First published in February 2023 and republished on 4 April 2024; updated 21 September 2026 to reflect the UN’s 2024 projections and current research on why projection results differ.

Every person has the right to decide freely whether and when to have children, with access to the information and services they need and freedom from pressure or coercion. Population projections are estimates of possible demographic change: they can inform planning, but they cannot determine anyone’s choices or rights. The United Nations Population Division publishes projections through World Population Prospects (WPP); the 2024 revision estimates a world population of 8.2 billion in 2024 and projects about 10.3 billion in the mid-2080s under its medium scenario, followed by a slight decline to about 10.2 billion in 2100. The UN estimates an 80 per cent likelihood that world population will peak within this century, with timing uncertain.

The UN’s Past Population Growth Projections

The UN actually have a pretty good record with past projections, specifically their median scenario. In 1968 they projected 5.44b in 1990 – the actual figure was 5.38bn (0.06bn difference). In 2000, the population projected for 2020 was a little under 8bn and we hit 7.8bn (0.2bn out). 

World Population Prospects (WPP) 2024 is the UN Population Division’s latest official estimates and projections edition. It estimates 8.2 billion people in 2024 and, in its medium scenario, projects a peak of about 10.3 billion in the mid-2080s before a gradual decline to 10.2 billion in 2100. The UN gives an 80 per cent probability that the global population will peak this century; its prediction interval spans a wide range of possible outcomes.

So, not perfect, but they have certainly stood up to scrutiny so far. This is notable given the number of variables involved.

There are a lot of variables, however. Demographers use different approaches to forecast how these variables will affect future population growth. There is strong consensus about our trajectory. We expect continued growth into the second half of the century, followed by a peak and eventual decline. What the numbers will be, and when that peak will occur, however, is a matter of greater debate.

International Institute of Applied Systems Analysis (IIASA)

IIASA is an international research institute based near Vienna, Austria, and the Intergovernmental Panel on Climate Change uses its modeling.

Employing a different methodology than the UN, IIASA produces a range of scenarios known as Shared Socioeconomic Pathways (SSP). These scenarios are based on possible developments in factors known to influence fertility rates. The primary factors considered are family planning use and education.

Their most recent scenarios and projections, Demographic and human capital scenarios for the 21st century, published in collaboration with the European Commission in 2018 are; 

• SSP1, ‘Sustainability/Rapid Social Development’ in which great progress in education, use of modern family planning and economic security leads to falling family size; 
• SSP2, ‘Continuation/medium population’ which is more or less our current trajectory; 
• SSP3, ‘Fragmentation/Stalled Social Development’ in which things don’t go as we hope they will.

In that 2018 model vintage, SSP2 peaked at about 9.7 billion around 2070, while SSP3 exceeded 13 billion and was still rising in 2100. IIASA’s later WIC2023 release uses a 2020 reference year and seven SSP-related scenarios across 200 countries. Its SSP2 reference scenario peaks at 10.13 billion in 2080 and reaches about 9.88 billion in 2100. These results use different release dates, assumptions and methods, so they are not direct like-for-like comparisons with the UN’s WPP2024 medium scenario.

Institute of Health Metrics and Evaluation at the University of Washington (IHME)

This research institute working in the area of global health statistics and impact evaluation at the University of Washington, Seattle, is the new kid on the block.

Published in the Lancet in 2020, IHME unpacks the factors driving fertility and produces a range of scenarios. Its main projection sees population growth peaking at 9.7bn in 2064 at a similar rate to the UN’s projections, then declining to 8.8 billion by the end of the century. 

Going beyond that, the difference seems dramatic. However, long-term projections come with significant uncertainty. At the top end of the main scenario’s “95% interval,” IHME projects a global population of 11.8 billion in 2100—more than a billion higher than the UN’s current medium variant projection. Its medium projection for a “worst case” scenario is actually higher than IIASA’s – 13.6bn by the end of the century. 

The IHME scenarios include lower numbers, however. In particular, it models how meeting the Sustainable Development Goals (SDGs) would impact fertility. The SDGs are the UN’s framework for securing human development and planetary health by 2030 – a set of 17 areas of activity, such as Zero Hunger, Gender Equality and Climate Action, each broken down into specific targets. 

Long-range results are especially sensitive to assumptions about future change. The modelled intervals quantify some uncertainty within each study’s framework; they do not capture every possible future event or guarantee the actual population will fall inside them.

ARE THERE OTHERS?  

Jorgen Randers is a quite genuine prophet of ecological collapse, having been an author of the hugely influential Limits to Growth report in 1972, which modelled future trajectories of economic activity, resource use and planetary boundaries and concluded that the then path was unsustainable. 

Back then, Randers predicted a global population of 13bn by 2030 but by 2012, he had revised it substantially: “The world population will never reach nine billion people… It will peak at eight billion in 2040, and then decline.” The level and timing of that peak are at odds with almost every other projection of population growth. 

Today though, even Randers no longer endorses it. In 2021, his projection estimated a peak of around 9.5 billion people in 2050, followed by a steep decline to around 6 billion by the end of the century. This would return the population to the same level it was at the start of the century, just twenty-odd years ago.

WHO’s most likely to be right?

People widely use the UN’s projections because of their longevity and, as we’ve seen, their relative accuracy.  They remain the most authoritative and useful.

The main messages to take from all the projections are these:

• The population is still rising and will likely continue to do so until the second half of the century
• We are unlikely to have a lower population by 2100 than we did in 2000.
• We have the power to change how big the peak in numbers will be, and how soon it happens.

Population projections are tools for understanding possible demographic change. They can inform planning, but they cannot determine people’s choices or rights, and no single scenario describes a certain future.

United Nations Population Division, World Population Prospects 2024: Sources: The Lancet IHME analysis; IIASA SSP 2023; population facts and figures; the UN 2024 release analysis; demographic momentum explainer.