Polar ice sheets may retreat much faster than previously thought

Norway’s ancient ice sheet may have retreated by more than 600 metres per day at the end of the last glaciation.

This is 10 times faster than previous estimates, suggesting that modern ice sheets might melt at the sea floor at far greater speeds than scientists had previously suspected, says Christine Batchelor at Newcastle University in the UK.

“Potentially, it shows us that ice sheets are physically capable of retreating at speeds that are an order of magnitude higher than anything we’ve seen [before],” she says. “It kind of provides a warning for what could happen if we continue our trajectory – or particularly if we end up on an upwards trajectory – of warming over the coming decades.”

Ice sheets attach to the bedrock of the sea floor until rising sea temperatures break down that bond by causing the ice-ocean junction to melt at the sea floor, leaving that section of ice floating as an ice shelf. The boundary of the ice attached to bedrock is called the grounding line.

Over the past 50 years, scientists have used satellite data to track the position of the grounding lines in ice sheets in Antarctica and Greenland. Retreating grounding lines have so far raised global sea levels by an estimated 0.7 millimetres each year since the 1990s.

But Batchelor and her colleagues suspected that satellite data might not be telling the whole story. They decided to take a closer look at the history of grounding-line changes at the end of the last glacial period, between 15,000 and 19,000 years ago. At the time, the Scandinavian ice sheet began retreating as the climate started to warm as part of a natural heating and cooling cycle, says Batchelor.

She and her colleagues realised that retreating ice sheets left long-lasting traces called corrugation ridges in the sediment on the sea floor. These features form well below the ocean’s waves and thus can remain undisturbed for tens of millennia, providing a record of the history of glacial melting, she says.

Batchelor and her team used ship-based devices to map the topography of the sea floor across 30,000 square kilometres of the mid-Norwegian continental shelf, including 7678 corrugation ridges.

They found evidence of ancient grounding-line retreat of up to 610 metres per day, with 70 per cent of the regions showing retreats of more than 100 metres per day – at least during “pulse” periods that might last days to months. The values far exceed all previously reported rates of grounding-line retreat from satellite and marine geological records, says Batchelor.

The rate of this retreat was particularly high along flatter areas of the sea floor, she adds.

Applied to the present day, the findings suggest that if the West Antarctic ice sheet thins at a rate of several tens of centimetres per day – which could occur under certain climate change scenarios – this could cause the grounding line to retreat of up to several hundreds of metres per day, especially across low-gradient areas.

That could be a particular problem with Antarctica’s Thwaites glacier, which has a central trunk grounded on a very flat bed.

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“This has widespread implications for key regions of Antarctica currently undergoing retreat that are of concern for future sea level rise,” says Adam Sproson at the Japan Agency for Marine-Earth Science and Technology.

“Although such extreme rates of grounding-line retreat have not been observed in the modern [period], these results from the geological record, along with numerical models of ice sheet behaviour, suggest rapid and extensive grounding-line retreat is possible under current climate conditions in Antarctica,” says Sproson.

“This could potentially have a significant impact on future global sea level rise if key regions of Antarctica that are currently observing an increase in retreat rates, such as in the Amundsen Sea region, speed up to the levels recorded by Batchelor [and her team].”