Efforts to predict the size of cities have proven less accurate than projections of national and regional populations. While the world's total urban population has been projected with some accuracy, projections of individual cities' populations and of cities' relative sizes have been much less reliable.
These shortcomings are partly due to the incompleteness of census information prior to the 1980s. But they also reflect the complexity of urban population dynamics. Like national populations, the size of cities depends on fertility, mortality and external migration rates, but it also depends on the differentials between urban and rural fertility and mortality and on migration within countries. Population movements are far more sensitive to regional variations in economic opportunities than are fertility and mortality.
It is also hard to predict changes in the spatial dimension of urban systems. In some cases, the population becomes increasingly less concentrated in central cities and more dispersed into urban systems of varying size with different economic specializations. Such patterns are difficult to predict.
In São Paulo, for example, the 1991 census showed both the population and the growth rate to be much smaller than anticipated. This reflected a large net outmigration, partly due to the relocation of industry to smaller cities, and a lower than expected fertility level.
Other unanticipated economic developments have upset expectations more dramatically. In 1982, Shanghai was the only Chinese city that the United Nations projected to be among the world's 20 largest urban centres in the year 2000, with an expected population of 13.5 million. By 1994, however, Shanghai, Beijing and Tianjin were all projected to be in the top 12 by 2000 (all three are currently in the top 15) and Shanghai's projected population has grown to 17.2 million.
The unreliability of forecasting is clearest in regard to single cities. Of the cities projected in 1982 to be the 15 largest in 2000, the total population was expected to be 233.8 million. This is close to the 1994 projection of 230.1 million in the top 15 cities in 2000.
However, the lists are not the same; the 15 cities in the 1982 list are currently projected to total 215 million people by 2000 18 million below the 1982 projection. Some cities were very poorly estimated. The current projection for Tokyo in 2000 is 10.8 million people larger than was projected in 1982; Mexico City's population in 2000 is now expected to be 9.9 million smaller than was projected in 1982.