The equation seems fairly simple: The more the world's population rises, the greater the strain on dwindling resources and the greater the impact on the environment.
The solution? Well, that's a little trickier to talk about.
Public-health discussions will regularly include mentions of voluntary family planning as a way to reduce unwanted pregnancies and births. But, said Jason Bremner of the Population Reference Bureau, those policies can also pay dividends for the environment.
"And yet the climate-change benefits of family planning have been largely absent from any climate-change or family-planning policy discussions," he said.
Bremner was speaking Tuesday at an event hosted by the Woodrow Wilson Center, which has formed a working group of scientists and officials working on climate change and family planning to try to cross the gap between the two. Even as the population passes 7.2 billion and is projected by the United Nations to reach 10.9 billion by the end of the century, policymakers have been unable—or unwilling—to discuss population in tandem with climate change.
But there's been little or no funding for such programs, and the discussions tend to stall before getting into meaty policy. And despite the United Nations holding a special session on population and development a day before its September climate-change summit, academics lamented a lack of cross talk.
Why? Talking about population control requires walking a tightrope: There's nuance between encouraging access to birth control and a China-style one-child policy, but that doesn't always translate in the retelling, and it can all too easily sound like a developed world leader telling people in the developing world that they should stop having children—especially because much of the population boom is coming from regions like sub-Saharan Africa.
And there's a coalition of critics ready to pounce on any speaker who slips, or even to twist the words of those who don't.
Just look at what happened to Hillary Clinton in 2009, when as Secretary of State she acknowledged the overpopulation issue during a discussion with Indian environment minister Jairam Ramesh. Clinton praised another panelist for noting "that it's rather odd to talk about climate change and what we must do to stop and prevent the ill effects without talking about population and family planning."
"And yet, we talk about these things in very separate and often unconnected ways," Clinton added.
Right-wing critics pounced, with the Alex Jones-run Info Wars calling her comments "Malthusian."
Oddly enough, Clinton has found herself criticized for falling on the other side of issue as well. In 2013, a line mentioning overpopulation was edited out of an essay for a Clinton-backed publication written by Bindi Irwin, the then-14-year-old daughter of the late "Crocodile Hunter" Steve Irwin (The Daily Mail chronicled the bizarre spat here).
But the Woodrow Wilson Institute researchers are quick to point out that they're not talking about touchy "population control" strategies like forced sterilization (prominently employed and quickly abandoned in India in the 1970s) or China's one-child policy. Rather, they're focusing on "voluntary family planning"—emphasis on the "voluntary"—programs that countries are already looking at. Sex education has been linked to reduced fertility rates, as has distribution of contraception.
"Achieving universal access to family planning throughout the world would result in fewer unintended pregnancies, improve the health and well-being of women and their families and slow population growth, all benefits to climate-compatible development," reads a draft statement from the group, which is crafting a full report to be released at the end of the year.
The connection isn't necessarily new: Plenty of academics have made it, and Natural Resources Defense Council President Frances Beinecke has blogged about it in the past. The U.N. Intergovernmental Panel on Climate Change has noted the burden of "population pressures" on natural resources, while separately looking at the health impact of family planning. Some national climate-adaptation plans around the world have brushed on family planning.
A study from the Futures Group said it would cost $3.7 billion a year to provide such services to countries that don't yet have them. The public-health benefits of reducing unplanned pregnancies are clear for both mother and child, especially in high-risk impoverished areas.
A 2010 study published in the Proceedings of the National Academy of the Sciences looked at the link between policies that help women plan pregnancies and family size and global emissions (the study also looked at aging and urbanization trends). The researchers predicted that lower population growth could provide benefits equivalent to between 16 and 29 percent of the emissions reduction needed to avoid a 2 degrees Celsius warming by 2050, the warning line set by international scientists.
But the benefits also come through easing the reduced resources that could result from climate change. The U.N. IPCC report notes the potential for climate-related food shortages, with fish catches falling anywhere from 40 to 60 percent and wheat and maize taking a hit, as well as extreme droughts. With resources already stretched in some areas, the IPCC laid out the potential for famine, water shortages and pestilence.
Still, the link remains a "very sensitive topic," said Karen Hardee, director of the Evidence Project at the nonprofit Population Council.
"At the global policy level you can't touch population … but what's been heartening is that over the last few years it's not just us, but people from the countries themselves talking about this," Hardee said.
Hardee conducted a review of 41 National Adaptation Programs of Action (the vulnerability documents required of low-income nations by the U.N.) and found that 37 mentioned population as a climate-related problem. Of those, six recognized family planning as a possible mitigation technique.
But none had funding for family-planning programs. And inclusion in a country-level plan doesn't translate to discussion at the international level, which is where the researchers say policy needs to be taking hold.
The very idea of intervention has even gotten some recent pushback. A study published this month from two researchers at Australia's University of Adelaide also cast doubt on whether population control could even stem growth, projecting that even drastic measures like a one-child policy would result in a population between 5 and 10 billion by century's end.
But the members of the Woodrow Wilson Center working group—which includes members from the WorldWatch Institute and USAID, as well as climate scientists—say their work can make waves, especially as world leaders prepare to meet for the U.N. climate summit in Paris next year. With countries already engaged on family planning, they say it's just a small leap to tie environmental concerns together.
"We want to achieve agreement on what the climate commitments are from individual countries," said Alexander Ochs of the Worldwatch Institute. "There's a new opportunity here, a new approach that takes a bottom-up look at what countries want to bring to the table. … We're just focused now on getting over the stumbling blocks."