Population and the Environment Research Paper

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Examples throughout history reveal that the simple formula—more people equals more environmental disruption—does not always apply. Nonetheless, in most circumstances, population growth has brought accelerated environmental change and continues to do so. Since the mid-twentieth century, as human population growth approached its maximum rate, the relationship between population growth and the environment has been the subject of popular and scholarly debate.

The relationship between human population and environment, contrary to popular belief, is anything but simple. In the past half-century, as concern over environmental degradation mounted, popular discussion more often than not emphasized the simple, appealing equation: more population equals more environmental degradation. Although this is true in many circumstances, it is by no means invariably so. Scholars have expended great energy trying to tease out the relationship, but with limited success. The question, in both popular and scholarly debate, has been for fifty years and remains to this day a highly political one, with deeply felt principles involved.

History of Human Population

Efforts to count the number of people in a given territory began in ancient times. The first effort to take a census for an entire polity (Tuscany) was made in 1427. Reasonably reliable censuses date from about 1800 and for most of the world from about 1950. So, reconstructing the history of the entire human population inevitably involves a hefty amount of inference and educated guesswork. Opinions vary, although there is remarkable consensus on the general trajectory.

Although it is unclear exactly when humans became humans, when they did there were few of them. Living as hunter-gatherers they were constantly on the move. Carrying multiple small children was a great burden, so early humans checked their fertility by prolonged breast-feeding (which reduces a woman’s fertility) and probably checked population growth via infanticide and abandonment. In any case, population growth was extremely slow by today’s standards, although it should be understood that today’s standards are a bizarre anomaly: for most of human history population growth in net terms was close to zero, and that population declined almost as often as it grew.

With the shift to food production and more sedentary ways of life, the chief constraint on population growth, the difficulty of carrying around small children, eased. The origins of agriculture date to about ten thousand years ago, at which time there were 4 million (more cautiously, 2 to 20 million) people on the Earth. Where agriculture first took root, in southwestern Asia and the tropical lowlands of the Americas, population growth accelerated somewhat. Birthrates climbed, and although death rates eventually did as well, they did not keep pace. Death rates eventually climbed because agricultural societies acquired new diseases, most of them transfers from herd animals such as pigs, cattle, and camels, that quickly killed people, especially young children. Crowding helped spread such diseases rapidly and promoted others that flourished where human beings lived amid their own wastes.

Gradually agriculture spread throughout much of the suitable terrain on Earth, and most of the human population lived in agrarian societies in which villages formed the social nucleus. Irrigation, especially in Egypt, southern Asia, and East Asia, allowed more productive farming and still denser populations. By 3500 BCE cities began to emerge, first in Mesopotamia. The efficiency of agriculture is extremely variable, depending on soils, crops, tools, and other factors, but as a general rule it can support ten times the population density that hunting and gathering can. For this and other reasons agricultural societies spread fairly rapidly at the expense of the less populous communities of hunter-gatherers.

In agrarian societies, children from the age of about five could perform useful labor, such as tending chickens or weeding gardens. Without the requirement of constant migration, children were more an economic asset than a liability, and so, except in conditions of land scarcity (and even sometimes in such conditions), people tended to marry young and reproduce prolifically. Fertility (here presented in the form of the crude birthrate) reached levels of perhaps 50 per 1,000 per year (about four times as high as the current birthrate in the United States), although rates of 35–40 per 1,000 were probably more typical. Even so, reproductive exuberance barely kept up with the toll from disease and famine, which every now and then reached catastrophic levels, pruning back the population growth of happier years. This, in broad strokes, was the demographic regime of agrarian society, the majority experience of humankind from at least 3000 BCE until 1800 CE.

During that time population grew much faster than it had during pre-agricultural times, although still slowly in comparison to today’s growth rates. And there were times when population declined. On local and regional scales, epidemics and famines produced such catastrophes fairly regularly, normally at least once or twice within every generation. On the global scale there were at least two great catastrophes, each of which probably brought global population decline (although the figures are not reliable enough to say with assurance). The first of these was the great pandemic of the fourteenth century known as the Black Death, probably a result of the spread of bubonic plague throughout most of Asia, Europe, northern Africa, and perhaps parts of sub-Saharan Africa. It reduced the population of Europe, Egypt, and southwestern Asia by perhaps one-quarter or one-third and on a global level by perhaps one-seventh or one-tenth. In Europe the population took 150 years to recover from the plague’s ravages. The second great catastrophe came when the population of the Americas was exposed to Eurasian and African diseases in the wake of voyages made by Christopher Columbus and other explorers. Estimates of the population loss for the period from 1500 to 1650 range from 50 to 90 percent. Because there are no good data on the size of the pre-Columbian population in the Americas, it is impossible to know how large the global effect of this disaster may have been. It might have lowered total global population, although more likely, because there were far more people in Eurasia and Africa than in the Americas, the total effect repressed world population growth without forcing it below zero.

Accelerating Growth

During the eighteenth century human population embarked on its current spectacular expansion. In several parts of the world epidemics and famines started to recede, and death rates fell. The reasons behind this remain uncertain, although ecological adjustment among pathogens (agents of disease) and their human hosts was surely part of it, as were improvements in food supply and famine management. In some places birthrates also rose slightly. During the nineteenth century world population almost doubled, and then in the twentieth century it almost quadrupled as death rates tumbled. Better sanitation, vaccines, and antibiotics lowered the toll from disease, and much more productive agriculture increased the food supply sharply. In Europe by the 1890s families responded by consciously limiting the number of births, a reaction that occurred later but more quickly in most other parts of the world. When and where birthrates shrank, population growth slowed; when and where birthrates remained robust, as in much of Africa, Central America, and parts of southern and southwestern Asia, population growth spurted after 1950. Globally, the growth rate peaked around 1970 at about 2.1 percent per year, and population increased by another billion every 12 to 15 years. By 2009 annual growth had declined to 1.1 percent per year, or about 73 million additional people annually. Demographers now expect the world’s population to reach 9 or 10 billion around 2050.

For most of history roughly three-fourths of humanity lived in Eurasia. That remains the case today, but the proportion living in the Americas grew sharply after 1750, and the proportion living in Africa leapt upward after 1950.

Population Policy

People have voiced concern that humanity is too thick on the ground since at least 1600 BCE. Such concern, however, has been rare until recently. Political authorities, when they gave any consideration at all to population, generally took the view that the more people there were within their borders, the better. All the major religions favored population growth, too. This is not surprising: until the last 250 years survival was so precarious that maximizing births was usually a sensible insurance policy against disaster. But in the mid- and late-twentieth century a few governments began to see matters differently. India and China, by far the two most populous countries, committed themselves to birth control, and in China’s case to stern restrictions that between 1978 and 2009 kept the population 300 million lower than it would otherwise have been (it was 1.33 billion in 2009). Other countries in the twentieth century, especially in Europe, sought to increase their birthrates but to very little effect.

Population and Environment

At all times and places the relationship between population and environment is one of mutual interaction. Environmental conditions affect population’s trajectory and population growth (or decline) affects the environment.

Historically, the environmental conditions that most influenced population were climate, disease, and agriculture. Major climate shifts, such as the waxing and waning of ice ages, strongly affected human population by changing the proportion of the Earth that was habitable and by changing the biological productivity of the parts not covered with ice. The onset of the last ice age presumably reduced human population, and its end encouraged population growth. Since the end of the last ice age ten thousand years ago, climate change has played only a small role in determining human population size on the global level.

As noted, the human burden of disease became markedly heavier when and where people took up cultivation, especially settled cultivation, especially when domesticated animals were involved. In tropical lands, except at high altitudes, sustaining dense settled populations was difficult because so many more disease organisms flourished in the warmth. The emergence of cities also created lethal disease environments, mainly because people lived cheek by jowl, communicating infections daily, and because few cities adequately disposed of wastes. So, cities generally, perhaps invariably, were black holes for humanity, sustained only by recurrent migration from healthier, rural landscapes. This situation remained in force until the end of the nineteenth century and in many countries until the middle of the twentieth century. Eventually, mainly through scientific sanitation after 1880, cities became even healthier than rural areas, and one of the great historic constraints on population growth, the lethality of city life, was lifted.

Changes in agricultural conditions also helped to regulate human population by affecting the food supply. Irrigation agriculture, as noted, could support more people than could rainfall agriculture. But irrigation often generated salinization (the accumulation of salts, harmful to plant growth), which over centuries could spoil farmland, as in Mesopotamia, where environmental degradation probably played a role in episodes of depopulation around 1900 BCE, around 1375 BCE, and before 1250 CE. Over centuries, even over decades, soil erosion could also significantly lower the productivity of agricultural land, which, if not otherwise compensated for, could reduce population. Salinization and erosion could easily affect local and regional populations severely, although at the global level their impacts have always been negligible.

A more recent change in agricultural conditions, the Green Revolution, has had impacts at every scale. Since the 1950s agronomists (agricultural scientists dealing with field-crop production and soil management) have bred new strains of most of the major food crops of the world, making them responsive to heavy doses of fertilizer and timely doses of irrigation water, more resistant to crop diseases and pests, and more suited to machine harvesting. As a result, modern chemicalized agriculture doubled and quadrupled crop yields. The global effect as of 2000 was to increase the world’s food supply by about one-third, an essential component of the contemporary surge in world population.

Population growth, or decline, also affects the environment. Just how much it does depends on many factors, including rates of growth, existing densities of population, resilience and stability of ecosystems, the technologies available, and just which aspect of the catch-all phrase the environment one chooses to measure. The amount of nuclear wastes around the world, for example, has had little to do with population levels or growth rates but everything to do with technology and politics. Conversely, urban sprawl has resulted directly from population growth (although other factors have been involved).

The circumstances under which population growth proved maximally disruptive to environments were probably those in which initial levels of population were either zero or very low, the growth rate vigorous, and the transformative technologies at hand powerful. The history of New Zealand provides an apt illustration. New Zealand was long isolated from outside influence and had no human population for millions of years—a sanctuary for species from the Cretaceous era. People first arrived around 1300 CE (perhaps as early as 1000), at first probably only a few. But the Maori—New Zealand’s original settlers—found ample resources in seals, mollusks, and large flightless birds (moas), which they hunted. They burned the forest to make better forage for creatures they hunted and to make room for crops. In the course of a few centuries, they drove the moa and a few other species to extinction and reduced the forest cover of New Zealand by about one-third or one-half. Similar dramatic changes followed upon initial human settlement of other isolated islands such as Madagascar (c. 400 CE) and Iceland (c. 870 CE). Presumably the impacts of initial human settlement in the pre-agricultural past were less pronounced, although the occupation of Australia (about sixty thousand years ago) and the Americas (about fifteen thousand years ago) may have brought on—this is the majority view but by no means a consensus view—numerous extinctions of large and midsized mammals. Even without much technology beyond spears and fire, humans, when entering landscapes in which species had no prior experience of human ways, proved highly disruptive.

With more powerful technologies at hand, population growth could be even more disruptive. After 1769, and especially after 1840, New Zealand acquired another settler population, primarily from Great Britain. These settlers had metal tools, which the Maori had not had, grazing animals, and eventually steam engines and the entire panoply of industrial machines. In the span of two centuries, New Zealand’s population went from less than 100,000 to about 3 million, almost all of whom used modern technologies. New Zealand lost most of the rest of its forest cover and many more of its indigenous species (mostly birds), and most of the landscape (discounting inhospitable extremes) became pastureland. Population growth alone, of course, was not responsible for this transformation of New Zealand, although it was crucial. Also crucial was the existence of overseas markets for wool, mutton, and butter, on which New Zealand’s pastoral economy rested.

Population growth has been least disruptive where heavy labor has been required to stabilize an environment. The best examples of this concern soil erosion. Farmers, when working on slopes, inevitably risk rapid soil erosion unless they can construct and maintain terraces. But that is extremely labor intensive. In the Machakos Hills district of Kenya, for example, early in the twentieth century farmers caused high erosion rates by tilling the soil. They did not have enough people to undertake the backbreaking labor of terrace construction. But by the 1960s population growth had changed that. Farmers built and maintained terraces, stabilizing their soils. A reduction in population density in terraced mountain environments can bring accelerated soil erosion because too few people remain to keep the terracing in place. This happened in the twentieth century in the mountain regions of southern Europe when birthrates fell and young people emigrated. The endless terraces of Java or southern China would be difficult to maintain without dense population.

Population decline destabilized other landscapes as well. In eastern Africa, for example, by the nineteenth century people had learned that to keep sleeping sickness at bay they had to burn off the bush (which reduced the habitat for the tsetse fly that carries sleeping sickness). Sleeping sickness killed cattle more easily than people; it was an economic as well as a health problem. But bush control required labor, and when lethal epidemics broke out at the end of the nineteenth century and the beginning of the twentieth, one result was to make it difficult for people to control the vegetation surrounding their villages. Thus a costly ecological change proceeded: more bush, more tsetse fly, more sleeping sickness. This example, like the terraces of southern Europe, is a case in which an environment, already modified by human action and in a more or less stable state, was disrupted by population decline.

These examples show that a simple formula (more people equals more environmental disruption) does not necessarily apply. Nonetheless, in most circumstances, population growth has brought accelerated environmental change and continues to do so. In the context of the last half-century, when human population growth reached its maximum rate, the role of population has probably been greater than in previous times (with the exception of local- and regional-scale examples such as New Zealand’s initial settlement). Cropland has increased by one-third since 1950, a process mainly driven by population growth. The proportion of land occupied by roads and buildings has grown roughly in step with population and chiefly because of population. The transformation of habitats, including deforestation, the extension of cropland, pastureland, and developed land, has put heightened pressure on many species lately, especially in tropical forests. This pressure, one of the signal environmental changes of modern history, has been driven in part by population growth, although it is difficult to specify how large that part might be.

Population growth also has been a factor in the increased pollution loads of modern history. In cases such as water pollution derived from human wastes, it has been a large factor indeed. But in other cases, such as the pollution of the stratospheric ozone layer by chlorofluorocarbons, population growth played only a little role, and technological change (the invention of chlorofluorocarbons) a much larger one. Thus, among types of pollution, as among environmental changes in general, the degree to which population growth can logically be held responsible varies tremendously from case to case.

In the future it is likely that population will decline in importance as a variable in shaping environmental change. This is partly because the extraordinary pulse of population growth of the past century, and especially the past half-century, will sooner or later come to an end. But this is also because technology looms ever larger as a mediator between people and their environments, and the pace of technological change seems unlikely to slow any time soon. If global population stabilizes after 2050, as many demographers suppose it will, population shifts locally and regionally will still exert pressures of one sort or another. And it remains possible that because there are already so many people on Earth the addition of another 2 or 3 billion will have a much stronger impact than the addition of the last 2 or 3 billion. That is, there may be nonlinear effects with respect to the consequences of population growth, thresholds that, if surpassed, bring major changes. Observers have predicted catastrophic consequences from population growth for millennia but most consistently (and most plausibly) in the last forty years. It has not happened yet. If it does, it will happen within the next fifty years.

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