The graph shows the estimated and projected trends of world population growth based on three fertility variants — low, medium, and high. The population projections are as follows:

2050 — constant variant: 10.5 billion; low variant: 8 billion; medium variant: 9.5 billion; high variant: 11 billion

2100 -- constant variant: 15.5 billion; low variant: 6 billion; medium variant: 10 billion; high variant: 25 billion

2150 — constant fertility variant: around 21 billion

The constant fertility variant continues to increase at the same rate over the time span shown

The high fertility variant continues to increase after 2020 and increases sharply after 2050

The medium variant increases at first but then levels off at about 10 billion by about 2070

The low variant increases up to 2040, plateaus for a few decades and then declines at a steady rate

Graph A shows the total projected global population growth, which will grow from 6.9 billion in 2010 to 10 billion in 2100 based on the medium fertility variant. Graph B shows how that growth of about 3.1 billion between 2010 and 2100 will be distributed by world regions. At almost 2.6 billion, population growth in Africa will account for most of this growth likely because it has the most in built population momentum.

Many cultural, economic, and demographic factors influence population growth by affecting fertility rates — i.e., the number of children a woman has in her lifetime. But these pronatalist factors play out differently in different countries, and often in ways that are not always knowable or predictable. The evidence suggests that fertility rates are dropping across all world regions but they are not falling at the same rate. Thus it is not entirely clear what will happen to fertility 90 years out. It is possible that global fertility rates will level off at replacement (the medium variant) but it is also possible that they might remain slightly higher (the high variant) or fall slightly lower (the low variant).

Currently Africa has the most youthful population in the world. This implies an in-built population momentum, in other words the population of Africa will continue to grow for some time (even if fertility rates are at replacement) as more people will be reproducing in the future than now. In addition, many pronatalist factors in Africa still favor higher fertility rates relative to other parts of the world. For example, the desired family size tends to be highest in African countries like Niger and Uganda, infant mortality rates are generally highest in African countries, and the unmet need for contraceptives is highest in Sub-Saharan Africa.