Carbon on the Land and in the Oceans: The modern carbon cycle
On land, the major exchange of carbon with the atmosphere results from photosynthesis and respiration. During the daytime in the growing season, leaves absorb sunlight and take up carbon dioxide from the atmosphere. In parallel, plants, animals and soil microbes consume the carbon in organic matter and return carbon dioxide to the atmosphere. When conditions are too cold or too dry, photosynthesis and respiration cease along with the movement of carbon between the atmosphere and the land surface. The amounts of carbon that move from the atmosphere through photosynthesis, respiration, and back to the atmosphere are large and produce oscillations in atmospheric carbon dioxide concentrations (see Keeling curve). Over the course of a year, these biological fluxes of carbon are over ten times greater than the amount of carbon introduced to the atmosphere by fossil fuel burning.
Carbon Dioxide in the atmosphere has been steadily rising since regular measurements began in 1958. The graph above shows both the long-term trend and the seasonal variation. (Graph by Robert Simmon, based on data from the NOAA Climate Monitoring & Diagnostics Laboratory)
Fire also plays an important role in the transfer of carbon dioxide from the land to the atmosphere. Fires consume biomass and organic matter to produce carbon dioxide (along with methane, carbon monoxide, smoke), and the vegetation that is killed but not consumed by the fire decomposes over time adding further carbon dioxide to the atmosphere.
Over periods of years to decades, significant amounts of carbon can be stored or released on land. For example, when forests are cleared for agriculture the carbon contained in the living material and soil is released, causing atmospheric carbon dioxide concentrations to increase. When agricultural land is abandoned and forests are allowed to re-grow, carbon is stored in the accumulating living biomass and soils causing atmospheric carbon dioxide concentrations to decrease.
In the oceans, carbon dioxide exchange is largely controlled by sea surface temperatures, circulating currents, and by the biological processes of photosynthesis and respiration. carbon dioxide can dissolve easily into the ocean and the amount of carbon dioxide that the ocean can hold depends on ocean temperature and the amount of carbon dioxide already present. Cold ocean temperatures favor the uptake of carbon dioxide from the atmosphere whereas warm temperatures can cause the ocean surface to release carbon dioxide. Cold, downward moving currents such as those that occur over the North Atlantic absorb carbon dioxide and transfer it to the deep ocean. Upward moving currents such as those in the tropics bring carbon dioxide up from depth and release it to the atmosphere.
Life in the ocean consumes and releases huge quantities of carbon dioxide. But in contrast to land, carbon cycles between photosynthesis and respiration vary rapidly; i.e., there is virtually no storage of carbon as there is on land (i.e., tree trunks and soil). Photosynthetic microscopic phytoplankton are consumed by respiring zooplankton (microscopic marine animals) within a matter of days to weeks. Only small amounts of residual carbon from these plankton settle out to the ocean bottom and over long periods of time represent a significant removal of carbon from the atmosphere.