evidence: Glaciers and ice sheets, large land-based formations of ice, are melting as global temperatures rise. That meltwater drains into the sea, increasing the ocean's water volume and global sea level. Melting ice has caused about two-thirds of the rise in sea level to date, one-third from land ice in Greenland and Antarctica and one third from melting ice on mountains. Ice sheets and glaciers in Greenland and Antarctica melt three ways: from above due to warming air, from the sides as they break off into the sea, and from below due to warming ocean water where the ice extends over the sea. Because of this, the rate of ice melt varies from place to place as conditions change. The Arctic is warming more quickly than the Antarctic, which explains why the ice there is thinning more quickly. However, recent research suggests that the melting of Antarctica’s ice shelves may be unstoppable—although the process may take centuries.