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6 over the past several centuries, changing water tables in numerous regions, earthquakes, the retreat of glaciers, the accumulation of "greenhouse" gases and ozone-depleting chemicals in the atmosphere, the appearance of the Antarctic "ozone hole", acidification of some soils and lakes, and the reduction in genetic diversity of animals and plants. The earth system is very dynamic and these changes occur on all time and geographic scales. The Earth itself holds testimony of ancient steaming bogs and crushing ice sheets, variations far beyond those known to modern civilization. Many of these changes are the result of a variety of interrelated natural processes, including changes in the climate system, in solar processes, in the earth's orbit, in volcanic processes, and in the distribution of biological species and land masses that may have been ongoing for centuries. Although human activities may have the potential to alter the earth system, it is clear that variations occur naturally over a wide range. The broad study of all of these interrelated earth processes constitutes global change research. What is Man's Role in Changing the Environment? Most humans experience changes as seasonal-to-decadal regional weather and climate changes (e.g., last summer's drought). For this reason these changes, and the influences that human activities might have on them, have dominated public concern. In the past, policymakers have understandably focused on needs perceived as the most immediate, such as weather forecasting, urban smog, and acid rain. In recent years, the attention of both scientists and policymakers has extended to more global-scale, longer-term changes, such as the question of global warming which may occur when additional heat radiated from the earth is trapped by increases in atmospheric "greenhouse" gases.
