M. Santini/Getty Images An iceberg detached from the Jakobshavn glacier in western Greenland. New research suggests the island's enormous ice sheet is more unstable than scientists previously believed.

If the ice sheet that blankets most of Greenland should melt, it would be catastrophic because sea levels would rise more than 20 feet. This would submerge coastal cities worldwide and threaten land that’s home to at least 15 million Americans.

Scientists had believed that the world’s second largest ice body had been relatively stable during the recent geological past. Even during periods of warming, the massive mantle of ice had remained largely intact, they thought.

But new research suggests that this assumption may be grossly off the mark.

Joe Raedle/Getty Images A researcher observes a canyon created over time by a meltwater stream on the Greenland's ice sheet, July 16, 2013.

The country’s ice sheet appears to have melted nearly completely at least once in the not-too-distant geological past, according to a groundbreaking study published on Thursday in the journal Nature.

By studying a sample of bedrock taken from beneath Greenland’s ice deposit, researchers found that the autonomous Danish territory was ice-free (or “deglaciated”) for “extended periods during the Pleistocene epoch,” stretching from 2.6 million years to 11,700 years ago. Researchers estimate that the ice sheet shrunk to less than 10 percent of its current size and stayed that way for at least 280,000 years.

Greenland’s mighty ice sheet, it seems, may not be as impregnable as previously believed.

“Unfortunately, this makes the Greenland ice sheet look highly unstable,” said study co-author Joerg Schaefer, a paleoclimatologist at Columbia University’s Lamont-Doherty Earth Observatory, in a news release.

M. Santini/Getty Images Icebergs that have broken off the Sermeq Kujalleq ice sheet, Qaasuitsup, Greenland.

Interestingly, another study published this week in the same edition of Nature revealed a significantly different picture of the ice sheet’s past. Also a result of bedrock testing but from a sample taken from a different location, the paper concluded that Greenland had maintained at least some ice cover for the past 7.5 million years.

Scientists say, however, that the two studies aren’t necessarily contradictory ― and may in fact inform the other. Nearly all of Greenland’s ice could’ve melted at certain times in the past, but some ice might’ve remained in areas of higher elevation.

“Both studies show that there’s the potential for the ice sheet to be quite dynamic and change over time,” University of Vermont professor Paul Bierman, lead author of the second study, told Time magazine.

Joe Raedle/Getty Images Water is seen on part of Greenland's glacial ice sheet, July 2013.

Authors of both papers agree that more research needs to be done to better understand the ice sheet’s history. And global warming should be taken as a real and present threat to the future of the ice body ― and the planet as a whole.

“We do what we’re doing with the atmosphere right now at our own risk,” Bierman told Time. “We’re dealing with an incredibly complex system on Earth and we don’t know the half of it. There are surprises lurking out there.”

Schaefer had a more direct warning. “We have to be prepared that this ice sheet might go again, and it might go again soon,” he said, according to MIT Technology Review.

“Soon,” of course, is relative to geological time. It could take centuries or even millennia for all that ice to melt.

But there’s still plenty of reason to be concerned. Greenland currently contributes about a quarter of the three millimeters that global sea levels rise every year ― and climate change could greatly accelerate the speed of the ice sheet’s melting. “Projections of sea-level rise during this century hover around 3 or 4 feet, but many, including the one from the Intergovernmental Panel on Climate Change, do not take Greenland into account,” states a blog post on Columbia’s website.

The impacts of climate change on Greenland are already being seen.

The island’s ice mantle is melting five times faster than it was in the 1990s, according to a 2012 study.

A Google Earth timelapse of northeast Greenland between 1984 and 2016 shows just how dramatic the melt has been on some parts of the country (use the scrollbar to move backwards and forwards in time):

Clarification: The article previously referred to Greenland as a “Northern European” country. The reference has been removed since geographically it is part of North America, although it is now politically a part of Denmark.