Introducing carrying capacity-based normalisation in LCA: framework and development of references at midpoint level

There is currently a weak or no link between the indicator scores quantified in life cycle assessment (LCA) and the carrying capacity of the affected ecosystems. Such a link must be established if LCA is to support assessments of environmental sustainability and it may be done by developing carrying capacity-based normalisation references. The purpose of this article is to present a framework for normalisation against carrying capacity-based references and to develop average normalisation references (NR) for Europe and the world for all those midpoint impact categories commonly included in LCA that link to the natural environment area of protection. Carrying capacity was in this context defined as the maximum sustained environmental intervention a natural system can withstand without experiencing negative changes in structure or functioning that are difficult or impossible to revert. A literature review was carried out to identify scientifically sound thresholds for each impact category. Carrying capacities were then calculated from these thresholds and expressed in metrics identical to midpoint indicators giving priority to those recommended by ILCD. NR was expressed as the carrying capacity of a reference region divided by its population and thus describes the annual personal share of the carrying capacity. The developed references can be applied to indicator results obtained using commonly applied characterisation models in LCIA. The European NR are generally lower than the global NR, mainly due to a relatively high population density in Europe. The NR were compared to conventional normalisation references (NR') which represent the aggregated interventions for Europe or the world in a recent reference year. For both scales, the aggregated intervention for climate change, photochemical ozone formation and soil quality were found to exceed carrying capacities several times. The developed carrying capacity-based normalisation references offer relevant supplementary reference information to the currently applied references based on society's background interventions by supporting an evaluation of the environmental sustainability of product systems on an absolute scale. Challenges remain with respect to spatial variations to increase the relevance of the normalisation references for impact categories that function at the local or regional scale. The sensitivity of NR to different choices, e.g. threshold value, should be quantified with the aim of understanding and managing uncertainties of NR. For complete coverage of the midpoint impact categories, normalisation references based on sustainability preconditions should be developed for those categories that link to the areas of protection human health and natural resources.