A carbon sink is a reservoir that accumulates and stores carbon. Natural carbon sinks include the oceans and the photosynthesis of plants.
The process whereby these sinks remove carbon from the atmosphere is known as carbon sequestration. This is an important concept for global warming because it is becoming increasingly urgent that we find ways to store the extra CO2 that is being added to the environment because of human activities.
Forests are carbons sinks. Trees take in CO2 as part of the photosynthesis cycle and then store the carbon in the plant and soil. In fact, scientists have determined that about half of a tree's weight is carbon. When trees live for many years, this carbon is effectively stored for long periods of time. It is estimated that vegetation stores 600 billion tons of CO2. Based on research at NASA using high-resolution maps of carbon storage, they determined that the forests in the United States, Europe, and Russia stored almost 700 million metric tons of carbon each year during the 1980s and 1990s.
The U.S. Environmental Protection Agency (EPA) is currently focusing attention on carbon sequestration efforts through agriculture and forestry in order to help prevent global climate change. They are focusing on maintaining existing forests in order to keep the carbon already there safely stored. According to the EPA, "Forests and soils have a large influence on atmospheric levels of CO2. Agricultural and forestry activities can both contribute to the accumulation of greenhouse gases in the atmosphere, as well as be used to help prevent climate change by avoiding further emissions and by sequestering additional carbon." A second approach is to increase the carbon storage capacity in the United States by planting more trees and by modifying some agricultural practices, such as having farmers convert from conventional tillage to conservation tillage methods. They also support the substitution of bio-based fuels and products for fossil fuels (coal and oil) because these alternative fuels produce less CO2 when used.
The oceans also function as a carbon sink. Carbon dioxide dissolves in the ocean from the air, and marine animals also extract CO2 to support their own needs. Depending on the temperature and pressure of the water, the ocean is able to absorb and dissolve large amounts of CO2. Scientists have estimated that 1,020 trillion tons (925 trillion metric tons) of CO2 is stored by the surface waters, and 40 billion tons (36 billion metric tons) in the deep waters.
Carbon dioxide is also stored in the Earth's soil and rocks, although not as much is stored here as in the oceans and vegetation. Methods of sequestration are being looked at more seriously as one way to counter the effects of global warming.
The U.S. National Energy Technology Laboratory's (NETL) Carbon Sequestration Program overseen by the U.S. Department of Energy (DOE) is helping to develop technologies to capture, purify, and store CO2 in order to reduce greenhouse gas emissions without having an adverse impact on energy availability and use or harming economic growth. NETL sees this as an important issue because they have determined that worldwide CO2 emissions generated from human activity have increased from insignificant levels 200 years ago to more than 33 billion tons (30 billion metric tons) today. In fact, the U.S. Energy Information Administration (EIA) predicts that if no action is taken, the United States will emit 7,550 million tons (6,850 million metric tons) of CO2 by 2030, which means an increase in emission levels from 2005 of more than 14 percent.
It is NETL's goal to have in place by 2012 technologies that provide safe, cost-effective, commercial-scale greenhouse gas capture, storage, and mitigation. Technologies being developed include injecting CO2 into geological formations, increasing carbon uptake on mined lands, using no-till agriculture, reforestation, rangeland improvement, wetlands recovery, and riparian restoration. The NETL is also involved in research into high-speed computing, simulations, and modeling as tools for designing, optimizing, analyzing, and better understanding the chemical and physical processes that take place in carbon sequestration.
An international climate change initiative has been established, the Carbon Sequestration Leadership Forum (CSLF), composed of 21 countries and the European Community. Its goal is to develop improved, cost-effective technologies for CO2 capture and long-term storage. Justin R. Swift, deputy assistant secretary for international affairs for fossil energy at the DOE, says that "CSLF members also work to make the technologies broadly available internationally, to help developing countries learn about and apply the technologies, and to identify and address regulatory and policy issues that relate to carbon capture and storage." According to the CSLF, "Joint projects are already increasing knowledge in areas that include technology, economics, health, safety, and the environment, and also demonstrate a wide range of CO2 capture, transport, and storage research and activities."
Swift says, "The CSLF has recognized ten projects worldwide. Two of these are: (1) the Weyburn CO2 Monitoring and Storage Project, a collaboration among the United States, Canada, the European Commission, and Japan; and (2) the Frio Brine Pilot Experiment, a joint project between the United States and Australia." In the Weyburn Project, the Weyburn oil field in southern Saskatchewan uses a CO2-enhanced oil recovery method to extract the oil. They acquire CO2 from the Great Plains Gasification Plant in North Dakota and pump it into the ground at Weyburn. Injecting the CO2 serves two purposes: The injection of CO2 reduces the oil's viscosity and expands its volume, allowing more oil to be withdrawn, the injected CO2 stays safely underground.
According to S. Julio Friedmann, who heads the Carbon Storage Initiative at the DOE Lawrence Livermore National Laboratory in California, "Weyburn combines carbon sequestration and enhanced oil recovery." This project also is allowing the development of a cutting-edge monitoring and tracking technique leading to a better understanding of CO2 movement in the new CO2 storage reservoir.
About the Frio Brine Pilot, Judd Swift says, "Its purpose is to ensure safe storage in a saline reservoir. We consider it a key U.S. project for CO2 injection. Saline reservoirs show particular promise worldwide for carbon sequestration." Frio Brine researchers drilled a well in 2004 and injected 1,904 tons (1,727 metric tons) of CO2 4,921 feet (1,500 m) underground at the South Liberty oil field near Dayton, Texas. The goal of this project is monitoring and research to gain an understanding of CO2 storage for the future in saline aquifers.
According to Julio Friedmann, "The Frio formation is an enormous aquifer on the Texas gulf coast. They wanted to demonstrate that they could store CO2 in that aquifer and, based on what they learned, demonstrate the potential for a large number of carbon storage projects along the Texas gulf coast."
Large-scale injection of CO2 for the sole purpose of removing it from the atmosphere is thus far only taking place in Norway, at the off shore Sleipner facility, which is run by Statoil, the oil company owned and run by Norway. Statoil strips excess CO2 from gas and injects it into an aquifer 2,625 feet (800 m) below the seabed. Operational since 1996, they have injected at least 1 million tons of CO2 a year at a low cost. This offers hope for the future.
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