Environment News Network
Thursday, December 23, 1999
By Gila Reckess
Wind currents around the Arctic Basin fluctuate in strength and intensity. This pattern, known as the Arctic Oscillation phenomenon, extends from the Earth's immediate atmosphere straight up into the stratosphere.
"It is sort of a rocking back and forth of the atmosphere like a canoe being hit by waves," explains Mark Baldwin of Northwest Research Associates. "CO2 and greenhouse gas increases tend to tilt the canoe to one side, but it still rocks back and forth."
Climate trends over the past 30 years worry scientists. Recently, wind currents have been increasingly strong, "tightening the spinning ring of air above the pole," according to David Thompson of the Joint Institute for the Study of the Atmosphere and Oceans. "This tightening is linked to changes in weather systems at lower altitudes," he explains.
For example, northern Europe and Asia have experienced consistently milder winters while Greenland and Newfoundland have been frozen due to an influx of colder continental gusts. Winter storms have also shifted northward in many regions.
"This change could conceivably be natural due to a long-term climate oscillation involving the ocean. If that's the case, conditions should eventually return to normal," says John Wallace, professor of atmospheric sciences at the University of Washington.
Since 1995, winters have been less mild than in the previous few decades. Wallace points out that the recent skew could simply be part of a larger pattern. Without comprehensive long-term data, this possibility cannot be ruled out.
Large volcanic eruptions are known to cause temporary shifts in the Arctic Oscillation and may therefore contribute to the changes. Volcanoes emit sulfur dioxide into the atmosphere. This gas reaches the stratosphere during larger eruptions, where it is converted into aerosols. Aerosols in turn absorb radiation, producing a warming effect.
But, according to Wallace, volcanic activity cannot be solely responsible for the long-term weather shifts found in the Northern Hemisphere.
"The shifting state of the Arctic Oscillation could be a sign of human-induced climate change," says Baldwin. And Wallace agrees. "We have a record going back to the late part of the 19th century - there's no big trend until 1970 and then you see a big swing from 1970-1995, extreme to the point that it's noticeably impacted the climate in places such as Europe."
Greenhouse gases warm the troposphere and cool the stratosphere. However, there is a disproportionately large amount of cooling in the stratosphere over higher latitudes. Cooling, therefore, greatly affects the winds that circle the pole and may shift the Arctic Oscillation. This has been shown in one model of climate change published in the May issue of Nature magazine.
Others have successfully simulated the shift by modeling the effects of ozone depletion on atmospheric winds. While ozone depletion has, until now, been focused around the Southern Hemisphere, current weather trends in the north could lead to severe depletion around the Arctic.
Regardless of the cause, long-term changes in Arctic Oscillation pose a potential threat to the Earth's environment. "If the trend continues, this might be the first case where we have compelling evidence that humans are changing the climate on Earth's surface — convincing to a broader range of people than the subtle changes in global mean temperatures," says Wallace.