The present paper is written with the aim to reconsider the relationship between carbon taxes and a marginal abatement cost curve. A marginal abatement cost curve is deduced from a marginal profit curve which shows the extra profit made by expanding output by one unit.
Figure 11 (which should be compared with Figure 9) tells us that a firm does not abate the carbon dioxide emissions on the marginal abatement cost curve. We should define it in the context of the theory of the firm.