The Environmental Protection Agency 2012 greenhouse gas (GHG) inventory shows that cows and other ruminants produce more "anthropogenic" methane than natural gas systems.

Methane emissions from enteric fermentation by ruminants has increased 2.3 percent since 1990 and now represents 25 percent of U.S. methane emissions. Natural gas systems have reduced their emissions almost 17 percent since 1990, but are still a major source, representing 22 percent of U.S. methane emissions.

EPA attributes the natural gas systems' emissions reduction to improved technology used in production and distribution. Natural gas production emissions increased by 24.9 percent from 1990 through 2006 due primarily to increases in hydraulically fractured well completions and workovers, and then declined by 20 40.4 percent from 2006 to 2012. The recent decline is attributed to industry adoption of new technology.

Should we attribute the increase in emissions from cows to the fact that there are more well-paid industry workers eating steak instead of chicken?

The chart below shows the methane emissions by source.

The inventory reports on CO2, CH4, N2O, and several other fluorine-containing halogenated substances, so there is much more to read. The complete draft 1990-2012 Greenhouse Gas Emissions Inventory and instructions on how to submit comments on the draft are available here. Comments are due by March 26, 2014.