Study Confirms Antarctica Warming

SEATTLE, Washington, September 6, 2006 (ENS) - New research suggests that Antarctica has been getting gradually warmer for the past 150 years, despite indications that the continent cooled considerably during the 1990s.

The warming trend is not identifiable in the short meteorological records and was masked at the end of the 20th century by large temperature variations, according to the study.

The study, published in the current edition of "Geophysical Research Letters," sheds some light on recent cooling in Antarctica and on the complexities of the icy continent's climate.

For the study, scientists collected ice cores from five areas and studied oxygen and hydrogen isotopes in the cores to develop the first reconstruction of Antarctic temperature records for the last 150 years.

The reconstructed temperature record that shows average Antarctic temperatures have risen about two-tenths of a degree Celsius, or about one-third of a degree Fahrenheit, in 150 years.

That might not sound like much, but the overall increase includes a recorded temperature decline of nearly 1 degree Celsius in the 1990s, said David Schneider, a University of Washington postdoctoral researcher in Earth and space sciences.

"Even if you account for the cooling in the '90s, we still see that two-tenths of a degree increase from the middle of the 1800s to the end of the 20th century," said Schneider, the lead author of the paper detailing the work.

Schneider and his colleagues found evidence in the ice cores that the cooling of Antarctica during the 1990s was due to a natural phenomenon called the Antarctic Oscillation - or Southern Annular Mode.

The Antarctic Oscillation is so named because atmospheric pressure in far southern latitudes randomly oscillates between positive and negative phases. During the positive phase, a vortex of wind is tightly focused on the polar region and prevents warmer air from mixing with the frigid polar air, which keeps Antarctica colder, Schneider explained.

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Anarctica, the world's southernmost continent, is 98 percent ice. (Photo courtesy NOAA)
Although it typically alternates between phases about every month, in the 1990s the positive phase occurred much more often. Absent that influence, Schneider said, it is likely the Antarctic would have shown the same kind of warming as the rest of the Southern Hemisphere.

Prior to 1975, Antarctica appears to have warmed at about the same rate as the rest of the hemisphere, about 0.25 degree Celsius per century, the researchers noted, but since 1975, while the Antarctic showed overall cooling, the Southern Hemisphere has warmed at a rate of about 1.4 degrees per century.

"The second half of the 20th century is marked by really large variability," Schneider said. "The periods of cooling correspond with a very strong positive Antarctic Oscillation. The caution is that we don't fully understand the feedbacks between overall climate warming and the Antarctic Oscillation. But having the 200-year record is what convinces us that there is a relationship between Southern Hemisphere temperature changes and Antarctic temperature changes."

Schneider added that other research has suggested that ozone depletion in the Southern Hemisphere is keeping the Antarctic Oscillation in its positive phase for longer periods.

The cooling trend in the Antarctica is at odds with other signs that the continent is feeling the effect of a warming climate.

Last year scientists reported that 90 percent of the glaciers of the Antarctic Peninsula are in retreat and melting into the sea.

In March, a study confirmed that the Antarctic ice sheet has lost significant mass in the past three years. The ice sheet, which contains 90 percent of the planet's ice, is losing some 36 cubic miles of ice annually.