Global material cycles - Encyclopedia of Earth
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چکیده
The earth is a constant recipient of an enormous flux of solar radiation as well as considerable infall of the cosmic debris that also arrives periodically in huge, catastrophe-inducing encounters with other space bodies, particularly with massive comets and asteroids. Nothing else comes in, and the planet's relatively powerful gravity prevents anything substantial from leaving. Consequently, life on the earth is predicated on incessant cycling of water and materials that are needed to assemble living bodies as well as to provide suitable environmental conditions for their evolution. These cycles are energized by two distinct sources: Whereas the water cycle and the circulations of carbon, nitrogen, and sulfur, the three key life-building elements, are powered by solar radiation, the planet's grand tectonic cycle is driven by the earth's heat. For hundreds of thousands of years, our species had no discernible large-scale effect on these natural processes, but during the 20th century anthropogenic interferences in global biospheric cycles became a matter of scientific concern and an increasingly important, and controversial, topic of public policy debates.
منابع مشابه
A Mooney and Dr Josep G Canadell in Encyclopedia of Global Environmental Change
Volume 2, The Earth system: biological and ecological dimensions of global environmental change, pp 461–465
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تاریخ انتشار 2007