By Holly Moeller on October 6, 2014

Last week, a high-profile study using the latest United Nations data revisited predictions of global population size. The news wasn’t good: Updated estimates using new statistical analyses suggest the world’s population will hit nearly 11 billion by 2100. There’s some uncertainty in this measure because birth and death rates may be changed by political and social dynamics. Still, the study’s authors wrote that there’s a four in five chance the world’s population will be between 9.6 and 12.3 billion by the end of the century.

I was glad to see several media outlets pick up the story. But while most of the reports alluded to the challenges of feeding and employing additional billions of humans, almost none acknowledged the fundamental issue with human population size.

There are already too many people on the planet, and this overpopulation drives the ongoing environmental crisis.

It’s no wonder we shy away from open discussion of this issue. First, “overpopulation” is hard to quantify. It’s obvious that the present-day human population is too large to sustainably support on the planet. For example, modern agriculture relies on the chemical fixation of nitrogen for fertilizers, which experts believe allowed Earth’s population to grow beyond 4 billion. Yet this fertilizer production requires energy from fossil fuels, a non-renewable resource. In other words, more than 3 billion people on the planet survive because of an unsustainable energy subsidy.

But how many is too many? Experts struggle to quantify the maximum sustainable human population size (called the “carrying capacity”) of the planet. Their calculations are limited both by uncertainty about Earth’s capacity to maintain its human life support systems (e.g., freshwater, breathable air and so on) and by the lifestyle each person chooses to maintain. For example, Americans have relatively high-impact habits that consume more natural resources than sub-Saharan Africans living below the poverty line. Ethically, we can agree that every human deserves a certain standard of living. But the higher that that standard rises in terms of energy and other resource consumption, the fewer people Earth can support indefinitely.

Meanwhile, our global population continues to swell, as do our environmental impacts. Each individual requires sustaining resources including food, housing and energy. No matter how small the average person’s resource demands are, each additional person adds to the human burden on the Earth. In the past 40 years, the human population has increased by 40 percent, while, as a consequence, the world’s wildlife population has been cut in half.

Indeed, the Intergovernmental Panel on Climate Change notes that population growth and economic development are the two most important drivers of increasing carbon dioxide emissions. In the United States, every child born increases a mother’s carbon legacy more than sixfold. Worse, present day population growth is occurring in impoverished areas where large families, though themselves contributing relatively little per person to global emissions and climate change, are more likely to be vulnerable to its impacts as well as the usual suite of natural disasters.

Why, then, do we fixate on technological fixes like solar power and carbon capture, rather than addressing global population growth?

According to former United Nations Population Division officials, the reasons stem primarily from political fear. Talking about population control necessarily brings up contentious moral issues like birth control and reproductive rights. Policymakers and government officials shy away from these thorny topics: Even the UN has spent the last 20 years focusing on women’s reproductive health and rights while ducking larger questions about population size.

Still, sociology tells us this is not a bad strategy. Educating women, and providing them with access to reliable birth control methods, is perhaps the most reliable way to reduce population growth. Educated women add value to families as breadwinners: the cost of having children now includes not only the infant’s direct expenses, but also the mother’s lost wages. As a result, these women are choosing to have fewer children.

The UN, which is about to miss its 2015 Millennium Development Goal of universal access to reproductive health, notes that 25 percent of sub-Saharan African women of reproductive age want to delay or avoid pregnancy but still have no access to reliable birth control.

This statistic should tell us something: Here is a need that we can fill, with a big impact on human population growth. The greatest uncertainties in population projection estimates center around sub-Saharan Africa, where investments in female education and family planning can make the biggest difference both in the lives of the individual women and in the sustainability of the global human population. Additionally, economic analyses show that these investments are more cost-effective at reducing carbon emissions than technological fixes like cleaner, greener power plants. So many birds can be killed with the same stone.

Rather than fearing discussion of human overpopulation, we should embrace it. That’s a lot less scary than rocketing blindly towards 10 billion with no plan for the environmental consequences when we get there.

Contact Holly Moeller at hollyvm ‘at’ stanford.edu.