The world's population is projected to surge past 9 billion before 2050 and then reach 10.1 billion by the end of the century if current fertility rates continue at expected levels, according to United Nations figures unveiled Tuesday.

Most of the increase will come from so-called "high fertility countries," mainly in sub-Saharan Africa but also in some nations in Asia, Oceania and Latin America, the figures reveal.

The 2010 Revision of World Population Prospects, prepared by the Population Division at the UN's Department of Economic and Social Affairs (DESA), shows that a small variation in fertility could lead to major long-term differences in the size of the global population.

Based on the medium projection, the number of people in the world – currently close to 7 billion – should pass 8 billion in 2023, 9 billion by 2041 and then 10 billion at some point after 2081.

But a small increase in fertility could mean a global population of as much as 15.8 billion by 2100, while a small decrease could result in an eventual overall decline in population to 6.2 billion by the end of the century.

Hania Zlotnik, Director of the Population Division, told journalists Tuesday that the populations of many countries are ageing and will continue to do so as their fertility rates decline.

The population of countries classed as low-fertility or intermediate-fertility – including China, Russia and many countries in Europe – would thus peak well before the end of the century.

Life expectancy is expected to rise across all categories of countries, particularly as better treatment for HIV/AIDS cuts early deaths in many sub-Saharan African countries. Global life expectancy is projected to increase from 68 years to 81 by the years 2095 to 2100.

Zlotnik noted also that the world population will surpass 7 billion at some point later this year.