The latest global population report from the United Nations estimates the number of people on the planet will peak at 10.9 billion by the end of the century. The vast majority of population growth is expected to come from sub-Saharan Africa, with declining population growth predicted in Asia, Europe and Latin America by 2100.

Underpinning the UN estimates is the observation of declining global fertility rates. At the beginning of the 20th century average global fertility rates sat around six births per woman. By 1990 this had fallen to 3.2, and in the latest report is currently averaged at 2.5 births per woman. It is predicted average fertility will continue to drop over the course of the century, to 2.2 in 2050, and finally 1.9 births per woman by 2100, ultimately suggesting a global population decline by the next century.

Of course, these birth rates are not even across the world, and more than 90 countries are already registering birth rates at below replacement levels (less than two births per woman). Sub-Saharan Africa, on the other hand, is still averaging 4.6 births per woman, leading the UN report to estimate overall population in the area to double by 2050.

While this general estimate that the global population should peak at under 11 billion is better news than some earlier higher estimates that it would reach as high as 15 billion by the end of the century, the researchers behind the study suggest the vast majority of population growth coming over the course of this century will occur in the world's poorest countries.

Nigeria, for example, is set to grow from a current population of around 200 million, to the world's third most populous nation by 2100 with over 700 million people. Thomas Spoorenberg, one of the researchers from the UN Population Division generating this report, says these projections are an urgent reminder than global policy and planning must account for these major looming demographic changes.

"Many of the fastest growing populations are in the world's poorest countries," says Spoorenberg. "In these countries, population growth is a real challenge for efforts to eradicate poverty and inequality, combat hunger and malnutrition, strengthen the coverage and quality of education and health systems, and improve access to basic services."

And conversely, countries with declining and aging populations must account for their own fundamental shifts as well. It is generally thought that the ratio of working age people (25 to 64) to those over 65 should be at least 3 to 1. This is referred to as the potential support ratio, and the UN report notes this ratio is falling as aging populations increase.

Japan currently has the lowest potential support ratio in the world, at 1.8, and 29 countries are already below 3. The UN report predicts by 2050 there will be 48 countries with potential support ratios below 2, meaning there may be significant pressure on those countries to balance a declining labor market against increasing costly economic protections for older citizens.

"Persons aged 65 or over already make up the world's fastest growing age group and virtually all countries can expect the percentage of older persons in their populations to increase," explains Spoorenberg. "Countries need to plan now for population aging to ensure the well-being of older persons, the protection of their human rights, their economic security, access to appropriate health services and lifelong learning opportunities, and formal and informal support networks."