Global warming means smoggier autumns in U.S. Southeast

GAINESVILLE, Fla. -- The drier, warmer autumn weather that’s becoming more common due to climate change may extend summer smog well into the fall in the Southeastern U.S. in the years ahead, according to a study published on Monday.

Research published in the Proceedings of the National Academy of Sciences also suggests a culprit for the smog that many people might not expect: It’s the lush woodlands that give much of the South a lovely green canopy. That’s because of a natural defense mechanism trees use to protect their leaves from drought conditions.

And since climate models predict more hot, dry Octobers in coming decades, we should expect these late-season smog, or ozone, events to happen more often, according to climatologists at the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology.

“It’s not going to happen every year, but when it happens it will be worse than in the summer time,” said Yuhang Wang, who co-authored the study with Yuzhong Zhang. “We are likely to have record ozone days in the fall, and we need to prepare for that.”

The research isn’t conclusive, but it presents enough evidence to raise concerns about the indirect consequences of global warming, said other scientists who weren’t involved in the study.

“Climate change​ is about more than sea ice and polar bears,” Laura Rehrmann, a fellow at the Center for Climate and Energy Solutions, said in an email. “It is also about whether the air will be safe so your children can go play outside.”

The climatologists​ looked deeply into a rare October smog spike in the Southeast in 2010. That was a drought year, with unseasonably high fall temperatures, and smoggier in October than it had been in July, when smog usually peaks.

Discovering what caused that spike was important to public health: A longer season for smog raises health risks for millions of people, especially the elderly and children whose lungs are more vulnerable. Smog also can stunt crops.

It has been known for decades that trees and other plants send more smog-causing emissions​ into the atmosphere than the pollution humans send directly from tail and factory pipes. This study points to an indirect consequence of the global warming humans have caused.

During hot Southern summers, trees usually have humidity to help them cope. But after the humidity breaks in the fall, they’re more vulnerable to hot days, which used to be more rare, they found.

The October 2010 smog spike, seen in bright red and orange splotches on an air pollution​ map, confused Wang and Zhang at first.

“We used to do all this research focusing on the summer, and this October case was very intriguing because we didn’t expect to see the red and oranges in the Southeast, with not nearly as much in the Northeast and in California,” Wang said.

Moreover, smog-causing emissions from cars and industry have actually declined in recent decades in the Southeast and elsewhere because of tighter emissions requirements under the Clean Air Act​. So why were the region’s ozone numbers that month higher than nearly everywhere else in the nation?

They looked at all the sources they could think of, and couldn’t find the culprit until they turned to the trees. Trees emit isoprene to protect their leaves during warm, dry weather. Isoprene combines with sunlight to produce smog. Only when they plugged this factor into their computer simulation did the results add up.

Chris Field, director of the Department of Global Ecology at the Carnegie Institution for Science at Stanford University, said the study doesn’t prove the cause of the October 2010 smog spike, but it does suggest that climate change threatens air quality in ways few people expected.

“The argument is that climate change affects the trees, the trees release a chemical that may help them cope with harsh conditions, and the chemical produced by the trees leads to an increase in ozone,” Field said in an email.

“For me, the most interesting feature of the results is something we see quite often, that the risks of damages from a changing climate​ often come through indirect mechanisms.”