Sequestration is the process of isolating a substance and storing it away from its primary environment, effectively moving it into a long-term reservoir. This concept applies broadly across different scientific disciplines, including chemistry, geology, and biology. The fundamental action involves intentionally diverting a material from an active cycle, such as the atmosphere, into a more stable, slow-moving pool. This mechanism is primarily known today for its application in managing atmospheric carbon dioxide.
The Science of Isolation
Understanding sequestration requires distinguishing between the methods used to achieve this isolation, which generally fall into biological, physical, or chemical categories. Biological sequestration relies on living organisms to absorb and store a substance, most commonly seen in the carbon cycle where plants use photosynthesis to draw carbon dioxide from the air. This process converts the atmospheric gas into organic matter, such as wood, leaves, and roots, locking the carbon into biomass.
Physical sequestration involves trapping a substance in a defined space, preventing its escape back into the surrounding environment. An example of this geological approach is the injection of compressed carbon dioxide into deep, sealed underground rock formations, where the surrounding strata act as a permanent barrier. Chemical sequestration involves reacting the substance with another material to create a new, stable compound. This can be seen when carbon dioxide reacts with certain minerals to form solid, inert carbonate salts, effectively converting a gas into a stable rock-like structure.
Nature’s Storage Units
The planet naturally sequesters vast amounts of carbon through interlocking ecological and geological mechanisms. Terrestrial ecosystems, primarily forests and soils, function as major carbon reservoirs, absorbing carbon dioxide from the atmosphere through the growth process of plants. Trees store carbon in their woody biomass, including their trunks, branches, and roots, where it can be held for decades or centuries.
Soil also represents an immense carbon pool, containing approximately 75% of the carbon stored on land. Carbon enters the soil as organic matter, such as fallen leaves, dead roots, and decomposing animal matter, where microbes convert it into stable soil organic carbon. Land management practices, such as reduced tillage and cover cropping, can enhance the soil’s ability to retain this carbon, preventing its release back into the atmosphere.
The oceans are another natural sink, absorbing about one-quarter of the carbon dioxide released by human activities. Oceanic sequestration is driven by two main processes: the biological pump and the solubility pump. The biological pump is driven by tiny marine plants called phytoplankton, which perform photosynthesis in surface waters and incorporate carbon into their bodies. As these organisms die or are consumed, the carbon-rich material sinks as “marine snow” toward the deep ocean, where it is sequestered for hundreds to thousands of years.
The solubility pump is a physical-chemical mechanism that transports dissolved inorganic carbon from the surface to the deep ocean. Carbon dioxide dissolves more readily in colder water, and when cold, dense water sinks at high latitudes, it carries the dissolved gas down into the deep ocean currents. This process is slower than the biological pump but moves enormous quantities of carbon, storing it away from the atmosphere for millennia.
Engineered Solutions
Beyond natural processes, technologies are designed to capture and sequester substances, primarily focusing on carbon dioxide from industrial sources. Carbon Capture and Storage (CCS) involves three distinct steps: capturing the carbon dioxide, transporting it to a storage site, and then securely injecting it deep underground.
Capture often occurs at large industrial point sources, such as power plants or cement factories, where concentrated flue gas streams are treated with chemical solvents to absorb the carbon dioxide. Once captured, the gas is compressed into a liquid-like, supercritical fluid state and transported, often via pipelines, to a suitable geological storage location. These storage sites are typically deep saline aquifers, which are porous rock formations filled with saltwater, or depleted oil and gas reservoirs.
Another approach is Direct Air Capture (DAC), which extracts carbon dioxide directly from the ambient air rather than an industrial source. DAC uses large fans to draw air across chemical materials, either liquid solvents or solid sorbents, which selectively bind with the low-concentration carbon dioxide. The captured gas is then released in a concentrated stream by applying heat. Furthermore, mineralization techniques offer permanent storage by reacting captured carbon dioxide with abundant minerals to form stable, solid carbonate materials, effectively creating synthetic rock.
Environmental Importance
The ability to sequester carbon, both through natural and engineered means, has become central to global strategies for mitigating climate change. Human activities, particularly the burning of fossil fuels, currently release tens of billions of metric tonnes of carbon dioxide into the atmosphere annually. This scale of emission necessitates a dual approach that reduces ongoing emissions while simultaneously removing excess carbon already accumulated in the atmosphere.
Sequestration directly addresses the need for carbon removal, which is viewed as a necessary complement to emission reductions, especially for industries that are difficult to decarbonize. Achieving net-zero emissions, the point where the amount of greenhouse gas released equals the amount removed, depends heavily on scaling up these sequestration efforts.