The global increase in methane emissions over the past decade has been more the result of naturally occurring biogenic sources in wetlands and landfills than from fossil fuel operations, such as hydraulic fracturing (fracking), according to British-led research published this month in the American Geophysical Union's journal, Global Biogeochemical Cycles.
The results contradict the thinking of many U.S. and other global policymakers that fossil fuels are a primary source of rising methane emission levels worldwide. The researchers acknowledge that they are upsetting conventional thinking.
Researchers at Royal Holloway, University of London, concluded that increased levels of methane have been driven mostly by biological sources, such as swamp gas, cow burps and rice fields, as opposed to fossil fuel emissions. "Fossil fuel emissions have not been the dominant factor driving the increase," the researchers said.
"Consequently, while agricultural emissions are likely to be increasing, as postulated by [a study earlier this year], and probably have been an important component in the recent increase, we find that tropical wetlands are likely the dominant contributor to recent growth," said the researchers, led by Euan Nisbet, a professor at the University of London earth sciences department.
Nisbet and his colleagues said that atmospheric methane was one of the most potent greenhouse gases in the 20th century, and methane emissions increased throughout most of the century, driven mostly by emissions from the natural gas and coal industries.
At the beginning of the 21st century the situation for methane emissions "stabilized," only to shift into an increase mode since 2007. The focus of the latest research is 2007 through 2014, during which the rise was in the atmospheric sources of methane.
"The year 2014 was extreme [globally], with the growth rate doubling, and large increases seen across the globe," said Nisbet, whose research has shown that the most pronounced growth has been centered on the tropics.
The British researchers have been working with the U.S. National Oceanic and Atmospheric Administration, assessing measurements and air samples from various geographic locations, including the Canadian Arctic, a UK territory in the south Atlantic, and Cape Point, South Africa.
The research led by Nisbet has included an international team, including collaborators from France, Canada and South Africa, along with the University of Colorado's Institute of Arctic and Alpine Research.