World Is Soon Half Urban
The United Nations projects that sometime
in 2008 more people will live in cities than in
rural areas.1 Over the past half-century, the
world’s urban population has increased nearly
fourfold, from 732 million in 1950 to 3.15 billion
in 2005.2 (See Figure 1.) People living in
cities accounted for 49 percent of the total population
of 6.46 billion in 2005.3
The bulk of future population increase—
88 percent of the growth from 2000 to 2030—
is projected to occur in cities of the developing
world.4 Asia and Africa, the most rural continents
today, are set to double their urban populations
to some 3.4 billion by 2030.5
Urbanization has slowed considerably in
North America and Europe, where by 1950
more than half the population already lived in
cities.6 Latin America, at 77 percent urban, has
also gone through this demographic
transition.7 Growth in that region’s
“megacities”—urban agglomerations
with more than 10 million inhabitants—has
slowed, although large slum populations
continue to grow, thanks to the world’s highest
levels of economic and social inequality.8
Africa, currently only 38 percent urban,
already has nearly 350 million city dwellers—
more than the populations of Canada and
the United States combined.9 (See Figure 2.)
Urbanization there is more recent and more
rapid because of higher population growth,
rural poverty, and wars that drive people into
cities.10 Lack of infrastructure for the poor, followed
by rapid urban growth, has produced
large slum populations at high risk of disease
and environmental hazards like flooding.11
Worldwide, roughly 1 billion urban dwellers
live in slums, defined as areas where people live
without one or more of life’s basic necessities:
clean water, sanitation, sufficient living space,
durable housing, or secure tenure.12
Asia, the world’s most populous region, is
roughly 40 percent urban.13 Pacific Asia—the
coastal region from Japan to Southeast Asia—has
undergone a remarkable economic transformation
over the past generation, and China is now
the site of 16 of the world’s 20 most polluted
cities.14 In western China, South Asia, and interior
Asia, urbanization is also rapid, but economic
growth has been slower, and poverty burdens
nearly a third of India’s urban population.15
Since 1975, more than 200 urban agglomerations
in the developing world have grown past
1 million inhabitants, so local governments are
facing greater sanitation, housing, transportation,
water, energy, and health care needs.16 By
2005, 15 of these were megacities (see Figure
3), although these areas account for only about
9 percent of the total urban population.17 Just
over half of the world’s city dwellers live in settlements
with fewer than 500,000 inhabitants.18
More than half of the rise in urban population
is caused by natural increase.19 But
migration is also a leading factor, as economic
opportunities and improvements in sanitation
and clean water have made city life more desirable.
Yet the benefits of urban prosperity are
shared unequally, and the poor public health
conditions of slums still sicken and kill on a
large scale.20
The environmental challenges that cities face
vary with the level of economic activity.21 The
poorest cities and their slums typically have the
worst local hazards, such as diseases spread by
dirty water and lack of toilets.22 As a city industrializes,
problems at the metropolitan scale,
such as air pollution from industry and traffic,
tend to worsen first and then improve as economic
growth allows for cleaner technologies.23
But a city’s burden on the global environment
often increases with economic growth as residents
buy more cars, bigger houses, and other
consumer goods.24
Yet the economies of scale possible with
high-density settlement provide a crucial
opportunity to create living patterns in harmony
with nature’s rhythms. Urban planners
are beginning to embrace the concept of “circular
metabolism,” in which much of the waste
from the water, food, fuels, and materials that
course through cities is reused or recycled.25
Architects are beginning to apply this idea to
buildings: the 15-story IBM headquarters in
Kuala Lumpur, Malaysia, for example, uses
plantings on its exterior to capture water that
would otherwise be wasted.26
Prospects 2005 (New York: 2006), also available
online at esa.un.org/unup. This Vital Sign is based
on Kai N. Lee, “An Urbanizing World,” in Worldwatch
Institute, State of the World 2007 (New York:
W. W. Norton & Company, 2007), pp. 3–21. Molly
O’Meara Sheehan also contributed to this research.
2. Figure 1 from ibid.
3. Ibid.
4. Ibid.
5. Ibid.
6. Ibid.
7. Ibid.
8. Megacities from ibid.; National Research Council
(NRC), Cities Transformed: Demographic Change and
Its Implications in the Developing World (Washington,
DC: National Academies Press, 2003), pp. 95–99.
9. Africa and Figure 2 from U.N. Population Division,
op. cit. note 1.
10. NRC, op. cit. note 8, pp. 99–102.
11. Ibid.
12. UN-HABITAT, State of the World’s Cities 2006/7 (London:
Earthscan, 2006), p. 16.
13. U.N. Population Division, op. cit. note 1.
14. NRC, op. cit. note 8, pp. 102–06; polluted cities in
China from World Bank, cited in “A Great Wall of
Waste—China’s Environment,” The Economist, 21
August 2004.
15. India’s urban poverty from UN-HABITAT, op. cit.
note 12, p. 11.
16. U.N. Population Division, op. cit. note 1, viewed
August 2006.
17. Figure 3 and share of total from ibid.
18. Ibid.
19. NRC, op. cit. note 8, p. 107.
20. UN-HABITAT, op. cit. note 12.
21. For analysis of this trend, see Gordon McGranahan
et al., The Citizens at Risk: From Urban Sanitation to
Sustainable Cities (Sterling, VA: Earthscan, for Stockholm
Environment Institute, 2001), chapter 4.
22. Kirk R. Smith and Majid Ezzati, “How Environmental
Health Risks Change with Development: The
Epidemiologic and Environmental Risk Transitions
Revisited,” Annual Review of Environment and
Resources, November 2005, pp. 291–333.
23. Xuemei Bai and Hidefumi Imura, “A Comparative
Study of Urban Environment in East Asia: Stage
Model of Urban Environmental Evolution,” International
Review for Environmental Strategies, summer
2000, pp. 135–58; McGranahan et al., op. cit.
note 21.
24. Millennium Ecosystem Assessment, “Summary for
Decision-Makers,” in Ecosystems and Human Well-
Being: Synthesis (Washington, DC: Island Press,
2005), p. 1; McGranahan et al., op. cit. note 21.
25. Herbert Girardet, Cities People Planet (Chichester,
U.K.: John Wiley & Sons, 2004), pp. 123–25; Herbert
Girardet, The Gaia Atlas of Cities (London: Gaia
Books, 1992), pp. 22–23.
26. Ken Yeang, Bioclimatic Skyscrapers (London: Ellipsis
London Press, 2000).