A similar heat wave that hit the U.S. Midwest last year damaged the corn and soy crops, and 739 people died in a head wave that broiled Chicago in 1995.

Using a new computer model that takes into account increasing levels of greenhouse gases such as carbon dioxide, Gerald Meehl and Claudia Tebaldi of the National Center for Atmospheric Research in Boulder, Colorado, found heat waves might become more common as global warming heats the Earth.

Writing in the journal Science, they said they tried to see if other pollutants such as sulfur dioxide might reflect sunlight away from the planet and perhaps offset some of the heat-trapping properties of carbon dioxide. But their model shows no such effects.

Regions already prone to heat waves, such as the U.S. Midwest and Southeast and Europe's Mediterranean areas, will suffer even more, and longer, the model predicts.

The average Paris heat wave lasting eight to 13 days, they predict, will last 11 to 17 days. In Chicago, heat waves will last on average a day longer, from eight days to nine days, and there will be two a year by 2080 instead of about one.

"But other areas (e.g. northwest United States, France, Germany and the Balkans) could see increases of heat wave intensity that could have more serious impacts because these areas are not currently as well adapted to heat waves," the researchers wrote.

For their study, Meehl and Tebaldi used data from 1961 to 1990 to predict future weather patterns in 2080 to 2099. They assumed there will be few policy changes to affect global warming.

During the Paris and Chicago heat waves, atmospheric pressure was higher than usual over Lake Michigan and Paris, producing clear skies and hot days, with little relief when the sun went down.

A COOLING SOLUTION

Another team of scientists said that governments can turn this pattern around right now, if they choose to.

Stephen Pacala and Robert Socolow of Princeton University identified 15 technologies, from wind, solar and nuclear energy to conservation techniques, that could each help reduce global warming.

Their report, also published in Science, counters the common argument that a major new technology needs to be developed before greenhouse gases can be controlled, said Pacala.

"It certainly explodes the idea that we need to do research for a long time before getting started," Pacala said in a statement.

"If we decide to act, we will need to reduce carbon emissions across the whole global economy," added Socolow.