Urban systems are special cases of socio-ecological systems (SES) that are generated by the sustained interaction of two distinct typologies of complex adaptive autopoietic systems: human societies and ecosystems. As a consequence, urban systems are subject to external constraints that are imposed by processes beyond human control and that determine the feasibility of the urban system: cities must be able to stabilize the required flow of inputs (food, energy, water, minerals, and imported products) and to dispose of the resulting flow of wastes and emissions while expressing a set of functions that guarantees a satisfactory life style for their citizens. The recent explosion of the urban population at the global scale has been possible because of the takeover of fossil energy as the main supply of energy in modern society. The industrial revolution has allowed us to break the original circular economy, entirely supported by natural cycles and powered by the conversion of food into human work. With the introduction of new systems of production, the economy managed to generate a quantity of jobs and a quality of life-style that was simply inconceivable in pre-industrial rural economies.
Within this general framework, urban systems can be perceived as highly specialized organs of human societies characterized by: (i) an extremely high spatial density of both fund (humans, technology and infrastructure) and flow elements (energy, food and material); and (ii) an extremely high degree of openness (heavily dependent on the import of biophysical flows and export of wastes). The extreme openness of the metabolic pattern of cities makes it difficult to define the boundaries of the urban system. The effect of globalization has accentuated this boundary problem. The massive trade of commodities across distant countries has made the urban metabolism (and especially that of megacities) virtually independent from the economic and biophysical processes taking place in the immediate rural surroundings.
Multi-Scale Integrated Analysis of Societal and Ecosystem Metabolism (MuSIASEM) provides insight in the metabolic pattern of urban systems by characterizing fund (land-use, population time use, power capacity) and flow elements (food, energy, water), the degree of openness of the system, and the dependence on external resources. Two examples are presented to illustrate this methodological approach: (1) A simple study of the feasibility, viability and desirability of scenarios of agricultural development in the Mauritius Islands based on the analysis of the nexus between food, energy, water and land use; (2) An alternative reading of the present global economic crisis recognizing the existence of planetary boundaries that are slowing down the pace of economic growth.