Global sea levels are currently rising, in part from thermal expansion of the seawater in a warmer climate, and partly as a result of the melting of the Greenland and Antarctic ice sheets. The Greenland and Antarctic ice sheets have some significant differences that cause them to respond differently to changes in air and water temperatures. The Antarctic ice sheet is about 10 times as large as the Greenland ice sheet, and since it sits on the South Pole, Antarctica dominates its own climate. The surrounding ocean is cold even during summer, and much of Ant arctica is a cold desert with low precipitation rates and high evaporation potential. Most meltwater in Antarctica seeps into underlying snow and simply refreezes, with little running off into the sea. Antarctica hosts several large ice shelves fed by glaciers moving at rates of up to 1,000 feet (305 m) per year. Most ice loss in Antarctica is accomplished through calving and basal melting of the ice shelves, at rates of 10-15 inches (25-38 cm) per year.
In contrast, Greenland's climate is influenced by warm North Atlantic currents and by its proximity to other landmasses. Climate data measured from ice cores taken from the top of the Greenland ice cap show that temperatures have varied significantly in cycles of years to decades. Greenland also experiences significant summer melting, abundant snowfall, has few ice shelves, and its glaciers move quickly at rates of up to miles per year. These fast-moving glaciers are able to drain a large amount of ice from Greenland in relatively short amounts of time.
The Greenland ice sheet is thinning rapidly along its edges, losing an average of 15-20 feet (4.5-6 m) in the past decade. In addition, tidewater glaciers and the small ice shelves in Greenland are melting an order of magnitude faster than the Antarctic ice sheets, with rates of melting between 25-65 feet (720 m) per year, a rate that is apparently increasing. About half of the ice lost from Greenland is through surface melting that runs off into the sea. The other half of ice loss is through calving of outlet glaciers and melting along the tidewater glaciers and ice shelf bases.
These differences between the Greenland and Antarctic ice sheets lead them to play different roles in global sea level rise. Greenland contributes more to the rapid short-term fluctuations in sea level, responding to short-term changes in climate. In contrast, most of the world's water available for raising sea level is locked up in the slowly changing Antarctic ice sheet. Antarctica contributes more to the gradual, long-term sea-level rise.
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