Sources and amounts of emissions of methane—an extremely potent greenhouse gas—from human activities are clouded in uncertainty, says a report by a National Academies of Sciences, Engineering & Medicine expert panel. These uncertainties have challenged efforts to reduce emissions and curb the impact of climate change, notes the report.
Consequently, the committee recommends that the U.S. develop a new, comprehensive system to measure human-made methane emissions. Such a system could serve as an international model, says James W. C. White, committee chair and University of Colorado, Boulder, geology professor.
Methane contributes but 9% of total anthropogenic greenhouse gases; carbon dioxide accounts for most of the rest. However, methane has 86 times the global warming potential of CO2 in the first 20 years after it is emitted. Recently, scientists used spectroscopic observations of atmospheric methane to show directly that methane’s warming effect on Earth’s climate has been increasing since 2007 (Nat. Geosci. 2018, DOI: 10.1038/s41561-018-0085-9). Many in the science community believe it is crucial to reduce methane emissions now because of its potentially critical impact on climate in the next 10 to 20 years.
A comprehensive national system to measure methane emissions should be based on a fine spatial grid that takes into account methane emission quantities, sources, times, and locations, the National Academies report says. It would be similar to what the nation’s scientists have developed to enhance weather predictions, White says.
Such a system for methane has a long way to go. For instance, the oil and natural gas drilling and production sector, the top U.S. methane source, emits about one-third of all anthropogenic methane emissions, according to the report. However, federal agencies differ even on the number of operating production wells—and the differences are huge. The Energy Information Administration places total U.S. active wells at 574,530, while the Environmental Protection Agency estimates them to be 421,893. Because each production unit has many individual valves, pipes, and other parts that could leak, methane could be escaping from millions of components.
The report also looks at methane released from landfills, manure-management systems, coal mining, and enteric fermentation from domestic ruminant animals, such as cattle. Together with oil and gas production, these make up 94% of anthropogenic methane emissions.
“The report is certainly needed,” says Rob Jackson, chair of the department of earth system science at Sanford University, “and it is important for policymakers to hear this. The most immediate issue, though, isn’t research; it’s whether EPA will roll back regulatory safeguards even industry supports to reduce emission and improve human health.”
The report did not address regulations. It did, however, emphasize the need to resolve measurement disagreements between two major approaches to estimate methane emissions. Several studies have found significant differences between “top-down” measurements that sample atmospheric concentrations of methane by aircraft above potentially leaking sources, such as natural gas fields or cattle farms, and “bottom-up” approaches that measure on-the-ground methane releases at specific sources and sites.
White envisions a greater use of a combination approach in which measurements are taken atmospherically and at the same time at a surface site.
“It isn’t just more measurements that we need. We need them to be coordinated and continuous. We need to close the loop,” he says.
New and currently available technologies could help solve these problems, White says.
“Over the last five or 10 years, technologies for measuring methane and other trace gases, such as ethane, have gotten better, smaller, and cheaper,” he says. “The pieces are fitting together.”
Among these new technologies is a portable laser-based field measurement device that continuously measures methane releases (Optica 2018, DOI: 10.1364/OPTICA.5.000320). It can detect methane leaks as small as one-quarter of a human exhalation from nearly a mile away—giving the technique the potential to detect frequent, infrequent, small, or large emissions from distant sites.
Such a tool could overcome one of the primary difficulties in past on-the-ground measurements: gaining access to privately operated methane sources. According to the developer, the instrument could sit on a mobile platform in an area surrounded by hundreds of oil and gas operations or methane-emitting animals. It swivels 360°, sending out carefully-tuned laser beams. The beams pass through the gas plume, which absorbs some of the light. Small mirrors reflect the light back to a detector for measurement. Coupled with atmospheric measurements and models, researchers could put together a more complete assessment of methane emissions.
The National Academies committee also noted that the U.S. Greenhouse Gas Inventory, the primary inventory of human-caused greenhouse gas emissions, is outdated and fails to reflect the past decade of methane studies. The committee recommends creating an advisory panel to improve the inventory.
Jeff Johnson is a science writer based in Washington, D.C.