Life is built on the conversion of carbon dioxide into the carbon-based organic compounds of living organisms. The carbon cycle illustrates the central importance of carbon in the biosphere. Different paths of the carbon cycle recycle the element at varying rates. The slowest part of the cycle involves carbon that resides in sedimentary rocks, where most of Earth’s carbon is stored. When in contact with water that is acidic (pH is low), carbon will dissolve from bedrock; under neutral conditions, carbon will precipitate out as sediment such as calcium carbonate (limestone). This cycling between solution and precipitation is the background against which more rapid parts of the cycle occur.

Short-term cycling of carbon occurs in the continual physical exchange of carbon dioxide (CO₂) between the atmosphere and hydrosphere. Carbon dioxide from the atmosphere becomes dissolved in water (H₂O), with which it reacts to form carbonic acid (H₂CO₃), which dissociates into hydrogen ions (H⁺) and bicarbonate ions (HCO₃⁻), which further dissociate into hydrogen and carbonate ions (CO₃²⁻). The more alkaline the water (pH above 7.0 is alkaline), the more carbon is present in the form of carbonate, as is shown in the following reversible reactions:

At the same time, carbon dioxide in the water is continually lost to the atmosphere. The exchange of carbon between the atmosphere and hydrosphere links the remaining parts of the cycle, which are the exchanges that occur between the atmosphere and terrestrial organisms and between water and aquatic organisms.

The biological cycling of carbon begins as photosynthetic organisms assimilate carbon dioxide or carbonates from the surrounding environment. In terrestrial communities, plants convert atmospheric carbon dioxide to carbon-based compounds through photosynthesis (see above The photosynthetic process). During this process, plants cleave the carbon from the two oxygen molecules and release the oxygen back into the surrounding environment. Plants are thus primarily responsible for the presence of atmospheric oxygen. In aquatic communities, plants use dissolved carbon in the form of carbonates or carbon dioxide as the source of carbon for photosynthesis. Once carbon has been assimilated by photosynthetic organisms, as well as by the animals that eat them, it is released again in the form of carbon dioxide as these organisms respire. The release of carbon dioxide into the atmosphere or hydrosphere completes the biological part of the carbon cycle.

The pathways of the global carbon cycle, however, are never completely balanced. That is to say, carbon does not move in and out of all parts of the biosphere at equal rates. Consequently, over time some parts of the biosphere accumulate more carbon than others, thereby serving as major accessible carbon reservoirs. In preindustrial times the major reservoirs of carbon were the deep and shallow portions of the ocean; the soil, detritus, and biota of the land; and the atmosphere. The oceans were, and still are, the greatest reservoirs of carbon. Because marine phytoplankton have such short life cycles, the carbon in the ocean cycles rapidly between inorganic and organic states.

In terrestrial environments, forests are the largest carbon reservoirs. Up to 80 percent of the aboveground carbon in terrestrial communities and about a third of belowground carbon are contained within forests. Unlike the oceans, much of this carbon is stored directly in the tissues of plants. High-latitude forests include large amounts of carbon not only in aboveground vegetation but also in peat deposits. Forests at high and low latitudes particularly are important reservoirs of carbon. An estimated one-half of the carbon in forests occurs in high-latitude forests, and a little more than one-third occurs in low-latitude forests. The two largest forest reservoirs of carbon are the vast expanses in Russia, which hold roughly 25 percent of the world’s forest carbon, and the Amazon basin, which contains about 20 percent.

Until recent centuries, the equilibrium between the carbon in the world’s forests and in the atmosphere remained constant. Samples of carbon dioxide trapped in ice during the past 1,000 years and direct measurements of carbon dioxide in the atmosphere had remained fairly constant until the 18th century. However, the cutting of much of the world’s forest is, along with the increase in consumption of fossil fuels attendant on the Industrial Revolution, has resulted in a disruption of the balance between the amount of carbon dioxide in the forests and in the atmosphere. The concentration of atmospheric carbon dioxide has been increasing steadily (Figure 4); currently the rate of increase is about 4 percent per decade (see global warming and climatic variation and change). If human activities continue to alter the relative sizes of the carbon reservoirs worldwide, they are likely to have large effects on the carbon cycle and other biogeochemical cycles. Large-scale deforestation in Russia and the Amazon basin are likely to have particularly significant effects on global carbon storage and cycling.

The warming of global temperatures also is changing which ecosystems act as long-term sinks for carbon and which act as sources for carbon dioxide in the atmosphere. For example, the Arctic tundra, with large amounts of carbon stored in its soils, has been a net sink for carbon dioxide during long periods of geologic time. The recent warming of many Arctic regions, however, has accelerated the rate of soil decomposition, transforming these Arctic areas into potential sources of atmospheric carbon dioxide.

The complex cycle of carbon and the varying sizes of carbon reservoirs illustrate some of the reasons it has been so difficult to predict the effects that increased atmospheric carbon will have on global change.