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As President Bush tours Midwest flood zones today, a new administration report on extreme weather warns that human-induced climate change is making heavy downpours more intense, with storms that used to occur every 20 years projected to occur every six by the end of the century.

Photo: Lightning rolls across the sky of Tyler, Texas as powerful thunderstorms stretch across Texas to Tennessee on Monday night, Feb. 5, 2008. (Dr. Scott M. Lieberman/AP)

"As greenhouse gasses increase, the faster they increase, the more extreme weather and climate events we'll be seeing," said Thomas Karl, co-editor of the report and director of the National Climatic Data Center at the National Oceanic and Atmospheric Administration.

Weather and Climate Extremes in a Changing Climate offers the administration's first major compilation of existing scientific research examining the present and future impact on the U.S. from more frequent heat waves, more intense rainfall and flooding, potentially stronger hurricanes, drought and even wildfires. The 182-page report to Congress calls the extremes "the most serous challenges to society in coping with a changing climate."

One of clearest trends in observed records is an increase in the number and intensity of heavy precipitation events, the report says. Over the last century, for example, days where it has rained more than four inches in the upper Midwest have jumped 50 percent.

Scientists say as humans warm the atmosphere, it holds more moisture.

"Increases in atmospheric greenhouse gases due to human activities have caused the global temperature to rise," Karl said. "That rise in global temperature has led to an increase in water vapor. That increase in water vapor is absolutely necessary for the production of heavy and extreme precipitation events."

Photo: Gary Cochran worked on the roof of Trinity Lutheran Church last week in Victorville, Calif. (Irfan Khan, LA Times, via AP)

The report also says abnormally hot days and nights are likely to become more frequent, and there will be fewer colder days.

"A day so hot that it is currently experienced only once every 20 years would occur every three years by the middle of the century over much of the continental U.S. and every five years over most of Canada," the report states.

Droughts are likely to become more severe in the southwestern part of the U.S. as rainfall totals drop in winter. Warmer air also will help evaporate moisture from the ground, making droughts worse.

Wildfires in the American West are "strongly associated with increased spring and summer temperatures and correspondingly earlier spring snowmelt in the mountains," the report said.

There is less clarity in the report when it comes to how much influence humans are having when it comes to hurricanes, reflecting the ongoing discussions taking place among scientists in peer-reviewed science journals.

Photo: Lightning rolls across the sky of Tyler, Texas as powerful thunderstorms stretch across Texas to Tennessee on Monday night, Feb. 5, 2008. (Dr. Scott M. Lieberman/AP)

"Over the past 50 years there has been a strong statistical connection between tropical Atlantic sea surface temperatures and Atlantic hurricane activity," the report reads. "This evidence suggests a human contribution to recent hurricane activity. However, a confident assessment of human influence on hurricanes will require further studies using models and observations, with emphasis on distinguishing natural from human-induced changes."

Computer climate models, however, project that for every 1.8 degrees Fahrenheit increase in ocean temperatures due to human-induced warming, rainfall from hurricanes will go up 6 to 8 percent. Hurricane wind speeds could increase by 1 to 8 percent.

"These are fairly significant numbers," Karl said.

Democratic lawmakers in Congress say the report highlights the need to curb greenhouse gasses.

"There is no safe haven. There is no place you can live that won't suffer the consequences of global warming," said Rep. Ed Markey, D-Mass., chairman of the House Select Committee on Energy Independence and Global Warming. "People have been watching huge floods, droughts, storms that are otherwise unaccountable and historic in their nature. And now people, I think, will have the dots connected."

Photo: Receding water on Sunday revealed the widespread damage caused by a record flood crest, but other Iowa areas face rivers that were still rising. Burlington is expecting a crest in the Mississippi River within the next couple of days. Visitors to Burlington's riverfront district stand along the edge of a flooded Front Street June 15, 2008 in Burlinigton, Iowa. (Julie Jacobson/AP)

Some scientists, such as NASA climatologist James Hansen, believe the dots have been connected for a long time. Next week, Hansen will mark the 20th anniversary of a 1988 appearance before a Senate hearing in which he made the case that humans, not natural causes, were responsible for the warming of the planet. He has largely been proven right by an enormous body of subsequent scientific research.

"My conclusions in 1988 were built on a wide range of inputs from basic physics, planetary studies, observations of on-going changes and climate models," Hansen told ABC News in an e-mail. "The evidence was strong enough that I could say it was time to 'stop waffling'. I was sure that time would bring the scientific community to a similar consensus, as it has."

He stated, "The next President and Congress must define a course next year in which the United States exerts leadership commensurate with our responsibility for the present dangerous situation."

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