Spreading awareness that the human population is in overshoot of the carrying capacity of the planet has led to a number of attempts to calculate what the true carrying capacity might be. My objective here is not to provide another calculation, but to explore some issues that need to be faced to address the question properly.
To start thinking about the problem, I am choosing as a point of reference the global population of about 1 billion that existed in 1800 before the main thrust of the industrial revolution. I choose this number for several reasons. Since that time, humanity has depleted the most easily extracted fossil energy and other nonrenewable materials that made industrial civilization possible. Due to the diminishing resource base, the global industrial economy, while still growing in some places, overall has begun to contract. I see every indication that depletion of strategic materials will continue until they become too scarce for most purposes, and the carrying capacity, at least regarding available energy, is back to where it was in 1800.
As one can see from table 1, the highest technology level at that time included tools, machines and small firearms made of iron and other metals. They were technologies that could be created using available energy sources, and were designed to support human activities using those same energy sources: biomass burning, human and animal power, and wind and flowing water to directly operate machines. In other words, every type of energy came, directly or indirectly, from current sunlight.
Can we therefore expect a return to the population level of the pre-industrial civilization of 1800? Relying mostly on solar energy, human society at that time used only 1/7 the energy that the world uses today. Other factors being equal, available energy is a primary determinant of carrying capacity. So it is reasonable, as access to fossil fuel declines, to consider a likely decline in global population from about 7 billion today to 1/7, or 1 billion, which is about the population in 1800. Based on a requirement of 0.5 ha per capita for an adequate food supply, and significant use of renewable solar energy technologies, Pimentel et al (1994) calculated an optimal carrying capacity of 1-2 billion. However, the question is complicated by a number of issues; these are the subject of the rest of this essay.
Carrying capacity (CC) is an essential concept for thinking in scenarios about a future in which access to key resources is declining. Carrying capacity refers in the first instance not just to a population level; it is the maximum indefinitely supportable ecological load. It is important to view the ecological load in terms of material resource consumption and strain on essential ecosystem services that the existing or desired quality of life requires, often measured in combination as the per capita ecological footprint. So it is first the level of sustainable resource consumption/strain (SRC) that a particular landscape or resource base can support, which in turn determines the mix of population level and per capita resource consumption/strain or ecological footprint. Thus the equation for sustainable resource consumption is
SRC = population × resource consumption/strain per capita
which makes clear that sustainable carrying capacity in terms of the actual number of people it will support depends on the level of individual consumption:
CC (sustainable population) = SRC ÷ resource consumption/strain per capita
As an example, if some people burn more than their share of firewood, others may not survive the winter.
Because the population that a given resource base will support depends in the first instance on the level of material consumption and its distribution, let us suppose an equal distribution, which would support the maximum population for a given resource base. How does that affect our one billion reference point? By 1800 the ecological footprint, both within societies and globally, had been extremely unequal for millennia, and has become more unequal since. So it may seem unrealistic to assume equal distribution of resources. Still, it allows us to consider that under a different resource distribution policy there might have been a higher population than one billion in 1800, and might still be in the future, at least with a more equal distribution of resources.
If our reference point is the beginning of the 19th century, a second question is whether, on average, the global population was already in overshoot at that time. Wood and farmland were still primary strategic resources, and dense populations in Europe and Asia were experiencing repeated crises of famine and wood scarcity. In fact such scarcities had contributed heavily to the demise of numerous civilizations for millennia. This history has led some ecologists to put the beginning of population overshoot soon after the development of agriculture and the rise of civilization itself. An extended exploration of the question is beyond the scope of this essay, but must be an important element in the assessment of carrying capacity.
It is essential to understand that because of delayed effects, populations can persist for a time at levels above CC before experiencing decline. Catton’s notion of phantom CC is useful here. Incorporated in the accompanying graph, it shows how reliance on temporarily available materials like fossil energy, or unsustainably harvested renewables like wood or fish can allow populations to temporarily exceed the sustainable ecological load. The graph shows how this “drawdown” of the resource base steals from the future because it erodes real CC and finally causes population collapse. The temporary success creates the illusion of permanence, whence the term “phantom CC”. An example of temporary success is the estimated tripling of global population since the invention of synthetic fertilizers, due almost entirely to gains in agricultural productivity from those energy-intensive fertilizers. This population increase represents phantom CC because the fertilizer production relies on fossil energy, a temporary resource.
There is another reason that 1800, its population level and material standard of life is a useful reference point. A number of energy scientists have made compelling arguments that the potential of renewable energy to replace fossil fuels is low, a 20% replacement in the most optimistic estimates. They say that the progress to date in producing such renewables as wind and solar electricity is misleading about their potential because it necessitates the continued existence of an industrial base built with yesterday’s cheap energy, but now in inexorable decline. Hence the rising cost of raw materials that would be required to build and maintain alternative energy systems has foreclosed any window of opportunity to create them on a scale necessary to continue the current level of industrial civilization. If we cannot even maintain essential infrastructure like roads and bridges, they claim, how can an economy in permanent contraction afford a new solar-electric transportation system to replace one that is totally dependent on oil? If these arguments are accurate, the energy available to support human society will decline eventually to levels available circa 1800. In that case much of industrialization and the population it supports will disappear.
If the energy available in the future is potentially comparable to energy consumption in 1800, what features of the natural resource base today and in the future are not comparable with the state of the planet and its CC at that time, and may lead to a different assessment of future population? Pre-industrial society already relied on a number of minerals like copper, now more scarce, that will reduce CC compared to 1800. Also, industrial economies have destroyed much of the biological wealth that supported world population two centuries ago. Loss of fisheries, land species populations and biodiversity, and water supplies has been well documented along with the increase in polluted waters and land acreage. It has become clear to scientists that a significant part of that loss is permanent because it has reshaped ecosystems and climatic systems in hard-to-alter ways. All this suggests that once humanity no longer enjoys cheap energy and the crutch of a panoply of energy-intensive technologies that supports phantom CC, a global population of even 1 billion may be unsustainable on the planet’s depleted natural resource base.
The age of industrial exuberance (Catton’s term) has created a vast built environment, much of which will not be usable for its original ends in a lower energy society. Will the leftovers to salvage from that built environment allow a higher CC? Presumably materials like metals, cut stone and glass, not needing mining and processing, would permit higher populations in some locations. The gain from salvage could slow the population decline. It would be temporary however, for according to the law of entropy, nothing is infinitely recyclable.
The accumulation of knowledge in the last two centuries is another part of the industrial heritage to salvage. How might that knowledge positively influence CC? Medical knowledge that requires little energy and other resource-intensive technology, in its application to sanitation for example, has reduced the threat of many diseases that used to limit populations. The Cuban health care system demonstrates that the level of health care, which rivals the US, has more to do with the social structure of that care (doctors living in neighborhoods, making house calls) than with expensive technologies and pharmaceuticals.
Knowledge of ecological systems has made possible the design of extremely low input but highly productive and regenerative agroecosystems, which could raise global CC per acre if they became widespread. Ironically, some of the best examples of these systems have existed for many centuries, before the advent of ecological science. The Aztec chinampas, wet rice-based mixed agriculture in the Pacific rim lands, peri-urban French intensive gardening, and wet meadow-based agriculture in the English lowlands and in riparian communities of colonial New England are some examples.
Where renewable energy can be produced at small scale with simple materials, and where it does not compete with land for food or create significant pollution, knowledge of these systems can add to the total energy supply and potentially boost CC. Small scale biogas production, for example, fits these requirements when it processes manure as part of an integrated small farming system.
In summary, the end of the oil age and industrial civilization as we know it suggests a return to the pre-industrial global population of about one billion. Whether that number represents the world’s carrying capacity in humans, on either the resource base of today or that of 1800, depends on a number of other considerations. Had technological development and resource depletion put humanity into overshoot already in 1800? Since then, how much has continued ecological damage reduced CC? How long will the salvage of leftovers from the built environment of the industrial age maintain population levels? Have we learned enough ecology to potentially manage ecosystems for higher CC than did many pre-industrial civilizations? Does our species have the ability for the long-term, big-picture thinking that management of such complex systems demands? Can we create a political economy capable of managing natural resources for the common good?
As stated at the outset, my goal was not to fully answer the question, “How many people can the world really hold?” I hope instead that I have raised awareness of some of the issues to think about in the quest for answers about true carrying capacity. Some that I have not even mentioned, like climate change and nuclear radiation (from war or from inability to control meltdown of waste from nuclear weapons or utility plants) could easily reduce the world’s human carrying capacity to zero. On the bright side, if our species has burnt through enough of the world’s nonrenewable resources to eliminate the possibility of another industrial “age of extravagance” with its population bubble and subsequent die-off and its threat of human extinction, that could give the species another shot at building a society that stays more or less within carrying capacity.
 Catton, William R. Jr. 1982. Overshoot: The Ecological Basis of Revolutionary Change.
 Pimentel, D. et al. (1994) Natural resources and an optimum human population. Population and the Environment 15(5): 347-369.
 Catton. Op cit