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Residents of bulging metropolises around the world should brace for an increase in stagnant, polluted air that hangs around for days as a result of climate change-related shifts in wind and rainfall patterns, according to a new study.

The findings highlight one way global warming can compromise human health, which is a major thrust behind the U.S. Environmental Protection Agency's recently proposed plan to curb power plant carbon emissions 30 percent by the year 2030, said Janice Nolen, an assistant vice president at the American Lung Association in Washington.

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Poor air quality is related to a range of heart and lung complications that the World Health Organization estimates cause 3.7 million premature deaths a year.

"As we are doing pollution planning, we have to begin to make sure that we are including the climate change impacts," said Nolen, who was not part of the new study but was asked to comment on its implications.

The research should keep the pressure on cities such as Los Angeles, Phoenix, and Salt Lake City to clean up pollutants from cars, trucks, and wood-burning stoves — the sources of ozone and tiny airborne particles that make air unhealthy to breathe, she said.

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"Air stagnation by itself is not necessarily a bad thing for public health, for us," said Daniel Horton, a research fellow in earth system science at Stanford University in California and the study's lead author. "You still need the pollutant component in there."

What is air stagnation?

Stagnant air is a natural meteorological phenomenon driven by a convergence of three atmospheric patterns: light winds near the surface, light winds higher in the atmosphere, and a lack of rain, Horton explained. Without rain or wind, the tiny particles that sear lungs, clog arteries, and turn eyes red hang in the air.

Horton and his colleagues fine-tuned an ensemble of global climate models to accurately simulate air stagnation events in the past and then ran them to get a view of air stagnation as this century progresses under a so-called business-as-usual scenario in which greenhouse gases continue to accumulate in the atmosphere at the current rate.

The models project that increased air stagnation by the end of the 21st century will affect areas covering about 55 percent of the world's population. Some regions will see up to 40 additional days a year of stagnant air.

"Potential impacts over India, Mexico, and the western US are particularly acute owing to the intersection of large populations and increases in the persistence of stagnation events, including those of extreme duration," Horton and colleagues write in the journal Nature Climate Change.

Los Angeles air quality to worsen

The models used in the new study, Horton said, lack the fine-scale resolution needed to describe impacts to specific metropolitan regions, where local features such as the mountains influence weather patterns.

The findings do, however, dovetail with preliminary results from a project that is examining climate change impacts on Los Angeles, a city known for poor air quality, according to Alex Hall, a climate scientist at the University of California, Los Angeles.

His group's work shows air temperatures in the Los Angeles region increase more aloft than at the surface, which creates a "lid" on the air near the surface. "Emissions of pollutants into this pool of air would become more concentrated, reducing air quality," he said in an email.

"Some version of this story would occur throughout the world, especially in the tropics and subtropics" as predicted by Horton and his colleagues, he said.

To lessen the impact, Horton said, "Limiting the amount of carbon dioxide emitted would certainly help."