Saturday, March 16, 2002

By ROWAN HOOPER

Staff writer

Common tropical plants emit large amounts of methyl chloride, one of the most important natural ozone-depleting chemicals, according to researchers at the National Institute for Environmental Studies in Tsukuba, Ibaraki Prefecture.

Despite intense research into compounds that deplete ozone, scientists had been unable to account for a large proportion of methyl chloride in the atmosphere.

The Tsukuba team published its results earlier this week in the science journal Nature. They have accounted for the missing chemical and identified its source.

Working in the Tropical Rainforest Glasshouse in Tsukuba Botanical Garden, the team, led by Yoko Yokouchi, found that about 15 percent of atmospheric methyl chloride comes from certain types of ferns and tropical plants in a group called Dipterocarpaceae, teak trees common in Southeast Asian forests.

Their discovery balances the atmospheric books.

"The most likely main source of methyl chloride was previously thought to be oceanic emission, with biomass burning the second-largest source," Yokouchi said in an interview conducted by e-mail.

"But we measured methyl chloride in the tropical rain forest glasshouse and discovered high concentrations in the air. We realized that some of the plants were emitting methyl chloride."

The ozone layer, between 10 km and 50 km above the surface of the Earth, shields living things from the sun's ultraviolet radiation.

A decline in the thickness of the ozone layer has led to higher levels of UV rays reaching the Earth, causing cataracts and skin cancers, reducing crop yields and disrupting the marine food chain.

In the Carboniferous Period, some 330 million years ago, before flowering plants evolved, ferns dominated the environment. Yokouchi's work suggests that this prehistoric atmosphere was rich in methyl chloride.

"If the huge amount of ferns in the Carboniferous period also emitted methyl chloride, there would have been less stratospheric ozone," she said.

Scientists estimate that if humans stop releasing chlorofluorocarbons that destroy the ozone layer, ozone levels will return to natural levels by 2050.

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