Fertility, mortality, migration, and population scenarios for 195 countries and territories from 2017 to 2100: a forecasting analysis for the Global Burden of Disease Study
Level 5 - mechanism / opinion, no new human data
Level 5 by design analogy; demographic forecasting and simulation modeling study using historical observational data.
OpenAlex W3043365850 · doi:10.1016/s0140-6736(20)30677-2
What was done
Researchers developed statistical forecasting models to project population trajectories from 2017 to 2100 across 195 countries and territories. Completed cohort fertility at age 50 was modeled as a time-series random walk driven by educational attainment and met need for contraception. Age-specific mortality was modeled using underlying mortality, risk factor scalars, and ARIMA models. Net migration was modeled based on the Socio-demographic Index, crude population growth rate, war and disaster deaths, and ARIMA time series. The study evaluated a reference scenario and alternative policy scenarios (accelerating education and contraception access) and estimated future gross domestic product alongside forecast uncertainty intervals (UIs).
What was found
In the reference scenario: - Global total fertility rate (TFR) was forecasted to decline to 1.66 (95% UI 1.33–2.08) by 2100. - Global population was projected to peak at 9.73 billion (95% UI 8.84–10.9) in 2064 and decline to 8.79 billion (95% UI 6.83–11.8) in 2100. - TFR was forecasted to drop below replacement (<2.1) in 151 countries by 2050 and in 183 countries by 2100. - In 2100, the most populous nations were projected to be India (1.09 billion [0.72–1.71]), Nigeria (791 million [594–1056]), China (732 million [456–1499]), the USA (336 million [248–456]), and Pakistan (248 million [151–427]). - Global age structure was projected to invert, with 2.37 billion (1.91–2.87) individuals aged >65 years versus 1.70 billion (1.11–2.81) aged <20 years in 2100. - 23 countries (including Japan, Thailand, and Spain) were projected to see population declines >50%, with China declining by 48.0% (-6.1 to 68.4). - China was forecasted to become the largest economy by 2035, with the USA regaining the top rank by 2098. - Under an alternative scenario meeting Sustainable Development Goals for education and contraception, global population in 2100 was projected at 6.29 billion (4.82–8.73).
Why it matters
This study provides an alternative demographic trajectory to standard UN projections, predicting an earlier global population peak and widespread sustained sub-replacement fertility. These shifts imply profound long-term challenges for labor forces, elderly support ratios, and geopolitical standing.
Limits
Projections spanning over 80 years accumulate large compounding uncertainties, reflected in wide uncertainty intervals (e.g., global population in 2100 ranging from 6.83 to 11.8 billion). The model relies heavily on historical relationships between fertility, education, and contraception that may not hold under future social, economic, climate, or technological conditions. Unmodeled policy interventions (such as pronatalist incentives or major shifts in immigration policy) could substantially alter these trajectories.
Cited by
- contradicts The global human population is projected to begin a steep decline by the year 2050.
- supports Society is currently experiencing a massive drop in fertility and birth rates are well below replacement level.