Slow, then very fast: For most of history the human population grew slowly. Since about 1800 it has shot up in a rapid growth curve.
The points to remember
- The world's population has followed a rapid growth curve: slow for thousands of years, then very fast.
- About 1 billion in 1800, 2.5 billion in 1950, 8.2 billion in 2024.
- Each extra billion came faster: 123 years from 1 to 2 billion, about 12 years for each recent one.
- The growth rate peaked at about 2% a year in the 1960s; it is now below 1%.
- But the base is so large that about 70 million people are still added every year.
- About 60% of people live in Asia; growth is fastest in sub-Saharan Africa.
Remember it as: Slow for ages, then the J: from 1 billion to 8 in about 220 years.
Real example: India passed China in 2023 to become the country with the most people, about 1.43 billion. China's population had begun to fall in 2022.
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Nobody can count the future, so the UN uses projection models.
The UN's three scenarios
- The UN uses projection models to predict the world's future population.
- They give three scenarios: low, medium and high.
- The scenarios differ in their future fertility rates: about half a child per woman less or more than the medium.
- Medium (2024): a peak of about 10.3 billion in the mid-2080s, then 10.2 billion in 2100.
- The lines stay close until about 2050, then spread apart.
| Scenario | Fertility it assumes | World population |
|---|---|---|
| High | About 0.5 child per woman above medium | Keeps rising past 2100 |
| Medium | The UN's best guess | Peak about 10.3 billion, mid-2080s |
| Low | About 0.5 child per woman below medium | Peaks sooner, then falls |
Real example: the UN's 2024 medium scenario is about 700 million lower for 2100 than its projection of ten years earlier, because fertility fell faster than expected.
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The scenarios spread apart because the future is uncertain. A reason for the uncertainty must be specific: 'predicting is hard' is not a reason.
Why projections are uncertain
- Future fertility is the biggest unknown: it is why there are three scenarios.
- Different models use different figures and get different answers.
- Governments may change their population policies, raising or lowering births.
- Disasters, disease and war cannot be predicted.
- Migration does not change the world total: people only move between countries.
Fertility
- South Korea's TFR fell to 0.72 by 2023, far faster than anyone forecast.
Policy
- China's one-child policy (1980-2015) cut births by law; it was then dropped.
Shocks
- COVID-19 killed millions in 2020-2021: no projection saw it coming.
Real example: even the UN's own projections change: the 2024 figure for 2100 is 700 million lower than the 2014 one.
Not migration, for the world: For a country, migration is a big unknown. For the whole world it changes nothing: migrants only move from one country to another.
Each concept and model helps predict growth, but each has a weakness. A [9] weighs them up. Two models met later in this topic also count: the age-sex pyramid and the demographic transition model.
How well can models predict?
- Rates (birth, death, natural increase): exact for today, but they change every year.
- Doubling time assumes the growth rate stays the same, which it rarely does.
- Fertility rate: the best single guide to the future, but nobody knows how it will change.
- Age-sex pyramids show how many future parents there are: good for the next 20-30 years.
- The DTM shows the usual path as countries develop, but not when each stage comes.
- UN scenarios give a range, not one number, and are revised every few years.
They help because
- Today's rates and structure are measured accurately
- Many young women today = many births soon
- The UN range shows how sure we can be
They are limited because
- Fertility, policies and shocks change
- Models assume rates stay the same
- Different models give different answers
Real example: Nigeria has many young people, so its pyramid predicts growth for decades, but how fast depends on how quickly its fertility falls.
The conclusion: A fair judgement: the models predict the next 20-30 years well, because the parents are already born, but the long-term future still depends on fertility, which nobody knows.
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How this comes up: Read a year or a value from a projection graph [1]; one reason why projections are uncertain [1]; outline the UN scenarios [4]; to what extent models can predict population growth [9].
The UN projects that the world's population will peak at about 10.3 billion in the mid-2080s.
To what extent do the various concepts and models associated with human population dynamics enable us to predict human population growth?
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