As greenhouse-gas emissions rise, North America is likely to experience more droughts and excessive heat in some regions even as intense downpours and hurricanes pound others more often, according to a report issued yesterday 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.

Coming at a time when record flooding is ravaging the Midwest, the new report paints a grim scenario in which severe weather will exact a heavy toll. The report warned that extreme weather events "are among the most serious challenges to society in coping with a changing climate." 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 NOAA's National Climatic 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 found that the last decade has seen fewer cold snaps than any other 10-year period in the historical record dating back to 1895. Under a middle-range scenario of future greenhouse-gas emissions, climate models indicate that by mid-century, extremely hot days that now occur only once every 20 years 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 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 heavy downpours, which 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 bouts of precipitation that might have occurred once every 20 years will take place every five years. The researchers, from both the federal and private sectors, reached more tentative conclusions about the connection between greenhouse-gas emissions and hurricanes.

The report noted that the intensity of hurricanes and tropical storms, as measured by an index that combines wind strength, duration and frequency, has shown a "substantial" increase since 1970 and that "there has been a strong statistical connection between tropical Atlantic sea surface temperatures and Atlantic hurricane activity." But the scientists said this suggestion of a connection to human activity is not conclusive.

NOAA research meteorologist Thomas R. Knutson, who contributed to the report and recently published an article in the journal Nature saying that it is too early to attribute more intense hurricane activity to a detectable human influence, said the synthesis reflects the current disagreement among scientists on the question of hurricanes. "This is a report that is a consensus document, where you have a number of authors who may not agree on all things," Knutson said.

(Por Juliet Eilperin, Washington Post, 20/06/2008)