Findings and Recommendations
In the previous chapters, the Mitigation Panel has discussed various options for responding to the emission of greenhouse gases to the atmosphere. Here the panel has organized that material within the framework of the charge it received to evaluate the effectiveness of various policies that could potentially mitigate greenhouse warming. No attempt has been made to judge whether action to mitigate greenhouse warming should be taken. If through the political process, however, the United States decides to attempt to mitigate greenhouse warming, it should do so as efficiently as possible, with a broad appreciation of the alternatives available, their potential effectiveness, and the implications of their implementation. This means (1) taking a global perspective with respect to possible actions, (2) assembling the best information available about the cost per ton of CO2-equivalent reductions, and (3) evaluating other costs and benefits of prospective actions.
The panel again emphasizes that substantial uncertainties cloud all the numerical estimates summarized in this chapter. The degree of uncertainty varies greatly, but in many important instances such as the large-scale "geoengineering" alternatives, it is so large that even relative judgments must be made tentatively. More generally, the assembly of information in this report should be regarded as useful primarily for comparing large families of options, and not as specific recommendations of steps to be taken without additional analysis, research, or empirical study.
U.S. Mitigation Policy
United States policy toward greenhouse gas mitigation is important for a number of reasons. First, the United States is currently the largest emitter
of greenhouse gases. As such, should greenhouse warming require active intervention, the United States has a responsibility to do its part to reduce greenhouse emissions, and unilateral action could contribute significantly to a reduction in the rate of emission growth. However, the U.S. role in greenhouse warming, although large (approximately 20 percent of worldwide CO2-equivalent emissions), is not so large that unilateral action could stabilize global climate. At least a 60 percent reduction in current worldwide CO2-equivalent emissions would be needed for stabilization, according to the Intergovernmental Panel on Climate Change (Intergovernmental Panel on Climate Change, 1991). As discussed in Chapter 28, geoengineering options may be able to reduce the amount of reduction required, but international agreement and participation in such actions would be necessary in order to undertake such action on a planetary scale.
Second, U.S. policy and technology will affect the inclination and capability of other nations to respond to greenhouse warming. The U.S. policy is of instrumental importance, both in meeting our potential national responsibilities within the world community and in leading constructive change in that community. The large magnitude and long time scale of potential adjustments imply that any response will require a coherent and sustained commitment on a global scale. What is needed is not a single national policy, but a long-term strategic perspective on greenhouse warming and its implications for the world economy.
Third, developing countries are unlikely to be able to respond to the potential threat of greenhouse warming at the same level as industrialized countries. The United States should not focus exclusively on interventions within its own boundaries because greenhouse warming is a global issue and emission reduction in one country could be as beneficial as in another. It may be appropriate for the United States and other industrial economies to seek low-cost opportunities for reducing greenhouse gas emissions in developing countries, or to provide economic and technological support, through the political process, should these countries decide that such actions are warranted.
Three basic premises are central to the panel's comparison of different mitigation policy options.
• First, possible responses to greenhouse warming should be regarded as investments in the future of the nation and the planet. That is, the actions needed would have to be implemented over a long time. They should be evaluated as investments, in comparison with other claims on the nation's resources, bearing in mind their often widespread implications for the economy.
• Second, cost-effectiveness is an essential guideline. The changes in energy, industrial practice, land use, agriculture, and forestry that might be implemented to limit greenhouse gas emissions, or the use of geoengineering options, imply an investment effort lasting several generations and large enough to affect the macroeconomic profile of the country. Costs of climate policy therefore need to be considered as a central element. A sensible guideline is cost-effectiveness: obtaining the largest reductions in greenhouse gas emissions at the lowest cost to society. Positive or negative effects of any mitigation option on societal factors not related to greenhouse warming must also be taken into account.
• Third, a mixed strategy is essential. The magnitude of the economic changes at stake, together with the need to pursue a cost-effective approach, implies that a mixed strategy, employing a variety of measures, would be required. This simple observation complicates the task of analysis and policy design, however, because a mixed strategy that is cost-effective can be designed and implemented only through comparisons of activities in different sectors of the economy.
Categories of Mitigation Options
A brief description of the mitigation options analyzed by the Mitigation Panel is shown in Table 29.1. In its comparison of different options, the panel examined several factors. The first of these was cost-effectivenesshow much reduction the United States can get in greenhouse warming for each dollar spent. By using the index of dollars per ton of CO2 equivalent, the panel was able to contrast not only the mitigation options that affect CO2 emissions, but also those that address emissions of halocarbons, N2O, and CH4.1 As noted in Chapter 20, cost-effectiveness was evaluated using four different discount rates: 3, 6, 10, and 30 percent. Options that do not involve emission reductions, but would seek to reduce the level of greenhouse gases already in the atmosphere or to compensate for their climatic effects, were also reviewed. These "geoengineering options" have been converted to CO2 emission reduction equivalence so that they can be compared with emission reduction options. The comparison is made by using the objective of climate stabilization rather than emission reduction per se. In the case of the geoengineering options, the cost-effectiveness was annualized for comparison purposes but was not discounted. This is because the panel felt that these options were so futuristic in nature that discounting would provide these "back of the envelope" cost estimates a degree of accuracy not available at the current time.
Assessment of the anticipated magnitude of climatic effects and appraisal of their impact on society have been the work of the Effects and Adaptation Panels (Parts Two and Four). That work builds a basis for informed judgments of the appropriate magnitude and rate of mitigation.
Barriers to implementing these mitigation options are also a major concern. Although it may be technically possible to achieve emission reduction
TABLE 29.1 Brief Descriptions of Mitigation Options Considered in This Study for the United States
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to a given level, the necessary actions might not be taken for any number of social, economic, or political reasons. For example, some actions that are economically sensible will not be undertaken by households and firms because they involve up-front costs and the households or firms face liquidity constraints that make them unwilling or unable to undertake the investment. In these cases, it could be desirable to find some institutional mechanism to overcome the constraints. In the case of energy efficiency, for example, the natural focus for such changes is builders, manufacturers, and utilities (i.e., the providers of electric power and natural gas). They are, in principle, in a position to make credit available, directly or indirectly, to purchasers who are constrained in their decisions by inadequate liquidity. Yet institutions may also face obstacles to moving to best practice or encouraging their customers to do so. Thus building codes are typically out of date and may limit local builders from incorporating the latest proven energy-saving materials in construction. In most states, public
utility pricing formulas reward public utilities for building new power plants, but not for investments that conserve energy. Examples exist in many areas. It should be remembered, however, that some of these ''constraints" serve other social objectives. For example, there are many reasons people drive their cars to work instead of taking mass transit, and many reasons they select large, less-efficient vehicles. The reasons may lie in time saved, safety, or personal flexibility, but in each case energy conservation is not the only social objective that enters into the decision process. Altering long-standing practices is rarely easyeven if such changes may bring economic as well as potential climatic benefits.
Many of the potential climate interventions, of course, have effects other than merely reducing atmospheric CO2 or its equivalent. Some of these effects will be positive, others negative, and some will have different effects on different parts of society. It is also important to remember that these inquiries occur on a planet where the population is still increasing and most of the inhabitants aspire to a higher standard of living. Therefore another important factor in analyzing various mitigation policies involves the costs and benefits (beyond implementation) that are likely to occur should the mitigation action be taken. Thus some low-cost options will be unattractive on other grounds, while some high-cost actions will provide additional benefits.
In the present study the Mitigation Panel has barely touched on issues such as the barriers to implementation and the social, environmental, and economic implications of the strategies investigated. The panel hopes that the mention of these issues in this analysis through the use of the categories described below will contribute to their visibility and raise them to a higher level of consideration in more detailed studies later. The panel has tried to be qualitatively sensitive to these issues and to discourage direct dollar comparisons of options with widely different external implications. These three factorscost-effectiveness, implementation obstacles, and other costs and benefitssuggested the categories for analysis.
The three categories of options are as follows:
• Category 1 options: "Best-practice" mitigation options available at little or no net cost that are not fully implemented due to various implementation obstacles.
• Category 2 options: Mitigation options that are either relatively costly or face implementation obstacles not fully represented in the implementation cost. They may also have other benefits and costs not fully represented.
• Category 3 options: Mitigation options that appear to be feasible with the current, limited state of knowledge. They may, with additional investigation, research, and development, provide the ability to change atmospheric concentrations of greenhouse gases, or radiative forcing, and the ultimate impact of greenhouse warming on a substantial scale.
In none of the categories have full institutional costs been estimated. This is important because most Category 1 options, particularly the ones estimated to produce net savings, require institutional changes before they become available to buyers and sellers of goods and services that release greenhouse gases. The costs of changing these institutional barriers are unknown. Without an appraisal of institutional costs, any comparison is incomplete; such an appraisal has not been done in this report. The panel believes it has provided a framework within which these appraisals can be made in future studies. A major rationale for discussing options under three categories is that the panel not believe the full range of options should be compared on a simple monetary scale.
In Tables 29.2 to 29.7 the panel summarizes the mitigation strategies that have been reviewed using the three categories. Although the menu of options reviewed is not intended to provide an inventory of all possibilities, it seeks to identify the most promising options. The panel hopes that it provides the beginnings of a structure and a process for identifying those strategies that could appropriately respond to the prospect of greenhouse warming.
Category 1 Options
Every progressive society finds its economic activities on average falling short of best practice in most areas. This is because new practices are being contrived continually and it takes time for them to diffuse throughout the economy. Thus every progressive society enjoys opportunities for improving its overall situation by reducing the gap between average practice and best practice. Obstacles to more rapid diffusion of better practice include lack of information, lack of opportunity (e.g., if stores do not stock the improved products), political resistance, capital investment, risk aversion, and simple human inertia. Cost of replacement is also an obstacle, but one that disappears as old equipment wears out and renewal becomes necessary. Within organizations, better practices may not be introduced because of divisions in responsibility, for example, if those making the decisions to go ahead do not get credit for the benefits that flow from the new practice (e.g., "maintenance" pays for the light bulbs, but "operations" pays the electric bills).
Thus there are typically many improvements that "ought" to be undertaken, and most of them will be undertaken, eventually, because they are in the interests of those undertaking them. These decisions can be hastened by providing information and opportunity. Heightened awareness will encourage stores to stock improved products, top management to review the division of responsibilities within their firms, and so on.
The general proposition that economic activities fall short of best practice
applies to every area of the economy. Many opportunities for reducing greenhouse gas emissions will also improve economic well-being, because they are more efficient than prevailing practices, judged in conventional terms. These "no-regrets" actions show up in Table 29.2 as measures with a net savings or very low cost. This negative cost does not imply that no expenditure is required to implement these actions, but rather that the real rate of return to the initial investment in making the change exceeds the common societal discount rates. However, as discussed in Chapters 21 and 22, households and firms do not have perfect information, and they are often observed to behave as if a 30 percent rate of return were needed to invest in one of these options. Therefore a column with a 30 percent discount rate has been added to illustrate what some households or firms seek in the marketplace prior to investment. As shown here, even at a 30 percent discount rate (well above their rate of return for other investment opportunities), firms and households will still receive a benefit for investment in many of these options. In other words, these "movement to best practice" actions involve attractive rates of return and would be undertaken voluntarily in many cases. However, as discussed in Chapters 21, 22, and 23, the timing can be accelerated if information, technical assistance, and financing can be provided.
Category 2 Options
As shown in Table 29.3, there are actions to reduce greenhouse gas emissions, or compensate for their climatic effects, that are either economically costly in the sense that the nation or the world must reduce its future income to reduce the potential for climate change, must face implementation obstacles, or will encounter additional benefits and costs not fully reflected in the implementation cost. The panel has tried to make rough estimates of the costs of reducing carbon in the atmosphere through various actions. The range of costs is wide, varying from well under $1/t CO2 equivalent reduction to over $500/t.
As discussed in Chapter 25, reduction in CFC consumption can also help reduce stratospheric ozone depletion. In another case of issues beyond current cost, the information in Chapter 24 indicates that solar energy is relatively costly now, but anticipated technological developments may lower the price substantiallyperhaps making it a cost-effective option. Nuclear power faces not only cost obstacles, but also implementation obstacles because of public concerns about nuclear plant safety and management of radioactive waste. The replacement of coal power plants with natural gas also faces implementation obstacles as utilities concerned about an uncertain natural gas supply resist investment in plants with a 30-year lifetime. Chapter 24 discusses each of the energy options in more depth.
TABLE 29.2 Category 1 Mitigation Options by Source
TABLE 29.3 Category 2 Mitigation Options by Source
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The point of Category 2 is that the relative attractiveness of the various options is not adequately captured in the simple index of cost per ton of CO2 equivalent. The relative appeal of Category 2 options is greatly affected by other social, environmental, and economic externalities.
Category 3 Options
Category 3 options (Table 29.4), mainly geoengineering options, are those that appear to be feasible with the limited information now available, which maywith additional investigation, research, and developmentprovide the ability to change atmospheric concentrations, or radiative forcing, and the ultimate impact of greenhouse warming on a substantial scale. By and large, they deal with the symptoms rather than the causes. Some of these actions could be initiated after a deleterious climate change was clearly identifiable, if research and development had been completed earlier. Near-term research of the Category 3 options as a "backstop" measure is likely to be beneficial and relatively inexpensive. In the end, as discussed in Chapter 28, some of these options could be inexpensive, safe, and reversible.
Many of the Category 3 options appear relatively inexpensive from an implementation standpoint but have large unknowns as to their environmental or carbon cycle side effects should they be implemented. For example, increasing phytoplankton growth through addition of iron to the oceans may be a feasible mitigation option, but the impact of tinkering with the oceanic balances of iron, carbon, oxygen, and other nutrients is unknown at this time. These options should be investigated further and should be well understood before implementation is considered seriously.
Comparing the Different Mitigation Options
Tables 29.5, 29.6, and 29.7 summarize the information in Tables 29.2, 29.3, and 29.4 by adding up the maximum potential emission reduction available from each sector and placing the net cost of the various options in categories. Categorization of cost numbers helps to illustrate that a great deal of uncertainty is, of course, associated with many of these numbers. Because this is a "first-order" analysis, the Mitigation Panel has used information from many sources, most of which were not intended to be used for comparative cost analysis. Improvement of the cost estimates will undoubtedly modify the priority ordering of options; at this time, therefore, categories are an appropriate way to compare alternatives. In addition, cost categorization allows comparison of costs of different options relative to a wide-ranging policy instrument such as a carbon tax and its impact on fuel prices.
Figure 29.1 illustrates how mitigation options might be ranked both on
TABLE 29.4 Category 3 Mitigation Options by Source
TABLE 29.5 Cost-Effectiveness Ordering of Category 1 Mitigation Options
TABLE 29.6 Cost-Effectiveness Ordering of Category 2 Mitigation Options
(Table 29.6 is on page 484)
TABLE 29.7 Cost-Effectiveness Ordering of Category 3 Mitigation Options
their cost-effectiveness and on their potential for reducing emissions. This supply curve is the goal of the analysis discussed in Chapter 20. In Figure 29.1, Category 1 and 2 options are ranked by cost estimates for mitigation measures in that family. Starting with the cheapest option (based on the middle costs estimate) at the left of the graph, the options are added together, so that the incremental cost of implementing the mitigation steps is plotted as a function of the total reduction in CO2-equivalent emissions.
Figure 29.1 shows three supply curves, one each for the low, middle, and high cost estimates. The middle cost is the average mitigation cost for that option as determined from the information in Tables 29.2 to 29.4. The panel's judgment as to the uncertainty range surrounding the middle cost estimate is then shown as low and high cost.
Note that the order in which options are implemented in the figure is based upon the middle cost estimates, a satisfactory approach for this preliminary analysis with highly uncertain data. In later studies, drawing upon better information, the supply curves should be recalculated for each family of cost estimates to ensure that the lowest-cost alternative remaining is the next one to be selected for implementation.
Several important factors should be noted. First, some options show negative costs, a point discussed above. A second concern is to avoid "double counting" in compiling the supply curve. For example, the nuclear and natural gas mitigation options replace the same coal-fired power plants. Furthermore, improvements in end-use efficiency will also reduce demand for electricity and therefore CO2-equivalent emissions. This means that some of the options are not additive. Therefore the potential emission reduction for the electricity supply options was determined by subtracting the emission reduction already obtained from end-use and power plant efficiency improvements (which cost less than new power) from the CO2-equivalent emissions currently generated by power plants.
Further, this method combines the Category 1 and Category 2 options into one figure, which means that the additional considerations identified as "other factors" in Table 29.3 are not considered. Such additional cost and benefits may move these options either up or down on this chart relative to societal preferences.
Another major limitation is that some options with significant potential for reducing CO2-equivalent emissions have not been included in Figure 29.1 because cost estimates are not readily available (e.g., industrial process technology and efficiency improvements). Many of these options are likely to be more cost-effective than some of those represented in Figure 29.1.
Finally, another important factor to remember is that these reductions include not only CO2, but also CFCs, CH4, and N2O, all which have a higher impact per molecule on greenhouse warming. Therefore Figure 29.1 shows the reduction of CO2-equivalent emissions, not just CO2. In addition, one geoengineering optionreforestationdoes not reduce emissions
FIGURE 29.1 Low-mid-high mitigation cost comparison, assuming 100 percent implementation.
from a source but acts as a sink. Thus this figure should be interpreted with extreme caution; however, the Mitigation Panel believes it provides a useful picture of the way in which many different mitigation options may be compared with one another.
Implementing Response Programs
Figures 29.2 to 29.5 place Figure 29.1 in the context of other cost estimates and the limitations associated with actually implementing a mitigation response program. To do this, the panel examined the sensitivity to the extent to which these programs are implemented by society. Figure 29.2 displays the Figure 29.1 supply curve for threed different levels of implementation of the emission reduction strategies: 25, 50, and 100 percent. As illustrated here, the effectiveness of CO2-equivalent emission reduction changes greatly as a function of the potential achieved. Figure 29.3 shows the range of technological costing cost estimates. The lower curve is the most optimistic estimate100 percent implementation of the option as described in Table 29.1, with the lower-cost bound. The upper curve is a more pessimistic estimate25 percent implementation of the option as described, with the upper-cost bound. Mitigation costs will not be known perfectly; an approach of the kind illustrated in Figure 29.3, which develops bounding cases, can be useful in developing mitigation plans.
Figure 29.4 is a compilation of a number of energy modeling estimates of the cost of CO2-equivalent reduction. Details on the compilation of this curve are provided in Appendix R. The energy modeling estimates differ in two important characteristics from the technological costing analyses in this report. First, energy modeling estimates are comprehensive energy sector models. That is, they include a consistent accounting of the demand, supply, and resources used in the countries or regions studied. In this respect, they differ from the approach in this report, which looks at the possibilities for greenhouse gas reductions from individual technologies and attempts to make the estimates mutually consistent by manual calculations. One difficulty with the technological costing approach is that calculations are on a constant cost basis; that is, they do not account for how implementation of these measures may affect the cost of the measure. A major surge in the number of natural gas plants, for example, will likely increase demand for and therefore the price of natural gas. This would increase the cost of the mitigation optionperhaps changing its ranking. The same is true for energy efficiency measures: as demand is reduced, the price of electricity would change. This would change the savings available from that energy efficiency measure. The energy modeling approach takes these supply and demand impacts into account, while
the technological costing approach is based on the margin of the current economy and therefore does not.
A second important difference between the energy modeling approach and the approach taken by the Mitigation Panel is that the models surveyed estimate the cost function for reducing greenhouse gas emissions beginning from the point at which all "negative-cost" options have been employed. In most economic models, the market equilibrium is this point; in one model, where market failures are allowed, the results have been recast so that cost estimates begin from the point at which the market failures have been allowed for. It is important to note then, that the negative-cost part of the cost function, should that exist, is excluded from energy modeling analyses. A full description of the development of this curve is provided in Appendix R.
Finally, Figure 29.5 combines the technological costing mitigation curves in Figure 29.3 and the energy modeling curves in Figure 29.4. The energy modeling curves fall roughly between the bounding estimates of the technological costing approach. The primary difference, as mentioned earlier, is that the energy modeling curves do not include the financial benefits from efficiency measures. At the current limited state of knowledge the panel believes that the actual implementation costs of mitigating greenhouse gas emissions (excluding costs beyond those needed directly for implementation) are likely to fall within the range provided by the technological costing method. It is important to note that while the panel believes that the technological costing approach is better suited to evaluating the comparative advantages and disadvantages of specific mitigation options because current economic models do not have the specificity needed for such an analysis, there are reasons to be skeptical of the degree to which such option-driven assessments can incorporate social responses (including market responses) to alternative courses of action.
A review of Figures 29.2 to 29.5 indicates that it would not be unreasonable to expect that a roughly 25 percent reduction in U.S. greenhouse gas emissions (i.e., 2 Gt CO2 equivalent) might be achieved at a cost of less than $10/t CO2 equivalent. This is, in more commonly used terms, roughly an additional $22 per short ton of coal, $4.75 per barrel of oil, $0.60 per million cubic feet of natural gas, $0.11 per gallon of gasoline, or 0.7 cents/kWh for the current U.S. electricity mix.
A wide array of policy instruments is available for implementing mitigation options. Two categories are direct regulation and incentives. Direct regulation instruments mandate action and include controls on consumption (bans, quotas, required product attributes), production (quotas on products or substances), factors in design or production (efficiency, durability, processes), and provision of services (mass transit, land use). Incentive instruments are designed to influence decisions by individuals and organizations,
and include taxes and subsidies on production factors (carbon tax, fuel tax), taxes on products and other outputs (emission taxes, product taxes), financial inducements (tax credits, subsidies), and transferrable emission rights (tradeable emission reductions, tradeable credits). The choice of policy instrument depends on the objective to be served.
Interventions at all levels of human aggregation could effectively reduce greenhouse warming. For example, individuals could reduce energy consumption, recycle goods, and reduce consumption of deleterious materials. Local governments could control emissions from buildings, transport fleets, waste processing plants, and landfill dumps. State governments could restructure electric and gas utility pricing structures and stimulate a variety of efficiency incentives. National governments could pursue action in most of the policy areas of relevance. International organizations could coordinate programs in various parts of the world, manage transfers of resources and technologies, and facilitate exchange of monitoring and other relevant data.
Although the analysis of mitigation options in this report does not include all possibilities, the Mitigation Panel is hopeful that it does identify the most promising options considered here. The panel feels confident that it provides the beginnings of a structure and a process for identifying those strategies that could appropriately mitigate the prospect of greenhouse warming.
Whatever policies the United States follows in order to truly address greenhouse warming, it will eventually be necessary to achieve broader international consensus in action. Many of the cost-effective options appropriate for the United States are also applicable in other countries, including developing nations. A range of other measures are also relevant, such as removing or reducing market-distorting subsidies that encourage greenhouse gas emissions. Effective participation of developing countries in the reduction of emissions will require political actions by those nations, as well as international negotiations that deal with the availability of financial and technical resources and with competing requirements for current economic growth.
As discussed in Chapter 26, population growth, largely taking place in developing countries, is a basic contributor to the increase in greenhouse gas emissions. This will become even more relevant in the future, as those countries improve their economies with accompanying increased energy consumption. Limiting growth in population is central to limiting future energy consumption and, therefore, to future stabilization of greenhouse gas emissions. Limiting population growth may not be financially costly, but it is beset with political, social, and ideological obstacles. Similarly, as discussed in Chapter 27, reducing or reversing net deforestation as a means of
reducing greenhouse gases raises a host of nontechnical issues that are not evident from a financial standpoint.
The international negotiations on greenhouse issues that will be required to lead to common action on these and related matters, and to avoid "free riders" (where one nation benefits from the costly actions of others), will be difficult and will necessarily involve matters of great political and economic concern. International studies and analyses are currently under way in an impressive number of settings, with the recent experience of the Montreal Protocol and Law-of-the-Sea negotiations as guides to approaches that are useful and those that should be avoided. Hard choices remain in the future, however, because negotiations will be more difficult than any of the predecessors in the environmental area. However, the necessarily deliberative nature of those negotiations should not obscure the conclusion that unilateral actions by the United States, or common actions by currently large greenhouse gas emitters among industrialized countries, can be useful on their own. They can reduce emissions below their expected levels in the short term, delay the onset of warming (if warming materializes), and create a precedent that could help lead to coordinated international action.
The Mitigation Panel has attempted to outline a perspective that should be pursued relative to mitigation policy. First, the United States needs to realize that although unilateral actions can contribute significantly to the reduction of greenhouse gases, the greenhouse warming phenomenon is global, and national efforts alone would not be sufficient to eliminate the problem. This means that the nation should take a global perspective with respect to possible actions. Second, cost-effectiveness should be a primary guide in making greenhouse warming mitigation policy as efficient as possible.
Therefore the Mitigation Panel has tried to bring together informed judgments of the cost of greenhouse gas reduction, as well as other costs and benefits of prospective actions. It should be emphasized that the analysis the panel conducted was "cross-sectional" as opposed to a longitudinal analysis of options over time. There was no attempt, for example, to project future levels of economic activity and their implications for greenhouse gas emissions. This study does account, however, for future consequences of current actions. In particular, the direct effects of each option on greenhouse gas emissions are assessed. The panel has not attempted to examine those options under the different overall emission rates that might occur at future times. Its analysis must therefore be seen as an initial assessment of mitigation options in terms of their return on investment under current conditions. A subsequent analysis might consider appropriate strategies under changing
conditions. Furthermore, the time required to implement these mitigation options is not considered. Some options, such as those in energy efficiency, can be implemented immediately if the noneconomic obstacles are overcome. Others, such as changes in electricity production, might take considerably longer, on the order of decades. The rates at which these mitigation options are implemented depends on the decision makers in a wide range of firms, households, and governmental units throughout the United States.
Once the cost-effectiveness and mitigation potential of each option were determined, the Mitigation Panel categorized these options. The best-practice (Category 1) options have significant potential for mitigating greenhouse warming at negative or low net implementation cost; however, information and incentive mechanisms are needed to hasten these reductions. Although no firm quantitative estimates of the net contribution of these policies can be given, it is not unreasonable to believe that U.S. greenhouse gas equivalent emissions could be reduced 25 percent from 1990 levels through use of these relatively low cost options alone. The second category of options (Category 2) entails additional costs and benefits not included in the cost-effectiveness estimate. The United States and other countries are already working to reduce CFC emissionsproviding a major contribution to the reduction of greenhouse gas emissions at a relatively low cost (in addition to the benefits to the stratospheric ozone layer). Perhaps one of the surprises of this analysis is the relatively low cost at which some of geoengineering options (Category 3) might be implemented. However, it will require further inquiry to decide if geoengineering options can produce the targeted responses without unacceptable additional efforts. The level at which science is currently able to evaluate the cost-effectiveness of engineering the global mean radiation balance leaves great uncertainty in both the areas of technical feasibility and environmental consequences. This analysis does suggest that further inquiry is appropriate.
Finally, greenhouse warming is an international problem that the United States cannot solve alone. Slowing worldwide population growth may be necessary to achieve a significant change in worldwide emissions of greenhouse gases. However, the panel's analysis indicates that reducing population growth alone may not reduce emissions of greenhouse gases if there is continued economic growth. Reduction of deforestation may provide another significant contribution to mitigating greenhouse gas emissions. Due to domestic concerns, however, candidate countries may find these options difficult to implement. The United States can make contributions to international efforts, and such action might significantly slow greenhouse warming at a cost that is less expensive than the cost of options implemented in the United States.
The uncertainties in all of the mitigation alternatives underscore the central role of learning. This is not the usual academic call for more research. It is instead a recommendation that policy actions be treated as opportunities
to learn and that they be designed and executed so that learning is enhanced. This implies the need for more and better policy analysis. The world being altered by greenhouse warming is one whose geophysical and social character is imperfectly understood. Errors are inevitable. Large errors will be costly and painful. Accordingly, the United States must seek to use small errors as a source of learning, so as to lessen the possibility of serious mistakes.
For example, the time dimension is an important part of formulating a greenhouse warming mitigation strategy. It can have important consequences for determining the optimal timing and quantity of any intervention. This is true if that decision is based on what society gets in the form of lesser global climate change vis-à-vis what it gives up in terms of current satisfaction and the enhanced ability to accommodate future adaptation. In this, fully accounting for all the positive aspects of mitigationreduced speed of change, reduced total exposure to damage, and final level of global climate changeis important. Each has separate effects on the consequences of societal interest such as rise in sea level, agricultural productivity, and changes in ecological systems. They can also differ in their effects on the distribution of consequences over time and geography. Different instruments may lead to outcomes that diverge from those expected when only tons reduced and costs are considered. Application of the relationships discussed here requires an understanding of the physical relationships among flows, stock, and global climate change that lies beyond current knowledge. It also requires complex judgments about the trade-offs among sometimes competing policy goals.
Political processes will, in the end, determine whether and when these particular mitigation options should be undertaken. The results of this analysis indicate that the United States could make an important contribution to slowing greenhouse warming through adoption of some of these mitigation options. Some options might even provide a net savings to the U.S. economy. Using this analysis and information from the other two panels, the Synthesis Panel judges the extent to which these options should be pursued.
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. 1991. Climate Change: The IPCC Response Strategies. Covelo, Calif.: Island Press.
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