Its authors fear a major factor in the climate-change equation has been badly overlooked.

"Carbon sinks" -- the famous ability of vegetation to absorb carbon dioxide (CO2), the principal greenhouse gas -- are being damaged by ozone, they say.

As a result, more CO2 will build in the atmosphere instead of being taken up by the land, which in turn will stoke global warming and thus worsen climate change.

In the stratosphere, a thin, naturally-occurring level of ozone is a vital shield for life on Earth, providing a shield against DNA-damaging ultraviolet.

But at ground level, it is a man-made pollutant, brewed in a reaction between fossil-fuel gases and sunlight.

Ozone has long been known to be a risk to health by damaging the airways, but recent research has also highlighted its damaging effect on vegetation.

The gas enters plants through respiratory pores, called stomata, in the leaves. It then produces byproducts that crimp efficiency in photosynthesis, leaving a plant that is weak and undersized.

Efforts to figure out how fast-rising levels of ozone will affect forests have been hampered by a nasty confounding factor.

High levels of CO2 and ozone cause stomata to close, which means the plant takes in less of the CO2 that it needs to grow -- but also less of the ozone that damages it.

Published in the British scientific journal Nature, the new study seeks to unravel these intertwining factors.

British researchers built a computer model to simulate the response of carbon sinks around the world in response to ozone levels, on a timescale running from 1901 to 2100.

They used two scenarios, depending on whether plants were deemed to have high or low sensitivity to ozone.

These scenarios were vetted for reliability by comparison with an experiment in which trees and shrubs in a Swiss field were exposed to artificially high levels of CO2 and ozone for seven years.

Under the "high" plant-sensitivity scenario, ozone diminished land carbon capture by a massive 23 percent over the two centuries. Under the "low" scenario, the fall was 14 percent.

Lead researcher Stephen Sitch of the Hadley Centre, part of Britain's Met Office, told AFP that the study did not estimate the effect of ozone for the 21st century specifically.

But, he said, it was clear that there would be a major contributory effect to global warming by 2100 as less airborne CO2 will be captured by the land.

"Existing calculations of the carbon cycle haven't factored in the negative effect of ozone," he said.

A rough calculation is that ozone could indirectly add "somewhere in the range of 0.5 to 1.25 degrees Celsius" (0.9-2.25 degrees Fahrenheit) in warming, according to Sitch.

By comparison, global surface temperatures rose by 0.74 C (1.33 F) from 1906 to 2005, eroding glaciers and alpine snow cover and forcing permafrost into retreat, according to the latest report, issued this year, by the UN's Intergovernmental Panel on Climate Change (IPCC).

By 2100, global average surface temperatures could rise by between 1.1 C (1.98 F) and 6.4 C (11.52 F) compared to 1980-99 levels, the IPCC calculates.

But this prediction is based on concentrations of greenhouse-gases and does not factor in the indirect effect of ozone.

Unlike CO2, which spreads around the planet's atmosphere, ground-level ozone pools nearer to its source, with North America, Europe, China and India high on the list of polluted regions.

In pre-industrial times, ozone was 17 parts per billion (ppb). Today, it is 35 ppb and is on course for 54 ppb by the end of the century, said Sitch.

Damage to plants starts to occur from 40-50 ppb but the levels vary greatly depending on the season, local topography and weather. Observational research in the US grainbelt has found spikes as high as 120 ppb.

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