Understanding the future course of the world’s human population is an important aspect of any predictions concerning consumption of natural resources, carbon emissions, biodiversity, and the like. The best projection from the United Nations 2004 report is the middle curve here, the orange one:
Here’s what the report says (emphasis ours):
Long-range population projections are reported to 2300, covering twice as long a period as ever covered in previous United Nations projections. These projections are not done by major area and for selected large countries (China and India), as was the previous practice, but for all countries of the world, providing greater detail. In these projections, world population peaks at 9.22 billion in 2075. Population therefore grows slightly beyond the level of 8.92 billion projected for 2050 in the 2002 Revision, on which these projections are based. However, after reaching its maximum, world population declines slightly and then resumes increasing, slowly, to reach a level of 8.97 billion by 2300, not much different from the projected 2050 figure.
Europe and Africa will be particularly out of phase. Europe will hit its low point in growth in 2050, Africa not till 80 years later, after all other major areas. From 2000 to 2100, Europe’s share of world population is cut in half, 12.0 to 5.9 per cent, while Africa's almost doubles, from 13.1 to 24.9 per cent. While shares of world population for major areas will rise and fall over the following two centuries, the distribution by 2300 will resemble that in 2100. Smaller regions within continents exhibit divergent patterns. For instance:
Three African regions — Eastern Africa, Middle Africa, and Western Africa — will grow unusually fast in comparison to every other region through 2100, even though total fertility will be close to replacement by 2050.
Southern Africa is seeing a decline in life expectancy to a lower level than anywhere else, but life expectancy will rebound, rise quite rapidly, and overtake other African regions.
Asian regions will grow fastest to the west, slowest to the east, but in every case with growth rates, at least up to 2100, below Eastern, Middle and Western Africa. By 2100, Asia, instead of being four-and-a half times as populous as Africa, will be only 2.2 times as populous.
Latin America and the Caribbean is the most homogeneous major area, with most of its regions following relatively parallel fertility and life expectancy paths.
Europe, like Asia, will experience higher growth to the west, lower growth to the east. Eastern Europe stands out with low life expectancy, and even in the long run does not catch up with other regions.
According to the 2010 Revision of the World Population Prospects of the United Nations Department of Economic and Social Affairs (UNDEP press release ) the world population is to reach 10 billion by 2100 if fertility in all countries converges to replacement level:
From the website Analytical Figures (Fig 3.) at WPPUndep2010:
In particular Africas population is currently growing rapidly. From the website Analytical Figures (Fig 2.) at WPPUndep2010:
Hence on average there is some correlation between economic performance and population growth. A quick glance on the respective maps displays however that cultural and political and social dispositions may also play a big role. Alone the examples of China versus the US or Saudi Arabia versus Russia displays this very visible:
Image: Population growth rate, as listed on CIA factbook (2006 estimate)
Image: GDP (PPP) Per Capita based on 2008, from the IMF