Reflections on human carrying capacity

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Adapted from a chapter called "Population Trends and the Environment" published in 2012 in the

Praeger Handbook of Environmental Health. (Editor R. H. Friis), Westport, CT, USA. 

Introduction

It is obvious that humans depend on the environment that exists here on Earth, in the habitable "Goldilocks" zone around our sun. Indeed, the region of the galaxy thought suitable to form terrestrial planets and old enough to allow biological evolution of complex multicellular life is comparatively limited.1 This nurturing milieu of Earth, atmosphere, and solar energy provides space, breathable air, liquid water, food, energy, and other resources necessary, useful, and desirable for humanity. Human life also depends on nonhuman life-forms. Not only did humans evolve from earlier forms of life, but all of the food we eat was once alive. Many other species contribute to 'ecosystem services' that benefit humans.2 Less obviously, many microscopic forms of life coexist with, on, and within humans (e.g., in the human gut); though some of these are harmful, many are benign, but the majority may be symbiotic and thus beneficial.

As humans have increased in number, so too has their impact on the environment.3 This is true even if additional people live in poverty. Poor people still require food, clothing, housing, and fuel. However the environmental impact of wealthy people is much greater, whether one considers diet (higher on the food chain, greater wastage of calories), housing (quality and size) or energy usage (electricity, travel).

Technological improvements (e.g. electricity powered by renewable energy, the driving of hybrid electric cars) only partially alleviate this impact. Increased efficiency is lowered by Jevons paradox, the tendency to increase the use of products if they become more efficient (and hence often less costly).

Global human population is now approaching seven billion and continues to climb by a million people every few days, as it has done now for several decades. Between 2000 year ago and AD 1650, global population increased by only 200 million, during which time two serious setbacks occurred, the Justinian plague in the sixth century and the Black Death in the fourteenth century. In 1900 global population was only 1.6 billion; it is now well over 7 billion. The rate of global population growth peaked in the late 1960 but in absolute term it is still very high increasing by at least 70 million a year, and probably over 80 million. Almost all of this growth is among poor people in low-income countries.

In comparison to the size of a person, the surface of the earth is enormous. The fact that surface area is fixed suggests that the expansion in human numbers will one day be forced to slow. However, well before space limitations force any such tapering many resources upon which humans depend will become scarce, depleted or degraded. This will necessitate radically new ways of life or a slower rate of population increase. It may even cause a steep decline in global population size.

A conscious human understanding of the relationship between population and environment is likely to be many thousands of years old and to far predate written records. This is difficult to prove, but considerable evidence can be detected. Writing consistent with this understanding survives from ancient Babylon, Rome, Greece, and China.4 Ecclesiastes (5:11) records that "when goods increase they are increased that eat them."5 In China, Hung Liang-Chi (1744-1809), independently of Thomas Malthus (1766-1834), wrote about population outstripping food supply, survival of the fittest, and reliance on the natural "checks" of flood, drought, plague, pestilence, and warfare to limit population growth.6

An unconscious understanding of this relationship can be deduced by consideration of the phenomena of migration and ancient forms of fertility restriction. Hundreds of thousands of years ago human ancestors and species related to humans, such as Neanderthals, migrated from Africa. Today, large-scale human migrations continue, including from Africa. The reasons for ancient and modem migration are easily understandable: people were and are looking for opportunity and reward, and they thought, hoped, and believed that this might exist over the horizon. Often they were correct.

Written records survive from comparatively recent times of abundant food species available to humans, especially in marine and coastal areas, lightly populated by modern standards. For example, stocks of cod were so rich and dense in some parts of the North Atlantic that they seemed as though they could be walked on.7 The paleontologist Tim Flannery describes, but also laments, how human populations often mismanaged and even squandered abundant resources upon arriving in an uninhabited ecosystem. A good example is the moa, a large flightless bird that existed by the thousands when Polynesians first migrated to Aoetoroa (New Zealand) around 1200.8 Within a hundred years humans were well-established in that country, but the moa was extinct. Indeed, throughout the Pacific, many species of bird perished, fairly soon after human colonization.9 Flannery characterizes behavior such as overhunting of the moa as "future eating" because such profligacy reduced the resources available to future human generations.8

ln some cases local elimination of species may have been intended, such as the retreat of large carnivores that threatened humans and their domesticated animals. In other cases, such as the "megafauna," extinction may have been unintended but till inevitable, given the growing sophistication of hunting and the fragility and scale of the environmental resources required to support them.

Nonetheless, future historians are likely to lament the wastefulness of our own time, especially in the way that we are consuming fossil fuels and several other nonrenewable resources.10 We are also critically reducing biodiversity (including highly prized species used for food, such as fish)2 and polluting, to the cost of our descendants, the atmosphere and oceans, with greenhouse gases, particulate matter, and other toxic substances such as mercury, plastics, and persistent organic pollutants.11

The Debate: Are Humans the Exceptional Animal?

Readers with backgrounds in ecology, geography, or anthropology may find the introduction above entirely unexceptionable. In fact, debate about the significance of the human impact on the environment has become in recent decades extremely contentious, especially in the social sciences. For example, in 1982 two demographers summarized the issue of population in the then recent demography literature by stating: "From a high point some 10-15 years ago, intellectual concern about population has steadily waned to a position where it falls now somewhere between ocean mining and acid rain."12

For much of the following two decades a widespread view prevailed, not only in demography but also in economics, that the issues raised by those concerned about limits to population and economic growth were invalid. A representative view was expressed in The Economist:

“It is argued that predictions of ecological doom, including recent ones, have such a terrible track record that people should take them with pinches of salt instead of lapping them up with relish. For reason of their own, pressure groups, journalist · and fame-seekers will no doubt continue to peddle ecological catastrophes at an undiminishing speed. These people, oddly, appear to think that having been invariably wrong in the past makes them more likely to be right in the future. The rest of us might do better to recall, when warned of the next doomsday what ever became of the last one.13

This lessening of anxiety permeated public opinion. A global backlash flared against family planning, also fueled by the clumsy, corrupt, and coercive practices of many "family planners."14 Yet in the scientific literature, apprehension about the danger of too many people abated only slightly. Some writers continued to warn that a high rate of population growth, especially among poor and badly governed nations, was harmful to economic development, 15 although this debate was far less evident after 1980 compared to its heyday in the 1950s.16 17

Perhaps the most significant evidence for this ongoing fear was that in 1992 more than 1,700 leading scientists, including over 100 Nobel Laureates, signed the World Scientists' Warning to Humanity, which argued that human population growth threatened the viability of the global environmental support mechanisms for humanity and called for urgent action to' slow human population growth.18 In contrast to the urgency of this warning and to the stature of the signatories, the paucity of press attention paid to it was shameful, revealing, and unsettling.

In 1993 fifty-eight science academies issued a similar statement, nominating human population growth as a critical issue if civilization is to flourish. The African Academy of Science dissented: Whether or not the Earth is finite will depend on the extent to which science and technology is able to transform the resources available for humanity. There is only one earth-yes, but the potential for transforming it is not necessarily finite (emphasis added). 19

Yet in 1994 lethal genocidal conflict erupted in Rwanda, then (as now) the most densely populated nation in Africa. Within a few months about 800,000 people were killed, representing almost 10 percent of the Rwandan popuJation.20 Though many analysts have focused on the rivalry between the two main ethnic group (Hutu and Tutsis) as the "cause" of this conflict that is a simplistic explanation. A perfect storm occurred to ignite this genocide. Land scarcity in Rwanda21 combined with a scarcity of urban employment opportunities and declining prices for its major export, coffee.22 Unlike many developing countries, Rwanda received very few remittances (i.e., foreign exchange transferred home by Rwandans working internationally). The high fertility rates in this largely Catholic nation led to a 'bulge of youth23 many of whom were unemployed, unmarried, and squashed together in Rwanda's main city, Kigali.

These young men became the main implementers of the genocide. It is widely accepted that male youth are the most violent of any human age group, but of course youth bulges elsewhere have not led to conflict on any scale similar to that of Rwanda at that time. Without excusing the "genociders" for their collective responsibility, the Rwandan butchery can also be viewed as a systemic phenomenon, an example of an "irruptive" population trajectory. This phenomenon is well known in ecology, a pattern of a comparatively steep rise in population followed by a sharp decline. This was first described for some herbivores in ecosystems that lack predators, such as wolves.24 It is also common for some invertebrate populations, especially for introduced species.25

However, the possibility and application of irruption to human societies and populations is far less accepted, for several reasons. One such reason is that most humans do not like to consider themselves as animals. We prefer to think of ourselves as a special case, liberated from the biological constraints that may have burdened our ancestors. Our collective capacity to reason, to invent, and to migrate helps to disguise and hide human dependence upon environmental (including ecological) resources. But this dependency remains absolutely fundamental and unchangeable. Indeed, the more we collectively assert our independence from nature, the more vulnerable we are.

Many authors point to a deep human capacity for denial, perhaps one that is "hard-wired."26 Such denial is also evident in the human capacity to ignore the suffering of people who are far away in distance or time or unlike us in other ways. However, humans also have a disconcerting capacity to become conditioned to ignore the suffering of people close to them;27 this is unsettling in the context of the future.

Many other examples exist of conflict among humans that have a basis partially derived from resource scarcity; in fact, some might argue that it is hard to find situations of human conflict that do not have an environmental basis, whether the Israeli-Palestinian struggle,28 Darfur (Sudan),29 the 2003 invasion of Iraq, or the German invasion of Russia in World War II, when the Nazis desperately strove to secure the oilfields around Stalingrad. On the other hand, a vigorous literature denies any substantial relationship between resources and conflict, instead focusing on political religious, and ethnic elements that are claimed to be unrelated to control over resources. A prominent recent example of this disconnect is the 2003 invasion of Iraq, when the U.S. government, and some of its allies, strenuously and repeatedly denied any oil-related motivation for the invasion.

The History of Contraception

In recent history, and perhaps even now, there has been a widespread perception that contraception is a relatively recent addition to human knowledge.30 In fact, there is substantial evidence of ancient forms of restricting fertility, that is, well before the era of oral contraception, and even before widespread knowledge of the "fatal secrets" (including withdrawal and condoms) that underpinned the first marked decline in fertility in modem Europe, observed in France in the eighteenth century.31 Coitus interruptus was not invented by the French. There are numerous references to this practice in Jewish, Christian, and Islamic texts, but this method appears to have lost favor in the early Christian period. Mohammed approved the use of al-azl, mentioning that the man's wife should also give her permission.32 Its practice was widespread in Europe, though officially frowned upon. To this day it retains its Latin name, at least in English. Herbs have also been used for millennia, both as contraceptives and to induce abortion, though with varying success.32, 33

Many customs have been followed that do not rely on pills, potions, or douches. In the seventeenth century in England (where excellent records survive), the average age of marriage reached thirty, and it was higher among poorer than wealthy families. 34 Western societies had no monopoly on spacing out and sparing children. The Koran recognizes that prolonged lactation delays fertility.32 More than 1,000 years after that book was written, Charles Darwin observed that prolonged breastfeeding likely contributed to small families.32 But where birth could not be prevented, infanticide and neglect were common.32

The anthropologist Virginia Abernethy describes a variety of customs and practices that helped to regulate human populations, including in isolated valleys of New Guinea, keeping the populations within the limits of "human carrying capacity." In some valleys with a comparative scarcity of people, widows could easily marry; in others, with a comparative oversupply of people, various taboos limited opportunities for sexual intercourse.33 Abernethy also describes how the Roman Empire, under Constantine (the first Christian emperor), turned away from pronatalist legislation, at a time when unemployment was rising, agricultural production was falling, and there was a growing sense that "the world was full."35 In many parts of Africa prolonged breastfeeding (up to four years among

the Bushmen), postpartum abstinence, and pathological sterility modified fertility.36 In parts of the Himalayas and Tibet, polyandry has long been practiced, another custom that reduces fertility.

Perhaps the most extraordinary method of reducing human fertility is sub-incision, a traditional ritual practice long observed among adult indigenous men in the most arid and harsh environment of Australia. Early European commentators developed rather fanciful psychoanalytic interpretations for this custom,37 and there is no clear evidence that Indigenous men understood its purpose, beyond that of bonding and initiation. However, it is almost certain that sub-incision had a powerful contraceptive effect, by causing an artificial hypospadius (a condition in which the urethra opens along the shaft of the penis).38 The coincidence of this custom with the driest part of Australia provides very strong circumstantial evidence that sub-incision coevolved in the human population as a mechanism to limit human environmental impacts and to maintain prolonged and tolerable human existence.39

Malthus, the Green Revolution, and the Cornucopians

In the West the debate that concerns Limits to Growth40 and the tension between population and food supply is often summarized as Malthusianism, an eponymous term derived from Thomas Robert Malthus. However, Malthus was but one of many intellectuals who recognized the general principle. Western forebears of Malthus included Benjamin Franklin, Adam Smith, and the pioneering political scientist Giovanni Botero (1544-1617). Similar principles were recognized by such non-Westerners as Japanese, Chinese, and Arabic scholars.6

Simply expressed, the principle that often bears Malthus's name states that population growth is constrained by the availability of food and that, in the absence of deliberate checks on population growth (such as delayed marriage or the use of contraception), other checks" will occur, like famine, epidemic. or war. Though Malthus appear to have referred only to food, his thinking can be applied to resources in general, such as land, water, energy and other raw materials required to grow and distribute food. Caldwell notes that Jevons in the mid-nineteenth century indeed did extend the ideas of Malthus beyond food.41

Such thinking has long been controversial in both West and East. Many nations saw power and prestige flowing from large populations irrespective of the health, wealth, and vitality of those populations. Mao Tse Tung, the first Communist ruler of China, held such views,42 at least prior to the terrible famine from 1959 to 1962, in which about 30 million people perished.43 Remnants of this "mercantile" thinking persist today. For example, Ugandan President Museveni is on record as recently calling for a larger Ugandan population, which he equates with success.44 Yet such high population growth in Uganda and some other countries of East Africa is likely to delay achievement of the Millennium Development Goals45,46 and may, if it continues long enough, lead to other examples of irruptive population trajectories.

Even in his day, Malthus was controversial. Malthusian thinking influenced British economic and population policy, including its response to the Irish famine of the hungry 1840s. Marx was very critical of Malthus, starting a long tradition of suspicion among the Left of any advocate or policy that stresses population rather than distribution or production. Malthusian thinking began to be challenged in Europe as early as the "optimistic 1860s" but persisted far longer in less-industrialized, low-income, and famine-prone countries such as India.47

However, support for Malthus periodically strengthened, including from Lord Maynard Keynes, the most influential economist of his period, from the 1930s until his death soon after he had cofounded the World Bank. Bowen, defending Malthus in 1930, wrote: "It did not occur to those who contributed to this torrent of abuse that a fire which could not be put out must be the very fire of truth itself."48 A chief motivation used by Hitler for the expansion of the German Reich was his internally popular call for more Lebensraum, or living space. Hitler also craved oil security. However, Hitler's policies had two fatal flaws. The first is that Lebensraum could only be acquired at the expense of the living space of other people, such as the Poles and the Russians. The second flaw is that, in fact, the resources available in Germany in the late 1930 were adequate for a relatively high standard of living.

Although the expansion of the European empires preceding the Third Reich was not justified in terms of Lebensraum or carrying capacity, a concept that had not been applied at that time to human populations,24 there should be little doubt that territorial expansion and conquest both provided a relief valve for millions of Europeans and enabled the appropriation of vast extra-European resources from the colonies. This contributed to a rapid increase in European living standards.

In the first decades after World War II, concern about the Malthusian check of famine rose. In 1959 the then comparatively new Food and Agricultural Organization of the UN (FAO) warned that population was outstripping food. Such concerns were widely held, not only by Paul Ehrlich (whose book title, The Population Bomb,49 was taken by permission from a pamphlet earlier published by the heir of the Dixie Cup fortune, William Draper14), but also by U.S. President Lyndon B. Johnson and many other writers.

Yet as global population increase crested (as a proportion) at just over 2 percent in 1969, relief was in sight, in the form of the Green Revolution. Though much attacked by its critics, the Green Revolution led to a remarkable increase in the productivity of much cultivated land area, for example by the use of new strains such as dwarf wheat. It is true that these new crops were comparatively homogenous, fertilizer-hungry, and pesticide reliant. There is also no doubt that some human actors used the Green Revolution to increase inequality, for example, by encouraging indebtedness and perhaps by overcharging for fertilizer. Further, the enormous and rapid increase in per capita food supply did not eliminate global undernutrition, though it did lay the foundation for a reduction in both proportion and absolute terms, which continued until about 2000. But the failure of global society to achieve a steeper and more substantial decline in hunger is certainly not the fault of the technologies used in the Green Revolution, but of the failure of numerous human actors to collectively remedy the numerous other factors that are associated with and help to perpetuate inequality. A co-factor for the failure to better capitalize on the Green Revolution was the continued high rate of population increase, especially in the 1980s, the lost "decade of development." The lagging demographic transition not only reduced the global per person abundance of food but also hindered economic development in low-income countries,46 of which Rwanda is the most tragic example.

In 1970 the agricultural scientist Norman Borlaug was awarded the Nobel Prize for Peace for his seminal role in fostering the Green Revolution. Borlaug (later a signatory of the World Scientists' Warning to Humanity) was well aware of the consequences of rapid population growth.  In his acceptance speech, he warned:

“The Green Revolution has won a temporary success in man’s war against hunger and deprivation; it has given man a breathing space. If fully implemented, the revolution can provide sufficient food for sustenance during the next three decades. But the frightening power of human reproduction must also be curbed; otherwise the successes of The Green Revolution will be ephemeral only. (emphasis added)"50

It is sobering to realize that in late 1984, almost midway through the period of respite forecast by Borlaug, President Ronald Reagan became the first U.S. president in a series to specifically deny the importance of population.51 The Catholic U.S. President John F. Kennedy has been credited with advancing the global agenda to slow population growth in the early 1960s, particularly through U.S. support for key UN resolutions on population.52 Between Kennedy and Reagan, U.S. Presidents Johnson, Nixon, and Carter were explicitly Malthusian in their outlook. In 1968 Johnson authorized the emergency shipment of grain to India, then facing a famine, on condition that India increase its family planning program.53

Toward· a Theory for Human Carrying Capacity

The science of ecology largely concerns the interaction of many species, a mixture of predators and prey, all sharing different degrees of competition and cooperation in the context of always limited resources. All ecologists recognize that there are limits to the growth of any one species and also of the total biomass supported by any area and climate. Yet in ecology, the theory of carrying capacity has fallen from favor, perhaps because more precision was anticipated from the concept than could be delivered.24 Ideas about human carrying capacity (the maximum population supportable by a region for an indefinite period) are even more contested not least because the concept can and has been used to justify coercion,24 invasion, and "ethnic cleansing." Nevertheless, problems with the precise definition of human carrying capacity do not justify abandonment of the entire concept.

This is because its denial would obviate limits to population growth, which is untenable, despite legitimate debate concerning the proximity of these limits.

One way to conceptualize global human carrying capacity is as an emergent phenomenon arising from the interaction of different forms of wealth. Human will remain perpetually in need of resources from the physical environment, such as food, water, and energy. But people are not ciphers, programmable to live in perfect harmony, irrespective of their social milieu. Though some groups (such as the citizens of Hong Kong or Tokyo) are able to live largely peaceful, cooperative lives in intense proximity, these forms of group behavior cannot easily or quickly be transplanted to the entire globe.

Ample evidence for this includes the high fraction of wealth used for military purposes, largely insulated from spending cuts even during recession. History teaches that war and conflict are part of the human condition. Natural capital (both nonrenewable resources such as fossil fuels or uranium and renewable resources such as forests, water, and fisheries), and the degree of social cohesion are important determinants for the human carrying capacity of any region. However, the resources that generate human carrying capacity can be used not only to support a given number of humans, but also to provide wealth for any given population size. For example, the carrying capacity of Rwanda at the time of the genocide in 1994 would have been higher if there had been only one main ethnic group. But total homogeneity would not have indefinitely postponed the Rwandan calamity. The ethnically homogenous (and also largely Catholic) country of Ireland suffered an even greater proportional population crash between 1848 and 1852, following the cool, wet summer of 1848, when the potato blight Phytopthora infestans flourished. A more plausible mechanism by which genocide may have been deferred in Rwanda is via an earlier introduction of education, health care, and family planning supported by the Rwandan Catholic Church and promoted by good government. This in turn is likely to have facilitated economic takeoff, generating a virtuous circle leading to the establishment of export industries, more tourism, and increased skills enabling significant remittances. Instead, the people of Rwanda were effectively trapped, with diminishing marginal productivity that heralded the barbarity of its genocide.

In addition to the foundations of natural and social capital human carrrying capacity can be expanded by ingenuity and by inherited wealth: the quality and quantity of built and financial resources. Another example concerns the Caribbean island of Hispaniola, of which about two-thirds is allocated to the Dominican Republic, with the balance allocated to the more densely populated Haiti. The average income in the larger eastern nation is about seven times that of Haiti. There are, of course multiple interlocking reasons for this difference, but an important component is the rate of population growth.

With a sufficient expansion in resources (such as an infusion of capital), both nation could support more people at a higher living standard. The tiny, natural capital-poor island of Hong Kong shows that ingenuity, cooperation, and high human built and financial resources can support and sustain a densely settled population at a high living standard. However, Hong Kong cannot be extrapolated to the entire world. Many of its natural resources are supplied from beyond its boundaries, purchased with capital generated by the sale of manufactured goods and of ideas and services that are exported. Similarly, the wealth and total population size of Europe expanded in—the nineteenth century, due in part to the European import of natural capital from its numerous colonies; again a strategy that could not be applied globally.

The concept of human carrying capacity is necessarily imprecise; it does not mean that any simple number exists for how many people the earth could support. Such an answer depends not only on the human, financial, natural, social, and built resources available to any society, but also on any particular society's preference for wealth, numbers, and population growth rates.

A clearer understanding of these issues will reduce the chance that nations fruitlessly pursue wealth via a high birth rate. It also reduces the chance of population crashes, not only on a local and regional scale, but perhaps even on a global or subglobal scale.

Conclusion

Doctors, like ecologists, also recognize that endless growth is impossible. Indeed, trends in that direction, beyond an optimum, are harmful to a patient, whether as obesity or cancer. Some of our most eminent economists, including John Stuart Mill (I 806-l873), have recognized that in the future limits to growth will impose a steady state economy.54 Yet the prospect of Limits to Growth continue to be vigorously resisted. Its denial and that of the fundamental dependency of human well-being on limited environmental and ecological resources is every day leading the world closer to an eco-social precipice. Climate change, costlier energy, and the consequences of food insecurity and inequality could well push us over this precipice.

If we continue on this pathway, we risk a massive irruptive population trajectory, manifesting via a "blowback" world,55 in which nuclear-armed terrorist groups and rogue states strike out, leading to confusion in the "fog of war." A more plausible future, however, is the gradual intensification of our existing "enclave world," in which "seas" of order increasingly strengthen the barriers-financial, legal, military, and psychological against the "islands" of disorder. The size and influence of these islands are continuing to grow, whether manifest as pirates from Somalia, favelas in Rio de Janeiro, terrorists from Afghanistan, or train-derailers in the growing chaos and insurgency of northeast India.

To prevent these futures, we need to work toward a genuinely fairer world, in which we "muddle through" to a viable future. This is not a utopian vision of "Health for All," and the term "muddle through" should not be taken to mean a relaxed, laissez faire pathway. But it recognizes that, even if we need a massive technological campaign to accelerate the technological and sustainability transition, we will still need considerable agility and some luck. Irruptions are inevitable but need not become overwhelming.  Complacency will, however, be a fatal error.

Notes

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35. V. D. Abernethy, Population politics: The choices that shape our future (New York: Plenum Press, 1993).

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37. P. Singer and D. E. Desole, The Australian subincision ceremony reconsidered: Vaginal envy or kangaroo bifid penis envy, American Anthropologist 69 (1967): 355-358.

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