World population will likely peak in 2064 around 9.7 billion, and then decline to about 8.8 billion by 2100 – about two billion lower than some previous estimates, according to a study published in The Lancet.

It stands today at 7.8 billion.

The study projects that by the end of the century, most people won’t be replacing themselves.

From 195 countries, 183 will have total fertility rates (TFR) below the replacement level of 2.1 births per woman.

The TFR is is the rate “which represent the average number of children a woman delivers over her lifetime”.

Without immigration to offset the low fertility rates, those countries will shrink in number.

For the climate, this is good news.

It’s also good news for women and girls.

The eventual population decline projected in the study will bring some problems, but it reflects a much better lot for about half of the world’s population.

According to a statement from The Lancet, the “widespread, sustained” decline in population is a consequence of “improvements in access to modern contraception and the education of girls and women”.

The study predicts “huge shifts” in the global age structure, with an estimated 2.37 billion people over 65 years globally in 2100, compared with 1.7 billion under 20 years, “underscoring the need for liberal immigration policies in countries with significantly declining working-age populations”.

The Lancet editor-in-chief Richard Horton urged world leaders to heed the study’s findings.

This important research charts a future we need to be planning for urgently,” Dr Horton said.

Key projections outlined in the study include:

• The global TFR is predicted to steadily decline, from 2.37 in 2017 to 1.66 in 2100 – well below the minimum rate (2.1) considered necessary to maintain population numbers (replacement level) – with rates falling to around 1.2 in Italy and Spain, and as low as 1.17 in Poland
• Much of the anticipated fertility decline is predicted in high-fertility countries, particularly those in sub-Saharan Africa where rates are expected to fall below the replacement level for the first time – from an average 4.6 births per woman in 2017 to just 1.7 by 2100
• In Niger, where the fertility rate was the highest in the world in 2017 – with women giving birth to an average of seven children – the rate is projected to decline to around 1.8 by 2100
• Nevertheless, the population of sub-Saharan Africa is forecast to triple over the course of the century, from an estimated 1.03 billion in 2017 to 3.07 billion in 2100 – as death rates decline and an increasing number of women enter reproductive age
• North Africa and the Middle East is the only other region predicted to have a larger population in 2100 (978 million) than in 2017 (600 million)
• Many of the fastest-shrinking populations will be in Asia and central and eastern Europe
• Populations are expected to more than halve in 23 countries and territories, including Japan (from around 128 million people in 2017 to 60 million in 2100), Thailand (71 to 35 million), Spain (46 to 23 million), Italy (61 to 31 million), Portugal (11 to 5 million), and South Korea (53 to 27 million)
• An additional 34 countries are expected to have population declines of 25 to 50 per cent, including China (1.4 billion in 2017 to 732 million in 2100)
• People over 80 will outnumber children under five by two to one
• The model predicts that some countries with fertility lower than replacement level, such as the US, Australia and Canada, will probably maintain their working-age populations through net immigration. Although the authors note that there is considerable uncertainty about these future trends.

A disappearing workforce

The modelling research – from the Institute for Health Metrics and Evaluation (IHME) at the University of Washington’s School of Medicine – used data from the Global Burden of Disease Study 2017 to project future global, regional and national population.

The study used “novel methods for forecasting mortality, fertility and migration” – and the findings sit at odds with “continuing global growth” from the United Nations Population Division.

“Continued global population growth through the century is no longer the most likely trajectory for the world’s population,” said IHME director Dr Christopher Murray, who led the research.

This study provides governments of all countries an opportunity to start rethinking their policies on migration, work forces and economic development to address the challenges presented by demographic change.”

The authors note some important limitations, including that “while the study uses the best available data, predictions are constrained by the quantity and quality of past data”.