U.S. EPA's methane emissions inventory underestimates the releases nationwide by as much as 50 percent, according to a new study.
Part of the discrepancy is due to the oil and gas sector, which is likely emitting about five times more methane in Texas and Oklahoma than is currently assumed, according to the study.
"There are very large methane emissions out of south-central United States, parts of Texas and Oklahoma, and the emissions from those regions seem to be at odds with the existing estimates of emissions that we have," said Scot Miller, a graduate student in the Department of Earth and Planetary Sciences at Harvard University and lead author of the study.
The study, published yesterday in Proceedings of the National Academy of Sciences, adds to the body of work on emissions from the oil and gas sector. Natural gas is usually promoted as a bridge to a post-carbon world, but there are concerns that large emissions of methane, a potent greenhouse gas that traps 70 times more heat in the atmosphere than carbon dioxide over a 20-year time scale, could negate the benefits. EPA estimates that the oil and gas sector is the largest emitter of methane in the United States.
In September, scientists from the University of Texas, Austin, and the Environmental Defense Fund analyzed emissions from natural gas drilling, specifically from pneumatic controllers, green completions and other processes. Their results were widely interpreted as confirmation that the EPA inventory of the oil and gas sector is accurate (EnergyWire, Sept. 17; EnergyWire, Nov. 6).
The present study suggests this may not be the case.
David Allen, author of the UT Austin study, stressed that his research focused only on specific processes of natural gas drilling. In contrast, the Harvard study released yesterday looks at emissions across entire regions.
Experts have said that both types of studies -- inventories at the ground level, as done by UT Austin, and broader methane measurements, as done by Harvard and scientists at the National Oceanic and Atmospheric Administration -- are needed to resolve how much methane is emitted by the industry.
"They conclude that methane emissions from ... fossil fuel production and processing are underestimated in current emission inventories," Allen said. "A logical follow-up question is which sources within these sectors are responsible for the emissions. Some emission sources may be more important than others."
The Harvard study used measurements of methane taken by aircraft and a network of towers as tall as the Empire State Building. The data are collected by the Department of Energy and NOAA throughout the United States.
Miller and his colleagues used 12,700 measurements made in 2007 and 2008, before the natural gas drilling boom really accelerated. They used wind and weather modeling to trace the methane back to the source.
In contrast, EPA scientists use an accounting method in which they count the numbers of cows, pipelines, wellheads, landfills and other sources and list how much each must emit. From that, they extrapolate to generate an inventory for the United States as a whole.
This inventory does not leave room for real-world scenarios, such as malfunctions at landfills or unanticipated blowouts at well pads, that suddenly spit out large amounts of methane.
"The total emissions [in the atmosphere] are not in line with the total coming from these accounting-based methods," said Anna Michalak, a professor in the Carnegie Institution for Science's Department of Global Ecology at Stanford University and a co-author of the study. "This really means we need to go back and work on those inventories to see what is missing."
The modeling method employed by the scientists is not used by EPA because of the complexity of the effort, Michalak said.
Moreover, EPA's inventory method is the best way to get detailed information about hundreds of subcategories of emitters, which would be needed for policymaking and regulation, said David McCabe, an atmospheric scientist with the Clean Air Task Force.
Livestock, oil and gas
The discrepancy between the Harvard study and EPA's inventory is due to livestock and, predominantly, the oil and gas sector, said Marc Fischer, a scientist at the Lawrence Berkeley National Laboratory and a co-author of the study.
"Even if we made emissions from livestock several times higher than inventory estimates would suggest for the southwest, you still don't get enough to cover what's actually being observed," he said in a statement. "That's why it looks like oil and gas are likely responsible for a large part of the remainder."
The scientists found that Texas and Oklahoma emitted more methane than is currently assumed. The study found the region emits an average of 3.7 terragrams of carbon in the form of methane every year.
That is higher than the 0.75 terragrams of carbon recorded in the EDGAR methane inventory of the European Commission. EDGAR is roughly similar to EPA's inventory, Miller said.
The scientists also found propane -- which is emitted only during fossil fuel extraction -- in the atmosphere, which strongly suggested that the emissions recorded in the region were coming from the oil and gas industry, said McCabe of the Clean Air Task Force.
"This adds up to a pretty clear qualitative signal that oil and gas is a significant portion of the undercount of methane," he said.
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