The concept of an urban metabolism provides a means of understanding the sustainable development of cities by drawing analogy with the metabolic processes of organisms. The parallels are strong: “Cities transform raw materials, fuel, and water into the built environment, human biomass and waste” (Decker et al. 2000). In practice the study of an urban metabolism (in urban ecology) requires quantification of the inputs, outputs and storage of energy, water, nutrients, materials and wastes.
Urban metabolism can be defined as “the sum total of the technical and socio-economic processes that occur in cities, resulting in growth, production of energy, and elimination of waste” (Kennedy et al. 2007). The metabolism of an ecosystem involving the production, via photosynthesis, and consumption, by respiration, of organic matter is often expressed by ecologists in terms of energy. A few studies of urban metabolism have focused on quantifying the embodied energy in cities, while others have more broadly included fluxes of nutrients and materials, and the urban hydrologic cycle.
First applied by Wolman to a hypothetical American city of 1 million people, there have been about ten metabolism studies of actual cities worldwide. These include studies in the 1970s of Tokyo, Brussels and Hong Kong, and then from the 1990s: Vienna, Sydney, London, Toronto, Cape Town and part of the Swiss lowlands. The regions of these studies are typically greater metropolitan areas, including rural or agricultural fringes around urban centers.
There are a variety of practical reasons for studying the urban metabolism. Firstly, the metabolism parameters provide suitable measures of the magnitude of resource exploitation and waste generation to be used as sustainability indicators. The metabolism provides measures of resource efficiency and the degree of circularity of resource streams, and may be helpful in identifying opportunities to improve theses measures. As well as providing a comprehensive accounting of the stocks and flows through cities, urban metabolism also provides a context to understand critical processes such as rising or falling groundwater tables, urban heat islands, accumulation of nutrients, and the long-terms impacts of hazardous materials stored in the building stock. It is pertinent for urban policy makers to understand the metabolism of their cities, to consider to what extent their nearest resources are close to exhaustion and, where necessary, develop appropriate strategies to slow exploitation.
Several factors influence the metabolism of cities. Urban form, including density and morphology, and the evolution of transportation technology can influence both energy and material flows. Sprawled, low-density cities have higher per capita transportation energy requirements than compact cities. Climate also has an impact on the metabolism; for example, cities with interior continental climates will expend more energy on winter heating and summer cooling than those with temperate climates. Application of technology, appropriate use of vegetation, policies such as building codes, and the costs of energy may also influence energy consumption. Similarly, climate, choice of technology, e.g., for recycling, policies supporting demand management, and social attitudes will impact the flows of water and nutrients. The age of a city, its overall infrastructure, and its stage of industrial development might also impact its urban metabolism.
Studies from cities around the world show that urban metabolism is increasing. Certainly this is the case in absolute terms, where the population of cities is increasing with urbanization. Even in per capita terms, trends are generally upwards. In Hong Kong, for example, per capita food, water and materials consumption increased by 20%, 40% and 149%, respectively from 1971 to 1997. Studies also show that Hamburg and Vienna have become more material intensive. Water and wastewater flows were typically greater for cities in the 1990s than those in the early 1970s. Energy inputs to Sydney increased over the same period, although in Toronto per capita energy use showed signs of leveling off during the 1990s. Changes in waste streams are mixed. Emissions of sulfur dioxide (SO2) and particulates have decreased in several cities, while nitrogen oxides (NOx) has increased. Through implementation of large-scale recycling, many cities have seen reductions in residential waste disposal, in absolute terms, but outputs of commercial and industrial waste are possibly on the increase. The implication of increasing metabolism, with today’s predominant technologies, is accelerated consumption of natural resources; greater loss of farmland, forests and species diversity; as well as more traffic and more pollution.
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- Decker, H., S. Elliott, F.A. Smith, D.R. Blake, and F. Sherwood Rowland (2000). Energy and material flow through the urban ecosystem. Annual Review of Energy and the Environment, 25:685-740.
- Kennedy, C.A., J. Cuddihy, and J. Engel Yan (2007). The changing metabolism of cities, Journal of Industrial Ecology. May. [doi:10.1162/jiec.0.1107][www.mitpressjournals.org/doi/abs/10.1162/jiec.0.1107].
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- Wolman, A. (1965). The metabolism of cities. Scientific American 213(3):179-190.