(CNN)As the US and Europe worked for decades to reduce air pollution for the sake of public health and the planet, scientists found an unintended and challenging consequence: an increase in tropical storms in some regions.
Reducing harmful air pollution has led to a surprising effect — more hurricanes in the North Atlantic
A new study published Wednesday in the journal Science Advances found that over the past four decades, a 50% decrease in aerosols — tiny particles of air pollution — over North America and Europe led to a 33% increase in the number of tropical cyclones in the North Atlantic.
On the other side of the world, the study found that a 40% increase in aerosol pollution in China and India over the same time period sparked a 14% decline in the number of tropical cyclones in the western North Pacific. Air pollution surged significantly in China and India during that time due to the countries' economic and industrial growth.
"Decreasing aerosol emissions is something that's good for human health; but on the other hand, we found there are some bad effects when we reduce aerosol emissions — and that is hurricane activity," Hiro Murakami, the lead author of the study and a scientist at the National Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics Laboratory, told CNN.
Aerosols aren't like greenhouse gases. They are tiny particles of pollution that float in the air and — unlike carbon dioxide or methane, which absorb sunlight and lead to warming — reflect sunlight back to space, which has a cooling effect. There are natural aerosols, but much of the pollution in the early to mid-20th century came from sources like industrial smokestacks and car exhaust.
Murakami found that as aerosol pollution decreased in the decades following the United States' Clean Air Act and similar actions in Europe, the ocean could absorb more sunlight, leading to warmer sea surface temperatures that fueled more storms.
Murakami warns his results don't mean we should stop controlling air pollution. Reducing aerosol emissions is kind of like quitting smoking, he said. When a person stops smoking, they improve their health and can avoid cancer. But in some cases, quitting also comes with side effects, including gaining weight and feeling stressed.
"Aerosol decreasing is actually similar," he said. "Aerosol decrease may lead to good health, but on the other hand, hurricane risk increases. This is where good things accompany bad things. It's kind of like pros and cons."
Jim Kossin, a senior hurricane scientist at the Climate Service who reviewed the research, said this study is important to help distinguish how storms respond to air pollution versus greenhouse gases.
"Tropical cyclones are fairly random animals, and they respond to the random nature of the atmosphere at any given time," Kossin told CNN. "But certainly, this steady warming of the ocean that's been happening in the Atlantic because of the combination of greenhouse gas increases and the particulate pollution decreases, that has a profound effect — and the changes to the particulate pollution have a much more dramatic effect on the hurricanes."
Other scientists not involved with the study told CNN the findings are in line with what they know about the complex nature of air pollution, and that they add to the growing body of research on how the climate crisis may be influencing hurricanes.
"This study shows very nicely that the impact of aerosols is not isolated to the Atlantic, but involves a global shift in the distribution of tropical cyclones," Gabriel Vecchi, a professor of climate and geosciences at Princeton University, told CNN. "Aerosols are among the most uncertain elements of the climate system, so I think that there should — and I predict there will — be follow-on studies that explore the sensitivity of the results to a range of aerosol-related uncertainties."
Tom Knutson, a senior scientist at NOAA's Geophysical Fluid Dynamics Laboratory who was not involved in the study, said aerosol pollution is another important way that humans have altered hurricane activity over the past 40 years.
"It's like the aerosols created some kind of a vacation from hurricanes in the Atlantic, but then when we reduced the aerosol forcing, then it kind of springs back," Knutson told CNN. "We have several things that we think are going on the Atlantic and this paper is among those that are trying to sort of tease out the relative influence of these different things."
Murakami said he predicts that aerosol pollution will remain stable, so greenhouse gas emissions will start to have a stronger influence on hurricanes over time — particularly on their intensity.
"Climate science is very complex and it's a work in progress, especially for hurricane activity," Murakami said. "What we saw in the past 40 years may not be applied to the future, so we may see something much different."