Humanity after the peak: long-run population and human-capital pathways for global environmental change

Population and human-capital pathways are central to global environmental change research, yet most widely used demographic projections end in 2100. This creates a time-horizon problem for slow-moving environmental processes, including sea-level rise, land-use change, long-lived infrastructure, food-system transformation and adaptation across generations. We extend Shared Socioeconomic Pathways population and human-capital scenarios to 2300 and examine how peak population gives way to long-run demographic decline, aging and regional redistribution. In the middle-of-the-road pathway, the global population declines to about 4.2 billion by 2300. In the sustainability pathway, it approaches 1 billion. These are not extinction scenarios, but they describe a profound change in the social and demographic setting in which environmental demand, exposure, adaptation and policy operate. The results show that the post-2100 period cannot be treated as a simple continuation of twentieth-century population growth. A smaller, older and more educated world may reduce some overall pressures while intensifying others, especially around labor, care, migration and uneven regional change. Long-run environmental analysis, therefore, needs population pathways that extend beyond 2100 and include not only population size, but also age, education, spatial distribution and demographic momentum.