al CO2 emissions roughly doubled (Figure 19). Almost all of these emissions originated from the burning of fossil fuels: 44 percent from oil and oil products, 37 percent from coal, and 19 percent from natural gas. Small amounts of CO2 are also emitted from other activities, including cement production (about 0.6 percent), landfills, and domestic sewage. Figure 20 breaks down U.S. emissions from fossil fuel burning.

Some of the CO2 emitted from human activities remains in the atmosphere, while some is absorbed by the ocean or is stored in soils, vegetation, and other carbon "sinks." In U.S. forests, carbon uptake currently exceeds emissions. The removal of trees by timber harvesting, land-use conversion, and fuelwood use emits roughly 1,300 MMTs of CO2. (All estimates in forestry and agriculture are assigned an error range of +/- 50 percent because they are so uncertain. Research to improve these estimates continues.)

Other sources of carbon emissions not yet quantified include emissions due to flooding of land, conversion of grasslands to cultivated lands, and forest death and decline from air pollution. U.S. forests absorbed an estimated 1,690 MMTS of CO, in 1988, bringing net CO2 uptake removal from forests to 188-563 MMTs of CO2 (4-11 percent of fossil energy emissions). In contrast, land conversion globally, primarily tropical deforestation, has led to a net average annual flux of 2,200-9,500 MMTs of CO2 (0.6-2.6 billion metric tons of carbon) to the atmosphere over the past decade (IPCC 1992a). (This last estimate does not include all carbon uptake in terrestrial systems.)

Methane

CH4 is produced primarily through the anaerobic (without oxygen) decomposition of organic matter. Table 8 shows the contributions from each major U.S. source in 1988 and the large range of uncertainty in the estimates. It does not include net changes in CH, emissions due to net changes in the acreage of wetlands.

Nitrous Oxide

For 1988, U.S. N2O emissions were estimated to be 0.34-1.21 MMTs, but this estimate is highly uncertain. Anthropogenic (produced by human activities) NO emissions come from the use of nitrate ammonium and organic fertilizers, fossil fuel burning, biomass burning, and soil emissions from agricultural management (e.g., irrigation) and land conversion. Fertilizer nitrates can also leach into ground water, which releases N2O to surface-water systems. Industrial processes are another large source of N2O emissions. For example, the United States produces about one-third of the world's nylon, which resulted in 0.25 MMTS of N2O emissions in 1988 (U.S. EPA 1992). Fertilizer use contributed an estimated 0.05-0.85 MMTs, while energy burning for transportation accounted for 0.04-0.11 MMTs. Data on emissions from fossil fuel burning in stationary sources (such as electric utilities) and land conversion are not currently available, but are expected in 1993.

Chlorofluorocarbons

CFCs are solely a product of the chemical industry. They do not occur naturally in the environment. CFCs are used for a wide range of purposes, including refrigeration, solvent cleaning, sterilization, and insulation. CFC-11 and CFC-12, the most abundant halocarbons in the global atmosphere, are used in blowing plastic foams, in aerosol cans, and in refrigeration (Lashof and Tirpak 1990).

In 1988, the United States emitted roughly 0.7 MMTs of halocarbons, including CFCs (Table 9). Under the Clean Air Act Amendments of 1990, the United States will phase out CFC production by 1996.

Source: U.S. Government 1991

Nitrogen Oxides

X

NO, is an important greenhouse gas precursor because it contributes to the formation of ozone, greenhouse gas, in the lower atmosphere (troposphere). NO, is formed primarily when fuel is burned at high temperatures. In 1988, the United States emitted 19.6 MMTs of NO,, 38 percent of which came from electric utilities. Fuel use in automobiles and trucks accounted for 32 percent, and industrial energy use for 16 percent. Because NO emissions, along with CO and nonmethane VOC emissions, are important components of air pollution and precursors of photochemical smog, they have been controlled in the United States during the past several decades. As a result, the emissions of these gases per unit of industrial activities and per vehicle miles driven in the United States are among the lowest in the world. From 1970 to 1980, NO, emissions from electric utilities and highway vehicles grew by 50 percent. Though emissions from electric utilities stayed constant over the next decade, those from highway vehicles fell by over 40 percent.

Carbon Monoxide

Over 90 percent of the 53.4 MMTS of CO emitted in 1988 can be attributed to burning fuel. When fuel is not completely burned, a portion is emitted as CO, rather than CO2. For industrial sources, with generally efficient equipment, the emission of CO relative to CO2 is very small. The largest contributor of CO emissions is transportation (about three-fourths of all emissions). Half of transportation CO emissions come from gasoline use in passenger cars. More efficient and less polluting engines have reduced the ratio of CO emissions to vehicle miles driven by half over the past decade (U.S. EPA 1991). Residential fuel use accounts for about 12 percent of CO emissions, and industrial processes account for the remainder.

Nonmethane VOCS

Human-induced nonmethane volatile organic compounds are emitted by the evaporation of fuels and of solvents in products used by industry, commercial

Table 9

CFC-11

Carbon Methyl

Source: U.S. Government 1991

establishments, and consumers.2 During 1988, industrial processes accounted for 51 percent of anthropogenic nonmethane VOC emissions. The largest contributors include surface coating, petroleum product storage and transfer, production of organic chemicals, and petroleum refining.

Energy use for transportation accounted for 43 percent of human-induced nonmethane VOC emissions

four-fifths of which came from gasoline use in passenger cars. Motor vehicle controls have reduced these emissions by two-thirds since 1970. Between 1981 and 1990, emissions from highway vehicles decreased by 34 percent, despite a 37 percent increase in vehicle miles traveled during this period (U.S. EPA 1991). The remaining nonmethane VOC emissions were due to other energy combustion processes.

The U.S. greenhouse gas inventory for 1988, produced in 1991, provides a starting point for developing a more detailed and comprehensive national inventory. The U.S. government expects to update and improve the estimates presented in this chapter over time. Knowledge of the sources, emission rates, and removal of greenhouse gases is evolving rapidly. Priorities for further analysis are:

• Improved understanding about how emissions are produced. More scientific research is needed to develop methods for estimating emissions from new source and sink categories and to refine existing methodologies. Sources and sinks of methane emissions may be the most important area needing research, particularly emissions from waste management, ruminant animals, and rice production. Also needed is additional study of how different types of land conversion and management affect net emission fluxes, including emissions from soils.

• Refinement of estimates of the rate of emissions or removal from a given activity. Current estimates of rates of emission or removal are often based on measurements under a narrow, nonrepresentative set of conditions. New measurement efforts are needed in the following areas: the rates of CH, released under dif

ferent landfill conditions, NO formation from energy combustion and fertilizer applications, leakage from oil and gas systems, emission changes due to land conversion, the carbon content of different. soils and vegetation, and the CH, content in the coal seam before mining.

• Better information on activities producing emissions. Important areas where data need improvement include the amount and types of land undergoing specific types of conversion; equipment type and emission control technology in oil and gas systems; and the fates of harvested carbon (for paper, housing, etc.) and its effects on emissions or storage over time. Complete estimates in all source categories. Not all sources and sinks of emissions have yet been quantified, either because data are incomplete or because methods for estimating emissions do not yet exist. Priorities include emissions from some types of land conversion, such as those due to flooding of land, conversion of grasslands to cultivated lands, and forest death and decline from air pollution; CH, emissions from certain oil and gas production activities; CO2 emissions from oil and gas wells and from some industrial processes; and NO emissions from fertilizer leaching and runoff.