At the country level, much of the overall increase between now and 2050 is projected to occur either in high-fertility countries, mainly in Africa, or in countries with large populations. During 2015-2050, half of the world’s population growth is expected to be concentrated in nine countries: India, Nigeria, Pakistan, Democratic Republic of the Congo, Ethiopia, United Republic of Tanzania, United States of America, Indonesia and Uganda, listed according to the size of their contribution to the total growth.
According to the medium variant of the 2015 Revision, global fertility is projected to fall from 2.5 children per woman in 2010-2015 to 2.4 in 2025-2030 and 2.0 in 2095-2100. Steep reductions are projected for the least developed countries, from 4.3 in 2010-2015 to 3.5 in 2025-2030 and 2.1 in 2095-2100. However, for countries with high fertility there is significant uncertainty in the projection of fertility, even in the 15-year horizon of the post-2015 development agenda, and more so in the long-term projection to 2100. Slower-than-projected fertility declines would result in much higher population totals in all subsequent time periods. For example, a scenario in which all countries had a fertility rate that was consistently half a child higher than in the medium variant would produce a population of 16.6 billion in 2100, more than 5 billion higher than the medium-variant projection.
Globally, life expectancy at birth is projected to rise from 70 years in 2010-2015 to 77 years in 2045-2050 and to 83 years in 2095-2100. Africa is projected to gain about 19 years of life expectancy by the end of the century, reaching 70 years in 2045-2050 and 78 years in 2095-2100. Such increases are contingent on further reductions in the spread of HIV, and combating successfully other infectious as well as non-communicable diseases. Both Asia and Latin America and the Caribbean are projected to gain 13-14 years of life expectancy by 2095-2100, while Europe, Northern America and Oceania are projected to gain 10-11 years.
Why World population will likely be higher than 9.75 billion in 2050
1. The UN has been revising its population projections higher for the last 15-20 years. They have kept expecting the birthrate in Africa to decline faster than it actually has
2. The population census in African and many other poorer countries tend to be out of date. Which means the most recent census is old and inaccurate. This tends to mean an undercount. China likely has an undercount of 10-30 million because of the second and third child being hidden because of the one child policy. World population is likely already 7.5 billion and not 7.3 billion.
3. China will move completely away from population control by 2020 and shift to encouraging more children. This will likely mean that China's population will not fall much after 2030.
4. There will be more improvements in medicine that will further increase life expectency.
Technology Review also has a world population graphic.
SOURCES - United Nations, Technology Review