8.1 Human populationsIB Environmental Systems and Societies SL: Revision notes

A human population changes through inputs and outputs. Births and immigration are inputs. Deaths and emigration are outputs. If inputs exceed outputs the population grows; if outputs exceed inputs it shrinks. Net migration is immigration minus emigration.

Population dynamics can be measured using four indicators.

• Total fertility rate (TFR): the average number of children a woman would have in her lifetime at current age-specific birth rates. About 2.1 is the replacement level.
• Life expectancy: the average number of years a newborn is expected to live.
• Natural increase rate = (birth rate − death rate) ÷ 10, giving a percentage when rates are per 1000. Migration is not included.
• Doubling time ≈ 70 ÷ annual growth rate (%) (the rule of 70).

In calculations, show the formula, the working and the units.

The global population has followed a rapid growth curve: about 1 billion in the early 1800s, 3 billion in 1960 and 8 billion by late 2022. The growth rate peaked in the 1960s at around 2% a year and has since slowed. Models, such as United Nations projections, predict future population; they usually forecast a peak of around 10 billion in the 2080s, but they depend on assumptions about fertility, so they are uncertain.

Direct policies act on births, deaths or migration: the one-child policy in China (1979–2015), pronatalist incentives in some low-fertility countries, and immigration quotas.

Indirect policies change economic, social, health or development conditions: girls' education, family-planning services, healthcare, pensions and women's employment. Iran and Thailand reduced their TFR sharply this way.

Direct policies act quickly but raise ethical concerns; indirect policies are voluntary but slower.

An age-sex pyramid shows the numbers or percentages of males and females in each age band. A wide base and narrow top means high fertility and a growing population. Straight sides suggest a stationary one. A narrow base with large older bands shows a declining, ageing population (e.g. Japan). Pyramids can be compared between countries or over time.

The DTM describes changing birth and death rates as a country develops.

• Stage 1: high birth and death rates, so slow growth.
• Stage 2: death rate falls (healthcare, sanitation, food) while births stay high: rapid growth.
• Stage 3: birth rate falls (education, contraception, urbanisation): growth slows.
• Stage 4: low birth and death rates: low growth or stable.

Some versions add a stage 5 with births below deaths.