The result is more rainfall and more damage to buildings as hurricanes hover over population centers for longer periods of time. The change is even more dramatic in storms that have made landfall from the North Atlantic - they're moving 20 percent slower.
This isn't about how powerful a storm's winds are, just how fast it chugs along.
"It is plausible to say that the local rainfall impacts, the impacts from slowing, are equal to and possibly greater than the impacts from increased water vapor in the atmosphere", he said.
Christina Patricola, a scientist with the climate and ecosystem sciences division of California's Lawrence Berkeley National Laboratory, called Kossin's work "important and new" and says she found it "pretty convincing". Wind speeds within the storm remain high, but the whole system itself moves slower across the landscape, allowing punishing rains to linger longer over communities.
"Nothing good comes out of a slowing storm", Dr Kossin told National Geographic.
Dr Kossin said more rain was also falling during cyclones, and there was evidence that tropical cyclones were migrating more towards the poles.
That means that storms farther from land in the earlier part of the study may not have had their speeds included in the study.
But contrary to what you might think that means, the finding is actually a bad thing.
"These trends are nearly certainly increasing local rainfall totals and freshwater flooding, which is associated with very high mortality risk", he said.
But there are probably more variables at play than a warmer climate putting the brakes on tropical cyclones. "There's been a sea change there in terms of what's unsafe".