Increasing greenhouse gas emissions are warming our planet at an unprecedented rate and scale. While anthropogenic warming has no direct historical parallel, warm episodes in Earth’s history can offer clues as to the future.
A team of ice core scientists led by Cambridge University wanted to find out what happened to the West Antarctic Ice Sheet during the Last Interglacial, over 100,000 years ago; a period when the polar regions were about 3°C warmer than present and sea levels were significantly higher.
“The warming during the Last Interglacial was similar to what we might see within decades,” said Cambridge Earth Science’s Eric Wolff. “Studying the vulnerability of this ice sheet during the Last Interglacial could help us better forecast how the continent’s ice will respond in future.”
The West Antarctic Ice Sheet holds enough fresh water to raise sea level by around 3-4 metres. This ice sheet is particularly vulnerable to warming because its bowl shape allows seawater to flow under its rim and slowly melt its base. The Ronne Ice Shelf stretches out from the ice sheet into the Atlantic Ocean, forming a floating platform that acts like a buttress that holds back and protects glaciers in-land.
Scientists haven't been able to say for certain whether the West Antarctic Ice Sheet and its adjacent ice shelves were lost entirely during the Last Interglacial. “It’s surprising that an event as big as a whole ice sheet going missing is something that we haven’t been able to see in our climate records very easily,” said Wolff.
In search of answers, Wolff and the team collected a 651-metre-long ice core from Skytrain Ice Rise, a coastal dome next to the Ronne Ice Shelf. The site is close enough to the West Antarctic Ice Sheet to witness its shrinkage whilst remaining intact. The researchers spent two months in 2019 drilling in sub-zero temperatures and blizzard conditions.
The drilling and living tents at Skytrain Ice Rise. Credit: Eric Wolff.
“We think the site chosen for the drilling is one of the best places in Antarctica to look back at the state of the West Antarctic Ice Sheet in the Last Interglacial period. It will lead us to a critical understanding of the stability of this vast ice sheet, and its contribution to sea level rise, to the warming global temperatures predicted for the next 100 years,” said study co-author Robert Mulvaney from the British Antarctic Survey.
It has taken several years for the team to decode the climate signals preserved in the ice core. Their results, published today in the journal Nature, provide the first direct evidence of what happened to the ice sheet during this warm period.
By analysing the composition of water isotopes in the ice core, they found that the West Antarctic Ice Sheet was partially lost during the Last Interglacial, shrinking to perhaps half its modern-day mass.
Scientists running the ice analysis system in the laboratories at the British Antarctic Survey. Credit: University of Cambridge / British Antarctic Survey.
They also measured sea salt contents of the core to gauge the amount of sea spray and therefore the site’s proximity to the coast. That dataset showed that the Ronne Ice Shelf covered a similar extent to today, “we’re seeing that the ice sheet retreated, but not with the added disappearance of the Ronne Ice Shelf,” said Wolff.
Although the ice sheet shrunk significantly, this isn’t the catastrophic ice sheet collapse that some reconstructions have suggested could have happened at this time. But, said Wolff, “we still know that sea level was significantly higher at this time.”
Scientists have clear evidence that sea level was several metres above present during last interglacial. The loss of the West Antarctic Ice Sheet had been thought to be the most likely contributor.
“The ice melt for that sea level rise had to come from somewhere, so now we need to ask where else it could have come from,” said Rachael Rhodes, study co-author from Cambridge Earth Sciences.
“It might be the case that we need to reappraise whether the East Antarctic Ice Sheet was also an important source of freshwater,” Rhodes added. Even though the East Antarctic Ice Sheet holds much more water than the West Antarctic Ice Sheet (enough to raise sea level by around 52 metres), it is generally thought to be more stable and resilient to warming.
The researchers say their findings will help improve forecasts of what could happen to ice sheets as our climate warms. The largest uncertainty in long-term sea level projections, including those made by the IPCC, is the fate and stability of the West Antarctic Ice Sheet.
"Data like those from Skytrain Ice Rise are invaluable for modellers trying to understand how the West Antarctic Ice Sheet might behave under future warming. They offer a window into what actually happened in the past, allowing the sensitivity of our models to be accurately calibrated. Without this kind of information, our projections for the future will never be as accurate as we'd like them to be,” said co-author Nick Golledge from the Victoria University of Wellington.
These latest results follow on from an earlier study by the team, using data from the same ice core. During a more recent warm period, around eight thousand years ago—after the end of the Last Ice Age—they found that the West Antarctic Ice Sheet shrunk suddenly and dramatically, shrinking by around 450 metres (more than the height of the Empire State Building) in just under 200 years.
Reference: Wolff, E. W. et al. The Ronne Ice Shelf survived the last interglacial. Nature https://doi.org/10.1038/s41586-024-08394-w (2025).