In
this analysis we used a spatially explicit, simplified bottom-up
approach, based on animal inventories, feed dry matter intake, and
feed intake-based emission factors to estimate county-level enteric
methane emissions for cattle and manure methane emissions for cattle,
swine, and poultry for the contiguous United States. Overall, this
analysis yielded total livestock methane emissions (8916 Gg/yr; lower
and upper 95% confidence bounds of ±19.3%) for 2012 (last census
of agriculture) that are comparable to the current USEPA estimates
for 2012 and to estimates from the global gridded Emission Database
for Global Atmospheric Research (EDGAR) inventory. However, the spatial
distribution of emissions developed in this analysis differed significantly
from that of EDGAR and a recent gridded inventory based on USEPA.
Combined enteric and manure methane emissions from livestock in Texas
and California (highest contributors to the national total) in this
study were 36% lesser and 100% greater, respectively, than estimates
by EDGAR. The spatial distribution of emissions in gridded inventories
(e.g., EDGAR) likely strongly impacts the conclusions of top-down
approaches that use them, especially in the source attribution of
resulting (posterior) emissions, and hence conclusions from such studies
should be interpreted with caution
