Global warming can affect hurricanes, in part because warmer oceans offer more energy to fuel them. But this is not the only factor at play. A study released on Wednesday (11) confirms that, in terms of hurricane frequency, the effects of air pollution are even greater.
Over the past four decades, new research demonstrates, the decline in pollution in the form of tiny aerosol particles generated by transportation, energy production and industry in North America and Europe has accounted for the highest number of hurricanes and other forms of tropical cyclones in the North Atlantic.
In the same period, increasing pollution generated by the booming economies of India and China had the opposite effect, reducing hurricane activity in the western Pacific Ocean, according to the study.
A growing body of research demonstrates the link between tropical cyclones and global warming, which results from emissions of carbon dioxide and other greenhouse gases generated by human activities.
A study published in 2020, for example, used observational data to demonstrate that hurricanes became stronger and more destructive from the 1980s onwards because of global warming and the absorption of more heat by the oceans.
The new study assesses the number but not the strength of such storms. Author Hiroyuki Murakami said the work demonstrates that reducing or increasing anthropogenic aerosols “is the most important component” in determining storm frequency.
James Kossin, a scientist at The Climate Service, a consultancy that assesses climate risks for companies, and author of the 2020 study, said Murakami’s research was consistent with other work showing that “warming caused by reduced regional pollution had a more profound effect on hurricane activities” than ocean warming caused by the greater volume of greenhouse gases. The new study “attempts to provide a more global context to demonstrate how regional changes in climate are occurring,” he said.
The study was published on Wednesday by the scientific journal Science Advances.
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 practically impossible in the real world: isolate the effects of pollutants such as sulfur dioxide. They form aerosols, small particles that, as a component of air pollution, are harmful to human health. They also have the ability to block the sun and prevent its light from reaching the Earth’s surface.
In recent decades, aerosol pollution has decreased by perhaps as much as 50% in North America and Europe as a result of laws and regulations that have reduced emissions from sources such as vehicles and power plants. The North Atlantic hurricane seasons were more active over the same period, with more storms than in past decades.
In the North Atlantic, Murakami found, the decline in aerosols has led to warming that has had two effects on tropical cyclones: First, the reduction in pollution has resulted in greater ocean warming, which means there is more energy for storms to form.
The decline in pollution has led to warming on land, too, and the combined warming has affected atmospheric circulation, weakening the winds in the upper part of the atmosphere. This, in turn, has reduced windshear, the changes in wind speed and direction that can affect the way in which cyclonic storms develop. A smaller wind gradient means that storms form more easily.
Murakami’s simulations demonstrate a different mechanism at work in the Pacific. There, he found that the increase in aerosol pollution, largely derived from India and China, leads to a cooling of the earth’s surface. This reduces the temperature difference between land and ocean, weakening the monsoon winds that develop in the area, which leads to fewer tropical cyclones — including typhoons, the equivalent of hurricanes in the Pacific region.
Adam Sobel, a climate scientist at Columbia University, said the new study demonstrated something that other studies had already pointed out: that in the western Pacific, “cooling caused by aerosols is offsetting warming caused by greenhouse gases.” As with North America and Europe, this is likely to change as governments in Asia begin to act to reduce pollution because of its adverse health effect.
Murakami said his work points to the difficulties governments will face when they decide to act to reduce pollution, because doing so is likely to lead to more storms.
