Projections of global human population are generally based on birth rates and death rates, and since these are difficult to predict very far into the future, forecasts of global population numbers and growth rates have changed over time.
20th century Walter Greiling projected in the 1950s that world population would reach a peak of about nine billion in the 21st century and then stop growing after an improvement in
public health in
less developed countries. In 1983, astrophysicist
Brandon Carter developed the
Doomsday Argument, a probabilistic argument that postulates it is unlikely for humans born today to be randomly selected at an early position in the ordering of all humans who will ever live. In the original formulation, there is a 95% chance that the total number of humans will be less than 1.2 trillion and that extinction will occur before the year 9120. In 1992, the United Nations published five projections of long-term world population growth. According to their medium projection, the world population would grow to 10.0 billion by 2050, 11.2 billion by 2100, and 10.8 billion by 2150.
21st century Estimates published in the early 2000s tended to predict that the population of Earth would stop increasing around 2070. For example in a 2004 long-term prospective report, the
United Nations Population Division projected that world population would peak at 9.2 billion in 2075 and then stabilize at a value close to 9 billion out to as far as the year 2300.
Jørgen Randers, one of the authors of the seminal 1972 long-term simulations in
The Limits to Growth, offered an alternative scenario in a 2012 book, arguing that traditional projections insufficiently take into account the downward impact of global urbanization on fertility. Randers' "most likely scenario" predicted a peak in world population in the early 2040s at about 8.1 billion people, followed by decline. In 2012, the UN changed its prediction to the effect that no maximum would likely be reached in the 21st century, and that by the year 2100 world population would increase to somewhere in the range 9.6 to 12.3 billion with 10.9 billion being the midpoint of that range. The main reason for the revision was a recognition that the high fertility rate in
Africa was not declining as fast as had been previously assumed. File:Human population since 1800.png|alt=|thumb|260px|World population since 1800 in billions. Data from the United Nations projections. Another 2014 paper by demographers from several universities, using data from the UN's 2014 report and their own statistical methods, forecast that the world's population would reach about 10.9 billion in 2100 and continue growing thereafter. In 2017 the UN predicted that global population would reach 11.2 billion by 2100 and still be growing then at the rate of 0.1% per year. In 2019 it was updated to 10.9 billion by 2100 and still growing. The 2022 revision of the UN's
World Population Prospects report represents a departure from the pattern of the previous ten years, it was the first to project a peak in the 21st century. It expected that a slowing of the population growth rate will lead to a population peak of 10.4 billion in 2086 in the medium scenario, after which it would then begin to slowly fall. This shift from earlier projections of peak population and predicted date of zero population growth comes from a more rapid drop in Africa's birth rate than previous projections had expected. For example, the 2012 report predicted that the population of Nigeria, Africa's most populous country, would rise to 914 million by 2100; the 2022 report lowers that to 546 million, a reduction of 368 million; the 2024 report lowered that further to 477 million, a reduction of 69 million. A table based on UN World Population Prospects reports, using the medium fertility scenario: == Drivers of population change ==