Climate change worsening cities' air

The threats from climate change are many: extreme weather, shrinking snowpack, altered ecosystems and rising and more acidic seas, to name a few.

Another lesser-known issue may hit especially close to home for city dwellers. In the world’s already smoggy metropolises, pollution is likely to grow worse, a phenomenon scientists have taken to calling the climate penalty. Ozone is a key culprit.

This lung-damaging compound, often formed from chemical reactions involving sunlight and automobile exhaust and other pollution, plagues major cities around the globe. As the climate heats up, it is projected that more ozone will form in polluted areas on sweltering days. “You have a hot summer, you’re going to get a lot of ozone,” said Daniel Jacob, a professor of atmospheric chemistry and environmental engineering at Harvard.

The explanation lies in chemistry. Ozone, formed by a sunlight-aided reaction of volatile organic compounds with nitrogen oxides, is created more quickly at higher temperatures, as was evident during the European heat wave of 2003. Climate change will also make the air more stagnant in some areas like the East Coast of the United States, Dr. Jacob said, because with the Arctic getting warmer more quickly than the tropics, air circulation between those two regions will slow.

In a warmer world, plants may also produce more emissions that are precursors to ozone.

In a 2009 paper in the journal Atmospheric Environment, Dr. Jacob and another researcher found that “climate change alone will increase summertime surface ozone in polluted regions by 1-10 parts per billion over the coming decades, with the largest effects in urban areas and during pollution episodes.”

Climate penalty

But the projections for ozone are not uniformly bad. Scientists predict that the climate penalty will mainly affect already polluted cities, where ozone is formed locally. But because a warmer climate means more airborne water vapor, which can dismantle ozone through a series of chemical reactions, the background level of ozone — that not created by man — at the earth’s surface is expected to fall. This means that sparsely populated areas, which produce less pollution, may escape the climate penalty.

In Europe, for example, southern areas are expected to see climate change lead to higher ozone, whereas the thinly populated Nordic region could feel no impact or even see improvements, according to Joakim Langner, an associate professor at the Swedish Meteorological and Hydrological Institute. In China, a similar regional split is expected to emerge.

Eastern China, home to megacities like Shanghai and Beijing, is likely to see an increase in ozone problems, whereas western China can expect lower levels, scientists project. The ozone in western China is largely produced elsewhere, allowing water vapor in the atmosphere an opportunity to dismantle the ozone through a series of chemical reactions.

The phenomenon of an increase in climate-linked ozone is likely to hold true for other big, polluted Asian cities, like those in Japan or India, said Amos Tai, an assistant professor in the Earth System Science Program at the Chinese University of Hong Kong.

Scientists are most certain of the trajectory of ozone, but other health-damaging air pollutants may also be affected by climate change. Attention is turning to soot and other fine particles, which can lodge in the lungs and cause long-term harm.In areas prone to drought, the climate may also worsen soot-spewing wildfires.

In China, where fine particles are already a huge problem for big cities, a change in winter monsoon patterns could bring less clean air from eastern Siberia, and “that would favour more pollution over eastern China,” said Yuxuan Wang, an assistant professor of atmospheric chemistry at Texas A&M University, Galveston. A changing monsoon could also affect South Asia’s pollution distribution, she said.

However, the future of monsoon patterns is uncertain under existing climate models, Dr. Wang said.

Even on ozone further study is needed, said Ruth Doherty, a reader in atmospheric sciences at the University of Edinburgh in Scotland. “Ozone is still a key area of research in the context of climate change impacts because it is strongly coupled to meteorological conditions and hence climate,” she said.

But with the threat of a climate penalty looming, of course, added impetus is on the world’s nations to reduce their emissions of air pollutants from factories and motor vehicles.