The study focuses not on averages but on extremes that are expected to happen if worldwide greenhouse gases keep increasing.

"It's the extremes, not the averages, that cause the most damage to society and to many ecosystems," said Claudia Tebaldi, lead author of the report by the National Center for Atmospheric Research in Boulder, Colo., Texas Tech University and Australia's Bureau of Meteorology Research Center.

"In the future, rising frequency, intensity and duration of temperature extremes ... are likely to have adverse effects on human mortality and morbidity," according to the report, "Going to Extremes," which will be published in the December issue of the peer-reviewed journal Climatic Change. "Changes in precipitation-related extremes such as heavy rainfall and associated flooding also have the potential to incur significant economic losses and fatalities."

The federally funded analysis is among the first to use advanced, super-computer simulations developed in the United States, Japan, France and Russia for a global committee of scientists, the Intergovernmental Panel on Climate Change.

The climate scenarios were based on three projections in volumes of greenhouse gas emissions, which come largely from the burning of gasoline, coal and other fossil fuels.

Even under the lowest emissions scenario, more extreme events were predicted, although the trend was significantly weaker. That means that if greenhouse gases are reduced, it will lower the risk of severe heat waves and heavy storms.

Also, the report contained some potentially optimistic news for the American Southwest: The increase in dry spells occurred only under the highest emission scenario, so if global emissions begin to decline, there may be no higher frequency of droughts. The West is in the eighth year of its most severe drought since record-keeping began in 1895.

Wetter climate was one of the most significant and consistent patterns that showed up in the modeling, the study shows.

Extra precipitation is tied to global warming because warm oceans evaporate more and warm air holds more moisture.

The higher latitudes, above 40 degrees north — in the United States, north of Reno, Nev., Denver and Philadelphia — are expected to feel the most effects of more extreme precipitation.

"We see increases in precipitation intensity almost everywhere, but particularly at higher latitudes," said co-author Gerald A. Meehl, a scientist in the National Center for Atmospheric Research's climate and global dynamics division.

Along with more precipitation, more days will pass between rain events in the Southwest, he said.

"The reason these can both happen simultaneously is that you can have longer dry spells between rainfall events, but when it does rain, it rains harder," Meehl said.

The researchers took 10 international agreed-upon indices that measure climate extremes — five that deal with temperature and five with precipitation — and ran computer models for the world through the year 2099. What Tebaldi called the scariest results had to do with heat waves and warm nights. Everything about heat waves — their intensity, length and occurrence — worsens.

"The changes are very significant there," Tebaldi said. "It's enough to say we're in for a bad future."

The measurement of warm nights saw the biggest forecast changes. Every part of the globe is predicted to experience a tremendous increase in the number of nights during which the low temperature is extremely high. Those warm night temperatures that should happen only once every decade will likely occur at least every other year by the time we reach 2099, if not more frequently, Tebaldi said.

Warm nights are crucial because Chicago's 1995 heat wave demonstrated that after three straight hot nights, people start dying, Meehl said.

However, heat-wave deaths are decreasing in the United States because society has learned to adapt better, using air conditioning, noted University of Alabama at Huntsville atmospheric sciences professor John Christy. He is one of a minority of climate scientists who downplay the seriousness of global warming.

Similarly, the days when the temperature drops below freezing will plummet worldwide. That's not necessarily a good thing, because fewer frost days will likely bring dramatic change in wildlife, especially bug infestation, Tebaldi said.

"It's a disruption of the equilibrium that's been going for many centuries," Tebaldi said. But she noted that a lengthier growing season in general is good.

"This notion of the greening of the planet ... generally is a positive benefit," Christy said.

Christy, who did not participate in the study but acknowledges that global warming is real and man-made, said an increase in nighttime low temperatures makes much more sense than the rain-and-drought forecasts of the paper.

One of the larger changes in precipitation predicted is in the intensity of rain and snowfall. That means, Tebaldi said, "when it rains, it rains more" even if it doesn't rain as often.

Tebaldi's assessment jibes with the National Climatic Data Center's tracking of extreme events in the United States, said David Easterling, chief of the center's scientific services. Easterling's group has created a massive climate extreme index that measures the weather in America. Last year, the United States experienced the second most extreme year in 95 years; the worst year was in 1998.

LEARN MORE

U.S. government's climate extreme index:

http://www.ncdc.noaa.gov/oa/climate/research/cei/cei.html

The Los Angeles Times and The Associated Press contributed to this report.