The upward trend threatens to leave even more people in poorer countries further behind, as governments struggle to provide enough classrooms and jobs for a rapidly growing number of youth and food insecurity becomes an even more urgent problem.
Over the next three decades, the population of Nigeria is expected to soar from 216 million this year to 375 million, the U.N. says. That will put the West African nation in a tie for third place with the United States after India and China.
“We are already overstretching what we have — the housing, roads, the hospitals, schools. Everything is overstretched," said Gyang Dalyop, an urban planning and development consultant in Nigeria.
Nigeria is among eight countries the U.N says will account for more than half the world's population growth between now and 2050 — along with fellow African nations the Democratic Republic of Congo, Ethiopia and Tanzania.
“The population in many countries in sub-Saharan Africa is projected to double between 2022 and 2050, putting additional pressure on already strained resources and challenging policies aimed to reduce poverty and inequalities,” the U.N. report said.
It projected the world's population will reach around 8.5 billion in 2030, 9.7 billion in 2050 and 10.4 billion in 2100.
The U.N.'s Day of 8 Billion milestone Tuesday is more symbolic than precise, officials are careful to note in a wide-ranging report released over the summer that makes some staggering projections.
In the Democratic Republic of Congo's capital, Kinshasa, where more than 12 million people live, many families struggle to find affordable housing and pay school fees. While elementary pupils attend for free, older children's chances depend on their parents' incomes.
“My children took turns" going to school, said Luc Kyungu, a Kinshasa truck driver who has six children. "Two studied while others waited because of money. If I didn't have so many children, they would have finished their studies on time.”
According to the U.N., the population in sub-Saharan Africa is growing at 2.5% per year — more than three times the global average. Some of that can be attributed to people living longer, but family size remains the driving factor. Women in sub-Saharan Africa on average have 4.6 births, twice the current global average of 2.3.
Families become larger when women start having children early, and 4 out of 10 girls in Africa marry before they turn 18, according to U.N. figures.
There are also often cultural and political reasons for large families in sub-Saharan Africa.
In Tanzania, former President John Magufuli, who ruled the East African country from 2015 until his death in 2021, discouraged birth control and family planning programs, saying that a large population was good for the economy. Additionally, children are generally seen as a blessing and a source of support for their elders.
Rapid population growth means more people vying for scarce water resources and leaves more families facing hunger as climate change increasingly impacts crop production in many parts of the world.
Experts say a big threat to the environment is consumption, which is highest in developed countries not undergoing big population increases.
"Over the past 25 years, the richest 10% of the global population has been responsible for more than half of all carbon emissions,” said Poonam Muttreja, executive director of the Population Foundation of India.
Charles Kenny, a senior fellow at the Center for Global Development in Washington, says environmental concerns surrounding the 8 billion mark should focus on consumption, particularly in developed countries.
"Population is not the problem, the way we consume is the problem — let’s change our consumption patterns,” he said.