After the world population increased more than 400% over the 20th century, population growth has slowed considerably. Over the course of the 21st century world population will likely only rise by 50% and reach around 11 billion by 2100. Peak world population growth rate was already reached in the late 1960s, and it has been falling since.
There are two primary determinants of population growth: life expectancy and fertility rates. The global improvement in life expectancy works to increase the world population, but it is more than offset by the fall in fertility. The global average fertility rate was 5 children per woman until the end of the 1960s and has halved since then. While the world population increased by 2% annually in the late 60s it has now slowed to an increase of just about 1%. From previous UN projections of population growth rates we have learned that the accuracy of the projections is very high.
# Empirical View
# World maps of population growth at the peak of the global population growth rate (between 1965 and 1970) and the latest available data (2005-10) – Max Roser1
Full screen view Download Data
Annual population growth rate (%) and population growth rate projections by world regions, 1 million years BCE until 2100 CE – Max Roser2
# The Two Drivers of World Population Growth: The World Fertility Rate & The World Life Expectancy
Population growth is driven by two different factors: fertility and life expectancy.3 It is important to have an idea of how the two drivers changed and how they are interlinked to understand how the forecasts are possible and to assess their credibility.
As the fertility rate measures the children per woman, a rate of 2 would mean that every woman gives birth to (on average) one girl. Therefore, the current generation of women would just replace itself by a younger generation of girls. The fertility rate that would create this scenario is called the replacement rate. In reality the replacement rate is higher than 2 because not all girls reach the age to give birth. This means that the replacement rate for populations with low mortality is close to 2 and for populations with a high mortality is considerably over 2. As mortality differs widely between different nations, the replacement rate for different countries ranges from below 2.1 for rich countries to almost 3.5 for Sierra Leone according to a research paper by Espenshade, Guzman and Westoff (2003).4 Globally the study found a replacement rate of 2.335.
The population increases not only when more children are born but also when less people die. This makes life expectancy the second driver of population growth. Furthermore, the replacement rate is a function of life expectancy as increasing life expectancy means that more daughters reach the age to become mothers themselves.
Before we look at the forecast for the world population over the next century, we look at the two drivers of population growth: world total fertility rate and world life expectancy.
# The World Fertility Rate
In the late 1960s the fertility rate in less developed regions of the world started falling. It is this important change that reduced the world fertility rate from almost 5 (in the 1950s and 60s) to around 2.5 children per woman in the present world. The UN forecasts that this downward trend will continue and that over the course of this century the world total fertility rate will continue to fall from 2.5 to just over 2. As world fertility approaches the replacement level, it ceases to be the driver behind world population growth in the near future.
# Total fertility by socio-economic region, 1950-2010 and UN forecasts (Medium-fertility Variant) (2010-2100) – Max Roser5
The high population growth rates are a modern development. They were non-existent in the pre-modern world–but not for lack of trying. Previously birth rates were very high, but the horrific child mortality rates in the pre-modern world prevented the population from growing. This lack of population growth implies that during this time period the average family size was small.
With the onset of modernity, as living standards rose and health improved, more and more children survived, which led to unprecedented growth of the population. This episode of large families was short and only lasted for few generations. As a consequence of further development, the family size began decreasing around the world — this time not due to high child mortality, but due to low birth rates. How rapidly the world changed is hard to grasp, but a comparison highlights this transformation: In 1950, 44% of the world population had a total fertility of 6 or more children. By 2010 the world has massively changed and 48% now have less than 2.1 children! This development of a decreasing family size around the world is depicted in the following graph.
Fertility is driven by the socio-economic development of the population, especially the status and wellbeing of women. Accordingly, the early developing populations started the transition much earlier than world regions that remained poor for a longer time. Europeans and Northern Americas reached the replacement rate at a time when fertility only started to decline in Asia and Latin America. Africa, where poverty and lack of education persisted even longer, was the last continent to experience a decreasing fertility rate.
# UN estimates and forecasts of total fertility by major regions, 1950-2100 (children per woman) – UN7
# The World Life Expectancy
As health is rapidly improving around the world, life expectancy is also increasing rapidly. You can read more about life expectancy at the our life expectancy data entry.
# Life expectancy at birth, 1950-2010 and UN forecasts, 2010-2100 (both sexes combined, medium variant) – Max Roser8
# Forecast of World Population Growth
The most discussed forecasts of the world population are those by the Population Division of the United Nations that are discussed here. But there are also other institutions working on population forecasts like the Austria-based Institute for Applied Systems Analysis (IIASA) that can be found here. We can see below that there are stark differences in the projected world population depending on the fertility rate. If the rate is low, then the world population will start to decline. However, if it is high, then the population will continue to increase rapidly with no decrease in sight.
# UN estimates and forecasts of the world population (billions), all variants 1950-2100 – Max Roser9Full screen view Download Data
The following graph shows how this increase in world population is shared amongst different world regions. We can see that presently China, India and Africa are the regions that have the largest world population and are predicted to stay that way by 2100. But, if you click ‘expanded’ and change the graph from absolute numbers to shares of world population, we can see that India and Africa’s share of world population is continued to increase, while China’s is expected to decrease, likely due to falling fertility rates.Full screen view Full screen view of the same graph with more detailed world regions Download Data
The following chart displays projections of population by country to the year 2100. It is based on the medium growth scenario (SSP2) developed by the International Institute of Applied Systems Analysis (IIASA).
The History Database of the Global Environment (HYDE) – the data source of the population density estimates in the following maps – visualised the estimates of population density on an interactive world map here.
# Correlates, Determinants & Consequences
As shown above, changes in life expectancy and fertility rates determine population growth. The data entry on past world population growth discusses the demographic transition as the central concept that explains population growth.
# Data Quality & Definition
In a 2001 paper, Nico Keilman assed the forecasts of world population the UN published between 1951 and 1998.13 The next graph shows the estimates of the actual world population in retrospect along with the UN forecasts between 1950 and 1980. With one exception (1950 I) that can be explained by the low data quality – especially for the populous country of China – the forecasts are remarkably accurate. Even the 1950 III forecast that was made with better information on China after the 1953 Census is not far off for the population size half a century later.
Nevertheless the direction of the under- and overestimation of the world population is interesting and telling. Over the first four decades the UN forecasts underestimated world population growth and this reversed over the last 10 years when the world population size and growth rates were overestimated. Why this happened becomes clearer if we look at the last two graphs – the true and forecasted world fertility rate and life expectancy.
Keilman (2001) also studies the two drivers of population growth separately and this can help to explain why the UN first underestimated the population growth and then overestimated the population. As shown in the following two graphs he finds that the UN firstly underestimated the rapid fall of world fertility and that secondly the UN “projection makers have been too pessimistic about future mortality” and that “predicted life expectancy levels that were too low on average – much too low in many cases”.
On a regional level the study finds that ‘Not surprisingly, problems are largest in pre-transition countries, and especially in Asia’. He highlights that the stagnation or even decline of life expectancy in the former USSR before its dissolution has not been unforseen and caused the forecasts to be inaccurate.
# Estimates of the world fertility rate compared with the UN forecasts, since 1950 – Keilman (2001)15
# Estimates of the world life expectancy compared with the UN forecasts, since 1950 – Keilman (2001)16
# Data Sources
Data on population growth – including projections – can be found in the data section of the entry on world population growth.