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Recent indications notwithstanding that showed that Antarctica had cooled considerably during the 1990s, new evidence has suggested that the icy continent has been getting warmer for the last 150 years. David Schneider, from the University of Washington’s Department of Earth and Space Sciences said the trend was not identified in the short meteorological records and had been effectively masked at the end of the 20th century by large-scale temperature variations. "Even if you account for the cooling in the 1990s, 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, in his paper in the Geophysical Research Letters. For his research, Schneider studied numerous ice cores collected from five areas to reconstruct a temperature record. Findings revealed that the average Antarctic temperatures rose about two-tenths of a degree Celsius, or about one-third of a degree Fahrenheit, in 150 years, including the temperature decline of nearly one degree in the 1990s. He said the main reason Antarctica appeared to have cooled during the 1990s was due to a natural phenomenon called the Antarctic Oscillation, or Southern Annular Mode, which was largely in its positive phase during that time. 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. Typically the Antarctic Oscillation alternates between phases about every month. But in the 1990s the positive phase occurred much more often, Schneider said, adding that without the influence of the Antarctic Oscillation, it was likely the Antarctic would show the same kind of warming as the rest of the Southern Hemisphere. "Before 1975, Antarctica appears to have warmed at about the same rate as the rest of the hemisphere, about 0.25 degree C per century. 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. The periods of cooling correspond with a very strong positive Antarctic Oscillation," he said. He further said: "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". "We have pretty good confidence that we’re right, though some of the details might have to be refined," he added. —ANI
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