Hurricane Helene at sunset on Sep. 26, 2024, as the storm was closing in on the Florida coast as a Cat 4 with 130 mph winds. (Image credit: NOAA/RAMMB-CIRA Satellite Library)After a spectacular burst of rapid intensification, Hurricane Helene made landfall just east of the mouth of the Aucilla River, about 10 miles west-southwest of Perry, Florida, at about 11:10 p.m. EDT Thursday. Top sustained winds were estimated at 140 mph, making Helene a Category 4 hurricane at landfall. We’ll have much more on Helene’s many impacts—some still unfolding on Friday—in our next Eye in the Storm post.
Helene’s landfall gives the U.S. a record eight Cat 4 or Cat 5 Atlantic hurricane landfalls in the past eight years (2017-2024), seven of them being continental U.S. landfalls. That’s as many Cat 4 and 5 landfalls as occurred in the prior 57 years. The only comparable beating the U.S. has taken from Category 4 and 5 landfalling hurricanes occurred in the six years from 1945 to 1950, when five Category 4 hurricanes hit South Florida.
With the U.S. taking such a beating from extreme hurricanes in recent years, it’s worth reviewing how climate change is contributing to making hurricanes worse.
Landfalling U.S. Cat 4s and 5s
The eight Cat 4 and 5 landfalls since 2017: Harvey (2017 in Texas), Irma (2017 in Florida), Maria (2017 in Puerto Rico), Michael (2018 in Florida), Laura (2020 in Louisiana), Ida (2021 in Louisiana), Ian (2022 in Florida), Helene (2024 in Florida).
The eight Cat 4 and 5 landfalls in the prior 57 years: Charley, 2004; Andrew, 1992; Hugo, 1989; Celia, 1970; Camille, 1969; Betsy, 1965; Carla, 1961; Donna, 1960.
Climate change makes the strongest hurricanes stronger
Hurricanes are heat engines that take heat energy out of the ocean and convert it to the kinetic energy of their winds. A hotter ocean will allow hurricanes to grow more powerful, assuming that the other factors that power hurricanes, including low wind shear and a moist atmosphere, are present.
As far back as 1987, MIT hurricane scientist Kerry Emanuel theorized that the wind speeds in hurricanes can be expected to increase about 5% for every increase of one degree Celsius (1.8°F) in tropical ocean temperature, assuming that the average wind speed near the surface of the tropical oceans does not change. Computer modeling has found a slightly smaller magnitude (4%) for the increase.
A 4-5% increase in hurricane winds may not seem like a big deal, but damage from a hurricane increases exponentially with an increase in winds. For example, according to NOAA, a Category 2 hurricane with 100 mph (161 kph) winds will do 10 times the damage of a Category 1 hurricane with 75 mph (121 kph) winds. This includes damage not only from winds but also from storm surge, inland flooding, and tornadoes. Bottom line: A 4-5% increase in winds yields about a 40-50% increase in hurricane damage (Figure 1).
According to NOAA’s Coral Reef Watch, sea surface temperatures along Helene’s path through the western Caribbean and eastern Gulf of Mexico were about 1-2 degrees Celsius (1.8-3.6°F) above the long-term average. Using the theoretical results above, this increase in sea surface temperatures equated to a 50-100% increase in Helene’s destructive power.
Not all of this year’s unusual warmth in the Gulf of Mexico can be confidently attributed to human-caused global warming. If we restrict ourselves to looking at just the long-term trend in August sea surface temperatures in the Gulf, these have warmed about 0.9 degree Celsius (1.62°F) per century, and 1.0 degree Celsius since 1910, according to NOAA (Fig. 2), which is close to about a 1.3-degree Celsius increase since preindustrial times (using the year 1880 as a reference). That is similar to the long-term trend for the global oceans, which have warmed about 1 degree Celsius since preindustrial times in the late 1800s.
