This document provides information on green house gas sources and sinks, and estimates of emissions and removals for the United States for 1990 1993, as well as the methods used to calculate these estimates, and the uncertainties associated with them. Although estimates are provided for all four years, the 1990 estimates are considered the base year, since under the Framework Convention on Climate Change, countries are to submit inventories of greenhouse gas emissions for the year 1990.
The emission estimates presented here were calculated using the IPCC Draft Guidelines for National Greenhouse Gas Inventories (IPCC/OECD, 1994) to ensure that the emission inventories submitted to the FCCC are consistent and comparable across sectors and between nations. In order to fully comply with the IPCC Draft Guidelines, the United States has provided a copy of the IPCC reporting tables in Annex D of this report. These tables include the data used to calculate emission estimates using the IPCC Draft Guidelines. The United States has followed these guidelines, except where more detailed data or methodologies were available for major U.S. sources of emissions. In such cases, the United States expanded on the IPCC guidelines to provide a more comprehensive and accurate account of U.S. emissions. These instances have been documented, and explanations have been provided for diverging from the IPCC Guidelines (IPCC/OECD, 1994).
The current U.S. greenhouse gas inventory for 1990-93 is summarized in Table ES-1. For the 1990 base year, total U.S. emissions were 1,444 MMTCE. To be consistent with the IPCC-recommended guide lines, this estimate excludes emissions of 22.6 MMTCE from international transport. Changes in CO2 emissions from fossil fuel consumption had the greatest impact on U.S. emissions from 1990 to 1993. While U.S. emissions of CO2 in 1991 were approximately 1.2 percent lower than 1990 emission levels, in 1992 they were about 1.5 percent over 1991 levels, thus returning emissions to about 1990 levels. This trend is largely attributable to changes in total energy consumption resulting from the economic slowdown in the U.S. economy and the subsequent recovery. Based on preliminary data for 1993, the upward trend since 1991 has continued, with 1993 CO2 emissions from fossil fuel combustion approximately 2.4 percent greater than 1990.
CH4, N20, and HFCs and PFCs represent a much smaller portion of total emissions than CO2. Overall, emissions of these gases remained relatively constant from 1990 to 1992. Methane emissions from coal mining declined slightly due to small decreases in coal production and increases in coalbed methane recovery. N20 emissions remained relatively constant, while HFC emissions increased slightly, due to increased production of HCFC-22, which increased by-product emissions of HFC-23. Emissions of PFCs have remained constant over the period.
U.S. emissions were partly offset by an uptake of carbon in U.S. forests of 119 MMTCE. This increase was due to intensified forest management practices and the regeneration of forest land previously cleared for cropland and pasture.
Figure ES-l illustrates the relative contribution of the primary greenhouse gases to total U.S. emissions in 1990. Due largely to fossil fuel consumption, CO2 emissions accounted for the largest share of U.S. emissions -- 85 percent. These emissions were partially offset by the sequestration that occurred on forested lands. Methane accounted for 11 percent of total emissions, including contributions from landfills and agricultural activities, among others. The other gases were less important, with N20 emissions comprising about 2 percent of total U.S. emissions, HFCs accounting for slightly over 1 percent, and PFCs about 0.3 percent. The emissions of the photochemically important gases CO, NOx, NMVOCs, and SO2 are not included in Figure ES-1 because there is no agreed upon method to estimate their contribution to climate change. These gases only affect radiative forcing indirectly. Also, any gases covered under the Montreal Protocol are not included in this figure because their use is being phased out, and the IPCC Guidelines (IPCC/OECD, 1994) recommend excluding gases covered by the Montreal Protocol.
The following sections present the anthropogenic sources of greenhouse gas emissions, briefly discuss the emission pathways, summarize the emission estimates, and explain the relative importance of emissions from each source category.
Table ES-2 summarizes U.S. emissions and uptake of carbon dioxide, while the remainder of this section presents detailed information on the various anthropogenic sources and sinks of carbon dioxide in the United States.
Fossil Fuel Consumption. In 1990, the United States emitted a total of 1,335 MMTCE from fossil fuel combustion. (Bunker fuels, or fuels used in international transport, accounted for an additional 22.6 MMTCE.) The energy-related activities producing these emissions included heating in residential and commercial buildings, the generation of electricity, steam production for industrial processes, and gasoline consumption in automobiles and other vehicles. Petroleum products across all sectors of the economy accounted for about 44 percent of total U.S. energy related carbon dioxide emissions; coal, 36 percent; and natural gas, 20 percent.
Industrial Sector
The industrial sector accounts for 34 percent of U.S. emissions from fossil fuel consumption, making it the largest end-use source of CO2 emissions (see Figure ES-3). About two-thirds of these emissions result from the direct consumption of fossil fuels in order to meet industrial demand for steam and process heat. The remaining one-third ofindustrial energy needs is met by electricity for such uses as motors, electric furnaces and ovens, and lighting.
The industrial sector is also the largest user of nonenergy applications of fossil fuels, which often store carbon. Fossil fuels used for producing fertilizers , plastics, asphalt, or lubricants can store carbon in products for very long periods. Asphalt used in road construction, for example, stores carbon indefinitely. Similarly, the fossil fuels used in the manufacture of materials like plastics also store carbon, releasing this carbon only if the product is incinerated.
Transportation Sector
The transportation sector is also a major source of CO2 accounting for about 31 percent of U.S. emissions. Virtually all of the energy consumed in this sector comes from petroleum-based products. Nearly two-thirds of the emissions are the result of gasoline consumption in automobiles and other vehicles, with other uses, including diesel fuel for the trucking industry and jet fuel for aircraft, accounting for the remainder.
Residential and Commercial Sectors
The residential and commercial sectors account for about 19 and 16 percent, respectively, of CO2 emissions from 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 U.S. relies on electricity to meet a significant portion of its energy requirements. In fact, as the largest consumers of fossil fuels, electric utilities are collectively the largest producers of U.S. CO2 emissions (see Figure ES-4). Electric utilities generate electricity for such uses as lighting, heating, electric motors, and air conditioning. Some of this electricity is generated with the lowest C02-emitting energy technologies, particularly nonfossil options, such as nuclear energy, hydropower, or geothermal energy. However, electric utilities rely on coal for 55 percent of their total energy requirements and account for about 85 percent of all coal consumed in the United States.
Fuel Production and Processing. CO2 is produced via flaring activities at natural gas systems and oil wells. Typically, the methane that is trapped in a natural gas system or oil well is flared to relieve the pressure building in the system 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 carbon dioxide. In 1990, the amount of CO2 from the flared gas was approximately 1.8 MMTCE, or about 0.1 percent of total U.S. CO2 emissions.
Biomass and Biomass-Based Fuel Consumption. Biomass fuel is used primarily by the industrial sector in the form of fuelwood and wood waste. Biomass-based fuel use, such as ethanol from corn or woody crops, occurs mainly in the transportation sector. Ethanol and ethanol blends, such as gasohol, are typically used to fuel public transport vehicles, such as buses or centrally fueled fleet vehicles.
Biomass, ethanol, and ethanol-blend fuels do release carbon dioxide. However, in the long run, the carbon dioxide they emit does not increase total atmospheric carbon dioxide 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. As a result, carbon dioxide emissions from biomass have been estimated separately from fossil fuel-based emissions and, as recommended in the IPCC Draft Guidelines, are not included in national totals.
For 1990, CO2 emissions from biomass consumption were approximately 48 MMTCE, with the industrial sector accounting for 73 percent of the emissions and the residential sector, 25 percent. Carbon dioxide emissions from ethanol use in the United States are generally declining, due to a combination of low gasoline prices and limited ethanol supply. In 1990, total U.S. CO2 emissions from ethanol were estimated to be 1.2 MMTCE, and were emitted mostly in the South and Midwest, where the majority of ethanol is produced and consumed.
Cement Production (8.9 MMTCE). Carbon dioxide is produced primarily during the production of clinker, an intermediate product from which finished Portland and masonry cement are made. Specifically, carbon dioxide is created when calcium carbonate (CaC03) is heated in a cement kiln to form lime and carbon dioxide. This lime combines with other materials to produce clinker, while the carbon dioxide is released into the atmosphere.
Lime Production (3.2 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) in a kiln, creating calcium oxide (quicklime) and carbon dioxide, which is normally emitted to the atmosphere.
Limestone Consumption (1.4 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, such as iron. In this case, limestone heated in a blast furnace reacts with impurities in the iron ore and fuels, generating carbon dioxide as a by-product. It is also used in flue gas desulfurization systems to remove sulfur dioxide from the exhaust gases.
Soda Ash Production and Consumption (1.1 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 carbon dioxide is generated as a by-product. In addition, carbon dioxide is released when the soda ash is consumed. Of the two states that produce natural soda ash, only Wyoming has net emissions of carbon dioxide, because producers in California recover the CO2 and use it in other stages of production. U.S. CO2 emissions from soda ash production were approximately 0.4 MMTCE in 1990, while U.S. soda ash consumption generated about 0.7 MMTCE.
Carbon Dioxide Manufacture (0.3 MMTCE). Carbon dioxide 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, carbon dioxide used in these applications will eventually be released into the atmosphere.
In the United States improved forest-management practices and the regeneration of previously cleared forest area have actually increased the amount of carbon stored on U.S. Ands. This uptake of carbon 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. The regeneration of forest land greatly increases carbon storage in both standing biomass and soils, and the impacts of these land-use changes are still affecting forest carbon fluxes in the East. In addition to land-use changes in the early part of this century, forest carbon fluxes in the East are affected by a trend toward managed growth on private land in recent decades, resulting in a near doubling of the biomass density in eastern forests since the early 1950s. More recently, 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 activities, combating soil erosion, and converting marginal cropland to forests.
As a result of these activities, the net CO2 flux from standing biomass and vegetative cover in 1990 was estimated to have been an uptake (sequestration) of 119 MMTCE. The Northeast, North Central, and South Central regions of the United States accounted for 99 percent of the uptake of carbon, largely due to high growth rates that are the result of intensified forest management practices and the regeneration of forest land previously cleared for cropland and pasture. Western states are responsible for a small net release of carbon, reflecting mature forests with a near balance between growth, mortality, and removals.
There are considerable uncertainties associated with the estimates provided for the net carbon flux from U.S. forests, however. Four major uncertainties are presented briefly below:
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, carbon dioxide, and stabilized organic materials (materials that cannot be decomposed further). Methane emissions from landfills are affected by such factors as waste composition, moisture, and landfill size.
Methane emissions from U.S. landfills in 1990 were 60 MMTCE, or about 37 percent of total U.S. methane emissions. Emissions from U.S. municipal solid waste landfills, which received over 70 percent of the total solid waste generated in the United States, accounted for about 90 to 95 percent of total landfill emissions, while industrial landfills accounted for the remaining 5 to 10 percent. Currently, about 10 percent of the methane emitted is recovered for use as an energy source.
Enteric Fermentation in Domestic Livestock (34.9 MMTCE). In 1990, enteric fermentation was the source of about 22 percent of total U.S. methane emissions, and about 68 percent of methane emissions from the agricultural sector. During animal digestion, methane is produced through enteric fermentation, a process in which microbes that reside in animal digestive systems break down the feed consumed by the animal. Ruminants, which include cattle, buffalo, sheep, and goats, have the highest methane emissions among all animal types because they have a rumen, or large "fore-stomach," in which a significant amount of methane-producing fermentation occurs. Nonruminant domestic animals, such as pigs and horses, have much lower methane emissions than ruminants because much less methane-producing fermentation takes place in their digestive systems. The amount of methane produced and excreted by an individual animal also depends upon the amount and type of feed it consumes.
Manure Management (13. 7 MMTCE). The decomposition of organic material in animal manure 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 (e.g., lagoons, ponds, tanks, or pits) tend to produce a significant quantity of methane. However, when manure is handled as a solid or when it is deposited on pastures and rangelands, it tends to decompose aerobically and produce little or no methane. Higher temperatures and moist climate conditions also promote methane production.
Emissions from manure management were about 8 percent of total U.S. methane emissions in 1990, and about 27 percent of methane emissions from the agricultural sector. Liquid-based manure management systems accounted for over 80 percent of total emissions from animal wastes.
Rice Cultivation (2. 6 MMTCE). Most of the world's rice, and all of the rice in the United States, Is grown on flooded fields. When fields are flooded, anaerobic conditions in the soils develop, and methane is produced through anaerobic decomposition of soil organic matter. Methane is released primarily through the rice plants, which act as conduits from the soil to the atmosphere.
Rice cultivation is a very small source of methane in the United States. In 1990, methane emissions from this source were less than 2 percent of total U.S. methane emissions, and about 5 percent of U.S. methane emissions from agricultural sources.
Field Burning of Agricultural Wastes (0.5 MMTCE). Large quantities of agricultural crop wastes are produced from farming systems. 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 carbon dioxide, methane, carbon monoxide, nitrous oxide, and oxides of nitrogen. Crop residue burning is not considered to be a net source of carbon dioxide emissions because the carbon dioxide released during burning is reabsorbed by crop regrowth during the next growing season. However, burning is a net source of emissions for the other gases. Because this practice is not common in the United States, it was responsible for only 0.3 percent of total U.S. methane emissions in 1990, and 0.9 percent of emissions from the agricultural sector.
Produced millions of years ago during the formation of coal, methane is trapped within coal seams and surrounding rock strata. When coal is mined, methane is released to the atmosphere. The amount of methane released from a coal mine depends primarily upon the depth and type of coal, with deeper mines generally emitting more methane (U.S. EPA, 1993a). Methane from surface mines is emitted directly to the atmosphere as the rock strata overlying the coal seam are removed.
Methane is hazardous in underground mines because it is explosive at concentrations of 5 to 15 percent in air. Therefore, all underground mines are required to remove methane by circulating large quantities of air through the mine and venting this air into the atmosphere. At some mines, more advanced methane-recovery systems may be used to supplement the ventilation systems and ensure mine safety. The practice of using the recovered methane as an energy source has been increasing in recent years.
Over the past two centuries, human activities have raised atmospheric concentrations of nitrous oxide by approximately 8 percent. The main anthropogenic activities producing N20 are soil management and fertilizer use for agriculture, fossil fuel combustion, adipic acid production, nitric acid production, and agricultural waste burning. The relative share of each of these activities to total U.S. nitrous oxide emissions is shown in Figure ES-7, and U.S. nitrous oxide emissions by source category for 1990 are provided in Table ES-4.
Other agricultural soil management practices, such as irrigation, tillage practices, or the fallowing of land, can also affect N20 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.
For example, catalytic converters installed to reduce air pollution resulting from motor vehicles have been proven to promote the formation of nitrous oxide. As catalytic converter-equipped vehicles have in creased in the U.S. motor vehicle fleet, emissions of nitrous oxide from this source have also increased (EIA, 1994g). Mobile emissions totalled 6.8 MMTCE in 1990 (22.4 percent of total N2O emissions), with road transport accounting for approximately 95 percent of these N2O emissions. Nitrous oxide emissions from stationary sources were 2.6 MMTCE in 1990.
Forestry activities may also result in fluxes of nitrous oxide, since dry soils are a source of N2O emissions. However, the effects of forestry activities on fluxes of these gases are highly uncertain; therefore, they are not included in the inventory at this time. Similarly, the U.S. inventory does not account for several land-use changes because of uncertainties in their effects on trace gas fluxes, as well as poorly quantified land-use change statistics. These land-use changes include loss and reclamation of freshwater wetland areas, conversion of grasslands to pasture and cropland, and conversion of managed lands to grasslands.
In 1990, the use of CFC and HCFC substitutes was minimal. Thus, emissions of HFCs and PFCs were quite small, and were largely the result of 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. While the use of such ozone-depleting substances as methyl chloroform, CFC-12, and HCFC-22 is declining, consumption of HFCs is increasing markedly. Emissions of HFCs and PFCs should continue to rise as their use as replacements increases.
These criteria pollutants are generated through a variety of anthropogenic activities, including fossil fuel combustion, solid waste incineration, oil and gas production and processing, industrial processes and solvent use, and agricultural crop waste burning. Table ES-6 summarizes U.S. emissions from these sources for 1990. The United States has annually published estimates of criteria pollutants since 1970. Table ES-6 clearly shows that fuel consumption accounted for the majority of emissions of these ~gases. In fact, motor vehicles that burn fossil fuels comprise the single largest source of CO emissions in the United States, contributing about two-thirds of all U.S. CO emissions in 1990. Motor vehicles also emit about one-third of total U.S. NOX 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, NOX, and NMVOC
