Making affordable contraception available to women who want it would be a cheap, effective way to curb climate change and fast-rising risks in a hotter world, scientists said on Tuesday.
Worldwide each year, close to 100 million pregnancies are unintended, or 44% of the total, scientists at the New York-based Population Council wrote in an opinion piece in the BMJ Sexual & Reproductive Health medical journal.
Because each new person born needs food, energy for cooking and warmth, transport and other resources, population increases generally add to climate-changing emissions, so curbing unwanted births could help contain their rise.
But efforts to expand access to contraception run into obstacles, from objections by some faith leaders to worries about medical side-effects, said John Bongaarts, a vice president at the Population Council.
The international non-profit, with offices around the world, conducts research to develop contraceptives and to better understand issues related to contraceptive access and policy.
Bongaarts said objections to expanding access to contraception cross political lines, with some women's rights activists, for instance, fearful that encouraging smaller families could turn into stronger pressure to restrict births.
"The one-child policy in China is something nobody ever wants to repeat, so there's significant worry that some government will take an approach that is too coercive," the demographer told the Thomson Reuters Foundation.
But with the world's current population of 7.7 billion expected to hit 10.9 billion by 2100 - with the fastest hikes in sub-Saharan Africa and South Asia - finding ways to reduce unwanted births should be a priority, the scientists said.
That is already happening in countries such as Ethiopia, where thousands of young women have been trained as community health workers. They travel from village to village, offering basic healthcare, advice and free contraception, Bongaarts said.
Contraceptive use is now close to 40% in the country, he noted, and the lower birth rate has cut demand for construction of more schools, roads, clinics and other government facilities.
But other countries like Nigeria still have very low rates of contraceptive use, Bongaarts said. There, some Islamic ethnic groups in the north are opposed to birth control.
Policies that allocate income from the country's oil resources by numbers of people also give its provinces little incentive to hold down population, though some are trying, he added.
The United Nations estimates Nigeria's population is likely to rise to about 750 million by 2100, from 180 million today.
Across the region, sub-Saharan Africa's population could grow from 1 billion now to about 4 billion by the turn of the century unless birth rates fall, Bongaarts said.
That raises the prospect of greater hunger, migration and unrest as young people fail to find enough work and governments struggle to provide services, particularly as climate pressures such as droughts and harvest failures grow.
"I think there will be many, many problems in sub-Saharan Africa and they will spill over to the rest of the world" if populations soar, Bongaarts predicted.
Funding to improve access to contraception is widely available, he said, with major donors from the Bill and Melinda Gates Foundation and the Buffett Foundation to the British government's aid programme making it a focus.
Ann Starrs, who is director of family planning at the Gates Foundation and did not contribute to the BMJ commentary, noted that "helping women and adolescent girls everywhere access and use contraception is crucial to achieving a more prosperous and sustainable future for all of us".
She said contraceptive access improved women's health, gave them more power in their communities and boosted their economic situation.
What is largely missing in expanding that access, Bongaarts said, is backing from top political and community leaders with the standing to promote new ideas - something that has happened in nations from Bangladesh to Rwanda.
"Leaders have to say this is important. That's how social norms change," he said.