The rise in methane concentration in the atmosphere may reflect the growth of agriculture to feed Asia's booming population, according to a study published Thursday in the journal Science.
Rice paddies in Southeast Asia and livestock in India and China are probably behind the increase, according to researchers. The study was led by Hinrich Schaefer, an atmospheric scientist at the National Institute of Water and Atmospheric Research in Wellington, New Zealand. The findings were based on a chemical analysis of methane in the atmosphere.
Other scientists, however, challenged the results, arguing that the fracking-driven U.S. oil and gas boom is more likely to be the cause. Scientists have been trying to discover why methane levels in the atmosphere started rising in 2007 after holding steady for nearly a decade. As a greenhouse gas, methane is 84 times more potent than carbon dioxide over 20 years. Reducing methane emissions is considered crucial to slowing global warming.
"If we want to get serious about reducing methane emissions, we now know better where we have to start working," Schaefer said.
The study focused on unique isotopic signatures of methane from different sources. Methane from natural gas leaks in oil and gas production, for example, has a different signature from methane generated by bacteria in a cow's stomach or similar methane-producing bacteria found in rice paddies or other wetlands. The different signatures are based on the ratio of carbon-12 to carbon-13, two forms of carbon that are in methane. This ratio varies from source to source, allowing researchers to make inferences about the origins of the gas.
"If you see changes in the carbon-12 to carbon-13 ratio of methane in the atmosphere, you can draw conclusions about how different methane sources change over time," Schaefer said.
Other scientists, however, said that analysis is too simplistic.
"When you have eight or nine or 10 different sources of methane, each with a range of ratios, there is no way to calculate where it is coming from," said Robert Howarth, a Cornell University professor who studies methane emissions. "If you had a little bit of melting of permafrost and a big increase in natural gas production, you could get a pattern that these people are interpreting as cows in India."
The study also drew on previously published research based on satellite data that suggested the region including India, China and Southeast Asia was the source of increased emissions. The combined information led the researchers to conclude that the additional emissions were from agriculture, not from oil and gas or melting permafrost.
Daniel Jacob, a professor of atmospheric chemistry and environmental engineering at Harvard University, questioned how such a large increase could come from livestock in Asia.
"You could say we have more livestock because we have more people to feed and people eat more meat, but you look at the increase in the number of head of livestock, and that doesn't really account for the increase in methane," he said.
Jacob co-authored a separate study based on satellite data and surface observations last month in the academic journal Geophysical Research Letters. The study found that U.S. methane emissions could account for 30 to 60 percent of the global growth of atmospheric methane over the past decade.
Jacob's work doesn't pinpoint the source of the emissions but suggests leaks from the oil and gas industry may be the cause. The study notes that other researchers have recently observed increases in atmospheric concentrations of ethane. Ethane is a component of natural gas. If both methane and ethane are rising, natural gas is likely the source, Jacob said.
A peer-reviewed, satellite-based study published in 2014 found a significant increase in methane emissions from North Dakota and Texas where oil and gas production from the Bakken and Eagle Ford formations had been expanding rapidly. The study provides further evidence that the methane increase is from the oil and gas industry, Howarth said.
If the magnitude of the recent increase in U.S. emissions is correct, that would call into question the conclusion that agriculture in Asia is responsible, Jacob said.
"Thirty to 60 percent leaves room for something else, but still, that could be a tall order," Jacob said. "The jury is still out."