The Greenland and Antarctica ice sheets are melting, but the amounts that will melt and the time it will take are still unknown, according to Richard Alley, Evan Pugh professor of geosciences, Penn State. In the past, the Greenland ice sheet has grown when its surroundings cooled, shrunk when its surroundings warmed and even disappeared completely when the temperatures became warm enough. If the ice sheet on Greenland melts, sea level will rise about 23 feet, which will inundate portions of nearly all continental shores. However, Antarctica, containing much more water, could add up to another 190 feet to sea level.