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.”

Camp Century sub-ice sample processed at the University of Vermont.

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.