It's not often that the pollution generated by industrial activity can be cast in a positive light, but a recent study suggests a potential benefit of the asthma-causing, air-clouding aerosols released by burning fossil fuels.
In the study, published yesterday in the journal Nature Geoscience, researchers suggest that aerosols released in the 20th century decreased the frequency of tropical storms in the North Atlantic Ocean. They also show that when the emissions of aerosols were reduced, the frequency of such storms increased.
Researcher Nick Dunstone, of England's Met Office Hadley Centre, explained how such tiny particles can influence major storm systems like those in the north Atlantic.
Aerosols, because they reflect radiation, can have a cooling effect on the Earth. A classic example of this is the cooling that can occur for a season after a major volcanic eruption.
They also cool in other ways, noted Dunstone. "When aerosols interact with clouds, they basically make them brighter and live longer," he said.
Such aerosols, released by fossil fuel burning as the United States and Europe industrialized in the late 19th and 20th centuries, cooled off the north Atlantic Ocean. This cooling caused the Hadley circulation, which circulates air around the equator, to shift southward.
"The Hadley circulation moves toward the warmer atmosphere, which is, in this case, the south," said Dunstone. "And basically, this sets up unfavorable conditions in the tropical north Atlantic which don't favor hurricane formation." Industrial activity goes up, storms go down
Dunstone's paper shows graphs that roughly correlate decreased periods of tropical storm activity with rapid industrial buildups, such as the pre-World War I years or the post-World War II economic expansion.
The reverse is also true with increased periods of North Atlantic tropical storms during depressions and after the passage of air quality regulations.
Since 1990, aerosol levels in the region have decreased as both the United States and Europe have implemented more stringent pollution controls. The Hadley circulation has moved northward, and storm activity in the north Atlantic has increased.
Dunstone was interested in aerosols' effect on tropical storms because, decade to decade, the frequency of such storms changes a lot. Scientists did not know whether this was due to natural variability or some other factor.
By comparing model simulations with aerosol emissions against simulations without aerosol emissions, the researchers were able to see that aerosols appeared to be the main force behind tropical storm frequency.
"We are sort of raising the possibility that this might be an important mechanism," Dunstone said.
Pier Luigi Vidale, a climate scientist at the University of Reading, said the study brought together "an impressive range of scales and phenomena."
"We cannot explain the climate record without taking into account both natural variability and anthropogenic influences. This study allows us to better understand the role of each," Vidale said.
Johannes Quaas, a meteorologist at the University of Leipzig in Germany who reviewed the paper, said the researchers provided "convincing evidence for a significant influence of anthropogenic aerosols on tropical storm activity over the north Atlantic in the 20th century." What new computer models need to know
Aerosols have been linked to other changes in climate patterns, too, Quaas noted. Researchers have previously reported a connection between dust outbreaks in the Sahara and reduced hurricane activity in the north Atlantic. Aerosols have also been fingered as a cause for drought in Africa's Sahel region in the 1970s and 1980s, and for weakening the Indian monsoon.
Dunstone pointed out that eventually, as the Earth warms, greenhouse gas emissions may overtake aerosols as a main force behind the frequency of tropical storms. Some studies have suggested that in the future, climate change might lead to fewer, but more intense, Atlantic hurricanes.
The researcher also noted that since Asian countries are experiencing such rapid industrialization, studying aerosols' impact in the Pacific Ocean could be a more urgent area of research.
"Especially the impact of Chinese aerosols and Indian aerosols on tropical cyclones in Japan and the west Pacific, that might be a quite interesting thing to look at," he said.
Quaas cautioned that this sort of statistical attribution could benefit from verification with future modeling efforts. Higher-resolution models might be able to offer additional support for the hypothesis, although running a model at such a fine scale would require significant computing power.
Dunstone agreed, saying he would like to extend the work with a new, higher-resolution model.
"It's really important for us to say that these processes are very uncertain," he said. "Even trying to represent aerosols in models is very difficult. Trying to represent clouds in models is very difficult. And then you try and put these two really uncertain things together."
Reprinted from ClimateWire with permission from Environment & Energy Publishing, LLC. 202-628-6500.E&E Publishing is the leading source for comprehensive, daily coverage of environmental and energy issues. Click here to start a free trial to E&E's information services.
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