World Population Day 2015
The UN predicts that there will be between 9.5 and 13.3 billion people in 2100. Marcos Brindicci/Reuters

A new study has predicted that the world's human population could grow by 40% to around 11.2 billion by the end of the century. John R. Wilmoth, the director of the United Nations (UN) Population Division, told a session focused on demographic forecasting at the 2015 Joint Statistical Meetings (JSM 2015) in Seattle, that the population could surge to 9.7 billion by 2050 before topping out at between 9.5 and 13.3 billion people in 2100. The population right now is 7.2 billion.

Wilmoth said that the primary reason for the surge will come from the rapid rise in the amount of humans in Africa, where it is predicted that the current number of 1.2 billion is expected to jump to between 3.4 billion and 5.6 billion people by the end of this century, which will be spurred on by the persistently high levels of fertility and the recent slowdown in the rate of fertility decline.

Africa's total fertility rate (TFR) has been in decline over the last decade, but it is still only a quarter of the decline rate experienced in Asia, Latin America and the Caribbean during the 70s. Asia, which is the most populous continent with 4.4 billion residing there, is set to peak at around 5.3 billion in the middle of the century, before falling to 4.9 billion by 2100.

The research from the UN also studied the levels of ageing in countries around the globe. Wilmoth said that one way to measure this is to look at the potential support ratio (PSR), which indicates how the population is ageing, and the burden placed on the working population to support the elderly. The calculation takes the number of people aged 20-64 and divides that by the number of people aged 65 and over.

For example, Japan currently has the lowest ratio at 2.1 meaning that for every 210 people aged 20-64, there are 100 people over 65, signalling an ageing population. On the other hand, Bangladesh's current PSR is 11.2 which shows that for every 1,120 people under 65, there are 100 over 65, showing a youthful population, although this is expected to drop to 1.6 by the turn of the century.

In the US, the PSR is expected to fall from 4.0 to 1.9 by the end of the century, with Germany falling from 2.9 to 1.4, China from 7.1 to 1.4, and Mexico from 8.7 to 1.4. Only five countries will have a PSR above 5.0 by 2100, according to the UN; Niger, Somalia, Nigeria, Gambia, and Angola - with Niger having the highest at 6.5.