The second half of the 1960s was a boom time for nightmarish visions of what lay ahead for humankind. In 1966, for example, a writer named Harry Harrison came out with a science fiction novel titled “Make Room! Make Room!” Sketching a dystopian world in which too many people scrambled for too few resources, the book became the basis for a 1973 film about a hellish future, “ Soylent Green .” In 1969, the pop duo Zager and Evans reached the top of the charts with a number called “ In the Year 2525 ,” which postulated that humans were on a clear path to doom. No one was more influential — or more terrifying, some would say — than Paul R. Ehrlich , a Stanford University biologist. His 1968 book, “The Population Bomb,” sold in the millions with a jeremiad that humankind stood on the brink of apocalypse […]