Is climate change really making hurricanes worse?

The deadly and destructive hurricanes this year have torn through enormous swaths of the country and reached places where people have never experienced such disasters in their lives. Beryl, Debbie, Francine, Helene, and Milton all made landfall in the continental United States in a season that’s shaping up to be well above average and may set all-time records for hurricane activity. The storms have stunned and alarmed experts who have been tracking these cyclones for decades. And there’s still more than a month to go in the season.

Several factors converged to make 2024 so fertile for tropical storms. Hurricanes feed on warm water, and the Atlantic Ocean and the Gulf of Mexico provided ample nourishment as they reached record-high temperatures. Wind shear — where air currents change speed and direction with altitude — tends to rip apart tropical storms before they can form hurricanes, but there was little of that this year due in part to the ripple effects of the shift to La Niña in the Pacific Ocean.

In a warming world, it’s reasonable to ask how climate change is affecting the storms we’ve seen: How much of the damage can we trace back to our ravenous appetite for fossil fuels? And how much worse are storms like these going to get?

There are strengthening links between climate change and the most deadly aspects of hurricanes, but climate change isn’t the only way humanity is enhancing the devastation of these disasters.

Some types of extreme weather have a robust connection to climate change. As average temperatures rise, heat waves get hotter, for example. But severe weather events like hurricanes are more complicated, arising from local, regional, and global ingredients, making humanity’s specific role harder to discern.

One way to model a hurricane is as a heat engine, a device that harnesses a temperature gradient to do work. Your car likely has a heat engine that uses gasoline to heat air inside a cylinder that presses on a piston to turn your wheels. Hurricanes are heat engines that use hot water to move air.

When warm water evaporates from the surface of the ocean, it cools down the surrounding water, similar to how evaporating sweat cools your skin. As low atmospheric pressure settles over an area of hot ocean water, convection elevates that evaporating water up to high, chilly altitudes, where it then condenses and warms the air around it, forming a convective storm. The warm air surrounded by cooler air creates a temperature gradient that generates wind. If it’s large enough, the Earth’s rotation induces a spin into the storm.

The stronger the contrast between the hot and cool portions of the storm, the more intense the cyclone.

Higher temperatures across the planet from the rising concentrations of greenhouse gasses in the atmosphere mean oceans can accumulate more heat, and warmer air can hold onto more moisture, encouraging more evaporation.

“The more greenhouse gas, the more potential there is for stronger winds and hurricanes,” said Kerry Emanuel, an emeritus professor of atmospheric science at the Massachusetts Institute of Technology and one of the most prolific researchers studying the links between climate change and hurricanes. “For every degree centigrade that you raise the ocean temperature, the wind speed in the hurricane goes up between 5 and 7 percent.”

The other way that climate change can influence hurricanes is through water. While we rank hurricanes by their wind speed, the flooding that they leave in their wake is what tends to cause the most fatalities and property damage.

Warming across the planet is causing ice on land to melt, increasing the quantity of water in the oceans. The water is also expanding as it heats up. These two phenomena cause sea level rise. So when a hurricane makes landfall, particularly with high winds, it can push water further inland and cause storm surges to reach greater heights.

And again, warmer air can hold onto more moisture. For every 1.8 degrees Fahrenheit (1 degree Celsius) increase in air temperature, air can retain 7 percent more water. That means when a major storm occurs, it can dish out more rain and cause greater flooding.

So climate change can increase the destructive potential of hurricanes. But having all the raw materials present needed to build a dangerous hurricane doesn’t mean that one will be assembled. Compared to weather events like thunderstorms and heat waves, hurricanes are relatively rare; there are only a couple dozen tropical storms in the Atlantic Ocean in a given year and a smaller subset of them ever reach hurricane strength.

Hurricanes demand a precise sequence of actions to form, and there’s a lot that can disrupt this choreography, such as wind shear, atmospheric stability, or dust from the Sahara Desert.

And scientists still aren’t clear what governs the total number of cyclones across Earth. Even in 2024, when there was ample high-octane fuel for hurricanes all season in the Atlantic, there was a lull in cyclone activity before gargantuan storms like Helene and Milton erupted.

“Conditions have to be kind of perfect for that to happen,” Emanuel said. “But they do become perfect once in a while.”

The world has already heated up by about 1.9 degrees Fahrenheit (1.1 degrees Celsius) since the dawn of the Industrial Revolution. There’s some evidence of this warming playing out in various hurricane traits, but there are also places where it’s absent.

“There’s never been a consensus among the scientific community who actually studies hurricanes about the overall frequency of hurricanes when it comes to climate change,” Emanuel said.

The Intergovernmental Panel on Climate Change, an international team of climate scientists convened by the United Nations, put out their latest comprehensive assessment of the body of climate science last year. It was the sixth iteration of the report, but for the first time, authors included a chapter on weather extremes. When it comes to tropical cyclones like hurricanes, they found that there wasn’t an increase in the overall number of these storms, particularly when looking at the ones that made landfall in the United States.

“A subset of the best-track data corresponding to hurricanes that have directly impacted the USA since 1900 is considered to be reliable, and shows no trend in the frequency of USA landfall events,” according to the IPCC report.

Hurricanes are relatively infrequent weather events, and their numbers naturally vary year to year and decade to decade, making it harder to suss out a specific trend due to climate change. There has been an increase in hurricane activity in the Atlantic since the 1980s, with more cyclones and an increase in stronger hurricanes. However, it’s not clear how much of this is due to a normal pattern of high and low activity versus human-caused warming.

The IPCC authors also note that there isn’t great data on hurricanes going back over the whole time humans have been burning fossil fuels since the mid-18th century. “This should not be interpreted as implying that no physical (real) trends exist, but rather as indicating that either the quality or the temporal length of the data is not adequate to provide robust trend detection statements, particularly in the presence of multi-decadal variability,” they wrote. Which is to say, scientists need more observations and more time to confirm whether climate change is having any influence on the number of hurricanes.

That said, the IPCC report does show hurricanes changing in ways beyond their overall numbers. One is that hurricanes in recent decades have likely been shifting toward the poles, farther away from their normal habitat in the tropics. That makes sense knowing that hurricanes need warm water, around 80 degrees Fahrenheit, so hotter oceans mean these storms can have a greater range.

Another changing trait is that hurricanes appear to be moving slower. That means the storms that make landfall spend more time parked over a given region, forcing the area to endure more wind and rainfall. Hurricane Harvey in 2017 was an exemplar of this as it sauntered along the Texas coast at 5 miles per hour and drenched Houston.

Rapid intensification is a climate change hallmark as well. This is where a tropical storm gains 35 miles per hour or more in windspeed in 24 hours. Hurricane Milton rapidly intensified from a tropical storm to a Category 5 hurricane with winds up to 180 miles per hour in just one day. The IPCC found that the global frequency of rapid intensification in tropical cyclones has likely increased over the past 40 years to an extent that can’t be explained by natural variability alone.

One of the strongest signals of climate change in hurricanes is rain. The average and maximum rainfall rates from hurricanes are increasing, largely a function of rising water and air temperatures. More recently, some researchers have begun to connect the increase in rainfall from individual hurricanes to climate change. The World Weather Attribution research group analyzed the rain from Hurricane Helene. They found the precipitation from the storm was 10 percent heavier due to climate change and that such extreme rain is now 40 to 70 percent more likely because of warming. Looking at Hurricane Milton, the researchers reported that heavy one-day rain events like those spawned from the storm are at least 20 to 30 percent more probable.

Emanuel, however, is hesitant to read too much into the precision of these direct attribution studies for individual storms, particularly when it comes to rainfall. For one thing, rain is much harder to measure than temperature. The amount of rainfall can vary a lot over a short distance and rain gauges aren’t spread out evenly, so ground-based measurements are patchy. Meanwhile, radar is a coarse measure of precipitation.

“The theory is crystal clear, and if you treat this as a problem of risk, undoubtedly these storms have produced more rain than they would have if the atmosphere were cooler. It’s a slam dunk,” Emanuel said. “To be able to say in a particular storm, ‘So much of the rain was caused by climate change,’ I think that’s going out on a limb.”

However, as average temperatures continue to rise, many of these hurricane trait knobs will continue to turn to higher levels and more robust signs of humanity’s role will likely emerge. There’s an upper limit to how strong a hurricane can get, but the Earth hasn’t hit that ceiling yet. “The potential intensity can still go up,” Emanuel said. “We might see records being broken 50 years from now, we might have 220-mile-per-hour hurricanes.”

If you expand the question of “How does climate change affect hurricanes?” to “How do people affect hurricanes?” there are other variables to consider as well.

One factor is aerosols, tiny particles suspended in the air. These aerosols can come from natural sources like dust, but they can also rise out of smokestacks and tailpipes attached to generators and engines burning fuels like coal and diesel. In the atmosphere, their presence can suppress hurricane formation. They can also block out enough sunlight to cause a measurable drop in the temperature below. Higher concentrations of aerosols can thus lead to fewer hurricanes.

As governments across North America and Europe implemented new air pollution regulations over the past 40 years, the amount of aerosols over the Atlantic Ocean declined. That helped drive the rise in tropical cyclones in the Atlantic over this time period, according to a 2022 study in the journal Science. More recently, the International Maritime Organization imposed new pollution regulations on ships in 2020 that led to a 10 percent drop in sulfur dioxide aerosol pollution around the world. That may have helped push the Atlantic Ocean to record high temperatures.

The other major variable is that a growing number of people and properties are now in the paths of these storms, even far away from coastlines. That means when a hurricane makes landfall, it threatens to kill more people and destroy more homes. The rising exposure to extreme weather coupled with inflation means that extreme weather events in general are extracting a far higher price in lives and livelihoods.

Conversely, it shows there are ways to reduce the harm from hurricanes, even as they spool up faster, pour out more rain, and linger longer. It demands careful planning and sound policies, as well as some difficult decisions about where people can live. And it remains prudent to curb climate change by reducing greenhouse gas emissions as much as possible, as fast as possible.