The study was released Wednesday by the Natural Resources Defense Council and the Rocky Mountain Climate Organization. The research concludes storms that led to flooding that swamped Cedar Rapids in 2008 and that forced the U.S. Army Corps of Engineers to blow up a Mississippi River levee to save Cairo, Ill., in 2011 are part of a growing climate trend.
Between 1961 and 2011, Iowa had a 32 percent increase in storms that brought 3 or more inches of rain in 24 hours, said the report, titled, “Doubled Trouble: More Midwestern Extreme Storms.”
“Global studies already show that human-caused climate change is driving more extreme precipitation, and now we’ve documented how great the increase has been in the Midwest and linked the extreme storms to flooding in the region,” said Stephen Saunders, president of the Rocky Mountain Climate Organization and the report’s lead author.
Saunders said a threshold may have already been crossed so that major floods in the Midwest should no longer be considered purely natural disasters, but instead mixed natural/unnatural disasters. If emissions continue to increase, the forecast is for more extreme storms, he added.
In reaction to the report, state climatologist Harry Hillaker said there is not much doubt that since the late 1950s Iowa has had more rain, which has been accompanied by more storms of all types. The wettest stretch was between 2007 and 2010, which he described as “four very wet years in a row.”
But Iowa has had periods of significant precipitation in the past, Hillaker said. He noted that from the late 1840s to the early 1880s, Iowa was “very, very wet, even wetter than what the recent period has been.”
“If you go far back enough in time, you will start seeing similar statistics as far as rainfall amounts and such,” Hillaker said.
Jeff Zogg, a hydrologist with the National Weather Service in Johnston, pointed to a federal report released in 2009 that concluded global warming is unequivocal and is primarily human induced.
The research also found that during the past 50 years, the amount of precipitation falling during the heaviest rains increased nearly 20 percent. The largest increases were in the Midwest and the Northeast.
“That definitely indicates an increase in the intensity of rainfall events here,” Zogg said.
Here are some key findings of Wednesday’s report:
• Since 1961, the Midwest has had more large storms. The largest of storms, those of 3 inches or more of precipitation in a single day, increased by 103 percent through 2011. Storms of at least 2 inches but less than 3 inches in a day were up 81 percent. Storms producing 1 to 2 inches of rain showed a 34 percent increase.
• The rates for all large storms accelerated over time, with 2001 to 2010 showing the largest increases.
• The frequency of extreme storms grew so much in recent years that the first 12 years of this century included seven of the nine top years for the most extreme Midwest storms since 1961.
• With more frequent extreme storms, the average time between them has become shorter.
The report said the two worst years in the Midwest for storms of 3 inches or more per day were 2008 and 1993. Those years produced the worst floods in about 80 years, causing $16 billion and $33 billion in damage, respectively, and ranking among the nation’s worst natural disasters.
“This report confirms what most of us in the Midwest have known for a while: Violent storms are becoming more frequent. And the nation’s crumbling water infrastructure just makes the problem worse,” said Karen Hobbs, a senior policy analyst for the National Resources Defense Council.