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Global Warming May Favor Certain Weather Patterns


Scientists looking for signs of global warming should spend more time scrutinizing Earth's weather circulation patterns, a new study suggests.

Researchers who have studied northern winters since mid-century found that a weather pattern favoring typically milder winters was two to three times more likely to arise in the mid-1990s than around 1950.

The findings suggest that global warming blamed on human industry is manifesting itself not just by causing a global rise in temperatures but by also favoring certain established weather patterns.

"This is telling us that we shouldn't expect to see unusual patterns we've never seen before. That's something the atmosphere is very reluctant to do. You'll just get certain patterns becoming more likely and other patterns being less likely," said Tim N. Palmer of the European Centre for Medium-Range Weather Forecasts near Reading, England.

Palmer and two colleagues from the Interuniversity Computing Centre in Bologna, Italy, studied atmospheric circulation data for winters in the Northern Hemisphere from 1949 to 1994.

They charted air circulation patterns into four "clusters" of high and low pressure systems, finding that the grouping favoring mild winters arose more frequently while patterns causing severe winters occurred less often.

The findings were published in the April 29 issue of the journal Nature.

They mean climatologists should devote more time to scrutinizing shifts in the natural variability of weather circulation patterns to understand global warming, said Tom Karl, director of the National Climate Data Center in Asheville, N.C.

"Some of these patterns now seem to have the odds loaded in their favor of occurring with more frequency and intensity," Karl said. "This points out the importance of looking more closely at the patterns of climate circulation that affect the day-to-day weather we all experience."

The Intergovernmental Panel on Climate Change, a U.N.-sponsored network of more than 2,000 scientists monitoring the global climate, has concluded that there is a "discernible human influence" on the climate through the burning of fossil fuels, which produce greenhouse gases.

Kevin Trenberth, who heads the climate analysis section at the National Center for Atmospheric Research in Boulder, Colo., said Palmer's findings don't prove or disprove whether human activity is affecting the climate. But he said evidence that the frequency of some weather patterns is changing shouldn't be overlooked.

"It's certainly a step along the way in terms of making people think of things in a different way and ask different types of questions," he said. "Global warming won't be a simple warming of the Earth. It's bound to be more complex than that."

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