(April 2004) Human beings have become an increasingly powerful environmental force over the last 10,000 years. With the advent of agriculture 8,000 years ago, we began to change the land.1 And with the industrial revolution, we began to affect our atmosphere. The recent increase in the world's population has magnified the effects of our agricultural and economic activities. But the growth in world population has masked what may be an even more important human-environmental interaction: While the world's population is doubling, the world's urban population is tripling. Within the next few years, more than half the world's population will be living in urban areas.2
The level and growth of urbanization differ considerably by region (see Figure 1). Among developing countries, Latin American countries have the highest proportion of their population living in urban areas. But East and South Asia are likely to have the fastest growth rates in the next 30 years. Almost all of future world population growth will be in towns and cities. Both the increase in and the redistribution of the earth's population are likely to affect the natural systems of the earth and the interactions between the urban environments and populations.
Population Living in Urban Areas
Source: UN, World Urbanization Prospects: The 2003 Revision (2004).
The best data on global urbanization trends come from the United Nations Population Division and the World Bank.3 The UN, however, cautions users that the data are often imprecise because the definition of urban varies country by country. Past projections of urbanization have also often overestimated future rates of growth. Therefore, it is important to be careful in using urbanization data to draw definitive conclusions.
The Dynamics of Urbanization
In 1800 only about 2 percent of the world's population lived in urban areas. That was small wonder: Until a century ago, urban areas were some of the unhealthiest places for people to live. The increased density of populations in urban areas led to the rapid spread of infectious diseases. Consequently, death rates in urban areas historically were higher than in rural areas. The only way urban areas maintained their existence until recently was by the continual in-migration of rural people.4
In only 200 years, the world's urban population has grown from 2 percent to nearly 50 percent of all people. The most striking examples of the urbanization of the world are the megacities of 10 million or more people. In 1975 only four megacities existed; in 2000 there were 18. And by 2015 the UN estimates that there will be 22.5 Much of the future growth, however, will not be in these huge agglomerations, but in the small to medium-size cities around the world.6
The growth in urban areas comes from both the increase in migration to the cities and the fertility of urban populations. Much of urban migration is driven by rural populations' desire for the advantages that urban areas offer. Urban advantages include greater opportunities to receive education, health care, and services such as entertainment. The urban poor have less opportunity for education than the urban nonpoor, but still they have more chance than rural populations.7
Urban fertility rates, though lower than rural fertility rates in every region of the world, contribute to the growth of urban areas. Within urban areas, women who migrated from rural areas have more children than those born in urban areas.8 Of course, the rural migrants to urban areas are not a random selection of the rural population; they are more likely to have wanted fewer children even if they had stayed in the countryside. So the difference between the fertility of urban migrants and rural women probably exaggerates the impact of urban migration on fertility.
In sub-Saharan Africa, the urban fertility rates are about 1.5 children less than in rural areas; in Latin America the differences are almost two children.9 Therefore, the urbanization of the world is likely to slow population growth. It is also likely to concentrate some environmental effects geographically.
Environmental Effects of Urbanization
Urban populations interact with their environment. Urban people change their environment through their consumption of food, energy, water, and land. And in turn, the polluted urban environment affects the health and quality of life of the urban population.
People who live in urban areas have very different consumption patterns than residents in rural areas.10 For example, urban populations consume much more food, energy, and durable goods than rural populations. In China during the 1970s, the urban populations consumed more than twice as much pork as the rural populations who were raising the pigs.11 With economic development, the difference in consumption declined as the rural populations ate better diets. But even a decade later, urban populations had 60 percent more pork in their diets than rural populations. The increasing consumption of meat is a sign of growing affluence in Beijing; in India where many urban residents are vegetarians, greater prosperity is seen in higher consumption of milk.
Urban populations not only consume more food, but they also consume more durable goods. In the early 1990s, Chinese households in urban areas were two times more likely to have a TV, eight times more likely to have a washing machine, and 25 times more likely to have a refrigerator than rural households.12 This increased consumption is a function of urban labor markets, wages, and household structure.
Energy consumption for electricity, transportation, cooking, and heating is much higher in urban areas than in rural villages. For example, urban populations have many more cars than rural populations per capita. Almost all of the cars in the world in the 1930s were in the United States. Today we have a car for every two people in the United States. If that became the norm, in 2050 there would be 5.3 billion cars in the world, all using energy.13
In China the per capita consumption of coal in towns and cities is over three times the consumption in rural areas.14 Comparisons of changes in world energy consumption per capita and GNP show that the two are positively correlated but may not change at the same rate.15 As countries move from using noncommercial forms of energy to commercial forms, the relative price of energy increases. Economies, therefore, often become more efficient as they develop because of advances in technology and changes in consumption behavior. The urbanization of the world's populations, however, will increase aggregate energy use, despite efficiencies and new technologies. And the increased consumption of energy is likely to have deleterious environmental effects.
Urban consumption of energy helps create heat islands that can change local weather patterns and weather downwind from the heat islands. The heat island phenomenon is created because cities radiate heat back into the atmosphere at a rate 15 percent to 30 percent less than rural areas. The combination of the increased energy consumption and difference in albedo (radiation) means that cities are warmer than rural areas (0.6 to 1.3 C).16 And these heat islands become traps for atmospheric pollutants. Cloudiness and fog occur with greater frequency. Precipitation is 5 percent to 10 percent higher in cities; thunderstorms and hailstorms are much more frequent, but snow days in cities are less common.
Urbanization also affects the broader regional environments. Regions downwind from large industrial complexes also see increases in the amount of precipitation, air pollution, and the number of days with thunderstorms.17 Urban areas affect not only the weather patterns, but also the runoff patterns for water. Urban areas generally generate more rain, but they reduce the infiltration of water and lower the water tables. This means that runoff occurs more rapidly with greater peak flows. Flood volumes increase, as do floods and water pollution downstream.
Many of the effects of urban areas on the environment are not necessarily linear. Bigger urban areas do not always create more environmental problems. And small urban areas can cause large problems. Much of what determines the extent of the environmental impacts is how the urban populations behave — their consumption and living patterns — not just how large they are.
Health Effects of Environmental Degradation
The urban environment is an important factor in determining the quality of life in urban areas and the impact of the urban area on the broader environment. Some urban environmental problems include inadequate water and sanitation, lack of rubbish disposal, and industrial pollution.18 Unfortunately, reducing the problems and ameliorating their effects on the urban population are expensive.
The health implications of these environmental problems include respiratory infections and other infectious and parasitic diseases. Capital costs for building improved environmental infrastructure — for example, investments in a cleaner public transportation system such as a subway — and for building more hospitals and clinics are higher in cities, where wages exceed those paid in rural areas. And urban land prices are much higher because of the competition for space. But not all urban areas have the same kinds of environmental conditions or health problems. Some research suggests that indicators of health problems, such as rates of infant mortality, are higher in cities that are growing rapidly than in those where growth is slower.19
Urban Environmental Policy Challenges
Since the 1950s, many cities in developed countries have met urban environmental challenges. Los Angeles has dramatically reduced air pollution. Many towns that grew up near rivers have succeeded in cleaning up the waters they befouled with industrial development. But cities at the beginning of their development generally have less wealth to devote to the mitigation of urban environmental impacts. And if the lack of resources is accompanied by inefficient government, a growing city may need many years for mitigation. Strong urban governance is critical to making progress. But it is often the resource in shortest supply.20 Overlapping jurisdictions for water, air, roads, housing, and industrial development frustrate efficient governance of these vital environmental resources. The lack of good geographic information systems means that many public servants are operating with cataracts. The lack of good statistics means that many urban indicators that would inform careful environmental decisionmaking are missing.21
When strong urban governance is lacking, public-private partnerships can become more important.22 These kinds of partnerships can help set priorities that are shared broadly, and therefore, implemented. Some of these public-private partnerships have advocated tackling the environmental threats to human health first. "Reducing soot, dust, lead, and microbial disease presents opportunities to achieve tangible progress at relatively low cost over relatively short periods," concluded conferees at a 1994 World Bank gathering on environmentally sustainable development.23 But ultimately there are many other urban environmental priorities that produce chronic problems for both people and the environment over the long term that also have to be addressed.
Much of the research that needs to be done on the environmental impacts of urban areas has not been done because of a lack of data and funding. Most of the data that exist are at a national level. But national research is too coarse for the environmental improvement of urban areas. Therefore, data and research at the local level need to be developed to provide the local governments with the information they need to make decisions. Certainly the members of the next generation, the majority of whom will be living in urban areas, will judge us by whether we were asking the right questions today about their urban environments. They will want to know whether we funded the right research to address those questions. And they will also want to know whether we used the research findings wisely.
Barbara Boyle Torrey is a writer and consultant who serves on PRB's board of trustees.
- M. Gordon Wolman, "Population, Land Use, and Environment: A Long History," in Population and Land Use in Developing Countries, ed. Carole L. Jolly and Barbara Boyle Torrey, Committee on Population, Commission on Behavioral and Social Sciences and Education, National Research Council (Washington, DC: National Academies Press, 1993).
- United Nations, World Urbanization Prospects: The 2003 Revision (New York: UN, 2004).
- World Bank, World Development Report 2002: Building Institutions for Markets (New York: Oxford University Press for the World Bank, 2002).
- Nathan Keyfitz, "Impact of Trends in Resources, Environment and Development on Demographic Prospects," in Population and Resources in a Changing World, ed. Kingsley Davis et al. Stanford, CA: Morrison Institute for Population and Resource Studies, 1989).
- United Nations, World Urbanization Prospects.
- National Research Council, Cities Transformed: Demographic Change and Its Implications in the Developing World, ed. Mark R. Montgomery et al., Panel on Urban Population Dynamics, Committee on Population, Commission on Behavioral and Social Sciences and Education, National Research Council (Washington, DC: National Academies Press, 2003).
- United Nations, World Urbanization Prospects: 193.
- Martin Brockerhoff, "Fertility and Family Planning in African Cities: The Impact of Female Migration," Journal of Biosocial Science 27, no. 3 (1995): 347-58; and Robert Gardner and Richard Blackburn, "People Who Move: New Reproductive Health Focus," Population Reports Vol. 24, no. 3 (Baltimore, MD: Johns Hopkins School of Public Health, Population Information Program, November 1996).
- Estimates calculated from 90 Demographic and Health Surveys as reported in National Research Council, Cities Transformed: Demographic Change and Its Implications in the Developing World.
- Jyoti K. Parikh et al., Indira Gandhi Institute of Development Research, "Consumption Patterns: The Driving Force of Environmental Stress" (presented at the United Nations Conference on Environment and Development, August 1991).
- Jeffrey R. Taylor and Karen A. Hardee, Consumer Demand in China: A Statistical Factbook (Boulder, CO: Westview Press, 1986): 112.
- Taylor and Hardee, Consumer Demand in China: 148.
- U.S. Census Bureau, Statistical Abstract of the United States: 2003 (Washington, DC: Government Printing Office, 2003).
- Taylor and Hardee, Consumer Demand in China: 125.
- Gretchen Kolsrud and Barbara Boyle Torrey, "The Importance of Population Growth in Future Commercial Energy Consumption," in Global Climate Change: Linking Energy, Environment, Economy and Equity, ed. James C. White (New York: Plenum Press, 1992): 127-42.
- Andrew S. Goudie, The Human Impact on the Natural Environment, 2d ed. (Cambridge, MA: MIT Press, 1987): 263.
- Goudie, The Human Impact on the Natural Environment: 265.
- Kolsrud and Torrey, "The Importance of Population Growth in Future Commercial Energy Consumption": 268.
- Martin Brockerhoff and Ellen Brennan, "The Poverty of Cities in Developing Regions," Population and Development Review 24, no. 1 (March 1998): 75-114.
- Eugene Linden, "The Exploding Cities of the Developing World," Foreign Affairs 75, no. 1 (1996): 52-65.
- Organisation of Economic Co-operation and Development (OECD), Better Understanding Our Cities, The Role of Urban Indicators (Paris: OECD, 1997).
- Ismail Serageldin, Richard Barrett, and Joan Martin-Brown, "The Business of Sustainable Cities," Environmentally Sustainable Development Proceedings Series, no. 7 (Washington, DC: The World Bank, 1994).
- Serageldin, Barrett, and Martin-Brown, "The Business of Sustainable Cities": 33.