Storage in terrestrial and marine environments

Soils

Soils represent a short to long-term carbon storage medium, and contain more carbon than all terrestrial vegetation and the atmosphere combined. Plant litter and other biomass accumulates as organic matter in soils, and is degraded by chemical weathering and biological degradation. More recalcitrant organic carbon polymers such as cellulose, hemi-cellulose, lignin, aliphatic compounds, waxes and terpenoids are collectively retained as humus. Organic matter tends to accumulate in litter and soils of colder regions such as the boreal forests of North America and the Taiga of Russia. Leaf litter and humus are rapidly oxidized and poorly retained in sub-tropical and tropical climate conditions due to high temperatures and extensive leaching by rainfall. Areas where shifting cultivation or slash and burn agriculture are practiced are generally only fertile for 2–3 years before they are abandoned. These tropical jungles are similar to coral reefs in that they are highly efficient at conserving and circulating necessary nutrients, which explains their lushness in a nutrient desert. Much organic carbon retained in many agricultural areas worldwide has been severely depleted due to intensive farming practices.

Grasslands contribute to soil organic matter, stored mainly in their extensive fibrous root mats. Due in part to the climactic conditions of these regions (e.g. cooler temperatures and semi-arid to arid conditions), these soils can accumulate significant quantities of organic matter. This can vary based on rainfall, the length of the winter season, and the frequency of naturally occurring lightning-induced grass-fires. While these fires release carbon dioxide, they improve the quality of the grasslands overall, in turn increasing the amount of carbon retained in the retained humic material. They also deposit carbon directly to the soil in the form of char that does not significantly degrade back to carbon dioxide.

Forest fires release absorbed carbon back into the atmosphere, as does deforestation due to rapidly increased oxidation of soil organic matter.

Organic matter in peat bogs undergoes slow anaerobic decomposition below the surface. This process is slow enough that in many cases the bog grows rapidly and fixes more carbon from the atmosphere than is released. Over time, the peat grows deeper. Peat bogs inter approximately one-quarter of the carbon stored in land plants and soils.

Under some conditions, forests and peat bogs may become sources of CO₂, such as when a forest is flooded by the construction of a hydroelectric dam. Unless the forests and peat are harvested before flooding, the rotting vegetation is a source of CO₂ and methane comparable in magnitude to the amount of carbon released by a fossil-fuel powered plant of equivalent power.

Regenerative agriculture

Current agricultural practices lead to carbon loss from soils. It has been suggested that improved farming practices could return the soils to being a carbon sink. Present worldwide practises of overgrazing are substantially reducing many grasslands performance as carbon sinks. The Rodale Institute says that Regenerative agriculture, if practiced on the planet’s 3.5 billion tillable acres, could sequester up to 40% of current CO₂ emissions. They claim that agricultural carbon sequestration has the potential to mitigate global warming. When using biologically based regenerative practices, this dramatic benefit can be accomplished with no decrease in yields or farmer profits. Organically managed soils can convert carbon dioxide from a greenhouse gas into a food-producing asset.

In 2006, U.S. carbon dioxide emissions from fossil fuel combustion were estimated at nearly 6.5 billion tons. If a 2,000 (lb/ac)/year sequestration rate was achieved on all 434,000,000 acres (1,760,000 km²) of cropland in the United States, nearly 1.6 billion tons of carbon dioxide would be sequestered per year, mitigating close to one quarter of the country's total fossil fuel emissions.

Oceans

Oceans are at present CO₂ sinks, and represent the largest active carbon sink on Earth, absorbing more than a quarter of the carbon dioxide that humans put into the air. On longer timescales they may be both sources and sinks - during ice ages CO₂ levels decrease to ~180 ppmv, and much of this is believed to be stored in the oceans. As ice ages end, CO₂ is released from the oceans and CO₂ levels during previous interglacials have been around ~280 ppmv. This role as a sink for CO₂ is driven by two processes, the solubility pump and the biological pump.^] The former is primarily a function of differential CO₂ solubility in seawater and the thermohaline circulation, while the latter is the sum of a series of biological processes that transport carbon (in organic and inorganic forms) from the surface euphotic zone to the ocean's interior. A small fraction of the organic carbon transported by the biological pump to the seafloor is buried in anoxic conditions under sediments and ultimately forms fossil fuels such as oil and natural gas.

At the present time, approximately one third of human generated emissions are estimated to be entering the ocean. The solubility pump is the primary mechanism driving this, with the biological pump playing a negligible role. This stems from the limitation of the biological pump by ambient light and nutrients required by the phytoplankton that ultimately drive it. Total inorganic carbon is not believed to limit primary production in the oceans, so its increasing availability in the ocean does not directly affect production (the situation on land is different, since enhanced atmospheric levels of CO₂ essentially "fertilize" land plant growth). However, ocean acidification by invading anthropogenic CO₂ may affect the biological pump by negatively impacting calcifying organisms such as coccolithophores, foraminiferans and pteropods. Climate change may also affect the biological pump in the future by warming and stratifying the surface ocean, thus reducing the supply of limiting nutrients to surface waters.

In January 2009, the Monterey Bay Aquarium Research Institute and the National Oceanic and Atmospheric Administration announced a joint study to determine whether the ocean off the California coast was serving as a carbon source or a carbon sink. Principal instrumentation for the study will be self-contained CO₂ monitors placed on buoys in the ocean. They will measure the partial pressure of CO₂ in the ocean and the atmosphere just above the water surface.

In February 2009, Science Daily reported that the Southern Indian Ocean is becoming less effective at absorbing carbon dioxide due to changes to the regions climate which include higher wind speeds.