Carbon capture and sequestration (or storage)—known as CCS—is a physical process that involves capturing manmade carbon dioxide (CO 2 ) at its source and storing it before its release to the atmosphere. CCS could reduce the amount of CO 2 emitted to the atmosphere despite the continued use of fossil fuels. An integrated CCS system would include three main steps: (1) capturing CO 2 and separating it from other gases; (2) purifying, compressing, and transporting the captured CO 2 to the sequestration site; and (3) injecting the CO 2 in subsurface geological reservoirs or storing it in the oceans. As a measure for mitigating global climate change, CCS has attracted congressional interest and support because several projects in the United States and abroad—typically associated with oil and gas production—are successfully capturing, injecting, and storing CO 2 underground, albeit at relatively small scales. The oil and gas industry in the United States injects nearly 50 million metric tons of CO 2 underground each year to help recover oil and gas resources (a process known as enhanced oil recovery, or EOR). Potentially, much larger amounts of CO 2 produced from electricity generation—approximately 2.1 billion metric tons per year, about 40% of the total CO 2 emitted in the United States from fossil fuels (see Table 1 )—could be targeted for large-scale CCS. Fuel combustion accounts for 94% of all U.S. CO 2 emissions. Electricity generation contributes the largest proportion of CO 2 emissions compared to other types of fossil fuel use in the United States ( Table 1 ). Electricity-generating plants are among the most likely initial candidates for capture, separation, and storage or reuse of CO 2 because they are predominantly large, stationary, single-point sources of emissions. Large industrial facilities, such as cement-manufacturing, ethanol, or hydrogen production plants, that produce large quantities of CO 2 as part of the industrial process are also good candidates for CO 2 capture and storage. This report is a brief summary of what CCS is, how it is supposed to work, why it has gained the interest and support of some Members of Congress, and what some of the challenges are to its implementation and deployment across the United States. Several other CRS reports cover different aspects of CCS in more detail, such as capture technology, challenges to siting and infrastructure, policy challenges to a comprehensive climate policy, and others. These are listed in the last section of this report, titled " Other CRS Reports ." This report covers only CCS and not other types of carbon sequestration activities whereby CO 2 is removed from the atmosphere and stored in vegetation or soils, such as forests and agricultural lands. The first step in CCS is to capture CO 2 at the source and produce a concentrated stream for transport and storage. Currently, three main approaches are available to capture CO 2 from large-scale industrial facilities or power plants: (1) post-combustion capture, (2) pre-combustion capture, and (3) oxy-fuel combustion capture. For power plants, current commercial CO 2 capture systems could operate at 85%-95% capture efficiency. The capture phase of the CCS process, however, may be 80% or more of the total costs for CCS. A detailed description and assessment of these capture technologies is provided in CRS Report R41325, Carbon Capture: A Technology Assessment . This process involves extracting CO 2 from the flue gas following combustion of fossil fuels or biomass. Several commercially available technologies, some involving absorption using chemical solvents, can in principle be used to capture large quantities of CO 2 from flue gases. U.S. commercial electricity-generating plants currently do not capture large volumes of CO 2 because they are not required to and there are no economic incentives to do so. Nevertheless, the post-combustion capture process includes proven technologies that are commercially available today. This process separates CO 2 from the fuel by combining the fuel with air and/or steam to produce hydrogen for combustion and a separate CO 2 stream that could be stored. The most common technologies today use steam reforming, in which steam is employed to extract hydrogen from natural gas. This process uses oxygen instead of air for combustion and produces a flue gas that is mostly CO 2 and water, which are easily separable, after which the CO 2 can be compressed, transported, and stored. The U.S. Department of Energy's (DOE's) flagship CCS demonstration project, FutureGen, plans to retrofit an existing power unit with an oxy-fuel combustion unit. Pipelines are the most common method for transporting CO 2 in the United States. Currently, approximately 4,100 miles of pipeline transport CO 2 in the United States, predominately to oil and gas fields, where it is used for EOR. Transporting CO 2 in pipelines is similar to transporting petroleum products like natural gas and oil; it requires attention to design, monitoring for leaks, and protection against overpressure, especially in populated areas. Using ships may be feasible when CO 2 must be transported over large distances or overseas. Ships transport CO 2 today, but at a small scale because of limited demand. Liquefied natural gas, propane, and butane are routinely shipped by marine tankers on a large scale worldwide. Rail cars and trucks can also transport CO 2 , but this mode would probably be uneconomical for large-scale CCS operations. Costs for pipeline transport vary, depending on construction, operation and maintenance, and other factors, including right-of-way costs, regulatory fees, and more. The quantity and distance transported will mostly determine costs, which will also depend on whether the pipeline is onshore or offshore, the level of congestion along the route, and whether mountains, large rivers, or frozen ground are encountered. Shipping costs are unknown in any detail, however, because no large-scale CO 2 transport system (in millions of metric tons of CO 2 per year, for example) is operating. Ship costs might be lower than pipeline transport for distances greater than 1,000 kilometers and for less than a few million metric tons of CO 2 (MtCO 2 ) transported per year. Even though regional CO 2 pipeline networks currently operate in the United States for enhanced EOR, developing a more expansive network for CCS could pose numerous regulatory and economic challenges. Some of these include questions about pipeline network requirements, economic regulation, utility cost recovery, regulatory classification of CO 2 itself, and pipeline safety. Three main types of geological formations are being considered for carbon sequestration: (1) depleted oil and gas reservoirs, (2) deep saline reservoirs, and (3) unmineable coal seams. In each case, CO 2 would be injected in a supercritical state—a relatively dense liquid—below ground into a porous rock formation that holds or previously held fluids. When CO 2 is injected at depths greater than 800 meters in a typical reservoir, the pressure keeps the injected CO 2 in a supercritical state (dense like a liquid, fluid like a gas) and thus it is less likely to migrate out of the geological formation. Injecting CO 2 into deep geological formations uses existing technologies that have been primarily developed and used by the oil and gas industry, and that could potentially be adapted for long-term storage and monitoring of CO 2 . Other underground injection applications in practice today, such as natural gas storage, deep injection of liquid wastes, and subsurface disposal of oil-field brines, can also provide valuable experience and information for sequestering CO 2 in geological formations. The storage capacity for CO 2 in geological formations is potentially huge if all the sedimentary basins in the world are considered (see discussion below of storage capacity estimates for the United States). The suitability of any particular site, however, depends on many factors, including proximity to CO 2 sources and other reservoir-specific qualities like porosity, permeability, and potential for leakage. For CCS to succeed, it is assumed that each reservoir type would permanently store the vast majority of injected CO 2 , keeping the gas isolated from the atmosphere in perpetuity. Pumping CO 2 into oil and gas reservoirs to boost production (EOR) is practiced in the petroleum industry today. The United States is a world leader in this technology, and oil and gas operators inject approximately 48 MtCO 2 underground each year to help recover oil and gas resources. Most of the CO 2 used for EOR in the United States comes from naturally occurring geologic formations, however, not from industrial sources. Using CO 2 from industrial emitters has appeal because the costs of capture and transport from the facility could be partially offset by revenues from oil and gas production. Carbon dioxide can be used for EOR onshore or offshore. To date, most CO 2 projects associated with EOR are onshore, with the bulk of U.S. activities in west Texas. Carbon dioxide can also be injected into oil and gas reservoirs that are completely depleted, which would serve the purpose of long-term sequestration, but without any offsetting benefit from oil and gas production. Depleted or abandoned oil and gas fields, especially in the United States, are considered prime candidates for CO 2 storage for several reasons: oil and gas originally trapped did not escape for millions of years, demonstrating the structural integrity of the reservoir; extensive studies for oil and gas typically have characterized the geology of the reservoir; computer models have often been developed to understand how hydrocarbons move in the reservoir, and the models could be applied to predicting how CO 2 could move; and infrastructure and wells from oil and gas extraction may be in place and might be used for handling CO 2 storage. Some of these features could also be disadvantages to CO 2 sequestration. Wells that penetrate from the surface to the reservoir could be conduits for CO 2 release if they are not plugged properly.