It is almost impossible to overemphasize just how dramatically the human population has grown recently. Figure 1 plots changes in the human population over the past 2000 years. Although the curve’s shape looks superficially similar to exponential growth, the growth rate for humans has—until recently— actually increased over time, leading to a steeply rising curve. It took all of human history to reach a population size of 1 billion in 1804, and only 123 years to reach 2 billion. Most recently, it took only 12 years to add 1 billion people to the population to reach 7 billion.
Consider that until your grandparents’ generation, no individual had lived long enough to see the human population double. But many members of the generation born in the early 1920s lived to see the population triple. And to drive home the impact of continued population doubling, consider this: If you were given a penny on January 1, then $0.02 on January 2, $0.04 on January 3, and so on, you would be handed $10,737,418.00 on January 31.
The astonishing growth rate of the human population has raised a sense of alarm that we will overshoot (or have already overshot) the carrying capacity of our planet—and this concern is not new. For example, Thomas Robert Malthus’s book, An Essay on the Principle of Population, was widely influential in the early 1800s. Malthus warned that the population cannot continue to increase unchecked—competition for resources will
eventually slow population growth, whether by famine, war, or voluntary reduction of family size. Malthus was an important source of inspiration for Charles Darwin in his formulation of the theory of evolution by natural selection.