tampabay.com

Scientists blame global warming on rice

By ASSOCIATED PRESS
Published May 2, 2007


BANGKOK, Thailand - As delegates to a climate conference here debate how to reduce greenhouse gases, one of the problems - and a possible solution - lies in the rice fields that cover much of Thailand, the rest of Asia and beyond.

Methane emissions from flooded rice paddies contribute to global warming just as coal-fired power plants, automobile exhausts and other sources do with the carbon dioxide they spew into the atmosphere.

In fact, the report from the Intergovernmental Panel on Climate Change meeting this week in Bangkok concludes that rice production was a main cause of rising methane emissions in the 20th century. It calls for better controls.

"There is no other crop that is emitting such a large amount of greenhouse gases, " said Reiner Wassmann, a climate change specialist at the International Rice Research Institute in the Philippines.

It's the bacteria that thrive in flooded paddies that produce methane, by decomposing manure used as fertilizer and other organic matter in the oxygen-free environment. The gas is emitted through the plants or directly into the atmosphere.

A molecule of methane is 21 times more potent than a molecule of carbon dioxide as a heat-trapping gas. Although carbon dioxide is still the bigger problem, representing 70 percent of the warming potential in the atmosphere, rising levels of methane now account for 23 percent, reports the U.S. Environmental Protection Agency.

After decades of atmospheric buildup, methane - also emitted naturally from wetlands and from other man-made sources, such as landfills and cattle farming - has leveled off in the past few years. Some scientists credit changes in rice production, and some also trace it to repairs in oil and gas storage facilities that can leak methane.

A 2005 study by U.S. scientists focused on China, which produces a third of the world's rice and where rice fields have shrunk by 24-million acres in the past decade as farmers shifted to other crops and abandoned marginal land. The study also found that nitrogen-based fertilizer has replaced manure, and many Chinese farmers are using less water on their fields.

For Asians, modifying rice production might prove easier and cheaper than some of the other fixes proposed in the IPCC draft report, such as switching from coal to solar, wind power or other renewable energy sources.

But despite the recent leveling off, the EPA projects that global methane emissions will rise again, as rice fields expand with growing populations.

"In the developing world, you really have to think first and foremost about providing population with food, " said Pete Smith of the University of Aberdeen in Scotland, lead author of the IPCC report's section on agriculture. "You can't start thinking about climate mitigation if you have to feed your family."