Last week I was triggered to search for the facts in the discussion about the current human population on our planet. With unprecedented population size and population growth, it is an important matter to investigate the limits of our world. Do we need to seriously start worrying about our population growth? Did we already surpass the maximum sustainable population size? And if so, what is then the estimated maximum population? And finally, does the world need a ´Massive Ebola’ to get rid of its excess inhabitants?
According to Wikipedia, the definition of overpopulation is when the number of individuals of a species is bigger than the maximum carrying capacity. The maximum carrying capacity can be simplified to the maximum number of mouths a region can feed. But this would severely underestimate the complexity of the topic, as the maximum carrying capacity is by far not only dependent on the maximum amount of food available. Other resources can act directly and indirectly as a constraint on the carrying capacity as well, for example fresh water, biologically accessible nitrogen, soil, minerals, etc. Next to that, the maximum carrying capacity can also be dependent on time, since we are able to use energy from the past (fossil) and make all kinds of products with it to increase the carrying capacity temporarily. It should not be misunderstood that nowadays the maximum carrying capacity of a region can be way higher than the ‘natural’ carrying capacity of that same region. Of course it is silly to not take into account that we all share one planet with the whole human population. Eventually, we are forced to respect this natural carrying capacity of the earth. Once we exceed this capacity, our population would (partially) starve and decrease until the number of population reaches a sustainable number again.
So to be concise, let’s try to take a look in how many seats the world has to offer us. There are different ways of calculating the maximum human carrying capacity of the world, all with different outcomes. Without hard numbers it is hard to address the problem effectively. One of the vital parameters to the survival of humans is access to fresh water. We could divide the total amount of renewed fresh water per time period by the amount of water an average human uses in that same time period. Same can be done with food, but food production is related to the availability of fresh water and other resources as well. Different studies have used different methods leading to different results. The intertwining of parameters and the uncertainty in the further development of technologies makes it a rather slippery terrain. Still some hard numbers can be found in literature; the result of a meta-analysis of almost 70 studies led to a best point estimate maximum carrying capacity of 7.7 billion humans, the lower and upper bounds be 0,65 billion and 98 billion, respectively.
What does it mean when the United Nations project the human population to reach 9,3 billion in 2050 and 10.1 billion in 2100? Are we close to entering a zone where limits of the carrying capacity on earth may be encountered? It is safe to answer confirmative on this question, as those projections lie significantly above the best point estimate of 7.7 billion in the meta-analysis of Van Den Bergh (2004). The prospect will not be pleasant at all, as thousands of people would be forced to move as economic, warfare of environmental refugees. Thousands would starve and suffer malnutrition, not to speak of the possible effect of water shortages and poisoning. But wait, is this not already happening?
Is it the rotten economic model we adapted in our modern world or is it the approaching human carrying capacity of the host planet leading to the millions suffering from basal problems today? Can we not provide clean water, electricity and food to the poorest of sub-Saharan Africa (for example) because those resources aren’t available sufficiently? Or because ‘they’ cannot pay for it but ‘we’ can? Can it be true that in a world with enormous economic and wealth inequality the maximum carrying capacity is reached? Let me be clear on this: personally I do sincerely believe that population growth should be slowed down, stopped and preferably reversed in the coming years. To opt for more babies in a house that isn’t remodeled yet sounds like a bad plan to me. But it is not the Earth's maximum carrying capacity that causes millions to suffer from the lack of basic human needs, it is the way how we design our economy. This is the house we live in.
Remodeling the metaphorical house would in my eyes mean: transform all processes where resources are involved from linear models to circular models. If done right, the only resource that should be added constantly is energy. And guess what, we do have a pretty nice source of renewable energy for the coming 5 billion years: the sun. A short calculation with the corrected and average energy budget of 240W/m² and the average disk surface of the earth leads to a total irradiance on the total surface of 3,06*1016W. In the case that we are able to cover 5% of the planet with technologies that harvest 5% of the energy available with a human population of 10 billion people, we would have 7650W available per person. This is a number that can be compared to the TOTAL energy use of a European citizen nowadays (6000W).  With other words, using these assumptions, it is possible for 10 billion people to live on this planet with the life standards of modern European citizens.
The house needs to be remodeled before more people can move in, or the house will collapse. To opt for an epidemic will not cure the decease that prevails, how ironic and ambiguous that may sound. A sudden decrease of human population will just lead to a moratorium on execution, namely an insurmountable mismatch between human needs and planets gifts. We don’t need a ´Massive Ebola’. We need a complete and ruthless transition towards circular economies driven by renewable energy.
https://en.wikipedia.org/wiki/Human_overpopulation, visited on 1-12-15
 Cohen, Joel E., (1995). “Population growth and Earth’s Human Carrying Capacity”, Science, New Series, Vol 269, No. 5222, 341-346.
 Van Den Bergh, Jeroen C. J. M.; Rietveld, Piet (2004). "Reconsidering the Limits to World Population: Meta-analysis and Meta-prediction". BioScience 54 (3): 195.
"World Population Prospects:The 2010 Revision Press Release" (PDF). Population Division of the Department of Economic and Social Affairs of the United Nations Secretariat. May 2011.
http://www.treehugger.com/corporate-responsibility/you-are-entitled-to-2000-watts-use-them-wisely-updated.html, visited on 2-12-2015
Ryerson, W. F. (2010). "Population, The Multiplier of Everything Else". In McKibben, D. The Post Carbon Reader: Managing the 21st Century Sustainability Crisis. Watershed Media