Places around the nation are facing more frequent, more extreme precipitation over time — a reality laid bare once again by the record-shattering rains and catastrophic flooding in eastern Kentucky and St. Louis last week.
The warming atmosphere is supercharging any number of weather-related disasters — wildfires, hurricanes, crippling heat waves. But as it also fuels once-unthinkable amounts of rain in single bursts, the problem of so much water arriving so quickly is posing serious challenges in a nation where the built environment is not only outdated but increasingly outmatched.
“The infrastructure we have is really built for a climate we are not living in anymore,” said Andreas Prein, a scientist at the National Center for Atmospheric Research (NCAR) who studies extreme precipitation.
From populated cities to rural outposts, the United States has long struggled with antiquated sewage and wastewater networks, outdated bridges and crumbling roads and culverts. But as more water falls from the sky more quickly in many places, those challenges have become only more urgent.
“What happened was way more than the system — any system — can handle,” Sean Hadley, spokesman for the Metropolitan St. Louis Sewer District, said of the recent storms that dumped more than 9 inches of rain there in a matter of hours, shattering the previous daily record from 1915.
“It was just too much water,” Hadley said.
An analysis of weather data by the nonprofit group Climate Central found that nearly three-quarters of locations the group examined around the country have experienced an increase in the amount of rain falling on their annual wettest day since 1950 — particularly along the Gulf Coast and Mid-Atlantic. The numbers show that 2021 was a record-setting year for extreme rainfall events, with dozens of places logging their wettest day in generations.
A separate Climate Central report this spring found that of 150 locations the group analyzed, 90 percent now experience more average rainfall per hour, compared with 1970. Those increasing bursts of extreme precipitation carry profound economic and human health risks, the likes of which have been on display most recently in eastern Kentucky.
Jen Brady, a data analyst for Climate Central, said many places around the country are getting roughly the same, or in some cases, less rain annually than in the past. But it is the sudden, relentless rainfalls that are contributing to flash floods and other problems.
“The damage that’s happening doesn’t show up when you just look at [annual] precipitation records. It matters if you get 2 inches a day, versus 2 inches an hour,” Brady said. “Our infrastructure is not designed to hold that much water in that much time.”
Scientists say there is little doubt about what is driving the shift toward more frequent, more devastating rains: climate change.
“Individual events happen all the time and have happened all the time in our historical record. We need to be aware that just because we have an event doesn’t mean it represents something unusual,” said Kenneth Kunkel, an atmospheric sciences professor at North Carolina State University.
But while it remains difficult for researchers to outline the precise climate fingerprint on specific summer thunderstorms and other heavy-rain events, they are increasingly able to detail the climate impact on massive tropical cyclones such as Hurricane Harvey. What’s more, after decades of observing and analyzing precipitation metrics around the country, Kunkel says the numbers tell a clear story of change.
“There’s no doubt that the frequency and intensity of the extreme rainfall events is increasing,” Kunkel said, adding that the trend is especially strong in the eastern and central United States.
“When I started 30 years ago, a [climate] signal was emerging,” he said. “That signal has gotten stronger and stronger. … The data are pretty definitive in showing that.”
The explanation boils down to what Kunkel calls “basic physics.” For every degree Fahrenheit that the air temperature increases, the atmosphere can hold about 4 percent more water.
The world already has warmed more than 1.8 degrees Fahrenheit (1 degree Celsius) since preindustrial times. That increased heat means more moisture in the air — in the United States, much of which comes off the Gulf of Mexico — and more fuel for more intense rainstorms.