Hurricane Ian grew from a tropical storm to a Category 3 storm in a single day. That's happening more often as oceans warm.

• Hurricane Ian strengthened from a tropical storm to a major hurricane in a single day.
• Research suggests this "rapid intensification" happens more often as global temperatures rise.

Hurricane Ian gathered 125 mph winds on Tuesday morning, making it a Category 3 storm — a major hurricane — just a day after it first became classified as a hurricane with 74 mph winds.

That speedy strengthening is endangering millions of people in Cuba and Florida, as the storm churns past the island and toward southwest Florida. Forecasts are changing and uncertain, but if Ian makes landfall in Tampa Bay, it will be the first major storm to directly strike the area in 100 years.

To get that powerful, Ian underwent rapid intensification — a process in which a tropical cyclone's maximum sustained winds increase by at least 35 mph in just 24 hours.

Rapid intensification is usually fueled by warm water. Scientists expect it to happen more often as climate change leads to higher temperatures in the oceans, especially the Gulf of Mexico. Ian is passing through water that's warmer than 85 degrees Fahrenheit on its way to Florida, according to meteorological data.

In June, scientists warned that this summer's intense heat could fuel more furious hurricanes.

"That's really the biggest impact, the potential for a rapid intensification," Matthew Rosencrans, lead seasonal hurricane forecaster at the National Oceanic and Atmospheric Administration's Climate Prediction Center, told NBC News at the time. "So as [a] storm gets close to the coast, it's just going to have that much more energy to work with."

More storms are likely to intensify rapidly

In an especially drastic example, Typhoon Noru swelled from the equivalent of a Category 1 hurricane to the equivalent of a Category 5 in just six hours between Saturday and Sunday, before slamming into the Philippines, according to CNN.

In a 2020 study, researchers at the National Oceanic and Atmospheric Administration found that each new decade over the last 40 years has brought an 8% increase in the chance that a storm turns into a major hurricane. In the North Atlantic, where hurricanes impact the Caribbean, US, and Canada, that probability of a storm becoming a major hurricane has increased 49% each decade.

"Rapid intensification just before landfall is likely to become increasingly frequent and severe as the globe warms," climate scientist Kerry Emmanuel wrote in a 2017 analysis. "As rapid intensification is difficult to forecast, there is a risk of an increased frequency of poorly anticipated, high-intensity landfalls, leading to higher rates of injury and death."

It's just one of the deadly ways climate change is supercharging cyclones. Warmer air helps them gather and release more water as rainfall. Research suggests rising temperatures slow the storms' forward progress, allowing them to churn over an area longer and do more damage.

Hurricanes form in warm water and air

Hurricanes are vast, low-pressure tropical cyclones with wind speeds over 74 mph. They form over warm water near the equator, when sea surface temperature is at least 80 degrees, according to the National Hurricane Center.

That's because when warm moisture rises from the ocean, it releases energy and forms thunderstorms. As more thunderstorms appear, the winds spiral upward and outward, creating a vortex. Clouds then form in the upper atmosphere as the warm air condenses, and an area of low pressure forms over the ocean's surface. Then hurricanes just need low wind shear — a lack of prevailing wind — to form their cyclonic shape.

Cyclones start out as tropical depressions, with sustained wind speeds below 39 mph. Once winds pass that threshold, the cyclone becomes a tropical storm. Then above 74 mph, the storm is considered a Category 1 hurricane on the Saffir-Simpson scale.

The Atlantic hurricane season generally runs from June through November, with storm activity peaking around September 10. On average, the Atlantic sees six hurricanes during a season, with three of them developing into major hurricanes (Category 3 or above).

Hurricane Ian is a jolt in a relatively sleepy hurricane season. Until this month, the Atlantic Ocean had produced no major storms. Now, Ian comes on the heels of Hurricane Fiona, which grew to Category 4 and devastated Puerto Rico and parts of Canada.