A multi-level framework for metabolism in urban energy systems from an ecological perspective

Cities have become the main centres of consumption and transformation of resources. As cities are still growing in terms of physical size, number of inhabitants and total energy consumption, unique challenges present themselves in order to safeguard a healthy urban living environment, while preventing resource shortages and pollution. The concept of urban metabolism may contribute to sustainable growth of cities as it can be used to understand emergent patterns in flows of energy and materials in the urban environment. The concept of urban metabolism draws upon an analogy with the biological meaning of metabolism, as it occurs in organisms and ecosystems. Here we present a review of the interpretation of urban metabolism in the context of urban energy dynamics and assess the validity of the proposed analogy with biology. Based on this analogy, we propose a conceptual framework for the role of urban metabolism in urban energy systems. Our review highlights that urban energy systems show a hierarchical organization comparable to ecological systems. However, the emergent patterns that result from the dynamics within urban systems differ from those occurring within ecological systems. We suggest that, in contrast to biological systems, urban energy systems lack energetic constraints at the lowest level of organisation that could enable the resource-efficient regulation of energy requirements at larger scales. The proposed framework highlights that low-level resource supply regulations may contribute to introduce scale-dependent relationships that increase energy use efficiency as cities grow.