The population story ... so far.

• Author: Nierenberg, Danielle

A generation ago, human population growth became an explosive issue. Since then, it has largely disappeared from the media. But the consequences of still-rising population colliding with fast-rising resource consumption have in some respects worsened, and are bringing a whole new set of concerns.

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Forty years ago, the world's women bore an average of six children each. Today, that number is just below three. In 1960, 10-15 percent of married couples in developing countries used a modern method of contraception; now, 60 percent do.

To a considerable extent, these simple facts sum up the change in the Earth's human population prospects, then and now. In the mid-1960s, it was not uncommon to think about the human population as a time bomb. In 1971, population biologist Paul Ehrlich estimated that if human numbers kept increasing at the high rates of the time, by around 2900 the planet would be teeming with sixty million billion people (that's 60,000,000,000,000,000). But the rate of population rise actually peaked in the 1960s and demographers expect a leveling-off of human numbers this century.

Every couple of years the United Nations Population Division issues projections of human population growth to 2050. In 2002, UN demographers predicted a somewhat different picture of human population growth to mid-century than what the "population bombers" thought likely a generation ago. World population, growing by 76 million people every year (about 240,000 people per day), will pass 6.4 billion this year. The latest UN mid-range estimate says there will be about 8.9 billion people on Earth by 2050. And, according to this new scenario, total population will begin to shrink over the next hundred years.

These numbers are leading some people to say that the population bomb has been defused. A few nations, such as Italy and Japan, are even worried that birth rates are too low and that their graying populations will be a drain on the economy. (Some studies suggest that China, the world's most populous country, may also "need" more people to help support the hundreds of millions who will retire in coming decades).

We're not out of the woods yet. While the annual rate of population growth has decreased since 1970--from about 2 percent to 1.3 percent today--the rate is applied to a much larger population than ever before, meaning that the added yearly increments to the population are also much larger. These numbers show that the largest generation in history has arrived: 1.2 billion people are between 10 and 19. In large measure, it will be their choices--those they have, and those they make--that determine where the global population meter rests by mid-century.

Population X Consumption

Potential for catastrophe persists. In many places, population growth is slowly smoldering but could turn into a fast burn. Countries as diverse as Ethiopia, the Democratic Republic of Congo, and Pakistan are poised to more than double their size by 2050 even as supplies of water, forests, and food crops are already showing signs of strain and other species are being squeezed into smaller and smaller ranges. Arid Yemen will likely see its population quadruple to 80 million by 2050. The UN estimates that populations in the...