In 2019, a team of scientists glimpsed an ancient, shrubby landscape at the base of a long-forgotten ice core—rare evidence that Greenland wasn’t always completely covered in ice. Now, they have pinpointed the age of that ecosystem, and the implications are disturbing—Greenland’s ice sheet, the finds show, could melt at any time, contributing to catastrophic sea level rise.
“There’s way more Moon rocks on the Earth, in terms of mass, than there is this stuff from the bottom of the ice sheet.”
In the 1960s, U.S. scientists helping to build a top-secret military base at Camp Century in northwest Greenland drilled a core through 1,389 meters (4,560 feet) of ice. They hit the bottom and kept going, collecting 4 meters (12 feet) of gravel and sludge, before blowing a bearing and calling it a day.
The core was lost for decades, stored in a poorly labeled box at the back of a walk-in freezer. When it was rediscovered, those last 4 meters proved to be scientific dynamite.
“There’s way more Moon rocks on the Earth, in terms of mass, than there is this stuff from the bottom of the ice sheet,” said co–lead author Paul Bierman, a geologist from the University of Vermont. “It’s that rare.”
A Lost Ecosystem
The gravels contained a snapshot of a much greener Greenland. “There are perfectly preserved fossils of a tundra ecosystem—twigs, leaves, mosses, these little midges that fly around Greenland getting in your hair,” Bierman said.
The team reported the find in 2021 but at the time were unable to accurately date the deposits. Doing so, said coauthor Tammy Rittenour, a paleoclimatologist with Utah State University, is crucial for “understanding the conditions at which you can melt the Greenland ice sheet.”
The team, which included 21 scientists from around the world, used luminescence dating to assess the last exposure to sunlight of the subglacial samples. Minerals in sand grains absorb electrons from the surrounding rock at a steady rate, creating a natural nuclear clock. When exposed to sunlight, those electrons are ejected, resetting the clock. When reburied again, the minerals slowly start to “recharge.” By blasting quartz and feldspar in the Camp Century core with infrared or blue-green light and measuring the electrons ejected, Rittenour and her colleagues determined how long it’s been since the grains last saw sunlight.
These records are “the history of the world, written on a grain of sand,” Rittenour said.
The researchers pinned the last exposure at around 416,000 years ago, during the interglacial period called marine isotope stage 11 (MIS 11).
In Bierman’s lab, a group including glacial geomorphologist and co–lead author Andrew Christ measured isotopes of aluminum and beryllium in quartz crystals from the Camp Century ice core. The ratios of these isotopes revealed that the sediments at Camp Century had been exposed to cosmic radiation—free of ice cover—for around 16,000 years.
The two techniques combined, Christ said, to produce “the first direct terrestrial evidence that the ice sheet was smaller during MIS 11.” The team’s work was recently published in Science.
“We’ve Got This Dilemma”
MIS 11 has long intrigued climate scientists. Though patterns in Earth’s orbit were broadly similar to today and carbon dioxide (CO2) concentrations in the atmosphere were similar to what they were just before the Industrial Revolution, sea levels were, nonetheless, up to 13 meters (42 feet) higher.
“We’ve got this dilemma,” said Tom Cronin, a senior geologist with the U.S. Geological Survey, who has also studied MIS 11 but was not involved in this research. “Why are so many proxies suggesting warmer than pre-Industrial [era temperatures], higher sea level and reduced sea ice in the Arctic, when CO2 levels were not higher?”
Climate variability driven by the El Niño–Southern Oscillation, North Atlantic Oscillation, and other patterns, Cronin said, could have driven Earth’s climate, ocean circulation, and ice dynamics to tipping points not predicted by current CO2 modeling.
“I think this variability represents aspects of Earth’s climate system that we don’t fully understand or appreciate,” Cronin said.
“MIS 11 wasn’t particularly warm,” added Christ, “but it was warm for a very long time [perhaps 25,000-30,000 years]. And we think that duration of warmth was key to melting the Greenland ice sheet.”
Inherent Warning
For years, evidence has been mounting that parts of Greenland were, to some extent, ice free during MIS 11. In 2007, DNA in ice cores hinted at a now vanished conifer forest, and another team found pollen in sediments off the island’s southern coast. In 2014, geologist Alberto Reyes from the University of Alberta led a study that recorded a break in silt deposits on the seafloor—indirect evidence of deglaciation.
Reyes and his team were forced to look for clues to Greenland’s past in the ocean because of the extreme difficulty of accessing terrestrial sediments from under the ice cap. The subfossil record from the Camp Century core is therefore invaluable, Reyes said. He was not involved in the Camp Century study.
“This study really puts the nail in the coffin” of the idea that Greenland was entirely covered by an ice sheet during MIS 11, Reyes said. “There should be no debate anymore that Greenland hasn’t had some pretty substantial ice-free conditions during stage 11.”
Although it is not yet possible to say whether Greenland was completely ice free during this period, the scale of the loss was immense. At Camp Century, “nearly a mile of ice disappeared,” Bierman said. “It means that nature on its own, without fossil fuel emissions in the atmosphere, removed a significant portion of the ice sheet.”
At the very minimum, the melting detected at Camp Century would have added around 1.4 meters (5 feet) to sea level. If the entire Greenland ice sheet melted, it would have added 7 meters (23 feet). If that happened today, it would devastate most of the world’s coastal towns and cities.
Christ said the long duration of MIS 11 carries an inherent warning. “Looking forward, we have a scary situation, because carbon [from fossil fuels] is going to stay in the atmosphere for tens of thousands of years before it’s geologically sequestered. And that’s going to keep temperatures elevated for a very long time.”
—Bill Morris, Science Writer
Correction, 8 September 2023: This story has been updated to correct the dating technique (luminescence) used by the researchers, as well as the estimated time period during which MIS 11 was warm (25,000 years to 30,000 years). We regret the errors.