8.1 Human populationsIB Environmental Systems and Societies HL: Revision notes

Births and immigration are inputs to a human population; deaths and emigration are outputs. Dynamics are quantified using:

• Total fertility rate (TFR): average children per woman over her lifetime; replacement is about 2.1.
• Life expectancy: average years a newborn is expected to live.
• Natural increase = birth rate − death rate (migration excluded).
• Doubling time ≈ 70 ÷ growth rate (%).

Always show the formula, working and units.

The global population has followed a rapid growth curve, from about 3 billion in 1960 to 8 billion in late 2022. Models, such as United Nations projections, predict a peak around 10 billion in the 2080s. They depend on assumptions about fertility, so they carry uncertainty and are best used as scenarios.

Direct policies target births, deaths or migration directly (the one-child policy, pronatalist bonuses, immigration quotas). Indirect policies work through economic, social, health or development measures (girls' education, healthcare, family planning, pensions). Direct policies work quickly but raise ethical issues; indirect policies are voluntary but slower.

An age-sex pyramid models the composition of a population by age and sex: a wide base shows high fertility; a narrow base and large older bands show an ageing, declining population.

The demographic transition model (DTM): stage 1, high birth and death rates; stage 2, death rate falls, rapid growth; stage 3, birth rate falls; stage 4, low rates; some versions add stage 5 with natural decrease.

Dependency ratio = (number aged 0-14 + number aged 65 and over) ÷ number aged 15-64 × 100. It shows the number of dependants per 100 working-age people. A high ratio can place a burden on the workforce and on public services.

Population momentum: a population keeps growing after TFR reaches replacement because a large share of people are young and entering their reproductive years. It can take decades before births and deaths balance.

Patterns of structure and growth are explained by comparing countries at different DTM stages.

• Niger (stage 2): TFR above 6, about half the population under 15, births far exceed deaths, rapid growth, very wide-based pyramid.
• Germany (late stage 4/5): TFR about 1.5, median age about 45, deaths exceed births, an ageing pyramid; immigration keeps the population from falling.

Use any suitable pair of countries, with their TFR, age structure and the reasons (education, healthcare, economic development, policy).

Rapid growth from 3 billion (1960) to 8 billion (2022) has increased demand for food, water, energy and land and added to stress on Earth's systems: six of nine planetary boundaries are transgressed. Impact also depends on affluence and technology: footprints per person vary greatly.

Environmental migration is movement caused by environmental issues such as climate change, drought and land degradation. Example: around Lake Chad, shrinkage of the lake has removed livelihoods and pushed people to towns and camps. Receiving areas face pressure on services and resources.