text: The explanation for the convex abatement cost penalty curve is as follows. With a high level of participation the least cost abatement options are used first. However, if there is less participation, some low cost options are not available, so higher cost options have to be used to achieve the same emissions reductions. Figure 1 shows the ratio by which the abatement cost would increase for less than full participation. For example, at 50% participation, the cost penalty would be a factor of 3.5 (i.e. 250%) higher than with full participation (ref. A Question of Balance , Ch VI, pp. 116-122 ).