Geographers study where and why people live in particular locations. Neither people nor resources are distributed uniformly across Earth. In regards to population growth, geographers emphasize three elements: the population size, the rate of increase of world population, the unequal distribution of population growth. Geographers seek to explain why these patterns exist.

The subject of overpopulation can be highly divisive, given the deep personal views that many people hold. Human geography emphasizes a geographic perspective on population growth as a relative concept. Human-environment interaction and overpopulation can be discussed in the contexts of carrying capacity, the availability of Earth’s resources, as well as the relationship between people and resources.

The study of the human population has never been more critical than it is today. There are over 7 billion people on the planet, but the majority of this growth has occurred in the last 100 years, mostly in developing nations. Humans do not live uniformly around the world, but rather in clusters because of earth’s physical geography. Environments that are too dry, wet, cold, or mountainous create a variety of limiting factors to humans. Two-thirds of the world’s population is located within three significant clusters: East Asia (China), South Asia (India and Indonesia, and Europe, with the majority in East and South Asia.

Demographers, scientists that study population issues, and other scientists say there is more to the story than pure population growth. Ecologists believe that humans have outgrown the Earth’s carrying capacity. There is not enough of the world’s resources to give every human a standard of living expected by most Americans. If all the people on the planet lived the average American lifestyle, it would require over three Earths. At this level of consumption, the earth cannot sustain a population of 7 billion, though we are expected to reach 9 billion by 2100.

Distribution of the World’s Population

Economist Jeffrey Sachs, director of the Earth Institute at Columbia University, believes that there are two reasons why the global population and extreme poverty occur where they do:

• Capitalism distributes wealth to nations better than socialism or communism
• Geography is a significant factor in population distribution in relationship to wealth

For example, the population tends to be lower in extreme environments such as arid climates, rainforests, polar or mountainous regions. Another example is a nation that has a large body of water within its boundaries or has large mineral deposits or resources that are likely to have more wealth and a larger population.

Humans only occupy five percent of the Earth’s surface because oceans, deserts, rainforests, and glaciers cover much of the planet (Figure 2.1.). The term for areas where humans permanently settle is ecumene. Population growth and technology dramatically increase the ecumene of humans, which affects the world’s ecosystems.

It is argued that the world cannot support all the humans on the planet. On some level, that’s true, and on another, it is not. For example, we could pack all 7 billion humans in California, but that is not desirable, sanitary, or sustainable. The reality is that humans cannot live in many parts of the world due to moisture, temperature, or growing season issues. For example, 20 percent of the world is too dry to support humans. This mostly has to do with high-pressure systems around 30 degrees north and south of the equator where constant sunny conditions have created some of the world’s largest deserts. Some of these include the Sahara, Arabian Peninsula, Thar, Takla Makan, and Gobi deserts. Most deserts do not provide enough moisture to support agriculture for large populations.

Regions that receive too much moisture also cause problems for human settlement. These are tropical rainforest regions located between the Tropic of Cancer (23.5 degrees North) and the Tropic of Capricorn (23.5 degrees South). The problem with these regions of the world has to do with the soil erosion due to high precipitation. High levels of precipitation greatly hinder agricultural production because nutrients in the soil are quickly washed away. This is partly why slash-and-burn agriculture occurs in these regions. Locals will burn part of the forest to put nutrients back into the ground. This only works for a short period because the precipitation washes away nutrients within a few years, so farmers move on to other parts of the forest with their slash-and-burn practices.

Additionally, regions that are too cold pose problems for large population clusters and food production. The cold Polar Regions have a short growing season, and many of the Polar Regions have limited amounts of moisture because they are covered by high- pressure systems (much like the desert regions). Thus, cold polar regions are defined by temperature and lack of moisture, despite access to snow, ice, and glaciers. Mountainous and highland regions lack population clusters due to steep slopes, snow and ice cover, and short growing seasons.

Population Profiles

Demographers use various ways to measure and analyze population density. The arithmetic density, also called population density, of a population, is the total number of people in proportion to the area of land. This may not be the best indicator of actual population density because there are many environments humans cannot live comfortably in, including deserts, arctic, tropical forests, and mountainous regions. It also does not consider if the ground is used for producing food. The physiological density of a population is the total population in proportion to the area of arable land suited for agriculture. Even more specifically, agricultural density refers to the number of farmers available compared to arable land. A high agricultural density suggests that the available agricultural land used for farming and the farmers who are capable of producing and harvesting food is reaching its limit for that region. If the demand for food continues or rises, the risk is that there will not be enough arable land to feed their people. In contrast, an area with a low agricultural density has a higher potential for agricultural production. Economically, a low agricultural density would be favorable for future growth.

To understand these methods, let us look at an example. Let us say we have City X, which is home to 10,000 people, 6,000 of whom are farmers, and has a square area of 10,000 kilometers and a farmable square area of 4,000 kilometers. If we look at the arithmetic density, we come up with a population density of 1 person per kilometer (10,000 people/10,000 kilometers). If we look at the agricultural density, we come up with 1.5 people per kilometer (6,000 farmers/4,000 kilometers of farmable land).

Finally, if we look at the physiological density, we come up with 2.5 people per kilometer (10,000 people/4,000 kilometers of farmable land). Each of these numbers tells us something different.

Of these three methods, physiological density is considered the best way to measure population density because it is most reflective of population pressure on arable land. Arable land is any land that is suitable for growing crops. The higher the population density we find from this method, the faster the arable land is going to be used up or reach its output limit. That means there will not be enough land for the people that are coming into the area. In our example, if 100,000 more people moved to the same area, we would end up with a physiological density of 27.5 people per square kilometer (110,000 people/4,000 kilometers of farmable land)

A useful tool used by scientists that focus on demographics is a population profile, also called a population pyramid. A population profile visually demonstrates a particular region’s demographic structure concerning males and females and is often expressed in numbers or percentages.

The following are some characteristics of population profiles:

• A bell-shaped graph will indicate that a country has experienced high population growth in the past but is experiencing a slight decrease.
• Narrow triangles show countries with high population growth.
• As a country’s population boom begins to age, a strange profile shape can develop with a broader top and a narrower base.
• Populations that have stabilized have profiles where the bulge of past high birth rates migrates to older populations moderately and not quickly, while the base has a reasonably smaller but not dramatic base.
• When a country has a large immigrant population, specifically “guest workers” that usually tend to be men, the male side of the graph will be dramatically wider than the women’s side of the graph.
• If a country has experienced war, a catastrophic disaster, or a genocide that eliminates an entire generation, that generation will have a smaller number or percent than the generations before or after. For example, a significant war may cause a reduction in populations in their mid-20s and 30s, which would appear on the profile graph.

Global Population Trends

A region’s population will grow as long as their crude birth rates are higher than their crude death rates. A crude birth rate (CBR) is the total number of live births for every 1,000 people in a given year. So, a crude birth rate of 10 would mean ten babies are born every year for every 1,000 people in that region. Crude death rates (CDR) are the total number of deaths per 1,000 people in a given year.

When comparing CBRs to CDRs, a region’s natural increase rate can be determined. A natural increase rate (NIR) is the percent a population will grow per year, excluding annual migration. Usually, an NIR of 2.1 is required to maintain or stabilize a region’s population. Any more than that and the population will grow, any less than a NIR of 2.1 causes population contraction. The reason why the NIR percent is 2.1 and not 2.0 for stability is because not every human will pair up and have a child because of genetics, choice, or death before childbearing years. Once we know the NIR, we can determine the doubling time. Doubling time is how many years it would take for a defined population to double in size, assuming that NIR stays the same over time. Currently, about 82 million people are added to the world’s global population every year.

Key Factors Influencing Population Change

Three key factors to understand when trying to predict or analyze population change are the total fertility rate, infant mortality rate, and life expectancy at birth. Total fertility rate (TFR) is the average number of children a woman would be expected to have during childbearing years (between 15-49 years old). The global average for TFRs is about 2.5, but in less developed countries, it is as high as 5.0 or higher, and in more developed countries, it is as low as 2.0 or less. Fertility patterns can vary widely within countries. Racial and ethnic minorities may have higher fertility rates than the majority, and families with low incomes or low levels of education typically have more children than those that are affluent or well-educated. Women who work outside the home typically have fewer children than those who stay home, and rural families tend to have more children than city dwellers. In 2016, the number of births per 1,000 people worldwide was 20, with extremes ranging from a low of 8 or 9 (mainly in Northern and Western Europe and Hong Kong), to 60 or more in a few West African nations (Population Reference Bureau, 2016 World Population Data Sheet, pp. 10-19).

Mortality is the second significant variable that shapes population trends. A population’s age structure is an essential factor influencing its death rate. Death rates are highest among infants, young children, and the elderly, so societies with many older adults are likely to have more deaths per 1,000 people than those where most citizens are young adults. Developed countries with excellent medical services have more people in older age brackets than developing countries, so the developed societies can have higher death rates even though they are healthier places to live overall. Infant mortality rate (IMR) is determined by calculating how many children die before the age of 1 per 1,000 live births annually. The highest IMRs are in less developed countries where rates can be as high as 80 or more. Conversely, in a place like Europe, it is as low as 5 percent.

Life expectancy at birth is straightforward—it is an average of how many years a newborn is expected to live, assuming that mortality rates stay consistent. In more developed countries, the average life expectancy is over 80 years old, and in less developed countries, it is only around 40 years (Figure 2.6). When we compare CBRs, CDRs, and TFRs, we find that the world has a large population of youth with the most substantial percent in less developed countries. This causes high stress on the education systems and, to some extent, the health care systems in poorer countries. However, more developed countries tend to have older demographics, which tends to cause stress on the health care and social safety nets of those countries. The dependency ratio discussed later in this chapter, is used to understand these stresses and is the number of people who are too young or too old to work compared to the number of people who are in their “productive years.” The larger the ratio, the greater the economic stress on those nations.