Ice Cores Reveal First Evidence of Rapid Antarctic Ice Loss 8,000 Years Ago
Ice cores of compacted snow offer clues to ice shrinkage in the past, and what we could face in the future.
The first direct indication of the rapid shrinking of the West Antarctic Ice Sheet at the end of the Last Ice Age has been discovered in ice cores; in one location in just under 200 years, the ice sheet thinned by 450m – more than the height of the Empire State Building.
Scientists worry that today’s rising temperatures could weaken parts of the ice sheet in the future before passing a tipping point and inducing a runaway collapse. In a paper published in Nature Geoscience, researchers from the University of Cambridge and the British Antarctic Survey (BAS) investigate how quickly Antarctic ice could melt if temperatures continue to rise.
Going back in time
Melting of the Antarctic ice sheets could release enough freshwater to raise global sea levels by around 57m and the West Antarctic Ice Sheet is considered particularly vulnerable because much of it sits on bedrock below sea level; models predict that a large part of it could disappear in the next few centuries, causing sea levels to rise.
“We now have direct evidence that this ice sheet suffered rapid ice loss in the past,” said Professor Eric Wolff, senior author from Cambridge’s Department of Earth Sciences. “This scenario isn’t something that exists only in our model predictions and it could happen again if parts of this ice sheet become unstable.”
But exactly when and how quickly is uncertain. To find out, researchers studied ice cores to determine what happened to the West Antarctic Ice Sheet at the end of the Last Ice Age, when temperatures were rising, but at a slower rate than current anthropogenic warming. “Using ice cores we can go back to that time and estimate the ice sheet’s thickness and extent,” says Dr Isobel Rowell, study co-author from the BAS.
In 2019, researchers drilled a 651-metre-long ice core from Skytrain Ice Rise, a mound of ice at the edge of the ice sheet, near the point where grounded ice flows into the floating Ronne Ice Shelf. Ice cores consist of layers of snow buried and compacted into ice crystals over thousands of years. Trapped within each layer are bubbles of ancient air and contaminants that mixed with each year’s snowfall; this can provide clues to the changing climate and ice extent.
Researchers analysed the core to reconstruct the ice thickness. They measured stable water isotopes, which gave an indication of the temperature at the time the snow fell and pressure of air bubbles trapped in the ice – both of which vary with elevation. Warmer temperatures and higher-pressure air bubbles meant lower-lying, thinner ice.
The measurements revealed that ice thinned rapidly 8,000 years ago, probably triggered by warm water seeping underneath the edge of the West Antarctic Ice Sheet. This likely untethered a section of the ice from bedrock, which floated away to form what is now the Ronne Ice Shelf and allowed neighbouring Skytrain Ice Rise, unrestrained by grounded ice, to thin rapidly. “Once the ice thinned, it shrunk really fast,” said Wolff, “this was clearly a tipping point — a runaway process.”
Ice retreat
The sodium content of the ice also increased about 300 years after the ice thinned, indicating the ice shelf shrunk back so the sea was hundreds of kilometres nearer to their site. Models already showed that ice thinned around this time, but the retreat could have been anywhere between 12,000 and 5,000 years ago and scientists couldn’t say how quickly it happened. “We now have a very precisely dated observation of that retreat that can be built into improved models,” said Rowell.
The West Antarctic Ice Sheet retreated quickly 8,000 years ago, stabilising at its current size. “It’s now crucial to find out whether extra warmth could destabilise the ice and cause it to start retreating again,” says Wolff.