The West Antarctic Ice Sheet has been observed to be moving more rapidly toward the sea now than 15 years ago. Higher ocean temperatures erode the underside of the glacier that is cantilevered out over the water from its grounding point on the Antarctic land mass. Whether the rise in ocean temperatures in this region is caused by global warming or natural cycles has not been determined. Kerr (2004) reviews recent research that underscores the need to better understand these glacial processes, since a destabilized West Antarctic Ice Sheet that broke off would raise sea level by 5-7 m. Currently this event is not considered likely but more measurements of ice movement are needed to better assess the possible time scale of such an event.
Reference
Kerr, R.A., 2004: A bit of icy Antarctica is Sliding Toward the Sea. Science, 305, 1897.