How many people can the earth support? (A) Introduction There is widespread concern about the deterioration of the global environment. Future food supplies are threatened by widespread topsoil depletion, falling water tables and Stalinization of irrigated soils. Human waste products are causing severe pollution of soils, rivers, lakes and coastal waters. Gaseous emissions are causing global warming resulting in an increase of severe weather events, and are predicted to lead to the loss of large areas of densely populated yet agriculturally valuable land through rising sea levels. Large areas of natural ecosystems such as tropical rainforests are being destroyed, leading to what many consider to be the opening phase of a mass extinction of species which might rival mass extinction episodes of the prehistoric past. Not surprisingly, the United Nations Environmental Programmed has concluded that there used to be a long time horizon for undertaking major environmental policy initiatives. Now time for a rational, well-planned transition to a sustainable system is running out fast. In some areas, it has they say, already run out. Here are two examples from recent studies which illustrate the situation. The first deals with a specific problem in one part of the world, the second looks at many factors in the world as a whole. Drechsel et al studied soil nutrient depletion in 36 countries of sub-Saharan Africa. Soil fertility is considered to be the main biophysical fact limiting per capita food production on the majority of African small farms. The depletion is caused by crop harvest and removal of residues, erosion, run-off and leaching. When soil nutrients are lost faster than nutrients are added by natural means and by man, a negative balance occurs. A strong negative correlation was found between soil nitrogen balance and rural human population density. Fallow periods allow for natural soil regeneration. But fallows have been increasingly encroached upon in the attempt to increase food production. And there was a positive correlation between nitrogen balance and percentage of land under fallow. Marginal lands, not really suitable for agriculture, have increasingly been used and protected areas encroached upon. Sharing farms between sons has led to reduction of farm size to the point where size is inadequate and many people become landless. The authors conclude “it appears that Malthusian mechanisms are at work”. No amount of innovative management will lead to sustainable use of resources under conditions of continuous population increase and farm size reduction, i.e. without “out-migration” and population growth limiting measures. The second study, by Tilman and others examined the trajectories over the last 35 years or more of several environmental indicators: annual rates of application of nitrogenous fertilizer; annual rates of application of phosphate fertilizer; total area of irrigated crop land; total area of pastureland; total area of crop land; global pesticide production rates; expenditures on pesticide imports. Surprisingly each trajectory was a linear and almost equally strong function of time, population and GDP. Assuming agriculture continues on these trajectories, the authors prepare forecasts for the next 50 years. According to the forecasts there will be a world wide loss of 109 hectares of natural ecosystems (converted to agriculture) -an area larger than the USA. This could mean the loss of about one third of remaining tropical and temperate forests. This loss would be accompanied by a roughly two and a half fold increase in eutrophication of terrestrial, freshwater and near-shore marine ecosystems, and a comparable increase in pesticide use which would have adverse effects on wildlife and on human health through continued accumulation in food chains. Now many have argued that mans innovative and regulatory abilities (for example plant breeding) will enable mankind to cope effectively with any future population growth. The second paper above throws doubt on this since the trajectories studied include in them the impact of past technological developments, changes in consumer choices and environmental regulation. We see here confirmation of the concern expressed by the Royal Society (UK) and the National Academy of Sciences (USA) in their 1992 joint statement “Population Growth, Resource Consumption and a Sustainable World” which encapsulates many concerns. The statement argued that if human population growth continued as predicted, and patterns of human activity on the planet remained unchanged, it may not be possible for science and technology to prevent either irreversible degradation of the environment or continued poverty for much of the world. All these considerations lead us to conclude that human population growth is the underlying multiplier, some times the chief cause of, global environmental problems. Other causes are however also important. So we agree with one of the conclusions of the recent United Nations report “Population, environment and development”. Asserting that environmental problems are largely the result of human activities, they vary in the degree to which they can be linked directly to population size, growth and distribution. The report gives as one example: increases in some types of pollution which it says are primarily the by-product of rising per capita production and consumption in richer economies where population has generally been growing only slowly. We think that if one draws together the various strands of argument presented so far, it is reasonable to conclude with William Rees (8) as follows. The situation we are in is that both human population and average consumption are increasing while the total area of productive land and stocks of ‘natural capital’ are fixed or declining. In other words, the total ‘load’ of the human economy is increasing. So a fundamental question is: will the physical output of remaining ecosystems and the waste assimilation capacities of the ecosphere be adequate to sustain the anticipated increased load of the human economy? (B) Is population growth always such a serious problem? Economists have for a long time pointed out that population growth can bring economic benefits. Thus as population grows, markets for goods become larger, which leads to bigger manufacturing plants that may be more efficient than smaller ones, with longer production runs and hence lower set-up costs per unit of output. A larger market makes possible a greater division of labor. If markets for goods are small, a firm will buy a machine that can be used in the production of several kinds of product. If the market is larger, the firm can afford to buy a separate more specialized machine for each operation. And a greater population density makes social investment (e.g. railways, irrigation) more profitable. Could not such increases in economic efficiency benefit the environment, and thus have a positive effect on carrying capacity changes? Some economists however go much further, none more so than Julian Simon. He argues that in terms of non-renewable resources, to think in terms of population growth leading to depletion, what he calls a ‘closed system’ is the wrong way to look at things. This approach focuses on the conservation of resources rather than the creation of such resources. He supports the following paradigm: Pursuit of some particular resource leads in the short term to falling availability and consequent rise in prices. This however has two effects. First, it stimulates people to develop better extraction technology; second, it stimulates people to find/develop substitutes for the non- renewable resource. The result is that this leaves us better off than if the original problem had never arisen. Simon says that on his view – and contrary to general expectations of what is happening – the result should be, in the long time, a fall in prices of raw materials measured by prices relative to consumer goods prices or relative to wages. He claimed that this was the way that prices had gone in the long run for most non-renewable resources. We see here one aspect of a more general point of view which is shared by people who are optimistic about the future of mankind despite continued population growth: man has always managed in the past to surmount his problems through technological innovation, so there is no reason to think he will not continue to do so. However, William Rees argues the major difference between mankind and other species is that in addition to out biological metabolism, we have developed an industrial metabolism which requires a continuous flow of energy and material from and to the environment. Mainstream economic thought, working through monetary analysis, largely ignores these flows (‘externalities’). We may think of things in terms of the Second Law of Thermodynamics: complex dynamic systems such as industrial economies are in a non-equilibrium state through the continuous dissipation of available energy and material extracted from the host environments. They thus require a constant input of energy/matter to maintain their internal order in the face of spontaneous entropic decay. This relationship implies that beyond a certain point, the continuous growth of the economy can only be purchased at the expense of increasing disorder or entropy in the world ecosystem (ecosphere) which supports it. Now many economists have sought to ‘internalise’ environmental costs, by resource pricing and pollution charges/taxes. Unfortunately, prices merely reflect current availability, they do not reflect the size of natural capital stocks, and many “ecological goods” and life support services (for example, the ozone layer) remain unprimed. In the opinion of W. Rees, “because of such non-trivial losses of information, commoditizing nature is misleading and potentially dangerous”. (C) Carrying Capacity One focus for discussion of population growth–environmental deterioration issues is the concept of carrying capacity, a term taken up from ecology and applied ecology. We will begin an exploration of this concept in the present essay. ( i ) Carrying capacity in plant and animal populations Imagine a small population of an organism living in an environment unlimited in extent and containing unlimited resources. The population grows at an increasing rate, and the population growth curve is the exponential curve, the curve which represents growth in the absence of restraints (curve a in the figure). We might try to reproduce this type of growth in a laboratory, but we know that in nature, such ever-accelerating growth does not take place. The growth of a population might start off approximating to the exponential curve, but increasingly departs from it and the resulting curve, sigmoid in shape (curve b), shows how growth is restricted by the environment in which an organism lives. We say that growth is limited by the carrying capacity (K) of the environment . The carrying capacity concept then implies the existence of limits to growth. This sigmoid curve is discussed in textbooks of ecology. The carrying capacity concept has been taken up in applied ecology and definitions here are often along the following lines: The maximum population of a given species that can be supported indefinitely in a given area, that is without permanently damaging the ecosystem on which it is dependent. By ecosystem we mean the community of all living organisms in a given area together with its physical environment. This definition will initially serve our purposes in this essay. An example from applied ecology illustrates the carrying capacity concept and the difficulties of applying it. The example concerns raising cattle in the Kalahari of Southern Africa. The Kalahari is a savanna region with low and very variable annual mean rainfall (gradually decreasing towards the south-west), high daytime summer temperatures, no surface water, and a deep sand substrate. The Kalahari has supported a plant population of scattered trees and shrubs and a cover of herbs (mainly grasses) interspersed with much bare sand. This habitat was ideal for supporting large herds of big herbivores such as the Wildebeest which migrated over large distances to places where the grazing was better at particular times. Cattle were introduced, supported by borehole-supplied water. It became important to establish how many cattle a given area could support. If there were too many cattle, this would cause overgrazing and a consequent reduction of carrying capacity. However, it was a complicated business to try to determine cattle carrying capacity. Many factors are relevant – amount of rainfall, its seasonal distribution, water loss by evaporation from the sand and by transpiration from plant leaves, depth of water penetration, sand temperature, total weight of grasses in a given area and depth of plant roots, the nitrogen content, palatability and digestibility of grasses, etc. Things were complicated further by the fact that the annual rainfall is extremely variable. On top of this, the Kalahari is subject to a 20 year rainfall/drought cycle, so a series of mainly good years is followed by a series of predominantly poor rainfall years. The value of such calculated carrying capacities is consequently very limited. The best that could be said is that the estimates show the general order of the number of cattle that could be raised when specified for either good or poor rainfall years. (ii) Carrying capacity of human populations. We have seen that carrying capacity applied to animal populations can be quite a complicated matter. We might expect that when the concept is applied to human populations, things would also be complicated, probably more so. For man has the ability to alter the environment deliberately, for good or for worse. And his technological innovations can greatly alter mans impact on the environment. So it should come as no surprise that one worker noted that estimates of global human carrying capacity varied from less than one billion to more than 1,000 billion, although most estimates fell within the range 2 to 14 billion (remember that the present population is a little over 6 billion). Further the same author alleged that many estimates of human carrying capacity, have a “cloak of quantification”, but are probably more in the nature of political instruments than they are dispassionate analyses. And one demographer concluded that the carrying capacity idea has so many conceptual difficulties that it is virtually useless for practical purposes. We do not share this latter view, so we will proceed with our analysis. We briefly mention two factors that must be taken into account, at the same time accepting that these factors make it difficult to estimate present global capacity, and especially estimates of future capacity. These two factors are technological development and lifestyles (or affluence). We are all aware how improved technology can increase the energy efficiency of a given operation (and so reduce energy resource consumption and harmful emissions produced by the operation). We only have to think of increased fuel efficiency in cars, or reduction of pollution from factory chimneys or incinerators. In general terms we know that technology is more advanced in developed countries than in developing countries. Yet the greater part of the human population is found in the developing world and it is here that most future population growth will take place. To what extent will more advanced technology be purchased and installed in the developing world? It is impossible to say. Such technology may reduce costs in the long term, but it is expensive to buy and install. To what extent will developing countries have the money to make use of such technology? Perhaps more importantly, what priority will they give to making this compared with say arms purchase? Now to lifestyles. It is clear that in determining carrying capacity of a nation or the world, we must specify the lifestyles of the people or if you like, their degree of affluence. It has often been pointed out that that any given area of the world could support far fewer people with the lavish lifestyles of the USA, than it could people with the lifestyle of the average citizen of a country like India. Now a quarter of the world's population lives in extreme poverty and many millions more have an unacceptably low standard of living. It is important to try to raise the standard of living in developing countries. But to what extent will this be possible? And if people eat more and better food, will this not increase the adverse effect of agriculture on the environment? Clearly the earths carrying capacity, whatever that might currently be, is likely to change. It would be wise to adopt the precautionary principle, widely accepted as being an essential part of any strategy to achieve sustainable development. One implication of this is that we should make every effort to reduce future population growth. Meanwhile, it remains important to try to estimate the present carrying capacity of the earth, despite the difficulties. This could then be used as a baseline for various projection scenarios, taking into account past trajectories as we saw in the work of Tilman and associates earlier. So we return to the question. Is it possible to determine the earth's carrying capacity? If so, how do we go about it? We believe it is possible through the study of what is termed “ecological footprints”, a concept developed by William Rees and Mathis Wackernagel. Consider any city. It is clear that it does not sustain itself just from resources within its boundary. Most of the food and raw materials will come from outside. And the waste products of the city will not all end up within the city's boundary. For example, carbon dioxide emissions will enter the atmosphere which is no respecter of boundaries. The city then depends for its life on a lot of land outside its boundaries, land on which food is grown and from which raw materials are taken. And forests are needed to absorb the carbon dioxide emissions to mitigate global warming. This total area of land needed by the city is called its ecological footprint. In one publication the question was posed “what is 120 times the size of London?” The answer given was the land area or ecological footprint required to supply London’s environmental needs. Calculating ecological footprints then involves calculating land areas for different requirements (e.g. land for crops). It’s an old question. Two hundred years ago, Thomas Malthus said population would race ahead of food supply, but he wasn't the first. The early Christian writer Tertullian said (around AD 200, in De Anima): "We are burdensome to the world, the resources are scarcely adequate for us... Truly, pestilence and hunger and war and flood must be considered as a remedy for nations, like a pruning of the human race becoming excessive in numbers." That was when the population of the whole planet was maybe 100 million or so. We reached the first billion mark by about 1850. By 1950, it was about 2.5 billion. In less than one short lifetime, this figure doubled. It passed six billion in the late 1990s. Note that: humans took 150,000 years to get to the first billion. The most recent billion arrived in just 12 years. Nobody knows how many people the planet could hold. The UN predicted this week that fertility would decline and longevity would increase until the global population stabilized at nine billion in 2300. Some optimists have argued that the planet could support 1,000 billion; others look at what is happening right now and wish that it had stayed at ancient Roman levels. Joel Cohen, the Rockefeller University population biologist, argues in a 1995 book (How Many People can the Earth Support?) that it isn't a question like "How old are you?" which only has one answer at any one time. Cohen argues that you could fit one billion people each a meter apart, into a field 32km square. So everybody in the world would fit easily into Yorkshire. But it takes 900 tones of water to grow a tone of wheat, and there is only so much water, so much land and so much sunshine. Human action has its own "ecological footprint"; there has to be so much land to provide food, clothing, and shelter, medicines, building material, fresh air and clean water for any one human. It takes, according to some calculations, 2.1 hectares of land and water to provide for one average human. The important word is: average. The American footprint is about 10 hectares. So if all humans lived at US standards, we'd need another four Earths.
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