Future emissions of trace gases are intrinsically linked to economic growth and abatement policies, which in turn, are governed by expectations of greenhouse damages. Trace gas indices that depend upon future emissions can be calculated either on the basis of emissions scenarios, such as those devised by the IPCC, or using optimal control techniques where the trade off between damages and abatement costs is made explicit. The scientific and economic issues of multiple gas abatement policies and trace gas indices are comprehensively addressed.