“The growth in urban areas is going to be astonishing,” Bongaarts said. “Most of these people are going to end up in slums. That’s not good news.”

African countries are expected to experience a huge migration from rural to urban areas that don’t have the necessary infrastructure to support them.

“This is not good news,” said John Bongaarts, a vice president of the Population Council , an international nongovernmental research organization based in New York. “In the poorest countries, fertility rate declines have slowed down and in some cases are even stalling.”

“The concentration of population growth in the poorest countries presents its own set of challenges, making it more difficult to eradicate poverty and inequality, to combat hunger and malnutrition and to expand educational enrollment and health systems,” said John Wilmoth, the director of the population division in the U.N.’s Department of Economic and Social Affairs.

Nigeria, currently the seventh-largest country by population, is projected to surpass the United States and become the world’s third largest by 2050. In that time, 28 African countries could more than double in size, and 10 — including Angola, Somalia and Uganda — will more than quintuple.

Africa is expected to account for more than half the world’s population growth through 2050, to 9.7 billion from 7.3 billion today. India will likely surpass China and become the most populous country by 2022.

Fueled by population growth in some of the poorest African countries, the number of people on earth is expected to grow faster than previously expected, reaching 8.5 billion in 15 years and 11.2 billion by the end of this century, according to United Nations projections released Wednesday .

But there is some good news. The AIDS epidemic is under control, and infant mortality has declined. But fertility rates haven’t, which is an issue of concern to experts. More births, coupled with increasing life expectancy, are contributing to astounding population gains.

Some African countries, such as Ethiopia and Rwanda, have successfully expanded access to contraceptives and family planning education. “I do think we’re starting to see political commitment to these issues,” said Jason Bremner, an associate vice president at the Population Reference Bureau in Washington, D.C.

An international family planning summit in London in 2012 helped galvanize the governments of developing nations to focus on fertility issues, he said.

The U.N. projections show significant increases in life expectancy among the poorest countries, from 56 years for those born in the early 2000s to 62 years for those born this decade.

Worldwide the pace of population growth has slowed because of lower fertility rates. But living longer has created an age imbalance in countries dealing with a swelling elderly population and not enough young people to support them.

In Europe, more than a third of the population is expected to be more than 60 years old by 2050. In Latin America, the Caribbean and in Asia, the population will go from about 11 percent over 60 to more than 25 percent by 2050.

“Eventually, there will be very small numbers of labor force participants to support the booming numbers of elderly,” Bongaarts said. “It’s a very serious problem.”

In the U.S. and other aging countries such as Japan and Italy, the strain on pension funds is already being felt. “They’re going to run out of money,” he said. “We have to increase taxes, or we have to reduce benefits. Something has to give.”

Bremner expects fertility rates to eventually inch up in Europe.

U.N. projections are based largely on two factors: births and deaths. Earlier models relied on expert opinion from around the world to project trends. The new forecast combines all available data — from government statistics to expert forecasts on mortality rates, fertility rates and international migration — to come up with a probability model.

Populations in many parts of the world are still young. In Africa, children under 15 account for 41 percent of the population today, and those 15 to 24 make up an additional 19 percent. There are 2.8 billion children and young adults in Latin America, the Caribbean and Asia.

These future workers and parents need health care, education and employment opportunities to “help build a brighter future for their countries,” the report said.