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Balancing on a Planet The Future of Food and Agriculture

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Eating Stardust

Population, Food, and Agriculture on Planet Earth

1. Introduction

Imagine that you are zooming outward from the chair you are sitting on while reading this book. You see the place where you were sitting recede into its continent, and then the curvature of our planet Earth appears, growing smaller and smaller, joined by the other planets of our star, the Sun. Our solar system, too, grows smaller and seems to hover in emptiness against the background of our galaxy, the Milky Way. And then, as you continue zooming through the Milky Way it too becomes a speck and disappears, lost in the vastness of the universe.

Our solar system is about 12 × 109 km in diameter, but the next closest star is just over four light years away (Murphy 2006)-a distance of over three thousand times that diameter. The Milky Way consists of about two hundred billion stars and is about one hundred thousand light years (~9.5 × 1017 km) in diameter. The farthest point of the universe we can see is about 13.5 billion light years away and contains a vast number of stars-and perhaps a vast number of planets? Our planet, and our Sun that provides the energy for life here, are truly insignificant at the scale of the universe. But how unique are humans as a form of intelligent life capable of thinking about our place in the universe?

Humans appear to have been fascinated for a long time by the possibility of extraterrestrial intelligent life-from our earliest ancestors' animations of star formations and planets to present-day astrobiologists quantifying the possibilities of life-supporting planets in the universe, such as NASA's Kepler mission, "a search for habitable planets" (Kepler 2013). Kepler will explore about 156,000 nearby stars, and as of the end of 2012 it had confirmed the existence over sixty planets.

It would be easy to jump to the conclusion that there must be intelligent life out there, given the vastness of the universe. However, Howard Smith, an astronomer at Harvard University, argues that available data suggest that the number of "even vaguely suitable stars with possible habitable planets" is very small, and the probability of intelligent life evolving on them is "much slimmer" (Smith 2011). He concludes that even if we waited for one hundred human generations, any signals reaching us would not be from further than 1,250 light years away, which is a tiny portion of the universe. So, even if the universe were infinitely large and lasted infinitely long, making the probability of other intelligent life evolving very high, the chances of us Earthlings detecting it, much less interacting with it, are pretty darn slim. We humans are most likely not going to get help from another planet-we are on our own in figuring out how to get out of the food and agriculture problems we have gotten ourselves into.

Nor will we likely get help from contemplating the meaning of it all because there is no evidence that the universe itself has any meaning or purpose that we can discover. As Steven Weinberg, the Nobel Prize-winning physicist, famously remarked, "The more the universe seems comprehensible, the more it seems pointless" (Weinberg 1994). Yet I think that seeing our human problems in deep time and space-in the context of the evolution of the universe-can help us to put things in perspective. It suggests to me that it is up to us-as individuals, as members of local communities, and ultimately as a global community-to define our purpose in the universe, including how we treat our planet and one another, and how we feed ourselves. We can't expect that the universe will provide a purpose, or that intelligent beings from another planet will come to our aid in helping us discern one-or in solving our food crisis.

Whether we do solve the world food crisis depends on our ability to understand the problem and conceive of solutions. It also depends on consciously deciding, individually and collectively, that coming up with a solution is our goal, and then committing the resources needed to achieve that goal. As a step toward understanding, this chapter takes a broad view of the relationship between the demand and supply sides of our agrifood systems, including basic principles of life in the context of our planet and universe, how human populations and their impacts on the Earth grow, the Malthusian conflict of potentially limitless growth of population and consumption and the limited human carrying capacity of the Earth, and finally, the options for human response when we enter the zone where human impact exceeds human carrying capacity.

2. The origins of the universe and life on Earth

Although humans are a relatively intelligent life form living on a tiny speck of stardust in the vastness of the universe, we are subject to the same forces that govern the rest of the universe, including the laws of thermodynamics. The second law states that entropy-a dispersion and loss of order (information) in matter and energy-always increases, as confirmed by observation of the universe since its beginning in the big bang almost fourteen billion years ago. Yet life temporarily resists the second law in small areas of space-time in its interaction with geochemical cycles (Kleidon 2010), as in James Lovelock's Gaia hypothesis (Lovelock 1986); it pumps order in the form of energy and matter from the outside environment into the living organism, extracting order to build and maintain life, and sheds degraded (higher-entropy, less-ordered) energy and matter as waste products. These waste products can have major negative impacts for future life-a prime example being our export of carbon dioxide to the atmosphere, which is driving global warming. Agriculture is a strategy for supporting human life, increasing the flow of information and resources through human bodies and human-managed systems. It is a strategy that has supported increasing levels of human population and consumption, with major impacts on the environment and society.

Of course, as part of the universe, life is an endgame-eventually the second law will conquer all. But we humans exist at some point in time between the beginning and the end of the universe, and this is where all the fun is-figuring out what to do with this amazing opportunity.

2.1. Biological and sociocultural evolution

Understanding the opportunity we humans have requires understanding biological and sociocultural evolution and the relationship between these processes, which do not always work in the same direction. Biological evolution may be defined as cumulative genetic change over generations resulting from the selection of phenotypic (physical) traits with some heritable basis. Biological fitness is a measure of the number of copies of genes passed on to the next generation-that is, of how successful genes, organisms, or populations are in reproducing themselves (Wilson and Wilson 2008). Beginning at the time it first comes into existence, a population (or species) must increase in order to survive. A population constantly decreasing in size will eventually become extinct. The population of any biologically successful population or species tends to increase until limited directly or indirectly by the environmental carrying capacity for that organism.

Biological fitness is nonteleological. In other words, it is not future or goal oriented and is not an absolute, but a relative, value; it is defined by the specific environment in which an allele (a form of a gene), organism, or population exists (Wilson and Wilson 2008). The environment includes physical components such as temperature, soil, and moisture; other species including food, competitors, hosts, and parasites; and the demographic and genetic structure of the population being considered, or to which the individual organism of interest belongs (Hedrick 2005:204ff.). In addition, fitness trends at different scales may be inconsistent, so that increasing fitness values at one level can be coupled with decreasing fitness values at an adjacent level (Wilson and Wilson 2008). For example, a population of organisms with high relative fitness may evolve in response to changing selection pressures of a particular microenvironment, resulting in reduced fitness of that population in comparison with others in the larger environment, leading to reduced size or extinction of that population (Bergelson and Purrington 2002, Endler 1986:43-44).

Ecologists have described the adaptation of population growth rates via evolution in terms of r and K selection (MacArthur and Wilson 1967:149ff.). Where resources are variable and abundant, r selection favors species with high, unregulated growth rates. In environments with less variable and scarcer resources, K selection favors species with growth rates that decrease as the population grows in response to feedback from the environment indicating resource scarcity. These species' growth rate is described as density dependent, as it slows with increasing population density, allowing the species to survive and persist over the longer term. Selection operates against species that do not limit growth under such conditions. This shows that the criteria of biological fitness can vary through time as the nature of the resources organisms depend on changes.

However, the evolution of phenotypes via selection by their natural environment over generations depends entirely on existing information, not predictions for future conditions. This means that evolutionary biological success is a measure that can look only from the present backward, into the past. Whatever drives organisms to multiply can be modified only by current environmental conditions; there is no mechanism whereby probable future environmental conditions can affect the reproductive drive of present organisms. Current conditions themselves may be "predictors" of future conditions when they are part of a trend-for example, if there is a tendency over time for a decrease in a required nutrient, selection may favor an organism adapted to lower availability, which may be preadapted to even lower future availability.However, present conditions are not always good predictors of future conditions; for example, when a threshold is passed, triggering an acceleration in rate of change or a state change, adaptation to the present may result in reduced fitness under future conditions. In contrast, the human capacity to anticipate the future with some accuracy allows us unique influence over our future biological success, depending on the time frame we are interested in.

We humans are an extraordinary species in the extent to which we have devised strategies to successfully direct more and more of the Earth's resources to our consumption. In a sense, through our cognitive abilities, we have expanded the environmental space for r selection for Homo sapiens, creating what appear to be unlimited resources. Our evolutionary biological success has functioned as it does in other species-it has been based on current environmental conditions, not on likely future ones. The diversion of energy and matter for the expansion of our population has increased for generations, especially in some parts of the world, with most of this increase based on assessments of our environment today, not of the future environmental impacts of that diversion. Thus, to a great extent, our cognitive abilities have been applied to supporting human biological fitness in the present, not to overcoming the inherent shortsightedness of biological selection and fitness.

This leads to what I call the fitness paradox-humans have been successful in increasing our numbers and rates of consumption by dominating the Earth's resource cycles to a greater degree over a shorter time span than any other species with individuals of similar size. The paradox is that this past success threatens our future survival. That is, biological success as defined under r selection has a threshold beyond which it turns into biological failure, because of the conflict between an increasing number of organisms and their impact on the limited resources needed to support them. As that resource threshold is approached, success is increasingly determined by the logic of K selection. The human species, much more than any other, has the ability to predict the future based on knowledge of past events and to change our behaviors and institutions based on solid predictions. So the question is: Will humans be able to transition from an r-selection to a K-selection mode of growth via sociocultural evolution?

The economist Kenneth Boulding described a situation analogous to r and K selection in terms of sociocultural evolution: he contrasted unlimited or open systems, where growth-centered "cowboy economics" is successful in the short term, with limited or closed systems, where steady-state-centered "spaceman economics" is required to avoid disaster (Boulding 1968). "Spaceman economics" implies that humans can incorporate predictions about the future and about the effects of their present actions on that future in decision making. It also suggests that humans have goals for how they want the future to be, as well as ideas about what actions are needed to achieve those goals (see chapter 3 for more details). The critical questions are: What mechanisms are responsible for the transition from "cowboy" to "spaceman" economics? And what kind of life does that transition lead to-for example, a stable population size with short average life span or a stable population size with long average life span (Cohen 1995); a stable level of consumption equitably distributed or a stable level of consumption with a few living in extreme luxury and the rest impoverished?

Thus, for Homo sapiens, incorporated within the fitness paradox is a cognitive paradox-the human cognitive abilities that helped to create the fitness paradox also offer a solution-we can resolve the cognitive paradox and think our way out of the fitness paradox! In other words, while evolution blindly evolved cognitive traits to increase short-term biological fitness, these traits included behavioral plasticity and the ability to consciously control thoughts and behaviors. Consciousness allows me to reflect on "myself" in this sentence, and it allows you to do the same when you read it (Hofstadter 2007), in order to make decisions about the future. For example, some tendencies-such as empathy, sociality, and altruism-can be consciously encouraged at the individual or group level, while other traits-such as territoriality, materialism, and greed-can be subdued, or vice versa. Our decisions, active or passive, about what characteristics should be and are promoted will affect the long-term fitness of Homo sapiens (Wilson et al. 2009). Our sociocultural evolution will have a major impact on the future of our biological evolution and sucess.

No matter how we choose to address our problems, it is helpful to recognize that humans will not persist indefinitely. Of course, there will be periods of relative homeostasis where forces tending to cause increase or decrease of a population or species in a given environment are balanced. However, over evolutionary time, homeostasis is not the rule-populations and species come into being and some time later become extinct. Evolutionary success is always temporary; no individual, population, or species lives forever, even those with the ability to evolve rapidly in response to changing conditions. One estimate of the average life span of a species based on the fossil record is about three million years (Gilinsky et al. 1989, Gould et al. 1987). This is analogous to the situation within individual organisms where physiological systems tend to be homeostatic during the life of the individual, yet all individual organisms eventually die. It is not clear at this point what the balance will be between our ability to change our environment and our ability to change our behavior to avoid extinction. However, as is true for a finite individual organism, there are different ways for us as a species to approach the opportunity we have for life and different choices we can make about how we experience it.

2.2. Competition in resource and energy cycles

Through time and space, different individual organisms, populations, and species on Earth cooperate and compete to obtain access to a limited quantity of matter and energy. For now, the matter is limited to the planet Earth, and energy is effectively limited to that derived directly from the Sun and converted to heat or chemical energy via photosynthesis by green plants, or to electrical energy via photovoltaic cells. Other sources of energy are those derived indirectly from the Sun (wind, waves, tides), or from nuclear fission (and potentially fusion) and geothermal sources, and all have associated costs-some quite high, as with nuclear fission, as seen in disastrous accidents such as those at Chernobyl on April 26, 1986, and Fukushima Daiichi on March 11, 2011. However, even if there were a source of unlimited "clean" energy, like all energy it could be useful to organisms only by energizing matter, so life on Earth is ultimately limited, both globally and locally, by the amount, structure, and distribution of physical matter of the planet itself. This means that there is no such thing as energy with no environmental impact, because even if it could be generated with no impact, energy can be used only by affecting the environment. Most energy controlled by humans is used to convert natural resources into humans or things humans use, which means a decrease in resources available to maintain natural ecosystems, on which humans, and all other life, ultimately depend.

Much of the history of life on Earth is the history of changing the locations of critical elements such as carbon, oxygen, hydrogen, nitrogen, calcium, and phosphorus (Raven, Andrews, et al. 2005, Raven, Handley, et al. 2004) across a wide spatial scale, from their position in chemical compounds to the distribution of their abundance on Earth, from the molten core to the upper atmosphere, from oceans to mud puddles. Nutritional biochemistry in living organisms-including humans-reflects the early geochemical evolution of the Earth (Fedonkin 2009). In other words, human evolution has been constrained by the evolution of chemical and biochemical pathways, and today the Earth's biogeochemistry is embedded in human physiology, with cellular metabolism repeating to some extent the "main events of the coevolution of geochemical and biotic processes in the early biosphere-life remembers the youth of the Earth" (Fedonkin 2009:1321).

At very local levels, the relative abundance of elements and compounds controls the growth and multiplication of organisms. For example, phosphate ions in the rhizosphere (the root zone) are required for the growth of crops such as sorghum, and sorghum is used to make porridge, which will determine the number of people that can live in a West African village. Populations and species have blindly evolved the ability to organize the matter and energy in their internal environments in ways that tend to optimize their growth and reproduction under current external conditions, while they also affect those external conditions. For example, photosynthesizing plants helped to increase the oxygen content of the atmosphere (see below). (In chapter 8 I discuss biogeochemical cycles in relation to climate change and diet.)

At the global level, too, resources become limited in relationship to the number of organisms and the demand they place on resources. For example, the more than seven billion humans living on Earth in 2013 were equivalent to about 350 billion kg of matter (at 50 kg per person)-matter that is temporarily unavailable to other organisms. And this does not include the resources in crops, domestic animals, and human-made capital, such as buildings, cars, and roads-but I don't know if this calculation has been attempted. In terms of the total photosynthetic output of green plants (net primary product, or NPP), Vitousek et al. have made a widely cited estimate that humans use 38 percent of our planet's NPP, of which 4 percent is used directly for food and an additional 34 percent is either co-opted for nonedible products or destroyed (Vitousek et al. 1986). Recent studies support the estimate of 38 percent of NPP used by humans, while pointing out that of the remaining 62 percent of NPP, only about 15 percent might be available in forms humans can use; given projected growth in demand, humans will deplete this available NPP within decades (Running 2012).

2.3. Does life, and agriculture, have a purpose?

This overview of humans in the context of the evolution of the universe helps provide the necessary context for understanding our agrifood system-its historical development, current accomplishments and problems, and possible futures. Many behaviors and institutions were selected for their survival value during the long course of human biological, and then sociocultural, evolution. But many of those were selected for during a time when human impact on the Earth was relatively small, and they seem maladaptive under current conditions. That is, they can lead to a reduction in human biological fitness in the environments we are creating on Earth. For our continued survival, humans need to rapidly evolve socioculturally to encourage the development of alternative ideas, behaviors, and social structures (Gowdy et al. 2010).

A key to solving the world food crisis and creating more sustainable agrifood systems is replacing the biological criteria of success based on blind, short-term biological selection for fitness in response to current conditions with sociocultural criteria that modify current trends to create alternative futures, while at the same time being adapted to those futures. Of course, agreeing on the vision for the future is critical and will not be easy (see chapter 3). However, empirical evidence from the natural sciences makes it clear that it will require replacing the biological criterion of increased physical growth in numbers with the sociocultural criterion of increased cognitive growth, and replacing unlimited per capita consumption with a focus on nonmaterial changes to improve our lives (after basic physical needs are met) (Jackson 2011).

So, what should be the goal of agriculture? As we saw in the introduction, the mainstream approach to the food crisis tends to assume that the goal of agriculture is to increase the food supply and decrease hunger-that is, to nourish human bodies. But this avoids going beyond the biological purpose of food. The ultimate goal of agriculture must be the same as that of human life, which in turn must be subjectively decided by humans-it cannot be simply to keep humans alive and multiplying. That is tautological nonsense.

The purpose of human life is defined by many different cultures and in different ways as what can be interpreted as subjective happiness. The Japanese advocate of natural farming Masanobu Fukuoka is often quoted: "The ultimate goal of farming is not the growing of crops, but the cultivation and perfection of human beings" (Fukuoka 1978:119). In his book Eating Animals, novelist Jonathan Safran Foer captures one example of this deeper meaning of food and human life in a story about his Jewish grandmother's struggle to survive while hiding from the Nazis in World War II. Even though she was starving, she refused to eat pork because it is prohibited by the Jewish religion: "If nothing matters, there's nothing to save" (Foer 2009:16-17).

Reaching at least a partial consensus on the purpose of human life is a key part of defining our goals for agrifood systems, as we shall see in chapters 3 and 4. The country of Bhutan has institutionalized the idea of happiness being a central goal in its Index of Gross National Happiness (Ura 2008). In an effort to reduce the subjectivity of such a goal, neurologist Sam Harris has suggested instead of "happiness" the term "human welfare," which he argues can be objectively measured in terms of electrical patterns in the brain (Harris 2010). Whatever we even partially agree upon as the purpose of human life, the unique human opportunity is the possibility of creating a better world in the dynamic tension between physically based biological evolution and cognitively based sociocultural evolution.

3. Population growth

Our success as a species at increasing our numbers and our per capita consumption, and the effects of this on society and the environment, are central to thinking about food and agriculture. In this section we will look further into population growth as a driver of human impact.

3.1. How do populations grow?

Human population growth is similar to the growth of populations of other organisms. What determines population growth rate? On a local scale:

r = BR − DR + (IR − ER), where

r = growth rate

BR = birth rate

DR = death rate

IR = immigration (migration in) rate

and ER = emigration (migration out) rate (fig. 1.1)

On a global scale, immigration and emigration are irrelevant, so that when BR is greater than DR, there is positive growth or natural increase, which means the potential for infinite population size, limited only by the resources needed to support it, and when BR is less than DR there is negative growth, leading eventually to extinction. It is obvious that for population size to be stable, BR and DR must, on average, equal each other.

Fig. 1.1 Global birth rates and death rates per 1,000 population.

So, depending on the relationship between BR and DR over time, the shape and properties of the growth curve of a population will be different. There are several kinds of growth that populations can have, and it is important to understand human population in terms of these alternatives. They differ in terms of the combination of relative growth rates (the numbers added during each time interval as a proportion of the population at the end of the previous interval) and the absolute growth rate (the number added during each time interval). Note that I discuss growth here as positive, but everything applies to negative growth as well.

Linear growth (also called arithmetic growth) is change by a constant amount (the absolute growth rate stays the same), which means that the relative growth rate (r) is decreasing, since the base population is growing, but the number of individuals added each year remains constant.

In sigmoidal, logistic, or density-dependent growth, growth is linked to carrying capacity (K), such that the absolute growth rate is determined by the difference between K and population size P at any given time. As P approaches K, both the absolute and relative growth rates decrease toward zero, so that P never exceeds K.

Superexponential growth is change by an increasing relative growth rate due to DRs decreasing or BRs increasing or both. Of course, the absolute rate will also increase.

Exponential growth is change by a constant relative rate, which means that absolute rates will increase. This has long been recognized as a special type of growth, and it is commonly used today as a model for growing populations. The following are different forms of the equation for exponential growth.

Pt = P0 (er t)

P0 = (Pt ∕ er t)

r = (lnPtlnP0) ∕ t

t = (lnPt− lnP0) ∕ r, where

P0 = population at time 0 (i.e., the starting population size)

Pt = population at time t

e = the base of the natural logarithm (2.71828 . . . )

r = the relative growth rate

t = time between P0 and Pt

ln = natural logarithm

One convenient way to think about exponential growth is in terms of doubling time (DT), or the time it takes a population to double in size-and the time period during which doubling will occur will remain constant as long as the exponential growth rate remains constant. DT is often used by demographers as an intuitively appealing way to express the growth rate of a population and to compare populations. When a population doubles, Pt = 2P0, and therefore, by substitution in the equation for Pt, 2P0 = P0 (er t), and ert = 2, and taking the natural log of both sides gives us rt = 0.693, and the time for the population to double = 0.693∕r. Therefore, a quick way of estimating the DT of a population undergoing exponential growth is

DT ≈ 0.7∕r, or when r is expressed as a percent, DT ≈ 70∕r

There are many tales illustrating the astonishing result of continued doubling that is exponential growth during a given time period. For example, according to the legend of the Ambalapuzha Temple in Kerala in southern India, the Hindu deity Krishna once appeared in the form of a sage and challenged the king of the region to a game of chess. For the prize if he won, the sage said he wished only a few grains of rice-one grain for the first square of the square chess board, two for the next, four for the next, and so on. The king was disappointed that the sage requested so little, but when he lost he soon discovered his dilemma. The sixty-four squares of the board would require 263 grains of rice = 9,223,372,036,854,780,000, or more than 9,000 trillion grains of rice, equal to over 230 billion metric tons, nearly ninety times the size of the total world cereal harvest in 2011 (FAO 2013b)! Krishna then appeared in his true form and told the king he could pay off the debt over time in the form of paal payasam, a sweet rice dish, served for free to all pilgrims visiting the temple. This story gives us a feeling for the power of doubling to increase the original number rapidly, and for the consequences of not understanding what current trends mean for future conditions.

The next example gives a feeling for the dramatically increasing absolute rate of increase in exponential growth in relation to deciding how to deal with it. A pond has a single lily pad growing on it, which grows to two the second day and continues to double every day. After thirty days the pond will be completely covered, extinguishing all life living within it. The creatures living in the pond are aware of this, but given how busy they are with finding food, eating one another, settling disputes, and reproducing, they decide to wait until the pond is half covered before addressing the problem. What day will they finally meet to resolve the problem of the lily pads? The answer: the twenty-ninth day-too late!

3.2. How does the human population grow?

Understanding how the human population has grown in the past can help us to predict how it might grow in the future and to develop plans to deal with this growth. Joel Cohen, a population ecologist at Rockefeller University, has done a comprehensive review of the history of human population growth in relation to carrying capacity (Cohen 1995). He showed that the Earth's human population has grown at different rates-at very low exponential rates for foragers (gatherer-hunters), and then at increasing rates with the advent of agriculture about twelve thousand years ago, becoming super exponential. Since the mid-1960s, when the world's annual population growth rate peaked at 2.06 percent, the rate has been decreasing.

However, the absolute growth rate has remained fairly high and stable because the base population is so large. It has taken since the origin of our species, Homo sapiens, until about the year 1800 for our population to reach one billion, only 130 years to add the second billion, 30 years for the third, and only 12 to 14 years for the fourth, fifth, sixth, and seventh (in 2011) (PRB 2011). In 2012 the world population of 7.1 billion was growing at a rate of 1.2 percent per year, which means a net increase of about 231,000 more people every day and over 84 million per year, and it is estimated that another billion people will be added by 2025 (PRB 2012).

The absurdity of continuing our current rate of growth is shown by the following calculation. In 2012 there was an annual world population growth rate of 1.2 percent and there were about 4640 people per km2 of land in the world. However, with a world land area of 1.53 × 1012 m2, it would take only 448 years for there to be one million people per km2-that is, one person per every square meter of land in the world, which is obviously not sustainable, no matter how you define it.

t = (lnPtlnP0) / r

t = (ln 1.53 × 1012 ln 7.1 × 109) / 0.012

t = 448 years

In more personal terms, this presents humans with a dramatic choice-we cannot have a long average lifespan (low DR), a high BR, and a stable population size at same time (Cohen 1995:18). If we want long lives and a stable population (r = 0), then on average each individual can have only enough children to replace themselves-only one! Any growth rate greater than zero for the Earth's human population is ultimately unsustainable because the population would increase indefinitely, and so if birth rates don't fall, death rates will have to increase to maintain a stable population size.

3.3. What is the future of the human population?

As we can see, the current rate of population growth has to change, and population projection is the field of demography concerned with predicting how different factors affecting population growth-birth, death, and immigration rates, population age structures-will affect the growth rates and future sizes of human populations. Given past and present changes, most population growth in the next decades will occur in the Third World, but with continued slowing of the rate of growth there as birth rates continue decreasing faster than death rates. As a result, these Third World populations will begin to resemble more and more the age structure of industrial populations (fig. 1.2). The narrowly based age pyramids of populations with low birth and death rates mean higher proportions of people in less productive older years, increasing the overall dependency ratio.

Fig. 1.2. Population pyramids in the industrial and Third worlds.

Another important demographic trend is increasing urbanization. As of 2010 the majority of the human population lived in urban areas for the first time in human history. Thinking of the world food crisis in light of growing urbanization and dependency ratios emphasizes the necessity of switching from a short-term biological evolution-based criterion of success to one based on a combination of the most objective assessment of the biophysical world and agreement on the goals of the agrifood system to guide our sociocultural choices and changes.

4. The Earth's human carrying capacity

We have seen that we humans have a remarkable biological potential for reproducing ourselves and an equally remarkable potential for controlling the environment to support this growth. We have also seen that the potential for growth is limited-but if we are to use our cognitive abilities to anticipate and adapt to these limits, we also have to understand a bit more about those limits. In this section we focus on human carrying capacity as the constraint to population growth and other aspects of human impact, both locally and globally.

4.1. Malthus's argument

Thomas R. Malthus was an English cleric, famous for bringing to the attention of nineteenth-century Europeans the conflict between the potential for humans to increase their numbers "geometrically" (i.e., exponentially) without limits, while the food supply on which they depend is limited by the limited supply of land and other agricultural inputs and can only increase "arithmetically" (i.e., linearly) (Malthus 1992 [1803]:17-19). He believed that the incompatibility of these two rates of growth inevitably resulted in checks on human population growth. In the first edition of An Essay on the Principle of Population, published in 1798, he emphasized two checks-positive and preventive (Malthus 1992 [1803]:15, Winch 1992:xiv, xvi). Malthus's positive checks are negative in terms of their effect on human happiness-they function to reduce impact via reduction in consumption and increase in death rates, causing misery and fear of misery. The possibility for preventive checks was not a very happy alternative either, in Malthus's view. For him, preventive checks function to prevent impact by reducing birth rates due to "vice," meaning sex outside of marriage and other unnamed "vicious" customs, including prostitution and contraception.

Malthus is often represented as a pessimist who believed that humans would unavoidably sink into misery as a result of population growth. What is not so often appreciated, however, is that beginning with the 1803 second edition of the Essay, Malthus emphasized the preventive check of moral restraint, by which he meant delayed or no marriage, and celibacy outside of marriage: "It is not the duty of man simply to propagate his species, but to propagate virtue and happiness; and that, if he has not a tolerably fair prospect of doing this, he is by no means called upon to leave descendants" (Malthus 1992 [1803]:271). He believed that humans were unique among animals in their ability to exercise this check due to their "distinctive superiority" in "reasoning faculties, which enables [them] to calculate distant consequences" (Malthus 1992 [1803]:21).

Thus, Malthus laid out the human predicament due to the conflict between food supply and a growing human population as one that would inevitably lead to pervasive misery and suffering, except that humans have the ability to foresee and plan for the future and to change their behavior accordingly, either in the manner he described, or through what is today more broadly available and socially acceptable safe birth control methods, an example of behavioral change arising from sociocultural change.

As I briefly described in the introduction, food supply and human numbers have been increasing at similar overall rates, beginning with the Neolithic and continuing through the present. To some, this means that Malthus was wrong (e.g., Evans 1993). However, if you accept the idea that there are limits on the extent to which humans can divert energy and matter from natural ecosystems, and social limits on the extent to which humans can organize society to produce more food, then over the longer term Malthus was right. His fundamental observation seems incontrovertible-humans, like all organisms, tend to increase exponentially with no intrinsic biological limit to the ability to reproduce, while there are physical limits that prevent food supply from increasing exponentially except over relatively short periods of time. This means that the necessity of a sustainable agrifood system is Malthusian: food production, and therefore human numbers, are ultimately limited by the physical limits of the Earth, and sustainable agrifood systems must be adapted to this situation.

4.2. Human carrying capacity

I use the term human carrying capacity (HCC) to refer to the number of people a biophysical environment can support (culturally, socially, biophysically) without decreasing its ability to support the same number in the future. Human impact (HI) is the product not only of population size (N), which I have emphasized so far in this chapter, but also per capita consumption (C), and the technology (T) used by humans to provide what is consumed (HI = NCT) (following Daily and Ehrlich 1992:762), and a sustainable HI means that it does not reduce HCC over the long term (see section 1.5).

While there are many estimates of the Earth's HCC, most evidence suggests that with current growth rates it is finite in the relatively short term (i.e., in terms of the human evolutionary time scale), and it may be exceeded already (Cohen 1995, Giampietro et al. 1992), or will be very soon (Running 2012). Cohen reviewed more than sixty-five estimates of HCC made from the seventeenth century through 1994 and found a huge range, from one billion to a billion billion (1018). While there was no trend in the average estimate, there was an increase in range, although more than 50 percent of the estimates (based on the upper bound when a range was given) were between 4 and 16 billion (Cohen 1995: 212 ff.). If only the short-term production potential of the planet is considered, very large estimates are possible-for example, 282 billion if all land area is used for crops, with the same yields as the highest yielding crops (Franck et al. 2011). However, when you consider the longer-term limits of the biophysical environment of the Earth, subject to the laws of thermodynamics, then the estimates of HCC are much lower (Moran et al. 2009, Pereira 2009, Gilland 2006).

So far, we have assumed that HCC is defined in terms of the biophysical environment. However, it can also be defined as "social" carrying capacity (Daily and Ehrlich 1992:762) or, more correctly, sociocultural carrying capacity, to emphasize the role of subjective values in its definition and estimation. Biophysical and sociocultural dimensions cannot be separated, because biophysical limits can be determined only based on human values about the kinds of lives we want, or are willing to live, and the risks we are willing to take. Cohen concluded that because no estimates of HCC have included these factors, "taking into account the diversity of views about their answers in different societies and cultures, no scientific estimates of sustainable human population size can be said to exist" (Cohen 2005a). Sayre agrees, believing that carrying capacity has been used in an uncritical manner that does not acknowledge that "limits are rarely static or quantifiable, let alone predictable and controllable," often to advance ideological positions (Sayre 2008:132). His conclusion that the concept is not very useful, however, ignores the concept's potential for stimulating questions and research, as discussed in the rest of this chapter.

4.3. Organisms affect their environments: Limits of human carrying capacity

While some other species make use of technology to enhance their evolutionary success, humans alone have excelled at this, and have increased HCC dramatically through modifying the environment and other species to increase our food supply.

Ecologists have traditionally thought of causality between organisms and their environments as being unidirectional-namely, organisms change under the influence of other organisms and the environment. But organisms also affect their environments in ways that in turn affect their adaptation (biological and cultural) to those environments in an interlocking system (Corenblit et al. 2011; Fedonkin 2009). This has been modeled by Odling-Smee et al. as

dO ∕ dt = f(O, E), dE ∕ dt = g(O, E)

which states that changes in organisms over time (dO dt) are a function of both the organisms and the environments (f(O, E)), and changes in the environment over time (dE dt) are a function of both organisms and the environment (g(O, E)) (Odling-Smee et al. 2003:18).

Thanks to humans' unique cognitive and technological capabilities, which are much greater than those of other organisms, humans can uniquely modify their environments with the goal of increasing their carrying capacity, and human activities have increased to a level that is now having dramatic effects on the Earth's atmosphere, climate, hydrology, soils, and other species. Graphs that track changes since the beginning of the industrial period show a dramatic similarity in the superexponential increase in human activity (e.g., population size, fertilizer consumption, and McDonald's fast food outlets), on the one hand, and the effects of this activity on the Earth's biogeochemical systems (e.g., global warming, loss of forests, and species extinctions), on the other hand (Steffen et al. 2011: Figs. 1, 3; see also Vince 2011). These changes will have a negative impact on the biophysical HCC, but also on the sociocultural HCC, because of increased conflict, negative psychological consequences, displacement from homelands, and loss of cultural identity (World Bank 2012b:55ff).

Many scientists believe that we are affecting the biophysical nature of the Earth so dramatically that the consequences of human impact today will be evident thousands or even millions of years in the future, and so they have suggested naming the period beginning with the industrial age about 150 years ago the Anthropocene epoch (Vince 2011). Such an impact raises the possibility that the Earth could be leaving behind the relatively stable biogeochemical cycles of the Holocene on a "one-way trip to an uncertain future" (Steffen et al. 2011:757); indeed, that these cycles might be approaching a tipping point that would change the Earth dramatically, beyond anything ever experienced by humans before (Barnosky et al. 2012; World Bank 2012b).

5. Dealing with "the zone"

Joel Cohen has aptly described the space where human population and the Earth's HCC overlap as the "zone" (Cohen 1995), and I add consumption and technology to population in comprising human impact (HI) (fig. 1.3). Based on his exhaustive review of estimates of HCC described above, Cohen believes that humans have entered the zone.

Fig. 1.3. The zone.

Being in the zone means that there is an existing or imminent imbalance between the demands of humans for goods and services and the ability of the environment and society to meet these demands. As I have mentioned, the good news is that humans are unique compared with other species in that we have the ability of self reflection, so unlike nonhuman biological evolution, our sociocultural evolution can be teleological-that is, it can be consciously directed toward achieving goals in the future (Richerson and Boyd 2005). Therefore, humans have the ability to adapt as we approach the zone in ways that slow our approach and even reverse our direction. However, in order to adapt, several key ingredients are required-adequate knowledge of the zone and the consequences of being in it, the value-based social decision to do something, and the will and perseverance to actually do it. Similarly, based on his review of societal collapses and perseverance in the face of environmental limits exacerbated by human activity, Diamond sees the two biggest challenges to avoiding collapse as "long-term planning, and willingness to reconsider core values" (Diamond 2005:522).

Experience doesn't serve as a very good guide to dealing with the zone, since at the global scale we have never had to deal with it before. Biogeochemical changes (such as global warming) and their effects on agrifood systems (such as increased variability in crop yields) and human society (such as increased food insecurity and social unrest) are unpredictable. We can't simply extrapolate past trends into the future, especially because some of these changes will be nonlinear, and even pass biophysical thresholds, resulting in entirely new states we may have no knowledge of, as may be the case with climate change (see chapter 8). It's like anticipating an unknown guest coming for a meal that we must prepare using ingredients we've never seen before.

For example, if farmers in a village notice that the soil in their fields is eroding at the same time that yields are declining, they will expect further yield declines unless erosion is checked. However, if erosion has been going on for a long time, and yields have remained stable, how can farmers know that the topsoil in their fields will eventually pass a threshold beyond which yields will decline rapidly to very low levels? Thus, the challenge of teleological sociocultural evolution is to predict possible futures based on understanding current trends and predictions of potential dramatic deviation from trends, and to take action to direct the future toward desired outcomes. For this reason, the human ability to analyze the present as the basis for predicting the future is critical.

The question for sustainable agrifood systems is not only what biophysical mechanisms mediate between increasing demand for food and increasing agricultural production, but how we can detect when HCC and HI are going to approach or intersect each other and how we can respond. In the simplest terms, the choice is either increasing HCC or decreasing HI; once HCC is maximized under current knowledge and technology, decreasing HI is the only option. HI can be decreased by slowing or reversing population growth (by decreasing birth rates and/or increasing death rates), decreasing consumption rates, or increasing the efficiency while decreasing the negative environmental impact of technology. Therefore, given our understanding of human population growth and the Earth's finite resources, the critical question is how we are going to reduce HI to below HCC.

Following Meadows et al. (Meadows et al. 1992), I discuss this in terms of four basic scenarios for human response to the zone (fig. 1.4). The first scenario assumes that HCC can be increased indefinitely in response to increasing impact. The other three assume that the ability to increase HCC is limited.

Fig. 1.4. Future scenarios for HI and HCC.

5.1. Human ingenuity increases growth of human carrying capacity: A Boserupian scenario

Ester Boserup's first book on population and agriculture, The Conditions of Agricultural Growth (1965), was influential at the time it was published in 1965, and it remains so today (Malakoff 2011). It sounded a positive note about population growth at a time when there was growing concern. Following World War II there was a surge in population growth as death rates fell much faster than birth rates, and many were concerned about the possibility of food crises (Cohen 1995:67-68). Indeed, the 1960s was a period of unprecedented growth rates in the world population (peaking at 2.06 percent per year in 1965). It was also a period of widespread public concern about the effects of population growth, spurred by sensationalist predictions of the dire consequences of not slowing growth, including Paul Ehrlich's Population Bomb, which received wide publicity (Ehrlich 1968).

The Conditions of Agricultural Growth appeared just one year after Theodore Schultz's seminal book, Transforming Traditional Agriculture (Schultz 1964). Both Boserup and Schultz rejected the then-popular belief among agricultural economists and agricultural development professionals that small-scale farmers are irrational. Instead, they cited evidence suggesting that farmers are capable of responding in economically rational ways to the external forces of the marketplace, and in Boserup's work, of population pressure as well. They differed, however, in a key assumption. Schultz believed that increasing modernization of traditional agriculture was the only way of increasing output. Boserup believed that Schultz's conclusion failed to distinguish variation in intensity of land use in traditional farming systems (Boserup 1965:15-16, 1990:278-279). Boserup thought that even though intensification would be avoided when possible, because it leads initially to reduced production per unit labor input (diminishing returns), the pressure of population growth, and the stimulation of markets, would spur intensification and technological innovation that would lead to higher production, and eventually to higher returns to labor as well (fig. 1.4a).

There are many data supporting a positive correlation between population growth and increased HCC. Undoubtedly the best-known example of a Boserupian scenario is the World Bank-funded study in the 1980s of Machakos District, Kenya. This study showed that an agricultural environment that had been severely degraded as population grew, and that had been written off as irrecoverable, was reclaimed to productivity as a result of agricultural intensification, including extensive terracing and new market crops, transforming it into a poster child for Boserup's theory and the World Bank development policies (Mortimore and Tiffen 1994, Tiffen and Mortimore 1993, Tiffen et al. 1994). There have been many critiques of the study for its oversimplification and unfounded conclusions (e.g., Siedenburg 2006).

There is no doubt that humans can increase HCC, as they did dramatically with the invention of fire, tools, and agriculture. By looking selectively at that range of environments and time periods where carrying capacity can be increased, at least in the short to medium term, the Boserup hypothesis can be supported. Thus, a good deal of the controversy about the applicability of Boserupian views may be due to the failure to specify the range of conditions in terms of environmental HCC for each case study (Carr et al. 2009), precisely because it is assumed that an environmental HCC does not exist.

5.2. Negative feedback leads to low growth rate of human impact: A Malthusian scenario

As we have seen, Malthus's preferred strategy for dealing with the mismatch between exponential human population growth and arithmetic growth in food supply was preventive checks (moral customs) that reduce population growth when food is scarce, resulting in the logistic growth curve of HI in fig. 1.4b. In other words, awareness of the approach to or entrance into the zone can motivate humans to reduce HI and avoid the positive check of increasing death rates, analogous to K selection discussed in section 2.2.

The Kusasi of northeast Ghana before the advent of colonialism present us with an example of a Malthusian scenario, at least over the short term. I lived for eighteen months in Zorse, a village of Kusasi farmers, carrying out research for my dissertation, described in the preface (Cleveland 1980). The research was part of a project investigating the drought in the Sahelian region of West Africa in the 1970s. The Kusasi homeland, known as Kusaok, consists of Bawku District, Ghana, and neighboring areas of Burkina Faso and Togo. The Kusasi make their living in this environment with short-handled hoes, wooden flails, grinding stones, and an occasional ox plow. In good years, the rainy season between May and October can produce the illusion of prosperity, with thick green stands of millet and sorghum towering above the dispersed mud-and-thatch homesteads. The dry season, however, brings dramatic contrast, with the only relief from the barren, dusty fields being the gardens in dry streambeds watered by buckets carried from hand-dug wells (fig. 0.1).

Food production in Kusaok is labor intensive, so parents desire lots of children because children become net household producers by age ten, contributing directly to family welfare. Children also contribute to household food production indirectly-for example, by caring for younger siblings, herding goats, scaring birds from ripening crops, and gathering fuel and food-so that adults have more time for the heavier work in the fields and house of producing and processing food (fig. 1.5). Therefore, there is a high demand for children, which is rational in terms of optimizing household welfare, with fertility regulated not to maximize the number of live births, but the number of surviving children.

Fig. 1.5. Kusasi child gathering millet stalks for the cooking fire.

Traditionally, the high demand for children was balanced by fertility regulation embedded in community management of agricultural resources (see chapter 5), such as fields, pasture, and forests, and households helped one another in many tasks (Cleveland 1986b). Fertility was tied to the HCC of Kusaok via two social mechanisms. First was postpartum sexual abstinence-husbands and wives abstained from intercourse after the wife gives birth until the youngest child was old enough to remain healthy when her mother became pregnant again. Health is affected by nutrition (food supply) and disease (including water quality), so the period of abstinence increased as food supply and water quality decreased during periods of drought. In addition, inter- and intratribal hostilities meant that the next-to-youngest child had to be big enough to run away if attacked while in the field with its mother. The net result was long birth intervals of three or more years.

The other mechanism controlling fertility was age at marriage, which was limited by the availability of bride-wealth cattle. The bride-wealth tradition requires that each young man give four cows to his future bride's family, and the would-be husband depended on his own extended family (father, uncles, grandfathers, and so on) for these cattle. But the supply of cattle is limited by their food supply, and by crop harvests, since good harvests allowed households to trade for cattle. Both of these, in turn, are controlled by the carrying capacity of the environment.

Thus, even though there was a high demand for children, Kusasis traditionally limited their fertility via feedback from the HCC of their environment by using Malthusian preventive moral checks-delayed marriage and postpartum abstinence.

5.3. Human impact exceeds human carrying capacity: A neo-Malthusian scenario

This is the scenario that Malthus dreaded, in which humans are either unable to understand that they are approaching or in the zone, unable to agree on a plan to do something about it, or unable to organize effectively to carry out the plan. The result is that HI overshoots HCC and causes environmental collapse, which in turn forces a reduction in HI through Malthus's positive checks-increasing death rates, misery, and the fear of misery, as food becomes scarcer (fig. 1.4c). As discussed above (section 4), there is much evidence that HI has already overshot HCC in many local situations, and perhaps globally as well.

The indigenous agricultural and demographic regime that created a Malthusian balance between HI and HCC in Zorse as described in the previous section changed dramatically and rapidly with the invasion and military dominance of European colonialists. Population grew despite substantial labor migration south to work in the colonial economy-between 1948 and 1970 the Kusasi population grew by nearly 50 percent. This growth can be partially explained by reduced mortality stemming from public health measures and improvements in transportation that permitted freer movement of people, cash, and food (Cleveland 1991). However, I found that it was also the result of a 44 percent increase in the total fertility rate (or TFR, the average number of births anticipated over the lifetime of a woman in a particular population) since 1943 due to a breakdown in traditional institutions of fertility control (Cleveland 1986b).

The first cause of increased fertility was shorter birth intervals resulting from shorter periods of postpartum abstinence. One reason for this was improved child health, which was a consequence of increased transportation of food in time of famine and public health interventions, especially after the mid eighteenth century (vaccinations, disease vector control, cleaner drinking water, curative medicine) (fig. 1.6). The combined result was that postpartum abstinence was no longer dependent on the ability of the local environment to produce food and healthy conditions. The reduced infant and child mortality rates meant that parents could reduce birth intervals by decreasing the period of postpartum abstinence, thereby increasing the number of surviving children and improving household welfare (table 1.1, fig. 1.7).

Fig. 1.6. Birth intervals and child mortality.

Table 1.1. Zorse demographic variables.

Fig. 1.7. Value of children and fertility in Zorse.

Another reason for reduced periods of postpartum abstinence was European colonial rule-first by Germans, soon replaced by the British-in northern Ghana and Zorse beginning in the early twentieth century. The British military occupation led to decreased local hostilities among neighboring groups, diminishing the importance of mobility for mothers with young children, one reason for long birth intervals. Of course, the main purpose of the application of British colonial military power was extracting profit from farmers, which increased violence overall, due to British military dominance and coercive tactics such as forced labor.

The second cause of increased fertility rates was the decreasing age at which women married, a result of young men's becoming able to acquire bride-wealth cattle from outside of the village (Cleveland 1991). The colonial strategy was to break up largely subsistence-based agricultural communities, which were locally self-sufficient, in order to extract labor. At first this was done by physical coercion, and then by imposing a tax on each household, payable in British currency, which could be earned only by labor migration to work in commercial agriculture, mining, and public works in the south. These sectors were directly tied to the European economy for the benefit of Britain. Low wages and poor working conditions encouraged most migrants to return to their savanna villages when they were sick, injured, or too old to work. When Ghana gained its political independence from Britain, this new pattern of migration had become firmly established and was maintained by the changes in the social, economic, and transport systems.

The resulting increase in migration of young men to the south led to their economic independence from their families and community. They could now obtain bride wealth and marry without their family's help, without depending on the local environment to support the accumulation of bride-wealth cows. Along with their increasing exposure to outside values (e.g., more liberal attitudes toward sex), this led to the weakening of traditional values and the authority of elders. Therefore, marriage age (and thus fertility) was no longer connected to the local community and resource base (HCC), but to the international economy. The net result of this migration was a younger age at marriage, which in turn increased fertility (table 1.1).

At any one time about 50 percent of working-age males and 15 percent of working-age females from Zorse and the Upper Region were migrants in southern Ghana for a period of a year or more. Significantly increased dependency ratios mean that as a result of this migration all remaining working-age adults had to support themselves plus four dependents, instead of supporting only three dependents, as would be the case without migration. Remittances by Zorse migrants are equal to only a small fraction of the value of their lost productive labor: 50 kg of grain is required per consumer per year, but the average remittance from migrants was enough to buy only 2.5 kg. Evidence suggests that the net effect of migration on Zorse has been negative; neither labor productivity nor land productivity is likely to compensate for the higher dependency ratio.

What my research in Zorse revealed was a classic neo-Malthusian scenario-HI exceeding the HCC, HCC in turn declining under increasing HI, but HI continuing to increase in terms of population growth (table 1.1). The incentive to have more children was reinforced because increasing population densities were accompanied by a decrease in land productivity-due to erosion and lack of organic matter, labor shortages during periods of critical farm activity, reduction in natural vegetation, and increasingly inadequate food supplies-which led to chronic malnutrition and further eroded the quality of the labor force.

Examples from Zorse and elsewhere, and theories of common property management (chapter 7), support the hypothesis that fertility behavior can be adjusted to HCC in traditional societies. The industrial and scientific revolution led to the illusion of unlimited resources, which has encouraged delinking HI and HCC, under Boserupian assumptions.

5.4. Human carrying capacity and human impact gradually reach stability: A slow learner scenario

In the slow learner or overshoot and oscillation scenario, HI and HCC fluctuate by increasingly smaller amounts, then stabilize with HI below HCC (fig. 1.4d). This scenario implies that humans learn to regulate HI only when the consequences of not doing so are experienced, as consecutive increases in HCC due to innovation are followed by surges in HI, which decrease HCC.

Norman Borlaug, who received the 1970 Nobel Peace Prize for his leading role in creating the Green Revolution, argued for more rapid learning. He stated in his acceptance address that any "breathing space" won by the huge increase in yields and production resulting from the high-yielding crop varieties, fertilizers, irrigation, and other inputs of the Green Revolution should be taken advantage of by human beings to lower their birth rates: "the frightening power of human reproduction must . . . be curbed; otherwise the success of the green revolution will be ephemeral only." (Borlaug n.d.:30-31).

It now seems clear to many that at the global scale the Boserupian scenario has reached its limits, and the Malthusian scenario has been left behind. We are in an Anthropocene neo-Malthusian scenario-there is overwhelming evidence that HI exceeds HCC. Therefore, we can strive only to successfully speed up the fourth scenario-to be faster learners. It seems that a key part of this learning will be to replace the dominant, empirically unsupported assumption that increasing HI based on unlimited economic growth is possible, as well as its supporting value-based assumption that doing so is a good thing (see chapter 3).

6. Calculating human carrying capacity: Water for rice for energy

In the previous sections we saw how different sets of assumptions about HI and HCC affect beliefs about the zone and about what human response should be. In this section I provide a simple illustration of how to calculate HCC by dividing available resources by the human requirement for those resources, using the example of water needed to grow irrigated rice to provide calories for human energy. By highlighting the critical role of efficiencies and assumptions, this exercise helps demonstrate both the range of possibilities for dealing with the zone and the arbitrariness of any given estimate of HCC.

In its most basic form, the estimate of HCC is:

HCC (number of people) =

(Availability of resources for food production ∕ time period)

(Requirement per capita for food, in terms of agricultural resources needed to produce it ∕ time period)

However, it would be very difficult to obtain the data needed for this formula. It is much easier to calculate HCC for a critical resource and a critical requirement.

HCC (number of people) =

(Availability of the critical resource for producing the critical human-required nutrient ∕ time period)

(Requirement per capita for the critical nutrient in amount of critical resource needed to produce it ∕ time period)

Figure 1.8 illustrates the variables used for calculating HCC.

Fig. 1.8 Estimating HCC: general method.

6.1. Efficiencies

The HCC calculated here is the result of dividing the requirement for a critical resource into the availability of that resource, subject to three key efficiencies affecting the movement of the resource from its origin to its final human output. These efficiencies are: the resource-delivery efficiency (RDE) of delivering the resource to the crop (e.g., water from river to root zone of the crop plants), the resource-use efficiency (RUE) of the crop plants in converting the resource to required food (e.g. water into calories contained in rice), and the human-use efficiency (HUE) of humans converting food into the output desired by humans (e.g., calories in harvested rice to work energy) (fig. 1.9).

Fig. 1.9. Three key efficiencies affecting HCC.

The base availability of the resource has to be adjusted by the RDE, and the base requirement has to be adjusted by the HUE and by the RUE:


(Base availability of the critical resource per time period, for producing critical human required nutrient) × (RDE)

[(Base human requirement for the critical nutrient per time period) ∕ HUE] ∕ (RUE)

Efficiencies are ratios of output to input, and for RDE and HUE, because output and input are measured in the same units, the maximum efficiency is 100 percent-it is impossible to extract more of a given resource than you put in. To use an estimate of efficiency in a formula it must be in decimal form (e.g., 70 percent = 0.7). However, when output and input are in different units, such as RUE, efficiencies cannot be expressed as a percent, but the ratios for different scenarios can be compared. HCC decreases as efficiencies decrease, either as a result of decreasing resource availability in the numerator or increasing requirement in the denominator.

6.2. Assumptions

Even though estimates of HCC can be useful in providing insights into the determinants of human-environment interactions, they are based on assumptions that make them quantitatively ambiguous:

• that the diversion of natural resources in the amounts used in the calculation do not negatively affect ecosystems or society in the short or long term, when if fact this is difficult to determine;

• that consumption of food is done in a way that does not negatively affect ecosystems or society-in other words, that it is equitably distributed and consumed in forms that are nutritious and satisfying; and

• that the resource chosen, the nutrient chosen, and the crop chosen (irrigation water, energy, and rice in the example) are representative of other resources, nutrients, and crops. However, we know that many resources are required for food production, that many nutrients are required for human health, and that crop species and varieties differ in the amount of nutrients they contain and their efficiency in converting resources into those nutrients.

6.3. An example

In this and the following sections I show how to calculate the HCC illustrated with the example of water needed to grow irrigated rice to provide kilocalories for human energy (fig. 1.10) and how changing the values of key variables can dramatically change the final estimation of HCC.

HCC (billions of people) =

(Availability of water (km3) for irrigating rice year-1)

(Requirement of water (m3) to grow rice providing kilocalories required person-1 year-1)

Fig. 1.10. Estimating HCC for one key food production resource: irrigation water for growing rice for human energy requirements.

6.4. Availability of resources

The availability of resources for production is estimated based on those available given current technology, infrastructure, and social organization. Increasing their quantity in the future depends on changing one or more of these parameters. The movements and modifications of that resource to make it immediately available to plants will necessarily reduce its quantity or quality or both, in turn reducing the potential for future food production. This is measured as the resource-delivery efficiency (RDE):


Amount of resource delivered to plant

Amount of resource extracted from natural system.

For irrigation, RDE is referred to as the irrigation efficiency (IE), and because it is always less than 1, it reduces availability from base (extracted) levels.

Availability = (Base availability yr-1) × (IE), where IE =

water delivered to root zone

water extracted from river, lake, or groundwater aquifer for irrigation

The total volume of water on Earth is about 1,400 million km3, of which only 2.5 percent, or about 35 million km3, is freshwater. For this example I will use an arbitrary estimate of 9,683 km3 of water per year available for withdrawal for agriculture globally. The two main components of IE typically used are conveyance efficiency (CE) and application efficiency (AE). Examples of factors that decrease CE are evaporation and leakage from canals, breakage of canals, and extraction from canals for other uses, such as household washing. We can assume for this example that CE also includes efficiency of extraction. Factors that decrease AE include unevenness of the field, so that there are high spots that don't get enough water and low spots that get too much, loss of water below the root zone, in animal burrows, and evaporation. Multiplying efficiencies results in lower overall efficiency-for example, if CE = 0.5 and AE = is 0.6 (typical for small-scale, traditionally based irrigation), then IE = 0.3.

Availability = (Base Availability yr-1) × [IE = (CE) × (AE)]

= (9,687 km3 yr-1) × (0.5) × (0.6)

= 2,906 km3 yr-1

6.5. Requirement for resources: Human nutritional requirement

On the requirement side, we have to first determine what individual (per capita) requirements are for food in terms of the recommended dietary allowances (RDAs) for critical nutrients-most often energy (measured as kilocalories, or kcal) and protein (measured in grams). This is a scientifically complex topic with many political and ethical aspects, and hence it is very controversial.

For calculating HCC we will use the conservatively low round number of 2,000 kcal per person per day. Then we adjust the gross human requirement for the critical resource by the efficiency with which the harvest is transported from the field; processed, cooked, and eaten; and ultimately converted to usable nutrients by the human body. Since all of these steps reduce the overall quantity and sometimes the quality of the nutrient, this process will increase the effective requirement and ultimately decrease the HCC. We refer to this as the human-use efficiency (HUE):


Amount of food (nutrient) metabolized by humans

Amount of food (nutrient) harvested

Factors that contribute to reducing the amount of a nutrient that is metabolized, proceeding from harvest to human metabolism, include:

• harvest and postharvest losses: grain, tubers, and so on left in the field, eaten by insects, rodents, or microorganisms

• losses in transportation

• losses in processing and storage

• processing that reduces nutrient availability

• health-related reductions in digestive and metabolic efficiency

• conversion of plants to animal food

It is conceptually useful to divide HUE into two components: from field to consumption (field to mouth), and from consumption to final use by the body (metabolism). To estimate the effect of HUE on resource requirement, we divide the base requirement by HUE, which increases the requirement.

Requirement person-1 for food, in terms of critical nutrient

= (Base human requirement for the critical nutrient) / (HUE)

For our example, if we assume that the HUEs for field to mouth and for human metabolism are both 0.75:

Requirement (kcal)

= [(2,000 kcal day-1 person-1) × (365 days yr-1)] ∕ [(0.75) × (0.75)]

= 1,297,778 kcal person-1 yr-1

6.6. Requirement for resources: Conversion of resources to food

The third major step before calculating the HCC is to estimate the amount of a resource required to produce a particular amount of human nutrient. We do this by working backward, converting the human requirement for food in terms of the critical nutrient into the amount of critical resource required to produce it-for example, by converting the requirement for energy in kcal into kg of water required to produce those kcal in harvested rice grain. Once the resource is spatially available to the plant-for example, water or nutrients in the root zone-it must be taken up and converted into the food required by humans, the resource-use efficiency (RUE).


Amount of food (nutrient) produced by the plant

Amount of resource used by the plant

To estimate the RUE we typically begin by calculating the amount of the nutrient in a kg of total aboveground dry matter (TDM), then divide this by the amount of water required to produce 1 kg TDM, which we assume is 500 kg water. To convert TDM to grain we multiply by the harvest index (HI), a measure of the proportion of TDM that is grain (edible portion). For our example, if we assume that HI = 0.5, then 1 kg TDM = 0.5 kg grain. If there are 3,500 kcal per kg of grain, then 1 kg TDM = 1.750 kcal.

kcal (kg TDM)-1 = [(1 kg TDM ) (HI = 0.5) (3500 kcal kg-1)

kcal (kg TDM)-1 = 1750 kcal

Finally, we divide the kcal contained in 1 kg of TDM by the 500 kg of water required to grow 1 kg TDM:


= [1750 kcal (kg TDM)-1] ∕ [500 kg water (kg TDM)-1]

= 3.50 kcal kg-1 water

6.7. Calculating human carrying capacity

Putting the three parts of the equation together:


Availability per year

Requirement per year [(Human nutritional requirement) ∕ (Conversion efficiency of resource to nutrient)]

For our example:


= 2,906 km3 water yr-1


= [(1,297,778 kcal person-1 yr-1) ∕ 3.5 kcal kg-1 water)]

= 371 m3 water person-1 yr-1


= (2,906 km3 water yr-1 available) ∕ (371 m3 water person-1 yr-1 required)

= 7.8 billion people

6.8. Sensitivity analyses

Table 1.2 summarizes the above calculation in the "Text example" column, with "Alternate scenarios" columns illustrating the change in HCC due to modifying the assumptions of the text example for the highlighted variables. This sensitivity exercise shows that HCC varies widely with seemingly small differences in estimates of the values of variables in the equation. The first scenario assumes that 33 percent of the required calories are consumed as animal foods, which are produced by converting the calories in rice to calories in animal foods with only 25 percent efficiency. The large reduction in HCC points out the large contribution that animal foods make to total HI, and the potential for diet change to reduce impact, as discussed further in chapter 8. The second scenario shows the importance of water management and the need for effective physical and social means of efficiently delivering and using resources, as discussed further for irrigation in chapter 7. Increasing harvest index is the changed assumption in the third scenario, which has been a major factor in the ongoing domestication and breeding of crops, and especially important in the Green Revolution, as discussed in chapters 2 and 5. The last scenario shows the dramatic decrease in HCC that results from a 20 percent decrease in the availability of water and highlights a major challenge of global climate change for agrifood systems, as discussed in chapter 8.

These estimates of HCC and the way they change with differences in assumptions about the current and future realities are useful even though they are based on uncertain data and arbitrary assumptions. They provide us with some conceptual understanding of how to estimate the number of people the Earth can support, reinforcing the notion that there is a finite amount of resources available to support the human population. They also give us insight into the efficiencies in the relationships between variables determining the HCC, including how HCC varies with change in these variables. Finally, estimating HCC can help to increase awareness of the zone and stimulate discussion to figure out how we want to deal with it, given that the growth of HI is so great that just increasing efficiencies will probably not be sufficient to help us step back from the zone (see chapters 8 and 9).

Table 1.2. Calculating the HCC