A well-kept mystery seems to have been solved: During the last interglacial period, the West Antarctic Ice Sheet apparently melted completely, as long suspected, causing global sea levels to rise by several meters. The DNA of a small octopus provided an interdisciplinary team of scientists with the decisive clues.
For decades, scientists have been searching for evidence of a collapse of the West Antarctic Ice Sheet (WAIS) during the last interglacial period – an increasingly pressing question. However, the geological studies to date have only been able to provide indications.
In a novel approach, a small octopus, Pareledone turqueti, has now helped an international team of molecular biologists and geologists to provide empirical evidence for the melting of the WAIS during this warm period around 129,000 to 116,000 years ago. At that time, the average temperature of the earth was about 0.5°C higher than today and the sea level was five to ten meters higher.
In view of the current rapid warming of our planet, it is predicted that it will not be many decades before the global average temperature reaches this level again. It is already at least 1.1°C higher than in pre-industrial times. In the current issue of the journal Science, the team concludes that the WAIS could soon reach its tipping point and melt irreversibly – even if the Paris climate target of a maximum of 1.5°C warming is met.
“What makes the WAIS important is that it is also Antarctica’s current biggest contributor to global sea level rise. A complete collapse could raise global sea levels by somewhere between 3 and 5 metres. Understanding how the WAIS was configured in the recent past, when global temperatures were similar to today, will help us improve future global sea level rise projections,” Prof. Jan Strugnell explains, lead researcher at James Cook University and senior author of the study, in a press release by the Alfred Wegener Institute.
Analyses of the DNA of P. turqueti from the Weddell Sea, the Amundsen Sea and the Ross Sea enabled the researchers to conclude that these three marginal seas must have been connected at that time. They tested various scenarios using demographic models of octopus populations and found that a gene exchange of the previously isolated populations coincides with the interglacial period. This connection would not have been possible with a more or less intact WAIS. It was the melting of the ice sheet that opened up waterways and raised the sea level. P. turqueti, which occurs in the Southern Ocean circumpolar, was able to migrate back and forth from all three regions over thousands of years and exchange its genetic material, which is still reflected in the DNA of the re-isolated populations today.
“DNA holds a history of its past and can be used to look back in time to pinpoint when different populations of animals were mixing and exchanging genetic material,” Sally Lau, an evolutionary geneticist at James Cook University and lead author of the study, said in a British Antarctic Survey press release.
The DNA samples come from 96 octopuses collected over a period of 33 years, many of them as bycatch from fishing vessels.
In an accompanying article published in the same issue of Science Andrea Dutton, a geologist at the University of Wisconsin-Madison, and Robert M. DeConto, Professor and Director of the School of Earth & Sustainability at the University of Massachusetts Amherst, both of whom were not involved in the study, asked further questions: How fast would sea levels rise if the WAIS collapsed? Would it rise slowly and gradually or in one or more rapid jumps if vulnerable areas of the ice sheet collapsed? They also emphasize that understanding how ice loss occurred in the past is the basis for forecasting future sea level rise. These in turn are of crucial importance for coastal planners.
“This is telling us that we need to take this bigger picture seriously,” says Dutton. “We can’t just kick the can down the road and wait to make emissions cuts for another 5 years, another 10 years. It really demands that we do it now.”
Julia Hager, PolarJournal
Link to the study: Sally C. Y. Lau et al. ,Genomic evidence for West Antarctic Ice Sheet collapse during the Last Interglacial.Science382,1384-1389(2023).DOI:10.1126/science.ade0664
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