The Rocky Mountain Climate Organization (RMCO) and Natural Resources Defense Council (NRDC) have released a new report, Extreme Storms in Michigan, which documents an 89 percent increase in the annual frequency of extreme precipitation events--defined as 2 inches or more in a day—across the state over the last 50 years. In southern Michigan, home to most of the state's residents, there has been a whopping 128% increase over the past half century.

The NRDC-RMCO news release on the report is available here.

What Michigan has at stake as extreme storms become more frequent is illustrated by two recent storms--an August 2014 storm that deluged Detroit with up to six inches of rainfall in eight hours, and widespread April 2013 storms of more than two inches of rainfall in a day. Both storms caused major flooding that prompted federal disaster designations and from which the areas are still recovering. The Detroit flooding led to $1.1 billion in damages.

The RMCO-NRDC report is based on an analysis of daily precipitation records from 37 weather stations over 50 years. It is the most comprehensive analysis yet of daily precipitation trends in Michigan, and by including data through 2013 has the most recent data of any analysis of precipitation in the Midwest.

“The report digs deep into the data to confirm something most Michiganders already guessed—dangerous, extreme storms are on the rise across the state,” said Stephen Saunders, the president of the Rocky Mountain Climate Organization and the report’s lead author. “Global studies already show climate change is driving more extreme precipitation, and now we’ve documented how great the increase has been in Michigan, where aging infrastructure makes the resulting floods even worse.”

Intense storms overwhelm communities where infrastructure intended to capture these deluges cannot keep up, leading not only to property losses from flooding but also to contamination of water supplies, wells, rivers and the Great Lakes. A particular vulnerability that Michigan has to extreme storms stems from the state’s reliance on combined sewer systems, an older type of system which carries both storm water and sewage in the same pipes. Michigan has 46 combined sewer systems, the third-largest total in the nation. Combined sewer overflows (CSOs) can spread untreated human and industrial wastes, and people can become sick by drinking or being exposed to the contaminated water. The report includes a new analysis of CSOs, finding that 76 percent of the worst incidents in recent years resulted from storms of two inches or more per day—the extreme storms tracked in the report’s precipitation analysis.

The study also notes that private drinking water wells are at risk from flooding, and that more people in Michigan rely on them than in any other state, with over one million households drinking water from private wells.

“As Michigan faces more of these big storms, there is a lot to be done in shifting the state to clean energy and away from dirty fuels that produce more global warming pollution,” said Theo Spencer of the NRDC. “On the bright side, more renewables like solar and wind, and more green infrastructure projects to better handle runoff mean more jobs in Michigan."

The increase in extreme storms is much greater than that of smaller storms. In Michigan over the past half century, the annual frequency of storms with less than one inch of precipitation in a day increased by only 12 percent, and that of storms of one to two inches increased by 24 percent. So changes in the frequency of different storms are linked to their size, with larger storms becoming more frequent at a larger rate. This is consistent with a general global pattern of increasing frequency of large precipitation events, which scientists have concluded is a result of human-caused climate change. Climate change drives this pattern primarily because, under the basic laws of physics, warmer air can hold more moisture.