World Population: From One Billion Humans to Demographic Saturation

An exponential leap since the Industrial Revolution

In 1800, the world human population is estimated at about 900 million. Two centuries later, in 2020, it exceeds 7.8 billion. This dizzying growth is the result of an exponential dynamic fuelled by medical, agricultural and technological progress. This growth is modelled by the equation: \( N(t) = N_0 \cdot e^{r t} \) where \( N(t) \) is the population at time \( t \), \( N_0 \) the initial population, and \( r \) the average growth rate. Until the early 21st century, the parameter \( r \) reached values close to \( 0.02 \), i.e. growth of 2% per year. This evolution is explained by a sharp reduction in mortality without an initial decline in fertility — a desynchronisation characteristic of phase 2 of the demographic transition.

Towards a global saturation?

According to the 2024 revision of the United Nations (World Population Prospects 2024, published on 11 July 2024), the world population should continue to grow for another five to six decades, crossing 9 billion around 2037 and 10 billion around 2061, before reaching a peak of about 10.3 billion individuals around the mid-2080s (around 2084), then slowly declining to 10.2 billion in 2100. This levelling off is explained by the global demographic convergence: the majority of countries are experiencing or will experience a decline in fertility towards the replacement level (\( \sim 2.1 \) children/woman), which the global average (2.25 children/woman in 2024) should reach around 2036. From that point, natural growth becomes negative in an increasing number of countries, unless offset by migration flows or pro-natalist policies.

The probability that the world population will actually peak within this century is estimated by the UN at about 80%, this peak could theoretically occur anywhere between the mid-2060s and 2100 depending on assumptions about fertility, mortality and migration. These projections therefore remain probable scenarios, not certainties.

This asymptotic dynamic also reflects the depletion of resources, water stress, rising inequalities, and negative feedback related to climate change.

Demographic momentum: growth continues despite falling fertility

An apparent paradox deserves explanation: global fertility has been declining for decades, yet the population continues to grow strongly. This phenomenon has a name: demographic momentum. When a population has experienced a long period of high fertility, it inherits a very young age structure, with a large number of women of childbearing age. Even if each of these women now has fewer children than before, the absolute number of births remains high simply because the number of potential mothers is large. Growth therefore continues for several decades after fertility has fallen to replacement level, until that large age cohort passes through the life cycle.

According to the United Nations, this momentum alone would explain about 79% of the increase in the world population by 2054, i.e. about 1.4 billion additional people. Conversely, in countries where fertility has remained persistently very low and where the population has already peaked — 24 countries are in this situation in 2024 — a return to fertility of 2.1 children per woman or more in the next thirty years is considered highly unlikely by UN demographers.

A future polarised between Africa and global ageing

From a spatial point of view, the population growth of the 21st century is strongly polarised. Most of the increase by 2100 will be concentrated in sub-Saharan Africa, the only major region still engaged in the early phases of the demographic transition. Conversely, many regions (Europe, East Asia) are already experiencing natural decline. This heterogeneity creates strong geopolitical, economic and migratory tensions.

Furthermore, the global demographic transition is accompanied by rapid ageing: as early as 2080, people aged 65 and over are expected to outnumber children under 18 globally — a historic inversion of economic balances and social models. This corresponds to a transition of the demographic system towards a new dynamic equilibrium, marked by the structural ageing of populations.

Population mondiale en temps réel

Estimated world population in 2026

Estimated world population in 1800

A tightening forecast: the evolution of UN projections

Global demographic projections are not fixed: they are regularly revised by the UN Population Division as new census data and new fertility trends become available. This succession of revisions well illustrates the normal functioning of forecasting science, based on the gradual updating of models rather than fixed certainties.

The 2019 revision did not project a peak within the century, with the world population still growing to 10.9 billion in 2100. The 2022 revision was the first to anticipate a peak during the 21st century, around 10.4 billion around 2086. The 2024 revision, the most recent, revised this peak slightly downward and slightly earlier: 10.3 billion around the mid-2080s. This adjustment stems largely from lower fertility than expected in several large countries, first and foremost China, whose fertility rate reached only 1.01 children per woman in 2024.

Dynamics of a global demographic system

The world population growth between 1800 and 2100 illustrates the behaviour of an open system coupled to its environment: a phase of rapid growth under external energy and technological input, then saturation and bifurcation towards a new stationary state, or even a declining one. Future equilibria will depend on many coupled parameters: education, public policies, climate, reproductive technologies, and inequalities.