Christopher Kennedy is an associate professor in the Department of Civil Engineering at the University of Toronto in Toronto, Canada.
The Changing Metabolism of Cities
Article first published online: 8 FEB 2008
Journal of Industrial Ecology
Volume 11, Issue 2, pages 43–59, April 2007
How to Cite
Kennedy, C., Cuddihy, J. and Engel-Yan, J. (2007), The Changing Metabolism of Cities. Journal of Industrial Ecology, 11: 43–59. doi: 10.1162/jie.2007.1107
John Cuddihy, former graduate student in the Department of Civil Engineering at the University of Toronto in Toronto, Canada.
Joshua Engel-Yan, former graduate student in the Department of Civil Engineering at the University of Toronto in Toronto, Canada.
- Issue published online: 8 FEB 2008
- Article first published online: 8 FEB 2008
- global cities;
- industrial ecology;
- materials flow analysis (MFA);
- sustainable cities;
- urban environment;
- urban metabolism
Data from urban metabolism studies from eight metropolitan regions across five continents, conducted in various years since 1965, are assembled in consistent units and compared. Together with studies of water, materials, energy, and nutrient flows from additional cities, the comparison provides insights into the changing metabolism of cities. Most cities studied exhibit increasing per capita metabolism with respect to water, wastewater, energy, and materials, although one city showed increasing efficiency for energy and water over the 1990s. Changes in solid waste streams and air pollutant emissions are mixed.
The review also identifies metabolic processes that threaten the sustainability of cities. These include altered ground water levels, exhaustion of local materials, accumulation of toxic materials, summer heat islands, and irregular accumulation of nutrients. Beyond concerns over the sheer magnitudes of resource flows into cities, an understanding of these accumulation or storage processes in the urban metabolism is critical. Growth, which is inherently part of metabolism, causes changes in water stored in urban aquifers, materials in the building stock, heat stored in the urban canopy layer, and potentially useful nutrients in urban waste dumps.
Practical reasons exist for understanding urban metabolism. The vitality of cities depends on spatial relationships with surrounding hinterlands and global resource webs. Increasing metabolism implies greater loss of farmland, forests, and species diversity; plus more traffic and more pollution. Urban policy makers should consider to what extent their nearest resources are close to exhaustion and, if necessary, appropriate strategies to slow exploitation. It is apparent from this review that metabolism data have been established for only a few cities worldwide, and interpretation issues exist due to lack of common conventions. Further urban metabolism studies are required.