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Study Predicts Longer Heat Waves in 21st Century - 2004-08-23

Summer days may seem hot now, but just wait until the end of the century. Using a state-of-the-art computer model, atmospheric scientists predict summer heat waves will get longer and hotter over the coming decades.

Summer temperatures will sizzle for longer periods of time in the later part of the 21st century, according to scientists at the National Center for Atmospheric Research in Boulder, Colorado. They base their prediction for dramatic rises in global temperatures on studies of extended heat waves in the United States in 1995 and in Europe in 2003. They focused in on one city in each case, Chicago and Paris.

Gerald Meehl, a senior scientist at the National Center for Atmospheric Research says "for a heat wave, you have to have kind of a dome of high pressure sitting over a region for a number of days; you have clear skies and the sun's beating down and the heat just builds. So, we just used the Chicago and Paris locations for illustrative purposes just to show the processes that are involved in what produces a given heat wave in a given location, but you can extend that to other locations, as well."

Mr. Meehl says investigators focused primarily on data from the Chicago and Paris heat waves, because the records in those two cities are among the best kept, making it easier to predict future conditions.

Based on current trends, scientists predict the number of scorching hot days in Chicago will increase from five during the big heat wave of 1995 to an average of nine days at the end of the century. And in Paris, the computer model predicts the average number of heat wave days would rise from an average of 11 days last year to 17 days in the future.

Since the start of the last century, the global mercury has increased more than a half-degree Celsius, a trend that is expected to continue over the next hundred years.

Scientists say an accumulation of greenhouse gases, such as carbon dioxide, in the upper atmosphere, contribute in part to the increasing heat.

The climate predictions of Mr. Meehl's team appeared in the journal Science.