Melting Antarctic ice could actually slow sea level rise

Rising land beneath Antarctica’s ice sheet could slow ice loss and reduce sea-level rise in coming centuries. However, if emissions continue to rise, the effect could raise sea levels even more than the melting ice alone.

The finding comes from a model that simulates the mantle – the layer beneath Earth’s crust – in more detail than ever before. As melting ice reduces the weight of Antarctica, the elastic mantle below rebounds, raising the land above it. As melting ice reduces the weight of the continent, Earth’s elastic mantle rebounds, raising the land above it. The rebounding land may in turn slow the flow of the ice sheet where it meets the sea. This “sea level feedback” mainly happens because the rising land reshapes the seabed in a way that limits the thickness of the ice sheet at its edge – thinner ice there reduces the overall flux of ice into the sea.

Researchers have long thought this effect would play some role in slowing ice loss. But it wasn’t clear when this effect would kick in, or how it would vary at different parts of the ice sheet.

Natalya Gomez at McGill University in Canada and her colleagues modelled the relationship between the melting ice and rebounding land, including a simulation of the mantle that captured differences in viscosity beneath the continent. East Antarctica sits above a more viscous mantle and thicker crust, while West Antarctica’s rapidly melting glaciers lie atop a less viscous mantle and thinner crust. This more detailed picture of the interior Earth is based on decades of precise measurements of changes in the elevation of the ice sheet, as well as data about the mantle below Antarctica from seismic waves produced by earthquakes. “This is something that’s been hard earned,” says Gomez.

Under a very low-emissions scenario, the researchers found rebounding land reduced Antarctica’s contribution to global average sea level rise by over half a metre by 2500, compared with a model that treated the ground beneath the ice as rigid. This effect was less significant under a moderate emissions scenario, but it still led to a substantial reduction in sea level rise, which kicked in as soon as 2100.

However, under a very high emissions scenario, the team found that rebounding land led Antarctica to contribute an additional 0.8 metres to sea level rise by 2500. This happened because the ice sheet receded faster than the land rebounded, and because the rising seafloor displaced more water into the rest of the ocean.

“From a modelling perspective it’s a very big advance,” says Alexander Bradley at the British Antarctic Survey. He says rebounding land was always assumed to reduce sea level rise, but this higher-resolution modelling shows that the effect depends on emissions. “The changes that take place in the 21st and 22nd century are really baked in by what we do now,” says Bradley.

Alexander Robel at the Georgia Institute of Technology in Atlanta says “it’s a very good simulation”, but the scenario where rebounding land increases sea level rise is based on worst-case assumptions about emissions as well as the rate at which the ice sheet retreats.

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