That's an optimistic scenario, one that assumes the worldwide average birthrate, now 2.5 children per woman, will decline to 2.1.
If birthrates stay where they are, the population is expected to reach 11 billion by midcentury — akin to adding three Chinas.
Under either forecast, scientists say, living conditions are likely to be bleak for much of humanity. Water, food and arable land will be more scarce, cities more crowded and hunger more widespread.
On a planet with 11 billion people, however, all those problems will be worse.
The outcome hinges on the cumulative decisions of hundreds of millions of young people around the globe.
The relentless growth in population might seem paradoxical given that the world's average birthrate has been slowly falling for decades. Humanity's numbers continue to climb because of what scientists call population momentum.
So many people are now in their prime reproductive years — the result of unchecked fertility in decades past, coupled with reduced child mortality — that even modest rates of childbearing yield huge increases.
"We're still adding more than 70 million people to the planet every year — which we have been doing since the 1970s," said John Bongaarts, a leading demographer and vice president of the nonprofit Population Council in New York. "We're still in the steep part of the curve."
Think of population growth as a speeding train. When the engineer applies the brakes, the train doesn't stop immediately. Momentum propels it forward a considerable distance before it finally comes to a halt.
U.N. demographers once believed the train would stop around 2075. Now they say world population will continue growing into the next century.