The United Nations projects the world's population will reach eight billion on Tuesday.

In July, the U.N. released its "Day of Eight Billion" report, predicting the "milestone in human development."

"While it took the global population 12 years to grow from 7 to 8 billion," the report reads, "it will take approximately 15 years—until 2037— for it to reach 9 billion, a sign that the overall growth rate of the global population is slowing." The report outlines that roughly 70% of the world's births occurred in low-income and lower-middle-income countries.

On Friday, the U.N. secretary-general, António Guterres, wrote in a USA Today op-ed that while the world's population continues to rise, it grows "more divided."

"Billions of people are struggling; hundreds of millions are facing hunger and even famine. Record numbers are on the move seeking opportunities and relief from debt and hardship, wars and climate disasters."

But he adds, "I never bet against human ingenuity, and I have enormous faith in human solidarity."

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