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this a matter of special concern here is that the value placed on the specific mitigation effects of instruments depends crucially on the mix of goals sought.

Goals of Mitigation of Global Climate Change

The mitigation of global climate change is not a one-dimensional phenomenon such that all possible benefits of policy actions are achieved simultaneously. This complicates the ranking of instruments on a cost per ton basis. To take different components into account in making rankings, it is first necessary to decompose the bundle of potential desired goals and then to determine how each possible policy instrument furthers or hinders the satisfaction of each. Furthermore, before the instruments can be put on a common basis it is also necessary to form some judgments about the terms of the acceptable trade-offs among mitigation goals. A similar process is required to compare the cost incurred in pursuing the use of a mitigation instrument with the cost of adaptation or of meeting non-climate-change goals such as faster economic growth or increased consumption.

Three component subgoals of global climate change mitigation can be posited. The first is to reduce the rate of change in the stock of greenhouse gases, on the twin premises that the speed of global climate change is sensitive to relatively small changes in the stock and that damage wrought is an increasing function of the rate of change. This goal would stress the avoidance of sudden increases in the flow of greenhouse gases, for example, even at the cost of giving up some reductions in the long-term level of the stock.

The second subgoal is to reduce the total amount of global climate change experienced between now and some future time, with the endpoint defined either arbitrarily or as the point where the global climate system is again in equilibrium. The presumption here is that the damage to be mitigated arises from the integral of global climate change over each year between now and the endpoint selected. This might be loosely termed the ''total damage borne" measure, and a proxy for its mitigation is the sum by years of the augmentation of the stock of greenhouse gases avoided.

The third subgoal is to reduce the ultimate level of global climate change at the chosen endpoint. Pursuit of this goal presumes that the time path of global climate change is of little consequence as long as the policy instruments result in an acceptably low level of ultimate change. As a proxy for this goal, the target is the ultimate level of the stock of greenhouse gases. Total benefits of mitigation would presumably be maximized by some optimal combination in achieving all three of these subgoals.

Figure B.1 illustrates these concepts in a schematic way. It shows the stock of greenhouse gases that is taken as a proxy for global climate change.

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