Long-lost Greenland ice core suggests potential for disastrous sea level rise

Originally Published: 20 JUL 23 14:00 ET

By Laura Paddison, CNN

(CNN) — A recently discovered ice core taken from beneath Greenland’s ice sheet decades ago has revealed that a large part of the country was ice-free around 400,000 years ago, when temperatures were similar to those the world is approaching now, according to a new report – an alarming finding that could have disastrous implications for sea level rise.

The study overturns previous assumptions that most of Greenland’s ice sheet has been frozen for millions of years, the authors said. Instead, moderate, natural warming led to large-scale melting and sea level rise of more than 1.4 meters (4.6 feet), according to the report published Thursday in the journal Science.

“When you look at what nature did in the past, as geoscientists, that’s our best clue to the future,” said Paul Bierman, a scientist at the University of Vermont and a lead author of the study.

What it indicates is “frightening,” he told CNN.

Levels of carbon dioxide in the atmosphere are 1.5 times higher now than they were 400,000 years ago, and global temperatures keep climbing.

If Greenland’s ice sheet saw rapid melting during a period of moderate warming, it “may be more sensitive to human-caused climate change than previously understood – and will be vulnerable to irreversible, rapid melting in coming centuries,” the study authors said in a statement.

This would have significant impacts on sea level rise. If Greenland’s ice sheet were to melt completely, sea levels would rise by about 7 meters (23 feet) causing devastation to the billions who live along the world’s coasts.

To complete the research, Bierman and a team of international scientists spent years analyzing frozen sediment from an ice core collected in 1966 at Camp Century, a US army base in northwest Greenland. Scientists had drilled through more than 4,500 feet of ice to pull up a 12-foot-long soil and rock sample from beneath the ice sheet.

At the time, there wasn’t the technology to understand the sediment very well and so it was lost in a freezer for decades, Bierman said. Then, in 2017, it was rediscovered in Denmark.

Bierman went to Copenhagen and brought two samples back to the University of Vermont to test. As the scientists started to sieve it to separate out the sediment, they were surprised to see twigs, mosses, leaves and seeds.

“We have a fossilized frozen ecosystem here,” said Bierman, “And what that meant, of course, is the ice sheet had gone away because you can’t grow plants under a mile of ice.”

The scientists still needed to figure out how long ago the plants had been growing. To establish the time frame, samples were passed to a team at Utah State University, which uses luminescence technology – a technique which allows them to date the last time the sediment was exposed to daylight.

The scientists calculated that the sediment was deposited in an ice-free environment roughly 416,000 years ago.

“It’s really the first bulletproof evidence that much of the Greenland ice sheet vanished when it got warm,” Bierman said. “Greenland’s past, preserved in 12 feet of frozen soil, suggests a warm, wet, and largely ice-free future for planet Earth,” he added.

The potential implications for sea level rise are enormous, Tammy Rittenour, a professor from Utah State University and study co-author said in a statement. “We are looking at meters of sea level rise, probably tens of meters. And then look at the elevation of New York City, Boston, Miami, Amsterdam. Look at India and Africa – most global population centers are near sea level.”

As well as contributing to sea level rise, the loss of the ice also accelerates global warming, as white ice, which reflects the sun’s energy away from the Earth’s surface, is replaced with darker rock and vegetation, which absorbs the sun’s energy.

“There’s a feedback that sets in once you start to get rid of the ice sheet where we warm up even faster,” Bierman said.

Andrew Shepherd, head of geography and environmental sciences at Northumbria University in the UK who was not involved in the study, said the research was important because it “increases our confidence in predictions of how much melting we can expect in a warmer climate.”

Jason Box, professor in glaciology at the Geological Survey of Denmark and Greenland who was also not involved in the study, said the results could force a reevaluation of established thinking.

“The current greenhouse gas emission-driven warming may reduce the Greenland ice sheet faster than forecast,” he told CNN.

For Bierman, it all adds up to evidence that Greenland’s ice sheet is fragile.

Unless the world takes radical action to bring levels of planet-heating pollution to zero, and simultaneously works to remove the carbon pollution already in the atmosphere, he said “we’re dooming the Greenland ice sheet, and a lot of that sea level rise is going to come quickly.”

“Geologists don’t usually get very upset about what we find,” he said. “But this is really upsetting.”

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