The ecological footprint (EF) “was designed as a readily comprehended indicator of the sustainability of the human economy vis-à-vis the Earth’s remaining “natural” capacity to supply resources (sometimes considered equivalent to the planet’s terrestrial “carrying capacity”). It groups and calculates material and energy requirements of nations (or regions) for a limited number of consumption functions, converts these metabolic flows into the ecologically productive land area required to produce the resources used in these activities, and compares the required areas to available regional, national, and global ecologically productive areas. Existing studies have typically been restricted to the ecological resource output potential of terrestrial areas.” (Daniels and Moore 2001)

“Particularly, the application of EF at city level has been conducted by several scholars. Such applications usually can be categorized into two kinds, namely, the top-down compound and the bottom-up component methods (Moore et al., 2013). The compound method uses national per capita ecological footprint data that is scaled to reflect the city as much as possible. The advantage to the compound method is that total national production, import and export data for key sectors are readily available and easier to locate than city-specific data. However, this method has limited ability to reflect the impacts of local policy and action (Chamberset al., 2000). The component method starts with local data that reflect the study population’s consumption activities and therefore can better assess the local development performances; however, such a method requires more accurate local data, which may be unavailable in some regions (Barrett et al., 2002). Two sub-approaches were proposed for the component method, namely the input output analysis (Bicknell et al., 1998) and the direct estimates of material and energy throughput using local data (Moore et al., 2013).” (Geng et al. 2014, 4)

“Although widely used, EFs have also been criticized considerably and the two main criticisms are still difficult to properly incorporate in footprint calculations (van den Bergh and Verbruggen, 1999; van Kooten and Bulte, 2000; McDowell, 2002). First, the original formulation of EFs assumes spatial homogeneity, which is rarely the case over the spatial scales relevant to urban ecosystem service withdrawal. Second, the aggregation of all ecosystem services ignores the fact that several services may be provided by the same surface area. Based on these and related problems, much discussion has ensued about the applicability of EFs [...] and solving these problems is an area of active research (Wackernagel et al., 2004).” (Jenerette et al. 2006)

Since the EF is actually a measure of biologically productive land area and therefore does not fall under the scope, it could also not have been included in the review. However, as can be seen from the case studies and as should be evident from the description now, materials are first quantified before they are translated into global hectares and there are often other methods applied within a study as well, which warranted a closer look. For more information about the history of EF, ways of calculating it and critique, the paper of McDonald and Patterson (2004) is recommended.

For the sake of completeness, two members of the EF are here also mentioned, as they emerged in the review. First, the energy ecological footprint (EEF), which is “utilized to characterize the pressure of energy consumption on the ecological environment” (Yang and Fan 2019). As there was only one case study that made use of it and its scope (energy within EF) does not directly pertain to urban material accounting, it was excluded from further review. Second, the Sustainable Process Index (SPI) is also a member of the EF family and “is calculated as the ratio of total land area required to sustainably manufacture a product or provide a service to the average available land area per individual, specific to the location of the production facility” (Doble and Kruthiventi 2007) converted from material and energy flows (Gwehenberger and Narodoslawsky 2007). Although this may not be suitable for the urban system as a whole, it is worthwhile to mention it, because it could either be used for the demonstration projects or the sector level of CityLoops.