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.
A few factors help account for the shift, including the warming waters — fueled by climate change — that give hurricanes more energy to release through crushing winds and pounding waves. Climate scientists suspect the slow movement of storms like Ian also stems from global warming, giving them a greater opportunity to strengthen and destroy as long as day-to-day conditions remain ripe.
Since 2017, an unprecedented number of storms rated Category 4 or stronger have lashed the US 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 m.p.h. 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 m.p.h. winds Monday to a Category 3 storm with 125 m.p.h. winds Tuesday. Then, as it approached Florida on Wednesday, its winds surged even faster, going from 120 m.p.h. around 2 a.m. to 155 m.p.h. winds 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 m.p.h., 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 4 m.p.h. each decade over the past 30 years.
The Intergovernmental Panel on Climate Change’s most recent assessment came to a similar conclusion, that tropical cyclones are likely 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 m.p.h. in a day, the same storm would increase by 48 m.p.h. 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 been warming at record rates because of human-emitted greenhouse gases, bringing this threshold closer within 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 bathtub-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 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, a risk assessment consultant and former scientist at NOAA who has led studies on hurricane intensification.