Future Heat Waves Will Be Hotter, Longer, More Frequent

BOULDER, Colorado, August 13, 2004 (ENS) – Heat waves in North America and Europe will become more intense, more frequent and longer lasting during this century, scientists said on Thursday.

A new modeling study shows 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.

Gerald Meehl is a specialist in tropical climate variability and projections of future climate. He plays a key role on the Intergovernmental Panel on Climate Change, serving as lead author for both the 1995 and 2000 assessments. (Photo courtesy UCAR)

Heat waves can kill more people in a shorter time than almost any other climate event.

According to records, 739 people died as a result of Chicago's July 1995 heat wave, and 15,000 Parisians are estimated to have died from heat in August 2003.

For the study Meehl and NCAR colleague Claudia 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.

Heat from the Sun is being trapped close to the planet by a blanket of greenhouse gases. (Photo courtesy NOAA)

In the model, atmospheric pressure increases even more during heat waves in both regions as carbon dioxide accumulates in the atmosphere.

The researchers note that during the Paris and Chicago heat waves, atmospheric pressure rose to values higher than usual over Lake Michigan and Paris, producing clear skies and prolonged heat.

Health experts say the most severe health impacts during the 1995 Chicago heat wave resulted from the lack of cooling relief several nights in a row.

Parisians take refuge from the summer heat in the Tuileries Gardens. (Photo by Ian Britton courtesy FreeFoto)

The average number of heat waves in the Chicago area increases in the coming century by 25 percent, from 1.66 per year to 2.08.

Chicago's present heat waves last from 5.39 to 8.85 days - future events are predicted to increase to between 8.5 and 9.24 days long.

In Paris, the average number increases 31 percent, from 1.64 heat waves per year to 2.15.

Present day heat waves in Paris persist from 8.33 to 12.69 days. In future decades they are predicted to last between 11.39 and 17.04 days.

"This study provides significant insight into the complex response of global climate to possible future worldwide economic and regulatory policies," said Cliff Jacobs, program director in the National Science Foundation’s division of atmospheric sciences, which funded the research. "The societal implications of this study need to be further explored."

The findings of the study appear in today’s issue of the journal “Science.”