Think the world is crowded now? The population will probably hit 11 billion by 2100, according to new estimates. That’s 2 billion more people than original United Nations estimates, which showed population peaking in 2050 and then falling. It’s the first forecast based on modern science, as opposed to older ways of projecting population growth. Most of the growth will be in Africa, and it could lead to shortages of food, fuel and housing if economic and social development isn’t accelerated there. “Population, which had sort of fallen off the world's agenda, remains a very important issue,” says Adrian Raftery of the University of Washington, who worked on the new forecast published in the journal Science.

Populations across Africa will triple from about 1 billion people to 4 billion by the beginning of the next century, the University of Washington team says. Asia — home to population leaders India and China — will see growth fall off after peaking at 5 billion people in 2050. North America, South America, Europe and the Caribbean will all stay relatively uncrowded with populations below 1 billion. But many countries will have growing populations of elderly people, with ever-fewer young adults to support them. The world's population didn't reach 1 billion until 1804. It took 123 years to hit the 2 billion mark in 1927. This doubled to 4 billion in 1974 and it hit 7 billion a year ago.

IN-DEPTH

— Maggie Fox