Appendix A. Estimation Methods

The organization of this year’s report differs from that of earlier reports in the series. Previously, the body of the report contained a discussion of estimated emissions of greenhouse gases, combined with a discussion of the methods used to estimate emissions. This year, information on emissions estimation methods have been compiled into this appendix. The organization of the appendix generally follows the organization of the body of this report: the discussion is divided by greenhouse gas and by emissions source.

Carbon Dioxide

Most U.S. anthropogenic carbon dioxide emissions come from energy consumption. Energy production contributes a small amount during the flaring of natural gas at oil and gas wells, and a number of industrial processes also emit carbon dioxide through noncombustion processes. The largest single source of emissions from these processes is the calcination of limestone in cement production. Other sources include lime manufacture, limestone and dolomite consumption, soda ash manufacture and consumption, industrial carbon dioxide manufacture and aluminum production. Lastly, some small adjustments are made to reach the total for national emissions. Each emissions source for carbon dioxide, the estimation method used, and the data sources are described here.

Several emissions sources are excluded from the carbon dioxide emissions presented in this report, either due to the uncertainty of estimates or because they are based on biomass combustion (which is assumed to be consumed sustainably with a net flux of carbon dioxide to the atmosphere equal to zero). Should the energy use of biomass fuels result in a long-term decline in the total carbon embodied in standing biomass (e.g., forests), the net release of carbon would be treated as a land use issue (see Chapter 7).

Energy Consumption

Most U.S. commercial energy is produced through the combustion of fossil fuels such as coal, natural gas, and petroleum. Chemically, the main components of fossil fuels are hydrocarbons, made up of molecules containing hydrogen and carbon atoms. When these fuels are burned, atmospheric oxygen combines with the hydrogen atoms to create water vapor, and with the carbon atoms to create carbon dioxide. In theory, if the amount of fuel burned and the amount of carbon in the fuel is known, the volume of carbon dioxide emitted into the atmosphere can be computed with a high degree of precision. In practice, however, a combination of real-world complexities can reduce the precision of the estimate. These complexities will be discussed further in this appendix. Nonetheless, energy-related carbon dioxide emissions are known with greater reliability than other greenhouse gas emissions sources, and the uncertainty in the estimate is probably 10 percent or less. Appendix C, “Uncertainty in Emissions Estimates,” contains an extended discussion of the nature and sources of uncertainty in the estimates presented in this report.

One real-world complexity is that not all of the carbon in fuel is perfectly combusted. About 1.5 percent of the carbon in fossil fuels is emitted in the form of carbon monoxide, which swiftly decays into carbon dioxide in the atmosphere. Another 1 percent is emitted in the form of volatile organic compounds (including methane), which also eventually decay into carbon dioxide. The carbon dioxide emissions reported in Chapter 2 include all “potential” carbon dioxide emissions from the sources covered, including both carbon dioxide emitted directly and carbon emitted in other forms (such as carbon monoxide) that rapidly decay into carbon dioxide in the atmosphere.

Estimation Method

Carbon emissions in this report were calculated by multiplying energy consumption for each fuel type by an associated carbon emissions coefficient. The result was then modified by subtracting carbon sequestered by nonfuel use. This section describes the derivation of information on energy consumption, emissions coefficients, and carbon sequestered by nonfuel use.

Consumption Data

The Energy Information Administration (EIA) collects a wide variety of information from primary suppliers on a frequent basis and from energy consumers less often, but still in a timely manner. Thus, levels of energy consumption in the United States are fairly well known by end-use sector and detailed fuel type [112]. To estimate carbon dioxide emissions, the EIA uses annual data from the four end-use sectors (residential, commercial, industrial, and transportation) and all of the fossil fuels (coal, natural gas, and the full slate of petroleum products). The petroleum products include asphalt and road oil, aviation gasoline, distillate fuel, jet fuel, kerosene, liquefied petroleum gases (LPGs), lubricants, motor gasoline, residential fuel, and other petroleum products. Definitions and chemical characteristics of the fossil fuels are documented in the Appendixes of the EIA’s recurring reports: the Annual Energy Review (AER) and State Energy Data Report (SEDR), as well as Petroleum Supply Annual, Coal Industry Annual, and Natural Gas Annual. Using this approach to estimate emissions provides detailed information about trends in sources of emissions.

Information about consumption of “other petroleum” is derived from unpublished EIA data. In recent years, these products included crude oil, naphtha <401^oF, other oil >=401^oF, motor gasoline blending components, miscellaneous products, petroleum coke, pentanes plus, still gas, special naphthas, waxes, and unfinished oils.

Emissions Coefficients

The amount of carbon released when a fossil fuel is burned depends on the density, carbon content, and gross heat of combustion of the fuel [113]. Most of the coefficients for major fuels are assumed to be constant over time. However, for motor gasoline, liquefied petroleum gases (LPG), jet fuel, and crude oil, EIA developed annualized carbon emissions coefficients to reflect changes in chemical composition or product mix over the years. Appendix B contains a more detailed discussion of the methodology for developing the coefficients. Table B1 presents a full listing of all factors for crude oil, natural gas, and the complete slate of petroleum products.

Carbon Sequestration: Nonfuel Use of Fossil Fuels

After energy consumption was multiplied by the emissions coefficients, carbon sequestered through nonfuel use was then deducted from gross carbon emissions. Estimates of nonfuel use of fossil fuels were based on data provided in EIA’s Annual Energy Review 1995, Table 1.15 [114]. Table A1 lists nonfuel use of fossil fuels by type. Most nonfuel use occurs in the industrial sector. It accounted for about 5 quadrillion Btu of consumption in 1995 (Table A1).

Not all nonfuel use of fossil fuels results in carbon sequestration. For example, natural gas (predominantly methane, or CH₄) is used as a feedstock to make ammonia (NH₄). The carbon in the methane is reformed into carbon dioxide and emitted into the atmosphere. On the other hand, petrochemical feedstocks, such as ethane, are made into ethylene and then into polyethylene plastics and numerous other products. The carbon in these products ultimately is sequestered in landfills.

Ideally, nonfuel use of fossil fuels would be divided into its constituent applications, and each application would be studied to determine the ultimate fate of the carbon in the fossil fuel. Thus far it has not been possible to collect sufficient information to adopt such a detailed approach. Instead, the EIA uses the Intergovernmental Panel on Climate Change (IPCC) methods and information specific to U.S. industry to approximate how much carbon was sequestered by each product shown in Table A1 [115]. A “proportion of nonfuel use sequestered” was assumed for each product, usually based on IPCC recommendations but with EIA assumptions for those products for which no IPCC recommendation was available or for which more precise information could be obtained. These assumptions are shown in Table A2. The rationale for the assumptions is:

• Asphalt and Road Oil. All carbon in the 1.18 quadrillion Btu of asphalt and road oil was presumed to be sequestered.

• Liquefied Petroleum Gas. This use is derived from sales of LPG (i.e., ethane, propane, butane, isobutane, and pentanes plus) to the chemical industry. For this amount (approximately 1.82 quadrillion Btu), the EIA assumed that 80 percent of the carbon was sequestered [116].

• Petrochemical Feedstocks. Petrochemical feedstocks include naphtha and other oils used as petrochemical feedstock, still gas not used as refinery fuel, and synthetic gas. Together, feedstocks accounted for about 1.2 quadrillion Btu of industrial energy consumption in 1995. Since most of these fuels are used to make products like plastic and rubber, in which carbon will be sequestered for long periods of time, the EIA assumed that 80 percent of the carbon in the feedstocks is sequestered.

• Natural Gas. Nonfuel use of natural gas is based on estimates of natural gas feedstocks supplied to the chemical industry to manufacture ammonia (for fertilizer) and other chemicals. This use was about 0.55 quadrillion Btu in 1995. All the natural gas used to manufacture ammonia was assumed to be oxidized during the production process, while the carbon in other chemicals was assumed to be sequestered. Since the mix of these products can change from year to year, the effective sequestration rate also varies. In earlier EIA reports, this was assumed to be a constant 33 percent, based on the general recommendations of the IPCC, but actually it ranges from 41 to 55 percent in the United States, depending on changes in production levels of ammonia and other chemicals.

To recapitulate, the quantity of nonfuel use of fossil fuels shown in Table A1 was multiplied by the emissions coefficients in Table B1 and the proportion sequestered shown in Table A2 to determine the amount of carbon sequestered by nonfuel use. In 1995 nonfuel use of fossil fuels resulted in the sequestration of 2 million metric tons of carbon. The results for other recent years are shown in Chapter 2, Table 7.

Carbon Sequestration: Fraction Combusted

There is also a very small amount of carbon sequestration associated with the combustion of fossil fuels. Using IPCC assumptions, this report assumes that oxidation of liquid and solid fuels during combustion is 99 percent complete, and that 1 percent of the carbon remains sequestered. Oxidation of gaseous fuels (LPG and natural gas) is assumed to be 99.5 percent complete [117]. Conceptually, fuel may be “lost” before combustion due to evaporation, leaks, or spills; it may be subject to incomplete combustion and vented to the atmosphere in the form of volatile organic compounds or particulates; or it may remain at the site of combustion in the form of carbon-containing ash or soot.

Data Sources

Fossil Fuel Consumption: (1980-1993): Energy Information Administration, State Energy Data Report: Consumption Estimates, DOE/EIA-0214(93) (Washington, DC, July 1995). (1994 and 1995): Energy Information Administration, Monthly Energy Review, DOE/EIA-0035(96/07) (Washington, DC, July 1996), and Petroleum Supply Annual 1995, DOE/EIA-0340(95)/1 (Washington, DC, May 1996). Energy Information Administration, Natural Gas Annual 1994, DOE/EIA-0131(94) (Washington, DC, October 1995).

Non-Fuel Use of Energy and Biofuels Consumption (1980-1995): Energy Information Administration, Annual Energy Review 1995, DOE/EIA-384(95) (Washington, DC, July 1996), and Petroleum Supply Annual 1995, DOE/EIA-0340(95)/1 (Washington, DC, May 1996).

See also http://www.eia.doe.gov.

Adjustments to U.S. Energy Consumption

In recent years, there have been several estimates of U.S. carbon emissions, some of which differ by as much as 5 percent. Two significant reasons for the differences in emissions estimates (beyond those associated with differences in coefficients) are the definitions of “energy consumption” and “the United States” employed by researchers. Subtle differences in definition can produce variations of several percentage points in reported energy consumption, and hence in carbon emissions. Some estimates include U.S. territories while others exclude them. If consumption is estimated as “apparent consumption” based on production plus imports minus exports plus stock change, then statistical discrepancies will be included in consumption. International bunkers are sometimes counted as domestic consumption, and sometimes as exports. This section describes how each of these adjustments is accommodated in the EIA estimates.

U.S. Territories

EIA’s energy data for the United States cover only the 50 States and the District of Columbia. In contrast, energy data produced by the International Energy Agency for the United States cover the 50 States plus U.S. territories, including Puerto Rico, the U.S. Virgin Islands, and Guam, American Samoa, Micronesia, and Wake Island. Annual energy consumption in the U.S. territories is only about 0.5 quadrillion Btu (Table A3). Because all the U.S. territories are islands, their consumption consists primarily of petroleum products. For the territories as a group, oil consumption ranges between 200,000 and 250,000 barrels per day, and coal consumption averages about 300,000 short tons per year, mostly in Puerto Rico.

Estimation Method

Energy consumption for U.S. territories was converted to carbon emissions using the same emissions coefficients applied to U.S. energy data. Carbon emissions for U.S. territories ranged from 10 to 12 million metric tons per year (see Table 15 in Chapter 2). Because a large portion of reported energy consumption in U.S. territories was from “other petroleum,” there is a degree of uncertainty about the correct emissions factor to be used in this area, as well as the reliability of underlying data.

Data Sources

1980-1992: Energy Information Administration, International Energy Annual, DOE/EIA-0219 (Washington, DC, various years), and unpublished estimates for Wake Island, American Samoa, and the Pacific Trust Territories, which are included as “Other” in the Far East and Oceania region in the International Energy Annual. 1993-1995: EIA estimate.

International Bunker Fuels

Emissions Sources

The term “international bunker fuels” refers to fuel purchased by merchant ships in U.S. ports and by international air carriers in U.S airports. By convention, trade statistics treat sales of bunker fuels as exports by the selling country, because the purchaser promptly hauls the fuel outside national boundaries. This convention is followed by organizations that prepare international energy statistics, such as the United Nations and the International Energy Agency.

Bunker fuels, however, are an export without a corresponding import, because the purchasing ship generally burns the fuel on the high seas. EIA energy statistics, which are based on domestic sales of products, treat bunker fuels sales in the same way as sales of other fuels, i.e., as domestic energy consumption. Carbon emissions from bunker fuels are, therefore, already counted in the domestic energy consumption of the United States—primarily as transportation-related consumption of residual oil. Those who wish to understand the differences between emissions inventories based on international energy statistics and EIA data will, however, need to know the amount of energy consumption and the amount of carbon emissions associated with international bunkers. Table A3, therefore, shows U.S. international bunker fuel usage [118]. The amount is about 1.1 quadrillion Btu (or 500,000 barrels per day), largely of residual oil. It accounts for emissions of about 19 to 24 million metric tons of carbon annually (see Table 15 in Chapter 2).

Estimation Method

The appropriate carbon coefficient was applied to estimated annual consumption for several fuels (including residual and distillate fuels, as well as kerosene-type jet fuels) with the assumption that 99 percent of the fuel is combusted.

Data Sources

Distillate and Residual Fuels: (1980-1990): Energy Information Administration, International Energy Annual, DOE/EIA-0219 (Washington, DC, 1980-1990). (1991-1994): Energy Information Administration, Fuel Oil and Kerosene Sales, DOE/EIA-0535 (Washington, DC, 1991-1994). Jet Fuels (1980-1994): Oak Ridge National Laboratory, Transportation Energy Data Book (Oak Ridge, TN, various years).

Unmetered Natural Gas Consumption

Emissions Source

The “balancing item” in natural gas statistics produced by EIA represents the difference between reported supply and disposition of the gas. On an annual basis, the volume of natural gas distributed by suppliers has always been larger than that reportedly consumed. This discrepancy can be attributed to the effects of measurement errors, data reporting problems, pipeline leakage, and unreported consumption or stock changes.

Repairing leaks has become a priority in pipeline operations, due to safety and liability concerns. For this reason, only 1 percent of natural gas marketed can be attributed to pipeline leakage. Leaked gas enters the atmosphere in the form of methane. (Estimates of methane emissions from natural gas leakage can be found in Chapter 3 of this report.) While measurement errors and data reporting problems certainly exist in the natural gas industry, these errors ought not to be “tilted” in the direction of gas supply unless there is unreported consumption. The EIA believes that the amount of gas in the “balancing item” less the amount lost to leakage is more likely than not to reflect unreported consumption.

Estimation Method

Emissions from this source were estimated by first converting the volume of unmetered consumption into Btu, then multiplying by a carbon emissions coefficient. Annually, emissions from unreported natural gas consumption tend to fall in the range of 1 to 4 million metric tons (Table A4).

Data Sources

Gas Consumption and Balancing Item: (1980-1994): Energy Information Administration, Natural Gas Annual, DOE/EIA-0131 (Washington, DC, various years). (1995): Energy Information Administration, Natural Gas Monthly, DOE/EIA-0130(96/05) (Washington, DC, May 1996). Estimated Gas Leakage: Estimates presented in Chapter 3 and based on methodology for methane emissions in the oil and gas production industry, found later in this appendix.

Industrial Sources

U.S. energy production also generates small volumes of carbon dioxide emissions. The two principal sources of these emissions are flaring of natural gas and venting of the carbon dioxide that is produced in conjunction with natural gas [119]. When a field is developed for petroleum extraction, any natural gas associated with that field may be flared if its use is not economically justifiable. This is typically the case with a remote site or when the gas is of poor quality or minimal volume. During natural gas production, flaring may be used for disposal of waste products (e.g., hydrogen sulfide), capacity testing, or as a result of process upsets.

Estimation Method

The method for estimating emissions from natural gas flaring is based on the volume of vented and flared gas reported to the EIA by each State. This composite volume is scaled by a State-specific flaring percentage to ascertain the amount of natural gas flared in that State. The percent flared value is taken from a 1990 Department of Energy study that determined the relative split between venting and flaring for each State [120]. To calculate carbon emissions, the State figures are aggregated, converted into Btu, and then multiplied by the emissions coefficient equal to 14.92 million metric tons of carbon per quadrillion Btu.

As estimates presented in Chapter 2 indicate, natural gas flaring is a minor source of emissions, accounting for only about 1.5 to 2.0 million metric tons of carbon annually. There is some uncertainty associated with these estimates, given that operators in the field are not required to meter the amount of gas that is vented or flared. Further, methods used by States to determine vented and flared gas statistics are not uniform.

Data Sources

Venting and Flaring: (1980-1994): Energy Information Administration, Natural Gas Annual, DOE/EIA-0131 (Washington, DC, various years). (1995): Natural Gas Monthly, DOE/EIA-0130(95/06) (Washington, DC, May 1996).

Industrial Processes

In addition to energy-related emissions, carbon dioxide is also produced during certain industrial processes. The primary source of industrial emissions is limestone (CaCO₃) calcination to create lime (CaO). These two compounds are basic materials in a variety of manufacturing processes, particularly cement, iron and steel, and glass. Other sources of industrial emissions include the production and use of soda ash (Na₂CO₃), the manufacture of carbon dioxide, and aluminum production.

For this source category, emissions estimates are based on the compound used in the industrial process. Table A5 presents activity data for industrial processes. By multiplying the amount of production or consumption of the compound by a carbon coefficient (the relative amount of carbon in that compound), a process-specific estimate is derived. In recent years, industrial sources have accounted for about 17 to 18 million metric tons of carbon annually. Each industrial process, emissions source, and estimation method is discussed below.

Cement Manufacture

More than half of the carbon dioxide emissions from industrial sources originate from cement manufacturing (see Chapter 2).

Emissions Sources. Four basic materials are required to make cement: calcium, silicon, aluminum, and iron. Substrates of these materials are ground into a powder and heated in a kiln. While in the kiln, limestone (the predominant source of calcium) is broken down into carbon dioxide and lime. The carbon dioxide is driven off into the atmosphere. After the kilning process has been completed, cement clinker is left.

Estimation Method. One mole of calcined limestone produces one mole of carbon dioxide and one mole of lime. Since virtually all of the lime produced is absorbed into the clinker, the lime content of clinker is assumed to be representative of the amount of carbon dioxide that is emitted. In order to estimate emissions from cement manufacture, a carbon coefficient must be calculated. The EIA has adopted the IPCC recommendation that 64.6 percent of cement clinker is lime [121]. Multiplying this lime content factor by the ratio of carbon produced to lime produced yields the coefficient for cement clinker. A separate coefficient is necessary for estimating emissions from the additional lime used to produce masonry cement. In this case, the amount of lime not accounted for as clinker is assumed to be 3 percent [122]. This factor is then multiplied by the same production ratio of carbon to lime, generating the carbon coefficient for masonry cement.

Lime Manufacture

Lime is an important chemical with a variety of industrial, chemical, and environmental applications.

Emissions Sources. Lime production involves three main stages: stone preparation, calcination, and hydration. Carbon dioxide is generated during the calcination stage, when limestone is roasted at high temperatures, just as it is released during clinker production. The carbon dioxide is driven off as a gas and normally exits the system with the stack gas.

Estimation Method. Based on the ratio of the molecular weight of carbon dioxide to the weight of calcium carbonate, the EIA assumes that 785 metric tons of carbon dioxide, or 214 metric tons of carbon, are released for every 1,000 metric tons of lime produced. This factor is applied to annual levels of lime manufacture to estimate potential emissions. The EIA does not account for the relatively minor instances in which the carbon dioxide is recovered or reabsorbed, due to lack of data at the present time.

Limestone and Dolomite Consumption

These are basic raw materials used by a wide variety of industries, including the construction, agriculture, chemical, and metallurgical industries.

Emissions Sources. Limestone (including dolomite) can be used as a flux or purifier in metallurgical furnaces, as a sorbent in flue gas desulfurization (FGD) systems in utility and industrial plants, or as a raw material in glass manufacturing. Limestone is heated during these processes, generating carbon dioxide as a byproduct.

Estimation Method. Assuming that limestone has a carbon content of 12 percent and dolomite 13.2 percent, the EIA applies the appropriate factor to the annual level of consumption in the iron smelting, steelmaking, and glass manufacture industries, and in flue gas desulfurization systems that use this sorbent. This amounts to 120 metric tons of carbon for every 1,000 metric tons of limestone consumed, or 132 metric tons of carbon for every 1,000 tons of dolomite consumed (when dolomite is distinguished in the data).

Soda Ash Manufacture and Consumption

Commercial soda ash (sodium carbonate) is used in many familiar consumer products, such as glass, soap and detergents, paper, textiles, and food.

Emissions Sources. Two methods are used to manufacture natural soda ash in the United States. The majority of production comes from Wyoming, where soda ash is manufactured by calcination of trona ore in the form of naturally occurring sodium sesquicarbonate. For every mole of soda ash created in this reaction, one mole of carbon dioxide is also produced and vented to the atmosphere. The other process used to manufacture soda ash is carbonation of brines; however, the carbon dioxide driven off in this process is captured and reused.

Once manufactured, most soda ash is consumed in glass and chemical production. Other uses include water treatment, flue gas desulfurization, soap and detergent production, pulp and paper production, etc. As soda ash is processed for these purposes, additional carbon dioxide may be emitted if the carbon is oxidized. Because of the limited availability of specific information about such emissions, only certain uses of soda ash are considered in this report. Sodium silicate and sodium tripolyphosphate are included in this category as chemicals manufactured from soda ash and components of detergents.

Estimation Method. For soda ash manufacture, in order to ensure that carbon dioxide from the carbonation of brines is not included in emissions estimates, the calculations in this report are derived solely from trona ore production figures. Approximately 1.8 metric tons of trona ore are required to yield 1 metric ton of soda ash. This amounts to 97 million metric tons of carbon for every 1,000 tons of trona ore produced annually. For soda ash consumption, the EIA applies a factor of 113 metric tons for every 1,000 metric tons of soda ash consumed in glass manufacturing or in flue gas desulfurization.

Carbon Dioxide Manufacture

Emissions Source. Carbon dioxide is produced from a small number of natural wells and as a byproduct of chemical (i.e., ammonia) manufacturing. The Freedonia Group has determined that the United States exhibits an 80 to 20 percent split between carbon dioxide produced as a byproduct and carbon dioxide produced from wells [123]. Emissions of byproduct carbon dioxide are incorporated into the natural gas energy consumption estimates as nonfuel, nonsequestered carbon and therefore are not included here to avoid double-counting.

Most carbon dioxide produced from wells is injected back into the ground for enhanced oil recovery. This process sequesters the carbon dioxide, at least in the short run. Conceptually, only carbon dioxide produced from wells and diverted to industrial use is emitted to the atmosphere.

Estimation Method. The Freedonia Group estimates that nonsequestering industrial use of carbon dioxide resulted in emissions of 1.3 million metric tons of carbon in 1993 [124]. If 20 percent of this industrial use is supplied by wells, emissions can be estimated at 0.26 million metric tons of carbon. Based on the Freedonia report, the 1994 estimate is calculated assuming an annual 4.2 percent increase, implying emissions of 0.29 million metric tons of carbon.

Aluminum Manufacture

Aluminum is an element used in alloys. Because it is light in weight, malleable, and not readily corroded or tarnished, it is used as a principal material for kitchen utensils, aircraft, some automobiles, bicycles, and other manufactured products. The United States is a major producer of aluminum and also an importer, depending on market conditions.

Emissions Sources. Carbon dioxide is emitted during the aluminum production process when alumina (aluminum oxide) is reduced to aluminum. The aluminum oxide (Al₂O₃) is exposed to an anode of carbon, forming aluminum (Al) and carbon dioxide.

Estimation Method. Research indicates that 1.5 to 2.2 metric tons of carbon dioxide are emitted per metric ton of aluminum produced [125]. The EIA uses the midpoint of this range for estimating emissions.

Data Sources for Industrial Processes

Cement and Clinker Production: (1980-1994): U.S. Department of the Interior, Bureau of Mines, Cement Annual Report (Washington, DC, various years). (1995): U.S. Department of the Interior, U.S. Geological Service, Office of Minerals, Faxback Service.

Lime Manufacture: (1980-1994): U.S. Department of the Interior, Bureau of Mines, Mineral Commodity Summaries (Washington, DC, various years). (1995): U.S. Department of the Interior, U.S. Geological Service, Office of Minerals, Faxback Service.

Limestone Consumption: Iron Smelting, Steelmaking, and Glass Manufacture: (1980-1994): U.S. Department of the Interior, Bureau of Mines, Crushed Stone Report (Washington, DC, various years). (1995): EIA estimate. Flue Gas Desulfurization (1980-1995): Energy Information Administration, unpublished survey data, Form EIA-767, “Steam Electric Plant Operation and Design Report” (Washington, DC, various years).

Soda Ash: Manufacture and Consumption in Glass Making: (1980-1994): U.S. Department of the Interior, Bureau of Mines, Soda Ash Report (Washington, DC, various years). (1995): U.S. Department of the Interior, U.S. Geological Service, Office of Minerals, Faxback Service. Flue Gas Desulfurization (1980-1995): Energy Information Administration, unpublished survey data, Form EIA-767, “Steam Electric Plant Operation and Design Report” (Washington, DC, various years). Sodium Silicate and Sodium Tripolyphosphate: (1980-1994): Chemical Manufacturers Association, U.S. Chemical Industry Statistical Handbook 1994 (Washington, DC, September 1995). (1995): Chemical and Engineering News, “Growth of Top 50 Chemicals Slowed in 1995 from Very High 1994 Rate” (April 1996).

Carbon Dioxide: Freedonia Group, Inc., Carbon Dioxide, Business Research Report B286 (Cleveland, OH, November 1991), and Carbon Dioxide, Industry Study 564 (Cleveland, OH, February 1994).

Aluminum: (1980-1994): U.S. Department of the Interior, Bureau of Mines, Aluminum Report (Washington, DC, various years). (1995): U.S. Department of the Interior, U.S. Geological Service, Office of Minerals, Faxback Service.

Methane

Energy Sources

Oil and Gas Production, Processing, and Distribution

Emissions Sources

Natural gas may be released from the oil and gas system at several points, including oil wells, oil refineries, natural gas wellheads, gas processing plants, and gas transmission and distribution pipelines. Because methane is the principal constituent of natural gas (representing about 95 percent of the mixture) releases of natural gas lead to methane emissions.

Oil and Gas Production and Processing. As natural gas extracted at the wellhead is transferred to processing plants through gathering pipelines, leakage from valves, meters, and flanges occurs. Pneumatic valves, pressurized with natural gas, will emit gas when reset. Natural gas also escapes when gathering pipelines are emptied for maintenance. After the gas reaches the processing plant, additional emissions occur as a result of leakage, maintenance operations, and system upsets. System upsets result from sudden increases in pressure that require the release of gas as a safety measure or, failing that, result in a system rupture. Such events are uncommon in the U.S. oil and gas system and contribute only a minor amount to overall emissions.

Gas Transmission and Distribution. High-pressure transmission pipelines are used to transport natural gas from production fields and gas processing facilities to distribution pipelines. Gas pressure is lowered at gate stations before it enters the local distribution system. Natural gas may escape through leaky pipes and valves. It also may be released as part of compressor exhaust, while resetting pneumatic devices, and during routine maintenance.

Oil Refining and Transportation. Methane leaks from equipment when methane and oil are separated during the refining process. When oil is transferred to storage tanks at the refinery methane is emitted via vapor displacement. Methane not destroyed during flaring operations will also be vented to the atmosphere. Vapor displacement emissions also occur during loading and unloading of oil barges and tankers.

Gas Venting. When an oil reservoir is developed for extraction there will often be associated natural gas produced at the wellhead. If the flow of associated gas is too small or intermittent to be of value the gas will be vented or flared. Associated gas with an insufficient heat content to marketed may also be vented or flared. If a site lacks the necessary gathering and processing facilities for associated gas, that gas may be vented or flared. When gas is flared, its methane content is converted to carbon dioxide (see emissions estimates in Chapter 2 and methods above), but when it is vented methane is released directly to the atmosphere.

Estimation Methods

Oil and Gas Production and Processing. Estimates of emissions from oil and gas wells are scaled to the number of wells in operation, emissions from gathering pipelines are pegged to pipeline miles, and emissions from gas processing plants vary with gas throughput. Activity data for the oil and gas system are shown in Table A6. Estimates of methane emissions from these sources are scaled to point-in-time estimates appearing in a study prepared jointly by the U.S. Environmental Protection Agency and the Gas Research Institute (EPA/GRI) [126]. With a more representative sample, and more recent data, the EPA/GRI study improved substantially on previous estimates of emissions from the natural gas system. This report uses emissions factors developed on the basis of that study, replacing factors developed from earlier studies. A comparison of the new emissions factors and those used previously appears in Table A7.

Gas Transmission and Distribution. Methane emissions from transmission and distribution pipeline and gate stations were also estimated using emissions factors from the joint EPA/GRI study. These emissions estimates were scaled to pipeline mileage, with separate emissions factors for plastic and non-plastic pipeline (Table A7).

Oil Refining and Transportation. Estimates of emissions from this source were calculated using emissions factors from a 1992 Radian Corporation report [127] in conjunction with refinery data collected by the Energy Information Administration.

Gas Venting. The EIA collects State-level data on the volume of gas either vented or flared. The portion of venting versus flaring is not collected. However, a 1990 Department of Energy study estimated the share of gas vented and flared in each State [128]. These shares were applied to annual State venting and flaring data, and the results were aggregated to estimate national emissions.

Data Sources

Oil and Gas Production and Processing. Natural gas wellheads, gross gas withdrawals, and gas processing plant throughput can be found in EIA’s Natural Gas Annual, DOE/EIA-0031 (various years). Numbers of operating oil wells are available annually in the February issue of the World Oil journal.

Gas Transmission and Distribution. Transmission and distribution pipeline data are published annually by the American Gas Association in Gas Facts.

Oil Refining and Transportation. Data on volume of crude oil refined and volume of crude oil transported on marine vessels can be found in EIA’s Annual Energy Review 1995, DOE/EIA-0384(95) (Washington, DC, July 1996), and Petroleum Supply Annual, DOE/EIA-0340 (Washington, DC, various years).

Gas Venting. State data on gas venting and flaring can be found in EIA’s Natural Gas Annual 1995, DOE/EIA-0131(95) (Washington, DC, November 1995).

Coal Mining

Emissions Sources

As coal is formed from organic material by natural chemical and physical processes, methane is also produced. The methane is stored in the pores (open spaces) of the coal itself and in cracks and fractures within the coalbed. As coal is mined, the pressure surrounding the stored methane decreases, allowing much of it to be released into the operating coal mine (in the case of an underground mine) or into the atmosphere (in the case of a surface mine). The methane remaining in the coal pores is emitted as the coal is transported and pulverized for combustion. There are five avenues for methane emissions from coal mines:

Ventilation Systems in Underground Mines.

Degasification Systems in Underground Mines.

Surface Mines.

Post-Mining Emissions.

Methane Recovery for Energy.

Estimation Method

Ventilation Systems in Underground Mines. Emissions from this source were segregated into two classes: emissions from “gassy” mines, and emissions from “non-gassy” mines [130]. Because methane concentrations and airflow in gassy mines are carefully monitored by the Mine Safety and Health Administration (MSHA), a fairly reliable set of data can be derived for emissions from ventilation systems in gassy mines. However, because MSHA data are voluminous and in inconsistent format, it is difficult to compile and available for only a subsample of years: 1980, 1985, 1988, 1990, and 1993. Thus, the available data are used in conjunction with coal production data for those years to develop emissions factors per ton of coal mined on a basin-by-basin level. Emissions factors for non-sample years are interpolated or extrapolated. The resulting emissions factors are then multiplied by production data to estimate emissions from this source (for detailed production data, see Table A8).

Emissions from non-gassy mines make up less than 2 percent of all emissions from underground mines [131]. Basin-level emissions factors for non-gassy mines were established by dividing 2 percent of each basin’s estimated emissions from non-gassy mines for 1988 by that year’s production levels. The resulting emissions factors are applied to annual production data.

Degasification Systems in Underground Mines. Degasification emissions are not monitored by any regulatory agency. Where degasification does occur, the method of disposition (e.g., venting, flaring, sale for energy) may not be tabulated. Emissions from degasification systems are estimated by multiplying annual production in mines known to have a degasification system in place by a per-ton emissions factor.

Surface Mines. Emissions from U.S. surface mines have not been systematically measured. However, studies on surface coal mines in the United States, England, France, and Canada suggest a range of 0.3 to 2.0 cubic meters per metric ton of coal mined [132]. This report adopts the central value of that range and multiplies it by U.S. surface coal production.

Post-Mining Emissions. Like emissions from surface mines, post-mining emissions are not measured systematically. Thus, global average emissions factors must be relied upon. Post-mining emissions for coal mined from the surface are estimated to be very low, between 0.0 and 0.2 cubic meters per metric ton of coal mined. In contrast, post-mining emissions from underground coal are estimated to be significant, between 0.9 and 4.0 cubic meters of methane per metric ton of coal mined [133]. The central values of these ranges are adopted and multiplied by annual production data for this report.

Methane Recovery for Energy. Methane recovery for energy is restricted to a small sample of mines that typically meter their gas sales. Thus, total methane recovery can be estimated from the volume and heat content of these sales.

Data Sources

Ventilation Systems in Underground Mines. Coal mine ventilation data for the approximately 200 gassiest U.S. mines was drawn from a database prepared by the Department of Interior’s Bureau of Mines for the years 1980, 1985, 1988, 1990, and 1993. Coal production data are reported to the Energy Information Administration on Form EIA-7A, “Coal Production Report.” Basin-level emissions for non-gassy mines in 1988 were calculated by the U.S. Environmental Protection Agency. See U.S. Environmental Protection Agency, Office of Air and Radiation, Anthropogenic Methane Emissions in the United States: Estimates for 1990 (Washington, DC, April 1993), pp. 3-19 - 3-24.

Degasification Systems in Underground Mines. Emissions factors for this source are derived from estimates of 1988 emissions from degasification systems prepared by the U.S. Environmental Protection Agency. See U.S. Environmental Protection Agency, Office of Air and Radiation, Anthropogenic Methane Emissions in the United States: Estimates for 1990 (Washington, DC, April 1993), pp. 3-19 - 3-24. Annual production figures are reported to the Energy Information Administration on Form EIA-7A, “Coal Production Report.”

Surface Mines. Emissions factors for surface mines are found in the Intergovernmental Panel on Climate Change (IPCC), Greenhouse Gas Inventory Reference Manual. Coal production data are reported to the Energy Information Administration on Form EIA-7A, “Coal Production Report.”

Post-Mining Emissions. Emissions factors for post-mining emissions are found in the Intergovernmental Panel on Climate Change, Greenhouse Gas Inventory Reference Manual. Coal production data are reported to the Energy Information Administration on Form EIA-7A, “Coal Production Report.”

Methane Recovery for Energy. Volumes of methane recovered were obtained from a report prepared by the U.S. Environmental Protection Agency. See U.S. Environmental Protection Agency, Office of Air and Radiation, Identifying Opportunities for Methane Recovery at U.S. Coal Mines: Draft Profiles of Selected Gassy Underground Coal Mines (Washington, DC, September 1994), p. 6-6.

Stationary Combustion

Emissions Sources

The principal products of fuel combustion are carbon dioxide and water vapor. When fuel combustion is incomplete, methane may also be released. The volume of methane released varies according to the efficiency and temperature of the combustion process. Most stationary sources are large, comparatively efficient boilers, such as those found in the industrial and utility sector, and thus have low levels of methane emissions. However, a significant amount of wood is consumed in residential woodstoves and fireplaces, which are typically inefficient combustion chambers. Wood combustion in these devices produces most methane emissions from stationary sources.

Estimation Method

An emissions factor based on fuel type (e.g., coal, wood, natural gas) and combustion technology (e.g., utility boiler, industrial boiler, woodstove) is applied to consumption data for each fuel and technology type.

Data Sources

Emissions coefficients for stationary fuel were obtained from the U.S. Environmental Protection Agency, Office of Air Quality Planning and Standards, Compilation of Air Pollutant Emission Factors, AP-42, Supplement D (Research Triangle Park, NC, September 1991), and the Intergovernmental Panel on Climate Change, Greenhouse Gas Inventory Reference Manual, IPCC Guidelines for National Greenhouse Gas Inventories, Vol. 3 (Paris, France, 1994). Fuel consumption data were drawn from EIA’s State Energy Data Report 1993: Consumption Estimates, DOE/EIA-0214(93) (Washington, DC, May 1995), for 1980-1993; and Monthly Energy Review, DOE/EIA-0035(96/07) (Washington, DC, July 1996). Residential wood fuel consumption data were derived from EIA’s Renewable Energy Annual 1995, DOE/EIA-0603(95) (Washington, DC, December 1995), p. 18.

Mobile Combustion

Emissions Sources

Methane emissions from mobile combustion are, like those from stationary combustion, the result of incomplete fuel combustion. In automobiles, methane emissions result when oxygen levels in the combustion chamber drop below levels sufficient for complete combustion. The effects of incomplete combustion may be moderated somewhat by post-combustion emissions controls, such as catalytic converters. Methane emissions are also generated by fuel combustion in other modes of transport, including aircraft, ships, and locomotives. There is, however, some evidence that jet airplane engines may consume ambient methane during flight, reducing their net emissions [134].

Estimation Methods

Methane emissions from highway vehicles such as automobiles, light-duty trucks, motorcycles, buses, and heavy-duty trucks are estimated by applying a per vehicle mile traveled emissions factor to vehicle use data. Because of improving technology and more stringent environmental regulations, these emissions factors vary by vehicle type and decline over time. For non-highway sources, emissions coefficients in terms of volume of fuel consumed are applied directly to consumption data without year-to-year modifications.

Data Sources

Emissions factors for all vehicles are provided by the Intergovernmental Panel on Climate Change, Greenhouse Gas Inventory Reference Manual, IPCC Guidelines for National Greenhouse Gas Inventories, Vol. 3 (Paris, France, 1994), pp. 1.64-1.68.

The EIA collects data on miles traveled in personal transportation vehicles (cars and light-duty trucks) as part of its tri-annual Residential Transportation Energy Consumption Survey (RTECS): Energy Information Administration, Household Vehicles Energy Consumption 1994, DOE/EIA-0464(94) (Washington, DC, July 1996, and previous years). This survey contains data for the years 1983, 1985, 1988, 1991, and 1994. Emissions for intervening years were estimated by interpolating between the weighted average estimates of survey years. Vehicle miles traveled for non-household vehicles (fleets, rental cars, etc.), motorcycles, buses, and heavy-duty trucks were obtained from the U.S. Department of Transportation, Federal Highway Administration, Highway Statistics 1994 (Washington, DC, 1995).

Data on fuel consumption for ships, locomotives, farm equipment, and construction equipment is available in EIA’s Fuel Oil and Kerosene Sales 1994, DOE/EIA-0535(94) (Washington, DC, September 1995). Fuel consumption data for jet and piston-powered aircraft are contained in EIA’s Petroleum Supply Annual 1995, DOE/EIA-0340(95)/1 (Washington, DC, May 1996). Data on fuel consumption by recreational boats are taken from S.C. Davis and S.G. Strang, Transportation Energy Data Book, Edition 15, ORNL-6856 (Oak Ridge, TN: Oak Ridge National Laboratory, Center for Transportation Analysis, May 1995).

Landfill Methane Emissions

Emissions Sources

After organic wastes (e.g., food, paper, yard waste) are placed in landfills they begin to decompose. Aerobic bacteria, consuming oxygen, convert organic material to carbon dioxide, heat, and water. When available oxygen is depleted, anaerobic bacteria, including methanogens, begin digesting the waste and producing methane. Methanogenic anaerobes are highly sensitive to temperature, pH, and moisture levels. Because U.S. sanitary landfills are essentially closed systems designed to minimize entry and exit of moisture, conditions within a landfill are largely a product of the composition of the waste it contains. Thus, methane is likely to be produced at different rates and volumes both across different landfills and within a single landfill.

The biogas produced in a landfill is typically between 35 and 50 percent methane. At these levels, methane is highly explosive. Often, landfill operators will put a methane control system in place to prevent migration of high concentrations to buildings. Methane captured by control systems may be vented to the atmosphere or flared. However, captured methane is a potentially valuable energy resource. Where landfills produce steady, large volumes of methane and landfill gas-to-energy prices are competitive with other energy alternatives, recovered gas may be used as an energy resource. In most cases, the gas is converted to electricity and used for on-site energy needs or sold to local utilities. In some cases, the gas is transported via pipeline to a local end-user.

Estimation Methods

Data on methane emissions from landfills are limited to those landfills with methane recovery systems in place. For more than 100 U.S. landfills with gas recovery systems in place, Thorneloe et al. have measured or estimated methane emissions during 1992. Methane emissions from these landfills were estimated at 2.1 million metric tons for 1992 [135]. Methane emissions from landfills without gas recovery systems have not been measured, and even their number is subject to considerable uncertainty. Emissions from a given landfill are largely the product of the composition of the waste it contains and an array of site-specific factors. Waste composition data on a landfill-specific basis are nonexistent. However, national-level waste flow and waste composition data are available, and their reliability has improved over time. Thus, for this report, all waste not disposed of in a landfill with measured emissions is treated as if it has flowed to one, very large, national landfill.

To estimate methane emissions from all waste not disposed of in a landfill with measured emissions, waste volumes are subjected to a slightly modified version of the EMCON Methane Generation Model [136]. This model divides the waste into three categories: readily decomposable, moderately decomposable, and slowly decomposable, each with its own set of emissions characteristics. The EMCON model provides both a high methane yield scenario and a low methane yield scenario. For each category of decomposable waste, a time lag until methane generation begins is estimated, as well as a time constant during which the methane yield of the waste is realized. The methane yield represents the total amount of methane that a given amount of waste will produce over its lifetime. For example, under a low methane yield scenario, slowly decomposing waste will begin producing methane after a 5-year lag and will continue emitting over a 40-year period. Table A9 shows the EMCON methane generation model parameters.

Waste flows were estimated from 1940 through 1995. Waste in place in the Nation’s landfills was assumed to represent the waste stream for all previous years plus the current year’s additions. Those landfills examined in the Thorneloe et al. study contained 9.4 percent of the waste estimated to be in place in the Nation’s landfills during 1992. This report assumes that the share of waste in these landfills and the share in all other landfills remained constant over time. Thus, the EMCON model was applied to 90.6 of the waste generated each year.

To estimate emissions from those landfills with measured data for 1992 but no data for other years, the EMCON model was recalibrated to produce the 2.1 million metric tons of measured emissions in 1992. The recalibrated model, with methane yields almost twice as large as the original, was then applied to 9.4 percent of the waste stream for all years. These much higher yields are not unexpected, as gas recovery systems are most economically employed in high-emitting landfills.

Data Sources

Franklin Associates provides estimates of municipal solid waste landfilled beginning in 1960: Franklin Associates, Ltd., Characterization of Municipal Solid Waste in the United States (annual updates prepared for the U.S. Environmental Protection Agency, Office of Solid Waste and Emergency Response). Biocycle magazine provides estimates of waste generated, including construction and demolition waste and sludge for 1987-1994, in its “Nationwide Survey: The State of Garbage in America” (various years). Because Biocycle data include several categories of waste that are excluded from the Franklin Associates data, it systematically shows larger volumes of waste generated and landfilled. On average these numbers are 1.43 times those in the Franklin data. To develop a consistent data series back to 1960, the Franklin data were multiplied by 1.43 for all years (Table A10). To further extend waste generation estimates back to 1940, a regression equation relating waste generation to GNP and population was developed, and 1940-1959 waste streams were backcast.

Methane recovery data were estimated based on the measured recovery data provided by S.A. Thorneloe, M.R.J. Doorn, L.A. Stefanski, M.A. Barlaz, R.L. Peer, and D.L. Epperson, “Estimate of Methane Emissions from U.S. Landfills,” prepared for U.S. Environmental Protection Agency, Office of Research and Development (April 1994). While there were 105 known landfills with methane recovery systems in place in 1992, 130 landfills were identified as having recovery systems in place by 1994. Electricity generating capacity from these landfills was reported at 300 megawatts in 1992 and 360 megawatts in 1994 (J. Pacey, S.A. Thorneloe, and M.Doorn, “Methane Recovery from Landfills and an Overview of EPA’s Research Program for Landfill Gas Utilization,” presented at the 1995 Greenhouse Gas Emission and Mitigation Research Symposium, U.S. Environmental Protection Agency, Washington, DC, June 27-29, 1995). This report assumed that methane recovery and electricity capacity maintained a constant ratio and interpolated growth in intervening years lacking data points.

Domestic and Commercial Wastewater Treatment

Emissions Sources

Emissions of methane from the treatment of wastewater occurs when liquid waste streams containing high concentrations of organic materials are treated anaerobically (in the absence of oxygen). Anaerobic processes used in the United States are anaerobic digestion, anaerobic, and facultative (combining aerobic and anaerobic processes) stabilization lagoons, septic tanks, and cesspools [137]. Treatment of wastewater solids using anaerobic digestion is the most obvious potential source of methane emissions. However, emission of significant quantities of methane from this process depends on the digester gas being vented rather than recovered or flared. Anaerobic and facultative lagoons involve retention of wastewater in impoundments where the organic materials in the wastewater undergo bacterial decomposition. The growth of algae, which absorb carbon dioxide and release oxygen as a result of photosynthesis, sustains aerobic conditions at least near the surface of the lagoon. However, the bacteria deplete oxygen at the bottom of the lagoon, producing conditions suitable for methanogenic bacteria. The extent of the resulting anaerobic zone and the associated methane generation depend on such factors as organic loadings and lagoon depth. In facultative lagoons, unlike anaerobic lagoons, a significant aerobic zone persists.

Nearly 75 percent of U.S. households are served by sewers that deliver domestic wastewater to central treatment plants. Septic tanks or cesspools treat domestic wastewater from most of the remaining households (24 percent) [138]. Anaerobic digestion is frequently used to treat sludge solids at U.S. municipal wastewater treatment plants. However, anecdotal evidence suggests that neither recovery nor flaring of digester gas is common in the United States, and equipment for recovery and flaring of digester gas is poorly designed or maintained, allowing most of the methane produced to be released to the atmosphere [139].

Estimation Method

Insufficient information is available to develop separate estimates of methane emissions from each of the sources discussed above. Information on the type of treatment used by the thousands of municipal and industrial treatment facilities is simply not available. For instance, no reliable statistics were found for the use of anaerobic digestion at municipal treatment facilities. Knowledge regarding the emissions of methane from lagoons, septic systems, and cesspools is limited. Another difficulty is the overlap between the municipal and industrial treatment systems. Many industrial concerns discharge wastewater, which may or may not have been treated, into municipal systems. Therefore, it is necessary to base the current estimate of methane emissions from wastewater treatment on the highly simplified approach recommended by the Intergovernmental Panel on Climate Change (IPCC) [140]:

The IPCC’s simplified approach [141] assumes that each person in a developed nation contributes 0.5 kg of BOD₅ to domestic wastewater, and 15 percent of this wastewater is treated anaerobically, yielding 0.22 kg of methane per kg of BOD₅ in the wastewater [142]. It was assumed that recovery of methane at municipal wastewater treatment facilities is negligible.

Data Source

U.S. Census Bureau, estimate of resident population on July 1 of each year.

Agricultural Sources

Enteric Fermentation in Domesticated Animals

Emissions Sources

The breakdown of carbohydrates in the digestive track of herbivores (including insects and humans) results in the production of methane [143]. The volume of methane produced from this process (enteric fermentation) is largest in those animals that possess a rumen, or forestomach, such as cattle, sheep, and goats. The forestomach allows these animals to digest large quantities of cellulose found in plant material. This digestion is accomplished by microorganisms in the rumen, some of which are methanogenic bacteria. These bacteria produce methane while removing hydrogen from the rumen. The majority (about 90 percent) of the methane produced by the methanogenic bacteria is released through normal animal respiration and eructation. The remainder is released as flatus.

Estimation Method

The level of methane emissions from enteric fermentation in domesticated animals is a function of several variables, including quantity and quality of feed intake, the growth rate of the animal, its productivity (reproduction and/or lactation), and its mobility. To estimate emissions from enteric fermentation, the animals are divided into distinct, relatively homogeneous groups. For a representative animal in each group, feed intake, growth rate, activity levels, and productivity are estimated. An emissions factor per animal is developed based on these variables. The factor is then multiplied by population data for that animal group to calculate an overall emissions estimate. The method for developing these factors differs somewhat for cattle as opposed to all other animals.

Cattle. Because emissions from cattle account for about 95 percent of U.S. emissions from enteric fermentation, they are given particular scrutiny. The U.S. cattle population is separated into dairy and beef cattle. Dairy cattle are then divided into replacement heifers 0-12 months old, replacement heifers 12-24 months old, and mature cows. Beef cattle are divided into six classes: replacements 0-12 months old, replacements 12-24 months old, mature cows, bulls, steers and heifers raised for slaughter under the weanling system, and steers and heifers raised for slaughter under the yearling system. These populations are then multiplied by emissions factors developed for each category of cattle within the U.S. population as it was composed in 1990 [144]. Because characteristics critical in determining energy intake and thus emissions rates for cattle (such as growth rates and milk production) change annually, an effort to scale emissions factors to these changes is made. Emissions rates were pegged to average slaughter weights for the calves and adult cattle respectively (Table A11) [145].

Other Animals. For sheep, pigs, goats, and horses, populations are not disaggregated below the species level. Emissions factors for each animal group are multiplied by that group’s population. Emissions factors are drawn from the work of Crutzen et al. [146].

Data Sources

Population and slaughter weight data for cattle, and population data for sheep and swine are provided by the U.S. Department of Agriculture (USDA), National Agricultural Statistics Service, Livestock, Dairy, and Poultry Branch [147]. Population data for goats and horses are extrapolated using the information in U.S. Department of Commerce, Economics and Statistics Administration, Bureau of the Census, Census of Agriculture, United States Summary and State Data, Vol. 1, ”Geographic Area Series,” Part 51 (Washington, DC, 1982, 1987, and 1992).

Solid Waste of Domesticated Animals

When the solid waste of animals is allowed to decompose under anaerobic conditions, methane is produced. The volume of methane produced varies according to the amount of organic material susceptible to decomposition within the waste (volatile solids) and the manner in which the waste is handled. Liquid-based waste management systems, in addition to providing a suitable anaerobic environment, provide the moisture necessary for methanogenic bacterial cell production and acid stabilization [148]. Thus, they result in substantially higher methane emissions than dry management systems.

Estimation Method

Methane emissions from the solid waste of domesticated animals are estimated by linking emissions to the volume of solid waste produced by a given animal, the volatile solids in that waste, and the manner in which the waste is handled. The volume of waste produced is controlled by the animal’s size, diet, and energy requirements. As a proxy for these variables, typical animal mass as estimated in a 1990 inventory of livestock and poultry prepared by the U.S. Environmental Protection Agency [149] is used to determine waste production per animal. These animal sizes are adopted directly for all animals except cattle, whose masses are adjusted annually based on live slaughter weights as reported by the U.S. Department of Agriculture. Volatile solids produced per kilogram of animal weight and the maximum methane-producing capacity of each animal’s waste are adopted from the work of Safley et al. [150]. For all animals except dairy cattle, the share of waste handled in each management system is also drawn from Safley et al.

Because the methods for handling the waste of dairy cattle in six States (Arizona, Florida, Nevada, North Carolina, North Dakota, and Texas) have changed since 1990 when estimates were prepared by Safley et al., the estimation method for dairy cattle differs slightly from other animals. The national average distribution of waste management techniques was applied to all dairy cattle except those in the six States listed. Individual annualized distributions were used for each of the six States. The individual States’ waste management system shares were obtained from the EPA report, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994 [151]. After calculating emissions from dairy cattle in each of the six States, the States were totaled and added to the emissions estimate for the dairy cattle in the rest of the United States.

Data Sources

Population and slaughter weight data for cattle and population data for sheep, poultry, and swine were provided by the U.S. Department of Agriculture (USDA), Agricultural Statistics Board, in Livestock Slaughter Estimates: Annual Summary (Washington, DC, various years), and were obtained via the internet at www.mannlib.cornell.edu. Average broiler chicken populations for each year were estimated by multiplying the estimated number of broiler chickens slaughtered annually by 0.1425, based on their 7-week life cycle, as recommended by the USDA’s Economic Research Service (personal communication, May 1993). Population data for goats and horses were extrapolated using information from the U.S. Department of Commerce, Economics and Statistics Administration, Bureau of the Census, Census of Agriculture, United States Summary and State Data, Vol. 1, ”Geographic Area Series,” Part 51 (Washington, DC, 1982, 1987, and 1992).

Rice Cultivation

Emissions Sources

As organic material decays under anaerobic conditions in flooded rice fields, methane is produced. Between 60 and 90 percent of the methane generated is oxidized by other bacteria in the soil, and an additional portion leaches into groundwater. The majority of the methane that remains moves through rice plants by diffusive transport to reach the atmosphere. A smaller amount of methane reaches the atmosphere by bubbling from the soil and by diffusion through the water column.

Estimation Method

A daily emissions rate range was developed using studies of rice fields in California [152], Louisiana [153], and Texas [154]. The high and low ends of the range, 0.1065 to 0.5639 grams of methane per square meter of land cultivated, were applied to the growing season length and the harvested area for each State that produces rice. In States with a second or ”ratoon” crop, the additional area harvested was incorporated into estimates.

Data Sources

Rice area harvested and length of growing season data were obtained from the U.S. Department of Agriculture, National Agricultural Statistics Service, Crop Production (annual reports).

Burning of Crop Residues

Emissions Sources

Crop residues contain substantial shares of carbon, between 40 and 50 percent of dry matter [155]. When crop residues are burned for fodder, land supplementation, and fuel, incomplete combustion results in methane emissions [156].

Estimation Method

In keeping with the methods recommended by the Intergovernmental Panel on Climate Change, [157] this report assumed that 10 percent of all crop residues are burned annually. The dry weight and carbon content of each crop was then determined and used in conjunction with estimated combustion efficiencies to derive the volume of methane emissions.

Data Sources

Sizes of crops harvested were obtained from the U.S. Department of Agriculture, National Agricultural Statistics Service, Crop Production (annual reports). Factors used to estimate emissions were derived using the sources outlined in Table A12.

Industrial Processes

Chemical Production

Emissions Sources

A wide variety of organic compounds (those containing carbon) are used as feedstocks in chemical production. High temperatures are often used to ”crack” the molecular bonds of the compound, with differing temperatures producing specific chemicals. The process of cracking produces a number of chemical byproducts, including methane.

Estimation Method

The Intergovernmental Panel on Climate Change has published emissions factors for methane emitted during the manufacture of ethylene, ethylene dichloride, styrene, methanol, and carbon black [158] (Table A13). Production figures for the chemicals were multiplied by those emissions factors.

Data Sources

Chemical production figures are provided by the Chemical Manufacturers Association in U.S. Chemical Industry Statistical Handbook (Washington, DC, various years).

Iron and Steel Production

Emissions Sources

Coke, sinter, and pig iron are the principal material inputs for the production of iron and steel. Coke is produced by heating coal in the absence of oxygen. One of the gaseous byproducts of this process is methane. During the next step in the production process, coke, iron ore, and flux materials are combined to form sinter. The coke is burned to create heat, causing the sinter to agglomerate. During agglomeration methane is released. Coke and iron are then added to flux materials in a blast furnace and reduced into iron, slag, and exhaust gases. Methane is one of the exhaust gases.

Estimation Method

The Intergovernmental Panel on Climate Change has published emissions factors for methane emitted during the production of coke, sinter, and pig iron [159]. Production figures for iron and steel inputs were multiplied by those emissions factors.

Data Sources

Coke, sinter, and pig iron production data are published annually by the American Iron and Steel Institute in its Annual Statistical Report (Washington, DC, various years).

Nitrous Oxide

Most anthropogenic nitrous oxide emissions in the United States can be attributed to agricultural and energy-related sources. In particular, fertilizer use (which amplifies the natural flux of nitrous oxide from soil) and vehicular fuel combustion combine to account for approximately 70 percent of estimated emissions (although the range of uncertainty associated with emissions from fertilizer use is quite large). Emissions estimates in this report include fertilizer application; burning of crop residues; mobile source combustion from passenger cars, buses, motorcycles, trucks, and other minor sources; stationary source combustion from residential, industrial, and electric utility energy use; and industrial production of adipic acid and nitric acid.

Energy Use

Mobile Combustion

Emissions Sources

Nitrous oxide emissions are produced as a byproduct of fuel combustion. During combustion, nitrous oxide is produced as a result of chemical interactions between nitric oxide and other combustion products. With most conventional combustion systems, high temperatures limit the quantity of nitrous oxide that escapes; therefore, emissions from these systems are typically low. Mobile sources of fuel combustion include passenger cars, buses, motorcycles, light-duty and other trucks, air, rail, and water transportation sources, and farm and construction equipment.

Estimation Method

See section on methane emissions from mobile combustion, above.

Stationary Combustion

Emissions Sources

As with mobile sources, nitrous oxide emissions are produced as a byproduct of fuel combustion. The three fuels of primary importance burned by stationary sources are coal, fuel oil, and natural gas. Combustion systems powered by coal produce the most nitrous oxide, approximately 76 percent of annual emissions. As a sector, electric utilities consistently account for more than one-half of total emissions. Other important sources are commercial facilities, industrial facilities, and residences.

Estimation Method

Nitrous oxide emissions from stationary combustion are estimated by multiplying fuel consumption figures for each fuel type and stationary source by emissions factors for each type of fuel. The emissions factors used in this report differ from those used in previous years; therefore, emissions estimates may also be different from those presented in last year’s report. Emissions were estimated by applying emissions factors for coal, oil, and natural gas to EIA’s consumption data for each of those fuels in the commercial, residential, industrial, and electric utility sectors.

Data Sources

Fuel consumption data are from the Energy Information Administration, State Energy Data Report 1993, DOE/EIA-0214(93); and Monthly Energy Review, DOE/EIA-0035(96/03) (Washington, DC, March 1996).

The emissions factors used in this report are those recommended by the IPCC as derived from studies of numerous conventional systems: G.G. De Soete, “Nitrous Oxide from Combustion and Industry: Chemistry, Emissions and Control,” in A.R. van Amstel (ed.), International IPCC Workshop Proceedings: Methane and Nitrous Oxide, Methods in National Emissions Inventories and Options for Control (Bilthoven, Netherlands: RIVM, 1993), pp. 287-337.

Agriculture

Nitrous oxide uptake and emissions occur naturally as a result of nitrification and denitrification processes in soil. When nitrogen-based fertilizers are added to the soil, emissions generally increase, unless application precisely matches plant uptake and soil capture [160]. A variety of other factors, including certain soil properties and moisture content, are known to influence the rate of emissions. Although these factors have been identified, they have not been systematically quantified, and we are not aware of data that would allow them to be incorporated into emissions estimates.

Estimation Method

Emissions factors ranging in order of magnitude from 0.001 to 0.1 grams of nitrogen (in nitrous oxide) per gram of nitrogen in fertilizer were applied to the nitrogen content of fertilizer consumed annually in the United States, producing low, median, and high estimates. In 1995, the median estimate (which assumes that 1 percent of the nitrogen in fertilizer is emitted as nitrous oxide—also the percentage recommended by the IPCC [161]) indicates that 167,000 metric tons of nitrous oxide was released into the atmosphere as a result of fertilization practices.

Data Sources

Estimates of total U.S. fertilizer consumption were obtained from reports from the Tennessee Valley Authority Fertilizer Research Center for various years through 1994: J.T. Berry et al., Commercial Fertilizers (Muscle Shoals, AL: Tennessee Valley Authority, Fertilizer Research Center, Reports for 1986-1991, 1993-1994).

Estimates for prior years have been modified from those in last year’s report to represent the nitrogen content of annual fertilizer consumption for the calendar year. The emissions factor for nitrous oxide is taken from A.R. Mosier, “Nitrous Oxide Emissions from Agricultural Soils,” in A.R. van Amstel (ed.), International IPCC Workshop Proceedings: Methane and Nitrous Oxide, Methods in National Emissions Inventories and Options for Control (Bilthoven, Netherlands: RIVM, 1993), p. 281.

Crop Residue Burning

Emissions Sources

Crop residue is commonly disposed of by incorporation into the soil, spreading over the soil surface to prevent erosion, as animal bedding, or through burning. Burning crop residue releases nitrous oxide into the atmosphere. The burning of crop residue occurs throughout the United States, although it is illegal in certain areas. There are no accurate estimates of the amount of crop residue burned in the United States.

Estimation Method and Data Sources

See section on methane emissions from burning crop residues, above.

Industrial Processes

Emissions of nitrous oxide from production of adipic acid are calculated by multiplying adipic acid production figures by nitrous oxide emissions coefficients. For every metric ton of adipic acid produced, 0.3 metric ton of nitrous oxide is created [162]. Currently, two plants (accounting for approximately 77 percent of total production) control emissions by thermally decomposing the nitrous oxide, and 98 percent of the potential emissions from those plants are eliminated by this technique [163].

Data Sources

Adipic acid production figures are from Chemical and Engineering News, annual report on the “Top 50 Industrial Chemicals” (April issue, various years).

The adipic acid emissions coefficient is from M. Thiemens and W. Trogler, “Nylon Production: An Unknown Source of Atmospheric Nitrous Oxide,” Science, Vol. 251 (February 22, 1991), p. 932.

Nitric Acid Production

Measurements at a DuPont plant indicate emissions factors of 2 to 9 grams of nitrous oxide per kilogram of nitric acid manufactured [164]. The emissions estimates presented in this report were calculated by multiplying the annual quantity of nitric acid produced by the midpoint (5.5 grams nitrous oxide per kilogram of product) of the emissions range determined at the DuPont plant. There is, however, a considerable degree of uncertainty associated with this estimate, because the emissions factor for the DuPont plant may not in fact be generalizable across the industry.

Data Sources

Nitric acid production figures are from the Chemical Manufacturers Association, Chemical Industry Statistical Handbook (Washington, DC, 1995). The nitric acid emissions coefficient is from the Intergovernmental Panel on Climate Change, Greenhouse Gas Inventory Reference Manual, IPCC Guidelines for National Greenhouse Gas Inventories, Vol. 3 (Paris, France, 1995), p. 2.9.

Halocarbons and Other Gases

Emissions Sources

Halocarbons and other gases have hundreds of uses, but the bulk of emissions come from a few broad categories of use:

• As a refrigerant or working fluid in air conditioning and refrigeration equipment

• As a solvent in various industrial processes

• As a blowing agent for making insulating foams

• As a fugitive emission from various industrial processes, including the manufacture of halocarbons.

The emissions profile differs for each emissions source. Refrigerants are used in a closed cycle inside cooling equipment, and they tend to leak out when the equipment is scrapped or serviced. Some portion of the refrigerants is captured and recycled or destroyed, rather than emitted, when equipment is serviced. Halocarbons in solvent applications are often recycled, but net consumption (after recycling) is probably a good indicator of emissions. Halocarbons used as blowing agents can be characterized by the type of foam manufactured: halocarbons used to make “open cell” foam are released to the atmosphere immediately, while halocarbons used to make “closed cell” foam are trapped within the foam for the life of the foam, which can vary (depending on the use) from a few weeks to many years.

Estimation Method

In general, the EIA has relied on estimates of halocarbon emissions published by the EPA. However, the EPA has not prepared estimates for all years and all gases. The EIA has therefore extended EPA emissions estimates using various methods.

In general, estimating emissions of halocarbons begins with determining the level of annual consumption of the halocarbon and distributing consumption across the principal end-use applications. Emissions from each end-use application are then computed, based on the assumed emissions characteristics of the application. Alternatively, industrial emissions from large corporations can be determined directly for some chemicals by reference to companies’ reporting under the EPA’s Toxics Release Inventory (TRI).

In a few important cases, emissions factors for fugitive emissions from industrial processes have been developed: HFC-23 emissions from HCFC-22 manufacture (4 percent of HCFC-22 production) and perfluoromethane and perfluoroethane emissions from aluminum smelting (0.6 kg CF₄ per metric ton aluminum and 0.06 kg C₂F₆ per metric ton aluminum). In these cases, emissions can be calculated by multiplying the underlying activity factor by the emissions factor.

Finally, emissions of HFC-134a were estimated by multiplying the number of vehicles manufactured that use HFC-134a as a refrigerant by estimated charge sizes and leakage rates provided by personal communications with Ford and General Motors.

Data Sources

EPA estimates of emissions of halocarbons and other gases can be found in U.S. Environmental Protection Agency, Inventory of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994, EPA-230-R-96-006 (Washington, DC, November 1995).

Information on halocarbon production, consumption, and sales is spotty. Information on production and sales of some compounds is provided in U.S. International Trade Commission, Synthetic Organic Chemicals: United States Production and Sales, 1994, USITC Publication 2933 (Washington, DC, November 1995). The Alternative Fluorocarbons Environmental Acceptability Study (AFEAS) provides information on “world” and “northern hemisphere” production, sales, and emissions of certain halocarbons, as well as a breakdown of sales by anticipated end use: Alternative Fluorocarbons Environmental Acceptability Study, Production, Sales and Atmospheric Release of Fluorocarbons Through 1994 (Washington, DC, October 1995). The end-use share data can be used to (crudely) estimate U.S. consumption for particular types of end uses. Large industrial emitters of certain halocarbons are required to report emissions, destruction, and recycling of these compounds. This information is published in U.S. Environmental Protection Agency, 1994 Toxics Release Inventory: Public Data Release (Washington, DC, July 1996).

Emissions factors for HFC-23, perfluoroethane, and perfluoromethane can be found in U.S. Environmental Protection Agency, Inventory of U.S. Greenhouse Gases Emissions and Sinks: 1990-1993, EPA-230-R-94-014 (Washington, DC, September 1994), p. 38.

The estimates presented in this report are taken directly from the Environmental Protection Agency’s report, National Air Pollutant Emission Trends: 1990-1994, EPA-454/R-95-011. Chapter 6 of the EPA report summarizes the methodologies used in estimating emissions and revisions to these methodologies as they have occurred. Since the information is too extensive to be included here, only a simplified description is provided below.

Emissions were calculated either for individual sources or for many sources combined, using basic activity data as indicators of trends. The national activity data used to calculate the individual source categories were obtained from many different sources. Activity data are used in conjunction with emissions factors, which relate the quantity of emissions to the activity, and assumptions about control efficiency. Emissions factors are generally available from the Environmental Protection Agency’s Compilation of Air Pollutant Emission Factors, AP-42, often referred to as the AP-42 emissions factors. The EPA currently derives the overall emissions control efficiency of a source category from a variety of sources, including published reports, the 1985 NAPAP (National Acid Precipitation and Assessment Program) emissions inventory, and other EPA databases.

Data Sources

U.S. Environmental Protection Agency, National Air Pollutant Emission Trends, EPA-454-R-95-011 (Research Triangle Park, NC, October 1995). See also: http://www.epa.gov/oar/emtrnd94/emtrn d94.html.

Land Use Issues

A large amount of carbon, on the order of 100 million metric tons, is removed from the atmosphere (sequestered) by forest land in the United States each year. An additional 12 million metric tons is locked into long-term storage in wood products, and approximately 15 million metric tons of carbon enters landfills in the form of wood and paper waste, where processes that cause the carbon to be released to the atmosphere may take up to 60 years or longer.

Less well understood or quantifiable are the impacts of the various types of land uses on other greenhouse gases. Chapter 7 and this section also briefly discuss methane emissions from wetlands, as well as methane and nitrous oxide emissions (and uptake) from forest land, grassland, cropland, pasture, and rangeland. It appears that methane and nitrous oxide emissions attributable to changes in land use in the United States are relatively small, making a negligible contribution to GWP-weighted concentrations of greenhouse gases in the atmosphere. However, there is considerable uncertainty associated with estimates of land-use-related emissions of these gases.

Carbon Sequestration

Carbon Cycling in Forests

Sequestration/Emissions Sources

Every year in the United States and throughout the world a large amount of carbon dioxide is removed from the atmosphere and sequestered into biomass. At the same time, carbon is released to the atmosphere from vegetative respiration, combustion of wood as fuel, degradation of manufactured wood products, consumption of biomass by herbivores, and the natural decay of expired vegetation.

Changes in forest land have a more important impact on U.S. anthropogenic contributions to greenhouse gas emissions than changes in any other type of land. Forests sequester atmospheric carbon in biomass and soil. Photosynthesis is responsible for sequestering carbon into live vegetation, while mortality of roots, foliage, and stems and branches adds carbon to the soil.

On average, the amount of carbon stored in U.S. forests is 17.7 kilograms per square meter of forest land (using an estimate by Forest Service Researcher Richard Birdsey). The range in forest storage across States is large: from 9 kilograms per square meter in Nevada to 26 kilograms per square meter in Alaska, according to Birdsey [165].

The opposite of photosynthesis is respiration, the release of carbon to the atmosphere through decomposition of dead biomass and as a byproduct of internal mechanisms within living plants. Trees add new cell layers each year. Old and new cells require energy for maintenance. Growth and increased maintenance cause respiration to increase. Eventually, because of limitations to total tree foliage area, the rate of photosynthesis cannot continue to outpace respiration, and trees pass through a stage of rough equilibrium between photosynthesis and respiration. When respiration exceeds photosynthesis, mortality follows. This causes the cycle to shift into reverse, as stored carbon in individual trees is released through decay (which is utilized by new trees that emerge in place of the dead and dying ones). The cycle is somewhat different at the forest level, where continual additions to soil carbon from decaying biomass can result in the forest remaining an active carbon sink.

Carbon is also sequestered in wood products and landfills. Forests produce a number of wood products, most notably lumber and paper. Carbon in lumber may remain sequestered for decades or centuries, depending on the end use of the product. Carbon in paper products and wood products that are landfilled may remain sequestered for long periods of time, although no accurate national estimates have been made on the retention time of carbon in landfilled waste.

Estimation Method

Estimates of carbon uptake and release by U.S. forests are made by multiplying biomass volume growth rates from compilations of national forest inventories and ecosystem studies by associated carbon sequestration rates based on estimates of the carbon content of the biomass of various forest types. Forest statistics include acreage and age of distinct forest types, and carbon sequestration rates are based on biomass equations developed in several ecosystem studies conducted throughout the United States. National forest acreages, ages, and forest types are obtained from periodic assessments conducted by the U.S. Department of Agriculture.

The carbon flux estimates presented in this report are based on statistics for the coterminous United States, thus excluding Alaska and Hawaii, for which adequate statistics are lacking. Carbon sequestration estimates include carbon flux by live trees and other vegetation, “dead” flux of carbon entering the soil, and fluxes of carbon to wood products and landfills. A considerable amount of carbon enters the soil, the forest floor, and understory vegetation, which, aside from living trees, are the other major repositories of organic carbon in forest ecosystems. Estimates of carbon storage in trees were based on periodic forest inventories designed to provide estimates of timber volume, growth, removals and mortality [166]. Above-ground tree biomass was calculated by multiplying estimated timber volumes by conversion factors derived from the national biomass inventory [167].

Birdsey and Heath’s estimates of total acreages of distinct types of forest land are obtained from the U.S. Department of Agriculture, which is required to conduct comprehensive assessments of all forest and range land resources on both public and private lands under the Forest and Rangeland Renewable Resources Planning Act (RPA) of 1974. The Forest Service collects information on the Nation’s timber resources from four regions. Each region is composed of subregions, for a total of nine subregions nationally. Each subregion periodically collects local estimates of forest resources. The average cycle of periodic surveys nationally is 10 years. Because the Forest Service produces RPA assessment updates every 5 years, it is assumed that approximately one-half of the information contained in each update is new, and one-half is old. The last RPA update was published in 1992; accordingly, the estimates of carbon sequestration in Chapter 7 are for 1992. The next RPA assessment will occur in 1997, and the report will not be available until some time in 1998. Hence, carbon sequestration estimates for years after 1992 are not likely to be available until 1998 or 1999 (although projections based on previous trends have already been made).

The estimate of total carbon flux from U.S. forests is somewhat higher in this report than in a similar report released by the U.S. Environmental Protection Agency [168]. The EPA, also citing Birdsey and Heath, estimated total flux at 125 million metric tons, excluding the soil carbon flux estimated by Birdsey and Heath.

The amount of carbon sequestered in wood products and landfills has also been estimated by Birdsey and Heath, although the estimates are sensitive to assumptions about recycling, age of trees at harvest, and other factors that affect the amount of wood and the retention periods in various pools. In addition, the estimates of carbon sequestration into these pools understate actual sequestration by an unknown, but probably large, degree, as Birdsey and Heath’s estimates include only wood product and landfill sequestration from biomass produced on private timberland. Inclusion of biomass from other timberland, such as Federally owned timberland, would raise total carbon sequestration estimates considerably.

Other Land Use Changes Affecting the Carbon Budget

It is difficult to be specific about how much carbon might be gained or lost through transformations of grasslands and pasturelands to croplands. Typical estimates of the amount of soil carbon lost when pastureland or grassland is converted to cropland are approximately 30 percent of the amount in place at the time of conversion. These losses can be expected to take place over a period of 20 years, or longer, following conversion. Although the amount of carbon in a square meter of forest might be on the order of 9 to 26 kilograms, depending on the condition of the forest and the age and type of trees growing, typical estimates of carbon storage in cultivated lands range from 1 to 8 kilograms per square meter, and estimates for uncultivated (but cultivatable) lands range from 2 to 10 kilograms per square meter [169]. Thus, there is less carbon to be gained or lost, and the range of possible outcomes per unit of land is consequently smaller. Soils initially very low in carbon tend to gain slight amounts of carbon after cultivation, but richer soils tend to lose at least 20 percent of their carbon after cultivation begins.

A study commissioned by the EPA estimated average soil carbon content for an area of 272 million acres of farmland in the United States at 4.8 to 7.9 kilograms per square meter [170]. The study estimated that 1.0 billion to 1.6 billion metric tons of soil carbon had been lost from the farmland since it had been placed in cultivation, equivalent to 16 percent of the estimated original carbon content of the soil. The study also noted, however, that land with a soil carbon content of less than 4 kilograms per square meter was generally not being cultivated at the time of the study.

It would not be surprising if the least fertile farmland were the most likely to be removed from cultivation. Therefore, assuming that no trees are planted or naturally regenerate, the carbon gains from idling cropland are likely to be small. However, converting farmland to forest produces relatively large carbon gains, both through the addition of biomass (i.e., carbon stored in trees) and through the accretion of carbon into the soil, as dead limbs, trees, and roots gradually decay above and below ground.

Data Sources

The primary researchers who have combined State and national level forestry statistics with biomass growth equations to determine total national carbon fluxes are USDA Forest Service researchers Richard Birdsey and Linda Heath. Their findings have been presented in the following technical reports, and are one of the two sources of data for carbon flux estimates presented in this report: R.A. Birdsey and L.S. Heath, “Carbon Changes in U.S. Forests,” in L.A. Joyce (ed.), Productivity of America’s Forests and Climate Change, General Technical Report RM-GTR-271 (Fort Collins, CO: USDA Forest Service, 1995); and R.A. Birdsey, Carbon Storage and Accumulation in United States Forest Ecosystems and “Changes in Forest Carbon Storage from Increasing Forest Area and Timber Growth,” in R.N. Sampson and D. Hair (eds.), Forests and Global Change, Vol. 1, “Opportunities for Increasing Forest Cover, American Forests” (Washington, DC).

Birdsey and Heath’s estimates of total acreages of distinct types of forest land are obtained from the U.S. Department of Agriculture: K.L. Waddell, D. Oswald, D. Daniel, Powell, and S. Douglas, Forest Statistics of the United States, 1987, USDA Forest Service Resource Bulletin PNW-RB-168 (Portland, OR, 1989).

Carbon storage and flux by distinct forest types have been estimated by several Forest Service researchers: N.D. Cost, J. Howard, B. Mead, et al., The Biomass Resource of the United States, USDA Forest Service General Technical Report WO-57 (1990); D.S. Powell, J.L. Faulkner, D.R. Darr, et al., Forest Resources of the United States, 1992, USDA Forest Service General Technical Report RM-234 (Fort Collins, CO, 1993); and K.L. Waddell, D. Oswald, D. Daniel, Powell, and S. Douglas, Forest Statistics of the United States, 1987, USDA Forest Service Resource Bulletin PNW-RB-168 (Portland, OR, 1989).

Heath and Birdsey are also the source of EIA’s estimate of carbon flux to wood products and landfills: L.S. Heath, R.A. Birdsey, and C. Row, “Carbon Pools and Flux in U.S. Forest Products,” in The Role of Forest Ecosystems and Forest Resource Management in the Global Carbon Cycle, NATO ASI Series (Berlin, Germany: Springer-Verlag, 1995).

For information on carbon storage after converting land to cultivated land, see L.K. Mann, “Changes in Soil Carbon Storage After Cultivation,” Soil Science, Vol. 142, No. 5 (November 1986), p. 279; and W.H. Schlesinger, “Changes in Soil Carbon Storage and Associated Properties with Disturbance and Recovery,” in J. Trabalka and D. Riechle (eds.), The Changing Carbon Cycle: A Global Analysis (New York: Springer-Verlag, 1986), p. 12.

Changes in Land Use

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Appendix A Data Tables

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Appendix B. Carbon Coefficients Used in This Report

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