4.4 The Human Population

Joni Baumgarten

4.4 The Human Population

Section Goals:

Apply population growth models to the current global population.
Understand how age structure of a population relates to predicted future population size.
Consider population growth in different countries.

The Human Population

Concepts of animal population dynamics can be applied to human population growth. Humans are not unique in their ability to alter their environment. For example, beaver dams alter the stream environment where they are built. Humans, however, have the ability to alter their environment to increase its carrying capacity, sometimes to the detriment of other species. Earth’s human population and their use of resources are growing rapidly, to the extent that some worry about the ability of Earth’s environment to sustain its human population.

Graph plots the world population growth from 1000 AD to the present. The curve starts out flat and then becomes increasingly steep. A sharp increase in population occurs around 1900 AD. In 1000 AD the population was around 265 million. In 2000 AD it was around 6 billion. — Figure 1: Human population growth has been exponential since 1000 AD.

Long-term exponential growth carries with it the potential risks of famine, disease, and large-scale death, as well as social consequences of crowding such as increased crime. Please note that there is an uncomfortable and often ignored history of overconcern from members of “upper” society with the populations of people viewed as “less-than”, e.g., the people who believed in eugenics. Therefore, this topic is one where careful thought and additional reading are highly encouraged to ensure that structural inequalities are not perpetuated as we as a global society deal with the consequences of climate change.

Human technologies, most particularly our harnessing of the energy contained in fossil fuels, have caused unprecedented changes to Earth’s environment, altering ecosystems to the point where some may be in danger of collapse. There is overwhelming scientific consensus on this fact. Human-caused changes on a global scale including depletion of the ozone layer, desertification and topsoil loss, and global climate change. These challenges will be addressed more directly in the second half of this textbook (starting with Chapter 6). Because the way societies use resources is linked to individual choice, the fact that the population has accelerated since 1960 (Figure 2) is of note.

Human Population Growth

Bar graph shows the number of years it has taken to add each billion people to the world population. By 1800, there were about one billion people on Earth. It took 130 years, until 1930, for the population to reach two million. Thirty years later, in 1960, the population reached three billion, and 15 years after that, in 1975, the population reached four billion. The population reached five billion in 1987, and six billion in 1999, each twelve years apart. Currently, the world population is nearly seven billion. The population is projected to reach eight billion in 2028, and nine billion in 2054. — Figure 2: The time between the addition of each billion human beings to Earth decreases over time. (credit: modification of work by Ryan T. Cragun)

The fundamental cause of the acceleration of growth rate for humans in the past 200 years has been the reduced death rate due to changes in public health and sanitation. Clean drinking water and proper disposal sewage has drastically improved health in areas where it is implemented. Also, medical innovations such as the use of antibiotics and vaccines have decreased the ability of infectious disease to limit human population growth.

In the past, diseases such as the bubonic plaque of the fourteenth century killed between 30 and 60 percent of Europe’s population and reduced the overall world population by as many as one hundred million people. Naturally, infectious disease continues to have an impact on human population growth. For example, life expectancy in sub-Saharan Africa, which was increasing from 1950 to 1990, began to decline after 1985 largely as a result of HIV/AIDS mortality.

Technological advances of the industrial age have also supported population growth through urbanization and advances in agriculture. These advances in technology were possible, in part, due to the exploitation of fossil fuels. The increase in agricultural yields and the stability of the global food system since the widespread implementation of inorganic fertilizers, called the Green Revolution, also have contributed to human population growth by supporting the growth (see Chapter 8).

Age Structure, Population Growth, and Economic Development

The age structure of a population is an important factor in population dynamics. Age structure is the proportion of a population in different age classes. Models that incorporate age structure allow better prediction of population growth, plus the ability to associate this growth with the level of economic development in a region. Countries with rapid growth have a pyramidal shape in their age structure diagrams, showing a preponderance of younger individuals, many of whom are of reproductive age (Figure 3). This pattern is most often observed in underdeveloped countries where individuals do not live to old age because of less-than-optimal living conditions, and there is a high birth rate. Age structures of areas with slow growth, including developed countries such as the United States, still have a pyramidal structure, but with many fewer young and reproductive-aged individuals and a greater proportion of older individuals. Other developed countries, such as Italy, have zero population growth. The age structure of these populations is more conical, with an even greater percentage of middle-aged and older individuals.

For the four different age structure diagrams shown, the base represents birth and the apex occurs around age 70. The age structure diagram for stage 1, rapid growth, is shaped like a deflated triangle that starts out wide at the base and rapidly decreases to a narrow apex, indicating that the number of individuals decreases rapidly with age. The age structure diagram for stage 2, slow growth, is triangular in shape, indicating that the number of individuals decreases steadily with age. The age structure diagram for stage 3, stable growth, is rounded at the top, indicating that the number of individuals per age group decreases gradually at first, then increases for the older portion of the population. The final age structure diagram, stage 4, widens from the base to middle age, then narrows to a rounded top. The population type indicated by this diagram is not given, as this is part of the art connection question. — Figure 3: Typical age structure diagrams are shown. The rapid growth diagram narrows to a point, indicating that the number of individuals decreases rapidly with age. In the slow growth model, the number of individuals decreases steadily with age. Stable population diagrams are rounded on the top, showing that the number of individuals per age group decreases gradually, and then increases for the older part of the population.

Long-Term Consequences of Exponential Human Population Growth

Many dire predictions have been made about the world’s population leading to a major crisis called the “population explosion.” In the 1968 book The Population Bomb, biologist Paul R. Ehrlich wrote,

“The battle to feed all of humanity is over. In the 1970s hundreds of millions of people will starve to death in spite of any crash programs embarked upon now. At this late date nothing can prevent a substantial increase in the world death rate.”

While these predictions obviously didn’t bear fruit, the laws of exponential population growth are still in effect, and unchecked human population growth cannot continue indefinitely. Efforts to moderate population control led to the one-child policy in China, which imposes fines on urban couples who have more than one child. Due to the fact that some couples wish to have a male heir, many Chinese couples continue to have more than one child. The effectiveness of the policy in limiting overall population growth is controversial, as is the policy itself. Moreover, there are stories of female infanticide having occurred in some of the more rural areas of the country. These results are not the goal. However, the United Nations estimates the future world population size to be 11.2 billion people by the year 2100. It is hard to imagine how that number of individuals will be supported at full health given the challenges we already see happening.

Another consequence of population growth that can be avoided without direct control of human population is the change and degradation of the natural environment. Many countries have attempted to reduce the human impact on climate change by limiting their emission of greenhouse gases. However, a global climate change treaty remains elusive, and many underdeveloped countries trying to improve their economic condition may be less likely to agree with such provisions without compensation if it means slowing their economic development. Furthermore, the role of human activity in causing climate change has become a hotly debated socio-political issue in some developed countries, including the United States, despite the overwhelming scientific evidence. Thus, we enter the future with considerable uncertainty about the interaction of human population growth with the environmental impact predicted should less developed countries consume resources at a rate similar to that in the United States.

The actual growth rates in different countries are shown in Figure 4, with the highest rates tending to be in the less economically developed countries of Africa and Asia. Again, to reiterate the point at the beginning of this section: This topic has a history steeped in judgmental views (e.g., white supremacy). Please do not use this figure to judge or to perpetuate simplified perceptions of people.

Percent population growth, which ranges from zero percent to three plus percent, is shown on a world map. Europe, Northern Asia, Greenland, and South Africa are experiencing zero percent population growth. The United States, Canada, the southern part of South America, China, and Australia are experiencing zero to one percent population growth. Mexico, the northern part of South America, and parts of Africa, the Middle East and Asia are experiencing one percent population growth. Most of Africa and parts of the Middle East and Asia are experiencing two percent population growth. Some parts of Africa are experiencing three percent population growth. — Figure 4: The percent growth rate of population in different countries is shown.

Attribution

Human Population Growth by OpenStax is licensed under CC BY 4.0. Modified from the original by Joni Baumgarten.

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