Ozone (O3) is bad for people and plants. When lingering near the top of the atmosphere, the O3 molecule provides cover from energetic radiation but closer to the ground it can choke lungs and blight leaves. In addition, O3 is a greenhouse gas, helping to trap heat and warm the earth, and new research shows that it plays an even larger role in global warming by destroying plants' ability to use extra carbon dioxide.

Climatologist Stephen Sitch of the Met Office Hadley Centre for Climate Prediction and Research in Exeter, England, and his colleagues used a climate model to examine the impact of rising O3 levels. "If ozone continues to increase, vegetation will take up less and less of our carbon dioxide emissions, which will leave more CO2 in the atmosphere, adding to global warming," Sitch says.

Ground-level O3 produced by the burning of fossil fuels chokes plants, which "breathe" by inhaling CO2 through stomata—pores on their leaves —and exhaling oxygen. But O3 can also get into a plant through these stomata and, once in, shutter the opening. In fact, ground-level O3 is blamed for as much as $3 billion in crop damages annually in the U.S.

The researchers plotted O3 levels that would increase to an average of 40 parts per billion (ppb) everywhere—and 70 ppb in Eurasia, North America, Brazil and East Asia by 2100—against CO2 emissions that would quintuple by that time. Because CO2 can aid the growth of plants—and close down stomata as well to protect against O3 invasion—it was unclear how vegetation worldwide would respond to such an increase in emissions of both CO2 and O3.

While plants did grow more under these conditions, they did not grow as much as they would have in the absence of higher O3 levels, the researchers report in this week's Nature. In effect, O3 delivers global warming via two routes: the 0.35 watt-per-meter-squared (w/m2) extra heat it traps directly and the as much as 15 percent less vegetation that grows worldwide as a result of O3 damage. This translates into more CO2 in the atmosphere and therefore more warming, with estimates ranging from 0.62 w/m2 to 1.09 w/m2. "The indirect effect makes ozone the second most important anthropogenic greenhouse gas after carbon dioxide," Sitch says.

The researchers have predicted that increasing smog would prevent as much as 263 billion metric tons of carbon from being taken out of the atmosphere by plants over the past and coming century, though this depends on how tropical plants respond to O3 pollution. Studies have only been conducted on the effects of O3 on temperate plants thus far. "The CO2 fertilization of photosynthesis, which seems to be largely responsible for the global land carbon sink, could be heavily suppressed by O3 increases," Sitch notes, especially if tropical plants are as sensitive as temperate ones to smog. And that makes controlling O3 pollution from fossil fuel burning as important for climate change as it is for human and plant health.