Urbanization is the study of the social, political, and economic relationships in cities, and someone specializing in urban sociology would study those relationships. In some ways, cities can be microcosms of universal human behavior, while in others, they provide a unique environment that yields their own brand of human behavior. There is no strict dividing line between rural and urban; there is a continuum where one bleeds into the other. However, once a geographically concentrated population has reached approximately 100,000 people, it typically behaves like a city regardless of its designation. There are three prerequisites for the development of a city.

First, a good environment with fresh water and a favorable climate; second, advanced technology, producing a food surplus to support non-farmers; and third, strong social organization to ensure social stability and a stable economy. Most scholars agree that the first cities were developed somewhere in ancient Mesopotamia, though there are disagreements about exactly where. Most early cities were small by today’s standards, and the largest city around 100 CE was most likely Rome, with about  650,000 inhabitants. The factors limiting the size of ancient cities included a lack of adequate sewage control, limited food supply, and immigration restrictions. For example, serfs were tied to the land, and transportation was limited and inefficient. Today, the primary influence on cities’ growth is economic forces.

Growth of Urban Populations

Urbanization levels are affected by two things – migration and natural increase. Migration is the movement of the population from one area to another. Some migrations are forced, voluntary, permanent, temporary, international, and regional. Rural-to-urban migration is the movement of people from the countryside to city areas. This type of migration happened in developed countries from the 18th century onwards on a large scale and has gradually slowed down. However, many developing countries are experiencing massive rural-to-urban migration, mainly of young males, into the major cities. The major reasons for migration can be classified into push and pull factors.

A push factor can force or encourage people to move away from a country. Push factors include famine (as in Ethiopia in the 1980s), drought, flooding, a lack of employment opportunities, population growth, overpopulation, and civil war. A pull factor encourages people to move to a city. Pull factors include the chance of a better job, better access to education and services, and a higher standard of living. These factors have contributed to millions of people in developing countries moving to cities, creating mass urbanization. Natural increase (a population increase due to more births and fewer deaths) also significantly affects urbanization rates. Natural increase is stimulated by better access to medical care, improved water supplies, sanitary conditions, and wealth.

Suburbs and Exurbs

As cities grew and became more crowded (and often more impoverished and costly), more and more people began to migrate back out of them. But instead of returning to rural small towns (like they had resided in before moving to the city), these people needed close access to the cities for their jobs. In the 1850s, suburbs developed as the urban population greatly expanded and transportation options improved. Suburbs are the communities surrounding cities, typically close enough for a daily commute but far enough away to allow for more space than city living affords. The bucolic suburban landscape of the early 20th century has largely disappeared due to sprawl.

Urban sprawl contributes to traffic congestion, which contributes to commuting time. Commuting times and distances have continued to increase as new suburbs developed farther and farther from city centers. Simultaneously, this dynamic contributed to an exponential increase in natural resource use, like petroleum, which sequentially increased pollution in the form of carbon emissions (negative aspects of urban sprawl will be explored further in the following section).

As the suburbs became more crowded and lost their charm, those who could afford it turned to the exurbs. These communities exist outside the ring of suburbs and are typically populated by even wealthier families who want more space and have the resources to lengthen their commute. It is interesting to note that, unlike U.S. cities, Canadian cities have always retained a fairly large elite residential presence in enclaves around the city centers, a pattern that has been augmented in recent decades by patterns of inner-city resettlement by elites (Caulfield 1994; Keil and Kipfer 2003). As cities evolve from industrial to postindustrial, this practice of gentrification becomes more common. Gentrification refers to members of the middle and upper classes entering city areas that have been historically less affluent and renovating properties. At the same time, the poor urban underclass is forced by resulting price pressures to leave those neighborhoods. This practice is widespread, and the lower class is pushed into increasingly decaying portions of the city.

The city centers, suburbs, exurbs, and metropolitan areas combine to form a metropolis. New York was the first North American megalopolis, a huge urban corridor encompassing multiple cities and their surrounding suburbs. The Toronto-Hamilton-Oshawa and Calgary-Edmonton corridors are similar megalopolis formations. These metropolises use vast natural resources and are a growing part of the North American landscape.


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Introduction to Environmental Sciences and Sustainability Copyright © 2023 by Emily P. Harris is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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