Boston, Oct 30: The spread of ozone, a greenhouse gas, could inflict serious damage on vegetation in many places, cutting up to 12 percent off the value of global crops by 2100 and hurting the world economy, a study said.
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 yesterday.
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 John Reilly, associate director of MIT's Joint Program on the Science and Policy of Global Change.
During photosynthesis -- the process through which plants absorb energy and produce oxygen -- carbon dioxide enters plants through tiny pores called stomata.
''When crops are fertilized, their stomata open up, and they suck in more air. And the more air they suck in, the more ozone damage occurs'' in the crops, Reilly said. ''It's a little like going out and exercising really hard on a high-ozone day.'' The effects of ozone typically include yellowing leaves, mottled markings or a bronzed appearance.
Greenhouse gases such as carbon dioxide are widely blamed for global warming. Scientists say average temperatures will rise by between 2-6 degrees Celsius by the end of the century, causing droughts, floods and violent storms.
Without emissions' curbs, forest and pasture yields will decline slightly or in some cases grow because of the warmer climate and carbon dioxide effects, but crop yields would fall by nearly 40 per cent worldwide, the study said.
That does not translate directly into economic losses. The world would adapt by expanding the amount of land for crops, Reilly said.
But the cost of doing so would shave 10-12 per cent off the total value of crop production, he added.
The damage varies by region. Reilly estimated the United States, China and Europe would need to import more food, while supplying those imports would benefit tropical countries.
According to the July study in Nature, ozone has doubled since the mid-19th century due to chemical emissions from vehicles, industrial processes and the burning of forests. 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.