the figure of 2100 are estimates , explain why it is difficult to predict the population numbers for future
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5 Future Population Growth
In practice, future population growth is determined by improvements in mortality and the pattern and extent of migration, as well as by the future trends in fertility decline and by the current age distribution. The population growth that results from urbanization of a highly rural population in which the rural sector has higher fertility than in the urban sector is called spatial momentum (Rogers and Willekens 1978, Schoen and Kim 1993). Modern projection/prediction studies show that as the world's populations complete demographic transitions, the momentum inherent in the current young age distribution is the major component of future population growth in most developing countries (Bongaarts and Bulatao 1999). The other components of population growth—the spatial momentum, and the growth due to changes in fertility and mortality—under realistic assumptions is shown to be less than the age momentum (Bongaarts and Bulatao 1999, Brockerhoff 1999). Thus the growth associated with the young current age distribution in developing countries will be the major contributor to the future growth of the world population.
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