… Scientists suggest that the greater contribution to skyrocketing methane levels has more to do with biological sources of the gas. Methane molecules are made of carbon and hydrogen atoms, and the carbon in biological methane tends to be slightly lighter than the carbon in methane associated with fossil fuels. And over the past decade or so, the proportion of lighter methane in the atmosphere compared to heavier methane has been rising. “I think this perspective is basically right,” said Martin Helmann, of the Max Planck Institute for Biogeochemistry, in Jena, Germany, in an email. Helmann was not involved in the research.
The authors of the Science paper have some ideas about why biological sources of methane may be increasing. “In the southern hemisphere especially,” Nisbet said, “but also in the northern tropics, a series of really wet years has caused wetlands to expand”—and vegetation decomposing in swamps and shallow lakes is a well known source of natural methane emissions. Another is cows, which generate methane as they digest their food, then belch it out into the air.
These explanations, however, aren’t at all definitive — another key point Nisbet and his co-authors make in the Science paper. “The measurements we make in the air are direct,” he said. “Estimates of where methane is coming from, by contrast, is much less reliable. You estimate the contributions from gas leaks, count up the cows, estimate the emissions from wetlands. There’s obviously going to be a lot of error.”
And in fact, there is: the estimates of how much methane should be going into the atmosphere are greater than what actually ends up there. Tracking methane emissions more accurately is crucial, said the scientists, and not just as an academic exercise.
“If we want to control greenhouse-gas emissions,” Nisbet said, “it’s obviously important to know where the emissions are coming from.”
Global vegetation decomposition, wetlands, the oceans, rice paddys, rain forests, patio hibachis, your neighbor’s annoying little yapping kikmi dog and Humpback whales are all part of the mix.