Estimating Earth's Human-Carrying Capacity

Estimates of Earth’s human carrying capacity published in the past century ranged from less than 1 billion to more than 1,000 billion people. These political numbers were intended to persuade people either that too many humans were already on Earth or that there was no problem with continuing rapid population growth. The vast disparities in estimates arose because Earth's capacity to support people is determined partly by processes that the social and natural sciences have yet to understand, and partly by choices that we and our descendants have yet to make. Most published estimates of Earth’s human carrying capacity uncritically assumed answers to one or more of these questions: What will future humans desire and what will they accept as the average level and distribution of material well-being? What technologies will they use? What domestic and international political institutions will be used to resolve conflicts? What economic arrangements will provide credit, regulate trade, set standards, and fund investments? What social and demographic arrangements will influence birth, health, education, marriage, migration, elder care, urban planning, and death? What physical, chemical and biological environments will people want to live in? What level of variability will people be willing to live with? (If people do not mind seeing human population size drop by billions when the climate becomes unfavorable, they may regard a much larger population as “sustainable” when the climate is favorable.) What level of risk are people willing to live with? (Are mud slides, hurricanes or floods acceptable risks? The answer will influence the area of land viewed as habitable.) What time horizon is assumed? (Sustainability for how long?) Finally, what will people’s values and tastes be in the future? In particular, how much will each person value the well-being of other contemporary and future individuals of his own and other species? The difficulty of answering these questions should not prevent using available knowledge and resources to solve today’s problems and to prepare the next generation to solve tomorrow’s problems. That incremental strategy depends very little on knowing Earth’s human carrying capacity.