Carbon sequestration means putting carbon dioxide, the leading global warming pollutant, somewhere other than into the atmosphere. There are two basic methods of sequestering carbon: geologic sequestration and terrestrial sequestration.

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Geologic sequestration takes carbon dioxide from large emissions sources such as coal-fired power plants and pumps it in a nearly liquid state deep into the earth. The geologic formation then traps the carbon dioxide. As stated by the Intergovernmental Panel on Climate Change: “For well-selected, designed and managed geological storage sites, the vast majority of the CO₂ will gradually be immobilized by various trapping mechanisms and, in that case, could be retained for up to millions of years.”

Geologic sequestration has the most potential to help stabilize the concentration of global warming pollutants in the atmosphere. But it comes with certain hazards:

• Fractured rock formations, faults, and seismic activity could provide an avenue for CO₂ leakage.
• Pressure from CO₂ injection could trigger small earthquakes.
• The cement caps usually placed on the wells could deteriorate when exposed to carbonic acid, which can form when CO₂ interacts with saline formations.
• Abandoned oil and gas wells that were not sealed to today’s standards could leak. A sudden and large release of CO₂ could pose immediate dangers to people in the vicinity.
• Elevated CO₂ concentrations in the shallow subsurface could have lethal effects on plants and subsoil animals, and could contaminate groundwater.
• Carbon-laden liquids could mobilize toxic metals and organics and contaminate groundwater.

Despite these hazards, and despite substantial talk on the part of the coal and electric utility industry saying it wants to start pumping carbon dioxide into geologic formations, there are no regulations defining geologic sequestration, and no guarantee that carbon dioxide will be pumped into the earth in a safe and effective manner.

Many people mistakenly say that geologic sequestration is the same things as enhanced oil recovery (EOR). EOR pumps carbon dioxide or other gases into an oil or gas field in order to access more oil and gas. If done properly, EOR can result in geologic sequestration, but the two terms are not synonymous. They are done for different purposes; the permanent storage of the CO₂ is not the priority of EOR. Additional modeling, monitoring, and verification will be necessary if an EOR well is to subsequently be used for the permanent sequestration of carbon dioxide.

For more information:

• Climate Change 2007: Mitigation of Climate Change, A Summary for Policymakers.  A report by the Intergovernmental Panel on Climage Change.
  
• "The Future of Coal: An Interdisciplinary MIT Study" (© 2007) This report, the future of coal in a carbon-constrained world, evaluates the technologies and costs associated with the generation of electricity from coal along with those associated with the capture and sequestration of the carbon dioxide produced coal-based power generation.
  
• Special Report: "Dirty king coal" (5/31/07 from The Economist print edition). "Scrubbing carbon from coal-fired power stations is possible but pricey."
• "Future of 'Clean Coal' Power Tied to (Uncertain) Success of Carbon Capture and Storage" (by David Biello, Scientfic American, March 14, 2007). A new report confirms that coal has a large role to play in meeting the world's energy demands, but to avoid runaway climate change, technologies to sequester its carbon need to advance quickly.
• Carbon Sequestration Technology Roadmap and Program Plan 2006 (NOTE:  THIS PDF IS 8.7 MB).  A U.S. Department of Energy document that  (1) defines the current status of carbon dioxide (CO₂) capture and sequestration technology, (2) identifies research pathways that lead to achievement of the Carbon Sequestration Program goal, and (3) describes efforts that the DOE program is pursuing along priority pathways.
• Summary for Policyimakers: An Intergovernmental Panel on Climate Change (IPCC) special report on carbon dioxide capture and storage (September 2005)