Expert Commentary

Anthropogenic emissions of methane in the United States

A 2013 study published in the Proceedings of the National Academy of Sciences seeking to better quantify the current levels, regions and sectors that are the key sources of methane in the United States.

Anthropogenic methane emissions (
Anthropogenic methane emissions (

The fight against global warming is most closely associated with efforts to reduce carbon emissions, and with good reason: Of all the greenhouse gases, carbon is estimated to make the largest contribution to climate change. Its sources in the United States — primarily power generation, transportation and industrial use — are also relatively easy to target, even if finding the political and public will remains difficult. The U.S. Environmental Protection Agency’s proposed 2013 limits on pollution from new power plants squarely target carbon, and the agency’s new standard for fuel economy, 54.5 mpg by 2025, effectively does the same, as cars and trucks generate up to 28% of the greenhouse gases produced by the United States.

The contribution of individual greenhouse gases to climate change is not just a function of their presence in the atmosphere and the amount emitted, however — another factor is their lifespan. Methane is less present in the atmosphere than carbon, but has up to 72 times the global-warming potential over 20 years. Nitrous oxide is rarer still, yet has 289 times the warming potential over the same time frame.

Methane is of particular concern for two reasons. First, it’s a major component of natural gas, a low-carbon fuel that’s on the rise because of concerns about climate change and the ability of hydraulic fracturing to open up deposits that were previously locked away. Natural-gas prices have consequently plummeted, and with the EPA’s proposed rules, the majority of new power plants are expected to be gas-fired. But as natural-gas extraction rises, so do methane emissions, be it at the wellhead or throughout the transportation and distribution chain. Second, methane is a byproduct of agriculture, especially livestock raised for meat or dairy production — up to 33% of global methane emissions, according to the IPCC Fourth Assessment Report. Other sources of significant methane emissions include landfills and coal mines.

A 2013 study published in the Proceedings of the National Academy of Sciences (PNAS), Anthropogenic Emissions of Methane in the United States,” seeks to better quantify the current levels, regions and sectors that are its key sources. The study’s research team — based at Harvard, Stanford, the University of Colorado, NCAR, Lawrence Berkeley National Laboratory and the E.U.’s Institute for Environment and Sustainability — preface their work by noting that previous estimates have varied widely: “Disparities among bottom-up and top-down studies suggest much greater uncertainty in emissions than typically reported,” they write. “For example, EPA cites an uncertainty of only ±13% for the United States. Independent assessments of bottom-up inventories give error ranges of 50% to 100%.”

The question of establishing current emissions is not an abstract one: Greenhouse-gas reduction targets are necessarily based on estimates of current levels — for example, California’s AB 32, signed into law in 2006, sets targets of 7% reductions by 2020 and up to 16% in 2035 for different regions of the state. But if the actual levels are different than the estimates on which law is based, its very structure is called into question. And given that methane is a relatively long-lived greenhouse gas, its presence and emissions rate factor heavily in the overall pace of climate change.

For the PNAS study, the authors analyzed samples of ambient air at a wide range of locations across the United States — 4,984 samples from tall towers and 7,710 from aircraft. Samples were taken in 2007 and 2008. The study’s findings include:

  • Current U.S. methane emissions appear to be significantly higher than previous estimates — 1.5 times greater than those by the U.S. Environmental Protection Agency and 1.7 times greater than data from the European Environmental Agency’s Emission Database for Global Atmospheric Research (EDGAR).
  • Emissions related to livestock operations are more than twice current estimates.
  • The discrepancy in U.S. methane emissions is most pronounced in the south-central states of Texas, Oklahoma and Louisiana; total regional emissions are approximately 2.7 times greater than most estimates and 4.9 times greater than EDGAR. The region accounts for 24% of U.S. methane emissions (the 95% confidence interval is plus or minus 3%) and 3.7% of total U.S. greenhouse gas emissions.
  • In the south-central United States, fossil-fuel extraction and refining have much larger greenhouse-gas impacts than previously estimated, and contribute 45% of the detected emissions (95% confidence interval, plus or minus 13%). “Texas and Oklahoma were among the top five natural gas producing states in the country in 2007, and aircraft observations … indicate that the natural gas and/or oil industries play a significant role in regional [methane] emissions.”
  • Current atmospheric concentrations of methane, approximately 1,800 parts per billion, are nearly triple the levels found in the preindustrial era — 680 to 715 parts per billion. Human activity accounts for 50% to 65% of global methane emissions.

“These results cast doubt on the U.S. EPA’s recent decision to downscale its estimate of national natural gas emissions by 25% to 30%,” the authors write. “We find that [methane] data from across North America instead indicate the need for a larger adjustment of the opposite sign.”

The EPA has reported that petroleum and natural gas systems, including fracking, constituted the second largest sector in terms of greenhouse gas emissions (see the EPA interactive map to locate these facilities). A study in the journal Nature has reported, some wellheads appear to be losing as much as 9% of their total output in methane leakage. A study in PNAS suggests lower loss rates, but only looks at the production part of the process. A 2014 study in Science indicates that a small number of “super-emitters” are responsible for much of the methane leakage in the United States, and targeting them would significantly reduce U.S. emissions.

Related research: A 2014 paper in the journal Science, “Methane Leaks from North American Natural Gas Systems,” reviews 20 years of literature on natural-gas emissions in the United States and Canada to improve understanding of leakage rates. The research shows that a small number of leaks contribute a significant portion of escaped natural gas. This “presents an opportunity for large mitigation benefits if scientists and engineers can develop reliable (possibly remote) methods to rapidly identify and fix the small fraction of high-emitting sources.” Two major 2013 research studies add to our overall understanding of the climate issue more broadly: The Intergovernmental Panel on Climate Change’s Fifth Assessment Report; and the National Academy of Sciences’ “Abrupt Impacts of Climate Change” report.

Keywords: fossil fuels, global warming, fracking, animal husbandry, agriculture, livestock, @leightonwalter, @journoresource

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