Carbon Sequestration Essay ⋆ Environment Essay Examples ⋆ EssayEmpire

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Carbon sequestration occurs when carbon is removed from the atmosphere, or captured directly from industrial emission sources, and then stored (sequestered) where it cannot readily reenter the atmosphere. Most atmospheric carbon is present in the form of carbon dioxide (CO2) and methane. Although these gases constitute a minor component of the total atmosphere (less than 0.1 percent of all atmospheric gases), they are among the primary greenhouse gases and contribute directly to global warming.

Burning fossil fuels and converting forest lands to agriculture release large amounts of carbon to the atmosphere and are leading to a rise in atmospheric CO2 and methane levels. Stabilizing atmospheric greenhouse gas concentrations will require either a reduction in the amount of carbon being released or an increase in the rate of carbon sequestration. Many countries are pursuing both strategies.

There are two different approaches to carbon sequestration: Biological and technological. Both focus on CO2, which is removed either directly from the atmosphere or at the point of emission and sequestered in a form or location where it is not radiatively active (that is, where it will not contribute to global warming). In some cases, sequestered carbon remains out of the atmosphere for hundreds or thousands of years, while in others the period of sequestration may be relatively short (years to decades).

Biological carbon sequestration takes advantage of the natural carbon cycle, and is primarily concerned with the uptake of atmospheric CO2 and its storage as organic matter. Whenever photosynthesis (CO2 uptake) is greater than respiration (CO2 release), the result is carbon storage.

On land, green plants take up CO2 through photosynthesis and store that carbon in leaves, stems, and roots. When leaves are shed or a plant dies, that stored carbon is either released back into the atmosphere relatively quickly through the process of decomposition, or it may be sequestered for much longer periods; for example, undecomposed tree trunks, wood products such as lumber, or soil organic matter. The longer the sequestration time, the more valuable the process is for climate change mitigation.

In the ocean, most photosynthesis is done by single-celled algae called phytoplankton, floating near the surface. Phytoplankton grows and dies quickly and much of the stored carbon is released back into the atmosphere when the algal cells decompose. If dead algae sink below the depth where decomposition occurs, however, their stored carbon can be sequestered for long periods as organic matter in marine sediments.

Measurements and Prediction

To predict changes in atmospheric greenhouse gas concentrations as a result of fossil fuel burning and deforestation, we need accurate measurements of global biological carbon sequestration. The greater the rate of carbon sequestration, the less rapid the expected rise in atmospheric CO2 . For individual countries and municipalities, quantifying biological carbon sequestration may be important for negotiating carbon emission standards or for validating carbon trading schemes. A variety of methods are used to calculate rates of biological carbon sequestration; these include measurements of carbon in wood, soil, and ocean sediments, measurements of CO2 concentrations in the air and water, and computer simulations of the carbon cycle.

Strategies to increase biological carbon sequestration by land plants usually focus on increasing the amount of carbon in organic matter that decays slowly. Allowing forests to regrow or planting new forests results in carbon sequestration in wood and in soil. In agricultural lands, conservation tillage (or no-till soil) management practices result in higher amounts of soil carbon. The amounts of carbon that might be sequestered through improved forestry and agricultural practices are potentially quite large, with estimates for the United States of up to 50 percent of its annual fossil fuel emissions. Biotechnology also may play a role in efforts to enhance biological carbon sequestration. For example, the chemical structure of wood can be bioengineered to slow the natural process of decomposition, thereby lengthening the time carbon may remain sequestered in this form.

Technological carbon sequestration involves capture of CO2 at the point of emission (its conversion to a form that can be transported in pipelines) and its long-term storage underground or under water. Much of the CO2 emitted through fossil fuel burning comes from stationary sources such as power plants, oil refineries, and other energy intensive industries. Existing technology can be used to remove CO2 from industrial flue gases and prevent it from entering the atmosphere. Chemical or physical solvents are used to trap CO2 and CO2 can also be separated from other gases cryogenically by cooling and condensation. Most CO2 capture technologies now in place work best with high CO2 concentration flue gases, however, and may need to be modified to work with the more dilute flue gases from many CO2 emission sources.

Long-term sequestration of CO2 captured in this way can be achieved via injection into underground geological formations or into the ocean. Candidate sites for geological sequestration are deep saline aquifers, old oil and gas fields, and coal beds. The potential CO2 storage capacity of these geological formations worldwide is very large. Due to the costs involved in transporting captured CO2, however, it is important that potential geological sequestration sites be relatively close to emission sources. Studies suggest that these conditions are common enough that carbon sequestration through capture and storage underground can be an important CO2 emissions mitigation strategy. Carbon sequestration in ocean waters is theoretically possible, either by piping highly concentrated CO2 below 1,000 meters where it would remain trapped by the overlying salt water, or by piping it into shallower waters where it would dissolve and disperse. In either case, possible impacts on marine life would be an important concern. For both biological and technological carbon sequestration, the feasibility of any particular approach rests on a complete accounting of the costs (monetary, energetic, and environmental) and benefits (amount of carbon stored and its sequestration time).

Bibliography:

David Gerard, Carbon Capture and Sequestration Integrating Technology, Monitoring, Regulation (Blackwell Publishers, 2007);
W.M. Post, R.C. Izaurralde, D. Jastrow, B.A. McCarl, J.E. Amonette, V.L. Bailey, P.M. Jardine, T.O. West, and J. Zhou, “Carbon Sequestration Enhancement in U.S. Soils,” BioScience (v.54, 2004);
W.H. Schlesinger, Biogeochemistry: An Analysis of Global Change (Academic Press, 1997).