Glacial ice on Greenland's coasts is calving into the sea, but that's just the tip of the iceberg, so to speak. By 2012, ice loss on Greenland's massive ice sheet had accelerated to a rate nearly four times what it was in 2003, and it may have now reached a "tipping point," according to research led by Michael Bevis of Ohio State University. Focusing on Greenland's southwest—an area "not generally considered" a key source of ice loss, per the New York Times—the study finds more than 400 billion tons of ice was lost in 2012 (a stretch defined by what the Times calls "greatly accelerated melting"), followed by a yearlong pause in melting. Scientists attribute this to a recurring weather cycle known as the North Atlantic Oscillation, which brings warmer air, more sunlight, and less snowfall to west Greenland in the negative phase. Ice loss paused with a shift into the positive phase.
Until 2000, the North Atlantic Oscillation wasn't thought to have much of an impact on Greenland's rate of ice loss. But "if a relatively minor cycle can cause massive melting ... it means you've reached a point of amazing sensitivity" to warmer temperatures, says Bevis. As average temperatures continue to climb, "large amounts of ice mass are going to leave as meltwater, as rivers that flow into the sea," he says in a release on the study in the Proceedings of the National Academy of Sciences. "It's too late for there to be no effect. This is going to cause additional sea level rise. We are watching the ice sheet hit a tipping point," Bevis tells CNN. "The only question is: How severe does it get?" he adds, per the Guardian. The only course of action? "Adapt and mitigate further global warming," Bevis tells CNN. (Antarctica has the same problem.)