Urbanization—the transformation of social life from rural to urban settings—is the seminal process in defining the course of civilization. Urban life evolved approximately ten thousand years ago as a result of sophisticated agricultural innovations that led to a food supply of sufficient magnitude to support both the cultivators and a new class of urban residents. These agricultural innovations, mainly irrigation-based public works, required a more complex social order than that of an agrarian village. Urbanization at its base is thus distinguished from the settled agrarian life that preceded it in two important respects: it embodies a multifaceted social hierarchy and relies on sophisticated technologies to support the activities of daily living (Childe 1936). These uniquely urban characteristics consistently define both urbanization and civilization from the past to the present.
Just as civilization emerged from urbanization in the past, so too does the future course of civilization hinge on our ability to incorporate the reality of contemporary urbanization into our responses to twenty-first-century challenges that include climate change, the elimination of severe poverty, ecological balance, the conquest of communicable diseases, and other pressing social and environmental problems. This is the case because since 2007 more than half of the world’s population resides in urban settings—a historic first.
Urban life can be defined, following Louis Wirth (1938), as life in permanent dense settlements with socially diverse populations. According to Lewis Mumford (1937), the city plays a critical role in the creation and maintenance of culture and civilization. Finally, following Henri Pirenne (1925), the crucial role of trade and production should be stressed. Thus urbanization involves an ongoing threefold process: (1) urbanization geographically and spatially spreads the number and density of permanent settlements; (2) settlements become comprised of populations that are socially and ethnically differentiated; and (3) these urban populations thrive through the production and exchange of a diverse array of manufactured and cultural products.
THE CONTEMPORARY SPREAD OF URBANIZATION
Although city life extends back at least ten millennia, the shape and size of modern urban settlements have roots that extend back only about 250 years to the Industrial Revolution, which marked a significant transformation in the role of cities as loci of critical productive activity and not just as cultural and political centers for a surrounding agrarian countryside. The present characterization of the world as predominantly urban is the cumulative result of this urbanization-industrialization trend. Industrial urbanization emerged first in the countries of the West, then in the countries of Latin America and the Caribbean, and is now strongly evident in Asia and Africa (Garau et al. 2005).
According to United Nations projections, by 2030 the increase of 2.06 billion in net global population will occur in urban areas. Over 94 percent of that urban total (1.96 billion) will be in the world’s less-developed regions (UN Population Division 2004). This means that virtually all of the additional needs of the world’s future population will have to be addressed in the urban areas of the poorest countries.
UN-Habitat, the United Nations agency responsible for promoting sustainable urban development, estimates that roughly one-third of the current urban population of about three billion live in places that can be characterized as slums. The UN classification schema for slums is a fivefold measure: lack of access to safe drinking water, lack of access to sanitation, inadequate shelter, overcrowding, and lack of security of tenure. If a place of residence meets any one of these measures, it is classified as a slum. By the year 2030, if nothing is done, the proportion of the urban population living in slums will rise to 43 percent (1.7 billion in an urban population of 3.93 billion). All of these people will be living in the urban slums of countries in the developing world, mainly in sub-Saharan Africa and Southeast Asia (UN-HABITAT and Global Urban Observatory 2003).
DENSITY, DIVERSITY, AND URBANIZATION
Urbanization is most powerfully observed through physical density (comparatively large numbers of people living in comparatively small areas). The ratio of population size to land area is the standard metric for evaluating density. It is a precise measurement that is not easily interpreted because neither the numerator (population size) nor the denominator (land area) is static. Political boundaries are only marginally helpful in defining the effective size of an urban settlement because at any moment changes in communications and transport technology alter the size of the relevant space over which urban residents live and work. Until the end of the eighteenth century, cities were spatially compact places with a radius of about one to two miles—the distance an individual could comfortably walk in carrying out daily activities. With the arrival of industrialization, effective urban size spread rapidly through the nineteenth and twentieth centuries. Contemporary urban or, more properly, metropolitan settlements easily encompass radii of 50 miles or more. This is a result of the continual improvement in rapid overland and even air travel modes and digital and wireless communications technology. The exact spatial configuration of any metropolitan settlement is a compromise between activities that must remain within walking distance and those for which residents are willing to travel (Schaeffer and Sclar 1980).
These spatially widening metropolises are not uniform in terms of their residential population densities. Hence density measurements alone tell us little about the quality of urban life. This quality can vary widely within the comparatively small confines of any metropolitan area. It is especially important to understand that high density per se is not an indicator of compromised living conditions. Metropolitan New York, which includes both the central city and the surrounding suburbs, has an average population density of over 5,300 people per square mile (ppm2), but the wealthiest part of the region, Manhattan Island, has a density that exceeds 66,000 ppm2. By way of comparison Nairobi, Kenya, has a city-wide density of over 1,400 ppm , but its centrally located slum, Kibera, considered the largest in Africa, has an estimated population density of at least 100,000 ppm . While Kibera’s density significantly exceeds Manhattan’s, the major determinant of the differences in quality of life relate to the quality of shelter and the ancillary urban services, such as water, sanitation, public safety, and most importantly transportation. Very poor urban residents must exchange life in high-density, poorly serviced places for the ability to walk to the places in the urban center where they earn a livelihood.
The major urban challenge of the twenty-first century concerns the ability of governments to effectively provide adequate shelter and to plan and deliver services for metropolitan-wide areas in developing countries. The difficulty in meeting the challenge is rooted in part in the fact that the historical political boundaries of the central city and suburban (i.e., satellite city) subunits of government typically derive from an earlier century, before contemporary transport and communications technology redefined effective spatial relationships. The urban economies of modern metropolises now run beyond the legal jurisdictions of the subunits of government responsible for infrastructure and public services. The insistence of international financial agencies and donors on governmental decentralization in developing countries has only served to exacerbate this problem because it has left these governmental subunits with the responsibility but without either the technical ability or revenue sources. The result is that necessary regional planning and infrastructure investment to address the challenges of urbanization are often stymied.
Urban population growth is largely migration driven. On one side there is the push of rural poverty and on the other the pull of urban opportunity. This migration-driven growth is further exacerbated by natural rates of urban population increase (birth rates that exceed death rates). Social life in urban settlements is thus more complex than its village counterpart. The transactions of daily living in villages are governed by a social economy where goods and services are exchanged on the basis of social roles and rules of reciprocity rooted in longstanding customs and religious observances. The transactions of urbanization that confront the new arrivals are, in whole or in part, defined by the impersonal exchange relationships typical of a market economy. This transition is never a simple one-for-one exchange.
Because urban populations are continually in flux and often simultaneously expanding, they are often characterized by a multiplicity of informal and formal social relationships and institutions in a similar state of flux. The variations among an informal social economy and a formal market economy in any city at any moment in time are highly reflective of the larger external forces, such as globalization and migration, that are continually redefining the roles of different cities in a world of complex trading and production relationships. In the slum of Kibera, many of the activities of daily life, including the provision of vital public services such as water, sanitation, and public safety, are governed by an informal local, but powerful, social economy (Lowenthal 1975). In contrast, life in the working-class neighborhoods of cities in developed countries is typically an amalgam of informal social institutions imbedded in formal mechanisms of municipal public-service delivery. For the wealthiest residents of these same cities, virtually all the services they consume are provided via the formal institutions of government or market exchanges.
URBANISM AND THE GLOBAL ENVIRONMENT
Many of the patterns of contemporary urban development are extensions of those set in place in the nineteenth and twentieth centuries. These patterns were based on three assumptions: (1) energy was relatively inexpensive; (2) safe drinking water was abundant; and (3) the environment could absorb all the waste products of urbanization. None of these assumptions is any longer valid. Consequently, urbanization in the twenty-first century will have to be reconceived in both social and environmental terms. The world cannot afford the political instability and social costs of massive pockets of urban slum dwellers, nor can it accommodate urban growth through a further spatial spread that relies on urban transport powered by carbon-based energy sources and the discharge of waste products into both the local and global atmosphere. A healthy and vibrant environment is now a scarce but vital good.
Environmental problems are principally generated by the disorderly sprawl of urban settlements into the surrounding countryside. In the developed world, metropolitan areas organized around private automobile travel among low-density suburbs and tied to a central business district generate high volumes of automobile travel in the absence of tight land-use regulation and good public transport. This development pattern has led to increased mobile source pollution within the metropolitan areas and significant greenhouse gas emissions that endanger the whole planet. Sprawl requires an ever-increasing spread of impermeable (i.e., paved) ground surfaces. This in turn leads to the runoff of polluted waters into the groundwater supplies. The paved surfaces absorb heat from the sun and create urban heat islands that require more energy consumption and the emission of pollution and greenhouse gases to cool homes and offices. In addition, there are inadequate landfills to collect all the refuse of these high-consumption urban centers.
In the developing world, the problems are similar but more acute in their direct manifestation. The high rates of population growth lead to a pattern in which urban settlement runs ahead of infrastructure improvement. This leads to the establishment of informal settlements (i.e., slums) characterized by an absolute lack of safe drinking water and sanitation. The lack of adequate public transport and public health protection systems leads to a congestion of private cars and informal transports in the center of cities, which exacerbates the air quality problems and greenhouse gas emissions. The social costs of the lack of these services fall disproportionately on the poorest residents of these burgeoning metropolitan areas. These costs take the form of excessive mortality and morbidity rates, low rates of labor productivity, and the reinforcement of an ongoing trap of urban poverty.
Climate change generated by greenhouse gas emissions adds yet another layer of special urgency to these pressing social problems. It is the very concentrated nature of cities—their population densities and their centrality in social functioning—that makes them and their residents so vulnerable to the hazards and stresses that climate change is inducing. Rising sea levels and warming water make serious climatic assaults on cities more frequent. Devastating storms and floods that hit once in a century now occur in far shorter cycles. The impacts are not equitably distributed. The poorest urban residents tend to live in the riskiest portions of the urban environments—flood plains, unstable slopes, river basins, and coastal areas.
Although the challenges of urbanization are formidable, the technical knowledge for their solutions exists. The question for the twenty-first century involves the ability of the international community, nations, and local governments to create institutions of urban planning and democratic governance that can effectively apply these solutions at a sufficiently broad scale that they can make a measurable difference.