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Carbon dioxide sink

Carbon dioxide sink

Overview
A carbon sink is a natural or artificial reservoir
Reservoir
A reservoir , artificial lake or dam is used to store water.Reservoirs may be created in river valleys by the construction of a dam or may be built by excavation in the ground or by conventional construction techniques such as brickwork or cast concrete.The term reservoir may also be used to...

 that accumulates and stores some carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

-containing chemical compound for an indefinite period. The process by which carbon sinks remove carbon dioxide (CO2) from the atmosphere is known as carbon sequestration. Public awareness of the significance of CO2 sinks has grown since passage of the Kyoto Protocol
Kyoto Protocol
The Kyoto Protocol is a protocol to the United Nations Framework Convention on Climate Change , aimed at fighting global warming...

, which promotes their use as a form of carbon offset
Carbon offset
A carbon offset is a reduction in emissions of carbon dioxide or greenhouse gases made in order to compensate for or to offset an emission made elsewhere....

.

The main natural sinks are:
  • Absorption of carbon dioxide
    Carbon dioxide
    Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

     by the ocean
    Ocean
    An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...

    s via physicochemical
    Solubility pump
    In oceanic biogeochemistry, the solubility pump is a physico-chemical process that transports carbon from the ocean's surface to its interior.-Overview:...

     and biological
    Biological pump
    In oceanic biogeochemistry, the biological pump is the sum of a suite of biologically-mediated processes that transport carbon from the surface euphotic zone to the ocean's interior.-Overview:...

     processes
  • Photosynthesis
    Photosynthesis
    Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

     by terrestrial plants


Natural sinks are typically much larger than artificial sinks.
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Encyclopedia
A carbon sink is a natural or artificial reservoir
Reservoir
A reservoir , artificial lake or dam is used to store water.Reservoirs may be created in river valleys by the construction of a dam or may be built by excavation in the ground or by conventional construction techniques such as brickwork or cast concrete.The term reservoir may also be used to...

 that accumulates and stores some carbon
Carbon
Carbon is the chemical element with symbol C and atomic number 6. As a member of group 14 on the periodic table, it is nonmetallic and tetravalent—making four electrons available to form covalent chemical bonds...

-containing chemical compound for an indefinite period. The process by which carbon sinks remove carbon dioxide (CO2) from the atmosphere is known as carbon sequestration. Public awareness of the significance of CO2 sinks has grown since passage of the Kyoto Protocol
Kyoto Protocol
The Kyoto Protocol is a protocol to the United Nations Framework Convention on Climate Change , aimed at fighting global warming...

, which promotes their use as a form of carbon offset
Carbon offset
A carbon offset is a reduction in emissions of carbon dioxide or greenhouse gases made in order to compensate for or to offset an emission made elsewhere....

.

The main natural sinks are:
  • Absorption of carbon dioxide
    Carbon dioxide
    Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

     by the ocean
    Ocean
    An ocean is a major body of saline water, and a principal component of the hydrosphere. Approximately 71% of the Earth's surface is covered by ocean, a continuous body of water that is customarily divided into several principal oceans and smaller seas.More than half of this area is over 3,000...

    s via physicochemical
    Solubility pump
    In oceanic biogeochemistry, the solubility pump is a physico-chemical process that transports carbon from the ocean's surface to its interior.-Overview:...

     and biological
    Biological pump
    In oceanic biogeochemistry, the biological pump is the sum of a suite of biologically-mediated processes that transport carbon from the surface euphotic zone to the ocean's interior.-Overview:...

     processes
  • Photosynthesis
    Photosynthesis
    Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

     by terrestrial plants


Natural sinks are typically much larger than artificial sinks. The main artificial sinks are:
  • Landfill
    Landfill
    A landfill site , is a site for the disposal of waste materials by burial and is the oldest form of waste treatment...

    s
  • Carbon capture and storage
    Carbon capture and storage
    Carbon capture and storage , alternatively referred to as carbon capture and sequestration, is a technology to prevent large quantities of from being released into the atmosphere from the use of fossil fuel in power generation and other industries. It is often regarded as a means of mitigating...

     proposals


Carbon sources include:
  • Fossil fuels
  • Farmland; there are proposals for improvements in farming practices to reverse this.

Kyoto Protocol


Because growing vegetation absorbs carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

, the Kyoto Protocol
Kyoto Protocol
The Kyoto Protocol is a protocol to the United Nations Framework Convention on Climate Change , aimed at fighting global warming...

 allows Annex I countries with large areas of growing forests to issue Removal Units
Removal Units
A Removal Unit is a tradable carbon credit or 'Kyoto unit' representing an allowance to emit one metric tonne of greenhouse gases absorbed by a removal or Carbon sink activity in an Annex I country....

 to recognise the sequestration of carbon. The additional units make it easier for them to achieve their target emission levels.

Some countries seek to trade emission rights in carbon emission markets, purchasing the unused carbon emission allowances of other countries. If overall limits on greenhouse gas emission are put into place, cap and trade market mechanisms are purported to find cost-effective ways to reduce emissions. There is as yet no carbon audit regime
Carbon audit regime
A carbon audit regime is a means of accounting for quantifiable greenhouse gas control efforts. It establishes that the claimed reductions in emissions or enhancements of carbon sinks, has actually occurred and is stable....

 for all such markets globally, and none is specified in the Kyoto Protocol. National carbon emissions are self-declared.

In the Clean Development Mechanism
Clean Development Mechanism
The Clean Development Mechanism is one of the "flexibility" mechanisms defined in the Kyoto Protocol . It is defined in Article 12 of the Protocol, and is intended to meet two objectives: to assist parties not included in Annex I in achieving sustainable development and in contributing to the...

, only afforestation
Afforestation
Afforestation is the establishment of a forest or stand of trees in an area where there was no forest. Reforestation is the reestablishment of forest cover, either naturally or artificially...

 and reforestation
Reforestation
Reforestation is the natural or intentional restocking of existing forests and woodlands that have been depleted, usually through deforestation....

 are eligible to produce certified emission reduction
Certified Emission Reduction
Certified Emission Reductions are a type of emissions unit issued by the Clean Development Mechanism Executive Board for emission reductions achieved by CDM projects and verified by a DOE under the rules of the Kyoto Protocol...

s (CERs) in the first commitment period of the Kyoto Protocol (2008–2012). Forest conservation activities or activities avoiding deforestation
Deforestation
Deforestation is the removal of a forest or stand of trees where the land is thereafter converted to a nonforest use. Examples of deforestation include conversion of forestland to farms, ranches, or urban use....

, which would result in emission reduction through the conservation of existing carbon stocks, are not eligible at this time. Also, agricultural carbon sequestration is not possible yet.

Soils


Soils represent a short to long-term carbon storage medium, and contain more carbon than all terrestrial vegetation and the atmosphere combined. Plant litter
Plant litter
Plant litter, leaf litter or tree litter is dead plant material, such as leaves, bark, needles, and twigs, that has fallen to the ground. Litter provides habitat for small animals, fungi, and plants, and the material may be used to construct nests. As litter decomposes, nutrients are released to...

 and other biomass
Biomass
Biomass, as a renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel....

 accumulates as organic matter
Soil organic matter
Organic matter is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds...

 in soils, and is degraded by chemical weathering and biological degradation
Biodegradation
Biodegradation or biotic degradation or biotic decomposition is the chemical dissolution of materials by bacteria or other biological means...

. More recalcitrant organic carbon polymers
Biopolymer
Biopolymers are polymers produced by living organisms. Since they are polymers, Biopolymers contain monomeric units that are covalently bonded to form larger structures. There are three main classes of biopolymers based on the differing monomeric units used and the structure of the biopolymer formed...

 such as cellulose
Cellulose
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....

, hemi-cellulose, lignin
Lignin
Lignin or lignen is a complex chemical compound most commonly derived from wood, and an integral part of the secondary cell walls of plants and some algae. The term was introduced in 1819 by de Candolle and is derived from the Latin word lignum, meaning wood...

, aliphatic compounds, waxes and terpenoids are collectively retained as humus. Organic matter tends to accumulate in litter and soils of colder regions such as the boreal forests of North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

 and the Taiga
Taiga
Taiga , also known as the boreal forest, is a biome characterized by coniferous forests.Taiga is the world's largest terrestrial biome. In North America it covers most of inland Canada and Alaska as well as parts of the extreme northern continental United States and is known as the Northwoods...

 of Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...

. Leaf litter and humus are rapidly oxidized and poorly retained in sub-tropical and tropical climate
Climate
Climate encompasses the statistics of temperature, humidity, atmospheric pressure, wind, rainfall, atmospheric particle count and other meteorological elemental measurements in a given region over long periods...

 conditions due to high temperatures and extensive leaching by rainfall. Areas where shifting cultivation
Shifting cultivation
Shifting cultivation is an agricultural system in which plots of land are cultivated temporarily, then abandoned. This system often involves clearing of a piece of land followed by several years of wood harvesting or farming, until the soil loses fertility...

 or slash and burn
Slash and burn
Slash-and-burn is an agricultural technique which involves cutting and burning of forests or woodlands to create fields. It is subsistence agriculture that typically uses little technology or other tools. It is typically part of shifting cultivation agriculture, and of transhumance livestock...

 agriculture are practiced are generally only fertile for 2–3 years before they are abandoned. These tropical jungles are similar to coral reefs in that they are highly efficient at conserving and circulating necessary nutrients, which explains their lushness in a nutrient desert. Much organic carbon
Soil carbon
Soil carbon is the generic name for carbon held within the soil, primarily in association with its organic content. Soil carbon is the largest terrestrial pool of carbon. Humans have, and will likely continue to have, significant impacts on the size of this pool...

 retained in many agricultural areas worldwide has been severely depleted due to intensive farming practices.

Grasslands contribute to soil organic matter
Soil organic matter
Organic matter is matter that has come from a once-living organism; is capable of decay, or the product of decay; or is composed of organic compounds...

, stored mainly in their extensive fibrous root mats. Due in part to the climactic conditions of these regions (e.g. cooler temperatures and semi-arid to arid conditions), these soils can accumulate significant quantities of organic matter. This can vary based on rainfall, the length of the winter season, and the frequency of naturally occurring lightning-induced grass-fires
Wildfire
A wildfire is any uncontrolled fire in combustible vegetation that occurs in the countryside or a wilderness area. Other names such as brush fire, bushfire, forest fire, desert fire, grass fire, hill fire, squirrel fire, vegetation fire, veldfire, and wilkjjofire may be used to describe the same...

. While these fires release carbon dioxide, they improve the quality of the grasslands overall, in turn increasing the amount of carbon retained in the retained humic material. They also deposit carbon directly to the soil in the form of char
Biochar
Biochar or terra preta is charcoal created by pyrolysis of biomass. Biochar is under investigation as an approach to carbon sequestration via bio-energy with carbon capture and storage. Biochar thus has the potential to help mitigate climate change, via carbon sequestration...

 that does not significantly degrade back to carbon dioxide.

Forest fires release absorbed carbon back into the atmosphere, as does deforestation due to rapidly increased oxidation of soil organic matter.

Organic matter in peat
Peat
Peat is an accumulation of partially decayed vegetation matter or histosol. Peat forms in wetland bogs, moors, muskegs, pocosins, mires, and peat swamp forests. Peat is harvested as an important source of fuel in certain parts of the world...

 bogs undergoes slow anaerobic decomposition below the surface. This process is slow enough that in many cases the bog grows rapidly and fixes
Carbon fixation
In biology, carbon fixation is the reduction of carbon dioxide to organic compounds by living organisms. The obvious example is photosynthesis. Carbon fixation requires both a source of energy such as sunlight, and an electron donor such as water. All life depends on fixed carbon. Organisms that...

 more carbon from the atmosphere than is released. Over time, the peat grows deeper. Peat bogs inter approximately one-quarter of the carbon stored in land plants and soils.

Under some conditions, forests and peat bogs may become sources of CO2, such as when a forest is flooded by the construction of a hydroelectric dam. Unless the forests and peat are harvested before flooding, the rotting vegetation is a source of CO2 and methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

 comparable in magnitude to the amount of carbon released by a fossil-fuel powered plant of equivalent power.

Regenerative agriculture


Current agricultural practices lead to carbon loss from soils. It has been suggested that improved farming practices could return the soils to being a carbon sink. Present worldwide practises of overgrazing are substantially reducing many grasslands' performance as carbon sinks. The Rodale Institute says that Regenerative agriculture, if practiced on the planet’s 3.5 billion tillable acres, could sequester up to 40% of current CO2 emissions. They claim that agricultural carbon sequestration has the potential to mitigate global warming. When using biologically based regenerative practices, this dramatic benefit can be accomplished with no decrease in yields or farmer profits. Organically managed soils can convert carbon dioxide from a greenhouse gas into a food-producing asset.

In 2006, U.S. carbon dioxide emissions from fossil fuel combustion were estimated at nearly 6.5 billion tons. If a 2,000 (lb/ac)/year sequestration rate was achieved on all 434000000 acres (1,756,337.2 km²) of cropland in the United States, nearly 1.6 billion tons of carbon dioxide would be sequestered per year, mitigating close to one quarter of the country's total fossil fuel emissions.

Oceans



Oceans are at present CO2 sinks, and represent the largest active carbon sink on Earth, absorbing more than a quarter of the carbon dioxide that humans put into the air. On longer timescales they may be both sources and sinks - during ice age
Ice age
An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...

s levels decrease to ~180 ppmv, and much of this is believed to be stored in the oceans. As ice ages end, is released from the oceans and levels during previous interglacials have been around ~280 ppmv. This role as a sink for CO2 is driven by two processes, the solubility pump
Solubility pump
In oceanic biogeochemistry, the solubility pump is a physico-chemical process that transports carbon from the ocean's surface to its interior.-Overview:...

 and the biological pump
Biological pump
In oceanic biogeochemistry, the biological pump is the sum of a suite of biologically-mediated processes that transport carbon from the surface euphotic zone to the ocean's interior.-Overview:...

. The former is primarily a function of differential CO2 solubility in seawater
Seawater
Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% . This means that every kilogram of seawater has approximately of dissolved salts . The average density of seawater at the ocean surface is 1.025 g/ml...

 and the thermohaline circulation
Thermohaline circulation
The term thermohaline circulation refers to a part of the large-scale ocean circulation that is driven by global density gradients created by surface heat and freshwater fluxes....

, while the latter is the sum of a series of biological processes that transport carbon (in organic
Organic chemistry
Organic chemistry is a subdiscipline within chemistry involving the scientific study of the structure, properties, composition, reactions, and preparation of carbon-based compounds, hydrocarbons, and their derivatives...

 and inorganic
Inorganic chemistry of carbon
Scientists once thought that organic compounds are or could only be created by living organisms. Over time, human have learned how to synthesize organic compounds. There is an immense number of distinct compounds that contain carbon atoms. Some sources suggest that there are about ten million known...

 forms) from the surface euphotic zone
Photic zone
The photic zone or euphotic zone is the depth of the water in a lake or ocean that is exposed to sufficient sunlight for photosynthesis to occur...

 to the ocean's interior. A small fraction of the organic carbon transported by the biological pump to the seafloor
Seabed
The seabed is the bottom of the ocean.- Ocean structure :Most of the oceans have a common structure, created by common physical phenomena, mainly from tectonic movement, and sediment from various sources...

 is buried in anoxic
Anoxic sea water
Anoxic waters are areas of sea water or fresh water that are depleted of dissolved oxygen. This condition is generally found in areas that have restricted water exchange....

 conditions under sediments and ultimately forms fossil fuel
Fossil fuel
Fossil fuels are fuels formed by natural processes such as anaerobic decomposition of buried dead organisms. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years...

s such as oil
Petroleum
Petroleum or crude oil is a naturally occurring, flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights and other liquid organic compounds, that are found in geologic formations beneath the Earth's surface. Petroleum is recovered mostly through oil drilling...

 and natural gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...

.

At the present time, approximately one third of human generated emissions are estimated to be entering the ocean. The solubility pump is the primary mechanism driving this, with the biological pump playing a negligible role. This stems from the limitation of the biological pump by ambient light and nutrients required by the phytoplankton
Phytoplankton
Phytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...

 that ultimately drive it. Total inorganic carbon
Total inorganic carbon
The total inorganic carbon or dissolved inorganic carbon is the sum of inorganic carbon species in a solution. The inorganic carbon species include carbon dioxide, carbonic acid, bicarbonate anion, and carbonate. It is customary to express carbon dioxide and carbonic acid simultaneously as CO2*...

 is not believed to limit primary production
Primary production
400px|thumb|Global oceanic and terrestrial photoautotroph abundance, from September [[1997]] to August 2000. As an estimate of autotroph biomass, it is only a rough indicator of primary production potential, and not an actual estimate of it...

 in the oceans, so its increasing availability in the ocean does not directly affect production (the situation on land is different, since enhanced atmospheric levels of CO2 essentially "fertilize" land plant growth). However, ocean acidification
Ocean acidification
Ocean acidification is the name given to the ongoing decrease in the pH and increase in acidity of the Earth's oceans, caused by the uptake of anthropogenic carbon dioxide from the atmosphere....

 by invading anthropogenic CO2 may affect the biological pump by negatively impacting calcifying
Calcium carbonate
Calcium carbonate is a chemical compound with the formula CaCO3. It is a common substance found in rocks in all parts of the world, and is the main component of shells of marine organisms, snails, coal balls, pearls, and eggshells. Calcium carbonate is the active ingredient in agricultural lime,...

 organisms such as coccolithophore
Coccolithophore
Coccolithophores are single-celled algae, protists, and phytoplankton belonging to the division of haptophytes. They are distinguished by special calcium carbonate plates of uncertain function called coccoliths , which are important microfossils...

s, foraminiferans and pteropods. Climate change
Climate change
Climate change is a significant and lasting change in the statistical distribution of weather patterns over periods ranging from decades to millions of years. It may be a change in average weather conditions or the distribution of events around that average...

 may also affect the biological pump in the future by warming and stratifying
Stratification (water)
Water stratification occurs when water masses with different properties - salinity , oxygenation , density , temperature - form layers that act as barriers to water mixing...

 the surface ocean, thus reducing the supply of limiting nutrients to surface waters.

In January 2009, the Monterey Bay Aquarium Research Institute
Monterey Bay Aquarium Research Institute
The Monterey Bay Aquarium Research Institute is a not-for-profit oceanographic research center in Moss Landing, California affiliated with the Monterey Bay Aquarium. It was founded in 1987 by David Packard of Hewlett-Packard fame...

 and the National Oceanic and Atmospheric Administration
National Oceanic and Atmospheric Administration
The National Oceanic and Atmospheric Administration , pronounced , like "noah", is a scientific agency within the United States Department of Commerce focused on the conditions of the oceans and the atmosphere...

 announced a joint study to determine whether the ocean off the California
California
California is a state located on the West Coast of the United States. It is by far the most populous U.S. state, and the third-largest by land area...

 coast was serving as a carbon source or a carbon sink. Principal instrumentation for the study will be self-contained CO2 monitors placed on buoy
Buoy
A buoy is a floating device that can have many different purposes. It can be anchored or allowed to drift. The word, of Old French or Middle Dutch origin, is now most commonly in UK English, although some orthoepists have traditionally prescribed the pronunciation...

s in the ocean. They will measure the partial pressure of CO2 in the ocean and the atmosphere just above the water surface.

In February 2009, Science Daily reported that the Southern Indian Ocean is becoming less effective at absorbing carbon dioxide due to changes to the regions climate which include higher wind speeds.

Forests



Forests are carbon stores, and they are carbon dioxide sinks when they are increasing in density or area. In Canada's boreal forests
Boreal forest of Canada
Canada's boreal forest comprises about one third of the circumpolar boreal forest that rings the northern hemisphere, mostly north of the 50th parallel. Other countries with boreal forest include Russia, which contains the majority, and the Scandinavian and Nordic countries . The boreal region in...

 as much as 80% of the total carbon is stored in the soils as dead organic matter. A 40-year study of African, Asian, and South American tropical forests by the University of Leeds, shows tropical forests absorb about 18% of all carbon dioxide added by fossil fuels. Tropical reforestation
Reforestation
Reforestation is the natural or intentional restocking of existing forests and woodlands that have been depleted, usually through deforestation....

 can mitigate global warming until all available land has been reforested with mature forests. However, the global cooling effect of carbon sequestration by forests is partially counterbalanced in that reforestation can decrease the reflection of sunlight (albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

). Mid-to-high latitude forests have a much lower albedo
Albedo
Albedo , or reflection coefficient, is the diffuse reflectivity or reflecting power of a surface. It is defined as the ratio of reflected radiation from the surface to incident radiation upon it...

 during snow
Snow
Snow is a form of precipitation within the Earth's atmosphere in the form of crystalline water ice, consisting of a multitude of snowflakes that fall from clouds. Since snow is composed of small ice particles, it is a granular material. It has an open and therefore soft structure, unless packed by...

 seasons than flat ground, thus contributing to warming. Modeling that compares the effects of albedo differences between forests and grasslands suggests that expanding the land area of forests in temperate zones offers only a temporary cooling benefit.

In the United States in 2004 (the most recent year for which EPA statistics are available), forests sequestered 10.6% (637 teragrams) of the carbon dioxide released in the United States by the combustion of fossil fuels (coal, oil and natural gas; 5657 teragrams). Urban trees sequestered another 1.5% (88 teragram
Teragram
Teragram may refer to:* 1012 grams, equivalent to a megatonne. See Orders of magnitude * Teragram Corporation...

s). To further reduce U.S. carbon dioxide emissions by 7%, as stipulated by the Kyoto Protocol
Kyoto Protocol
The Kyoto Protocol is a protocol to the United Nations Framework Convention on Climate Change , aimed at fighting global warming...

, would require the planting of "an area the size of Texas
Texas
Texas is the second largest U.S. state by both area and population, and the largest state by area in the contiguous United States.The name, based on the Caddo word "Tejas" meaning "friends" or "allies", was applied by the Spanish to the Caddo themselves and to the region of their settlement in...

 [8% of the area of Brazil] every 30 years". Carbon offset
Carbon offset
A carbon offset is a reduction in emissions of carbon dioxide or greenhouse gases made in order to compensate for or to offset an emission made elsewhere....

 programs are planting millions of fast-growing trees per year to reforest tropical lands, for as little as $0.10 per tree; over their typical 40-year lifetime, one million of these trees will fix 0.9 teragrams of carbon dioxide. In Canada, reducing timber harvesting would have very little impact on carbon dioxide emissions because of the combination of harvest and stored carbon in manufactured wood products along with the regrowth of the harvested forests. Additionally, the amount of carbon released from harvesting is small compared to the amount of carbon lost each year to forest fires and other natural disturbances.

The Intergovernmental Panel on Climate Change
Intergovernmental Panel on Climate Change
The Intergovernmental Panel on Climate Change is a scientific intergovernmental body which provides comprehensive assessments of current scientific, technical and socio-economic information worldwide about the risk of climate change caused by human activity, its potential environmental and...

 concluded that "a sustainable forest management strategy aimed at maintaining or increasing forest carbon stocks, while producing an annual sustained yield of timber fibre or energy from the forest, will generate the largest sustained mitigation benefit". Sustainable management practices keep forests growing at a higher rate over a potentially longer period of time, thus providing net sequestration benefits in addition to those of unmanaged forests.

Life expectancy of forests varies throughout the world, influenced by tree species, site conditions and natural disturbance patterns. In some forests carbon may be stored for centuries, while in other forests carbon is released with frequent stand replacing fires. Forests that are harvested prior to stand replacing events allow for the retention of carbon in manufactured forest products such as lumber. However, only a portion of the carbon removed from logged forests ends up as durable goods and buildings. The remainder ends up as sawmill by-products such as pulp, paper and pallets, which often end with incineration (resulting in carbon release into the atmosphere) at the end of their lifecycle. For instance, of the 1,692 teragrams of carbon harvested from forests in Oregon and Washington (U.S) from 1900 to 1992, only 23% is in long-term storage in forest products.

Oceans



One way to increase the carbon sequestration efficiency of the oceans is to add micrometre-sized iron particles in the form of either hematite
Hematite
Hematite, also spelled as haematite, is the mineral form of iron oxide , one of several iron oxides. Hematite crystallizes in the rhombohedral system, and it has the same crystal structure as ilmenite and corundum...

 (iron oxide) or melanterite
Melanterite
Melanterite is a mineral form of hydrous iron sulfate: FeSO4·7H2O. It is the iron analogue of the copper sulfate chalcanthite. It alters to siderotil by loss of water. It is a secondary sulfate mineral which forms from the oxidation of primary sulfide minerals such as pyrite and marcasite in the...

 (iron sulfate) to certain regions of the ocean. This has the effect of stimulating growth of plankton
Plankton
Plankton are any drifting organisms that inhabit the pelagic zone of oceans, seas, or bodies of fresh water. That is, plankton are defined by their ecological niche rather than phylogenetic or taxonomic classification...

. Iron is an important nutrient for phytoplankton
Phytoplankton
Phytoplankton are the autotrophic component of the plankton community. The name comes from the Greek words φυτόν , meaning "plant", and πλαγκτός , meaning "wanderer" or "drifter". Most phytoplankton are too small to be individually seen with the unaided eye...

, usually made available via upwelling along the continental shelves, inflows from rivers and streams, as well as deposition of dust suspended in the atmosphere
Earth's atmosphere
The atmosphere of Earth is a layer of gases surrounding the planet Earth that is retained by Earth's gravity. The atmosphere protects life on Earth by absorbing ultraviolet solar radiation, warming the surface through heat retention , and reducing temperature extremes between day and night...

. Natural sources of ocean iron have been declining in recent decades, contributing to an overall decline in ocean productivity (NASA, 2003). Yet in the presence of iron nutrients plankton populations quickly grow, or 'bloom', expanding the base of biomass
Biomass
Biomass, as a renewable energy source, is biological material from living, or recently living organisms. As an energy source, biomass can either be used directly, or converted into other energy products such as biofuel....

 productivity throughout the region and removing significant quantities of CO2 from the atmosphere via photosynthesis
Photosynthesis
Photosynthesis is a chemical process that converts carbon dioxide into organic compounds, especially sugars, using the energy from sunlight. Photosynthesis occurs in plants, algae, and many species of bacteria, but not in archaea. Photosynthetic organisms are called photoautotrophs, since they can...

. A test in 2002 in the Southern Ocean
Southern Ocean
The Southern Ocean comprises the southernmost waters of the World Ocean, generally taken to be south of 60°S latitude and encircling Antarctica. It is usually regarded as the fourth-largest of the five principal oceanic divisions...

 around Antarctica suggests that between 10,000 and 100,000 carbon atoms are sunk for each iron atom added to the water. More recent work in Germany (2005) suggests that any biomass carbon in the oceans, whether exported to depth or recycled in the euphotic zone, represents long-term storage of carbon. This means that application of iron nutrients in select parts of the oceans, at appropriate scales, could have the combined effect of restoring ocean productivity while at the same time mitigating the effects of human caused emissions of carbon dioxide to the atmosphere.

Because the effect of periodic small scale phytoplankton blooms on ocean ecosystems is unclear, more studies would be helpful. Phytoplankton have a complex effect on cloud formation via the release of substances such as dimethyl sulfide
Dimethyl sulfide
Dimethyl sulfide or methylthiomethane is an organosulfur compound with the formula 2S. Dimethyl sulfide is a water-insoluble flammable liquid that boils at and has a characteristic disagreeable odor. It is a component of the smell produced from cooking of certain vegetables, notably maize,...

 (DMS) that are converted to sulfate aerosols in the atmosphere, providing cloud condensation nuclei
Cloud condensation nuclei
Cloud condensation nuclei or CCNs are small particles typically 0.2 µm, or 1/100 th the size of a cloud droplet ) about which cloud droplets coalesce. Water requires a non-gaseous surface to make the transition from a vapour to a liquid. In the atmosphere, this surface presents itself as tiny...

, or CCN. But the effect of small scale plankton blooms on overall DMS production is unknown.

Other nutrients such as nitrates, phosphates, and silica as well as iron may cause ocean fertilization. There has been some speculation that using pulses of fertilization (around 20 days in length) may be more effective at getting carbon to ocean floor than sustained fertilization.

There is some controversy over seeding the oceans with iron however, due to the potential for increased toxic phytoplankton growth (e.g. "red tide
Red tide
Red tide is a common name for a phenomenon also known as an algal bloom , an event in which estuarine, marine, or fresh water algae accumulate rapidly in the water column and results in discoloration of the surface water. It is usually found in coastal areas...

"), declining water quality due to overgrowth, and increasing anoxia in areas harming other sea-life such as zooplankton, fish, coral, etc.

Soils


Since the 1850s, a large proportion of the world's grasslands have been tilled and converted to croplands, allowing the rapid oxidation of large quantities of soil organic carbon. However, in the United States in 2004 (the most recent year for which EPA statistics are available), agricultural soils including pasture land sequestered 0.8% (46 teragrams) as much carbon as was released in the United States by the combustion of fossil fuels (5988 teragrams). The annual amount of this sequestration has been gradually increasing since 1998.

Methods that significantly enhance carbon sequestration in soil include no-till farming
No-till farming
No-till farming is a way of growing crops from year to year without disturbing the soil through tillage. No-till is an agricultural technique which increases the amount of water and organic matter in the soil and decreases erosion...

, residue mulching, cover crop
Cover crop
Cover crops are crops planted primarily to manage soil fertility, soil quality, water, weeds, pests, diseases, biodiversity and wildlife in agroecosystems , ecological systems managed and largely shaped by humans across a range of intensities to produce food, feed, or fiber.Cover crops are of...

ping, and crop rotation
Crop rotation
Crop rotation is the practice of growing a series of dissimilar types of crops in the same area in sequential seasons.Crop rotation confers various benefits to the soil. A traditional element of crop rotation is the replenishment of nitrogen through the use of green manure in sequence with cereals...

, all of which are more widely used in organic farming
Organic farming
Organic farming is the form of agriculture that relies on techniques such as crop rotation, green manure, compost and biological pest control to maintain soil productivity and control pests on a farm...

 than in conventional farming. Because only 5% of US farmland currently uses no-till and residue mulching, there is a large potential for carbon sequestration. Conversion to pastureland, particularly with good management of grazing, can sequester even more carbon in the soil.

Terra preta
Terra preta
Terra preta is a type of very dark, fertile anthropogenic soil found in the Amazon Basin. Terra preta owes its name to its very high charcoal content, and was indeed made by adding a mixture of charcoal, bone, and manure to the otherwise relatively infertile Amazonian soil, and stays there for...

, an anthropogenic, high-carbon soil, is also being investigated as a sequestration mechanism.
By pyrolysing
Pyrolysis
Pyrolysis is a thermochemical decomposition of organic material at elevated temperatures without the participation of oxygen. It involves the simultaneous change of chemical composition and physical phase, and is irreversible...

 biomass, about half of its carbon can be reduced to charcoal
Charcoal
Charcoal is the dark grey residue consisting of carbon, and any remaining ash, obtained by removing water and other volatile constituents from animal and vegetation substances. Charcoal is usually produced by slow pyrolysis, the heating of wood or other substances in the absence of oxygen...

, which can persist in the soil for centuries, and makes a useful soil amendment, especially in tropical soils (biochar
Biochar
Biochar or terra preta is charcoal created by pyrolysis of biomass. Biochar is under investigation as an approach to carbon sequestration via bio-energy with carbon capture and storage. Biochar thus has the potential to help mitigate climate change, via carbon sequestration...

or agrichar).

Savanna


Controlled burn
Controlled burn
Controlled or prescribed burning, also known as hazard reduction burning or Swailing is a technique sometimes used in forest management, farming, prairie restoration or greenhouse gas abatement. Fire is a natural part of both forest and grassland ecology and controlled fire can be a tool for...

s on far north Australian savanna
Savanna
A savanna, or savannah, is a grassland ecosystem characterized by the trees being sufficiently small or widely spaced so that the canopy does not close. The open canopy allows sufficient light to reach the ground to support an unbroken herbaceous layer consisting primarily of C4 grasses.Some...

s can result in an overall carbon sink. One working example is the West Arnhem Fire Management Agreement, started to bring "strategic fire management across 28,000 km² of Western Arnhem Land". Deliberately starting controlled burns early in the dry season results in a mosaic of burnt and unburnt country which reduces the area of burning compared with stronger, late dry season fires. In the early dry season there are higher moisture levels, cooler temperatures, and lighter wind than later in the dry season; fires tend to go out overnight. Early controlled burns also results in a smaller proportion of the grass and tree biomass being burnt. Emission reductions of 256,000 tonnes of CO2 have been made as of 2007.

Artificial sequestration


For carbon to be sequestered artificially (i.e. not using the natural processes of the carbon cycle) it must first be captured, or it must be significantly delayed or prevented from being re-released into the atmosphere (by combustion, decay, etc.) from an existing carbon-rich material, by being incorporated into an enduring usage (such as in construction). Thereafter it can be passively stored or remain productively utilized over time in a variety of ways.

For example, upon harvesting, wood (as a carbon-rich material) can be immediately burned or otherwise serve as a fuel, returning its carbon to the atmosphere, or it can be incorporated into construction or a range of other durable products, thus sequestering its carbon over years or even centuries. One ton of dry wood is equivalent to 1.8 tons of carbon dioxide.

Indeed, a very carefully designed and durable, energy-efficient and energy-capturing building has the potential to sequester (in its carbon-rich construction materials), as much as or more carbon than was released by the acquisition and incorporation of all its materials and than will be released by building-function "energy-imports" during the structure's (potentially multi-century) existence. Such a structure might be termed "carbon neutral" or even "carbon negative". Building construction and operation (electricity usage, heating, etc.) are estimated to contribute nearly half of the annual human-caused carbon additions to the atmosphere.

Natural-gas purification plants often already have to remove carbon dioxide, either to avoid dry ice
Dry ice
Dry ice, sometimes referred to as "Cardice" or as "card ice" , is the solid form of carbon dioxide. It is used primarily as a cooling agent. Its advantages include lower temperature than that of water ice and not leaving any residue...

 clogging gas tankers or to prevent carbon-dioxide concentrations exceeding the 3% maximum permitted on the natural-gas distribution grid.

Beyond this, one of the most likely early applications of carbon capture is the capture of carbon dioxide from flue gas
Flue gas
Flue gas is the gas exiting to the atmosphere via a flue, which is a pipe or channel for conveying exhaust gases from a fireplace, oven, furnace, boiler or steam generator. Quite often, the flue gas refers to the combustion exhaust gas produced at power plants...

es at power station
Power station
A power station is an industrial facility for the generation of electric energy....

s (in the case of coal, this is known as "clean coal
Clean coal
Historically used to refer to technologies for reducing emissions of ash, sulfur, and heavy metals from coal combustion; the term is now commonly used to refer to carbon capture and storage technology...

"). A typical new 1000 MW coal-fired power station
Coal-fired power station
A coal-fired power station produces electricity, usually for public consumption, by burning coal to boil water, producing steam which drives a steam turbine which turns an electrical generator...

 produces around 6 million tons of carbon dioxide annually. Adding carbon capture to existing plants can add significantly to the costs of energy production; scrubbing costs aside, a 1000 MW coal plant will require the storage of about 50 Moilbbl of carbon dioxide a year. However, scrubbing is relatively affordable when added to new plants based on coal gasification
Coal gasification
Coal gasification is the process of producing coal gas, a type of syngas–a mixture of carbon monoxide , hydrogen , carbon dioxide and water vapour –from coal...

 technology, where it is estimated to raise energy costs for households in the United States using only coal-fired electricity sources from 10 cents per kW·h to 12 cents.

Carbon capture



Currently, capture of carbon dioxide is performed on a large scale by absorption of carbon dioxide onto various amine-based solvents
Amine gas treating
Amine gas treating, also known as gas sweetening and acid gas removal, refers to a group of processes that use aqueous solutions of various alkylamines to remove hydrogen sulfide and carbon dioxide from gases...

. Other techniques are currently being investigated, such as pressure swing adsorption
Pressure swing adsorption
Pressure swing adsorption is a technology used to separate some gas species from a mixture of gases under pressure according to the species' molecular characteristics and affinity for an adsorbent material. It operates at near-ambient temperatures and so differs from cryogenic distillation...

, temperature swing adsorption, gas separation membranes
Gas separation
Gas mixtures can be effectively separated by synthetic membranes. For other methods see adsorption, absorption, cryogenic distillation.Membranes are employed in:* separation of nitrogen or oxygen out of air...

, and cryogenics. Recent pilot studies include flue capture
Flue gas
Flue gas is the gas exiting to the atmosphere via a flue, which is a pipe or channel for conveying exhaust gases from a fireplace, oven, furnace, boiler or steam generator. Quite often, the flue gas refers to the combustion exhaust gas produced at power plants...

 and conversion to baking soda and use of algae
Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. They are photosynthetic like plants, and "simple" because their tissues are not organized into the many...

 for conversion to fuel or feed.

In coal-fired power stations, the main alternatives to retrofitting amine-based absorbers to existing power stations are two new technologies: coal gasification combined-cycle
Integrated Gasification Combined Cycle
An integrated gasification combined cycle is a technology that turns coal into gas—synthesis gas . It then removes impurities from the coal gas before it is combusted and attempts to turn any pollutants into re-usable byproducts. This results in lower emissions of sulfur dioxide, particulates, and...

 and oxy-fuel combustion. Gasification first produces a "syngas
Syngas
Syngas is the name given to a gas mixture that contains varying amounts of carbon monoxide and hydrogen. Examples of production methods include steam reforming of natural gas or liquid hydrocarbons to produce hydrogen, the gasification of coal, biomass, and in some types of waste-to-energy...

" primarily of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...

 and carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...

, which is burned, with carbon dioxide filtered from the flue gas. Oxy-fuel combustion burns the coal in oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...

 instead of air, producing only carbon dioxide and water vapour, which are relatively easily separated. Some of the combustion products must be returned to the combustion chamber, either before or after separation, otherwise the temperatures would be too high for the turbine.

Another long-term option is carbon capture directly from the air using hydroxides. The air would literally be scrubbed of its CO2 content. This idea offers an alternative to non-carbon-based fuels
Fossil fuel
Fossil fuels are fuels formed by natural processes such as anaerobic decomposition of buried dead organisms. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years...

 for the transportation sector.

Examples of carbon sequestration at coal plants include converting carbon from smokestacks into baking soda, and algae-based carbon capture, circumventing storage by converting algae into fuel or feed.

Oceans


Another proposed form of carbon sequestration in the ocean is direct injection. In this method, carbon dioxide is pumped directly into the water at depth, and expected to form "lakes" of liquid CO2 at the bottom. Experiments carried out in moderate to deep waters (350–3600 m) indicate that the liquid CO2 reacts to form solid CO2 clathrate hydrate
Clathrate hydrate
Clathrate hydrates are crystalline water-based solids physically resembling ice, in which small non-polar molecules or polar molecules with large hydrophobic moieties are trapped inside "cages" of hydrogen bonded water molecules...

s, which gradually dissolve in the surrounding waters.

This method, too, has potentially dangerous environmental consequences. The carbon dioxide does react with the water to form carbonic acid
Carbonic acid
Carbonic acid is the inorganic compound with the formula H2CO3 . It is also a name sometimes given to solutions of carbon dioxide in water, because such solutions contain small amounts of H2CO3. Carbonic acid forms two kinds of salts, the carbonates and the bicarbonates...

, H2CO3; however, most (as much as 99%) remains as dissolved molecular CO2. The equilibrium would no doubt be quite different under the high pressure conditions in the deep ocean. In addition, if deep-sea bacterial methanogens that reduce carbon dioxide were to encounter the carbon dioxide sinks, levels of methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...

 gas may increase, leading to the generation of an even worse greenhouse gas.
The resulting environmental effects on benthic life forms of the bathypelagic, abyssopelagic and hadopelagic zones are unknown. Even though life appears to be rather sparse in the deep ocean basins, energy and chemical effects in these deep basins could have far-reaching implications. Much more work is needed here to define the extent of the potential problems.

Carbon storage in or under oceans may not be compatible with the Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter
Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter
The Convention on the Prevention of Marine Pollution by Dumping of Wastes and Other Matter 1972, commonly called the "London Convention" or "LC '72555" and also barbie abbreviated as Marine Dumping, is an agreement to control pollution of the sea by dumping and to encourage regional agreements...

.

An additional method of long-term ocean-based sequestration is to gather crop residue
Crop residue
There are two types of agricultural crop residues:Field residues are materials left in an agricultural field or orchard after the crop has been harvested. These residues include stalks and stubble , leaves, and seed pods...

 such as corn stalks or excess hay into large weighted bales of biomass and deposit it in the alluvial fan
Alluvial fan
An alluvial fan is a fan-shaped deposit formed where a fast flowing stream flattens, slows, and spreads typically at the exit of a canyon onto a flatter plain. A convergence of neighboring alluvial fans into a single apron of deposits against a slope is called a bajada, or compound alluvial...

 areas of the deep ocean basin. Dropping these residues in alluvial fans would cause the residues to be quickly buried in silt on the sea floor, sequestering the biomass for very long time spans. Alluvial fans exist in all of the world's oceans and seas where river deltas fall off the edge of the continental shelf
Continental shelf
The continental shelf is the extended perimeter of each continent and associated coastal plain. Much of the shelf was exposed during glacial periods, but is now submerged under relatively shallow seas and gulfs, and was similarly submerged during other interglacial periods. The continental margin,...

 such as the Mississippi alluvial fan
Mississippi Delta
The Mississippi Delta is the distinctive northwest section of the U.S. state of Mississippi that lies between the Mississippi and Yazoo Rivers. The region has been called "The Most Southern Place on Earth" because of its unique racial, cultural, and economic history...

 in the gulf of Mexico
Gulf of Mexico
The Gulf of Mexico is a partially landlocked ocean basin largely surrounded by the North American continent and the island of Cuba. It is bounded on the northeast, north and northwest by the Gulf Coast of the United States, on the southwest and south by Mexico, and on the southeast by Cuba. In...

 and the Nile alluvial fan
Nile Delta
The Nile Delta is the delta formed in Northern Egypt where the Nile River spreads out and drains into the Mediterranean Sea. It is one of the world's largest river deltas—from Alexandria in the west to Port Said in the east, it covers some 240 km of Mediterranean coastline—and is a rich...

 in the Mediterranean Sea
Mediterranean Sea
The Mediterranean Sea is a sea connected to the Atlantic Ocean surrounded by the Mediterranean region and almost completely enclosed by land: on the north by Anatolia and Europe, on the south by North Africa, and on the east by the Levant...

. A downside, however, would be an increase in aerobic bacteria growth due to the introduction of biomass, leading to more competition for oxygen resources in the deep sea, similar to the oxygen minimum zone
Oxygen minimum zone
The Oxygen minimum zone , sometimes referred to as the shadow zone, is the zone in which oxygen saturation in seawater in the ocean is at its lowest. This zone occurs at depths of about 200 to 1,000 metres, depending on local circumstances....

.

Geological sequestration


The method of geo-sequestration or geological storage involves injecting carbon dioxide directly into underground geological formations. Declining oil field
Oil field
An oil field is a region with an abundance of oil wells extracting petroleum from below ground. Because the oil reservoirs typically extend over a large area, possibly several hundred kilometres across, full exploitation entails multiple wells scattered across the area...

s, saline aquifer
Aquifer
An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...

s, and unminable coal seams have been suggested as storage sites. Caverns and old mines that are commonly used to store natural gas are not considered, because of a lack of storage safety.

CO2 has been injected into declining oil fields for more than 40 years, to increase oil recovery. This option is attractive because the storage costs are offset by the sale of additional oil that is recovered. Typically, 10-15% additional recovery of the original oil in place is possible. Further benefits are the existing infrastructure and the geophysical and geological information about the oil field that is available from the oil exploration. Another benefit of injecting CO2 into Oil fields is that CO2 is soluble in oil. Dissolving CO2 in oil lowers the viscosity of the oil and reduces its interfacial tension which increases the oils mobility. All oil fields have a geological barrier preventing upward migration of oil. As most oil and gas has been in place for millions to tens of millions of years, depleted oil and gas reservoirs can contain carbon dioxide for millennia. Identified possible problems are the many 'leak' opportunities provided by old oil wells, the need for high injection pressures and acidification which can damage the geological barrier. Other disadvantages of old oil fields are their limited geographic distribution and depths, which require high injection pressures for sequestration. Below a depth of about 1000 m, carbon dioxide is injected as a supercritical fluid, a material with the density of a liquid, but the viscosity and diffusivity of a gas.
Unminable coal seams can be used to store CO2, because CO2 absorbs to the coal surface, ensuring safe long-term storage. In the process it releases methane that was previously adsorbed to the coal surface and that may be recovered. Again the sale of the methane can be used to offset the cost of the CO2 storage. Release or burning of methane would of course at least partially offset the obtained sequestration result – except when the gas is allowed to escape into the atmosphere in significant quantities: methane has a higher global warming potential
Global warming potential
Global-warming potential is a relative measure of how much heat a greenhouse gas traps in the atmosphere. It compares the amount of heat trapped by a certain mass of the gas in question to the amount of heat trapped by a similar mass of carbon dioxide. A GWP is calculated over a specific time...

 than CO2.

Saline aquifers contain highly mineralized brines and have so far been considered of no benefit to humans except in a few cases where they have been used for the storage of chemical waste. Their advantages include a large potential storage volume and relatively common occurrence reducing the distance over which CO2 has to be transported. The major disadvantage of saline aquifers is that relatively little is known about them compared to oil fields. Another disadvantage of saline aquifers is that as the salinity of the water increases, less CO2 can be dissolved into aqueous solution. To keep the cost of storage acceptable the geophysical exploration may be limited, resulting in larger uncertainty about the structure of a given aquifer. Unlike storage in oil fields or coal beds, no side product will offset the storage cost. Leakage of CO2 back into the atmosphere may be a problem in saline-aquifer storage. However, current research shows that several trapping mechanisms immobilize the CO2 underground, reducing the risk of leakage.

A major research project examining the geological sequestration of carbon dioxide is currently being performed at an oil field at Weyburn
Weyburn, Saskatchewan
Weyburn is a city in southeastern Saskatchewan, Canada. It is located on the Souris River southeast of the provincial capital of Regina and is north of the border with the United States. The name is reputedly a corruption of the Scottish "wee burn," referring to a small creek. The city is...

 in south-eastern Saskatchewan
Saskatchewan
Saskatchewan is a prairie province in Canada, which has an area of . Saskatchewan is bordered on the west by Alberta, on the north by the Northwest Territories, on the east by Manitoba, and on the south by the U.S. states of Montana and North Dakota....

. In the North Sea
North Sea
In the southwest, beyond the Straits of Dover, the North Sea becomes the English Channel connecting to the Atlantic Ocean. In the east, it connects to the Baltic Sea via the Skagerrak and Kattegat, narrow straits that separate Denmark from Norway and Sweden respectively...

, Norway's Statoil
Statoil
Statoil ASA is a Norwegian petroleum company established in 1972. It merged with Norsk Hydro in 2007 and was known as StatoilHydro until 2009, when the name was changed back to Statoil ASA. The brand Statoil was retained as a chain of fuel stations owned by StatoilHydro...

 natural-gas platform Sleipner
Sleipner
Sleipnir is an eight-legged horse in Norse mythology. Sleipnir or Sleipner may also refer to:Ships and offshore* Sleipner gas field** Sleipner A, an offshore platform...

 strips carbon dioxide out of the natural gas with amine solvents and disposes of this carbon dioxide by geological sequestration. Sleipner reduces emissions of carbon dioxide by approximately one million tonnes a year. The cost of geological sequestration is minor relative to the overall running costs. As of April 2005, BP
BP
BP p.l.c. is a global oil and gas company headquartered in London, United Kingdom. It is the third-largest energy company and fourth-largest company in the world measured by revenues and one of the six oil and gas "supermajors"...

 is considering a trial of large-scale sequestration of carbon dioxide stripped from power plant emissions in the Miller oilfield
Miller oilfield
The Miller oilfield is a deep reservoir under the North Sea, 240 kilometres northeast of Peterhead in UKCS Blocks 16/7b and 16/8b. It was discovered in 1982 by BP in a water depth of 100 metres. Production from Miller field started in June 1992, and plateau production was from late 1992 to 1997 at...

 as its reserves are depleted.

In October 2007, the Bureau of Economic Geology at The University of Texas at Austin received a 10-year, $38 million subcontract to conduct the first intensively monitored, long-term project in the United States studying the feasibility of injecting a large volume of CO2 for underground storage. The project is a research program of the Southeast Regional Carbon Sequestration Partnership (SECARB), funded by the National Energy Technology Laboratory
National Energy Technology Laboratory
The National Energy Technology Laboratory is a science, technology, and energy laboratory owned and operated by the U.S. Department of Energy . As part of DOE's national laboratory system, NETL supports DOE's mission to advance the national, economic, and energy security of the United States...

 of the U.S. Department of Energy (DOE)
United States Department of Energy
The United States Department of Energy is a Cabinet-level department of the United States government concerned with the United States' policies regarding energy and safety in handling nuclear material...

. The SECARB partnership will demonstrate CO2 injection rate and storage capacity in the Tuscaloosa-Woodbine geologic system that stretches from Texas to Florida. Beginning in fall 2007, the project will inject CO2 at the rate of one million tons per year, for up to 1.5 years, into brine up to 10000 feet (3,048 m) below the land surface near the Cranfield oil field about 15 miles (24.1 km) east of Natchez, Mississippi
Natchez, Mississippi
Natchez is the county seat of Adams County, Mississippi, United States. With a total population of 18,464 , it is the largest community and the only incorporated municipality within Adams County...

. Experimental equipment will measure the ability of the subsurface to accept and retain CO2.

Mineral sequestration


Mineral sequestration aims to trap carbon in the form of solid carbonate
Carbonate
In chemistry, a carbonate is a salt of carbonic acid, characterized by the presence of the carbonate ion, . The name may also mean an ester of carbonic acid, an organic compound containing the carbonate group C2....

 salts. This process occurs slowly in nature and is responsible for the deposition and accumulation of limestone
Limestone
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate . Many limestones are composed from skeletal fragments of marine organisms such as coral or foraminifera....

 over geologic time. Carbonic acid
Carbonic acid
Carbonic acid is the inorganic compound with the formula H2CO3 . It is also a name sometimes given to solutions of carbon dioxide in water, because such solutions contain small amounts of H2CO3. Carbonic acid forms two kinds of salts, the carbonates and the bicarbonates...

 in groundwater slowly reacts with complex silicates to dissolve calcium
Calcium
Calcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...

, magnesium
Magnesium
Magnesium is a chemical element with the symbol Mg, atomic number 12, and common oxidation number +2. It is an alkaline earth metal and the eighth most abundant element in the Earth's crust and ninth in the known universe as a whole...

, alkalis and silica and leave a residue of clay minerals
Clay minerals
Clay minerals are hydrous aluminium phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations. Clays have structures similar to the micas and therefore form flat hexagonal sheets. Clay minerals are common weathering products and low...

. The dissolved calcium and magnesium react with bicarbonate
Bicarbonate
In inorganic chemistry, bicarbonate is an intermediate form in the deprotonation of carbonic acid...

 to precipitate calcium and magnesium carbonates, a process that organisms use to make shells. When the organisms die, their shells are deposited as sediment and eventually turn into limestone. Limestones have accumulated over billions of years of geologic time and contain much of Earth's carbon. Ongoing research aims to speed up similar reactions involving alkali carbonates.

Several serpentinite deposits are being investigated as potentially large scale CO2 storage sinks such as those found in NSW, Australia, where the first mineral carbonation pilot plant project is underway. Beneficial re-use of magnesium carbonate from this process could provide feedstock for new products developed for the built environment and agriculture without returning the carbon into the atmosphere and so acting as a carbon sink.

One proposed reaction is that of the olivine-rich rock dunite
Dunite
Dunite is an igneous, plutonic rock, of ultramafic composition, with coarse-grained or phaneritic texture. The mineral assemblage is greater than 90% olivine, with minor amounts of other minerals such as pyroxene, chromite and pyrope. Dunite is the olivine-rich end-member of the peridotite group...

, or its hydrated equivalent serpentinite
Serpentinite
Serpentinite is a rock composed of one or more serpentine group minerals. Minerals in this group are formed by serpentinization, a hydration and metamorphic transformation of ultramafic rock from the Earth's mantle...

 with carbon dioxide to form the carbonate mineral magnesite
Magnesite
Magnesite is magnesium carbonate, MgCO3. Iron substitutes for magnesium with a complete solution series with siderite, FeCO3. Calcium, manganese, cobalt, and nickel may also occur in small amounts...

, plus silica and iron oxide (magnetite
Magnetite
Magnetite is a ferrimagnetic mineral with chemical formula Fe3O4, one of several iron oxides and a member of the spinel group. The chemical IUPAC name is iron oxide and the common chemical name is ferrous-ferric oxide. The formula for magnetite may also be written as FeO·Fe2O3, which is one part...

).

Serpentinite sequestration is favored because of the non-toxic and stable nature of magnesium carbonate. The ideal reactions involve the magnesium endmember components of the olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....

 (reaction 1) or serpentine (reaction 2), the latter derived from earlier olivine by hydration and silicification (reaction 3). The presence of iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...

 in the olivine or serpentine reduces the efficiency of sequestration, since the iron components of these minerals break down to iron oxide and silica (reaction 4).

Serpentinite reactions


Reaction 1

Mg-olivine + carbon dioxide → magnesite + silica + water
Mg2SiO4 + 2CO2 → 2MgCO3 + SiO2 + H2O


Reaction 2

Serpentine + carbon dioxide → magnesite + silica + water
Mg3[Si2O5(OH)4] + 3CO2 → 3MgCO3 + 2SiO2 + 2H2O


Reaction 3

Mg-olivine + water + silica → serpentine
3Mg2SiO4 + 2SiO2 + 4H2O → 2Mg3[Si2O5(OH)4


Reaction 4

Fe-olivine + water → magnetite + silica + hydrogen
3Fe2SiO4 + 2H2O → 2Fe3O4 + 3SiO2 + 2H2

Zeolitic imidazolate frameworks



Zeolitic imidazolate frameworks is a metal-organic framework
Metal-organic framework
Metal-Organic Frameworks are crystalline compounds consisting of metal ions or clusters coordinated to often rigid organic molecules to form one-, two-, or three-dimensional structures that can be porous. In some cases, the pores are stable to elimination of the guest molecules and can be used for...

 carbon dioxide sink which could be used to keep industrial emissions of carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...

 out of the atmosphere
Atmosphere
An atmosphere is a layer of gases that may surround a material body of sufficient mass, and that is held in place by the gravity of the body. An atmosphere may be retained for a longer duration, if the gravity is high and the atmosphere's temperature is low...

.

Trends in sink performance


According to a report in Nature magazine, (November, 2009) the first year-by-year accounting of this mechanism during the industrial era, and the first time scientists have actually measured it, suggests "the oceans are struggling to keep up with rising emissions—a finding with potentially wide implications for future climate." With total world emissions from fossil fuels growing rapidly, the proportion of fossil-fuel emissions absorbed by the oceans since 2000 may have declined by as much as 10%, indicating that over time the ocean will become "a less efficient sink of manmade carbon." Samar Khatiwala, an oceanographer at Columbia University
Columbia University
Columbia University in the City of New York is a private, Ivy League university in Manhattan, New York City. Columbia is the oldest institution of higher learning in the state of New York, the fifth oldest in the United States, and one of the country's nine Colonial Colleges founded before the...

 concludes that the studies suggest "we cannot count on these sinks operating in the future as they have in the past, and keep on subsidizing our ever-growing appetite for fossil fuels." However, a recent paper by Wolfgang Knorr indicates that the fraction of absorbed by carbon sinks has not changed since 1850.

See also



  • Biochar
    Biochar
    Biochar or terra preta is charcoal created by pyrolysis of biomass. Biochar is under investigation as an approach to carbon sequestration via bio-energy with carbon capture and storage. Biochar thus has the potential to help mitigate climate change, via carbon sequestration...

  • Bio-energy with carbon capture and storage
    Bio-energy with carbon capture and storage
    Bio-energy with carbon capture and storage is a greenhouse gas mitigation technology which produces negative carbon emissions by combining biomass use with geologic carbon capture and storage....

  • Carbon capture and storage
    Carbon capture and storage
    Carbon capture and storage , alternatively referred to as carbon capture and sequestration, is a technology to prevent large quantities of from being released into the atmosphere from the use of fossil fuel in power generation and other industries. It is often regarded as a means of mitigating...

  • Carbon cycle
    Carbon cycle
    The carbon cycle is the biogeochemical cycle by which carbon is exchanged among the biosphere, pedosphere, geosphere, hydrosphere, and atmosphere of the Earth...

  • Carbon flux
  • Carbon offset
    Carbon offset
    A carbon offset is a reduction in emissions of carbon dioxide or greenhouse gases made in order to compensate for or to offset an emission made elsewhere....

  • Carbon sequestration
  • Center for the Study of Carbon Dioxide and Global Change
    Center for the Study of Carbon Dioxide and Global Change
    The Center for the Study of Carbon Dioxide and Global Change is a 501 non-profit organization based in Arizona in the United States...

  • Iron fertilization
    Iron fertilization
    Iron fertilization is the intentional introduction of iron to the upper ocean to stimulate a phytoplankton bloom. This is intended to enhance biological productivity, which can benefit the marine food chain and remove carbon dioxide from the atmosphere. Iron is a trace element necessary for...

  • North American Carbon Program
    North American Carbon Program
    The North American Carbon Program is one of the major elements of the Strategic Plan for the U.S. Climate Change Science Program.The central objective of NACP is to measure and understand carbon stocks and the sources and sinks of carbon dioxide , methane , and carbon monoxide in North America...

  • Carbon sequestration in terrestrial ecosystems
    Carbon Sequestration in Terrestrial Ecosystems
    Carbon sequestration by terrestrial ecosystems is the net removal of carbon dioxide from the atmosphere or the avoidance of carbon dioxide emissions from terrestrial ecosystems into the atmosphere. The removal process includes CO2 uptake from the atmosphere by all chlorophyllous plants, through...


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