Carbon sinks threatened by increasing ozone

Pollutant poisons plants and hampers photosynthesis.

Beijing smog is a sign of rising ozone pollution. Credit: R. KRAUSE/REUTERS

Rising levels of ozone pollution over the coming century will erode the ability of plants to absorb carbon dioxide from the atmosphere, a new climate-modelling study predicts.

Ozone is already known to be a minor greenhouse gas, but the new calculations highlight another, indirect way in which it is likely to influence global warming by 2100. High levels can poison plants and reduce their ability to photosynthesize, says Stephen Sitch of the UK Met Office's Hadley Centre for Climate Prediction and Research in Exeter.

Whereas ozone high in the stratosphere shields Earth from solar ultraviolet rays, high levels of ozone closer in are toxic to plants and animals. Ozone is generated when oxides of nitrogen — chiefly from vehicle exhausts and fossil-fuel power stations — react with other chemicals in the air.

Many of the world's most polluted areas routinely endure ozone concentrations higher than 40 parts per billion, enough to damage plant life. By the end of the century, virtually all the world's populated areas are predicted to be above this threshold.

Plant growth is a vital carbon sink, estimated to suck up around a quarter of the CO₂ emitted into the atmosphere. “This study combines knowledge about ecosystem function with atmospheric chemistry, and that's never been done before,” says Sitch. “No one has considered the detrimental effect of ozone on plants.”

The issue is complicated by the intricate interplay between rising CO₂ levels, which boost plant growth, and increasing ozone, which stunts it. Some ecosystem models had predicted that the rise in CO₂ expected over the coming century might be offset by the expected boost in the growth of forests and swamps. But factoring in ozone means that it looks less likely that this carbon sink will grow fast enough to keep pace with the increasing emissions.

Eva Pell, who studies the effects of ozone on plants at Pennsylvania State University in University Park and who was not involved in the study, finds the results credible. “There is no doubt that ozone reduces CO₂ fixation,” she says. “It makes sense that the adverse effects of ozone would be dampened by elevated CO₂, and the ability of plants to serve as carbon stores in an elevated CO₂ environment would be reduced by elevated ozone.”

In 1901, according to the researchers' calculations, plant growth was responsible for storing 113 billion tonnes of carbon worldwide. By 2100, this figure is predicted to be 171 billion tonnes — without ozone it would be more than 200 billion tonnes, they say. Their results are published online this week (S. Sitch et al. Nature doi:10.1038/nature06059; 2007).

Different plant species vary in their sensitivity to ozone, and these figures reflect predictions based on a fairly high average level of toxicity, says team member Bill Collins of the Met Office. But even in the researchers' low-sensitivity calculations, there's likely to be a dent of around 15 billion tonnes in overall carbon sequestration as a result of the effects of ozone.

“The bottom line is that ozone is a greenhouse gas, so it's known to contribute to the greenhouse effect. Our study says you should double that predicted contribution,” says Collins.

Unlike most greenhouse gases, ozone is a short-lived, regional pollutant that can be tackled at an individual level using catalytic converters, for example, to reduce precursors to ozone, Sitch suggests.