Using fossil fuels releases much more of the potent greenhouse gas methane than previously thought — possibly 25 to 40 percent more, new research suggests. The finding could help scientists and policy makers target how and where to reduce these climate-warming emissions, researchers report February 19 in Nature.
The amount of methane released from geologic (rather than biological) sources is from 172 to 195 teragrams (trillions of grams) per year. Those geologic methane sources include not only the oil and gas industry, but also natural vents such as onshore and offshore gas seeps. Researchers previously had estimated that the natural portion of those geologic emissions released between 40 to 60 teragrams of methane each year, with the remainder coming from fossil fuels.
But new analyses of over two centuries of methane preserved in ice cores suggest that natural seeps — both in the past and in modern times — send far less methane into the atmosphere than once thought. That means that modern human activities are responsible for nearly all of the current geologic emissions of methane, atmospheric chemist Benjamin Hmiel of the University of Rochester in New York and his colleagues conclude.
Methane has about 80 times the atmosphere-warming potential of carbon dioxide — but only on short timescales, because methane only lingers in the atmosphere for 10 to 20 years, while CO2 can linger for hundreds of years. “So the changes we make to our [methane] emissions are going to impact the atmosphere much more quickly,” Hmiel says.
Coal mining, natural gas and other fossil fuel sources pushed atmospheric methane levels upward through the 20th century. Those emissions tapered off in the first few years of the 21st century. However, beginning in 2007, atmospheric methane began to increase again, and is now at a level not seen since the 1980s.
What’s causing the post-2007 buildup of the gas isn’t clear. Previous research points to some combination of amped-up microbial activity in wetlands — possibly linked to changes in temperature and rainfall — and more cow burps and leaky pipelines (SN: 11/18/15). Less methane is also getting broken down in the atmosphere (SN: 4/20/17).
If methane emissions continue rising, meeting the greenhouse gas reduction goals of the 2015 Paris Agreement (SN: 11/26/19) will be difficult, says Euan Nisbet, a geochemist at Royal Holloway, University of London, who was not involved in the new study. So identifying the portion of the methane bump that’s linked to the oil and gas industry offers opportunities for targeted reductions.
To calculate today’s methane emissions from all geologic sources, scientists first need to establish a baseline for preindustrial methane emissions from natural sources like seeps and mud volcanoes. One way to distinguish biological from geologic sources of methane is by using the radioactive isotope carbon-14, a version of the element. Biological sources produce methane with relatively high carbon-14 levels, while methane from geologic sources tends to be very old, so that the carbon-14 has long since decayed away.
To separate human-caused from natural geologic sources, researchers need to look into the past. So, in the new study, the team turned to methane preserved in ice cores from Greenland dating from 1750 to 2013.
Before the Industrial Revolution, the team discovered, methane emissions from geologic sources were around 1.6 teragrams per year on average — and no more than 5.4 teragrams per year at their highest. That’s an order of magnitude smaller than previous estimates.
Subtracting that amount from total methane emissions today, the researchers calculate that nearly all of the nonbiological methane measured today, from 172 to 195 teragrams per year, is coming from anthropogenic sources. That’s about 38 to 58 teragrams higher per year than previously estimated, an increase of 25 to 40 percent.
“Paradoxically, that’s actually a hopeful finding,” Nisbet says. Stopping gas leaks and reducing coal mine emissions are relatively easy ways of cutting greenhouse gas emissions, he says. So reducing methane emissions offers “an even bigger opportunity” for reducing greenhouse gases overall.
But such ice core–based work is not yet proven to be the most accurate technique to estimate natural geologic emissions, says Stefan Schwietzke, an environmental scientist with the Environmental Defense Fund who is based in Berlin. The ice core information is useful because it gives an immediate global snapshot of methane emissions, but “it has the challenge of interpretation and a lot of very complex analysis,” Schwietzke says.
Direct measurements of methane emitted from different seeps or over mud volcanoes suggest much larger natural emissions, he adds. The problem with this method, however, is that it’s difficult to scale up from local measurements to a global number. “To really understand the magnitudes, these two methods need to be reconciled. That hasn’t happened yet.”
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Schwietzke and other researchers have proposed using airborne remote sensing to try to reconcile the two techniques. Airborne measurements can give a bigger-picture estimate, while also identifying local hot spots. Scientists have already been using this work to identify sources such as leaking pipelines, landfills or dairy farms (SN: 11/14/19). Similar projects are tracking methane emission hot spots in Arctic permafrost.
Still, Schwietzke adds, this debate over the technique doesn’t change the fact that human-caused emissions, including fossil fuels, are responsible for the dramatic rise of atmospheric methane over the last century. “It is very large. And reducing those emissions will reduce warming.”