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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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Introduction Total U.S. emissions of greenhouse gases in 1997 increased by 1.4 percent from their 1996 level. Overall, U.S. emissions are now about 10 percent higher than they were in 1990. The expansion in 1997 is a return to earlier trends after the unusual growth in 1996 emissions (up by a revised 2.8 percent from the 1995 level),(1) which was caused primarily by severe weather in 1996 (see Table ES1). Since 1990, U.S. emissions have increased at a compounded annual rate of about 1.3 percent, slightly faster than population (1.1 percent) but more slowly than energy consumption (1.7 percent), electricity consumption (2.0 percent), or gross domestic product (GDP) (2.3 percent). Table ES1 shows trends in emissions of the principal greenhouse gases, measured in million metric tons of gas. In Table ES2, the value shown for each gas is weighted by its global warming potential (GWP), which is a measure of "radiative forcing." This concept, developed by the Intergovernmental Panel on Climate Change (IPCC), provides a comparative measure of the impacts of different greenhouse gases on global warming, with the effect of carbon dioxide being equal to 1 (see "Units for Measuring Greenhouse Gases" on page 6).(2) The GWPs for other greenhouse gases are considerably higher (see discussion in Chapter 1). Most (82 percent) of U.S. greenhouse gas emissions are caused by the combustion of fossil fuels such as coal, petroleum, and natural gas. Consequently, U.S. emissions trends are largely caused by trends in energy consumption. In recent years, national energy consumption, like emissions, has grown relatively slowly, with year-to-year fluctuations caused (in declining order of importance) by weather-related phenomena, business cycle fluctuations, and developments in domestic and international energy markets. Other U.S. emissions include carbon dioxide from noncombustion sources (2 percent of total U.S. greenhouse gas emissions), methane (9 percent), nitrous oxide (5 percent), and other gases (2 percent) (Figure ES1). Methane and nitrous oxide emissions are caused by the biological decomposition of various waste streams, fugitive emissions from chemical processes, fossil fuel production and combustion, and many smaller sources. The other gases include hydrofluorocarbons (HFCs), used primarily as refrigerants, perfluorocarbons (PFCs), released as fugitive emissions from aluminum smelting and also used in semiconductor manufacture, and sulfur hexafluoride, used as an insulator in utility-scale electrical equipment. The Kyoto Protocol, drafted in December 1997, raised the public profile of climate change issues in the United States in general, and of emissions estimates in particular. Emissions inventories are the yardstick by which the success or failure in complying with the Kyoto Protocol would be measured. This report, required by section 1605(a) of the Energy Policy Act of 1992, provides estimates of U.S. emissions of greenhouse gases, as well as information on the methods used to develop the estimates. Carbon Dioxide Carbon dioxide accounts for 84 percent of U.S. greenhouse gas emissions. Carbon dioxide emissions originate almost entirely from fossil energy consumption and are influenced by the interaction of three factors:
Figure ES2 illustrates some recent U.S. trends in these areas. Emissions per dollar of GDP and emissions per capita are crude measures of the carbon intensity of the use of energy services. U.S. emissions per capita, which declined in the early 1980s, have risen in the 1990s, although at a relatively low rate. Emissions per dollar of GDP have declined almost every year. On the other hand, some of the indicators of carbon intensity have begun to increase, particularly, emissions per kilowatthour of electric power generation. During the early 1990s, several unrelated factors combined to lower the carbon intensity of power generation, including the expansion of natural-gas-fired generation caused by relatively low natural gas prices and better nuclear power plant operating rates. Over the past 2 years, however, the trends for some of those factors have reversed. Several nuclear power plants have been shut down since 1995, and nuclear generation declined by about 7 percent between 1996 and 1997; natural gas prices have risen, with the result that utilities have turned increasingly to existing coal plants for power generation. Figure ES3 illustrates trends in carbon dioxide emissions by energy consumption sector. Emissions from the industrial sector dropped substantially in the early 1980s as higher energy prices induced industry to adopt energy-efficient technologies. Emissions from other sectors also dropped slightly in the early 1980s. In the late 1980s, however, emissions rose consistently as energy prices dropped dramatically and the economy grew. In 1990, somewhat higher energy prices induced an economic slowdown that was felt most strongly in 1991, with the result that emissions fell. Since 1991, emissions have grown consistently in all sectors, with the largest increases in the transportation and electric power sectors. Emissions in the industrial sector have grown relatively slowly, even during a vigorous economic expansion, due to energy efficiency improvements and low growth in energy-intensive industries. Methane Methane accounts for about 9 percent of U.S. GWP-weighted greenhouse gas emissions. In contrast to carbon dioxide, there is no clear trend in methane emissions: they appear to have remained roughly constant through the 1990s, or perhaps to have declined slightly. Methane emissions estimates are more uncertain than those for carbon dioxide, however, and correspondingly less confidence can be placed in the apparent trends. Methane emissions come from three categories of sources, each accounting for approximately one-third of U.S. methane emissions, or about 3 percent of the Nation's total greenhouse gas emissions. The largest of the three sources is the anaerobic decomposition of municipal solid waste in landfills (Figure ES4). Emissions from this source are declining (although very slowly) as a consequence of a reduction in the volume of waste landfilled and a gradual increase in the volumes of landfill gas captured for energy or flared. Methane is also emitted as a byproduct of fossil energy production and transport. Methane can leak from natural gas production and distribution systems and is also emitted as a consequence of coal production. Finally, domestic livestock management causes emissions from the animals and also from the anaerobic decomposition of their waste. Nitrous Oxide Nitrous oxide accounts for about 5 percent of U.S. GWP-weighted greenhouse gas emissions. Emissions estimates for nitrous oxide are more uncertain than those for either carbon dioxide or methane. The emissions estimate for nitrous oxide in this year's report is three times larger than the estimate last year, as a result of the implementation of the new IPCC methods for estimating nitrous oxide emissions. Estimated nitrous oxide emissions have been roughly constant in the 1990s, with no clear trend. The revised estimates of nitrous oxide emissions include one large class of sources and two small classes (Figure ES5). Agriculture is the major source, and agricultural emissions are dominated by emissions from nitrogen fertilization of agricultural soils. The emissions factor for this source has been raised somewhat, but the bulk of the increase has been caused by the counting of nitrogen in manure used as fertilizer and nitrogen in crop residues, in addition to nitrogen from chemical fertilizers. Also, following the revised IPCC guidelines, secondary nitrous oxide emissions from nitrogen in agricultural runoff into streams and rivers have been incorporated. Motor vehicles equipped with catalytic converters also emit significant amounts of nitrous oxide. Certain chemical processes, fuel combustion, and wastewater treatment plants are smaller sources of nitrous oxide emissions. Halocarbons and Other Gases The Kyoto Protocol specifies that emissions of several classes of engineered gases be limited: hydrofluorocarbons (HFCs), perfluorocarbons (PFCs), and sulfur hexafluoride (SF6). Emissions of these three classes of gases account for about 2 percent of U.S. GWP-weighted emissions. There are several other categories of chemicals that also qualify as greenhouse gases but are excluded from the Framework Convention on Climate Change and the Kyoto Protocol because they are already controlled under the Montreal Protocol on Ozone-Depleting Substances. They include chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and several solvents. Emissions of the gases included in the Kyoto Protocol have increased rapidly in the 1990s, but emissions of all of them are very small (at most a few thousand metric tons). On the other hand, many of the gases have atmospheric lifetimes measured in the hundreds or thousands of years, and consequently they are potent greenhouse gases with global warming potentials hundreds or thousands of times higher than that of carbon dioxide per unit of molecular weight. Land Use and Forestry Forest lands in the United States are net absorbers of carbon dioxide from the atmosphere. According to U.S. Forest Service researchers, U.S. forest land absorbs about 200 million metric tons of carbon, equivalent to almost 15 percent of U.S. carbon dioxide emissions. Absorption is enabled by the reversal of the extensive deforestation of the United States that occurred in the late 19th and early 20th centuries. Since then, millions of acres of formerly cultivated land have been abandoned and have returned to forest. The regrowth of forests is sequestering carbon on a large scale. The process is steadily diminishing, however, because the rate at which forests absorb carbon slows as the trees mature. The extent to which carbon sequestration should be included in emissions inventories generally, and the extent to which sequestration would "count" under the Kyoto Protocol, are still being determined. The Kyoto Protocol specifically limits "countable" effects for countries like the United States to anthropogenic afforestation, deforestation, and reforestation that has occurred "since 1990," and only if it is "measurable and verifiable." Each clause would probably limit the applicability of carbon sequestered as a result of land use changes and forestry.
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