The formula for how much water a hurricane drops on you is pretty simple: how much rain is falling per hour times how many hours the storm is overhead. While this won't account for things like storm surges, it can give a strong sense of the problems inland areas will face. Hurricane Harvey took this formula to an extreme when it got stuck over Houston for several days, dumping incredible amounts of rain all the while.

Alterations in hurricane behavior due to climate change have been much dissected, from projections of stronger storms in a warming world to the unavoidable fact that a warmer atmosphere can carry more moisture. But there's also a second part to that simple formula—could hurricanes linger longer, adding to rainfall totals?

That question is complex, but a new study by NOAA’s James Kossin takes a look at one portion of it—whether hurricanes are moving more slowly than they did in the past.

Slow down

Kossin looked at hurricane-tracking data going back to 1949. While most measurements of hurricane conditions improved dramatically in the satellite era, tracking the position of each storm’s eye has been effective for much longer. He averaged the movement rate of all the tropical cyclones in each year to create an almost 70-year-long record.

While there are year-to-year wiggles in that record, it also showed a significant 10-percent drop in speed. While tropical cyclones once chugged along at around 19 kilometers per hour, the average in recent years was about 17.5 kilometers per hour.

Regionally, there are important differences. The northern Indian Ocean, for example, saw no significant change, while storms in the western North Pacific slowed twice as much as the global average. And the movement of storms after landfall is also noteworthy in several places—20-percent slower along the North Atlantic and in Australia and 30-percent slower along the western North Pacific.

Talking about the reasons for these changes gets a little tricky. First of all, this study doesn’t include any of the work it would take to say how much of a role global warming played in this. It simply shows how average tropical cyclone behavior has changed during a period of warming. But this is not the first piece of research to suggest a link between those two things.

What hit the brakes?

Another study, published two months ago, found that simulated tropical cyclones slowed down (even as their wind speed increased) in a warmer climate. The cause of this probably has to be bigger than the storm itself, since tropical cyclones are pushed along by the prevailing winds. That means that any changes to the larger-scale atmospheric circulation will similarly affect individual storms.

A number of studies have found that warming should weaken the summertime circulation of the atmosphere in the tropics. Evidence for slowing tropical storms over the past 70 years supports that explanation nicely.

Hurricane Harvey was a different beast—its movement stalled because of high pressure regions that essentially blocked its path. It’s not clear whether we’ll see that specific situation more commonly as the world warms. Other ways in which climate change contributed to Harvey’s impact—like warmer ocean water and warmer air holding more water vapor—are more obvious.

But more generally, this study shows that we should add another item to the list of worrying tropical cyclone trends—taking their sweet time as they endanger the people beneath them.

Nature, 2018. DOI: 10.1038/s41586-018-0158-3 (About DOIs).