As humans emit more greenhouse gases, North America is likely to experience more droughts and excessive heat even as intense downpours and hurricanes increase, according to a report issued today by the U.S. Climate Change Science Program.

    The 162-page study, which was led by the National Oceanic and Atmospheric Administration, provides the most comprehensive assessment yet of how global warming has helped to transform the climate of the United States and Canada over the past 50 years, and how it may do so in the future.

    Warning that extreme weather events "are among the most serious challenges to society in coping with a changing climate," the report finds that "recent and projected changes in climate and weather extremes have primarily negative impacts."

    Coming at a time when record floods are ravaging the Midwest, the new report paints a grim scenario in which unpredictable and severe weather will exact a heavy toll. While the Southwest is likely to face even more intense droughts, the scientists wrote, heavy downpours will become more frequent in some other parts of the country because of increased water vapor in the air.

    "This report addresses one of the most frequently asked questions about global warming: What will happen to weather and climate extremes?" said one of the report's two co-chairs, Thomas R. Karl, who directs of NOAA's National Climate Data Center in Asheville, N.C.. He added that the report, which synthesizes the findings of more than 100 academic papers, "concludes that we are now witnessing and will increasingly experience more extreme weather and climate events."

    The authors charted observed changes in the temperature record -- finding that the last decade has seen fewer cold snaps than any other 10-year period in the historical record dating back to 1895, and that six of the last 10 years have had annual average temperatures that fall in the top 10 percent of all years on record for the United States .

    They also projected how these changes may intensify in the future. Climate models indicate that even under a middle range scenario of future greenhouse gas emissions, by mid-century extremely hot days that now occur only once every 20 years in the continental U.S. will occur every three years.

    Richard Moss, vice president and managing director for climate change at the World Wildlife Fund, said in an interview that the report was prepared by "an A-list of authors" and is "really frightening" in light of the estimated costs associated with the recent flooding in the Midwest. "It's a clear signal of the costs of inaction," Moss said.

    In a conference call with reporters, Karl and the other co-chair, Gerald A. Meehl, senior scientist at the National Center for Atmospheric Research, said there is no doubt that human-generated heat-trapping gases have helped intensify both the Southwest's current drought and the heavy downpours that have been increasing at a rate three times that of average precipitation over the past century.

    "That's a certainty," Karl said. "People aren't questioning whether there's been an increase in heavy downpours."

    By the end of the century, he added, models predict that intense precipitation bouts that might have occurred once every 20 years will take place every five years.

    The researchers, who hail from both the federal and private sector, reached more tentative conclusions about the connection between industrial greenhouse gas emissions and hurricane intensity.

    The report notes that the intensity of hurricanes and tropical storms, as measured by the Power Dissipation Index, has had a "substantial" increase since 1970 and that "there has been a strong statistical connection between tropical Atlantic sea surface temperatures and Atlantic hurricane activity, this suggestion of a connection to human activity is not conclusive.

    "In terms of connecting that to human activity, it's a little more difficult because we don't have the right modeling tools," Meehl told reporters.