The difference between Greenland and Antarctica is that Greenland is not floating ice. Below the ice is bedrock. As the ice melts, it creates a destabilizing film between the ice and the rock. This destabilization, similar to a hydroplaning car, has resulted in a dramatic increase in "glacial earthquakes" with readings of up to 5.1 on the Richter scale in Greenland.
These earthquakes have resulted in a tremendous amount of ice falling off Greenland into the sea. "There's an amount of fresh water melting off of Greenland now that's bigger than the whole annual flow of the Colorado River times 10," Gore says.