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Executive Summary
Total U.S. emissions of greenhouse gases in 1999, estimated at 1,833 million metric tons carbon equivalent, were 0.8 percent higher than the estimated 1998 level of 1,818 million metric tons carbon equivalent. The increase was slightly lower than the 1.1-percent growth rate that has characterized total U.S. greenhouse gas emissions during the 1990s but significantly higher than the 0.1-percent growth between 1997 and 1998. By comparison, U.S. real gross domestic product (GDP) grew by 4.1 percent from 1998 to 1999. The relatively moderate increase in estimated greenhouse gas emissions in 1999 is attributable primarily to warmer-than-normal winter weather and to an increase in electricity generation from nuclear power plants. Overall, 1999 U.S. greenhouse gas emissions were about 10.7 percent higher than 1990 emissions, which are estimated at 1,655 million metric tons carbon equivalent. The 1.1-percent average annual growth in U.S. greenhouse gas emissions from 1990 to 1999 compares with average growth rates of 1.0 percent for the U.S. population, 1.5 percent for energy consumption, 2.2 percent for electric power generation, and 3.1 percent for real GDP. 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 one.1 U.S. greenhouse gas emissions generally follow trends in U.S. energy consumption. In 1999, for example, some 82 percent of U.S. greenhouse gas emissions consisted of carbon dioxide released by the combustion of energy fuels—coal, petroleum, and natural gas (Figure ES1). In recent years, national energy consumption, like emissions, has grown relatively slowly, with year-to-year fluctuations in the growth rate of energy consumption largely caused by variations in weather patterns, business cycles, fuel use for electricity generation, and domestic and international energy markets. Other 1999 U.S. greenhouse gas emissions include carbon dioxide from non-combustion sources (2 percent of total U.S. greenhouse gas emissions), methane (9 percent), nitrous oxide (6 percent), and other gases (2 percent) (Figure ES1). Methane and nitrous oxide emissions are caused by the biological decomposition of various waste streams and fertilizers, 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 (SF6), used as an insulator in electrical transmission and distribution equipment and as a cover gas in magnesium production and processing. Figure ES1. U.S. Greenhouse Gas Emissions by Gas, 1999 The Kyoto Protocol, drafted in December 1997 under the auspices of the United Nations Framework Convention on Climate Change, raised the public profile of climate change issues in the United States in general, and of emissions estimates in particular. Although the Protocol has yet to be ratified by enough nations, including the United States, to “enter into force,” emissions inventories are the common yardstick by which success or failure in reducing emissions—an objective of the Protocol and the Framework Convention on Climate Change— 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. U.S. carbon dioxide emissions in 1999 are preliminarily estimated at 1,527 million metric tons carbon equivalent—1.3 percent higher than in 1998 and accounting for 83 percent of total U.S. greenhouse gas emissions. Figure ES2 illustrates some recent U.S. trends in carbon dioxide emissions and energy consumption. Carbon dioxide emissions per dollar of GDP and carbon dioxide emissions per capita are approximate measures of the “carbon intensity” of energy use. Per capita carbon dioxide emissions declined in the early 1980s but rose in the 1990s at a relatively low rate of 0.3 percent per year. Carbon dioxide emissions per dollar of GDP have declined almost every year, averaging an annual 1.7-percent rate of decline during the 1990s. During the early 1990s, several unrelated factors—including improved nuclear power plant operating rates and relatively low natural gas prices leading to an expansion of natural-gas-fired electricity generation—combined to lower the carbon intensity of electric power generation (carbon dioxide emissions per unit of net generation). Figure ES3 illustrates trends in carbon dioxide emissions by energy consumption sector. In general, emissions have increased in each of the four sectors since 1990. An exception to the general upward trend was 1990-1991, when economic recession and higher oil prices following the Iraqi invasion of Kuwait led to a 0.9-percent decrease in national carbon dioxide emissions in 1991. Average annual growth rates in carbon dioxide emissions by sector during the 1990s were 1.8 percent for the commercial sector, 1.6 percent for the transportation sector, and 1.5 percent for the residential sector, all higher than the 1.4-percent average for total U.S. carbon dioxide emissions during the decade. For the industrial sector, however, annual growth in carbon dioxide emissions averaged only 0.6 percent during the 1990s. Industrial sector carbon dioxide emissions, which are relatively sensitive to economic fluctuations, declined by 2.4 percent in 1991 during the economic recession and dipped again in 1998 in the wake of the Asian economic slowdown. Figure ES3. U.S. Carbon Dioxide Emissions by Sector, 1990-1999 Carbon dioxide emissions from the U.S. electric power sector in 1999 are estimated at 614 million metric tons carbon equivalent—1.0 percent higher than the 1998 level. The 1999 increase is less than half the 1990-1999 average increase of 2.1 percent per year. Contributing to the relatively small increase in 1999 was an 8.1-percent increase in electricity generation from nuclear power plants relative to their 1998 output.2 Electricity-related emissions in the residential sector were 1.2 percent lower than in 1998, and in the commercial sector they were 1.4 percent lower.3 Although summer cooling degree-days were 2.9 percent above normal in 1999, air conditioning usage was lower than in 1998, when cooling degree-days were 18.2 percent above normal. In addition to electricity-related emissions, direct use of energy fuels in the residential, commercial, industrial, and transportation sectors produces carbon dioxide emissions. In the residential and commercial sectors, consumption of winter heating fuels, particularly natural gas, was higher in 1999 than in 1998 as a result of winter weather that was 7.4 percent colder than in 1998.4 Carbon dioxide emissions from the direct combustion of fuels (primarily natural gas) increased by 3.9 percent in the residential sector and by 2.6 percent in the commercial sector. Overall, carbon dioxide emissions in the residential and commercial sectors, at a combined 534 million metric tons carbon equivalent and 35 percent of total emissions, grew by 0.1 percent in 1999. Energy-related carbon dioxide emissions in the industrial sector in 1999 are estimated at 481 million metric tons carbon equivalent—0.2 percent higher than in 1998. The relatively small increase is noteworthy because, historically, industrial energy consumption and carbon emissions have been more sensitive to economic growth than to the weather, and 1999 was a year of rapid economic growth (4.1 percent). Industrial energy consumption and emissions are concentrated in a few industries, however, and their performance may have more influence on emissions than does the performance of the industrial sector as a whole. Six industry groups—petroleum refining, chemicals and related products, primary metals, paper, food, and stone, clay and glass—collectively account for 87 percent of manufacturing energy consumption and 81 percent of carbon dioxide emissions in the industrial sector. In 1999 the six energy-intensive industry groups appeared to be still recovering from downturns from their 1997 growth rates. Their 1999 annual growth rates were lower than those for the overall economy (4.1 percent), the industrial sector (3.6 percent), and the manufacturing component of industrial production (4.3 percent). For the six energy-intensive industries, 1999 growth rates were 0.8 percent (primary metals), 2.1 percent (chemicals), 1.1 percent (paper), 2.9 percent (stone, clay and glass), 1.3 percent (petroleum), and 0.7 percent (food). The industries that grew rapidly in 1999 were primarily those with lower energy intensities, including computer equipment, which grew by 57 percent, and semiconductors and related components, which grew by 48 percent.5 In addition, the relatively mild weather in 1999 moderated energy consumption in industries whose energy consumption is affected by the weather, such as farming. Carbon dioxide emissions in the transportation sector, at 496 million metric tons carbon equivalent, were 2.9 percent higher in 1999 than in 1998. Gasoline consumption, which accounted for 60 percent of transportation sector emissions, grew by 2.1 percent. Emissions from jet fuel use grew by 3.1 percent, and emissions from residual fuel (used mostly by oceangoing ships) grew by 14.6 percent. Emissions from distillate use increased by 3.8 percent, as a healthy U.S. economy led to greater consumption of diesel fuel by freight trucks. U.S. emissions of methane in 1999 were 1.8 percent lower than in 1998, at 28.8 million metric tons of methane or 165 million metric tons carbon equivalent. The decline resulted primarily from an increase in methane recovery for energy use at landfills and, to a lesser extent, from reductions in emissions from animal waste, coal mining, and petroleum systems.6 To be eligible for the tax credit included in Section 29 of the Windfall Profits Tax Act of 1980, methane recovery systems at landfills must have been operational by June 30, 1998. As the last recovery projects installed by the tax credit deadline came on line full time in 1999, methane recovery for energy at U.S. landfills rose from 1.7 million metric tons in 1998 to 2.1 million metric tons. In addition, total U.S. coal production fell in 1999 for the first time in 5 years due to a drop in coal exports and nearly flat demand from electric utilities,7 lowering methane emissions from coal mining and post-mining activities by about 0.2 million metric tons. Domestic oil production also declined in 1999, decreasing methane emissions from petroleum systems. The drop in methane emissions from the solid waste of domesticated animals was the result of a decrease in swine populations. Methane emissions come from four categories of sources, three major and one minor. The major sources are energy, waste management, and agriculture, and the minor source is industrial processes. The three major sources accounted for 37, 32, and 31 percent, respectively, of total 1999 U.S. emissions of methane, or approximately 9 percent of the Nation’s total carbon-equivalent greenhouse gas emissions. The largest of the three major sources was energy, followed by waste management (Figure ES4). Emissions from the anaerobic decomposition of municipal solid waste in landfills, part of the waste management source category, had been declining slowly before 1999 as a consequence of a reduction in the volume of waste landfilled and a gradual increase in the volumes of landfill gas captured. Emissions of methane resulting from waste management declined by 3.7 percent in 1999. Figure ES4. U.S. Methane Emissions by Source, 1990-1999 (Million Metric Tons Methane) 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 during coal production. Energy-related methane emissions fell by 1.2 percent in 1999. Agricultural emissions have several sources but are dominated by emissions from domestic livestock, including the animals themselves and the anaerobic decomposition of their waste. Agricultural emissions fell by about 0.4 percent in 1999. Methane emissions estimates are more uncertain than those for carbon dioxide. U.S. methane emissions do not necessarily increase with growth in energy consumption or the economy. Energy-related methane emissions are strongly influenced by coal production from a relatively restricted number of mines; agricultural emissions are influenced in part by the public’s consumption of milk and beef and in part by animal husbandry practices; and livestock and municipal waste emissions are influenced by husbandry and waste management practices. U.S. nitrous oxide emissions increased by 0.1 percent in 1999 compared with 1998, although the rounded total remained at 103 million metric tons carbon equivalent. Nitrous oxide accounts for 6 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, because nitrous oxide is not systematically measured and many sources of nitrous oxide emissions, including nitrogen fertilization of soils and motor vehicles, require a significant number of assumptions to arrive at estimated emissions. U.S. nitrous oxide emissions include one large class of sources and two small classes (Figure ES5). Agricultural sources account for about 71 percent of nitrous oxide emissions, and emissions associated with nitrogen fertilization of soils account for about three-quarters of agricultural emissions. In 1999, estimated nitrous oxide emissions from nitrogen fertilization of soils fell by 0.5 percent. Emissions associated with fossil fuel use account for another 23 percent of nitrous oxide emissions, of which about 82 percent comes from mobile sources, principally motor vehicles equipped with catalytic converters. The balance of nitrous oxide emissions are caused by certain chemical manufacturing and wastewater treatment processes. The most striking trend in U.S. nitrous oxide emissions has been a 51-percent decline from 1996 levels of industrial emissions of nitrous oxide after the implementation of emissions controls at an adipic acid plant operated by the DuPont Corporation. Figure ES5. U.S. Nitrous Oxide Emissions by Source, 1990-1999 Hydrofluorocarbons, Perfluorocarbons, and Sulfur Hexafluoride The Kyoto Protocol specifies that greenhouse gas emissions, including several classes of engineered gases— HFCs, PFCs, and SF6—be reduced. Emissions of these three classes of gases account for about 2 percent of U.S. GWP-weighted emissions of greenhouse gases. At 37.6 million metric tons carbon equivalent in 1999, their emissions were about 5 percent lower than in 1998, the first drop in emissions of such gases since a decline from 24.0 million metric tons carbon equivalent in 1990 to 22.1 million metric tons carbon equivalent in 1991. The 1999 decline in emissions of the engineered gases was caused almost entirely by a drop in emissions of two gases, HFC-23 and SF6. HFC-23 is emitted primarily as a byproduct during the production of HCFC-22; SF6 is used as an insulator for electrical transmission and distribution equipment and as a cover gas in magnesium production and processing. The drop in 1999 may also be attributable in part to maturing markets for chlorofluorocarbon substitutes, stagnant markets for key high-GWP gases, and increasing awareness of the potential for recycling these gases. Emissions of the high-GWP gases specified in the Kyoto Protocol are very small (at most a few thousand metric tons). On the other hand, some of the gases (including PFCs and SF6) have atmospheric lifetimes measured in the hundreds or thousands of years, and consequently they are potent greenhouse gases with GWPs hundreds or thousands of times higher than that of carbon dioxide per unit of molecular weight. However, some of the commercially produced HFCs (134a, 152a, 4310, 227ea), which are used as chlorofluorocarbon (CFC) replacements, have shorter atmospheric lifetimes of 1 to 36 years. At 21.2 million metric tons carbon equivalent, emissions of HFCs make up the majority of this category, followed by SF6 at 8.9 million metric tons carbon equivalent and PFCs at 4.9 million metric tons carbon equivalent. Another group of engineered gases, consisting of other HFCs, other PFCs, and perfluoropolyethers (PFPEs), includes HFC-152a, HFC-227ea, HFC-4310mee, and a variety of PFCs and PFPEs. They are grouped together in this report to protect confidential data. In 1999 their combined emissions totaled 2.6 million metric tons carbon equivalent. With the exception of the “other” group, emissions of the three major classes were lower in 1999 than in 1998, with HFCs falling by 3.8 percent, PFCs by 4.2 percent, and SF6 by 12.7 percent. Since 1990, HFC emissions from U.S. sources have increased by 116.6 percent, PFC emissions have decreased by 25.1 percent, and SF6 emissions have increased by 18.1 percent. 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 211 million metric tons of carbon annually, equivalent to almost 14 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, with the regrowth of forests 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, and because the rate of reforestation has slowed. Over the past several years there has been increasing interest in the United States regarding carbon sequestration in agricultural soils through changes in agricultural practices. Proponents suggest that changes in tillage practices can cause agricultural soils to move from being net sources to net sinks of carbon dioxide, and that the amounts of carbon that might be absorbed by these changes could be significant at the national level. At present, the Energy Information Administration does not have sufficient information to permit reliable estimation of national-level emissions or sequestration from this source. As more reliable information becomes available, soil carbon estimates will be included in future reports. If you would like to receive any information relating to any of our greenhouse gas reports via e-mail, click here and subscribe by entering your e-mail address.
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