Weakening ice shelf has caused crucial Antarctic glacier to accelerate

A large and fast-melting glacier in West Antarctica has sped up dramatically since 2017. This may be a sign that the floating ice shelf in front of it is no longer helping to hold back the ice.

Pine Island glacier is the fastest-flowing glacier in Antarctica and the largest contributor to sea-level rise of all Antarctic glaciers. It is a key part of the West Antarctic ice sheet, which holds enough ice to raise the global sea level by 5.3 metres if melted completely.

The Pine Island ice shelf lies in front of the glacier and juts out over the ocean. It is thought to play a crucial role in holding back the inland ice and shielding it from warm water, buttressing an amount of ice equivalent to 51 centimetres of sea-level rise.

The instability of Pine Island glacier and the neighbouring Thwaites glacier, nicknamed the Doomsday glacier, poses a major threat to the long-term viability of the broader West Antarctic ice sheet.

Sarah Wells-Moran at the University of Chicago and her colleagues tracked the movement of Pine Island glacier using imagery from the Copernicus Sentinel-1 Satellite and observations going back to the early 1970s.

The glacier’s velocity increased from 2.2 kilometres per year in 1974 to 4 kilometres per year by 2008. Then, between 2017 and 2023, it jumped to nearly 5 kilometres per year, a 20 per cent increase over six years and a 113 per cent increase since 1973.

Between 1973 and 2013, the rate of ice discharge from Pine Island glacier increased by more than three-quarters.

These changes led to a dramatic retreat of the glacier’s grounding line, the point at which the ice shelf begins to float rather than rest on the seafloor, by more than 30 kilometres.

The team compared these observations with computer models and concluded that the rapid acceleration has occurred due to the thinning and fracturing of the ice shelf as warmer sea water reaches further along its underside. The sides of the ice shelf have become detached from the surrounding ice, “unzipping” the margins of the shelf, write Wells-Moran and her colleagues.

They conclude that Pine Island ice shelf “now provides negligible buttressing to the ice upstream”, which has accelerated the loss of ice from West Antarctica.

Sue Cook at the University of Tasmania in Australia says calving – the break-up of ice at the front of the ice shelf – isn’t enough to explain the glacier’s acceleration. “Most likely the cause is increased damage in the shear margins of the glacier,” she says. “This study helps to confirm that mechanism.”

Ted Scambos at the University of Colorado says warm ocean water may be reaching the margins of the ice shelf where it juts into Pine Island Bay, a glacial carved fjord. “With the loss of the ice shelf, it is likely that ocean circulation in the fjord will speed up, and the intensity of the circulation near the point where the glacier is grounded on the bedrock will increase,” says Scambos.

Nerilie Abram at the Australian Antarctic Division says the study helps demonstrate how much and how quickly Pine Island ice shelf is failing. “There is no doubt that ice loss from this region will continue to impact the world’s coastlines over the coming decades and centuries,” says Abram.

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