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(but these are ignored here as well). Also relevant is the hypothesis that past increases in greenhouse gases have yet to be fully reflected in observed global climate change. This suggests that stabilization of the stock of greenhouse gases would still leave additional global climate change in the system; a reduction of the stock would be required to stabilize the climate itself.

Highly simplified, the basic relationships are illustrated in Figures B.1a and B.1b, which describe stocks and flows, respectively. The vertical scales differ enormously between the two diagrams; in each, the scale is broken to exaggerate the changes relative to the base. Line 2 in Figure B.1b describes the BAU trend of flows of greenhouse gases net of BAU sequestration. It is associated with the BAU trend of greenhouse gas stocks previously described in Figure B.1a. Line 1 of Figure B.1b shows net flows kept constant at current rates, just as line 1 of Figure B.1a shows constant stocks at the current level. The BAU lines of Figure B.1 are used in subsequent diagrams as the base case to which the effects of classes of instruments are compared. Actual future trends, in reality, will probably be nonlinear (i.e., curve) with respect to time, but the trend line is shown here as linear to illustrate the principles involved in characterizing different policy instruments.

Characterization of Classes of Policy Instruments

The conclusion that follows from the above discussion is that the time dimension is a useful addition to the evaluation criteria used to choose among policy instruments. Their relative attractiveness depends on more than their resource costs and the number of tons removed:

• It depends on when the costs are incurred (the later, the better) and when the benefits are felt (the sooner, the better).

• It depends on the effect of the instrument on the speed with which the climate change occurs (the slower, the better).

• It depends on the effect of the instrument on the total global climate change experienced, as summed over the years from the present to the endpoint (the smaller the total, the better).

• It depends on the effect of the instrument on the ultimate level of global climate change imposed on the future at the chosen endpoint (the lower, the better).

Major classes of policy instruments are discussed below with reference to the above time-related criteria.

Temporary Reduction in Greenhouse Gas Flows: Class 1

One class of instruments yields a temporary reduction of greenhouse gas flows. Different cases are illustrated in Figure B.2. Figure B.2b shows