Global warming can affect hurricanes, in part because a warmer ocean provides more energy to fuel them. But it is not the only factor at play: a study published on Wednesday confirms that, due to the frequency of hurricanes, the effects of air pollution by particles are even greater.
Over the past four decades, the new research shows, declines in pollution in the form of tiny aerosol particles from transportation, energy production and industry in North America and Europe were 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, the study found.
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 warmed and the oceans absorbed more heat.
The new study looked at the numbers, not the strength, of these types 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 at The Climate Service, which looks at climate risks to businesses, and an author of the 2020 study, said Dr. Murakami’s research was consistent with other studies showing that “warming by reduction in Regional pollution has a much more profound effect on hurricane activity” than ocean warming from increased greenhouse gases. The new study “tries to provide a more global context in which 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, New Jersey, used computer simulations to do something that would be virtually impossible 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 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 percent, in North America and Europe as a result of 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 greater numbers of storms, than in previous decades.
In the North Atlantic, Dr. Murakami found, declining aerosols caused warming that had two effects on tropical cyclones. First, less pollution resulted in more warming of the oceans, which meant there was more energy for storms to form.
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The decrease in pollution also led to warming of the earth, and the combined warming affected atmospheric circulation, weakening the winds in the upper atmosphere. That, in turn, led to 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 in the Pacific. There, he found, increased aerosol pollution, mainly from China and India, led to cooling of the Earth’s surface. This reduced the temperature difference between land and ocean, weakening the monsoon 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 offsetting warming greenhouse gases.” As in North America and Europe, that will likely change as governments in Asia take steps to reduce pollution due to its health effects.
Dr. Murakami said his work highlights the difficulties those governments will face as they move to reduce pollution, as this is very likely to lead to increased numbers of storms.