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. All seven qualify as “rapid intensification events,” when a storm’s wind speeds increase by at least 35mph (56kph) within 24 hours.
These kinds of storms have increased in number in recent decades. 16 of the 20 hurricanes over the past two seasons in the Atlantic basin have undergone rapid intensification.
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.
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 57mph (92kph), 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 4mph (6kph) 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.
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˚F (26˚C) 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˚F (30˚C).
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
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.
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.
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.
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 (1.5 metres) 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.