The ice shelves surrounding the Antarctic coastline were retreating at up to 50 metres a day at the end of the last Ice Age, at least an order of magnitude faster than has been observed in even the most sensitive parts of Antarctica today, a new study has found.
This provides a clear indication of how quickly massive ice sheets can disappear into the ocean, the researchers say, and thus also a warning for the future.
“Should climate change continue to weaken the ice shelves in the coming decades, we could see similar rates of retreat, with profound implications for global sea level rise,” says Julian Dowdeswell from the Scott Polar Research Institute at the University of Cambridge, UK, which led the study.
The findings, published in a paper in the journal Science, are based on analysis of wave-like ridges on the Antarctic seafloor made during regional deglaciation roughly 12,000 years ago.
The ridges are about a metre high and 20 to 25 metres apart. The researchers believe they formed at what was then the grounding line – the zone where grounded ice sheet begins to float as an ice shelf.
The study was carried during the Weddell Sea Expedition in 2019, and the images of “unprecedented resolution” were taken by an autonomous underwater vehicle operating about 60 metres above the seabed.
“We knew these features were there, but we’ve never been able to examine them in such great detail before,” Dowdeswell says.
From that examination they infer that the ridges were caused by the ice moving up and down with the tides, squeezing the sediment into well-preserved geological patterns, looking a little like the rungs of a ladder, as the ice retreated.
Assuming a standard 12-hour cycle between high and low tide, and measuring the distance between the ridges, they calculated that the ice was retreating as much as 40 to 50 metres per day at the end of the Ice Age, a rate that equates to more than 10 kilometres per year.
In comparison, modern satellite images show that even the fastest-retreating grounding lines in Antarctica today, for example in Pine Island Bay, are moving at only about 1.6 kilometres per year.
Massive floating ice shelves skirt about 75% of the Antarctic coastline, acting as a buttress against ice flow from inland.
They are thinning, the researchers say, because relatively warm water currents are eating away at them from below, while they also are melting on top as summer air temperatures rise.