Cities have a metabolism — an in and out flow of materials and energy. The pace and scale of this metabolism help to determine how sustainable cities are. At least, this is a popular idea among academics. I spent last weekend at a workshop at the YMCA in Estes Park, where urban metabolism was front and center. I was participating in the NSF-funded Research Coordination Network on Sustainable Cities. The workshop brought together engineers, modelers, transportation experts, social scientists, and practitioners to discuss the research and practice needs for improving urban sustainability, particularly in the areas of energy, water and climate change.
Going into the workshop I was familiar with the idea of an urban metabolism but afterward I decided that I wanted to learn a bit more. One of the leading articles on the topic is written by Chris Kennedy and colleagues and published in the Journal of Industrial Ecology. They call urban metabolism “the sum total of the technical and socioeconomic processes that occur in cities, resulting in growth, production of energy, and elimination of waste.”
There are three things I learned from reading this. The first isn’t that surprising. Kennedy et al. say that the metabolism of cities is increasing — and they mean that cities’ per capita consumption of water, materials, energy, and nutrients is getting larger. More materials and energy are coming in to cities than are leaving cities, and these materials are coming from farther and farther away: “in some respects the city is like a plant stretching its roots out further and further until its resource needs are met.”
The second interesting idea is that metabolism accounting isn’t only able to track what goes in out of cities. A city’s metabolism also includes what stays in cities: what accumulates in aquifers, buildings, soils, and people. For example, 60% of all phosphorous and nitrogen inputs to Hong Kong never leave the city. These nutrients may be recycled, but the vast majority accumulate. The ability of cities to act as reservoirs for nutrients may be contributing to decreasing soil fertility in agricultural lands. An unanswered question in the paper is whether cities should be actively finding ways to “give back” nutrients to surrounding farm areas — from waste water or food waste for example. This used to be done but now cities handle waste much differently, relying on landfills and incinerators. Could urban-to-rural nutrient transfer practices be used again?
The last point I want to make about this article, and about the concept of urban metabolism, is the assumption that metabolism is for growth. Frankly, I was surprised to see this as part of the definition used by Kennedy and his colleagues. Water, materials, energy and nutrients are needed by cities that are shrinking or in a steady state. The same goes for people — we eat and burn energy and expel waste even when we are done growing. Urban metabolism studies often claim to be useful to decision makers by helping them pinpoint inefficiencies or over-extraction of resources. I think it would also be useful to think about how a metabolism that supports growth is different from one that doesn’t, or what kinds of growth can be supported with different “modes” of metabolism. This would help make urban metabolism studies even more useful in the policy and planning processes.