Definition
A population projection is a forecast of a population's future size and structure, built up from age-stratified fertility rates, mortality rates, and (for sub-national work) migration. The standard reference forecasts are produced by the UN Population Division and updated every two years.
Rosling stressed that because the world is past peak child, the projection out to 2100 — about 11 billion — is now mostly arithmetic: it follows from the children alive today moving up the age pyramid, not from speculative future fertility.
Why it matters
How it works
The cohort-component method is the workhorse: divide the population into age brackets and sexes, apply age-specific birth and death rates each year, age the cohorts up one step, and repeat. With reasonable fertility and mortality inputs the method projects forwards mechanically.
Three things make global projections more robust than national ones. First, migration cancels out (you can leave a country but not the planet). Second, the largest cohorts are already born — for any year before about 2040, the adults of that year are already alive today. Third, fertility transitions follow a recognisable S-curve across countries, so even the assumption-heavy parts of the model have many completed cases to anchor on.
Where genuine uncertainty bites is in the late tail of fertility decline in Sub-Saharan Africa and a handful of holdout regions. The UN's high and low variants differ mostly on how fast those final transitions complete; the medium variant lands on roughly 11 billion in 2100 and a plateau thereafter.