Valuing the impacts of CO2 emissions to the atmosphere has been widely debated, with various studies suggesting very different costs when expressed in units of dollar per tonne of carbon emitted, There are many complex issues involved, and it is often difficult to determine the reasons for the wide range of costs proposed, In this paper, models based on two very different approaches, the PAGE and Intera models, are considered, By applying these models to calculating the marginal impact of CO2 emissions it is shown that what appear initially to be divergent estimates can be reconciled, This process illustrates some of the key issues in this area, and a perspective is provided on which marginal costs are appropriate for different policy decisions.