Hurricanes are moving more slowly over both land and water, and that's bad news for communities in their path.
In the past 70 years, tropical cyclones around the world have slowed down 10 percent, and in some regions of the world, the change has been even more significant, according to a study published Wednesday in the journal Nature.
That means storms are spending more time hanging out, battering buildings with wind and dropping more rain.
"The slowdown over land is what's really going to affect people," says James Kossin, the author of the study and a tropical cyclone specialist at the National Oceanic and Atmospheric Administration. He points to Hurricane Harvey's effect on Houston as an example of what slower storms can mean for cities.
"Hurricane Harvey last year was a real outlier in terms of the amount of rain it dropped," he explains. "And the amount of rain it dropped was due, almost entirely, to the fact that it moved so slowly."
Harvey dumped rain on Southeast Texas for more than a week after it made landfall in August 2017. As much as 60 inches fell on some parts of the Houston area. At least 93 people died as a result of the storm.
The new research suggests slow-moving tropical storms such as Harvey are becoming more common because of global climate change. Climate change is causing the poles to become warmer, which in turn affects the atmospheric pressure. There is less and less difference in pressure between the poles and the tropics, and that causes the big currents of wind between the two areas to slow down.
Storms ride on those currents of wind, like a boat in a stream. "The wind itself that carries the tropical cyclone within it is slowing down, and therefore the tropical cyclones are slowing down," Kossin explains.
"This paper is very timely," says Christina Patricola, a climate scientist at Lawrence Berkeley National Laboratory who was not involved in the research. She says people who study hurricanes have traditionally focused on storm frequency and wind speed — but that rainfall is an increasingly important piece of the puzzle.
"This is really important for regional totals for rainfall," she says. "The implication here is that if tropical cyclones are moving more slowly they have the potential to leave more rainfall."
Another study, published last month by scientists at the National Center for Atmospheric Research, analyzed 22 recent hurricanes and found similar results, predicting that the storms of the future will spin more quickly, move sideways more slowly and bring more rain.
A storm slowing down even 10 percent can lead to twice as much rain as previous storms might have brought to a particular area, because warmer air and warmer water mean storms are carrying more water vapor than they used to.
And Kossin says slower storms are dangerous for other reasons.
"If you think about it, all the things about a tropical cyclone visiting your neighborhood, they're all bad. We don't want them to last very long," he explains.
"If the wind's blowing very hard against the structure and it blows a few more hours than it would have, the likelihood of knocking that structure down increases. You get more rainfall, you get more wind damage. You also get more storm surge. Slower storms will have a tendency to push a larger wall of water in front of them. So it's really a triple threat."
ARI SHAPIRO, HOST:
We're almost a week into hurricane season, and places like Houston, Texas, still have not recovered from last year's storms. Scientists say part of the reason last year's season was so devastating is that the hurricanes themselves are changing. NPR's Rebecca Hersher reports.
REBECCA HERSHER, BYLINE: Hurricane Harvey hit the Texas coast on August 25, 2017, and then, for nearly a week, it made its way real leisurely across the southeastern part of the state.
(SOUNDBITE OF NEWS MONTAGE)
SHAPIRO: The situation in Houston is getting more dire by the hour...
RACHEL MARTIN, BYLINE: Hurricane Harvey has brought unprecedented levels of flooding...
DAVID GREENE, BYLINE: Twenty inches of rain in some areas so far...
WADE GOODWYN, BYLINE: A trillion gallons, like three feet of rain. This is one of the greatest weather catastrophes in the nation's history.
HERSHER: In Houston, it was a new kind of hurricane disaster - a massive urban inland flood. James Kossin studies global hurricanes at the National Oceanic and Atmospheric Administration.
JAMES KOSSIN: Hurricane Harvey obviously was a real outlier in terms of the amount of rain it dropped. And the amount of rain it dropped was due almost entirely to the fact that it moved so slowly.
HERSHER: And Kossin says that's getting more common for tropical cyclones around the world.
KOSSIN: They seem to have slowed down just about everywhere.
HERSHER: The reason - climate change. As the poles get warmer, the big currents of wind between the poles and the tropics slow down. Storms ride on that wind, kind of like a boat in a stream.
KOSSIN: Yeah, the wind itself that carries the tropical cyclones within it is slowing down and therefore the tropical cyclones are slowing down.
HERSHER: In a study published today in the journal Nature, Kossin found storms all over the world have slowed down 10 percent in the last 70 years. Hurricanes moving over land in North America specifically have slowed down even more than that on average. Kossin says that's all really bad news.
KOSSIN: If you think of it, all of the things that come along with a tropical cyclone visiting your neighborhood - they're all bad. And you don't want them to last very long. If the wind's blowing really hard and it blows a few more hours than it would have, the likelihood of knocking that structure down increase, so you get more rainfall, you get more wind damage, you also get greater storm surge. Slower storms will have a tendency to push a larger wall of water in front of them. So it's kind of a triple threat.
HERSHER: He also says these storms could be more deadly because we're not used to having storms hang out over inland cities.
KOSSIN: People don't necessarily evacuate from inland locations, and so they get a lot of freshwater flooding. And then you also get these compound events of mudslides, and that's really where we get a really large loss of life.
HERSHER: He warns a lot of places need to reassess their hurricane risks. That will probably mean new evacuation plans, flood maps and emergency systems for a lot of cities. Rebecca Hersher, NPR News. Transcript provided by NPR, Copyright NPR.