No decade in history has experienced such an increase in urban population as the last. From Tokyo-Yokohama, the world's largest urban area (population: 37 million) to Godegård, Sweden, which may be the smallest (population: 200), urban areas added 700 million people between 2000 and 2010.

Nearly one in 10 of the world's new urban residents were in the fastest growing metropolitan regions (see: Definition of Terms used in "The Evolving Urban Form" Series), which added nearly 60 million residents. They ranged from a an estimated increase of more than 8.5 people in Karachi (Note 1) to 3.9 million people in Mumbai (Figure 1). The average population growth in these 10 metropolitan regions was 6 million, approximately the population of Dallas-Fort Worth or Toronto, which were fast-growers on their own in comparison to other high income world cities.

By comparison, the largest growth over any single decade over the past half century in US metropolitan areas has been less than one half of the 6 million average: 2.43 million in New York (1920s) and 2.37 million in Los Angeles (1950s). Only Tokyo-Yokohama (1960s) and Shenzhen (1990s) have added more than 5 million people in a single decade before the last decade.

Growth has been overwhelmingly concentrated outside the urban cores (Note 2) in these 10 fastest growing metropolitan region. Excluding Karachi (for which sufficient data is unavailable), approximately 85 percent of the growth was outside the urban cores (A 42 million increase in the suburbs and 8 million in the urban cores).

Dispersion in World Megacities

This is consistent with the findings of The Evolving Urban Form series, which is now two years old. These analyses have generally demonstrated that urban spatial expansion (pejoratively called "sprawl") is world-wide and contrary to some perceptions, not limited to the United States. Cities expand geographically as they add population, though this organic tendency is sometimes contained by urban planning. Peripheral growth is virtually always at lower densities than in urban cores, which means that as cities grow they tend to become less dense (Note 3).

This process ironically is sometimes accelerated by planning decision-making. London's greenbelt ---which banned the extension of housing into the near periphery of the city --- has result in even greater sprawl to far outside the principal urban area. This trend since World War II, has forced commuters to travel longer times and distances to the urban core (All of metropolitan London's growth has been suburban for 100 years, with a loss of 1.8 million in inner London, while the suburbs and exurbs grew by 10.5 million).

The Evolving Urban Form has now covered 23 of the world's 28 megacities (Note 4). As the Table indicates, population growth has been strongly oriented away from the urban cores and toward more suburban areas

In US examples, New York and Los Angeles, 95 percent and 85 percent of growth respectively of their corresponding metropolitan region growth has occurred outside the core municipalities since 1950. But these US regions are joined by middle income Buenos Aires and Mexico City where all growth has been outside urban core since 1950. In lower income Manila, 95 percent of the growth has been outside the urban core since 1950.

The world's largest metropolitan region, Tokyo-Yokohama, has experienced a virtual monopoly of suburban growth over the past 50 years, as has Japan's second largest metropolitan region, Osaka-Kobe-Kyoto.

Over the past quarter century, all of Istanbul's growth has been outside the urban core. The urban expansion has been going on for much longer, as is illustrated over the past 60 years (Figure 2). Cairo's urban expansion is similarly substantial (Figure 3). In one of the developing world's poorer megacities, nearly all population growth in the Mumbai region has been outside the urban core for 50 years

For the last 20 years, more than 115 percent of the growth in the Seoul-Incheon metropolitan region has been outside the core city. In the world's second largest urban area, Jakarta (Jabotabek), growth is also strongly suburban, accounting for 85 percent of growth over the past two decades. In Kolkata suburban growth has been 95 percent over the same two decades.

The same tendency is evident in the other megacities. Over the past decade or two, nearly all population growth in China's four megacities (Shanghai, Beijing, Guangzhou-Foshan and Shenzhen), Delhi and Rio de Janeiro has been outside the urban cores.

Dispersion in Other Large Urban Areas

The Evolving Urban Form has also examined smaller urban areas. The same pattern of dispersal is evident there as well even in traditionally compact cities. Zürich, for example has had all of its growth outside the core city since 1950. All of the growth in Barcelona and Milan has been outside the core cities for 40 years. Even high density Hong Kong has experienced all of its growth outside the urban core for three decades. Low income Addis Abeba indicates a pattern of urban expansion is not unlike that of Istanbul or Cairo (Figure 4). In megacity wannabe Chicago (1.4 million short), 125 percent of growth since 1950 has been outside the core; this number reflects that the central city has been shrinking even as the periphery expands. Even in fast-growing Dallas-Fort Worth, more than 80 percent of population growth over the past 60 years has been outside the city of Dallas (which itself is largely suburban in form, see Suburbanized Core Cities).

The one notable exception to the peripheral growth model is Quanzhou (Fujian, China), which is developing under an even more dispersed pattern, described by Yu Zhu, Xinhua Qi, Huaiyou Shao and Kaijing He at Fujian Normal University. Typically, urban areas expand from an urban core on the periphery. Quanzhou is experiencing "in situ" urbanization, the spontaneous conversion of rural areas into urban development that does not expand from the urban core. The result is a sparsely developed urban area (especially for China), with plenty of land for potential infill development in the future.

The Future of Urbanization

It is likely that urban areas will continue to expand as they grow larger, consistent with what appears to be both economic pressures and market preferences for lower cost, more spacious housing. For example, fast growing Ho Chi Minh City is expected to see virtually all of its population increase over the next 15 years outside the urban core. Not surprisingly Shlomo Angel, Jason Parent, Daniel Civco, Alexander Blei and David Potere at the Lincoln Land Institute project significant expansions of urban land by mid-century. And, Angel, in his Planet of Cities, notes how important it is to allow the expansion, in order to improve the quality of life for the majority of people, who deserve to live as well as people in the West.

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Note 1: Incomplete results of the 2011 Pakistan census have been reported by media in both Pakistan and India. However, no official announcement of the results has been identified from Pakistan census authorities. The Karachi population increase would be the largest metropolitan region 10 year rate of increase in history.

Note 2: Urban cores are generally the core historical jurisdiction, which often contains substantial non-core areas, even outside the United States. Core district data within these jurisdictions is used where available. Thus, this estimate over-states the urban core population increase.

Note 3: The driving factor in declining densities is principally transportation advances. Substantial urban expansion began with the coming of mass transit in the 19th century. However an even greater expansion began occurring with the availability of the automobile. As automobile orientation replaces transit orientation, densities tend to decline until it nearly all travel is by automobile. Even among automobile oriented urban areas, there can be large differences in urban densities. For example, transit's market share in the Boston urban area is substantially greater than in the Los Angeles urban area. Yet the Los Angeles urban area has a population density of 7000 per square mile (2,700 per square kilometer), more than three times that of the Boston urban area, at 220 per square mile (850 per square kilometer). The difference is that in Los Angeles residential development has largely occurred densities determined by the market, with single-family housing being typically built on 1/4 acre lots. In Boston, suburban lot sizes were forced higher by urban planning requirements for large lot zoning. The result is much greater land consumption than would have occurred if people's preferences (the market) had driven development. If Los Angeles had been developed at the same low density as Boston, its urban land area would equal that of the state of Connecticut.

Note 4: Megacities are urban areas with more than 10 million population. Five megacities remain to be described in The Evolving Urban Form (Karachi, Lagos, Nagoya, Paris and Teheran). Corresponding metropolitan regions are used for this analysis, since historic urban area data (areas of continuous urban development) is not available for most nations.

Wendell Cox is a Visiting Professor, Conservatoire National des Arts et Metiers, Paris and the author of “War on the Dream: How Anti-Sprawl Policy Threatens the Quality of Life.”

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Photo: New detached housing, suburban Tokyo-Yokohama (by author).