These slower speeds don't mean the winds circling the eye of the storm are slowing down.
On average, the storms moved 2 kilometers per hour slower in 2016 compared with 1949-a change of 10% during a period when Earth warmed by 0.5⁰C.
"The unprecedented rainfall totals associated with the "stall" of Hurricane Harvey over Texas in 2017 provide a notable example of the relationship between regional rainfall" and hurricane speed, wrote the study's author, James P. Kossin of the NOAA's Center for Weather and Climate in Madison, Wisconsin.
In the Atlantic basin, the slowdown is just 6 percent.
The slower a cyclone moves over the ocean, the more moisture and intensity it gathers; the slower it moves over the land, the more time it spends drenching it.
"Long-duration or slower-moving storms, even when weaker, can have exacerbated impacts through prolonged wind exposure [in addition to] flooding", according to Colin Zarzycki, a project scientist with the National Center for Atmospheric Research who was not involved with the study.
Kossin published his findings Wednesday in the journal Nature.
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These two trends ought to work in tandem to make today's storms much worse rainmakers.
"Every one of the hazards that we know tropical cyclones carry with them, all of them are just going to stick around longer", Kossin said. "Not quite like a cork in a stream, but similar", he said.
"What we're seeing nearly certainly reflects both natural and human-caused changes", Kossin said.
Kossin would actually agree on that point.
"I went in with that hypothesis and looked at the data, and out popped the signal that was much bigger than anything I was expecting", Kossin said.
But there are probably more variables at play than a warmer climate putting the brakes on tropical cyclones.
That's the real risk of a slower storm. "And when you start getting more and more lines of evidence that all point in the same direction, you get more confident in the answers".