MADABA, Jordan (AP) — The "new mantra" of the U.N. Population Fund is to keep girls in school until age 18 "in every nook and cranny of the world" as the best way to slow population growth, the head of the agency said.

Last month, the U.N. said it expects the world's population to reach 8.5 billion by 2030, 9.7 billion by 2050 and more than 11 billion by 2100.

Babatunde Osotimehin, the executive director of the population fund, said that key to slowing growth is ensuring that girls stay in school through their teens because women tend to have fewer children if their first one is born after they've turned 18.

Pushing for primary education for girls is no longer sufficient, he told The Associated Press on Friday, on the sidelines of a conference in Jordan. "We know that primary education is not the solution we are looking for," he said. "What we are looking for is secondary education."

"So our new campaign and our new mantra is to make sure that in every nook and cranny of the world, we get girls to go to school and stay in school," he said.

U.N. member states have endorsed "sustainable development goals" for 2030, including reproductive rights for women that are to be ratified next month at the U.N. General Assembly. Osotimehin said that "as we go into new sustainable development goals, we must up our game."

The goals are ambitious. Osotimehin noted in March that one in three girls is married before age 18 and that 225 million women have no access to modern contraception.

The agency chief acknowledged that there's still resistance in parts in the world to enshrining women's reproductive rights, but that "we have been able to get women to accept family planning just about everywhere in the world, even in the Arab world."

"I know for a fact that in many parts of the Arab world, if you talk about birth spacing, people will accept it," he said.

The world population is not growing as quickly as it used to, but growth rates are still high in the developing world because of its young population base, he said.

Those rates should not be singled out as the main threat to human survival, he said. Population growth is sluggish in the developed world, but higher production and consumption there "drive climate change faster than anything else that we know," he said.

Climate change, in turn, has caused natural disasters that led to large-scale migration, he said. "Now that's urgent, and that is something we have to deal with," he said.

Associated Press writer Edith M. Lederer at the United Nations contributed to this report.