WORLD Population Day was last Sunday. It dawned on about 5.5 billion people -- nearly 100 million more than on July 11, 1991. By the year 2000 the world's population will have passed the 6 billion mark. Will population ever stop growing? If so, when, and at what number?
Most people recognize that, because the Earth is finite, it cannot support endless population growth. The world's population must stop growing someday. But, paradoxically, even though the average family is smaller than in the past -- and in some developed countries has fallen below the two-children-per-family "replacement level" -- the world's population continues to grow ever larger. In the same way that compound interest makes financial savings mount, the dynamic of population growth rates causes the number of people in the world to rise higher and higher.
This dynamic helps explain why world population is growing more today than ever before. It took 100 years for the world population to grow from 1 billion to 2 billion, a level that was reached in 1930.
But it took only 30 years to add the next billion, as world population reached 3 billion in 1960. Four billion was reached in only another 15 years in 1975, and the world passed the 5 billion mark 12 years after that, in 1987.
What of the future? The effects of different population growth rates are dramatic. Consider two different patterns of childbearing: If a couple had four children, and these children and their children in turn each had four children, the couple would have 64 great-grandchildren (4x4x4=64). If, instead, this couple had only two children, and they and their children each had two children, the couple would have only eight great-grandchildren (2x2x2=8).
While we cannot predict the future, it is certain that even small differences in childbearing patterns in the 1990s will translate into large differences in population size within a few more decades. In fact, each 20-year delay in reaching replacement-level family size will add at least 1 billion people to the world's population when it eventually stops growing.
Barring catastrophe, the world's eventual population size will depend primarily on how soon the two-children-per-family average is reached. Even if this level had been reached in 1990, the total world population would still rise to over 7 billion by 2025 and would reach over 8 billion in 2100, because of the growth momentum created by so many women in their childbearing years, the result of previous high rates of childbearing.
If, instead, childbearing rates remained at current level -- now averaging about 3.4 children per couple -- world population would soar to 109 billion in 2100, and continue growing rapidly thereafter.
A more realistic expectation is for the replacement level to be achieved by 2100, with a world population of about 11 billion in that year, according to the United Nations "medium" projection. Although the United States, Europe and most other developed areas are approaching the two-children-per-couple replacement level or have already passed it, developing countries have not.
In developing countries, where most of the world's people live, the average family has about four children. While this is a substantial drop from the six-child average of the 1960s, it is still far above the two-child average. Thus, although average family size is falling, dramatically so in some countries, population continues to grow rapidly in many countries.
No one knows just how many people the Earth's resources can support, or what impact ever larger populations will have on social and economic conditions around the globe. Some argue that the current level of 5.5 billion people is already creating intolerable environmental conditions. Few of us can imagine a world with 100 billion people -- and we do not want to learn the hard way.
One thing is clear: Surveys reveal that millions of couples around the world want to have fewer children than they are having, but many lack the information or the high-quality family planning services necessary to avoid unwanted or mistimed births.
Providing this information and these services is one of the best ways to avoid lower standards of living in the future, as more and more people try to share a finite world, and instead help to assure the brighter future that parents want for their children.
Bryant Robey is senior writer in the Population Information Program at Johns Hopkins School of Hygiene and Public Health. He was the founding editor of American Demographics Magazine.