While hotter temperatures and increases in carbon dioxide from fossil fuels could help vegetation in northern temperate regions, those changes would be undermined by damage to world crops from higher ozone levels, the researchers said.

Levels of ozone, a form of oxygen that pollutes the atmosphere, has been growing near Earth's surface since 1850.

Without curbs on emissions, growing fuel combustion worldwide will push global average ozone up 50 per cent by 2100, said the Massachusetts Institute of Technology scientists whose research was published in November's journal Energy Policy.

'That increase will have a disproportionately large impact on vegetation,' the study said.

The work follows a British study published in the journal Nature in July that said 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 economic cost of the damage will be moderated by changes in land use and by agricultural trade, with some regions more able to adapt than others. But the overall economic consequences will be considerable,' the study said.

A mix of economic, climate and agricultural computer models shows that ozone levels tend to be highest in regions where crops are grown, said Dr John Reilly, associate director of MIT's Joint Program on the Science and Policy of Global Change. -- REUTERS