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 fuel them. But it’s not the only factor in play: A study released on Wednesday confirms that, for the frequency of hurricanes, the effects of particulate air pollution are even greater.

Over the past four decades, the new research shows, 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 increased numbers of hurricanes and other tropical cyclones in the North Atlantic.

Over the same period, increasing pollution from the growing economies of India and China had the opposite effect, reducing hurricane activity in the Western North Pacific, the study found.

A growing body of research has shown links between tropical cyclones and global warming, which is the result of human-caused emissions of carbon dioxide and other greenhouse gases. A 2020 study, for example, used observational data to show that hurricanes have become 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 numbers, not the strength, of these kinds of storms. Its author, Hiroyuki Murakami, said it shows that reducing or increasing anthropogenic aerosols “is the most important component” affecting frequency.

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

The study was published Wednesday in the journal Science Advances.

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

In recent decades aerosol pollution has declined, perhaps by as much as 50 percent, in North America and Europe as a result of laws and regulations that reduce emissions from sources like vehicles and power plants. Hurricane seasons in the North Atlantic over roughly the same period have been more active, with a greater number of storms, than in previous decades.

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

The pollution decline led to warming of the land as well, and the combined warming affected atmospheric circulation, weakening winds in the upper atmosphere. That in turn led to less wind shear, the changes in wind speed and direction that can affect how cyclonic storms develop. Less wind shear meant that storms formed more readily.

Dr. Murakami’s simulations showed a different mechanism at work in the Pacific. There, he found, increasing aerosol pollution, largely from China and India, led to cooling of the land surface. This reduced the temperature difference between the land and ocean, weakening the monsoonal winds that develop there. That, in turn, led to fewer tropical cyclones, including typhoons, the Pacific equivalent of hurricanes.

Adam Sobel, a climate scientist at Columbia University, said the new study showed what other studies have shown, that in the Western North Pacific, “aerosol cooling has been compensating for greenhouse gas warming.” Just as it did in North America and Europe, that will likely change as governments in Asia move to reduce pollution because of its effects on health.

Dr. Murakami said his work points up the difficulties that those governments will face as they move to cut pollution, since that will quite likely lead to increased number of storms.