SINGAPORE (Reuters) ? Atmospheric levels of methane, 20 times more powerful than carbon dioxide (CO2) at trapping heat, stayed steady for two decades to 2006 on wider fertilizer use to grow rice or a surge in natural gas demand, according to two separate studies in the journal Nature.
Climate researcher Fuu Ming Kai from the Massachusetts Institute of Technology's Singapore research center said in one study that methane output from rice fields in the Northern Hemisphere dropped during the period as fertilizers replaced manure and because of reduced water use.
In the second study, Murat Aydin at the University of California, Irvine, concluded that a drop in methane emissions from more efficient burning of fossil fuels and a surge in natural gas demand.
The studies aim to solve a puzzle that has confounded climate scientists for years: why did methane levels in the atmosphere, after rising steadily for many years, taper off in the mid-1980s in a dip lasting two decades?
Solving the puzzle is crucial because methane levels have risen more than 150 percent since the start of the industrial revolution, compared with CO2's 40 percent increase, and are on the rise again.
While the studies reach different conclusions, both studies point to human activities as the reason for the slowdown.
"In general most of the methane sources come from the Northern Hemisphere," Fuu told Reuters.
The main methane sources come from burning fossil fuels, rice paddies, coal mines, livestock and clearing and burning of tropical forests.
"We looked at the isotope data to see how it's changed over the past 20 to 30 years. And what we saw is a trend in the isotope signature and especially in the Northern Hemisphere."
Fuu said long-term data and comparing methane levels between the both hemispheres helped researchers conclude that about half the decrease in Northern Hemisphere methane emissions could be explained by reduced emissions from rice agriculture in Asia over the past three decades.
"It is important to know what the mechanism is behind the slow down. If you know this, you can adopt a suitable policy to reduce methane emissions," Fuu said.
Aydin concluded the drop coincided with rapid natural gas production as the fuel became increasingly price competitive with oil and other fossil fuel, instead of flaring it off. The gains came even though overall fossil fuel use increased as cleaner burning technologies helped keep methane emissions in check, he said.
"We speculate that the rising economic value of natural gas during the late 20th Century and the deployment of cleaner technologies led to sharp reductions in the release of light hydrocarbons into the atmosphere," the study says.
(Editing by Ed Lane)
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