The material footprints of cities and importance of resource use indicators for urban circular economy policies: A comparison of urban metabolisms of Nantes-Saint-Nazaire and Gothenburg

Material consumption has been increasing steadily since the beginning of the 20th century. The urban metabolism field of research is one of the fields that focuses on understanding and measuring this increase at the city level. Many studies have been carried out to calculate the material consumption at the domestic scale. But it is also important to include the non-domestic scale to account for the amount of materials extracted outside the city that were needed along the supply chains to produce the final products consumed in the city. This is referred as the material footprint, which provides a consumption-based indicator of resource use. The objective of this study was to develop a method to measure the material footprint of the cities of Nantes-Saint-Nazaire (France) and Gothenburg (Sweden), both port cities and pioneers in the implementation of urban policies targeting a circular economy. The methodology combines urban material flow analysis with multi-regional input-output database to extend the urban metabolism beyond the administrative boundaries of cities. We then calculated the absolute and per capita material footprints of the two cities and its material disaggregation. We compared these results with domestic material consumption. Further analysis of the urban material footprint was performed by spatializing the flows in the global economy to understand the extent of consumption due to cities. The results show that on average the material footprint is 2.4 times larger than the domestic material consumption in Gothenburg and 1.9 times larger in Nantes-Saint-Nazaire. A decoupling between material footprint and domestic material consumption can be observed, as the material footprints grew much faster than the domestic material consumption. Regarding the material disaggregation, the most significant category is non-metallic minerals, which weighs more than 50% on average of the total material footprint balance sheet and also increased the most. In conclusion, future work should thus better integrate material footprint, as there is a need to better understand the externalization of urban metabolism and to identify what aspects urban circular economy policies should focus on.