WASHINGTON (Reuters) - Floods like those that inundated the U.S. Midwest are supposed to occur once every 500 years but this is the second since 1993, suggesting flawed forecasts that do not take global warming into account, conservation experts said on Tuesday.

Grain from a silo floats in floodwaters after the Meyer levee broke near Canton, Missouri, June 19, 2008. REUTERS/Frank Polich

“Although no single weather event can be attributed to global warming, it’s critical to understand that a warming climate is supplying the very conditions that fuel these kinds of weather events,” said Amanda Staudt, a climate scientist with the National Wildlife Federation.

Warmer air can carry more water, Staudt said in a telephone briefing, and this means more heavy precipitation in the central United States. Big Midwestern storms that used to be seen every 20 years or so will likely occur every four to six years by century’s end, she said.

The idea that certain places along the Mississippi River and its tributaries will only flood once every 500 years may be based on mistaken assumptions that flood patterns do not change over time, said Nicholas Pinter of Southern Illinois University.

Pinter said these assumptions are contained in an analysis on Mississippi River flooding in the upper Midwest, led by the U.S. Army Corps of Engineers, which among other things builds and maintains river levees.

In the last 35 years, there have been four floods in the Mississippi River basin that qualified as 100-year floods or higher according to the Army Corps’ analysis, Pinter said.

BIGGER, MORE FREQUENT FLOODS

“It is an impossibility that those numbers can be correct,” Pinter told reporters. “These are not random events. We’re getting a systematic pattern of floods larger and/or more frequent than currently estimated by those calculations.”

The Army Corps’ analysis rejects any kind of climate change -- human-generated or naturally occurring -- as a mechanism that could alter flood patterns along the Mississippi over the last century, Pinter said.

He said the analysis also rejects the effects of land use and navigation construction over that period.

“We suggest the current flood, sadly, is a confirmation that ... these numbers are probably invalid, underestimating the occurrence of floods up and down this river for a variety of mechanisms,” Pinter said.

Given the impact of this year’s Midwest floods, the National Wildlife Federation, a non-profit conservation group, called on Congress to hold immediate hearings to revise the National Flood Insurance Reform and Modernization Act.

In a letter to chairmen and ranking members of the Senate Banking Committee and House Financial Services Committee, federation president Larry Schweiger noted that there was significant rebuilding in flood plains along the Mississippi after the 1993 floods.

“While there may have been an expectation that such floods would only happen every 500 years, scientists now warn that climate change will make such floods far more frequent,” Schweiger wrote.