This paper assesses the current state of knowledge about the average greenhouse gas (GHG) footprints of coal and natural gas-fired electricity in the United States today, how the growing share of natural gas production from shale formations could change this GHG footprint at the margin, and what the findings imply for policymakers, investors, and the environment. The first part of the paper examines recent bottom-up life-cycle analyses to provide context for the top-down analysis. The second part of the paper conducts the top-down life-cycle analysis of GHGs from natural gas and coal-fired electricity in 2008 using the U.S. Environmental Protection Agency's revised 2011 estimates as well as other publically available government data.

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