To understand how biological and physical processes interact to govern CO₂ levels in the air, we must first look at how carbon is distributed among its various reservoirs, the places where carbon is found on the Earth. As shown in the purple boxes in Fig. 25.6, reservoirs of carbon include organisms, the atmosphere, soil, the oceans, and sedimentary rocks, and both biological and geologic processes cycle carbon back and forth among these pools.

How much carbon is stored in each of Earth’s major reservoirs? If we add up all the carbon contained in the total mass of organisms living on land, that amount of carbon is just a bit smaller than the amount of carbon stored as CO₂ in the atmosphere. Soil, by comparison, stores as much carbon as do land organisms and the atmosphere combined, mostly as slowly decaying organic compounds. In the oceans, the amount of carbon contained in living organisms is actually very small. Much more resides as inorganic carbon dissolved in the water—most of it in the deep oceans as CO₂ and bicarbonate and carbonate ions. Perhaps fortunately for us, the oceans have sopped up some of the CO₂ generated by human activities.

The biggest carbon reservoir of all, however, lies beneath our feet, within sediments and sedimentary rocks. Calcium carbonate minerals (CaCO₃), which form limestone, and organic matter preserved in sedimentary rocks dwarf all other carbon reservoirs combined by three orders of magnitude. Coal, petroleum, and natural gas make up only a small part of the organic matter stored in sedimentary rocks, but as we have seen, they play an important role in the modern carbon cycle.

Fluxes are the rates at which carbon flows from one reservoir to another. The sensitivity of different reservoirs to change depends on the relative sizes of the reservoir and of the amount of movement of material into and out of it. When fluxes are large relative to the size of the reservoir, reservoir size can change rapidly. As shown by the arrows in Fig. 25.6, the amount of carbon stored as CO₂ in air is not that much larger than the annual fluxes into and out of the atmosphere. For this reason, atmospheric CO₂ abundance can be influenced by a number of processes at work in the carbon cycle.

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