The ice sheets have changed in size over the millennia, in response to changes in Earth’s climate, getting larger when the climate cools and smaller when the climate warms. The changes in the ice sheets cause changes in the amount of water in the oceans. Antarctica, on the other hand, is a frozen continent and has not melted all that much since most of the ice that is lost flows back into the ocean. As the velocity of an ice sheet changes, so does its size. When the ice sheet grows, sea level decreases, and when the ice sheet shrinks, sea level rises.
Listen to Dr. Robert A. Bindschadler talk more about why we should care.