Climate change is making tropical storms and hurricanes stronger, slower, and wetter

Hurricane Idalia has intensified rapidly and is projected to swell to a Category 3 hurricane on Wednesday morning. 

Scientists can't definitively say whether climate change caused any single storm without conducting further analysis, but overall, warming makes hurricanes more devastating than they would otherwise be.

That's because higher water temperatures lead to sea-level rise, which increases the flooding from storm surges. Warmer air also holds more atmospheric water vapor, which enables tropical storms to strengthen and unleash more precipitation.

Here's what to know about why storms are getting so much stronger, wetter, and slower.

How a hurricane forms

Hurricanes are vast, low-pressure tropical cyclones with wind speeds over 74 mph.

In the Atlantic Ocean, the hurricane season generally runs from June through November, with storm activity peaking around September 10.

On average, the NHC says the Atlantic sees seven hurricanes during a season, with three of them developing into major hurricanes.

The storms form over warm ocean water near the equator, when sea surface temperature is at least 80 degrees Fahrenheit, according to Penn State's College of Earth and Mineral Sciences.

As warm moisture rises, it releases energy, forming thunderstorms. As more thunderstorms form, the winds spiral upward and outward, creating a vortex. Clouds then form in the upper atmosphere as the warm air condenses.

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As the winds churn, an area of low pressure forms over the ocean's surface. At this point, hurricanes need low wind shear — or a lack of prevailing wind — to form the cyclonic shape associated with a hurricane.

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

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The National Oceanic and Atmospheric Administration (NOAA) projected a 60% chance that the 2023 Atlantic hurricane season would see above-normal activity.

NOAA projects that before the end of the hurricane season in November, we'll likely see 14-21 storms with at least 39 mph wind speeds, with six to 11 of those becoming major hurricanes with wind speeds of 74 mph or greater.

Hurricanes are moving more slowly and dropping more rain

Hurricanes use warm water as fuel, so once a hurricane moves over colder water or dry land, it usually weakens and dissipates.

However, because climate change is causing ocean and air temperatures to climb — summer 2023 is shaping up to be the hottest summer on record —  hurricanes are getting wetter and more sluggish.

Over the past 70 years or so, the speed of hurricanes and tropical storms has slowed about 10% on average, according to a 2018 study.

Over land in the North Atlantic and Western North Pacific specifically, storms are moving 20% to 30% more slowly, the study showed.

A slower pace of movement gives a storm more time to lash an area with powerful winds and dump rain, which can exacerbate flood problems. So its effects can wind up feeling more intense.

Hurricane Harvey in 2017 was a prime example of this: After it made landfall, Harvey weakened to a tropical storm, then stalled for days. That allowed the storm to dump unprecedented amounts of rain on the Houston area — scientist Tom Di Liberto described it as the storm that "refused to leave."

Climate scientist Michael Mann wrote on Facebook in 2017 that Hurricane Harvey — which flooded Houston, killed more than 100 people, and caused $125 billion in damages — "was almost certainly more intense than it would have been in the absence of human-caused warming, which means stronger winds, more wind damage, and a larger storm surge."

To make matters worse, a warmer atmosphere can hold more moisture, so a 10% slowdown in a storm's pace could double the amount of rainfall and flooding that an area experiences, James Kossin, a tropical cyclone expert, told Stormwater Report in 2018.

The peak rain rates of storms have increased by 30% over the past 60 years —  that means up to 4 inches of water can fall in an hour.

"Precipitation responds to global warming by increasing," Angeline Pendergrass, a project scientist at the University Corporation for Atmospheric Research at the time, said in 2018.

Storms are also getting stronger

As ocean temperatures continue to increase, we're also likely to see more severe hurricanes because a storm's wind speed is influenced by the temperature of the water below.

A 1-degree Fahrenheit rise in ocean temperature can increase a storm's wind speed by 15 to 20 mph, according to Yale Climate Connections.

"With warmer oceans caused by global warming, we can expect the strongest storms to get stronger," James Elsner, an atmospheric scientist at Florida State University, told Yale in 2016.

That can also mean that storms are able to intensify and develop into powerful hurricanes in a shorter time span.

Generally, a strong storm brings a storm surge: an abnormal rise of water above the predicted tide level. This wall of water can flood coastal communities — if a storm's winds are blowing directly toward the shore and the tide is high, storm surges can force water levels to rise as rapidly as a few feet per minute.

Higher sea levels, of course, lead to more destructive storm surges during a hurricane. Such surges are likely to become a more regular threat, since even if we were to cut emissions dramatically starting today, some sea-level rise is inevitable.

The planet's oceans absorb more than 90% of the extra heat that greenhouse gases trap in the atmosphere, and water (like most things) expands when it's heated.

This story has been updated. It was originally published on September 30, 2022. Aylin Woodward, Kevin Loria, and Jeremy Berke contributed to an earlier version of this story.