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 about a mile long and 12 feet of frozen sediment underneath.
“It’s a very miraculous engineering feat that has been very difficult to replicate,” said geoscientist Andrew Christ, who recently completed a postdoctoral fellowship at the University of Vermont.
The sample was the first deep ice core scientists ever collected, and in the decades that followed, the ice was the subject of intense scientific research, providing important clues about the history of Earth’s climate. The same cannot be said for the deposits, which were mostly overlooked before disappearing completely.
In 2017, deposits were rediscovered in a Danish freezer. Now, a study of frozen samples is shedding new light on Greenland’s past and perhaps providing ominous warnings for the future. The findings, published Thursday in the journal Science, suggest that about 400,000 years ago, the site of Camp Century in northwestern Greenland was temporarily ice-free. These studies add to accumulating evidence that the Greenland ice sheet has not been stable over the past 2.5 million years, as scientists once assumed.
“The big message that comes out of this is that Greenland is fragile.” says Paul Biermann, a geoscientist at the University of Vermont and author of the new study. “Ice sheets have melted in the past and may melt again.”
Dr. Biermann and an international team first started studying the sediment several years ago and quickly made some startling discoveries. The top layer of the sample, which was expected to be just a jumble of compacted rocks, was full of vegetation, including twigs, leaves and tiny pieces of moss. The findings, published by scientists in 2021, suggest the region wasn’t always covered in ice.
“But the question we weren’t able to answer at the time was how old these plants and sediments in this ice-free terrain are,” said Dr. Christo, who is also author of the new analysis. “This new study in science tells us when it happened, That was 400,000 years ago. ”
To arrive at that date, scientists used a technique known as luminescence dating. When minerals are in the ground, they are exposed to environmental radiation and accumulate free electrons. These electrons build up over time, but exposure to sunlight essentially wipes them out, much like a washing machine removes a layer of dirt that builds up on your clothes during a week-long camping trip, Christ said.
By measuring the signal emitted by the accumulated electrons, the researchers were able to calculate when the top layer of the sediment was last exposed to the sun, or how long the site had been ice-free.
(Utah State University geoscientist Tammy Littner, who led this part of the study, had to analyze the samples in the dark to avoid “resetting” the electronic clock.)
After estimating an approximate date for the thaw, the scientists modeled various scenarios where ice-free extraction sites could have existed 400,000 years ago and calculated that the ice sheet would have to melt sufficiently for sea levels to rise by at least 4.5 feet.
This is “significant sea-level rise,” says Dr Christ. “And that’s what we really need to consider as a worst-case scenario for future climate change.”
At that time, temperatures were not much warmer than they are today, and atmospheric carbon dioxide levels were much lower, he said.
Still, many uncertainties remain about how the ice sheet will respond to continued warming, said Elizabeth Thomas, a geologist at the University of Buffalo and author of the new study. And it’s hard to extrapolate from her one sampling location, which is “near the edge of the ice sheet, not even a particularly sensitive part of the ice sheet,” she said.
Samples taken from parts of the ice sheet known to be less stable could provide more information about what happens as the planet warms, she said.
“We have these wonderful samples collected in the 1960s,” said Dr. Thomas. “It’s so cool, we can work on them.” Still, “it would be nice to go back in time and say, ‘Hey, First Ice Core Team, can you pick another location?'” she added.