8.1 Human Populations

8.1.3's calculation skill (natural increase and doubling time) lives in the Population Inputs & Outputs tab, built into the live simulator. This tab covers the subtopic's other named skill: using secondary data to test a hypothesis with a suitable statistical tool.

Test a hypothesis with secondary data

Use secondary data from sources such as Gapminder, World Bank and Our World in Data to test a hypothesis about the relationship between a socio-economic indicator and a demographic factor, using a suitable statistical tool such as an online statistics calculator. The three scenarios below walk through the real judgment calls this skill actually requires.

Crude birth rate (CBR)

The number of live births per 1,000 people in a population per year.

Crude death rate (CDR)

The number of deaths per 1,000 people in a population per year.

Immigration rate

The number of immigrants entering a population per 1,000 population per year.

Emigration rate

The number of emigrants leaving a population per 1,000 population per year.

Total fertility rate (TFR)

The average number of births per woman of childbearing age in a population.

Life expectancy

The average number of years a person can be expected to live, usually measured from birth, assuming current demographic conditions remain unchanged.

Natural increase

Birth rate minus death rate, expressed as a percentage; the rate at which a population grows or shrinks from births and deaths alone, excluding migration.

Doubling time

The number of years it would take a population to double in size at its current growth rate, calculated with the rule of 70 (70 divided by the growth rate as a percentage).

Anti-natalist policy

A direct population policy aimed at lowering birth rates, such as China's one-child policy.

Pro-natalist policy

A direct population policy aimed at raising birth rates, such as Singapore's Baby Bonus Scheme.

Indirect population management

Economic, social, health or development policy that changes births, deaths or migration as a side effect, without directly targeting population size.

Age-sex pyramid

A chart showing the proportion of a population, by gender, in each age group, used to compare population structures between places or over time.

Expansive pyramid

An age-sex pyramid with a wide base of young people narrowing sharply toward old age, typical of a rapidly growing population such as Niger's.

Constrictive pyramid

An age-sex pyramid narrower at the base than in the middle or top, typical of a population with falling birth rates and a growing elderly share, such as Japan's.

Demographic transition model (DTM)

A model describing how birth and death rates change through five stages as a population develops over time, moving from high fluctuating rates to low or declining rates.

Dependency ratio

The ratio of dependents (under 15 and over 64) to the economically productive population (15-64), showing how much of a population must be supported by those of working age.HL

Population momentum

The tendency of a population to keep growing for a generation or more after its fertility rate falls to replacement level, because a large number of women are already of, or approaching, reproductive age.HL

Doughnut economics

A framework (Kate Raworth) picturing a safe and just space for humanity between a social foundation of minimum human needs and an ecological ceiling of planetary boundaries.HL

Planetary boundaries

Nine measurable Earth-system limits (including climate change and biosphere integrity) beyond which environmental change becomes destabilizing; as of 2023, six of the nine are assessed as transgressed.HL

Biocapacity

The capacity of an area of land or water to regenerate the resources a population consumes and absorb its waste, measured in global hectares.HL

Earth Overshoot Day

The calendar date by which humanity's demand on nature for the year exceeds what Earth's ecosystems can regenerate in that year; 24 July in 2025.HL

Environmental migration

Migration driven substantially by an environmental issue such as climate change, drought or land degradation, whether through a sudden onset event or a slow onset event.HL

Sudden onset event

An abrupt environmental hazard, such as a flood, forest fire or intensified storm, that can trigger rapid displacement.HL

Slow onset event

A gradual environmental change, such as desertification, sea-level rise or saltwater intrusion, that makes an area progressively less habitable over years or decades.HL

Rapid human population growth has increased stress on the Earth's systems. Today's global population is about 8.2 billion. The UN's medium-fertility projection puts this at about 9.7 billion by 2050 and about 10.2 billion by 2100, likely peaking around 10.3 billion near 2084. The low and high scenarios diverge far more by 2100, from roughly 7.0 to 14.4 billion, because small differences in assumed future fertility compound across generations, the same three-scenario uncertainty covered in 8.1.4.

That stress is often described through biocapacity disparity and the doughnut economics model. Earth Overshoot Day, the calendar date by which humanity's yearly demand on nature exceeds what Earth can regenerate that year, fell on 24 July in 2025: humanity is using resources at roughly 1.8 times the rate Earth's ecosystems can replace them. That demand is deeply uneven. The world's average ecological footprint is about 2.6 global hectares per person against an available biocapacity of about 1.5 global hectares per person, but a high-consumption country such as Qatar runs a footprint of roughly 14 global hectares per person, around 17 times that of a low-consumption country such as the Democratic Republic of the Congo, at roughly 0.8 global hectares per person.

As of the most complete assessment (Stockholm Resilience Centre, 2023), six of the nine planetary boundaries are judged to be transgressed:

Population growth is not the only driver of overshoot, consumption per person matters at least as much, as the Qatar-versus-DRC contrast above shows, but a larger population raises total demand on a ceiling that has already been crossed six times over.

Age-sex pyramids can be used to determine the dependency ratio and population momentum. The dependency ratio depends on the number of people in the ages defined as dependent (under 15 and over 64) against those defined as economically productive (15 to 64). It tends to be high in populations with either a very high or a very low fertility rate, exactly the Niger-and-Japan pattern from the previous tab: Niger's ratio (96.8%) is driven by children, Japan's (70.1%) is driven overwhelmingly by retirees.

Population momentum is why a population keeps growing even after its fertility rate falls to replacement level. It happens because population growth depends not only on how many children each woman has, but on how many women of reproductive age already exist. Niger illustrates this directly: 46.2% of its population is currently under 15. Even in a hypothetical scenario where Niger's fertility rate fell to replacement level tomorrow, its population would keep growing for a generation or more, simply because that huge cohort of girls is about to enter, or is already in, its reproductive years.

The reasons for patterns and trends in population structure and growth can be understood using examples of two countries in different stages of the demographic transition model, considering the past (30 or more years ago), the present, and the future (30 or more years ahead), and the historical, cultural, religious, economic, social and political reasons behind each.

Past to future, both countries: Niger's population rose from about 9.7 million (1995) to about 27.0 million (2024), projected to reach about 52.5 million by 2050 under the UN's medium scenario. Japan's population fell from about 125.5 million (1995) to about 124.0 million (2024), projected to fall to about 105.1 million by 2050. Both 2050 figures are medium-variant projections, not certainties.

Environmental issues such as climate change, drought and land degradation are causing environmental migration, increasingly through slow onset events such as desertification, sea-level rise and saltwater intrusion, rather than only sudden onset events like floods and storms. Tuvalu is the clearest real case of slow-onset migration.

Two real migration pathways now exist. New Zealand's Pacific Access Category has offered Tuvalu a ballot of 75 residence places a year for many years; Tuvaluan-heritage population in New Zealand grew steadily from 2,625 (2006 census) to 6,585 (2023 census), a real, measured outcome, not just a quota. In November 2023, Australia and Tuvalu signed the Falepili Union treaty, the first bilateral treaty explicitly linking climate change to a migration pathway: up to 280 Tuvaluans a year can gain Australian permanent residence through its mobility visa. The treaty's first ballot in 2025 drew about 2,474 primary registrations, roughly a third of Tuvalu's population once family members are counted, and the full 280 places were granted.

Tuvalu's government has also pursued the Future Now Project, an effort to preserve statehood and culture digitally, including a "digital nation" archive, alongside the regional Rising Nations Initiative, both aimed at securing Tuvalu's legal and cultural continuity even under the most severe land-loss projections.