Why sub-Saharan Africa might exceed its projected population size by 2100

The UNPD's 1998 revision projected sub-Saharan Africa's population in 2050 at 1·52 billion; this projection was revised to 1·75 billion in 2008 and 2·12 billion in 2019.3,4 One key factor might be low age at first birth in sub-Saharan Africa, which reduces the intergenerational gap.5,6 At the same levels of fertility, mortality, net migration, contraceptive use, age structure, and size, a population in which all women start childbearing at age 20 years will be at least 20% larger in 100 years than another in which all women start childbearing at age 25 years, all other parameters remaining exactly the same.7 Sub-Saharan Africa is the only region where the SDG scenario forecast was substantially lower than the lower bounds of the UI of the reference scenario in the new IHME projections. Murray and colleagues did not ask how existing rates of population growth in countries such as Nigeria could affect their ability to sustain investments in human capital development. The authors also ignore increases in life expectancy, which are known to increase population growth and are likely to have an effect on the projected size of sub-Saharan Africa's population by 2100.8 The Article reporting these new models is a complex paper that raises important issues on the implications of declining population sizes in many parts of the world and will probably reframe discourses on population issues to centre on the challenges associated with population decline.