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Unit 5: Human Population Dynamics // Section 1: Introduction

Human population trends are centrally important to environmental science because they help to determine the environmental impact of human activities. Rising populations put increasing demands on natural resources such as land, water, and energy supplies. As human communities use more resources, they generate contaminants, such as air and water pollution and greenhouse gas emissions, along with increasing quantities of waste.

Population interacts with several other factors to determine a society’s environmental impact. One widely-cited formula is the "I = PAT" equation, proposed by Paul R. Ehrlich and John P. Holdren in 1974 (footnote 1).

Environmental Impact = Population x Affluence (or consumption) x Technology

For generations people have tried to estimate Earth's carrying capacity, or the maximum population that it can support on a continuing basis. This is a slippery undertaking. Estimates of human carrying capacity over the past four centuries have varied from less than one billion people to more than one trillion, depending on how the authors defined carrying capacity. Some studies cast the issue solely in terms of food production, others as the availability of a broader set of resources.

In fact, the question depends on assumptions about human preferences. What standard of living is seen as acceptable, and what levels of risk and variability in living conditions will people tolerate? Many of these issues are not just matters of what humans want; rather, they intersect with physical limits, such as total arable land or the amount of energy available to do work. In such instances nature sets bounds on human choices (footnote 2).

Measuring Earth's carrying capacity at the global level obscures the fact that resources are not allocated equally around the world. In some areas such as the Sahel in West Africa (the transition zone between the Sahara desert and more humid woodlands to the south), population growth is putting heavy stresses on a fragile environment, so food needs are outstripping food production (Fig. 1). Other regions have better balances between populations and resources.

Gully erosion from over-cultivation, Sahel, West Africa

Figure 1. Gully erosion from over-cultivation, Sahel, West Africa
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Source: Courtesy United States Geological Survey, National Center for Earth Resources Observation Systems International Program.

Demography, the science of human population (or more specifically, the study of population structure and processes), draws together research from a number of disciplines, including economics, sociology, geography, public health, and genetics. In addition to the environmental impacts of population growth, population science also considers questions such as:

This unit discusses basic population dynamics, including birth and death rates and factors that influence demographic change. It then summarizes the history of world population growth and projections through mid-century, with a focus on rising urbanization and the aging of the global population. Next we examine the environmental, economic, and institutional implications of population growth and some actions that governments can take to maximize benefits from population growth and limit harmful impacts. Finally, we consider whether nations' demographic patterns are becoming more similar, in spite of their different historic, cultural, and economic legacies, taking note of some regions that do not fit this general pattern.

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