BANGKOK (Reuters) - Across the globe, chickens and pigs are doing their bit to curb global warming. But cows and sheep still have some catching up to do.
The farm animals produce lots of methane, a potent greenhouse gas that gets far less public attention than carbon dioxide yet is at the heart of efforts to fight climate change.
Government policies and a U.N.-backed system of emission credits is proving a money-spinner for investors, farmers and big polluters such as power stations wanting to offset their own emissions of greenhouse gases, particularly carbon dioxide (CO2).
The reason is simple: methane is 23 times more potent than carbon dioxide in trapping heat in the atmosphere and it is relatively simple to capture the gas from animal waste, landfills, coal mines or leaky natural gas pipes.
"A fifth of all greenhouse gas-induced global warming has been due to methane since pre-industrial times," said climate scientist Paul Fraser of Australia, where ruminant farm animals belch out vast amounts of the gas.
Methane concentrations have increased about 150 percent in the air since 1750 and now far exceed the natural range of the past 650,000 years, the U.N.'s climate panel says. And human activities are largely to blame.
The panel will be focusing on ways to curb methane and other greenhouse gas emissions when it releases a major report on mitigating the effects of climate change in Bangkok in early May.
"It's been argued that the reductions from methane are potentially cheaper than from carbon dioxide," said Bill Hare, climate policy director for Greenpeace and a lead author of the mitigation report.
"A lot of policy discussion in the United States has focused on methane rather than more difficult problems such as CO2 from coal," he added.
This is because capturing methane from landfills, mines, or from fossil fuel production or natural gas lines is pretty straight forward and makes economic sense. Methane is a major component of natural gas and can be burned to generate power.
Agriculture was a greater challenge, Hare said.
"There are more difficult areas for methane from livestock and from rice agriculture where, at best, longer time scales are required to change practices in agriculture than you might need in industrial areas," Hare said.
Rice paddies and other irrigated crops produce large amounts of methane, as do natural wetlands. Vast amounts of methane are also locked up in deposits under the ice in sub-polar regions, in permafrost or under the sea.
Hare said there are lots of options being looked at, such as additives for cattle and sheep to cut the amount of methane in their burps and moving away from intensive livestock feed lots to range-fed animals.
"And for example in rice, just changing the timing and when and how you flood rice paddies has great potential to reduce methane emissions."
For the moment, the amount of methane in the atmosphere is steady after leveling off around 1999, said Fraser, leader of the Changing Atmosphere Research Group at Australia's government-funded Commonwealth Scientific and Industrial Research Organization.
This is thought to be because the drying out of tropical wetlands seems to canceling out a rise in emissions from the oil and gas industry. But how long this lasts is anyone's guess.
"Most people would agree that some time in the future methane is going to start growing again, just because of the world demand for natural gas, rice and cattle," Fraser said.
All the more reason why chicken manure and pig waste are hot commodities.
Under the U.N.'s Kyoto Protocol, a system called the Clean Development Mechanism allows rich countries to keep within their emissions limits by funding projects that soak up greenhouse gases in poor countries, getting carbon credits in return.
This has made huge pig farms in South America and poultry farms in India attractive investments.
The waste is put into digesters and the methane extracted and burned to generate electricity or simply flared to create CO2 -- not perfect, but a lesser greenhouse gas evil.
And interest is growing in these kinds of projects, said N. Yuvaraj Dinesh Babu of the Singapore-based Carbon Exchange, which trades Kyoto carbon credits and helps broker emissions off-setting deals.
The Kyoto system of emissions credits has proved popular and the U.N. Framework Convention on Climate Change, which administers it, says dozens of methane-abatement projects have been approved in recent years with more being considered.
But Stephan Singer of conservation group WWF thinks this is not the complete solution. He believes more attention should be paid to controlling carbon dioxide emissions and the sources of methane not so easily controlled.
Only about 50 percent of all methane emissions are being controlled, namely from landfills, coal mines and the oil and gas industry, said Singer, head of WWF's European Energy and Climate Policy Unit.
"What worries me is the increased methane coming out of the stomachs of ruminants, mainly for increased beef consumption within an increasingly wealthy world. The diet of the West has a big impact on the atmosphere."
In the United States, cattle emit about 5.5 million tonnes of methane per year into the atmosphere, accounting for 20 percent of U.S. methane emissions, the Environmental Protection Agency says. In New Zealand, emissions from agriculture comprise about half of all greenhouse gas emissions.
But what worries Singer most is a rapid release of methane stored in sub-polar permafrost or in huge methane hydrate deposits under the sea. While this has not happened, some scientists suggest it might occur in a warmer world.
"If methane hydrates leak, then we're gone, then it's over."