In the United States, the increase in heat wave severity will be greatest in the West and the South. The findings appear in the August 13 issue of the journal Science.

Gerald Meehl and Claudia Tebaldi, both of NCAR, examined Earth's future climate using the Parallel Climate Model, developed by NCAR and the U.S. Department of Energy (DOE).

NCAR's primary sponsor, the National Science Foundation, and the DOE funded the study, with additional support from NCAR's Weather and Climate Impact Assessment Initiative.

Model results show that an increase in heat-absorbing greenhouse gases intensifies an unusual atmospheric circulation pattern already observed during heat waves in Europe and North America.

As the pattern becomes more pronounced, severe heat waves occur in the Mediterranean region and the southern and western United States. Other parts of France, Germany, and the Balkans also become more susceptible to severe heat waves.

"It's the extreme weather and climate events that will have some of the most severe impacts on human society as the climate changes," says Meehl.

Heat waves can kill more people in a shorter time than almost any other climate event. By a measure of excess death reports, 739 people died as a result of Chicago's heat wave July 14-20, 1995. Fifteen thousand are estimated to have died from heat in France in August 2003, along with thousands of farm animals.

For the study, Meehl and Tebaldi compared present (1961-1990) and future (2080-2099) decades to determine how greenhouse gases and sulfate aerosols might affect future climate in Europe and the United States, focusing on Paris and Chicago. They assumed little in the way of policy intervention to slow the buildup of greenhouse gases.

During the Paris and Chicago heat waves, changes in atmospheric pressure produced clear skies and prolonged hot conditions at the surface. In the model, similar atmospheric pressure changes are enhanced during heat waves in both regions as carbon dioxide accumulates in the atmosphere.

Model Results

Heat waves will become more severe

During the Chicago heat wave, the most severe health impacts resulted from the lack of cooling relief several nights in a row, according to health experts.

In the model, the worst three-day heat waves show a rise of more than 3 degrees Celsius (5.4 degrees Fahrenheit) in minimum nighttime temperatures in the western and southern United States and the Mediterranean region of Europe.

They will occur more often

The average number of heat waves in the Chicago area increases in the coming century by 25%, from 1.66 per year to 2.08. In Paris, the average number increases 31%, from 1.64 per year to 2.15.

They will last longer

Chicago's present heat waves range from 5.39 to 8.85 days; future events increase to between 8.5 and 9.24 days. For Paris, present-day heat waves range from 8.33 to 12.69 days; they stretch to between 11.39 and 17.04 days in future decades.

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