Antarctica: Observed Climate Changes
Antarctica: Observed Climate Changes
Antarctica holds 90% of the world's freshwater in the form of a vast ice sheet thousands of feet thick. This ice covering also extends out to sea in the form of floating ice tongues or shelves connected to the land. The Antarctic climate is cold, with winter temperatures as low as -130°F (-90°C). The continent's climate has changed repeatedly in the distant past due to many factors, including cyclic changes in Earth's orbit. It is also showing effects from present-day human-caused climate change.
In contrast to the north polar region, which has suffered more rapid warming than the rest of the world, much of Antarctica has shown relatively little warming over thelasthalf-century. This maybechanging, however, as ice shelves many thousands of years old collapse, glaciers accelerate their flow to the sea, surface melting is observed on hitherto unknown scales, and satellite measurements show that the Antarctic ice sheet is losing mass faster than it gains mass from snowfall. Antarctica's climate remains difficult to characterize in detail because of the region's size and remoteness, and scientists caution that there are still many uncertainties about what changes are happening and are likely to happen.
Historical Background and Scientific Foundations
Observation of Antarctica's weather, ice bodies, and ecosystems began in earnest during the International Geophysical Year (IGY), a period of enhanced government-funded scientific observation of Earth that lasted from July 1957 to December 1958. In the years leading up to the IGY, scientific bases were established in Antarctica by the United States, the Soviet Union, and other governments. Continuous scientific monitoring of Antarctic climate from the ground, sea, and air has been supplemented since the 1970s by satellite observations.
Information about Antarctica's climate can be roughly divided into three categories: 1) the deep geological past; 2) the last 1 million years; and 3) the last half-century. Information about Antarctica's deep past is obtained from fossils and other geological evidence. Antarctica was once located near the equator. Fossils show that from about 250 to 65 million years ago, the continent was lushly forested, with a tropical climate, dinosaurs and amphibians, and tree ferns. Continental drift moved Antarctica toward the South Pole, after which it froze over and accumulated an ice cap.
Detailed records of Antarctic climate have been preserved for the last million years by its unique location. Despite the abundance of water on Antarctica's surface—the ice sheet is 1.5 mi (2.5 km) thick on average and contains about 7 million mi3(29 million km3) of ice—it actually receives less annual precipitation per unit of area than any other continent and is considered, technically speaking, a desert. This apparent contradiction is explained by the fact that although little water falls on Antarctica, it tends to stay there. Each year's snowfall is relatively small (about 4 in [10 cm] is typical for a winter at the South Pole), but it does not melt. The snow packs down and freezes into a thin layer. Evaporation is slight because of the intense cold. Each new layer builds on the last. This accumulated ice is removed from the continent only by geologically rare melting episodes or by the slow creep of glaciers to the sea. Ice cores—2-mile-long cylinders of layered ice—have been drilled from Antarctic ice. These cores sample 800,000 continuous years of snowfall. Each layer contains air bubbles that were trapped when the layer was formed, so the core contains a continuous series of time-ordered samples of Earth's atmosphere.
Much information about both Antarctic climate and global climate is gathered from these cores. Atmospheric concentrations of dust and of the major greenhouse gases (carbon dioxide, methane, and nitrous oxide) are measured directly, and the overall temperature of Earth in each year is deduced from the abundance of certain isotopes of hydrogen and oxygen in the ice.
Ice-core data show that over long periods of time, global and Antarctic climates warm and cool in response to changes in Earth's orbit. These climate cycles, called Milankovitch cycles after one of their discoverers, Russian physicist Milutin Milankovitch (1879–1958), last for about 100,000 years each. During the cold part of each cycle, ice expands from the Poles to cover much of the world; during the warm part of each cycle, the ice retreats. As of 2008, Earth is in a warm (interglacial) cycle that began between 18,000 and 10,000 years ago. This has caused massive melting of Antarctic ice. Over the last 20,000 years, the ice sheet covering the western part of Antarctica has shrunk by two-thirds, raising sea levels worldwide by 33 ft (10 m).
Comparing ice cores from the Greenland ice cap and Antarctica shows that on shorter time-scales—thousands of years rather than tens or hundreds of thousands of years—Antarctica's climate is interlocked with that of the Northern Hemisphere through a complex mechanism known as the thermal bipolar seesaw. That is, as the north polar region warms, Antarctica tends to cool, and vice versa. This effect is caused by changes to the Atlantic Meridional Overturning Circulation (also sometimes termed the Great Conveyor Belt or thermohaline circulation). Scientists are still trying to understand the implications of the bipolar seesaw for today's climate changes.
For the last 50 years, ice core data are not needed because direct observations have been made. As might have been guessed from the bipolar seesaw, although the north polar region recently has been warming rapidly, Antarctica as a whole has not. The climate picture over Antarctica is a patchwork: the high-altitude plateau at the center of the continent is dominated by a temperature inversion, in which cold air is found closer to the surface with warmer air occurring at higher altitudes. (Usually it is the other way around with valleys tending to be warmer than mountain-tops.) The edges of the continent, especially the Antarctic Peninsula, are affected more by weather systems circulating around the continent in the polar vortex. As a result, the center of the continent has experienced slight cooling over the last 40 to 50 years, while the Antarctic Peninsula region has experienced significant warming.
Impacts and Issues
Warming of the Antarctic Peninsula is globally significant partly because it accelerates melting of the West Antarctic ice sheet, which raises sea levels. In 1995, a portion of the floating Larsen ice shelf called Larsen A, near the tip of the Antarctic Peninsula on the eastern side, broke up and melted in the sea. In 1998, scientists noticed early signs that the Larsen B ice shelf, much larger and older than Larsen A, might also be breaking up, undermined by warm ocean currents. These signs persisted, but scientists were shocked when, in 2002, the entire ice shelf—an area of ice 1,253 mi2(3,250 km2, the size of the state of Rhode Island), 720 ft (220 m) thick, weighing on the order of 500 billion tons—dis-integrated in a one-month period.
WORDS TO KNOW
ATLANTIC MERIDIONAL OVERTURNING CIRCULATION: The north-south circulation that dominates the movement of water in the Atlantic Ocean and is a key part of the global meridional thermohaline circulation and of the climate system. Moves warm waters toward the Arctic, where they cool, sink, and move southward along the ocean floor.
BIPOLAR SEESAW: The tendency of climate to warm at the North Pole while it cools at the South Pole and vice versa. Paleoclimatic data show that this seesaw effect has occurred many times in the geological past and is related to changes in ocean circulation, but scientists do not yet completely understand the mechanism of the seesaw.
GREAT CONVEYOR BELT: The overturning circulation of the world's seas, driven by temperature and salinity differences between the poles and tropics; also called the thermohaline circulation or meridional overturning circulation. Because the great conveyor belt transports thermal energy from the tropics toward the poles, it is a central component of Earth's climate machine.
GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.
MILANKOVITCH CYCLES: Regularly repeating variations in Earth's climate caused by shifts in its orbit around the sun and its orientation (i.e., tilt) with respect to the sun. Named after Serbian scientist Milutin Milankovitch (1879–1958), though he was not the first to propose such cycles.
Although floating ice shelves do not raise sea levels when they melt, in 2003 scientists showed that after the 1995 breakup of Larsen A, glaciers on land that had been dammed up by the shelf accelerated their flow to the sea. A similar acceleration was found in 2004 for half a dozen glaciers in West Antarctica.
In 2006, scientists from the British Antarctic Survey (BAS) announced that they had established direct ties between anthropogenic global warming and the breakup of the Larsen B ice shelf. In 2007, BAS scientists announced that they had measured 300 West Antarctic glaciers and found that all were accelerating their rate of flow to the sea. Average glacier speed had increased by 12% from 1993 to 2003 as a result of more rapid melting where the glaciers meet the sea. Glacial ice does raise sea levels when it passes from the land to the sea.
The Intergovernmental Panel on Climate Change (IPCC) noted in early 2007 that it could not forecast a specific upper limit for sea-level rise due to Antarctic melting over the next several centuries because the ability of anthropogenic global warming to melt Antarctic ice is not well-enough understood. Complete melting of Antarctica's ice cover would raise sea levels by about 200 ft (61 m), but this much melting is unlikely. Complete melting of the West Antarctic ice sheet alone would raise sea levels by about 20 ft (6 m).
Primary Source Connection
Researchers often study Antarctica to observe changes in the global climate. Both the ozone layer over Antarctica and the continent's vast ice sheets respond to changes in atmospheric conditions. In this article scientists describe how global climate change may be contributing to the surprising growth of one of Antarctica's ice sheets.
Irene Brown is a science correspondent for United Press International.
RESEARCHERS CHART SURPRISING GROWTH OF ICE
PASADENA, Calif., Jan. 17 (UPI)—In a surprising discovery, researchers using satellite radar pictures have found the Western Antarctic Ice Sheet may no longer be shrinking and actually is growing due to blocked up rivers of ice.
Roughly 25 percent of the accumulation of snow is not being discharged in the ice streams, increasing the overall mass of the ice sheet by about 1.5 inches a year, say researchers in an article to be published in the Jan. 18  issue of the journal Science.
“We know from geological evidence that the ice sheet has been retreating, but we think the mass balance has switched recently,” said Slawek Tulaczyk, an assistant professor of Earth Sciences at University of California-Santa Cruz. “We think we may have happened to capture a reversal of a geologic event.”
Ian Joughin, with NASA's Jet Propulsion Laboratory in Pasadena, Calif., and Tulaczyk used Canada's Radarsat spacecraft to make maps charting ice flows in a 447,000-square mile tract in Western Antarctica. The velocity of the ice flows indicates how much ice is being discharged. Researchers use the data in conjunction with snowfall measures to determine trends in ice accumulation.
Until the satellite radar images were used to make measurements, researchers did not have an accurate portrayal of ice flow dynamics, Tulaczyk said.
“People have been going around digging holes and measuring (snow) accumulation rates over past decade. We know that pretty well. But the output has always been much more difficult to determine.—It's kind of hard to run around Antarctica and measure when everything is moving,” said Tulaczyk.
“We now can make maps with extremely accurate ice displacement measurements,” he added. “The satellite data really revolutionized the way we could look at ice sheets.”
The scientists attribute the ice sheet growth to two events: the total blockage of a primary ice stream, which dissipated about 18 cubic miles of ice per year until about 150 years ago; and an ongoing slowdown in another ice stream, which blocks off the equivalent of about half the Missouri River. Tulaczyk said he expects that stream to come to a complete halt later this century.
The findings may be related to an overall global warming trend, said Reed Scherer with Northern Illinois University's Department of Geology and Environmental Sciences.
“You tend to think of warming in terms of melting, but in Antarctica warming can actually bring increased snowfall because the ocean is warmer and it puts more moisture in the air,” Scherer said.
Tulaczyk says his research is not inconsistent with global warming trends.
IN CONTEXT: ARID ANTARCTICA
Deserts are areas that receive little moisture. Although the most familiar image of a desert involves hot sand, the arctic north and Antarctica are also deserts, as they also receive very little moisture, usually in the form of snow. An arid climate is a desert-like dry climate. Arid climates receive less than 10 inches of rainfall in an entire year.
“What has been the major driver of changes in ice mass— the production of water and how fast the ice moves— occurs at the base, not at the top of the ice sheet. If the climate is changing now, maybe in several thousand years it will affect the dynamics of water,” he said.
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