The United Nations has published new predictions on the size and age of the world's population 300 years from now.
Countries like India will continue to be the most heavily populated
Its medium-case scenario forecasts a rise from the current 6.3 billion people to around 9 billion in 2300.
One startling projection based on present fertility levels is for 134 trillion inhabitants - although the UN concedes this is an impossible outcome.
The UN says its forecasts help agencies and governments assess the policy implications from population change.
The 134 trillion figure is used merely as a demonstration that present fertility levels are unsustainable.
The latest predictions are published by the population division of the UN's Department of Economic and Social Affairs.
The report suggests that if fertility levels stabilise at around two children for every woman the population increase will be more manageable, reaching just over nine billion people in three centuries' time.
The continuing rise in this case can be explained by improving longevity.
The number of people over 60 would rise from 10% to 38%, and those over 80 from 1% to 17%.
But even small variations in fertility levels either side of this could radically alter these figures, the report said.
An average fertility of 1.85 children per woman would result in a population of just 2.3 billion, whereas an average of 2.35 would yield 36.4 billion.
The report says that whatever the overall increase, the world's population is likely to be significantly older in 300 years.
Almost a quarter of this population will live in Africa while India, China and the United States will continue to be the most heavily populated countries of the world.
Low fertility: 2.3 billion
Medium fertility: 9 billion
High fertility: 36.4 billion
At current fertility levels - 134 trillion