The U.S. Environmental Protection Agency (EPA) released its annual update to the 2016 U.S. Inventory of Greenhouse Gas Emissions (GHGI) recently, showing a slight decrease in total greenhouse gas emissions from 2015 to 2016.
Methane, a potent greenhouse gas and the main constituent of natural gas, is responsible for roughly a quarter of global warming we see today. The GHGI estimates that 2016 methane emissions from the oil and gas industry were 8.37 million metric tons, down just over one percent from 2015 levels.
This decrease is misleading – it’s too slight a dip to indicate that industry is getting a better handle on methane. Studies also suggest that the inventory misses vast amounts of emissions from abnormal process conditions, or super-emitters.
EPA updates are positive but incremental
EPA finally included methane emissions from abandoned oil and gas wells. Emissions were estimated from measurement data collected by recent studies. Critically, EPA recognized that the activity data for this category is incomplete, since many abandoned wells predate recordkeeping requirements. In addition to the two million known wells, EPA estimated emissions from approximately 1.2 million more abandoned wells without records.
Proper plugging of abandoned wells can minimize methane emissions in additional to protecting groundwater resources. EPA’s estimate of abandoned well emissions is only about three to four percent of their total estimate of oil and gas emissions, but it’s still a substantial amount – equivalent to the annual natural gas consumption of about 200,000 homes.
Overall, EPA has made progress with this year’s GHGI, continuing their incorporation of recent data sources such as the Greenhouse Gas Reporting Program (GHGRP) and peer-reviewed science. At the source-level, though there still are some categories that should be updated.
For example, EPA has updated their methods for pneumatic controllers to estimate the number of devices from GHGRP data, but continue to use the 1990s emission factors (EFs) to estimate their emissions. Measurements indicate that these EFs are inaccurate, overestimating the emissions from some types of controllers, but failing to account for high-emitting malfunctioning controllers. If EPA updated their pneumatic controller calculation methodology to distinguish emissions from operational and malfunctioning devices, then they could more accurately estimate emissions and facilitate more effective mitigation.
EPA methodology should be updated with top-down data
As part of the country’s obligations under the United Nations Framework Convention on Climate Change, EPA releases a report every April that estimates greenhouse gas emissions from anthropogenic sources.
Emissions are estimated using a bottom-up, or component-level, approach, which multiplies activity data (e.g., number of wells) by emission factors (e.g., average emissions per well). The GHGI does not use a top-down approach, which estimates total emissions from an area based on measured increases in atmospheric methane concentrations and modeling.
The top-down approach tends to provide a more complete picture since the measurements are inclusive of all emissions; in contrast, the bottom-up approach can miss individual sources. Numerous studies have shown that basin- and site-level top-down estimates are higher than traditional, bottom-up inventories. For example, our research in the Barnett Shale found that site-level well pad estimates were 50 percent higher than component-level estimates.
NAS recommendations for greenhouse gas inventories
Although EPA’s updates this year are valuable and based on scientifically-rigorous data, the GHGI continues to underestimate total oil and gas industry emissions due to fundamental issues with the methodology.
Recently, the National Academy of Sciences, Engineering, and Medicine (NAS), a group of the nation’s most distinguished scientists, released their report on greenhouse gas emissions, including anthropogenic methane emissions. NAS made four recommendations for agency collaboration to improve the accuracy of emission estimates for oil and gas and other methane sources. Here, we discuss the implications of the NAS recommendations for EPA’s inventory.
NAS Recommendation #1: NOAA and NASA should continue and enhance current atmospheric methane observations and advance models and assimilation techniques used by top-down approaches.
NAS recommends expanded efforts to measure emissions with top-down approaches. EPA should coordinate with NOAA, NASA, and other groups to facilitate data collection in priority areas such as oil and gas basins. EDF continues to encourage the development of aerial and other top-down measurement technologies through initiatives such as the Mobile Monitoring Challenge, and recently announced plans for MethaneSAT, which would provide new insights for basin-level methane measurements.
NAS Recommendation #2: The EPA in collaboration with the scientific research community, the DOE, NOAA, USDA, and NASA should establish and maintain a spatially and temporally explicit (e.g. gridded) inventory of U.S. anthropogenic methane emissions.
NAS’s primary message is that emission inventories should be verifiable. Since the GHGI reports national emissions, it is difficult to use top-down data, which comes from smaller areas like basins or sites, to assess its accuracy. The solution is for EPA to develop a gridded inventory that accounts for variation in emissions across space and time. This allows a direct comparison of bottom-up and top-down emission estimates, like EDF performed previously in the Barnett Shale. EPA has already made progress on this front by partnering with Harvard University to grid the 2016 GHGI. EPA should prioritize routinely updating the gridded inventory.
NAS Recommendation #3: The EPA, DOE, NOAA, and USDA should promote a sustainable process for incorporating the latest science into the GHGI and regularly review U.S. methane inventory methodologies.
NAS recommends a formal process for updating the GHGI methods, such as a scientific advisory panel. Typically, EPA has convened workshops, published memos, and solicited comments on proposed updates to the GHGI. Overall, this process has been transparent and effective for implementing incremental changes, but it is not well-suited for making fundamental revisions to the methodology. We suggest that EPA convenes a series of workshops and/or solicits comments related to the use of top-down data and a gridded inventory to update the GHGI. A scientific advisory panel could also be valuable, as long as the panel is balanced by including representatives from academia and public institutions, and not just industry representatives.
NAS Recommendation #4: The United States should establish and maintain a nationwide research effort to improve accuracy, reliability, and applicability of anthropogenic methane emissions estimates at scales ranging from individual facilities to gridded regional/national estimates.
EDF realized the urgency of the oil and gas methane issue when we launched our series of research studies in 2012. We continue to study methane both in the U.S. and internationally, such as end use emissions from U.S. power plants and fugitive emissions from Alberta well pads. There still are many questions about oil and gas methane emissions that needed to be addressed, such as the most common cause of super-emitters. Further research of U.S. methane emissions will not only lead to a more accurate inventory but also facilitate more effective mitigation to reduce the climate impacts of oil and gas.
By David Lyon