Agriculture, including animal husbandry, seems “to be a major, possibly dominant, cause” of the jump in methane emissions. Photo: Brendon ThorneRice paddies and flatulent cows appear to be behind a surge in the potent greenhouse gas methane over the past decade, a shift that threatens to counter efforts to curb global warming, scientists say.
After growing at the rate of half a part per billion annually for the first six years of this century, atmospheric levels of methane have “experienced puzzling dynamics”, increasing more than 10-fold since 2007, researchers said in a paper published Monday in Environmental Research Letters.
The team, including Pep Canadell from the CSIRO, said expanded agriculture including rice fields and animal husbandry seems “to be a major, possibly dominant, cause” of the jump in emissions.
The finding has implications for the ability to reduce humans’ impact on the climate, as well as “the need to balance food security and environmental protection”, the paper said.
“Keeping global warming below 2 degrees is already a challenging target, with most of the attention placed on carbon-dioxide emissions,” the paper said.
“Such a target will become increasingly difficult if methane reductions are not also addressed strongly and rapidly.”
Methane’s impact on warming can be as much as 100 times more than CO₂ over a couple of decades and 34 times the warming potential over a century.
Although CO₂ is about 200 times more abundant, methane has contributed about one-sixth of recent warming, NASA says.
While a consensus on the source of the extra methane remains elusive, the paper said agriculture, fossil-fuel related emissions and a decrease in biomass burning were likely the biggest contributors. (See chart below in annual methane emissions by region for the 2003-2012 decade.)
“While methane is not warming the planet for centuries like CO₂, slowing down the impacts in the nearer future will depend on us adopting a less meat-rich diet, and a transition to renewables rather than investments into gas as a temporary ‘transition fuel’,” said Malte Meinshausen, director of Melbourne University’s Climate & Energy College.
“Luckily, those switches can come with a number of benefits,” Professor Meinshausen said. “For one, a healthier diet, and in addition, the avoidance of sunken investments into a gas energy infrastructure that does not fit well with a zero-emissions future sketched by the Paris [climate] agreement.”
Natural methane sources include wetlands, particularly in the tropics. “Some scientists think tropical wetlands have gotten a bit wetter and are releasing more gas,” NASA said in an online methane feature.
Humans do not have a direct influence on methane sinks, unlike CO₂, in which extra tree planting can take some of that gas out of the atmosphere, Professor Meinshausen said.
Methane also has a positive feedback loop, in which the higher the concentration of the gas, the longer its atmospheric lifetime, he said.
While atmospheric CO₂ levels have increased about 30 per cent since the Industrial Revolution began, methane levels have roughly doubled. (See NASA chart below, showing levels now exceed 1800 parts per billion.)
Another feedback is expected to come if a warming world triggers, as expected, a thawing of methane-rich permafrost.
For now, though, the paper found “no significant increase from Arctic regions”.
Research agencies are stepping up efforts to get a better fix on the sources of the extra methane, with the Franco-German Methane Remote Sensing Lidar Mission satellite – dubbed Merlin – due for launch in 2020.
Dimitri Lafleur, a former Shell engineer and PhD researcher at Melbourne University, said debate continues about the contribution of methane from the fossil fuel sector, with emissions apparently varying widely from basin to basin.
“When US fossil-fuel related emissions can be seen on satellite images then the potential differences on a basin level become quite clear,” Mr Lafleur said.
“It would be really valuable if we would have more measurements in Australia to understand the methane emissions from the fossil fuel industry and agriculture,” he added. “Currently we don’t have a good understanding who is responsible for how much.”
Follow Peter Hannam on Twitter and Facebook.