Leeuwenhoek became a bit obsessed after that. Though his tiny peephole gave him privileged access to a never-before-seen microscopic universe, he spent an enormous amount of time looking at spermatozoa, as they’re now called. Oddly enough, it was the milt he squeezed from a cod one day that inspired him to estimate, almost casually, just how many people might live on Earth.
Nobody then really had any idea; there were few censuses. Leeuwenhoek started with an estimate that around a million people lived in Holland. Using maps and a little spherical geometry, he calculated that the inhabited land area of the planet was 13,385 times as large as Holland. It was hard to imagine the whole planet being as densely peopled as Holland, which seemed crowded even then. Thus, Leeuwenhoek concluded triumphantly, there couldn’t be more than 13.385 billion people on Earth—a small number indeed compared with the 150 billion sperm cells of a single codfish! This cheerful little calculation, writes population biologist Joel Cohen in his book How Many People Can the Earth Support?, may have been the first attempt to give a quantitative answer to a question that has become far more pressing now than it was in the 17th century. Most answers these days are far from cheerful.
Historians now estimate that in Leeuwenhoek’s day there were only half a billion or so humans on Earth. After rising very slowly for millennia, the number was just starting to take off. A century and a half later, when another scientist reported the discovery of human egg cells, the world’s population had doubled to more than a billion. A century after that, around 1930, it had doubled again to two billion. The acceleration since then has been astounding. Before the 20th century, no human had lived through a doubling of the human population, but there are people alive today who have seen it triple. Sometime in late 2011, according to the UN Population Division, there will be seven billion of us. (Pictures: Population 7 Billion.)
And the explosion, though it is slowing, is far from over. Not only are people living longer, but so many women across the world are now in their childbearing years—1.8 billion—that the global population will keep growing for another few decades at least, even though each woman is having fewer children than she would have had a generation ago. By 2050 the total number could reach 10.5 billion, or it could stop at eight billion—the difference is about one child per woman. UN demographers consider the middle road their best estimate: They now project that the population may reach nine billion before 2050—in 2045. The eventual tally will depend on the choices individual couples make when they engage in that most intimate of human acts, the one Leeuwenhoek interrupted so carelessly for the sake of science.
With the population still growing by about 80 million each year, it’s hard not to be alarmed. Right now on Earth, water tables are falling, soil is eroding, glaciers are melting, and fish stocks are vanishing. Close to a billion people go hungry each day. Decades from now, there will likely be two billion more mouths to feed, mostly in poor countries. There will be billions more people wanting and deserving to boost themselves out of poverty. If they follow the path blazed by wealthy countries—clearing forests, burning coal and oil, freely scattering fertilizers and pesticides—they too will be stepping hard on the planet’s natural resources. How exactly is this going to work?
THERE MAY BE SOME COMFORT in knowing that people have long been alarmed about population. From the beginning, says French demographer Hervé Le Bras, demography has been steeped in talk of the apocalypse. Some of the field’s founding papers were written just a few years after Leeuwenhoek’s discovery by Sir William Petty, a founder of the Royal Society. He estimated that world population would double six times by the Last Judgment, which was expected in about 2,000 years. At that point it would exceed 20 billion people—more, Petty thought, than the planet could feed. “And then, according to the prediction of the Scriptures, there must be wars, and great slaughter, &c.,” he wrote.
As religious forecasts of the world’s end receded, Le Bras argues, population growth itself provided an ersatz mechanism of apocalypse. “It crystallized the ancient fear, and perhaps the ancient hope, of the end of days,” he writes. In 1798 Thomas Malthus, an English priest and economist, enunciated his general law of population: that it necessarily grows faster than the food supply, until war, disease, and famine arrive to reduce the number of people. As it turned out, the last plagues great enough to put a dent in global population had already happened when Malthus wrote. World population hasn’t fallen, historians think, since the Black Death of the 14th century.
In the two centuries after Malthus declared that population couldn’t continue to soar, that’s exactly what it did. The process started in what we now call the developed countries, which were then still developing. The spread of New World crops like corn and the potato, along with the discovery of chemical fertilizers, helped banish starvation in Europe. Growing cities remained cesspools of disease at first, but from the mid-19th century on, sewers began to channel human waste away from drinking water, which was then filtered and chlorinated; that dramatically reduced the spread of cholera and typhus.
Moreover in 1798, the same year that Malthus published his dyspeptic tract, his compatriot Edward Jenner described a vaccine for smallpox—the first and most important in a series of vaccines and antibiotics that, along with better nutrition and sanitation, would double life expectancy in the industrializing countries, from 35 years to 77 today. It would take a cranky person to see that trend as gloomy: “The development of medical science was the straw that broke the camel’s back,” wrote Stanford population biologist Paul Ehrlich in 1968.
Ehrlich’s book, The Population Bomb, made him the most famous of modern Malthusians. In the 1970s, Ehrlich predicted, “hundreds of millions of people are going to starve to death,” and it was too late to do anything about it. “The cancer of population growth … must be cut out,” Ehrlich wrote, “by compulsion if voluntary methods fail.” The very future of the United States was at risk. In spite or perhaps because of such language, the book was a best seller, as Malthus’s had been. And this time too the bomb proved a dud. The green revolution—a combination of high-yield seeds, irrigation, pesticides, and fertilizers that enabled grain production to double—was already under way. Today many people are undernourished, but mass starvation is rare.
Ehrlich was right, though, that population would surge as medical science spared many lives. After World War II the developing countries got a sudden transfusion of preventive care, with the help of institutions like the World Health Organization and UNICEF. Penicillin, the smallpox vaccine, DDT (which, though later controversial, saved millions from dying of malaria)—all arrived at once. In India life expectancy went from 38 years in 1952 to 64 today; in China, from 41 to 73. Millions of people in developing countries who would have died in childhood survived to have children themselves. That’s why the population explosion spread around the planet: because a great many people were saved from dying.
And because, for a time, women kept giving birth at a high rate. In 18th-century Europe or early 20th-century Asia, when the average woman had six children, she was doing what it took to replace herself and her mate, because most of those children never reached adulthood. When child mortality declines, couples eventually have fewer children—but that transition usually takes a generation at the very least. Today in developed countries, an average of 2.1 births per woman would maintain a steady population; in the developing world, “replacement fertility” is somewhat higher. In the time it takes for the birthrate to settle into that new balance with the death rate, population explodes.
Demographers call this evolution the demographic transition. All countries go through it in their own time. It’s a hallmark of human progress: In a country that has completed the transition, people have wrested from nature at least some control over death and birth. The global population explosion is an inevitable side effect, a huge one that some people are not sure our civilization can survive. But the growth rate was actually at its peak just as Ehrlich was sounding his alarm. By the early 1970s, fertility rates around the world had begun dropping faster than anyone had anticipated. Since then, the population growth rate has fallen by more than 40 percent.
THE FERTILITY DECLINE that is now sweeping the planet started at different times in different countries. France was one of the first. By the early 18th century, noblewomen at the French court were knowing carnal pleasures without bearing more than two children. They often relied on the same method Leeuwenhoek used for his studies: withdrawal, or coitus interruptus. Village parish records show the trend had spread to the peasantry by the late 18th century; by the end of the 19th, fertility in France had fallen to three children per woman—without the help of modern contraceptives. The key innovation was conceptual, not contraceptive, says Gilles Pison of the National Institute for Demographic Studies in Paris. Until the Enlightenment, “the number of children you had, it was God who decided. People couldn’t fathom that it might be up to them.”
Other countries in the West eventually followed France’s lead. By the onset of World War II, fertility had fallen close to the replacement level in parts of Europe and the U.S. Then, after the surprising blip known as the baby boom, came the bust, again catching demographers off guard. They assumed some instinct would lead women to keep having enough children to ensure the survival of the species. Instead, in country after developed country, the fertility rate fell below replacement level. In the late 1990s in Europe it fell to 1.4. “The evidence I’m familiar with, which is anecdotal, is that women couldn’t care less about replacing the species,” Joel Cohen says.
The end of a baby boom can have two big economic effects on a country. The first is the “demographic dividend”—a blissful few decades when the boomers swell the labor force and the number of young and old dependents is relatively small, and there is thus a lot of money for other things. Then the second effect kicks in: The boomers start to retire. What had been considered the enduring demographic order is revealed to be a party that has to end. The sharpening American debate over Social Security and last year’s strikes in France over increasing the retirement age are responses to a problem that exists throughout the developed world: how to support an aging population. “In 2050 will there be enough people working to pay for pensions?” asks Frans Willekens, director of the Netherlands Interdisciplinary Demographic Institute in The Hague. “The answer is no.”
In industrialized countries it took generations for fertility to fall to the replacement level or below. As that same transition takes place in the rest of the world, what has astonished demographers is how much faster it is happening there. Though its population continues to grow, China, home to a fifth of the world’s people, is already below replacement fertility and has been for nearly 20 years, thanks in part to the coercive one-child policy implemented in 1979; Chinese women, who were bearing an average of six children each as recently as 1965, are now having around 1.5. In Iran, with the support of the Islamic regime, fertility has fallen more than 70 percent since the early ’80s. In Catholic and democratic Brazil, women have reduced their fertility rate by half over the same quarter century. “We still don’t understand why fertility has gone down so fast in so many societies, so many cultures and religions. It’s just mind-boggling,” says Hania Zlotnik, director of the UN Population Division.
“At this moment, much as I want to say there’s still a problem of high fertility rates, it’s only about 16 percent of the world population, mostly in Africa,” says Zlotnik. South of the Sahara, fertility is still five children per woman; in Niger it is seven. But then, 17 of the countries in the region still have life expectancies of 50 or less; they have just begun the demographic transition. In most of the world, however, family size has shrunk dramatically. The UN projects that the world will reach replacement fertility by 2030. “The population as a whole is on a path toward nonexplosion—which is good news,” Zlotnik says.
The bad news is that 2030 is two decades away and that the largest generation of adolescents in history will then be entering their childbearing years. Even if each of those women has only two children, population will coast upward under its own momentum for another quarter century. Is a train wreck in the offing, or will people then be able to live humanely and in a way that doesn’t destroy their environment? One thing is certain: Close to one in six of them will live in India.
I have understood the population explosion intellectually for a long time. I came to understand it emotionally one stinking hot night in Delhi a couple of years ago… The temperature was well over 100, and the air was a haze of dust and smoke. The streets seemed alive with people. People eating, people washing, people sleeping. People visiting, arguing, and screaming. People thrusting their hands through the taxi window, begging. People defecating and urinating. People clinging to buses. People herding animals. People, people, people, people. —Paul Ehrlich
In 1966, when Ehrlich took that taxi ride, there were around half a billion Indians. There are 1.2 billion now. Delhi’s population has increased even faster, to around 22 million, as people have flooded in from small towns and villages and crowded into sprawling shantytowns. Early last June in the stinking hot city, the summer monsoon had not yet arrived to wash the dust from the innumerable construction sites, which only added to the dust that blows in from the deserts of Rajasthan. On the new divided highways that funnel people into the unplanned city, oxcarts were heading the wrong way in the fast lane. Families of four cruised on motorbikes, the women’s scarves flapping like vivid pennants, toddlers dangling from their arms. Families of a dozen or more sardined themselves into buzzing, bumblebee-colored auto rickshaws designed for two passengers. In the stalled traffic, amputees and wasted little children cried for alms. Delhi today is boomingly different from the city Ehrlich visited, and it is also very much the same.