In addition to the projection for the medium scenario based  on probabilistic methods which provide 80 and 95 per cent prediction intervals, the 2024 Revision includes includes thirteen different deterministic projection scenarios (see table below) that convey the sensitivity of the medium scenario projection to changes in the underlying assumptions, and to explore the implications of alternative future scenarios of population change. The results for these additional projection scenarios are available from the Download Center for the standard projection results in Excel file format as extra worksheets in each workbook file), or CSV file format. Probabilistic projection results are available as a separate set of files, see Download > Probabilistic Projections and Figures.

Medium scenario projection: in projecting future levels of fertility, mortality and international migration, probabilistic methods were used to reflect the uncertainty of the projections based on the historical variability of changes in each variable. The method takes into account the past experience of each country, while also reflecting uncertainty about future changes based on the past experience of other countries under similar conditions.  The medium scenario projection corresponds to the mean fertility and mortality and median net migration of several thousand distinct trajectories of each demographic component derived using the probabilistic model of the variability in changes over time. Prediction intervals reflect the spread in the distribution of outcomes across the projected trajectories and thus provide an assessment of the uncertainty inherent in the medium scenario projection.

Fertility scenarios: Eight of those scenarios differ only with respect to the level of fertility, that is, they share the same assumptions made with respect to sex ratio at birth, mortality and international migration. The eight fertility scenarios are: low, medium, high, constant-fertility, instant-replacement-fertility, no fertility below age 18 years, accelerated decline of adolescent birth rate (ABR), accelerated decline of ABR with recovery. A comparison of the results from these eight scenarios allows an assessment of the effects that different fertility assumptions have on other demographic parameters. The high, low, constant-fertility and instant-replacement scenarios differ from the medium scenario only in the projected level of total fertility. In the high scenario, total fertility is projected to reach a fertility level that is 0.5 births above the total fertility in the medium scenario. In the low scenario, total fertility is projected to remain 0.5 births below the total fertility in the medium scenario. In the constant-fertility scenario, total fertility remains constant at the level estimated for 2024. In the instant-replacement scenario, fertility for each country is set to the level necessary to ensure a net reproduction rate of 1.0 starting in 2024 (after taking into account the survival up to reproductive ages). Fertility varies slightly over the projection period in such a way that the net reproduction rate always remains equal to one, thus ensuring the replacement of the population over the long run. For the 2024 revision, three new scenarios were introduced to consider the potential impact of changes in fertility rates among adolescent women and girls: (1) The no fertility below age 18 years scenario assumes that fertility rates at ages below 18 years will immediately fall to zero in 2024 and remain at zero throughout the remainder of the century. (2) The accelerated decline of adolescent birth rate (ABR) scenario considers that fertility rates at ages below 20 decline by 20 per cent annually, beginning in 2024, until the adolescent birth rate falls below 10 births per thousand women aged 15 to 19 years. (3) The accelerated decline of ABR with recovery scenario assumes the same accelerated fertility decline as in the second scenario, but also that half of the reduction in fertility among women and girls younger than 20 years is recovered once those cohorts have aged 10 years (i.e., half of the reduced fertility among women aged 17 is recovered 10 years later among women aged 27).

Mortality scenarios: A constant-mortality scenario, and a “no change” scenario (i.e., both fertility and mortality are kept constant) are available. The constant-mortality scenario uses the same fertility assumption (medium fertility), and international migration assumption as the medium scenario. Consequently, the results of the constant-mortality scenario can be compared with those of the medium scenario to assess the effect that changing mortality has on various population quantities.

Migration scenarios: similarly, the zero-migration scenario differs from the medium scenario only with respect to the underlying assumption regarding international migration. Therefore, the zero-migration scenario allows an assessment of the effect that non-zero net migration has on various population quantities. The instant-replacement zero-migration scenario combines the fertility assumption of the instant-replacement scenario and migration assumption from the zero-migration scenario. The “no change” scenario has the same assumption about international migration as the medium scenario but differs from the latter by having constant fertility and mortality. When compared to the medium scenario, therefore, its results shed light on the effects that changing fertility and mortality have on the results obtained.

The momentum scenario illustrates the impact of age structure on long-term population change(United Nations, 2017). The scenario combines elements of three existing scenarios: the instant-replacement-fertility scenario, the constant-mortality scenario, and the zero-migration scenario.

In comparison to one another, deterministic scenarios permit a decomposition of the projected future populations according to the contributions of the four demographic determinants of change: fertility, mortality, migration, and momentum. The difference between the population projected with the “zero-migration” scenario and that in the medium variant results in a change in the future population owing to net international migration. The difference between the “instant-replacement zero-migration” scenario and the “zero-migration” scenario indicates the change in population due to changes in fertility. The difference between the “momentum” scenario and the “instant-replacement zero-migration” scenario gives the change in population due to changes in mortality. Finally, the difference between the “momentum” scenario and the population at the beginning of the projection period is the change in population due to the momentum intrinsic to the population age structure. The Summary of Results, which accompanies the 2024 revision of the World Population Prospects, includes several examples and illustrations of this decomposition by demographic components of population change (United Nations 2024).

 

For further details, see also the report World Population Prospects 2024: Methodology of the United Nations Population Estimates and Projections.