Investigating the inventory and characterization aspects of footprinting methods. Lessons for the classification and integration of footprints

Inventory and characterization schemes play different roles in shaping a variety of footprint indicators. This paper performs a systematic and critical investigation of the hidden inventory aspect and characterization aspect of selected footprints with implications for classification and integration of those footprints. It shows that all of the carbon, water, land and material footprints have two fundamentally distinct versions, addressing the environmental exchange of substances in terms of emissions and/or extractions either at the inventory level or at the impact assessment level. We therefore differentiate two broad categories of footprints, namely the inventory-oriented footprints and the impact-oriented footprints. The former allow for a physical interpretation of human pressure by inventorying emissions and extractions and aggregating them with value-based weighting factors, whereas the latter assess and aggregate the inventory results according to their potential contributions to a specific environmental impact using science-based characterization factors, with the recognition that these contributing substances are too different to be compared by mass, volume or area. While both categories have individual strengths and weaknesses, the impact-oriented footprints have a better performance than the inventory-oriented footprints on the integration of footprints into a single-score metric in support of policy making. Resembling the general procedure for life cycle impact assessment, we formulate a three-step framework for characterization, normalization and weighting of a set of impact-oriented footprints to yield a composite footprint index, which would allow policy makers to better assess the overall environmental impacts of entities at multiple scales ranging from single products, organizations, nations, even to the whole economy. The main value added of this paper is the establishment of a unified framework for structuring, categorizing and integrating different footprints. It may serve as a starting point for clearing the footprint jungle and for facilitating the ongoing discourse on a truly integrated footprint family