A vital measure for global warming policy is the marginal impact of a tonne of carbon emitted to the atmosphere. In economic terms, this value corresponds to the carbon tax level needed to internalize the externalities associated with climate change. This study re-evaluates the marginal impact of CO2 emissions in the light of new scientific and economic understanding of the cooling effects of sulphate aerosols and ozone depletion, the regional distribution of global warming damages, non-linearity in damage as a function of temperature rise and the appropriate discount rate.