When it comes to climate change, we know where the most important warming agent — carbon dioxide — is coming from. Most of it is coming from the burning of fossil fuels, with additional contributions from deforestation and other causes.
But the second-most potent greenhouse warming agent — the hard-hitting, if short-lived, gas known as methane — presents more of a mystery. There has clearly been an alarming uptick in atmospheric methane in recent years, following a flattening of concentrations from 2000 to around 2007. But the cause of this particular pattern has been hotly debated, with some blaming the fracked natural gas boom (natural gas is primarily composed of methane) and others pointing to causes such as agriculture.
Now, new research published Thursday in the journal Carbon Balance and Management by three scientists with the Joint Global Change Research Institute, a center of the University of Maryland and the Pacific Northwest National Laboratory, point the finger at agriculture once again. And more specifically, at cattle and other livestock.
“Just from livestock methane emissions, our revisions resulted in 11 percent more methane in a recent year than what we were previously estimating,” said Julie Wolf, lead author of the study who completed the work while a postdoc at the institute and now works at the Department of Agriculture. “It’s not the biggest contributor to the annual methane budget in the atmosphere, but it may be the biggest contributor to increases in the atmospheric budget over recent years.”
Here’s what that increase looks like:
Cows and other ruminant animals release methane into the atmosphere as a result of a process called “enteric fermentation” — a technical term that basically refers to the digestive chemistry in the animals’ stomachs. As the Environmental Protection Agency explains, the methane produced in this process “is exhaled or belched by the animal and accounts for the majority of emissions from ruminants.”
Furthermore, the animals’ waste also fills the atmosphere with methane depending on how it is handled, meaning that “manure management” is categorized as a separate source of methane emissions.
The new study found that a variety of guidelines introduced by the United Nations’ Intergovernmental Panel on Climate Change (IPCC) in 2006 to estimate methane emissions needed to be updated. That’s because livestock are being bred to be larger than before (and are being fed more), and their manure is being managed differently — more often in huge “anaerobic” waste lagoons that give off large volumes of methane.
Once the study updated the methodology, it found that for 2011, global emissions were 8.4 percent higher from enteric fermentation and 36.7 percent higher from manure management, compared with research by the IPCC.
“In most developed regions, the livestock have been bred to be larger — they are more productive, especially with dairy cows,” Wolf said. “And that will result in a larger estimate of methane emitted by each animal.”
The changes broadly reflect the ongoing industrialization of agriculture, she said. “These large operations, they’re very efficient.”
“This study improves estimated livestock emissions, taking into account where cattle are, how big they are, and how their feed and waste are managed,” said Rob Jackson, a methane expert at Stanford University who was not involved in the work.
The real question, though, is whether these changes are sufficient to account for rising atmospheric methane concentrations — something that the new study asserts.
“Our results suggest that livestock methane emissions, while not the dominant overall source of global methane emissions, may be a major contributor to the observed annual emissions increases over the 2000s to 2010s,” it says.
“We believe our approach, because of its uniqueness, has helped provide some clarity on some of the changes that we see over the recent decade,” said Ghassem Asrar, one of the study co-authors and director of the Joint Global Change Research Institute.
Daniel Jacob, an atmospheric scientist at Harvard who commented on the work for The Washington Post, agreed that was plausible.
“This work shows that global methane emissions from livestock are larger than previously thought by IPCC and could have made a major contribution to the observed rise in methane over the past decade, supporting the results from past studies using methane isotope data,” he said.
“The livestock emission estimates for the U.S. given in this work are in good agreement with the EPA methane inventory,” he added, “and this gives credit to the excellent work by EPA scientists in making estimates of greenhouse gas emissions in the U.S.”
But Jackson wasn’t so convinced when it came to the global picture. “It’s unlikely that livestock are responsible for the rapid rise in methane concentrations over the past couple of years,” he said. “Long-term, though, agricultural emissions are the largest source of methane from all human activities.” (In addition to livestock, for instance, rice paddies also emit large amounts of methane.)
“This doesn’t necessarily explain all of the observations,” added Drew Shindell, a professor at Duke University who studies methane emissions, although he said that the new findings are “likely part of the story” when it comes to growing atmospheric methane.
“I don’t think this clearly lets fossil fuels, in particular the U.S. fracking boom, off the hook for the growth in methane,” Shindell said.
Most important, Shindell noted, cattle methane emissions can be controlled — such as by changing the animals’ diets or, for that matter, cracking down on voluminous human food waste.
“The more of the total that comes from those potentially controllable sources the better, assuming we actually work to control those emissions,” Shindell said.
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