Human Population Growth and the Ecological Footprint

Human population growth is the increase in the number of people on Earth, which passed 8 billion in 2022, and the ecological footprint is a measure of the area of productive land and sea needed to supply the resources that a person, city or country consumes and to absorb its waste. In brief: population growth depends on birth and death rates, and its speed is described by the growth rate and the doubling time (approximately 70 ÷ percent growth rate, the “rule of 70”). The total impact of people depends on both the number of people and the consumption per person.

For most of history the human population grew very slowly, but it rose from about 1 billion around 1800 to over 8 billion in the early 2020s. This guide describes the pattern of growth and the demographic transition, explains age structure and the rule of 70, defines the ecological footprint and biocapacity, compares countries in a table, works through a doubling time and footprint calculation and corrects common mistakes.

How has the human population grown?

The human population reached 1 billion around 1800, 2 billion in the 1920s, 4 billion in the 1970s and 8 billion in November 2022, according to United Nations estimates. The number added per year peaked at about 90 million in the late 1980s, and the rate of growth has declined since the 1960s, from about 2.1 percent per year to around 0.9 percent in the early 2020s. UN projections suggest that the population could reach a peak of about 10.3 billion in the 2080s, but the projections are uncertain and depend on future fertility.

The growth was made possible by falling death rates, particularly of children, due to better food supply, sanitation, vaccines and medicine, and not by a large rise in the birth rate.

How is population growth calculated?

• Crude birth rate: births per 1,000 people per year. Crude death rate: deaths per 1,000 per year.
• Natural increase = (birth rate − death rate) ÷ 10, as a percentage per year. Growth also includes net migration.
• Total fertility rate (TFR): the average number of children born to a woman in her life. The replacement level is about 2.1 in countries with low child mortality, which is a bit more than 2 because some children do not survive to adulthood and slightly more boys than girls are born.
• Doubling time: the time needed for the population to double at a constant growth rate. The rule of 70 estimates it as 70 ÷ growth rate (in percent per year). This comes from the exponential growth formula, since ln 2 ≈ 0.693.
• Exponential and logistic growth: unlimited growth is exponential, but real populations eventually face limits, described by the logistic model with a carrying capacity. The carrying capacity for humans is hard to define, because technology, diet and consumption change what the Earth can support.

What is the demographic transition?

The demographic transition model describes the change in birth and death rates as a country develops:

1. Stage 1: high birth and death rates, with a stable small population.
2. Stage 2: death rates fall because of better health and food, but birth rates stay high, so the population grows rapidly.
3. Stage 3: birth rates fall, as women gain education and access to contraception, urban living makes children more costly and child survival improves. Growth slows.
4. Stage 4: low birth and death rates, with a stable or slowly growing population.
5. Stage 5 (proposed): birth rates below death rates, and the population declines, as in Japan, Italy and several Eastern European countries, unless immigration compensates.

The timing differs by country. The fertility rate has fallen below replacement level in a large number of countries, including China, South Korea (below 1 child per woman), and much of Europe.

What does age structure show?

An age-sex pyramid shows the proportion of the population in each age group. A wide base (many young people) is typical of rapidly growing populations, such as those in Niger or Chad, and a narrow base with a bulge in middle and old ages shows an aging population, as in Japan. Because many young people will become parents, a population with a young structure has “population momentum”: it continues to grow for decades after fertility drops to replacement level.

What is the ecological footprint?

The ecological footprint, developed by William Rees and Mathis Wackernagel in the 1990s, is a measure of the biologically productive area needed to produce the resources consumed and absorb the carbon dioxide emitted, expressed in global hectares (gha). It is compared with biocapacity, the productive area available. If the footprint is bigger than the biocapacity, there is an ecological deficit (overshoot), which is met by depleting stocks (fish, forests, groundwater) or by accumulating carbon dioxide in the atmosphere.

• The components are cropland, grazing land, forest products, fishing grounds, built-up land and the carbon footprint, which is typically the largest part.
• Impact = Population × Affluence × Technology (the IPAT equation, proposed by Ehrlich and Holdren in 1971): the total environmental impact depends on the number of people, the consumption per person and the environmental effect per unit of consumption.
• Footprint estimates by the Global Footprint Network indicate that humanity uses resources at a rate equal to roughly 1.7 Earths. The calculation includes assumptions, and critics point out that it does not cover all environmental effects, such as biodiversity loss or toxic pollution.

Footprint per person varies greatly: people in high-income countries such as the United States, Canada, Australia and Gulf states have footprints several times the global average, and in many low-income countries the footprint is below the average.

Population impact factors compared

Worked example: doubling time and footprint

Part A: doubling time. Country X has a growth rate of 2.0 percent per year, and country Y has 1.0 percent per year.

1. Rule of 70, X: 70 ÷ 2.0 = 35 years.
2. Rule of 70, Y: 70 ÷ 1.0 = 70 years.
3. Exact check for Y: ln 2 ÷ ln 1.01 = 69.7 years, so the rule gives a good approximation.
4. Projection for the world: if 8.0 billion people grew at 0.9 percent per year for 10 years, then N = 8.0 × 1.009¹⁰ = 8.75 billion (about 0.75 billion more).

Part B: footprint (illustrative numbers). Suppose that a country has an average footprint of 2.7 gha per person and a biocapacity of 1.6 gha per person.

1. Ecological balance: 1.6 − 2.7 = −1.1 gha per person, an ecological deficit.
2. Ratio: 2.7 ÷ 1.6 = 1.69, so the country uses about 1.7 times its own biocapacity, and must import resources or deplete its stocks.

The values are only illustrative, and not data for a real country.

How have ideas about population changed?

Thomas Malthus argued in 1798 that population grows exponentially, while food supply grows only linearly, which would lead to famine. Technology, such as the Haber-Bosch process for fertilizer and the Green Revolution crops led by Norman Borlaug, has increased food production far more than he expected. Paul Ehrlich’s The Population Bomb (1968) predicted widespread famine, which did not occur on the scale he forecast, though population growth contributed to environmental pressure. The 1994 Cairo conference placed emphasis on women’s education and health. Wackernagel and Rees published the ecological footprint method in 1996.

Which mistakes are common?

Mistake 1: Saying that the population is growing faster and faster

The number of people is increasing, but the percentage growth rate has fallen since the 1960s.

Mistake 2: Blaming population size alone

Impact depends on consumption as well. A person in a high-income country can have a footprint many times larger than one in a low-income country.

Mistake 3: Using the rule of 70 for changing rates

The rule assumes a constant growth rate. Real rates change.

Mistake 4: Treating carrying capacity as a fixed number

For humans it depends on diet, technology and the level of consumption, so estimates vary widely.