The Earth’s population doubled between 1960 and 1999, increasing from three billion to six billion people. During that period, human induced changes in the global environment accelerated in unprecedented fashion. Given continued population growth and environmental degradation, it has become paramount that we deepen our understanding of the role played by human population dynamics in environmental change. Drawing from the scientific literature, this essay presents a synthesis of what is known about the role played by human population factors in environmental change. Specifically, the report discusses the following:
• The relationship between population factors—size, distribution, and composition—and environmental change.
• The primary forces that mediate this relationship: technology, the institutional and policy contexts, and cultural factors.
• Two specific aspects of environmental change that are affected by population dynamics: climate change and land-use change.
• Implications for policy and further research.\
ENVIRONMENTAL IMPLICATIONS OF POPULATION SIZE, DISTRIBUTION, AND COMPOSITION
Population size is inherently linked to the environment as a result of individual resource needs as well as individual contributions to pollution. However, no simple relationship exists between population size and environmental change. Sheer human numbers in some instances have a direct impact on the environment. More often, however, the environmental implications of population size are ultimately determined by complex interactions among many forces, including technology, political and institutional contexts, and cultural factors. However, as global population continues to grow, limits on such global resources as land and water have come into sharper focus. For example, only in the latter half of the twentieth century has the unavailability of land become a potentially limiting factor in global food production. Assuming constant rates of production, per capita land requirements for food production now fall within the range of estimated available cultivable land. Likewise, continued population
growth occurs in the context of an accelerating human thirst for water: Global water consumption rose sixfold between 1900 and 1995, more than double the rate of population growth. Population size also influences pollution levels in complex ways. Though again this interaction is difficult to gauge, researchers have tried to calculate the relationship between population growth and pollution increases. Studies of air pollution in California, for instance, suggest that a 10 percent increase in population at the
county level produces an emissions increase of 7.5 to 8 percent for pollutants associated with automobile exhaust, largely because local population growth is important as a determinant of the volume of consumption. Greater numbers of people, for instance, typically imply more cars.
“Population distribution” refers to the dispersal and density of population. During the past 40 years, two trends have powerfully influenced the distribution of humans around the globe. First, continued
high fertility rates in many developing regions, coupled with low fertility in more-developed regions, have resulted in ever-increasing shares of the global population residing in less-developed countries.
According to UN estimates, 80 percent of the world population in 1999 lived in developing nations. Second, the Earth’s population is increasingly concentrated in urban areas. As recently as 1960, only
one-third of the world’s population lived in cities. By 1999, the percentage had increased to nearly half (47 percent). This trend is expected to continue well into the twenty-first century.
The distribution of people around the globe has three main implications for environmental change. First, as less-developed regions cope with an increasing share of global population, pressures will intensify on already dwindling resources within many of these areas. Second, the redistribution of population through migration shifts the relative pressures exerted on local environments, perhaps easing the strain in some areas and increasing it in others. Finally, the trend toward urbanization poses particularly complex environmental challenges. The rapid pace of urbanization often hinders the development of adequate infrastructure and regulatory mechanisms for coping with pollution and other byproducts of growth, often resulting in high levels of air and water pollution and other environmental
ills. Furthermore, urbanization can alter local climate patterns. Concentrations of artificial surfaces, such as brick and concrete, can create “heat islands.” In cities with more than 10 million people, the mean annual minimum temperature may be as much as 4 degrees Fahrenheit higher than in nearby rural areas. In addition, poorly planned urban development—“sprawl”—can result in loss of agricultural land and natural habitat.
“Population composition” refers to the characteristics of a particular group of people. Age and socioeconomic composition, for instance, have environmental implications. As for age composition, owing to the population boom of recent decades and increased longevity across the globe, today’s human population has both the largest cohort of young people (age 24 and under) and the largest proportion of elderly in history. Understanding population characteristics helps illuminate some of the mechanisms through which population dynamics affect environmental conditions. For example, migration propensities vary by age. Young people are more likely than their older counterparts to migrate, primarily as they leave the parental home in search of new opportunities. Given the relatively large younger generation, we might anticipate increasing levels of migration and urbanization and, therefore, intensified urban environmental concerns.
Income is an especially important demographic characteristic relevant to environmental conditions. Across nations, the relationship between economic development and environmental pressure resembles an inverted U-shaped curve; nations with economies in the middle-development range are most likely to exert powerful pressures on the natural environment, mostly in the form of industrial emissions. By contrast, the least-developed nations—because of low levels of industrial activity—are likely to exert relatively lower levels of environmental pressure. In addition, at highly advanced development stages, environmental pressures should subside due to improved efficiencies.
Within countries and across households, the relationship between income and environmental pressure is different. Environmental pressures can be greatest at the lowest and highest income levels.
Population growth and poverty often interact to produce unsustainable levels of resource use. Furthermore, higher levels of income tend to correlate with increased levels of production and consumption.
MEDIATING FACTORS: TECHNOLOGY, INSTITUTIONAL AND POLICY CONTEXTS, AND CULTURAL FACTORS
Several factors mediate the relationship between human population dynamics and the natural environment. Aspects of society relating to current technology, institutions, policy, and culture alter the ways in which demographic and environmental factors interact.
Technological factors have always influenced the relationship between population dynamics and environmental change. In some cases, technological advancements have caused greater environmental change than demographic trends alone would have led us to expect. The agricultural revolution of the seventeenth and eighteenth centuries, for instance, enabled demographic shifts that otherwise could not have occurred because it permitted food production sufficient to feed the world’s growing population.
The technological changes that have most affected environmental conditions relate to energy use. In particular, the consumption of oil, natural gas, and coal increased dramatically during the twentieth century. Until about 1960, developed nations were responsible for most of this consumption. Since then, however, the newly developing nations have experienced increasing levels of industrialization, resulting in greater reliance upon resource-intensive and highly polluting production processes. Obviously, improved energy efficiencies could greatly diminish the environmental impacts from energy consumption in both developed and developing nations. Institutional and Policy Contexts Institutions and policy responses are significant mechanisms through which humans react to environmental change and, in so doing, affect subsequent environmental change. These mechanisms can operate for good or ill. For example, following the Montreal Protocol of 1987, limits were established for emissions of chlorofluorocarbons (CFCs), which cause ozone depletion. The ozone layer shields humans from the sun’s high-energy ultraviolet radiation. As a result of the emission policy, CFC consumption has fallen by nearly 70 percent, and the ozone layer is expected to return to normal by the middle of this century. While demographic factors were not the only environmentally destructive forces in this example, population size influenced environmental conditions by being a market for CFCproducing goods. People buy refrigerators. In this instance, however, it was the policy response that ultimately defined the relationships among technology, consumption, population, and environmental change.
Although policy actions can ameliorate environmental decline, on occasion misguided policy may become a powerful force behind degradation. The ecological and social dilemmas facing the Aral Sea basin offer one extreme example of the effects of policies regarding resource use. The sea basin in central Asia is shared by several nations, mainly Uzbekistan and Kazakhstan. Since 1960, the Aral Sea has shrunk forty percent and has become increasingly contaminated.
Although some of the decline and contamination stems from natural variations, research has demonstrated that human forces have been the primary cause behind the ecological destruction—in particular, irrigation policies of the former Soviet Union appear to blame. In this case, the role of local population in environmental decline was shaped by policies regarding water use.
Cultural factors can also play a role in how population dynamics affect the environment. As examples, cultural differences with respect to consumption patterns and attitudes toward wildlife and conservation are likely to affect how populations interact with the environment. For instance, one study demonstrated distinctive patterns with regard to attitudes, knowledge, and behavior toward wildlife across three industrial democracies. While Americans and Germans express broad appreciation for a variety of animals, Japanese culture emphasizes the experience of nature in controlled, confined, and highly idealized circumstances (e.g., bonsai, rock gardening, flower arranging). These cultural variations in turn influence conservation strategies, because public support for various policy interventions will reflect societal values.
TWO SPECIFIC ARENAS OF POPULATION-ENVIRONMENT INTERACTION: GLOBAL CLIMATE CHANGE AND LANDUSE PATTERNS
Two specific areas of inquiry help to illustrate the challenges of understanding the complex influence of population dynamics on the environment: global climate change and land-use patterns. On the
other hand, these examples also demonstrate the growing body of scientific evidence that illuminates the interrelationships between demographics and environmental context.
Global Climate Change
Recent years have been among the warmest on record. Evidence suggests that temperatures have been influenced by growing concentrations of greenhouse gases, such as carbon dioxide, which absorb solar radiation and warm the Earth’s atmosphere. To what extent can climate change be attributed directly to demographic factors? A growing body of evidence suggests that many of the changes in atmospheric gas are human-induced. The demographic influence appears primarily in three forms: contributions to CO2 emissions stem from fossil fuel use related to industrial production and energy consumption; land-use changes, such as deforestation, also affect the exchange of carbon dioxide between the Earth and the atmosphere; and other consumption-related processes, such as rice paddy cultivation and livestock production, are responsible for greenhousegas releases to the atmosphere, particularly methane.
Research has shown that population size and growth are important factors in the emission of greenhouse gases. One study concludes that population size and growth will account for 35 percent of the global increase in CO2 emissions between 1985 and 2100 and 48 percent of the increase from developing nations during that period. However, as population growth slows during the next century, its contribution to emissions is expected to decline. This decline will be especially large in the context of developing nations. While population-driven emissions from developed nations are estimated to contribute 42 percent of CO2 emissions between 1985 and 2020, they are expected to contribute only 3 percent between 2025 and 2100.
Fulfilling the resource requirements of a growing population ultimately requires some form of land-use change, whether to expand food production through forest clearing, to intensify production on already cultivated land, or to develop the infrastructure needed to support increased population. Indeed, it is humans’ ability to manipulate the landscape that has allowed for the rapid pace of contemporary population growth.
Agriculture and deforestation are two prominent forms of humaninduced land-use change. During the past three centuries, the amount of Earth’s cultivated land has grown by more than 450 percent, increasing from 2.65 million square kilometers to 15 million square kilometers. At the same time, the world’s forests have been shrinking. Deforestation is closely linked to agricultural land-use change, because it often represents a consequence of agricultural expansion. A net decline in forest cover of 180 million acres occurred during the 15-year interval 1980–1995, although changes in forest cover vary greatly across regions.
Changing land use and deforestation in particular have several ecological impacts. Agriculture can lead to soil erosion, while overuse of chemical inputs can also degrade soil. Deforestation also increases soil erosion, in addition to reducing rainfall due to localized climate changes, lessening the ability of soils to hold water, and increasing the frequency and severity of floods. Land-use change in general results in habitat loss and fragmentation—the primary cause of contemporary species decline. It has been suggested that if current rates of forest clearing continue, a quarter of all species on Earth could be lost within the next 50 years.
IMPLICATIONS FOR POLICY
The environmental implications of demographic dynamics are complicated and can sometimes be controversial. While some view population growth in developing regions as the primary culprit in environmental decline, others focus on the costly environmental effects of overconsumption among the developed nations. Such differing emphases can lead to a disagreement over the most effective and equitable policy solutions—slow population increase in lessdeveloped nations or lessen destructive production and consumption patterns of the more-developed nations?
Such a debate, however, presumes that a one-step solution to the complex realities of the relationship between population and the environment exists. Both population growth and consumption play a role in environmental change and are among the many factors that should be considered and incorporated into realistic policy debate and prescriptions. Other demographic factors, such as population distribution and composition, are also relevant.
Some specific implications for policy that emerge from reviewing the scientific literature include the following:
• Family planning policies that enable couples to avoid unwanted pregnancies would reduce fertility and rates of population growth, therefore reducing pressure on environmental resources. This would be particularly beneficial in areas already characterized by resource scarcity.
• Rural development policies could reduce rural-to-urban migration, perhaps easing pressure on urban infrastructure. This would particularly benefit areas where rural resource shortages or lack of opportunities fuel rapid urban growth.
• More equitable land-tenure policies could ease resource pressures and, therefore, reduce agricultural expansion and rural-tourban migration. This would especially benefit areas with subsistence agriculture where individuals lack access to land.
• Policies encouraging sustainable intensification of land resources could increase yields, thereby lessening the need for agricultural extensification. This would be especially beneficial in areas characterized by arable land shortages.
• Policies providing incentives for the development of sustainable production processes could ease environmental pressure. Both developed and developing regions could benefit from the application of improved technological efficiencies.
• Policies providing education and encouragement for sustainable consumption could ease environmental pressure. This is particularly true in areas where consumption and production processes are environmentally intensive.
• Careful planning must accompany change in local population densities. This is true both in rapidly growing megacities as well as in less-densely populated areas receiving large influxes of migrants. Population policies represent only one of the many possible responses to the environmental implications of demographic dynamics. Yet population does matter, and increased attention to the associations between the environment and population size, distribution, and composition can improve policy capacity to respond to contemporary environmental change.