CO2 increase boosts methane emissions.

Plants around the world can slow the buildup of carbon dioxide pollution by absorbing tons of this greenhouse gas. But this apparent blessing comes at a price. The fertilizing effects of carbon dioxide on plants can cause some regions to increase emissions of methane, an even more potent heat-trapping gas.

These conclusions emerge from a study of wetland plants performed at the Smithsonian Institution's Environmental Research Center in Edgewater, Md. It suggests that wetlands and similar environments could be amplifying the greenhouse power of carbon dioxide gas, says John W.H. Dacey of the Woods Hole (Mass.) Oceanographic Institution. Dacey, Bert G. Drake of the Smithsonian, and Michael J. Klug of Michigan State University report their results in the July 7 NATURE.

For the last 8 years, Drake has studied how increasing carbon dioxide alters the growth of plant species in specially designed enclosures at the Smithsonian center near the Chesapeake Bay. Half the chambers have carbon dioxide concentrations of 690 parts per million, almost twice the atmosphere's current amount. The other enclosures have regular air and serve as controls.

During July 1991, Dacey measured the amount of methane escaping from the plots of land in the enclosures. He found that chambers enriched with carbon dioxide produced 80 percent more methane gas on average than the control chambers.

Methane in wetlands comes from soil bacteria that consume organic plant matter. Because carbon dioxide stimulates plant growth, wetland sedges in the experimental enclosures produced more organic matter for the methane-producing bacteria than did the plants in the control chambers.

"This study confirms that elevated carbon dioxide concentrations are likely to have quite complex and far-reaching effects on ecosystems," Drake says.

L. Hartwell Allen, a crop physiologist with the U.S. Department of Agriculture in Gainesville, Fla., finds similar methane enhancements in studies of rice. Plots growing in double the current concentration of carbon dioxide emitted roughly twice as much methane as did those subjected to lower carbon dioxide amounts, he told SCIENCE NEWS.

Although they cover only a limited part of the globe, wetlands and rice paddies account for 30 to 40 percent of methane emissions into the atmosphere. By enhancing methane production in these environments, the buildup of carbon dioxide could significantly boost atmospheric concentrations of methane, Dacey says.

Indeed, methane amounts are rising, having more than doubled since preindustrial times. Researchers blame most of this increase on coal mining, use of natural gas, rice cultivation, raising of cattle and sheep, and burning of vegetation. But the new study suggests that the buildup of carbon dioxide may have caused roughly 15 percent of the methane increase by stimulating growth in wetlands and rice paddies, says Dacey.

Climate experts are concerned about methane because each molecule can trap about 20 times more heat than carbon dioxide can.