If world population exceeds the world’s carrying capacity, it will result in misery, privation, and death. Unfortunately, the world’s carrying capacity is not well known.

In the previous post, I reviewed some data regarding world population. In this post I will discuss reasons that population is an essential concern for environmentalists, and try to draw out some consequences of the data in the previous post. In doing so, I am going to concern myself with world population size, not with how the population is distributed among countries, states, age ranges, genders. races, or ethnicities.

Only two things affect the population of the world: the number of births and the number of deaths. Births add to the population, and deaths subtract from it. As long as they match each other, the population of the world will neither rise nor fall. But if more people are born or fewer people die, then population will grow. On the other hand, if fewer people are born or more people die, then population will shrink.

The world has a carrying capacity, that is, a maximum population that it can sustain over time. This carrying capacity is limited by the availability of food, water, habitat, and other necessities. If the population exceeds the carrying capacity, then some of the people will not be able to obtain the necessities they need. The result will be misery, illness, and even death.

Figure 1. An S-Curve Describes Population Growth Over Time. Source: Brodnick 2016.

Figure 1 shows an s-curve, which is the curve demographers use to describe population growth. At the start, the population is low, so there are very few adults to reproduce. Consequently, population grows slowly. As the population grows, however, more and more adults are available to reproduce, so the population grows faster and faster. The growth rate continues to accelerate until the population becomes so large that competition for necessities increases. As it becomes harder and harder to gain life necessities, more and more individuals are unable to obtain them, and population growth slows. There is an eventual point above which the population cannot be sustained over time, and this is the carrying capacity.

This rather bland description hides a terrible fact: population growth slows because individuals have difficulty obtaining the necessities they need to survive. The result is hunger, disease, malnutrition, and squalor – in short, misery, privation, and even death. The idea here is that for any given level of resources, the population will expand to the point where one of two things will occur: either individuals will not be healthy enough to reproduce, lowering the birth rate, or the death rate will increase to match the birth rate. Either is terrible to contemplate.

The world is never without misery and privation. So long as population is below the world’s carrying capacity, however, it results from our inability (or unwillingness) to distribute life’s necessities to all people. On the other hand, if the world’s population exceeds its carrying capacity, the misery and privation will occur because there is not enough to go around. The amount of misery and privation will skyrocket. It is even possible that such a large population would cause so much environmental stress that it would cause a general environmental collapse, resulting in a world that had a much diminished capacity to support life. Such an outcome is not certain, but it is possible.

Figure 2. Data source: U.S. Census Bureau

Figure 2 is repeated from the previous post. It shows historical world population going back to 10,000 BCE. It is easy to see that the general shape of the curve matches the left hand side of Figure 1. The implication is that at some point, the increase in population could result in a scarcity of resources, resulting in an increase in misery, privation, and death on a global scale.

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Figure 3. Snowshoe Hare and Wolf Population Dynamics. Source: Swatski 2010.

Overpopulation, leading to a population crash, is a dynamic that has been seen with many animals: in times of plenty they multiply beyond the carrying capacity of their local environment, and then there is a huge die-off. One example would be snowshoe hares, as shown in Figure 3. The blue line represents the population of hares, which repeatedly spikes and crashes. The red line shows the population of wolves, which prey on snowshoe hares. Shortly after the hare population spikes, so does the wolf population. Shortly after the hare population crashes, so does the wolf population. The hares multiply because there are so few of them that food is abundant, and they thrive. As their number spikes, however, tragedy occurs: they consume all the available food and then starve; they get crowded together and diseases break out; and there are more wolves to eat them. The wolves multiply because their food source, the hares, becomes abundant. Nothing eats the wolves, however. They die off because their food supply (the hares) has crashed, and they starve. Also, the increased crowding makes them vulnerable to diseases, as it did the hares.

Humans, of course, consume more than food. We need many different kinds of necessities to sustain our modern way of life. Thus, our population strains the environment in many, many ways. Air pollution, water pollution, climate change, water scarcity, habitat destruction, species extinction, desertification, forest fire – all are made worse by the size of our population. The concern is that, if we have exceeded the carrying capacity of the earth, humanity will suffer a die-off

People have been trying to estimate the carrying capacity of the earth for more than 200 years, and their estimates have ranged from less than 500 million to 1 sextillion. Methods of estimation have grown in sophistication over the years, and most modern estimates put it between 8 and 16 billion.

Figure 4. Planetary Boundaries. Source: Steffen et al, 2015.

On the other hand, the Stockholm Resilience Centre has identified 9 essential ecological systems that must remain within certain limits. If the planet remains inside these boundaries, humanity can continue to develop and thrive. If the planet does not, they believe, the earth’s ability to support life will be greatly degraded. Their assessment of the planet’s status is shown in Figure 4. The 9 systems are like wedges of a pie. Inside the green circle represents the safe area, and the red circle represents the maximum limit that must not be exceeded. We have already violated the boundaries in 2 of the 9 areas: biogeochemical flows, and biosphere integrity. We are approaching the boundary in 2 others: land-system change and climate change. The scientists at the Stockholm Resilience Center believe that the 9 areas interact and are mutually dependent: if one of them goes beyond its boundary, it will cause an imbalance that will draw the others out of their boundaries, too. Thus, according to their research, we have already gone too far, and need to go back.

Whether their work is precisely correct or not, it requires no genius to understand that the world is experiencing unprecedented environmental stress. It is occurring despite the fact that population has yet to reach 8 billion. Thus, I feel that estimates of the worlds carrying capacity tend to be overly optimistic. They may be something like theoretical estimates that cannot be achieved in real life.

The fact that we are so severely stressing the world suggests that there are already too many of us. What the right number would be, I think nobody knows. But the above discussion certainly illustrates why population is an important concern for environmentalists. Our well being depends on not exceeding the earth’s carrying capacity, but we may have done so already.

Sources:

Brodnick, Robert. 2016. Curves that Matter: S-Curve. Robert Brodnick. https://www.linkedin.com/pulse/curves-matter-s-curve-robert-brodnick.

Malthus, Thomas. 1798. An Essay on the Principle of Population. London, England: J.Johnson. Available online at The Electronic Scholarly Publishing Project, http://www.esp.org/books/malthus/population/malthus.pdf.

Steffen, Will, Katherine Richardson, Johan Rockstrom, Sarah Cornell, Ingo Fetzer, Elena Bennett, R. Biggs, Stephen Carpenter, Wim de Vries, Cynthia de Wit, Carl Folka, Dieter Gerten, Jens Heinke, Georgina Mace, Linn Persson, Veerabhadran Ramanathan, B. Reyers, and Sverker Sorlin. 2015. “Planetary Boundaries: Guiding Human Development on a Changing Planet.” Science, 1/16/2015. Downloaded 8/13/2017 from https://stockholmuniversity.app.box.com/s/v8q2noqkkwk60o3uikyuy3txr7ifycc8.

Swatski, Rob. “Boom-and-Bust Cycles.” BIOL 101 Chp 53: Population Ecology. Downloaded 8/14/2017 from https://www.slideshare.net/robswatski/biol-101-chp-53-population-ecology.

United Nations Environment Program, Global Environmental Alert Service. 2012. One Planet, How Many People? A Review of Earth’s Carrying Capacity. Downloaded 8/11/2017 from https://na.unep.net/geas/archive/pdfs/GEAS_Jun_12_Carrying_Capacity.pdf.

U.S. Census Bureau. World Population: Historical Estimates of World Population. https://www.census.gov/population/international/data/worldpop/table_history.php.

Wikipedia. List of Famines. Viewed online 8/14/2017 at https://en.wikipedia.org/wiki/List_of_famines.