Carbon sequestration is the conversion of environmental carbon in the form of carbon dioxide to non-gaseous forms such as calcium carbonate. Natural processes that perform this conversion are called carbon sinks because they bind more carbon in a durable form than other processes which eventually re-release bound carbon as CO2 through decomposition.
The natural cycle for most terrestrial life is a series of conversions of carbon from one form to another. Plants use solar energy to convert atmospheric carbon dioxide into plant material through photosynthesis. Some of this material will die and decompose; some will be eaten by animals and converted into animal and eventually human bodies. Living animals use the solar energy stored by the plants. The chemical processes which allow animals to use this energy release carbon dioxide back to the atmosphere through respiration. When both plants and animals die, other animals -mostly microscopic-continue the process of breaking the organic carbon compounds down into inert chemical compounds and releasing carbon dioxide back into the atmosphere. This trip by carbon from atmospheric CO2 through plants and animals back to atmospheric CO2 is usually referred to as the carbon cycle.
Carbon sinks remove carbon from this cycle, thus "sequestering" it. The largest on-going natural carbon sink process occurs in the oceans as atmospheric carbon dioxide is taken up by phytoplankton, consumed through the food chain and is finally bound into the shells of crustaceans. The greatest mass of these end up in safe inert form as calcium carbonate or limestone on the ocean floors. Almost all natural limestone and marble found on the planet was at one time the shell of a crustacean, deposited on the ocean floor. As major geological events caused rapid mass extinctions of both plant and animal life over the eons of planetary evolution, a massive amount of carbon was removed from the cycle as the plant material was buried and converted to coal and animal material was converted to petroleum.
As human activities release huge quantities of carbon that was sequestered over millions of years in the form of oil, coal and peat back into the atmosphere over a period of decades, rapid changes in atmospheric carbon dioxide are occurring. Rapid deforestation also releases bound carbon back into the atmosphere as CO2 much quicker than the natural cycle of aging, death and decomposition would. Most environmental scientists agree that a rapid increase in the level of CO2 in the atmosphere will accelerate the greenhouse effect and thus contribute to global warming and the associated climate changes.
In order to mitigate this rapid increase in atmospheric CO2, scientists are trying to find means to bind the CO2 into other forms of carbon. One of the most effective means of temporary carbon sequestration is through forestation. This is considered temporary because trees die and release their carbon over time. Even the wood from trees converted to lumber or furniture will eventually decompose, releasing carbon dioxide.
Other promising technologies are being investigated which use chemical processes to bind atmospheric CO2 into calcium carbonate. One such process envisions "forests" of artificial trees which would result in a harvest of stable carbon which could be stored safely, perhaps in the oceans. Another idea is to freeze CO2 into dry ice and store it deep in the ocean bed, where temperature and pressure would keep it bound.
The main problem with developing new methods of carbon sequestration is that each process requires energy to make the process work. The process must be efficient enough so that the carbon emissions caused by generating the energy to run the process, as well as in the creation of the components, are less than the carbon removed by the process.