"The western U.S. and the Southwest, in particular, seem to be in for a rough ride, if you believe in these models," said Claudia Tebaldi, a researcher with the National Center for Atmospheric Research in Boulder, Colo.

"It is the extremes, not the averages, that cause the most damage to society and to many ecosystems," she said.

Tebaldi led the research team that said nine major climate models agree: The world will be warmer and wetter at the end of the 21st century with more extreme weather, such as floods, hurricanes and heat waves - all of which can have enormous impacts.

They zeroed in on 10 indicators of climate change. They projected the changes in extreme weather events from 2080 to 2099, assuming greenhouse gasses built up in the atmosphere at three rates, low, moderate and high. In all three scenarios, they found:

* The dry spells that produce and intensify droughts are likely to lengthen, especially if the emissions associated with climate change grow to the highest projected levels;

* More days of heavy rain and snow can be expected in areas like Salt Lake City that are around 40 degrees latitude and above;

* There will be more extremely warm nights and longer heat waves around the world; and

* One positive impact could be longer growing seasons worldwide.

Also, the data suggests that steps taken to reduce greenhouse gasses that contribute to climate change will help reduce the risk that the most severe changes will happen, Tebaldi said.

The research team of three Americans and an Australian took advantage of new data and tools developed for the next International Panel on Climate Change report that is expected early next year. Although many past studies have looked at how much average rainfall and temperatures might change as global warming gasses grow, this one focused on trends in extreme weather.

Tebaldi also noted that climate models continue to be "relatively coarse" when it comes to forecasting trends in specific geographical areas, such as the Wasatch Front.

Nonetheless, the models "are saying the same thing" about extreme weather. The more greenhouse gasses, the higher the likelihood of more and more intense extreme weather.

Jim Steenburgh, associate professor of meteorology at the University of Utah, said the study added a positive, new dimension to our understanding of climate change.

"It's trying to get out how people are going to be impacted," he said. "This is a good first step going down that road."

Steenburgh added that the study would have been more helpful if it had looked at summer heat waves and winter precipitation extremes, two areas of extra concern in Utah.

A warm spell in the winter does not pose the same hazards as one in the already-hot summer, he noted. And, because winter snow stores water for the summer here, it would be easier to project Utah's year-round water availability if more is known about winter storm trends.

fahys@sltrib.com