Antarctica, the fifth-largest continent, is centered on the South Pole and is almost entirely covered with a thick sheet of ice. The Antarctic ice sheet is 1.5 mi (2.5 km) thick on average and contains about 7 million mi3 (29 million km3) of ice. Antarctica's ice holds 90% of the world's freshwater, enough to raise sea level worldwide by 200 ft (61 m), if it
were all to melt. Although total melting is highly unlikely, some melting is occurring as global warming allows temperatures to rise above freezing across much of Antarctica during the summer months of the Southern Hemisphere. Scientists predicted as recently as 2001 that global warming might actually increase the amount of water stored on the Antarctic ice sheet by increasing snowfall, yet scientists announcedin 2006 that the Antarcticice sheet is, infact, melting faster than it is being replenished by snowfall. Overall, Antarctic climate remains poorly understood as of 2008.
Historical Background and Scientific Foundations
Until the nineteenth century, Antarctica was not visited by human beings. A series of exploratory voyages starting in 1820 confirmed the presence of land near the South Pole, an idea that had long been a subject of speculation. In the early twentieth century, overland expeditions reached the South Pole, and in the 1930s airplanes began to be used to explore the continent. Year-round scientific stations have been maintained in Antarctica since the 1950s, and knowledge of the continent has been greatly increased by satellite observations since the 1970s.
Antarctica's inaccessibility, size, and thick ice covering make it difficult to study, and its role in global climate is only partly understood. In recent years there has been uncertainty about whether Antarctica was gaining overall ice mass or losing it, and about whether global warming would cause Antarctica to gain ice mass through increased snowfall faster than it lost ice mass by melting, or the reverse. Antarctica, unlike the other continents, has shown little change during the recent period of rapid global warming.
In 2005, data from NASA's QuikScat satellite, which can distinguish between frozen and melted water by bouncing radar pulses off Earth's surface, showed that an area thesizeof Californiahad experienced surface snowmelt in January of that year. This was the largest Antarctic snow-melt seen in some three decades of satellite observation. Although the 2005 melt event was temporary and did not directly contribute to overall loss of ice mass, scientists analyzing the QuikScat data asserted that it proved that parts of Antarctica are showing the first impacts of global warming.
In 2006, new evidence convinced many scientists that the Antarctic ice sheet is melting faster than it is growing. In 2005, scientists analyzed three years of data from satellites measuring the force of gravity over the Greenland and Antarctic ice sheets. Loss of ice mass due to melting, or gain due to snowfall, can be measured as decreases or increases in gravitational pull. Gravitational measurements showed that both the Greenland ice sheet and the Antarctic ice sheet are melting faster than had been thought. The Antarctic ice sheet is losing 36 mi3(152 km3) of ice per year, despite increasing ice thickness over East Antarctica. If this trend is sustained over the long term, it will overturn the prediction, made by the Intergovernmental Panel on Climate Change (IPCC) in 2001, that Antarctica should gain mass during the twenty-first century.
Impacts and Issues
In 2006, taking the latest gravimetric and altitude-measurement data into account, the IPCC said that it was highly likely (more than 90% probable) that melting of the Greenland and Antarctic ice sheets had contributed to sea level rise from 1993 to 2003. Scientists also point out that uncertainties about the present speed and future course of melting remain, although these uncertainties are decreasing.
The main result to be feared from ice-sheet melting is sea-level rise, which could endanger coastal settlements around the world. Sea levels have been stable for most of the last 3,000 years, but began to rise at 0.04–0.08 in (1–2 mm) per year in the late 1800s. Since 1993, the average rate of rise has been faster, about 0.1 in (3 mm) per year, but as of 2007 it was still uncertain whether this acceleration would prove to be long-term or short-term.
WORDS TO KNOW
GRAVIMETRIC: Having to do with the measurement of gravity. Gravimetric data from satellites can reveal changes in large masses of ice on Earth's surface: in 2006, such data confirmed that Greenland's ice cap was melting faster than expected.
ICE SHEET: Glacial ice that covers at least 19,500 square mi (50,000 square km) of land and that flows in all directions, covering and obscuring the landscape below it.
SEA LEVEL: The datum against which land elevation and sea depth are measured. Mean sea level is the average of high and low tides.
IN CONTEXT: MELTING AT EARTH'S POLES
Polar melting is a problem in both northern and southern polar regions. In August 2007, a month before annual Arctic sea-ice melting normally peaks, a record had already been set for shrinkage of the north polar ice cap. No summer since 1979, when satellites first allowed accurate tracking of the annual melting, recorded such a large loss of ice.
William Chapman, an expert in the region at the University of Illinois, Urbana-Champaign, said that the melting rate in June and July of 2007 had been incredible. Arctic sea-ice melting is predicted to have many impacts, including a much warmer Arctic region, increased Arctic coastal erosion, declines in polar bear populations, damage to traditional hunting practices of indigenous Arctic peoples, disruption of large-scale ocean circulations, and more.
Over the last 20,000 years, the West Antarctic ice sheet has lost two-thirds of its mass and raised sea levels by 33 ft (10 m). Complete melting of the sheet, where most of Antarctica's ice loss is presently happening, would take many years, but could raise sea level by another 20 ft (6 m).
Cazenave, Anny. “How Fast Are the Ice Sheets Melting?” Science 314 (2006): 1250–1252.
Eilperin, Juliet. “Antarctic Ice Sheet Is Melting Rapidly.” Washington Post (March 3, 2006): A01.
Shepherd, Andrew, and Duncan Wingham. “Recent Sea-Level Contributions of the Antarctic and Greenland Ice Sheets.” Science 315 (2007): 1529–1532.
Velicogna, Isabella, and John Wahr. “Measurements of Time-Variable Gravity Show Mass Loss in Antarctica.” Science 311 (2006): 1754–1756.
“Mission News: NASA Finds Vast Regions of West Antarctica Melted in Recent Past.” National Aeronautics and Space Administration (NASA), May 15, 2007. < http://www.nasa.gov/vision/earth/lookingatearth/arctic-20070515.html> (accessed August 10, 2007).