A population projection is a conditional path

Many people alive in the next few decades are already born, which gives near-term population projections unusual inertia. Farther out, small differences in fertility accumulate, while mortality, migration and policy alter national paths. The United Nations therefore publishes variants and uncertainty intervals, not a revealed headcount for the end of the century.

Calling one medium projection “the population in 2100” erases those conditions. It also hides regional divergence: populations may age or decline in some places while growing in others. Scenarios are still useful for schools, pensions and infrastructure when their assumptions stay attached. The honest sentence begins “if”—if fertility follows this course, if survival changes so, if migration is treated this way.