Two recent studies have suggested that Greenland’s vast ice sheet may be more dynamic, and thus more vulnerable, than has been assumed. Scientists had thought that the ice sheet had remained stable, even during the warming periods of Earth’s geological past, but new research suggests otherwise. The stability of Greenland’s ice sheet is highly relevant in the era of climate change, since the sheet melting would result in sea levels rising more than 20 feet, threatening the homes of at least 15 million Americans.

One study, published Thursday in the journal Nature, shows that the ice sheet has melted at least once in the relatively recent past, for “extended periods during the Pleistocene epoch,” which lasted from 2.6 million to 11,700 years ago. The researchers studied a sample of bedrock taken from below the ice sheets, which showed that the sheet melted to less than 10 percent of its current area for about 280,000 years in this period. This information suggests that Greenland’s ice sheet may be more vulnerable to changes than was previously thought.

Joerg Schaefer, a paleoclimatologist at Columbia University’s Lamont-Doherty Earth Observatory, and one of the co-authors of the study, said, “Unfortunately, this makes the Greenland ice sheet look highly unstable.”

To complicate matters, another study published in the same issue of Nature concluded that Greenland consistently maintained some amount of ice cover over the last 7.5 million years. However, scientists have suggested that the two studies may be more complementary than contradictory. The second study used bedrock testing of a sample from a different location, suggesting ice may have been more consistent at higher elevations.

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

Both papers also conclude that more research is necessary into the history of the ice sheets.

Bierman, speaking to Time magazine, said:

“We do what we’re doing with the atmosphere right now at our own risk, 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.”

According to one 2012 study, Greenland’s ice sheets are melting at a rate five times faster than during the 90s.

Schaefer took a much more urgent tone, saying “We have to be prepared that this ice sheet might go again, and it might go again soon.”

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