Ullrika Sahlin

The climate impact of new cars in Sweden 2009 has been evaluated by the Swedish Transport Administration. Their report takes into account reduction factors to attribute the positive impact of renewable fuels on CO2 emissions. The Swedish Transport Administration recommends the public to buy cars that can run on biofuels. Besides acknowledging prevailing uncertainties for many of the input parameters to the index of new cars' climate impact, reduction factors are based on calculations from point estimates of input parameters. A probabilistic re-assessment of the index is presented to find out the importance of these uncertainties and to assess whether the point estimated recommendation might be misguiding. Probabilistic reduction factors for CO2 emissions were derived with the same deterministic model proposed by the Swedish Transport Administration, were Bayesian probability distributions or intervals assigned by expert judgements were used to describe uncertainty in the model input parameters. The use of biofuels most likely reduces CO2 emissions. Probabilistic modelling indicated a CO2 reduction for E85 as a fuel of 30% (95% credibility interval = 10-52%) in the same order as the 20% given by the Swedish Transport Administration. The best estimate of 28% decrease for gas cars (95% credibility interval = 3-44%) and is lower than the originally proposed reduction of 42%, but still within a similar range. The difference is due to the large extent of optimistic values used in the assessment by the Swedish Transport Administration. The CO2 emissions from the production of the biofuel had most influence on the model results. We conclude that the recommendation of the Swedish Transport Administration to consumers is still valid after probabilistic recalculation. (C) 2011 Elsevier Ltd. All rights reserved.