Seismologists at Harvard University and Columbia University have found an unexpected offshoot of global warming: “glacial earthquakes” in which Manhattan-sized glaciers lurch unexpectedly, yielding temblors up to magnitude 5.1 on the moment-magnitude scale, which is similar to the Richter scale. Glacial earthquakes in Greenland, the researchers found, are most common in July and August, and have more than doubled in number since 2002.

Scientists Göran Ekström and Victor C. Tsai at Harvard and Meredith Nettles at Columbia reported on Greenland’s glacial earthquakes in the journal Science. Ekström, Nettles, and colleagues first described glacial earthquakes in 2003, but that report did not recognize the seasonality or growing frequency of the phenomenon.

“People often think of glaciers as inert and slow-moving, but in fact they can also move rather quickly,” says Ekström, professor of geology and geophysics in Harvard’s Faculty of Arts and Sciences. “Some of Greenland’s glaciers, as large as Manhattan and as tall as the Empire State Building, can move 10 meters in less than a minute, a jolt that is sufficient to generate moderate seismic waves.”

As glaciers and the snow atop them gradually melt, water seeps downward. When enough water accumulates at a glacier’s base, it can serve as a lubricant, causing blocks of ice some 10 cubic kilometers in size to lurch down valleys known as “outlet glaciers,” which funnel all of Greenland’s glacial runoff toward the surrounding sea.

“Our results suggest that these major outlet glaciers can respond to changes in climate conditions much more quickly than we had thought,” says Nettles, a postdoctoral researcher at Columbia’s Lamont-Doherty Earth Observatory. “Greenland’s glaciers deliver large quantities of fresh water to the oceans, so the implications for climate change are serious. We believe that further warming of the climate is likely to accelerate the behavior we’ve documented.”