evidence: Greenland’s summer melt season now lasts 70 days longer than it did in the early 1970s. Every summer, warmer air temperatures cause melt over about half of the surface of the ice sheet – although recently, 2012 saw an extreme event where 97 percent of the ice sheet experienced melt at its top layer. [...] Beginning with the deployment of research buoys in the waters around Greenland this summer, NASA is embarking on a three-year airborne and ship-based campaign to answer precisely these questions. OMG, Oceans Melting Greenland, seeks to understand the role of ocean currents and ocean temperatures in melting Greenland’s ice from below – and therefore better predict the speed at which the ice sheet will raise sea level. [...] For the West Antarctic Ice Sheet, which largely rests on a bed that lies below sea level, the main driver of ice loss is the ocean. The waters of the Southern Ocean are layered: on top and at the bottom, the temperatures are frigid, but the middle layer is warm. The westerlies, the winds that spin the ocean waters around Antarctica, have intensified during the last decade, pushing the cold top layer away from the land. This allows the warmer, deeper waters to rise and spill over the border of the continental shelf, flowing all the way back to the base of many ice shelves. As the ice shelves weaken from underneath, the glaciers behind them speed up.