That's simply because in climate history, warm climate means small ice sheets, cold climate comes with big ice sheets, and sea level has changed accordingly. (realclimate.org)
The ice field represents the remnant of an even bigger ice sheet from the last glacial period, and now feeds dozens of glaciers across the continent. (theexpeditioner.com)
It is to cause even more global warming, so much so that we lose not only West Antarctica, but also the even bigger ice sheet of Greenland (which is generally considered the second - most - unstable major ice sheet). (washingtonpost.com)