MIAMI -- In the last 40 years, the East Coast, including Florida, has been hit by dozens of hurricanes.
New NOAA research suggests human pollution may have increased the likelihood of those Atlantic basin storms, but not in the way you might expect.
A decrease in aerosol pollution over the last 40 years, along with a couple of volcanic eruptions, played the largest role in the increase in hurricanes, said lead author Hiroyuki Murakami, a climate researcher at NOAA's Geophysical Fluid Dynamics Laboratory.
Climate change also played a role, although it was "outperformed" in the Atlantic basin specifically by aerosols and volcanoes, Murakami said.
This is the latest in a line of research that seeks to disentangle the complex relationship between climate change and natural variability in hurricane formation.
"At this point, there's no event that is 100% naturally driven and there's no event that's 100% climate change. It's all shades of gray," said Phil Klotzbach, a research scientist at Colorado State University not involved with the study.
The study by the National Oceanic and Atmospheric Administration examined every storm from 1980 to 2018 and found that the buildup of greenhouse gases in the atmosphere, along with changes in other human pollution, has changed how often storms form in certain locations. Some spots, like the Atlantic basin, saw a "substantial increase" in storms, but other spots, like the southern Indian Ocean, saw far fewer.
Volcanic eruptions from El Chichon in Mexico in 1982 and Mount Pinatubo in the Philippines in 1991 also cooled the atmosphere nearby and shifted storm activity. But NOAA scientists said the impacts of those eruptions dissolved by 2000 and didn't impact the next 18 years of activity.
For the Atlantic basin, the birthplace of the storms that threaten Florida, Murakami's team found that lower levels of aerosol pollution played a large role in the frequency of storms. Aerosols are small particles in the air and can be naturally occurring, like dust or sand, or human-caused like the thick smoke caused by burning diesel fuel. Clouds of air pollution shade ocean waters and keep temperatures down, making it harder for hurricanes to strengthen.
"When you have more aerosols and dust, especially in the Caribbean, you tend to have a quiet overall hurricane season," Klotzbach said.
With fewer particles clogging up the air, the sun-warmed ocean was the perfect conduit for strong storms, especially when paired with increasing greenhouse gas pollution, which traps heat in the atmosphere.
But although the study found that climate change played a role in shifting storms toward and away from certain spots on the planet, it didn't affect the overall number of storms that formed. However, the research showed that as the planet continues to heat up it could eventually lead to fewer hurricanes overall. But the ones that do form are more likely to be powerful Category 3, 4 and 5 hurricanes.
"In the future, we predict a decrease in tropical storms," Murakami said. Yet "we still predict increasingly strong tropical cyclones."
Climate change's role in forming storms remains complicated and not well understood. Scientists are careful not to make absolute statements like "climate change caused X hurricane," because that's not what the research shows.
Attribution science, as the field is known, is about discovering if climate change makes something more or less common. Murakami said that's simpler to do over a long period of time, like the 40 years analyzed in the study.
"Statistically speaking we can find some significant trends," he said. "But when you look at a specific tropical cyclone it's really difficult to figure out how climate change affected it."
Some connections, like hotter oceans fueling more powerful storms, are simple enough. As sea levels rise, hurricanes have more water to shove ashore, making storm surge higher and deadlier.
But as the air above the oceans warms, it actually makes the atmosphere more stable and complicates storm formation. That's why the NOAA study found that toward the end of the century the average annual number of tropical cyclones around the world could drop from 86 to 69.
And then there's the question of whether climate change will impact the number of El Ninos and La Ninas, weather systems that affect how and where hurricanes form around the globe.
"That's a big question too. There's no consensus there," Klotzbach said. "There's so many questions that need to be answered."
Hurricane season starts June 1, although a disturbance passing through the Florida Straits could strengthen into the first named storm of the season -- Arthur -- over the weekend.
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