The concept of optimum population size is nothing new, but in recent decades the methodology used to calculate it has seen little development. In a recently published book, We Zijn Met Te Veel (Dutch for We Are Too Many), a new approach is proposed for calculating optimum population size.
by Fons Jena
In the available literature on calculating optimum population size one finds a common approach. Researchers all start with an assumption about the impact of a desirable standard of living, then calculate optimum population size by dividing the available carrying capacity by the average individual impact. This is a logical method when you try to determine the maximum number of people who can live on the planet at an acceptable living standard, but it is hardly a population size that can be called ‘optimal’. To find the optimal population size, we need to ask where the balance lies between the disadvantages of having too few people and those of having too many.
I defined this ‘optimal condition’ in a previous blog post with the help of the SNQ model. This model states that the optimum is a condition where three parameters are maximized: the quality of life of all individuals, sustainability (a sustainable balance between environment and economy), and the room available for wild nature. The model uses four factors that define these three parameters: technology, culture, environment, and scale (which stands for population size). The impact of the scale factor on the three parameters is shown in figure 1. Sustainability and available room for nature are shown with a decreasing line as population size increases. Quality of life is a less straightforward relation, as arguably greater numbers and social complexity can provide material and social benefits that improve it — up to a point. But after achieving a certain population pressure, the disadvantages of scale start to outweigh the benefits and quality of life begins to decrease.
According to the SNQ model, optimum population size is the condition where quality of life is maximized, but with minimal impact on the environment such that the sustainability and room for nature parameters also are maximized. This point is represented in figure 1 by the letter A. The letter B represents our current condition, where population pressure is already reducing quality of life and biodiversity in most of the world.
The SNQ criteria for optimum population size
This theoretical ‘SNQ optimum’ must be translated into practical guidelines that we can use for calculating a corresponding optimum population size. For this I have defined four criteria. The first requirement, maximizing quality of life, means that a community should be large enough so that every member can obtain the material benefits they want, yet it must remain small enough so that social dynamics and the built environment remain on a human scale. This first requirement can be stated thus:
(Criterion 1) The population size of a community is limited to the minimal size necessary to provide each inhabitant of that community with all the necessary goods, services, and space that contribute to a higher quality of life.
This first criterion defines the population size of a single community; now we need to find the ideal number of communities. One community would be very sustainable (and it would leave lots of room for nature) but it has two main shortcomings. First, humanity would be very fragile; it could be wiped out if that single settlement was hit by a meteor or a pandemic. Second, the cultural and ethnic diversity within our species would be very limited. This diversity is a prerequisite for a higher quality of life because it enriches both the cultural and technological factors of the SNQ model. A third shortcoming is that not all the materials needed for a modern lifestyle are present in one location, hence some long-distance trade enhances material living standards. This leads to the following criterion:
(Criterion 2) The number of communities is at least the minimum number necessary to access sufficient geographically distributed resources and maintain the remaining diversity within the human species and ensure the long-term survival of that diversity.
These two criteria translate into the ‘quality of life’ parameter; now the ‘room for nature’ and sustainability parameters must be translated into additional requirements that the optimal population size must meet. Translating the ‘room for nature’ parameter is straightforward because it corresponds to the physical area other species need to survive and thrive:
(Criterion 3) The number, distribution and location of human communities are limited in such a way that they do not reduce biodiversity and render plant or animal species vulnerable to extinction.
Finally, the sustainability parameter can be translated into a criterion that uses the concepts of ecological footprint and carrying capacity. To further ensure the sustainability of communities, their essential resources must be acquired locally, without violating the previous three criteria:
(Criterion 4) The size and distribution of a community are limited so that its impact remains well below the carrying capacity of the local environment in terms of food, energy, renewable resources and ecosystem services.
Calculating the optimal population size
These four criteria may still be too abstract to be used for calculating an optimum population size, but they can be approximated with available data. Each criterion represents a step in the calculation process.
Step 1: defining the optimal size of one community (about 300.000 citizens)
For calculating the optimal size of a single community, it is modeled as a single urban center with a rural belt. Urban centers are necessary for certain economic and cultural activities and rural areas are equally important, as they not only provide material resources for the urban centers, but are important for quality of life and the mental and physical health of all residents. For the optimum size of an urban center I have found a consensus in the literature that 100.000 inhabitants is the upper limit, as such a size seems to combine most of the sociological, economic and political benefits of bringing people together while preventing as much as possible the disadvantages of increasing population pressure, such as crime, social disintegration, congestion, filth and pollution. For the ratio between the urban and rural population I refer to the spatial distribution of the European population prior to the twentieth century population explosion, when about 1/3 of the population lived in urban centers. With these two figures we can determine the optimum size of one community, which would be about 300.000 inhabitants.
Step 2: defining the minimal number of communities (about 4000 communities)
The second criterion needs to specify the minimal number of communities that are necessary for maximizing quality of life. For this second criterion I used the UN’s list of cities with a population higher than 100.000 inhabitants. This list contains about 4000 urban centers. Using each of these urban centers as the center of one community, we can say that the optimum population size of the world is 1,2 billion.
Step 3: calculating available carrying capacity (57 million km²)
This provisional value for the optimum global population size must now be checked against the other two criteria: sustainability (balance between environment and economy) and room for nature. The third criterion could be approximated by the Half Earth principle advocated by E.O Wilson, meaning that only half of the world’s biologically productive land area could be used intensively by humanity. Most conservation biologists agree that reserving this much habitat for other species is necessary to avoid a mass extinction. Using ecological footprint data from the Global Footprint Network (GFN) and halving it, we obtain an available productive land surface of 5,7 billion ha (which is 37% less than what is used today). This means that with 1,2 billion people restraining themselves to use only about half of available biological productivity, we would have 4,75 ha available per capita.
Step 4: checking the sustainability of the optimum population size
To check if the proposed optimum population of 1,2 billion would not violate the fourth SNQ criterion, we must check if 5,7 billion ha would be enough to supply all necessary material resources. Estimating the ecological footprint of a person in a sustainable world is difficult, but data from the GFN can provide a rough approximation. First, the land surface necessary for CO2-compensation would be dramatically reduced as fossil fuels would not be used. Second, all products and resources that are now produced by fossil fuels would have to be produced by biomass (such as hemp, wood, and cotton). This could increase the average ecological footprint considerably. In my calculation I used an average individual ecological footprint of 3 ha as an approximation. Since this number is well below the available 4,75 ha at 1,2 billion, we may conclude that the calculated optimum population size of 1,2 billion meets all SNQ criteria. That this population size is sustainable should not surprise us, since it is equal to the world population around 1850, when the use of fossil fuels was very limited.
Notes and further refinement
Since this method for calculating the optimum population size is a work in progress, it has some shortcomings that must be addressed. First, using a list of recent cities with populations higher than 100.000 for the second criteria is arbitrary and does not necessarily assume an optimal situation for maintaining cultural and ethnic diversity. In my book, I argue that the current global population size is an unwanted side effect of a positive economic and social evolution (the demographic transition), which means that the population size and distribution prior to the population explosion should be used as a reference. Using data from around 1850 would result in a much lower optimum population size, but this could violate the redundancy criterion (criterion 2).
I also made an abstraction of the exact distribution of the 4000 communities. Since a sustainable community must be able to find its energy, food and other basic resources in its local environment, it is likely that many existing communities are located in unfavorable locations that do not meet the third and fourth criteria.
Though the Half Earth principle for land sharing has some scientific foundation, it is used here more as a moral imperative. Ethically, I find it hard to argue that humanity has a right to use more than half the world’s biological productivity and resources, especially since doing so would extinguish numerous unique forms of life. It could also be argued that the share for other species should be greater than half. In the optimal scenario calculated above, the actual share for nature is higher than half, since only 3 ha per capita of the 4,75 ha available is used.
The approximation used for ecological impact in an ideal world can be further refined. I haven’t found any calculations for ecological footprint in a fossil-fuel free world, which I find curious. It would be interesting to know what percentage of Earth’s surface would have to be devoted to sustaining people if all fossil fuel-based products were replaced by sustainable alternatives. The same can be asked about available carrying capacity, where I used GFN’s data because they are easily available, although their calculations are based in a fossil-fueled world.
The above method can also be used to calculate the optimum population size of specific regions, down to the optimal size of a single community. In these cases some additional shortcomings must be addressed. As an example, in the book I used this method to calculate the optimum population for Belgium, but since it has far too many cities for its available land surface, I had to further limit city size or the urban-rural ratio. You can decrease the population of the urban center to 75.000 inhabitants and have an equal rural population, so that the optimal size of one community is only 150.000 instead of 300.000 people. Other possibilities to reconcile reality with the ideal world involve loosening the third and fourth conditions, so that the Half Earth and basic resource self-sufficiency principles are not fully met for a certain region. It can be argued that not every community or region must be fully self-sufficient in its basic needs, or that on a local scale nature may not need half of the available productive land area. Also, for calculating regional optimum population size the calculation could be reversed, so that you start by calculating the available land surface and then check how many people have to be distributed between the existing cities.
Finally, it can also be argued that the translation from the theoretical SNQ model to the more practical SNQ criteria needs some adjustments. The four criteria seem logical to me, but maybe they can be stated differently, or perhaps additional criteria are required. For example, one could add about 400 ‘megacities’ of 500.000 inhabitants each, since many people feel attracted to larger cities. This would increase the calculated optimum population size to 1,4 billion and still not violate the other criteria.
This blog post summarizes the second appendix of the book ‘We Zijn Met Te Veel’ (We Are Too Many). It was added to the book as an impetus for further study and refinement, so any feedback is welcome. The book is currently only available in Dutch, but the author wishes to publish an English translation in 2022. For our Dutch-speaking audience the book can be ordered through www.wezijnmetteveel.be. If you are interested in helping with the English translation, or finding a publisher, you can contact the author via email@example.com.
 A summary of some different studies on optimum population size can be found here: https://overpopulation-project.com/what-is-the-optimal-sustainable-population-size-of-humans/.
 As mentioned in my previous blog post, in reality this relation isn’t strictly linear, as the available technology can both increase or decrease an individual’s impact on sustainability and room for nature.
 Kirkpatrick Sale. Human Scale Revisited. Chelsea Green Publishing: White River Junction, 2017. The concept of ‘slow cities’ uses a limit of 50.000 inhabitants. Since I want to maximize sustainability and room for nature, I could have used an urban population size of 75.000 instead of 100.000. I have chosen the upper limit just to be sure to have a sufficiently capable community.
 ‘How much wild nature do people need?’, www.natureneedshalf.org; ‘The plan to turn half the world into a reserve for nature’ (19 March 2020), www.bbc.com; ‘Half of all land must be kept in a natural state to protect Earth’ (19 April 2019), www.nationalgeographic.com; E.O. Wilson, Half-Earth: Our Planet’s Fight for Life. W. W. Norton & Company: New York, 2016.
 David Pimentel, et al., 2010, “Will Limited Land, Water and Energy Control Human Population Numbers in the Future?” obtains a land use of 4 ha per capita of which 1,5 ha is used for food, 1 ha for wood products and 1,5 ha for energy. If we limit the use of biomass for energy production for car fuel, then the 1,5 ha can be a significantly reduced, so I use a total footprint of 3 ha as an approximation.