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In 1966, scientists at Camp Century, a now abandoned U.S. military base in the Arctic, drilled deep into the Greenland ice sheet, extracting a cylinder of ice nearly a mile long along with 12 feet of the frozen sediment that sat beneath it.

“That was a pretty miraculous engineering feat that has been really hard to repeat,” said Andrew Christ, a geoscientist who recently completed a postdoctoral fellowship at the University of Vermont.

The sample was the first deep ice core that scientists had ever collected, and over the decades that followed, the ice became the subject of intense scientific study, providing critical clues about the planet’s climate history. The same could not be said for the sediment, which was largely overlooked before vanishing completely.

In 2017, the sediment was rediscovered in a freezer in Denmark. Now, a study of the frozen samples is shedding new light on Greenland’s past and, perhaps, providing an ominous warning for the future. The findings, which were published in Science on Thursday, suggest that roughly 400,000 years ago the Camp Century site in northwestern Greenland was temporarily ice-free. They add to accumulating evidence that Greenland’s ice sheet has not been stable for the last 2.5 million years, as scientists once assumed.

“The big take-home message from this is Greenland is vulnerable,” said Paul Bierman, a geoscientist at the University of Vermont and an author of the new study. “The ice sheet has melted in the past, and therefore it can melt again.”

Dr. Bierman and an international team of collaborators first began studying the sediment several years ago, and they quickly made a surprising discovery. The top layer of the sample, where they had expected to find little more than a jumble of compressed rock, was full of plant matter: twigs, leaves, tiny pieces of moss. The discovery, which the scientists published in 2021, suggested that the area had not always been covered in ice.

“But the question we didn’t answer at that time was how old were these plants and the sediment from this landscape that didn’t have ice on it?” said Dr. Christ, who is also an author on the new analysis. “This new study in Science is telling us when that happened, which was 400,000 years ago.”

To arrive at that date, the scientists used a technique known as luminescence dating. As minerals sit in the ground, they are exposed to environmental radiation and accumulate free electrons. Those electrons build up over time, but exposure to sunlight essentially sweeps the electrons away, as a washing machine might remove the layers of dirt that build up on an item of clothing over the course of a weekslong camping trip, Dr. Christ said.

By measuring the signal that the accumulated electrons were giving off, the researchers were able to calculate the last time that the top layer of sediment had been exposed to the sun — and thus, how long ago the site had been ice-free.

(Tammy Rittenour, a geoscientist at Utah State University who led this part of the study, had to analyze the samples in the dark to avoid “resetting” the electron clock.)

Once the scientists had estimated the approximate date of the thaw, they modeled various scenarios that could have resulted in an ice-free sampling site 400,000 years ago, calculating that the ice sheet would have to have melted enough to increase sea levels by at least four and a half feet.

That “is a lot of sea-level rise,” Dr. Christ said. “And that is something that we need to really consider as a worst-case scenario for future climate change.”

The temperature at the time was not much higher than it is now, he noted, and the carbon dioxide levels in the atmosphere were much lower.

Still, many uncertainties remain about how the ice sheet will respond to continued warming, said Elizabeth Thomas, a geologist at the University at Buffalo and an author of the new study. And it is difficult to extrapolate from that one sampling site, which is “close to the edge of the ice sheet and is also not in a particularly sensitive part of the ice sheet,” she said.

Samples from parts of the ice sheet that are known to be less stable may be more informative about what could happen as the planet warms, she said.

“We have these amazing samples that were collected in the 1960s,” Dr. Thomas said. “It’s so cool that we get to work on them.” Still, she added, it would be nice to “go back in time and say, ‘Hey, first ice-core drilling team, can you please choose a different site?’”

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