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Emissions of Greenhouse Gases in the United States 2003 3. Methane Emissions Overview
U.S. anthropogenic methane emissions totaled 26.2 million metric tons of methane (601.9 million metric tons carbon dioxide equivalent)53 in 2003, up by 0.1 million metric tons from 2002 levels (Table 14). Small increases in methane emissions from landfills and coal mines more than offset decreases in methane emissions from mobile sources and rice cultivation. Estimated U.S. emissions of methane in 2003 were 4.6 million metric tons below the 1990 level, a decrease equivalent to 105.9 million metric tons of carbon dioxide, or 1.5 percent of total U.S. anthropogenic greenhouse gas emissions. In addition to a 4.2 million metric ton decrease in methane emissions from landfills since 1990, there was also a 1.4 million metric ton decrease in methane emissions from coal mines during the same period (Table 15). The 32.4-percent decline in emissions from coal mining was the result of a 189-percent increase in methane recovery from coal mines and a shift in production away from gassy mines. Overall, methane emissions account for about 8.7 percent of total U.S. greenhouse gas emissions when weighted by methane’s global warming potential factor. Methane emissions from industrial landfills are included for the first time in this annual report. As a result, the overall estimated levels of methane emissions from 1990 through 2002 are 0.8 million metric tons higher each year than they would have been in the absence of the revision. Methane emissions from industrial landfills have been included in this year’s report because the use of more rigorously gathered data on landfilled municipal solid waste54 reduces the likelihood that methane emissions from industrial landfills will be double counted in estimates of emissions from municipal solid waste (MSW). 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.55 Thus, methane emission estimates must often rely on proxy measurements. Estimated U.S. anthropogenic methane emissions for 2003 are based on incomplete data for several key sources; thus, the overall estimate is likely to be revised. Emissions from three of these sources—coal mining, natural gas systems, and landfills—represented about three-fifths of all U.S. methane emissions. Thus, comparisons between 2002 and 2003 numbers are more likely to be valid in the context of directional change rather than magnitude of change. For example, because 2003 data on waste generation are not yet available, waste generation has been scaled to economic output as a proxy. 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 were estimated at 11.1 million metric tons in 2003 (256.1 million metric tons carbon dioxide equivalent), less than 0.1 million metric tons higher than 2002 levels and 0.8 million metric tons below 1990 levels (Figure 2). The drop in methane emissions from energy sources since 1990 can be traced primarily to decreased emissions from coal mines and, to a lesser extent, to lower emissions from petroleum systems and stationary combustion. Methane emissions from coal mines dropped by 32.4 percent (1.4 million metric tons) between 1990 and 2003. 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 2003, the share of coal production represented by underground mines declined from 41.2 percent to 32.9 percent. Methane emissions from underground mines tend to have higher emission rates per ton of coal mined than surface mines 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 1.3 million metric tons in 1990 to 1.0 million metric tons in 2003. A decrease of 0.2 million metric tons in estimated methane emissions from stationary combustion made a smaller contribution to the overall drop in emissions from energy sources between 1990 and 2003. Together, the declines in emissions from coal mining, petroleum systems, and stationary combustion more than compensated for the increase of 1.1 million metric tons in emissions from the natural gas system, attributed to increasing U.S. consumption of natural gas between 1990 and 2003. Coal Mining The preliminary estimate of methane emissions from coal mines for 2003 is 2.9 million metric tons (Table 15), an increase of 2.2 percent from the 2002 level. This increase can be traced largely to a 0.1 million metric ton decrease in methane recovery from coal mines, more than offsetting a decline in emissions that resulted from lower levels of coal production. U.S. coal production declined to 1.07 billion short tons in 2003, down for the second consecutive year from the record 2001 level of 1.13 billion short tons. The decline occurred despite an increase in coal consumption driven by increased demand from the electric power sector. The shortfall in production was offset by a drawing down of end-of-year coal stocks during 2003.56 Between 1990 and 2003, methane emissions from coal mines dropped by 32.4 percent from the 1990 level of 4.2 million metric tons. The decline is attributed to three important trends: (1) methane recovery from active coal mines for use as an energy resource increased from 0.3 million metric tons in 1990 to about 0.8 million metric tons in 2003; (2) methane drainage from degasification in active mines decreased by nearly 0.2 million metric tons between 1990 and 2003; and (3) methane emissions from ventilation systems at gassy mines dropped by about 0.7 million metric tons between 1990 and 2003 (Table 15).57 Abandoned coal mines represent a significant source of additional emissions that have not been incorporated into the overall estimate of methane emissions in this report because of uncertainties associated with the data. The text box on page 36 contains a discussion of those uncertainties and the potential magnitude of additional emissions.
Natural Gas Systems At 6.7 million metric tons, 2003 estimated methane emissions from natural gas production, processing, and distribution were unchanged from the revised estimate of 6.7 million metric tons for 2002 (Table 16). The 2003 estimate is preliminary, however, because pipeline data for 2003 have not been finalized as of the publication of this report. The estimated 2003 emissions level is 19.4 percent above the 1990 level, 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 gas production.58 Petroleum Systems Methane emissions from petroleum systems are estimated at 1.0 million metric tons in 2003, nearly unchanged from 2002 levels and down by 21.4 percent from 1.3 million metric tons in 1990. Domestic oil production in 2003 was 78.0 percent of the 1990 level, accounting for the decline in methane emissions from this source. Approximately 96.3 percent of all emissions from petroleum systems occur during exploration and production. Of the roughly 1.0 million metric tons of methane emissions annually from this source, 90.2 percent was traced to venting, of which nearly half is attributable to venting from oil tanks (Table 17). A much smaller portion of methane emissions from petroleum systems can be traced to refineries and transportation of crude oil. Stationary Combustion U.S. methane emissions from stationary combustion in 2003 were 361 thousand metric tons, down by 0.1 percent from the 2002 level and 35.9 percent below 1990 levels (Table 18). 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 residential wood combustion have fallen by 39.8 percent since 1990, although these estimates are very uncertain.59 The universe of wood consumers is large and heterogeneous, and 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. Updated data on residential wood consumption for calendar year 2003 will be available from the 2005 RECS. Mobile Combustion Estimated U.S. methane emissions from mobile combustion in 2003 were 188 thousand metric tons, down by 4.3 percent from 2002 levels and 22.6 percent lower than the 1990 level (Table 19). 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 downwards from previous reports, reflecting a change in the source of data for vehicle miles traveled and a related adjustment in the emission factors for light duty trucks. The difference in estimates of methane emissions from mobile sources from those previously reported grows steadily for the period 1990 through 2002, reaching 45 thousand metric tons in 2002.60 Waste Management Methane emissions from waste management accounted for 26.6 percent of U.S. anthropogenic methane emissions in 2003 (Figure 2), down from 36.1 percent in 1990. Landfills represent 90.4 percent of the 7.0 million metric tons of methane emissions from waste management and are the second largest source, after natural gas systems, of U.S. anthropogenic methane emissions (Table 14). The remainder of emissions from waste management 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 methodology for calculating the 2003 estimates of methane emissions from waste management has changed from that used in previous years. The new methodology, which incorporates data on landfilled waste published in Biocycle,61 has reduced the estimated amount of municipal solid waste reaching landfills by more accurately accounting for construction and demolition waste mixed with municipal solid waste. EIA’s estimates have also been revised to reflect the inclusion of methane emissions from organic decomposition in industrial landfills, which based on a methodology developed by EPA are estimated to represent 7 percent of emissions from municipal solid waste landfills.62 The combined effects of these revisions have reduced emissions estimates by between 0.3 and 0.7 million metric tons for the years 1990 through 2003.
Landfills Due to increased levels of waste disposed in landfills, estimated methane emissions from landfills rose to 6.3 million metric tons (144.9 million metric tons carbon dioxide equivalent) in 2003, 1.3 percent above the 2002 level of 6.2 million metric tons but still 4.2 million metric tons (40.1 percent) below 1990 levels (Table 20). The dramatic decrease in methane emissions since 1990 is directly attributable to a 5.0 million metric ton increase in methane captured at landfills that otherwise would have been emitted to the atmosphere. Of the 6.3 million metric tons of methane believed to be captured from this source in 2003, 3.0 million metric tons was recovered for energy use, and 3.3 million metric tons was recovered and flared. In 2003, 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.63 Estimates of methane recovered for energy are drawn from data collected by the EPA’s Landfill Methane Outreach Program,64 while estimates of methane recovered and flared are based on data collected from flaring equipment vendors.65 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, this tax credit expired on June 30, 1998. As part of the recently enacted 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. To be eligible for the credit, a landfill gas-to-energy project must be put in service between October 22, 2004, and December 31, 2005. 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. These regulations require all landfills with more than 2.5 million metric tons of waste in place and annual emissions of nonmethane volatile organic compounds (NMVOCs) 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 0.8 percent between 2002 and 2003 to 0.67 million metric tons— about 16.6 percent above the 1990 level of 0.58 million metric tons (Table 14). Methane emissions from industrial wastewater treatment are discussed in the text box on page 39. 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 2.6 percent of all U.S. methane emissions in 2003. Thus, emissions are estimated using a default per-capita emissions factor and U.S. population data. Agricultural Sources Estimated agricultural methane emissions decreased by 0.3 percent between 2002 and 2003 due mainly to small decreases in emissions from rice cultivation. At an estimated 7.9 million metric tons of methane (182.8 million metric tons carbon dioxide equivalent), methane emissions from agricultural activities in 2003 represent 30.4 percent of total U.S. anthropogenic methane emissions (Table 14) and approximately 2.6 percent of total U.S. greenhouse gas emissions. Ninety-four percent of methane emissions from agricultural activities result from livestock management. Sixty-six percent of these emissions can be traced to enteric fermentation in ruminant animals, and the remainder is attributable to the anaerobic decomposition of livestock wastes. A small portion of U.S. methane emissions result from crop residue burning and wetland rice cultivation. Enteric Fermentation in Domesticated Animals In 2003, estimated methane emissions from enteric fermentation in domesticated animals declined 0.3 percent to 5.0 million metric tons (Table 21). Because cattle account for 95.1 percent of all emissions from enteric fermentation, trends in emissions correlate with trends in cattle populations. Between 2002 and 2003, cattle populations were nearly constant, with small declines in beef cattle populations. Estimated methane emissions from enteric fermentation in 2003 are 4.7 percent below 1990 levels.
Solid Waste of Domesticated Animals Estimated methane emissions from the solid waste of domesticated animals increased from 2.50 million metric tons in 2002 to 2.52 million metric tons in 2003 (Table 22). The increase reinforced a larger trend over the past decade: in 2003, emissions from the solid waste of domesticated animals were 0.6 million metric tons above 1990 levels, an increase of 30.2 percent. Between 1990 and 2003 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.66 Rice Cultivation Estimated methane emissions from U.S. rice cultivation declined to 0.41 million metric tons in 2003 from 0.44 million metric tons in 2002 (Table 14). The drop was the result of a 6.9-percent decrease in the number of acres harvested.67 All U.S. rice producing states saw decreases in acres harvested during 2003. Methane emissions from rice cultivation in 2003 were 2.5 percent higher than in 1990. Burning of Crop Residues Crop residue burning, the smallest contributor to agricultural greenhouse gas emissions, represents 0.2 percent of total U.S. methane emissions. Estimated 2003 methane emissions from the burning of crop residues were 0.05 million metric tons, up 5.2 percent from 2002 levels and 8.5 percent above 1990 levels (Table 14). The increase between 2002 and 2003 is attributable mainly to large increases in corn and wheat production. Industrial Sources Chemical Production The preliminary estimate of methane emissions from U.S. chemical production in 2003 is 65.6 thousand metric tons, unchanged from 2002. Methane emissions from chemical production in 2003 were 18.1 percent above their level in 1990. The increase is attributable to increased production of carbon black, ethylene, and styrene, which more than offset a drop in methanol production (Table 23).68 Iron and Steel Production With production of pig iron and sinter at their lowest levels since the early 1980s,69 methane emissions from iron and steel production are estimated at 46.5 thousand metric tons, 24.7 percent below the 1990 level of 61.7 thousand metric tons (Table 23).
Released: December 2004 |
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