October 7, 2016
When trying to figure out why atmospheric concentrations of methane — a potent greenhouse gas driving climate change — have been rising continuously since 2007, fingers often point to North America’s shale oil and gas boom over the last decade.
The study shows that even though fossil fuels development has polluted the atmosphere with up to 60 percent more methane than scientists previously thought, the main culprits behind the rise in global methane levels are wetlands, landfills, rice fields and belching cows.
Methane is about 34 times as potent as carbon dioxide in warming the climate over the span of a century. Global methane emissions have risen and fallen since the 1980s, but have been rising steadily for the last nine years.
Using an emissions database 100 times larger than those used before, the scientists set out to update estimates of global methane emissions. They used the differences in methane isotopes to determine their sources — fossil fuels, biomass burning, microbes, agriculture and others.
They found that biological sources such as cattle, landfills and agriculture account for up to 67 percent of total human-caused methane emissions.
Study lead author Stefan Schwietzke, a research scientist at NOAA’s Earth System Research Laboratory in Boulder, Colo., said the team hasn’t been able to determine which of those biological sources are primarily driving the methane spike. More research is needed, he said.
The team found that while fossil fuel development is responsible for more methane pollution than previously thought, it accounts for just 20 to 25 percent of all the world’s methane emissions.
“We recognize the findings might seem counterintuitive — methane emissions from fossil fuel development have been dramatically underestimated — but they’re not responsible for the increase in total methane emissions observed since 2007,” Schwietzke said.
Over the last decade, the oil and gas industry has clamped down on methane leaks from wells and pipelines, cutting methane emissions by up to 8 percent. But a huge boom in oil and gas production during the same time has balanced those cuts, keeping the industry’s methane emissions roughly constant, the study says.
“Our study shows that leaks from oil and gas activities around the world are responsible for a lot more methane than we thought,” said study co-author Lori Bruhwiler, a NOAA research scientist. “The good news is that fixing leaking oil and gas infrastructure is a very effective short-term way to reduce emissions of this important greenhouse gas.”
Schwietzke said it will be difficult to cut methane emissions from biological sources until more research is conducted that pinpoints their specific sources, he said.
“If the microbial methane increase is mainly coming from cows or agriculture, then we could potentially do something about it,” he said, adding that if wetlands are the source, global warming itself could be driving those emissions.
For example, research has shown that thawing Arctic permafrost is a major source of methane emissions.
Robert Howarth, a Cornell University biogeochemist who studies methane emissions and is unaffiliated with the research, said the NOAA study is an improvement over previous studies because it uses a much larger set of emissions data than previous studies.
He said the study is one of several recent papers that have tried to explain the cause of the global methane spike, but their conclusions have all been different.
For example, a research team led by scientists in New Zealand published a study in March suggesting that agriculture is the primary cause of the rise in global methane emissions. A Harvard study published in February used satellite data to suggest that the U.S. alone could be responsible for up to 60 percent of the global increase in methane possibly because of U.S. oil, gas and cattle operations.
Eric Kort, an assistant professor of climate and space sciences at the University of Michigan, said the NOAA study is valuable because of the large size of the database the research team used and their use of methane isotopes to determine the emissions sources.
The study adds clarity to the conflicting results of other research, and it shows that because fossil fuels are a large source of methane emissions, there is a major opportunity to reduce humans’ impact on climate change by cutting oil and gas emissions, he said.
“Fossil fuel emissions are much larger than previously thought, and thus there is larger mitigation potential than previously appreciated,” Kort said.