Global debates about population policy are confusing. One side argues that rising human populations threaten our environment and prosperity. Land, water, energy and biodiversity all seem to be under greater stress than ever, and population growth appears to be a major source of that stress.
The other side of the debate, mainly in rich countries, argues that households are now having so few children that there won't be enough to care for aging parents.
Those who fret about population growth have the better argument. Issues confronting Europe, Japan, and to a lesser extent the US and some middle-income countries concerning aging populations are manageable. Moreover, the benefits of slower population growth outweigh the adjustment costs.
By contrast, if global populations continue to rise rapidly, the stresses on the world's resources will worsen. Governments should therefore refrain from deliberate policies to raise birthrates, even in places where birth rates are low.
Part of the confusion of the public debate reflects different population trends in different parts of the world. The fastest population growth is taking place in the poorest regions. Poor people, especially poor people living on farms, tend to have the most children (often six or more per woman), and therefore the highest rates of population growth.
Poor farm families rely on their children for farm chores and for security when parents reach old age. Poor families lack access to contraception and family planning. Finally, poor families have many children as a kind of insurance policy against high child mortality rates.
As a result of high fertility rates in Africa, the UN Population Division predicts a doubling of Africa's population from around 900 million today to around 1.8 billion in 2050. Rapidly growing populations are also young populations, because of the high number of children per household. In Africa, the median age is now a mere 19 years and is projected to rise to around 28 years in 2050.
In Europe, the trends run in the other direction. The UN projects a decline in population to around 630 million in 2050, from around 725 million people today. With few children and longer life expectancy, the median age of the population rises sharply in this forecast, from 39 years in 2005 to around 48 years in 2050.
For the world as a whole, population is expected to continue to grow by another 2.5 billion people from 2005 to 2050. All of that growth will be in the developing world: 1.3 billion more people in Asia, 900 million more in Africa, the rest in Latin America and other regions.
Adding another 2.5 billion people to the planet will put enormous strains not only on societies with rising populations, but on the entire planet. Total energy use is soaring, reflecting the combined effect of rising incomes -- and thus rising per capita energy use -- and population growth.
Higher energy use is already changing the world's climate in dangerous ways. And the strains of increased global populations, combined with income growth, are leading to rapid deforestation, depletion of fisheries, land degradation, and the loss of habitat and extinction of a vast number of animal and plant species.
Population growth in developing regions -- especially Africa, India, and other parts of Asia -- needs to slow. Public policies can play an important role by extending access to family planning services, expanding social security systems, reducing child mortality through public health investments, and improving education and job opportunities for women.