Ozone Pollution to Worsen Under Climate Change

Medical doctors argue over whether there is such a thing as a "safe" threshold for ozone, but the World Health Organization says sustained exposure to concentrations over 50 parts per billion is dangerous. The United States Environmental Protection Agency's standard for air quality is 75 parts per billion.

Ozone is not emitted directly from car tailpipes or power plant smokestacks. Instead, it forms when nitrogen oxides interact with other commonly emitted gases like methane, carbon monoxide and volatile organic compounds (VOCs) in the presence of sunlight. As a result, local concentrations vary a lot depending on how much pollution is in the immediate area.

This makes assessing the global effects of climate change on ozone difficult. Some projections for warming predict the planet will heat up by two to three degrees Centigrade by the middle of the century. Humidity is also due to increase as the planet warms, which typically suppresses ozone formation.

But other factors are also at work, Pavan Racherla of Carnegie Mellon University said. For instance, some plants naturally emit a VOC called isoprene. On its own isoprene is harmless, but when combined with nitrogen oxides it readily forms ozone. Warming temperatures will lead to large spurts in plant growth in some parts of the world, and isoprene emissions will add up.

Other parts of the globe are already drying out, and wildfires appear to be intensifying, adding yet another source of ozone to the mix. In 2007, severe wildfires roared through the United States, scorching 13 million acres of land, of which 1.3 million was in California. In a study published this week in Geophysical Research Letters, researchers found that the California fires tripled the number of times ozone in the region exceeded the EPA health standard.

And climate change projections for the 21st century forecast increasingly hot and dry weather for much of North America, exacerbating the threat of fire.

"With climate change and increasing temperature, the effect from wildfires will reach up in magnitude," Gabriele Pfister of the National Center for Atmospheric Research in Colorado, said. "You'll have an earlier snow melt, more drought and a longer fire season."

In Africa and Southeast Asia, for example, biomass burning has already degraded air quality in regions that can hardly afford to get much worse.

"Biomass burning in Africa, and biomass burning coupled with emissions from megacities in Southeast Asia routinely cause ozone episodes of 100 parts per billion and higher," Fowler said. He believes the growing threat is all the more reason to curtail all human-related pollutants as quickly as possible.

"Greenhouse gas reduction will also decrease the amount of oxidants like ozone in the atmosphere," he said. "These problems are all connected, but they are largely ignored by policy makers. We shouldn't pretend the connections don't exist."

Related Links:

World Health Organization

Global Warming: What You Need to Know

Discovery Earth Live