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, can alter their environment to increase their carrying capacity, sometimes to the detriment of other species. Earth’s human population and its use of resources are growing rapidly, to the extent that some worry about the ability of Earth’s environment to sustain its human population. 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.

Human technology, particularly our harnessing of the energy contained in fossil fuels, has caused unprecedented changes to Earth’s environment, altering ecosystems to the point where some may be in danger of collapse. Changes on a global scale, including depletion of the ozone layer, desertification and topsoil loss, and global climate change, are caused by human activities.

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 since 1000 AD is exponential.

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 the human growth rate 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 of sewage have drastically improved health in developed nations. Also, medical innovations such as the use of antibiotics and vaccines have decreased the ability of infectious diseases to limit human population growth. In the past, diseases such as the bubonic plague of the fourteenth century killed between 30 and 60 percent of Europe’s population. They reduced the world population by as many as one hundred million. Naturally, infectious disease continues to impact human population growth, especially in poorer nations. For example, life expectancy in sub-Saharan Africa, which increased from 1950 to 1990, began declining after 1985, largely due to HIV/AIDS mortality. The reduction in life expectancy caused by HIV/AIDS was estimated to be seven years in 2005. 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.

Click through this interactive view of how human populations have changed over time.

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 incorporating age structure allow better prediction of population growth and the ability to associate this growth with a region’s economic development level. 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 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. The actual growth rates in different countries are shown in Figure 4, with the highest rates tending to be in Africa and Asia’s less economically developed countries.

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.

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 the population in different countries is shown. Notice that the highest growth occurs in less economically developed countries in Africa and Asia.

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 starved to death despite any crash programs embarked upon now. Nothing can prevent a substantial increase in the world death rate at this late date.”

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 with more than one child. Because 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 in some of the country’s more rural areas. Family planning education programs in other countries have positively affected limiting population growth rates and increasing living standards.

Despite population control policies, the human population continues to grow. The United Nations estimates the world population will be 11.2 billion by 2100. There is no way to know whether human population growth will moderate to the point where the crisis described by Dr. Ehrlich will be averted. Another consequence of population growth 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 our ability to curb human population growth and protect our environment to maintain the carrying capacity of the human species.

Visit this website and select “Launch the movie” for an animation discussing the global impacts of human population growth.


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Introduction to Environmental Sciences and Sustainability Copyright © 2023 by Emily P. Harris is licensed under a Creative Commons Attribution 4.0 International License, except where otherwise noted.

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