In 1966, scientists at Camp Century, now an abandoned US military base in the Arctic, drilled deep into the Greenland ice sheet, uncovering a cylinder of ice nearly a mile long with 12 feet of frozen sediment sitting beneath it.
“That’s a pretty miraculous feat of engineering that’s 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 collected by scientists, and in the following decades, the ice became the subject of intense scientific study, providing critical clues about the planet’s climate history. The same cannot be said for the sediment, which is largely overlooked before disappearing completely.
In 2017, sediment was rediscovered in a freezer in Denmark. Now, a study of frozen samples sheds new light on Greenland’s past and, perhaps, provides an ominous warning for the future. The findings, published in Science on Thursday, suggesting that around 400,000 years ago the Camp Century site in northwest Greenland was temporarily ice-free. They add to the accumulating evidence that Greenland’s ice has not stabilized in the last 2.5 million years, as scientists had thought.
“The big message from this is that Greenland is vulnerable,” said Paul Bierman, a geoscientist at the University of Vermont and an author of the new study. “The ice has melted in the past, and therefore it can melt again.”
Dr. Bierman and an international team of collaborators first began studying the sediment a few years ago, and they quickly made a startling discovery. The top layer of the sample, where they expected to find little more than a mix of compressed rock, was full of vegetation: branches, leaves, small pieces of moss. The discovery, published by scientists in 2021suggested that the area was not always covered with ice.
“But the question we didn’t answer at the time was how old are these plants and the sediment from this ice-free landscape?” said Dr. Christ, who is also an author on the new review. “This new study in Science tells us when that happened, that was 400,000 years ago.”
To arrive at that date, scientists used a technique known as luminescence dating. As minerals sit in the ground, they are exposed to atmospheric radiation and accumulate free electrons. Those electrons build up over time, but exposure to sunlight essentially sweeps those electrons away, as a washing machine can remove layers of dirt that accumulate on an item of clothing during a weeklong camping trip, Dr. Christ.
By measuring the signal that the accumulated electrons are giving off, the researchers calculated the last time the top layer of sediment was exposed to the sun – and thus, how long the site was ice-free.
(Tammy Rittenour, a geoscientist at Utah State University who led this part of the study, had to study the samples in the dark to avoid “resetting” the electron clock.)
Once the scientists estimated the approximate date of the melt, they modeled various scenarios that could have resulted in an ice-free sampling area 400,000 years ago, calculating that the ice sheet should have melted enough to raise sea levels by at least four and a half feet.
That’s “a lot of sea level rise,” said Dr. Christ. “And that’s something we have to consider as a worst-case scenario for future climate change.”
The temperature at that time was not much higher than it is now, he says, and the levels of carbon dioxide in the atmosphere were lower.
Still, many uncertainties remain about how the ice will respond to continued warming, said Elizabeth Thomas, a geologist at the University at Buffalo and an author of the new study. And it’s hard to extrapolate from a sampling site, which is “close to the edge of the ice sheet and also not in a particularly sensitive part of the ice sheet,” he said.
Samples from parts of the ice sheet known to be less stable could be more instructive about what might happen as the planet warms, he said.
“We have amazing samples that were collected in the 1960s,” said Dr. Thomas. “It’s so cool we’re going to make them.” Still, he added, it’s nice to “go back in time and say, ‘Hey, first ice-core drilling team, can you pick a different site?'”