Wolfgang Lutz is one of the world's experts in projecting global population levels and demography. I contacted him to help understand the differences between projections based on demography like his and the United Nations' versus systemic ones like in Limits to Growth. He gave a comprehensive overview of who projects and how, at least as much as can be covered in under an hour. Some highlights: Who projects based on demography: the UN, International Institute for Applied Systems Analysis (IIASA), and the Wittgenstein Center, among others. He described what and how demographers project: Assumptions, methods, variables of age, sex, education, migration, fertility rate, mortality rate. He consistently repeated the importance of education. On Limits to Growth, he pointed out that systems analyses include feedback mechanisms, but their demographics tend to be less sophisticated, for example lacking age structure or effects of education. Demographers don't take them seriously because of their oversimplification. I asked how demographers include feedback. He described a few ways, including asking experts and translate their responses into different scenarios. What about big events like fish or aquifers depleting? He pointed out extreme events are hard to predict, though humanity's historical resilience suggests we'll figure out ways to level their effects. Demographers also include probabilistic models for tipping points, disease, and such, and report levels of variance. The results of his research and projections: Human population peaking somewhere around 2080 at around 10 billion then declining. It may reach about 3 to 4 billion by 2200, which could be long-term sustainable, though the transition is uncertain. Humanity could reach a healthy, wealthy, more equal, more resilient, and well educated future, but not given. Potential problems: heat waves, drought, floods, sea level rise. Humans can solve to some degree, but we have to prepare. What to focus on: since population changes slowly, behavior, technology, and migration first, then education especially of women in the long term since its effects happen more slowly. Also family planning, women's health, contraception, and sexual equality. We covered a lot, though scratched the surface, gives understandable overview of demographics and global population projections. I put greater weight on difficult-to-predict extreme uncertain events. At least I'd make the uncertainty go down more than the symmetry I see, but our conversation was about learning and understanding, not debate. I've learned a lot each time I've listened to this episode. It's dense with information, but on an important subject.