"Leading demographers, including those at the United Nations and the U.S. Census Bureau, are projecting that world population will peak at 9.5 billion to 10 billion later this century and then gradually decline as poorer countries develop. But what if those projections are too optimistic?"

Carl Haub, who was senior demographer at the Population Reference Bureau for three decades, wants you to think the unthinkable. What if the vaunted "demographic transition" that's supposed to be an inevitable part of countries' economic and social development doesn't materialize?

This transition has happened in all developed countries, leading birth rates to go to around two children per couple or less — the point at which a country's population becomes stable or shrinks. If it happens worldwide, world population stabilizes. If it doesn't, well, can the earth really handle more than 10 billion people?

What if population continues to soar, as it has in recent decades, and the world becomes home to 12 billion or even 16 billion people by 2100, as a high-end UN estimate has projected? Such an outcome would clearly have enormous social and environmental implications, including placing enormous stress on the world’s food and water resources, spurring further loss of wild lands and biodiversity, and hastening the degradation of the natural systems that support life on Earth.

Some countries are already showing signs of bucking the demographic transition, including Jordan, Indonesia, and parts of sub-Saharan Africa.

Often fertility rates might decline from a higher level and then “stall” for a time, not continuing their downward trajectories to the two-child family, resulting in a higher-than-projected population. […] The real possibility of fertility decline stopping before the two-children level is reached requires demographers, policy makers, and environmentalists to seriously consider that population growth in the coming century will come in at the high end of demographic projections.

Haub asks how sub-Saharan Africa could support five times its current population (the number projected by one model), especially as the climate warms and resources become scarce. The answer, of course, is simple: Poor and rural populations will reach their local carrying capacity, and external factors like food availability will limit them thereafter. Basically, things keep getting more and more crowded until people start dying off. That's not a future anyone wants to envision, however, especially to the extent that it's already playing out in countries like Somalia.