But if fertility levels in the developing world remain at today's levels, the global population would reach 244 billion in 2150 and 134 trillion in 2300, according to the report, "World Population in 2300”. At present, there are 6.3 billion people.
The report released on Tuesday forecast that the Japanese would live to the age of 108. Africa’s population would explode while the Europeans could turn into a dwindling species, it said.
"It's like the Titanic with an iceberg ahead," said Joseph Chamie, director of the population division. "You sink because the rates are so low or you simply grow too rapidly because the rates are too high. Either way you have to change course."
Even small changes could make a huge difference, he said. The nine billion estimate is based on a two-child family but one-quarter of a child more per family could boost the population in 2300 to 36.4 billion.
The reason for the 300-year forecast was because policymakers struggling with climate change, agriculture production and immigration needed long-term projections to take corrective action.
"You sink because the rates are so low or you simply grow too rapidly because the rates are too high"
director, population division, UN
People in rich countries would live much longer. Americans, Swedes and Japanese can expect life expectancies of more than 100 years on average. And in China people are expected to live until 85, Chamie said.
The good news, according to Chamie, was a trend toward smaller families seen in a variety of nations. He noted that two children were the norm in such countries as Iran, Brazil, Indonesia, Mexico and Thailand.
But in Europe, Japan, Australia or Canada, the families are too small. The report warns that at current levels of 1.4 children per family and no increase in immigration, there would only be 232 Europeans in a 100 years for every 1,000 today.
Russia, Italy and Spain would only have about 1% of their current size if they did not increase the number of children by 2300. And the population in all of Germany would be down to the current size of Berlin, Chamie said.
In Africa, the population will double to 2.3 billion people, from 13% of the world's people today to 24% in 2300, assuming effective widespread treatment for AIDS.
Latin America and the Caribbean would remain the same or decline slightly. And Asia is expected to decrease, from 61% of the world's population to 55% by 2300.