The human population nearly tripled from 2.5 billion people in 1950 to 7.3 billion today and will continue growing through 2070, according to two recent demographic projections. After that, demographers disagree on whether populations will begin to shrink or continue to rise into the next century, wrote Worldwatch Institute Senior Fellow Robert Engelman and Research Assistant Yeneneh Terefe in the Institute's latest Vital Signs Online article.

Two population projections----one from the United Nations Population Division, the other from the International Institute for Applied Systems Analysis (IIASA)---- agree on how population has grown until now. But their future scenarios document a breakdown in consensus.

U.N. demographers rely on a methodology that applies past behavior and expert opinion about the future to assign quantified probabilities to various population outcomes. Defying a widespread media and public perception that a stationary world population of 9 billion in 2050 is a near certainty, the U.N. analysts report that the most likely long-term future is for continued growth into the 22nd century.

Demographers associated with IIASA, based in Laxenburg, Austria, however, differ with this analysis. They foresee world population peaking around 2070 at 9.4 billion people and then gradually shrinking to 8.9 billion by the century's end.

The disagreement between these two respected groups of population researchers lies in their varying assumptions, mostly regarding two topics: Africa and the future of education. The U.N. demographers point to recent surveys showing that human fertility (defined as the average number of children that women in a population give birth to over their lifetimes) is not falling in some countries as earlier projections had assumed they would.

The IIASA demographers, by contrast, focus largely on educational trends. In every region of the world, including Africa, the proportion of young people enrolled in school has generally been rising, and these rates are likely to continue to rise, the analysts argue. Because even moderately high levels of educational attainment are associated with reductions in fertility, fertility even in high-fertility countries is likely to fall more than current fertility trends on their own suggest, the demographers reasoning.

Two Australian environmental scientists, Corey J. A. Bradshaw and Barry W. Brook, recently published another set of population projections---- with a twist. They add scenarios in which humanity experiences increases in the deaths of children due to climate change or outright demographic catastrophes due to "global pandemic or war." In their most extreme scenario, 6 billion people die in the early 2040s, in which case human population would decline to about 5 billion by 2100.

The Australian analysts are non-demographers engaging in a one-off thought exercise. But the significant differences among the various projections tell us something important about population and the human future. Despite general perceptions that demographers confidently forecast future population, no one knows when population will stop growing or the level at which it will peak. Moreover, the future of population growth may respond to decisions made today, so ideally these decisions would support a reduced incidence of unintended pregnancy (now about 40 percent of all pregnancies globally) rather than allow environmental and social conditions to deteriorate until death rates reverse their historic decline.