3. Methane Emissions

Overview

U.S. Anthropogenic Methane Emissions, 1990-2004 Printer Friendly Version   Methane Carbon Dioxide Equivalent Estimated 2004 Emissions (Million Metric Tons) 27.8 639.5 Change Compared to 2003 (Million Metric Tons) 0.2 5.6 Change from 2003 (Percent) 0.9% 0.9% Change Compared to 1990 (Million Metric Tons) -3.6 -81.9 Change from 1990 (Percent) -11.4% -11.4%

U.S. anthropogenic methane emissions in 2004 totaled 639.5 million metric tons carbon dioxide equivalent (MMTCO₂e),⁶³ or 27.8 million metric tons of methane, representing 9.0 percent of total U.S. greenhouse gas emissions. U.S. methane emissions in 2004 were 0.9 percent (5.6 MMTCO₂e) higher than their 2003 level of 633.9 MMTCO₂e (Table 15), primarily as a result of an increase in emissions from landfills and smaller increases in emissions associated with animal waste, rice cultivation, and coal mining.

U.S. emissions of methane in 2004 were 11.4 percent (81.9 MMTCO₂e) below their 1990 level of 721.4 MMTCO₂e (Figure 2). In addition to a reduction of 74.4 MMTCO₂e (29 percent) in methane emissions from landfills since 1990, there was also a decrease of 29.5 MMTCO₂e (30 percent) in methane emissions from coal mines over the same period (Table 16). The 30-percent decline in emissions from coal mining was the result of a 150-percent increase in methane recovery from coal mines and a shift in production away from gassy mines.

Methane emission estimates are much more uncertain than carbon dioxide emission estimates. Methane emissions usually are accidental or incidental to biological processes and may not be metered in any systematic way.⁶⁴ Thus, methane emission estimates must often rely on proxy measurements.

Principal Sources of U.S. Anthropogenic Methane Emissions, 1990-2004 Printer Friendly Version Source Million Metric Tons CO2e Percent Change 1990 2004 1990- 2004  2003- 2004 Energy 275.04 256.31 -6.8% * Waste Management 270.21 198.19 -26.7% 2.4% Agriculture 173.42 182.25 5.1% 0.5% Industrial Processes 2.70 2.70 0.2% 4.1%

Estimated U.S. anthropogenic methane emissions for 2004 are based on incomplete data for several key sources; thus, the overall estimate is likely to be revised. Because emissions from three of these sources—coal mining, natural gas systems, and landfills—represented more than three-fifths of all U.S. methane emissions, comparisons between 2003 and 2004 numbers are more likely to be valid in terms of their direction than their magnitude. For example, because 2004 data on waste generation are not yet available, waste generation has been estimated from a simple regression equation with economic output as the independent variable. Less critical but still important data are also unavailable for natural gas systems, such as miles of gas transmission and distribution pipeline.

Energy Sources

U.S. methane emissions from energy sources in 2004 are estimated at 256.3 MMTCO₂e, equivalent to 40 percent of total methane emissions. The 2004 emission level is nearly unchanged from the 2003 level of 256.4 MMTCO₂e. Total methane emissions from energy sources in 2004 were 18.7 MMTCO₂e below their 1990 level of 275.0 MMTCO₂e.

The drop in methane emissions from energy sources since 1990 can be traced primarily to an overall reduction in emissions from coal mines and secondarily to lower emissions from petroleum systems and stationary combustion. Methane emissions from coal mines dropped by 30 percent (29.5 MMTCO₂e) between 1990 and 2004. This decline resulted partly from the increased capture and use of methane from coal mine degasification systems and a shift in production away from some of the Nation’s gassiest underground mines in Central Appalachia. Also, between 1990 and 2004, the share of coal production represented by underground mines declined from 41 percent to 34 percent. Methane emissions from underground mines tend to be higher than emissions from surface mines per ton of coal mined, because coal mined from the surface has been subjected to lower pressures and methane in the seams of surface mines has had earlier opportunities to migrate to the surface through cracks and fissures.

Methane emissions from petroleum systems dropped from 29.9 MMTCO₂e in 1990 to 23.2 MMTCO₂e in 2004. A decrease of 5.0 MMTCO₂e in estimated methane emissions from stationary combustion (from 13.0 MMTCO₂e in 1990 to 8.0 MMTCO₂e in 2004) made a smaller contribution to the overall drop in emissions from energy sources between 1990 and 2004. Together, the declines in emissions from coal mining, petroleum systems, and stationary combustion more than compensated for the increase of 23.7 MMTCO₂e in emissions from natural gas systems, attributed to increasing U.S. consumption of natural gas between 1990 and 2004.

Coal Mining

U.S. Methane Emissions from Coal Mining, 1990-2004 Printer Friendly Version   Methane Carbon Dioxide Equivalent Estimated 2004 Emissions (Million Metric Tons) 3.0 68.2 Change Compared to 2003 (Million Metric Tons) * 1.0 Change from 2003 (Percent) 1.5% 1.5% Change Compared to 1990 (Million Metric Tons) -1.3 -29.5 Change from 1990 (Percent) -30.2% -30.2%

The preliminary estimate of methane emissions from coal mines for 2004 is 68.2 MMTCO₂e (Table 16), an increase of 1.5 percent (1.0 MMTCO₂e) from the 2003 level of 67.2 MMTCO₂e. This increase can be traced primarily to a 3.7-percent increase in coal production in 2004, centered predominantly in underground mines (up by 6.8 percent). U.S. coal production rose to 1.11 billion short tons in 2004, up from 1.07 billion short tons in 2003. The increase in coal production resulted from robust economic growth in 2004 that was accompanied by higher demand for coal to produce electricity and by record levels of coal exports.⁶⁵

Methane emissions from coal mines have dropped by 30 percent, from 97.7 MMTCO₂e in 1990 to 68.2 MMTCO₂e in 2004. The decline is attributed to three important trends: (1) methane recovery from active coal mines for use as an energy resource increased from 6.1 MMTCO₂e in 1990 to about 15.2. MMTCO₂e in 2004; (2) methane emissions from degasification systems were reduced by nearly 8.5 MMTCO₂e, from 28.9 MMTCO₂e in 1990 to 20.4 MMTCO₂e in 2004; and (3) decreases in coal production from gassy mines, combined with enhanced methane recovery though degasification, caused methane emissions from ventilation systems at gassy mines to drop by about 12.8 MMTCO₂e, from 48.9 MMTCO₂e in 1990 to 36.1 MMTCO₂e in 2004 (Table 16).⁶⁶

Abandoned coal mines represent a significant source of additional emissions that has not been incorporated into the overall estimate of methane emissions in this report because of uncertainties associated with the data. The text box on "Methane Emissions from Abandoned Coal Mines" provides a discussion of those uncertainties and the potential magnitude of additional emissions.

Natural Gas Systems

U.S. Methane Emissions from Natural Gas Systems, 1990-2004 Printer Friendly Version   Methane Carbon Dioxide Equivalent Estimated 2004 Emissions (Million Metric Tons) 6.6 152.6 Change Compared to 2003 (Million Metric Tons) * -0.5 Change from 2003 (Percent) -0.3% -0.3% Change Compared to 1990 (Million Metric Tons) 1.0 23.7 Change from 1990 (Percent) 18.4% 18.4%

U.S. Methane Emissions from Petroleum Systems, 1990-2004 Printer Friendly Version   Methane Carbon Dioxide Equivalent Estimated 2004 Emissions (Million Metric Tons) 1.0 23.2 Change Compared to 2003 (Million Metric Tons) * -0.1 Change from 2003 (Percent) -0.4% -0.4% Change Compared to 1990 (Million Metric Tons) -0.3 -6.7 Change from 1990 (Percent) -22.5% -22.5%

U.S. Methane Emissions from Stationary Combustion, 1990-2004 Printer Friendly Version   Methane Carbon Dioxide Equivalent Estimated 2004 Emissions (Million Metric Tons) 0.3 8.0 Change Compared to 2003 (Million Metric Tons) * -0.5 Change from 2003 (Percent) -6.3% -6.3% Change Compared to 1990 (Million Metric Tons) -0.2 -5.0 Change from 1990 (Percent) -38.7% -38.7%

At 152.6 MMTCO₂e, 2004 estimated methane emissions from natural gas systems were down by 0.3 percent from the 153.1 MMTCO₂e emitted in 2003 (Table 17). The 2004 estimate is preliminary, however, because pipeline data for 2004 have not been finalized as of the publication of this report. The estimated 2004 emissions level is 18 percent (23.7 MMTCO₂e) above the 1990 level (128.9 MMTCO₂e), with more than three-fifths of the increase attributable to increased mileage of transmission and distribution pipelines and almost two-fifths attributable to increases in natural gas production.⁶⁷

Petroleum Systems

Methane emissions from petroleum systems in 2004 are estimated at 23.2 MMTCO₂e, nearly unchanged from their 2003 level and down by 22 percent (6.7 MMTCO₂e) from their 1990 level of 29.9 MMTCO₂e. The decline in emissions from this source is almost exclusively due to a 26-percent reduction in domestic oil production from 1990 to 2004. Approximately 92 percent (21.4 MMTCO₂e) of all U.S. emissions from petroleum systems occur during oil exploration and production (Table 18). A much smaller portion of methane emissions from petroleum systems can be traced to refineries (0.6 MMTCO₂e) and transportation of crude oil (1.1 MMTCO₂e).

Stationary Combustion

U.S. methane emissions from stationary combustion in 2004 were 8.0 MMTCO₂e, down by 6.3 percent from their 2003 level of 8.5 MMTCO₂e and 39 percent below their 1990 level of 13.0 MMTCO₂e (Table 19). Residential wood consumption typically accounts for about 85 percent of methane emissions from stationary combustion. Methane emissions are the result of incomplete combustion, and residential woodstoves and fireplaces provide much less efficient combustion than industrial or utility boilers. Estimates of emissions from residential wood combustion have fallen by 43 percent, from 11.8 MMTCO₂e in 1990 to 6.7 MMTCO₂e in 2004, although these estimates are very uncertain.⁶⁸

The universe of wood consumers is large and heterogeneous, and the Energy Information Administration (EIA) collects data on residential wood consumption only at 4-year intervals in its Residential Energy Consumption Survey (RECS). The most recently published EIA data on residential wood consumption are from the 2001 RECS.⁶⁹ Updated data on residential wood consumption for calendar year 2004 will be available from the 2005 RECS.

Mobile Combustion

Estimated U.S. methane emissions from mobile combustion in 2004 were 4.4 MMTCO₂e, up by 0.7 percent from the 2003 level but 22 percent lower than the 1990 level of 5.6 MMTCO₂e (Table 20). Methane emissions from passenger cars have declined since 1990 as older vehicles with catalytic converters that are less efficient at destroying methane have been taken off the road. Estimates of methane emissions from mobile sources have been revised downward in the last two annual editions of this report, reflecting a change in the source of data for vehicle miles traveled and a related adjustment in the emission factors for light-duty trucks.⁷⁰

Waste Management

U.S. Methane Emissions from Mobile Sources, 1990-2004 Printer Friendly Version   Methane Carbon Dioxide Equivalent Estimated 2004 Emissions (Million Metric Tons) 0.2 4.4 Change Compared to 2003 (Million Metric Tons) * * Change from 2003 (Percent) 0.7% 0.7% Change Compared to 1990 (Million Metric Tons) -0.1 -1.2 Change from 1990 (Percent) -22.1% -22.1%

Figure Data

U.S. Methane Emissions from Landfills, 1990-2004 Printer Friendly Version   Methane Carbon Dioxide Equivalent Estimated 2004 Emissions (Million Metric Tons) 7.9 182.6 Change Compared to 2003 (Million Metric Tons) 0.2 4.5 Change from 2003 (Percent) 2.5% 2.5% Change Compared to 1990 (Million Metric Tons) -3.2 -74.4 Change from 1990 (Percent) -29.0% -29.0%

U.S. Methane Emissions from Domestic and Commercial Wastewater Treatment, 1990-2004 Printer Friendly Version   Methane Carbon Dioxide Equivalent Estimated 2004 Emissions (Million Metric Tons) 0.7 15.6 Change Compared to 2003 (Million Metric Tons) * 0.2 Change from 2003 (Percent) 1.0% 1.0% Change Compared to 1990 (Million Metric Tons) 0.1 2.4 Change from 1990 (Percent) 18.0% 18.0%

Methane emissions from waste management, at 198.2 MMTCO₂e, accounted for 31 percent of U.S. anthropogenic methane emissions in 2004 (Figure 2). Emissions from this source have fallen by 27 percent (72.0 MMTCO₂e) from their 1990 level of 270.2 MMTCO₂e. Landfills represent 92 percent (182.6 MMTCO₂e) of the methane emissions from waste management in 2004, and they are the largest single source of U.S. anthropogenic methane emissions (Table 15). The remainder of emissions from waste management (15.6 MMTCO₂e) is associated with domestic wastewater treatment. Estimated emissions from waste management would increase if sufficient information were available to develop a reliable estimate of emissions from industrial wastewater treatment.

EIA’s estimates of methane emissions from landfills include emissions from both municipal solid waste landfills and industrial landfills. Estimated methane emissions from industrial landfills are based on a methodology developed by the EPA and represent 7 percent of emissions from municipal solid waste landfills.⁷¹

EIA has revised its estimates of methane recovered for energy from landfills. Previous editions of this report erroneously included the avoided emissions of carbon dioxide from fossil fuel combustion displaced by landfill gas-to-energy operations in the estimate of methane recovered. This resulted in double counting of the impacts of fossil fuel displacement by landfill gas-to-energy. For this year’s report, the avoided emissions have been removed from estimates going back to 1990. Similarly, estimates of methane recovered and flared have been lowered to eliminate some potential double counting associated with methane recovery for energy projects, as flares are typically used for backup systems at plants that recover methane for energy. Together, these revisions have raised the EIA estimates of overall methane emissions from waste management by increments ranging from 8 MMTCO₂e for 1990 to 21 MMTCO₂e for 2003.

Landfills

Due to increased levels of waste disposed in landfills, estimated methane emissions from landfills rose to 182.6 MMTCO₂e in 2004, 2.5 percent (4.5 MMTCO₂e) above the 2003 level of 178.1 MMTCO₂e but still 29 percent (74.4 MMTCO₂e) below the 1990 level of 257.0 MMTCO₂e (Table 21). The dramatic decrease in methane emissions since 1990 is directly attributable to a 100.4 MMTCO₂e increase in methane captured at landfills that otherwise would have been emitted to the atmosphere. Of the 122.1 MMTCO₂e of methane believed to be captured from this source in 2004, 63.3 MMTCO₂e was recovered for energy use, and 58.9 MMTCO₂e was recovered and flared. In 2004, methane recovery for energy increasingly took the form of direct use of medium-Btu gas in industrial boilers. The acceleration of this practice was driven by higher natural gas prices, which made landfill gas more competitive.⁷²

Estimates of methane recovered for energy are drawn from data collected by the U.S. Environmental Protection Agency’s (EPA’s) Landfill Methane Outreach Program.⁷³ Estimates of methane recovered and flared are based on data collected from flaring equipment vendors, in conjunction with data reported on Form EIA-1605.⁷⁴ There is less uncertainty in the estimate of methane recovered and used for energy, and it is likely that estimates of methane flared are biased downward due to a lack of comprehensive industry data.

The rapid growth in methane recovery has been aided by a combination of regulatory and tax policy. The Federal Section 29 (of the Internal Revenue Code) tax credit for alternative energy sources, added to the tax code as part of the Crude Oil Windfall Profits Act of 1980, provided a subsidy roughly equivalent to 1 cent per kilowatthour for electricity generated from landfill gas; however, the tax credit expired on June 30, 1998. As part of the American Jobs Creation Act of 2004, a tax credit for electricity generation using landfill gas was added to Section 45 of the Internal Revenue Code. The credit was augmented under the Energy Policy Act of 2005, which extended the credit period—previously 5 years from the original date of service—to 10 years from the original date of service. To be eligible for the credit, a landfill gas-to-energy project must be placed in service between October 22, 2004, and December 31, 2007. Those facilities that qualify are eligible for a 5-year tax credit valued at 0.9 cent per kilowatthour.

Increases in methane recovery have also resulted from the implementation of the EPA’s New Source Performance Standards and Emission Guidelines. The regulations require all landfills with more than 2.5 million metric tons of waste in place and annual emissions of nonmethane organic compounds (NMOCs) exceeding 50 metric tons to collect and burn their landfill gas, either by flaring or for use as an energy source.

Domestic and Commercial Wastewater Treatment

With the U.S. population growing slowly, methane emissions from domestic and commercial wastewater treatment are estimated to have grown by nearly 1.0 percent between 2003 and 2004 to 15.6 MMTCO₂e—about 18 percent above the 1990 level of 13.2 MMTCO₂e (Table 15). Methane emissions from industrial wastewater treatment are discussed in the text on "Methane Emissions from Industrial Wastewater Treatment".

Methane emissions from domestic and commercial wastewater treatment are a function of the share of organic matter in the wastewater stream and the conditions under which it decomposes. Wastewater may be treated aerobically or anaerobically. Because aerobic decomposition does not yield methane, whereas anaerobic decomposition does, the method of treatment is a critical determinant of emissions; however, there is little information available on wastewater treatment methods. Data on flaring or energy recovery from methane generated by wastewater are also sparse. EIA believes that emissions from this source are relatively small, representing less than 3 percent of all U.S. methane emissions in 2004. Thus, emissions are estimated using a default per-capita emissions factor and U.S. population data.

Agricultural Sources

Estimated methane emissions from agricultural sources, at 182.3 MMTCO₂e in 2004, represent 28.5 percent of total U.S. anthropogenic methane emissions (Table 15). Agricultural methane emissions increased by less than 1 percent (0.9 MMTCO₂e) from 2003 to 2004, as a small decrease in emissions from enteric fermentation was offset by increases in emissions from animal waste management, rice cultivation, and crop residue burning. Of total estimated methane emissions from agricultural activities, 93 percent (170.0 MMTCO₂e) results from livestock management, of which 67.8 percent (115.2 MMTCO₂e) can be traced to enteric fermentation in ruminant animals and the remainder (54.7 MMTCO₂e) to anaerobic decomposition of livestock wastes. A small portion of U.S. agricultural methane emissions result from crop residue burning and wetland rice cultivation.

Enteric Fermentation in Domesticated Animals

U.S. Methane Emissions from Enteric Fermentation in Domesticated Animals, 1990-2004 Printer Friendly Version   Methane Carbon Dioxide Equivalent Estimated 2004 Emissions (Million Metric Tons) 5.0 115.2 Change Compared to 2003 (Million Metric Tons) * -0.9 Change from 2003 (Percent) -0.8% -0.8% Change Compared to 1990 (Million Metric Tons) -0.2 -4.4 Change from 1990 (Percent) -3.7% -3.7%

In 2004, estimated methane emissions from enteric fermentation in domesticated animals declined by 0.8 percent to 115.2 MMTCO₂e (Table 22). Because 95 percent of all emissions from enteric fermentation is attributable to cattle (109.9 MMTCO₂e), trends in emissions correlate with trends in cattle populations. Between 2003 and 2004, cattle populations were nearly constant, with small declines in all population categories offset somewhat by increases in populations of beef cattle on feedlots. Estimated methane emissions from enteric fermentation in 2004 are 3.7 percent below their 1990 level of 119.6 MMTCO₂e.

Solid Waste of Domesticated Animals

Estimated methane emissions from the solid waste of domesticated animals increased from 54.2 MMTCO₂e in 2003 to 54.7 MMTCO₂e in 2004 (Table 23). The increase reinforced a larger trend over the past decade: in 2004, emissions from the solid waste of domesticated animals were 11.2 MMTCO₂e above their 1990 level of 43.5 MMTCO₂e, an increase of 26 percent. Between 1990 and 2004, there was a shift in livestock management to larger facilities, which are believed to be more likely to manage waste using liquid systems that tend to promote methane generation.⁷⁵

Rice Cultivation

Estimated methane emissions from U.S. rice cultivation increased to 11.0 MMTCO₂e in 2004 from 9.8 MMTCO₂e in 2003 (Table 15). The rise was the result of a 12-percent jump in the number of acres harvested.⁷⁶ All U.S. rice-producing States saw increases in acres harvested during 2004, and total methane emissions from rice cultivation in 2004 were 18 percent (1.7 MMTCO₂e) higher than in 1990.

Burning of Crop Residues

U.S. Methane Emissions from Solid Waste of Domesticated Animals, 1990-2004 Printer Friendly Version   Methane Carbon Dioxide Equivalent Estimated 2004 Emissions (Million Metric Tons) 2.4 54.7 Change Compared to 2003 (Million Metric Tons) * 0.5 Change from 2003 (Percent) 0.9% 0.9% Change Compared to 1990 (Million Metric Tons) 0.5 11.2 Change from 1990 (Percent) 25.8% 25.8%

Crop residue burning, the smallest contributor to U.S. methane emissions, represents less than 1 percent of total U.S. methane emissions. Estimated 2004 methane emissions from the burning of crop residues were 1.3 MMTCO₂e, up by 12 percent from 2003 and 27 percent above their 1990 level of 1.0 MMTCO₂e (Table 15). The increase from 2003 to 2004 is attributable mainly to large increases in corn and soybean production.

Industrial Processes

Chemical Production

The preliminary estimate of methane emissions from U.S. chemical production in 2004 is 1.6 MMTCO₂e, up by 6 percent from the 2003 level of 1.5 MMTCO₂e. Methane emissions from chemical production in 2004 were 25  percent above their 1990 level of 1.3 MMTCO₂e. The increase is attributable to increased production of carbon black, ethylene, and styrene, which more than offset a drop in methanol production (Table 24).⁷⁷

Iron and Steel Production

With production of pig iron rebounding from a 2-decade low in 2003,⁷⁸ methane emissions from iron and steel production rose to an estimated 1.1 MMTCO₂e in 2004, a 2.2-percent increase from 2003 but still 23 percent below their 1990 level of 1.4 MMTCO₂e (Table 24).

Notes and Sources

Tables 15-24