The bulk of the recent rise in atmospheric methane, a potent greenhouse gas, is linked to emissions from the oil and gas industry, a new study led by the National Aeronautics and Space Administration (NASA) scientists suggests.
Methane emissions in the atmosphere currently stand at around 550 teragrams a year, with an increase of about 25 teragrams every year.
A long-standing puzzle – now solved thanks to the research led by John Worden of NASA’s Jet Propulsion Laboratory in California – involved a mismatch between the 25 teragram figure and estimates of emissions from two known sources of increase: oil and gas industry and microbial production in wet tropical environments like marshes and rice paddies.
A press release issued on the occasion said that a new calculation of emissions from global fires had helped solve the puzzle.
“Methane emissions have been rising sharply since 2006. Different research teams have produced viable estimates for the oil and gas industry, and microbial production in wet tropical environments. But when these estimates were added to estimates of other sources, the sum was considerably more than the observed increase,” the statement said.
The team reconciled the difference [between the level of methane in the atmospheric and the estimated emissions from all the various sources] by accurately assessing the impact of reduction in wood fires over the last two decades.
Using observations by NASA’s Moderate Resolution Imaging Spectrometer satellite instrument, the research team found that between early 2000s and 2014, surface area burned across the globe had decreased by about 12 percent each year.
“The logical assumption would be that methane emissions from fires have decreased by about the same percentage,” the statement said, adding that the real decrease in methane emissions was, in fact, almost twice as much that assumption would suggest.
When this large decrease in emissions was deducted from the sum of all emissions, the methane budget balanced correctly, said the NASA statement.
In his comments on the method used to ascertain the exact source of different methane molecules in the atmosphere, Worden said, “tracking down their sources is a detective job involving multiple lines of evidence: measurements of other gases, chemical analyses, isotopic signatures, observations of land use, and more. A fun thing about this study was combining all this different evidence to piece this puzzle together.”
The team determined the various sources by studying carbon isotopes in methane molecules. Of the three methane sources examined, the statement said, emissions from fires contained the largest percentage of heavy carbon isotopes. “Microbial emissions had the smallest carbon isotopes, and fossil fuel emissions are in between,” it said.
The research team used carbon monoxide and methane data from the Measurements of Pollutants in the Troposphere instrument on NASA’s Terra satellite and the Tropospheric Emission Spectrometer instrument on NASA’s Aura to quantify fire emissions of methane. The results showed that these emissions had been decreasing much more rapidly than expected.
“Combining isotopic evidence from ground surface measurements with the newly calculated fire emissions, the team showed that about 17 teragrams per year of the increase is due to fossil fuels, another 12 is from wetlands or rice farming, while fires are decreasing by about 4 teragrams per year. The three numbers combine to 25 teragrams a year — the same as the observed increase,” the statement said.
The research has been published in the journal Nature Communications.