Broad background to the physical environment
The physical environment is the set of physical and biological conditions that surround human beings at the Earth's surface. This book is broadly based on a systems approach to the physical environment. A system may be defined as 'a structured set of components of variables (i .e. phenomena that are free to assume variable magnitudes) that exhibit discernible relationships with one another and operate together as a complex whole, according to some observed pattern' (Chorley & Kennedy 1971). The Earth can be viewed as a system since virtually no part of the system operates in isolation from another, and thus changes in one part of the Earth system can affect other parts. A systems approach provides a rational set of procedures for subdividing the physical environment into subsystems, or units that demonstrate strong internal connections. For example, the physical environment consists of four main subsystemsthe atmospheric environment (or atmosphere), the hydrological environment (or hydrosphere), the geomorphic environment (part of which is often referred to as the lithosphere), and biological environment (or biosphere)-and the relationships between them (Figure 1.1). As Figure 1.1 shows, the subsystems interact within an open environmental system with inputs of energy, materials, and human activity, and outputs of specific environmental impacts.