What a destructive, deadly, wet and expensive decade it's been.
This year marks the fourth consecutive Atlantic hurricane season with above-average activity, and wraps up a decade that saw some of the most devastating hurricanes to hit the United States in recorded history.
But as destructive as 2019's Atlantic hurricane season was, it lacked the high-dollar damage, high number of fatalities and high rainfall that have characterized much of this decade of storm activity. Scientists are studying whether a link exists between this storm activity and climate change — an area likely to expand in the coming decade — but experts say it has been challenging to draw conclusions with limited data.
While many scientists agree that global warming is responsible for more rainfall in these storms, and has no effect on storm frequency, there is no consensus on a link between warming and the storm intensification and wind strength.
The high-activity hurricane era in the Atlantic that started in 1995 isn’t slowing down anytime soon, experts say.
According to the National Oceanic and Atmospheric Administration (NOAA), high-activity eras last 20 to 40 years, and are marked by the number of above-normal hurricane seasons. With 18 named storms, six hurricanes and three major hurricanes, NOAA declared 2019 an above-normal season. While the six hurricanes and three major are near-normal numbers, only eight other times since 1851 have there been 18 or more named storms.
Gerry Bell, a hurricane climate specialist and research meteorologist at NOAA's Climate Prediction Center, said that this is the 17th above-normal season in the last 25 years. The previous high-activity era in the Atlantic lasted from around 1950 until the 1970s.
Climate change and hurricanes
The intensity of hurricanes’ rainfall and storm surge in recent years, as was seen with Dorian in September 2019, Florence in 2018 and Irma in 2017, have focused attention on the possible impact of climate change on hurricanes.
The link between hurricanes and global warming is an area of intense study and little consensus.
“By far, this is the most contentious area in the field,” said Phil Klotzbach, an atmospheric scientist at Colorado State University in Fort Collins. “If you were to poll 50 scientists to see how climate change has affected hurricanes, you would get so many different answers.”
By far, this is the most contentious area in the field
Phil Klotzbach, Colorado State University scientist
Scientists agree that climate change is causing storms to be rainier. As global temperatures rise, the subsequently warmer atmosphere can hold more water vapor, said Jennifer Francis, a senior scientist at the Woods Hole Research Center in Falmouth, Massachusetts.
“This means that any storm, no matter what it is, is going to produce more rainfall because it has that much more water to work with,” she said.
Two of the five named storms with the highest rainfall totals in the United States occurred in the last decade: Hurricane Harvey, which dumped 60.58 inches of rain over Nederland, Texas, in 2017 and Hurricane Lane, which soaked Hilo, Hawaii, with 58 inches of precipitation in 2018.
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.
“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.
Bell said that the large number of storms that formed in the Gulf of Mexico this year is what led to so many storms making landfall. But while storms that form over the Gulf typically make landfall, they also don’t tend to last long.
The main factor in how long a storm lasts is an extensive area of weak wind shear that helps strengthen them into hurricanes. Bell said that weak wind shear present throughout Hurricane Dorian’s path, which brushed the banks of North Carolina, is how the storm reached Category 5 wind speeds.
This year’s hurricane season saw weaker wind shear, more monsoons and warmer temperatures in the Atlantic Ocean; factors that lead to longer storms such as Dorian, a storm that lasted more than two weeks, Bell said.
However, inhibiting factors led to the large spike in short storms in 2019, Bell said. Jet stream patterns in the north slowed storms that attempted to move across the Caribbean Sea.
“In that type of situation, the storms don’t live very long,” he said.
While 2019 may have had more short storms, it also had the most named storms since 2012, and Bell said he doesn’t see the high activity era slowing down. In fact, this year gave Bell more indication that we’re still in it.
“Nobody wants to see a lot of major hurricanes striking landfall, so having a little bit of a temperate season is always a good thing,” Bell said.