| Carbon Dioxide | Methane | Nitrous Oxide | Halocarbons and Other Gases | Criteria Pollutants | Land Use Issues |
The organization of this appendix generally follows the organization of the body of the report: the discussion is divided
by greenhouse gas and by emissions source.
Most U.S. anthropogenic carbon dioxide emissions result from energy consumption. Energy production contributes
a small amount from 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 methods used, and the data sources are described here.
Several emissions sources are excluded from the carbon dioxide emissions presented in this report, due either 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 longterm 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).
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 amountof 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 realworld complexities can reduce the precision
of the estimate. These complexities are discussed further in this appendix. Nonetheless, energy-related carbon dioxide
emissions are known with greater reliability than are other greenhouse gas emissions sources, and the uncertainty of
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 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.
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.(127) To estimate carbon dioxide emissions, the EIA uses annual data
from the four end-use sectors (residential, commercial, industrial, and transportation) and for all 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 the Petroleum Supply Annual, Coal Industry Annual, and Natural Gas Annual. This approach to estimate emissions
enables EIA to provide 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 have included crude oil, naphtha <401oF, other oil 401oF, 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.(128) 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 employed for developing the coefficients. Table B1 in Appendix B presents a
full listing of all factors for crude oil, natural gas, and the complete slate of petroleum products.
Corrections to Energy Consumption Information
While, in general, emissions can be estimated simply by multiplying fuel consumption by the appropriate emissions
coefficients, several small adjustments to EIAenergy statistics are necessary to eliminate double counting or
miscounting of emissions. Usually the adjustments amount to less than 0.1 percent of energy-related carbon emissions.
They include:
Carbon Sequestration: Nonfuel Use of Fossil Fuels
Gross emissions can be estimated by multiplying fossil fuel consumption by an emissions factor embodying the
estimated carbon content of the fuel. However, portions of the fossil fuels consumed are not actually combusted but
are used as chemical feedstocks, construction materials, lubricants, solvents, or reducing agents (Table A1).
The EIA estimates "nonfuel" use of fossil fuels annually in Table 1.16 of the Annual Energy Review.(129) For this report, EIA has gone one step further and determined
the fate of the carbon in fuels used for nonfuel purposes (Table A2). In some cases, the carbon winds up in the
atmosphere; in other cases, it does not.
The principal nonfuel uses of fossil fuels, the methods of estimating nonfuel consumption, and the fate of the carbon are listed below.
Future research on the fate of the carbon in feedstocks for other chemical industry uses will probably gradually
reduce the 100-percent sequestration share currently assumed.
Carbon Sequestration: Fraction Combusted
A small amount of carbon sequestration is associated with the combustion of fossil fuels. Using IPCC assumptions,
EIA 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.(131) 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.
Fossil Fuel Consumption (1980-1994): Energy Information Administration, State Energy Data Report, DOE/ EIA-0214(94) (Washington, DC, September 1996). (1995-1996): Energy Information Administration, Monthly Energy Review,
DOE/EIA-0035(97/07) (Washington, DC, July 1997); Petroleum Supply Annual 1996, DOE/EIA-0340(96) (Washington, DC, June 1997); Natural Gas Annual 1995,
DOE/EIA-0131(95) (Washington, DC, October 1996); and Renewable Energy Annual 1996, DOE/EIA-0603(96)
(Washington, DC, March 1997).
Nonfuel Use of Energy and Biofuels Consumption: Energy Information Administration, Annual Energy Review 1996,
DOE/EIA-384(96) (Washington, DC, July 1997); Manufacturing Energy Consumption Survey 1991, DOE/EIA-0512(91)
(Washington, DC, December 1994); and preliminary material from the 1994 MECS posted on the EIA web site
(www.eia.doe.gov); American Petroleum Institute, Sales of Natural Gas Liquids and Liquefied Refinery Gas (various years);
U.S. International Trade Commission, Synthetic Organic Chemicals, USITC Publication 2933 (various years through
1994); and Intergovernmental Panel on Climate Change, Greenhouse Gas Inventory Reference Manual, IPCC Guidelines
for National Greenhouse Gas Inventories, Vol. 3 (Paris, France, 1995), p. 1.27.
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 adjustment is accommodated in the EIA
estimates.
Emissions Sources
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, 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 Methods
Energy consumption for U.S. territories is converted to carbon emissions by using the same emissions coefficients
applied to U.S. energy data. Carbon emissions for U.S. territories ranges 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 is 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
1980-1995: 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. 1996: EIA estimate.
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 under-stand 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.(132) 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 Methods
The appropriate carbon coefficient is 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). (1991-1996): Energy Information Administration, Fuel Oil and Kerosene Sales,
DOE/EIA-0535 (Washington, DC, 1991-1996). Jet Fuels (1980-1995): Oak Ridge National Laboratory, Transportation
Energy Data Book (Oak Ridge, TN, various years).
Carbon dioxide emissions from "industrial sources" are industrial emissions that are not caused by the combustion or
feedstock use of commercial fossil fuels. These emissions typically are created either by the combustion of waste
products containing fossil carbon (natural gas flaring) or by chemical reactions with carbon-containing minerals (for
example, calcining sodium carbonate [limestone] to make lime or cement).
Emissions Sources
U.S. energy production also generates small volumes of carbon dioxide emissions. The two principal sources are flaring
of natural gas and venting of carbon dioxide produced in conjunction with natural gas.(133) 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
for remote sites 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 Methods
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. The 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.(134) To calculate carbon emissions, the State
figures are aggregated, converted into Btu, and then multiplied by an emissions coefficient of 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 this estimate, given that operators in the field are not required to meter 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 (1980-1995): Energy Information Administration, Natural Gas Annual, DOE/EIA-0131
(Washington, DC, various years). (1996): Natural Gas Monthly, DOE/EIA-0130(96/06) (Washington, DC, May 1997).
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 A4 shows 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 Methods. 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 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.(135) 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.(136)
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 Methods. 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 doesnot account for the instances in which the carbon dioxide is recovered or reabsorbed. Representatives of the
National Lime Association believe that 10 to 20 percent of the carbon dioxide emitted in lime manufacture is recovered
for industrial use or reabsorbed from the atmosphere by chemical reactions induced by the use of lime.
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 Methods. 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, and pulp and paper production. 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 as chemicals manufactured from soda ash and components of detergents.
Estimation Methods. 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 of carbon 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, in the United States, there is an 80 to 20
percent split between carbon dioxide produced as a byproduct and carbon dioxide produced from wells.(137) 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 Methods. The Freedonia Group estimates that nonsequestering industrial use of carbon dioxide resulted in emissions of 1.3 million metric tons of carbon in 1993.(138) 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 1996 estimate is calculated by assuming an annual 4.2-percent increase, implying emissions of 0.30 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. As part of the primary aluminum smelting process, alumina (aluminum oxide, Al2O3) is vaporized
by a powerful electric current. Emissions from the electricity used to generate the current are included with emissions
from industrial electricity consumption and are not counted separately. The current also vaporizes a carbon "sacrificial
anode," typically manufactured from petroleum coke. This is a nonsequestering nonfuel use of a fossil fuel. To avoid
double counting, 50 percent of nonfuel use of petroleum coke is carried as "sequestering." Thus, process emissions from
aluminum smelting can be considered as a deduction from the sequestering portion of nonfuel use of petroleum coke.
Estimation Methods. In previous years, following the work of Abrahamson, EIA has used the midpoint of a range of
emissions factors of 1.5 to 2.2 metric tons of carbon dioxide (0.41 to 0.60 metric tons of carbon) emitted per metric ton
of aluminum smelted.(139) The 1994 MECS indicated that nonfuel use of fuels by aluminum smelters (SIC 3334) totaled
40 trillion Btu in 1994.(140) The composition of nonfuel use by fuel type has been withheld for confidentiality purposes,
but it is probable that most of the 40 trillion Btu consists of petroleum coke, which would imply an emissions factor
of about 0.338 metric tons of carbon per metric ton of aluminum smelted (0.04 quadrillion Btu of coke × 27.85 million
metric tons of carbon per quadrillion Btu / 3.295 million metric tons of aluminum smelted in 1994). For this year's
report, EIA used an emissions factor of 0.4 metric tons carbon per metric ton of aluminum smelted, which is at the low
end of Abrahamson's range and also equals the mass balance for a "typical" aluminum smelter from another source.(141)
Data Sources for Industrial Processes
Cement and Clinker Production (1980-1995): U.S. Department of the Interior, Bureau of Mines, Cement Annual Report
(Washington, DC, various years). (1996):
U.S. Department of the Interior, U.S. Geological Service, Office of Minerals, Faxback Service.
Lime Manufacture: (1980-1995): U.S. Department of the Interior, Bureau of Mines, Mineral Commodity Summaries
(Washington, DC, various years). (1996): U.S. Department of the Interior, U.S. Geological Service, Office of Minerals,
Faxback Service.
Limestone Consumption in Iron Smelting, Steelmaking, and Glass Manufacture: (1980-1995): U.S. Department of
the Interior, Bureau of Mines, Crushed Stone Report (Washington, DC, various years). (1996): EIA estimate. Limestone
Consumption in 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 Soda Ash Consumption in Glass Making: (1980-1995): U.S. Department of the Interior,
Bureau of Mines, Soda Ash Report (Washington, DC, various years). (1996): U.S. Department of the Interior, U.S.
Geological Service, Office of Minerals, Faxback Service. Soda Ash Consumption in 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.
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 occurs from valves, meters, and flanges. Pneumatic valves, pressurized with
natural gas, emit gas when they are 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 in transport through leaky pipes and valves. It also may
be released as part of compressor exhaust, while pneumatic devices are being reset, 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, associated natural gas is often produced at the
wellhead. If the flow of associated gas is too small or intermittent to be of value, the gas is vented or flared. Associated
gas with an insufficient heat content to be marketed may also be vented or flared. If a site lacks the necessary gathering
and processing facilities for associated gas, that gas may also 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 A5. 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).(142) 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.
Gas Transmission and Distribution. Methane emissions from transmission and distribution pipeline and gate stations
are also estimated by using emissions factors from the joint EPA/GRI study. The emissions estimates are scaled to
pipeline mileage, with separate emissions factors for plastic and nonplastic pipeline.
Oil Refining and Transportation. Estimates of emissions from this source are calculated by using emissions factors
from a 1992 Radian Corporation report(143) in conjunction with refinery data collected by the EIA.
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 shares of gas vented and
flared in each State.(144) These shares are applied to annual State venting and flaring data, and the results are 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 EnergyReview, DOE/EIA-0384) (Washington, DC, various years), 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, DOE/EIA-0131(95)
(Washington, DC, various years).
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 when the coal is transported and pulverized for combustion. There are five
avenues for methane emissions from coal mines:
Ventilation Systems in Underground Mines. Methane in concentrations over 5 percent is explosive and presents a
mortal danger to coal miners. To meet safety standards set by the Mine Safety and Health Administration (MSHA)
requiring levels of methane concentration to be maintained well below the 5 percent threshold, mine operators use
large fans to provide a steady airflow across the mine face and ventilate the mine shaft. Typically, these ventilation
systems release substantial quantities of methane in the fan exhaust.
Degasification Systems in Underground Mines. When the volume of gas in underground mines is too high to be
practically reduced to safe levels by standard ventilation techniques, degasification systems are employed.
Degasification may take place before mining or may take the form of gob-wells or in-mine horizontal boreholes.
Methane captured by degasification systems may be vented, flared, or recovered for energy. Twenty-four U.S. mines
are currently known to have operating degasification systems in place, 12 of which recover gas for energy use.
Surface Mines. Because coal mined from the surface has formed at lower temperature and pressure than coal from
underground mines, its methane content is lower. Further, because the coal is located near the
surface, methane has had ample opportunity to migrate to the atmosphere before mining. Thus, while methane
emissions from surface mines are heterogeneous in nature, they are systematically smaller than emissions from
underground mines.
Post-Mining Emissions. Methane that remains in coal pores after either underground or surface mining will desorb
slowly as the coal is transported (typically by train) to the end user. Because coal that is consumed in large industrial
or utility boilers is pulverized before combustion, methane remaining in the coal pores after transport will be released
prior to combustion.
Methane Recovery for Energy. In some cases (for example, in some mining degasification systems), methane is emitted
from coal mines in sufficiently high volumes and concentrations to permit commercial recovery of the gas as either
pipeline gas or fuel for electric power generation. Since coal mine methane recovered commercially is combusted, the
quantities recovered are subtracted from estimates of total coal mine methane emissions.
Estimation Methods
Ventilation Systems in Underground Mines. Emissions from this source are segregated into two classes: emissions
from "gassy" mines and emissions from "nongassy" mines.(145) Because methane concentrations and airflow in gassy
mines are carefully monitored by the MSHA, a fairly reliable set of data can be derived for emissions from ventilation
systems in gassy mines. However, MSHA data are voluminous inconsistent in format, and difficult to compile, and
they are available for only a subsample of years (1980, 1985, 1988, 1990, 1993, and 1994). 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.(146) Emissions factors for nonsample 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 A6). For 1996, a subsample of ventilation emissions from the 80 gassiest U.S. mines was
obtained. Emissions from the remaining gassy mines are estimated by assuming that
they represent the same share of total emissions from gassy mines as they did in 1994.
Emissions from nongassy mines make up less than 2 percent of all emissions from underground mines.(147) Basin-level
emissions factors for nongassy mines were established by dividing 2 percent of each basin's estimated emissions from
nongassy 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
degasification systems in place by a per-ton emissions factor. Because mine-by-mine production data are not yet
available for the current year, emissions from degasification systems are scaled to increases in underground coal
production.
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.(148) 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 applied. 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 more significant, between 0.9 and 4.0 cubic meters of
methane per metric ton of coal mined.(149) 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 sales.
Data Sources
Ventilation Systems in Underground Mines. Coal mine ventilation data for the approximately 200 gassiest U.S. mines
were drawn from a database prepared by the Department of Interior's Bureau of Mines for the years 1980, 1985, 1988,
1990, and 1993. Ventilation data for 1994 and a subsample of data for 1996 were obtained from the U.S. Environmental
Protection Agency (EPA), Atmospheric Pollution Prevention Division, Coalbed Methane Outreach Program. Coal
production data are reported to the EIA on Form EIA-7A, "Coal Production Report." Basin-level emissions for nongassy mines in
1988 were calculated by the EPA's Office of Air and Radiation, in Anthropogenic Methane Emissions in the United States:
Estimates for 1990 (Washington, DC, April 1993), pp. 3-193-24.
Degasification Systems in Underground Mines. Emissions factors for this source are derived from estimates of 1988
emissions from degasification systems prepared by the EPA's Office of Air and Radiation, in Anthropogenic Methane
Emissions in the United States: Estimates for 1990 (Washington, DC, April 1993), pp. 3-193-24. Annual production figures
are reported to the EIA on Form EIA-7A, "Coal Production Report."
Surface Mines. Emissions factors for surface mines are found in the IPCC Greenhouse Gas Inventory Reference Manual.
Coal production data are reported to the EIA 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 EIA on Form EIA-7A, "Coal Production Report."
Methane Recovery for Energy. Volumes of methane recovered during 1993 were obtained from 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. Recovery volumes for 1995
were obtained from the EPA's Coalbed Methane Outreach Program. Recovery for 1994 was interpolated from 1993 and
1995 data, and recovery for 1996 was assumed to remain stable at 1995 levels.
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 sectors, 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 of the U.S. methane emissions from stationary sources.
Estimation Methods
An emissions factor based on fuel type (for example, coal, wood, natural gas) and combustion technology (for example,
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 EPA's Office of Air Quality Planning and Standards,
Compilation of Air Pollutant Emission Factors, AP-42, 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 1994: Consumption Estimates, DOE/EIA-0214(94) (Washington, DC, October 1996), for 1980-1994; and Monthly Energy Review, DOE/EIA-0035(97/07)
(Washington, DC, July 1997). Residential wood fuel consumption data were derived from EIA's Renewable Energy
Annual 1996, DOE/EIA-0603(96) (Washington, DC, April 1997), p. 10.
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 in automobiles 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.(150)
Estimation Methods
Methane emissions from highway vehicles, such as automobiles, light-duty trucks, motorcycles, buses, and heavy-duty
trucks, are estimated by applying emissions factors (per vehicle mile traveled) to vehicle use data. The emissions factors
vary by vehicle type. Because of improving technology and more stringent environmental regulations, they have
declined over time. For nonhighway 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 nonhousehold
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 1995 (Washington, DC, 1996).
Data on fuel consumption for ships, locomotives, farm equipment, and construction equipment are available in EIA's
Fuel Oil and Kerosene Sales 1996, DOE/EIA-0535(95) (Washington, DC, September 1997). Fuel consumption data for jet
and piston-powered aircraft are contained in EIA's Petroleum Supply Annual 1996, DOE/ EIA-0340(96)/1 (Washington,
DC, May 1997). 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).
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 tominimize 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 put methane control systems in place to prevent migration of high concentrations
to buildings. Methane captured by control systems may be vented to the atmosphere or flared, but it is also 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. measured or estimated methane
emissions at 2.1 million metric tons for 1992.(151) Methane emissions from landfills without gas recovery systems have
not been measured, and even the number of landfills 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.(152) 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 A7 shows the EMCON
methane generation model parameters.
Waste flows were estimated from 1940 through 1996. 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. The landfills examined by Thorneloe et al.
contained 9.2 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 90.8 percent 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 9.2 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 on waste generated and landfilled for the period 1988 through 1996 (Table A8) were drawn from Biocycle
magazine.(153) These data were not collected by Biocycle prior to 1988. Waste generated and landfilled for the period 1960
through 1987 was estimated from data produced by Franklin Associates. On behalf of the EPA's Office of Solid Waste
and Emergency Response, Franklin Associates have estimated municipal solid waste (MSW) generated and landfilled
for the years 1960 through 1996.(154) In contrast to the Biocycle data, which include all waste going to landfills, including
construction and demolition (C&D) waste and sludge, the Franklin data include only MSW going to landfills.
In order to account for categories of waste other than MSW going to landfills between 1960 and 1987, an average ratio
of waste generation estimated by Biocycle and waste generation estimated by Franklin Associates for 1988 through 1996
was calculated. The annual average ratio during this period was 1.47 to 1. Thus, all Franklin estimates for 1960 through
1987 were multiplied by 1.47 to estimate overall waste generation and landfilling for those years. To further extend
waste generation estimates back to 1940, a regression equation relating waste generation to GNP and population was
developed.
Methane recovery data were estimated from 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). There were
105 known landfills with methane recovery systems in place in 1992, and 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 the 1993
estimate of recovery and flaring. The 1995 and 1996 estimates were extrapolated from the 1992 to 1994 trend.
Emissions Sources
Emissions of methane from the treatment of wastewater occur 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, aerobic, and facultative (combining aerobic and anaerobic processes)
stabilization lagoons, septic tanks, and cesspools.(155) Treatment of wastewater solids using anaerobic digestion is the
most obvious potentialsource of methane emissions; however, emission of significant quantities of methane from this
process requires that the digester gas be 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. 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).(156)
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 that
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.(157)
Estimation Methods
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 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 IPCC,(158) which assumes that each person in a developed nation contributes 0.5 kilogram of BOD5
to domestic wastewater annually, and 15 percent of this wastewater is treated anaerobically, yielding 0.22 kilogram
of methane per kilogram of BOD5 in the wastewater.(159) It is assumed that recovery of methane at municipal wastewater
treatment facilities is negligible.
Data Source
Estimate of the U.S. resident population on July 1 of each year were obtained from the U.S. Census Bureau.
Emissions Sources
The breakdown of carbohydrates in the digestive track of herbivores (including insects and humans) results in the
production of methane.(160) 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 Methods
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 animals, their productivity (reproduction and/or
lactation), and their 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 from these variables. The factor is then
multiplied by population data for the 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 represent 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.(161)
Because characteristics critical in determining energy intake, and thus emissions rates, for cattle--such as growth rates
and milk production--change annually, an effort is made to scale emissions factors to these changes. Emissions rates
are pegged to average slaughter weights for calves and adult cattle respectively (Table A9).(162)
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 their respective populations. Emissions factors are drawn from the
work of Crutzen et al.(163)
Data Sources
Population and slaughter weight data for cattle and population data for sheep and swine were obtained from the U.S.
Department of Agriculture, National Agricultural Statistics Service, Livestock, Dairy, and Poultry Branch, web site
www.mannlib.cornell.edu. Population data for goats and horses were extrapolated from the U.S. Department of
Commerce Census of Agriculture for the years 1982, 1987, and 1992.(164)
Emission Sources
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 managed. 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.(165) Thus, they result in the most substantial methane emissions.
Estimation Methods
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
managed. 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
EPA(166) is used to determine emissions per animal. Animal sizes are adopted directly for all animals except cattle, whose
masses are adjusted annually on the basis of live slaughter weights as reported by the U.S. Department of Agriculture.
Volatile solids produced per kilogram of animal weight, maximum methane-producing capacity of each animal's waste,
and the share of waste managed in each management system are adopted from the work of Safley et al.(167) For all
animals except dairy cattle, the share of waste managed in each management system is also drawn from Safley et al.
Because the methods for managing the waste of dairy cattle in Arizona, Florida, Nevada, North Carolina, North
Dakota, and Texas have changed since 1990 (when estimates were prepared by Safley et al.), emissions for this group
are analyzed separately for the years 1990-1996. To do so, national-level methane conversion factors and waste
management system distribution are applied only to national population data after netting out dairy cattle in these six
States, which are then examined individually on the basis of shifts in waste management techniques since 1990. The
updated methane conversion factors and waste management system shares are obtained from the EPA report, Inventory
of U.S. Greenhouse Gas Emissions and Sinks: 1990-1994.(168) Emissions from dairy cattle in these six States are totaled and
added to the emissions estimates for 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 obtained from
the U.S. Department of Agriculture (USDA), National Agricultural Statistics Service, Livestock, Dairy, and Poultry
Branch, from web site 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.(169) Population data for goats and horses were
extrapolated from the U.S. Department of Commerce's Census of Agriculture for the years 1982, 1987, and 1992.(170)
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 diffusion through
the water column.
Estimation Methods
A daily emissions rate range has been developed from studies of rice fields in California,(171) Louisiana,(172) and Texas.(173)
The high and low ends of the range, 0.1065 to 0.5639 grams of methane per square meter of land cultivated, are applied
to the growing season length and the harvested area for each State that produces rice. In States with a second ("ratoon")
crop, the additional area harvested is incorporated into the 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).
Emissions Sources
Crop residues contain substantial shares of carbon, between 40 and 50 percent of dry matter.(174) When crop residues
are burned for fodder, land supplementation, or fuel, incomplete combustion results in methane emissions.(175)
Estimation Methods
In keeping with the methods recommended by the IPCC,(176) this report assumes that 10 percent of all crop residues are
burned. The dry weight and carbon content of each crop are determined (Table A10) and used in conjunction with
estimated combustion efficiencies to derive 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).
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 compounds, with different temperatures producing
specific chemicals. The process of cracking produces a number of chemical byproducts, including methane.
Estimation Methods
The IPCC has published emissions factors for methane emitted during the manufacture of ethylene, ethylene
dichloride, styrene, methanol, and carbon black(177) (Table A11). Production figures for the chemicals are multiplied by
those emissions factors.
Data Sources
Chemical production figures were obtained from the Chemical Manufacturers Association, U.S. Chemical Industry
Statistical Handbook (Washington, DC, various years).
Emissions Sources
Coke, sinter, and pig iron are the principal material inputs for the production of iron and steel. Coke isproduced
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 to iron, slag, and exhaust gases. Methane is one of the exhaust
gases.
Estimation Methods
The IPCC has published emissions factors for methane emitted during the production of coke, sinter, and pig iron.(178)
Production figures for iron and steel inputs are 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).
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 motor vehicle fuel
combustion combined to account for approximately 65 percent of 1996 estimated emissions of nitrous oxide (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.
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 Methods and Data Sources
See section on methane emissions from mobile combustion, above.
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 75 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 Methods
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 are estimated by applying emissions factors for coal, oil, and natural gas to EIA's
consumption data for each fuel in the commercial, residential, industrial, and electric utility sectors.
Data Sources
Fuel consumption data are from the EIA's State Energy Data Report (EIA-0214(94)) database. The emissions factors used
in this report are those recommended by the IPCC as derived from studies of numerous conventional systems.(179)
Emissions Sources
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.(180) 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 EIA is not aware of data that would allow them to be incorporated into
emissions estimates.
Estimation Methods
Emissions factors ranging in magnitude from 0.001 to 0.1 gram nitrogen (in nitrous oxide) per gram of nitrogen in
fertilizer are applied to the nitrogen content of fertilizer used annually in the United States, producing low, median,
and high estimates. In 1996, 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(181)) indicates that 141,000 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 by 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 and 1993-1994)-- and from the Association of
American Plant Food Control Officials for 1995 and 1996.
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 was 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.
Emissions Sources
Crop residue is commonly disposed of by incorporation into the soil, spreading over the soil surface to
preventerosion, 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 Methods and Data Sources
See section on methane emissions from burning crop residues, above.
Emissions Sources
Manufacture of adipic acid is one of the two principal sources of nitrous oxide from industrial processes. Adipic acid
is used primarily in the manufacture of nylon fibers and plastics in carpet yarn, clothing, and tire cord. Other uses of
adipic acid include production of plasticizers for polyvinyl chloride and polyurethane resins, lubricants, insecticides,
and dyes. In the United States, three companies, which operate a total of four plants, manufacture adipic acid by
oxidizing a ketone-alcohol mixture with nitric acid. Creation of nitrous oxide is an intrinsic byproduct of this chemical
reaction.
Estimation Methods
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.(182) 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.(183)
Data Sources
Adipic acid production figures were obtained Chemical and Engineering News, annual report on the "Top 50 Industrial
Chemicals" (April issue, various years). For 1996, U.S. total adipic acid production was estimated
from information on adipic acid production and industry characteristics, obtained from the Dupont Company, as well
as information reported on DuPont's Form EIA-1605, "Voluntary Reporting of Greenhouse Gases." Dupont's 1996 Form
EIA-1605 submission contained data on nitrous oxide emissions for 1992 through 1995. The adipic acid emissions
coefficient was taken from M. Thiemens and W. Trogler, "Nylon Production: An Unknown Source of Atmospheric
Nitrous Oxide," Science, Vol. 251, No. 4996 (February 22, 1991), p. 932.
Emissions Sources
Nitric acid is a primary ingredient in fertilizers. The process for manufacturing this acid involves oxidizing ammonia
(NH3) with a platinum catalyst. Nitrous oxide emissions are a direct result of the oxidation.
Estimation Methods
Measurements at a DuPont plant indicate emissions factors of 2 to 9 grams of nitrous oxide per kilogram of nitric acid
manufactured.(184) The emissions estimates presented in this report are 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 were based on data published by the U.S. Department of Commerce, Bureau of the
Census, in its annual and quarterly Current Industrial Reports on fertilizer materials. See also web site
www.census.gov/cir/www/mq28a.html. The nitric acid emissions coefficient was taken 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 have hundreds of uses, but the bulk of emissions come from a few broad categories of use:
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.
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 from 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 kilogram CF4 per metric ton aluminum and 0.06 kilogram 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 are 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.
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).
EPA estimates for 1995 are published in U.S. Department of State, Climate Action Plan, Publication 10496 (Washington,
DC, July 1997), pp. 71-72.
Information on halocarbon production, consumption, and sales is spotty. Information on production and sales of some
compounds, up to 1994, 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
1995 (Washington, DC, January 1997). 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, 1996 Toxics Release Inventory: Public Data Release (Washington, DC, June 1997).
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 EPA report, National Air Pollutant Emission Trends:
1900-1995, EPA-454/R-96-007 (Research Triangle Park, NC, October 1996) (see also web site www.epa.
gov/airprogm/oar/emtrnd/index.html. Chapter 6 of the EPA report, published in October 1995, 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 either for individual sources or for many sources combined were calculated from 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
EPA'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.
A large amount of carbon, on the order of 170 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.
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.(185)
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 in nonmanaged systems 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 cause the forest to remain an active carbon sink.
Carbon is also sequestered in wood products and landfills. Forests produce a number of wood products, most notably
paper and lumber. 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 of the retention time of carbon in landfilled
waste.
Estimation Methods
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 (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 forest floor and 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 are based on periodic forest inventories designed to provide estimates of timber volume,
growth, removals and mortality.(186) Above-ground tree biomass is calculated by multiplying estimated timber volumes
by conversion factors derived from the national biomass inventory.(187)
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 contains subregions (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;(188) 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 EPA.(189) 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.
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.
The primary researchers who have combined State and national 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).
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, C. Row, and A.J. Plantinga, "Carbon Pools and Fluxes 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.
Estimates of the total U.S. land area occupied by different types of land use form part of the basis for estimating
greenhouse gas emissions and sequestration from changes in land use. The primary source of information used for
EIA's land use figures is the Economic Research Service (ERS) of the U.S. Department of Agriculture. The ERS has
regularly inventoried the major uses of land in the United States at intervals coinciding with the censuses of agriculture
since 1945. The latest inventory was conducted in 1992.
There is an unquantified amount of error associated with national-level land use statistics. Data are typically obtained
from surveys differing greatly in scope, methods, definitions, and other characteristics. Individual sources account for
only a limited part of the total land area. The available data contain conflicts and overlap that must be reconciled or
removed. The ERS addresses these problems to the extent feasible, but there is undoubtedly some error associated with
combining and normalizing land use data encompassing approximately 2.3 billion acres.
The ERS compiles land use data in its periodic report, Major Uses of Land in the United States. The estimates in the ERS report are from a series of land-use inventories, based on available land-use data from a wide variety of sources, conducted by the ERS and predecessor agencies. See, for example, A.B. Daugherty, Major Uses of Land in the United States, 1992, USDA Economic Research Service Agricultural Economic Report Number 723 (Washington, DC, 1995).
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