National Academy of Sciences of the
United States of America
Anthropogenic emissions of methane in the United States
November 25, 2013 - Successful regulation of greenhouse gas
emissions requires knowledge of current methane emission sources.
Existing state regulations in California and Massachusetts
require ∼15% greenhouse gas emissions reductions from current levels
However, government estimates for total US methane emissions
may be biased by 50%, and estimates of individual source sectors are
even more uncertain.
This study uses atmospheric methane observations to reduce
this level of uncertainty.
We find greenhouse gas emissions from agriculture and fossil
fuel extraction and processing (i.e., oil and/or natural gas) are
likely a factor of two or greater than cited in existing studies.
Effective national and state greenhouse gas reduction
strategies may be difficult to develop without appropriate estimates
of methane emissions from these source sectors.
This study quantitatively estimates the spatial distribution of
anthropogenic methane sources in the United States by combining
comprehensive atmospheric methane observations, extensive spatial
datasets, and a high-resolution atmospheric transport model.
Results show that current inventories from the US
Environmental Protection Agency (EPA) and the Emissions Database for
Global Atmospheric Research underestimate methane emissions
nationally by a factor of ∼1.5 and ∼1.7, respectively.
Our study indicates that emissions due to ruminants and
manure are up to twice the magnitude of existing inventories.
In addition, the discrepancy in methane source estimates is
particularly pronounced in the south-central United States, where we
find total emissions are ∼2.7 times greater than in most inventories
and account for 24 ± 3% of national emissions.
The spatial patterns of our emission fluxes and observed
methane–propane correlations indicate that fossil fuel extraction
and refining are major contributors (45 ± 13%) in the south-central
This result suggests that regional methane emissions due to
fossil fuel extraction and processing could be 4.9 ± 2.6 times
larger than in EDGAR, the most comprehensive global methane
These results cast doubt on the US EPA’s recent decision to
downscale its estimate of national natural gas emissions by 25–30%.
Overall, we conclude that methane emissions associated with
both the animal husbandry and fossil fuel industries have larger
greenhouse gas impacts than indicated by existing inventories.
Proceedings of the National Academy of Sciences