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Methane Capture: Options for Greenhouse Gas Emission Reduction
array of economic sectors, with electric power generation and industrial consumption accounting
for 28% of total consumption; residential use, 21%; and commercial use, 13%.19
Global Warming Potential
Global warming potential (GWP) is an estimate of how much a greenhouse gas affects climate
change over a quantity of time relative to CO2, which has a GWP value of 1. Methane is a potent
greenhouse gas with a global warming potential of 25.20 Over a 100-year timeframe, methane is
25 times more effective than CO2 at trapping heat in the atmosphere. In other words, it takes 25
tons of CO2 to equal the effect of 1 ton of CH4. Methane has a relatively short atmospheric
lifetime (approximately 12 years) when compared to the atmospheric lifetime of carbon dioxide;
thus efforts to capture methane from anthropogenic sources provide more near-term climate
change abatement than capturing or reducing comparable amounts of CO2, but less multi-decadal
Once methane or other greenhouse gases are converted, using GWP or other methods, they can be
expressed in a common unit of measurement: carbon dioxide-equivalent (C02-eq. or CO2e). CO2e
both takes into account the potency of each gas and expresses the quantity of the gas. Carbon
dioxide-equivalent has been adopted as a principal unit of measurement to aggregate or make
comparisons across greenhouse gases. CO2e expresses the tons of a greenhouse gas in the
equivalent effect of tons of CO2 on climate change (more specifically, on "radiative forcing").21
Once all gases are converted to CO2e, they can be compared or added together.
Sources of Methane
The top three anthropogenic sources of the roughly 585 million metric tons CO2e of methane
emitted in 2007 were enteric fermentation, landfills, and natural gas systems.22 These three
sources combined were responsible for about 64% of total U.S. methane emissions (see Figure
1). There are also natural sources of methane emissions, such as wetlands, and releases of natural
gas from geologic formations. Natural sources of methane are generally assumed to account for
30% of an annual methane emissions inventory that includes natural and anthropogenic sources. 23
19 For more information on market conditions for natural gas, see CRS Report R40487, Natural Gas Markets: An
Overview of 2008, by Robert Pirog; and the Energy Information Administration report Natural Gas Year-In-Review
2008, April 2009, http://www.eia.doe.gov/pub/oil-gas/natural-gas/feature articles/2009/ngyir2008/
20 The Intergovernmental Panel on Climate Change (IPCC) assigns methane a carbon dioxide equivalent, or global
warming potential, of 25. Intergovernmental Panel on Climate Change, Climate Change 2007: The Physical Science
Basis (2007), p. 212.
21 "Radiative forcing" is defined as the change in the difference between incoming and outgoing radiation at the top of
the troposphere. CO2e is not exactly equivalent to radiative forcing, but it is similar and easier to understand for policy
purposes than the main alternative, watts per square meter (W/m2).
22 1 teragram = 1 million metric tons. A Tg CO2e (teragram of carbon dioxide equivalent) is a principal unit of
measurement across greenhouse gases. See footnote 3 for the definition of enteric fermentation.
23 Kathleen Hogan, Current and Future Methane Emissions from Natural Sources, United States Environmental
Protection Agency, EPA 430-R-93-011, Washington, DC, August 1993.
Congressional Research Service
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Methane Capture: Options for Greenhouse Gas Emission Reduction, report, February 1, 2010; Washington D.C.. (digital.library.unt.edu/ark:/67531/metadc809768/m1/12/: accessed February 21, 2019), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.