Human Population: The Next Half Century
Joel E. Cohen (cohen@rockefeller.edu)
(Science Mag 302 p.1172 Nov 14 2003)
By 2050, the human population will probably be larger by 2 to 4 billion people, more slowly growing (declining in the more developed regions), more urban, especially in less developed regions, and older than in the 20th century. Two major demographic uncertainties in the next 50 years concern international migration and the structure of families. Economies, nonhuman environments, and cultures (including values, religions, and politics) strongly influence demographic changes. Hence, human choices, individual and collective, will have demographic effects, intentional or otherwise.
It is a convenient but potentially dangerous fiction to treat population projections as exogenous inputs to economic, environmental, cultural, and political scenarios, as if population processes were autonomous. Belief in this fiction is encouraged by conventional population projections, which ignore food, water, housing, education, health, physical infrastructure, religion, values, institutions, laws, family structure, domestic and international order, and the physical and biological environment. Other biological species are recognized explicitly only in the recent innovation of quantifying the devastating demographic impacts of HIV and AIDS. The absence from population projection algorithms of influential external variables indicates scientific ignorance of how external variables influence demographic rates rather than any lack of influence (1).
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Past Population
Earth's population grew about 10-fold from 600 million people in 1700 to 6.3 billion in 2003 (3). These and all demographic statistics are estimates; repeated qualifications of uncertainty will be omitted. It took from the beginning of time until about 1927 to put the first 2 billion people on the planet; less than 50 years to add the next 2 billion people (by 1974); and just 25 years to add the next 2 billion (by 1999). The population doubled in the most recent 40 years. Never before the second half of the 20th century had any person lived through a doubling of global population. Now some have lived through a tripling. The human species lacks any prior experience with such rapid growth and large numbers of its own species.
From 1750 to 1950, Europe and the New World experienced the most rapid population growth of any region, while the populations of most of Asia and Africa grew very slowly. Since 1950, rapid population growth shifted from Western countries to Africa, the Middle East, and Asia.
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Demographic Projections of the Next 50 Years
Projections of future global population prepared by the United Nations Population Division, the World Bank, the United States Census Bureau, and some research institutions assume business as usual (7–9). They include recurrent catastrophes to the extent that such catastrophes are reflected in past trends of vital rates, but exclude catastrophes of which there is no prior experience, such as thermonuclear holocaust or abrupt, severe climate change. The following summary relies mainly on the United Nations Population Division's urbanization forecasts (6) and World Population Prospects: The 2002 Revision (4). Alternative projections prepared by the UN include low, medium, high and constant-fertility variants. Estimates of present levels of demographic variables are projections based on measurements in recent years, rather than global current measurements.
According to the medium variant, the world's population is expected to grow from 6.3 billion today to 8.9 billion in 2050. Whereas the first absolute increase by 1 billion people took from the beginning of time until about 1800, the increase by one billion people from 6.3 billion to 7.3 billion is projected to require 13 to 14 years. The anticipated increase by 2050 of 2.6 billion over today's population exceeds the total population of the world in 1950, which was 2.5 billion.
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Because the populations of the poor countries have been growing more rapidly than those of the rich, they have a much higher fraction of people under the age of 15 years (33% versus 18% in 2000). By 2050, in the medium variant, these fractions will drop to 21% and 16% in poor and rich countries, respectively. The global fraction of the elderly population (aged 65 years or more) will rise from 7% in 2000 to 16% by 2050. Over the same period, the elderly fraction will rise from 5 to 14% in the presently poor countries and from 14 to 26% in the rich countries. Though the fraction of children in the population will decrease by more in the poor countries than in the rich, the fraction of elderly will increase by more in the rich countries than in the poor. Both shifts will have consequences for spending on the young and the old.
Slowly growing populations have a higher elderly dependency ratio (the ratio of the number of people aged 65 and older to the number aged 15 to 64), while rapidly growing populations have a higher youth dependency ratio (the ratio of the number of people aged 0 to 14 to the number aged 15 to 64). The elderly dependency ratio rose from 1950 to 2000 at a rapid rate in the more developed countries, slightly less rapidly in the United States, and still less rapidly in the world as a whole. The ratio rose only slightly in the less developed countries, and hardly at all in the least developed countries. After 2010, in the more developed countries, the United States, and the less developed countries, the elderly dependency ratio will increase sharply faster; this acceleration will be greater in the more developed countries and the United States. The least developed countries will experience a slow increase in the elderly dependency ratio after 2020 and, by 2050, will be approaching the elderly dependency ratio of the more developed countries in 1950.
Demographic Uncertainties: Migration and the Family
According to the United Nations Population Division, "International migration is the component of population dynamics most difficult to project reliably. This occurs in part because the data available on past trends are sparse and partial, and in part because the movement of people across international boundaries, which is a response to rapidly changing economic, geopolitical or security factors, is subject to a great deal of volatility" (11). The UN's 2002 medium variant posits migration from less to more developed regions of 2.6 million people annually during 1995–2000, declining to nearly 2.0 million by 2025–30, and remaining constant at that level until 2050. The United States is anticipated to increase annually by 1.1 million of these 2 million migrants, more than five times the number expected to be added annually to the next largest recipient, Germany (211,000). The major sending countries are expected to be China, Mexico, India, the Philippines, and Indonesia.
International migration is likely to remain important for specific countries, including the United States. In the mid-1990s, about 125 million people (2% of world population) resided outside of their country of birth or citizenship. In 1990, only 11 countries in the world had more than 2 million migrants, and they collectively had almost 70 million migrants. The largest numbers of migrants were in the United States (19.6 million), India (8.7 million), Pakistan (7.3 million), France (5.9 million), and Germany (5.0 million). The countries with the highest percentage of international migrants in the total population were countries with relatively small populations. In the United Arab Emirates, Andorra, Kuwait, Monaco, and Qatar, 64 to 90% of the population were immigrants.
If predicting international migration is difficult, predicting change in family structure is more difficult. Goldscheider (12) suggested that the fall in fertility during the demographic transition weakened the ties between men and women based on parenthood and that the rise in divorce and cohabitation is weakening the ties between fathers and children. Nonmarital births increased as a percentage of all births in the United States from 5.3% in 1960 to 33.0% in 1999. In 1999, the United States had 1.3 million births to unmarried women (13). In 1998, Iceland, Norway, Sweden, Denmark, France, United Kingdom, and Finland all had higher proportions of nonmarital births than the United States. By contrast, in Germany, Italy, Greece, and Japan, less than 15% of births were nonmarital (13). Among United States women aged 15 to 29 years at first birth, when that first birth was conceived before marriage, the fraction who married before the birth fell from 60% in 1960–64 to 23% in 1990–94 (14). By 1994, about 40% of children in the United States did not live with their biological father (12).
In the United States, the number of widowed males aged 55 to 64 per thousand married persons fell from 149 in 1900 to 35 in 2000, whereas the number of divorced males aged 55 to 64 per thousand married persons rose from 7 to 129. Divorced males became more frequent than widowed males between 1970 and 1980. Divorced females became more frequent than widowed females between 1990 and 2000. By 2000, the number of divorced and widowed persons aged 55 to 64 per thousand married persons was 164 males and 426 females (2.6 such females for each such male) (15). Remarriages and stepfamilies are becoming increasingly common.
Three factors set the stage for further major changes in families: fertility falling to very low levels; increasing longevity; and changing mores of marriage, cohabitation, and divorce. In a population with one child per family, no children have siblings. In the next generation, the children of those children have no cousins, aunts, or uncles. If adults live 80 years and bear children between age 20 and 30 on average, then the parents will have decades of life after their children have reached adulthood and their children will have decades of life with elderly parents. The full effects on marriage, child bearing, and child rearing of greater equality between the sexes in education; earnings; and social, legal, and political rights have yet to be felt or understood.
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