The process whereby an increasingly larger percentage of a given population lives in spatial clusters considered to be “towns” and “cities.” The criteria that qualify a settlement as “urban” frequently vary from country to country, meaning that what is considered an “urban place” in one location may not be defined as such in another nation-state. In the United States, the U.S. Bureau of the Census considers any place having a concentrated population greater than 50,000 inhabitants to be an “urban area,” and any population center of at least 2,500 residents outside of an urban area and having a community identity and name to be “urban.” Urbanization is typically associated with a number of related processes: industrialization, economic development, declining average family size, rising levels of literacy and education, and others. In some cases a single urban place will come to dominate a country’s urban geography in terms of both population size and economic function—this city serves as the primate city. In 2008 the United Nations estimated that for the first time in history, more than half of humanity lived in urban areas, and that the fastest rates of urbanization are occurring in the developing world. Urban places are not located on the landscape randomly. Many factors may affect the rate of growth of an urban area, including transportation linkages, natural resources located nearby, economic opportunities, and even the local physical geography can play a role in urbanization. Geographers have developed a number of approaches to explain the patterns of urbanization and to determine the characteristics that may influence urban growth and development. Central Place Theory holds that urban places are spatially arranged in a hierarchical framework, based on the sophistication of economic functions offered by each city or town.
Urbanization on a significant scale first took place in ancient Sumeria, along the course of the Tigris-Euphrates river system. The rich soils of these river valleys allowed for the production of an agricultural surplus, which in turn led to a differentiation of labor, development of social classes, and the emergence of more sophisticated economic and political systems than had previously been the case. The morphology of many early cities was haphazard and chaotic, but some urban areas in the ancient world were surprisingly organized and well planned. The city of Mohenjo Daro, established around 2500 BCE in the Indus River Valley, was clearly a planned settlement with streets and buildings constructed following a grid system, covered sewers, and a municipal garbage collection system! But in general, cities for much of history were places of squalor, congestion, and disease until the late 19th century. Pestilence typically spread quickly in urban areas, due to high population densities and poor sanitation. Industrialization and the agricultural revolution that accompanied it initiated a surge in urbanization that continues in many parts of the world, as thousands of rural workers migrated to the cities seeking employment. By the mid-20th century, zoning legislation, urban decentralization, the development of green belts, and other factors had lowered population densities and improved the quality of life in many urban areas in developed countries, while large cities in the developing world continued to struggle with the problems of pollution, overburdened transportation networks, and shortages of adequate housing and basic services.
The process of urbanization may be driven by numerous forces. The population of urban areas may grow via a relatively high rate of natural increase, or by the arrival of migrants from other regions. The qualities of the physical landscape may coincide with the presence of a break-of-bulk point, or trade flows may lead to the emergence of an entrepôt, either of which may stimulate economic opportunities and urban growth. This in turn will lead to a higher demand for labor, attracting immigrants seeking employment opportunities. Subsequently, urbanization is compounded by the effects of agglomeration, which increase the scale of economic development and bring additional residents to the urban place. A classic example of this process is the growth of Chicago’s population (along with other cities in the northern region of the United States) between 1910 and 1930, when hundreds of thousands of African Americans migrated from the American South to the industrial urban centers of the Midwest. The desire to escape discrimination in their home states drove many of these migrants northward, but the most important factor behind this movement was economic expansion in the north, resulting in the creation of tens of thousands of jobs. Chicago’s black population expanded by nearly 200,000 in these 20 years, or an increase of almost five times the figure of 1910. This domestic migration of American blacks to northern cities followed decades of immigration to those locations by Europeans, who mostly came seeking the economic advantages such large, growing urban places offered.
Geographers have developed various models to describe the developmental patterns of urbanization. The pre-industrial city model was first articulated in the work of Gideon Sjoberg in the early 1960s. Sjoberg held that prior to the industrial age, social and political authority was concentrated in the center of the typical city, and that many cities in the developing world continue to follow this pattern. In the decades prior to Sjoberg’s work, urban geographers had offered several theoretical perspectives on the spatial character of urban places, based on cities in the economically developed world. The first such effort was the concentric zone model, which was based on the development of Chicago in the 1920s. In this case, the urban space is divided into a series of rings around the Central Business District (CBD). Each ring is characterized by a specific type of economic use and activity. In 1939 Homer Hoyt presented a revised urban model, which became known as the Sector Model, or sometimes simply the Hoyt model. The sector model emphasizes the role of transportation linkages in the development of distinct urban functional zones, resulting in “sectors” of development that are associated with transport access and economic functionality. The multi-nuclei model, introduced after World War II, attempts to incorporate some elements from the previous models, while also accounting for the rise of extensive suburban development and other phenomena of urbanization that had appeared by the mid-1940s. All of these models are somewhat flawed, but nevertheless have contributed to our understanding of urbanization processes.
In the last decades of the 20th century, new trends in urbanization were identified, serving to highlight the complexity of the urbanization process. The emergence of so-called asylum suburbs is an example. In these locations residents live along the margins of urban space, enjoying the benefits of urban life from a distance and thereby avoiding many of the disadvantages of residing within the city proper, such as higher crime rates, congestion, higher taxes, higher levels of pollution, etc. In the 1970s and 1980s a trend of “counter-urbanization” was identified by urban geographers. Counter-urbanization occurred as urban populations voluntarily decentralized, with thousands of residents moving away from larger metropolitan areas, especially the emerging megalopolis along the eastern seaboard, into smaller communities lying outside the urban zone, or to smaller urban places located at some distance. More sophisticated and extensive transportation networks, including commuter rail linkages and expanded highways, provided the opportunity to live at a greater distance from the urban environment, but to retain employment there, as well as occasionally enjoying the entertainment and economic benefits the urban center typically offers. To offset the loss of residents to suburbs and rural areas, metropolitan authorities frequently use a strategy of gentrification, which may have the effect of attracting permanent residents, or revitalizing existing business districts to draw visitors for shopping, dining, and entertainment opportunities.
In many cities in the developing world, extremely rapid and unregulated urbanization has led to many problems, including congestion of transportation systems, high levels of pollution, poor and unsanitary housing conditions, and an inability to provide adequate services to the rapidly growing population. Mexico City is an example of a developing city that faces this spectrum of problems. Between 1940 and 1980 the population of the metropolitan area of Mexico City increased by almost 12 million, growing from just under 2 million to well over 14 million inhabitants. Most of this spectacular expansion was due to immigration from rural villages, as millions of poor Mexicans moved to the capital seeking greater employment opportunities, and settled in squatter settlements. The city’s industrial base expanded to take advantage of the increasing supply of cheap labor, especially in the cases of petrochemicals, steel manufacture, and durable goods. Pollution from the dozens of large factories associated with this expansion plagued the city’s residents for decades—from the 1960s to the 1990s Mexico City was frequently ranked as having the worst air quality in the world. The city’s housing market simply could not keep up with demand, and many of the new arrivals could not afford housing in the urban area anyway. The result was a ring of poor neighborhoods composed of shacks around the city, known locally as barrios. Many of these districts have no sewer systems, with electricity and treated water in short supply. Many other cities in developing countries suffer from similar problems, due to rates of urban growth that challenge the most dedicated urban planners.
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