Challenges in Urban Metabolism: Sustainability and Well-Being in Cities

The concept of urban metabolism dates back to the nineteenth century The term urban metabolism, as emphasized by Cspedes Restrepo and Morales-Pinzn ( Cities are complex ecosystems fully natural, even if we sometimes think human-made structures are not nature. Understanding cities is much easier if we are aware that they are natural ecosystems, obeying to the same laws (Lotka's maximum power principle, Darwin's evolution, natural selection) as all other compartments and species. Although cities cover <2% of the earth's surface, they consume about 78% of the energy available on the planet, to which one must add the amount of materials and products (food, construction minerals, metals, etc.) that indirectly also require energy consumption. This figure can be explained in economic and social terms, as cities offer residents new opportunities for business, education, health services, security, social life. Supporting these activities (and related benefits) requires huge flows of resources, that most often translate into an environmental stress both locally and globally. Therefore, assessing the resource metabolism of an urban system, although very difficult due to its complexity, becomes increasingly crucial, not only concerning the relation with the environment as a source or a sink, but also concerning the internal dynamics of resource exchange among city components and sectors as well as competition among cities for scarce resources. Over the last few years, there has been an increasing interest in cities and their energy and material use-driven in part by climate change concerns and in part by social problems determined by the social and organization problems generated by the huge migration flows from rural to urban areas. Cities have a central role to play in the reduction of CO 2 emissions and the fight against climate change, among the most serious challenges our society is presently facing Climate change is not the only problem determined by and affecting cities: the abandonment of rural areas, the demand for water to be diverted from natural pathways, the concentration of urban waste and the need for appropriate ways to dispose of them, the demand for transportation and the related space, energy and air pollution constraints, the huge amount of resources in support to urban lifestyles (education, health services, supply chain of food and manufactured goods, etc.), not only require energy and generate pollution, but are the potential source of new market dynamics, labor reorganization, conflicts, new governance tools and roadmaps, demand for community services and finally decline of previous cultural and social models and their replacement by new ones (or by nothing, even worse). Monitoring resource inflows and outflows and understanding how they relate to population, resource availability, and environmental carrying capacity is crucial for aware and concerned urban sustainability policies and governance. Reorganizing cities' structures linking