By Paul Rincon
Science reporter, BBC News
Ozone could be a much more important driver of climate change than scientists had previously predicted, according to a study in Nature journal.
Ozone is an important component of smog
The authors say the effects of this greenhouse gas - known by the formula O3 - have been largely overlooked.
Ozone near the ground damages plants, reducing their ability to mop up carbon dioxide (CO2) from the atmosphere.
As a consequence, more CO2 will build up in the atmosphere instead of being taken up by plants.
This in turn will speed up climate change, say the Nature authors.
"Ozone could be twice as important as we previously thought as a driver of climate change," co-author Peter Cox, from the University of Exeter, UK, told the BBC News website.
Scientists already knew that ozone higher up in the atmosphere acted as a "direct" greenhouse gas, trapping infrared heat energy that would otherwise escape into space.
Ozone closer to the ground is formed in a reaction between sunlight and other greenhouse gases such as nitrogen oxides, methane and carbon monoxide.
Greenhouse emissions stemming from human activities have led to elevated ozone levels across large tracts of the Earth's surface.
This study is described as significant because it shows that O3 also has a large, indirect effect in the lower part of the atmosphere.
Research into ground-level ozone has tended to concentrate on its harmful effects on human lungs.
But the gas also damages plants, reducing their effectiveness as a "carbon sink" to soak up excess CO2 from the atmosphere.
Co-author Stephen Sitch, from the Met Office's Hadley Centre, said: "Calculations of the efficiency of land ecosystems to take up carbon would be less efficient than we thought previously."
Furthermore, Peter Cox said: "The indirect effect is of a similar magnitude, or even larger, than the direct effect."
There are uncertainties, Professor Cox admits; but he added: "Arguably, we have been looking in the wrong place for the key impacts of ozone."
Ozone enters plants through pores, called stomata, in the leaves. Interfering with the reactions involved in photosynthesis, it leaves the plants weakened and undersized.
However, efforts to determine how rising levels of ozone will affect global plant growth are complicated by other factors.
High levels of both CO2 and O3 cause stomata to close. This means they take up less of the carbon dioxide they need for photosynthesis, but also absorb less of the harmful ozone.
The researchers built a computer model to estimate the impact of predicted changes in ozone levels on the land carbon sink over a period running from 1900 to 2100.
This model was designed to take into account the effect of ozone on plant photosynthesis and the interactions between O3 and CO2 through the closure of pores.
They used two scenarios, depending on whether plants were deemed to have high or low sensitivity to ozone.
Under the high scenario, ozone reduced plant productivity by 23%; under the low scenario, productivity was reduced by 14%.
"It's an interesting effect, and I don't think it has been introduced into a coupled [computer] model before so that the overall effect can be seen," said Dr Nathan Gillett, from the Climatic Research Unit at the University of East Anglia, UK, who was not involved in the study.
The results may have implications for global food production, particularly in vulnerable areas.