Researchers in a new NASA-led study say they have solved a puzzle involving the recent rise in atmospheric methane, a potent greenhouse gas (GHG), with a new calculation of emissions from global fires. The new study resolves what looked like irreconcilable differences in explanations for the increase and finds that emissions from the oil and gas industry and microbial production in wet tropical environments are behind the rise whereas emissions from global fires have decreased.
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Global methane (CH4) emissions have been rising sharply since 2006 and are increasing by about 25 teragrams (Tg) per annum, with total emissions currently around 550 Tg (550 million tonnes) annually. Different research teams have produced viable estimates for two known sources of the increase: emissions from the oil and gas industry, and microbial production in wet tropical environments like marshes and rice paddies.
But when these estimates were added to estimates of other sources, the sum was considerably more than the observed increase. In fact, each new estimate was large enough to explain the whole increase by itself.
US National Aeronautics and Space Administration (NASA) scientist John Worden at the Jet Propulsion Laboratory in Pasadena, California and colleagues at the National Center for Atmospheric Research, Boulder, Colorado; and the Netherlands Institute for Space Research and the University of Utrecht, both in Utrecht, the Netherlands, focused on fires because they are also changing globally.
Global fires decreasing
The area burned each year decreased about 12 percent between the early 2000s and the more recent period of 2007 to 2014, according to a new study using observations by NASA’s Moderate Resolution Imaging Spectrometer satellite instrument. The logical assumption would be that methane emissions from fires have decreased by about the same percentage.
Using satellite measurements of methane and carbon monoxide (CO), Worden’s team found the real decrease in methane emissions was almost twice as much as that assumption would suggest.
When the research team subtracted this large decrease from the sum of all emissions, the methane budget was balanced correctly, with room for both fossil fuel and wetland increases.
The research, “Reduced biomass burning emissions reconcile conflicting estimates of the post-2006 atmospheric methane budget“, is published in the journal Nature Communications.
Tracking different sources
Most methane molecules in the atmosphere do not have identifying features that reveal their origin. 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, Worden said.
Carbon isotopes in the methane molecules are one clue. Of the three methane sources examined in the new NASA-led study, emissions from fires contain the largest percentage of heavy carbon isotopes, microbial emissions have the smallest, and fossil fuel emissions are in between.
Another clue is ethane, which like methane is a component of natural gas. An increase in atmospheric ethane (C2H8) indicates increasing fossil fuel sources. Fires emit carbon monoxide (CO) as well as methane, and measurements of that gas are a final clue.
Fossil fuels largest source of the increase
Worden’s 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 show these emissions have 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 Tg (17 million tonnes) per annum of the increase is due to fossil fuels, another 12 Tg (12 million tonnes) is from wetlands or rice farming, while fires are decreasing by about 4 Tg (4 million tonnes) annually. The three numbers combine to 25 Tg (25 million tonnes) per annum – the same as the observed increase.
