"Natural" Carbon Cycle	Anthropogenic contribution to atmospheric carbon

The largest amount of CO₂ transferred in the carbon cycle is the atmosphere-biosphere (green plants-animals) exchange. [The illustration has been corrected from the one shown in lecture to reflect the relative amounts of carbon flowing in the pathways shown.]

A significant, though small, exchange occurs between the oceans and the atmosphere (the dark green arrows), not including the ocean biosphere (plankton). The carbon become buried in the Earth's crust as it settles out in sediments of carbonate compounds, then is eventually emitted as CO₂ through outgassing, such as volcanic eruptions. This cycle takes thousands or millions of years for the carbon to move through.

Fossil fuel is carbon that was once plant and animal life millions of years ago, and was buried and transformed to coal (elemental carbon), crude oil (complex hydrocarbon), and natural gas (CH₄).

Pulling fossil fuel out of the ground and burning it releases carbon dioxide to the air much more rapidly than would have happened naturally (the normal cycle would be on the order of millions of years). And, the natural environment does not have a sink rate large enough to handle this extra CO₂ load (the natural sinks will normally take care of only the "natural" CO₂ sources, mainly from respiration). The additional burden of CO₂ to the atmosphere has caused the accumulation of CO₂ since the beginning of the Industrial Revolution.

The largest anthropogenic source of CO₂ is fossil fuel combustion, mainly from coal combustion in electric power generation and from transportation (automobiles). Deforestation indirectly increases atmospheric CO₂ by decreasing the sink for CO₂ (green plants).

Cement has a high carbonate content. When mining the cement out of the ground and grinding it into a powder, the cement material heats up (the pressure of the grinding process raises the temperature), which causes the carbonates to release CO₂ into the atmosphere.