“The footprint methods can evaluate the direct and often indirect environmental effects of a UM for a particular indicator (e.g. global warming potential). As for the energy assessment methods, all flows between components of the assessed system must be considered to evaluate a footprint. Once this step is done, the goal is to add the environmental effects of those components and translate them into one type of environmental impact per habitant (i.e. footprint). The main advantage of the footprint methods is the simplicity of the message it offers to its target audience, that is to say, decision makers. However, this great advantage is counterbalanced by the lack of consideration for other relevant impacts (e.g. human health, biodiversity) that are also linked with the activities of UM. The ecological footprint and the carbon footprints are the two types of footprints that have been used to assess UM in the reviewed studies but others variants (e.g. water footprint) also exist.” (Beloin-Saint-Pierre et al. 2016)

The original idea behind the concept of the footprint was to account for the physical space that the production of a good or service “costs”, measured in global hectares (gha) (Galli et al. 2011). If that were still the case, then the footprint methods would not actually be included in this review as the focus is not on the quantification of land mass, but of materials. However, there are many other forms of footprint that steer away from the physical space and look at the amount (in mass) of a particular material that it takes, is embodied or was emitted during the production of a good, for example.

A multitude of these individual footprints that can be assessed, encompassed in the “environmental footprint family”. There are various approaches to determine these footprints, which can be grouped under EE-IOA, LCA (process-based) or a hybrid of the two (Vanham et al. 2019). Ecological, carbon and water footprints are the most prominent ones in terms of number of studies, followed by the analysis of energy, material and land footprint (Vanham et al. 2019).

Again, the footprint methods’ main strength lies in the fact that they are rather easy to understand and are therefore also very suitable for communication with decision-makers and the public. However, exactly herein also remains one of their largest weaknesses in that trade-offs or problem shifting to either other footprints or even concerns that are not expressed in a footprint, are not reflected by only looking at an individual footprint. This issue could be remedied by integrating multiple footprints, which a handful of authors have already called for (Galli et al. 2012; Vanham et al. 2019). This could either be done around the idea of the footprint family or by developing specific footprints around material groups. For example, Goldstein et al. (2017, 2) suggest that there may be a need to develop methods for the urban foodprint, “to capture the various elements of diverse resource consumption and environmental impacts associated with the production, processing, distribution, and waste generation of food demanded by urban residents. The foodprint may be measured in a variety of ways and include units of mass, embodied carbon, ecological footprint (EF), nutrient flows, or other relevant indicators.” Since CityLoops deals with organic waste as well, investigating an urban foodprint may be of interest.

In addition to the issue of integrating footprints, there is difficulty with this method category as well, as they “have so far been calculated using different methodological approaches” (Vanham et al. 2019). In their review of studies, Goldstein et al. (2017, 7) remarked that it was an obstacle to deal with “studies using equally distinct methodologies within assessment study categories (e.g., input output [I-O] vs. process), entity accounted (household vs. city), and data sources (national, regional, or city).”

Adding to that criticism, it could be argued that the single components of the footprint family are not actually methods on their own but indicators that are derived by applying other methods (e.g. EE-IOA or LCA). Since it is not the goal of this literature review to redefine or reclassify these various fields, it will remain in this list of methods here, as people know or understand it as such.

The three most studied footprints are ecological, carbon and water footprints.