Q: When could world population stop growing?
A: World population will stop growing when the birth rate equals the death rate; no one knows whether this will happen.
Demographers usually assume that the birth rate and the death rate will eventually reach equilibrium several decades after couples average two children each. This two-child average is called replacement level fertility, because each couple replaces themselves in the number of people in a population. The total fertility rate refers to the average number of children women are having. When the total fertility rate is at replacement (or 2.1 children per family), the two children born essentially replace the parents when they die. The decimal value accounts for child mortality. Because some children die before they grow up to have their own children, the average number of children born can still be slightly above two and fertility would be considered at replacement level. Therefore, the value for replacement level fertility could be higher in a country where mortality is higher.
While no one knows exactly when the population will stop growing, the United Nations and other organizations estimate that world population could continue to grow well into the 22nd century, reaching 9.8 billion by 2150. These outcomes are based on the medium projections, which assume (to varying degrees for different countries) that the downward trend of fertility rates will continue and stabilize at 2.1 children per woman. They also assume continued mortality declines. If fertility were to decrease at a much faster pace and stabilize at 1.6 children per women, world population could stop growing much sooner — by 2050 — at 7.3 billion. Given that scenario, the population would decline to 5.3 billion by 2150. On the other hand, slower declines in fertility could lead to a global population of 10.7 billion by 2050 and 16.2 billion in 2150, with fertility projecting to stabilize at 2.5 children per woman.
We do know that future population growth is inevitable. But the range of possible future population sizes varies dramatically. Five plausible projections published by the United Nations lead to outcomes ranging from 7.3 billion people to 10.7 billion people in 2050.
Terms
Birth rate (or crude birth rate): The number of live births per 1,000 population in a given year. Not to be confused with the growth rate.
Death rate (or crude death rate): The number of deaths per 1,000 population in a given year.
Mortality: Deaths as a component of population change.
Total fertility rate (TFR): The number of children women are having today. The average number of children that would be born alive to a women during her childbearing years if she conformed to the age-specific fertility rates of a given year.