Carbon sequestration

 The process of capture and long-term storage of atmospheric carbon dioxide (co2) (1)

and may refer specifically to:

The process of removing carbon from the atmosphere and depositing it in a reservoir. (2)

When carried out deliberately, this may also be referred to as carbon dioxide removal. (3)

  long-term storage of carbon dioxide or other forms of carbon to either mitigate or

defer global warming and avoid dangerous climate change. It has been proposed as a way to

slow the atmospheric and marine accumulation of greenhouse gases, which are released by

burning fossil fuels. (4) CO2 sequestration has the potential to significantly reduce the level of

carbon that occurs in the atmosphere as CO2 and to reduce the release of CO2 to the

atmosphere from major stationary human sources, including power plants and refineries. (5)

Before human-caused CO2 emissions began the natural processes that make up the

global “carbon cycle” (fig) maintained a near balance between the uptake of CO2 and its

release back to the atmosphere. However, existing CO2 uptake mechanisms (sometimes

called CO2 or carbon “sinks”) are insufficient to offset the accelerating pace of emissions

related to human activities. (6)

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What is carbon dioxide capture and sequestration?

Carbon dioxide (CO2) capture and sequestration (CCS) is a set of technologies that can

greatly reduce CO2 emissions from new and existing coal- and gas-fired power plants and

large industrial sources. CCS is a three-step process that includes:

Capture of CO2 from power plants or industrial processes

Transport of the captured and compressed CO2 (usually in pipelines).

Underground injection and geologic sequestration (also referred to as storage) of

the CO2 into deep underground rock formations. These formations are often a mile

or more beneath the surface and consist of porous rock that holds the CO2.

Overlying these formations are impermeable, non-porous layers of rock that trap

the CO2 and prevent it from migrating upward.

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. (7)

Why is it important?

Increases in atmospheric CO2 concentration may be generating increases in average

global temperature and other climate change impacts. Although some of the effects of

increased CO2 levels on the global climate are uncertain, most scientists agree that doubling

atmospheric CO2 concentrations may cause serious environmental consequences. Rising

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global temperatures could raise sea levels, change precipitation patterns and affect both

weather and climate conditions. (8)

In light of these potential impacts, strategies to help reverse these emission trends are

increasing in importance. Many state, national and international governments are taking steps

to more effectively manage and slow the growth of their carbon emissions. (9)

Carbon dioxide (CO2) capture and sequestration (CCS) could play an important role

in reducing greenhouse gas emissions, while enabling low-carbon electricity generation

from power plants. As estimated in the U.S. Inventory of Greenhouse Gas Emissions and

Sinks , more than 40% of CO2 emissions in the United States are from electric power

generation. CCS technologies are currently available and can dramatically reduce (by 80-

90%) CO2 emissions from power plants that burn fossil fuels.

The amount of GHG emissions avoided (with a 90% reduction efficiency) would be

equivalent to:

Planting more than 62 million trees, and waiting at least 10 years for them to grow.

Avoiding annual electricity-related emissions from more than 300,000 homes (10)

Sequestration methods

enhancing the storage of carbon in soil (soil sequestration);

enhancing the storage of carbon in forests and other vegetation (plant sequestration);

storing carbon in underground geological formations (geo sequestration);

storing carbon in the ocean (ocean sequestration); and

Subjecting carbon to chemical reactions to form inorganic carbonates (mineral carbonation).

Soil sequestration

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.

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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. (11)

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Plant sequestration

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.

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 re-vegetation, but the capacity is

limited and longevity of storage depends on the final fate of the timber or plant material.

However, carbon sequestration from re-vegetation and plantation programs could provide a

significant shorter-term contribution to climate change mitigation.

Geosequestration

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:

capture of CO2 either before or after combustion of the fuel

transport of the captured CO2 to the site of storage, and

injection and storage of the CO2.

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

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higher confidence in the longevity of storage; large capacity of potential storage sites; and

generally greater understanding of the mechanisms of storage.

Ocean sequestration

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.

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.

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Mineral sequestration

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. (12)

Why action is needed now?

Cumulative historical CO2 emissions from fossil fuels in the United States are

equivalent to more than the total amount of carbon stored in U.S. forests. If current trends

continue, cumulative U.S. emissions are projected to double by 2050 and increase by a factor

of three to four by 2100. According to the Intergovernmental Panel on Climate Change

Fourth Assessment Report of 2007, sequestration and reduction of emissions over the next

two to three decades will potentially have a substantial impact on long-term opportunities to

stabilize levels of atmospheric CO2 and mitigate impacts of climate change. (6)

Carbon Offsets

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. (12)

An example is planting trees to offset one’s gas emissions from driving a car to work every

day.

According to von Haggen and Burnett (2006), “a carbon offset project is one

implemented specifically to reduce the level of greenhouse gases in the atmosphere.” (13)

These projects have three elements:

1. Cancel out emissions

2. Reductions are documented in a greenhouse gas registry

3. The end offset is as though the cancelled emissions had not occurred (14)

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References

1) Roger, S. and Brent, S. (2012). Carbon Sequestration in Forests and Soils. Annual

Review of Resource Economics. 4, 127–144.

2) http://unfccc.int/essential_background/glossary/items/3666.php#C

3) http://en.wikipedia.org/wiki/Carbon_sequestration#cite_note-6

4) Chris, H. (2008). Squaring the Circle on Coal - Carbon Capture and Storage (CCS)

Claverton Group conference, Bath 24-26 October.

5) Pacala, S. and Socolow, R. (2004) Stabilization wedges-solving the climate problem

for the next 50 years with current technologies. Science. 305, 968-972.

6) http://pubs.usgs.gov/fs/2008/3097/pdf/CarbonFS.pdf

7) http://www.epa.gov/climatechange/ccs/

8) http://www.epa.gov/aml/revital/cseqfact.pdf

9) http://belfercenter.ksg.harvard.edu/analysis/stavins/?p=225.

10) MIT (2007) the Future of Coal: Options for a Carbon-Constrained World ,

Massachusetts Institute of Technology, 2007.

11) Post, W. M., Emanuel, W. R., Zinke, P. J. and Stangenberger, A. G. (1982). Soil

carbon pools and world life zones.  Nature. 298, 156–9.

12) Goodward, J. and Kelly, A. (2010). Bottom Line on Offsets. World Resources

Institute. Retrieved 2010-09-08.

13) Hagen, V.B. and Burnett, M. (2006). Emerging markets for carbon stored by

Northwest forests. In: Forests, carbon and climate change: a synthesis of science

findings. Portland, OR: Oregon Forest Resources Institute: 131–155.

14) Pacala, S. and Socolow, R. (2004). Stabilization wedges-solving the climate problem

for the next 50 years with current technologies: Science. 305, 968-972.

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