In addition to the projection for the medium scenario based on probabilistic
methods which provide 80 and 95 per cent prediction intervals, the 2022
Revision includes ten different 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 and mortality, 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 median 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: Five
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 five fertility scenarios are: low, medium, high,
constant-fertility and instant-replacement-fertility. A comparison of the
results from these five 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 2022. 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 2022. 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.
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.
Table. Projection scenarios in
terms of assumptions for fertility, mortality
and international migration
|
Projection scenarios
|
Assumptions
|
Fertility
|
Mortality
|
International
migration
|
Medium
(fertility)
|
Medium (based
on median
probabilistic fertility)
|
Medium (based
on median
probabilistic fertility)
|
Medium
|
Low fertility
|
Low
|
Medium
|
Medium
|
High fertility
|
High
|
Medium
|
Medium
|
Constant-fertility
|
Constant as of
2022
|
Medium
|
Medium
|
Instant-replacement-fertility
|
Instant-replacement
as of 2022
|
Medium
|
Medium
|
Constant-mortality
|
Medium
|
Constant as of
2022
|
Medium
|
No change
|
Constant as of
2022
|
Constant as of 2022
|
Medium
|
Zero-migration
|
Medium
|
Medium
|
Zero from
2022
|
Instant-replacement zero-migration
|
Instant-replacement
as of 2022
|
Medium
|
Zero from 2022
|
Momentum
|
Instant-replacement
as of 2022
|
Constant as of
2022
|
Zero
from 2022
|
|
|
|
|
For further details, see also the report
World Population Prospects 2022: Methodology of the United Nations Population Estimates and Projections.