In meteorological terms, 2020 has been a historic year: Subtropical Storm Theta has formed in the Atlantic Ocean, bringing the total number of named storms this season to 29—breaking the record set in 2005, with weeks still left in the season. By September the National Hurricane Center had already run through its alphabetical list of names and switched to Greek letters.
In the minds of Americans, storm seasons like this increasingly are linked to climate change. One CBS poll published last year found that 45 percent of respondents believe that climate change is contributing “a great deal” to severe hurricanes.
But when climate scientists are asked, many suggest that the explanation for this season’s activity isn’t so straightforward. They caution against saying a single storm or even a single season is a sign of climate change.
“Given everything we know about how climate change is warming oceans, and given that this season was hyperactive because it was so warm, it’s very likely it contributed to the active season,” says James Kossin, a climate scientist at the National Oceanic and Atmospheric Administration (NOAA). “But by how much? Now we’re getting into something that’s tougher.”
What we know about hurricanes and a warmer climate
The 2020 hurricane season has lived up to predictions that it would be more active than usual. Storms began forming early, and their number mounted at a record-breaking pace. Florida, Nicaragua and Honduras are still battling flood waters after this year’s 28th storm, Hurricane Eta, inundated the regions with rain. The season could even stretch into winter: In 2005, Tropical Storm Zeta, the 28th that year, formed in late December.
One reason there were so many storms this year was the formation of a La Niña weather pattern in the Pacific, which leads to less winds in the Atlantic that can stop storms from spinning up into hurricanes.
But a number of storms also showed characteristics that scientists associate with climate change: intensifying rapidly, moving slowly, and dumping large amounts of rain. All those things go back to heat.
“First and foremost for the season, the Atlantic Ocean has been anomalously warm, and there’s no question that has driven the hyperactive hurricane season,” says Kossin.
Climate change is definitely contributing to the anomalous warmth: Average sea surface temperatures have been steadily rising since the late 19th century. But NOAA attributes part of this year’s unusual warmth to a natural climate cycle called the Atlantic Multidecadal Oscillation (AMO), which makes the North Atlantic Ocean either warmer or cooler every few decades. Scientists aren’t certain which influence is greater—climate change or the AMO.
Either way, warm water acts as fuel for hurricanes, and can lead to a process called rapid intensification, in which a storm’s maximum wind speeds increase by at least 35 mph in under 24 hours. Eight of 2020’s storms underwent this rapid intensification over the warm waters of the Caribbean Sea and the Gulf of Mexico.
“You’re giving these storms more energy. They get their energy from the ocean, for the most part, and it accelerates how fast they spin,” says Kossin. “It’d be like someone modifying their car engine to go faster. It accelerates more quickly.”
Rapid intensification means a tropical storm can spin up into a hurricane, or a hurricane into major hurricane, more quickly. But that doesn’t mean it will necessarily travel faster over the sea and land; another force governs that. A hurricane is propelled forward by steering winds in the atmosphere. That’s why we saw Hurricane Laura zip over the Gulf Coast at 15 mph, while Hurricane Sally, which followed a similar path, advanced at a 3-mph crawl.
In his own work, Kossin has found that hurricanes may be trending toward moving more slowly, by an estimated 17 percent in the past 120 years. That too may be linked to climate change.
“The winds are driven by a temperature difference between the equator and the poles,” he says, As the Arctic warms at a faster rate than the tropics, that temperature difference is reduced and slows the speeds of the steering winds, Kossin and other scientists theorize.
Slower-moving hurricanes often dump more water, as Hurricane Harvey did over Houston in 2017, Hurricane Dorian did over the Bahamas in 2019, and Sally did over Alabama this year. The drenching effect of the sluggish movement is compounded by the fact that a warming atmosphere holds more water.
“When it comes to climate change, I think one of the most straightforward impacts is increased rainfall. A warmer atmosphere holds more moisture,” says Phil Klotzbach, an atmospheric scientist at Colorado State University. The increased moisture increases the odd that any given storm will dump more rain.
Not just climate change?
NOAA’s data set on tropical storms in the Atlantic shows a long-term increase in the number of storms since the 1880s. But researchers attribute that to improved technology: Satellites now detect short-lived storms that went unrecorded before, especially in far-off parts of the ocean.
That may not explain, however, the increase in tropical storms that has been observed since the 1980s. MIT climate scientist Kerry Emanuel says that more recent trend is real, not an observational artifact. But it may have to do with a different kind of air pollution than the carbon emissions that cause global warming—or rather, with the way we have cleaned that pollution up.
Aerosol pollution from cars, power plants, and factories is known to locally cool the surrounding environment. Those types of sulfate aerosols are highly effective at reflecting sunlight. East Coast summer days during the 1970s were often hazy, the result of sunlight being reflected away from Earth.
That same phenomenon suppresses storm formation, Emanuel says. And conversely, a study published in 2013 linked the passage of the Clean Air Act in 1970 to the subsequent decrease in aerosols, the rise in heat, and the increase in tropical storms that began in the late 1980s.
One challenge in verifying this theory, Emanuel notes, is that storm data are more limited before the 1970s, when weather satellites became common. Kossin calls the precise impact of aerosol pollution “one of the most important unanswered questions we have” about hurricanes.
There’s no evidence that climate change caused by carbon emissions is increasing the number of storms. “The number of storms—we don’t expect to see that go up [with climate change],” Klotzbach says. “It’s more the intensity of storms.” On the other hand, he says, this year’s strong La Niña weather pattern likely means the Atlantic will be active late into the season.
“I don’t think Eta is the last (hurricane) we’re going to see this year,” he says.
A matter of odds
Kossin and Emanuel advise us to think of climate change impacts on hurricanes in terms of probabilities.
As an example, Emanuel says, “We can say about Hurricane Harvey in 2017, the probability of that amount of rain was two to three times larger in 2017 than it was in 1970.”
Two studies published at the end of 2017 found that climate change made a slow, rainy storm like Hurricane Harvey three times more likely.
What goes for a single storm goes for a storm season, Kossin says. You can’t say climate change caused 2020’s roster of rapidly intensifying, slow-moving, rain-rich storms—only that it made such a year more likely: “This season we just saw is the kind of season we might expect to see more of under climate change.”
This story has been updated with the news of Subtropical Storm Theta.