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As humans warm the planet, the soaking rains and lashing winds that Hurricane Helene brought last month are becoming increasingly likely occurrences in the Southeastern United States, scientists said Wednesday.
Their assessment is a warning to Americans that Helene, the deadliest hurricane to hit the U.S. mainland in nearly two decades, was rare but no fluke. Instead, it represents the kind of storm that the nation can expect to experience more often as societies continue burning coal, oil and gas for energy.
The report came less than a day before Hurricane Milton made landfall in Florida and barreled across the state early Thursday, leaving millions without power and entire neighborhoods flooded.
After analyzing Helene, an international team of scientists estimated that the storm dumped 10 percent more rain than a similarly extreme storm would have done in cooler times. As it roared ashore, its winds were about 13 miles per hour more intense. And the ocean waters from which the storm drew energy were around 2.3 degrees Fahrenheit warmer.
Like all the most destructive hurricanes, Helene was not just a meteorological catastrophe: Its devastation also reflects factors such as local infrastructure and readiness.
“We need to accelerate our preparedness and our adaptation for these types of events that are just beyond what’s imaginable for an individual,” said Julie Arrighi, director of programs at the Red Cross Red Crescent Climate Center and one of the report’s authors. Many of today’s weather calamities are “beyond what we used to consider a once-in-a-lifetime event,” she said.
The report on Helene was produced by nearly two dozen researchers affiliated with World Weather Attribution, an initiative led by scientists at Imperial College London. Their analysis has not yet undergone academic peer review, though it relies on methods the group has used to study other extreme-weather events, including for peer-reviewed journals.
The findings about Helene line up with researchers’ predictions about how the warming climate might be affecting hurricanes and tropical cyclones across the globe.
Worldwide, the proportion of storms that becomes very intense is projected to increase as the planet continues heating up, according to the National Oceanic and Atmospheric Administration. Storms are also expected to become more likely to gather strength rapidly as they traverse warm ocean waters.
The bathtub-warm surface of the Gulf of Mexico last month helped Hurricane Helene intensify to Category 4 from Category 1 in less than a day. On Monday, Hurricane Milton reached Category 5 at an even faster clip.
Another way global warming is affecting hurricanes is that warmer air can take up more moisture than cooler air, allowing storms to drop bigger downpours. Helene effectively delivered two major payloads of rain to the Southeast, said Jay Cordeira, an atmospheric scientist at the Scripps Institution of Oceanography.
While the center of the hurricane was still over the Gulf of Mexico, the storm’s winds sent a huge amount of moist tropical air north, Dr. Cordeira said. This surge of moisture crashed into the Appalachian Mountains and a trough of cooler air over the Ohio River Valley, causing heavy showers in Georgia and the southern Appalachians.
Then the hurricane itself roared ashore, bringing even more rain. Dr. Cordeira said the rush of moisture preceding Helene resembled an atmospheric river, or a long, thin conveyor belt of air carrying water vapor out of the tropics.
In the United States, atmospheric rivers are best known for delivering heavy rain and snow to California. But they can shape the weather in other places as well, including the Southeast.
“This type of event, it’s not unprecedented,” said Dr. Cordeira, who wasn’t involved with World Weather Attribution’s analysis. But “in its location and in its intensity, it was definitely unprecedented,” he said.
When examining how climate change affects extreme weather, scientists face several challenges. By definition, extreme events don’t happen often. Weather data for some places go back less than a century. And the weather naturally varies quite a lot. So researchers need to undertake careful analysis to untangle how human-caused changes might be influencing extremes.
World Weather Attribution does this by using computer models to conduct what are effectively experiments on the global climate. For example, what would it look like if humans hadn’t warmed the atmosphere and oceans? And how does that compare to the climate as it is?
To study Hurricane Helene, the researchers used both models and weather observations to examine two-day and three-day rainfall peaks over two regions, Florida’s Big Bend and the Southern Appalachians.
When they estimated the extent to which changes in these peaks could be attributed to human-caused warming, not all of their results were statistically significant. Even so, past studies of other hurricanes, plus well-established principles of physics, gave the researchers “high confidence” in concluding that a storm as rare as Helene would have been less rainy in the preindustrial climate, two of the report’s authors, Ben Clarke and Friederike Otto, wrote in an email.
Climate scientists have now estimated the influence of the warming Earth on hundreds of severe-weather events worldwide. They can produce these estimates with ever-greater sophistication. This has led some researchers to ask whether their findings could, for instance, help governments determine how much high-polluting countries should compensate low-polluting ones for the damages from extreme weather.
Nations are establishing a fund for this purpose, but it has been slow to take shape. Some scientists say climate-change attribution studies still have significant limitations, particularly when it comes to translating hazards like heavy rainfall into monetary costs.
To Michael Wehner, a senior scientist at the Lawrence Berkeley National Laboratory, none of these limitations are deal breakers. When it comes to setting disaster compensation, “some information is better than no information,” Dr. Wehner said.
The other arena where climate change attribution is starting to play a role is the courtroom.
The Oregon nonprofit Our Children’s Trust used attribution analysis in two successful cases in Montana and Hawaii that sought to compel government action to reduce emissions. But Julia Olson, the group’s founder, said the burden of proof will be higher in cases that seek financial damages.
Those include the two dozen cases that state and local governments have brought against fossil fuel companies. Some of the plaintiffs in these lawsuits used studies linking climate change to specific events, such as Hurricane Maria in 2017.
“Some people have become more optimistic about success in those lawsuits because of the emergence of attribution science,” said Michael Gerrard, founder of the Sabin Center for Climate Change Law at Columbia University. None of those cases have gone to trial yet.
Karen Zraick contributed reporting.
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