Scientists estimate that the global potential of soil carbon sequestration is 0.4 to 1.2 GtC/yr, or an amount equal to roughly 5 to 15% of total man-made CO₂ emissions. To be most effective, CO₂ must be fixed into long-lived pools (or "sinks").
The process of photosynthesis uses sunlight as the energy necessary to convert carbon in the atmosphere into the organic compounds that constitute trees, grasses and other plants, and agricultural crops. CO₂ is released back into the atmosphere when trees are cut down, crops harvested or decayed, and when the soil is disturbed or eroded. A considerable amount of carbon originally contained in soils and vegetation has been released already as a result of deforestation and traditional agricultural practices. The goal of terrestrial sequestration is to reduce the amount of CO₂ that is released while enhancing the storage capability of soils, grazing and crop lands, and trees through changes in management practices.
Terrestrial sequestration can play an important role in stabilizing CO₂ concentrations, particularly in the next three to four decades. Scientists estimate that the terrestrial carbon pool is the third largest (after oceans and geologic processes), and contains about four times as much carbon as is held currently in the atmosphere. Terrestrial sequestration can be implemented rapidly, allowing time for the development of a more robust suite of carbon management technologies. Adoption of recommended management practices provides many ancillary, value-added environmental benefits by reducing soil erosion, improving soil fertility, creating new or improving existing habitats that encourage biodiversity, enhancing water retention and transmission, reducing fire risk and reclaiming degraded lands.
There are many uncertainties and knowledge gaps regarding the impact of climate change on terrestrial carbon pools. One area for research is the development of technologies for precise and reliable ways to quantify the amount of carbon stored in a given ecosystem on a regional or national level. Another key issue concerns permanence. How long can carbon sequestered in soil and plant life remains in these pools in relation to the specific land use and management? Despite the unknowns, however, many known low-cost, low-risk terrestrial approaches that are being implemented currently can make a substantial contribution to meeting any CO₂ reduction goals.
In general, terrestrial carbon sequestration is enhanced by:
- Minimizing soil disturbance and erosion
(for example, by using no-till farming and effective erosion control)
- Maximizing the return of crop
residues to soils
- Maximizing water and nutrient use efficiencies
in crop production
- Growing plants
with a large capacity to store carbon in above-ground and below-ground biomass
- Implementing reduced impact logging techniques
to minimize forest disturbance
(planting new forests) on degraded and marginal agricultural soils.
R&D efforts in terrestrial sequestration seek to increase the rate of CO₂ removal from the atmosphere by natural processes, while considering all the ecological, social and economic implications. There are two fundamental approaches: 1) protecting ecosystems that store carbon so that sequestration can be maintained or increased and 2) manipulating ecosystems to increase carbon sequestration beyond their current ability. Terrestrial sequestration research is focused on integrating measures for improving the full life-cycle carbon uptake of terrestrial ecosystems. It involves a search for ways to achieve greater, more rapid, and longer-lasting sequestration.
The following ecosystems offer significant terrestrial sequestration opportunities:
The focus of research in this area includes rain-fed and irrigated croplands with an emphasis on increasing long-lived soil carbon. Soil is a repository for decaying plant matter and the largest terrestrial storehouse of carbon. Soil management is of central importance. Soils currently are estimated to contain about 82 percent of all terrestrial carbon. Reducing "tillage" to minimally disturb soil before planting crops; returning crop residues to the soil, planting temporarily-retired land with grass to stabilize it; and integrating nutrient and pest management into diversified cropping systems, are approaches for CO₂ sequestration in croplands.
- Degraded soils.
Restoring degraded soils offers significant benefits and carbon sequestration potential.
- Grazing lands.
Grazing lands consist of pasture and range land. Such grasslands encompass large geographic areas. Planting appropriate species with deep root systems and managing grazing, soil fertility and fire frequency are important strategies for grazing lands. In addition to sequestering organic carbon, grazing lands in arid and semi-arid regions also have potential for sequestering inorganic carbon.
- Forest lands.
The focus of research includes below-ground carbon and long-term management. Reforestation (replanting trees where they have been harvested), afforestation (planting new forests) and a variety of forestry management practices present many sequestration opportunities. Practices that encourage trees to store more carbon include: thinning and selective harvesting (which also may reduce the risk of catastrophic forest fires that release carbon stored in trees and soils), the addition of fertilizers, less frequent rotation (harvest) of forests, and site management to reduce soil compaction.
The Midwest Regional Carbon Sequestration Partnership (MRCSP) is coordinating its carbon sequestration research with other groups of researchers. The Partnership's work involves many of the same leading scientists that participate in CSITE (Carbon Sequestration in Terrestrial Ecosystems) and CASMGS (Consortium for Agricultural Soils Mitigation of Greenhouse Gases). CSITE is a research consortium established by the US Department of Energy's Office of Science to examine basic science processes that govern terrestrial sequestration processes. CASMGS is a university-based consortium sponsored by the US Department of Agriculture to study applied aspects of terrestrial sequestration.