In the United States, methane emission data are generated by various entities including the U.S. Environmental Protection Agency (EPA), state and local governments, industry, and researchers from academia, national laboratories, and nongovernmental organizations (NGOs). The stakeholder community using these emission data is even broader and includes policymakers at various levels of government, industry, scientific communities, and the general public. The needs of these stakeholders are diverse, as are their academic backgrounds and their understanding of the generation and reporting of methane emission information. Careful consideration of the audience for any published methane emission data is a key step in generating products that will be scientifically valid and properly used. For the scientific community, presentation of results in peer-reviewed literature would be expected to facilitate dissemination of key technical findings to fellow researchers. Similarly, various governmental agencies, research institutes, industries, and NGOs also publish methane results and reports, but they may not have undergone the peer-review process. As a broader audience attempts to understand and apply the research findings, there is increasing potential for misinterpreting and incorrectly using the results.
Those generating methane information are trying to answer a specific question or otherwise fulfill a requirement or meet a need. Such needs include submission of the Greenhouse Gas Inventory (GHGI) to the United Nations Framework Convention on Climate Change or understanding the difference between bottom-up and top-down emission estimates for a specific region in the United States. Although research goals and end uses of the data may be different, there are some common approaches that could facilitate the presentation of study results for multiple uses. This chapter focuses on the presentation of methane emission data to facilitate comparisons among studies and to ensure that results are useful for policymaking.
Our knowledge of methane emissions is evolving, and there is both a large community generating new methane emission information and an even larger community interested in using the new methane information. Furthermore, the methods utilized
for quantifying methane emissions and the accuracy of those measurement methods are not always clearly communicated to policymakers. Producing quality research findings and generating useful reports start with a good study design. Encouraging active participation of stakeholders in the early design phase of any research program would enhance the usability of the final results by the larger community. Such early participation would allow sufficient time for stakeholders to provide important operational and policy-relevant input, allowing for useful and practical answers to policy-relevant scientific questions. The current peer-review process for scholarly journal articles also helps ensure that the methane information made publicly available is not only of appropriate quality, but that the usability and applicability of that information is available for end users of that information. Applying a similar peer-review process for any published studies or reports on methane could enhance the quality of the overall body of literature on this matter.
Chapters 2 and 3 highlighted the advantages and disadvantages of bottom-up and top-down studies, and the applicability and usefulness of results stemming from such studies could be considered in the design phase. For example, although offsite or ambient measurements of methane can provide methane estimates, there are limitations in using such techniques to attribute emissions to specific sources, which would be needed if the goal of the study were to develop sector-specific programs and policies. Inherent biases in measurement sample sites in bottom-up studies can be minimized through various strategies such as random selection of sites by the research team, review of operational records prior to site visit, and/or comparison of measurements at similar sites.
Additionally, the establishment of research networks can help standardize project design to make the end data more valuable to a wider range of users. For example, in the agriculture sector, the Greenhouse gas Reduction through Agricultural Carbon Enhancement network, was initiated by the U.S. Department of Agriculture (USDA) Agricultural Research Service (ARS). The goal was to quantify greenhouse gas (GHG) emissions from cropped and grazed soils under current management practices using standardized protocols that all sites would follow, as well as to identify and further develop improved management practices that would enhance carbon sequestration in soils, decrease GHG emissions, promote sustainability, and provide a sound scientific basis for carbon credits and GHG trading programs. Data collected by participating researchers are regularly uploaded into a main template that is then made publicly available, allowing further use of the data for activities such as model development and validation and providing greater value than the data collected by individual
researchers.1 Another example is the Conservation Effects Assessment Project, another USDA ARS project, focused on water quality, which provides data across 14 watersheds at 12 locations with all data made publicly available through the Sustaining the Earth’s Watersheds, Agricultural Research Data System database. In the petroleum and natural gas industry, a network2 was formed in 2012 among the Environmental Defense Fund and more than 100 universities, research institutions, and companies, who collaborated to investigate methane losses across the natural gas supply chain using a variety of different measurement techniques. In addition, the U.S. Department of Energy funded multiple methane measurement and monitoring projects across the natural gas systems. Similar to the networks noted above, one goal of this study was to make the latest methane information publicly available to a larger community. For landfill methane emissions, a current California study supported by the California Air Resources Board and the California Department of Resources Recovery and Recycling is systematically addressing statewide approaches for improved quantification of site-specific and regional emissions. This study is being conducted by the California Polytechnic State University and the University of California Irvine. Historically, methane emission research has been supported by the waste industry (i.e., the Environmental Research and Education Foundation, Waste Management, Inc., and Cooperative Research and Development Agreements with the EPA) as well as by public agencies such as the Los Angeles County Sanitation Districts and the Delaware Solid Waste Authority.
Research networks that help standardize project design through (1) consistent data collection methods and presentation within sectors and (2) the development and application of common protocols, if applicable, would facilitate better comparison of data between studies and generate more usable data for policymakers (e.g., consistent use of reporting units and measurements of key variables influencing emissions).
Although final decisions regarding study design or the scope and content of a report are ultimately made by the relevant study lead (whether a principal investigator or a government agency), it is incumbent upon those who generate methane emission information to ensure transparent reporting of the study design, assumptions, methodology, and results so that readers understand the data and their possible uses and
limitations. Transparent data reporting facilitates comparisons among studies, thereby promoting generation of information that is policy relevant. Similar conclusions have been reached by Heath et al. (2014) and Weber et al. (2013), who recommended standardization of definitions and assumptions when communicating methane emission data to policymakers and the general public.
Describing the Coverage (Scope and Spatial and Temporal Boundaries) of the Study
A clear description of the boundaries of a study (including not only the sources covered, but also the specific geographic and temporal scales) is essential to enable potential users of the data to interpret the results. The following questions, if considered when drafting reports, could help users know if and to what extent the results of a particular study can be used in their own work.
- Which processes or technologies are covered in the study and how are these defined? For example, if an estimate of methane emissions at a facility is reported, how is the facility defined? Which processes are included and excluded?
- What are the spatial and temporal boundaries of the study? The GHGI produces a national, annual emission estimate for each category in the United States. Other studies may present data for a specific unit or coal mine over the course of a week or two. Clearly articulating the geographic/spatial and temporal boundaries of the study helps users know the relevance of any resulting methane emission estimate, activity data, or emission factor for their own work or use. For example, a landfill study limited to the warm, dry summer months, a flyover of a natural gas production field during liquid unloading activities, or an estimate of methane emissions from a liquid manure storage operation during summer or winter may not be representative of national conditions over the course of the year for use in the GHGI. Researchers are encouraged to clearly document the spatial and temporal boundaries of a study, and users are likewise encouraged to be aware of this scope prior to using any data in their own work.
Knowing the technical coverage (types of processes covered) informs understanding of the completeness and applicability of the results of a study. Clarity on the spatial and temporal resolution is important because emissions are not uniformly distributed across the United States or over time, and without clear understanding of the boundaries used for a study, there is a risk of misinterpretation of the results.
When presenting results on methane emissions, clarity on the scope and spatial
and temporal boundaries is essential to enable potential users of the data to interpret the results.
The results from specific studies that are quoted and communicated to a broader audience could be misinterpreted as being widely applicable at multiple scales. As such, caution is warranted when study results from a specific bottom-up field study (or specific coordinated top-down and bottom-up studies) are extrapolated to arrive at estimates at larger regional- and national-level scales. This is especially important when measurements are done only at a small number of facilities, because they may not be representative of national conditions. On the other hand, when presenting the results of top-down studies of short duration, it is important to compare the results with the inventory of emission sources within the specific spatial area operating during the specific time period and with careful consideration of operating conditions of these emission sources. In the absence of these vital supporting data, top-down studies may be misleading and perhaps overestimate or underestimate emissions.
Clearly Articulating the Units of Measurement
Methane emissions are typically reported in the units of absolute mass per unit time. For example, the GHGI reports methane emissions in kilotons per year, but other units may be more relevant for other studies. The Committee observed the challenges of consistent reporting of metrics while striving to consistently document available data in this report. Different sectors were found to follow different conventions for units reported that seem to be influenced by the magnitude of emissions and timescales that are commonly reported (e.g., kilograms of methane per cow per year, grams of landfill methane per square meter per day, cubic meters per year from coal mines, and cubic feet per day from natural gas systems, to name a few). See Appendix F for a list of units that are used in this report and standard equivalents used to convert among units. As long as the units are commonly understood and transparently documented, conversion among units should be possible. If data are reported on a carbon dioxide equivalent (CO2eq) basis, it is important to document the global warming potential value that was applied to methane.
In addition to an absolute magnitude of emissions, other intensity metrics such as leakage rate, flux rates, emissions per unit of product, and emissions based on life-cycle analysis have also been used to compare methane emissions across studies. Expressing methane emissions in terms of a methane intensity metric provides a convenient comparison of various operating areas, companies, producers, or regions. Intensity metrics may also facilitate comparison with the GHGI for some categories
(e.g., enteric fermentation, as described below), but they could introduce confusion for others (e.g., petroleum and natural gas systems, where the GHGI does not always have a clear delineation of whether methane is released from a petroleum- or gas-producing well).
In the agriculture sector, for example, there may be other more valuable representations of the emission data that would benefit policymakers, industry, and consumers. For livestock, in addition to reporting total kilotons of methane produced per unit time, emissions could be further expressed as methane produced per head of livestock per unit time, per unit of feed dry matter intake (i.e., emission yield), and per unit of product such as energy-corrected milk or kilogram of daily gain or carcass weight (i.e., emission intensity). When data are presented in this manner, it becomes easier to see if overall efficiency of the industry is improving over time, that is, less methane emitted per unit of milk produced. As agricultural production, energy production, and waste generation are expected to increase along with a growing population, there will likely be inherent increases in total methane produced from these sectors; however, efficiencies can be improved that would be reflected only in the intensity of GHG production when expressed on a production basis.
Providing methane data on an absolute mass/time basis as well as other intensity metrics (e.g., leakage rate, flux rates, emissions per unit of product, and emissions based on life-cycle analyses) is useful for comparing results across studies.
There is often a wealth of underlying activity data and emission factor, parameter, and model information used to generate emission estimates, whether for the GHGI, the Greenhouse Gas Reporting Program (GHGRP), or other local, state, regional, or national methane studies. However, these data are not always publicly available. For example, researchers studying emissions from petroleum and natural gas regions often create their own regional gridded inventories (e.g., Barnett, Fayetteville) and have to rely on state petroleum and natural gas commissions or private databases (see Chapter 2). The public release of basic activity and emission factor data in a machine-readable format, as well as more detailed information on any models used, including the underlying methods, assumptions, and equations used, could facilitate transparency. This would also help improve national methane emission estimates while supporting improved gridded inventories.
Advances have been made in recent years to make data more publicly available. For example, the activity data and emission factors used in the GHGI for petroleum and
natural gas systems is now publicly available in electronic format.3 These efforts could be expanded to other sectors of the GHGI and to other groups publishing methane information. Nonconfidential data reported under the GHGRP also are made publicly available;4 however, the dataset is updated frequently as a result of EPA verification activities and resubmissions made by individual reporters to correct errors. Although there are release numbers associated with the EPA’s online data publication site,5 there is no archive to allow retrieval of previously available datasets that may have been used in published studies, and it is therefore difficult to assess studies based on these estimates.
Future studies and emission estimate comparisons would benefit from increased data transparency by making underlying data used in reports publicly available in machine-readable formats, subject to confidentiality concerns, and improved documentation and archiving of Greenhouse Gas Reporting Program data. The development of a library of data that is properly documented, archived, and publicly accessible would enhance data integrity and facilitate its use.
There is a value in convening multidisciplinary discussions with representatives of the research community, academia, industry, and policymakers that share an interest in methane emission measurement and reporting. Such representatives could be practitioners in the field, research scientists and academics, and those working on larger national and international policy issues and scientific questions. In recent years, the EPA has periodically convened stakeholder workshops to discuss possible forthcoming changes to the methane GHGI for petroleum and natural gas systems6 and the waste sector7 providing an opportunity for any interested stakeholders to offer feedback. These workshops are open to any interested individual and often focus on changes for the next GHGI submission. In addition to conducting these more informal workshops, establishing and maintaining a smaller, more formal advisory group composed
of experts from relevant research, scientific, and policy communities could help guide how evolving science could be incorporated into improving methane inventories in the short, medium, and long term. Such a group could consider questions such as
- Is there sufficient information available to justify updating existing emission factors or activity data?
- What other types of research efforts may be necessary before including data in the GHGI?
- Do the data reported at a facility level, or information generated by the top-down networks, suggest that updating current methods is warranted? Will such top-down information and facility-level data reduce uncertainty?
Any changes to the GHGI resulting from these activities should be transparently described and clearly communicated to the public.
An advisory group could help guide how new science should be incorporated into improving the methane portion of the Greenhouse Gas Inventory. Such an advisory group could be facilitated by the U.S. Environmental Protection Agency and the National Oceanic and Atmospheric Administration and comprised of experts from academia, industry, policymaking, other federal agencies, and nongovernmental organizations. Its goal would be to facilitate timely improvements in activity data and enhance characterization of emission sources and quantities.