How you store carbon depends very much on where you're storing it. For example, the Weyburn-Midale Carbon Dioxide Project compresses CO2 emissions from a coal-fired energy plant in Beulah, N.D., into liquid form and then runs this liquid through a 200-mile (321-kilometer) pipeline that stretches underground from the power plant to a pair of huge, empty oil fields in Midale, Saskatchewan. There, the liquid carbon dioxide is pumped into these empty holes deep in the ground at a rate of about 8,000 tons of carbon dioxide per day. Un-minable coal seams, deep pools of non-potable water, and porous deposits of basalt are natural geologic formations also being explored for their use in carbon sequestration.
Another proposed home for the atmosphere's excess carbon is the oceans. Like planting trees, proponents hope to create blooms of plantlike phytoplankton, which breath in CO2 and breathe out oxygen [source: Nature]. However, encouraging these blooms through, for example, the addition of iron-rich fertilizer may have unintended environmental impacts, including potentially lowering deep water oxygen levels, or the growth of algae types that harm sea life. While ocean sequestration via iron fertilization has been tried in experiments (for example, the 2009 LOHAFEX trial in the Southern Pacific), the ecological concerns leave the procedure in the realm of promise rather than practice.
Or take the case of peat bogs. Generally when a plant dies, it releases its carbon back to the atmosphere as its biomass decays -- but not if the plant dies and sinks into a peat bog, like into quicksand. In that case, the peat bog effectively mummifies the plant, keeping its carbon trapped inside.