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Prologue: Our Changing Planet The Earth is a changing place: over the past million years deserts, forests, and grasslands have migrated across the land, great ice sheets have appeared and disappeared, and wet and dry periods have come and gone. Until recent times, severe weather, seasonal extremes, and longer term climatological patterns of temperature and precipi- tation appeared to be driven by unknown forces. It was not easy to discern the influences of even familiar things, like vegetation, the oceans, or the Sun. The focus of human activity was on providing better shelter and improved agriculture. The industrialization of the planet in the 19th and 20th centuries has given rise to a new set of concerns, namely, that human activity may be adversely affecting the earth system. Recent events, such as the discovery of the Antarctic "ozone hole" and the 1988 North American drought, have led to calls for fundamental change in the economic and social policies of both industrialized and developing nations. In the past several decades, science has provided increased insight into how the earth and its global environment functions. Science and technology have now evolved sufficiently to begin to unravel the complex processes that dominate the life-sustain- ing earth, including how human activities may influence life on our planet. The capability to understand how the global earth system will evolve provides the opportunity for a new and more productive partnership with nature and a sound scientific basis for making policy decisions on global change issues.