Human population and associated industrial activities continue to increase rapidly, and have reached levels that put the environment under stress in many areas of the world. In addition natural fluctuations of the Earth's physical and biological systems, often occur in time frames that are not readily evident to man. Such fluctuations cause additional stress on the environment, and can result in changes that impact society in terms of diminished availability of clean water, unspoiled land and natural vegetation, minerals, fish stocks, and clean air. Human societies are making a rapidly increasing number of policy and management decisions that attempt to allow both for natural fluctuations and to limit or modify human impact. Such decisions are often ineffective, as a result of economic, political and social constraints, and inadequate understanding of the interactions between human activities and natural responses. Improved understanding of such issues is important in its own right, and will contribute to ameliorating economic, political and social constraints. Developing improved understanding of environmental change is within the realm of the natural sciences and is being addressed by the International Geosphere-Biosphere Programme (IGBP) and other programmes concerned with describing and understanding the Earth System. Natural variability, occurring over a variety …