EPIBuilding a Sustainable Future
Lester R. Brown

Chapter 10. Stabilizing Population by Reducing Fertility: Stopping at Two

You do not need to be a mathematician to understand that there is no long-term alternative to having only two children per couple, the number needed for replacement. Joel Cohen, population analyst at Rockefeller University, makes this point rather effectively. He notes that if the 1990 population growth rates in various regions had continued until 2150, there would be 694 billion people in the world. This compares with 6.1 billion people today. "No way," says Cohen. "Not enough water falls from the sky to satisfy the needs of such a vast human population."47

The basic arithmetic is not new. We have always known that a seemingly innocuous growth of 3 percent a year, a rate that has been common in many developing countries, would lead to a 20-fold increase in one century and a 400-fold increase in two centuries. Saudi Arabia today has 20 million people and a population that is expanding at this rate. If this were to continue throughout this century, it would have 440 million people in 2100
more than the current population of North America.

Or look at Nigeria, also growing at roughly 3 percent a year. A century from now, Nigeria's 114 million people would total 2.46 billion. Considering that all of Africa is supporting 800 million today, it is impossible to visualize 2.46 billion people in Nigeria alone. It is hard to argue with Cohen's basic point that the only viable long-term option is two children per couple. A population that is growing, however slowly, will eventually overwhelm its life-support systems. Conversely, a population that is declining, however slowly, will eventually disappear.

The growth in world population over the last half-century is sufficiently recent that we are still struggling to understand what it means. We may intuitively understand that a 20-fold increase in a century is not possible, but we have yet to come to terms with the reasons why. For some threats to our future we have designed response systems. For example, an outbreak of a deadly infectious disease such as the Ebola virus sets off programmed responses to contain and eradicate it. This response involves the World Health Organization, the U.S. Centers for Disease Control and Prevention, and the appropriate agencies in the government of the county affected. And if the currency of a country like Indonesia or Russia collapses, the international monetary system is programmed to respond to that threat. Such is not the case when population growth crosses key support system thresholds.

When the water use of a growing population surpasses the sustainable yield of an aquifer and the water table starts to fall, there is no alarm system that triggers a response in the councils of government. As a result, the gap between the demand for water and the sustainable yield of the aquifer widens. Each year, the drop in the aquifer is greater than the year before, setting the stage for an eventual dramatic reduction in the water supply as an aquifer is depleted and the amount pumped out is reduced to the recharge level. If overpumping is extensive, the drop in water supply could be traumatic, disrupting food production.

Unfortunately, no one regularly measures the water table level under the North China Plain, the Punjab in India, or the southern Great Plains of the United States, announcing when overpumping begins, how much water is left, and when the aquifer will be depleted. As a result, instead of societies planning for a soft landing by bringing the demand for water into balance with sustainable yield, they keep going until the inevitable crash occurs.

Societies with water demands surpassing the sustainable yield of the aquifers and desiring more water per person in the future will have to consider the possibility of reducing population size, a trend already under way in some European countries. This means shifting not to a two-child family, but to a one-child family.

In countries where rural populations continue to grow and holdings are divided among the children in each generation, the land per family eventually shrinks to the point where survival is threatened. Halting the fragmentation that is creating a nightmarish situation in many rural communities in Africa and Asia depends either on moving quickly to replacement-level fertility or accepting massive rural-urban migration.

Although population projections for the world have been available since the 1950s, remarkably little has been done to analyze the relationship between the size of current and future populations and the earth's capacity to satisfy people's needs for basic resources, such as water and cropland. Demographers who do the projections have long since abandoned this area as a field of research. In his 1996 book How Many People Can the Earth Support?, Joel Cohen analyzed the 1992 and 1993 annual meetings of the Population Association of America, where there were some 200 symposia. Not one of these panels attempted to analyze the relationship between projected population growth and the earth's natural resource base.48

The good news is that the world is making progress in achieving replacement-level fertility. Fifty-four countries have now reduced average family size to two children or less. (See Table 10-1.) Together these countries contain 2.5 billion people. Family size in China, at 1.8 children per couple, is now below the level in the United States (2.1). Even so, the large number of young people reaching reproductive age in China means that the population is still expected to reach 1.49 billion by 2038, before its numbers begin to decline, dropping to 1.46 billion in 2050. Some countries have seen fertility drop well below replacement level. For example, Russia's fertility rate is 1.2 children. As a result of this decline, and a rise in mortality over the last decade, Russia's population of 144 million is now declining by 900,000 per year. Other countries where population is beginning to decline include Bulgaria, the Czech Republic, Italy, and the Ukraine.49

Despite these trends, the threat of continuing population growth in more than a hundred developing countries is all too real. Perhaps the most dangerous educational gap is the lack of understanding of the relationship between family size, the longer-term population trajectory, and the future availability of resources per person. Filling this gap requires projections that link a range of family sizes—say, two, four, or six children—to the future availability of land, water, and other basic resources. Without this information, individuals may simply not understand the urgency of shifting to smaller families. And of even more concern, political leaders will not be able to make responsible decisions on population and related policies, such as investment in family planning services.

Discussions of future population growth in this chapter use the U.N. medium projections, those that have world population going from 6.1 billion at present to 9.3 billion by 2050. There is also a high projection, which has human numbers approaching 11 billion by 2050, and a low projection, which has population peaking at 7.9 billion in 2046 and then declining. (See Figure 10-3.)50

This low number assumes that the entire world will quickly move below replacement-level fertility to 1.7 children per couple. This is not only achievable, it is the only humane population option. Otherwise the land and water scarcity that is already increasing hunger and deaths in some countries could spread to many more.

Achieving this lower figure is the responsibility of national political leaders, but unless world leaders—the Secretary-General of the United Nations, the President of the World Bank, and the President of the United States—urge governments and couples everywhere to adopt a goal of two surviving children per couple, resource constraints will likely lead to economic decline. The issue today is not whether individual couples can afford more than two children, but whether the earth can afford for couples to have more than two children.


Table 10-1. Fertility Levels in Selected Countries in 2001
Average Number of Children Per Woman1
Population, Mid-2001
Countries with Fertility at or Below Replacement Level2    


United Kingdom
United States
Countries with Fertility Above Replacement Level2    
Saudi Arabia
Dem. Rep. of Congo
1The average number of children born to a woman in her lifetime is also known as the Total Fertility Rate. 2Replacement-level fertility is an average of 2.1 children per woman.
Source: Population Reference Bureau, 2001 World Population Data Sheet, wall chart (Washington, DC: 2001).

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47. Joel E. Cohen, How Many People Can the Earth Support? (New York: W.W. Norton & Company, 1996).

48. Ibid.

49. PRB, op. cit. note 27.

50. Figure 10-3 from United Nations, op. cit. note 1


Copyright © 2001 Earth Policy Institute