| Overview | Energy Sources | Waste Management | Agricultural Sources | Industrial Processes | Methane Data Tables |
| U.S. Anthropogenic Methane Emissions,
1990-1994 | ||
| Methane | Carbon Equivalent | |
| Estimated 1994 Emissions (Million Metric Tons) | 31.0 | 177.5 |
| Change Compared to 1993 (Million Metric Tons) | 0.5 | 3.0 |
| Change from 1993 (Percent) | 1.7 | 1.7 |
| Change Compared to 1990 (Million Metric Tons) | -0.3 | -1.9 |
| Change from 1990 (Percent) | -1.1 | -1.1 |
| Major Sources of U.S. Methane Emissions,
1990-1994 | ||||
| Source | Million Metric Tons Methane | Percent Change | ||
| 1990 | 1994 | 1990- 1994 | 1993- 1994 | |
| Energy | 11.96 | 11.42 | -4.6 | 4.1 |
| Waste Management | 10.96 | 10.34 | -5.7 | -2.3 |
| Agriculture | 8.29 | 9.11 | 9.9 | 3.7 |
| Industrial Processes | 0.12 | 0.13 | 11.6 | 5.6 |
Estimated U.S. anthropogenic methane emissions totaled 31.0 million metric tons in 1994, an increase of 530,000 metric tons from 1993, though still well below the total of 31.3 million metric tons emitted in 1990 (Table 16). In terms of tons of gas emitted, methane emissions are dwarfed by carbon dioxide emissions (a ratio of 1 ton of methane for every 175 tons of carbon dioxide). Yet, because the heat-trapping capacity of methane is some 21 times that of carbon dioxide, the overall effect of methane on global climate is significant. After weighting emissions by the global warming potential of each gas, methane represents 10 percent of all U.S. anthropogenic emissions. There is, however, substantial uncertainty in estimates of emissions from most U.S. sources, ranging from 25 percent to as high as several hundred percent. Thus, trends in emissions from each source tend to be more reliable than their overall estimated magnitude [30].
Between 1990 and 1994, total methane emissions declined by 1 percent. Methane emissions are dominated by three sources: energy production and consumption, waste management, and agriculture (Figure 6) [31]. Decreases in emissions from energy and agriculture were for the most part offset by increases in emissions from agriculture and industrial processes (Figure 6). Energy sources account for 37 percent of emissions, while waste management and agriculture contribute about one-third each. Emissions from industrial processes contribute less than 0.5 percent of the total.
Although methane emissions from energy sources increased between 1993 and 1994, they have declined by 550,000 metric tons or some 4.6 percent since 1990, mainly due to decreases in emissions at the Nation’s gassiest coal mines (Figure 7). Emissions from waste management (especially landfills) decreased by nearly 6 percent between 1990 and 1994 because of a continuing decline in the annual volume of waste being landfilled, combined with a steady increase in methane recovery. In contrast, emissions from agriculture have increased by 820,000 metric tons, or nearly 10 percent, since 1990. This was due to concurrent increases in average animal size and populations and a shift in the management of dairy cattle waste to higher emitting systems in several States.
Preliminary estimates of emissions for 1995 suggest that overall emissions can be expected to remain flat or decline slightly. Animal sizes and populations have stabilized, while venting of associated gas from oil wells appears to have declined, neutralizing two potential sources of emissions growth. Further, underground coal production in 1995 was down by 4 percent from 1994 levels, and the trend toward increased curbside recycling and landfill methane recovery continues.
In 1994 energy sources accounted for 11.4 million tons of methane emissions, including nearly 7 million tons from the largest source, oil and gas production and distribution; almost 4 million metric tons from coal mining; and the remainder from stationary combustion and transportation. Although methane emissions from energy sources increased by 4.1 percent between 1993 and 1994, they are nearly 5 percent lower than in 1990. Each source is discussed in the following sections, beginning with the largest source, oil and gas production and distribution.
| U.S. Methane Emissions from Oil and Gas
Production, Processing, and Distribution, 1990-1994 | |
| Estimated 1994 Emissions (Million Metric Tons Methane) | 6.89 |
| Change Compared to 1993 (Million Metric Tons Methane) | 0.12 |
| Change from 1993 (Percent) | 1.7 |
| Change Compared to 1990 (Million Metric Tons Methane) | 0.30 |
| Change from 1990 (Percent) | 4.6 |
Methane emissions from oil and gas operations have shown a slow upward trend over much of the past decade, rising at a rate of just over 1 percent a year between 1988 and 1994 (Table 17). About one-half of this increase is attributable to an increase in emissions from the venting of associated gas at oil wells. A large portion of the remaining growth has occurred as a result of fugitive emissions at gas processing plants and gate stations. While the growth in estimated emissions from gas processing plants and gate stations is the direct result of rising demand for natural gas, which has increased throughput at each stage of the gas distribution system, the cause of the increase in venting at associated gas wells is unknown [32]. Further, the growth in emissions from venting appears to have reversed itself in 1995 [33].
New Research Used To Improve Estimates of Methane Emissions from Oil and Gas OperationsA recently completed study jointly funded and managed by the U.S. Environmental Protection Agency (EPA) and the Gas Research Institute (GRI) indicates that methane emissions from the U.S. natural gas industry are substantially higher than previously estimated. This study estimated 1992 emissions from the U.S. natural gas industry, excluding gas vented at oil wells, at 5.83 million metric tons—more than double the estimate of 2.57 million metric tons provided in last year’s edition of Emissions of Greenhouse Gases in the United States, which was largely based on an earlier EPA study, Anthropogenic Methane Emissions in the United States: Estimates for 1990. In its earlier study, the EPA estimated 1990 methane emissions from the natural gas industry at about 3 million metric tons. Although the revised estimate is subject to less uncertainty than previous estimates, it still is only accurate to plus or minus 50 percent. (A more complete discussion of the uncertainty associated with this estimate appears in Appendix C.) The differences between the earlier EPA study and the more recent joint EPA/GRI effort center on data availability and sample size. The more recent study benefits from a more complete inventory of equipment components in the natural gas industry as well as a larger and more representative sample used to estimate emissions from each component. For example, the earlier study based emissions estimates from transmission and distribution pipelines on two leakage studies conducted by Pacific Gas and Electric and Southern California Edison. Because these systems use very little cast iron pipe, data collected on their leakage rates grossly underestimated more typical pipeline leakage rates. Using these leakage rates when making calculations of emissions from pipelines results in estimates substantially lower than those that result from the use of emissions factors derived from data in the more recent EPA/GRI study, which better represents the quality of the existing capital stock. (A comparison of the old and revised emission factors is provided in Appendix A.) The table below compares the estimated 1992 emissions from the oil and gas system appearing in last year’s edition of Emissions of Greenhouse Gases in the United States with this year’s estimate. Of the approximately 3.3 million metric ton difference in the estimates, more than 1.5 million metric tons can be attributed to previously unavailable data on emissions from pneumatic devices during production; emissions from compressors during processing, transmission, and storage; leakage from distribution pipeline; and emissions from metering and pressure regulating stations.
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Preliminary estimates show emissions from venting declining by 330,000 metric tons, or about 33 percent, between 1994 and 1995. This decrease is likely to be only partially offset by emissions growth in other parts of the oil and gas distribution system, leading to a more general decline in emissions from U.S. oil and gas operations. Estimates of emissions from portions of the natural gas system other than gas venting are scaled to measures of capital stock, such as pipeline mileage, and measures of production, such as throughput [34]. These measures are very unlikely to grow at a rate comparable to the decline in gas venting, leading to lower overall emissions estimates for 1995.
| U.S. Methane Emissions from Coal Mining,
1990-1994 | |
| Estimated 1994 Emissions (Million Metric Tons Methane) | 3.85 |
| Change Compared to 1993 (Million Metric Tons Methane) | 0.34 |
| Change from 1993 (Percent) | 9.7 |
| Change Compared to 1990 (Million Metric Tons Methane) | -0.79 |
| Change from 1990 (Percent) | -17.0 |
Despite record U.S. coal production of more than 1 billion short tons during 1994, methane emissions from coal mining remained well below their historic 1990 high of 4.64 million metric tons (Table 18). While coal production in most regions rebounded close to or above production levels seen prior to 1993, production in the gassiest basin, Warrior, continued to decline to a point nearly 20 percent below 1990 levels (see Appendix A) [35]. This trend away from production in the Nation’s gassiest mines is underscored by the decline in production at mines with degasification systems in place. Degasification systems are employed in mines where methane volumes are too high to be practically reduced with standard ventilation techniques. Thirty mines are known to have such systems in place, and emissions from these systems represent just under one-third of all emissions from U.S. coal mining operations. However, production in these 30 mines has declined by 27 percent since 1990, reducing estimated emissions by more than 440,000 metric tons [36]. Further, the quantity of methane recovered at coal mines increased by more than 200,000 metric tons between 1990 and 1994.
The one notable area of growth in emissions from coal mining is surface mining. Record levels of coal production were largely driven by surface mining, which produced at a rate 5 percent above earlier levels. This effective shift from production in the Nation’s gassiest underground mines to production from surface mines has significant emissions implications. A ton of coal mined in the Nation’s gassiest underground mines may result in methane emissions that are, on average, 50 times greater than a ton of coal mined on the surface. The shift from underground mining to surface mining can be credited to the vast advantages in productivity found in surface coal mining as well as the need for the lower sulfur coal produced in Western surface mines to comply with the provisions of the Clean Air Act Amendments of 1990.
| U.S. Methane Emissions from Stationary
Combustion, 1990-1994 | |
| Estimated 1994 Emissions (Thousand Metric Tons Methane) | 434 |
| Change Compared to 1993 (Thousand Metric Tons Methane) | -7 |
| Change from 1993 (Percent) | -1.6 |
| Change Compared to 1990 (Thousand Metric Tons Methane) | -26 |
| Change from 1990 (Percent) | -5.7 |
Methane emissions from stationary combustion result almost entirely from wood consumption for heating in the residential sector. Emissions declined slightly between 1993 and 1994 as the amount of wood consumed in the residential sector declined for the second consecutive year. Methane emissions are the result of incomplete combustion, and wood used in the residential sector is typically consumed in woodstoves and fireplaces, resulting in much less efficient combustion than fuels burned in an industrial or utility boiler. Thus, while the residential sector consumes less than one-third the amount of wood consumed in the industrial sector, residential methane emissions are some 30 times higher than emissions from the industrial sector.
There is, however, considerable uncertainty associated with this estimate [37]. Estimating wood combustion in the residential sector is difficult, given that the universe of consumers is large and heterogeneous and that wood may be obtained from sources outside the commercial economy. The EIA scales estimated residential wood consumption to heating degree-days, assuming that consumption increases as individuals need to heat their homes [38]. This method may not accurately capture changes in wood’s share of the home heating market.
The importance of emissions from residential wood consumption is underscored by the growth in methane emissions from 1994 to 1995 due to an estimated recovery in residential wood fuel consumption of just above 10 percent. Overall methane emissions from stationary combustion grew by 9.4 percent to 475,000 metric tons in 1995. Nearly all of this growth is attributed to emissions from residential wood combustion which increased from 368,000 metric tons in 1994 to 409,000 metric tons in 1995 (Table 19). Much smaller growth occurred in methane emissions from the combustion of natural gas in the industrial sector. The only area of decline in emissions from stationary combustion was emissions from the combustion of fuel oil, possibly reflecting a move toward natural gas as an industrial and utility energy source.
| U.S. Methane Emissions from Mobile
Combustion, 1990-1994 | |
| Estimated 1994 Emissions (Thousand Metric Tons Methane) | 239 |
| Change Compared to 1993 (Thousand Metric Tons Methane) | -6 |
| Change from 1993 (Percent) | -2.4 |
| Change Compared to 1990 (Thousand Metric Tons Methane) | -34 |
| Change from 1990 (Percent) | -12.5 |
Methane emissions from mobile combustion declined moderately from 239,000 metric tons in 1994 to 230,000 metric tons in 1995 (Table 20). The decrease in emissions from passenger cars was responsible for all of this reduction. Methane emissions from U.S. passenger cars are largely a function of the emissions control technology employed. Over time, the efficiency of catalytic converters in reducing methane emissions has improved. Thus, newer vehicles have lower emissions than older vehicles. As older vehicles leave the fleet and are replaced by new vehicles, overall emissions decline. Between 1988 and 1994, emissions from passenger cars declined each year, from a high of 173,000 metric tons of methane to the most recent 115,000 metric tons of methane. This trend can be expected to continue for the near future.
| U.S. Methane Emissions from Landfills,
1990-1994 | |
| Estimated 1994 Emissions (Million Metric Tons Methane) | 10.18 |
| Change Compared to 1993 (Million Metric Tons Methane) | -0.25 |
| Change from 1993 (Percent) | -2.4 |
| Change Compared to 1990 (Million Metric Tons Methane) | -0.63 |
| Change from 1990 (Percent) | -5.9 |
A continuing decline in the annual volume of waste being landfilled, combined with a steady increase in methane recovery, has led to a significant reduction in the level of U.S. methane emissions over the past 5 years (Table 21). While the volume of waste generated grew by about 10 percent between 1990 and 1994, the volume of waste being landfilled declined by some 20 percent. According to the EPA’s Office of Solid Waste, the share of municipal solid waste generated that eventually found its way into landfills declined from 82 percent in 1990 to only 61 percent in 1994 (see Appendix A) [39]. This decline is largely a function of the rapid spread of curbside recycling programs, which served 21 million Americans in 1989 and 101 million citizens in 1993. According to Biocycle magazine, some 23 percent of waste generated in the United States is now recycled (Figure 8). However, the rapid growth in curbside recycling may moderate as municipalities grow concerned about program costs and the economically viable market reaches its saturation point [40]. Whether the annual growth in recycling will continue to offset the 2-percent yearly growth in waste generation remains to be seen.
The volume of methane recovered at landfills also grew substantially between 1990 and 1994. In 1990, approximately 940,000 metric tons of methane was recovered for energy use, and an additional 300,000 metric tons of methane was recovered and flared. In 1994, these numbers had grown to 1.44 million metric tons and 600,000 metric tons, respectively, eliminating more than 2 million metric tons of potential emissions. Regulations implemented by the EPA during 1996 require all landfills with more than 2.5 million metric tons of waste in place and annual emissions of nonmethane organic compounds exceeding 50 metric tons to collect and combust (use or flare) their landfill gas. These New Source Performance Standards (NSPS) emissions guidelines will affect about 300 of the Nation’s largest landfills and should result in decreased methane emissions.
While the volume of methane recovered will likely rise, its future disposition is uncertain. About two-thirds of landfills that use recovered methane for energy burn it to generate electricity, and the remainder sell it to end-use customers or distribution companies via pipeline. Thus, the economics of landfill gas-to-energy projects are largely driven by wholesale electricity prices. With the pace of deregulation and restructuring in the electric power industry increasing, the future of wholesale electricity prices is less predictable.
Landfill gas-to-energy projects may be eligible for Federal tax credits, pursuant to the “Section 29” tax credit for alternative energy sources. This credit was recently expanded to include all landfill gas projects in place by December 31, 1996, and operating by June 30, 1998.
| U.S. Methane Emissions from Domestic
and Commercial Wastewater Treatment, 1990-1994 | |
| Estimated 1994 Emissions (Thousand Metric Tons Methane) | 157 |
| Change Compared to 1993 (Thousand Metric Tons Methane) | 1.5 |
| Change from 1993 (Percent) | 1.0 |
| Change Compared to 1990 (Thousand Metric Tons Methane) | 6.5 |
| Change from 1990 (Percent) | 4.4 |
Methane emissions from domestic and commercial wastewater treatment continued their slow but steady increase in 1994, rising from just over 155,000 metric tons in 1993 to almost 157,000 metric ton in 1994. Because the growth in estimated emissions is driven by increases in U.S. population, emissions can be expected to rise by about 1 percent annually for the foreseeable future. Methane emissions from this source were just over 158,000 metric tons in 1995.
Industrial WastewaterThis report does not present estimates of methane emissions from industrial wastewater treatment due to a lack of reliable data on waste treatment methods and volumes of wastewater generated for several important industries. Methane generation from wastewater is the result of anaerobic decomposition of organic matter in the wastewater. Thus, emissions are driven by the share of organic matter in the wastewater stream and the method under which it decomposes. Most industrial wastewater has a low organic content. However, wastewater from the pulp and paper industry and the food and beverage industry (particularly breweries) has a very high organic content. Currently, the EPA’s Office of Research and Development is conducting research into emissions from industrial sources. Depending on the treatment methods employed in each industry, emissions from industrial wastewater might amount to as much as several million metric tons of methane annually [41]. |
Methane emissions from agricultural sources totaled 9.1 million metric tons in 1994, about 30 percent of all U.S. anthropogenic methane emissions. There are two major agricultural sources of methane, both of which can be traced to the management of domesticated livestock. The first, enteric fermentation, occurs when carbohydrates are broken down in the digestive track of herbivores such as cattle, sheep, and goats. As microorganisms in the forestomach (rumen) of these animals assist in the digestion of the large quantities of cellulose found in plant material, they produce methane. Nearly all (90 percent) of this methane is released as part of normal animal respiration and eructation. The remainder is released as flatus. The second source is the solid waste of domesticated animals. As solid waste decomposes under anaerobic conditions methane is produced. Together, these two sources accounted for an estimated 8.5 million metric tons of methane emissions in 1994. Of that total, 5.6 million metric tons of methane was emitted through enteric fermentation and the remainder from solid wastes accumulated at farm facilities. The minor sources of agricultural methane emissions, rice cultivation and burning of crop residues, added an additional 600,000 metric tons to the emissions totals.
| U.S. Methane Emissions from Enteric
Fermentation in Domesticated Animals, 1990-1994 | |
| Estimated 1994 Emissions (Million Metric Tons Methane) | 5.62 |
| Change Compared to 1993 (Million Metric Tons Methane) | 0.15 |
| Change from 1993 (Percent) | 2.8 |
| Change Compared to 1990 (Million Metric Tons Methane) | 0.49 |
| Change from 1990 (Percent) | 9.5 |
Methane emissions from enteric fermentation in domesticated animals are estimated at 5.62 million metric tons in 1994, up by nearly 3 percent from 1993 and by nearly 10 percent from 1990 (Table 22). These increases reflect the continuing rise of beef cattle populations. The increases in population were accompanied by growth in the average size of the animals, as measured by their live weight at slaughter (see Appendix A). The growth in beef cattle sizes in 1994 came after a slight decrease in animal sizes in 1993. The 1993 decrease was the first decline in beef cattle sizes in over a decade. While the population of dairy cattle did not rise along with beef cattle in 1994, emissions per animal did increase, as milk production (and hence food energy consumption) rose [42].
Average cattle size in 1995 was slightly below that for 1994 but still above 1993 levels. Thus, small increases in population were offset by decreases in estimated emissions per animal. In contrast, populations of dairy cattle declined slightly in 1995, but levels of milk production per animal increased again over the previous year. The net result of these countervailing forces was that emissions for 1995 remained stable compared to 1994 at 5.62 million metric tons.
| U.S. Methane Emissions from Solid Waste
of Domesticated Animals, 1990-1994 | |
| Estimated 1994 Emissions (Million Metric Tons Methane) | 2.88 |
| Change Compared to 1993 (Million Metric Tons Methane) | 0.07 |
| Change from 1993 (Percent) | 2.5 |
| Change Compared to 1990 (Million Metric Tons Methane) | 0.25 |
| Change from 1990 (Percent) | 9.7 |
Methane emissions from the solid waste of domesticated animals were 2.9 million metric tons in 1994, up from 2.8 million metric tons in 1993, and well above the 2.6 million metric tons emitted from this source in 1990 (Table 23). Just over one-third of emissions from this source can be traced to dairy cattle, and another one-third is attributed to swine sold for human consumption. Nearly 75 percent of the increase in methane emissions between 1990 and 1994 can be traced to these two animal groups.
New Method for Estimating Methane Emissions from Dairy Cattle WasteIn its November 1995 publication, Inventory of U.S. Greenhouse Gas Emissions and Sinks, 1990-1994, the EPA identified six States—Arizona, Florida, Nevada, North Carolina, North Dakota, and Texas—where methods for handling the solid waste of dairy cattle changed between 1991 and 1992. In general, this shift moved dairy cattle waste into anaerobic lagoons from other management systems. Because anaerobic lagoons are particularly well suited for methane production, waste moved into lagoons produces 5 to 90 times the methane it would produce in another management system. To capture this shift, the EIA revised its method for estimating methane emissions from dairy cattle. For the period 1990 to 1995, emissions from dairy cattle in the six States listed above were calculated separately from emissions for all other cattle, and the subtotals were then aggregated (for a more detailed discussion of estimation methods, see Appendix A). These estimates were made using the new data on waste management systems provided by the EPA. The new estimation technique raised emissions estimates for dairy cattle by just under 10 percent and overall estimates of methane emissions from the solid waste of domesticated livestock by about 3 percent over the estimates produced with the method used in last year’s edition of Emissions of Greenhouse Gases in the United States. A comparison of estimates for the period 1990 to 1995 appears below.
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Due to growth in their population, emissions from market swine grew by 11 percent between 1990 and 1994. During that same period, emissions from dairy cattle grew by 9 percent. About one-half of the increase in estimated emissions from dairy cattle was the result of greater animal size and productivity. The remaining portion of the rise in emissions from dairy cattle is traced to a shift in the method used for handling the solid waste of dairy cattle in six States: Arizona, Florida, Nevada, North Carolina, North Dakota, and Texas. Waste management techniques in these States shifted toward liquid systems, especially anaerobic lagoons. Solid waste managed in anaerobic lagoons realizes a much larger share of its maximum potential methane production than waste managed in any other manner.
The growth in market swine populations accelerated in 1995, growing by 13 percent. This will further increase methane emissions from the solid waste of domesticated livestock. However, the size, population, and productivity of other animals appears to have stabilized, restricting emissions growth to swine.
| U.S. Methane Emissions from Rice Cultivation, 1990-1994 | |
| Estimated 1994 Emissions (Thousand Metric Tons Methane) | 463 |
| Change Compared to 1993 (Thousand Metric Tons Methane) | 62 |
| Change from 1993 (Percent) | 15.5 |
| Change Compared to 1990 (Thousand Metric Tons Methane) | 66 |
| Change from 1990 (Percent) | 16.5 |
As organic material decomposes under anaerobic conditions in flooded rice fields, methane is produced [43]. Methane emissions from flooded rice fields are estimated at 460,000 metric tons for 1994, up 15 percent from the levels of both 1990 and 1993 (Table 16). This growth was the result of highly favorable growing conditions across all the rice-producing States. Hectares harvested grew by 17 percent between 1993 and 1994, from 1.24 million hectares to 1.46 million hectares, the highest levels seen since the early 1980s (Table 24).
In 1995, methane emissions from U.S. rice fields declined to 430,000 metric tons as each of the rice-producing States experienced a decline in hectares harvested. The total area harvested moved back toward levels routinely seen over the past decade, about 1.35 million hectares.
| U.S. Methane Emissions from Burning of Crop Residues, 1990-1994 | |
| Estimated 1994 Emissions (Thousand Metric Tons Methane) | 147 |
| Change Compared to 1993 (Thousand Metric Tons Methane) | 33 |
| Change from 1993 (Percent) | 28.7 |
| Change Compared to 1990 (Thousand Metric Tons Methane) | 18 |
| Change from 1990 (Percent) | 13.7 |
In 1994, estimated methane emissions from the burning of crop residues reached their highest level in over a decade, as moderate summer temperatures and precipitation levels in the Midwest led to record levels of soybean and corn production. Corn production increased by 44 percent from 1993 to 1994, to 336 million metric tons, while soybean production soared by 38 percent to 69 million metric tons.
In 1995, corn production declined sharply to 258 million metric tons and soybean production receded to 58 million metric tons. As a result, preliminary estimates of 1995 methane emissions from crop residue burning show a drop to 120,000 metric tons, 10,000 metric tons below 1990 levels (Table 16).
| U.S. Methane Emissions from Chemical Production, 1990-1994 | |
| Estimated 1994 Emissions (Thousand Metric Tons Methane) | 72 |
| Change Compared to 1993 (Thousand Metric Tons Methane) | 6 |
| Change from 1993 (Percent) | 9.6 |
| Change Compared to 1990 (Thousand Metric Tons Methane) | 17 |
| Change from 1990 (Percent) | 29.8 |
During 1994, methane emissions from chemical production continued their slow but steady growth. Levels of methane emissions from chemical production largely reflect more general trends in the national economy. The growth in emissions from chemical production slowed during the 1991 recession, although emissions levels did not decline. Following the recession, emissions from chemical production grew rapidly, rising from 57,000 metric tons in 1991 to 72,000 metric tons in 1994, an annual growth rate of more than 8 percent. In 1995, emissions from chemical production remained unchanged at 72,000 metric tons (Table 25).
A significant portion (60 percent) of the growth in methane emissions from chemical production between 1991 and 1994 can be attributed to an increase in emissions from production of ethylene and styrene. Production of ethylene grew by more than 20 percent during the 1991-1994 time period, and production of styrene grew by nearly 40 percent. To a large extent, the production of styrene drives the production of ethylene, because ethylene is one of the principal feedstocks in producing styrene. Styrene is used to produce polystyrene, a transparent, hard solid with many popular applications such as refrigerator doors, air conditioner cases, clock and radio cases, toys, foam packaging, and water coolers. In 1995, production of styrene leveled off, and production of ethylene declined slightly.
| U.S. Methane Emissions from Iron and Steel Production, 1990-1994 | |
| Estimated 1994 Emissions (Thousand Metric Tons Methane) | 59 |
| Change Compared to 1993 (Thousand Metric Tons Methane) | 1 |
| Change from 1993 (Percent) | 1.0 |
| Change Compared to 1990 (Thousand Metric Tons Methane) | -3 |
| Change from 1990 (Percent) | -4.7 |
Like methane emissions from chemical production, levels of methane emissions from iron and steel production are largely linked to rates of economic activity. Emissions from iron and steel production reached a peak of 64,000 metric tons in 1989 but then declined by 16 percent to 54,000 metric tons during the 1991 recession (Table 25). While emissions from iron and steel production have yet to return to their peak level, they have grown slowly since 1991, reaching 61,000 metric tons in 1995.
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