Urbanization refers to a process whereby the building intensity of a high density human population increases or the number of people living in an urban area increase. Globally, this process has been on the rise for centuries; however, it has accelerated since the Industrial Revolution and was boosted again especially during the second half of the 20th century. The urbanization process was fueled mainly by migration from rural areas. The process usually comes to its end when urban population reaches a level of 80 to 90 percent of the total population of a country. Most developed countries reached this stage during the 20th century. Currently, at the beginning of the 21st century, urbanization is on the rise in most developing and less developed countries (Figure 1).
Despite the fact that the urbanization process in any country may come to a leveling off, the growth of urban population may continue due to natural growth or in=migration. The urbanization process leads to a rapid expansion of the urban built-up area. Such expansion results in massive urban sprawl and urban encroachment most often onto prime agricultural lands. Rapid urbanization process usually leads to the emergence of urban slums and other types of crime and poverty-stricken neighborhoods. It is often accompanied by unmanageable levels of environmental problems such as garbage collection and sewage treatment. Urbanization, its roots, and repercussions are studied by historians, geographers, sociologists, architects and city planners.
Current and future trends in urbanization
According to the United Nations Population Division, the world’s urban population reached 2.9 billion in 2000 and is expected to rise to 5 billion by 2030. Whereas 30 percent of the world population lived in urban areas in 1950, the proportion of urban dwellers rose to 47 percent by 2000 and is projected to attain 60 percent by 2030. Virtually all the population growth expected at the world level during 2000-2030 will be concentrated in urban areas (Figure 2). During that period the urban population is expected to increase by 2.1 billion persons, nearly as much as will be added to the world population, 2.2 billion.
Almost all of the population increase expected during 2000-2030 will occur in the urban areas of the less developed regions, where population will likely rise from approximately two billion in 2000 to just under four billion in 2030. The urban population of the more developed regions is expected to increase slowly, passing from 0.9 billion in 2000 to one billion in 2030.
Rural-urban migration and the transformation of rural settlements into cities are important determinants of the high population growth expected in urban areas of the less developed regions over the next thirty years. In combination with the universal reduction of fertility levels that is expected to occur in the future, these changes will lead to the eventual reduction of the rural population of the less developed regions, where growth rates will become negative in 2025-2030 for the first time. Therefore, it is expected that the rural population of the less developed regions will reach a peak around 2025 and then begin to decline just as the rural population of the more developed regions has done since 1950.
The process of urbanization is already very advanced in the more developed regions, where 75 percent of the population lived in urban areas in 2000. Nevertheless, the concentration of population in cities is expected to increase further so that, by 2030, 83 percent of the inhabitants of the more developed countries will be urban dwellers.
Environmental effects of urbanization
It is estimated that more than one billion people are exposed to outdoor air pollution annually. According to the World Health Organization, urban air pollution is linked to more than one million premature deaths and one million pre-natal deaths each year. Urban air pollution is estimated to cost approximately 2% of gross domestic product (GDP) in developed countries and 5% in developing countries. Rapid urbanization has resulted in increasing urban air pollution in major cities, especially in developing countries. Over 90% of air pollution in cities in these countries is attributed to vehicle emissions brought about by high number of older vehicles coupled with poor vehicle maintenance, inadequate infrastructure and low fuel quality. Noise pollution, chiefly from urban transportation sources, is a major environmental health issue, affecting the physical and psychological health of millions of people around the globe.
A principal adverse environmental effect of urbanization is the destruction of habitat, leading to biodiversity loss. In additon massive impacts to biodiversity and species richness are engendered by associated effects of the comcomitant habitat fragmentation.
Half the world's population lives within 60 kilometers of an ocean or sea, and three-quarters of all large cities are located on the coast. However, the seas and oceans are under increasing pressure from pollution. Much of this pollution comes from urban centers, and it creates environmental problems which threaten the viability of the cities themselves. According to the United Nations Development Program, every year sewage treatment facilities discharge 5.9 trillion gallons of sewage into coastal waters, and an estimated 160,000 factories dump between 41,000 to 57,000 tons of toxic organic chemicals and 68,000 tons of toxic metals into coastal waters.
By over-exploiting the seas as a source of food and as a location for waste disposal, coastal cities compromise the benefits offered by their location. Damage to mangroves, coral reefs and other coastal ecosystems can lead to erosion and siltation as well as render coastlines more vulnerable to storms and natural disasters. Since most of the Earth's water resources are shared by two or more countries, coordinated action locally, nationally, and internationally is equally important.