"The Earth is a changing place. Over the past million years deserts, forests, and grasslands have migrated across the land, seashores have advanced and retreated, and wet and dry periods have come and gone. The dramatic rise in industrial activities during the 19th and 20th centuries has produced a new set of concerns, namely, that human activities may be affecting the Earth system. Consequently, wise use of the Earth for human habitation has become an important political and scientific issue. World population growth, intense industrial and agricultural activities, and the need to maintain man's health and welfare require that each individual and country exercise appropriate environmental care and sensitivity. The global changes that may affect both human well-being and the quality of life on this planet include ozone depletion, global climate warming, sea level change, drought, deforestation, desertification, and reduction in biodiversity. While dramatic and complex in and of themselves, these discrete global environmental concerns cannot be fully understood unless they are addressed collectively."
U.S. Global Change Research Program Resource Library: http://www.library.globalchange.gov/