a
National ecological footprint [global ha/person] 2006
Cities and environmental change 2.3 12
Urbanization level in 2005
10
>90% 80–90%
8
70–80% 60–70%
6
50–60% 40–50%
4
30–40% 20–30%
2
0
<20%
0.2
0.4 0.6 Human development index 2007
0.8
1.0
b
Severity
Household sanitation Ambient air Carbon emissions
Wealth
Figure 2.3-2 Relationship between environmental problems, urbanization and development: (a) Ecological footprint as a function of development and urbanization; (b) Schematic representation of the correlation between ecological footprint and level of development (environmental Kuznets curve). Sourcen: UNEP, 2011b: 461 f.
also take on global dimensions. This footprint is a characteristic feature of a city’s condition and changes with its development. The internal dynamics of a city and its supply and disposal systems can be described using the analogy of the metabolism of an ecosystem, which exchanges energy and material (nutrients and waste products) with its environment. The first explicit use of the concept of metabolism in relation to cities goes back to Wolman (1965), who modelled the metabolism of a hypothetical US city. Wolman’s innovation was to depict the city as an ecosystem. This was a paradigm shift in the field of urban ecology. It was no longer the ecology in the city that was studied, but the ecology of the city; the purpose is
not to explain how ecological processes in the city differ from those in other environments, but how the city, as a dynamic system, exchanges energy and material with its environment (Figure 2.3-3). However, the city differs from natural ecosystems in that the material flows follow a ‘linear metabolism’, while the material flows in natural ecosystems can largely be seen as cyclical or closed. The challenge for cities, therefore, also lies in getting as close as possible to a circular model of metabolism, in which all kinds of waste are recycled, and making materials re-usable, in order to be sustainably embedded in the natural ecosystem in the long term. This is of great urgency because cities today are responsible for 70-80 % of
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