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 [see below].

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.

Here's the abstract of the article, "Indirect radiative forcing of climate change through ozone effects on the land-carbon sink" (sub. req.) by Sitch et al.:

The evolution of the Earth's climate over the twenty-first century depends on the rate at which anthropogenic carbon dioxide emissions are removed from the atmosphere by the ocean and land carbon cycles. Coupled climate–carbon cycle models suggest that global warming will act to limit the land-carbon sink, but these first generation models neglected the impacts of changing atmospheric chemistry. Emissions associated with fossil fuel and biomass burning have acted to approximately double the global mean tropospheric ozone concentration, and further increases are expected over the twenty-first century. Tropospheric ozone is known to damage plants, reducing plant primary productivity and crop yields, yet increasing atmospheric carbon dioxide concentrations are thought to stimulate plant primary productivity. Increased carbon dioxide and ozone levels can both lead to stomatal closure, which reduces the uptake of either gas, and in turn limits the damaging effect of ozone and the carbon dioxide fertilization of photosynthesis. Here we estimate the impact of projected changes in ozone levels on the land-carbon sink, using a global land carbon cycle model modified to include the effect of ozone deposition on photosynthesis and to account for interactions between ozone and carbon dioxide through stomatal closure. For a range of sensitivity parameters based on manipulative field experiments, we find a significant suppression of the global land-carbon sink as increases in ozone concentrations affect plant productivity. In consequence, more carbon dioxide accumulates in the atmosphere. We suggest that the resulting indirect radiative forcing by ozone effects on plants could contribute more to global warming than the direct radiative forcing due to tropospheric ozone increases.

This post was created for ClimateProgress.org, a project of the Center for American Progress Action Fund.