8.1 Human populations

Births and immigration add people to a population; a crude input rate makes the count comparable across populations of different sizes.

Use events ÷ total population × 1,000 when the question asks for a rate per 1,000 per year. Births happen inside the population; immigration crosses its boundary from outside.

25,000 births in 500,000 people gives (25,000 ÷ 500,000) × 1,000 = 50 births per 1,000 people.

50 per 1,000: (2,000 ÷ 40,000) × 1,000.

2,000 is an event count, not a rate. A rate needs the population denominator and its units.

Deaths and emigration remove people from a population; their crude rates express those annual outputs per 1,000 residents.

Use the same events ÷ population × 1,000 calculation at town, country or global scale. A crude death rate describes the population’s current output, not an individual’s lifetime risk.

15,000 deaths among 750,000 people gives 20 deaths per 1,000 people for that year.

The smaller population: 50 versus 10 per 1,000.

Do not compare raw deaths without population size; that confuses a count with a rate.

Each population metric answers a different question: TFR describes births per woman, life expectancy describes expected years, natural increase compares birth and death rates, and doubling time estimates growth speed.

Use doubling time ≈ 70 ÷ growth rate (%) only after converting the growth rate to a percentage. Use natural increase = birth rate − death rate; if rates are per 1,000, divide the result by 10 to express a percent.

At 2% annual growth, doubling time is about 35 years; birth rate 30 and death rate 10 per 1,000 gives natural increase 20 per 1,000, or 2%.

Doubling time, not life expectancy or TFR; use 70 ÷ percentage growth.

Do not put a per-1,000 rate directly into 70 ÷ r; the formula expects percent units.

The global human population has followed a rapid growth curve, but future projections are conditional scenarios rather than fixed predictions.

UN high, medium and low scenarios diverge mainly because different fertility assumptions compound across generations; mortality and migration assumptions also affect the path.

If fertility falls faster than assumed, the population path can move below an earlier medium scenario even though current population is unchanged.

Read each curve's fertility, mortality, migration and time assumptions, and treat the spread as evidence of uncertainty.

The middle scenario is not guaranteed, and uncertainty does not make every scenario equally plausible.

Direct population policies explicitly target births or migration: anti-natalist policies seek fewer births, pro-natalist policies seek more births, and migration policies alter immigration or emigration.

Evaluate intended rate, cultural and religious context, economic and social measures, rights, uptake, time lag and measured outcome.

Anti- or pro-natalist describes intent, not automatic success or ethical acceptability.

Indirect policies change economic, social, health or development conditions; these alter births, deaths or migration through intermediate choices and survival pathways rather than a population quota.

Gender equality, education, sanitation, public health, pensions and welfare can act through different pathways; effects depend on access, culture, employment and time.

An indirect influence is probabilistic: improved education or welfare does not produce the same demographic result in every society.

An age-sex pyramid is a snapshot of population composition: age groups run vertically, the two sexes sit on opposite sides, and width shows number or percentage.

Start with shape before causes. A broad base signals many young people; a wider upper section signals more older people. The diagram describes structure, not automatically a DTM stage or forecast.

A pyramid that narrows sharply above age 60 has fewer older people relative to the base; a column-like shape has more similar-sized age groups.

The population is relatively youthful; you cannot yet infer the cause or exact DTM stage.

Do not explain a pyramid’s cause before describing its axes and shape.

DTM stages describe how birth and death rates change; the gap between those rates, not the stage label, determines whether population grows, stabilizes or declines.

Stage 1 has high births and deaths; in stage 2 deaths fall first, stage 3 births fall, stage 4 both are low, and stage 5 may have births below deaths. Use the rate sequence to explain the curve.

When deaths drop from 30 to 10 per 1,000 while births stay at 35, the gap widens and growth accelerates; later, falling births narrow the gap.

Stage 2 pattern: the death rate has fallen first, so the positive gap is large.

DTM stages are a generalized pattern, not a universal timetable every country follows identically.

Rapid population growth can increase stress on Earth's systems, but total pressure also depends on per-person consumption, technology, distribution and governance.

Biocapacity is the ability of ecosystems to regenerate resources and absorb waste. The local textbook projection table rises to about 10.37 billion in 2075 and about 10.40 billion in 2100; projections beyond this are conditional on fertility, mortality, migration and policy.

Two cities with equal populations can create different pressure if one uses much more fossil energy per resident while the other has lower consumption but unmet basic needs.

Use the doughnut model to test both social foundations and planetary boundaries, then compare population, consumption and local biocapacity rather than population alone.

A projected population total is not a fixed outcome, and population size is not a single-cause explanation for environmental stress.

Dependency ratio compares dependent-age groups (commonly ages 0–14 and 65+) with the productive-age population aged 15–64; population momentum is continued growth caused by a large young cohort even after fertility falls.

Calculate (population aged 0–14 + population aged 65+) ÷ population aged 15–64 × 100. Momentum occurs because many people are already approaching reproductive ages, so today's age structure carries growth forward.

If 300 young and older dependents rely on 600 people aged 15–64, the dependency ratio is 50 dependents per 100 productive-age people; a large young cohort can still raise total births after fertility per woman drops.

Falling fertility does not necessarily stop growth immediately because cohort size and the number entering reproductive ages also matter.

The standard age bands are a model: schooling, retirement and employment vary among societies.

A DTM comparison uses birth and death rates and age structure to describe a pattern, then uses historical, cultural, religious, economic, social and political evidence to explain it.

Compare change at least 30 years into the past, the present and at least 30 years ahead; the DTM organizes the rate pattern but does not make development a single cause.

A stage label is not a value judgement or complete causal explanation; verify country evidence, projections and historical path.

Environmental migration occurs when climate change, drought or land degradation changes safety or livelihoods together with social, economic and political constraints.

Separate sudden-onset floods, droughts, forest fires and intensified storms from slow-onset desertification, sea-level rise and saltwater inundation; then trace housing, work, food, water or safety impacts.

Named example—people have migrated from Tuvalu to New Zealand as sea-level rise and saltwater inundation increase pressure on island homes, freshwater and livelihoods; ability to move still depends on resources and migration rules.

Name the environmental stress, onset timescale, livelihood pathway, origin and destination; avoid claiming one automatic cause.

Not every displaced person is a simple 'climate migrant': environmental pressure is mediated by vulnerability, support and choice.