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.
"Methane emissions are unique to rice," he said. "If Asian countries are exploring possibilities to reduce greenhouse gas, they have to look at rice production. I'm not saying it's the biggest source, but in Asia it's a source that cannot be neglected."
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 manmade 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.
Wassmann said few countries have followed China's example, instead ignoring such solutions as periodically draining their fields or shifting to locations that need less water.
Scientists say such measures pose the same challenge for poor countries as proposals to introduce environmentally friendly tilling methods or capping methane from livestock manure: Farmers often lack the funds and know-how to shift away from techniques in use for generations.
"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."
Thailand, the world's largest rice exporter, shows both the promise and limitations of trying to make the industry greener.
Most large mills here burn leftover rice husks for power — a more climate friendly source than coal or oil — and are increasingly selling excess power back to the state.
"Instead of letting it rot in the fields and produce bad gas, we burn it and make use of it," said Rut Subniran, executive chairman of the Patum Rice Mill and Granary outside Bangkok. "This is good for the country because it can reduce our oil imports. It's good for the environment."
But a few miles away, impoverished rice farmers have largely ignored government calls to periodically drain their fields to help reduce methane emissions.
Busy harvesting the latest crop, some blamed tradition and habit, but others said draining the fields was just too costly.
"The government has told us how rice paddies release methane," said Adisak Wantayachiwa, who farms 28 acres north of Bangkok. But most farmers "don't want to pay the cost of draining their fields," he said. "They would just rather keep them flooded."