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“Carbon sequestration” describes the process of capture and long-term storage of atmospheric carbon dioxide (CO2) in a stable state. This process can be direct or indirect, and can be biological, chemical, geological, or physical in nature. When inorganic CO2 is sequestered directly by plants through photosynthesis or through chemical reactions in the soil, this process is often called “carbon fixation”. Biological processes that occur in soils, wetlands, forests, oceans, and other ecosystems can store CO2. These uptake mechanisms are sometimes called “carbon sinks.”
Before the dramatic increase in carbon emissions during the Industrial Revolution, global carbon cycle, or “carbon flux” maintained a near balance between uptake of CO2 (sinks) and its release back into the atmosphere (sources). Today existing uptake mechanisms are insufficient to offset accelerating emissions – for example, USGS reports the United States emits about 1.6 gigatons (billion metric tons) of carbon but uptakes only about 0.5 gigatons, resulting in a net release of about 1.1 gigatons per year. You can learn more about global, national, and state emission trends from the Carbon Dioxide Information Analysis Center here. By applying best management practices to existing systems additional carbon inputs can be stored through activities such as peat production, reforestation, and wetland restoration. Non-biological processes are not detailed in this resource summary, which focuses on terrestrial and aquatic processes relevant to the conservation sector, but you can learn more about point source capture and geological storage processes here. The IPCC offers detailed guidance on the mitigating potential of carbon sequestration and storage interventions for the energy sector here.
When carbon is sequestered directly in the soil, inorganic chemical reactions convert CO2 into inorganic carbon compounds such as calcium and magnesium carbonates. Direct carbon sequestration occurs in plants as they photosynthesize atmospheric CO2 into biomass, which means it is stored in “sinks” instead of being released into Earth’s atmosphere. Subsequently, some of this plant biomass is also indirectly sequestered as soil organic carbon (SOC) during decomposition processes. The amount of carbon sequestered at a project site is determined by several factors, such as soil and forest types, successional stages, and productivity dynamics – it can be difficult to determine, but in general, the total sequestration calculation reflects the long-term balance between carbon uptake and release mechanisms.
As the Soil Science Society of America reports, many agronomic, forestry, and conservation practices, including best management practices, lead to a beneficial net gain in carbon fixation in soil, presenting global opportunities for a soil carbon economy. In the future, as accounting and offset exchanges develop, SOC projects may be viable sources of revenue for land trust managed conservation projects. Similar trends are expected for “Blue Carbon” projects, which refer to offsets from wetlands and aquatic systems such as seagrass habitats. Meanwhile, several conservation groups and land trusts are already taking advantage of existing voluntary carbon markets by implementing forest carbon projects.