The Greenland ice sheet is not as frozen as it appears. There is liquid water within and beneath the ice. Scientists are still a long way from knowing how much liquid water there is among the ice, but a big step forward is published today and described at BBC News:
The significance of the aquifer described in the article depends on how effectively it drains into the ocean — if it does so at all. Perhaps the water drains out slowly somehow, through channels that haven’t been mapped, but another possibility is that it is not currently draining at all and may lead to an “abrupt” sea level rise of some millimeters upon some future melt event. Regardless of questions of flow, the presence of any liquid water in the Greenland ice sheet reduces the mass of ice that remains to melt.