Human Population Growth and the Demographic Transition

Eight billion and counting

For most of human history, our population grew very slowly. Hunting, gathering and early farming societies had high birth rates, but also high death rates, especially among babies and children. Then, in about two centuries, everything changed. The United Nations estimates that the world reached 1 billion people around 1804, 2 billion in 1927, and 8 billion on 15 November 2022. In 2026, the world population is about 8.3 billion.

In this lesson you will apply your population models to the most important species for our future: us.

Why did human growth speed up?

The population model is the same: ΔN = births − deaths (migration does not change the world total). What changed were the rates:

• Agriculture (beginning about 10 000 years ago) produced more food per hectare, supporting more people.
• The Industrial Revolution (from the late 1700s) and modern agriculture (machinery, fertilizers, improved crop varieties) increased the food supply further, a way of raising K.
• Sanitation, clean water, vaccines and antibiotics caused death rates to fall, especially for infants and children.

Birth rates fell much more slowly than death rates, so for a long time births greatly exceeded deaths. The world growth rate peaked at a little over 2% per year in the 1960s. It has roughly halved since and is now below 1% per year. The number of people added each year is still large, because the population is so big.

For much of the 20th century human growth was even faster than exponential: the per capita growth rate itself was rising. That is now reversing.

Measuring human population change

Demographers use rates per 1000 people:

• Rate of natural increase (%) = (CBR − CDR) ÷ 10.

For example, CBR = 30 and CDR = 8 give (30 − 8) ÷ 10 = 2.2% per year. By the rule of 70, that population doubles in about 70 ÷ 2.2 ≈ 32 years.

The total fertility rate (TFR) looks ahead: the average number of children per woman. Replacement level is about 2.1 where child mortality is low (slightly above 2, because not every child survives to have children, and slightly more boys than girls are born). Canada's TFR was 1.25 in 2024, a record low according to Statistics Canada. With fertility well below replacement, Canada's growth now comes mostly from immigration. Statistics Canada estimated Canada's population at about 41.8 million in July 2026, after revising its earlier estimates upward in September 2026: a reminder that even official population numbers are estimates that get corrected.

The demographic transition model

As countries industrialize, their birth and death rates tend to change in a typical sequence:

1. Stage 1, pre-industrial: high birth rates and high death rates; slow, unsteady growth. (No country is fully in Stage 1 today.)
2. Stage 2, early transition: death rates fall (food, sanitation, medicine) while birth rates stay high: rapid growth.
3. Stage 3, late transition: birth rates fall (urban living, education, especially for girls and women, access to family planning, lower child mortality so families no longer need many children); growth slows.
4. Stage 4, post-industrial: low birth and death rates; slow or zero growth. Canada is here.
5. Stage 5 (proposed): birth rate falls below death rate; the population shrinks without immigration (for example Japan and Italy).

Link to the unit: the transition is like a population moving from r-like reproduction (many children, high early mortality) toward K-like reproduction (few children, high investment in each), and survivorship shifting from Type II or III toward Type I.

Age pyramids and population momentum

An age pyramid shows the population by age and sex.

• Expansive (wide base): many children, rapid growth; typical of countries in Stages 2 and 3, such as Niger.
• Stationary (straight sides): similar numbers in each age group until old age; slow growth.
• Constrictive (narrow base): fewer children than adults; an aging, possibly shrinking population, such as Japan. Canada's pyramid is becoming more constrictive: in recent years there have been more Canadians aged 65 and over than children under 15.

Population momentum: a country with a very young population keeps growing for decades even after its TFR drops to 2.1, because the large generation of children has not yet had its own children. This is one reason the UN projects that world population will keep growing until it peaks at about 10.3 billion in the mid-2080s (World Population Prospects 2024), even though global fertility is already falling.

What is Earth's carrying capacity for humans?

No one knows a single number, and scientists' estimates vary enormously, because human K depends on how we live. A planet of people eating mostly plants, using renewable energy and living in efficient homes can support far more people than a planet where everyone consumes as much as the average Canadian. Technology can raise K (more food per hectare) but can also deplete resources that future generations need. The next lesson, Consumption, Technology and the Ecological Footprint, gives a way to measure this.