20-Year Intense Downpours to Occur Every 6 Years

June 19, 2008 — -- In Burlington, Iowa, the rain-swollen Mississippi has swallowed part of downtown. It's the second record-breaking flood in the past 15 years — and the second time Dennis Standard's riverfront restaurant has been ruined.

"It's not supposed to happen. It's supposed to be every 500 years. I thought I'd be gone by now," Standard said, laughing. "But it's changing."

Scientists say he's right.

As President Bush toured the 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.

"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.

The prospect of more frequent, heavy storms is daunting news to Burlington City Manager Doug Worden.

"It's not a 500 year event; it's more frequent event," Worden said. "We need to be thinking about how we're going to address it if it happens again in 10 years or 15 years."

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."

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.

"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."

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."