Human Population Growth

Infinite growth is not possible on a planet with finite resources. This is true for all animal species, human and non-human alike. Often, when things such as carrying capacity and limiting factors are discussed, they are referring to non-human wildlife populations.

However, we frequently fail to acknowledge that humans (Homo sapiens) are also animals and are therefore subject to many of the same limiting factors as other animal species. So, let's talk about human population growth.

In the following article, we will look at human population growth, its impact, its history, the relevant mathematical models, and the various options to humanely limit unsustainable population growth.

Human population growth

For starts, let's define population growth.

It is estimated to have nearly octupled since 1800 when the global population was estimated to be around 1 billion. The population is projected to grow until the pressure of numerous limiting factors becomes too great, presumably at some point between the middle and end of the 21st Century. Scientists believe the population will peak at around 10-11 billion people. This exponential human population growth is unsustainable and is one of the primary drivers of climate change, resource depletion, the loss of biodiversity, and pollution.

The growth level results from several factors, particularly the dramatic medical and technological advances made during the 20th Century, which led to significantly increased life expectancy and decreased rates of premature death due to many diseases and environmental factors. Currently, the three countries with the largest populations are China, India, and the United States, though this is expected to change by the end of the century, when India is expected to overtake China and Nigeria is expected to overtake the United States. While many of the countries with the largest populations are projected to remain large, half of all population growth for the remainder of the 21st Century is projected to come from six countries: Angola, the Democratic Republic of the Congo, Ethiopia, Nigeria, Pakistan, and Tanzania (Fig. 1).

Human population growth chart

Figure 1: Past, present and projected future global human population. Source: Pew Research Center

As you can see, in 2100 India will have significantly exceeded China to become the world's single largest population, while China is projected to actually decrease its population by nearly 400 million. The population of the United States is expected to increase by more than 100 million, adding significant strain to natural resources due to the disproportionate amount the U.S. population consumes. Nigeria's population is projected to nearly quadruple and four other African nations are expected to enter the world's most populated countries-the Democratic Republic of Congo (DRC), Egypt, Ethiopia, and Tanzania. The reasons for this massive increase in Africa are complex and numerous, including reduced infant mortality rates, increased birth rates, limited access to contraceptives, and younger age of motherhood. In fact, in the DRC, around 61% of pregnancies are reportedly unwanted and there are significant barriers to contraceptive access. In some areas, this rapid population growth has resulted in the expansion of poverty and slums surrounding major cities, along with the associated environmental degradation and biodiversity loss. This also increases the risk of disease spread, which is particularly worrisome in DRC and Nigeria, where Ebola is present.

Human population growth rate

We can measure the human population’s growth rate using a simple mathematical equation, where the current population size is subtracted from the beginning population size and then divided by the beginning size (Figure 2). Another way to model the population’s growth rate is the logistic growth model using the logistic equation, where the growth rate of the population (rP) is multiplied by one minus the population size (P) divided by the population’s carrying capacity (K) (Figure 3).

While the growth rate has dropped from around 2.2% annually during the early 1960s to around 1.1% annually today, this growth remains unsustainable and, even if the current population size remained stable, the pressure on natural resources would be too great to persist indefinitely.

History of human population growth

Unsurprisingly, the deeper into history population estimates go, the less accurate they become. The earliest population estimates for modern humans, at the time of speciation approximately 200,000 years ago, range from 100,000 to 300,000 people. It is possible that modern humans were pushed to near extinction during prehistory. Some evidence suggests that modern humans may have been reduced to a population of less than 10,000 individuals globally around 70,000 years ago, following the catastrophic eruption of a supervolcano at what is currently Lake Toba in the Indonesian province of North Sumatra. This is known as the Toba catastrophe theory.

Throughout recorded human history there has been a mostly consistent increasing trend, despite occasional dips in the global population due to factors, such as natural disasters. One of the most notable causes of a large drop in the human population within recorded history was the Black Death plague, which is estimated to have killed 75 million people during the 14th Century. Since then, the population has been growing continuously, with the most dramatic growth occurring since the 1950s (Fig. 4).

Perhaps one of the most well-known names in the study of human population growth is Thomas Malthus (1766-1834), an economist and philosopher from Britain who promoted the theory that a population would experience continued growth until this growth was mitigated by factors such as disease and starvation. This became known as the Malthusian Theory. The Malthusian Theory has been criticized as it did not take into account the technological advances in agriculture that helped to increase the supply of food and other resources beyond natural limits.

An American biologist named Paul Ehrlich (1932-) predicted dire consequences for humanity due to overpopulation in his 1968 book The Population Bomb. The book proved quite controversial and some of his predictions proved to be premature or overly pessimistic, but much of what he predicted proved to be accurate, including widespread hunger and malnutrition in the developing world, anthropogenic climate change, loss of biodiversity, and increased spread of disease. Ehrlich suggested methods such as increased taxation on larger families as a means of mitigating population growth.

In 1979, China introduced a "one-child policy" in an attempt to control its population growth. By the beginning of the 21st Century, the policy had become less strict and was replaced by a "two-child policy" in 2015. This was itself removed in 2021 and currently there are no longer any official child limits in China.

Methods to mitigate human population growth

While there have been many controversial methods proposed to control the human population, many humane methods have been proposed. One potentially effective method is through the use of family planning, which consists of better sex education, increased access to contraception and abortion, and promotion of adoption. The promotion of "small families" as the cultural ideal, as opposed to the current ideal of "large families" is another method to cause a shift in cultural values towards a more sustainable population. Sexual equality on a global scale has also been proposed as an effective method, since this lessens the regulations of traditional gender roles and allows a greater number of women to enter the workforce, rather than being pressured to remain at home raising children. A more controversial method proposed involves imposes increased taxes on larger families, along with tax incentives for those with smaller families. The effectiveness and acceptability of such methods vary widely, depending on the local culture and religion.

Human population growth curve

There are two kinds of population growth, both resulting in different growth curves. Exponential growth, which produces a J-shaped curve, is rare in nature and always temporary. Logistic growth produces an S-shaped curve and occurs when population growth is gradually slowed due to limiting factors, until reaching carrying capacity and leveling off (Figure 5). The S-shaped curve is, by far, the most common in nature. The human population has, thus far, produced a J-shaped curve, indicating exponential population growth.

Human population growth worksheet

To finish off, let's look at an example of a worksheet involving population growth.

1.) Create your own human population growth curve utilizing the data below (Table 1). Remember to use the proper scaling (based on the data) and label both axes. Plot dots at each of the time periods and then draw your curve.

Table 1: Human population data (including future projections). Source: United Nations Population Division

2.) Based on the data above, between 1960 and the 21st Century, the human population ________?

a.) Tripled

b.) Doubled

c.) Increased by 50%

d.) Increased by 25%

3.) Based on the graph you created, humans have a ____________ growth curve.

a.) S-shaped

b.) U-shaped

c.) J-shaped

d.) M-shaped

4.) Based on the graph you created, humans have experienced ______________ growth.

a.) Exponential

b.) Decreased

c.) Logistic

d.) Sustainable