This report outlines actions to improve estimates of the amounts of anthropogenic methane emitted in the United States and to improve the utility and usability of methane emission inventories to governments, industries, academia, nongovernmental organizations, and the general public. Atmospheric observations are fed into atmospheric inverse models to compute emission fluxes, called the “top-down” approach to emission estimation. The more familiar “bottom-up” approach is based on scaling up of data for emissions by individual components or facilities.Top-down and bottom-up approaches can be tested against each other to improve the application of both approaches.
The traceable attribution of emissions to specific sectors, processes, and components is a key output of bottom-up inventories, which are thus uniquely suited for applications in the sphere of mitigation and societal interests. Currently, however, the U.S. Greenhouse Gas Inventory (GHGI) of methane emissions cannot be independently tested against top-down measurements, because both spatial and temporal attributes are missing, and thus expected atmospheric concentrations cannot be inferred. There are many benefits of building a strong link between atmospheric measurements of methane concentrations and methane emission inventories, including the discovery of missing sources or processes, improved confidence in the basic data that enter into decisions by companies and governments, and better capability to detect trends over time.
The effective interlinking of top-down and bottom-up approaches involves strengthening both approaches, as recommended in Box 6.1. These steps will enable improved accuracy, better attribution of emissions to specific sectors/processes, and detection of trends in sectoral emissions.
The four recommendations that follow are interlinked, and if they are to meet the goals outlined in this report, they should be viewed as interdependent and addressed systematically1 (Figure 6.1). The monitoring and verification of methane emissions is
1 The Committee identifies agencies to implement the recommendations where possible but recognizes that the list it provides may not be exhaustive, and that the agencies should have the flexibility to parse out the tasks as they think will best serve the interests and missions of their agencies and the country.
a science that is, at least in part, still in early stages of development. The United States should take bold action now to match monitoring and measurement efforts to the importance of the task.
Recommendation 1: The National Oceanic and Atmospheric Administration and the National Aeronautics and Space Administration should continue and enhance current atmospheric methane observations and advance models and assimilation techniques used by top-down approaches.
Atmospheric observations of methane are the foundation for understanding anthropogenic and natural changes in methane concentrations. Long-term observations, together with atmospheric models, are critical for estimating methane emissions and detecting large-scale trends. A monitoring program needs sufficient spatial and temporal coverage to be able to determine anthropogenic and natural emissions and track changes over time. Some examples of dedicated efforts are the European monitoring initiatives, such as the Integrated Carbon Observation System and Copernicus, providing dense continuous methane monitoring sites. The Committee recommends continuing the existing U.S. and global background observational networks and expanding measurements across multiple spatial scales, including in the vertical dimension. Multiscale observational strategies (e.g., aircraft, surface, tower, and satellite remote sensing) supply complementary information and can provide flux estimates from the facility scale through the local and regional scale, enabling direct comparison with gridded inventories (Recommendation 2).
Because atmospheric measurements reflect both natural and anthropogenic methane sources, accurate knowledge of the spatiotemporal distribution of natural methane emissions is also required but currently lacking, especially in regions with spatial overlap between different source categories. This limitation inhibits our abilities to accurately attribute observed methane emissions to source categories and thus processes.
Measuring a suite of tracer species that provide additional information on the attribution of emissions should also be expanded. Related species used to obtain insight into underlying source processes can include carbon monoxide, ethane,13CH4, CH3D, 14CH4, and multiply-substituted (clumped) methane isotopes. Geochemical fingerprinting of various methane sources could assist with this effort by identifying multiple gases and conditions capable of unambiguously tracking specific types of sources. Scientific studies are also needed to identify necessary measurements to close observational gaps that result in current ambiguities of our methane budget (sources and sinks; Chapters 3 and 4).
In addition to investing in comprehensive monitoring of atmospheric methane and associated species, improvements in our ability to accurately simulate these data in models should be undertaken. Current models cannot adequately represent small-scale processes that affect measured concentrations, such as planetary boundary layer mixing and diurnal growth, convection, and complex effects of local topography. Advances in atmospheric modeling are needed, as well as innovative model evaluation tools.
Recommendation 2: The U.S. Environmental Protection Agency, in collaboration with the scientific research community, the U.S. Department of Energy, the National Oceanic and Atmospheric Administration, the U.S. Department of Agriculture, and the National Aeronautics and Space Administration should establish and maintain a fine-scale, spatially and temporally explicit (e.g., gridded) inventory of U.S. anthropogenic methane emissions that is testable using atmospheric observations and update it on a regular basis.
The development of a spatially and temporally resolved anthropogenic inventory is required for independent verification using atmospheric observations. A gridded inventory, including values for the major sources within the grid, is a prerequisite for detailed comparisons between bottom-up and top-down approaches, enabling feedback into improved inventories at the national scale. The recent efforts to report the GHGI in a spatially and temporally resolved form (Chapter 2) should be continued and expanded.
The GHGI has increasingly been used for purposes for which it was not initially designed, including comparisons with top-down estimates of methane emissions in specific regions at specific times. The gridded inventory should be consistent with the national GHGI inventory for integrated total emissions per source and have sufficient documentation to allow the scientific and policy communities interested in regional methane emissions to reproduce and adapt the inventory. For example, providing the gridded datasets used to create the inventory would facilitate direct comparison of top-down analyses with not just the bottom-up product (flux), but also the bottom-up methodology. The spatial and temporal resolution should be at as fine a scale as possible (e.g., 0.1° × 0.1° or finer spatial resolution and monthly or finer temporal resolution), based on the data used to generate the inventory. Because expected uses of the inventory, such as comparison with top-down snapshots of daytime emissions, require a finer temporal resolution than the inventory would normally support, the inventory should also provide guidance on fine-scale temporal allocation of emissions.
Recommendation 3: The U.S. Environmental Protection Agency, U.S. Department of Energy, National Oceanic and Atmospheric Administration, and U.S. Department of Agriculture should promote a sustainable process for incorporating the latest science into the U.S. Greenhouse Gas Inventory and regularly review U.S. methane inventory methodologies.
The GHGI, in particular should be maintained at the federal level and data made publicly available on an annual basis. It is important to recognize, however, that the science is evolving. Some inventory methodologies become outdated; therefore, methodologies should be evolutionary to be consistent with the best scientific under-
standing and current engineering practice. Although emission estimates extending back to 1990 are needed for GHGI reporting to the United Nations Framework Convention on Climate Change, the need to back-cast to 1990 should not be a constraint in implementing new methods that improve emission estimates for current and future applications (Chapter 2).
Inventory improvements should be done in coordination with both the research community and stakeholders. However, the Committee recognizes that the inventory process evolves slowly. There is a need for an advisory group to guide how new science should be incorporated into improving the national GHGI (Chapter 5). Such a group could be facilitated by the U.S. Environmental Protection Agency and National Oceanic and Atmospheric Administration and composed of experts from academia, industry, policymaking, other federal agencies, and nongovernmental organizations. Their goal would be to facilitate timely improvements in activity data and enhance characterization of emission sources and quantities. Any resulting changes in the methodologies, process models, or underlying inputs should be transparently described and clearly communicated to the public.
Recommendation 4: The United States should establish and maintain a nationwide research effort to improve accuracy, reliability, and applicability of anthropogenic methane emission estimates at scales ranging from individual facilities to gridded regional/national estimates.
The Committee calls for a sustained coordinated national research effort to improve top-down and bottom-up estimation and monitoring methods and technologies. These efforts should include results from atmospheric observations, sustained spatial and temporal characterization of methane emissions for key sectors in the United States, improvements in models and assimilation techniques used by top-down approaches, and improvements in estimation techniques and process models. Results from such efforts could better support attribution of emissions to specific sectors or processes as well as trend detection. The improved monitoring network and recommended gridded national inventory (Recommendations 1 and 2) are necessary to conduct comparisons at the finest temporal and spatial scales that the data, models, and site access allow. The comparisons will enable stakeholders and the public to identify deficiencies in the gridded inventory as well as the atmospheric observations, catalyzing Recommendation 3 for continual improvement in all aspects of methane inventory development. The improvements should be informed by a national research effort (Recommendation 4) for which the guiding goal should be (1) better quantification and attribution of methane emission rates and trends over time and (2) identification of knowledge gaps and guidance for resolving those gaps (Chapters 3 and 4).