The globe's hurricanes have seen a striking slowdown in their speed of movement across landscapes and seascapes over the past 65 years, a finding that suggests rising rainfall and storm-surge risks, according to research reported Wednesday.
The study, published in the journal Nature, finds a 10 percent slowdown in storm speed between 1949 and 2016. It points directly to the example of Hurricane Harvey, whose catastrophic rains were enabled by the storm's lingering in the Houston area for such a long period.
Slower-moving storms will rain more over a given area, batter that area longer with their winds and pile up more water ahead of them as they approach shorelines, said Jim Kossin, a scientist with the National Oceanic and Atmospheric Administration and the study's author.
The question of hurricane speed, and whether it would change under global warming, has drawn little attention in the past in comparison with more headline-grabbing questions, such as whether storms are getting stronger overall.
Kossin decided to investigate it, based on the expectation that climate change is already altering the general, large-scale circulation of the atmosphere, within which hurricanes are embedded and by which they are steered. "Not quite like a cork in a stream, but similar," he said.
In particular, a slowing of circulation as the polar regions warm up faster than equatorial regions ought to slow down storm tracks, as well.
"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.
When it came to the storms' travel over land, the slowdown in some cases was even more pronounced than it was over the oceans or in general. In the Atlantic region, storms moved 20 percent slower over land, the study found.
The overall magnitude of the change in storm speed — a 10-degree reduction — was also striking in light of other changes expected in hurricanes under the amount of global warming that we have seen.
For instance, it is expected that hurricanes will rain about 7 to 10 percent more per degree Celsius (1.8 degrees Fahrenheit) of warming, as the atmosphere retains more water vapor, Kossin explained. But here was a 10 percent slowdown in storm movement speed with only a half-degree Celsius (.9 degrees Fahrenheit) of warming globally over the period he studied.
These two trends ought to work in tandem to make today's storms much worse rainmakers. Kossin thinks the slower speed of movement — which naturally adds more rainfall to any region the storm crosses — may be a bigger deal than the simple increase in rain overall.
"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.
Zarzycki did raise a few questions, though — although he says he is not sure how much they would change the result.
First, he noted that over the more than 60-year period of the study, there may be natural, decades-long cycles in the climate system that could affect the steering of storms and have little or nothing to do with global warming. So it is not clear just how much of the change Kossin found is attributable to human-induced climate change.
Kossin would agree on that point.
Zarzycki's second point is that our means of studying hurricanes have also changed. Indeed, after about 1980, we could observe them by geostationary satellite — before that, storms in the open ocean might have been missed completely and gone unrecorded, at least if they never encountered any vessel.
That means storms farther from land in the earlier part of the study may not have had their speeds included in the study.
Overall, while scientists will need to dissect and better understand the new findings, it is hard to mistake the implication that intense, torrential rainfall associated with hurricanes could be getting worse when they make landfall because the storms are, basically, dragging out the punishment they deliver to the places they strike.