Hurricanes suppressed by air pollutants

by Aimee Gillespie
Tuesday, June 25, 2013

Understanding how often devastating tropical storms like Superstorm Sandy occur, and how humans may play a role in their frequency, is a major goal among climate scientists. Now, a new study indicates that aerosols may suppress storm formation over the Atlantic. Thus, researchers say, more frequent storms at the end of the last century might have been an unintended side effect of cleaning up the air.

Burning coal and other fossil fuels, the hallmark of the industrialization and economic expansion that took place throughout the 19th and 20th centuries, releases (among other things) carbon dioxide and sulfur compounds into the air, the latter of which can form aerosols akin to particulates formed by volcanic eruptions. On average, these aerosols stay aloft for about two weeks before raining down as acid rain.

While they’re in the air, sulfate aerosols reflect nearly all the sunlight they intercept, which has a cooling effect on the planet. But things get more complicated when aerosols interact with clouds. Water droplets can condense on particles like sulfate aerosols, both creating clouds and changing cloud properties like their brightness and longevity, and making it harder for climate scientists to predict the overall effects of aerosols on climate. “Aerosols are quite difficult" to understand and model, and clouds are "quite difficult too,” making their interactions even more complex, says Nick Dunstone of the U.K.'s Met Office in Exeter, England, lead author of the study, published this week in Nature Geoscience.

To investigate the effects of aerosols and clouds on tropical storms in the Atlantic Ocean over the past century, Dunstone and his colleagues ran simulations on a collection of climate models. They ran the simulations multiple times, each time excluding a different factor: greenhouse gases, volcanic aerosols, ozone and anthropogenic aerosols. This allowed them to identify the impact of each individual factor on tropical storm frequency.

They discovered that anthropogenic aerosols lowered the frequency of tropical storms by altering the temperature of the ocean’s surface. (Sea-surface temperatures are a major factor in tropical storm formation.) The temperature difference caused atmospheric circulation to shift, leading to different patterns in precipitation and increased wind shear in some regions, which impairs storm intensification. Without anthropogenic aerosols, there might have been a lot more storms throughout the 20th century, the team reported. The models show that “aerosols have a bigger influence than we thought,” Dunstone says.

They also noticed that the number of storms increased in the 1980s and ’90s, possibly, they suggested, in response to U.S. and European clean air legislation — which abated air pollution and decreased aerosol concentrations over the Atlantic. 

The pronounced effect of aerosols, and their cleanup, is somewhat contrary to conventional wisdom regarding storm frequency, which suggests that storm frequency is primarily driven by natural climate variations, such as the Atlantic Multidecadal Oscillation. But this study “strongly calls into doubt still often-cited claims that recent increases in Atlantic [storm] activity can be attributed to an internal climate oscillation,” says Michael Mann, a climate scientist at Penn State University who was not involved in the study. 

There certainly could be natural oscillation going on, Dunstone says, but there may be a “larger role for humans than previously thought.”

While aerosols were the dominant factor in keeping down the number of hurricanes for much of the 20th century, their concentrations are now decreasing. “Looking to the future, [Dunstone and his colleagues] show that warming resulting from greenhouse gas emissions may outweigh any influence of anthropogenic aerosol reductions on storm activity,” wrote Johannes Quaas, a climate scientist at the Institute for Meteorology at the University of Leipzig in Germany, in an accompanying commentary in Nature Geoscience.

Over the short term, Dunstone and his team reported, greenhouse gases are likely to increase temperatures and thus potentially storm activity.

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