The world's population will be around 9 billion by 2300, the UN forecast in a new report yesterday.
But the UN Population Division said a small shift in fertility levels could have an enormous impact on the population of planet Earth, which is now 6.3 billion.
The 9 billion estimate is based on the two-child family, but as little as one-quarter of a child less or one-quarter of a child more per family would result in world populations in 2300 ranging from 2.3 billion to 36.4 billion, it said.
Even more startling is the projection of world population in 300 years if current fertility levels remain unchanged -- it would jump to 134 trillion people, with virtually all the increase in the developing world.
"Clearly, high fertility rates cannot continue indefinitely as they yield extremely large populations in the developing world," said Joseph Chamie, director of the Population Division.
By contrast, developed countries would see substantial declines in population -- with Russia, Italy and Spain only about 1 percent of their current size if their fertility rates remained constant until 2300, he said.
But Chamie said developed countries are taking steps to promote childbearing in Europe.
"The Italians and others, for example, are offering 1,000 euros (US$1,220) for the birth of a child, and one Italian mayor has reportedly increased this to 10,000 euros (US$12,200) for his town," Chamie said.
Chamie said the projections are "groundbreaking" because they look so far into the future. Previously, the furthest the UN looked forward was 150 years. And instead of making projections by continent, the UN did it by country, he said.
Previous long-range projections put the world population at 10 billion to 12 billion in 2200, he said. The new projections are lower, largely due to the continuing decline from high fertility rates in the developing world.
The report, World Population in 2300, also foresees far more older people.
The median age of the world today is 26 years. By 2300, it will nearly double to 50 years, Chamie said. Similarly, the world population of people aged 60 and over will jump from 10 percent to 38 percent by 2300 -- and the percentage over 80 will rise from just 1 percent today to 17 percent over the next 300 years.
The expected population growth will also result in a redistribution of the world's population. In the medium scenario, Africa's share will double from 13 percent today to 24 percent in 2300 while Europe's share will be halved from 12 percent to 7 percent, he said.
Asia will go from 61 percent today to 55 percent in 2100 and remain at 55 percent in 2200 and 2300, Chamie said. Latin America and the Caribbean, which account for 9 percent of global population today, will decline to 8 percent in 2100, and remain at 8 percent in 2200 and 2300.