Human behavior helped cause more tropical cyclones to form in the Atlantic Ocean over the last 40 years, according to a new study led by the National Oceanic and Atmospheric Administration.
The average annual number of cyclones has not changed across the world since 1980, researchers found, but the places where those storms form most have shifted. Among the potential causes is what might seem like a surprising culprit: a decrease in a specific form of particle pollution that blocks the sun’s rays. Lead author of the study and NOAA researcher Hiroyuki Murakami said other factors are at play, too, including fluctuations in greenhouse gas emissions and volcanic eruptions.
More cyclones have come especially to the North Atlantic — the basin above the equator in which tropical storms and hurricanes threaten Florida. Others, including the western Pacific Ocean and southern Indian Ocean, showed relatively fewer storms.
The study illustrates a vexing subject for climate scientists and serves as an example of the complicated interplay between pollution and the atmosphere. In some places, the concentration of aerosols, including those that emerge when people burn fuel in cars and power plants, have dropped because policies like the Clean Air Act are working. But having fewer aerosol particles creates a clearer path for the sun to warm seas, Murakami said. Hotter water fuels storms.
Man-made aerosols are pollutants and can be harmful for public health. Before the regulations of the last century, researchers believe aerosols counteracted and obscured some of the global warming caused by carbon dioxide emissions. Scientists say such greenhouse gases, which also come from burning fuel, heat the world and its oceans even further.
“Things we do can impact the climate, but I also don’t recommend starting to pollute a lot more just to cool the Atlantic,” said Phil Klotzbach, a Colorado State University atmospheric science researcher not involved in the study. Some aerosols, he said, are natural — like Saharan dust that blows over the ocean and suppresses hurricane formation. Changes in winds brought less dust over the Atlantic in recent decades than in the 70s or 80s, Klotzbach said.
The NOAA study showed that volcanic eruptions (in particular Mexico’s El Chichón in 1982 and Pinatubo in the Philippines nine years later) also released particles that led to cooling in areas, which helped shift storms in turn.
The research, published in Proceedings of the National Academy of Sciences of the United States of America, involved models based on observed climate data, Murakami said. The historical tropical cyclone record is limited.
Through its analysis, the NOAA-led team showed the patterns of cyclone formation were not just products of natural variations in climate across decades. Instead, the scientists said, human-induced changes had a measurable impact. The project did not address storm intensity.
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“It’s not a super straightforward process like some people would have you believe,” Klotzbach said. “There’s still a lot of stuff we just don’t know.”
The trajectory of the last four decades is not expected to linger forever.
The researchers predict the yearly world average of tropical cyclones will drop from 86 to about 69 by the end of the century because of climate change. They anticipate those storms will be generally more severe.
Greenhouse gases, Murakami said, could cause heating all around that limits temperature variability, or instability, between layers of the atmosphere. Hurricanes and tropical storms thrive on this instability. Still, water temperatures might rise, too, meaning that when cyclones do form, they’ll have energy to draw upon and grow bigger.
Brian Soden, a University of Miami atmospheric science professor not involved with the NOAA study, said climate change is creating other reasons for more destructive hurricanes. For one, he said, scientists believe storms are getting wetter with heavier rainfall that leads to more flooding. Rising seas could increase storm surges, too.
“Those are my two biggest concerns,” Soden said.
Hurricane season begins June 1.