Human Population Size

It took the whole of human history until around 1800 for the world to reach its first billion people. The eighth billion arrived in late 2022, just twelve years after the seventh. Every day the planet gains roughly 200 000 more people than it loses. Understanding why the human population changes so fast, and what that growth does to cities, resources and the environment, is one of the central questions in environmental management.

In this lesson you will learn the four levers that change a population: birth rate, death rate, immigration and emigration. You will calculate natural increase, follow a country through the demographic transition, and weigh the push and pull factors that drive people from the countryside into ever-larger cities. Then you will trace the real consequences of rapid growth, from informal settlements and megacities to pressure on water, energy, land and clean air.