China's policy of limiting parents to having only one child has been a heroic effort to combat the country's rising concern about becoming too populous to sustain itself. The same concern applies to the planet as a whole, where the global population is expected to rise from the current 6.68 billion to about 9.5 billion by 2050.

But, as China discovered, managing human population is complex, with many unpredictable and surprising consequences. Historically, however, population has been controlled by natural forces, continually rising to fill the space and food available. Europe's population growth was stabilizing in the 15th century until the discovery of the New World brought more productive crops and new places to colonize and populate. Emigration and disease actually decreased Europe's population during the "disease-ridden years" of the 18th century (Globe & Mail, Sept. 6/08). Two centuries later, when industrialization began to mass-produce food worldwide, populations everywhere rose to match the plentitude. And when food shortages threatened mass starvation in Asia and Africa in the mid-20th century, the so-called Green Revolution multiplied grain production – humans reflexively proliferated to match the supply.

This procreational trend cannot continue on a finite planet. At present fertility rates, the United Nations calculates the world's population would rise to an astounding 44 billion by 2100, 244 billion by 2150 and 1.34 trillion by 2300. The actual scenario is that human numbers will likely reach 9.5 billion by 2050 and then stabilize at about 9 billion by 2100 – if the planet is able to sustain even this many people.

Two forces will constrain population growth. The first is simply the availability of land and food resources – this constraint could be brutal. The second, surprisingly, is affluence. As people become more prosperous and educated, they reproduce less. A UN study of 30 countries found that women with no schooling bore an average of 6.9 children, while those with some schooling bore an average of only 3 children. In 51 modern countries with educated populations, their numbers will start falling as fertility dips below the maintenance level of 2.1 children per woman. And in 24 more, populations will only be maintained by immigration. Canada fits in this category. Its growth in citizens is now dependent on 300,000 immigrants per year – if this number doesn't increase, the population will start shrinking in about 20 years.

Global population is growing primarily because of high birth rates in the impoverished countries of Africa, Southeast Asia and South America – the crowding of people into mega-cities also assists this trend. Consequently, the poor are getting poorer and more numerous while the rich are getting richer and less numerous, a situation that is establishing the two population trends that will increasingly affect our lives and those of our children.

The first trend is the ascendance of seniors as a demographic group. In many Canadian communities, school populations are declining even as towns and cities continue to grow in total numbers. Couples are having fewer children and an increasing proportion of citizens are seniors. In Europe, for example, 1 in 5 are now older than 65, and 1 in 3 will be non-working persons by 2060. This trend poses increasing burdens on the working population to generate goods, provide services and support the elderly.

The second trend is the powerful pressure to allow the immigration that will sustain national economic activity and keep the social system functioning. But immigration brings its own dilemmas. Immigrants arrive with their own languages and cultures, thereby altering the character of their new country. Each recipient country must be prepared for a multinational character as its unique national identity is reshaped by immigration. "Politically speaking," notes the historian William H. McNeil, "one must expect considerable volatility in public responses to what is still a new and perhaps unstable demographic regime in the rich, urbanized countries of the earth" (Ibid.). The alternative to immigration for prosperous countries is shrinking populations, skewed demographics and the possibility of reduced political influence, economic strength, military power and cultural energy.

These are not the only impacts. A German demographer expects parts of eastern Germany, southern Italy, northern Spain and Bulgaria to be abandoned by 2050 as Europe's population of 447 million falls by 52 million. The same could occur in Japan and South Korea where populations are about to decline without immigration. If populations do decline, consider the effects on property values when dwellings far outnumber the people to live in them. Consider the impact on modern businesses and economies that are founded on the promise of perpetual expansion.

The demographic turbulence we are undergoing on our way to a stable population of a mixed-age distribution is fraught with incredible challenges, assuming our planet can even accommodate the demands of 9 billion people on a spiral of upward material consumption. James Lovelock, the father of modern ecology, thinks our Earth can comfortably sustain about 1 billion at the present living standard of developed countries; optimists think 2 billion is possible. Getting from 9 billion to even 2 billion will present us with issues of staggering social, political and environmental complexity. Staying at 9 billion may present us with even greater ones.

For a glimpse of these complexities, consider China's One Child policy – a solution that may not be gentle but is at least a beginning. Elsewhere on the planet, our future starts today. Whether or not we are ready, population is already presenting us with major complexities at almost every level of almost every society. Expect more.