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This story was originally published by The Guardian and appears here as part of the Climate Desk collaboration

Deep in the DNA of an Antarctic octopus, scientists may have uncovered a major clue about the future fate of the continent’s ice sheet — raising fears global heating could soon set off runaway melting.

Climate scientists have been struggling to work out if the ice sheet collapsed completely during the most recent “interglacial” period about 125,000 years ago, when global temperatures were similar to today.

The ice sheet holds enough water to raise sea levels by three to four metres with fears that global heating could soon push it towards runaway melting that would lock in rising sea levels over centuries.

In an ingenious approach, a team of 11 scientists — including biologists, geneticists, glaciologists, computer scientists and ice-sheet modellers — looked at the genetics of Turquet’s octopus — a species that has been living around the Antarctic continent for about four million years.

Genetic samples were taken from 96 octopuses collected over three decades from around the continent.

The octopus DNA carries a memory of its past, including how and when different populations were moving and mixing together, exchanging genetic material.

The scientists say they detected clear signs that, about 125,000 years ago, some octopus populations on opposite sides of the West Antarctic Ice Sheet had mixed together, with the only likely route being a seaway between the south Weddell Sea and the Ross Sea.

“That could only have happened if the ice sheet had completely collapsed,” said Dr. Sally Lau, a geneticist at James Cook University who led the research.

A clue to rising sea levels lies in the DNA of a four million-year-old octopus, scientists say. #ClimateChange #SeaLevelRise

The research is undergoing peer review at a journal but it has been made public, Lau said, because she wanted the scientific community to have early access and because of the urgent nature of the findings.

She said information on the changes in the DNA of the octopus can be used like a clock, allowing her to pinpoint the period when octopuses in the south Weddell Sea and the Ross Sea were mixing.

Topographic map Antarctica showing the Weddell Sea and Ross sea. Photo by Hugo Ahlenius, UNEP/GRID-Arendal

Prof. Nick Golledge, a co-author of the research from Victoria University of Wellington in New Zealand, said a major concern was that once the ice sheet reaches a tipping point, the melting becomes “self-sustaining” and would continue for centuries or longer.

He said the route the octopuses are thought to have used is about 1,500 to 2,000 metres below the top of the current ice sheet. That channel would have been about 1,000 metres deep but shallower nearer the edge.

“It’s a sizeable ocean segment and a significant seaway for organisms to traverse,” he said.

He said over the past two decades, the rate of ice loss from west Antarctica had been increasing.

According to the most recent United Nations climate assessment, temperatures during the last interglacial were between 0.5 C and 1.5 C warmer than the period just before the industrial revolution. Sea levels were between five and 10 metres higher than today.

The authors of the octopus research say their findings suggest that even under global heating of 1.5 C — the most ambitious goal under the global Paris climate agreement — the West Antarctic Ice Sheet could be consigned to collapse.

Prof. Nathan Bindoff, an oceanographer and Antarctic expert at the University of Tasmania, said with sea levels that high scientists strongly suspect a melting West Antarctic Ice Sheet must have contributed to those rising sea levels.

Bindoff, who was not involved in the research, said using octopus DNA was “the last way I would have thought of having evidence of large sea level changes coming from the collapse of the West Antarctic Ice Sheet.”

“The loss of that ice sheet would have very real consequences for the whole planet. If this [octopus research] is correct then there are sensitivities in the Earth system that lead to planetary-scale sea level rise.”

In the most recent UN climate report, Bindoff said one area of the greatest uncertainties on how high sea levels might get related to the West Antarctic Ice Sheet.

He said about 670 million people currently lived in low-lying areas around the world, with a further 65 million in small island states.

He said: “This paper is another piece of evidence that reduces that uncertainty of how this ice sheet has evolved in the past and that is critical for how we think about the future.”

Prof. Richard Alley, a leading ice sheet expert at Penn State University, said while there was evidence the ice sheet had collapsed millions of years ago, “we still aren’t sure whether the ice sheet deglaciated during the most recent interglacial.”

He described the octopus research as “interesting and important” and said it strengthened arguments for the loss of the ice sheet during the last interglacial period.

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