Many of the important issues facing modern society are the consequences-intended and unintended, positive and negative-of human modifications of the physical environment. So it is that the daily news media chronicles such things as the building of dams and aqueducts to bring water to semiarid areas, the loss of wildlife habitat, the reforestation of denuded hills, the depletion of the ozone layer, the ecological effects of acid rain, the reduction of air pollution in certain urban areas, and the intensification of agricultural production through irrigation.

Environmental modifications have economic, social, and political implications for most of the world’s people. Therefore, the geographically informed person must understand the reasons for and consequences of human modifications of the environment in different parts of the world.

Human adaptation to and modification of physical systems are influenced by the geographic context in which people live, their understanding of that context, and their technological ability and inclination to modify the physical environment. To survive people depend on the physical environment. They adapt to it and modify it to suit their changing needs for things such as food, clothing, water, shelter, energy, and recreational facilities. In meeting their needs, they bring knowledge and technology to bear on physical systems. Consequently, humans have altered the balance of nature in ways that have brought economic prosperity to some areas and created environmental dilemmas and crises in others. Clearing land for settlement, mining, and agriculture provides homes and livelihoods for some but alters physical systems and transforms human populations, wildlife, and vegetation. The inevitable by—products—garbage, air and water pollution, hazardous waste, the overburden from strip mining—place enormous demands on the capacity of physical systems to absorb and accommodate them.

The intended and unintended impacts on physical systems vary in scope and scale. They can be local and small-scale (e.g., the terracing of hillsides for rice growing in the Philippines and acid stream pollution in eastern Pennsylvania), regional and medium-scale (e.g., the creation of agricultural polderlands in the Netherlands and of an urban heat island with its microclimatic effects in Chicago), or global and large—scale (e.g., the clearing of the forests of North America for agriculture or the depletion of the ozone layer by chlorofluorocarbons).

Students must understand both the potential of a physical environment to meet human needs and the limitations of that same environment. They must be aware of and understand the causes and implications of different kinds of pollution, resource depletion, and land degradation and the effects of agriculture and manufacturing on the environment. They must know the locations of regions vulnerable to desertification, deforestation, and salinization, and be aware of the spatial impacts of technological hazards such as photochemical smog and acid rain. Students must be aware that current distribution patterns for many plant and animal species are a result of relocation diffusion by humans.

In addition, students must learn to pay careful attention to the relationships between population growth, urbanization, and the resultant stress on physical systems. The process of urbanization affects wildlife habitats, natural vegetation, and drainage patterns. Cities create their own microclimates and produce large amounts of solid waste, photochemical smog, and sewage. A growing world population stimulates increases in agriculture, urbanization, and industrialization. These processes expand demands on water resources, resulting in unintended environmental consequences that can alter water quality and quantity.

Understanding global interdependence begins with an understanding of global dependence-the modification of Earth’s surface to meet human needs. When successful, the relationship between people and the physical environment is adaptive; when the modifications are excessive the relationship is maladaptive. Increasingly, students will be required to make decisions about relationships between human needs and the physical environment. They will need to be able to understand the opportunities and limitations presented by geographical contexts and to set those contexts within the local to global continuum.