The US should take bold steps to improve measurement, monitoring, and inventories of methane emissions caused by human activities, a Mar. 27 report issued by the National Academies of Science, Engineering, and Medicine (NASEM) concluded. Better data on the greenhouse gas would help inform government policy decisions related to climate, economics, and human health, it suggested.
“Methane is getting more attention because it is a potent, short-lived greenhouse gas that is increasing,” said James WC White, a geological sciences professor at the University of Colorado at Boulder who chaired the committee that conducted the study and wrote the report. “There have been recent advances in our abilities to measure and monitor methane from its many sources, and now we need to strengthen and [link] these different approaches.”
The US Environmental Protection Agency, US Department of Energy, National Oceanic and Atmospheric Administration, and National Aeronautics and Space Administration sponsored the study.
Methane emissions sources from human activity span energy, agriculture, waste disposal, and other economic sectors, the report’s highlights noted. Natural origins include wetlands, coastal oceans, wildfires, and geologic sources, they said. “Given methane’s value as the main component of natural gas, monitoring in the energy, agriculture, and waste sectors is aimed at reducing losses or enhancing recovery of methane,” the summary said.
It identified two main approaches for measuring methane emissions:
• Bottom-up (or inventory development) estimates that measure emissions at the scale of individual methane emitters, such as natural gas wells or cattle farms. Methane inventories are developed for a variety of purposes and use a combination of activity data (the number of emitters) and emission factors (the amount of methane per emitter), which then are extrapolated to regional and national scales, it said. EPA’s Greenhouse Gas Inventory is the primary domestic measurement of human-caused GHG emissions, it said.
• Top-down estimates use observations of atmospheric methane concentrations and models that account for transportation from the emitter to the observation location to estimate how much emissions must have occurred. Such measurements are made via aircraft, surface, and tower and satellite remote sensing, the summary said.
Each approach has its strengths and limitations, the report said. “Bottom-up methods provide information about emissions from specific sources but may not account for all sources and may use uncertain or inaccurate activity data and emissions factors,” it explained. “In contrast, top-down estimates include emissions from all sources, both natural and anthropogenic, but may have difficulty in attributing emissions to specific sources or source categories.”
In some cases, the estimates that the two methods produce differ substantially, potentially revealing missing emission sources, for example, or problems with the atmospheric sampling, the report observed. To address such discrepancies, it recommended a national research effort to strengthen the methods to improve accuracy, better attribute emissions to specific sectors and processes, and detect trends.
The report pointed out that in recent years, EPA periodically convened webinars and workshops with stakeholders to discuss possible changes to its GHG inventory and get feedback.
“In addition, an advisory group could help guide how new science should be incorporated into improving the methane portion of [the GHG inventory],” it continued. EPA and NOAA could facilitate this advisory group of experts from academia, industry, government policymaking and other agencies, and nongovernment organizations, the report suggested. The group’s goal would be to facilitate timely improvements in activity data and enhance characterizations of emissions sources and quantities, it said.
