(March 2003) The newly released 2002 revision of the United Nations World Population Prospects shows that, by the year 2050, 75 percent of all countries in the less developed regions of the world will experience below-replacement fertility — that is, a fertility rate lower than 2.1 children per woman.

This estimate is the UN’s medium variant and highlights a lower world population in 2050 than the UN’s 2000 Revision did: 8.9 billion instead of 9.3 billion. About half of the 400 million difference in these projected populations results from an increase in the number of projected deaths, the majority stemming from higher projected levels of HIV prevalence. The other half of the difference reflects a reduction in the projected number of births, primarily as a result of lower expected future fertility levels.

World population, now at 6.3 billion, is growing at a rate of 1.2 percent annually, meaning an additional 77 million people each year. This is considerably slower than the peak annual growth rate of over 2 percent, reached in the early 1970s.

For the medium variant, UN demographers assume that the fertility rate for the entire world population, now estimated to be 2.8 children per woman over the course of her lifetime, will continue to decline, reaching 2.0 children per woman by 2050. This would be just below the level at which a population maintains its size over the long term. Already in 2003, China, Japan, almost all of Europe, and many parts of South and Southeast Asia have fertility rates below replacement level. If fertility declines at this rate, then world population in 2050 will likely be 8.9 billion.

The UN assumes that life expectancy in the more developed countries will rise from 74.8 years in the late 1990s to 81.6 years in the late 2040s. Already women in Japan and several European countries have longer life expectancies, so 81.6 years for the whole of the developed world does not seem too ambitious. But the UN projects a life expectancy of only 73.1 years for the less developed regions in 2045-2050. In other words, in 50 years, they still will not have caught up to where the rich countries of the world are today. Much of this pessimism is due to the long-lasting effects of the AIDS pandemic, a major cause of premature deaths. For 53 countries for which the UN makes AIDS projections, the population in 2050 will be nearly half a billion smaller due to the effects of AIDS than it would have been without AIDS deaths.

The medium-variant projections are not meant to show what will happen automatically. If the decline in fertility rates slows or stalls, such that the world average is 2.5 children per woman in 2050, the world population would likely be 10.6 billion, according to the UN. If the fertility decline is relatively fast, and the world average reaches 1.5 children per woman, then world population would be 7.4 billion in 2050. The reason that population would still be growing during the next four decades, despite a decline of fertility rates below replacement levels, is that a high proportion of people, especially in the developing countries, are just entering their childbearing years. Half a child per woman, more or less, is not such a large margin of error for projections of nearly five decades. Fertility rates in the United States rose and then fell by more than that during the baby-boom and baby-bust years of 1945 to 1975.

The world averages hide a huge range in demographic futures for different countries. At one extreme, Russia, where mortality rates have been increasing and fertility rates have remained at very low levels, is projected to decline in population, from 146 million in 2000 to 101 million in 2050. Several very poor African countries, such as Niger, Somalia, and Uganda, may see their populations quadruple in the same time period, under plausible assumptions. For these countries, the news that population growth averaged over the whole world is slowing down may not be much more cheering than the news that incomes averaged over the whole world are adequate.

The fertility decline of recent decades in most of the world was helped along by a major effort to make safe and effective means of contraception widely available. Whether the remaining high-fertility countries will see a decline in birth rates as rapid as the medium-variant projections assume will depend in part on whether this effort is kept up. A new generation of adolescents, the largest in human history, is now approaching the peak ages for marriage and fertility. The world population future depends on the choices they will make, and not even demographers can pretend to read the minds of teenagers.

John Haaga is director of domestic programs at PRB.

The United Nations Population Division estimates and projections can be found at www.un.org/esa/population/unpop.htm. A good discussion of projections, how they are produced, and what they mean, can be found in the PRB Population Bulletin, “World Population Futures.”