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The total volume of the ice sheet covering Antarctica is estimated at 29 million cu km (7 million cu mi), or about 90 percent of the world’s ice. (Greenland contains about 8 percent of the global ice mass, with most of the rest found as glaciers in mountainous regions around the world.) The Antarctic ice sheet holds about 70 percent of all the freshwater on Earth. If the entire ice sheet melted, the oceans of the world would rise by 60 m (200 ft). Some 11 percent of the ice sheet consists of ice shelves—massive floating slabs of permanent ice fringing the continent—that are anchored to the rock and extend into the surrounding ocean. The largest, Ross Ice Shelf, is about the size of France. The Antarctic ice sheet has an average thickness of 2,160 m (7,090 ft); its greatest recorded depth is more than 4,700 m (15,400 ft).
Antarctica’s ice sheet formed over millions of years. As new snow falls, it compresses the layers of older snow beneath it into ice. The physical characteristics of the ice sheet are constantly changing as new ice forms and slides outward toward the coasts. Large masses of moving ice known as glaciers move down the continent’s five major drainage systems in two ways. If there is a layer of water between the glacier and the bedrock beneath it, the whole glacier can slide under the force of gravity. Alternatively, the weight of the accumulated snow and ice can cause the ice crystals of the glacier to form into layers, which glide over one another. Glaciers flow either into ice shelves or directly out to the edges of the continent, where portions break off and form floating masses called icebergs. Carried by circumpolar currents and prevailing winds, these icebergs drift westward around the continent and then northward to the Antarctic Convergence before gradually breaking up and melting upon contact with warmer waters.
Icebergs that break away from Antarctica and melt raise sea level by adding water that was locked up as land ice. Increased snow fall that adds to the Antarctic ice sheet could offset the melting of icebergs and glaciers, however. The flow of glaciers to the sea has been accelerating in parts of West Antarctica. Large portions of ice shelves have also broken off. Since ice shelves float on water, they would not raise sea level when they melt. However, rising sea levels in the future would lift ice shelves and lead to more break ups and melting of ice that was once on land.
In addition to icebergs and ice shelves, ocean waters close to the continent contain floating sea ice. In contrast to icebergs and ice shelves, which are frozen fresh water, sea ice is frozen sea water. As sea water freezes, it expels salt, which collects as concentrated brine between the ice crystals. The briny water eventually drains from the underside of the sea ice, creating a downward flow of heavier salty water. As a result, sea ice that survives for a number of months can be virtually fresh. Some sea ice is attached to the land (fast ice) and some drifts with wind and currents (pack ice). Sea ice melts and freezes seasonally, covering up to 21 million sq km (8 million sq mi) in late winter and only 5 million sq km (2 million sq mi) in summer. Unlike melting glaciers and ice sheets, melting sea ice does not directly raise sea level. However, loss of sea ice exposes darker ocean water to sunlight and heats the ocean.
The maximum area of sea ice surrounding Antarctica each winter varies from year to year. A marked decline during the 1970s appears to have reversed in more recent decades, except in the Antarctic Peninsula area. This area has lost almost 40 percent of its sea ice since the start of the 1980s. Sea ice is important to marine life. Krill feed on algae that live under the sea ice and are released when the ice melts in spring and summer. In turn, many marine animals feed on krill. Emperor penguins breed on flat expanses of sea ice close to land during the winter. Seals give birth on sea ice in early spring.
Ice on land and on sea reflects sunlight back into space, which has a cooling effect on Earth’s climate. Shrinking ice shelves or reduced sea ice in summer allows the exposed dark ocean to absorb more heat, raising temperatures.