent, as bedrock cores show that some of its ice grew back after a more severe thaw thousands of years ago. The evidence suggests that Earth’s crust rebounding from under the diminishing ice, like a memory foam mattress, could slow glacial melt and sea level rise.
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| Satellite view of icebergs breaking away from a glacier in West Antarctica into the Amundsen Sea NASA Earth Observatory/Lauren Dauphin/Landsat 8 |
The ice sheet, which holds enough ice to raise oceans more than 3 metres, sits mostly below sea level. As advancing seawater flows down into this basin, it is expected to eat away at the underside of the sheet until it collapses. A recent study found that a particularly vulnerable part of the ice sheet called the Amundsen Sea Embayment has lost 3 trillion tonnes of ice in 25 years, and it has previously been suggested that this area of ice has gone into “irreversible retreat”.
But within the past 8000 years, ice in the embayment melted to become at least 35 metres thinner than now and thickened again, according to a new study by Greg Balco at the Berkeley Geochronology Center in California and his colleagues.
“It’s like pushing a car over the top of a hill,” says Balco. “What we’ve shown here is essentially that we got the car over the top of the hill once in the past, and it got back.”
The findings are based on carbon-14, a radioactive isotope produced by cosmic rays that can penetrate only a few metres of ice. Based on the amount of carbon-14 in ice cores, which were drilled 36 to 41 metres under the ice surface at three sites on a mountainside next to the Pope Glacier, the bedrock here must have once been covered by only 2 to 7 metres of ice. In modern times, thinning of the ice there has corresponded to the retreat of nearby glaciers.
These results align with recent evidence from microbial carbon in a subglacial lake, indicating that 6000 years ago the base of the ice sheet retreated at least 250 kilometres.
The ice sheet was later able to advance, Balco and others believe, because the land underneath was rising, lifting more of its massive edges out of the ocean. After glaciers melt, Earth’s crust rebounds from under their weight like a memory foam mattress returning to its original shape. Landmasses have been rising since the ice of the last glacial period melted away, but West Antarctica is lifting up faster than almost anywhere. That could speed up even more if much of the ice sheet melts.
“The effects for now are limited, but if it really starts to retreat, we could see a much faster rebound,” says Valentina Barletta at the Technical University of Denmark.
The question is whether that rebound will come soon enough to check West Antarctica’s melt and thus sea level rise, which could reach more than a metre by 2100 without deep cuts in carbon emissions. The rebound described in Balco’s study took at least 3000 years. “I expect roughly 10 per cent less sea level rise by 2100 due to glacial rebound,” says Torsten Albrecht at the Potsdam Institute for Climate Impact Research in Germany.
That would buy us a bit of time. But little-studied phenomena like collapsing ice cliffs mean we could still be underestimating sea level rise.
“There’s a massive uncertainty whether the rate of retreat of glaciers will be slow enough that these feedbacks can save them,” says glaciologist Edward Gasson at the University of Exeter in the UK.
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