20-Year Intense Downpours to Occur Every 6 Years

New government report details impacts of extreme weather on the U.S.

ByABC News
June 19, 2008, 1:25 PM

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.