Population decline begins quietly. Growth slows, the total reaches a plateau, and annual changes remain small. The underlying age structure may nevertheless be preparing a much faster contraction.
Four forces determine the pace: births, deaths, immigration, and emigration. Fertility affects births per woman, but the number of births also depends on how many women are at childbearing ages. Mortality affects both the number and timing of deaths. Migration can offset natural decrease or accelerate it. Age structure carries the history of all three forward.
A country with fertility modestly below replacement and a balanced age structure may stabilize gradually. A country with very low fertility, few young adults, and a large elderly population can decline faster as deaths rise and births remain few. The same TFR can produce very different annual population changes.
Migration changes the national arithmetic most quickly. It can add working-age adults directly and, because migrants are usually younger, add future births as well. It cannot make every country grow at once: international migration redistributes population rather than creating it. As immigrants and their children adapt to the host country’s family pattern, their fertility tends to converge too. A stable national total can conceal both an unrepaired birth deficit and a loss of young people somewhere else.
National turns precede the global turn
Countries cross from growth to decline at different times. The UN groups the world accordingly: some populations have already peaked, others are likely to peak in coming decades, and a smaller set is projected to grow through 2054. The continued growth of the latter can outweigh decline elsewhere in the global sum.
This produces a long period in which national contraction and global growth coexist. China, Japan, and several European countries can lose population while sub-Saharan African countries add many more people. The global peak arrives only when the combined losses exceed the combined gains.
After a peak, decline need not proceed at a constant percentage. It may accelerate as large cohorts enter ages of high mortality and small cohorts reach parenthood. It may slow if fertility recovers, longevity rises, or migration changes. War and economic crises can disturb the path, sometimes for years.
Forecasts are conditional trajectories. They show what current age structure and plausible assumptions imply; they cannot assign a date to an unavoidable future. The near term is constrained by people already alive. The distant pace depends increasingly on choices and conditions we cannot yet observe.
Citations
- United Nations, Department of Economic and Social Affairs, Population Division, World Population Prospects 2024: Summary of Results.
- United Nations, Department of Economic and Social Affairs, Population Division, World Population Prospects 2024: Methodology Report.
- Organisation for Economic Co-operation and Development, “Migrant family building: Recent evidence and implications”, in International Migration Outlook 2023, 2023.