New research published this week found that global atmospheric methane has risen sharply over the past decade, particularly in the last two years. The increase threatens to undo progress in cutting carbon dioxide emissions and is prompting researchers to intensify their focus on agriculture's role in combating climate change.
In 2014, methane concentrations rose 12.5 parts per billion, and in 2015, 9.9 parts per billion, compared to an annual rise of about 0.5 parts per billion a decade ago, according to a study published this week in Environmental Research Letters. Although the study acknowledges that methane sources come from a range of natural and man-made activities—oil extraction, natural gas leaks, wetlands, landfills, warming permafrost—researchers say they found less evidence that the boom in oil and gas production is the primary driver of the spike. Instead, they say the likely culprit is agriculture, especially in the tropics.
"The best evidence suggests an important contribution from biological sources, particularly agriculture," said Robert Jackson, chair of Stanford University's Earth System Science Department, who co-authored the study. "We see evidence for a smaller increase from the fossil fuel industry."
Agricultural sources include manure pits and cow burping from "enteric fermentation," which results from the digestive process of cattle. Flooded rice paddies also emit methane because water prevents oxygen from reaching the soil, triggering a build-up of methane-emitting bacteria.
Researchers trying to identify the sources of methane look at its carbon fingerprint—its isotopic signature. That signature looks different when the methane comes from a burning forest, for example, than from a cow's burp. But when researchers look at multiple sources of methane, and multiple carbon-13 signatures, it can become less clear where the methane is coming from, which the study's authors acknowledge.
"We have good, but certainly not great power for attributing methane sources," Jackson said. "We have information from isotopes. We get some information from latitudinal differences"—all consistent with agricultural sources, he said.
Robert Howarth, a professor of ecology and environmental biology at Cornell University, challenged the findings. He said the signature of shale gas—the source of the natural gas and fracking boom—can vary widely. Howarth also noted that some satellite data says the global methane increases have come from the U.S., where cattle population has dropped and rice production has mostly leveled off.
"They're assuming the [carbon-13] in all natural gas is constant, and that might have been true 10 years ago. But in the U.S. there's been a huge increase in shale gas, and the signature for shale gas can look a lot like the signature from a cow," Howarth explained. He concludes it's likely that more methane is actually coming from the energy sector.
Most methane emissions are caused by human activity—about 60 percent, as much as a third of which comes from agriculture. Carbon dioxide lasts longer, but methane's impact is more potent in the short term, which Jackson said represents a better opportunity to curb warming in the short term.
"Reducing methane emissions provides a more immediate response for climate change," he said. "Carbon dioxide has a longer reach, but doesn't start instantly warming the planet in quite the same way that methane does."
The research by Jackson and his colleagues isn't the first this year to blame agriculture for rising global methane levels. In April, researchers, led by New Zealand's National Institute of Water and Atmospheric Research, published a paper in Science, ruling out fossil fuels as the primary driver of worldwide methane over the past decade. (Two weeks later, the U.S. Environmental Protection Agency said that the oil and gas industries were the largest emitters in the U.S., accounting for one-third of methane emissions.)
That study suggested that rice and cattle production in China, India and Southeast Asia, are among the biggest sources of methane. This week's research also points to the tropics, noting that two-thirds of the rise in methane comes from those latitudes, suggesting that agriculture, rather than energy in the Northern Hemisphere, is likely to blame for the recent surge.
Cutting methane emissions from agriculture is more difficult than trimming or regulating methane emissions from energy. In California, where half the state's emissions come from livestock, lawmakers recently passed legislation that will eventually set targets for methane reductions, the most ambitious law of its kind in the country.
Environmental groups and researchers have long focused on controlling livestock emissions through the use of gas-trapping digesters that have the potential to convert manure methane into usable energy. But the price of the digesters is high, and even in California, which has subsidized the costs, farmers have found it difficult to make them profitable.
At the federal level, budget bills have prohibited the regulation of greenhouse gasses from large livestock operations since 2009.
More recently, research has focused on changing ruminants' diets to cut the methane in their belching. Australian scientists found that adding a certain seaweed to cattle feed could cut global methane emissions by 70 percent. A team of Spanish researchers found that a certain feed compound could cut methane emissions from dairy cows by 30 percent.
"The trick is finding incentives for changing people's behaviors," Jackson said. "You can establish carbon trading, such as offsets. You can provide financial incentives for certain types of feeds. I'm not advocating that, but one way or another we need economic incentives."