Scientists from the University of Cambridge and British Antarctic Survey have used ice core records to draw new conclusions about how Antarctica was affected by increased global temperatures over 100,000 years ago. The new paper, published today in the journal Nature, shows that large parts of the West Antarctic Ice Sheet were lost, contributing to significant sea level rise. However, the data also suggests that the nearby Ronne Ice Shelf – which climate models project could be lost under future warming scenarios – survived this period of global heating.
Greenhouse gas emissions are warming the Earth at an unprecedented speed and scale. While anthropogenic warming has no direct historical parallel, warm episodes in Earth's history can offer clues to the future.
A team of ice core scientists, led by Eric Wolff from Cambridge University, wanted to find out what happened to the West Antarctic Ice Sheet during the Last Interglacial, when the polar regions were about 3°C warmer than present and sea levels were significantly higher. This period of Earth's history is considered comparable to conditions we might see within decades.
Louise Sime, climate scientist at British Antarctic Survey and a contributing author of the new paper, explained:
"Solving the puzzle of how quickly and how much of the Antarctic ice sheet melted during the Last Interglacial has been a major goal for paleoclimate researchers for decades. By drilling and analysing their new West Antarctic SkyTrain ice core, it seems that Eric Wolff and his team have finally got their hands on key evidence that a large loss of ice really did occur 126,000 years ago.
It may be surprising to hear that, until now, we've been missing conclusive data showing these major changes to Antarctica in climate records. This information from the last time Antarctica was warmer than present day is crucial for predicting how and when the West Antarctic ice sheet will change under future warming."
The West Antarctic Ice Sheet holds enough fresh water to raise sea levels 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 inland.
In 2019, Wolff and the team collected a 651m 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 – meaning data from this site can offer a long term view on the stability of this vast ice sheet, and its contribution to sea level rise.
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. 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.
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. Lead author Eric Wolff summarised: "We're seeing that the ice sheet retreated, but not with the added disappearance of the Ronne Ice Shelf." 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.
Rachael Rhodes, study co-author from Cambridge Earth Sciences, commented on the new questions the research has raised:
"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. It might be the case that we need to reappraise whether the East Antarctic Ice Sheet was also an important source of freshwater."
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.
Nick Golledge, co-author from the Victoria University of Wellington, explained the significance of the work to future planning:
"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."
'The Ronne Ice Shelf survived the last interglacial', by Eric Wolff, is published today in Nature, https://doi.org/10.1038/s41586-024-08394-w
