As Hurricane Ian barreled toward Florida this week, it did what six other storms did over the past six years as they approached the United States: It intensified, quickly.
Since 2017, an unprecedented number of storms rated Category 4 or stronger have lashed the U.S. shoreline: Harvey, Irma, Maria, Michael, Laura, Ida and now Ian. They all qualify as “rapid intensification events,” when a storm’s wind speeds increase by at least 35 mph within 24 hours.
These kinds of storms have increased in number in recent decades. Sixteen of the 20 hurricanes over the past two seasons in the Atlantic basin have undergone rapid intensification.
“Especially in the near-coastal region where the hurricane is just ahead of landfall, what we are witnessing is that the hurricane intensification rates have been ramping up,” said Karthik Balaguru, a climate scientist at the Pacific Northwest National Laboratory. “And that is a bit disconcerting.”
Ian was only the latest case when its winds nearly doubled within a 24-hour period, going from a low-end hurricane with sustained 75 mph winds Monday to a Category 3 storm with 125 mph winds Tuesday. Then, as it approached Florida on Wednesday, its winds surged even faster, going from 120 mph around 2 a.m. to 155 mph by 7 a.m.
The series of intense hurricanes striking the United States since 2017 is “one of the busiest times for landfalling powerful hurricanes that we’ve seen historically,” said Phil Klotzbach, a senior research scientist at Colorado State University. One comparable period of hurricane activity came from 1945 to 1950, when five Category 4 hurricanes hit Florida in six years, making Klotzbach reluctant to call the series of intense storms since 2017 unprecedented.
And, Klotzbach added, a period of rapid strengthening is almost a prerequisite for a storm to become among the most powerful hurricanes. “The stronger the storm, the more likely it is to undergo rapid intensification,” he said.
Even so, recent research suggests that the rise in rapidly intensifying storms is having a profound impact.
One study published earlier this year found that since 1990, a steadily growing number of global tropical cyclones have undergone what the study called “extreme rapid intensification,” with winds increasing by at least 50 knots, or 57 mph, within a 24-hour period. Another study from 2018 focused on the Atlantic basin found that among cyclones that have strengthened the most rapidly, their rates of intensification have accelerated, growing by about 4 mph each decade over the past 30 years.
The Intergovernmental Panel on Climate Change’s most recent assessment came to a similar conclusion, finding that tropical cyclones are probably becoming more intense and prone to rapid intensification.
“I would say one of the most worrying things about climate change is a change in extremes,” said Balaguru. “The rapid intensification is a process that fits that category of extreme.”
For instance, Balaguru said, if a storm in the Caribbean Sea four decades ago intensified by 34 mph in a day, the same storm would increase by around 48 mph in today’s climate.
And a preliminary analysis of Ian’s rainfall released Thursday by Stony Brook University professor Kevin Reed and Lawrence Berkeley National Laboratory senior scientist Michael Wehner, based on previously peer-reviewed research, found that climate change is responsible for increasing Ian’s extreme rainfall rates by 10 percent.
A hotter ocean, along with low vertical wind shear, have helped drive the rapid intensification of recent storms.
Generally, ocean waters must be above 79 degrees Fahrenheit for a hurricane to develop and persist. In recent decades, the ocean has warmed at record rates because of human-emitted greenhouse gases, making this threshold easier to reach. As Ian was moving away from Cuba, sea surface temperatures were approaching 86 degrees.
Rising global air temperatures also mean that waters, especially in bodies like the bathub-esque Gulf of Mexico, are warming beyond just their surface. The deeper that warmth goes, the more fuel can flow to a slow-moving storm like Ian. The warm seawater evaporates and pumps moisture into the air, which can recondense into storms, clouds and rain.
“A storm can sit over this warm water almost for days and, if it’s deep enough, it’s not going to kill itself,” said James Kossin, a senior scientist at the Climate Service, risk assessment consultant and former scientist at the National Oceanic and Atmospheric Administration who has led studies on hurricane intensification.
Vertical wind shear — changing wind speeds and direction at different altitudes in a storm — is also a key influence on the intensity of hurricanes, although researchers are still deciphering any long-term trends. High wind shear can weaken a hurricane, while weaker shear can help a hurricane form and strengthen.
Wind shear has been relatively low in the western Atlantic since 2017, a factor that has contributed to the flurry of tropical cyclones since then, according to Klotzbach. It’s possible that in the long run, climate change could make this environmental condition more common. Scientists hypothesize that the jet stream, which creates strong wind shear, could be pushed north as global temperatures rise.
Climate change also may be increasing hurricanes’ potential for intensification — and destruction — by slowing them down, increasing the duration of damaging winds and flooding rainfall.
For example, while Hurricane Ian took a similar path, and with a similar intensity, as Hurricane Charley in 2004, Charley blew into Florida at 20 mph, while Ian only moved half as fast. That allowed Ian to dump as much as 20 inches of rain along its path, according to early estimates, already more than twice Charley’s rainfall.
Scientists hypothesize storms’ slow movement may stem from rapid warming at Earth’s poles, because this has narrowed the gap in temperature and pressure from the poles to lower latitudes. Those differences drive winds around the globe, pushing around weather systems, including hurricanes, like corks in a stream. So when they are minimized, that may be causing a broader slowdown in global weather systems.
The same phenomenon may be causing abnormal spells of extreme heat and blasts of polar frigidity, because the jet stream winds that normally break up weather patterns and drive storm systems are weaker.
“It just seems like the whole atmosphere is getting more sluggish, and consequently, the storms that are carried in it are moving more slowly,” Kossin said.
Slower-moving storms are capable of dropping massive amounts of rain. Hurricane Harvey showered more than 60 inches of rain in some parts of southeastern Texas because it stalled over the region for nearly two days.
Similarly, if a storm bearing hurricane-force winds remains over one spot for long enough, “it’s eventually going to flatten everything,” Kossin said.
Authorities have changed the way they issue forecasts and warn the public, meteorologists said, because of the danger posed by a storm’s sudden escalation.
The National Hurricane Center signaled fears about rapid intensification with Ian as soon as the storm developed into a tropical storm. On Friday morning, well before Ian approached the Cayman Islands and Cuba, forecasters warned it would likely feed off warm Gulf of Mexico waters and become a major hurricane approaching Florida within five days.
“We have a lot better tools to be able to predict this rapid intensification than we used to,” Klotzbach said. “The models are just better.”
