How Hurricane Ian Became So Strong

New data from NASA reveals how warm ocean waters in the Gulf of Mexico fueled Hurricane Ian to become one of the most powerful storms to hit the United States in a decade.

Sea surface temperatures were particularly warm off the southwest coast of Florida, allowing the storm to gain energy just before making landfall in the state north of Fort Myers.

The storm brought strong winds, incessant rain and devastating flooding to Southwest Florida. As it moved inland, it lost strength and was downgraded to a tropical storm, but is expected to redevelop into a hurricane as it travels across the warm Atlantic toward South Carolina.

Storms usually weaken as they move inland and lose access to their main source of moisture and energy.

Hurricane Ian was able, during its path, to draw a lot of energy from the ocean, which could keep it around longer than normal, said Christopher Slocum, a natural scientist with the National Oceanic and Atmospheric Administration.

September is the peak of hurricane season, spurred by Gulf temperatures that are warmer than other times of the year, experts say. The climate phenomenon known as La Niña has also contributed to more favorable conditions for hurricanes in the North Atlantic over the past three years.

However, waters off the coast were also two to three degrees Fahrenheit warmer than usual for this time of year, according to preliminary data from NASA.

And a few degrees can make a huge difference, said Karthik Balaguru, a climate scientist at the Pacific Northwest National Laboratory, because it provides extra energy for a storm.

Unlike the land or the atmosphere, “it’s very difficult to warm the ocean,” Dr. Balaguru said. A large amount of heat had to have been absorbed by the ocean just to raise temperatures by a tiny fraction of a degree, he said.

More than 90 percent of the excess heat from human-caused global warming over the past 50 years has been absorbed by the oceans, and most of it is stored in the top few hundred meters.

Scientists say that while climate change has not necessarily increased the number of hurricanes, it has made them more powerful, as warmer ocean waters strengthen and sustain these storms. The proportion of the most severe storms — Categories 4 and 5 — has increased since 1980, when satellite imagery began to reliably track hurricanes.

As the climate warms, more storms also undergo rapid intensification, which describes an increase of at least 35 miles per hour in maximum sustained winds over a 24-hour period. Ian quickly intensified several times, as have some of the most powerful Atlantic storms of the past decade. Hurricane Harvey in 2017 jumped from a Category 1 to a Category 4 hurricane overnight. In 2021, Ida strengthened from a Category 1 to a near Category 5 in less than 24 hours.

A warmer climate also allows hurricanes to unleash more rain, a consequence of an atmosphere that, with each degree Celsius of warming, can hold about 7 percent more water vapor that is then released as precipitation. In addition, storm surges rise on top of sea level, which can exacerbate coastal flooding.

Wetter, more intense hurricanes can have devastating consequences for the communities they hit, said Kristie L. Ebi, a professor at the University of Washington’s Center for Health and the Global Environment. For the most part, the greatest damage to life and property both during a hurricane and in its aftermath comes from flooding, not winds, he said.