Methane is more than 20 times as effective as CO₂ at trapping heat in the atmosphere—i.e., a global warming potential of 21. (The figure includes the direct effects and those indirect effects due to the production of tropospheric ozone and stratospheric water vapor.) Anthropogenic sources of CH₄ include natural gas and petroleum systems, agricultural activities, landfills, coal mining, wastewater treatment, stationary and mobile combustion, and certain industrial processes.

During the period from 2007 to 2011, gross natural gas production in the US increased from 24,663,656 to 28,479,026 million cubic feet, an increase of 15.5%, according to the US Energy Information Administration (EIA). Within that, from 2007 to 2011, natural gas production from fracking grew 327% from 1,990,145 million cubic feet to 8,500,983 million cubic feet.

Overall, EPA reported a slightly less than 1% drop in methane (CH₄) emissions from 2010 to 2011 (from 592.7 to 587.2 Tg or million metric tons CO₂ equivalent.) CH₄ emissions represented 8.8% of total reported US GHG emissions (6,702.3 Tg CO₂ eq) in 2011.

Natural gas systems—wells, processing facilities, transmission and distribution pipelines—were the largest anthropogenic source category of CH₄ emissions in the US in 2011 with 144.7 Tg CO₂ eq (25%).

The methane and non-combustion CO₂ emissions from natural gas systems are generally process related, with normal operations, routine maintenance, and system upsets being the primary contributors. Emissions from normal operations include: natural gas engines and turbine uncombusted exhaust; bleed and discharge emissions from pneumatic devices; and fugitive emissions (leak) from system components.

Routine maintenance emissions originate from pipelines, equipment, and wells during repair and maintenance activities. Pressure surge relief systems and accidents can lead to system upset emissions.

There are four major stages to natural gas systems, EPA noted: field production; processing; transmission and storage; and distribution.

• Field production. Wells withdraw raw gas from underground formations. Emissions arise from the wells themselves, gathering pipelines, and well-site gas treatment facilities such as dehydrators and separators. Emissions from pneumatic devices, gas wells with liquids unloading, and gas well completions and refracturing (workovers) with and without hydraulic fracturing account for the majority of CH₄ emissions, according to EPA, while flaring emissions account for the majority of the non-combustion CO₂ emissions.

  Emissions from field production accounted for approximately 37% of CH₄ emissions and about 33% of non-combustion CO₂ emissions from natural gas systems in 2011.

• Processing. In this stage, natural gas liquids and various other constituents from the raw gas are removed, resulting in “pipeline quality” gas, which is injected into the transmission system. Fugitive CH₄ emissions from compressors, including compressor seals, are the primary emission source from this stage.

  Processing plants account for about 14 percent of CH₄ emissions and approximately 66 percent of non-combustion CO₂ emissions from natural gas systems.

• Transmission and Storage. Natural gas transmission involves high-pressure, large-diameter pipelines that transport gas long distances from field production and processing areas to distribution systems or large volume customers such as power plants or chemical plants. Compressor station facilities move the gas throughout the United States transmission system. Fugitive CH₄ emissions from these compressor stations and from metering and regulating stations account for the majority of the emissions from this stage.

  CH₄ emissions from the transmission and storage sector account for approximately 30% of emissions from natural gas systems, while CO₂ emissions from transmission and storage account for less than 1% of the non-combustion CO₂ emissions from natural gas systems.

  CH₄ emissions from this source decreased by 11% from 1990-2011 due to increased voluntary reductions (e.g., replacement of high bleed pneumatics with low bleed pneumatics, replacement of wet seals with dry seals).

• Distribution. Distribution pipelines take the high-pressure gas from the transmission system at “city gate” stations, reduce the pressure and distribute the gas through primarily underground mains and service lines to individual end users. There were more than 1,231,000 miles of distribution mains in 2011, an increase of approximately 287,000 miles since 1990.

  Distribution system emissions, which account for approximately 19% of CH₄ emissions from natural gas systems and less than 1% of non-combustion CO₂ emissions, result mainly from fugitive emissions from gate stations and pipelines. An increased use of plastic piping, which has lower emissions than other pipe materials, has reduced emissions from this stage.

Some other significant trends in US emissions of CH₄ reported by EPA include:

• Emissions from enteric fermentation—the second largest anthropogenic source of CH₄ emissions in the United States—increased by 4.6 Tg CO₂ eq. (3.5%) in 2011 since 1990. This increase in emissions from 1990 to 2011 in enteric generally follows the increasing trends in cattle populations.

• Landfills—the third largest anthropogenic source of CH₄ emissions in the United States—accounted for 17.5% of total CH₄ emissions (103.0 Tg CO₂ eq.) in 2011. From 1990 to 2011, CH4 emissions from landfills decreased by 44.7 Tg CO2 Eq. (30.3%), with small increases occurring in some interim years.

• In 2011, CH4 emissions from coal mining were 63.2 Tg CO₂ Eq., a 9.2 Tg CO₂ eq. (12.6%) decrease under 2010 emission levels. The overall decline results from the mining of less gassy coal from underground mines and the increased use of CH₄ collected from degasification systems.

• Methane emissions from manure management increased by 65.3% since 1990, from 31.5 Tg CO2 Eq. in 1990 to 52.0 Tg CO₂ Eq. in 2011. The majority of this increase was from swine and dairy cow manure, since the general trend in manure management is one of increasing use of liquid systems, which tends to produce greater CH₄ emissions.

Resources

• Inventory of US Greenhouse Gas Emissions and Sinks: 1990 – 2011 (EPA 430-R-13-001)