Urban metabolism (UM) is a model to facilitate the description and analysis of the flows of the materials and energy within cities, such as undertaken in a material flow analysis of a city. It provides researchers with a metaphorical framework to study the interactions of natural and human systems in specific regions. From the beginning, researchers have tweaked and altered the parameters of the urban metabolism model. C. Kennedy and fellow researchers have produced a clear definition in the 2007 paper The Changing Metabolism of Cities claiming that urban metabolism is "the sum total of the technical and socio-economic process that occur in cities, resulting in growth, production of energy and elimination of waste." With the growing concern of climate change and atmospheric degradation, the use of the urban metabolism model has become a key element in determining and maintaining levels of sustainability and health in cities around the world. Urban metabolism provides a unified or holistic viewpoint to encompass all of the activities of a city in a single model.
History With deep roots in sociology, Karl Marx and fellow researcher Friedrich Engels may have been the first to raise concerns around issues which we would now call urban metabolism. Marx and Engels concentrated on the social organization of the harvesting of the Earth's materials by "analysing the dynamic internal relationships between humans and nature." Marx used the metaphor of metabolism to refer to the actual metabolic interactions that take place through humans' exertion of physical labour to cultivate the Earth for sustenance and shelter. In short, Marx and Engels found that when humans exerted such physical labour they ultimately altered the biophysical processes as well. This acknowledgement of altering the biophysical landscape is the first stepping stone for the creation of urban metabolism within social geography. They also used metabolism to describe the material and energy exchange between nature and society in as a critique of industrialization (1883) which created an interdependent set of societal needs brought into play through the concrete organization of human labour. Marx advocated that urban metabolism becomes a power in itself (like capitalism), and will control society unless society is able to control it. Later, in reaction against industrialization and coal use, Sir Patrick Geddes, a Scottish biologist, undertook an ecological critique of urbanization in 1885, making him the first scientist to attempt an empirical description of societal metabolism on a macroeconomic scale. Through his experimental study of urbanization he established a physical budget for urban energy and material throughput by way of an input output table.
"Geddes' table consisted of the sources of energy and materials transformed into products in three stages: (1) extraction of fuels and raw materials; (2) the manufacture and transport; and (3) exchange. The table also included intermediary products used for manufacture or transport of the final products; calculation of energy losses between each of the three stages; and the resultant final product; which was often surprisingly small, in material terms, compared with its overall material inputs." It wasn't until 1965 when Abel Wolman fully developed and used the term urban metabolism in his work, "The Metabolism of Cities" which he developed in response to deteriorating air and water qualities in American cities. In this study Wolman developed a model which allowed him to determine the inflow and outflow rates of a hypothetical American City with a population of 1 million people. The model allows the monitoring and documentation of natural resources used (mainly water) and the consequential creation and out-put of waste. Wolman's study highlighted the fact that there are physical limitations to the natural resources we use on a day-to-day basis and with frequent use, the compilation of waste can and will create problems. It also helped focus researchers and professionals of their time to focus their attention on the system wide impacts of consumption of goods and sequential production of waste within the urban environment. Working off of Wolman's pioneering work in the 60s, environmentalist Herbert Girardet (1996) began to see and document his findings in the connection between urban metabolism and sustainable cities. Girardet laid the foundation for the industrial ecology approach to urban metabolism in which it is seen as the "conversion of nature into society." Aside from being a great advocate and populariser for urban metabolism, Girardet significantly coined and drew the difference between a 'circular' and 'linear' metabolism. In a circular cycle, there is nearly no waste and almost everything is re-used. Girardet characterizes this as a natural world process. On the other hand, a 'linear' metabolism which is characterized as an urban world process has a clear resource in-put and waste out-put. Girardet emphasizes that the accelerated use of linear metabolisms in urban environments is creating an impending global crisis as cities grow. More recently the metabolism frame of reference has been used in the reporting of environmental information in Australia where researchers such as Newman have begun to link urban metabolic measures to and it has been suggested that it can be used to define the sustainability of a city within the ecosystems capacity that can support it. This research has stayed mainly at a descriptive level and did not reach into the political or social forces of urban form and stages of flow. From this research there has been a strong theme in present literature on urban sustainability is that of the need to view the urban system as a whole if we are to best understand and solve the complex problems.
Two main schools of approach
The energy method
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