The world’s population could reach 27 billion by 2100 if fertility rates continue at present rates in all countries, according to the United Nations’ Department of Economic and Social Affairs (DESA) Population Division.
The expected number is closer to 10.1 billion, on the assumption that fertility will continue to decline in countries that still have fertility above replacement level (that is, countries where women have, on average, more than one daughter) and an increase of fertility in the countries that have below-replacement fertility. In addition, mortality would have to decline in all countries.
The report says:
“Today, 42 per cent of the world population lives in low-fertility countries, that is, countries where women are not having enough children to ensure that, on average, each woman is replaced by a daughter who survives to the age of procreation (i.e., their fertility is below replacement level). Another 40 per cent lives in intermediate-fertility countries where each woman is having, on average, between 1 and 1.5 daughters, and the remaining 18 per cent lives in high-fertility countries where the average woman has more than 1.5 daughter.”
The population of Africa is expected to triple, though Asia will remain the most populous region peaking at 5.2 billion in 2052.
Overall, life expectancy is projected to increase from 68 years in 2005-2010 to 81 years in 2095-2100 because of developments in HIV / AIDS treatment in high fertility countries and improvements in lifestyle across all countries.