RESUMO
The model of demographic transition predicts that in the final stages of the transition, the population will become stationary. However, during the past 20 years or so, the birth rate in the majority of industrialized countries has fallen below the death rate, and a continuation of this trend will ultimately result in a declining population. In the UN recent projections of world population growth, an attempt was made to salve the transition model by assuming that when faced with a reduction in numbers, couples will wholeheartedly respond by increasing their family size. The author presents a theoretical model in which he showed that during the final stages of the transition, it is likely that there are built-in mechanisms which will result in low birth rates, and that it is unlikely that fertility will return to replacement level. In addition to the scenario presented by the UN, there is another in which the birth rate remains below the death rate indefinitely. This catastrophic scenario would ultimately result in the disappearance of the human species. The author presents and compares both of these scenarios. Using some calculations by J.N. Biraben, he calculates the number of humans that would have been born between the beginning and end of the human species, as described by the catastrophic scenario and estimates this to be at 100 billion. In order to end on a more upbeat note, he than imagines a new demographic transition in which biologists have succeeded in extending the duration of a woman's reproductive life. They could then engage in 2 fertile unions during their lifetimes and thus save the species.