Many methodological choices in a life cycle assessment (LCA) depend on the goal of the study, i.e. its application area. A better definition of the application areas and especially a clear distinction between retrospective and prospective applications allows a more unambiguous description of the methodology to apply for different applications.
The presentation argues that LCA methods should be selected in relation to the study’s application area. It identifies the affected products and interest groups, together with the temporal and geographical scope, as factors that shape those choices. A central distinction is between retrospective studies, which describe existing systems, and prospective comparisons, which examine possible changes between product systems. Stating which type of question the study addresses can make the methodological choices clearer and reduce ambiguity.
For a prospective comparison, the functional unit defines the substitution being assessed. The paper notes that this change may also affect processes elsewhere in the life cycle. Identifying those effects requires considering the market segment, the scale and duration of the change, product availability, and factors such as price and product information. When systems deliver different quantities of co-products, the presentation recommends system expansion to include the processes affected by the change; retrospective studies require allocation instead. It also links forecasting methods to the time horizon: short-term estimates may use extrapolation, while longer-term or broader system studies may require models or scenarios. Forecasts remain uncertain, but the paper argues that making assumptions explicit can help manage that uncertainty.
