Global human population growth is forecast mainly with cohort-component projections, while exponential and logistic growth models represent possible growth patterns. The demographic transition model (DTM) explains how development may alter birth and death rates, but does not independently produce a precise global forecast.
Population changes through births and immigration as inputs, and deaths and emigration as outputs. Models combine assumptions about these flows to project future population size. The core concepts are SL and HL in Subtopic 8.1; population momentum is HL-only depth.
How the Models Work
| Model | Use and limitation |
|---|---|
| Exponential growth | Produces a J-shaped curve at a constant proportional growth rate. It can represent rapid growth but assumes no limiting constraints. |
| Logistic growth | Produces an S-shaped curve as growth slows near carrying capacity. Global human carrying capacity is uncertain because technology, consumption, trade and distribution change it. |
| Cohort-component projection | Divides population by age and sex, then applies fertility, mortality and migration assumptions. United Nations World Population Prospects publishes variants based on different assumptions; these are projections, not certainties. |
| Demographic transition model | Links changing birth and death rates to socioeconomic development. It indicates likely growth direction, but countries do not necessarily follow identical pathways. |
Age-sex pyramids support projections by showing the size of reproductive-age groups. A youthful population can keep growing after fertility falls because many people will soon enter reproductive ages.
A common misconception is that one model gives an exact future population. Models are simplified representations of reality, so outputs depend on uncertain fertility, disease, migration, policy, conflict and environmental change.
Exam Technique
For an ESS explain question, identify the model, outline its mechanism, and connect births, deaths and migration to growth. For evaluate, weigh usefulness against limitations, use UN World Population Prospects as a named example, and conclude that projections are scenario-dependent rather than certain.
