The more catastrophic consequences of human population growth predicted by Paul Ehrlich have not yet materialized, in part because he did not anticipate the enormous increase in agricultural productivity enabled by fossil fuels. But Ehrlich’s population bomb will inevitably detonate when fossil fuels become too scarce and expensive to sustain a growing population.
The inexorable strain put on Earth’s life support system by human population growth can be illustrated by considering how many additional person years are ultimately caused when a person has children. The demographic legacy of a woman (or man) can be thought of as one half of the years lived by her children, plus one fourth of the years lived by her grandchildren, and so on, until her lineage dies out.
Based on analysis of the 2005 demographic data for the world’s ten most populous countries, the average number of person years added per child ranges from 136 (Nigeria, where fertility is high but life expectancy is short) to 470 (United States). In the United States, where female life expectancy is around 80 years, each child results, on average, in almost six (470 divided by 80) additional lifetimes spent on Earth over ensuing generations.
