Carbon sequestration is the general term used for the capture and long-term storage of carbon dioxide. Capture can occur at the point of emission (e.g. from power plants) or through natural processes (such as photosynthesis), which remove carbon dioxide from the earth's atmosphere and which can be enhanced by appropriate management practices. Sequestration methods include:
It is estimated that soils contain between 700 gigatonnes (Gt, 109 tonnes) and 3000 Gt of carbon, or more than three times the amount of carbon stored in the atmosphere as carbon dioxide. However, most agricultural soils have lost 50–70 per cent of the original soil organic carbon pool that was present in the natural ecosystem prior to clearing and cultivation. When forests are converted to agricultural land, the soil carbon content decreases. This happens because organic matter in the soil decomposes following the disturbance while, at the same time, less carbon enters the soil because the clearance has reduced the biomass above ground, and practices such as stubble burning will reduce it even more. Agricultural usages such as grazing, harvesting and tillage also tend to reduce soil carbon, as does increased erosion that often results.
Given the enormous carbon storage capacity of soils, it has been suggested that with appropriate changes in management practices, they could represent a significant sink for atmospheric CO2. Managing agricultural soils to increase their organic carbon content can also improve soil health and productivity by adding essential nutrients and increasing their water-holding capacity.
Management practices that can retain or increase the carbon content of soils include low-tillage or no tillage, use of manures and compost, conversion of monoculture systems to diverse systems, crop rotations and winter cover crops, and establishing perennial vegetation on steep slopes. These practices primarily affect the amount of labile carbon in the soil, or carbon with relatively high turnover time (<5 years). Labile carbon is released to the atmosphere as carbon dioxide through decomposition and microbial activity. The potential increase in storage through such methods is limited by soil type, which determines the carbon-holding capacity, and climate, which determines the rate of decomposition. Soil microbial activity increases with soil moisture and temperature, and increasing average temperatures due to climate change may be expected to increase the turnover rate of labile carbon in soils.
An alternative and promising approach, which is the subject of much current research, is the use of 'biochar' to increase the soil carbon sink. Biochar is a type of charcoal that results from heating organic materials such as crop residue, wood chips, municipal waste or manure in an oxygen-limited environment (a process known as 'pyrolysis'). This can occur in a dedicated facility that harnesses the resultant 'bioenergy' to produce electricity, and the biochar residue can be returned to the soil. As a more generally applicable process, biochar can be produced through replacement of conventional slash and burn practices with 'slash and char', where complete burning is inhibited, for example by dampening the fire with earth. Biochar is chemically stable and the carbon can remain in the soil for hundreds to thousands of years.
The properties of biochar will differ depending on the source of material used in its production and the conditions of pyrolysis. For example, different feedstocks (manure, wood waste, etc) will result in different nutrient levels and chemical stability of the resulting biochar. Different pyrolysis temperatures will affect the capacity of the biochar to adsorb or mop up toxic substances and help to rehabilitate contaminated sites, or to increase the water holding capacity of the soil.
The net agronomic benefits of biochar are still being investigated. Biochar production removes agricultural waste that may otherwise be returned to the soil as labile carbon and returns it instead as biochar. The relative impacts of this process are not yet well understood, but it is thought that biochar has the potential to significantly increase crop yields and improve soil health.
The diagram below illustrates the processes involved in low-temperature pyrolysis of biomass for bioenergy production and biochar sequestration. Typically, about half of the biomass input into the system is converted to biochar for long-term storage.
Source: CSIRO, 'Biochar', Fact sheet.
Plants use the energy of sunlight to convert CO2 from the atmosphere to carbohydrates for their growth and maintenance, via the process of photosynthesis. Natural terrestrial biological sinks for CO2 already sequester about one third of CO2 emissions from fossil fuel combustion. These natural sinks are a transient response to higher atmospheric CO2 concentration, which enhances the rate of photosynthesis. The uptake of CO2 by vegetation will decrease with time as plants grow to their full capacity and become limited by other resources such as nutrients, and regrowth potential in previously cleared or sparsely vegetated areas is fulfilled. Biological storage could be enhanced through agricultural and forestry practices and revegetation, but the capacity is limited and longevity of storage depends on the final fate of the timber or plant material. However, carbon sequestration from revegetation and plantation programs could provide a significant shorter-term contribution to climate change mitigation.
Geosequestration is the injection and storage of greenhouse gases underground, out of contact with the atmosphere. The most suitable sites are deep geological formations, such as depleted oil and natural gas fields, or deep natural reservoirs filled with saline water (saline aquifers). Geosequestration is part of the three-component scheme of carbon capture and storage (CCS), which involves:
This scheme is proposed as a means of reducing to near-zero the greenhouse gas emissions of fossil fuel burning in power generation and CO2 production from other industrial processes such as cement manufacturing and purification of natural gas. It is predominantly aimed at mitigating emissions of CO2, but geosequestration may also prove to be applicable to other greenhouse gases. The concept of CCS may also be applied to other long-term storage options (see ocean sequestration and mineral sequestration below). However, of the storage options, geosequestration is thought to be the most promising due to higher confidence in the longevity of storage; large capacity of potential storage sites; and generally greater understanding of the mechanisms of storage.
The diagram below illustrates the various options for geosequestration.
Source: IPCC, Special Report on Carbon Dioxide Capture and Storage, Working Group III of the Intergovernmental Panel on Climate Change [B. Metz, O. Davidson, H. de Coninck, M. Loos and L. Meyer (eds.)], Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 2005, Figure TS7, p. 32.
Though geosequestration has not yet been commercially demonstrated, there has been considerable knowledge gained through the widespread use in the oil industry of underground CO2 injection for enhanced oil recovery (EOR), which is directly applicable to geosequestration. EOR involves injecting CO2 into the oil-containing reservoir to pressurise the reservoir and improve the rate of flow of oil. The displaced oil is pushed through the well bore, while most of the CO2 remains underground in the reservoir.
Several demonstration sites have been established to investigate the feasibility and safety of geosequestration. The longest-running such site has been injecting and storing CO2 underground in the North Sea since 1996 (the Sleipner project), storing about one million tonnes of CO2 per year. The largest geosequestration demonstration project, the Weyburn project in Canada, uses CO2 injection for enhanced oil recovery and injects about 1.5 million tonnes of CO2 per year. In Australia, the Otway Project in Victoria commenced in April 2008 and is injecting about 4500 tonnes of CO2 per month into a depleted gas reservoir about two kilometres underground.
Australia's Cooperative Research Centre for Greenhouse Gas Technologies was established in 2003 to carry out research and development of technologies for carbon dioxide capture and geological storage, assess the potential for their application in Australia, and help the technologies become commercially viable. Research within the CRC has assessed several of Australia's geological basins for geosequestration suitability and identified storage opportunities both onshore and offshore in Victoria, Western Australia and Queensland.
The IPCC estimated in its Special report on carbon dioxide capture and storage that a new coal-fired power plant with modern efficiency standards (see Clean coal) employing CCS would use about 20 per cent more energy than an equivalent plant without CCS. Capital cost for a CCS plant would be about 40 per cent more, and the cost of electricity produced would increase by 20–55 per cent. However, with Australia's plentiful coal reserves and well-established supportive infrastructure, deployment of CCS is likely to become viable as carbon emissions become constrained by policy measures (especially the introduction of an emissions trading scheme) and carbon permit prices increase.
The ocean represents the largest carbon store on earth. Before the industrial revolution it contained 60 times as much carbon as the atmosphere and 20 times as much carbon as the land vegetation and soil. The ocean has been a significant sink for anthropogenic CO2 emissions of similar magnitude to the land sink but, as with the land sink, the ocean sink will decrease in strength. Increasing CO2 concentration in the upper layer of the oceans is also causing ocean acidification with potentially severe consequences for marine organisms and ecosystems (see Oceans are heating up and acidifying). CO2 dissolves in seawater by combining with carbonate ions, but the number of these ions is limited and as their concentration decreases this will limit the rate at which CO2 is taken up by the ocean. A possible slow-down in ocean circulation may also reduce the ocean sink capacity. In addition to the dissolution process, phytoplankton in the surface layers perform photosynthesis and incorporate CO2 into biological material but, as with terrestrial photosynthesis, there comes a saturation point where other factors restrict further photosynthesis.
It has been proposed to bypass the natural ocean CO2 uptake mechanism and inject CO2 directly into the deep ocean to utilise its enormous storage capacity. Models suggest that CO2 injected into the deep ocean would remain isolated from the atmosphere for several centuries, but on the millennial time scale it would recycle into the atmosphere. Considerable uncertainties exist in our understanding of deep ocean chemistry and biology and the potential adverse impacts on ocean ecosystems. In addition, despite many years of theoretical work and small-scale experiments, the feasibility of ocean storage has not been demonstrated and the technologies for deep ocean CO2 transport and dispersal are yet to be developed.
Another possible way to enhance the ocean carbon sink that has been proposed involves large scale ocean fertilisation with iron to stimulate phytoplankton growth and photosynthesis. This is one of several ambitious geo-engineering schemes that involve high uncertainty and risk but may provide quick and effective means to halt or significantly slow the rate of climate change.
Mineral sequestration (otherwise known as mineral carbonation) involves reaction of CO2 with metal oxides that are present in common, naturally occurring silicate rocks. The process mimics natural weathering phenomena, and results in natural carbonate products that are stable on a geological time scale. There are sufficient reserves of magnesium and calcium silicate deposits to fix the CO2 that could be produced from all fossil fuel resources. Though the weathering of CO2 into carbonates does not require energy, the natural reaction is slow; hence as a storage option the process must be greatly accelerated through energy-intensive preparation of the reactants. The technology is still in the development stage and is not yet ready for implementation; however, studies indicate that a power plant that captures CO2 and employs mineral carbonation would need 60–180 per cent more energy than an equivalent power plant without the capture and conversion process.
IPCC, Climate change 2007—the physical science basis, Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [S. Solomon, D. Qin, M. Manning, Z. Chen, M. Marquis, K. B. Averyt, M. Tignor and H. L. Miller (eds)], Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 2007.
W. M. Post, W. R. Emanuel, P. J. Zinke and A. G. Stangenberger, 'Soil carbon pools and world life zones', Nature, vol. 298, pp. 156–9, 1982.
European Communities, Climate change—can soil make a difference?, Conference Report, Brussels, 12 June 2008.
CSIRO, 'Managing Australia's soil and landscape assets (MASaLA)', 'Soil carbon—the basics', 'Factors which influence soil carbon levels', 'Biochar fact sheet'.
Biochar.org, Balance Carbon and Restore Soil Fertility.
CSIRO, Australian Soil Resource Information System (ASRIS)—Mapping of soil types, landforms, regolith, soil depth, water storage, permeability, fertility, carbon and erodability.
IPCC, Special report on carbon dioxide capture and storage, Working Group III of the Intergovernmental Panel on Climate Change [B. Metz, O. Davidson, H. de Coninck, M. Loos and L. Meyer (eds.)], Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, 2005.
Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC).
International Energy Agency Greenhouse Gas Research and Development Programme, 'CO2 capture and storage'.