The number of extreme rainstorms - deluges that dump 3 inches or more in a day - doubled in the U.S. Midwest over the last half-century, causing billions of dollars in flood damage in a trend climate advocates link to a rise in greenhouse gas emissions.
Across the Midwest the biggest storms increased by 103 percent from 1961 through 2011, a study released by the Rocky Mountain Climate Organization and the Natural Resources Defense Council reported on Wednesday.
States in the upper Midwest fared worse than those in the south part of the region, the study found, with the number of severe rainstorms rising by 203 percent in Wisconsin, 180 percent in Michigan, 160 percent in Indiana and 104 percent in Minnesota.
"The increase in extreme storms, because of the linkage to flooding, probably represents the Midwest's greatest vulnerability to climate change," said study author Stephen Saunders, president of the Rocky Mountain Climate Organization.
Overall annual precipitation for the region rose 23 percent between 1961 and 2011, the study found, using data from weather stations.
The worst flood year during the period was 2008, followed by 1993. Those two years saw the worst Midwest flooding since the 1930s, Saunders said by telephone.
In 2008, he noted, "You had these enormous storms, and they happened in quick succession. We ended up with federal disaster areas all over the Midwest, with $16 billion in damage."
Extreme rains became floods that washed out the city of Cedar Rapids, Iowa, in 2008; forced the U.S. Army Corps of Engineers to blow up Mississippi River levees to save Cairo, Illinois, in 2011; and, also in that year, sent the Missouri River over its banks for hundreds of miles.
A series of strong rainstorms in the first half of June 2008 was particularly damaging because the saturated ground failed to recover from one storm in time to absorb the water from the next, Saunders said.
He pointed to global studies projecting more extreme precipitation and floods as a result of climate change, which is a product of increased emissions of heat-trapping greenhouse gases like carbon dioxide. Carbon dioxide is emitted by natural and human sources, notably the burning of fossil fuels.
"A threshold may already have been crossed, so that major floods in the Midwest perhaps now should no longer be considered purely natural disasters but instead mixed natural/unnatural disasters," Saunders said. "And if emissions keep going up, the forecast is for more extreme storms in the region."
The study is available online.
(Reporting by Deborah Zabarenko, Environment Correspondent; Editing by Prudence Crowther)