Illustrative example of a relative and absolute environmental sustainability assessment.
Relative environmental sustainability assessment:
A diesel car and an electric train may be compared on the basis of the life-cycle impacts they cause while transporting a person 6000 km throughout a year, using a comprehensive list of impact categories. If transportation using electric train turns out to have the best environmental performance overall (for example, based on a weighted sum of impacts), it can then be identified as being more environmentally sustainable than transportation using a diesel car. The outcome of a relative assessment greatly depends on the choice of reference. For example, the electric train would perhaps not be the superior option if a bicycle was also considered in the assessment. Also, a relative assessment cannot evaluate whether any of the transportation modes performs ‘well enough’ to be part of an environmentally sustainable economy.
Absolute environmental sustainability assessment:
Instead of comparing the environmental impacts of different modes of transport to each other, an absolute assessment compares them to an external list of environmental carrying capacities. For example, the life-cycle climate impacts of a person’s annual commuting generated by using a diesel car, an electric train and a bicycle could be compared to a share of a carrying capacity derived from the 1.5 degree climate goal of the Paris Agreement (UNFCCC 2015). This carrying capacity share could be calculated using one or more sharing principles. For example, an ‘equal per capita’-principle could initially be used to assign a carrying capacity share to an individual, followed by another principle that captures the value of commuting relative to the other consumption activities in which that individual engages. It may then turn out that the bicycle is the only mode of transport whose climate impact does not exceed its assigned carrying capacity. Note that the ranking of performance between modes of transport is likely to be similar in a relative and absolute assessment (e.g. diesel car worst and bike best).
