Carbon cycle
Carbon dioxide gas exists in the atmosphere and is dissolved in water. Photosynthesis converts carbon dioxide gas to organic carbon, while respiration cycles the organic carbon back into carbon dioxide gas. Long-term storage of organic carbon occurs when matter from living organisms is buried deep underground and becomes fossilized. Volcanic activity and human emissions bring this stored carbon back into the carbon cycle.
Source
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"OpenStax College, Biogeochemical Cycles. October 17, 2013."
http://cnx.org/content/m44889/latest/Figure_46_03_04.jpg
OpenStax CNX
CC BY 3.0.
