Some experts worry that we have too many children; others that we have too few. Illustration by Nishant Choksi

On May 12, 1907, toward the end of the annual meeting of the German Bunsen Society, which was held that year in Hamburg, a distinguished chemist named Walther Nernst insulted a not so distinguished junior colleague named Fritz Haber. The topic of the put-down—the synthesis of ammonia at very high temperatures—was, even by Bunsen Society standards, abstruse, but the gibe was strongly worded, so everyone at the meeting understood Nernst’s intent. Haber, who suffered from a variety of nervous ailments, was mortified. When he returned home to Karlsruhe, his skin broke out in hives. Before Nernst’s attack, he hadn’t been all that interested in synthesizing ammonia. The insult had the unintended consequence of stiffening his resolve. Haber threw himself full time into proving that ammonia could indeed be cooked up in the laboratory, using hydrogen and ordinary nitrogen gas. The result of this effort, which eventually became known as the Haber-Bosch process, had unintended consequences of its own, some of which proved to be world-altering.

Nitrogen is a tease. It’s crucial to life but exists mostly as N 2 , a form that living things can’t make use of. Early in the history of agriculture, people realized—without, obviously, understanding the chemistry behind this insight—that when usable nitrogen ran low fields turned barren. Eight thousand years ago, farmers in the Middle East were already planting legumes, whose roots harbor nitrogen-fixing bacteria, in rotation with cereal crops, such as wheat. Later, Cato the Elder recommended that Romans “save carefully goat, sheep, cattle, and all other dung.” Bird shit is an excellent source of nitrogen, and in the early nineteenth century, when Europeans learned that there were mountains of the stuff on remote islands off Peru, the discovery inspired a guano rush; by the eighteen-fifties, Britain was importing four hundred million pounds of bird poop a year, and the United States a hundred and seventy million pounds. In 1856, the U.S. Congress passed the Guano Islands Act, which authorized Americans to lay claim to any deserted guano islands they could find. (Through the act, the U.S. did not come into much nitrogen; it did, however, acquire a host of minor territories, including Midway Island.)

By Haber’s day, the appetite for crop-friendly nitrogen was so huge that scientists had turned their attention skyward. Nitrogen is the most common element in the earth’s atmosphere—nearly four times more plentiful than oxygen and more than eighty times more plentiful than argon—but almost all of it is floating around in the intractable form of N 2 . When the humiliated Haber showed how to bust up N 2 to produce ammonia—NH 3 —he basically solved the problem. No more guano would be needed. Haber had, it was said, figured out how to turn air into bread.

The first industrial-scale factory to employ the new Haber-Bosch process opened almost exactly a century ago, near Ludwigshafen. It pumped out more than ten tons of ammonia a day; this was further processed into fertilizer and sold as quickly as it could be manufactured. When the First World War broke out, the plant was converted into a munitions factory; nitrogen, as it happens, is also critical for making explosives. Thanks to the Haber-Bosch process, the Germans were able to keep the bombs dropping even after their supplies of saltpeter had run low. (According to some historians, without the process the Second Reich would have collapsed as much as two years sooner.)

Since the end of the Second World War, nitrogen-based fertilizer production has increased at least twentyfold. Such are the quantities being churned out in factories from the U.S. to Uzbekistan that humans are now likely responsible for fixing more nitrogen than all terrestrial ecosystems combined. It’s been estimated that almost half of the world’s current population subsists on crops grown with the output of the Haber-Bosch process. These people—who may well include you and me—are eating bread made of air, and so, in a sense, are made of air as well.

In a 2007 best-seller, “The World Without Us,” Alan Weisman imagined a planet suddenly devoid of humans. His new book, “Countdown: Our Last, Best Hope for a Future on Earth?” (Little, Brown), represents a less radical thought experiment. Instead of eliminating people from the planet altogether, Weisman wants only to get rid of several billion of them. He argues that when Haber figured out how to make bread out of air, things took a turn for the worse. The circumventing of the nitrogen cycle allowed Homo sapiens to reproduce at an unprecedented pace. (E. O. Wilson has described the rate as “more bacterial than primate.”) Among the results of this explosive growth has been a buildup of greenhouse gases in the atmosphere, which now puts not only humans but also pretty much every other creature on earth at risk. Since it was the Haber-Bosch process that made the surge possible, the process also suggests a target for the abatement.

“Before artificial nitrogen fertilizer became widely available, the world’s population was around 2 billion,” Weisman observes. “When we no longer have it—or if we ever decide to stop using it—that may be a number to which our own naturally gravitates.” The alternative to an orderly global “countdown” is, he warns, pretty dire. “Whether we accept it or not, this will likely be the century that determines what the optimal human population is for our planet,” he writes. “Either we decide to manage our own numbers, to avoid a collision of every line on civilization’s graph—or nature will do it for us.”

There is, of course, a long tradition in English of grim, though never quite realized, predictions of this sort. Thomas Malthus published “An Essay on the Principle of Population” in 1798, around the time humanity was reaching the one-billion mark (not that Malthus would have had any way of knowing this). It stated that people would inevitably produce more mouths to feed than food to feed them, since population “increases in a geometrical ratio” while “subsistence increases only in an arithmetical ratio.” Even a “slight acquaintance with numbers,” Malthus wrote, was enough to appreciate the “immensity” of the mismatch. War was one way the population and the food supply might be kept in line; another was “epidemics, pestilence, and plague.” If none of these proved sufficient, then “gigantic inevitable famine” would come to the rescue and “with one mighty blow” solve the problem.

A century later, in 1898, the number of people on the planet had nearly doubled when William Crookes, a chemist who’d recently become president of the British Association for the Advancement of Science, warned of an imminent crisis. According to Crookes, wheat production was levelling off, even as the number of wheat eaters continued to increase exponentially. “England and all civilized nations stand in deadly peril of not having enough to eat,” he declared. He gave the “civilized” world three decades.

By 1968, there were three billion people on the planet. That year, Paul Ehrlich published “The Population Bomb,” which announced that “the battle to feed all of humanity is over.” According to Ehrlich, a professor of biology at Stanford University, nothing could be done to avert disaster: “in the 1970’s the world will undergo famines—hundreds of millions of people are going to starve to death in spite of any crash programs embarked upon now.” In the most optimistic scenario Ehrlich could envision, after the “major die-back” had ended, those countries left with functioning governments—the United States, Russia, and Canada among them—would embark on a program of agricultural development that would allow what remained of humanity to survive. Ehrlich challenged his readers to come up with a plausible but more upbeat possibility. “I won’t accept one that starts, ‘In early 1972, the first monster space ships from a planet of the star Alpha Centauri arrive bearing CARE packages,’ ” he wrote. Since Ehrlich issued this challenge, the global population has once again more than doubled, to 7.2 billion.