Today is Earth Day. For 45 years, the secular holiday has brought people—along with their ideas and enthusiasm—together to confront the world's environmental challenges. There will be speeches about sustainability, discussions about air quality, and pamphlets on how to reduce your carbon footprint. You might even learn how to help save some sub-Saharan elephants, but nobody will be addressing the elephant in the room. That's the fact that every single environmental solution is addressing the same, ugly problem: The world has to support a lot of hungry, thirsty, fertile people.

"No question, the human population is the core of every single environmental issue that we have," says Corey Bradshaw, an ecologist at the University of Adelaide in Australia. There are seven billion of us and counting. And though people are developing technologies, regulations, and policies to make humanity less of a strain on the Earth, a number of environmentalists believe that these fixes will never catch up to the population as long as it continues to grow. The only way to save the world is to stop making more (and more, and more, and more) humans.

This is not a new idea—but it has been driven underground for a time. Built on Malthusian foundations, and bolstered by books like The Population Bomb, reining in human reproduction was a major talking point at the first Earth Day, in 1970. The idea almost went mainstream in America, but extremists advocating for government regulation of fertility gave it a bad reputation. China's one-child policy, in 1980, didn't help.

But advocates for population stabilization—which they soften up by calling "family planning"—say mandatory family sizes are not the answer, never were. Instead, they say they'll get people to regulate their reproduction through other (more publicly palatable) goals: "We need to raise people out of poverty, give them better health care, and educate them," says Suzanne York, a spokesperson for the Institute of Population Studies in Berkeley, California.

Humanitarian sorcery? Nay. These goals work in service of something called demographic shift, a well established sociological principle. Basically, it's a way of looking at the survival instinct on a societal scale. See, in pre-industrial societies, people die often. OK, children die often. So people in pre-industrial societies tend to have a lot of children. As a society progresses—getting richer, healthier, and more educated—the child mortality rate drops. The birth rate stays high for a while, but eventually it too tapers off. (This lag period of fast population growth is what happened in Europe prior to the Industrial Revolution, it was the situation in China before One Child, and it's happening right now in parts of Africa.) Family planning simply makes sure that this gap between death and birth rates doesn't lead to a huge population boom.

Not everyone is behind family planning, though. To some, the term is still a euphemism for totalitarian population control, and it invokes the specter of intrusive, abusive government policies (such as China's, which has produced horror stories like forced abortions). And some people working on the population problem even think we need to be stabilizing the population by having more babies.

It's OK, you can take a moment to read that again. "I see people as the ultimate resource," says Steven Mosher, of the Population Research Institute in Virginia. According to Mosher, more people means more minds to contribute to solutions, and more competition leading to more innovation—innovation that can tackle the problems created by too many bodies. Eventually, society will reach demographic transition on its own, no birth control necessary. Mosher says this also frees people from having the terms of their societal transition dictated to them.

Other experts are skeptical that the population can balance itself out. "That idea is so wrong in so many ways that I don’t know where to begin," says Bradshaw. He says unregulated reproduction ignores an ecological principle called density feedback. "When you increase population in a finite space, you increase per capita aggression, and increase competition for resources. You see more conflict, more suffering, more pain, more death," he says.

Bradshaw agrees that it's important that societies that undergo demographic transition aren't denied the comforts of post-industrialization. But he says that limitless population growth will make conditions much much worse, before they ever (if ever) get better. This is because any technology allowing the planet to support more people has always lagged behind the rate of population growth. "We can’t even feed the people on the planet now," says Bradshaw, noting that there are nearly one billion hungry people on the planet.

The key, says Bradshaw, is to augment family planning with tech fixes that reduce the environmental impact of each individual. In the end, any number of population control techniques—from farcical mass abortion plots to widespread war and famine to reasonable strategies targeting unplanned pregnancies—won't stall global population growth, as Bradshaw noted in a paper he co-authored last year (with the give-away title "Human population reduction is not a quick fix for environmental problems").

In that paper, Bradshaw evaluated several methods of population control, and some of the best results came from targeting unplanned births—which number about 33 million a year. "Through education, if you could stop the unintended pregnancies that result in unplanned births, you’d have the same net effect as having a global one-child policy in place by 2100," says Bradshaw.

But even under these best of circumstances, with the best of outcomes, Bradshaw's projected global population in 2100 would be about the same as today: seven billion hungry, thirsty people. Maybe we should get to work on improving air quality after all.