Consequential life cycle assessment of automotive material substitution: Replacing steel with aluminum in production of north American vehicles

The substitution of aluminum for steel in vehicle body and closure components is a common strategy for reducing fuel consumption. In order to assess the greenhouse gas (GHG) consequences of this decision, the system must be examined using a life cycle approach. Furthermore, attributional life cycle assessment (ALCA) does not suffice for a number of reasons, including the fact that ALCA does not model the incremental system and that allocating the benefits of recycling inhibits the modelling of system-wide consequences caused by the decision studied. This study thus uses a consequential life cycle assessment (CLCA) framework. We examine the physical and economic processes that guide the North American light-duty vehicle fleet from its initial state in 2012 to a state in 2050. Industry projections are used to model the production and use phases. The system is expanded to include the scrap and material markets. This generates new insights regarding the environmental consequences of changes in scrap generation and recycling in automotive material substitution. The method is applied to the fleet in order to forecast if and when aluminum intensification constitutes net GHG reduction under various conditions. Using baseline parameter values compiled from public and industry data; we calculate a GHG payback period of 25 years, i.e. before a net reduction in emissions relative to a no change counterfactual is achieved. A local sensitivity analysis is performed, showing that the net GHG reduction may be achieved in a period as short as 12 years, or never be achieved at all. A global sensitivity analysis is performed using Monte Carlo simulation, where 16% of trials never reach a net reduction in GHG emissions. We also estimate which parameters contribute the most to variance in the model outcomes. The material replacement coefficient, or the amount of aluminum it takes to functionally replace one kilogram of steel, is the top contributor to the variance (29.8%). Overall, the results are most sensitive to parameters governing the amount of mass that can be replaced by each kilogram of additional aluminum, the GHG intensity of additional aluminum production, and the response of the aluminum scrap and material markets to additional aluminum scrap generation. We conclude that given the current lack of understanding of key parameters and their underlying uncertainties, it is not possible to definitively state that substituting aluminum for steel results in a net reduction in GHG emissions from a fleet of vehicles.