For a long while, more than a million years ago, Greenland wasn't covered in ice, scientists announced last week.
That may not seem like the biggest news in science, except for this: The Greenland ice sheet is the second largest ice cube on the planet, after the Antarctic ice sheet. If the Greenland ice sheet were to melt – if it is even possible for the ice sheet to melt – then it's also possible that the planet's oceans might rapidly rise five or six meters, or more than twenty feet, and wreak havoc on coastal cities worldwide.
Before now, scientists didn't know whether Greenland's ice sheet was so stable that it would just weather any climate changes, or if there were ever a period in which Greenland was, if not verdant, at least a bit rocky.
It turns out there was, in fact, a time when it was largely ice-free, perhaps for as long as 250,000 years, more than a million years ago.
Scientists were able to determine this because the bare rock during that time was exposed to cosmic rays in the atmosphere, says Marc Caffee, professor of physics and astronomy at Purdue, in a press release.
"We now have pretty conclusive evidence that for a time that ice wasn't there," Caffee says. "That's big. That's new. It's probably not much different in temperature now than it was then, so we shouldn't count on that ice sheet never melting again."
Caffee's lab, the Purdue Rare Isotope Measurement (PRIME) Laboratory, was able to determine that the bedrock of Greenland had been exposed to the atmosphere and cosmic rays from outer space by looking at rock samples that had been recovered by a U.S. scientific team from beneath nearly two miles of ice in 1993.
It was only in the past year that Purdue's PRIME lab developed techniques using a gas-filled magnet attached to a particle accelerator that was sensitive enough to detect the beryllium-10 and aluminum-26 atomic isotopes. These isotopes had been created by the cosmic rays striking the rock and had been hiding beneath the ice for more than a million years.
The results of all of this scientific sleuthing were published in this week's Nature.
The lead author on the Nature paper, Joerg Schaefer, a paleoclimatologist at Columbia University's Lamont-Doherty Earth Observatory, says it's possible that the Greenland ice sheet could go away again.
"Unfortunately, this makes the Greenland ice sheet look highly unstable," he said in a Columbia University news release. "With human-induced warming now well underway, loss of the Greenland ice has roughly doubled since the 1990s; during the last four years by some estimates, it shed more than a trillion tons [of ice]."
Additional authors on the paper include Richard Alley, Pennsylvania State University; Nicolas Young and Roseanne Schwartz, Columbia University; Greg Balco, the University of California-Berkeley; Jason Briner, University of Buffalo; and Anthony Gow, U.S. Army Cold Regions Research and Engineering Laboratory.