A major storm took hold across swaths of the central and southern United States on Wednesday unleashing extreme flooding and huge tornadoes from Arkansas up to Michigan. And conditions are expected to worsen on Friday as soils become saturated and water piles up: The National Weather Service is warning of a “life-threatening, catastrophic, and potentially historic flash flood event,” along with a risk of very large hail and more twisters. Eight people are so far confirmed dead, while 33 million are under flood watches across 11 states.

While scientists will need to do proper research to suss out exactly how much climate change is contributing to these storms, what’s known as an attribution study, they can say generally how planetary warming might worsen an event like this. It’s not necessarily that climate change created this storm — it could have happened independent of all the extra carbon that humanity has pumped into the atmosphere — but there are some clear trends making rainfall worse.

“In a world without the burning of fossil fuels, this event would happen once in a lifetime — that’s kind of what the National Weather Service is saying,” said Marc Alessi, a climate science fellow at the Massachusetts-based Union of Concerned Scientists. “But with the burning of fossil fuels, with more heat-trapping emissions, with a warming planet, this event will become more frequent.”

Rainfall is changing because Earth sweats. When the sun evaporates water off Earth’s surface, that moisture rises into the atmosphere, condenses, and falls as rain. But greenhouse gases trap heat up there, so the planet sweats more in response. In other words, it strikes an energy balance.

A warmer atmosphere also gets “thirstier”: For every 1 degree Celsius of warming, the air can hold 6 to 7 percent more water. That means more moisture is available to fall as rain: This weekend, the slow-moving storm is forecasted to dump as much as 15 inches of rainfall in some areas. “The sponge, which is the atmosphere in this case, will become bigger, which allows the sponge to hold more water and carry it from oceans farther inland,” Alessi said. “That could be tied into this event here.” 

The body of water in question here is the Gulf of Mexico. An outbreak of tornadoes and heavy rain is typical for this time of year as warming waters send moisture into the southern and central United States. And at the moment, the Gulf of Mexico is exceptionally warm. “There’s a lot more fuel for these rain-producing storms to lead to more flooding,” Alessi said. (The influence of climate change on tornadoes in the U.S., though, isn’t as clear.)

So a warmer Gulf of Mexico is not only producing more moisture, but the atmosphere is also able to soak up more of that moisture than it could before human-caused climate change. Indeed, the U.S. government’s own climate assessments warn that precipitation is already getting more extreme across the country, as are the economic damages from the resulting flooding. That’s projected to get worse with every bit of additional warming.

The problem is that American cities aren’t built to withstand this new atmospheric reality. Urban planners designed them for a different climate of yesteryear, with gutters and sewers that whisk away rainwater as quickly as possible to prevent flooding. With ever more extreme rainfall, that infrastructure can’t keep up, so water builds up and floods. And with storms that last for days, like those tearing through Arkansas and Kentucky right now, soils get saturated until they can hold no more water, exacerbating flooding even more. On Thursday, rescue crews in Nashville, Tennessee were scrambling to save people trapped by surging water levels. 

Now scientists will have to pick through the data to figure out, for instance, how much additional rain the storm dropped because of the sponge effect and warming of the Gulf of Mexico. But the overall trend is abundantly clear: As the planet warms, it doesn’t always get drier, but wetter, too.