Some scientists believe that global warming may be about to push the ice sheet over a threshold where the entire ice sheet will melt in less than a few hundred years. If the entire 2,850,000 cubic kilometers (683,751 cu mi) of ice were to melt, it would lead to a global sea level rise of 7.2 m (23.6 ft). This would inundate most coastal cities in the World and remove several small island countries from the face of Earth, since island nations such as Tuvalu and Maldives have a maximum altitude below or just above this number.

The melt zone, where summer warmth turns snow and ice into slush and melt ponds of meltwater, has been expanding at an accelerating rate in recent years. When the meltwater seeps down through cracks in the sheet, it accelerates the melting and, in some areas, allow the ice to slide more easily over the bedrock below, speeding its movement to the sea. Besides contributing to global sea level rise, the process adds freshwater to the ocean, which may disturb ocean circulation and thus regional climate.

Source: Wikipedia