Natural Sequestration in Forests and Oceans
Enormous amounts of carbon are naturally stored in the forest by trees and other plants, as well as in the forest soil. As part of photosynthesis, plants absorb carbon dioxide from the atmosphere, store the carbon as sugar, starch and cellulose, while oxygen is released back into the atmosphere. A young forest, composed of rapidly growing trees, absorbs carbon dioxide and acts as a sink. Mature forests, made up of a mix of various aged trees as well as dead and decaying matter may be carbon neutral above ground. In the soil however, the gradual buildup of slowly decaying organic material will continue to accumulate carbon thereby acting as a sink.
Oceans are natural carbon dioxide sinks, and as the level of carbon dioxide increases in the atmosphere, the level in the oceans also increases, creating potentially disastrous acidic oceans. Ocean water can hold a variable amount of dissolved CO2 depending on temperature and pressure. Phytoplankton in the oceans, like trees, use photosynthesis to extract carbon from CO2. They are the starting point of the marine food chain. Plankton and other marine organisms extract CO2 from the ocean water to build their skeletons and shells of the mineral calcite, CaCO3. This removes CO2 from the water and more dissolves in from the atmosphere. These calcite skeletons and shells along with the organic carbon of the organism eventually fall to the bottom of the ocean when the organisms die. The carbon or plankton cells have to sink to the deep water in 2000 to 4000 meter to be sequestered for ca. 1000 years. The sinking can be accelerated orders of magnitude when zooplankton prey on the cells and produce fast sinking fecal pellets or fecal strings, like the Antarctic krill. This process is called the biological pump. It has been theorized that the organic carbon within the accumulating ocean bottom sediments is how fossil fuels are created.
Methods of Enhancing natural sequestration
Forests are carbon dioxide stores, but the sink effect exists only when they grow in size: it is thus naturally limited. The rate at which forests can sequester carbon, given the available land, is far exceeded by the rate at which it is released by the combustion of fossilised forests (coal, oil and natural gas). It seems clear that the use of forests to curb climate change can only be a temporary measure. Even optimistic estimates come to the conclusion that the planting of new forests is not enough to counter-balance the current level of greenhouse gas emissions . To reduce U.S. carbon emissions by 7%, as stipulated in the Kyoto Protocol, would require the planting of "an area the size of Texas every 30 years", according to William H. Schlesinger, dean of the Nicholas School of the environment and earth sciences at Duke University, in Durham, N.C. .
One of the most promising ways to increase the carbon sequestration efficiency of oceans is to add micrometre-sized iron particles called hematite or iron sulfate to the water. This has the effect of stimulating growth of plankton. Iron is an important nutrient for phytoplankton, usually made available via upwelling along the continental shelves, inflows from rivers and streams, as well as deposition of dust suspended in the atmosphere. Natural sources of ocean iron have been declining in recent decades, contributing to an overall decline in ocean productivity (NASA, 2003). Yet in the presence of iron nutrients plankton populations quickly grow, or 'bloom', expanding the base of biomass productivity throughout the region and removing significant quantities of CO2 from the atmosphere via photosynthesis. A test in 2002 in the Southern Ocean around Antarctica suggests that between 10,000 and 100,000 carbon atoms are sunk for each iron atom added to the water
The carbon sequestration potential of soils (by increasing soil organic matter) is substantial; below ground organic carbon storage is more than twice above-ground storage. Soils' organic carbon levels in many agricultural areas have been severely depleted. Improving the humus levels of these soils would both improve soil quality and increase the amount of carbon sequestered in these soils.