To Simon, more people meant more good ideas about how to achieve a sustainable future. Technology-water-control techniques, hybridized seeds, management strategies-had revolutionized agriculture, and global crop yields were increasing. The economist Julian Simon took up Ehrlich’s challenge. “I challenge you to create one more optimistic,” Ehrlich wrote, adding that he would not count scenarios involving benevolent aliens bearing care packages. Only half a billion or so people die of starvation. Famine spreads, and countries teeter, but the major death wave ends in the mid-1980s.

In the “cheerful” scenario, population controls begin. Russia, China, and the United States are dragged into nuclear war, and the resulting environmental degradation soon extinguishes the human race. In his book, Ehrlich played out hypothetical scenarios that represented “the kinds of disasters that will occur.” In the worst-case scenario, famine rages across the planet. Populations exploded, blowing past the available resources, and then crashed. He knew that nature did not regulate animal populations delicately. Ehrlich was an accomplished butterfly specialist. It was cold, hard math: The human population was growing exponentially the food supply was not. Resource shortages would cause hundreds of millions of starvation deaths within a decade. In his 1968 best seller, The Population Bomb, Ehrlich insisted that it was too late to prevent a doomsday apocalypse resulting from overpopulation. On one side was the Stanford biologist Paul R.

The bet was on, and it was over the fate of humanity.

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