Where is this taking us? The coming half century will see huge shifts in the geopolitical balance of numbers, further declines in the number of children per woman, smaller but more numerous households, an increasingly elderly population, and growing and more numerous cities.

The United Nations Population Division anticipates 8 billion people by 2025, 9 billion by 2043 and 10 billion by 2083. India will have more people than China shortly after 2020, and sub-Saharan Africa will have more people than India before 2040.
In 1950, there were nearly three times as many Europeans as sub-Saharan Africans. By 2010, there were 16 percent more sub-Saharan Africans than Europeans. By 2100, according to the Population Division, there will be nearly five sub-Saharan Africans for every European.

In some ways, the growth in the numbers of people matters less than the growth in the numbers of households. If each household has its own refrigerator, air-conditioner, TV and car, the average energy demand for a given number of people goes up as the average number of people in a household goes down.

The urban population of developing countries is expected to grow by a million people every five days through at least 2030, while the rural population falls. Many cities will eat into prime agricultural land unless they grow in density, not extent. And nearly half of urban population growth by 2015 will occur in cities of fewer than half a million people.

The coming revolution in aging is well under way in the more developed countries. It will go global in the next half century. In 1950, for each person 65 and older, there were more than six children under 15. By 2070, elderly people will outnumber children under 15, and there will be only three people of working age (15 to 64) for every two people under 15 or 65 and older. Pressures to extend the "working age" beyond 65 will grow more intense.

Is economic development the best contraception? Or is voluntary contraception the best form of development? Does the world need a bigger pie (more productive technologies) or fewer forks (slower population growth through voluntary contraception) or better manners (fewer inequities, less violence and corruption, freer trade and mobility, more rule of law, less material-intensive consumption)? Or is education of better quality and greater availability a key ingredient of all other strategies?

All these approaches have value. However much we would like one, there is no panacea, though some priorities are clear: voluntary contraception and support services, universal primary and secondary education, and food for pregnant and lactating mothers and children under 5.

These priorities are mutually reinforcing, and they are affordable. Providing modern family planning methods to all people with unmet needs would cost about $6.7 billion a year, slightly less than the $6.9 billion Americans are expected to spend for Halloween this year. By one estimate, achieving universal primary and secondary education by 2015 would cost anywhere from $35 billion to $70 billion in additional spending per year.

David Fuller's view My guess is that the numbers can be managed, given the increasing rate of technical innovation affecting all aspects of our lives. However, this needs to be accompanied by improved governance, universal education, better management of our planet and its resources, and luck.

All but luck are possible as part of our evolution as a species but none are certainties.