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of so much water arriving so quickly is posing serious challenges in a nation where the built environment is not only outdated but increasingly outmatched.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.
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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cities to rural outposts, the United States has long struggled with antiquated sewage and wastewater networks, outdated bridges and crumbling roads and culverts.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.
The record-crushing rain in St. Louis inundated storm drains and creeks. Sewage backed up into homes. The River des Peres swelled beyond its banks. The area’s sprawling drainage systems, parts of which date to the 19th century, were quickly overwhelmed.
“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.”
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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.
“We’ve had an increase in the amount of atmospheric water vapor, … so we are seeing more of these heavy rainfall events,” said David Easterling, a scientist at the National Oceanic and Atmospheric Administration’s National Centers for Environmental Information. “This is all very consistent with the notion of a warming atmosphere.”
It’s not that St. Louis, for instance, hasn’t had heavy rainstorms in the past. But these days, Easterling said, that same storm likely has access to a lot more moisture that can become torrential rain.
“What was really highly unusual 100 years ago is not that unusual anymore,” he said.
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More heavy rains alone don’t automatically translate to more flooding. It matters whether the soil where rain falls is dry or already saturated, how populated the area is and whether the water has someplace to go.
In an urban area like St. Louis, the sheer amount of paved surfaces contributed to runoff that overwhelmed drainage systems. In eastern Kentucky, the steep terrain funneled cataclysmic amounts of water to flatter areas below, where most homes and people are.
No matter the geography, the more intense rains are posing a major planning, engineering and adaptation challenge on the ground.
One problem is that the country’s flood mapping and its collection of precipitation data are underfunded and outdated, and have long relied on “a very patchwork approach,” said Chad Berginnis, executive director for the Association of State Floodplain Managers.
That means engineers, planners and public works officials don’t always have access to the most accurate and up-to-date data about current risks — and those probably on the horizon.
Berginnis said some local governments with more resources — places such as Milwaukee, Nashville and Charlotte — have undertaken research to understand and plan for the water-related challenges they face. New York City also has invested in its own studies and in measures to better gird itself against heavier rains and rising seas.
“They are going to see less damage going forward,” Berginnis said. But not every place is so fortunate.
“In rural areas or places that have less capacity, they are stuck with the data that is nationally available, and it’s just not that good,” he said. “Unfortunately, it’s kind of the haves and the have-nots in a lot of cases.”
The problem of more frequent and extreme precipitation is not only national but also global. Europe saw deadly flooding after severe rains last summer. Parts of Australia have endured tremendous rainfall in recent days, putting Sydney on track for its wettest year on record. Parts of China have experienced devastating floods this summer, fueled by rainfall that, at least in one area, dumped 3.3 inches in a single hour.
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Across the globe, the torrents show few signs of slowing.
The federal government’s most recent National Climate Assessment found that, over the coming century, “observed increases in the frequency and intensity of heavy precipitation events in most parts of the United States are projected to continue.” The largest increases in intense precipitation events have occurred — and are expected to continue — in the Northeast and Midwest.
“These trends are consistent with what would be expected in a warmer world, as increased evaporation rates lead to higher levels of water vapor in the atmosphere, which in turn lead to more frequent and intense precipitation extremes,” scientists wrote.
That same assessment found that the nation’s water systems “face considerable risk even without anticipated future climate changes.” But with the changes, the risk will rise for dams and levees at risk of failure, for landslides and erosion on the West Coast, for more flooding in low-lying areas of the Midwest and Southeast, and more strain on old and overtaxed infrastructure in the Northeast.
For now, extreme precipitation events are likely to get only more extreme and more common unless the world makes rapid and drastic cuts in planet-heating emissions — something that has yet to materialize.
Prein, the NCAR scientist, said that even if the world halts warming at 2 degrees Celsius (3.6 degrees Fahrenheit) beyond preindustrial levels — a core aim of the Paris climate accord — rain and flood events are still likely to get worse in the near term.
“We cannot just shut down our greenhouse gas emissions immediately,” he said. “We will see these events become more intense in the next couple of decades, and there is very little we can do about that.”
That’s why investing in effective adaptation efforts and early warning systems is essential, he said. So is being more cautious about where and how humans build new developments and manage existing infrastructure. Because the heavy rains will come.
“It’s sad but true,” he said, “these kinds of events are our new normal.”
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