Cleaner air may have brought more storms

Pollution during the 20th century appears to have suppressed North Atlantic hurricanes

The Clean Air Act, which has benefited breathing in many American cities over the last few decades, may have worsened the weather in some places.

New climate simulations suggest that reducing the level of atmospheric aerosol particles produced by human activity might have been the main cause of a recent increase in tropical storm frequency in the North Atlantic.

Aerosol levels have increased since the industrial revolution began, but there have been periods when emissions stalled or fell, such as the Great Depression, World War II and after clean air legislation was enacted in Europe and the United States in the 1970s and 1980s.

The climate simulations suggest that these periods of low emissions eventually increased tropical storm frequency. “It seems the Clean Air Act in particular has led to an increased number of hurricanes over the last decade or so,” says Doug Smith of Met Office Hadley Centre in England, a coauthor of the research published June 23 in Nature Geoscience.

Aerosol particles come from fuels burned in power plants and cars, as well as from natural sources like volcanoes, sea spray and dust. Aerosols can cool the Earth’s surface because they scatter the sun’s energy back into space and they seed brighter and more long-lived clouds. The authors suggest that high levels of aerosols in the past cooled the surface of the North Atlantic. This cool patch of the ocean shifted the position of a major air current, suppressing the formation of hurricanes.

This mechanism is credible, says Venkatachalam Ramaswamy of the National Oceanic and Atmospheric Administration and Princeton University.  Ramaswamy finds the work impressive because it carefully incorporates information on aerosols’ effects on clouds. Such effects are emerging as a major influence on regional climate, he says.

In the 20th century, aerosols probably had more effect on storm frequency than did greenhouse gases like carbon dioxide, Smith says. But greenhouse gases hang around for many decades, while aerosols stay in the atmosphere only for weeks. The simulations suggest that by the end of the 21st century, greenhouse gases will reduce tropical storm frequency once more.

But in the near future, further improvements in air quality may lead to even more storms, Smith suggests. He cautions that atmospheric aerosols’ effects on storms are not a good reason to let them increase again because they are hazardous to human health.  “We don’t want to give the impression pollution is a good thing,” he says. Other researchers have suggested that decreased aerosols helped end the drought that devastated the Sahel region of Africa in the 1980s.

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