Global warming can affect hurricanes, in part because a warmer ocean provides more energy to fuel them. But it’s not the only factor at play: A study released 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 finds, the decline in pollution in the form of small aerosol particles from transportation, energy production and industry in North America and Europe has been responsible for the increased number of hurricanes and other tropical cyclones in the United States. North Atlantic Ocean. †
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 results from human-induced emissions of carbon dioxide and other greenhouse gases. For example, a 2020 study used observational data to show that since the 1980s, hurricanes have become stronger and more destructive as the world has warmed and the oceans have absorbed more heat.
The new study looked at the numbers, not the strength, of these types of storms. The author, Hiroyuki Murakami, said it shows that reducing or increasing anthropogenic aerosols is “the main component” that affects frequency.
James P. Kossin, a scientist at The Climate Service who analyzes climate risks to businesses, and an author of the 2020 study, said Murakami’s research was consistent with other studies showing that “warming from regional pollution reduction is a much more profound effect on hurricane activity” than ocean warming from increasing greenhouse gases. The new study “attempts 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.
Murakami, a physicist at the Geophysical Fluid Dynamics Laboratory at the National Oceanic and Atmospheric Administration 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 have been shown to be harmful to human health as a component of air pollution. They can also prevent some sunlight from reaching the Earth’s surface.
In recent decades, aerosol pollution in North America and Europe has decreased by perhaps 50% as a result of laws and regulations that reduce emissions from sources such as vehicles and power plants. Hurricane seasons in the North Atlantic over about the same period were more active, with a higher number of storms, than in previous decades.
In the North Atlantic, Murakami found, the decline in aerosols led to warming that had two effects on tropical cyclones. First, less pollution resulted in more ocean warming, meaning there was more energy to form storms.
The decrease in pollution also led to global warming, 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 storms formed more easily.
Murakami’s simulations showed a different mechanism at work in the Pacific Ocean. There he discovered that increasing pollution from aerosols, largely from China and India, was leading to cooling of the land surface. This reduced the temperature difference between the land and the ocean and weakened 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 is offsetting greenhouse gas warming.” As in North America and Europe, that is likely to change as governments in Asia take steps to reduce pollution due to its health effects.
Murakami said his work highlights the difficulties those governments will face in reducing pollution, as that will most likely lead to increased storm numbers.