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U.S. Emissions of Greenhouse Gases in Perspective Appendix A: Estimation Methods Appendix B: Carbon Coefficients Used in this Report Appendix C: Uncertainty in Emissions Estimates Appendix D: Emissions Sources Excluded Appendix E: Emissions of Energy-Related Carbon Dioxide in the United States, 1949-1997 Appendix F: Common Conversion Factors
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Overview
Estimated U.S. anthropogenic methane emissions totaled 29.1 million metric tons in 1997, down slightly from the 29.2 million metric tons estimated for 1996 and about 1.1 million metric tons below 1990 levels (Table 15). The 29.1 million metric tons of methane emitted in 1997 were equivalent to 167 million metric tons of carbon--about 9.2 percent of total U.S. greenhouse gas emissions--after accounting for the heat trapping capacity of the gas. Estimates of methane emissions are more uncertain than carbon dioxide emissions estimates. U.S. energy consumption is carefully tracked, and because most carbon dioxide emissions are the result of fossil fuel combustion, estimates of carbon dioxide emissions are likely to be accurate to within 3 to 5 percent. In contrast, methane emissions usually are accidental or incidental to biological processes and may not be metered in any systematic way.(24) Thus, methane emissions are difficult to calculate and often rely on proxy measurements. As a result, estimates of methane emissions are significantly more uncertain than estimates of carbon dioxide emissions. Because of limitations on data availability and the inability to develop plausible proxies, emissions from two known sources--associated gas at oil wells and industrial wastewater--are not estimated in this report. These sources may be significant, and their exclusion clearly biases overall U.S. estimates downward (see boxes on pages 30 and 33). Total estimated U.S. anthropogenic emissions for 1997 also include preliminary data for several key sources; thus, the overall estimate must be described as preliminary. Emissions from two of these sources, coal mining and oil and gas systems, are substantial, together accounting for almost one-third of all U.S. methane emissions. Coal production data on a mine-by-mine basis will not be available until December 1998. Estimates for emissions from coal mines are scaled to national-level production data for 1997 that are already available. Similarly, estimates of emissions from oil and gas systems in 1997 are scaled to total gas throughput for the year, because detailed industry activity data are not yet available.
There are four principal sources of U.S. methane emissions: energy production and consumption, waste management, agriculture, and industry. Emissions from energy sources, which represent just over one-third of all U.S. methane emissions (Figure 4), declined significantly between 1990 and 1997 as a result of a substantial drop in emissions from coal mining. Methane emissions from waste management account for another one-third of the total. Emissions from this source also decreased between 1990 and 1997 as the share of municipal solid waste generated that was recycled increased, and greater amounts of methane generated from the decomposition of waste in landfills was captured and used for energy production. Emissions from agriculture represent about 30 percent of all U.S. methane emissions. Driven by livestock populations, emissions from agriculture peaked in 1995 and have since begun to decline. Energy Sources U.S. methane emissions from energy sources were estimated at 10.0 million metric tons in 1997, up 100,000 metric tons from 1996 levels but 800,000 metric tons below the 10.8 million metric tons emitted in 1990. The 7.4-percent decrease between 1990 and 1997 resulted almost entirely from lower emissions from coal mines. Emissions from coal mines dropped by 26 percent or about 1.1 million metric tons between 1990 and 1997. The amount of methane recovered from coal mines and used as an energy resource more than tripled, and industry consolidation lowered emissions from the Nation's gassiest mines. A decrease of 120,000 metric tons in estimated emissions from stationary combustion made a smaller contribution to the overall drop in emissions from energy sources. Together, the declines in emissions from coal mining and stationary combustion overwhelmed the increase of 500,000 metric tons in emissions from the oil and gas system attributed to increasing U.S. consumption of natural gas between 1990 and 1997. Coal Mining The preliminary estimate of methane emissions from coal mines for 1997 is 3.1 million metric tons, an increase of 6.7 percent from the 1996 level. The increase is due primarily to a 7.6-percent increase in ventilation emissions from gassy mines. Between 1990 and 1997, methane emissions from coal mines dropped by more than 25 percent from the 1996 level of 4.26 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 250,000 metric tons in 1990 to more than 900,000 metric tons in 1997; (2) methane drainage from active mines decreased by almost 200,000 metric tons between 1990 and 1997; and (3) methane emissions from ventilation systems at gassy mines dropped by more than 250,000 metric tons (Table 16). The estimates of methane emissions from coal mining for 1990-1997 have been revised substantially downward from estimates that appeared in the October 1997 edition of Emissions of Greenhouse Gases in the United States. The revisions are based on new information published by the U.S. Environmental Protection Agency (EPA), Office of Air and Radiation, on methane recovery operations and drainage from degasification systems.(25) Emissions from degasification are estimated to be on the order of 300,000 metric tons lower than previously believed. Meanwhile, methane recovery has grown more rapidly than previously reported, largely due to significant growth in Virginia. The increase in methane recovery can be traced directly to new regulations promulgated by the Virginia Department of Oil and Gas in 1990 to encourage coalbed methane development. Beginning in 1992, CONSOL Coal Group began recovering gas at the Buchanan No. 1 mine. In 1993, CONSOL acquired Island Creek Coal and the very gassy VP No. 3, VP No. 5, and VP No. 6 mines, and later combined VP No. 5 and VP No. 6 into VP No. 8. CONSOL has since recovered gas from VP No. 3 and VP No. 8, in addition to Buchanan No. 1. In December 1995, CONSOL sold the gas rights from these mines to MCNIC Oil and Gas.(26) The methane recovered from these three mines is responsible for more than 0.5 million metric tons of the increase in methane recovery. Similar language aimed at bolstering coalbed methane development was included in the Energy Policy Act of 1992 and in legislation enacted in West Virginia during 1994. The decrease in ventilation emissions from gassy mines is a function of the ongoing consolidation of the coal industry. In the Warrior Basin, a two-thirds drop in production, and hence emissions, from the very gassy Blue Creek No. 5 mine lowered estimated emissions by 100,000 metric tons. In Central Appalachia, the consolidation of the very gassy Island Creek mines after their acquisition by CONSOL reduced ventilation emissions by 120,000 metric tons. In the Western Basin, the closure of the Golden Eagle mine early in 1997 was responsible for additional decreases of 35,000 metric tons in methane emissions from ventilation systems and 20,000 metric tons in emissions from degasification systems.
Oil and Gas Production, Processing, and Distribution Estimated methane emissions from U.S. oil and gas systems reached 6.16 million metric tons in 1997 (Table 17), more than 8 percent above 1990 levels. The steady growth in emissions from the oil and gas system can be traced to a general trend toward increased natural gas consumption between 1990 and 1997. During that period, total gas withdrawals grew by 12 percent, gas processed increased by 16 percent, and distribution main pipeline miles rose by 11 percent. About half the increase in emissions from 1990 to 1997 is attributable to the gas distribution system, which saw emissions rise from 1.36 million metric tons of methane in 1990 to 1.56 million metric tons in 1997. Emissions from gas production accounted for another one-third of the increase, growing from 1.47 million metric tons in 1990 to 1.59 million metric tons in 1997. Most of the remaining portion of the emissions escalation can be traced to emissions from gas processing, which grew from 650,000 metric tons in 1990 to 740,000 metric tons in 1997.(27) In contrast, emissions from transmission and storage remained stable, with a decline in transmission pipeline miles offset by increased withdrawals from gas storage. Methane emissions from oil wells dropped slightly between 1990 and 1997, but emissions from oil refining and transport rose by more than 8 percent during the period.
Stationary Combustion U.S. methane emissions from stationary
combustion in 1997 were 450,000 metric tons, well below the 1996 level of 593,000 metric
tons and 22 percent lower than 1990 levels (Table 18). These
declines are due primarily to a 27-percent drop in estimated consumption of wood in the
residential sector between 1996 and 1997, the result of an unusually warm winter in 1997
attributed to the "El Niņo" effect. Residential wood consumption typically
represents nearly 90 percent of all 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.
Thus, although the residential sector consumes about one-quarter the amount of wood that
the industrial sector consumes, emissions from wood consumption for the residential sector
are 29 times higher than those for the industrial sector.
Estimates of residential wood combustion are, however, highly uncertain (see Appendix C). The universe of wood consumers is large and heterogeneous, and wood for residential consumption is typically obtained from sources outside the documented economy. The EIA relies on the Residential Energy Consumption Survey (RECS) to estimate residential wood consumption. This survey includes only primary residences and thus systematically underestimates consumption by perhaps 5 percent. The last RECS was completed in 1993. Residential wood consumption since that time is estimated by scaling the 1993 number to heating degree days. Therefore, changes in estimated emissions are caused by changes in weather patterns.
Mobile Combustion Methane emissions from mobile combustion in 1997 were estimated at 240,000 metric tons, down 8,000 metric tons from both 1990 and 1996 levels (Table 19). Methane emissions from mobile sources declined slowly but steadily between 1980 and 1992, primarily because of a 27-percent decline in emissions from passenger cars. Catalytic converters, used on U.S. automobiles to control emissions, have grown more efficient in reducing methane emissions over time. Thus, as the U.S. fleet is replaced, the remaining automobiles have lower emissions profiles than their earlier counterparts. Emissions from passenger cars continued to decline between 1992 and 1995, dropping by another 16.9 percent. That decrease was more than offset, however, by an increase in emissions from the rapidly growing fleet of light-duty trucks, leading to a rise in overall emissions from mobile combustion between 1992 and 1995. In 1996 and 1997 the size of the light-duty truck fleet declined. In combination with continued declines in emissions from passenger cars, the decrease in emissions from light-duty trucks lowered emission levels by more than 5 percent between 1995 and 1997.
Waste Management Waste management activities are the single largest source of U.S. anthropogenic methane emissions. In 1997, emissions from waste management were 10.4 million metric tons or 36 percent of total U.S. methane emissions. Emissions from waste management have declined from 11.1 million metric tons in 1990. Ninety-eight percent of emissions from waste management are attributed to emissions from landfills, and the remainder are associated with domestic wastewater treatment. The landfill share would be somewhat lower if reliable data on emissions from industrial wastewater treatment were available and were added to the total (see box on page 33). A diminishing portion of municipal solid waste is landfilled each year as recycling programs grow (Figure 5). Also, an increasing amount of methane generated from the decomposition of waste in landfills is being captured and used as an energy resource. Together, these trends have lowered emissions from landfills and waste management overall. Landfills Estimated methane emissions from U.S. landfills continued their seven-year decline from a 1990 peak of 11.0 million metric tons. In 1997, an estimated 10.2 million metric tons of methane were emitted from U.S. landfills, down 160,000 metric tons from the 1996 levels and 6.8 percent from the 1990 level (Table 20). While municipal solid waste generation continues to increase slowly (along the lines of population growth), the share of waste being recycled or incinerated rather than landfilled grew from 16 to 39 percent between 1990 and 1997.(28) Thus, gross methane generation at landfills grew slowly (less than 1 percent annually) between 1990 and 1995, stabilized in 1996 and declined in 1997.
The estimated volume of methane recovery for flaring or energy use has nearly doubled since 1990. Absent the increase in methane recovery, net emissions of methane from U.S. landfills would have risen somewhat between 1990 and 1997. In 1990, approximately 940,000 metric tons of methane were recovered for energy use, and an additional 300,000 metric tons of methane were recovered and flared. By 1997, these numbers had grown to an estimated 1.7 million metric tons and 667,000 metric tons respectively, preventing almost 2.4 million metric tons of potential methane emissions. According to the EPA's Office of Solid Waste, municipal solid waste generation in the United States is expected to increase by about 4 percent between 1997 and 2000.(29) Per capita generation is expected to remain nearly unchanged, but a growing population will increase overall generation. The growth rate can be expected to bring the volume of waste generated in 2000 to approximately 340 million short tons. In contrast to waste generation, which is trending upward, the share of waste generated that will reach a landfill is expected to decline from 61 percent in 1997 to 55 percent in 2000. With waste combustion expected to remain stable as a share of waste generation at approximately 15 percent, predicted declines in landfilling are attributable to expected increases in curbside recycling. The growth in methane recovery for energy between 1990 and 1997 was driven largely by the Federal Section 29 tax credit for alternative energy sources. This credit provides a subsidy roughly equivalent to 1 cent per kilowatthour for electricity generated using landfill gas. Signed contracts are currently in place for an additional 200 landfill gas-to-energy recovery projects. In order to qualify for Section 29 tax credits scheduled to run through 2008, the projects must have been in operational condition by June 30, 1998. If all the projects were fully operational at maximum capacity, the emissions reductions expected are estimated at an additional 1.8 million metric tons; however, the number of projects that actually met the deadline has not yet been determined.(30) Increases in methane recovery and flaring after expiration of the Section 29 tax credits are expected to result from the implementation of the New Source Performance Standards (NSPS) and Emissions Guidelines administered by the EPA. 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 as an energy resource. In addition to the 600 landfills that currently flare gas, as many as 500 additional landfills could be required to flare gas under the NSPS and Emissions Guidelines regulations.(31) Those landfills with contracts in place that do not meet the June 30, 1998, operational deadline for Section 29 tax credits are likely to flare the gas, with significant additional emissions reductions beginning in 1999. Initial indications are that the majority of landfills potentially subject to the NSPS are able to demonstrate sufficiently low NMVOC emissions to be exempted.(32) Thus, a small number of additional flares are likely to come on line after 1999, further reducing methane emissions. Domestic and Commercial 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. If it is treated aerobically, methane emissions will be low. Under anaerobic conditions, methane emissions will be high. There is little data available on wastewater treatment methods. Data on flaring or energy recovery from methane generated by wastewater is also sparse. Thus, emissions are scaled to U.S. population data. With the U.S. population growing slowly, methane emissions from domestic and commercial wastewater treatment are estimated to have grown by less than 1 percent between 1996 and 1997 and by just over 7 percent since 1990.
Agricultural Sources Agricultural activities represent another important U.S. source of anthropogenic methane. Emissions from agriculture totaled 8.6 million metric tons in 1997, nearly 30 percent of all U.S. methane emissions (Table 15). This share of emissions has risen from 27 percent in 1990, largely because of significant drops in emissions from both energy sources and waste management practices. More than 94 percent of all methane emissions from agriculture are associated with livestock management. Almost two-thirds of emissions from livestock management can be traced to enteric fermentation, a process in which the digestion of carbohydrates in the forestomach of ruminant animals produces methane. Because cattle are the source of 95 percent of enteric fermentation emissions and 44 percent of emissions from animal waste, cattle populations are responsible for 75 percent of all emissions from agriculture and more than 22 percent of all U.S. anthropogenic methane emissions. Cattle populations grew between 1990 and 1994, raising methane emissions, then stabilized in 1995 and declined in 1996 and 1997. This decline was offset by a rapid rise in swine populations and an escalating concentration of swine on large industrial farms. Cattle population data back to 1980 suggest a cyclical pattern related to market conditions for cattle products. If livestock management practices remain unchanged, this pattern would suggest further declines in methane emissions from livestock over the next several years.
Enteric Fermentation in Domesticated Animals Estimated methane emissions from enteric fermentation in domesticated animals were 5.4 million metric tons in 1997, 110,000 metric tons below estimates for 1996 and nearly 5 percent below peak levels in 1994 and 1995 (Table 21). After escalating from 1990 through 1995, emissions from enteric fermentation have declined to their lowest levels since 1991. Because cattle represent more than 95 percent of all emissions from enteric fermentation, trends in emissions correlate with trends in cattle populations. Between 1990 and 1994, cattle populations grew, but by 1995 they had largely stabilized and in 1996 and 1997 populations decreased. Similarly, those variables that affect animal energy intake requirements, such as animal size and milk production, increased between 1990 and 1994 before declining from 1995 through 1997. Because methane emissions are a function of energy intake, emission trends attached to population changes have been exacerbated by simultaneous changes in energy intake requirements. For example, roughly two-thirds of the decline in emissions between 1996 and 1997 is attributable to decreased population and one-third to decreased energy intake requirements. For the first time in preparing estimates of methane emissions from enteric fermentation for Emissions of Greenhouse Gases in the United States, the EIA has used State-level dairy population data in conjunction with regional emissions factors rather than national populations and national average emissions factors. This change was made to capture regional variations in feed intake and cattle energy requirements. The effect of the change on the estimates presented has been to raise estimates by 200,000 to 600,000 metric tons annually between 1995 and 1997.
Solid Waste of Domesticated Animals Estimated methane emissions from the solid waste of domesticated animals were 2.8 million metric tons in 1997, up from 2.7 million metric tons in 1996. Emissions levels remain well above the 1990 level of 2.6 million metric tons (Table 22). The 1997 growth in emissions is attributable to large increases in swine waste, particularly from swine bred for market, and the shift of swine populations to large industrial farms that employ concentrated manure management techniques. In 1997, methane emissions from swine waste reached the highest levels in the past 17 years. In contrast to emissions from enteric fermentation, which are nearly all attributable to cattle, 49 percent of emissions from the solid waste of domesticated animals were attributable to swine waste in 1997 and only 44 percent to cattle waste. Populations of swine bred for market grew by 9 percent between 1996 and 1997. Absent this rapid growth, overall emissions would have shown a slight decline as cattle populations and related emissions dropped in 1997. Nearly three-fifths of the increase in emissions from solid waste between 1990 and 1997 can be credited to increased emissions from swine waste. About half the remainder is the result of increased emissions from cattle waste. The last portion of emissions growth is associated with increased poultry populations. Emissions from poultry waste accounted for just 7 percent of all emissions from this source in 1997, but they have increased by 26 percent since 1990.
Rice Cultivation Estimated methane emissions from rice cultivation were 426,000 metric tons in 1997, up from the 403,000 metric tons in both 1996 and 1990 (Table 23). The increase would have been even larger with the addition of emissions from Florida, which were included for 1990-1996 but excluded for 1997 due to a lack of available data; however, Florida contributes less than 1,000 metric tons annually to national emissions levels. Methane emissions from rice cultivation are, in part, a function of the area of land harvested. In 1996, the area harvested dropped to its lowest level since 1990. In 1997, the area of land harvested rebounded, increasing in 5 out of 7 rice-growing States (Arkansas, California, Louisiana, Mississippi, and Missouri). Area harvested, and thus emissions, rose by 17 percent in Arkansas alone. Only Texas reported a decline in area harvested. The overall area harvested in 1997 was more than 6 percent greater than in 1996 and 4.4 percent greater than in 1990. Burning of Crop Residues In 1997, fueled by large increases in corn, soybean, and pea production, methane emissions from the burning of crop residue grew to 41,500 metric tons, up by 6.1 percent from 1996 levels. This total was nearly 10 percent above the 1990 emissions of 37,900 metric tons. Nevertheless, this source accounts for less than 0.2 percent of total U.S. methane emissions. Methane emissions from crop residue burning are a function of crop production and the share of crop residues burned. Overall, 1997 crop production was up by 4.3 percent from 1996 and by 12.0 percent from 1990. Emissions from the burning of crop residues in California rice fields had decreased steadily from 1990 to 1996 as the share of residue burned declined from 99 percent to 50 percent. In 1997, however, a 13-percent increase in State production, accompanied by an end to reductions in the portion of crop residue burned, returned emissions from California rice fields to their 1995 levels. Industrial Sources Chemical Production In 1997, Methane emissions from chemical production were unchanged from 1996 levels at 74,000 metric tons (Table 24). This was still 18,000 metric tons above 1990 levels. Since 1990, emissions levels have increased along with production levels as a consequence of steady economic growth. Production of styrene has grown the most markedly, rising by 42 percent between 1990 and 1997. Because ethylene is a principal feedstock of styrene, its production has increased similarly, jumping 32 percent.
Iron and Steel Production Emissions from iron and steel production were essentially unchanged from 1996 levels, with a minimal increase of 150 metric tons due to higher pig iron production (Table 24). The 1997 total was 4.8 percent below 1990 levels. Iron and steel production dropped by about 13 percent during 1991, recovered about three-quarters of that loss by 1994, and since then has remained virtually flat.
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