Current urban demographic trends

While the process of urbanization has been going on for all of recorded human history, its pace has dramatically increased in the past hundred years. From 1900 to 1950 the world's urban population increased from approximately 220 million to 732 million, and then from 1950 to 2005 to about 3.2 billion. By 2030, demographers project the number will be about 4.9 billion. Sometime in 2005, for the first time in history, there were more people living in cities than in the countryside. By 2030, about 60 percent of the world's population will be living in cities (United Nations, 2006). In 1800, Beijing was the only city with a population of over a million people. From 1800 to 1990, the average size of

Table 4.1 World megacities* 1975, 2000, and 2015 (projected): population in millions

1975

2000

2015

Tokyo (19.8)

Tokyo (26.4)

Tokyo (26.4)

New York (15.9)

Mexico City (18.1)

Mumbai (26.1)

Shanghai (11.4)

Mumbai (18.1)

Lagos (23.2)

Mexico City (11.2)

Sao Paolo (17.8)

Dhaka (21.1)

Sao Paolo (10)

Shanghai (17)

Sao Paolo (20.4)

New York (16.6)

Karachi (19.2)

Lagos (13.4)

Mexico City (19.2)

Los Angeles (13.1)

New York (17.4)

Kolkata (12.9)

Jakarta (17.3)

Buenos Aires (12.6)

Kolkata (17.3)

Dhaka (12.3)

Delhi (16.8)

Karachi (11.8)

Metro Manila (14.8)

Delhi (11.7)

Shanghai (14.6)

Jakarta (11)

Los Angeles (14.1)

Osaka (11)

Buenos Aires (14.1)

Metro Manila (10.9)

Cairo (13.8)

Beijing (10.8)

Istanbul (12.5)

Rio de Janeiro (10.6)

Beijing (12.3)

Cairo (10.6)

Rio de Janeiro (11.9)

Osaka (11.0)

Tianjin (10.7)

Hyderabad (10.5)

Bangkok (10.1)

*Cities with populations s 10 million Source: United Nations Population Fund (2001).

*Cities with populations s 10 million Source: United Nations Population Fund (2001).

the world's hundred largest cities grew from around 200,000 to over 5 million (Hardoy etal, 2001). There now are more than 40 cities with populations of at least 5 million, and 19 with more than 10 million. The latter are referred to as megacities, the list of which has grown and will continue to do so dramatically in the coming decades (see Table 4.1).

These are actually no longer discrete metropolitan areas surrounded by well-defined rural areas, but urban agglomerations that typically include the original city, now represented as a central urban zone, surrounded by a mix of suburbs, semi-urban, and semi-rural areas, all of which are interlinked. These are in turn connected (via the central city) to a global transportation network facilitating the rapid flow of people, vectors, and pathogens globally. Some urban agglomerations, like Tokyo, Mexico City, New York, and the Ruhr, are composed of more than one central city and one municipal government, but in many ways function like a single social ecological entity. The population of Tokyo, Yokohama, Kawasaki, and Saitama is more than 34 million. The population of Mexico City, Nezahualcoyotl, Ecatepec, and Naucalpan is more than 22 million. Seoul can be thought of as including Bucheon, Goyan, Incheon, Seongnam, and Suweon, with a total population of about 22 million. Not only has uncontrolled urbanization produced "cities" with population sizes unimaginable two generations ago, it has also created a new geography in which large, small, and medium human settlements across large regions have coalesced to create regional landscapes qualitatively different from those of the past. A similar growth pattern is unfolding in hundreds of smaller urban areas throughout the developing and developed world.

Thus, cities like Bangkok are now either referred to as the original municipality, currently with around 6 million people, or as "greater" Bangkok, which encompasses the surrounding districts with which the urban "habitat" of the original administrative unit is now contiguous. Such contiguous municipalities effectively constitute a single pool of humans - and a potential disease reservoir - which, in the case of Bangkok, now exceeds 10 million people. Even if the physical infrastructure and thus urban human density are not contiguous, it often becomes effectively so from a human pathogen's standpoint. The connectivity and mixing of people made possible by the modern transportation infrastructure and commuter lifestyle makes this so.

Nearly 500 cities now have population sizes approaching or exceeding 500,000. This is just above what mathematical epidemiology has found to be the critical population size for disease persistence and recurrent epidemics - a key transition point beyond which the trend of increasing infectious disease re-emergence or emergence is much more difficult to reverse, as is described further below.

Most of the fastest growing of these cities are in the developing tropical zones, where climate, environmental, and social ecological conditions are favorable for the transmission of pathogens responsible for the vast majority of old and new infectious diseases. While these conditions and the disease patterns are an integral part of the history of the development of human settlements and civilization, that the scale and magnitude of the present era of human-induced environmental change is unprecedented is starkly illustrated by the figures on urban population growth in developing countries during the past half-century. From 1950 to 2005, the urban population of the developing nations increased from just over 308 million to about 2.25 billion (United Nations, 2006). Most of that growth has resulted from immigration from rural areas, with many people bringing their rural lifestyle to the city. Figure 4.2 illustrates the growth of urban populations from 1950 to 2030, and shows that the largest increase has been in the low- and moderate-income countries of the developing world.

4500 4000

c o

3500

E

3000

c

c

2500

o

2000

ΓΌ

Q.

1500

c

a

-Q

1000

i5

500

1950

r r 1960

r r 1970

r r 1980

r r 1990 Year r r 2000

r r 2010

r r 2020

r i 2030

Figure 4.2 Growth of urban populations, 1950-2030. Source: United Nations (2002); World Bank (2002).

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