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
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
TABLE 19.1 Global Greenhouse Gases with Their Anthropogenic Emission Sources
TABLE 19.2 Key Greenhouse Gases Influenced by Human Activity
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
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
TABLE 19.3 Global Greenhouse Gas Emissions from Human Activities
TABLE 19.4 Radiative Forcing Relative to CO2 per Molecule and per Unit Mass Change in the Atmosphere for Present-Day Concentrations
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
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.
TABLE 19.5 Carbon Dioxide Emissions from Fossil Fuel Burning and Cement Manufacture in the United States (Mt C/yr)
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)
TABLE 19.6 Global Warming Potentials of Several Greenhouse Gases
• 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. perspectivemethods 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.
TABLE 19.7 Estimate of Current U.S. CO2-Equivalent Emissions
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.
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).
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).
1. Throughout this report, tons (t) are metric; 1 Mt = 1 megaton = 1 million tons; and 1 Gt = 1 gigaton = 1 billion tons.
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.