Environmental Engineering Reference
In-Depth Information
Urban Areas, Impervious Surfaces and Energy Supply
Humans prefer to settle on fertile soils, and this means that urbanization and indus-
trialization have led to disproportionately large net losses of primary productivity.
O'Neill and Abson (2009) quantii ed this reality by analyzing the average potential
NPP in urban areas and comparing it to the average potential NPP of regions and
biomes containing the cities. They concluded that on the global scale, the potential
NPP of urban areas is about 20% higher than the NPP of their regional surround-
ings, and they also found an opposite phenomenon, namely, that the areas set aside
for protection as parks had a lower potential NPP. Taking a different approach,
Imhoff et al. (2004b) calculated that urbanization in the United States has reduced
the country's overall carbon sink capacity by about 1.6% of the pre-urban rate, a
reduction large enough to nearly offset the overall gain of 1.8% created by the
conversion to intensively farmed agricultural land.
Although cities have a history of more than i ve millennia, an overwhelming
majority of agricultural populations had always lived in small settlements, and the
high habitation densities of such villages or farmsteads (including sheds for animals
and storehouses for harvested crops) made for very small spatial claims compared
to the land needed for food and fuel production. Ancient and medieval cities also
had very high residential densities within their stone or brick walls, and because
low yields required all but a small fraction of the population to be engaged in food
production, city sizes remained restricted. For example, Rome, the largest city in
European antiquity, occupied an area of just 15 km
2
within the Aurelian walls, and
with its exceptionally high population total of about one million people, it averaged
nearly 700 people/ha, a high density even by modern urban standard (Smil 2010c).
The overall situation changed little even at the beginning of the early modern era:
historical reconstructions of urban populations indicate that in 1600, only about 5%
of people lived in cities, compared to about 4% in 1500 and about 2.5% in the year
1000 (Klein Goldewijk, Beusen, and Janssen 2010). In the Western world cities began
to expand beyond their medieval walls only with the onset of large-scale industrializa-
tion. In North America they grew with the rising immigration, industrialization, and
the westward settling of the continent during the latter half of the nineteenth century,
and in Latin America, Asia, and Africa the period of rapid urban growth began only
during the twentieth century, mostly after World War II, and in China (where migra-
tion to cities was restricted during the Maoist decades) only during the 1980s.
By 1900 urban residents made up 15% of the global population, and 100 years
later the share had reached about 47%. According to Klein Goldewijk, Beusen, and
