17 Entering the Zone

17 Entering the Zone - 17 Entering the Zone No species has...

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17 Entering the Zone No species has ever been able to multiply without limit. There are two biological checks upon a rapid increase in number—a high mortality and a low fertility. Unlike other biological organisms, man can choose which of these checks shall be applied, but one of them must be. —Harold F. Dorn 1962 1 RECAPITULATION The human population of the Earth now travels in the zone where a substantial fraction of scholars have estimated upper limits on human population size. These estimates are no better than present understanding of humankind's cultural, economic and environmental choices and constraints. Nevertheless, the possibility must be considered seriously that the number of people on the Earth has reached, or will reach within half a century, the maximum number the Earth can support in modes of life that we and our children and their children will choose to want. This conclusion emerges clearly from the three main elements of the book up to this point: human population history, scenarios of future population and estimates of the maximum number of people the Earth can support ( Figure 17.1 ). The history: since 1600, the human population increased from about half a billion to nearly six billion. The increase in the last decade of the twentieth century exceeds the total population in 1600. Compared to all human history prior to World War II, the world's population growth rate since 1950 has been and still is unprecedented. Within the lifetime of some people now alive, world population has tripled; within the lifetime of everyone over 40 years old, it has doubled— yet never before the last half of the twentieth century had world population doubled within the life span of any human. The future: human populations, like economies, environments and cultures, are highly unpredictable, and only conditional predictions are credible. In the United Nations' high projection published in 1992, if worldwide -367- Figure 17.1 Human population size 1600-1990, three United Nations scenarios of future population growth 1990-2150 and estimates of the Earth's maximum human population ("limits") by year of publication 1679-1994. The constant-fertility projection assumes that fertility in each region of the world remains at its level in 1990; in this scenario, the global average total fertility rate rises from 3.3 children per woman in 1990 to 5.7 children per woman in 2150 as the faster- growing regions become a larger share of world population. The instant-replacement projection assumes that the total fertility rate dropped to 2.06 children per woman in 1990 and remains at that level. The low-fertility projection assumes that the total fertility rate gradually moves to 1.7 children per woman everywhere. By the year 2050, according to these three projections, the
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world's population would number 21.2 billion, 7.7 billion and 7.8 billion. The plotted estimates of the Earth's maximum human population are the highest given when an author stated a range. SOURCE: for history, Appendix 2; for future scenarios,
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17 Entering the Zone - 17 Entering the Zone No species has...

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