Cities are areas of the greatest alteration of the environment, areas where almost all the effects of ecological modification as a result of development come together. Cities have their own type of climate, vegetation and surface relief; their own brand of pollution; and their own specialized demands for physical resources. Our world is being progressively urbanized, the number of people living in the towns and cities is rising at a much faster rate than the world population, and increasingly an artificial environment is created, replacing the natural one. Urbanization can be defined as the proportion of thed total population of a country or a region which lives in an urban settlement. The current rate of urbanization is especially high in the developing countries but is not so for the developed ones, although the percentage of people living in cities there is high from an earlier period of urban growth. By early next century, more than half the total population of the developing countries will be urban.
Although cities of all sizes are growing, the growth of megacities (those with a population exceeding 8 million) in the developing countries is extremely impressive. In the 1950s, most of the biggest cities were in the United States or Europe; currently such cities are located almost entirely in the developing countries. The projected future trends indicate that this pattern is going to continue. For example the fifteen largest cities of 1994 in order of their population were: