Before the U.S. military tried to hide nuclear missiles under the Greenland Ice Sheet during the Cold War, they asked scientists to determine the stability of the ice that would shelter the warheads. From the secret nuclear-powered Project Iceworm base, a maze of ice tunnels and caves spanning nearly two miles, they drilled down more than 4,000 feet to the ground, retrieving the world’s first deep ice core sample from a polar ice sheet in 1966. Today, those once-forgotten ice cores are advancing the understanding of a different, but no less global threat than nuclear war—climate change.

Since that first boring of the ice sheet, scientists have retrieved hundreds more ice cores to reconstruct an accurate record of the Earth’s climate for the past few million years. The layers of ice and frozen gas bubbles show the composition of the atmosphere when the ice formed, how much it snowed and how warm or cold it was. 

But the tools to make such a detailed analysis didn’t exist when that first core came to the surface, and climate change wasn’t yet a concern for polar scientists. The goal was to expand the tunnel network, potentially to thousands of miles, and to hide medium-range nuclear missiles that could quickly strike the Soviet Union. 

The missile plan was shelved and the base on and below the ice—known as Camp Century—was abandoned. The samples, first stored in freezers in Buffalo, then in Copenhagen, were mostly forgotten. But recently, American and Danish scientists rediscovered them when they were moving between labs. They said they were stunned to find bits and pieces of plants in the gravel layers sampled from the bottom of 4,000 feet of ice. 

Ancient Tundra

The twigs and foliage came from moss and shrubs that grew in tundra similar to that found in parts of Siberia and Alaska today. Finding the vegetation at the bottom of the core showed that the ice sheet, with enough frozen water to raise sea level by 20 feet, melted away completely at least once about 1 million years ago, during a climate shift similar to the pulse of warming caused by human greenhouse gas emissions now. The findings were published Monday in the Proceedings of the National Academy of Sciences.

“It’s one of the things that can help tell the story of how much warming you need before the ice sheet disappears,” said co-author Dorthe Dahl-Jensen, an ice physics researcher at the University of Manitoba and the University of Copenhagen. 

Until now, the strongest evidence showing a complete meltdown of Greenland’s ice sheet came from the warm mid-Pliocene geologic era, about 3 million years ago, said Pam Pearson, director of the International Cryosphere Climate Initiative, who was not involved in the study. But the new research strengthens evidence of a much more recent meltdown, brought on by less warming.

“This might indeed be new, since this core is from the highest point on the ice sheet, which one would expect to persist the longest,” she said. Unless temperatures 1 million years ago were warmer than thought, she said, the study suggests that the Greenland Ice Sheet is prone to melting in today’s temperatures. Greenland is already more than 3.6 degrees Fahrenheit warmer than in pre-industrial times, probably close to the threshold that will cause persistent long-term melting, she said.

University of Liége ice researcher Xavier Fettweis cautioned that that the new findings don’t enable a direct comparison between current and past climate conditions, because how the ice sheet warms may be just as important as how much it warms.

For one thing, the location where the ice core was taken is in a part of Greenland that could melt first, according to some climate models.

“That means that vegetation can appear in this area while the ice sheet could remain in some other areas,” he said. Ocean conditions and continent positions were different a million years ago and therefore, “the projected warming can not be directly compared with past conditions.”

And the current and anticipated warming of the Earth is driven by an increase in infrared radiation—the heat trapped by greenhouse gases—while incoming solar radiation is relatively constant. In the past, changing levels of incoming solar radiation, related to changes in Earth’s orbital cycles, drove wide, cyclical climate pulses that periodically warmed and cooled Greenland.

A ‘Frozen Ecosystem’

For a while, the old ice core was stashed in the U.S. Army Cold Regions Lab in Buffalo, New York, but in the mid-1990s, they were transferred, still frozen, to Denmark, where they continued to languish in storage until 2014, when University of Vermont geoscientist Paul Bierman met Dahl-Jensen at a conference.

“Dorthe said, ‘We have some more dirt in Copenhagen, you should analyze it,’” Bierman said. When they realized the core sample included about 15 feet of gravel from the base of the ice sheet, they knew they might have something unique and rare that would give them a chance to travel back in time a million years to roughly when Homo erectus and related species started dispersing from Africa.

There’s less of this stuff than moon rocks,” he said. “This is the richest archive that we’ve seen.”

The fact that there was organic debris mixed in was the icing on the cake. The sample isn’t just rock and sand, it’s a “frozen ecosystem,” he said. “What we really need to know is, is it measurable?” 

It turns out that, with modern gear, scientists can learn a lot from a handful of ancient twigs and slivers of leaves mixed into gravel, sand and mud.

“What’s exciting about looking at these sediments is that it’s a garbage dump of everything that was living on the landscape at the time,” said co-author Drew Christ, also a geologist at the University of Vermont. “We can analyze molecules from the wax coating on leaves and extract what the temperature was, and we can analyze the ancient DNA to get a holistic picture of what was there.”

Looking at the atoms in the material helped determine how long the sediments were exposed to cosmic rays before they were buried under the ice, and Christ was also able to study the luminescence of individual sand grains to establish when they were last exposed to light.

“Getting this sediment and using these new methods really is like using a time machine,” he said. That time machine shows that the Greenland Ice Sheet survived most of the warm periods between ice ages during the last million years, “But there is some threshold where it melts away, a change in the climate system at an uncontrolled rate, that pushes it toward melting.”

Bierman said the study is the best evidence yet that the Greenland Ice Sheet is at grave risk of irreversibly melting off.

“This is not a 20-generation problem, this is an urgent problem for the next 50 years,” he said.