A Gridded Inventory of Annual 2012-2018 U.S. Anthropogenic Methane Emissions

Abstract

Nationally reported greenhouse gas inventories are a core component of the Paris Agreement's transparency framework. Comparisons with emission estimates derived from atmospheric observations help identify improvements to reduce uncertainties and increase the confidence in reported values. To facilitate comparisons over the contiguous United States, we present a 0.1° × 0.1° gridded inventory of annual 2012-2018 anthropogenic methane emissions, allocated to 26 individual source categories, with scale-dependent error estimates. Our inventory is consistent with the U.S. Environmental Protection Agency (EPA) Inventory of U.S. Greenhouse Gas Emissions and Sinks (GHGI), submitted to the United Nations in 2020. Total emissions and patterns (spatial/temporal) reflect the activity and emission factor data underlying the GHGI, including many updates relative to a previous gridded version of the GHGI that has been extensively compared with observations. These underlying data are not generally available in global gridded inventories, and comparison to EDGAR version 6 shows large spatial differences, particularly for the oil and gas sectors. We also find strong regional variability across all sources in annual 2012-2018 spatial trends, highlighting the importance of understanding regional- and facility-level activities. Our inventory represents the first time series of gridded GHGI methane emissions and enables robust comparisons of emissions and their trends with atmospheric observations.

Keywords: UNFCCC; agriculture; coal; greenhouse gas; natural gas; petroleum; waste.

Figure 1

Schematic of the gridding methodology showing the gridding of enteric fermentation emissions as an example. (Step A) State emissions are allocated based on national emissions multiplied by the fraction of proxy data in each state. (Step B) County emissions are allocated based on state-level emissions multiplied by the fraction of proxy data in each county. (Step C) Gridded emissions are allocated based on county-level emissions multiplied by the fraction of proxy data within each grid cell.

Figure 2

Gridded 2018 CONUS methane emission fluxes, split among six aggregate source groups. CONUS totals for 2018 are given in subplot titles.

Figure 3

National gridded CONUS methane emissions: (a) absolute 2018 emission fluxes, (b) change in total emission fluxes between 2012 and 2018, and (c) illustration of regional changes in emission fluxes for specific source groups.

Figure 4

Comparison between the gridded GHGI, EDGAR v6, and CALGEM inventories. (a) Total anthropogenic methane emission fluxes from EDGAR v6 for 2018, (b) gridded GHGI–EDGAR difference for 2018, (c) 2012–2018 trend in EDGAR v6, (d) total livestock, oil and gas, landfill, and wastewater emissions in CALGEM for 2012, and (e) gridded GHGI–CALGEM difference for 2012. (d,e) use the same color scales as (a,b), respectively. Correlation coefficients and totals are calculated over cells with nonzero emissions in both inventories.

Figure 5

Error standard deviation curves optimized based on comparison of 2012 emissions from the gridded GHGI with the Barnett (solid) and CALGEM (dashed) inventories. Errors are shown as a function of resolution, with solid lines on the right representing the national-level GHGI errors.