Carbon sequestration is the process through which CO2 from the atmosphere is absorbed by various carbon sinks. Principal carbon sinks include agricultural sinks, forests, geologic formations, and oceanic sinks. Carbon sequestration and storage (CSS) occurs when CO2 is absorbed by trees, plants, and crops through photosynthesis and stored as carbon in biomass, such as tree trunks, branches, foliage, and roots, as well as in the soil. For a more in-depth discussion of the specifics of actual carbon sequestration storage methods, refer to chapter 8.
In terms of global warming and impacts to the environment, sequestration is very important because it has a large influence on levels of CO2 in the atmosphere. According to the IPCC, carbon sequestration by forestry and agriculture alone significantly helps offset CO2 emissions that contribute to global warming and climate change.
The amount of carbon that can be sequestered varies geographically and is determined by tree species, soil type, regional climate, type of topography, and even the type of land-management practice used in the area. For example, in agricultural areas, if conservation tillage practices are used instead of conventional tillage, this limits the introduction of CO2 into the atmosphere by sequestering larger amounts of CO2 in the soil. According to the EPA, switching from conventional to conservation tillage can sequester 0.11-0.33 tons (0.1-0.3 metric tons) of carbon per acre per year.
Carbon sequestration does reach a limit, however. The amount of carbon that accumulates in forests and soils will eventually reach a saturation point at which no additional carbon will be able to be stored. This typically occurs when trees reach full maturity or when the organic matter contained in soils builds up.
According to the EPA, the U.S. landscape currently functions as an efficient carbon sink, sequestering more than it emits. They do warn, however, that the overall sequestration amounts are currently declining because of increased harvests, land use changes, and maturing forests.
Regarding global sequestration, the IPCC has estimated that 110 billion tons (100 billion metric tons) of carbon over the next 50 years could be sequestered through forest preservation, tree planting, and improved, conservation-oriented agricultural management. In the United States, Bruce McCarl (professor of agricultural economics at Texas A&M University) and Uwe Schneider (assistant professor of the Research Unit Sustainability and Global Change Department of Geosci-ences and Economics at Hamburg University in Germany) have determined that an additional 55-165 million tons (50-150 million metric tons) of carbon could be sequestered through changes in both soil and forest management, new tree planting, and biofuel substitution.
Another positive aspect supporting carbon sequestration is that it also affects other greenhouse gases. In particular, methane (CH4) and nitrous oxide (N2O) can also be sequestered in agricultural activities such as grazing and the growing of crops. Nitrous oxide can be intro duced via fertilizers. Instead of using these fertilizers, which can have a negative effect environmentally, other practices could be used instead, such as rotational grazing. In addition, if forage quality is improved, livestock methane emissions should be significantly reduced. Nitrous oxide emissions could be avoided by eliminating the need for fertilizer. The EPA stresses that finding the right sequestration practices will help lessen the negative effects of all the greenhouse gases.
Other environmental benefits of carbon sequestration are that they enhance the quality of soil, water, air, and wildlife habitat. For instance, when trees are planted, they not only sequester carbon, they also provide wildlife habitat. When the rain forests are preserved, they keep both plant and animal species from becoming endangered and help control soil erosion. When forests are maintained they also cut down on overland water flow, soil erosion, loss of nutrients, as well as improving water quality.
The continuation of global warming, however, can have an impact on carbon sequestration. According to a 2001 report issued by the National Academy of Sciences, "Greenhouse gases are accumulating in the Earth's atmosphere as a result of human activities, causing surface air temperatures and subsurface ocean temperatures to rise." Besides temperature, they also say that human-induced climate change may affect the growing seasons, precipitation patterns and amounts, as well as the frequency and severity of extreme weather events such as the wildfires that currently plague the American Southwest.
According to the EPA: "In terms of global warming impact, one unit of CO2 released from a car's tailpipe has the same effect as one unit of CO2 released from a burning forest. Likewise, CO2 removed from the atmosphere through tree planting can have the same benefit as avoiding an equivalent amount of CO2 released from a power plant."
The experts at the EPA also caution, however, that even though forests, agriculture, and other sinks can store carbon, the process can also become saturated and slow down or stop the storage process (such as traditional agricultural cultivation), or the sink can be destroyed and completely stop the process (such as complete deforestation). Carbon sequestration processes can naturally slow down and stop on their own when they get older.
In addition, carbon sequestration can be a natural or man-made process. Research is currently in progress to perfect the methodologies that enhance the natural terrestrial cycles of carbon storage that remove CO2 from the atmosphere by vegetation and store CO2 in biomass and soils. In order to accomplish this, research of biological and ecological processes are under way by the EPA. Specific technical areas that are currently being researched include:
• Increasing the net fixation of atmospheric CO2 by terrestrial vegetation with emphasis on physiology and rates of photosynthesis of vascular plants;
• Retaining carbon and enhancing the transformation of carbon to soil organic matter;
• Reducing the emission of CO2 from soils caused by heterotrophic oxidation of soil organic carbon;
• Increasing the capacity of deserts and degraded lands to sequester carbon. Man-made processes include technologies such as geologic, mineral, and ocean sequestration.
In carbon sequestration, the main goal is to prevent CO2 emissions from power plants and industrial facilities from entering the atmosphere by separating and capturing the emissions and then securing and storing the CO2 on a long-term basis.
Currently, the EPA is involved in research in an attempt to separate and capture the CO2 from fossil fuels and from flue gases—both pre- and post-combustion processes. Underground storage facilities are also receiving large amounts of attention recently, and their potential is enormous. As an example, today more than 750 billion gallons (2.8 trillion liters) of both hazardous and nonhazardous fluids are disposed through a process called "underground injection."
There are certain risks associated with underground sequestration, however. Leakage from the storage reservoir is a concern that must be taken seriously when selecting the right location for storage. Because this is a relatively new technology, much still needs to be learned about appropriate safety measures. The potential risks of underground CO2 sequestration include escape of CO2 from the res ervoir through leakage, seismicity (shifting of the Earth's crust from seismic activity), ground movement, or displacement of brine (if present). Leakage is the largest of the risks, which could occur through or along abandoned wells and by cap rock failure. A cap rock is the "impermeable" rock formation above the deposit, keeping it naturally contained.
Another issue to consider is that diffusion of CO2 through the cement or steel casing it is contained in—caused by corrosion—may be a slow process. However, when it is sequestered for tens of thousands of years, the integrity of the well casing needs to be critically assessed. Other areas of potential concern are through high permeability zones in the cap rock above the deposit or through faults and fractures that extend into the cap rock. Those areas are considered the most important natural leakage pathway.
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