Diesel or Petrol ? An environmental comparison hampered by uncertainty

Diesel cars take up a growing share of the passenger car market, and for Belgium this goes up to 55%. In addition to improved diesel technology, diesel has been ‘promoted’ by Belgian fiscal policy because excises on diesel fuel are lower. This has sparked a debate on the environmental consequences of the shift to diesel. In this paper we examine whether diesels are better than petrol cars taking into account the underlying uncertainties and assumptions about their environmental effects. Although diesels emit less greenhouse gasses there are serious concerns about the public health effects of their PM emissions. The central question is therefore which pollutants are more important. This type of question is usually addressed by calculations of external costs, but the answers are likely to be uncertain. Therefore we have assembled information on the uncertainty of the parameters used in calculations of external costs with the ExternE methodology. A simplified-world model (ExTC) was built that can handle probability distributions in addition to single point estimates for each parameter. The propagation of the variability of all parameters through the model calculations results in a probability distribution of the final estimate. Calculations show that estimates of external costs are very uncertain and overlap for petrol and diesel cars. Most variation is attributed to the uncertainty on PM-related mortality and the global warming impacts of CO2. The comparison of petrol and diesel with paired assumptions strongly depends on local population densities. For Belgium, we find a consistent net environmental benefit for petrol cars in most areas and for a wide range of assumptions. The results shed some light on the benefits of both engine technologies from an environmental policy perspective and indicate which type of research in epidemiology or economics would bring the largest improvement to estimates of external costs.