Wolman first developed urban metabolism as a methodological framework in 1965 in response to observations of the declining environmental health in urban environments. The term metabolism is borrowed from ecology where it is used to describe how a living system performs critical functions by taking in material and energy and then outputting waste. Similarly, urban metabolism has been used to analyze the flows of four critical substances – material, water, energy, and nutrients – that are essential to the functioning of cities. A city is sustainable when, like a sustainable ecosystem, it can engage in the throughput of material and energy without accumulating waste.
Kennedy et al. describe how the metabolism of cities has increased, and this has threatened the sustainable development of cities. The changing metabolism of cities is a direct result of the underlying dynamics. The following are examples of how shifts in technology or socioeconomic dynamics impact the flow of material, water, energy, and nutrients:
- Material: Increased population growth and historical (and ongoing) investments in infrastructure that favor the use of automobile has resulted in an outward growth of the city, which entails more use of material in construction.
- Water: With increased construction, there is an increase in paving of land surfaces, which prevents rainwater from permeating into the soil and recharging aquifers.
- Energy: Paved surfaces also absorb heat, which increases the temperature of urban environments, resulting in an increase in the energy used for cooling.
- Nutrient: The development of the modern fertilizer industry (paired, most likely, with an improved political lobbying force) resulted in a decline in the use of treated sewage for agriculture.
In general, we’ve developed an improved capacity to draw resources from a broader context, thereby augmenting the local ecosystem’s sustaining capacity without consideration of the resulting waste and externalities.
While urban metabolism facilitates the identification of how a city operates unsustainably, it can also be used to identify critical processes that enable cities to operate more like natural ecosystems i.e. lowering the rates of metabolism. In general, the key processes entail a greater connection to the “hinterlands” and improved circularity in the consumption of resources. Examples given by Kennedy et al. include fertility exchange, or the use of sewage for agriculture, and recycling. In the same way that we can see how a disruption of an urban area’s ecosystem functioning translates to an increased metabolism and negative impacts to the environment, a restoration of the ecosystem functioning via a greater connection and understanding of local “place” will lead to a decreased metabolism and positive impacts for the environment.
Kennedy, C.A. J. Cuddihy, and J. Engel Yan (2007). The changing metabolism of cities, J. Industrial Ecology, 11(2), 43-59