Scientists have long wondered whether the West Antarctic Ice Sheet is a ticking time bomb in terms of sea level rise. New evidence from the DNA of a small octopus that lives in the Southern Ocean suggests that the ice sheet is indeed at risk of collapsing, according to a study published on Thursday in the journal Science.

The research doesn’t predict when this might happen, but it indicates that 1.5 degrees Celsius of global warming over the preindustrial global average, or perhaps even less than that, might be a tipping point for the ice sheet. The Earth is close to that temperature level now.

Several distinct populations of Pareledone turqueti, commonly known as Turquet’s octopus, live in the waters around Antarctica today. These octopuses crawl along the sea floor and generally don’t stray far from home. A few individuals or their eggs might occasionally drift on currents to neighboring groups, but populations in the Ross Sea and Weddell Sea are separated by the impassable West Antarctic Ice Sheet.

And yet, genetic analysis of octopuses from different locations around Antarctica show that these two populations were mingling and swapping DNA about 120,000 years ago. This was a time in Earth’s history called the Last Interglacial period, before the most recent ice age, when temperatures were similar to today.

The observed patterns in the octopus gene pool would only be possible if the West Antarctic ice sheet wasn’t there at the time and relatively open seaways across the continent allowed the octopuses to travel freely between the Ross and Weddell Seas, according to the researchers.

Scientists know that sea levels were several meters higher then. But whether the additional water came from West Antarctica is “the question that the geoscience community has been trying to answer for nearly 50 years,” said Sally Lau, a postdoctoral researcher at James Cook University in Townsville, Australia, and lead author on the new study.

Today, the global average temperature is about 1.2 degrees Celsius higher than it was from 1850 to 1900, when the burning of fossil fuels began warming the climate. During the Last Interglacial, the global average temperature was similarly about 0.5 to 1.5 degrees Celsius warmer than that preindustrial base line, but sea levels were five to 10 meters higher than today. If climate change melts the West Antarctic ice sheet entirely, sea levels could rise by an average of up to five meters, or 16 feet. (The East Antarctic Ice Sheet holds even more frozen water, but it is considered more stable.)

The researchers did not explicitly state whether today’s temperatures had already committed the planet to a complete collapse of the western ice sheet. “We’re still not able to say for certain, but that’s definitely the implication,” said Nicholas Golledge, a professor of glaciology at Victoria University of Wellington in New Zealand and another author of the study.

If the ice sheet has already reached a tipping point, estimates for how quickly it might melt range anywhere from 200 years to 2,000 years. “Our actions from this point on will still change the rate at which we get there,” Dr. Golledge said.

Unlike today, the Last Interglacial was part of an ongoing natural cycle of changes in the tilt of Earth’s axis and its orbit around the sun, and the resulting changes in the amount of sunlight the planet receives. These cycles happen gradually over tens of thousands of years. Our current greenhouse gas emissions are causing similar changes in temperature, but at a much faster rate.

Although the reasons behind past and current warming are different, the Last Interglacial is still one of the best analogues for present-day climate change, said Roger Creel, a postdoctoral scholar at the Woods Hole Oceanographic Institution. He was not involved in the study published Thursday but has contributed to estimates of sea level during that period.

“It’s such strong evidence from a completely different vantage point than the climate community often has,” Dr. Creel said of the new study.

Some of the octopus specimens Dr. Lau studied were collected more than 30 years ago, from fishing boats and scientific expeditions, and held in museums. Because DNA in dead animals degrades over time, this type of research using museum specimens hasn’t been possible until very recently with advances in genetic sequencing.

Other scientists have shown that the population genetics of land animals line up with past melting of the West Antarctic Ice Sheet. A 2020 study of springtails, small invertebrates that live in soil, also suggested that ice in the Ross Sea region had melted away during warm periods over the past 5 million years, including during the Last Interglacial.

Geoscientists can use mathematical models to reconstruct ice sheets and sea levels of the past, but the emerging biological evidence can help corroborate these reconstructions, said Ian Hogg, a research scientist at Polar Knowledge Canada, an agency that monitors polar regions, and an author of the study of springtails.

“As biologists, we know these patterns exist among populations,” he said. The challenge for biologists is explaining these observed patterns, while a challenge for geoscientists studying Antarctica has been gathering enough observational, physical evidence to validate their models.

“They have something that they’re providing us,” Dr. Hogg said. “And we have something that we can provide them.”

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