Correction appended.
The U.S. oil and gas industry has boomed in recent years, but the expansion has not resulted in a global spike in methane levels, finds a landmark new study.
The study comes on the heels of a new pledge by President Obama yesterday to expand regulations of the energy industry to cut down leaks of methane, a potent greenhouse gas that is the primary component of natural gas.
Methane warms the Earth at 86 times the rate of carbon dioxide over the two decades it stays in the atmosphere. The United States and Canada plan to cut methane emissions by 40 to 45 percent below 2012 levels by 2025, and the oil-sector regulations are a major piece of that effort (see related story).
But the research, published yesterday in Science, finds policymakers might make more progress on global warming if they focus on curbing emissions from agriculture or animal husbandry, primarily in the tropics.
"Currently increasing methane levels are caused not by fossil fuel production but rather by wetlands or, more likely, agriculture," said Hinrich Schaefer, an atmospheric scientist at the National Institute of Water and Atmospheric Research in Wellington, New Zealand, and lead author of the study.
"That means we have to find ways to reduce methane emissions from rice agriculture, beef and dairy farming while still feeding the world’s population if we want to mitigate climate change," he said.
Lori Bruhwiler, a physical scientist at the National Oceanic and Atmospheric Administration’s Earth System Research Laboratory, called the study "important."
"Many of us were waiting for a paper like this to come out," she said.
‘Circumstantial’ evidence of gas well contributions?
The study deals with one of the enduring puzzles about the planet’s atmosphere. Historical records show methane levels have been steadily rising since the Industrial Revolution. In the 1970s and 1980s, methane levels climbed by 12 parts per billion every year.
The rise abruptly stopped in the 1990s as nations stabilized their emissions. Then, around 2007, methane levels began growing once again. About 17 teragrams more methane is now emitted every year compared to the annual emissions between 2000 to 2006.
Scientists have been trying to identify the source of the methane, a challenging task because there are so many potential sources. Anaerobic bacteria in wetlands decompose vegetation and release methane. The gas also escapes from cows and manure lagoons on dairy farms, from submerged rice fields, from coal fields, and from leaky equipment on oil fields.
The latter source has seemed particularly suspicious. The United States has experienced an oil and gas boom since 2005 and vaulted over Russia in 2014 to become the world’s top producer.
"[People] are pointing at the U.S. because it is well-known that production in the U.S. has increased dramatically," said Bruhwiler. "It’s a little bit circumstantial."
To hammer down the contributions of the various sources, Schaefer and his colleagues used a fingerprinting technique. There are two types of methane in the air, one with a heavier carbon atom and the other with a lighter one. Scientists can look at the mix of heavy and light carbon atoms in a sample and deduce the origins of the gas.
Schaefer and his colleagues analyzed methane samples from around the world and from Earth’s history, embalmed as air bubbles in ice cores drilled from the poles.
Enviros say science doesn’t undercut policy
The samples collected since 2006 gave unexpected results. The analysis suggested that fossil fuel production is not the major driver of the recent methane uptick. Rather, emissions from other human activity — agriculture and dairy farming — are. Schaefer called the results "surprising."
Greenhouse gas inventories from U.S. EPA show that emissions from fossil fuel extraction have increased in recent years. But this has apparently not registered on the global scale. This is possibly because the U.S. energy industry contributes little to the overall burden of global fossil fuel emissions, Schaefer said.
He stressed, however, that all progress in curbing carbon emissions is desirable.
"I’d like to add that any methane emissions are a problem for climate change and that even if oil and gas exploration don’t contribute to rising methane levels, they certainly increase carbon dioxide, which stays in the atmosphere much longer," he said.
Steven Hamburg, chief scientist at the nonprofit Environmental Defense Fund, was cautious about the policy implications of the study. The advantages of reducing fossil fuel methane leaks are "not undercut in any way by the results here," he said.
All opportunities to curb emissions should be taken advantage of, he said. Moreover, other scientists are also studying the reasons behind the global rise in methane, and the knowledge base is still building.
"I wouldn’t treat any single [study] as definitive," he said.
Rice field microbes possible culprits
Officials with the oil and gas industry, meanwhile, embraced the findings.
"Even as oil and natural gas production has risen dramatically, methane emissions have fallen, thanks to industry leadership and investment in new technologies. These industry-led efforts are a proven way to reduce methane emissions from existing sources, and they are clearly working," said American Petroleum Institute Vice President of Regulatory and Economic Policy Kyle Isakower.
Other scientists have looked at the levels of other gases, such as ethane and propane, which are emitted with methane during oil and gas production. Their preliminary research has suggested large increases of those gases, and therefore methane, over North America.
Schaefer’s study suggests that something changed in the tropics around 2007. Microbes in submerged rice fields or in the stomachs of ruminants could be responsible. India, China and Southeast Asia are major rice and livestock producers and could be responsible, the authors suggest.
Bruhwiler said that wetlands could be a source, as well. When wetlands in the tropics and mid-latitudes are warm and wet, bacteria produce more methane. Climate change is expected to make these regions warmer and wetter — a feedback that may have already kicked in, she said.
"If we are ever going to control emissions to stave off climate change, we need to understand what the emissions are and how they are changing over time," she said.
Correction: An earlier version of this story misstated the rate of methane rise since 2007. It is 550 teragrams per year, 17 teragrams more than the rate between 2000 and 2006.
