Harvey, in particular, caused extensive flooding after the storm grew sluggish and stalled over southeast Texas, inundating Houston and the surrounding area with rain. Hurricane Dorian was also notable for how slow it moved, it averaged 1.3 mph over the Bahamas and was stationary for consecutive hours. The storm’s eyewall, where winds are fastest and weather most severe, battered the Grand Bahama island for 40 hours straight.

Francis said some evidence suggests slow-moving storms could become more common in the future, especially as the warming climate alters the atmospheric conditions that move storms around.

“The steering currents in the upper atmosphere are tending to move slower,” Francis said. “It’s related to the fact that the jet stream, which is really what steers storms, has been weakening, especially in the summer and fall.”

But David Nolan, a professor in the Department of Atmospheric Sciences at the University of Miami, said slow-moving hurricanes are not altogether unusual, and when these storms stop and churn over a populated area — as was the case with Dorian — it can be just a matter of bad luck.

Nolan said he agreed that global warming is driving increased rainfall but added that it has been harder to draw links between hurricanes and other effects of climate change.

For instance, it has been suggested that global warming could strengthen storms by making the strongest hurricanes even more intense. But so far, Nolan said, there has been no evidence that such a trend exists.

“You can look at temperature records around the world and see that there’s global warming, or look at sea level records and see that they are definitely increasing, but we just haven’t seen that kind of signal yet in hurricanes,” he said.

The hurricane data problem

Part of the problem is that there isn’t enough data. Category 5 storms — the strongest types of hurricanes, with sustained wind speeds of at least 157 mph — are relatively rare. Only 36 storms have reached Category 5 strength in the Atlantic basin since record keeping began in 1851.

“It would be awesome if we had 500 years of good hurricane data, but unfortunately we don’t,” Klotzbach said.

In the past four years, at least one Category 5 hurricane has formed each season, including two in both 2017 and 2019, according to NOAA data. Still, it’s not known if global warming plays a role in intensifying these storms, Klotzbach said.

Records also indicate that climate change has not affected how many tropical cyclones form each season in the Atlantic Ocean. But, one area that has garnered significant interest is whether climate change drives some storms to grow in strength faster.

A 2019 paper published in the journal Nature Communications that used computer simulations and climate models to study the formation and evolution of tropical cyclones from 1982 to 2009 found that climate change likely plays a role in what’s known as rapid intensification, when a storm’s wind speed increases by at least 35 mph in 24 hours.

This can be attributed to warmer ocean temperatures, which is providing some of the fuel needed for tropical cyclones to develop, according to Kieran Bhatia, one of the authors of the 2019 paper, who conducted the research as a postdoctoral associate at Princeton University.

Over the decade that started in 2010, several hurricanes such as Dorian in 2019 and Harvey and Maria in 2017 underwent rapid intensification. Dorian in particular saw its peak winds increase from 150 mph to 185 mph in the span of only nine hours.

But one of the biggest challenges is knowing which ingredients are most crucial to supercharge hurricanes, and how climate change alters this complex interplay.

“It’s like a recipe,” Bhatia said. “You definitely need flour when you’re making a cake, but there’s also eggs and sugar and other ingredients, and a cake is not going to come out without those things. With hurricanes, just changing sea surface temperature is not enough.”

For all the unknowns, Klotzbach, the Colorado State University scientist, said there’s already plenty of cause for concern.

“Everyone wants to tie things to what’s really dramatic, but unfortunately with these topics, the links are more challenging to pin down,” he said. “But there are things we have confidence in. Even if a hurricane isn’t going to be stronger but it brings 20 percent more rainfall, that would be really bad. Even if hurricanes don’t change at all, but sea levels continue to get higher and higher, that will mean more water penetrating inland.

“And even if you think climate change is a hoax, hurricanes are going to do more damage because we’ve built up coastlines and so there are more people in harm’s way.”

Short storms in 2019

2019 set a record for the number of storms that lasted a day or less, Klotzbach said. In all, half of 2019’s storms lasted fewer than three days.

"For the continental U.S., the most damaging storm was a weak, short-lived tropical storm that caused a tremendous amount of damage,” Klotzbach said. “[Tropical Storm Imelda] was named 90 minutes before it made landfall, so it was right on the border of not having a name, and yet it still caused all that damage."

Many of the storms in 2019 formed in the Gulf of Mexico. Storms that form there are more likely to make landfall and more likely to die fast, Bell, the NOAA meteorologist, said.