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Rice: methane risk rises.

Climate change analysts have estimated that rice paddies contribute about 14 percent of the methane -- a major greenhouse gas -- emitted into the atmosphere by human activities each year. But that calculation, based largely on Western data, may substantially underestimate the global warming threat posed by rice cultivation.

A new study suggests that Chinese fields--which produce about 185 million tons of rice annually, or 36 percent of the world's total -- may generate four to 10 times more methane than U.S. or European fields.

Scientists from the Oregon Graduate Center in Beaverton and Academia Sinica's Institute of Atmospheric Physics in Beijing, China, collaborated on a two-year study of methane emissions from rice paddies in TuZu, a rural village in the heart of the Sczhuan province. Alternating between morning and afternoon sampling periods, the researchers measured methane emissions from six sites in four separate fields every other day throughout two 120-day growing seasons. The roughly 13,000 separate readings indicate that the TuZu paddies emit roughly 60 milligrams of methane per square meter each hour.

This overall methane emission rate is five times higher than that of U.S. rice fields, 3.75 to 8.5 times higher than Italian fields and 15 times higher than Spanish fields, report M.A.K. Khalil and his coauthors in the May Environmental Science & Technology. The reasearchers note, however, that methane fluxes vary considerably throughout the growing season. The TuZu fields emitted about 15 percent more methane in the afternoon than during the morning, for instance, and their offgassing rate tended to increase as the growing season progressed. The rates plummeted once the plans reached maturity and outdoor temperatures fell.

As the collaborators continue their studies, they hope to tease out the extent to which climate, soil conditions, fertilizer type and rice variety affect a paddy's methane releases.
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Title Annotation:rice cultivation produces methane, contributing to global warming
Publication:Science News
Date:May 18, 1991
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