pathways further summarize the emission estimates, and explain the relative importance of emissions from each source category. Global Warming Potentials Gases in the atmosphere can contribute to the greenhouse effect both directly and indirectly. Direct effects occur when the gas itself is a greenhouse gas; indirect radiative forcing occurs when chemical transformations of the original gas produce a gas or gases that are greenhouse gases, or when a gas influences the atmospheric lifetimes of other gases. The concept of a Global Warming Potential (GWP) has been developed to compare the ability of each greenhouse gas to trap heat in the atmosphere relative to another gas. Carbon dioxide was chosen as the reference gas to be consistent with IPCC guidelines. Global Warming Potentials are not provided for the criteria pollutants CO, NO,, NMVOCs, and SO, because there is no agreed upon method to estimate the Box ES-2: Greenhouse Gas Emissions from Transportation Activities Motor vehicle usage is increasing all over the world, including in the United States. Since the 1970s, the number of highway vehicles registered in the United States has increased faster than the overall population, according to the Federal Highway Administration (FHWA). Likewise, the number of miles driven up 21 percent from 1990 to 1998 and gallons of gasoline consumed each year in the United States have increased relatively steadily since the 1980s, according to the FHWA and Energy Information Administration, respectively. These increases in motor vehicle usage are the result of a confluence of factors including population growth, economic growth, increasing urban sprawl, and low fuel prices. One of the unintended consequences of these changes is a slowing of progress toward cleaner air in both urban and rural parts of the country, Passenger cars, trucks, motorcycles, and buses emit significant quantities of air pollutants with local, regional, and global effects. Motor vehicles are major sources of carbon monoxide (CO), carbon dioxide (CO2), methane (CH,), nonmethane volatile organic compounds (NMVOCs), nitrogen oxides (NO), nitrous oxide (N20), and hydrofluorocarbons (HFCs). Motor vehicles are also important contributors to many serious air pollution problems, including ground-level ozone (ie., smog), acid rain, fine particulate matter, and global warming. Within the United States and abroad, government agencies have taken actions to reduce these emissions. Since the 1970s, the EPA has required the reduction of lead in gasoline, developed strict emission standards for new passenger cars and trucks, directed states to enact comprehensive motor vehicle emission control programs, required inspection and maintenance programs, and more recently, introduced the use of reformulated gasoline New vehicles are now equipped with advanced emissions controls, which are designed to reduce emissions of nitrogen oxides, hydrocarbons, and carbon monoxide. Table ES-4 summarizes greenhouse gas emissions from all transportation-related activities. Overall, transportation activities excluding international bunker fuels accounted for an almost constant 26 percent of total U.S. greenhouse gas emissions from 1990 to 1998. These emissions were primarily CO2 from fuel combustion, which increased by 11 percent from 1990 to 1998. However, because of larger increases in N20 and HFC emissions during this period, overall emissions from transportation activities actually increased by 13 percent. • Aircraft emissions consist of emissions from all jet fuel (less bunker fuels) and aviation gas consumption. "Other" CO2 emissions include motorcycles, construction equipment, agncultural machinery, pipelines, and lubricants. * Emissions from international Bunker Fuels include emissions from both civilian and miltary activities, but are not included in totals. • "Other" CH, and N20 emissions include motorcycles, construction equipment, agricultural machinery, gasoline-powered recreational, industrial, lawn and garden, light commercial, logging, airport service, other equipment; and diesel-powered recreational, industrial, lawn and garden, light construction, airport service. * Includes primanly HFC-134a Box ES-3: Greenhouse Gas Emissions from Electric Utilities Like transportation, activities related to the generation, transmission, and distribution of electricity in the United States result in significant greenhouse gas emissions. Table ES-5 presents greenhouse gas emissions from electric utility-related activities. Aggregate emissions from electric utilities of all greenhouse gases increased by 16 percent from 1990 to 1998, and accounted for a relatively constant 29 percent of U.S. greenhouse emissions during the same period. The majority of these emissions resulted from the combustion of coal in boilers to produce steam that is passed through a turbine to generate electricity. Overall, the generation of electricity results in a larger portion of total U.S. greenhouse gas emissions than any other activity. Carbon Dioxide Emissions The global carbon cycle is made up of large carbon flows and reservoirs. Hundreds of billions of tons of carbon in the form of CO2 are absorbed by oceans and living biomass (sinks) and are emitted to the atmosphere annually through natural processes (sources). When in equilibrium, carbon fluxes among these various reservoirs are roughly balanced. Since the Industrial Revolution, this equilibrium of atmospheric carbon has been altered. Atmospheric concentrations of CO, have risen about 28 percent (IPCC 1996), principally because of fossil fuel combustion, which accounted for 98 percent of total U.S. CO2 cmissions in 1998. Changes in land usc and forestry practices can also emit CO2 (e.g., through conversion of forest land to agricultural or urban use) or can act as a sink for CO2 (e.g, through net additions to forest biomass). Emissions from nonutility generators are not included in these estimates. Nonutilties were estimated to produce about 10 percent of the electricity generated in the United States in 1998 (DOE and EPA 1999). Figure ES-6 and Table ES-7 summarize U.S. sources and sinks of CO2. The remainder of this section then discusses CO2 emission trends in greater detail. Energy Energy-related activities accounted for almost all U.S. CO, emissions for the period of 1990 through 1998. Carbon dioxide from fossil fuel combustion was the dominant contributor. In 1998, approximately 85 percent of the energy consumed in the United States was produced through the combustion of fossil fuels. The remaining 15 percent came from other energy sources such as hydropower, biomass, nuclear, wind, and solar (see Figure As fossil fuels are combusted, the carbon stored in them is almost entirely emitted as CO2. The amount of carbon in fuels per unit of energy content varies significantly by fuel type. For example, coal contains the highest amount of carbon per unit of energy, while petroleum has about 25 percent less carbon than coal, and natural gas about 45 percent less. From 1990 through 1998, petroleum supplied the largest share of U.S. energy demands, accounting for an average of 39 percent of total energy consumption. Natural gas and coal followed in order of importance, accounting for an average of 24 and 22 percent of total energy consumption, respectively Most petroleum was consumed in the transportation sec tor, while the vast majority of coal was used by electric utilities, and natural gas was consumed largely in the industrial and residential sectors. Emissions of CO, from fossil fuel combustion increased at an average annual rate of 1.3 percent from 1990 to 1998. The fundamental factors behind this trend include (1) a robust domestic economy, (2) relatively low energy prices, and (3) fuel switching by electric util.ties. After 1990, when CO2 emissions from fossil fuel combustion were 1,320.1 MMTCE, there was a slight decline in emissions in 1991, due in large part to an economic recession, followed by a relatively steady in Table ES-7: U.S. Sources of CO2 Emissions and Sinks (MMTCE) Source Fossil Fuel Combustion Cement Manufacture Natural Gas Flaring Lime Manufacture Waste Combustion Limestone and Dolomite Use Soda Ash Manufacture and Consumption Carbon Dioxide Consumption Land-Use Change and Forestry (Sink) Total Emissions Net Emissions (Sources and Sinks) 3 (316.4) (316.3) 32.2 32.7 30.0 27.2 26.7 275 27.9 29.9 31.3 1,340.3 1,326.1 1,350.4 1,383.3 1,404.8 1,416.5 1,466.2 1,486.4 1,494.0 1,023.9 1,009.8 1,034.2 1,170.6 1,192.5 1,204.7 1,254:9 1,275.3 1,283.2 * Sinks are only included in net emissions total. Estimates of net carbon sequestration due to land-use change and forestry activities exclude ron-forest soils, and are based partially upon projections of forest carbon stocks. • Emissions from International Bunker Fuels are not included in totals Note: Totals may not sum due to independent rounding. crease to 1,468.2 MMTCE in 1998. Overall, CO2 emissions from fossil fuel combustion increased by 11 pcr. cent over the nine year period and rose by 0.5 percent in the final year. In 1998, mild weather and low petroleum prices comprised the major forces affecting emission trends. A very mild winter more than offset the effects of a slightly hotter summer, resulting in significantly lower fuel consumption for residential and commercial heating compared to previous years. Emissions from the combustion of petroleum products grew the most (11.5 MMTCE or 1.9 percent) duc in large part to low prices. Alone, emissions from the combustion of petroleum by electric utilities increased by 7.3 MMTCE (42 percent) from 1997 to 1998. Emissions from the combustion of coal in 1998 increased by 5.5 MMTCE (1 percent) from the previous year, driven almost entirely by increased emissions by electric utilities. These increases were offset by a decrease in natural gas combustion emissions in every sector (9.1 MMTCE or 3 percent). The four end-use sectors contributing to CO2 emissions from fossil fuel combustion include: industrial, transportation, residential, and commercial. Electric utilities also emit CO2, although these emissions are produced as they consume fossil fuel to provide electricity to one of the four end-use sectors. For the discussion below, electric utility emissions have been distributed to each end-use sector based upon their fraction of aggregate electricity consumption. This method of distributing emissions assumes that each end-use sector consumes electricity that is generated with the national average mix of fuels according to their carbon intensity. In reality, sources of electricity vary widely in carbon intensity. By giving equal carbon-intensity weight to each sector's electricity consumption, for example, emissions attributed to the residential sector may be overestimated, while emissions attributed to the industrial sector may be underestimated. Emissions from electric utilities are addressed separately after the end-usc sectors have been discussed. Emissions from U.S. territories are also calculated separately due to a lack of end-use-specific consumption data. Table ES-8, Figure ES-9, and Figure ES10 summanze CO2 emissions from fossil fuel combustion by end-use sector. Industrial End-Use Sector. Industrial CO2 emissions resulting from direct fossil fuel combustion and from the generation of electricity consumed by the sector accounted for 33 percent of U.S. emissions from fossil fuel combustion in 1998. About two-thirds of these emissions resulted from producing steam and process heat from fossil fuel combustion, while the remaining third resulted from consuming electricity for powering motors, electric furnaces, ovens, and lighting. Transportation End-Use Sector. Transportation |