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19
Introduction

Various technologies and policy options have the potential to mitigate greenhouse warming. The Mitigation Panel was given the task of evaluating the effectiveness of these interventions, with the following specific charge:

• The panel should examine the range of policy interventions that might be employed to mitigate changes in the earth's radiation balance, assessing these options in terms of their expected impact, costs, and at least in qualitative terms, their relative cost-effectiveness.

• Preliminary evaluation will help identify policy interventions for closer examination. These might include reducing emissions in primary energy production or industrial processes, transportation vehicles and systems, or agricultural processes. They might include policies aimed at reducing energy consumption or changing practices in agriculture, silviculture, or general land use. Novel global system interventions, such as removal of greenhouse gases from the atmosphere, blocking of incident radiation, or altering of the earth's albedo, should not be excluded.

• Attention should be given to factors affecting the design and implementation of potential programs at the international and regional levels, including, as explicitly as feasible, organizations that should be involved and practical impediments. In performing this task, the panel should take into account any major relationship between the particular intervention and ecological or other problems apart from global climate change.

The panel defines "mitigation policy" as including programs and specific interventions that might reduce either the rate at which the radiative balance is changing or the ultimate level at equilibrium, assuming one is reached. Mitigation policies include not only interventions designed to reduce the emission of greenhouse gases but also actions such as reforestation (or



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Page 157 19 Introduction Various technologies and policy options have the potential to mitigate greenhouse warming. The Mitigation Panel was given the task of evaluating the effectiveness of these interventions, with the following specific charge: • The panel should examine the range of policy interventions that might be employed to mitigate changes in the earth's radiation balance, assessing these options in terms of their expected impact, costs, and at least in qualitative terms, their relative cost-effectiveness. • Preliminary evaluation will help identify policy interventions for closer examination. These might include reducing emissions in primary energy production or industrial processes, transportation vehicles and systems, or agricultural processes. They might include policies aimed at reducing energy consumption or changing practices in agriculture, silviculture, or general land use. Novel global system interventions, such as removal of greenhouse gases from the atmosphere, blocking of incident radiation, or altering of the earth's albedo, should not be excluded. • Attention should be given to factors affecting the design and implementation of potential programs at the international and regional levels, including, as explicitly as feasible, organizations that should be involved and practical impediments. In performing this task, the panel should take into account any major relationship between the particular intervention and ecological or other problems apart from global climate change. The panel defines "mitigation policy" as including programs and specific interventions that might reduce either the rate at which the radiative balance is changing or the ultimate level at equilibrium, assuming one is reached. Mitigation policies include not only interventions designed to reduce the emission of greenhouse gases but also actions such as reforestation (or

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Page 158 reducing deforestation), removal of radiatively active gases from the atmosphere, and altering the earth's albedo in ways that affect the earth's radiative balance. Sources of Greenhouse Gas Emissions This section provides a very brief summary of the magnitudes and sources of greenhouse gas emissions in order to suggest targets for mitigation strategies and some indication of the magnitude of the effort required. It is not intended to be a critical review, but relies on the recent summary compiled by the Intergovernmental Panel on Climate Change (1990, 1991). More information is available in the report of the Effects Panel (Part Two). The greenhouse gases include carbon dioxide (CO2), chlorofluorocarbons (CFCs), methane (CH4), nitrous oxide (N2O), ozone (O3), and water vapor. Although water vapor continually cycles through the atmosphere, if there is a change in atmospheric temperature, the mean water vapor concentration could change and provide an important positive feedback (i.e., magnify the temperature change). Other gases such as carbon monoxide (CO) and nitrogen oxides (NOx) are involved in chemical reactions in the atmosphere and affect the concentrations of greenhouse gases (in this case, O3). Greenhouse gas emissions come from both anthropogenic (man-made) and natural sources (such as CH4 from wetlands). Table 19.1 lists the primary greenhouse gases, the anthropogenic sources, and the relative contribution of each gas toward greenhouse warming. As shown in this table, CO2 is the single most important greenhouse gas worldwide, but others also make a significant contribution. Table 19.2 shows the current rates at which greenhouse gases are increasing worldwide. Figure 19.1 breaks down the current worldwide contributions to radiative forcing by source sector emissions during the 1980s. As shown here, energy use that generates emissions of CO2 and other greenhouse gases is the major greenhouse emission source. Table 19.3 shows a recent projection of global emissions from different sources for the years 2000, 2015, and 2050. As CFCs are phased out (presumably), under present international agreements, emissions from energy use are likely to dominate the anthropogenic influence on greenhouse warming. Even though CO2 contributes about half of the radiative forcing from increased atmospheric concentrations of greenhouse gases, Table 19.4 shows that once in the atmosphere, each molecule of the other greenhouse gases contributes more to global warming than does each molecule of CO2. For example, CFC-11 has, per molecule, 12,400 times the capacity of CO2 to trap heat. Worldwide, the United States is at present the largest emitter of greenhouse gases (World Resources Institute, 1990). As shown in Figure 19.2, the use of energy in the form of coal, oil, and natural gas is the largest

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Page 159 TABLE 19.1 Global Greenhouse Gases with Their Anthropogenic Emission Sources Greenhouse Gas Greenhouse Gas Contribution During the 1980sa (%) Anthropogenic Emission Sources Carbon Dioxide 56 Combustion of coal, oil, natural gas, and wood for use in electric utilities and for industrial, residential, and commercial use     Combustion of gasoline, diesel fuel, and other hydrocarbon fuels for automobiles, trucks, trains, aircraft, and ships; calcining of limestone during cement manufacture     Deforestation, which leads to a net decrease in the mass of terrestrial organic matter Methane 15 Decomposition of waste in landfills     Fossil fuel use, which results in emissions during coal mining, during exploration, production, and transportation of oil and natural gas, and via incomplete combustion of natural gas     Agricultural sources, including biomass burning, animal husbandry (cattle), and rice cultivation Chlorofluorocarbons 24 CFCs, which are used to make rigid and flexible foam, and as aerosol propellants, refrigerants, and industrial degreasers     Halons, which are used in fire extinguishers and as sterilants for some medical applications Nitrous oxide 5 Agricultural biomass burning, including use of wood as a fuel and forest clearing     Use of nitrogenous fertilizers and probably inadvertent fertilization through atmospheric nitrate deposition Tropospheric ozone — Generated from nitrogen oxides and carbon monoxide emitted aThe greenhouse contribution shown is the fractional contribution to the greenhouse gas alteration of the earth's radiation balance due to atmospheric concentration during the 1980s. The percent contribution is based on data from the IPCC Working Group I report (Intergovernmental Panel on Climate Change, 1990). Greenhouse gas emissions come from both anthropogenic (man-made) and natural sources (such as methane from wetlands). The contribution of tropospheric ozone to greenhouse warming is unknown at this time, according to the IPCC.

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Page 160 TABLE 19.2 Key Greenhouse Gases Influenced by Human Activity   CO2 CH4 CFC-11 CFC-12 N2O Preindustrial atmospheric concentration 280 ppmv 0.8 ppmv 0 0 288 ppbv Current atmospheric concentration (1990)a 353 ppmv 1.72 ppmv 280 pptv 484 pptv 310 ppbv Current rate of annual atmospheric accumulation 1.8 ppmv (0.5%) 0.015 ppmv (0.9%) 9.5 pptv (4%) 17 pptv (4%) 0.8 ppbv (0.25%) Atmospheric lifetime (years)b (50–200) 10 65 130 150 NOTES: Atmospheric lifetimes are computed as the ratio of the atmospheric burden to net annual removal, which is estimated as emissions less atmospheric accumulation. Net annual emissions of CO2 from the biosphere not affected by human activity are assumed to be small, as are volcanic emissions. Release and uptake from the biosphere not affected by human activity are included under emissions deriving from human activity. Emission estimates of human-induced emissions from the biosphere are controversial. Ozone has not been included in the table because of lack of precise data. Here, ppmv = parts per million by volume; ppbv = parts per billion by volume; and pptv = parts per trillion by volume. aThe 1990 concentrations have been estimated on the basis of an extrapolation of measurements that go through 1988 or 1989, assuming that the recent trends remained approximately constant. bFor each gas in the table, except CO2, the "lifetime" is defined as the ratio of the atmospheric content to the total rate of removal. This time scale also characterizes the rate of adjustment of the atmospheric concentrations if the emission rates are changed abruptly. CO2 is a special case because it is a thermodynamically stable gas that equilibrates with oceanic and biospheric processes. The lifetime shown does not indicate the lifetime of the gas molecules, but rather of the perturbation of atmospheric concentrations. SOURCE: Intergovernmental Panel on Climate Change (1990). Reprinted by permission of Cambridge University Press. anthropogenic source of CO2 emissions in the United States. Cement production, gas flaring, and land use change are relatively minor sources. Table 19.5 shows the history of U.S. emissions of CO2 since 1950, indicating the U.S. percentage has been cut in half over the last 30 years, although the total has almost doubled. The major sources of CH4 emissions (Figure 19.3) are solid waste (gas emissions from landfills), natural gas pipeline

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Page 161 image FIGURE 19.1 Estimated global contribution to radiative forcing by sector, 1980 to 1990. SOURCE: Intergovernmental Panel on Climate Change (1991). leakage and livestock. The emissions of N2O are much more difficult to estimate, and the principal cause of its increasing atmospheric concentration is unknown, but a rough approximation puts these emissions at approximately1.4 Mt/yr.1 This is determined by taking worldwide N2O emissions and scaling those emissions by the land area of the United States. With a variety of greenhouse gases being emitted to the atmosphere, it would be useful to have a single index of the relative greenhouse impact of the various gases. This would allow comparison of the relative climatic benefits of mitigation measures that address the emissions of different gases or measures that reduce emissions of one gas at the expense of increasing emissions of another (e.g., if changing the working fluid in a refrigeration system results in a less energy-efficient refrigerator). Ultimately, such an index might also let us understand the relative importance of the emissions of gases that are not themselves greenhouse gases but that because of their involvement in chemical interactions in the atmosphere influence the abundance of greenhouse gases. Because such an index would have to involve not only the infrared absorptive capacities, concentrations, and concentration changes of individual gases, but also their spectral overlaps and atmospheric residence times, exact values will be scenario dependent. A single index that meets all of our needs may not even exist. It is clear that the

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Page 162 TABLE 19.3 Global Greenhouse Gas Emissions from Human Activities       Projections (Mt/yr)a     1985 Emissions (Mt/yr) 2000 Emissions 2015 Emissions 2050 Emissions CO2emissionsb           Commercial energy 18,854 (86) 25,633 (87) 33,508 (89) 57,168 (92)   Tropical deforestation 2,628 (12) 3,241 (11) 3,388 (9) 3,728 (6)   Other 438 (2) 589 (2) 753 (2) 1,243 (2)   TOTAL 21,900 29,463 37,650 62,139 CH4 emissionsc           Fuel production 58 (18) 88 (22) 124 (26) 228 (32)   Enteric fermentation 74 (23) 96 (24) 110 (23) 156 (22)   Rice cultivation 109 (34) 124 (31) 138 (29) 171 (24)   Landfills 29 (9) 40 (10) 48 (10) 100 (14)   Tropical deforestation 19 (6) 24 (6) 24 (5) 28 (4)   Other 29 (9) 28 (7) 33 (7) 36 (5)   TOTAL 320 400 477 711 CFC-11 and CFC-12 emissionsd           TOTAL 0.64 0.84 0.76 0.83 N2O emissionse           Coal combustion 1.0 (25) 1.5 (26) 2.0 (29) 3.2 (36)   Fertilizer use 1.5 (38) 2.6 (43) 3.1 (44) 3.7 (41)   Gain of cultivated land 0.4 (10) 0.3 (8) 0.6 (8) 0.5 (6)   Tropical deforestation 0.5 (13) 0.4 (11) 0.7 (10) 0.8 (9)   Fuel wood and industrial biomass 0.2 (5) 0.2 (4) 0.2 (3) 0.2 (2)   Agricultural wastes 0.4 (10) 0.5 (8) 0.5 (7) 0.5 (6)   TOTAL 4 6 7 9 NOTE: Numbers in parentheses are percentages of total. aMt = megatons = million metric tons. bProjection based on U.S. EPA (1989) Rapidly Changing World Scenario; assumed average annual growth rate = 1.6 percent. cProjection based on U.S. EPA (1989) Rapidly Changing World Scenario; assumed average annual growth rate = 1.2 percent. dCFC emission projection (from EPA) assumes no further controls beyond original Montreal Protocol; assumed average annual growth rate = 0.4 percent. eNitrous oxide projections (from EPA) assume an average annual growth rate of 1.2 percent. SOURCE: Data are from U.S. Department of Energy (1990).

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Page 163 TABLE 19.4 Radiative Forcing Relative to CO2 per Molecule and per Unit Mass Change in the Atmosphere for Present-Day Concentrations   Change in Radiative Forcing (dF) Relative to Change in Temperature (dC) Gasa per Molecule Relative to CO2 per Unit Mass Relative to CO2 CO2 1 1 CH4 21 58 N2O 206 206 CFC-11 12,400 3,970 CFC-12 15,800 5,750 CFC-113 15,800 3,710 CFC-114 18,300 4,710 CFC-115 14,500 4,130 HCFC-22 10,700 5,440 CCl4 5,720 1,640 CH3CCl3 2,730 900 CF3Br 16,000 4,730 Possible CFC substitutes     HCFC-123 9,940 2,860 HCFC-124 10,800 3,480 HFC-125 13,400 4,920 HFC-134a 9,570 4,130 HCFC-141b 7,710 2,900 HCFC-142b 10,200 4,470 HFC-143a 7,830 4,100 HFC-152a 6,590 4,390 aCO2 CH4 and N2O forcings are from 1990 concentrations. SOURCE: Intergovernmental Panel on Climate Change (1990). relative importance of different gases will be a function of the time interval over which one chooses to integrate, with the short-lived gases appearing more important over short integration times. Evolution of such an index has occurred rapidly over the past several years, and a useful index of global warming potential (GWP) has recently been described in the IPCC Working Group I document (Intergovernmental Panel on Climate Change, 1990). The GWP is not yet a mature concept, but it provides a preliminary basis for a simple comparison of the emissions of various greenhouse gases and has been adapted for use here. It is, by definition, "the time integrated

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Page 164 image FIGURE 19.2 Sources of U.S. CO2 emissions (1987) in megatons CO2. SOURCE: Adapted from Marland (1990). commitment to climate forcing from the instantaneous release of 1 kilogram of a trace gas expressed relative to that from 1 kg of carbon dioxide." The GWP has, in essence, units of degree years over degree years and varies considerably with the time interval of integration because of the different mean lifetimes of the gases. The indices of global warming potential for the most important gases are given in Table 19. The CO2-equivalentimpact of different greenhouse gases on greenhouse warming is computed by taking the emission of each greenhouse gas and simply multiplying that emission by its GWP. As shown here, CO2 is the least effective greenhouse gas per kilogram emitted, but its contribution to global warming is the largest. CH4 has an "indirect effect" because its ultimate decomposition products are CO2 and H2O. The Mitigation Panel has used thesame methodto determine the "CO2-equivalent" reduction of different greenhouse gas mitigation strategies. In addition, as discussed in Part two, the effects Panel has developed a method of comparing the relative impact on radiative forcing and temperature rise due to greenhouse warming from reducing the emissions of different greenhouse gases on a worldwide basis. By using the U.S. greenhouse gas emission estimates provided earlier and multiplying these emissions by the GWP of each gas, a rough estimate of U.S. emissions in CO2-equivalent emissions is shown in Table 19.7. This provides a baseline for mitigation of U.S. greenhouse gas emissions.

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Page 165 TABLE 19.5 Carbon Dioxide Emissions from Fossil Fuel Burning and Cement Manufacture in the United States (Mt C/yr) Year Total Solid Liquid Gas Cement Gas Flaring Per Capita Percentage of Global Total 1950 696.1 347.1 244.8 87.1 5.3 11.8 4.6 42.5 1951 716.8 334.5 262.2 102.7 5.7 11.7 4.6 40.4 1952 698.0 296.6 273.2 109.9 5.8 12.5 4.4 38.7 1953 714.5 294.3 286.6 115.5 6.1 11.9 4.5 38.7 1954 680.5 252.2 290.2 121.2 6.3 10.6 4.2 36.4 1955 746.0 283.3 313.3 130.8 7.2 11.4 4.5 36.4 1956 781.9 295.0 328.5 138.1 7.6 12.7 4.6 35.8 1957 775.1 282.7 325.8 147.6 7.1 11.9 4.5 34.0 1958 750.8 245.3 333.0 155.8 7.5 9.3 4.3 32.1 1959 781.4 251.5 343.5 169.9 8.1 8.4 4.4 31.6 1960 799.5 253.4 349.8 180.4 7.6 8.3 4.4 30.9 1961 801.9 245.0 354.1 187.4 7.7 7.7 4.4 30.8 1962 831.5 254.2 364.3 198.7 8.0 6.3 4.5 30.7 1963 875.6 272.5 378.8 210.3 8.4 5.6 4.6 30.7 1964 912.9 289.7 389.7 219.8 8.8 5.0 4.8 30.3 1965 948.3 301.1 405.6 228.0 8.9 4.7 4.9 30.1 1966 999.7 312.7 425.9 246.4 9.1 5.5 5.1 30.2 1967 1039.2 321.1 443.6 258.5 8.8 7.2 5.2 30.4 1968 1081.0 314.8 471.9 277.4 9.4 7.6 5.4 30.1 1969 1132.0 319.7 497.4 297.8 9.5 7.7 5.6 29.7 1970 1165.5 322.4 514.8 312.1 9.0 7.2 5.7 28.5 1971 1173.2 305.7 530.5 323.3 9.7 4.2 5.7 27.7 1972 1227.3 310.4 575.5 327.6 10.2 3.6 5.9 27.8 1973 1275.4 334.0 605.4 321.7 10.6 3.6 6.0 27.4 1974 1231.1 330.1 580.7 307.9 10.0 2.4 5.8 26.4 1975 1179.0 317.6 565.1 286.0 8.4 1.9 5.5 25.5 1976 1262.0 351.6 608.1 291.3 9.0 2.0 5.8 25.8 1977 1269.7 355.6 641.9 260.5 9.7 2.0 5.8 25.2 1978 1293.4 361.2 655.0 264.7 10.4 2.2 5.8 25.5 1979 1300.9 378.7 634.6 274.8 10.4 2.4 5.8 24.4 1980 1259.3 394.6 581.0 272.5 9.3 1.8 5.5 23.9 1981 1210.6 403.0 533.1 264.2 8.8 1.4 5.3 23.6 1982 1116.9 390.1 502.2 245.4 7.8 1.4 4.9 22.5 1983 1149.4 405.5 500.1 233.8 8.7 1.4 4.9 22.6 1984 1187.5 427.8 507.1 241.5 9.6 1.6 5.0 22.6 1985 1201.3 448.0 505.6 236.7 9.6 1.4 5.0 22.3 1986 1204.5 439.7 531.1 222.6 9.7 1.4 5.0 21.7 1987 1257.5 465.8 545.3 235.0 9.6 1.8 5.2 22.1 1988 1310.2 493.6 566.4 238.6 9.5 2.1 5.3 22.2 NOTE: Emission estimates are rounded and expressed in megatons of carbon; per capita estimates are rounded and expressed in tons of carbon. SOURCE: Marland (1990).

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Page 166 image FIGURE 19.3 Sources of U.S. CH4 emissions (1987) in megatons CH4. SOURCE: Adapted from Table 24.1 in World Resources Institute(1990). Structure of Part Three The following key questions are addressed by the Mitigation Panel in this part of the report: • Concerning the comparison of mitigation options: What technical and policy options are available to mitigate emissions and greenhouse warming? What are the costs, benefits, and distributional effects of the various policies? • Concerning the implementation of mitigation options: What are the disadvantages and advantages of different policies and the methods of implementing those policies? How should different policy methods be implemented? In answering these questions, the panel was charged not with deciding whether emissions should be reduced, but rather with evaluating which options have the greatest potential to mitigate greenhouse warming if the decision is made to do so. Chapter 20 discusses the panel's approach to evaluating options and the general advantages and disadvantages of different methods of implementing policies. In Chapters 21 through 28, the technical costs and potentials of some of the mitigation options deemed to be most suitable for reducing greenhouse gas emissions are estimated by source sector: • Residential and commercial energy management (Chapter 21) • Industrial energy management (Chapter 22)

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Page 167 TABLE 19.6 Global Warming Potentials of Several Greenhouse Gases   Time Horizon   20 Years 100 Years 500 Years CO2 1 1 1 CH4 (including indirect) 63 21 9 N2O 270 290 190 CFC-11 4500 3500 1500 CFC-12 7100 7300 4500 HCFC-22 4100 1500 510 NOTE: The global warming potentials (GWPs) show the relative contributions to radiative forcing (with respect to CO2) for instantaneous injection to the atmosphere of 1 kg of gas. Because the gases have different atmospheric lifetimes, their relative importance changes with the time interval over which the radiative impact is integrated. CH4 is thus seen to have a large impact over short times, but it is less important over longer times because of its short lifetime. The CH4 calculation recognizes that when CH4 is fully oxidized, CO2 is one of its products. SOURCE: Intergovernmental Panel on Climate Change (1990). • Transportation energy management (Chapter 23) • Energy supply systems (Chapter 24) • Nonenergy emission reduction (halocarbons, agriculture, landfill gas) (Chapter 25) • Population (Chapter 26) • Deforestation (Chapter 27) • Geoengineering (reforestation, sunlight screening, ocean fertilization, halocarbon destruction) (Chapter 28) It is important to note that the panel did not formulate or analyze specific scenarios projecting emission rates into the future. The panel felt that the accuracy of such projections so far in the future was questionable (as illustrated by the accuracy of projections made in the past). Rather, it assumed that the world of the future would be roughly like the world of today and focused on potential methods for reducing emissions as if they were being applied to current (1989) emission sources. It should also be noted that the panel looked at emission reductions and other measures from a U.S. perspective—methods by which U.S. emissions could be reduced and areas in which the United States could transfer technology, support research and development, or otherwise assist other countries in reducing their emissions.

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Page 168 TABLE 19.7 Estimate of Current U.S. CO2-Equivalent Emissions Pollutant Approximate U.S. Greenhouse Emissions (Mt/yr)a GWP (100 yr)b Approximate U.S. CO2-Equivalent Emissions (Mt CO2 equivalent/yr)a,c CO2 4800 1 4800 ± 10% CH4 50 21 1050 ± 20% CFC-11 0.08 3500 280 ± 30% CFC-12 0.14 7300 1020 ± 30% CFC-113 0.08 4200 340 ± 30% N2O 1.4 290 410 ± 60% Approximate total U.S. anthropogenic CO2-equivalent emissions 7900 ± 20% aMt = megatons = 1 million metric tons. bGWP = global warming potential. This is multiplied by the emission estimate to determine the CO2-equivalent emissions. The 100-year lifetime integration is used in these calculations. cQualitative indications are given for the relative emissions numbers. SOURCE: Marland (1990) (CO2); World Resources Institute (1990) (CH4); personal communication from F. H. Vogelsberg, Du Pont, to Deborah Stine, Committee on Science, Engineering and Public Policy, 1990 (CFCs); U.S. Department of Energy (1990) (all gases). SOURCE: Marland (1990) (CO2); World Resources Institute (1990) (CH4); personal communication from F. H. Vogelsberg, Du Pont, to Deborah Stine, Committee on Science, Engineering and Public Policy, 1990 (CFCs); U.S. Department of Energy (1990) (all gases). The discussion in each of Chapters 21 through 27 is divided into the following sections: • Recent Trends. Recent trends in emissions from the sector are described. For example, in the industrial sector, the level of energy intensity has decreased in recent years. The effectiveness of efforts to reduce emissions or improve efficiency in the sector is also discussed. • Emission Control Methods. Methods that can be used to reduce emissions from the sector are discussed. These can include technical actions both on the demand side (e.g., improving end-use energy efficiency) and on the supply side (e.g., reducing emissions from power plants). In addition, the potential emission reductions and the costs of implementing such methods are quantified. As discussed in Chapter 20, a ''supply curve" of the implementation cost (dollars per ton CO2 equivalent) and emission reduction (megatons of CO2 per year) is developed for each option if possible. These are "first-order" analyses, meant only to be a beginning point for determining the cost-effectiveness of various mitigation options and for demonstrating a method that could be used to evaluate options.

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Page 169 Specifically, second-order effects, including system adjustments that change costs of greenhouse warming emissions in other sectors, in other regions of the world, or at later points in time, are not included. In other words, the analysis presented here should not be viewed as the definitive assessment of each option. Rather, the intent is to describe a manner in which options could be evaluated and to illustrate the approach with the best estimates available. • Barriers to Implementation. The technical and policy barriers to achieving the potential emission reductions described in the previous section are discussed. For example, in many cases we can improve the energy efficiency with relatively short economic payback periods, and yet these energy measures have still not been fully implemented. What prevents us from achieving the energy reductions that are possible? • Policy Options. A number of policy options with differing levels of effectiveness can be used to encourage the reduction of greenhouse gas emissions in a particular sector. Each policy and the positive and negative aspects of implementing it are discussed. The policies described here are not all-encompassing but are some that the panel believes are most worthwhile to consider. A key resource used in this section is the Department of Energy's (DOE) report entitled A Compendium of Options for Government Policy to Encourage Private Sector Responses to Potential Climate Change (U.S. Department of Energy, 1989). The DOE report includes a more comprehensive look at the range of possible mitigation policies. Unfortunately, for many of the options, few data are available to evaluate the expected effectiveness. Evaluations of the effectiveness of comparable policies implemented in the past are sorely needed. In the absence of such studies it is difficult to determine how much of the potential reductions can actually be achieved. • Other Benefits and Costs. Uncounted in the implementation cost are nongreenhouse-related benefits and costs that might derive from a particular policy. For example, on the benefit side, when energy consumption is reduced, the emissions that cause urban air pollution are also reduced. On the other hand, reductions in coal consumption could have severe economic consequences for coal-mining communities. • Research and Development Needs. Research and development that is needed to remove or decrease technical and other barriers to reducing greenhouse gas emissions is described. For example, hydrogen would be an ideal transportation fuel on some counts, but technical barriers in terms of storage and infrastructure limit its application. In some cases, the barrier is cost. For example, photovoltaics could generate at least a portion of the energy supply, but high cost currently limits broad usage. In this case, continued research could improve the technology so that cost can be reduced. A key reference on research and development in the energy sector is a recent report by the Energy Engineering Board of the National Research Council entitled Confronting Climate Change: Strategies for Energy Research and Development (National Research Council, 1990).

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Page 170 Because the discussion in these chapters is at times highly technical, a glossary (Appendix S) has been provided for the reader's convenience. In addition, conversion tables (Appendix T) are provided for those who may be unfamiliar with the units of measurement used throughout this report. The final chapter of this part, Chapter 29, summarizes the results of individual analyses and draws some general conclusions regarding the relative merits of potential interventions. This analysis should not be interpreted as all-inclusive, but it does provide semiquantitative consideration of a wide sampling of potential approaches to mitigation. The principal findings and recommendations concerning the policy choices facing the country are found in the report of the Synthesis Panel (Part One). Note 1. Throughout this report, tons (t) are metric; 1 Mt = 1 megaton = 1 million tons; and 1 Gt = 1 gigaton = 1 billion tons. References Intergovernmental Panel on Climate Change. 1990. Climate Change: The IPCC Scientific Assessment, J. T. Houghton, G. J. Jenkins, and J. J. Ephraums, eds. New York: Cambridge University Press. Intergovernmental Panel on Climate Change. 1991. Climate Change: The IPCC Response Strategies. Covelo, Calif.: Island Press. Marland, G. 1990. Carbon dioxide emission estimates: United States. In TRENDS '90: A Compendium of Data on Global Change, T. A. Borden, P. Kanciruk, and M. P. Farrell, eds. Report ORNL/CDIAC-36. Oak Ridge, Tenn.: Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory. National Research Council. 1990. Confronting Climate Change: Strategies for Energy Research and Development. Washington, D.C.: National Academy Press. U.S. Department of Energy (DOE). 1989. A Compendium of Options for Government Policy to Encourage Private Sector Responses to Potential Climate Change. Report DOE/EH-0103. Washington, D.C.: U.S. Department of Energy. U.S. Department of Energy (DOE). 1990. The Economics of Long-Term Global Climate Change: A Preliminary Assessment. Report of an Interagency Task Force. Report DOE/PE-0096P. Washington, D.C.: U.S. Department of Energy. World Resources Institute (WRI). 1990. World Resources 1990–91. New York: Oxford University Press.