Greenland wasn't always covered in ice — and data from a once-forgotten core sample from under the ice suggests it could melt again, in a climate like today's.
Research published in the journal Science shows that around 400,000 years ago, the sediment under Greenland's ice was originally "deposited by flowing water in an ice-free environment."
The findings came from a core sample that was drilled out from under a U.S. military base in Greenland in 1966. It was then stored in various freezers until researchers from the University of Copenhagen investigated it in 2018.
What they found revises what scientists thought they knew about Greenland: It hasn't been frozen under ice for millions of years. At one point, it was warmer — and ice modeling suggests when Greenland last melted, it would have contributed to more than 4 feet of global sea level rise.
Scientists say at the time, global average air temperatures hovered around where they are today.
But today, there's some 40% more carbon dioxide in the atmosphere than there would have been back then, and emissions of the greenhouse gas are climbing.
SEE MORE: Study: Greenland's Ice Is Melting Faster Than Previously Thought
The findings suggest that Greenland's ice "may be more sensitive to human-caused climate change than previously understood — and will be vulnerable to irreversible, rapid melting in coming centuries," the scientists said in a statement.
Study co-author Paul Bierman, a geologist at the University of Vermont, told ABC: "This is the evidence that Greenland can vanish. It's not a model. It's not a hypothetical. We know that the ice sheet vanished and it vanished under much less extreme conditions than we're forcing the climate to right now."
There isn't a firm timeline for when Greenland might be ice-free again, but it's already headed in that direction.
Data from NASA satellites shows Greenland's ice load is steadily shrinking: It's lost 271 billion metric tons of ice per year since 2002, and has already contributed to significant sea level rise since 1993.
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