In 1950 the total world population was 2.5 billion. It is now 6.5 billion. It is expected to reach 8.9 billion by 2050 but may then decline to 5.5 billion by the end of the century.
There are two reasons for the predicted drop.
The birth rate has fallen from a world average of 6 children per woman in 1960 to 3 children per woman today. It is expected to drop below replacement level by 2050.
The other reason is AIDS.
Population trends are not the same everywhere. In some countries population is declining rapidly; the population of Russia is now 145 million, it is expected to drop to 100 million by 2050. Some countries – most of Europe and China – are stable. But in several very poor African countries the population is expected to quadruple – Somalia will have a larger population than Russia.