As the Arctic warms, Greenland’s fringe of glaciers is thinning and melting—but the future of the Greenland ice sheet remains a giant question mark. Until recently, that was also true of the ice sheet’s past: Scientists have long debated whether it might have shrunk away to nothing during Earth’s warmest periods. Now, a new study suggests that Greenland was entirely ice free at some point in the last 1.25 million years.“We should be worried about the Greenland Ice Sheet,” says Joerg Schaefer, a geochemist from Columbia University’s Lamont-Doherty Earth Observatory in Palisades, New York, and lead author of the findings, presented yesterday at the American Geophysical Union’s annual meeting here.