Too often, it seems, matters of population are overlooked in discussions of global sustainability. And this is true, despite some rather obvious points: A world of, say, 5 billion people is more likely to be sustainable than one of 10 billion; and a world of 1 billion is likely more sustainable still. All things being equal, a world with fewer people will allow for a more robust planetary ecosystem, and a higher quality of life for humans, than a world with more people. Few seem willing to state things this clearly, but I think few would contest it, if pressed. Currently the Earth is at roughly 7.7 billion, heading to 9.5 billion by 2050, and perhaps to 11 or 12 billion by 2100. Each year about 135 million babies are born; if we subtract the 55 million annual deaths, we get an annual growth rate of 80 million—or around […]