Background
Carbon dioxide is a simple molecule composed of one carbon and two oxygen atoms. This naturally-occurring molecule comprises less than 0.04 percent of the air humans breathe.

CO2 Molecule

Carbon dioxide is released to the atmosphere from both natural and anthropogenic (human) processes.

Natural sources:
The most common natural sources of CO2 include the respiration and decay of living things, forest fires, the melting of permafrost, and limestone weathering. Emissions from volcanoes also fall under this category. A limited amount of CO2 is emitted from hot springs and geysers. Natural CO2 is also common in petroleum and natural gas reservoirs. The CO2 in these reservoirs was generated along with the oil and gas through bacterial and thermogenic processes.

Anthropogenic sources:
The largest global source of anthropogenic or human-made CO2 is the combustion of fossil fuels such as coal, oil, and gas in power plants, automobiles, and industrial facilities. The anthropogenic CO2 component of the atmosphere has been growing steadily since the Industrial Revolution (1760-1850). Over the past 800,000 years, atmospheric levels of CO2 naturally fluctuated from 171 to nearly 300 ppm (Lüthi and others, 2008). As of March 2013, the monthly mean atmospheric CO2 concentration recorded at the Mauna Loa Observatory in Hawaii was 397.3 ppm (Tans, and Keeling, 2013) — higher than any observed over the past 800,000 years (Lüthi and others, 2008).

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Geologic CO2 Storage
To help reduce anthropogenic CO2 emissions, several alternatives have been proposed that would remove CO2 (and other gases) from the emissions stream. One of these alternatives is geologic CO2 storage, often termed carbon sequestration or carbon capture and storage (CCS). The long-term strategy of geologic storage is to collect and store CO2 that would otherwise be released to the atmosphere from point sources such as coal-fired power plants, chemical plants, refineries, and natural gas plants and compressor stations. In principle, CO2 is injected into the subsurface as a supercritical fluid, where it remains in place for an appropriate amount of time. There are numerous options for where and how to store CO2 within a geologic basin, including abandoned and unmineable coal seams, depleted or uneconomic petroleum reservoirs, and saline aquifers, defined as aquifers containing groundwater with greater than 10,000 mg/L total dissolved solids, which is significantly above the national standards of 500 mg/L for drinking water.

References:

Lüthi and others, 2008, High-resolution carbon dioxide concentration record 650,000–800,000 years before present: Nature, v. 453, p. 379–382.

Tans, P., and Keeling, R., 2013, Trends in atmospheric carbon dioxide, National Oceanic and Atmospheric Administration Earth System Research Laboratory, at http://www.esrl.noaa.gov/gmd/ccgg/trends/.