The rain fell fast and hard in Middle Tennessee over the weekend, harder than it has ever fallen before. Up to 17 inches of rain inundated parts of the state in less than 24 hours on Saturday. Streets turned into rivers. Water barreled through homes. At least 21 people died, and more are still missing.
It was another deadly example of climate change after a summer of climate-driven calamities. Flash flooding — when water rises very quickly and flows with enormous speed and power — is getting more common in many places as Earth heats up.
The deadly floods over the weekend were the second major flooding event in Tennessee this year. Torrential rain flooded parts of Nashville in March and killed at least four people.
Climate scientists have warned for decades that global warming would cause more heavy rain. Now, with Earth almost 2 degrees Fahrenheit hotter than it was in the late 1800s, scientists are observing that trend in real time. The amount of rain falling during the heaviest storms increased by almost a third in the Southeast U.S. between 1958 and 2016, according to the most recent National Climate Assessment.
That's because hotter air can hold more moisture.
Hotter air also dries out soil, which makes it less absorbent. When a lot of rain falls in a short period of time, it can't soak into the ground, and runoff overwhelms pipes, dams and other drainage infrastructure.
Floodwaters can knock down buildings and carry away cars. Just this summer, more than 180 people were killed in flash floods in Germany and Belgium after record-breaking rains. At least 25 people in central China drowned after being trapped by rising water, and more than 100 people in western India died after heavy rain triggered landslides and flash floods.
The Tennessee floods also underscore how climate change can drive dangerous flooding far from the coasts. Although Tennessee is landlocked, about a quarter million residents live in places that are prone to flooding, according to a 2015 analysis.
LEILA FADEL, HOST:
The floods in Tennessee are a deadly example of climate change. Rebecca Hersher from NPR's climate team is here to talk about this. Hi, Rebecca.
REBECCA HERSHER, BYLINE: Hi there.
FADEL: So this is the second time Tennessee has seen deadly flash floods this year. Floods in Nashville killed at least four people back in March. Is this a trend?
HERSHER: Yes. And, you know, it's a trend that's driven by global warming. So as the Earth gets hotter, we're seeing more heavy rain. And that's when a lot of rain falls in a very short period of time, as the sheriff was just describing.
HERSHER: And I think especially this time of year, a lot of people in the U.S. might imagine heavy rain from a hurricane, but there doesn't have to be a big storm for there to be catastrophic water. You know, thunderstorms can cause really serious flash flooding. And we've seen it outside the U.S. as well. So China, Germany, Belgium and India - they've all experienced deadly flash floods this year just from rainstorms.
FADEL: So are climate scientists able to measure exactly how much more rain is happening because of climate change?
HERSHER: Yeah, and that's actually one of the wild things about this current moment. So global warming has progressed so far that scientists can actually see it happening in real time. So in the Southeastern U.S., which includes Tennessee, the amount of water falling during the heaviest rainstorms has increased by almost a third between 1958 and 2016. So that's a lot more water, right?
HERSHER: And it's not just the Southeast. So heavy rain has increased by more than a third in the rest of the Eastern U.S. Even in the West, where the big story this summer has been drought, when it does rain, the rain is more likely to fall all at once and cause flooding. So we've seen that this summer with flash floods in Arizona and in Colorado.
FADEL: OK, wait. That's counterintuitive. States that are in the middle of a record-breaking drought can also experience climate-driven flooding? Why?
HERSHER: Yeah, it is counterintuitive. So it actually goes back to the same underlying forces that cause the deadly floods in Tennessee. So at a basic level, hotter air can hold more moisture. The Earth is almost 2 degrees Fahrenheit hotter than it was in the late 1800s. And hot air also dries out soil. So when we're talking about places with drought, that's certainly at play. It's also happening to a lesser extent in the Eastern U.S. And the problem with dry soil is that it's less absorbent. So when a lot of rain falls really, really quickly, like the sheriff was just describing, the water can't soak in. It runs off the surface. And all that runoff is way too much for drainage pipes and even for rivers, right? It overwhelms the landscape. And that's when you see this really fast-moving, really powerful water that can carry away cars. It can destroy houses. And it can kill people.
FADEL: So it seems like living with climate change means living with more of this devastating flash flooding. What can be done to protect people?
HERSHER: There are a lot of options, actually. And we're seeing towns and cities spend more and more money on this as the climate changes. So, for example, upgrading drainage systems, you know, putting in bigger pipes, slowing down the water by making sure that there are unpaved places for it to collect. Another option is to make sure people aren't living in the areas that are most prone to flash flooding and putting warning systems on roads so that people don't accidentally drive into deep water. And of course, the last thing that can be done is, you know, to reduce greenhouse gas emissions quickly. If the Earth stops heating up, heavy rain will stop accelerating as well.
FADEL: NPR's Rebecca Hersher. Thank you.
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