USGS Carbon Cycle
Carbon cycles through natural systems. Carbon dioxide is present in the atmosphere as a gas. Plants and animals take in and release carbon dioxide through respiration. Human activities involving fossil fuels, including manufacturing, transportation, and agriculture, release carbon dioxide into the atmosphere in large amounts. There are also natural sources of carbon dioxide, including volcanos and forest fires. Microorganisms break down dead organic matter. Carbon also dissolves into porous rocks and in warm ocean water. Carbon can be "stored" in natural systems over long time scales, called carbon sinks. Underground oil and gas reserves, carbon-rich ecosystems (such as forests and wetlands), and deep oceans are some of the largest carbon sinks.
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