Greenland's Ice Cap melted twice as fast in 2012, as this video shows. In addition, because of the weight of the ice, Greenland's center is actually below sea level. The solid land of Greenland looks like an attol. A few places where glaciers have been flowing out, and the ice tongues retreating more rapidly year by year, connect the surrounding ocean to this below sea level interior.
It seems that these areas of egress could turn into ocean entry points, where warmer ocean water would begin to erode the central ice cap. All of this suggests that the models of Greenland's contribution to the rising ocean level have been underestimates.