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U.S. Emissions of Greenhouse Gases in Perspective Appendix A: Estimation Methods Appendix B: Carbon Coefficients Used in this Report Appendix C: Uncertainty in Emissions Estimates Appendix D: Emissions Sources Excluded Appendix E: Emissions of Energy-Related Carbon Dioxide in the United States, 1949-1997 Appendix F: Common Conversion Factors
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Overview
More than 98 percent of U.S. carbon dioxide emissions can be traced to the combustion of fossil fuels. Less than 2 percent comes from other industrial sources, including the manufacture of cement and lime. Total estimated emissions increased by 1.5 percent (22 million metric tons of carbon), from 1996 values of 1,479 million metric tons to 1,501 million metric tons of carbon in 1997 (Table 6).(23) Compared with 1990 emissions levels, the increase is 145 million metric tons of carbon or 10.7 percent. U.S. carbon emissions have increased every year since 1991 (Figure 1). The substantial growth in U.S. carbon dioxide emissions between 1995 and 1996 can be traced principally to an increase in energy consumption due to weather-related factors. The more moderate increase from 1996 to 1997 is the product of different factors. First, robust 3.8-percent growth in the economy in 1997 led to an overall increase in end-use energy consumption of about 0.7 percent, including a 0.6-percent increase in demand for electricity. The increase in electricity use resulted in a 3.7-percent increment in emissions, because proportionally more fossil fuels were used for the generation of electricity than in 1996. Although hydroelectric generation was at historically high levels in 1997, as it had been in 1996, the increase was not enough to compensate for the temporary closing of several large nuclear power plants (about 2.6 gigawatts of generating capacity) and the permanent retirement of several smaller facilities (about 1.5 gigawatts). Compensation for the decrease in nuclear availability consisted largely of coal consumption, which was up by 2.9 percent in 1997 over 1996. Thus, carbon dioxide emissions from electricity generation rose much more rapidly than electricity consumption. In times of slow or negative economic growth, U.S. carbon dioxide emissions have lagged behind population growth, as was seen in 1990, 1991, and 1992 (Figure 2). In recent years, however, with strong economic growth, carbon dioxide emissions have tended to grow more rapidly than population, although not as rapidly as the U.S. economy. The patterns of growth in energy use and carbon dioxide emissions are similar but not identical. Residual differences are caused by changes in fuel mix. The emergence of natural gas as a fuel for electricity generation has curbed the growth in U.S. carbon dioxide emissions since 1990. In the past 2 years, however, excess coal-fired generating capacity in the Midwest has provided an inexpensive source of electricity generation for those regions with transmission access. With the expansion of nuclear generation limited to increased operation of existing facilities, over the next several years the demand for fossil-fuel-generated electricity is likely to increase by more than overall demand for electricity. Energy Consumption Sectoral Analysis and Trends The Energy Information Administration (EIA) partitions total energy consumption statistics into four end-use sectors: industrial, transportation, residential, and commercial. For all the sectors except transportation, a substantial portion of the energy used is consumed as electricity. The transportation sector uses mostly petroleum, smaller quantities of other fuels, and negligible amounts of electricity. In this report, emissions for each sector are defined as the sum of emissions resulting from the direct burning of fuels plus emissions associated with the production of electric power (including losses) used in the sector. This approach makes sectoral analysis more meaningful; for example, about two-thirds of the carbon dioxide emissions in the residential and commercial sectors are derived from electricity (Figure 3). Electric power consumption has increased proportionately with growing end-use energy demands. The growth is significant, because about 3 British thermal units (Btu) of energy are needed to deliver 1 Btu of end-use electricity.
Industrial Sector Industrial primary energy consumption was about 32.9 quadrillion Btu in 1997, up by only 0.013 quadrillion Btu from 1996. In the United States, about one-third of all end-use carbon dioxide emissions are accounted for by the industrial sector (Table 7), which comprises manufacturing (the largest part of the sector), along with mining, construction, agriculture, fisheries, and forestry. Fossil fuel consumption is dominated by the need for heat and power; however, a substantial share of industrial petroleum use is for petrochemical feedstocks and other nonfuel uses that sequester carbon (Table 8). Natural gas and electricity consumption each account for about one-third of the energy consumed in the industrial sector. Emissions of carbon dioxide from energy use in the industrial sector amounted to about 483 million metric tons of carbon in 1997, up by about 1 percent from the previous year (Table 9). Energy efficiency improvements, combined with low growth in energy-intensive industries, have moderated trends in carbon dioxide emissions at the same time that total industrial output has expanded. Between 1990 and 1997, emissions for this sector increased by 29 million metric tons of carbon, or about 6 percent--the lowest growth of any sector. In comparison, total industrial gross output grew by about 20 percent between 1990 and 1997. Transportation Sector Fuel use for transportation was about 24.8 quadrillion Btu in 1997. More than two-thirds of U.S. oil consumption is used for transportation, which accounts for about one-third of U.S. carbon dioxide emissions. Low fuel prices, low stock turnover of existing older vehicles, and stable average fuel efficiency in vehicles all contribute to expanding transportation energy consumption. In addition, of the vehicles purchased in recent years, a higher proportion of them have been sports utility vehicles or light trucks. Thus, even though fuel economy may be stable by category, the shift to less fuel-efficient vehicles has meant growing energy consumption and related emissions. Motor gasoline accounts for nearly two-thirds of transportation sector energy consumption. Total carbon dioxide emissions in 1997 were 473 million metric tons (Table 10). Transportation sector emissions have yielded about 41 million metric tons of carbon, or 31 percent, of the national increase in emissions from energy end-use sectors since 1990 (Table 8). This year, in keeping with international emissions accounting guidelines, carbon dioxide emissions from bunker fuels have been subtracted from the total for U.S. energy-related emissions for all years (see "Adjustments to U.S. Energy" in Table 6). Residential and Commercial Sectors In 1997, energy consumption in the residential and commercial sectors combined totaled 32.8 quadrillion Btu, up by 0.2 quadrillion Btu from 1996. The small increase represents the combination of different trends in the two sectors. Most of the emissions from the residential sector are associated with the use of natural gas and electricity for space heating and air conditioning and thus are subject to the vagaries of the weather. In 1996, emissions were up 5.7 percent over 1995 because of cold weather in regions that rely on natural gas. As a direct result, demand for natural gas increased, leading to a spike in natural gas prices, which further caused a shift from natural gas to coal where possible for electricity generation. The return to more moderate weather in 1997 brought a leveling in emissions growth. Although residential energy demand fell by about 1 percent to about 18 quadrillion Btu in 1997, carbon dioxide emissions from the residential sector rose slightly--by 0.9 million metric tons of carbon (Table 11)--as a result of the increase in coal use and decrease in nuclear power for electricity generation. In the 7 years since 1990, residential emissions have accounted for 33 million metric tons of carbon, or about 25 percent, of the increase in carbon dioxide emissions from all energy end-use sectors (Table 7). In contrast to the residential sector, energy consumption in the commercial sector was about 14.8 quadrillion Btu in 1997--up by about 3 percent over 1996. Again, because of the increase in coal use for electricity generation, the increase in carbon dioxide emissions was even larger, at 4.9 percent. With the overall economy growing by 3.8 percent in 1997 and the service component of the economy leading that growth, the commercial sector (which includes the service component) grew most rapidly. As a result, energy demand and related emissions from the commercial sector increased. Electricity use accounts for more than one-half of commercial energy demand, with lighting the largest single use. The commercial sector contributes the smallest share of carbon dioxide emissions among the end-use sectors, making up about 16 percent of the total. Because this sector includes all business establishments that are not engaged in transportation, manufacturing, or other industrial activities (agriculture, mining, or construction), most of the energy consumed is electricity and natural gas. Commercial sector carbon dioxide emissions increased by 11 million metric tons, from 226 million metric tons in 1996 to 237 million metric tons of carbon in 1997 (Table 12). Between 1990 and 1997, the commercial sector accounted for 30 million tons of carbon, or about 23 percent of the total increase in U.S. carbon dioxide emissions from the energy end-use sectors. Commercial emissions have risen the most on a percentage basis and are up a total of 15 percent over 1990. Electric Utilities Although end users create the demand for electricity, electricity producers (primarily electric utilities) make decisions about how to meet that demand, based on fuel prices and capacity availability. In 1997, sales of electric power increased by only 0.6 percent, but utility carbon dioxide emissions increased by about 3.7 percent (Table 13), because coal-fired generation met a larger share of the demand for electricity. Despite utility efforts to improve generation efficiency and demand-side management programs that have kept emissions lower than they otherwise would have been, carbon dioxide emissions from the burning of fossil fuels to meet end-use demand accounted for an increase of 55 million metric tons of carbon between 1990 and 1997. Coal, which in recent years has fueled about one-half of all U.S. electric power generation, produces more carbon dioxide emissions per kilowatthour of electricity generated than do other fossil fuels. The existence of underused coal-fired capacity in the United States may be contributing to the current increase in coal use.
Industrial Sources Recent Trends Industrial emissions not caused by the combustion of fossil fuels accounted for only about 1.9 percent of total U.S. carbon dioxide emissions in 1997. Emissions from these sources depend largely on the level of activity in the construction industries and on production at oil and gas wells. These sources include limestone and dolomite calcination, soda ash manufacture and consumption, carbon dioxide manufacture, and aluminum production.
Energy Production When fossil fuels are burned, carbon dioxide is emitted to the atmosphere as a combustion product. In addition, oil and gas production leads to emissions of carbon dioxide from sources other than the combustion of commercial fuels, including the following:
The EIA has always estimated emissions from flaring of natural gas. This year, carbon dioxide emissions have been revised upward (and methane emissions have been revised downward) as a result of the assumption that essentially all the gas reported as "vented and flared" is actually flared. Emissions from this source are less than 5 million metric tons of carbon annually. The EIA has also, for the first time this year, estimated the quantities of carbon dioxide scrubbed from natural gas and released to the atmosphere, by computing the difference between the estimated carbon dioxide content of raw gas and the carbon dioxide content of pipeline gas. Estimated emissions from this source are about 4 million metric tons of carbon annually. Emissions of carbon dioxide from enhanced oil recovery are excluded from this report because of a lack of information. The EIA believes that most of the carbon dioxide recovered with the oil is recycled, so that annual emissions are a fraction of the carbon dioxide recovered, which in turn is probably less than the volume of carbon dioxide injected. Emissions from this source may be included in future reports. Combustion of "off spec" gases and fuels is not covered as a separate line item in this report, but much of the emissions from this source may be included in the "flaring" category described above or as industrial consumption of "still gas" by refineries. Emissions from these energy production sources are conceptually separate, but they may overlap as a result of imprecision in the reporting of U.S. natural gas production and processing. Thus, emissions from these sources may include both undercounting and double counting to some degree, and the estimates should be considered more uncertain than those for emissions from reported energy consumption. Industrial Processes Carbon dioxide emissions from industrial processes account for about 17 to 19 million metric tons of carbon per year (Table 14). Since 1990, these emissions have increased, but higher emissions from cement manufacture and limestone consumption have been partially offset by lower emissions from aluminum manufacture. More than one-half of the emissions from industrial processes are from cement manufacture. When calcium carbonate is heated (calcined) in a kiln, it is converted to lime and carbon dioxide. The lime is combined with other materials to produce clinker (an intermediate product from which cement is made), and the carbon dioxide is released to the atmosphere. In 1997, the United States manufactured an estimated 81 million metric tons of cement, resulting in the release of carbon dioxide containing about 10 million metric tons of carbon into the atmosphere. There are numerous other industrial processes in which carbonate minerals are used in ways that release carbon dioxide into the atmosphere, including the use of limestone in flue gas desulfurization and the manufacture and some uses of soda ash. Approximately 5 million metric tons of carbon per year are contained in emissions from these sources. Carbon dioxide is also released during aluminum smelting, when carbon anodes (with the carbon derived from nonfuel use of fossil fuels) are vaporized in the presence of aluminum oxide. Adjustments to Energy Consumption Under the Framework Convention on Climate Change, parties to the agreement committed to providing information on emissions trends, using methods that would facilitate international comparison of emissions estimates. To support such comparisons, a generalized reporting format was adopted. The format differs slightly from that used in the preparation of U.S. national energy statistics, primarily with respect to the method of calculating U.S. energy consumption, as well as its scope with regard to U.S. territories and bunker fuels for international transport. EIA's energy data for the United States cover the 50 States and the District of Columbia but not the U.S. territories. Bunker fuels (fuel consumed by ships and aircraft fueled in the United States and engaged in international trade) are subsumed in EIA's transportation sector energy consumption data. In contrast, energy data used by the International Energy Agency for the United States include U.S. territories and exclude bunker fuels. Collectively, these differences in treatment can produce variations of several percentage points in reported energy consumption and, hence, in estimates of carbon dioxide emissions. Therefore, the EIA subtracts the bunker fuel emissions and adds the emissions of U.S. territories as "adjustments to energy" in order to make this estimate of emissions conform more closely to international emissions accounting guidelines. U.S. Territories In this report, carbon dioxide emissions for the U.S. territories (Puerto Rico, Virgin Islands, Guam, American Samoa, Micronesia, and Wake Island) are included as an adjustment. Their combined energy consumption is only about 0.5 quadrillion Btu and is concentrated on petroleum products; only Puerto Rico uses coal. Together, they are estimated to have emitted almost 12.3 million metric tons of carbon in 1997 (Table 6). Bunker Fuels In this report, emissions from bunker fuels are subtracted from the estimates of carbon dioxide emissions from energy consumption in keeping with the IPCC method. The estimate is based on purchases of fuel by ocean-going ships in U.S. ports and by international air carriers in U.S. airports. In 1997, bunker fuel emissions of carbon dioxide produced about 19 million metric tons of carbon (Table 6).
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