It’s the fear that having a kid in this day and age dooms that kid to a miserable life on a miserably hot planet.

n India, this has been fantastic, it has actually reduced the birth rate but kept it above that magic 2.1 number. 

So, the UN forecasting model inputs three things: fertility rates, migration rates, and death rates. It doesn’t take into account the expansion of education for females or the speed of urbanization (which are in some ways linked). The UN says they’re already baked into the numbers. But when I went and interviewed [the demographer] Wolfgang Lutz in Vienna, which was one of the first things we did, he walked me through his projections, and I walked out of the room gobsmacked. All he was doing was adding one new variable to the forecast: the level of improvement in female education. And he comes up with a much lower number for global population in 2100, somewhere between 8 billion and 9 billion.