Scientists pinpoint when Greenland was last green, adding 5 feet to sea levels from melting ice

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A new study has uncovered evidence that most of Greenland melted only about 400,000 years ago, suggesting the country may be more sensitive to climate change than previously thought.

The study, published in the journal Science on July 20 by researchers from around the world, used decades-old sediment that had been gathered from thousands of feet below the ice as part of a secret U.S. military mission during the Cold War.

While previous work has estimated that the ice on Greenland melted at least once in the last 1.1 million years, the researchers behind this latest study say much of the territory was in fact green 416,000 years ago, plus or minus 38,000 years.

"It's really the first bulletproof evidence that much of the Greenland ice sheet vanished when it got warm," scientist Paul Bierman, who co-led the study, said in a story from the University of Vermont where he is a professor.

"We had always assumed that the Greenland ice sheet formed about two and a half million years ago—and has just been there this whole time and that it's very stable," Utah State University scientist Tammy Rittenour, who served as a co-author of the study, said.

"Maybe the edges melted, or with more snowfall it got a bit fatter — but it doesn't go away and it doesn't dramatically melt back. But this paper shows that it did."

CAMP CENTURY

The sediment was taken in northwestern Greenland at Camp Century, a military base from the 1960s that the U.S. army claimed was an Arctic science station.

The camp, in fact, was used for an operation called Project Iceworm, which aimed to hide hundreds of nuclear missiles under Greenland's ice in close proximity to the Soviet Union.

Although the nuclear missile mission was a "bust," the researchers say the scientists there at the time drilled down almost a mile deep, collecting 12 feet of soil and rock from below the ice.

This sediment moved from a military freezer to the University at Buffalo in New York state in the 1970s, before moving to a freezer in Denmark in the 1990s.

STUDYING THE SEDIMENT

The ice core, which was rediscovered in 2017, was found to contain vegetation such as leaves and moss.

The scientists believe flowing water deposited the sediment during a period of moderate warming called Marine Isotope Stage 11, between 424,000 and 374,000 years ago.

During that time, sea levels rose at least five feet around the globe, the researchers say.

The scientists analyzed the core for a "luminescence signal," a process where the sediment is exposed to blue-green or infrared light, which releases trapped subatomic particles known as electrons.

By measuring the number of electrons that are released, the scientists could determine when the sediment was last exposed to the sun.

The researchers also studied quartz from the sediment. They say this quartz includes rare forms, or isotopes, of the elements beryllium and aluminium that build up when the ground is exposed to the sun.

Through this, the researchers say the sediment was exposed less than 14,000 years before it was buried under ice.

They add that understanding Greenland's history can help predict how quickly the ice sheet will melt in response to a changing climate.

With a melting Greenland expected to contribute about 23 feet to rising sea levels, the researchers say this will pose risks to major coastal cities around the world.

"Greenland's past, preserved in 12 feet of frozen soil, suggests a warm, wet and largely ice-free future for planet Earth, unless we can dramatically lower the concentration of carbon dioxide in the atmosphere," Bierman said.