Two decades ago, Hurricane Katrina spun up like a massive atmospheric engine, using warm ocean water as fuel. Making landfall as a Category 3 storm with maximum sustained winds of 125 mph, it devastated New Orleans — surging seawater over levees, killing nearly 2,000 people, and causing more than $150 billion in damage. Even though engineers have since significantly bolstered those levees, their ability to withstand climate-supercharged cyclones remains uncertain.
In the past 20 years, researchers have gotten ever better at determining how much human-caused climate change has contributed to extreme weather — a field called attribution science — thanks to more data and better modeling. On the 20th anniversary of Katrina, a new report from the research group Climate Central looks back and crunches the numbers, finding that the monster storm fed on waters made 0.9 degrees Celsius, or 1.6 degrees Fahrenheit, hotter by climate change. That boost of fuel accelerated the maximum sustained wind speed by 5 mph.
“As sea surface temperatures go up, the fuel sources provided are going up, and that allows the hurricane to spin faster,” said Daniel Gilford, a climate scientist at Climate Central. “We have the strong confidence now that we can say human-caused climate change is influencing hurricanes in very specific ways.”
Unfortunately for New Orleans and any other city that finds itself in the path of a tropical cyclone, the ocean is really good at absorbing heat: 90 percent of the warming from humanity’s emissions has gone into the seas. The Gulf of Mexico in particular has been running extra hot, creating an expansive pool of fuel for tempests, which grow in strength as water evaporates and transfers energy into the atmosphere. In a separate analysis released last year, Climate Central found that climate change worsened all 11 of 2024’s hurricanes by raising ocean temperatures, boosting maximum sustained wind speeds by 9 to 28 mph.
“Just a little bit of warming in the ocean — 1 degree C, for example — can make a huge impact in the amount of energy these storms can pick up,” said Zachary Zobel, associate director of the risk program at the Woodwell Climate Research Center, who wasn’t involved in the new report. “And I think Katrina was really the wake-up call.”
To be clear, no one is saying that climate change created these storms, just that it exacerbated them. And while ocean temperatures provide the fuel for a hurricane engine, they’re not the only component. They also need favorable atmospheric conditions, like the absence of vertical wind shear — or winds moving in different directions at different altitudes — to spin up.
They also require humidity, as dry air will discourage their formation. And the warmer the atmosphere gets, the more moisture it can hold, boosting humidity and increasing the amount of rain that can fall. “If the hurricane is like a sponge, and it’s squeezing out its water, you can imagine when it spins faster, the ringing of the sponge is more intense,” Gilford said. “So more rain can come out. Not only do you have more water to begin with, but you can more effectively squeeze it out of the storm.”
The data backs this up: In the two decades since Katrina, hurricanes have indeed been dumping more precipitation. According to a 2020 attribution study, climate change made Hurricane Florence — which made landfall in North Carolina in 2018 — produce about 5 percent more rain. “If Hurricane Katrina were to happen today, it would probably be even stronger and have maybe even more rain and storm surge associated with it, because the Earth has warmed over the last 20 years,” said Kevin Reed, an atmospheric scientist at Stony Brook University and lead author of the study.
This storm surge, in which winds shove a bulge of water ashore, is what makes hurricanes so dangerous. A surge might be a foot or two, which can do a whole lot of damage, but Katrina’s reached 28 feet. And since then, sea levels have steadily climbed, meaning there’s a higher baseline for a storm surge to layer on top of. Indeed, a 2021 report from Climate Central found that sea level rise helped Hurricane Sandy reach 36,000 more homes and 71,000 more people, accounting for $8.1 billion of the $62.7 billion in losses.
The other factor that’s making hurricanes more hazardous is “rapid intensification,” when a storm’s maximum sustained wind speeds increase by at least 35 mph within a day. A 2023 study found a significant rise in the number of such events close to shore, thanks to the supercharging effect of warmer ocean temperatures. If forecasters are predicting that a Category 3 hurricane is going to make landfall, and suddenly it transmogrifies into a Category 5, that can leave a coastal community unprepared.
Luckily, those forecasts have improved dramatically since Katrina, providing a more accurate picture of where a tropical cyclone is expected to come ashore, and at what intensity. “The models have improved tremendously — like leaps and bounds in 20 years,” said Brian McNoldy, a hurricane researcher at the University of Miami who wasn’t involved in the new Katrina report. “And hopefully we see that again in the next 20 years.”
The field of attribution science has also grown dramatically. For one, scientists have taken all kinds of measurements over those 20 years, like using satellites and buoys to get the temperature of the ocean. All that extra data goes into the models that researchers have honed over and over, allowing them to tease out how much climate change contributed to an event compared to “natural variability” — that is, the typical fluctuations you get in Earth’s systems. (There are a few ways to do this, but one is by running simulations of how a recent hurricane behaved, and comparing those to simulations of how it might have behaved in a climate before human-caused warming.)
Because the planet has continued to heat rapidly since Katrina, there’s also a clearer picture of what humans have contributed with their wanton burning of fossil fuels. “Twenty years ago, we had maybe 0.8 degrees [Celsius] of global warming or so, and now we are 1.3, and that makes a huge difference in terms of emerging signals,” said Friederike Otto, who leads the climate analysis group World Weather Attribution. “If it’s not a very strong signal, it’s much harder to tease it out with poor data.”
The field has also been zeroing in on standardization of attribution techniques, which have been thoroughly peer-reviewed, meaning a research group can use a method that’s already been vetted. So while getting these studies published used to take perhaps two years after an extreme weather event, that’s sped up dramatically. “As the discipline has matured and the methodologies have been refined, you start to see that there’s attribution statements that are made much quicker, within months, and sometimes within weeks, and sometimes within hours, actually,” Reed said.
All told, more robust and rapid attribution analyses, paired with better forecasting, are giving scientists and communities a better handle on hurricanes that will only get worse from here. Even 20 years ago, Katrina showed how many people will die without proper — and equitable — preparation to evacuate people.
“That was the first event where we have seen really strongly how unequal and unfair the impacts of climate change are, and how already marginalized communities are the ones suffering most,” Otto said. “No adaptation can work if you don’t include fighting inequality as well.”
