The carbon cycle 

The environmental cycling of carbon compounds includes a flux of above 2×10¹⁴ kg C per year, very much larger than for any other substance apart from water (approximately 5×10¹⁷ kg per year in the hydrological cycle). Compassionate the carbon cycle has become particularly urgent like the atmospheric CO₂ content is presently increasing, producing global warming via the trapping of IR radiation within the atmosphere. Figure 1 depicts a summary of the major processes, along with estimates of the reservoirs (square boxes) and annual fluxes (round-cornered boxes) in units of 10¹² kg C.

Fig. 1. The carbon cycle, showing reservoirs (square-cornered boxes) and annual fluxes (round-cornered boxes) in units of 10₁₂ kg C.

Atmospheric CO₂ is cycled in approximately equal amounts through two different processes: (i) the conversion into soluble bicarbonate HCO^-₃and the consequent regeneration of CO₂ while water evaporates; (ii) the conversion into biological carbon compounds through photosynthesis, and reoxidation to CO₂ through respiration. Anaerobic decay of vegetation, ruminant animals like cows and other natural processes make small amounts of CH₄ and CO that are oxidized within the atmosphere to CO₂. Several parts of the cycle operate along with much larger reservoirs of carbon, but also very much slowly: they involve the mixing of surface bicarbonate along with deep ocean waters, the creation of sedimentary carbonate rocks (mainly from CaCO₃ shells and skeletons of marine organisms) and the eventual decomposition of carbonates through heating deep within the crust to again generate CO₂.

Dissimilar parts of the natural cycle must be almost in balance, even though over a period of millions of years a number of organic carbon has been buried before reoxidation, providing fossil fuels consisting of reduced carbon in the crust. Dioxygen from photosynthesis have passed into the atmosphere, although over geological time most of it has been exhausted in oxidizing surface rocks (mainly Fe^II to Fe^III compounds, and sulfides to sulfates), just a small fraction remaining like free O₂. The burning of fossil fuels comprise opposed this natural tendency and currently transfers approximately 5×10¹² kg C per year into the atmosphere like CO₂. Parts of the cycle outside human control might be responding to start some of this additional input, although the ability of either surface ocean waters or life to hold it in the short term is extremely limited, and the burning of land vegetation gives to the problem through reducing photosynthesis. Even though excess CO₂ must finally return to the crust like carbonate minerals, which can take place only over time scales measured in thousands or even millions of years.