ABSTRACT
A prime objective for a sustainable world must be to preserve and, if possible, to enhance options. Options change with human perception of what is possible and what goods and services are available; they are both material and intangible and are anchored in both material and cultural foundations. Even though human ingenuity and the elasticity in the natural system have allowed the human-caused global material and energy flow to increase, the Earth is a closed system except for the input of solar energy. The Earth is not a static place; the geological record shows that the physical and ecological environments of the Earth have always been changing. Changes, both natural and anthropogenic, that may affect the Earth as a habitat are increases in amounts of greenhouse gases, climate change, sea level fluctuations, and ecosystem changes, including large-scale migration or extinction of fauna and flora.
