One such scientist, the eminent Harvard University sociobiologist Edward O. Wilson, bases his estimate on calculations of the Earth's available resources. As Wilson pointed out in his book "The Future of Life" (Knopf, 2002), "The constraints of the biosphere are fixed."

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Fortunately, we may be spared from entering the end-times phase of overpopulation and starvation envisioned by Malthus. According to the United Nations Population Division, the human population will hit 7 billion on or around Oct. 31, and, if its projections are correct, we're en route to a population of 9 billion by 2050, and 10 billion by 2100. However, somewhere on the road between those milestones, scientists think we'll make a U-turn.

UN estimates of global population trends show that families are getting smaller. "Empirical data from 230 countries since 1950 shows that the great majority have fertility declines," said Gerhard Heilig, chief of population estimates and projections section at the UN.

Globally, the fertility rate is falling to the "replacement level" — 2.1 children per woman, the rate at which children replace their parents (and make up for those who die young). If the global fertility rate does indeed reach replacement level by the end of the century, then the human population will stabilize between 9 billion and 10 billion. As far as Earth's capacity is concerned, we'll have gone about as far as we can go, but no farther.