By Deborah Zabarenko, Environment Correspondent

WASHINGTON, July 25 (Reuters) - The affects of greenhouse gas ozone, which has been increasing near Earth's surface since 1850, could seriously cut into crop yields and spur global warming this century, scientists reported on Wednesday.

Ozone in the troposphere -- the lowest level of the atmosphere -- damages plants and affects their ability to absorb carbon dioxide, another global warming gas whose release into the atmosphere accelerates climate change, the researchers wrote in the journal Nature.

While carbon dioxide is blamed for global warming, it also has a beneficial effect on plant growth, and ozone counteracts this effect, said Stephen Sitch, a climate researcher at Britain's Met Office, which deals with meteorology.

"As CO2 (carbon dioxide) increases in the atmosphere, that stimulates plant growth," Sitch said by telephone. He noted that many scientific simulations that predict the impact of global warming have included this effect but "they haven't included the other effect, the negative effect of ozone damaging productivity."

Plants and soil currently slow down global warming by storing about a quarter of human carbon dioxide emissions, but that could change if near-surface ozone increases, the researchers said.

Projections of this rise in ozone "could lead to significant reductions in regional plant production and crop yields," they said in a statement.

Carbon dioxide's fertilizing effect can be powerful, Sitch and his colleagues reported, pushing global plant productivity by 88.4 billion tons (tonnes) a year.

This figure does not take into account the depressing effect of ozone; with that factored in, the fertilizing power of carbon dioxide is 58.4 billion tons, the scientists wrote.

Without accounting for increased ozone, earlier simulations have underestimated the amount of carbon dioxide that will remain in the atmosphere, Sitch said.

Ozone's damaging effect on plants means they will suck up less carbon dioxide from the atmosphere, leaving more of this chemical to contribute to greenhouse warming, he said.

"Carbon dioxide is the largest greenhouse warming gas but ... (ozone) is reducing plant productivity by an appreciable amount," Sitch said.

Ozone has doubled since the mid-19th century due to chemical emissions from vehicles, industrial processes and the burning of forests, the British climate researchers wrote. Carbon dioxide has also risen over that period.

Unlike carbon dioxide, which is directly caused by these human-spawned emissions, ozone is a so-called secondary air pollutant, produced by reactions with other chemicals like nitrogen oxide and carbon monoxide.

Tropospheric ozone is different from stratospheric ozone, which contributes to a protective layer high above Earth's surface that guards against harmful solar radiation.

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