From Envirowiki

Carbon sequestration refers to the act of taking carbon-based greenhouse gasses from the air, and storing (sequestering) them somewhere permanent. Most commonly in public debate at the moment, carbon sequestration refers to capturing carbon from coal power stations, and burying it underground. This is also called carbon capture and storage. But there are a number of other methods for sequestering carbon from the atmosphere.

[edit] 1 Natural carbon sequestration

Natural carbon removal processes have been occuring for billions of years. These include photosynthesis of plants, and of phytoplankton. Plants store the carbon in their cells, and when they die, that some of that carbon either enters into the soil, into herbivorous animals, or, given the right circumstances, into the earth's crust, as coal. Phytoplankton in the ocean similarly sequester carbon, and this sometimes also ends up in the earth's lithosphere as oil.

[edit] 2 Human induced sequestration

As well as the natural methods, humans have devised a number of ways of sequestering carbon. The most well known is carbon capture and storage, which uses chemicals to capture the carbon either from powerstations, or the air, and then compresses and stores it in specific geological formations. These methods are fairly untested, and have some potential dangers in that they are possibly prone to quick failure, which could be catastrophic for efforts to reduce global warming.

Humans have also invented a number of methods of increasing the rate at which natural processes sequester carbon. The first is by planting trees. New plantations can seqester a lot of carbon, but the sequestration is only permanent if the plantation isn't later clear felled, as this would relase the carbon back into the atmosphere. Some would be stored in timber, potentially for a long time if used in buildings - archisequestration - but this too would eventually rot or burn, releasing the carbon. Even if the forest is preserved, and never logged again, in most cases, it would be on land that was previously forested, and would only be removing the carbon from the atmosphere that was put there by the clearing process in the first place.

Biochar takes this a step further, by burning the timber (or other plant matter) in a low-oxygen pyrolisis reaction, which produces charcoal. The charcoal is then buried in soil, where it is likely to stay unless disturbed, and where it has potential to increase soil fertility, and hence plant growth. This could be taken a logical step further, and buried in large amounts under many meters of soil, where it might remain undisturbed, and eventually form artificial coal deposits.