______Environment or ecosystem mantle of the solid earth
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familiar image shows the Earth hanging against the black marbled coldness of deep space. The blues sparkle, barely dulled by patches of brown. Swirling white whorls veil the bright sphere only slightly. Twenty-five years ago that image had not yet been seen. Furthermore, the idea of the Earth as an integral unit was not a prevalent one. Study of the planet proceeded at local or regional scales. Then plate tectonics began to weave regional studies into one planetwide dynamic model.
The past two decades have also seen the emergence of a new perspective in the earth sciences—or to use a more recent term, earth system science—emphasizing changes in the global environment that occur over spans of geological time. The changing environments leave geological evidence that permits investigation of a wide range of geographic, oceanographic, climatic, and biotic transitions. Such evidence includes information about environmental changes that cannot be directly observed today. The record of the rocks reveals that certain factors force changes in the global environment and that some ecosystems are more sensitive than others to those changes.
The surface has been changing for over 4.5-billion-years. Many of these changes are fluctuations within definable extremes. A familiar example is that of water. Sediments record changes in the hydrologic cycle during which rising seas engulfed extensive continental tracts and then drained away. Some of these changes resulted from ice ages that left wide swaths of continental shelf exposed to the air during glacial accumulation and sent torrents to the oceans during intervals of melting. Other cyclic fluctuations recorded in the geological record include geochemical exchanges through reservoirs: atmosphere, ocean, biomass, sediment, crust, and mantle. Ocean basins rise and sink and expand and contract in cycles, and mountain ranges thrust upward and then waste away.
While cyclic changes leave recognizable patterns in the geological record, they continually alter the components that are recycled. Inevita-
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Suggested Citation:"3 The Global Environment and Its Evolution." National Research Council. 1993. Solid-Earth Sciences and Society. Washington, DC: The National Academies Press. doi: 10.17226/1990.×
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bly, new conditions are created by these natural movements. The original state can never be exactly regained. The new conditions are the result of what is referred to as secular change—change with time.
The most obvious secular change that has taken place during earth history is the early transformation of its surface from a landscape of naked rock, barren seas, and toxic atmosphere to a landscape seething with life, with organisms that exist on a variety of scales and in a medley of forms. As the cycles have churned away and new secular changes have occurred, sporadic catastrophic events have thrown the whole dynamic system into chaos.
Geoscientists use an assortment of techniques and instruments to investigate the complex interactive systems that have created the surface environment. A 200-year-old tradition of field mapping and detailed description offers a solid foundation—called ground truth—for new technologies like remote sensing and for new conceptual models such as ones that explain how biological evolution has altered the chemistry of the atmosphere.