A population forecast provides estimates of the most likely future trends in population size and in demographic indicators such as population distribution by age and sex. A forecast is based on the current understanding of the roles played by various factors affecting population growth and on an appropriate, accepted methodology for calculating the effects of future changes in these factors. A variety of methodologies are available for making forecasts, ranging from the simple extrapolation of past trends to complex multiple-equation models involving dozens of demographic, socioeconomic, and environmental variables. In practice, most projections made in recent years rely on the so-called cohort-component method, which computes future demographic trajectories implied by assumptions (based on demographic transition theory) about future trends in birth, death, and migration rates.
The terms "forecast" and "projection" are often used synonymously, though they have slightly different technical meanings. A forecast for a population can involve more than one projection. For example, the most likely future trajectory is usually called the medium variant, while alternative higher and lower projections can give an indication of the uncertainty surrounding this trend. In the contemporary demographic literature, "forecast" is typically used to refer to medium variant projections.
Future population trends are of interest to a wide range of analysts, including policymakers, scientists, and planners in industry and government. Global and national trends in population size are used to estimate the future demand for food, water, and energy, as well as the environmental impact of rising consumption of natural resources. Subnational projections help planners decide where to build schools, hospitals, roads, and other infrastructure. Estimates of the number of retired people are essential to the optimal design of social security systems that provide pensions and health care. To address the needs of such a variety of potential users, projections for countries and regions within countries are usually made on a regular basis by national agencies (e.g., the Bureau of the Census in the United States). Global as well as national population projections for all countries are produced by the United Nations and the World Bank.
The projections prepared in 1998 by the Population Division of the United Nations predict the population of the world to reach 8.9 billion in 2050. This represents an increase of 2.8 billion over the 2000 population of 6.1 billion. Nearly all of this future growth will occur in the developing world—Africa, Asia (excluding Japan, Australia, and New Zealand), and Latin America—where population size is projected to rise from 4.9 to 7.8 billion between 2000 and 2050. In contrast, in the developed world—Europe, North America, Japan, Australia, and New Zealand—population size is forecast to remain virtually stable, growing from1.19 to 1.22 billion between 2000 and 2025, followed by a decline to 1.16 billion in 2050. Trends for the two principal regions in the developed world are expected to diverge between 2000 and 2050, with an increase from 0.31 to 0.39 billion in North America and a decline from 0.73 to 0.63 billion in Europe.
National Research Council, Commission on Behavioral and Social Science and Education, Committee on Population (2000). Beyond Six Billion: Projecting the World's Population, eds. J. Bongaarts and R. Bulatao. Washington, DC: National Academy Press.
United Nations, Department for Social and Economic Policy Analysis, Population Division (1999). World Population Prospects: The 1998 Revision. New York: United Nations.