Carbon sequestration is a geoengineering technique for the long-term storage of carbon dioxide or other forms of carbon, for the mitigation of global warming. Carbon dioxide is usually captured from the atmosphere through biological, chemical or physical processes. It has been proposed as a way to mitigate the accumulation of greenhouse gases in the atmosphere released by the burning of fossil fuels.
CO2 may be captured as a pure by-product in processes related to petroleum refining or from flue gases from power generation. CO2 sequestration can then be seen as being synonymous with the storage part of carbon capture and storage which refers to the large-scale, permanent artificial capture and sequestration of industrially-produced CO2 using subsurface saline aquifers, reservoirs, ocean water, aging oil fields, or other carbon sinks.
Sequestration techniques are not instantaneous and when considering their efficacy, consideration has to be given to the fact that they will therefore be acting on future (not current) CO2 levels. These levels are expected by the IPCC to be higher than today's.
Biosequestration or carbon sequestration through biological processes has a huge effect on the Global carbon cycle. Major climatic fluctuations have been driven by these processes in the past, such as at the Azolla event which started the current Arctic climate. Fossil fuel formation is as a result of such processes, as is the formation of clathrate or limestone. By manipulating such techniques, geoengineers seek to enhance sequestration. Methods such as ocean iron fertilization are examples of such geoengineering techniques.
Iron fertilization of the ocean to encourage plankton growth which removes carbon from the atmosphere on a temporary, or arguably permanent basis. This technique is controversial due to difficulties of predicting its effect on the marine ecosystem, and the potential for side effects or large deviations from expected efficacy. Such effects potentially include the release of nitrogen oxides, and disruption to the nutrient balance in the ocean. Iron fertilization is a natural process and it is the enhancement of this process which is the geoengineering technique.
Australian company Ocean Nourishment Corporation (ONC) plans to sink hundreds of tonnes of urea into the ocean, in order to boost the growth of CO2-absorbing phytoplankton, as a way to combat climate change. In 2007, Sydney-based ONC completed an experiment involving 1 tonne of nitrogen in the Sulu Sea off the Philippines.
Reforestation is the replanting of trees on marginal crop and pasture lands to transfer CO2 from the atmosphere to new biomass. For this process to be successful it is essential to ensure that the carbon does not return to the atmosphere from burning or rotting when the trees die. To this end, it would be important to either manage such forests in perpetuity or use the wood from them for biochar, BECS or landfill.
Modification of agricultural practices is a recognized method of carbon sequestration as soil can act as an effective carbon sink offsetting as much as 20% of carbon dioxide emissions annually.
Carbon emission reduction methods in agriculture can be grouped into two categories: reducing and/or displacing emissions and enhancing carbon removal. Some of these reductions involve generally increasing the efficiency of farm operations (i.e. more fuel-efficient equipment) while some involve interruptions in the natural carbon cycle. While a wide variety of carbon control techniques are possible within agriculture, not all have the same degree of effectiveness, and the degree of effectiveness can also vary considerably when the normal variations in farming such as location, climate, weather, plant varieties, etc. are considered. Also, some effective carbon sequestering techniques (such as the elimination of stubble burning) can negatively impact other areas of environmental concern (increased use of chemicals to control weeds not destroyed by burning).
Many of the carbon reduction techniques used in other industries are also applicable to agriculture. Also, any increases in the efficiency of farming methods which result in higher yields will generally result in reduced emissions as well, since more food is being produced for the same or less effort. This includes more accurate use of fertilizers, better irrigation, and the use of higher yield crop strains such as those bred for locally-beneficial traits and those which have been selected for increased yields.
Replacement of more energy intensive farming operations with less energy intensive versions which accomplish the same task can also reduce emissions. Reduced or no-till farming requires less machine tillage and correspondingly less fuel burned per acre of production. However, increased use of weed-control chemicals are usually required for no-till farming operations and the extra residue left on the soil surface is more likely to release its CO2 to the atmosphere as it decays, reducing the net carbon reduction.
In practice, most farming operations which incorporate post-harvest crop residues, wastes and byproducts back into the soil will provide a carbon storage benefit. This is particularly the case for practices such as field burning of stubble - rather than releasing almost all of the stored CO2 to the atmosphere, tillage incorporates the biomass back into the soil where it can be absorbed and a portion of it stored permanently.
All crops absorb CO2 during their growth and release it after their death as they are consumed, destroyed, or incorporated into the soil. The goal of agricultural carbon removal is to use the crop itself and its relation to the carbon cycle to permanently sequester carbon within the soil. This is done by selecting farming methods which return biomass to the soil and enhance the conditions in which the carbon within the plants will be reduced to its elemental nature and stored in a stable state. Methods for accomplishing this include:
Sequestration practices may have positive effects on soil, air, and water quality, be beneficial to wildlife, and expand food production. On degraded croplands, an increase of 1 ton of soil carbon pool may increase crop yield by 20 to 40 kilograms per hectare of wheat, 10 to 20 kg/ ha for maize, and 0.5 to 1 kg/ha for cowpeas.
The effects of soil sequestration can be reversed. If the sequestering soil is disrupted or tillage practices are abandoned, the soil becomes a net source of greenhouse gases as carbon is released back into the atmosphere. Typically after 15 to 30 years of sequestration, soil becomes saturated and ceases to absorb more carbon. This implies that there is a global limit to the amount of carbon that soil can hold.
It is difficult to asses the costs of carbon sequestration in soils as many factors affect costs including soil quality, transaction costs and various externalities such as leakage and unforeseen environmental damage. Furthermore, because reduction of atmosperic CO2 levels is a long-term concern, it is difficult to motivate farmers to voluntarily adopt more expensive agricultural techniques when there is not a clear crop, soil, or economic benefit. Governments such as Australia and New Zealand are considering allowing farmers to sell carbon credits once they document that they have sufficiently increased soil carbon content
Encouraging various layers of the ocean to mix can move nutrients and dissolved gases around and thus act as a geoengineering approach. This mixing may be achieved by placing large vertical pipes in the oceans to pump nutrient rich water to the surface, triggering blooms of algae which also store carbon when they die. This produces results somewhat similar to ocean iron fertilization. This technique may result in a short-term rise in CO2 in the atmosphere, which limits its attractiveness.
Biochar is charcoal created by pyrolysis of biomass. The resulting charcoal-like material is landfilled, or used as a soil improver to create terra preta. Biogenic carbon is recycled naturally in the carbon cycle. By pyrolysing it to biochar, it’s rendered inert and sequestered in soil. Further, the soil encourages bulking with new organic matter, which gives additional sequestration benefit.
The carbon contained in the soil is therefore unavailable for oxidation to CO2 and consequential atmospheric release. As a result, the radiative forcing potential of the avoided CO2 is removed from the planet’s energy balance. This technique is advocated by prominent scientist James Lovelock, creator of the Gaia hypothesis. According to Simon Shackley, "I would say people are talking more about something in the range of one to two billion tonnes a year."
The mechanisms related to the carbon sequestration properties of biochar, is referred to as bio-energy with carbon storage, BECS.
The term BECCS refers to Bio-energy with carbon capture and storage  – Burning biomass in power stations and boilers which utilise carbon capture and storage. Using this technology with sustainably produced biomass would result in net-negative carbon emissions, as the carbon sequestered during the growth of the biomass would be captured and stored, thus removing carbon dioxide from the atmosphere.
This technology is sometimes referred to as bio-energy with carbon storage, BECS, though this term can also refer to the carbon sequestration potential in other technologies, such as biochar.
Burying biomass (such as trees) directly, thus sequestering the carbon in the ground rather than allowing it to escape, mimicking the natural processes that created fossil fuels. Landfill of trash also represents a physical method of sequestration.
The production of fossil fuels is a natural process which often involves the ocean burial of biomass, often near river mouths which bring large quantities of nutrients and dead material from upriver into the ocean. Transporting material, such as crop waste, out to sea and allowing it to sink into deep ocean storage has been proposed as a means of sequestration of carbon. International restrictions on marine dumping may restrict or prevent use of this technique at present.
The first large-scale CO2 sequestration project (1996) is called Sleipner, and is located in the North Sea where Norway's StatoilHydro strips carbon dioxide from natural gas with amine solvents and disposed of this carbon dioxide in a deep saline aquifer until Spring 2008 when a leak was discovered and the facility shutdown. In 2000, a coal-fueled synthetic natural gas plant in Beulah, North Dakota, became the world's first coal using plant to capture and store carbon dioxide.
CO2 has been used extensively in enhanced crude oil recovery operations in the United States beginning in 1972. There are in excess of 10,000 CO2 wells in the state of Texas alone. The gas comes in part from anthropogenic sources, but is principally from large naturally-occurring geologic formations of CO2. It is transported to the oil-producing fields through a large network of over 5,000 kilometres (3,100 mi) of CO2 pipelines. The use of CO2 for enhanced oil recovery (EOR) methods in heavy oil reservoirs in the Western Canadian Sedimentary Basin (WCSB) has also been proposed. However, cost of transport remains an important hurdle. A similar CO2 pipeline system to that of Texas does not yet exist in the WCSB that could connect most of the sources for CO2 in Canada associated with the mining and upgrading operations in the Athabasca oil sands, with the subsurface heavy oil reservoirs that could most benefit from CO2 injection hundreds of kilometres to the south.
Carbon, in the form of CO2 can be removed from the atmosphere by chemical processes, and stored in stable carbonate mineral forms. This process is known as 'carbon sequestration by mineral carbonation' or mineral sequestration. The process involves reacting carbon dioxide with abundantly available metal oxides–either magnesium oxide (MgO) or calcium oxide (CaO)–to form stable carbonates. These reactions are exothermic and occur naturally (e.g., the weathering of rock over geologic time periods).
|Earthen Oxide||Percent of Crust||Carbonate||Enthalpy
These reactions are favored at low temperatures. This process occurs naturally over geologic time frames and is responsible for much of the surface limestone. The reaction rate can be made faster, for example by reacting at higher temperatures and/or pressures, or by pre-treatment of the minerals, although this method requires additional energy.
CO2 naturally reacts with peridotite rock in surface exposures of ophiolites, notably in Oman. It has been suggested that this process can be enhanced to carry out natural mineralisation of CO2.
Carbon dioxide sequestration in basalt involves the injecting of CO2 into deep-sea formations. The CO2 first mixes with seawater and then reacts with the basalt, both of which are alkaline-rich elements. This reaction results in the release of Ca2+ and Mg2+ ions forming stable carbonate minerals.
Underwater basalt offers a good alternative to other forms of oceanic carbon storage because it has a number of trapping measures to ensure added protection against leakage. These measures include “geothermal, sediment, gravitational and hydrate formation.” Because CO2 hydrate is denser than CO2 in seawater, the risk of leakage is minimal. And assuming the injected CO2 is added into the ocean at depths greater than 2,700 meters, the seawater will have a greater density than the carbon dioxide, causing it to sink.[47 ]
Possible injection site: Juan de Fuca plate
Researchers at the Lamont-Doherty Earth Observatory found that the Juan de Fuca plate of the western coast of the United States has a possible storage capacity of 208 Gtons of carbon. This could cover the entire U.S. carbon emissions for over 100 years. When speaking of the possibilities for basalt storage, David Goldberg of the Earth Observatory expressed the belief that while years of research are still required, there is no technical reason why this method of storage would not work.[47 ]
Traditional cement manufacture releases large amounts of carbon dioxide, but newly developed cement types from Novacem can absorb CO2 from ambient air during hardening. A similar technique was pioneered by TecEco, who have been producing EcoCement since 2002.
In Estonia, oil shale ash, generated by the oil shale-fired power stations could be used as sorbents for CO2 mineral sequestration. The amount of CO2 captured averaged 60–65% of the carbonaceous CO2 and 10–11% of the total CO2 emissions.
Various carbon dioxide scrubbing processes have been proposed to remove CO2 from the air, usually using a variant of the Kraft process. Carbon dioxide scrubbing variants exist based on potassium carbonate which can be used to create liquid fuels, or on sodium hydroxide. These notably include the artificial trees proposed by Klaus Lackner with the purpose to remove carbon dioxide from the atmosphere using chemical scrubbers.
Adding crushed limestone or volcanic rock to oceans to restore the solubility pump, which naturally tends to remove excess CO2 from the atmosphere. Various other scientists have explored this technique, and suggested a variety of different bases which may be added to the ocean. 
Chemically removing hydrochloric acid from the ocean by electrolysis and neutralize the acid through reactions with silicate minerals or rocks. But this may also contribute to carbon addition to the ocean if not carefully managed.