What is the maximum sustainable human population? 
My own view is that it's about 600 million.
Other estimates given by people who either provide a credible methodology or who are known as thinkers of some authority are:
<100 million - note 19
300 million - note 1
500 million - note 12
600 million - note 10
<1,000 million - note 14
Up to 1,500 million - note 11
1.5 to 2 billion - note 18 (optimum, not maximum)
Methodology - I arrive at my figure of 600 million by looking at the global population of our species before we began burning fossil fuels in significantly large quantities: it was about 1 billion around 1815. We do not assert that 1 billion was a ceiling which could not be penetrated without fossil fuels, nor that the increase from 1 billion to 6.8 billion has all been directly due to the use of fossil fuels, but we can say that directly and indirectly the vast majority of the increase has been underwritten in some way by the drawdown of "ancient sunlight"   .
However, we climbed to the 1 billion level by unsustainable 'mining' of forests for fuel, building materials (for ships, buildings, and structures like dykes and bridges) and crop land.  So let's say 900 million was the maximum sustainable - in that pre-consumerism world. That is, even by 1815, we were drawing down capital in the form of forests  - faster than they were being replenished - in order to support the full billion.
Since 1815, we have degraded, eroded and destroyed prime agricultural and horticultural land, polluted and paved over other land , depleted aquifers and introduced so much toxic waste to land, air and sea, that the complex matrix of "environmental services" that existed in 1815 when we numbered 1 billion has been so degraded that I estimate the 900 million (referred to above) has actually contracted to 600 million now . John Gray refers to this as our increasingly 'prosthetic environment'.
Discussion - Unfortunately, I have to qualify the 600 million as well. I see 600 million as sustainable only if we do not continue to deplete our ecosystem services or suffer serious effects of climate change. That is, Homo sapiens has to be on its best biosensitive behaviour to survive over the long term with 600 million. If we lived biosensitively, the question of the "maximum sustainable human population" would be of little interest, because we would be content to live well within our ecological limits. If we daydream for a moment and imagine a human population that was not only fully biosensitive, but in which all members actively strived to live and work in broad alignment with the principles of Permaculture, Biodynamics and other farming practices which focus on enriching the soil naturally, then the question of a maximum would also be irrelevant (because we would not allow ourselves to press up against the environmental limits - though with sound soil restoration practices these limits would be in excess of 1 billion, and possibly well over 3 billion). But let's not daydream for too long; this is not the way Homo sapiens behaves . Homo sapiens, like all other species, goes through cycles of boom  and bust and the boom leads inevitably to the bust. Garrett Hardin explained why this is so in his 1968 essay The Tragedy of the Commons. He also explained why this can only be so.
Further considerations - If James Lovelock is right about climate change, then even 600 million is a dream and Homo sapiens will be reduced to "a few breeding pairs" in the mosquito-ridden Arctic tundras by 2100. More recently, he has predicted that, because of climate change, by 2100 the human population will be "a billion or less".  This is a different take on the topic; most estimates on this page look at a maximum sustainable human population on the assumption that climate change, pollution and biodiversity loss will not have further serious effects on food production, soil quality, rainfall, human health, political and social stability, military action etc. Also that humans will not undertake risky geo-engineering projects to mitigate the effects of climate change which have unintended deleterious consequences and that civilization is not affected by peak oil (and 'peak everything' for that matter). When we look at the results of the ways humans think, it is difficult to imagine a realistic 'way out'.  Faith in human ingenuity and expectations of technical solutions assume our species will take informed, rational decisions for the long term good. 
Incidentally, when I look at the advance of antibiotic resistant bacteria , I foresee the beginning of a return to a pre-Florey world within a decade. For this reason before all others, our numbers will never reach the demographers' predicted 9 billion  in 2050  .
* * * *
1. Using a similar methodology to my own, in September 2007, the Misanthropic Principle site came up with a carrying capacity of 300 million. He takes an estimate of the North American population before the arrival of Europeans (30m), and extrapolates this population density to the whole planet (with a number of qualifications). Back to text
2. One reason for the boom, even among people today who understand and generally apply the scientific method, is that Homo sapiens appear to be hardwired into believing that problems have solutions that are emotionally, spiritually and ethically satisfactory for the human species. However, as Amory Lovins told us, "the chief cause of problems is solutions".
3. Visit the Wikipedia page on antibiotic resistance or search any of the following terms on Google News for the current situation: "MRSA", "Clostridium difficile", "VRSA", "VRE", "superbug", "CDAD", "Type 078", "ESBL", "NAP1", "pneumonic plague" etc.
4. See note 1 on this page for a fundamental flaw in demographic models.
5. Our ingenious dependence on fossil fuel use has been supplemented by our use of forests  without replacement, our exploitation of 'the frontier' in north America, Australia, and elsewhere (originally by the first Homo sapiens who slew the megafauna, then Western colonial powers) - what amounts, in biological terms, to a 'burnt earth' war against marine food sources, our use of the environment to absorb our waste etc. See this discussion of the ecological footprint. Back to text
6. Note I say 'forests', not trees. It is important to take into account the ecological richness of natural forest environments, where half of the biomass - many thousands of species comprising a dynamic forest ecosystem - may be under the ground (in the soil), and half of the biomass may be other than trees. Back to text
7. I am frequently asked this question and even though it's a topic peripheral to the purpose of the evfit.com website, I have thought about it. This page explains how I arrived at my answer - and provides qualifications and a link to a discussion (see note 1) that arrives at a total of 300 million and also links to other credible estimates. Back to top
8. It is paradoxical that human population density may have driven the blossoming of Homo sapiens culture and may also force its collapse. Back to text
9. A term we often hear is "demographic transition". It's not directly relevant to this article, but I need a place to park the following note: “Demography’s predominant theoretical preoccupation in the twentieth century was the theory of demographic transition. Despite its name, the demographic transition was not so much a theory as a set of generalizations from observed trends. The explanatory and predictive value of transition theory is quite limited, but it is the direct forebear of some of the contemporary discipline’s main theoretical interests.... The demographic transition refers to the movement of death and birth rates in a society, from a situation where both are high – in the pre-transition stage – to one where both are low – in the post-transition stage. The interval separating the two is the transition itself, during which substantial and rapid population growth often occurs, because births exceed deaths." From: Donald T. Rowland in "Demographic methods and concepts".
10. Pentti Linkola believes the human population should be "reduced to about ten percent of what it is now". (Page 192 of Can Life Prevail?, 2009). As Linkola was writing when the global human population was around 6 billion, this means he concludes that 600 million is the maximum sustainable population. Back to text
11. Folke Gunther uses a similar methodology to my own to conclude "Thus, it is not sure that the carrying capacity of the planet after the oil crash will even reach 1.5 billion people." Back to text
12. John Michael Greer, on 9 September 2009, suggests the sustainable carrying capacity to be around 500 million; he does not provide a methodology. See midway through this blogged post. Back to text
13. In a New Scientist interview published on 23 January 2009 Lovelock said: "I think it's wrong to assume we'll survive 2°C of warming: there are already too many people on Earth. At 4°C we could not survive with even one-tenth of our current population. The reason is we would not find enough food, unless we synthesised it. Because of this, the cull during this century is going to be huge, up to 90 per cent. The number of people remaining at the end of the century will probably be a billion or less." The interviewer commented: "It's a depressing outlook." To which Lovelock replied: "Not necessarily. I don't think 9 billion is better than 1 billion ..." Back to text
14. Climate scientist Hans Joachim Schellnhuber on 13 March 2009 told a conference that if global temperatures rise by 9ºF, less than one billion people could be sustained Back to top
15. Julian Cribb, of Science Alert, calculates that "if you added together all the world's cities and towns you'd cover an area of soil half the size of China or the United States with concrete and asphalt. At current rates of growth, the footprint of the "world city" will be larger than either the US or China by 2040. Since cities are, for the most part, located (for historical reasons) in fertile river valleys, it follows that they permanently eliminate some of the world's richest soils. The effect of their expansion is is to force agriculture farther and farther out into drier, more marginal, drought and erosion-prone country. In effect, one hectare of lost river valley land has to be replaced by 4 - 5 ha of dry land to grow the same amount of food." Putting this in the context of individual citizens and urban consumers he says "Next time you hop into the supermarket for a loaf of bread or a juicy steak, you need to consider the damage you are doing to the continent, the planet and your children's prospects on it with your tiny economic signal. Australia, the scientists say, is in a 'dust age' - a period of accelerated erosion brought about, mainly, by two things: the 10-year drought which had dried things out incredibly and the pressure which all farmers, all over the world, are being forced to put on the landscape thanks to the rotten prices they get paid in the globalized food system.  These prices compel many of them to take unwise decisions to overstock or overcrop, and when the drought comes, the country's soil blows. The price signal to do this goes straight from the affluent consumer, often complertely oblivious of where their food comes from or how agricultural markets work, to the world's 1.8 bn farmers. In all of human history, food has never been so cheap. The average Australian spends about half what their parents and grandparents did, in terms of disposable income, on food. That's how we can afford all those 4WDs, plasma televisions and electronic trinkets. The miracle has been achieved by farmers and scientists dramatically intensifying farming systems, raising farm productivity - and so reducing what it "costs" (in dollars and over the short-term) to produce food and by food corporations and supermarkets who play one country's farmers off against another in their efforts to extract the lowest possible price. Just to please you. What most people don't seem to get is that there is a real cost to this that every one of us is bound to pay sooner or later. It is coming out of the environment and at this stage there seems little chance of stopping it." (Canberra Times, 28 September 2009. Not available on line.)
16. Farmers who produce commodities have to compete with the cheapest producers worldwide: one year the price setter may be Argentina (after excellent rains), the next year it may be the US (after increased subsidies to farmers), the next year it may be Brazil (after slashing rainforests to get the one-off soil fertility bounty) - a race to the bottom. Farmers in the less advantaged regions are forced to damage (mine) their soils and environment to match each year's price setters.
17. As to Homo sapiens behaviour, we need to bear in mind human physiology, too, as well as drivers in culture and psychology. We now know that women who defer child bearing or who have no children are more likely to develop breast and other cancers than women who have children and breast feed them for a significant period. This points to the phyiological need to have at least one child. And this points to the need - if the population reduction endeavour is to be successful - to reverse global population growth at both ends: (a) limiting, but not eliminating births by individual women and (b) reducing the lifespan of existing humans. This will present a repulsive ethical choice for most people (together with the non sequitur response "well, you be the first to go!" - as if making a cheap debating point could be a solution to the human over population problem). As an aside, we can say that Homo sapiens physiology also expects a diet containing meat and fish. Given the contribution of ruminant animals - which species happen to be our cultural preference as food sources - to methane emissions and, thus, global warming, we are encouraged to adopt a vegetarian diet to protect the environment. Again, it is our failure to address human population numbers, not our food sources, which is leading us to this dilemma. Back to text
18. Paul Ehrlich wrote (in The Electronic Journal of Sustainable Development, 2009) "Our group's analysis of what the optimum population size might be like comes up with 1.5 to 2 billion, less than a third of what it is today. We attempted to find a number that would maximize human options ... even more important would be the ability to maintain sustainable agricultural systems and the crucial life support services from natural ecosystems that humanity is so dependent upon." Back to text
19. Jack Alpert has a video on YouTube in which he argues that the maximum sustainable human population on our planet is 100 million. Jack Alpert also has a website. Jack's methodology is similar to our own, but he uses an engineer's approach and so tends not to understand the importance and dynamic nature of living systems. Thus he assumes that factors that are limiting in one scenario are similarly limiting in all plausable scenarios. He takes an engineer's linear view of nature, when, in fact, natural systems are dynamic cycles working to many different rhythms. Alpert illustrates his methodology using soil quality as the limiting factor. He uses a linear model which shows him that minerals are lost from soil in the farming process 16 times faster than they are released and otherwise made available by natural processes and that therefore we need to hold our soils fallow for 15 years out of 16 to avoid a net loss in soil quality - that is, our present agricultural land can be only one-sixteenth as productive as today if it is to be sustainable. Here he focuses on soil minerals, not a living soil with mycorrhiza, bacteria and other microbes which recycle nutrients in a healthy soil. He also ignores the potential for Permaculture-inspired agricultural and societal arrangements to improve productivity without any depletion of soil quality or biodiversity. There is much more to this story which we can't go into here. Despite Alpert's low figure of 100 million, he does not ignore the complicating factors of social justice and Homo sapiens' psychology. It would be only a person who came to this video with set prejudices who could complain he was racist or that he "blamed the victims". Indeed, I highly recommend Jack's video as a thought-stimulating provocation on this important issue. Back to text
20. David Attenborough made a one hour video titled How many people can live on planet Earth? Unfortunately, Attenborough does not answer his provocative question directly. Instead he draws on the work of William Rees on the ecological footprint. Extrapolating from this model he claims that if we all lived like the average Indian Earth could sustain 15 billion. He adds that if we added like the average Rawandan, it would be 18 billion. Of like the average Englishman (2.5 billion) or the average American (1.5 billion). He adds that these figures are based on superseded data - so why did he use them? Nevertheless, early in the video Attenborough makes this useful contribution: "Humans are good at many things, but thinking about our species as a whole is not one of them." Back to text
21. There is a tendency among Homo sapiens to cornucopian thinking and an exaggerated assessment of human culture. These inclinations became hardwired into our brains during our evolutionary past when the human population was small, technology was relatively unsophisticated and had no lasting impact on the environment. These inclinations are at the foundation of our culture but their validity has now been superseded by the artefacts and impacts of human civilization, multiplied by the sheer number of Homo sapiens. The cornucopian perspective is comforting, comfortable and easy - and therefore seductive - but it is anachronistic in a world where the ecological footprint score exceeds 1.0. Biophysical reality will ultimately and always trump human culture. Back to text
22. See notes 9 and 10 in the page on human hardwiring. Back to text
What do you think is the maximum sustainable human population - and why? Let us know
Evfit home Back to John Gray's paper On to essays on the future of humanity in the light of evolutionary psychology On to ecological footprint
Page added 25 April 2009 Page updated 6 May 2010