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Greenhouse Gas Inventory

Central to any study of climate change is the development of an emission inventory that identifies and quantifies a country's primary sources and sinks of greenhouse gases. This inventory provides both (1) a basis for the ongoing development of a comprehensive and detailed methodology for estimating sources and sinks of greenhouse gases and (2) a common, consistent mechanism that enables all signatory countries to the United Nations' Framework Convention on Climate Change (FCCC) to estimate emissions and to compare the relative contributions of different emission sources and greenhouse gases to climate change. Moreover, systematically and consistently estimating national and international emissions is a prerequisite for evaluating the cost-effectiveness and feasibility of mitigation strategies and emission reduction technologies.

This chapter summarizes the latest information on U.S. greenhouse gas emission trends from 1990 to 1995, as presented in the draft EPA report Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1995. To ensure that the U.S. emissions inventory is comparable to those of other FCCC signatory countries, the estimates presented here were calculated using baseline methods similar to those recommended in Volumes 1-3 of the IPCC Guidelines for National Greenhouse Gas Inventories (IPCC/OECD/IEA/UNEP 1995). For U.S. emission sources related to energy consumption, forest sinks, and some CH₄ sources, the IPCC default methodologies were expanded, resulting in a more comprehensive procedure for estimating U.S. emissions. Details on how these estimates were developed are available in the 1995 Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994 (U.S. EPA 1995) and in the upcoming edition.

Recent Trends in U.S. Greenhouse Gas Emissions

Greenhouse gases include water vapor, carbon dioxide (CO₂), methane (CH₄), nitrous oxide (N₂O), and ozone (O₃). Chlorofluorocarbons (CFCs), a family of human-made compounds, and other compounds, such as hydrofluorocarbons (HFCs) and perfluorinated carbons (PFCs), are also greenhouse gases.

Other nongreenhouse, radiatively important gases--such as carbon monoxide (CO), oxides of nitrogen (NO_X), and nonmethane volatile organic compounds (NMVOCs)--contribute indirectly to the greenhouse effect. These are commonly referred to as "tropospheric ozone precursors" because they influence the rate at which ozone and other gases are created and destroyed in the atmosphere. For convenience, all gases discussed in this chapter are generically referred to as "greenhouse gases" (unless otherwise noted).

Although CO₂, CH₄, and N₂O occur naturally in the atmosphere, their recent atmospheric buildup is largely the result of human activities. Since 1800, atmospheric concentrations of these greenhouse gases have increased by 30, 145, and 15 percent, respectively (IPCC 1996). This buildup has altered the composition of the Earth's atmosphere and may affect future global climate.

Beginning in the 1950s, the use of CFCs increased by nearly 10 percent a year, until the mid-1980s when international concern about ozone depletion led to the signing of the Montreal Protocol. Since then, the consumption of CFCs has rapidly declined as they are phased out. In contrast, use of CFC substitutes is expected to grow significantly.

Figure 3-1 and Table 3-1 summarize the current U.S. greenhouse gas emissions inventory for 1990-95. They present the estimated sources and sinks in millions of metric tons of carbon equivalent (MMTCE), which accounts for the gases' global warming potentials.

The growth of U.S. greenhouse gas emissions has been erratic from 1990 to 1995. Emissions from anthropogenic sources dropped in 1991, increased steadily through 1994, and then slowed down in 1995. Over the five-year period, greenhouse gas emissions rose by 5.9 percent, representing an average annual increase of just over one percent. This trend is largely attributable to changes in total energy consumption resulting from the economic slowdown in the early 1990s and the subsequent recovery. U.S. energy consumption increased at an average annual rate of 1.5 percent over the same period (U.S. DOE/EIA 1996c). The increase in emissions from 1993 through 1995 was also influenced by generally low energy prices, which increased demand for fossil fuels (U.S. DOE/EIA 1996b).

Table 3-1
Recent Trends in U.S. Greenhouse Gas Emissions: 1990-1995
                  (MMTs of Carbon Equivalent)

Gases and Sources       Emissions-Direct and Indirect Effects
                        1990    1991    1992    1993    1994    1995

Carbon Dioxide (CO2)    1,228   1,213   1,235   1,268   1,291   1,305
Fossil Fuel Combustion  1,336   1,320   1,340   1,370   1,391   1,403
Industrial Processes and Other
                          17      16      17      18      19      19
Total                   1,353   1,336   1,357   1,388   1,410   1,422
Forests (sink)*         (125)   (123)   (122)   (120)   (119)   (117)
Methane (CH4)             170     172     173     171     176     177
Landfills                  56      58      58      60      62      64
Agriculture                50      51      52      52      54      55
Coal Mining                24      23      22      20      21      20
Oil and Natural Gas Systems
                           33      33      34      33      33      33
Other                       6       7       7       6       6       6
Nitrous Oxide (N2O)         36      37      37      38      39      40
Agriculture                17      17      17      18      18      18
Fossil Fuel Consumption    11      11      12      12      12      12
Industrial Processes        8       8       8       8       9       9
HFCs                       12      12      13      14      17      21
PFCs                        5       5       5       5       7       8
SF6                         7       7       8       8       8       8
U.S. Emissions          1,583   1,570   1,592   1,624   1,657   1,676
Net U.S. Emissions      1,458   1,447   1,470   1,504   1,538   1,559


Note: The totals presented in the summary tables in this chapter 
may not equal the sum of the individual source categories due to rounding.

Among the inventory's greenhouse gases, changes in CO₂ emissions from fossil fuel consumption had the greatest impact during the five-year period. In most cases, emissions from methane, N₂O, HFCs, PFCs, and sulfur hexafluoride (SF₆) have remained relatively constant or have increased slightly. For example, methane emissions increased by just over 4 percent The rise in HFC, PFC, and SF₆ emissions, although a small portion of the total, is significant because of their extremely high global warming potentials and, in the cases of PFCs and SF₆, their long atmospheric lifetimes. Greenhouse gas emissions were partly offset by carbon sequestration in forests.

Figure 3-2 illustrates the relative contributions of the primary greenhouse gases to total U.S. emissions in 1995, with CO₂ emissions accounting for the largest share. The largest change in methane estimates compared to earlier inventories is in the natural gas sector, where emissions have been adjusted upward by more than 75 percent due to improved estimation methods; however, these revised emissions have not changed significantly during 1990-95. Larger landfills, expanded animal populations, and more widespread use of liquid manure management systems increased methane emissions from waste management and agricultural activities. In contrast, improved methane recovery and lower coal production from gassy mines have reduced methane emissions from coal mining.

Nitrous oxide emissions rose by just under 10 percent during the period, primarily for two reasons. First, fertilizer use, which accounts for approximately 46 percent of total U.S. N₂O emissions, increased significantly during 1993-95 as farmers planted more acreage and worked to replace nutrients lost in the 1993 floods. And second, emissions from other categories grew slightly as the U.S. economy grew.

HFC, PFC, and SF₆ emissions are increasing, along with their expanded use as substitutes for CFCs and other ozone-depleting compounds being phased out under the terms of the Montreal Protocol and Clean Air Act Amendments. Two major contributors to the rise in HFC emissions since 1990 are the use of HFC-134a for mobile air conditioners and the emission of HFC-23 during the production of the refrigerant HCFC-22.

The following sections present the anthropogenic sources of greenhouse gas emissions, briefly discuss emission pathways, summarize the emission estimates, and explain the relative importance of emissions from each source category.

Carbon Dioxide Emissions

The global carbon cycle is composed of large carbon flows and reservoirs. Hundreds of billions of tons of carbon in the form of CO₂ are absorbed by the oceans or trees (sinks) or are emitted to the atmosphere annually through natural processes (sources). When in equilibrium, carbon fluxes among the various reservoirs are roughly balanced.

Since the Industrial Revolution, this equilibrium has been increasingly compromised. Atmospheric concentrations of CO₂ have risen about 30 percent, principally because of fossil fuel combustion, which accounts for 99 percent of total U.S. CO₂ emissions (Seki and Christ 1995). Changes in land-use and forestry activities can emit CO₂ (e.g., through conversion of forest land to agricultural or urban use) and can act as a sink for--or absorb--CO₂ (e.g., through improved forest management activities).

Table 3-2 summarizes U.S. sources and sinks of CO₂, while the remainder of this section discusses CO₂ emission trends in greater detail.

The Energy Sector

Energy-related activities account for roughly 87 percent of annual U.S. greenhouse gas emissions. Of that share, approximately 85 percent is produced through fossil fuel combustion, and the remaining 15 percent comes from renewable or other energy sources, such as hydropower, biomass, and nuclear energy (Figure 3-3). Energy-related activities other than fuel combustion also emit greenhouse gases (primarily methane), such as those associated with producing, transmitting, storing, and distributing fossil fuels.

Fossil Fuel Consumption

The amount of carbon in fossil fuels varies significantly by fuel type. For example, coal contains the highest amount of carbon per unit of energy, natural gas has about 45 percent less than coal, and petroleum has about 20 percent less.

In 1995, U.S. fossil fuel combustion emitted 1,403 million metric tons of carbon equivalent (MMTCE). Total consumption of fossil fuels during 1990-95 increased at an average annual rate of 1.2 percent, primarily because of economic growth and generally low energy prices.

Overall, emissions from fossil fuel consumption increased during the five-year period. While emissions of CO₂ in 1991 were approximately 1.2 percent lower than the 1990 baseline level, in 1992 they increased by about 1.6 percent above the 1991 levels, thus returning emissions to slightly higher than the 1990 baseline. By 1993, CO₂ emissions from fossil fuel combustion were approximately 2.5 percent higher than in 1990; in 1994, they were about 4.1 percent higher than 1990; and in 1995, they were about 5 percent higher. This trend is largely attributable to changes in total energy consumption resulting from the economic slowdown in the United States in the early 1990s and the subsequent recovery.

Despite the continued increase in natural gas and coal consumption in 1995, the total amount of petroleum used for energy production declined by about 0.2 percent, as somewhat higher prices for crude oil in 1995 led electric utilities and industry to decrease their consumption of petroleum by 32.0 and 1.9 percent, respectively, and to rely more heavily on natural gas, coal, nuclear electric power, and renewable energy. In contrast, consumption of petroleum increased 1.3 percent in the residential and commercial sectors, and about 1.6 percent in the transportation sector.

The energy-related sources of CO₂ emissions included steam production for industrial processes, gasoline consumption for transportation, heating in residential and commercial buildings, and generation of electricity. Petroleum products across all sectors of the economy accounted for about 42 percent of total U.S. energy-related CO₂ emissions; coal, 36 percent; and natural gas, 22 percent.

Industrial Sector. Industry accounts for the largest percentage of U.S. emissions from fossil fuel consumption (Figure 3-4). About two-thirds of these emissions result from producing steam and process heat, while the remaining third results from providing electricity for such uses as motors, electric furnaces, ovens, and lighting.

Transportation Sector. In the same league as the industrial sector, the transportation sector accounts for about 31 percent of U.S. CO₂ emissions from fossil fuel consumption. Virtually all of the energy consumed in this sector comes from petroleum-based products. Nearly two-thirds of the emissions result from gasoline consumption in automobiles and other vehicles. The remaining emissions stem from meeting other transportation demands, including the combustion of diesel fuel for the trucking industry and jet fuel for aircraft.

Residential and Commercial Sectors . The residential and commercial sectors account for about 19 and 16 percent, respectively, of CO₂ emissions from fossil fuel consumption. Both sectors rely heavily on electricity for meeting energy needs, with about two-thirds to three-quarters of their emissions attributable to electricity consumption. End-use applications include lighting, heating, cooling, and operating appliances. The remaining emissions are largely due to the consumption of natural gas and oil, primarily for meeting heating and cooking needs.

Electric Utilities. The United States relies on electricity to meet a significant portion of its energy requirements--e.g., lighting, electric motors, and heating and air conditioning. As the largest consumers of U.S. energy (averaging 28 percent), electric utilities are collectively the largest producers (approximately 35 percent) of U.S. CO₂ emissions (Figure 3-5).

The type of energy electric utilities consume directly affects the volume of CO₂ emitted. For example, some of this electricity is generated with low-emitting technologies, such as nuclear energy, hydropower, and geothermal energy. However, electric utilities rely on coal for over half of their total energy requirements and account for about 87 percent of all coal consumed in the United States. For this reason, changes in electricity demand can significantly affect coal consumption and associated CO₂ emissions.

Fuel Production and Processing

The methane trapped in natural gas systems or oil wells is typically flared to relieve the rising pressure or to dispose of small quantities of gas that are not commercially marketable. As a result, the carbon contained in the methane becomes oxidized and forms CO₂. In 1995, flaring activities emitted approximately 2 MMTCE, or about 0.1 percent of total U.S. CO₂ emissions. Emission trends from fuel production and processing are dictated by fossil fuel consumption.

Biomass and Biomass-Based Fuel Consumption

Biomass fuel is used primarily by the industrial sector in the form of fuelwood and wood waste, while the transportation sector dominates the use of biomass-based fuel, such as ethanol from corn or woody crops. Ethanol and ethanol blends, such as gasohol, are typically used to fuel public transport vehicles.

Although these fuels emit CO₂, in the long run, their emissions do not increase total atmospheric CO₂ because the biomass resources are consumed on a sustainable basis. For example, fuelwood burned one year but regrown the next only recycles carbon, rather than creating a net increase in total atmospheric carbon.

CO₂ emissions from biomass consumption were approximately 51 MMTCE in 1995, with the industrial sector accounting for 72 percent of the emissions, and the residential sector, 25 percent. CO₂ emissions from ethanol use in the United States have been rising in recent years due to a number of factors, including extension of federal tax exemptions for ethanol production, the Clean Air Act Amendments mandating the reduction of mobile source emissions, and the Energy Policy Act of 1992, which established incentives for increasing the use of alternative fuels and alternative-fueled vehicles. In 1995, total U.S. CO₂ emissions from ethanol were 2 MMTCE.

Industrial Processes

Emissions are often produced as a by-product of various nonenergy-related activities. For example, in the industrial sector raw materials are often chemically transformed from one state to another. This transformation often releases such greenhouse gases as CO₂.

The production processes that emit CO₂ include cement production, lime production, soda ash production and use, limestone consumption (e.g., in iron and steel making), and CO₂ manufacture. In 1995, CO₂ emissions from these sources were approximately 10.5, 3.7, 1.6, 1.2, and 0.4 MMTCE, respectively, for a total of 17.4 MMTCE, or about one percent of total U.S. CO₂ emissions. Since 1990, emissions from cement, lime, and CO₂ manufacturing have increased slightly; emissions from limestone use have fluctuated; while emissions from soda ash production remained constant from 1990 through 1994 and increased in 1995.

Cement Production (10.5 MMTCE)

Carbon dioxide is produced primarily during the production of clinker, an intermediate product from which finished Portland and masonry cements are made. Specifically, CO₂ is created when calcium carbonate (CaCO₃) is heated in a cement kiln to form lime and CO₂. This lime combines with other materials to produce clinker, while the CO₂ is released into the atmosphere.

Lime Production (3.7 MMTCE)

Lime is used in steel making, construction, pulp and paper manufacturing, and water and sewage treatment. It is manufactured by heating limestone (mostly calcium carbonate--CaCO₃) in a kiln, creating calcium oxide (quicklime) and CO₂, which is normally emitted to the atmosphere.

Soda Ash Production and Consumption (1.6 MMTCE)

Commercial soda ash (sodium carbonate) is used in many consumer products, such as glass, soap and detergents, paper, textiles, and food. During the manufacturing of these products, natural sources of sodium carbonate are heated and transformed into a crude soda ash, in which CO₂ is generated as a by-product. In addition, CO₂ is released when the soda ash is consumed.

Limestone Consumption (1.2 MMTCE)

Limestone is a basic raw material used by a wide variety of industries, including the construction, agriculture, chemical, and metallurgical industries. For example, limestone can be used as a purifier in refining metals. In the case of iron ore, limestone heated in a blast furnace reacts with impurities in the iron ore and fuels, generating CO₂ as a by-product. Limestone is also used in flue-gas desulfurization systems to remove sulfur dioxide from the exhaust gases.

Carbon Dioxide Manufacture
(0.4 MMTCE)

CO₂ is used in many segments of the economy, including food processing, beverage manufacturing, chemical processing, crude oil products, and a host of industrial and miscellaneous applications. For the most part, the CO₂ used in these applications will eventually be released into the atmosphere.

Changes in Forest Management and Land Use

How the Earth's land resources are managed can alter the natural balance of trace gas emissions. Everyday land-use decisions include clearing an area of forest to create cropland or pasture, restocking a logged forest, draining a wetland, or allowing a pasture to revert to a grassland or forest.

Forests, which cover about 295 million hectares (737 million acres) of U.S. land in the contiguous 48 states (USDA/USFS 1990), are also an important terrestrial sink for CO₂. Because approximately half the dry weight of wood is carbon, as trees add mass to trunks, limbs, and roots, carbon is stored in relatively long-lived biomass instead of being released to the atmosphere. Soils and vegetative cover also provide potential sinks for carbon emissions.

In the United States improved forest-management practices and the regeneration of previously cleared forest areas have resulted in a net uptake (sequestration) of carbon in U.S. forest lands. This uptake is an ongoing result of land-use changes in previous decades. For example, because of improved agricultural productivity and the widespread use of tractors, the rate of clearing forest land for crop cultivation and pasture slowed greatly in the late 19th century, and by 1920 this practice had all but ceased. As farming expanded in the Midwest and West, large areas of previously cultivated land in the East were brought out of crop production, primarily between 1920 and 1950, and were allowed to revert to forest land or were actively reforested.

Since the early 1950s, the managed growth of private forest land in the East has nearly doubled the biomass density there. The 1970s and 1980s saw a resurgence of federally sponsored tree-planting programs (e.g., the Forestry Incentive Program) and soil conservation programs (e.g., the Conservation Reserve Program), which have focused on reforesting previously harvested lands, improving timber-management, combating soil erosion, and converting marginal cropland to forests.

As a result of these activities, the net CO₂ flux in 1995 is estimated to have been an uptake of 117 MMTCE (which includes the carbon stored both in the U.S. wood product pool and in landfills). This carbon uptake represents an offset of about 8 percent of the 1995 CO₂ emissions from fossil fuel combustion during this period. The amount of carbon sequestered through changes in U.S. forestry and land-use practices continues to decline, however, as the expansion of eastern forest cover slows down.

Methane Emissions

Atmospheric methane (CH₄) is an integral component of the greenhouse effect, second only to CO₂ as an anthropogenic source. Methane's overall contribution to global warming is large because it is estimated to be twenty-one times more effective at trapping heat in the atmosphere than CO₂ over a 100-year time horizon (IPCC 1996).

Over the last two centuries, methane's concentration in the atmosphere has more than doubled. Scientists believe these atmospheric increases are largely due to increasing emissions from anthropogenic sources, such as landfills, agricultural activities, fossil fuel combustion, coal mining, the production and processing of natural gas and oil, and wastewater treatment (Figure 3-6 and Table 3-3).

Landfills

Landfills are the largest single anthropogenic source of methane emissions in the United States. Of the estimated 3,000 methane-emitting landfills in the United States, 1,300 account for about half of the emissions.

In an environment where the oxygen content is low or nonexistent, organic materials, such as yard waste, household waste, food waste, and paper, are decomposed by bacteria to produce methane, CO₂, and stabilized organic materials (materials that cannot be decomposed further). Methane emissions from landfills are affected by such specific factors as waste composition, moisture, and landfill size.

Methane emissions from U.S. landfills in 1995 were 63.5 MMTCE, a slight increase over the 60 MMTCE reported in the previous inventory. Emissions from U.S. municipal solid waste landfills, which received over 59 percent of the total solid waste generated in the United States, accounted for about 90-95 percent of total landfill emissions, while industrial landfills accounted for the remaining 5-10 percent. Currently, almost 15 percent of the methane released is recovered for use as energy, compared to 10 percent reported in the last inventory.

A regulation promulgated in March 1996 requires the largest U.S. landfills to collect and combust their landfill gas, to reduce emissions of nonmethane volatile organic compounds (VOCs). It is estimated that by the year 2000, this regulation will have reduced landfill methane emissions by more than 50 percent (6.2 MMT of methane, or 35.5 MMTCE).

Agriculture

The agricultural sector accounted for approximately 31 percent of total U.S. methane emissions in 1995, with enteric fermentation in domestic livestock (34.9 MMTCE) and manure management (17.1 MMTCE) together accounting for the majority (Figure 3-7). Other agricultural activities contributing directly to methane emissions include rice cultivation (2.8 MMTCE) and field burning of agricultural crop wastes (0.04 MMTCE).

Between 1990 and 1995, methane emissions from domestic livestock enteric fermentation and manure management increased by about 7 percent and 15 percent, respectively. During this same time period, methane emissions from rice cultivation increased by about 10 percent, while emissions from field burning fluctuated. Several other agricultural activities, such as irrigation and tillage practices, may contribute to methane emissions. However, since emissions from these sources are uncertain and are believed to be small, the United States has not included them in the current inventory. Details on the emission pathways included in the inventory follow.

Enteric Fermentation in Domestic Livestock (34.9 MMTCE)

During animal digestion, methane is produced through a process referred to as enteric fermentation, in which microbes that reside in animal digestive systems break down the feed consumed by the animal. In 1995, enteric fermentation was the source of about 20 percent of total U.S. methane emissions, and about 64 percent of methane emissions from the agricultural sector. This estimate of 34.9 MMTCE is the same as that reported in the previous inventory.

Manure Management (17.1 MMTCE)

The decomposition of organic animal waste in an anaerobic environment produces methane. The most important factor affecting the amount of methane produced is how the manure is managed, since certain types of storage and treatment systems promote an oxygen-free environment. In particular, liquid systems tend to produce a significant quantity of methane, whereas solid waste management approaches produce little or no methane. Higher temperatures and moist climate conditions also promote methane production.

Emissions from manure management were about 10 percent of total U.S. methane emissions in 1995, and about 31 percent of methane emissions from the agriculture sector. Liquid-based manure management systems accounted for over 80 percent of total emissions from animal wastes. The 17.1 MMTCE estimate reported here is slightly above the 13.7 MMTCE reported in the previous inventory because of larger U.S. farm animal populations and expanded use of liquid manure management systems.

Rice Cultivation (2.8 MMTCE)

Most of the world's rice, and all of the rice in the United States, is grown on flooded fields. The soil's organic matter decomposes under the anaerobic conditions created by the flooding, releasing methane to the atmosphere, primarily through the rice plants.

In 1995, rice cultivation was the source of less than 2 percent of total U.S. methane emissions, and about 5 percent of U.S. methane emissions from agricultural sources. Emission estimates from this source have not changed significantly since 1990.

Field Burning of Agricultural Wastes (0.04 MMTCE)

Farming systems produce large quantities of agricultural crop wastes. Disposal systems for these wastes include plowing them back into the field; composting, landfilling, or burning them in the field; using them as a biomass fuel; or selling them in supplemental feed markets.

Burning crop residues releases a number of greenhouse gases, including CO₂, methane, carbon monoxide, nitrous oxide, and oxides of nitrogen. Field burning is not considered to be a net source of CO₂ emissions because the CO₂ released is reabsorbed by crop regrowth during the next growing season. However, this practice is a net source of emissions for the other gases, since their emissions would not have occurred had the wastes not been combusted.

Because field burning is not common in the United States, it was responsible for only 0.02 percent of total U.S. methane emissions in 1995, and 0.07 percent of emissions from the agricultural sector. Estimates of emissions from this source have dropped significantly since the last inventory as a result of new research indicating that a smaller fraction of U.S. crop wastes is burned than previously assumed.

Oil and Natural Gas Production and Processing

Methane emissions vary greatly from facility to facility. In 1995, an estimated 31.2 MMTCE (or approximately 18 percent) of U.S. methane emissions were due to leaks, disruptions, etc., in the operation and maintenance of equipment in the U.S. natural gas system. This figure is significantly higher than previous estimates because of revised estimation methods that improved activity factors (i.e., equipment counts) and emission factors. As a result, natural gas systems are now ranked as the third largest source of U.S. methane emissions.

Natural gas is often found in conjunction with oil exploration. Methane is also released during the production, refinement, transportation, and storage of crude oil. During 1995, oil and gas production and processing facilities released 2.0 MMTCE of methane to the atmosphere, representing about one percent of total U.S. methane emissions.

Coal Mining

Produced millions of years ago during the formation of coal, methane is trapped within coal seams and surrounding rock strata. The volume of methane released to the atmosphere during coal-mining operations depends primarily upon the depth and type of coal being mined.

Methane from surface mines is emitted directly to the atmosphere as the rock strata overlying the coal seam are removed. Because methane in underground mines is explosive at concentrations of 5-15 percent in air, most active underground mines are required to circulate large volumes of air and vent the air into the atmosphere. At some mines, methane-recovery systems may supplement these ventilation systems to ensure mine safety. U.S. recovery of methane for energy has been increasing in recent years.

During 1995, coal mining, processing, transportation, and consumption activities produced an estimated 20.4 MMTCE of methane, or 12 percent of total U.S. methane emissions. This lower estimate is the result of improved mine-specific information and expanded methane recovery.

Other Sources of Methane

Methane is also produced from several other sources in the United States, including energy-related combustion activities, wastewater treatment, industrial processes, and changes in land use. The sources included in the U.S. inventory are fuel combustion and wastewater treatment, which accounted for approximately 4.6 and 0.9 MMTCE, respectively, in 1995. These emissions represent about 3 percent of total U.S. methane emissions. Additional U.S. anthropogenic sources of methane--such as ammonia, coke, iron, and steel production and land-use changes--are not included because little information on methane emissions from these sources is currently available.

Nitrous Oxide Emissions

Nitrous oxide (N₂O) is a chemically and radiatively active greenhouse gas that is produced naturally from a variety of biological sources in soil and water. While N₂O emissions are much lower than CO₂ emissions, N₂O is approximately 310 times more powerful than CO₂ at trapping heat in the atmosphere over a 100-year time horizon (IPCC 1996).

During the past two centuries, human activities, such as those presented in Figure 3-8 and Table 3-4 have raised atmospheric concentrations of N₂O by approximately 8 percent. While emissions from soil management and fertilizers remained relatively constant during 1990-93, they increased during 1994-95 because of intensified fertilizer applications to speed the recovery of nutrients lost to the 1993 floods. N₂O emissions from all other sources showed no significant changes.

Agricultural Soil Management and Fertilizer Use

In 1995, U.S. consumption of synthetic nitrogen and organic fertilizers accounted for 18.4 MMTCE, or approximately 46 percent, of total U.S. N₂O emissions. Other agricultural soil management practices, such as irrigating, tilling, or laying fallow the land, can also affect N₂O fluxes to and from the soil. However, because there is much uncertainty about the direction and magnitude of the effects of these other practices, only the emissions from fertilizer use and field burning of agricultural wastes are included in the U.S. inventory at this time.

Fossil Fuel Combustion

N₂O is a product of the reaction that occurs between nitrogen and oxygen during fossil fuel combustion. Both mobile and stationary sources of combustion emit N₂O, and the volume emitted varies according to the type of fuel, technology, or pollution control device used, as well as maintenance and operation practices.

For example, catalytic converters installed to reduce vehicular pollutants have unintentionally promoted the formation of N₂O. As the number of catalytic converter-equipped vehicles has risen in the U.S. motor vehicle fleet, so have emissions of N₂O from this source (DOE/EIA 1993b).

In 1995, N₂O emissions from mobile sources totaled 9.2 MMTCE (or 23 percent of total N₂O emissions), and total N₂O emissions from stationary sources were 3.0 MMTCE.

Adipic Acid Production

The vast majority of all adipic acid produced in the United States is used to manufacture nylon 6,6. N₂O is also used to produce some low-temperature lubricants and to add a "tangy" flavor to foods.

In 1995, U.S. adipic acid production generated 5.2 MMTCE of N₂O, or 13 percent of total U.S. N₂O emissions. By 1996, all adipic acid production plants in the United States. are expected to have N₂O controls in place that will reduce emissions up to 98 percent, compared to uncontrolled levels. (One-half of the plants had these controls in place and operating in 1995.)

Nitric Acid Production

Nitric acid production is another industrial source of N₂O emissions. Used primarily to make synthetic commercial fertilizer, this raw material is also a major component in the production of adipic acid and explosives.

Virtually all of the nitric acid manufactured commercially in the United States is produced by the oxidation of ammonia, during which N₂O is formed and emitted to the atmosphere. In 1995, about 3.6 MMTCE of N₂O were emitted from nitric acid production, accounting for 9 percent of total U.S. N₂O emissions.

Other Sources of N₂O

Other N₂O-emitting activities include the burning of agricultural crop residues and changes in land use. In 1995 agricultural burning contributed approximately 0.01 MMTCE of N₂O emissions to the atmosphere.

The U.S. inventory does not account for several land-use changes because of uncertainties in their effects on fluxes in N₂O and trace gases, as well as poorly quantified statistics on them. These changes include forestry activities, reclamation of freshwater wetland areas, conversion of grasslands to pasture and cropland, and conversion of managed lands to grasslands.

HFCs, PFCs, and SF₆ Emissions

Hydrofluorocarbons (HFCs) and perfluorinated compounds (PFCs) have been introduced as alternatives to the ozone-depleting substances being phased out under the Montreal Protocol and Clean Air Act Amendments of 1990. Because HFCs and PFCs are not directly harmful to the stratospheric ozone layer, they are not controlled by the Montreal Protocol.

However, these compounds, along with sulfur hexafluoride (SF₆), are powerful greenhouse gases. Therefore, they are considered under the United Nations' Framework Convention on Climate Change (FCCC). In addition to having high global warming potentials, SF₆ and most PFCs have extremely long atmospheric lifetimes, resulting in their essentially irreversible accumulation in the atmosphere.

From 1990 to 1993, U.S. emissions of HFCs and PFCs remained relatively constant, while SF₆ emissions increased slightly. Since 1993, the use and emissions of HFC substitutes have grown largely from an increase in the use of HFC-134a in mobile air conditioners. HFC and PFC emissions also result as by-product emissions from other production processes. For example, HFC-23 is a by-product emitted during the production of HCFC-22, and PFCs (CF₄ and C₂F₆) are emitted during aluminum smelting.

Sulfur hexafluoride (SF₆) is the most potent greenhouse gas the IPCC has ever evaluated. About 80 percent of the worldwide use of SF₆ is as an insulator in electrical transmission and distribution systems. SF₆ is also used as a protective atmosphere for the casting of molten magnesium.

Table 3-5 presents emission estimates for these gases. In 1995, U.S. emissions of HFCs and PFCs were estimated to be 29 MMTCE, and SF₆SF₆ emissions, approximately 8 MMTCE.

Emissions of Criteria Pollutants

In the United States, carbon monoxide (CO), nitrogen oxides (NO_X), nonmethane volatile organic compounds (NMVOCs), and sulfur dioxide (SO₂) are commonly referred to as "criteria pollutants." CO is produced when carbon-containing fuels are burned incompletely. Oxides of nitrogen (NO and NO₂) are created by lightning, fires, and fossil fuel combustion, and in the stratosphere from nitrous oxide. NMVOCs--

which include such compounds as propane, butane, and ethane--are emitted primarily from transportation and industrial processes, as well as from forest wildfires and nonindustrial consumption of organic solvents. And SO₂ can result from the combustion of fossil fuels, industrial processing (particularly in the metals industry), waste incineration, and biomass burning (U.S. EPA 1996).

Because of their contribution to the formation of urban smog (and acid rain in the case of SO₂), criteria pollutants are regulated under the 1970 Clean Air Act and its successive amendments. These gases also affect global climate, although their impact is limited because their radiative effects are indirect. That is, they do not directly act as greenhouse gases, but react with other chemical compounds in the atmosphere to form compounds that are greenhouse gases. Unlike other criteria pollutants, SO₂ emitted into the atmosphere affects the Earth's radiative budget negatively; therefore, it is discussed separately from the other criteria pollutants in this section.

The most important of the indirect effects of criteria pollutants is their role as precursors of tropospheric ozone. In this role, they contribute to ozone formation and alter the atmospheric lifetimes of other greenhouse gases. For example, CO interacts with the hydroxyl radical (OH)--the major atmospheric sink for methane emissions--to form CO₂. Therefore, increased atmospheric concentrations of CO limit the number of OH compounds available to destroy methane, thus increasing the atmospheric lifetime of methane.

Since 1970, the United States has published estimates of annual emissions of criteria pollutants. Table 3-6 shows that fuel consumption accounts for the majority of emissions of these gases. In fact, motor vehicles that burn fossil fuels contributed approximately 81 percent of all U.S. CO emissions in 1995. Motor vehicles also emit more than a third of total U.S. NO_X and NMVOC emissions. Industrial processes--such as the manufacture of chemical and allied products, metals processing, and industrial uses of solvents--are also major sources of CO, NO_X, and NMVOCs. ~~