If population growth is a measure of a species' success, then the 20th century was astonishingly successful for Homo sapiens. In just one long human lifetime we grew our living population from 1.6 to 6.1 billion, a net addition of 4 ½ billion people. Now in the 21st, we exceed 7 billion and demographic computer models, fed with national birth and death statistics from countries around the globe, reflect our slowing but still-climbing growth, advancing towards 9-10 billion by 2050 despite falling total fertility rates in much of the world.
This worries biologists and ecologists, like Stanford's Paul Ehrlich (author of the 1968 bestseller The Population Bomb and many books since) who have seen exponential population growth at first succeed –then fail spectacularly—many times. In natural ecosystems, exponential growth (often called a "J-curve", owing to its sharp upwards curve) is the hallmark of a boom-and-crash species. Snowshoe hares (Lepus americanus), for example, procreate inexhaustibly until a food shortage or disease triggers a crash, decimating not just the hares but also the more abstinent lynxes (Lynx canadensis) that eat them. Nature is rife with grim examples of population booms and crashes, which is why Ehrlich and others grew fearful by the 1970s, as our exponential growth charged on with no signs of stabilization in sight. This inspired some extreme birth-control measures in the developing world, such as China's one-child policy and targeted sterilization programs in India. Today, "population growth" still resides at the top of many peoples' lists when asked to name the most pressing challenges facing the world. For many, the thought of ten billion people sharing the planet by midcentury—some forty percent more than today –is truly dreadful to envisage.
Such fear is misplaced. Not because ten billion isn't a large number indeed, it is huge. Total populations of other large-bodied, top-level predators (bears, for example) usually number in the tens to hundreds of thousands, not billions. The ubiquitous mallard duck, one of the world's commonest birds, has a global population of perhaps thirty million. The quantities of water, food, fiber, arable land, metals, hydrocarbons and other resources needed to support ten billion people are titanic, and the prospect of looming shortages and violent competition for such resources is the straight-line link many people make between numeric population totals and prophecies of water shortages, famines, hydrocarbon wars, and societal collapse.
Such threats are real, but unlike natural ecosystems, they aren't driven by simple head count. Instead, extreme variations in consumption, both as compared between societies of differing means and cultures (think America vs. Afghanistan) and also within societies (think rural vs. urban China) dominate contemporary natural resource necessities. This is not to say that total population doesn't matter, just that lifestyle matters even more.
Consider, for example, how the material needs for electricity, plastics, rare earth metals, and processed food must leap by many multiples in order to meet the requirements of a modern urban consumer living in Shanghai, as opposed to that same individual toiling away as a low-technology agrarian peasant in the countryside. In China alone, the massive rural-to-urban migration now underway promises 1 billion new urban consumers by 2050, despite zero population growth in that country. Africa will have 1.2 billion, nearly a quarter of the world's urban population. My UCLA colleague Jared Diamond calculates that if everyone alive today were to adopt the current lifestyles of North Americans, Western Europeans, Japanese and Australians, global resource consumption would rise eleven-fold. It would be as if the world population suddenly rose from 7 billion to 72 billion.
That, to me, is scarier than a +40% increase in total population, or even the economic social "slowing pains" of falling total fertility rates (with a graying planet, the ratio of elderly to working-age people rises, together with strains on social safety-net and healthcare programs). But unfortunately, in assuaging one fear (population growth) I've raised another (prosperity growth) which, of course, is impossible to decry. The massive rural-to-urban migration now underway as the developing world modernizes—currently some 3 million people per week, equivalent to adding another Seattle to the planet every day—has lifted hundreds of millions from relentless, grueling poverty. Who among us doesn't applaud that?
Rather than worrying about world population, the smarter focus is on the real challenge—reconciling our contradictory desires to bring modernity and prosperity to all, while stabilizing the innumerable natural resource demands that they foreshadow for our planet. Ask yourself this: What do you (the modern, educated urban dweller who is most likely reading this essay) need to give up, to align your resource needs closer to those of someone cultivating rice on the Irawaddy delta? The good news is that buying a compact house, riding a bus, or eating lower down the food chain, are all easier to control than humanity's reproduction rate. All it takes is redefining our definition of success.