The Greenland Ice Sheet is a vast body of ice covering roughly 80% of the surface of Greenland. It is the second largest ice body in the world, after the Antarctic Ice Sheet. The ice sheet is almost 2,400 kilometres long in a north-south direction, and its greatest width is 1,100 kilometres at a latitude of 77° N, near its northern margin. The ice sheet, consisting of layers of compressed snow from more than a hundred thousand years, contains in its ice today's most valuable record of past climates. In the past decades, scientists have drilled ice cores up to three kilometres deep. With the ice cores, scientist have obtained information on (proxies for) temperature, ocean volume, precipitation, chemistry and gas composition of the lower atmosphere, volcanic eruptions, solar variability, sea-surface productivity, desert extent and forest fires. This variety of climatic proxies is greater than in any other natural recorder of climate, such as tree rings or sediment layers. The Greenland Ice Sheet has experienced record melting in recent years and is likely to contribute substantially to sea level rise as well as to possible changes in ocean circulation in the future.