Greenland: A Horrifying Thought

I was just talking with a coworker. The topic of Greenland came up. We joked that we ought to go speculate for land there, but Warming Island came up and the fact that we need to make sure that the land is land. Then the nasty thought hit me: do we really know how much of Greenland is land and how much of it is really a huge iceberg? After all, 10 km of ice disappeared to make Warming Island when it was thought to be a pennisula of the mainland! If the ice sheet is mostly sitting at or below sea level, we're in deep shibbit. Wikipedia is hardly an authorative source, but it does state most of the interior of Greenland under the ice sheet is at sea level...

*glyph of concern*