The Earths Carbon Cycle

The carbon cycle is important on Earth because carbon is the basic structural material for all cell life. Everything on Earth that is organic contains carbon. The carbon cycle involves the movement of carbon between the atmosphere, the oceans, the land, and living organisms.

Similar to the hydrologic cycle, where water can cycle long-term for thousands of years through groundwater or the oceans, or via short-term paths through streams and condensation, carbon also cycles on both a long- and short-term time frame. The long-term cycle—also referred to as the "geological scale"—cycles over time periods that encompass millions of years. The short-term cycle—referred to as "biological/physical scale"—operates in time frames ranging from several days to thousands of years.

Carbon cycles globally as it moves from one location to another. When carbon is being stored in a reservoir—either short- or long-term—it is referred to as a sink. There are multiple combinations of carbon "sources" and "sinks" on Earth that carbon cycles through—or exchanges between—over time. At any one time on Earth, there are an abundance of physical, chemical, biological, and geological processes occurring that involve carbon directly and influence its exchange and movement. Because the oceans cover three-fourths of the Earth's surface, they contain the largest active source of carbon near the Earth's surface. The carbon that is stored in the deeper levels of the ocean, however, does not get readily exchanged; it remains stored in the depths long term. There are four principal repositories, or reservoirs, of carbon on Earth, and they are exchanged through specific routes. The major conduits of carbon movement are as follows:

• terrestrial biosphere,

Carbon in the oceans is found in both living and nonliving marine biota as well as dissolved inorganic carbon. The terrestrial biosphere is the carbon found on the surface of the Earth's land. This includes living and nonliving organic material, the carbon that resides in soil, and carbon in freshwater systems. Sediments encompass geologic formations and also include fossil fuel deposits such as coal, oil, and gas. Because fossil fuels originated from buried organic matter (prehistoric plants and animals) formed under high pressure, they are high in carbon content. The atmosphere acts as a pathway for carbon to combine with oxygen as a gas and move through the cycle as CO2. In addition to CO2, there is also carbon in methane and chlorofluorocarbons, as explained in chapter 1. Therefore, carbon in the form of CO2, carbonates, and organic compounds are cycled between the main reservoirs of the Earth's bio-geochemical system.

Over various short- and long-term time scales, the global carbon cycle exists in dynamic equilibrium. Currently, the Earth's atmosphere stores around 827 billion tons (750 billion metric tons) of carbon in the form of CO2. Of this, the deep oceans store more than 42,000 tons (38,000 metric tons), and the surface of the ocean contains about 1,102 tons (1,000 metric tons).

Over the past 250 years or so, the Earth's carbon cycle has become unbalanced. Because human sources of CO2 have increased (such as from fossil fuels), the system is no longer in equilibrium. The atmospheric portion of the cycle has increased in far greater proportions than in other parts of the cycle. According to the IPCC, since pre-industrial times, atmospheric CO2 concentrations have increased roughly 27 percent from about 280 ppm to 383 ppm. Restoring the equilibrium is not easily done. Because carbon is exchanged on both short- and long-term scales, fairly rapid transfers occur between the atmosphere and the biosphere.

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