Constrained scenarios for twenty-first century human population size based on the empirical coupling to economic growth

Growth in the global human population this century will have momentous consequences for societies and the environment. Population growth has come with higher aggregate human welfare, but also climate change and biodiversity loss. Based on the well-established empirical association and plausible causal relationship between economic and population growth, we devised a novel method for forecasting population based on Gross Domestic Product (GDP) per capita. Although not mechanistically causal, our model is intuitive, transparent, replicable, and grounded on historical data. Our central finding is that a richer world is likely to be associated with a lower population, an effect especially pronounced in rapidly developing countries. In our baseline scenario, where GDP per capita follows a business-as-usual trajectory, global population is projected to reach 9.2 billion in 2050 and peak in 2062. With 50% higher annual economic growth, population peaks even earlier, in 2056, and declines to below 8 billion by the end of the century. Without any economic growth after 2020, however, the global population will grow to 9.9 billion in 2050 continue rising thereafter. Economic growth has the largest effect on low-income countries. The gap between the highest and lowest GDP scenarios reaches almost 4 billion by 2100. Education and family planning are important determinants of population growth, but economic growth is also likely to be a driver of slowing population growth by changing incentives for childbearing. Since economic growth could slow population growth, it will offset environmental impacts stemming from higher per-capita consumption of food, water, and energy, and work in tandem with technological innovation.