Emissions of planet-heating methane from fossil fuel production are between 20 and 60 percent higher than widely cited estimates, including those used by the Intergovernmental Panel on Climate Change (IPCC), the science body whose assessments influence climate action around the world.
That is the main finding of a peer-reviewed study published last week in the journal Nature. It is one of the most exhaustive analyses of long-term global methane emissions and methane carbon isotope records, with implications for climate policy worldwide. The two-year study was done by 11 researchers from the National Oceanic and Atmospheric Administration (NOAA) and the Cooperative Institute for Research in Environmental Sciences at the University of Colorado.
The study also found that biological sources—including flatulent cows and rotting landfills—are to blame for the ongoing massive methane spike first detected by NOAA in 2007.
"It was a substantial effort," said co-author Ed Dlugokencky, a methane expert at NOAA's Earth System Research Laboratory, "with a thorough analysis of uncertainties, to show that the fraction of atmospheric methane emitted from fossil sources (both anthropogenic and natural seeps) is greater than previously thought."
Methane is a short-lived climate pollutant that is dozens of times more potent than carbon dioxide. It is the main component of natural gas and leaches out of every stage of production, development, transportation and consumption. It also escapes from oil operations and coal mines.
Methane escaping from natural gas, oil and coal production accounts for 132 to 165 million tons of the 623 million tons emitted by all sources every year, according to the study.
That makes fossil fuel industries responsible for between 20 and 25 percent of the global methane problem. That's one-fifth higher than IPCC estimates, and as much as 60 percent higher than the estimates in the European Joint Research Centre's Emissions Database for Global Atmospheric Research (EDGAR).
Understanding the exact sources and scope of methane leaks is critical to clarifing the solutions required to rein in dangerous global warming under the Paris climate agreement and other climate-fighting policies. In the U.S., natural gas is displacing coal at a rapid pace, and research has shown that methane leaks from gas operations would erase the climate benefits of reducing coal use.
Studies from the Environmental Defense Fund (EDF) project to quantify methane leaks show emissions are much higher than the Environmental Protection Agency estimates—at least 90 percent in one gas drilling hot spot.
"This latest research published...indicated that we have also been consistently underestimating oil and gas methane emissions on a global scale as well," EDF scientist Steven Hamburg wrote in a blog post on the new study. "This research also adds an important data set to an ongoing scientific debate on the causes of the almost decade-long increase in atmospheric methane concentration."
The Nature study analyzed thousands of air samples taken over three decades (between 1984 and 2013) at 84 sites on every continent that are part of NOAA's Global Greenhouse Gas Reference Network.
To establish an accurate carbon isotope fingerprint for biological sources, the study analyzed a database with measurements from more than 500 wetlands across all seasons. It also looked at more than 250 rice-growing areas, dozens of landfills as well as samples from termite-nest and biomass burning areas.
To gauge methane from fossil fuel sources, the scientists used data from nearly 7,500 measurements at natural gas and oil wellheads, as well as coal mines, in 45 countries.
Those countries account for about 80 percent of the world's natural gas and coal production.
Previous studies, including those used by the IPCC, relied on a less extensive database, and in particular lacked data from the Tropics and the Southern Hemisphere.
Euan Nisbet, a methane researcher and professor of earth sciences at the University of London, said the new study is the best estimate yet of methane sources. "The main point is the comprehensiveness and the size of the sample," he said.
The analysis shows that methane emissions from the oil and gas industry dropped from 8 percent to 2 percent of production over the past 30 years. That's because companies have become better at identifying and stopping leaks and the technology has improved, the study said.
But because fossil fuel production soared during that time, the amount of methane emitted from fossil fuel activities stayed flat.
Other than fossil fuels, sources of methane include natural geologic seeps, microbial activity in wetlands and landfills, agriculture and biomass burning.
Scientists have long been able to distinguish between various sources of methane by analyzing its carbon isotopes. (Methane emitted from fossil fuel sources, for instance, has more carbon-12 isotopes than methane from natural sources.)
By analyzing the shift in the ratio of those isotopes over time, scientists were able to show how the sources of methane have shifted over time—and who's to blame.
"What is new is the ID of isotopic signatures," said lead author Stefan Schwietzke, a NOAA researcher. "That's the question that many other scientists have tried to answer in the past is, what's driving the change in the global methane budget? Is there a specific source that's responsible for that?"