Air pollution can mean more or fewer hurricanes. It depends where you live.

Global warming can affect hurricanes, in part because a warmer ocean provides more energy to power them. But that’s not the only factor at play: a study published Wednesday confirms that, for the frequency of hurricanes, the effects of particulate air pollution are even more important.

Over the past four decades, new research shows that the decline in pollution in the form of tiny aerosol particles from transportation, energy production and industry in North America and Europe was responsible for the increase in the number of hurricanes and other tropical cyclones in the North Atlantic. .

During the same period, increased pollution from the growing economies of India and China had the opposite effect, reducing hurricane activity in the western North Pacific, according to the study.

A growing body of research has shown links between tropical cyclones and global warming, which is the result of man-made emissions of carbon dioxide and other greenhouse gases. A 2020 study, for example, used observational data to show that hurricanes have gotten stronger and more destructive since the 1980s as the world has warmed and the oceans have absorbed more heat.

The new study looked at the number, not the strength, of these types of storms. Its author, Hiroyuki Murakami, said it shows that the reduction or increase in anthropogenic aerosols “is the most important component” affecting frequency.

James P. Kossin, a scientist with the Climate Service, which analyzes climate risks for businesses, and author of the 2020 study, said Dr Murakami’s research was consistent with other studies showing that “warming by reducing regional pollution has a much more profound effect on hurricane activity” than ocean warming due to increased greenhouse gases. The new study “attempts to provide a more global context in which regional climate change occurs,” he said.

The study was published Wednesday in the journal Science Advances.

Dr. Murakami, a physicist at the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory in Princeton, NJ, used computer simulations to do something that would be a practical impossibility in the real world: isolate the effects of pollutants like sulphur dioxide. These form aerosols, small particles which, as a component of air pollution, have been shown to be harmful to human health. They can also block some of the sunlight from reaching the Earth’s surface.

In recent decades, aerosol pollution has decreased, perhaps by as much as 50%, in North America and Europe due to laws and regulations that reduce emissions from sources such as vehicles and power plants. North Atlantic hurricane seasons over roughly the same period have been more active, with more storms, than in previous decades.

In the North Atlantic, Dr. Murakami found, falling aerosols led to warming that had two effects on tropical cyclones. First, less pollution led to more warming of the oceans, which meant there was more energy for storms to form.

The drop in pollution has also caused land to warm, and the combined warming has affected atmospheric circulation, weakening winds in the upper atmosphere. This in turn resulted in less wind shear, the changes in wind speed and direction that can affect the development of cyclonic storms. Less wind shear meant storms formed more easily.

Dr. Murakami’s simulations showed a different mechanism at work in the Pacific. There, he found, increasing aerosol pollution, much of it from China and India, has caused the earth’s surface to cool. This has reduced the temperature difference between the land and the ocean, weakening the monsoon winds that develop there. This, in turn, reduced the number of tropical cyclones, including typhoons, the peaceful equivalent of hurricanes.

Adam Sobel, a climatologist at Columbia University, said the new study showed what other studies had shown, that in the western North Pacific, “aerosol cooling offset the warming of greenhouse gases.” Just as was the case in North America and Europe, this will likely change as Asian governments take steps to reduce pollution due to its health effects.

Dr Murakami said his work highlights the challenges these governments will face as they work to reduce pollution, as it will most likely lead to an increase in storms.

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