An ice sheet is a mass of glacial land ice extending more than 50,000 sq km.
The two ice sheets on Earth today cover most of Greenland and Antarctica. During the last ice age, ice sheets also covered much of North America and Scandinavia.
Together, the Antarctic and Greenland ice sheets contain more than 99 per cent of the freshwater ice on Earth. The Antarctic ice sheet extends almost 14 million sq km — roughly the area of the United States and Mexico combined. The Greenland ice sheet extends about 1.7 million sq km — three times the size of Texas.
Ice sheets form in areas where snow that falls in winter does not melt entirely over the summer. Over thousands of years, the layers of snow pile up into thick masses of ice, growing thicker and denser as the weight of new snow and ice layers compresses the older layers.
Ice sheets are constantly in motion, slowly flowing downhill under their own weight. Near the coast, most of the ice moves through relatively fast-moving outlets called ice streams, glaciers, and ice shelves. As long as an ice sheet accumulates the same mass of snow as it loses to the sea, it remains stable.
Ice sheets contain enormous quantities of frozen water. If the Greenland Ice Sheet melted, scientists estimate that sea level would rise about 20 feet. If the Antarctic Ice Sheet melted, sea level would rise by about 200 feet.
The Greenland and Antarctic ice sheets also influence weather and climate. Large high-altitude plateaus on the ice caps alter storm tracks and create cold downslope winds close to the ice surface.