How large a carbon tax is justified by the secondary benefits of CO2 abatement?

The combustion of fossil fuels emits a range of damaging pollutants, the emissions of which are reduced if fossil fuel use is reduced in order to achieve CO2 abatement. These reductions are termed the secondary benefits of such abatement. The paper reviews estimates of the size of these benefits at current levels of emissions of the relevant pollutants. Although the estimates are few and uncertain, their mid-range suggests that the secondary benefits are of the same order of magnitude as the gross costs of medium to high levels of CO2 abatement, and are substantially larger than the (equally uncertain) estimates of the primary benefits of CO2 abatement, except where these benefits derive from consideration of damages from unabated global warming in the very long term. The paper then reviews these calculations in the light of the limits on SO2 emissions mandated by the Second Sulphur Protocol (SSP). It finds that the secondary benefits from abating SO2 alone beyond the limits of the SSP still provide a substantial offset to the costs of a carbon tax. The paper concludes that the existence of significant secondary benefits greatly reinforces the economic case for an aggressive policy of CO2 abatement.