The United Nations said the world population hit 8 billion people on Tuesday, although overall growth rates in the total population are slowing.

The 8 billionth person was born somewhere in the world on Tuesday, according to UN projections.

The population milestone was reached owing to a gradual increase in the average human lifespan, accomplished through improvements in medicine and public health.

It took just 12 years for the world population to grow from 7 billion to 8 billion people, but the UN projects it will take 15 years to reach 9 billion people, a milestone expected to be reached by 2037.

UN Secretary-General Antonio Guterres said in a statement the world should celebrate the milestone but also exercise caution in the face of great challenges like climate change.

“The milestone is an occasion to celebrate diversity and advancements while considering humanity’s shared responsibility for the planet,” he said.

It took all of human history to reach 1 billion people in the early 19th century, about 123 years to reach 2 billion and 33 years to reach 3 billion people in 1960.

But the global population is expected to decline toward the end of the century.

The UN estimated in a report this year the global population will peak at 10.4 billion around 2080, mostly because of declining birth rates.

In the next decade or so, Asia and Africa are expected to drive much of the population growth in the world.