Antarctica and climate change; Recent melting in the planet's icebox could spell trouble.

AuthorMonaghan, Andrew

Visiting Antarctica is an incredible, almost unearthly, experience. Icebergs bigger than cities. Glaciers with cracks large enough to swallow a football stadium. A floating ice shelf the size of France. An ice cap thicker in some regions than 10 Empire State Buildings stacked on top of each other. Along with the enormity come the extremes: of the seven continents, Antarctica is the coldest, highest, driest, windiest--and of course iciest--place on Earth. Add to that "remotest," and it becomes clear why teasing the climate record out of this vast continent has been a challenge.

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Geography and topography are paramount to the existence of the Antarctic ice sheets. Antarctica is positioned over the South Pole, which means it receives less of the Sun's energy over the course of a year than any other continent because of the way Earth is tilted with respect to the Sun. Additionally, about 80 percent of the sunlight that reaches Antarctica is reflected by the icy surface. It is buffered from the rest of the globe by the vast Southern Ocean, which surrounds it with an apron of water and sea ice and which enables a belt of perpetual westerly winds to encircle the continent like a gigantic curtain. The steep topography along much of the Antarctic coastal margins acts as a barrier to intense cyclones that would otherwise transport warm, moist air inland, and thus an immense polar desert--vast and incredibly dry for a place that consists of frozen water--exists in the interior of the continent.

Antarctica's shape is relatively symmetrical compared to other continents--a rough circle with the highest topography located near the middle, sloping down toward the ocean on all sides. The cold, thick ice acts as a heat sink, and the air at the uppermost elevations undergoes the most cooling and becomes denser than the air at lower elevations. In turn, the dense air generates persistent, gravity-driven winds that flow down the sloping terrain to the edges of the continent. Thus, Antarctica itself plays a major role in determining--and perpetuating--its own climate (see map). Yet that climate appears to be changing, and its fate is critical to the rest of the Earth as well.

Three Sheets to the Winds

Antarctica can roughly be divided into three regions: East Antarctica, West Antarctica, and the Antarctic Peninsula (see map). East Antarctica accounts for about 75 percent of the surface area and is slightly larger than the United States. It is covered by the East Antarctic Ice Sheet (EAIS, pronounced to rhyme with "fleece"). EAIS is astounding, reaching a height of more than 4,000 meters at its apex and holding the water equivalent of 60 meters of global sea level. (The deepest ice core on Earth was recently drilled on EAIS at Dome C, yielding an 800,000-year record of climate history.) Like all ice sheets, the total mass of frozen water that makes up EAIS is the result of a balance between the growth of ice due to snowfall, and the loss of ice due to melting at the edges and the calving of icebergs. Changes in the "mass balance" of ice are of great concern to scientists because of the important implications for sea level. If an ice sheet is growing, it tends to lower sea level due to the additional storage of frozen water; if it is shrinking, it tends to raise sea level. Collectively, recent estimates of the mass fluctuations of EAIS suggest that it is approximately in balance.

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The Greenland-sized West Antarctic Ice Sheet (WAIS; rhymes with "race") makes up about 20 percent of Antarctica's surface area and stores the frozen water equivalent of six meters of global sea level. Despite being only one-tenth the size of EAIS, WAIS is the ice sheet that has scientists chewing their fingernails. Why? First, the average elevation of WAIS is much lower than EAIS, and thus WAIS is more susceptible to...

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