Carbon Sequestration Issues
Carbon Sequestration Issues
Carbon Sequestration Issues
Carbon sequestration is the collection and storage of carbon dioxide (CO2) to keep it out of the atmosphere. CO2 can be sequestered in the oceans, underground reservoirs, carbon compounds, biomass, or soil. A variety of ways to sequester carbon have been proposed since the 1980s. Some, such as planting trees and changing agricultural practices to sequester carbon in soil, are for the most part uncontroversial but may not be able to sequester enough CO2 to stabilize global climate.
Other methods, such as carbon capture and storage (CCS), have not yet proved to be effective or affordable. Some, such as iron fertilization of phytoplankton (single-celled green plants) in the seas, have not yet proved to be effective and might, according to their scientific critics, do more ecological harm than good even if they are capable of sequestering large quantities of carbon. The leading candidate technology for large-scale carbon sequestration in the near future is CCS.
Historical Background and Scientific Foundations
Human activities have increased the atmospheric concentration of CO2 from 380 parts per million in 1750, before the widespread burning of fossil fuels, to 383 parts per million in 2007, a 37% increase. Other gases have increased as well, but CO2 is the most important greenhouse gas, accounting for 63% of anthropogenic (human-caused) global warming. The other 37% of warming is due to other greenhouse gases and to changes in albedo (global brightness).
About 84% of CO2 emissions were, as of 2007, from the burning of fossil fuels, with the other 16% coming mostly from deforestation. In the United States in 2006, CO2 emissions were about 86% of greenhouse pollution, and globally about 29.8 billion tons (27 billion metric tons) of CO2 were being emitted yearly. Many climate scientists agree that in order to stabilize global average temperature at no more than about 3.6°F (2°C) above its preindustrial value, CO2 emissions must be greatly reduced by mid-century. To limit warming this much, a 2003 study in Science stated that by 2050, 75% to 100% of total power demand would have to be met by non-CO2-releasing sources. Nuclear power, renewables, and energy efficiency have all been proposed as ways to meet energy demand, but many experts believe that energy generation from fossil-fuel point sources such as coal-fired power plants will continue to grow in the coming decades. As more such plants are constructed, their CO2 emissions are bound to increase unless sequestration is used.
Italian energy specialist E. Marchetti suggested geologic or capture-and-store sequestration of CO2 from-fossil-fuel burning in 1976. In 1977, he suggested the alternative possibility of pumping CO2 into deep ocean waters, where it would dissolve and remain for centuries until making its way to the atmosphere. However, no action was taken on Marchetti's ideas, as there did not yet seem any urgent need for sequestration of carbon. Analysis of Marchetti's ideas in the mid 1980s seemed to show that sequestration would be prohibitively expensive.
In the 1990s, public and scientific concern over anthropogenic climate change intensified. In a little over ten years, from about 1995 to 2005, carbon sequestration went from being an idea of interest to only a few specialists to a widely discussed possibility for mitigating climate change, with hundreds of millions of dollars of government funding for demonstration projects from Europe, the United States, and elsewhere. By the late 1990s, the United States was funding studies of CCS in various American geological formations. By 2007, several industrial-scale CCS pilot projects were underway.
Carbon sequestration occurs naturally. Plants extract carbon from the atmosphere to build their tissues, and this carbon may be sequestered as dead plant matter in soils (hundreds of billions of tons are locked up in this form in the permafrost soils of the Arctic region) or even turned, over millions of years, into trillions of tons of oil, coal, and gas deposits. The carbon now being released from fossil fuels was sequestered by green plants hundreds of millions of years ago. The oceans also sequester carbon by absorbing it from the atmosphere: about half of the CO2 emitted by human activities is absorbed in this way, preventing it from enhancing global warming but making the oceans significantly more acidic.
Today's debates over carbon sequestration focus on allowing natural sequestration to proceed (i.e., by cutting down fewer forests or planting new ones), enhancing natural sequestration processes (i.e., by feeding iron to the phytoplankton floating in ocean surface waters, which transport carbon to the deep ocean when they die and sink), and building CCS systems. Iron fertilization is the most controversial of these options and CCS is the most mature.
Carbon Capture and Storage (CCS)
All CCS technologies have three major parts. First is the capture of CO2, either before or after fuel is burned. Second is transportation of the captured CO2 to the site where it is to be sequestered. Third is the disposal or sequestration itself.
There are three ways of capturing CO2 from fossil fuels, namely pre-combustion, post-combustion, and oxy-fuel combustion. In the first method, pre-combustion capture, chemical reactions are used to extract CO2 from the fuel before it is burned. For example, reacting the fuel with steam (water, H2O) produces two molecules of hydrogen (H2) and one of CO2 for each atom of carbon in the original fuel. The hydrogen may then be burned, producing water as the only byproduct. A steam reaction of this type is already the standard industrial technique for manufacturing hydrogen from natural gas.
WORDS TO KNOW
ALBEDO: A numerical expression describing the ability of an object or planet to reflect light.
AMINE: One of a family of compounds, the amines, containing carbon and nitrogen. When combined with a carboxyl group (CO2H), an amine forms an amino acid. All living things build proteins by chaining together amino acids.
BIOMASS: The sum total of living and once-living matter contained within a given geographic area. Plant and animal materials that are used as fuel sources.
DEFORESTATION: Those practices or processes that result in the change of forested lands to non-forest uses. This is often cited as one of the major causes of the enhanced greenhouse effect for two reasons: 1) the burning or decomposition of the wood releases carbon dioxide; and 2) trees that once removed carbon dioxide from the atmosphere in the process of photosyn-thesis are no longer present and contributing to carbon storage.
ENHANCED OIL RECOVERY: Use of special technologies to increase the amount of oil that can be recovered from a given oil field. Enhanced oil recovery techniques include heating oil to make it more liquid or pressurizing it by injecting gas or liquid into the reservoir. At least 40% of oil remains in the ground even with enhanced recovery.
FOSSIL FUELS: Fuels formed by biological processes and transformed into solid or fluid minerals over geological time. Fossil fuels include coal, petroleum, and natural gas. Fossil fuels are non-renewable on the timescale of human civilization, because their natural replenishment would take many millions of years.
GREENHOUSE GASES: Gases that cause Earth to retain more thermal energy by absorbing infrared light emitted by Earth's surface. The most important greenhouse gases are water vapor, carbon dioxide, methane, nitrous oxide, and various artificial chemicals such as chlorofluorocarbons. All but the latter are naturally occurring, but human activity over the last several centuries has significantly increased the amounts of carbon dioxide, methane, and nitrous oxide in Earth's atmosphere, causing global warming and global climate change.
IRON FERTILIZATION: Controversial speculative method for removing carbon from the atmosphere, in which adding powdered iron to ocean surface waters would cause single-celled aquatic plants (phytoplankton) to increase greatly in numbers, breaking down CO2 to obtain carbon for their tissues. The dead organisms would then, ideally, sink to deep waters or the ocean floor, sequestering their carbon content from the atmosphere and so mitigating climate change.
PERMAFROST: Perennially frozen ground that occurs wherever the temperature remains below 32°F (0°C) for several years.
PHYTOPLANKTON: Microscopic marine organisms (mostly algae and diatoms) that are responsible for most of the photosynthetic activity in the oceans.
The second method of capturing CO2 from fuel, post-combustion capture, first burns the fuel, allowing CO2 to form, then uses chemical reactions to capture the CO2 from the flue gas (hot gas normally sent right up the chimney or flue). The usual method of post-combustion capture is amine scrubbing. In this technique, the flue gas is bubbled through a watery solution of one or more of the chemicals called amines. This amine solution absorbs the CO2 from flue gas (which consists mostly of nitrogen, because air itself is 78% nitrogen). When heated in another part of the machinery, the amine solution gives up the CO2 in pure form, which is then collected.
A technology for pre-combustion capture that many energy experts view as promising is the integrated gasification combined cycle (IGCC). In IGCC, fuel is mixed with oxygen and steam to produce a burnable gas consisting mostly of hydrogen and carbon monoxide (CO). This first mixture is then reacted with steam to make CO2 and hydrogen. This second mixture can then be burned as-is directly, or the CO2 can be separated, leaving the hydrogen. Typical IGCC heat-to-electric efficiency is about 40% today and could, in theory, be made much higher (around 60%), which would make the energy cost of CO2 capture more affordable. IGCC plants—several of which were already in operation as of 2007, though without carbon capture—cost at least 20% more to build than conventional power plants, but also produce much less air pollution than comparable coal-fired plants.
Oxy-fuel combustion, the third major option for carbon capture in fuel burning, combusts fuel with pure oxygen, which produces flue gas that is mostly CO2 and steam and which can then be separated. Oxy-fuel combustion is already used in a number of power plants today because it produces more efficient combustion, but is not yet used with carbon capture.
Carbon capture is the most expensive part of CCS, and depending on the process can consume a good deal of energy. For example, a typical large, centralized, electricity-generation plant converts about 30% of the heat released from its fuel into electricity; the other 70% is wasted to the atmosphere. The best conventional plants (e.g., IGCC plants) operate at about 40% conversion efficiency. CCS might decrease the efficiency of an efficient plant from 40% to about 30%.
After capture, transportation is the next major phase of CCS. CO2 is nontoxic, so the main danger with transporting large quantities of it is that it can displace air near accident scenes and so cause suffocation. Also, CO2 that is to be shipped through pipelines must contain almost no water vapor because water and CO2 combine to form carbonic acid (H2CO3), which corrodes ordinary metal parts.
The third phase is sequestration or storage. Once transported by pipeline or tanker ship to an appropriate location, CO2 can be pumped at high pressure into deep boreholes made by standard drilling equipment. There are four basic options for underground (geological) storage. The first is depleted oil and gas reservoirs. Having been emptied of their fossil fuels, these underground pockets can be refilled with CO2. The second option is enhanced gas and oil recovery. In this method, CO2 is injected into gas or oil wells to squeeze out new fuel. These operations typically pay for themselves through the market value of the recovered fuel.
The third option is storage in deep saline aquifers (bodies of porous rock permeated with salty water), either onshore or offshore. The fourth is methane (CH4) recovery from coal seams that are too deep to mine. Coal naturally exudes methane, the main ingredient of natural gas. This option, Enhanced Coal Bed Methane recovery, consists of pumping CO2 into deep coal seams, capturing the methane that is forced out through other boreholes, and burning that methane as a fuel. CCS could be used to capture the CO2 from the burning methane and inject it back into the ground to force out still more methane, and so on. Deep coal deposits in the United States are estimated to have storage potential for about 37 billion tons (33.5 billion metric tons) of CO2, or six years' worth of emissions. The capacity of deep saline aquifers is uncertain, but the high end of the range of figures quoted by the U.S. Department of Energy is 500 billion tons (454 billion metric tons).
There is little doubt that CCS with geological sequestration can work, at least for the geological short term. Since 1986, Chevron Oil's Rangely Field project in the United States has used CO2 injection into underground oil reservoirs to help force petroleum to the surface. As of 2006, the project had sequestered over 26.5 million tons (24 million metric tons) of CO2.
In 1991, Norway became the first country to impose a per-ton tax on carbon emissions. In 1996, in order to avoid paying this tax on CO2 pumped from its Sleipner natural gas well in the North Sea, the Norwegian oil company Statoil began disposing of CO2 by injecting it into an aquifer 3,280 ft (1,000 m) below the sea floor. The natural gas obtained from the Sleipner well is unusual in that it consists of 9% CO2, which has to be separated from the gas to make it marketable. The Norwegian tax on CO2 emissions is about US$50 a ton, but it costs Statoil only about US$30 a ton to inject the CO2 into the deep aquifer, making the operation profitable. Statoil sequesters 1milliontons of CO2 per year at Sleipner, enough to increase Norway's CO2 emissions by 3% if it were all released into the atmosphere. Statoil plans to dispose, all told, about 20 million metric tons of CO2 at the Sleipner well.
In 2007, this was still the largest project for sequestering CO2 that was not part of an enhanced-oil-recovery (EOR) scheme. The Weyburn project in Canada, an EOR project, was also injecting about 1 million tons (907,000 metric tons) of CO2 per year. At the In Salah natural gas well in Algeria, CO2 separated from the gas was being injected back into the ground much as at the Sleipner field. The In Salah plan called for the eventual sequestration of 17 million tons (15.4 million metric tons) of CO2.
The alternative to geological storage is ocean storage. There are two basic options. The first is dissolution-type storage, in which a pipeline is run out to sea at a depth of approximately 1 mi (1.5 km). Liquid or gaseous CO2 is then forced through the pipeline, after which it bubbles up through the ocean in a rising plume. The gas
dissolves in the ocean before it reaches the surface layer. An alternative form of dissolution-type storage is to run a pipe down to a depth of at least 2 mi (3 km) and dissolve a sinking plume of liquid CO2 in deeper water that will sequester the carbon for a longer period of time.
The second basic type of ocean storage is lake-type storage. In this method, liquid CO2 is deposited on the deep ocean floor at about 2.5 mi (4 km) or greater depth in the form of a cold pool. Some experiments indicate that a skin of stable ice-like hydrates may form over such a pool, slowing its eventual dissolution in the ocean. Ocean disposal of CO2 was in the research phase in the early 2000s.
Proposed methods of carbon capture other than CCS include the manufacture of carbonate minerals; reformed agricultural practices to encourage carbon uptake by soils; the manufacture of carbon powder and its addition to soils as a fertilizer; planting forests and preventing deforestation; the addition of powdered iron compounds to ocean surface waters to encourage the growth of phytoplankton that will then die and sink, transporting their carbon to deep waters; and more. Most of these methods depend on sequestering carbon after it has been released, rather than capturing it before release as CCS does. As of 2007, pilot projects were underway for all these technology-dependent sequestration schemes.
Impacts and Issues
The goal of carbon sequestration is to reduce the amount of CO2 entering (or staying in) the atmosphere. For any given method or combination of methods, how much carbon is captured depends on several factors, namely the fraction of CO2 captured, the increase in CO2 production needed to achieve a given real output (e.g., amount of electricity) due to lowered efficiency with CCS, leakage of CO2 during transport, and the fraction of CO2 that leaks out of storage over a given time period. Today's CCS technologies capture about 85–95% of CO2 from gasified fuel or flue gas. Depending on the type of combustion, power plants would need to produce 10–40% more energy to capture this amount of CO2 and compress it into liquid form for disposal, whether underground, at sea, or by some other method.
As for retention of CO2 in storage, the disposal of CO2 underground may, in many cases, sequester carbon from the atmosphere for many thousands of years. Disposal in ocean waters would be relatively temporary, because CO2 added to the ocean must eventually migrate into the atmosphere until atmospheric and oceanic CO2 are in equilibrium or balance. Scientists believe that CO2 injected in the oceans would equilibrate with atmospheric CO2 in 2,000 years or less.
All methods of sequestering CO2 have expert advocates and expert critics. However, some methods are more widely criticized in the scientific community than others. Iron fertilization of the oceans is probably the most criticized method, followed by oceanic CCS.Critics of iron fertilization charge that although tests have shown that adding finely ground iron particles to surface ocean waters can indeed cause plankton blooms, these tests have not shown that the method significantly increases the transport of carbon to deep waters, where it would be sequestered.
Concerns about CCS, the most mature of the carbon sequestration technologies, include doubts about whether the CO2 will leak out through old boreholes or other geological defects, perhaps cracks caused by earthquakes. Sudden release of a large amount of CO2 could be disastrous for local populations (as well as defeating the greenhouse-abatement goal of putting the CO2 underground originally). A natural demonstration of this possibility occurred in 1986, when a large bubble of CO2 escaped from a volcanic lake in Cameroon, Africa, killing 1,700 people by suffocation. Storing CO2 in aquifers may be unstable because CO2 combines with water to form carbonic acid, which can weaken rock over time. However, no CO2 leakage has yet been observed from any of the pilot CCS sequestration projects now being conducted worldwide.
Ocean storage of CO2 would, according to biologists, injure deep-sea life by making the deep ocean more acidic. Biological communities near injection sites would be devastated at once, while the effect on living things farther away would be more gradual. CO2 from the atmosphere is already making the oceans significantly more acidic. By the end of the twenty-first century, acid-ification of the oceans may make it difficult for shelly marine creatures such as corals and clams to build their shells at all, even without additional acidity from oceanic CO2 sequestration.
Despite doubts, carbon sequestration may prove to be an indispensable technology in mitigating climate change. As of 2007, the European Union was considering requiring geologic CCS systems for all new coal-burning power plants. In the United States, a government-industry partnership called FutureGen began considering proposals in 2006 for the construction of a $1 billion power plant that would produce hydrogen and 275 megawatts of electricity while using CCS to yield near-zero greenhouse emissions. The plant was supposed to start construction in 2009 and enter service in 2012, and would act as a demonstration model for the feasibility of CCS on a commercial scale. Other governments worldwide were also participating in various CCS demonstration projects.
Primary Source Connection
The Environmental Protection Agency (EPA) is the lead environmental agency of the United States government. Here, EPA research explores the feasibility and efficiency of various methods of carbon sequestration for reducing greenhouse gas emissions.
Geologic sequestration, a type of carbon dioxide (CO2) capture and storage (CCS) process, is a promising technology for stabilizing atmospheric greenhouse gas concentrations. Instead of releasing CO2 to the atmosphere, geologic sequestration involves separating and capturing CO2 from an industrial or energy-related source, transporting it to a storage location, and injecting it deep underground for long-term isolation from the atmosphere.
The goal of CO2 capture is to produce a concentrated stream of CO2 that can be readily transported to a geologic sequestration site. Capture of CO2 can be applied to large stationary sources such as power plants, cement or ammonia production or natural gas processing. Several technologies, in different stages of development, exist for CO2 capture. Although these technologies are currently used in a limited number of facilities, research is still needed to improve the efficiency and cost.
After the CO2 is captured from the source and compressed, it can be geologically sequestered on-site or transported to a separate injection site. CO2 can be transported as a liquid in ships, road or rail tankers, but pipelines are the most efficient and cost-effective approach for transporting large volumes of CO2. In the U.S., there is a network of CO2 pipelines that supply CO2 to oil and gas fields, where it is used to enhance oil recovery. The majority of the 40 Tg CO2(Tg = 109 kg = 106 metric tons = 1 million metric tons) transported in these pipelines today is produced from natural CO2 reservoirs; however, the same pipelines can carry CO2 captured from industrial facilities. In fact, a synfuels plant located in North Dakota (Dakota Gasification) has been transporting captured CO2 via pipeline to a sequestration site hundreds of miles away in Canada since 2000.
Injection and Sequestration
Once a suitable geologic formation has been identified through detailed site characterization, CO2 is injected into that formation at a high pressure and to depths generally greater than 2625 feet (800 meters). Below this depth, the pressurized CO2 remains “supercritical” and behaves like a liquid. Supercritical CO2 is denser and takes up less space. Once underground, the CO2 occupies pore spaces in the surrounding rock, like water in a sponge. Saline water which already resides in the pore space will compress under pressure and/or move to allow room for the CO2. Over time, the CO2 also dissolves in water and chemical reactions between the dissolved CO2 and rock can create solid carbonate minerals, more permanently trapping the CO2.
Suitable geologic storage sites have a caprock, which is an overlying impermeable layer that prevents CO2 from escaping back towards the surface. Target formations for sequestration include geologic formations, both on and off-shore, that can demonstrate their ability to retain CO2 for very long periods of time. Well-suited formations include the following:
- Deep saline formations, rock units containing water with a high concentration of salts, are thought to have the largest storage capacity.
- Depleted oil and gas reservoirs are also targeted for CO2 sequestration and have a history of retaining fluids and gases underground for geologic time-scales. There is also more data available on these formations which may help characterize and better predict the long-term fate of injected CO2.
- Unminable coal beds, which are either too thin or too deep to be mined economically, offer less storage capacity but they have the benefit of enhancing the production of methane, a valuable fuel source. Less is known about the efficacy of using these formations as targets for sequestration, but research is underway to evaluate them.
With proper site selection and management, geologic sequestration could play a major role in reducing emissions of CO2(IPCC, 2005). Current assessments indicate that the storage capacity of these geologic formations is extremely large and widespread, with a significant proportion of storage opportunities in the U.S. In the U.S., an evaluation of CO2 sources and potential storage sites suggests that 95% of the largest 500 point sources (i.e., power plants and other industrial facilities), accounting for 82% of annual CO2 emissions, are within 50 miles of a candidate CO2 reservoir….
There is limited experience with commercial-scale geologic sequestration today. However, closely related and well-established industrial experience and scientific knowledge can serve as the basis for appropriate risk management strategies. Key components of a risk management strategy include appropriate site selection based on thorough geologic characterization, a monitoring program to detect problems during or after injection, appropriate remediation methods if necessary and a regulatory system to protect human health and the environment….
Potential pathways exist for CO2 to migrate from the target geologic formation to shallower zones or back to the atmosphere. These conduits for CO2 leakage could be largely avoided through proper site characterization and selection. Pathways for CO2 leakage include escape through the caprock (if it is compromised by high pressures or chemical degradation), an undetected or reactivated fault or an artificial penetration such as a poorly plugged abandoned well. In addition to careful site selection, a proper monitoring program can help ensure that CO2 does not escape from the storage site. A monitoring system would detect movement of CO2 into shallower formations and allow significant time to take corrective action in order to reduce potential impacts to human health and the environment.
Ground water could be affected both by CO2 leaking directly into an aquifer and by saline ground water that enters an aquifer as a result of being displaced by injected CO2. The risk of these impacts can be minimized through appropriate management strategies. Underground injection of CO2 for the purpose of sequestration is regulated by the Underground Injection Control (UIC) Program under the Safe Drinking Water Act (SDWA). The UIC program ensures that injection activities are performed safely and do not endanger current or future sources of drinking water.
Existing and Planned Projects
Internationally, commercial-scale geologic sequestration (greater than 1 Tg CO2 per year) is occurring or planned in various locations. Projects that are underway include the Weyburn CO2 Flood Project (Canada), Sleipner (Norway), and In Salah (Algeria).
The Weyburn CO2 Flood Project in Canada is the first international CO2-enhanced oil recovery (EOR) project to be studied extensively. The CO2 source is the Dakota Gasification plant near Great Plains, North Dakota. Unlike traditional EOR operations, the Weyburn operator will not use conventional end of projects techniques, which can release CO2, but will maintain the site in order to test and monitor long-term sequestration.
Commercial-scale geologic CO2 sequestration is also occurring at the Sleipner West field in the North Sea. Sleipner West is a natural gas/condensate field located about 500 miles off the coast of Norway. The CO2 is compressed and injected via a single well into a 500 foot thick, saline formation located at a depth of about 2,000 feet below the seabed.
In 2004, a CO2 capture and storage project was launched at the In Salah gas field, in the Algerian desert. Approximately 10% of the produced gas is made up of CO2. Rather than venting the CO2, a common practice on projects of this type, this project is compressing and injecting it 5900 feet deep into a lower level of the gas reservoir where the reservoir is filled with water. Around one million tons of CO2 will be injected into the reservoir every year.
Additionally, commercial scale projects are planned throughout the world. More information can be found on international projects through the Carbon Sequestration Leadership Forum (CSLF), an international climate change initiative focused on the development of improved cost-effective technologies for the separation and capture of CO2 for its transport and long-term safe storage.
In the U.S., the Department of Energy (DOE) is the lead federal agency on research and development of geologic sequestration technologies. The Department of Energy's Fossil Energy program is developing a portfolio of technologies that can capture and permanently store greenhouse gases. As part of this portfolio, DOE and an industry alliance recently launched FutureGen, an initiative to complete the world's first near-zero emissions, coal-based power plant with sequestration by 2012. DOE is also sponsoring a number of small-scale CO2 pilot projects designed to learn more about how CO2 behaves in the sub-surface and answer practical technical questions on how to design and operate geologic sequestration projects.
epa. “geologic sequestration.” < http://www.epa.gov/climatechange/emissions/co2_geosequest.html> (accessed november29, 2007).
Berstein, Lenny, et al. “Carbon Dioxide Capture and Storage: A Status Report.” Climate Policy 6 (2006): 241–246.
Caldeira, Ken, et al. “Climate Sensitivity Uncertainty and the Need for Energy Without CO2 Emission.” Science 299 (2003): 2052–2054.
Holloway, Sam. “Storage of Fossil Fuel–Derived Carbon Dioxide Beneath the Surface of the Earth.” Annual Review of Energy and the Environment 26 (2001): 145–166.
Jackson, Robert B., et al. “Trading Water for Carbon with Biological Carbon Sequestration.” Science 310 (2005): 1944–1947.
Kintisch, Eli. “Report Backs More Projects to Sequester CO2 from Coal.” Science 315 (2007): 1481.
Lackner, Klaus S. “Carbonate Chemistry for Sequestering Fossil Carbon.” Annual Review of Energy and the Environment 27 (2002): 193–232.
Lackner, Klaus S. “A Guide to CO2 Sequestration.” Science 300 (2003): 1677–1678.
Lal, R. “Soil Carbon Sequestration Impacts on Global Climate Change and Food Security.” Science 3024 (2004): 1623–1627.
Schiermeier, Quirin. “Putting the Carbon Back: The Hundred Billion Tonne Challenge.” Nature 442 (2006): 620–623.
Schlesinger, William H. “Carbon Sequestration in Soils.” Science 284 (1999): 2095.
Metz, Bert H., et al. “IPCC Special Report on Carbon Dioxide Capture and Storage.” Intergovernmental Panel on Climate Change, 2005. < http://www.ipcc.ch/activity/srccs/SRCCS.pdf> (accessed November 6, 2007).